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Review

Heterometallic Complexes Containing the NiII-LnIII-NiII Moiety—Structures and Magnetic Properties

by
Catherine P. Raptopoulou
Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Aghia Paraskevi, 15310 Athens, Greece
Crystals 2020, 10(12), 1117; https://doi.org/10.3390/cryst10121117
Submission received: 19 November 2020 / Revised: 1 December 2020 / Accepted: 4 December 2020 / Published: 8 December 2020
(This article belongs to the Special Issue A 10 Years Journey: Chemical, Physical, and Biological Properties and Applications of Crystals)
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Abstract

:
This review summarizes the structural characteristics and magnetic properties of trinuclear complexes containing the NiII-LnIII-NiII moiety and also oligonuclear complexes and coordination polymers containing the same trinuclear moiety. The ligands used are mainly polydentate Schiff base ligands and reduced Schiff base ligands and, in some cases, oximato, β-diketonato, pyridyl ketone ligands and others. The compounds reported are restricted to those containing one, two and three oxygen atoms as bridges between the metal ions; examples of carboxylato and oximato bridging are also included due to structural similarity. The magnetic properties of the complexes range from ferro- to antiferromagnetic depending on the nature of the lanthanide ion.

Graphical Abstract

1. Introduction

The field of Molecular Magnetism emerged as an interdisciplinary field of research between chemists willing to understand the experimental techniques and theoretical tools needed to explain the magnetic properties of coordination compounds and physicists willing to comprehend the molecular structures and intra/intermolecular interactions responsible for the magnetic properties of the above solids. Coordination compounds of various nuclearities and dimensionalities have been the experimental objects for magnetic properties studies and classes of materials with specific properties have been reported, for example Single-Molecule Magnets (SMMs), Single-Ion Magnets (SIMs) and Single-Chain Magnets (SCMs). SMMs can be considered as single molecules behaving like the ‘magnetic domains’ of bulk magnets and each molecule can be magnetized and retaining its magnetization upon removal of the external magnetic field. Experimentally this can be detected by the presence of out-of-phase signals in ac magnetic susceptibility studies and it was reported for the first time for the ‘archetype SMM’ compound [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O, known as [Mn12] [1]. The complex consists of a cubane unit of four MnIV ions surrounded by a ring of eight MnIII ions resulting in a ground state of spin S = 10 compatible with the alignment of all MnIII spins up and all MnIV spins down [2,3]. Another well studied SMM in the beginning of Molecular Magnetism was [Fe8O2(OH)12(tacn)6Br7(H2O)]Br.8H2O (tacn = 1,4,7-triazacyclononane), known as [Fe8], with ground spin state S = 10 corresponding to six FeIII spins up and two FeIII spins down [4,5]. The term SMM is used for oligo/polynuclear coordination clusters, whereas the term SIM was established for mononuclear complexes whose magnetic properties are governed by a single ion. The first 4f SIM is a TbIII double-decker phthalocyanine complex reported in 2003 [6] and the first 3d SIM is a linear FeI complex reported in 2013 [7]. SCMs are ferromagnetic chains that do not interact magnetically with each other and were first realized in a 1D coordination polymer consisting of Co(hfac)2 and a radical ligand which behaves as 1D ferrimagnet [8].
Fundamental in Molecular Magnetism is the presence of unpaired electrons that must be interact with other unpaired electrons occupying p, d and f magnetic orbitals, originating from organic radical ions, transition metal ions and lanthanide metal ions, respectively. The versatile chemistry of transition metal ions offers structural variety of coordination compounds with a wide range of unusual magnetic properties, related to the various oxidation states and electronic properties of the cations, splitting of the d orbitals upon complexation and variability of coordination geometries. Lanthanides display unique magnetic properties because of the contributions from the spin and orbit motion of the electrons and magnetocrystalline anisotropy (single-ion anisotropy) associated with crystal-field and spin-orbit coupling. The chemistry of lanthanides differs significantly from that of transition metals due to the shielding of the f electrons by the external 5s2 and 5p6 shells. As a consequence, the f electrons do not contribute to bonding upon coordination and therefore, the interactions between the f electrons and ligand orbitals are very weak and only strong electronegative atoms are able to bind to a lanthanide ion. Another important aspect is that all the 4f ions are able to form complexes without substantial geometrical differences in the geometrical environment, which is extremely helpful in molecular magnetism in order to study complexes of different electronic structures albeit of similar geometrical characteristics. The interest on heterometallic transition metal (TM) and lanthanide (Ln) ions complexes is based on the possibility to achieve high spin ground states along with large single-ion anisotropy that can potentially provide SMM behavior.
The synthetic approaches to heterometallic TM/Ln complexes involve two strategies, the ‘metal complexes as ligands’ and the ‘one-pot.’ The first, uses mono- or di-nuclear complexes of TM (metalloligands) with uncoordinated O-donor sites that can react with the Ln ions. The second, requires a mixture of TM and Ln salts and organic ligands containing distinct coordination compartments (pockets) for the preferential binding of the metal ions. The HSAB model plays an important role in this approach because Ln ions are hard Lewis acids and prefer hard O-sites whereas TM are soft and borderline Lewis acids and prefer N-atoms or purely soft bases (i.e., S-atoms) [9]. For example, the mononuclear octahedral complex [Ni(mpko)2(mpkoH)] (mpkoH = methyl 2-pyridyl ketone oxime) was used as metalloligand according to the ‘metal complexes as ligands’ strategy to obtain dinuclear [NiLn(NO3)3(mpko)2(mpkoH)], trinuclear [Ni2Ln(mpko)6](NO3) and tetranuclear complexes [Ni2Ln2(NO3)4(mpko)6] (Ln = Gd-Er) [10,11]. Among the ligands containing coordination compartments for the ‘one-pot’ strategy, Schiff bases have been widely used because they can be easily prepared and modified with the desired substituents and functional groups. For example, the heterometallic complex [CuTbL(hfac)2]2 (H3L the Schiff base ligand 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxy-benzylidenamino)ethane, Hhfac = hexafluoroacetylacetone) was the first SMM containing Ln ions [12]. Soon after the observation of ferromagnetic interactions in a Cu2Gd complex [13], synthetic strategies were developed for the designed synthesis of dinuclear TM/Ln complexes in order to investigate the nature and magnitude of the magnetic interactions between the metal ions [14,15]. Trinuclear TM2Ln complexes have been also reported and some have been found to display SMM properties [16].
The present review emphasizes on the reported trinuclear Ni-Ln-Ni complexes, oligonuclear complexes and coordination polymers containing the Ni-Ln-Ni moiety, their structural characteristics and magnetic behavior which ranges from ferro-to antiferromagnetic depending on the nature of the lanthanide ion. The ligands used are mainly Schiff bases based on salicylaldehyde, o-vanillin and tripodal, dipodal aliphatic and aromatic amines; few examples are reported with diketonato or other type of ligands.

2. Discussion

The review describes the trinuclear complexes that contain the NiII-LnIII-NiII moiety classified according to the type of organic ligands used in Section 2.1. The oligonuclear complexes and the coordination polymers that contain the NiII-LnIII-NiII moiety are presented in Section 2.2 and Section 2.3. respectively. The structural characteristics of the complexes, magnetic exchange parameters, energy barriers and pre-exponential factors of the reported complexes are given in Table 1, Table 2 and Table 3.

2.1. Trinuclear NiII-LnIII-NiII Complexes

2.1.1. Tripodal Polydentate Schiff Base Ligands and Reduced Schiff Base Ligands

A new phosphorus-supported ligand H3L1 = (S)P[M(Me)N=CH-C6H3-2-OH-3-OMe]3 (Scheme 1), prepared by the condensation of (S)P[M(Me)NH2]3 and o-vanillin, was used to synthesize the trinuclear isomorphous complexes [(NiL1)2Ln](ClO4) (Ln = La-Er, except Pm, 1–11) [17]. The cation consists of three nearly linear metal ions, two terminal NiII and one LnIII in the center (Figure 1). Each of the terminal NiII is bound to the three imino nitrogen and the three phenolato oxygen atoms of one (L1)3−, thus describing an in situ formed [NiL1]− metalloligand. Two such metalloligands are bound to the central LnIII ion through the phenolato and methoxy oxygen atoms of the two ligands, describing a distorted icosahedral. The Ln-O bond distances show a gradual reduction in accordance with lanthanide contraction. The Ni2Gd (7), Ni2Dy (9) and Ni2Er (11) complexes display ferromagnetic interactions between the metal ions. The magnetic susceptibility measurements of 7 were interpreted by using the spin Hamiltonian H =   − 2 J ( S N i 1 S G d + S G d S N i 2 ) where SNi1 = SNi2 = 1 and SGd = 7/2. The best set of parameters obtained using this model is J/kB = +0.375 cm−1 and g = 2.04. The magnetization measurements of 7 as a function of the field show relatively rapid saturation of the magnetization at high fields and agree with an S = 11/2 spin ground state. Ac susceptibility measurements of the Ni2Dy complex (9) as a function of the temperature at different frequencies and also as a function of the frequency at different temperatures under zero and 3500 Oe dc field showed that 9 exhibits slow relaxation of the magnetization and observation of field induced single-molecule magnet behavior. The data were fitted to an Arrhenius law in order to estimate the energy gap of 10.8 K and pre-exponential factor τ0 = 2.3 × 10−5 s. The value of 10.8 K is in the range observed for similar complexes, however the value of τ0 is much larger than expected suggesting that the quantum pathway of relaxation is only partially suppressed by the applied field of 3500 Oe and hence that the energy gap of the thermally activated relaxation should be higher than 10.8 K. In any case, the magnetic study of 9 suggests SMM behavior generated by the high spin ground state of the complex and the magnetic anisotropy of the DyIII ion. All other complexes behave as simple paramagnetic systems. DFT calculations on the Ni2Gd (7) and Ni2La (1) complexes revealed good agreement between the experimental and computed J values, confirmed the ferro- and antiferromagnetic nature of the JNiGd and JNiNi interactions, respectively and gave information on the spin densities of the metal ions and bridging oxygen atoms [18].
The tripodal hexadentate Schiff base-phenolate ligand H3L2 = 2,2’-((1E)-((2-(((E)-(2-hydroxybenzylidene)amino)methyl)-2-methylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylid ene))diphenol (Scheme 1) was used to synthesize the neutral trinuclear complexes [(NiL2)2Ln(NO3)] (LnIII = Gd, Eu, Tb, Dy, 12–15) [19,20] which show a Ni-Ln-Ni angle of ca. 140° (Figure 2). All four complexes have similar structures with the two terminal NiII ions coordinated to the three imino nitrogen and three phenolato oxygen atoms of one (L2)3−. The coordination geometry of each NiII ion is distorted from a regular octahedron toward a trigonal prism with trigonal twist angle ϕ ~45° (trigonal prism = 0°, octahedron = 60°). The central LnIII ion is bound to the phenolato oxygen atoms of the ligands and to chelate nitrato ligand. All complexes exhibit 3D structures due to intermolecular π-π and CH-π interactions between neighboring molecules. The magnetic data for the Ni2Gd (12) complex are consistent with ferromagnetic coupling between the metal ions giving rise to a ground state with spin S = 11/2. The best-fit parameters to the experimental magnetic susceptibility data of 12 were g = 2.24, J(Ni-Gd) = +0.19 cm−1 and D = +2.1 cm−1. A ferromagnetic interaction is also suggested for the Ni2Tb (14) and Ni2Dy (15) complexes.
The complex [(NiL3)2Gd](NO3) (16) (H3L3 = 6,6’-((1E)-((2-(((E)-(2-hydroxy-3-methoxybenzylidene)amino)methyl)-2-methylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylid ene))bis(2-methoxyphenol), Scheme 1) contains also two [NiL3]− metalloligands bound to a GdIII ion in an almost linear arrangement (~178°) [21]. The asymmetric unit contains two different cationic entities, the first one possesses two slightly different NiII environments, while the second one is symmetry-related through the GdIII ion (Supplementary Materials Figure S1). The coordination geometry around each NiII ion is distorted octahedral with N3O3 chromophore. The GdIII ion is coordinated to twelve oxygen atoms, deprotonated phenoxo oxygens and neutral methoxy oxygens. The magnetic susceptibility data were interpreted in terms of the spin Hamiltonian H = − J N i G d ( S N i 1 S G d + S N i 2 S G d ) + D ( S z 2 N i 1 + S z 2 N i 2 ) considering two equivalent Ni1-Gd and Ni2-Gd exchange interactions J and identical ZFS terms D. The best-fit parameters are JNiGd = 0.91 cm−1, g = 1.98 and D = 4.5 cm−1. The magnetization measurements at 2 K in the range 0–5 T were satisfactorily simulated with this set of parameters and confirmed an S = 11/2 ground state due to ferromagnetic coupling between the metal ions.
The tripodal ligand H3L4 = 6,6’,6’’-((1E,1’E)-((nitrilotris(ethane-2,1-diyl))tris (azanylylidene)) tris(methanylylidene))tris(2-methoxyphenol) (Scheme 1) gave four heterometallic complexes, [(NiL4)2Ln](NO3) (LnIII = Gd, Tb, Dy, 17–19) and [(NiL4)2Dy](ClO4) (20) which contain linear Ni-Ln-Ni moieties (Figure 3) [22]. The GdIII ion exhibits rare seven-coordination to six bridging phenoxo oxygen atoms and one methoxy oxygen atom from one of the (L4)3− ligands and can be considered as an intermediate between the capped trigonal prism (CTPR-7, C2v) and the capped octahedron (CTPR-7, C3v). The LnIII ions in the remaining three complexes exhibit rare six-coordination which can be described as quasi trigonal antiprism (intermediate between octahedron OC-6, Oh and trigonal prism TPR-6 D3h). The magnetic susceptibility measurements of 17 were interpreted by using the spin Hamiltonian H ^ = − 2 J S ^ G d 1 ( S ^ N i 1 + S ^ N i 2 ) and gave the best-fit parameters g = 2.04, J = 0.64 cm−1 and zJ’ = 0.009 cm−1 (R = 6.99 × 10−3). The magnetization at 1.8 K increases upon increasing the magnetic field and reaches a value of 11.87 Nβ at 7 T consistent with an S = 11/2 ground spin state. The magnetic study of 18–20 is consistent with ferromagnetic coupling between the metal ions. Ac susceptibility measurements of 18–20 between 1.8–10.0 K and frequencies 3–969 Hz under zero dc field showed temperature- and frequency-dependent out-of-phase peaks for 19 and 20 suggesting SMM behavior. The relaxation time derived from the χ’’ peaks follows the Arrhenius law τ = τ0exp(Δ/kBT) with effective energy barriers of 14.17 K (τ0 = 1.09 × 10−6 s) for 19 and 11.13 K (τ0 = 6.72 × 10−6 s) for 20 under zero dc field. Complexes 19 and 20 constitute rare examples of SMMs containing six-coordinate DyIII ions.
The tripodal hexadentate amine phenol ligand H3L5 = 2,2′-(((2-(((2-hydroxybenzyl)amino)methyl)-2-methylpropane-1,3-diyl)bis(azanediyl))bis(methylen e))diphenol (Scheme 1), which is the reduced form of ligand H3L2, gave a series of isostructural complexes [(NiL5)2Ln(solv)x)](ClO4) (LnIII = La, Pr, Nd, Gd, Dy, Ho, Er, Yb; 21–28; x = 2, H2O/(MeOH)0.5/(EtOH)0.5 in Ni2La, 21; x = 2, H2O/MeOH in Ni2Dy, 25; x = 1, H2O in Ni2Yb, 28) [23]. The complexes consist of two [NiL5]− metalloligands bound around the LnIII ions with bent Ni-Ln-Ni moiety. The LaIII ion in 21 is eight-coordinate to two (L5)3− ligands which are tridentate with respect to the LaIII ion and hexadentate with respect to one NiII ion (Figure S2). Two solvate molecules complete the coordination of the LaIII ion which is described as a D4d square antiprism distorted toward a C2v bicapped octahedron. The DyIII ion is seven-coordinate to two [NiL5]− metalloligands, one bidentate and one tridentate and to two solvate molecules in a capped trigonal prismatic geometry. The YbIII ion is six-coordinate to two [NiL5]− metalloligands with a distorted octahedral geometry. The magnetic studies indicated that antiferromagnetic exchange coupling between the NiII and LnIII ions increases with decreasing size of LnIII.
The tripodal hexadentate amine phenol ligand H3L6 = 2,2′,2′′-(((nitrilotris(ethane-2,1-diyl))tris(azanediyl))tris(methylene))triphenol (Scheme 1) gave two series of isostructural complexes, [(NiL6)2Ln(MeOH)](NO3) (all LnIII except Ce and Pm, 29–41) and [(NiL6)2Ln(MeOH)](ClO4) (LnIII = La, Pr, Nd, Sm, Gd, Dy, Ho, Er, 42–49), which contain a bent Ni-Ln-Ni moiety with angles in the range ~139–144° [24,25]. In all structurally characterized complexes, the LnIII ion is seven-coordinate being bicapped by two tridentate [NiL6]− metalloligands and a methanol molecule in flattened pentagonal bipyramidal geometry. Each NiII ion is encapsulated by a full deprotonated ligand via four amine and two phenolato functions in approximately octahedral geometry (Figure S3). It is recognized that the coordination number of LnIII ions tends to decrease with increased atomic number, that is, as the ionic radius decreases. However, in the present case, the coordination number and geometry of the LnIII ions do not change along the entire Ln series plus LaIII. Magnetic studies indicated that ferromagnetic exchange occurs in the case of Ni2Ln with LnIII = Gd, Tb, Dy, Ho, Er.
The congener ligand H3L7 = 6,6′,6′′-(((nitrilotris(ethane-2,1-diyl))tris(azane diyl))tris(methyl ene))tris(2-methoxyphenol) (Scheme 1) gave two isomorphous complexes [(NiL7)2Ln)](ClO4) (LnIII = Gd, Dy, 50–51) which contain a bent Ni-Ln-Ni moiety with angle ~113° [26]. The three metal ions are arranged in an isosceles triangle manner (Figure 4). The two terminal NiII ions are coordinated to four amine and two phenolate groups, whereas the central LnIII ion is eight-coordinated to six bridging phenolato oxygen atoms and two methoxy oxygen atoms from the ligands presenting distorted dodecahedral geometry. The magnetic studies showed that both complexes exhibit ferromagnetic coupling between the metal ions. The magnetic susceptibility data of 50 were analyzed on the basis of the spin Hamiltonian H ^ = − 2 J ( S ^ N i 1 S ^ G d + S ^ G d S ^ N i 2 ) − 2 J ′ S ^ N i 1 S ^ N i 2 (J′ is assumed to be zero due to large distance between the NiII ions) and gave J/k = 1.02 cm−1 and g = 2.01. At 2 K, the magnetization is 11.04 Nβ at 5 T which agrees with the saturation value of 11.94 Nβ expected for an S = 11/2 system, confirming the ferromagnetic interaction between the NiII and GdIII ions. The magnetization at 5 T for 51 is 12.65 Nβ and is larger than the theoretical value of 12 Nβ due to the presence of the anisotropic DyIII ion. The Ni2Dy complex 51 exhibited very weak field-induced slow relaxation of magnetization.
The ligand H3L8 = 2,2′,2′′-((1,4,7-triazonane-1,4,7-triyl)tris(methylene))triphenol (Scheme 1) gave the trinuclear complexes [(NiL8)2Ln(MeCN)2](ClO4) (LnIII = La, Nd, Gd, Dy, Yb, 52–55) and [(NiL8)2Yb](ClO4) (56) [27]. The GdIII ion in 54 is eight-coordinate in square antiprism distorted to C2v bicapped trigonal prism geometry, bicapped by two deprotonated tridentate (NiL8)− metalloligands (Figure 5). Each NiII is distorted octahedral in N3O3 coordination. The Ni-Gd-Ni angle is ~142°. The YbIII ion in 56 is six-coordinated by six bridging phenolate oxygens (Figure 8). The decrease of the coordination number with increasing atomic number is common in LnIII complexes. The complex is linear with Ni-Yb-Ni angle ~176°. Magnetic studies indicated ferromagnetic interactions for LnIII = Gd, Dy, Yb and antiferromagnetic coupling for LnIII = Nd.
The ligand H3L9 = 6,6′,6′′-((1,4,7-triazonane-1,4,7-triyl)tris(methylene))tris(2-methoxy-3-methylphenol) (Scheme 1) gave a series of 13 linear trinuclear complexes [(NiL9)2Ln(solv)x](ClO4) (LnIII = Y, La, Ce-Lu except Pr, Pm, Yb; x = 1, H2O in Ni2Sm, Ni2Eu, Ni2Tb; x = 0 in all other complexes, 57–69) [28]. For complexes with LnIII = Sm, Eu, Tb, the central lanthanide is seven-coordinate showing monocapped trigonal prismatic geometry (Figure S4). For the rest of the complexes, the central lanthanide as well as the terminal NiII ions are six-coordinate with coordination geometry between trigonal antiprism and trigonal prism and distorted octahedral, respectively. The Ni-Ln-Ni angles are in the range 165–179°. For 57, 58 and 69 the χMT at 300 K is 2.48 cm3Kmol−1, 2.33 cm3Kmol−1 and 2.76 cm3Kmol−1, higher than the theoretically expected value of 2.0 cm3Kmol−1 because the diamagnetic LnIII ions may induce small geometric variations and different ligand fields at the NiII ions, thus affecting the magnetic properties of the three complexes. The reduced magnetization for 57 and 69 shows a splitting of the isofield lines, which indicates a zero-field splitting. The best fit leads to JNi-Ni = −0.294 cm−1, gNi = 2.09, DNi = 1.933 cm−1 and E/D = 0.154 for 57 and JNi-Ni = −0.129 cm−1, gNi = 2.18, DNi = 2.838 cm−1 and E/D = 0.468 for 69. The fit of the magnetic data of the Ni2Gd complex 63 gave JNi-Ni = −0.377 cm−1, JGd-Ni = −0.009 cm−1, gNi = 2.102, gGd = 1.974 and χTIP = 0.003 cm3Kmol−1. Paramagnetic nuclear magnetic resonance (NMR) spectroscopy showed that in solution all complexes are isostructural showing the expected D3 symmetry for all metal ions being six-coordinate. These complexes have small magnetic anisotropies and the NMR data agree with the solid state SQUID measurements.
The ligand H3L10 = 6,6′,6′′-((1,4,7-triazonane-1,4,7-triyl)tris(methylene))tris(2,3-dimethylphenol) (Scheme 1) was used to prepare the linear complex [(NiL10)2Tb](ClO4) (70) [29]. Each NiII ion is coordinated by three nitrogen and three phenolate oxygen atoms in octahedral geometry and the TbIII ion is bound to six phenolate oxygen atoms in octahedral geometry also (Figure S5). The magnetic studies revealed ferromagnetic interactions between the adjacent NiII and TbIII ions.

2.1.2. Other Schiff Base Ligands

The Schiff base ligand HL11 = (Z)-2-methoxy-6-((phenylimino)methyl)phenol (Scheme 2) derived from the condensation of o-vanillin with aniline, gave the linear trinuclear complexes [(NiL113)2Ln](NO3) (LnIII = La, Pr, Gd, Tb, 71–74) [30,31] which contain two terminal NiII ions in octahedral N3O3 coordination and a central LnIII ion bound to six phenolato and six methoxy oxygen atoms from six (L11)− ligands. The central LnIII ion sits on inversion center and displays distorted icosahedron geometry (Figure 6). The Ni2Gd complex 73 displays ferromagnetic coupling giving a ground spin state value of S = 11/2. Simultaneous fitting to χMT(T) and isothermal M(H) plots by using the spin Hamiltonian H = − 2 J 1 ( S N i 1 S G d 1 + S G d 1 S N i 1 A ) and considering a single unique Ni-Gd interaction J1, gave J1 = +0.54 cm−1 with fixed g = 2.01. Q-band EPR spectra of polycrystalline 73 at 5 K can be reproduced by a model with S = 11/2 and ZFS parameters D = −0.135 cm−1, E/D = 0.004 with g = 2.05. The magnetocaloric efficiency of the Ni2Gd cluster 73 was studied for the first time for a linear Ni2Gd cluster, via heat capacity and isothermal magnetization measurements which revealed a value of 13.74 Jkg−1K−1 for the magnetic entropy change at 4 K and ΔH = 7 T. Weak ferromagnetic exchange between the NiII ions is found in the Ni2La complex 71. The experimental data were fitted by considering the spin Hamiltonian H = − 2 J S N i 1 S N i 2 + 2 D N i S N i z 2 considering the zero-field splitting parameter D and gave J = +0.46 cm−1, g = 2.245, D = +4.91 cm−1. Dc magnetic susceptibility measurements revealed that the NiII ions are coupled ferromagnetically with the TbIII ion in 74 and antiferromagnetically with the PrIII ion in 72. Ac susceptibility measurements performed on the Ni2Tb complex 74 under zero dc field and under H = 0.5 T revealed the onset of frequency dependent χ’’M signals indicating the possibility of SMM behavior. The absence of clear maxima in the χ’’M(T) plots down to 2 K indicates fast magnetic relaxation or fast QTM or perhaps both.
The Schiff base ligand H2L12 = (Z)-2-(((2-(hydroxymethyl)phenyl)imino)methyl)-6-methoxyphenol (Scheme 2) gave, among others, the linear trinuclear complex [{Ni(HL12)2}2La(NO3)](NO3)2 (75) which contains two NiII ions in distorted octahedral N2O4 environment and a central LaIII ion bound to four phenolato and four methoxy oxygen atoms from four (HL12)− ligands and two oxygen atoms from a chelate nitrate (Figure S6) [32]. The coordination geometry around the LaIII ion is best described as sphenocorona JSPC-10 (CShM = 3.37915). Weak antiferromagnetic exchange between the NiII ions is found in the complex via the closed shell LaIII ion. The fit of the χMT data from 150 K to 2 K using a HDVV Hamiltonian yielded parameters J = −0.978 cm−1, g = 2.177, D = 3.133 cm−1.
The pentadentate Schiff base ligand H4L13 = (Z)-2-((2-hydroxy-3-(hydroxymethyl)-5-methylbenzylidene)amino)-2-methylpropane-1,3-diol (Scheme 2) was used to prepare a family of isostructural complexes [{Ni(H3L13)2}2Ln](NO3)3 (LnIII = Gd, Tb, Dy, Ho, 76–79) with linear metal arrangement (Figure 7) [33]. Each of the terminal NiII ions is distorted octahedral in N2O4 environment; the four oxygen atoms are derived from two phenolates and two pendant -CH2OH arms of the two different (H3L13)− ligands. The two imino nitrogen atoms around Ni are trans with respect to each other. The central LnIII ion is coordinated to eight oxygen atoms (four benzyl alcohol groups and four phenolato O-atoms) in square antiprismatic geometry. Each Ni2Ln complex interacts with four neighboring molecules through the -CH2OH groups of the ligands and the NO3− counteranions, leading to 2D H-bonded network along the ab plane. The static magnetic properties of all four complexes showed a predominant ferromagnetic interaction between the metal ions and only the Ni2Dy complex exhibited frequency dependent tails in the χ’’M vs T plots under zero dc field. The magnetic properties of complex 76 were analyzed by using the spin Hamiltonian H = − J e x ( S N i 1 S G d + S N i 2 S G d ) − D ( S z 2 N i 1 + S z 2 N i 2 ) assuming two equivalent Ni(O)2Gd bridging halves. The best-fit parameters are Jex = +0.67 cm−1, g = 2.117, D = 4.92 cm−1 (R = 7 × 10−7). The magnetocaloric properties of the Ni2Gd were estimated from the experimental isothermal field-dependent magnetization data yielding −ΔSm = 11.85 Jkg−1K−1 at 4 K and ΔH = 5 T.
The Schiff base ligand H4L14 = (Z)-2-((2-hydroxybenzylidene)amino)-2-(hydroxymethyl)propane-1,3-diol (Scheme 2) afforded four isostructural complexes with formula [{Ni(H3L14)2}2Ln(O2CMe)2](NO3)3 (LnIII = Sm, Eu, Gd, Tb, 80–83) with strictly linear metal arrangement (Figure 8) [34]. The two NiII ions display N2O4 distorted octahedral coordination and the central LnIII ion is coordinated to four phenolato oxygen atoms from four (H3L14)− ligands and four carboxylato oxygen atoms from two chelate acetates describing square antiprismatic geometry. The magnetic susceptibility data of 80 revealed weak antiferromagnetic coupling between the two NiII ions with best fit parameters J = −0.37 cm−1, g = 1.97 and TIP = 0.001 cm3mol−1. The χMT product of 81 at 300 K is higher than expected and can be explained by assuming that the first excited states for the EuIII ion are populated at r.t. because they are very close to the ground state. The magnetic susceptibility data of 82 revealed dominant ferromagnetic interactions between the NiII and GdIII ions and were fitted by using the spin Hamiltonian H = − 2 J N i G d ( S N i 1 S G d + S N i 2 S G d ) + D ( S N i 1 2 + S N i 2 2 ) yielding JNiGd = +0.42 cm−1, D = +2.95 cm−1 (gNi = gGd = 1.98), resulting in S = 11/2 spin ground state. The magnetocaloric effect of 82 was determined by isothermal magnetization measurements in the temperature range 2–12.5 K under applied magnetic fields up to 5 T with −ΔSm = 14.2 Jkg−1K−1 at 2 K. The magnetic susceptibility data for 83 are consistent with dominant antiferromagnetic interactions between the metal ions and/or thermal depopulation of the TbIII excited states.
The hexadentate Schiff base ligand H2L15 = 6,6′-((1E,1’E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2-ethoxyphenol) (Scheme 2) gave two isostructural trinuclear complexes [(NiL15)2Ln(NO3)2](NO3) (LnIII = La, Ce, 84–85) with bent Ni-Ln-Ni arrangement (Figure S7) [35]. The central LnIII ion is bound to four phenolato and four ethoxy oxygen atoms as well as to two chelate NO3− anions in distorted icosahedron geometry. Each of the terminal NiII ions is coordinated to the two imino nitrogen and two phenolato oxygen atoms of the ligand in square planar geometry. The LnIII ion is bridged to each of the NiII ions via two phenolato and two ethoxy oxygen atoms. Both complexes showed antimicrobial activity on cultures of E. coli, S. aureus and CA. The congener ligand H2L16 (Scheme 2) gave a similar Ni2Ce complex with Ni-Ce-Ni angle of ~62° [36].
The ligand H2L17 = 6,6′-((1E,1’E)-(propane-1,3-diylbis(azanylylidene))bis (methanylylidene))bis(4-bromo-2-methoxyphenol) (Scheme 2) gave four isostructural trinuclear complexes [(NiL17)2Ln(O2CMe)2(MeOH)2](NO3) (LnIII = La, Nd, Ce, Pr, 87–90) [37,38] which contain a central LnIII ion in an inversion center bound to four phenolato and four methoxy oxygen atoms as well as to two acetato oxygen atoms from two carboxylato ligands in pentagonal antiprismatic geometry (Figure 9). Each NiII ion occupies the N2O2 compartment of the ligand and has distorted octahedral geometry with MeOH and bridging acetato oxygen atoms in the apical positions. The LnIII ion is bridged to each of the NiII ions via the two phenolato oxygen atoms and the acetato group. Weak antiferromagnetic coupling between the NiII ions through the diamagnetic LaIII ion was found in the Ni2La complex 87. The magnetic susceptibility data were interpreted based on the isotropic Heisenberg model ( H = − 2 J S → N i 1 S → N i 2 ) and the best least-squares fit yielded J = −0.75 cm−1, g = 2.18. The susceptibility data of the Ni2Nd, Ni2Ce and Ni2Pr complexes 88, 89, 90 respectively obey the Curie-Weiss law with the Curie constant of C = 3.71 cm3 K mol−1 and the Weiss constant of θ = −7.4 K for 88, C = 3.23 cm3 K mol−1 and θ = −9.9 K for 89 and C = 4.05 cm3 K mol−1 and θ= −25.5 K for 90. The negative values of Weiss constant confirm the antiferromagnetic exchange coupling between the metal ions. For complexes 89 and 90, the crystal field parameters for the CeIII/PrIII ions and the exchange coupling constant were estimated by using the generalized van Vleck formalism. The best fit yielded JNiCe = −1.1(4) cm−1, gNi = 2.23(3), D = 6.3(4) cm−1, A02<r2> = −265(10) cm−1, A04<r4> = 291(6) cm−1 for 89 and JNiPr = −1.3(8) cm−1, gNi = 2.15(2), D = 7.1(4) cm−1, A02<r2> = −310(9) cm−1, A04<r4> = 2335(11) cm−1, A06<r6> = 80(8) cm−1 for 90.
The tetradentate Schiff base ligand H2L18 = 2,2′-((1E,1’E)-(propane-1,3-diylbis (azanylylidene))bis(ethan-1-yl-1-ylidene))diphenol (Scheme 2) gave a trinuclear complex [(NiL18)2Ce(NO3)3] (91) with a central CeIII ion bound to two terminal [Ni(L18)] metalloligands in a transoid orientation to the central lanthanide ion (Figure S8) [39]. The CeIII ion is ten-coordinate to four phenolato oxygen atoms and to three chelate nitrates, the coordination polyhedron can be described as distorted tetradecahedron. Each NiII ion is in N2O2 square planar geometry. The Ni-Ce-Ni moiety is bent with angle ~122.5°.
The ligand H2L19 = 6,6′-((1E,1’E)-((2,2-dimethylpropane-1,3-diyl)bis(azanyl ylidene))bis(methanyl ylidene))bis(2-methoxyphenol) (Scheme 2) gave the trinuclear complexes [{NiL19(H2O)}2Ln(H2O)](trif)3 (LnIII = Gd, Eu, 92–93; trif = triflate anion) which contain a nine-coordinate central LnIII ion bound to four phenolato and four methoxy oxygen atoms from two (L19)2− ligands and a water molecule (Figure 10) [21,40]. Each of the terminal NiII ions is five-coordinate, linked to the N2O2 site of the ligand in the equatorial plane and to a water molecule in the apical position. The magnetic susceptibility data of 92 were fitted considering two different J parameters for the Ni-Gd magnetic exchange and an equivalent D term for both nickel ions, according to the spin Hamiltonian H = − J N i G d ( S N i 1 S G d ) − j N i G d ( S N i 2 S G d ) + D ( S z 2 N i 1 + S z 2 N i 2 ) . The best fit yielded JNiGd = 4.8(3) cm−1, jNiGd = 0.05(2) cm−1, g = 2.03(1) and D = 0.03(1) cm−1 (R = 1 × 10−5). A very similar J value (0.5 cm−1) yielded when the data were fitted without j and D parameters. The observed magnetization of 9.3 Nβ at 5 T was explained considering a ferromagnetic Ni-Gd dinuclear unit plus a mononuclear pentacoordinate Ni ion having a large magnetic anisotropy due to zero field splitting. The magnetization curve was fitted with J = 5 cm−1, j = 0, D = 12.4 cm−1, gNi = 2.16 and gGd = 2.00. The magnetic susceptibility data of 93 is governed by at least two magnetic phenomena that is, the depopulation of the Stark levels of the EuIII ion and the zero-field splitting of the NiII ions at very low temperatures.

2.1.3. Miscellaneous Ligands

The dipodal Schiff base ligand H4L20 = 3,3′-((1E,1’E)-((((2-aminoethyl)azanediyl)bis(ethane-2,1-diyl))bis(azanylylidene))bis(methanylylidene)) bis(2-hydroxybenzoic acid) (Scheme 3) gave a family of isomorphous V-shaped trinuclear complexes, [{Ni(H2L20)(tren)2}2Ln](NO3)3 [LnIII = Gd, Dy, Er, Lu, 94–97; tren = tris(2-aminoethyl)-amine) [41]. The ligand was prepared in situ and this fact justifies the complexation of tren around the NiII ions (Figure 11). The LnIII ions are eight-coordinate by four phenolato and four carboxylato oxygen atoms from two different ligands in distorted square antiprismatic geometry. Each of the terminal NiII ions is bound to four nitrogen atoms from the tren ligand, one amino group and one carboxylato oxygen atom from the Schiff base ligand in distorted octahedral geometry. Magnetic studies on all four complexes suggest the presence of weak antiferromagnetic interactions between neighboring ions. The magnetic susceptibility data of the Ni2Gd complex 94 were interpreted considering the spin Hamiltonian H = − 2 J ( S N i 1 S G d + S G d S N i 2 ) + 4 [ D N i ( S z 2 − 1 3 S ( S + 1 ) ) + g N i μ B H S z ] . The best set of parameters obtained using this model is J/kB = −0.083 cm−1 and g = 2.03. In complex 97, the two NiII ions are linked by the diamagnetic LuIII ion and the magnetic susceptibility data were interpreted using the spin Hamiltonian H = 2 [ D ( S z 2 − 1 3 S ( S + 1 ) ) + g μ B H S z ] considering the axial single-ion zero-field splitting of the two NiII ions. The data were correctly fitted with D = 3.2(0) K and g = 2.19(2). The fit of the magnetic susceptibility data for 95 and 96 in the range 50–300 K to the Curie-Weiss law gave Curie constant C of 16.19 and 14.42 cm3 K mol−1 and Weiss temperature θ of −4.2 and −7.9 K for 95 and 96 respectively. The negative θ value indicates the presence of antiferromagnetic interactions within the NiII-LnIII-NiII moiety.
The ligand 8-hydroxyquinoline, HL21 (Scheme 3) gave the trinuclear complex [{Ni(L21)3}2La(L21)], 98 (Figure S11) [42]. The two NiII ions are six-coordinate with distorted fac-octahedral coordination geometry derived from three chelating (L21)− ligands (O,N). The LaIII ion is eight-coordinate with distorted square antiprismatic geometry bound to six bridging oxygen atoms of six (L21)− ligands and to a chelate (L21)− ligand through the nitrogen and oxygen atoms. The three metal ions form an angle of ~130°.
The ligand H2L22 = 7,7′-(ethane-1,1-diyl)bis(quinolin-8-ol) (Scheme 3) was formed in situ under the solvo(hydro)thermal conditions used to prepare complexes [{Ni(L22)1.5}2Ln(OH)] (LnIII = Eu, Tb, Gd, 99–101) from 8-hydroxyquinoline as proligand (Figure S12) [43]. All complexes are isomorphous and crystallize in the hexagonal space group P63/m. The LnIII ion has position occupation 0.16667 and presents tricapped trigon-prismatic geometry comprised six oxygen atoms from three ligands and three terminal OH− groups with 0.16667 position occupation. The one crystallographically independent NiII ion has position occupation 0.33333 and is coordinated to three oxygen and three nitrogen atoms from three ligands creating a regular trigon-antiprismatic geometry. Supramolecular C-H∙∙∙π interactions between neighboring molecules result in an overall 3D net. The dc susceptibility studies of the Ni2Tb complex 100 displayed paramagnetic behavior in the temperature range 300–12.5 K and below that temperature a slow decrease in the χMT product mainly due to the thermal depopulation of crystal field effect.
The ligand HL23 (Scheme 3) was formed in situ via transition metal promoted nucleophilic addition of methanol to a nitrile group of dicyanonitrosomethanide (dcnm) and gave two families of trinuclear complexes, (Me4N)[{(Ni(L23)3}2Ln(L23)2] (LnIII = La, Ce, Pr, Nd, Sm, 102–106) and (Et4N)2[{(Ni(L23)3}2Ln(dcnm)2](ClO4) (LnIII = La, Ce, 107–108) [44]. The two NiII octahedral metal sites are each coordinated by three (L23)− ligands which chelate through the nitrogen atoms of the nitroso and imine groups to form a [Ni(L23)3]− metalloligand. The central LnIII ion is bound to two [Ni(L23)3]− metalloligands and presents ten-coordination completed by two (L23)− ligands or two unreacted dcnm− ligands, all of them bound via the N,O atoms of the nitroso group (Figure 12). The coordination geometry around the LnIII ions is best described as sphenocorona JSPC-10. The Ni2Ln moiety is bent in both type of complexes with Ni-Ln-Ni angle of ~142 and ~133° for (Me4N)[{(Ni(L23)3}2Ln(L23)2] and (Et4N)2[{(Ni(L23)3}2Ln(dcnm)2](ClO4) complexes, respectively. Variable temperature magnetic susceptibilities on these complexes revealed practically zero Ni-Ni exchange coupling in the Ni2La complexes 102 and 107, possible weak antiferromagnetic coupling in Ni2Pr 104 and possible weak ferromagnetic coupling in the Ni2Ce complexes 103 and 108 but thermal depopulation and ligand-filed effects on the central LnIII ion, particularly for SmIII, make the unambiguous assignment of ferro- versus antiferromagnetic coupling rather difficult. In any case the magnetic behavior of 102–108 is similar to those of congener complexes.
The ligand 2,6-di(acetoacetyl)pyridine, H2L24 (Scheme 3) gave 18 trinuclear NiII2LnIII complexes with LnIII = La-Lu except for Pm, which crystallize in four different types: (A) [(NiL24)2Ln(NO3)2(MeOH)4](NO3) (LnIII = La, Ce, Pr, Nd, Sm, Eu, Gd, 109–115), (B) [(NiL24)2Ln(NO3)2(H2O)2(MeOH)2](NO3) (LnIII = Sm, Eu, Gd, 116–118), (C) [(NiL24)2Ln(NO3)3(MeOH)4] (LnIII = Gd, Tb, Dy, 119–121) and (D) [(NiL24)2Ln(NO3)2(H2O)(MeOH)3](NO3) (LnIII = Ho, Er, Tm, Yb, Lu, 122–126) [45]. All types of complexes are linear with Ni-Ln-Ni angles 180° (A), ~178° (B), ~173° (C) and ~179° (D). The two terminal NiII ions present O6 distorted octahedral geometry bound to the 1,3-diketonate sites from two (L24)2− ligands together with MeOH and H2O molecules. The central LnIII ion is coordinated to the 2,6-diacylpyridine site from two (L24)2− ligands and to two or three nitrate ions in an overall ten-coordinate environment (Figure 13). The coordination geometry around the LnIII ions is best described as hexadecahedron HD-10 in 109–115 and tetradecahedron TD-10 in 116–126. The magnetic studies revealed that the Ni-Ln interaction is weakly antiferromagnetic for Ln = Ce, Pr, Nd and ferromagnetic for Ln = Gd, Tb, Dy, Ho, Er. The magnetic susceptibility data for 109 and 126 which contain the diamagnetic LaIII and LuIII ions, respectively, were interpreted based on the isotropic Heisenberg model H ^ =   − 2 J S N i 1 S N i 2 . The best-fit parameters are J = −0.63 cm−1, g = 2.22 for 109 and J = −0.65 cm−1, g = 2.17 for 126. For the Ni2Gd complex, the spin Hamiltonian H ^ =   − 2 J ( S N i 2 S G d + S N i 2 S G d ) − 2 J ′ S N i 1 S N i 2 (J is the exchange integral between the adjacent NiII and GdIII ions and J’ is the exchange integral between the terminal NiII ions) was used to fit the susceptibility data and considering gNi = 2.20 and J’ = −0.64 cm−1 (the mean values for 109 and 126) the best-fit parameters are J = +0.79 cm−1 gGd = 2.02. The magnetic data of the remaining complexes were evaluated by adopting an empirical method taking into account the behavior of the congener Zn2Ln and Ni2La complexes by using the equation Δ χ M T =   χ M T ( N i 2 L n ) − χ M T ( Z n 2 L n ) − χ M T ( N i 2 L a ) , Δ χ M T > 0 for ferromagnetic and Δ χ M T < 0 for antiferromagnetic interaction.
The congener ligand 2,6-bis(acetobenzoyl)pyridine H2L25 (Scheme 3) gave the trinuclear complexes [(NiL25)2Ln(O2CMe)3(MeOH)x] (LnIII = Gd, Ce; x = 2 or 3, 127–128) and [(NiL25)2Ln(O2CPh)3(solv)x] (LnIII = Gd, solv = MeOH, x = 2, 129; Ce, solv = MeOH/H2O x = 2, 130) [46]. Each (L25)2− ligand is bound to the NiII sites through the 1,3-diketonate sites and to the central LnIII ion through the 2,6-diacylpyridine site. Two carboxylato ligands act as bidentate bridging between the NiII and the LnIII ions and the third act as chelate bidentate around the central lanthanide ion. The six-coordination around the NiII ions is completed by solvate molecules (Figure S13). The coordination number around the CeIII ion is 10, whereas for the GdIII is 9 or 10. The N2O7 nine-coordination around GdIII in 127 is described as HH-9 (hula-hoop) and the N2O8 ten-coordination around the LnIII ions in 128–130 is described as staggered dodecahedron SDD-10.
The ligand 2,6-dipicolinoylbis(N,N-diethylthiourea) H2L26 (Scheme 3) gave the trinuclear complex [(NiL26)2Pr(O2CMe)3(MeOH)2], 131 (Figure S14) [47]. The two NiII ions are bound to the S,O atoms from two (L26)2− ligands, one oxygen atom from the bridging acetato ligand and one methanol in distorted octahedral geometry. The PrIII ion is ten-coordinate and is bound to the O,N,O group of the 2,6-diacylpyridine site of each (L26)2− ligand, two bridging and one chelate acetato groups. The polyhedron around the PrIII ion is described as double-capped square antiprism.
The neutral trinuclear complexes [{Ni(piv)3(bpy)}2Ln(NO3)].MeCN (Hpiv = pivalic acid, bpy = 2,2′-bipyridine, LnIII = Gd (132), Sm (133)) are isomorphous and contain a bent Ni-Ln-Ni moiety with angles ~153° [48]. The central LnIII ion is bound to each of the terminal NiII ions through two syn,syn pivalato groups and one μ-O carboxylato oxygen which belongs to a chelate-monodentate bridging pivalato group. The distorted octahedral coordination around each NiII ion consists of four carboxylato oxygen atoms and two nitrogen atoms of the bpy. The magnetic susceptibility data of 132 were interpreted by using the spin Hamiltonian H = − 2 J N i G d ( S N i 1 S G d + S N i 2 S G d ) − 2 J N i N i ( S N i 1 S N i 2 ) . The best fit yielded JNiGd = 0.105(5) cm−1, JNiNi = −0.70(5) cm−1, gNi = 2.015(1), gGd = 2.00 (fixed), tip = 0.0001 (R2 = 1.28 × 10−5). The neutral trinuclear complexes [{Ni(piv)3(Hpiv)(MeCN)}2Ln(NO3)] (LnIII = La, Pr, Sm, Eu, Gd, 134–138) [48] are isomorphous and contain a Ni-Ln-Ni moiety with angles ~144°. The central LnIII ion is bound to each of the terminal NiII ions in similar fashion as in 134–138. The coordination of each NiII ion is completed by a monodentate Hpiv and MeCN molecules and is distorted octahedral. The coordination of the central LnIII consists of six carboxylato oxygen atoms and one chelate nitrato group and is described as single-capped pentagonal bipyramid. The magnetic susceptibility data of 138 were interpreted by using the spin Hamiltonian H = − 2 J N i G d ( S N i 1 S G d + S N i 2 S G d ) − 2 J N i N i ( S N i 1 S N i 2 ) . The best fit yielded JNiGd = 0.44(2) cm−1, JNiNi = −2.25(5) cm−1, gNi = gGd = 2.00 (fixed), molar content of the S = 1 impurity is 5.5% (R2 = 1.5 × 10−4). The magnetic susceptibility data of 134 can be interpreted by using either the exchange Hamiltonian H e x = − 2 J e x S 1 S 2 which yields Jex = −1.0(3) cm−1, g = 2.24(1) and zJ’ = +0.9(1) cm−1, tip = 2.4(9) × 10−4 (R2 = 2.6 × 10−4) or by considering the zero-field splitting of the NiII ions H z f s = D [ S z 2 − ( 1 3 ) S ( S + 1 ) ] which yields D = 6.0(5) cm−1, g = 2.227(1), zJ′ = +0.05(1) cm−1, tip = 3.6(5) × 10−4 (R2 = 2.5 × 10−3). The magnetic susceptibility data for 135–137 which contain PrIII, SmIII and EuIII ions can be interpreted by assuming that the exchange interactions between NiII and LnIII ions are absent, whereas the weak antiferromagnetic interactions between the NiII ions ‘through the lanthanide’ do exist.
The ligands HL27 = (Z)-1-(pyridine-2-yl)ethenone oxime and HL28 = (E)-N’-hydroxypyrimidine-2-carboximidamide (Scheme 3) gave the trinuclear complexes [{Ni(L27)3}2Tb](NO3) (139) [10,11] and [(Ni(HL28)3}2Tb](NO3) (140) [49], respectively. Both complexes contain strictly linear Ni-Tb-Ni moiety (Figure 14 and Figure S15). Each of the terminal NiII ions in 139–140 is coordinated to three pyridine and three oximato nitrogen atoms from three (HL27)− or (HL28)− ligands in distorted octahedral geometry. The central TbIII ion is coordinated to six oximato oxygen atoms from the three (HL27)− or (HL28)− ligands (Scheme 3) in distorted octahedral geometry. The χMT values of 139 at 300 and 2 K are 13.54 and 2.80 cm3 K mol−1, respectively. The field dependence of the magnetization at 2 K is 6.41 NμB at 2 K. The χMT value of 140 at 300 K is 11.74 cm3 K mol−1 which is slightly low with respect to the expected value for two NiII (S = 1) and one TbIII (S = 3, L = 3, g = 3/2) non-interacting ions. The χMT value decreases on lowering the temperature and reaches the value of 5.41 cm3 K mol−1 at 5 K. The decrease of the χMT value as the temperature decreases for 139 and 140 is governed by the thermal depopulation of the ground-state sublevels as result of the spin-orbit coupling and the low symmetry crystal field.
It is well known that ligand L29 = di(pyridine-2-yl)methanone or di-2-pyridyl ketone (Scheme 3) can undergo nucleophilic addition on the carbonyl carbon atom to form the hemiketal and/or the gem-diol ligands L29’ = ethoxydi(pyridine-2-yl)methanol and L29’’ = di(pyridin-2-yl)methanediol, respectively. Both L29’ and L29’’ are formed in situ in the presence of metal ions. Complexes [Ni2(L29’)3(L29’’)Ln(NO3)(H2O)](ClO4)2 (LnIII = Gd 141, Tb 142) and [Ni2(L29’)4Ln(NO3)(H2O)][Ln(NO3)5](ClO4)2 (LnIII = Tb 143, Dy 144, Y 145) consist of dications which contain triangular Ni-Ln-Ni moieties with angles ~54° (Figure 15) [50,51]. The metal ions are linked through one μ3-O atom and three μ2-O atoms from the ligands (Scheme 3). Each NiII ion is coordinated to three oxygen and two nitrogen atoms from three respective ligands in distorted octahedron. Each LnIII ion is coordinated to N2O6 chromophore which consists of three carbonyl O-atoms, three pyridine N-atoms, one chelate nitrato and one terminal aqua ligands. The magnetic susceptibility data of 141 revealed χMT values of 12.61 and 18.63 cm3 K mol−1 at 300 and 2 K respectively. The data were interpreted by using the spin Hamiltonian H = − J ( S ^ G d · S ^ N i 1 + S ^ G d · S ^ N i 2 ) − J ′ ( S ^ N i 1 · S ^ N i 2 ) and the best fit yielded J = +1.03(8) cm−1, J’ = +0.9(2) cm−1, g = 2.246(1). The magnetization measurement at 2 K revealed saturation under 5 T at 12.9 μβ, indicative of an S = 11/2 ground state with a g value larger than 2.0. The magnetic data of 143 revealed ferromagnetic exchange interactions between the metal ions. The magnetic susceptibility data of 145 which contains the diamagnetic YIII ion allowed the determination of the magnetic exchange interaction between the NiII ions according to the above spin Hamiltonian by considering J = 0. The best fit gave J’ = +8.0(2) cm−1 and g = 2.15(1).
The coordination geometry around the NiII ions in complexes 1–145 is distorted octahedral; the only exceptions are complexes 12–15 which can be more acutely described as trigonal prismatic. The bridging modes and the coordination around the NiII ions observed in complexes 1–145 is depicted in Scheme 4. The NiII ions is complexes 1–28, 52–74, 98–101 and 141–145 are coordinated to three nitrogen and three oxygen atoms. The configuration around the NiII ions consisting of N3O3 coordination sphere is chiral with either a Δ or a Λ configuration due to the screw coordination arrangement of the achiral tripodal ligands around the metal ion. When two chiral molecules associate, both homochiral (Δ-Δ or Λ-Λ) and heterochiral (Δ-Λ) pairs are possible. Since the above complexes crystallize in centrosymmetric space groups, molecules with Δ-Δ and Λ-Λ pairs coexist in the crystals to form racemic crystals. The NiII ions in complexes 29–51, 75–83, 91–93 and 109–131 are coordinated to four nitrogen and two oxygen atoms. The NiII ions in 89–90 and 132–138 are coordinated to two nitrogen and four oxygen atoms and in 94–97 are coordinated to five nitrogen and one oxygen atoms. The NiII ions in 102–108 and 139–140 show N6 coordination sphere. On the other hand, the coordination around the LnIII ions in 1–145 displays a variety of geometries from the rare octahedral and quasi trigonal prism for O6 coordination, capped trigonal prism and capped octahedron for O7 coordination, square antiprism and bicapped octahedron for O8 and NO7 coordination, tricapped trigonal prism for O9 coordination and distorted icosahedron for O12 coordination.

2.2. Oligonuclear Complexes Containing the NiII-LnIII-NiII unit

The Schiff base ligand H2L30 = 6,6’-((1E,1’E)-(propane-1,3-diylbis(azanylylidene)) bis(methanylylidene))bis(2-methoxyphenol) which resulted from the 2:1 condensation of 3-methoxysalicylaldehyde with 1,3-propanediamine (Scheme 4) gave complex [{{(NiL30(H2O))(N(CN)2)}2Pr}2(μ-NO3)](OH) (146) [52] which consists of an hexanuclear cationic moiety built up by two almost linear Ni2Pr subunits (Ni-Pr-Ni = 175.7°) that are linked by a disordered nitrato group bridging the two PrIII ions (Figure 16). Within each Ni2Pr subunit, the metal ions are bridged by two phenolato oxygen atoms from ligand (L30)2−. Each NiII ion is also coordinated to two imino nitrogen atoms form (L30)2−, one nitrogen atom from a monodentate dicyamido ligand and one aqua ligand in distorted octahedral geometry. The two PrIII ions present nine- or ten-coordination (depending on the nitrato ligand coordination, monodentate or chelating) and is completed by four phenoxo and four methoxy oxygen atoms from the two (L30)2− ligands. The χMT product at room temperature (7.18 cm3 K mol−1) corresponds to the theoretical value for uncoupled metal ions and decreases by lowering the temperature. The susceptibility data obey the Curie-Weiss law with negative Weiss constant θ = −5.01 K and Curie constant 7.34 cm3 K mol−1.
Ligand H2L24 (Scheme 3) was used to prepare the hexanuclear complex [{Ni2(L24)Gd(H2O)4}2(C2O4)3] (147) which is built by two almost linear Ni2Gd subunits (Ni-Gd-Ni = 178.6°) bridged by an oxalate ligand coordinated to the GdIII ions (Figure 17) [53]. The NiII ions are hosted into the 1,3-diketonate compartments of the two ligands, in O6 octahedral coordination completed by two aqua ligands. Each GdIII ion is coordinated to the O,N,O donor atoms of the inner pocket of two ligands and two oxalate ligands, one terminal and one bridging.
The Schiff base ligand H2L19 (Scheme 2) was used in complex [{Ni(L19)2}2Dy(H2O)Fe(CN)6]2 (148) which consists of two almost linear Ni2Dy subunits (Ni-Dy-Ni = 179.1°) bridged through two [Fe(CN)6]3− moieties via the cyanido ligands bound to the NiII ions (Figure 18) [54]. The NiII ions are five-coordinate with the N2O2 donor set from one (L19)2− in the equatorial plane and the cyano nitrogen atom in the apical position of the square pyramid. Each [Fe(CN)6]3− uses two cis-cyano ligands to coordinate to two NiII ions and is distorted octahedral. The DyIII ions are bridged with each NiII ion in the Ni2Dy subunits through two phenolato oxygen atoms and present O9 coordination (four phenolato and four methoxy oxygen atoms from two (L19)2− and one aqua ligand) which is close to a muffin (MFF-9) or a spherical-capped square antiprism (CSAPR-9). The magnetic susceptibility measurements revealed overall ferromagnetic interactions with the χMT values at room temperature close to the theoretical values for four high-spin NiII (S = 1), two DyIII (6H15/2) and two low-spin FeIII (S = 1/2) ions. The field dependence of magnetization reaches a value of 25.8 Nβ at 5 T without reaching saturation probably due to the zero-field splitting effect. The complex shows a strong frequency dependence of the out-of-phase signals under zero dc field. The fitting of the ln(τ) vs 1/T according to the Arrhenius law τ = τ 0 exp ( Δ k T ) yielded τ0 = 1.6 × 10−7 s and an effective energy barrier of Δ = 25.0 K.
The ligand H2L19 was also used to prepare a series of isostructural heterotrimetallic octanuclear complexes, [(Ni(L19)2}2Ln(H2O)Cr(CN)6]2 (LnIII = Y, Gd, Tb, Dy, 149–153), [(Ni(L19)2}2Ln(H2O)Fe(CN)6]2 (LnIII = Y, Gd, Tb, 154–156) and [(Ni(L19)2}2Ln(H2O)Co(CN)6]2 (LnIII = Dy, 157) [55]. The crystal structures of 149–157 are similar to that of 148 described above. Complexes 152 and 153 contain the same [Ni4Dy2Cr2] cyclic complex, however 153 contains also PPPO lattice molecules (PPPO = 4-(3-phenylpropyl)pyridine-1-oxide) and displays different lattice structure. The χMT values for 149–157 at 300 K are close to the theoretical values for [Ni4Ln2Cr2], [Ni4Ln2Fe2] and [Ni4Ln2] systems of uncoupled atoms. The magnetic susceptibility data for the two [Ni4Y2M2] complexes (MIII = Cr 149, Fe 154) indicate the presence of ferromagnetic interactions which can be analyzed by using the isotropic Hamiltonian H ^ = − 2 J N i M S ^ M 1 ( S ^ N i 1 + S ^ N i 2 ) (considering two Ni-M-Ni (M = Cr or Fe) units), by including zJ’ for the intermolecular magnetic interaction and the zero-field splitting of NiII ions. The fitting yielded JNiCr = 9.4(1) cm−1, zJ’ = −0.050(2) cm−1, g = 2.06(1) for 149 and JNiFe = 4.9(7) cm−1, zJ’ = −0.35(2) cm−1, g = 2.24(1) for 154. The positive JNiCr and JNiFe confirm the presence of ferromagnetic interaction between the cyanido-bridged FeIII/CrIII and NiII ions, whilst the negative zJ’ values suggest weak antiferromagnetic interaction between the Ni-M-Ni units. The magnetic susceptibility data of the [Ni4Gd2M2] complexes (MIII = Cr 150, Fe 155) were fitted according the isotropic Hamiltonian H ^ = − 2 J N i G d S ^ N i 1 S ^ G d 1 − 2 J N i G d S ^ N i 2 S ^ G d 1 − 2 J M N i S ^ M 1 S ^ N i 2 − 2 J M N i S ^ M 1 S ^ N i 1 A − 2 J N i G d S ^ N i 1 A S ^ G d 1 A − 2 J N i G d S ^ N i 2 A S ^ G d 1 A − 2 J M N i S ^ M 1 A S ^ N i 2 A − 2 J M N i S ^ M 1 A S ^ N i 1 (MIII = Cr with S = 3/2 or Fe with S =1/2). The fitting yielded JNiCr = 11.82 cm−1, JNiGd = 0.94 cm−1 and g = 2.04 for 150 and JNiFe = 10.58 cm−1, JNiGd = 1.24 cm−1 and g = 2.03 for 155. The magnetic susceptibility data of the [Ni4Tb2M2] (MIII = Cr 151 or Fe 156) and [Ni4Dy2M2] (MIII = Cr 152–153 or Co 157) complexes reveal the presence of ferromagnetic interactions. The decrease of the χMT values at low temperatures may be due to the zero-field splitting of TbIII and DyIII ions and/or the intermolecular antiferromagnetic interaction. Complexes 151–153 and 156–157 display strong frequency dependence of the out-of-phase signal below 5 K under zero dc field, indicating the presence of slow magnetic relaxation. The fitting of the ln(τ) vs 1/T according to the Arrhenius law τ = τ 0 exp ( U e f f k T ) yielded Ueff = 21.9 K (τ0 = 4.71 × 10−8 s) for 151, Ueff = 38.9 K (τ0 = 4.89 × 10−9 s) for 152, Ueff = 37.2 K (τ0 = 6.44 × 10−9 s) for 153, Ueff = 29.6 K (τ0 = 4.52 × 10−10 s) for 156 and Ueff = 24.4 K (τ0 = 4.94 × 10−7 s) for 157.
The ligand H2L19 also gave the isostructural heterotrimetallic octanuclear complexes, [(Ni(L19)2}2Ln(H2O)W(CN)8]2 (LnIII = Tb, Dy, Y, 158–160) and [(Ni(L19)2}2Tb(H2O)Co(CN)6]2 (161) [56]. The crystal structures of 158–161 are similar to those of complexes 148–157 described above; the only difference is the presence of the bridging [W(CN)8]3− (158–160) or [Co(CN)6]3− (161) moieties. The magnetic susceptibility data of 160 were fitted by using the spin Hamiltonian H = − J ( S N i 1 S W + S N i 2 S W ) and including zJ’ for intermolecular interaction and yielded J = 20.0(5) cm−1, zJ’ = −0.67(1) cm−1, gNi = 2.25 and gW = 2.0. The magnetic susceptibility data for 158 and 159 revealed ferromagnetic interactions with χMT values of 29.8 and 35.0 cm3 K mol−1 at 300 K, 34.3 and 38.1 cm3 K mol−1 at 15 K and 13 K, 8.97 and 22.4 cm3 K mol−1 at 2 K. Ac-susceptibility measurements for 158 and 159 under zero dc field showed frequency dependence of the χM’’ component. Analysis of the Cole-Cole plots yielded α parameters in the narrow range 0.05–0.15 for temperatures 3–5 K which indicate very narrow distribution width of the relaxation time and a single relaxation pathway. The fitting of the ln(τ) vs 1/T according to the Arrhenius law τ = τ 0 exp ( U e f f k T ) yielded Ueff = 23.0 K (τ0 = 2.57 × 10−7 s) for 158 and Ueff = 26.4 K (τ0 = 6.0 × 10−8 s) for 159. The magnetic behavior of 161 is consistent with ferromagnetic Ni-Tb interactions, however no signal for χM’’ down to 2 K under zero dc field is observed. This difference in the ac-susceptibility data between 158–159 and 161 is attributed to the magnetic interactions between the transition metal ions, that is, the two Ni2W moieties for 158–159, in contrast to the presence of the diamagnetic CoIII ions in 161. For 158 and 161 the magnetic anisotropy is provided by the NiII and TbIII ions which have identical geometry in both complexes (i.e., square pyramidal and distorted tricapped trigonal prismatic, respectively). Therefore, any effect due to modification of the TbIII geometry can be ruled out and similar relaxation features are expected. However, an energy barrier of 23 K is reached for 158 whereas for 161 the slow dynamic features are very similar to those of dinuclear NiLn or trinuclear Ni2Ln complexes.

2.3. Coordination Polymers Containing NiII-LnIII-NiII Subunits

2.3.1. 1D Coordination Polymers

The ligand H2L31 = 2,2′-((1E,1’E)-(propane-1,3-diylbis(axanylylidene))bis(methan ylylidene))diphenol (Scheme 4) was used to prepare complexes [{(NiL31)2Ln(NO3)3}pyz]n (pyz = pyrazine, LnIII = Gd, Tb, Dy, 162–164) [57]. The structure of 164 consists of {(NiL31)2Dy(NO3)3} subunits linked through pyrazine molecules and result in polymeric chains parallel to the (1–10) crystallographic plane (Figure 19). The asymmetric unit contains two trinuclear units with slightly different structural characteristics. The central DyIII of the Ni2Ln moiety is linked to each of the terminal NiII ions through two phenolato oxygen atoms of the (L31)2− ligand and two nitrato bridges, each one adopting different coordination mode (i.e., chelate bindentate and μ1,3-bridging). A third chelate nitrato ligand completes nine-coordination around the DyIII ion which is described as distorted monocapped square antiprism. The two NiII ions of the Ni2Dy moiety are hosted in the N2O2 pocket of the (L31)2− ligand and display distorted octahedral geometry which is completed by one nitrogen atom of the bridging pyrazine and one oxygen atom from bridging nitrato groups. The Ni-Dy-Ni moiety deviates from linearity with angles 149.6 and 153.5° for the two crystallographically independent units. The magnetic susceptibility data of 162 revealed the presence of ferromagnetic interactions between the two NiII and one GdIII ions. The data were fitted by using the spin Hamiltonian H ^ = − 2 J S ^ G d ( S ^ N i 1 + S ^ N i 2 ) , considering JGdNi1 = JGdNi2 = J and yielded 2J = +0.124 cm−1 and g = 2.044(1). The χMT values at 300 K for 163 and 164 are 14.4 and 16.8 cm3 K mol−1, close to the theoretical values for uncoupled one LnIII and two NiII ions. The χMT values decrease upon lowering the temperature and reach 7.35 and 11.5 cm3 K mol−1 at 2 K for 162 and 163, respectively. The behavior of 163 and 164 can be attributed to the depopulation of the Stark sublevels of the LnIII ions, the presence of zero-field splitting of the NiII ions and the exchange coupling between the metal ions. Ac magnetic susceptibility measurements for 163 and 164 did not show any signal for χM’’ under zero or 2000 Oe dc field down to 2 K.
The ligand H2L24 (Scheme 3) was used to synthesize the 1D helical chain complex [{(NiL24)2Dy(H2O)4(oxy-bbz)}(NO3)]n 165 (oxy-bbz = the dianion of 4,4′-oxy-bis(benzoic) acid) [58]. The molecular structure of 165 consists of trinuclear subunits {Ni(L24)2Dy(H2O)4}3+ which are bridged through the (oxy-bbz)2− molecules and form cationic chains which are balanced by uncoordinated nitrate counterions (Figure 20). The helical chains of 165 extend along the crystallographic b axis. Each of the NiII ions is accommodated in the outer 1,3-diketone pockets of two ligands and complete distorted octahedral geometry with two aqua ligands. The central DyIII ion is coordinated to the middle compartment of the two ligands and complete N2O8 coordination by two bidentate carboxylate groups of two different (oxy-bbz)2− molecules.
Complex Na2[Ni(L32)1.5]2 which contains the dianion of ligand H2L32 = 1,3-bis(pyrazine-2-carboxamide) benzene (Scheme 4) was used as building block to afford the 1D complexes [{[Ni(L32)1.5]2Tb(H2O)5}(CF3SO3)]n (166) and [{[Ni(L32)1.5)]2Ln(H2O)4(NO3)]n (LnIII = Gd, Tb, 167, 168) [59]. The structure of 166 consists of cationic chains extending along the [101] direction of the crystal. The TbIII ions link two dinuclear [Ni(L32)1.5]22− units through coordination to one of the amidate groups of two different (L32)2− ligands with angle Ni(1)-Tb(1)-Ni(2’) = 96.7° (Figure 21). The TbO7 coordination is completed by five aqua ligands, showing coordination polyhedron intermediate between pentagonal bipyramid (PBPY-7), capped trigonal prism (CTPR-7) and capped octahedron (COC-7). The two NiII ions exhibit distorted NiN6 coordination environment formed by three pyrazine and three amidate nitrogen atoms in a fac disposition. Compounds 167 and 168 are isostructural and consist of neutral chains (Figure S16). The main difference with the cationic chains in 166 is that in the former the LnIII ions are bound to a chelate NO3− leading to neutral chains and TbO8 coordination described as trigonal dodecahedron (TDD-8). The Ni(1’)-Ln-Ni(2) angles are ~99.3 o. The χMT products of 166–168 at r.t. are close to the expected values for two uncoupled NiII ions (S = 1, g = 2) and one LnIII ion and remain constant down to approximately 20 K and then increase sharply down to 2 K, confirming ferromagnetic exchange between the NiII and LnIII ions along the chains. Complexes 166 and 168 do not show out-of-phase signals above 2 K in spite of the ferromagnetic interactions along the chains and the magnetic anisotropy of the TbIII ions.
The ligand H2L33 = 1,3-bis(pyridine-2-carboxamide)benzene (Scheme 4) gave the cationic chains [{[Ni(L33)1.5]2Tb(H2O)5}(NO3)]n (169) and [{[Ni(L33)1.5]2Ln(H2O)6}(CF3SO3)]n (LnIII = Tb, Dy, Gd, 170–172) [60]. The structure of 169 consists of cationic chains {[Ni(L33)1.5]2Tb(H2O)5}+ extending along the (101) direction and is quite similar to the structure of 166 described above. The TbO7 coordination sphere is a pentagonal bipyramid. The Ni(1)-Tb-Ni(2’) (‘ = −0.5 + x, 0.5 − y, −0.5 + z) angle is 99.8°. The structures of 170–172 consist of cationic chains [{[Ni(L33)1.5]2Ln(H2O)6}+ and are similar to the structures of 166 and 169. The LnIII ions in 170–172 exhibit LnO8 coordination environment with dodecahedral geometry and Ni1-Ln-Ni2 angles of ~73°. It should be mentioned that the LnIII coordination number in 169–172 seems to depend on the counteranion present in the structure; TbO7 vs. TbO8 for nitrates and triflates, respectively. This has an impact on the crystal field potentials which has a direct influence on the depopulation of the Stark sublevels, the magnetic anisotropy and the magnetic properties. Compounds 169–171 do not exhibit magnetic slow relaxation despite the large anisotropy of the TbIII and DyIII ions and the existence of ferromagnetic interactions along the chains, mainly due to the weakness of the intrachain magnetic interactions.
The ligand H4L34 = (13Z,19Z)-bis(2-(2-hydroxyethoxy)ethyl) 6,7-dioxo-5,6,7,8,14a,15,16,17,18,18a-decahydrotribenzo[b,f,l] [1,4,8,11] tetraazacyclotetradecin e-13,20-dicarboxylate (Scheme 4) gave three isomorphous double-chain coordination polymers [{(Ni(H2L34))2Ln(H2O)3}(ClO4)3]n (LnIII = Pr, Sm, Gd, 173–175, Figure 22) which contain the repeating unit [{(Ni(H2L34))2Ln(H2O)3}3+ with almost linear NiII-LnIII-NiII moieties (~179.3°) [61]. The coordination mode of the (H2L34)2− ligands promotes the formation of rectangular Ln2(Ni(H2L34))2 subunits. Both NiII ions present slightly distorted N4 square planar coordination geometry. The central LnIII ion presents O9 coordination defined from two bidentate oxamido groups belonging to two different (H2L34)2−, two hydroxyls belonging to two different (H2L34)2− and three aqua ligands. The coordination geometry around the LnIII ion is described as distorted monocapped square antiprism. Compounds 173–175 were able to induce DNA cleavage.
The Schiff base ligand HL35 obtained from the condensation of 2-pyridylaldehyde with isonicotinic hydrazide N-oxide (Scheme 4) gave three 1D polymers with formula [{(Ni(L35)1.5)2Ln(HL35)(dmf)4}(ClO4)4]n (LnIII = Gd, Tb, 176–177) and [{(Ni(L35)2)2Dy(dmf)4}(ClO4)3]n (178) which are based on trinuclear Ni2Ln subunits albeit consist of corner-sharing squares Ni2Ln2 due to the coordination mode adopted by the ligand [62].

2.3.2. 2D Coordination Polymers

The ligands H2L36 = oxydiacetic acid (Scheme 4) and dpds = 4,4′-dipyridyl disulfide gave the 2D coordination polymer [{Ni(L36)1.5}2Eu(dpds)2(H2O)4]n (179) with NiII-EuIII-NiII moiety forming an angle of 145.7° [63]. The EuIII ion is coordinated to three ether and six carboxylate oxygen atoms from three (L36)2− ligands and shows tricapped trigonal prismatic geometry. Each of the NiII ions is bound to two aqua ligands, two pyridyl nitrogen atoms from two dpds ligands and two carboxylate oxygen atoms from two neighboring (L36)2− ligands in distorted octahedral geometry. The (L36)2− ligands display two coordination modes, half of them behave as pentadentate ligands chelating one EuIII ion and bridging two NiII ions and the other half act as tetradentate chelating one EuIII ion and bridging a NiII ion. The bridging action provided by the (L36)2− and dpds ligands results in the formation of an overall 2D layer structure in (100) plane (Figure 23).

2.3.3. 3D Coordination Polymers

Isonicotinic acid (HL37, Scheme 4) gave three 3D coordination frameworks of general formula [{Ni(L37)2.5(N3)(H2O)}2Ln(H2O)]n (LnIII = Gd, Pr, Nd, 180–182) [64,65]. The structures contain one LnIII and two NiII ions two unique azide ligands and five different isonicotinate anions. Th LnIII ions are coordinated to nine oxygen atoms, two doubly coordinated and four singly coordinated carboxylates and one aqua ligand. The two tridentate (L37)− anions bridge the LnIII ions into 1D zig-zag chain via the carboxylate groups and also bound to Ni(1) through the N atoms. Two alternating end-on and end-to-end azides connect the Ni(1) ions into chain which is further linked to neighboring LnIII ions via μ-O,O,N (L37)− ligands, resulting in 2D sheets. The three bidentate (L37)− ligands are coordinated to Ni(2) ions and also to Ni(1) and LnIII ions resulting in the formation of an overall 3D network. Magnetic susceptibility studies of 180 revealed the presence of ferrimagnetic coupling between the NiII and GdIII ions and the contribution of the alternating end-on and end-to-end azido system which corresponds to an alternating ferromagnetic-antiferromagnetic chain in the temperature range 40–300 K, as well as zero-field splitting of the NiII ions in the low temperatures regime. In the case of 181–182 the magnetic susceptibility data indicate the presence of dominant antiferromagnetic interactions between the metal ions.

3. Summary

Trinuclear heterometallic complexes containing the NiII-LnIII-NiII moiety, with Ni-Ln-Ni angles in the range ~54–180o, have been reported with LnIII = Y, La-Lu except Pm. In these complexes, the NiII ions display octahedral coordination geometry comprised of N3O3, N2O4, N4O2, N5O, NO5, N6, O6 and O2S4 chromophores. Rare examples containing square planar NiN2O2, NiN4 and square pyramidal NiN2O3, NiN3O2 coordination geometries have been also reported. The complexes that contain the N3O3 chromophore are chiral with either a Δ or a Λ configuration due to the screw coordination arrangement of the achiral ligands around the metal ion; however, the complexes reported herein crystallize in centrosymmetric space groups therefore the Δ-Δ and Λ-Λ pairs coexist in the crystals to form racemic crystals. The coordination geometry around the Ln ions range from the rare octahedral, quasi trigonal prism and trigonal antiprism for six-coordination; capped trigonal prism, capped octahedron and pentagonal bipyramid for seven-coordination; square antiprism, bicapped octahedron and single-capped pentagonal bipyramid for eight-coordination; tricapped trigonal prism and hula-hoop for nine-coordination; sphenocorna, pentagonal antiprism, tetradecahedron, hexadecahedron, dodecahedron and double-capped square antiprism for ten-coordination; and distorted icosahedron for twelve-coordination. The magnetic properties of the trinuclear complexes range from ferro- to antiferromagnetic depending on the lanthanide ion. All Ni2Gd complexes display weak ferromagnetic interactions between the metal ions leading to ground spin state S = 11/2. The magnetic exchange parameters of the Ni2Gd were found in the range +0.19 to +1.03 cm−1. Some of the Ni2Dy complexes showed zero-filed or field-induced SMM behavior with thermal barriers in the range 10–14 K and pre-exponential factors τ0 ~10−5–10−6 s. The trinuclear complexes were linked via inorganic and/or organic anions to form dimers of trimers. The use of bridging complex anions, such as [MIII(CN)6]3− (MIII = Fe, Cr, Co) and [WV(CN)8]3−, afforded cyclic heterometallic complexes [Ni2LnM]2 which exhibit SMM behavior under zero dc field with energy barriers in the range ~22–39 K and τ0 ~10−7–10−10 s. The trinuclear NiII-LnIII-NiII moiety was found as the repeating unit in 1D-, 2D and 3D-coordination polymers with interesting structures and physical properties.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/10/12/1117/s1, Figures S1–S16 showing the molecular structure of the complexes mentioned in the text.

Funding

This research received no external funding.

Conflicts of Interest

The author declares no conflict of interest.

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Crystals 10 01117 g001 550
Figure 1. The molecular structure of the cation [(NiL1)2Gd]+ in complex 7. Primed atoms are generated by symmetry: (‘) = −x, y, 1−z. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small, P grey large, S open large [17].
Figure 1. The molecular structure of the cation [(NiL1)2Gd]+ in complex 7. Primed atoms are generated by symmetry: (‘) = −x, y, 1−z. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small, P grey large, S open large [17].
Crystals 10 01117 g001
Crystals 10 01117 g002 550
Figure 2. The molecular structure of complex [(NiL2)2Gd(NO3)] (12). Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small [19].
Figure 2. The molecular structure of complex [(NiL2)2Gd(NO3)] (12). Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small [19].
Crystals 10 01117 g002
Crystals 10 01117 g003 550
Figure 3. The molecular structures of the cations [(NiL4)2Gd]+ (17, left) and [(NiL4)2Dy]+ (20, right). Color code as in Figure 2 [22].
Figure 3. The molecular structures of the cations [(NiL4)2Gd]+ (17, left) and [(NiL4)2Dy]+ (20, right). Color code as in Figure 2 [22].
Crystals 10 01117 g003
Crystals 10 01117 g004 550
Figure 4. The molecular structure of the cation [(NiL7)2Dy)]+ in complex 51. Color code as in Figure 2 [26].
Figure 4. The molecular structure of the cation [(NiL7)2Dy)]+ in complex 51. Color code as in Figure 2 [26].
Crystals 10 01117 g004
Crystals 10 01117 g005 550
Figure 5. The molecular structures of the cations [(NiL8)2Gd(MeCN)2]+ 54, (left) and [(NiL8)2Yb]+ 56, (right). Color code as in Figure 2 [27].
Figure 5. The molecular structures of the cations [(NiL8)2Gd(MeCN)2]+ 54, (left) and [(NiL8)2Yb]+ 56, (right). Color code as in Figure 2 [27].
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Crystals 10 01117 sch001 550
Scheme 1. The tripodal polydentate Schiff base and reduced Schiff base ligands used in the Ni2Ln complexes 1–70.
Scheme 1. The tripodal polydentate Schiff base and reduced Schiff base ligands used in the Ni2Ln complexes 1–70.
Crystals 10 01117 sch001
Crystals 10 01117 g006 550
Figure 6. The molecular structure of the cation [(NiL113)2Gd]+ in complex 73. Primed atoms are generated by symmetry (‘) 1−x, y, 0.5−z. Color code as in Figure 2 [30].
Figure 6. The molecular structure of the cation [(NiL113)2Gd]+ in complex 73. Primed atoms are generated by symmetry (‘) 1−x, y, 0.5−z. Color code as in Figure 2 [30].
Crystals 10 01117 g006
Crystals 10 01117 g007 550
Figure 7. The molecular structure of the cation [{Ni(H3L13)2}2Ln]3+ in complex 76. Color code as in Figure 2 [33].
Figure 7. The molecular structure of the cation [{Ni(H3L13)2}2Ln]3+ in complex 76. Color code as in Figure 2 [33].
Crystals 10 01117 g007
Crystals 10 01117 g008 550
Figure 8. The molecular structure of the cation [{Ni(H3L14)2}2Ln(O2CMe)2]+ in complex 83. Primed atoms are generated by symmetry: (‘) 1.5−x, 0.5−y, z. Color code as in Figure 2 [34].
Figure 8. The molecular structure of the cation [{Ni(H3L14)2}2Ln(O2CMe)2]+ in complex 83. Primed atoms are generated by symmetry: (‘) 1.5−x, 0.5−y, z. Color code as in Figure 2 [34].
Crystals 10 01117 g008
Crystals 10 01117 g009 550
Figure 9. The molecular structure of the cation [(NiL17)2Pr(O2CMe)2(MeOH)2]+ in complex 90. Primed atoms are generated by symmetry: (‘) 1−x, 1−y, 1−z. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small, Br open large [38].
Figure 9. The molecular structure of the cation [(NiL17)2Pr(O2CMe)2(MeOH)2]+ in complex 90. Primed atoms are generated by symmetry: (‘) 1−x, 1−y, 1−z. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small, Br open large [38].
Crystals 10 01117 g009
Crystals 10 01117 g010 550
Figure 10. The molecular structure of the cation [{NiL19(H2O)}2Eu(H2O)]3+ in complex 93. Color code as in Figure 2 [40].
Figure 10. The molecular structure of the cation [{NiL19(H2O)}2Eu(H2O)]3+ in complex 93. Color code as in Figure 2 [40].
Crystals 10 01117 g010
Crystals 10 01117 sch002 550
Scheme 2. The Schiff base ligands used in Ni2Ln complexes 71–93.
Scheme 2. The Schiff base ligands used in Ni2Ln complexes 71–93.
Crystals 10 01117 sch002
Crystals 10 01117 g011 550
Figure 11. The molecular structure of the cation [{Ni(H2L20)(tren)2}2Er]3+ in complex 96. Color code as in Figure 2 [41].
Figure 11. The molecular structure of the cation [{Ni(H2L20)(tren)2}2Er]3+ in complex 96. Color code as in Figure 2 [41].
Crystals 10 01117 g011
Crystals 10 01117 g012 550
Figure 12. The molecular structure of the anion [{(Ni(L23)3}2Nd(L23)2]− in complex 105. Primed atoms are generated by symmetry: (‘) 1/3 + y, −1/3 + x, 1/6 − z. Color code as in Figure 2 [44].
Figure 12. The molecular structure of the anion [{(Ni(L23)3}2Nd(L23)2]− in complex 105. Primed atoms are generated by symmetry: (‘) 1/3 + y, −1/3 + x, 1/6 − z. Color code as in Figure 2 [44].
Crystals 10 01117 g012
Crystals 10 01117 g013 550
Figure 13. The molecular structure of the cation [(NiL24)2Gd(NO3)2(H2O)2(MeOH)2]+ in complex 118. Color code as in Figure 2 [43].
Figure 13. The molecular structure of the cation [(NiL24)2Gd(NO3)2(H2O)2(MeOH)2]+ in complex 118. Color code as in Figure 2 [43].
Crystals 10 01117 g013
Crystals 10 01117 g014 550
Figure 14. The molecular structure of the cation complex [{Ni(L27)3}2Tb]+ 139. Primed atoms are generated by symmetry: (‘) –x + y, −x, z. Color code: Tb large octant, Ni small octant, N light grey, O dark grey, C open small, S open large [11].
Figure 14. The molecular structure of the cation complex [{Ni(L27)3}2Tb]+ 139. Primed atoms are generated by symmetry: (‘) –x + y, −x, z. Color code: Tb large octant, Ni small octant, N light grey, O dark grey, C open small, S open large [11].
Crystals 10 01117 g014
Crystals 10 01117 g015 550
Figure 15. The molecular structure of the dication complex [Ni2(L29’)3(L29’’)Gd(NO3)(H2O)]2+ 141. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small, S open large [50,51].
Figure 15. The molecular structure of the dication complex [Ni2(L29’)3(L29’’)Gd(NO3)(H2O)]2+ 141. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small, S open large [50,51].
Crystals 10 01117 g015
Crystals 10 01117 sch003 550
Scheme 3. The ligands used in the Ni2Ln complexes 94–145.
Scheme 3. The ligands used in the Ni2Ln complexes 94–145.
Crystals 10 01117 sch003
Crystals 10 01117 g016 550
Figure 16. The molecular structure of the cationic complex [{{(NiL30(H2O))(N(CN)2)}2Pr}2(μ-NO3)]+ (146). Primed atoms are generated by symmetry (‘) 1 − x, y, 0.5 − z. Color code: Pr large octant, Ni small octant, N light grey, O dark grey, C open small [52].
Figure 16. The molecular structure of the cationic complex [{{(NiL30(H2O))(N(CN)2)}2Pr}2(μ-NO3)]+ (146). Primed atoms are generated by symmetry (‘) 1 − x, y, 0.5 − z. Color code: Pr large octant, Ni small octant, N light grey, O dark grey, C open small [52].
Crystals 10 01117 g016
Crystals 10 01117 g017 550
Figure 17. The molecular structure of complex [{Ni2(L24)Gd(H2O)4}2(C2O4)3] (147). Primed atoms are generated by symmetry (‘) −1 − x, y, −0.5 − z. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small [53].
Figure 17. The molecular structure of complex [{Ni2(L24)Gd(H2O)4}2(C2O4)3] (147). Primed atoms are generated by symmetry (‘) −1 − x, y, −0.5 − z. Color code: Gd large octant, Ni small octant, N light grey, O dark grey, C open small [53].
Crystals 10 01117 g017
Crystals 10 01117 g018 550
Figure 18. The molecular structure of complex [{Ni(L19)2}2Dy(H2O)Fe(CN)6]2 (148). Primed atoms are generated by symmetry (‘) 2 − x, 1 − y, 1 − z. Color code: Dy large octant, Ni and Fe small octant, N light grey, O dark grey, C open small [54].
Figure 18. The molecular structure of complex [{Ni(L19)2}2Dy(H2O)Fe(CN)6]2 (148). Primed atoms are generated by symmetry (‘) 2 − x, 1 − y, 1 − z. Color code: Dy large octant, Ni and Fe small octant, N light grey, O dark grey, C open small [54].
Crystals 10 01117 g018
Crystals 10 01117 g019 550
Figure 19. Part of the 1D structure of 164. Primed atoms are generated by symmetry (‘) −1 + x, −1 + y, z. Color code: Ln large octant, Ni small octant, N light grey, O dark grey, C open small [57].
Figure 19. Part of the 1D structure of 164. Primed atoms are generated by symmetry (‘) −1 + x, −1 + y, z. Color code: Ln large octant, Ni small octant, N light grey, O dark grey, C open small [57].
Crystals 10 01117 g019
Crystals 10 01117 g020 550
Figure 20. Part of the 1D structure of 165. Color code as in Figure 19 [58].
Figure 20. Part of the 1D structure of 165. Color code as in Figure 19 [58].
Crystals 10 01117 g020
Crystals 10 01117 g021 550
Figure 21. Part of the 1D cationic structure of 166. Primed atom is generated by symmetry (‘) −0.5 + x, 0.5 − y, −0.5 + z. Color code as in Figure 19 [59].
Figure 21. Part of the 1D cationic structure of 166. Primed atom is generated by symmetry (‘) −0.5 + x, 0.5 − y, −0.5 + z. Color code as in Figure 19 [59].
Crystals 10 01117 g021
Crystals 10 01117 g022 550
Figure 22. Part of the double-chain structure of 175. Primed atom is generated by symmetry (‘) 1 − x, y, 1.5 − z. Color code as in Figure 19 [61].
Figure 22. Part of the double-chain structure of 175. Primed atom is generated by symmetry (‘) 1 − x, y, 1.5 − z. Color code as in Figure 19 [61].
Crystals 10 01117 g022
Crystals 10 01117 g023 550
Figure 23. Part of the 2D layer structure of 179. Color code: Eu large octant, Ni small octant, N light grey, O dark grey, C open small, S open large [63].
Figure 23. Part of the 2D layer structure of 179. Color code: Eu large octant, Ni small octant, N light grey, O dark grey, C open small, S open large [63].
Crystals 10 01117 g023
Crystals 10 01117 sch004 550
Scheme 4. Ligands used in the oligonuclear and polynuclear complexes 146–182 which contain the Ni2Ln moiety.
Scheme 4. Ligands used in the oligonuclear and polynuclear complexes 146–182 which contain the Ni2Ln moiety.
Crystals 10 01117 sch004
Table
Table 1. Structural characteristics of the crystallographically studied Ni2Ln complexes.
Table 1. Structural characteristics of the crystallographically studied Ni2Ln complexes.
ComplexNiII Coordination ModeLnIII Coordination ModeNi-Ln-Ni
Angle (o)		LnIIIRef.
[(NiL1)2Ln](ClO4)N3O3O12176.2–178.7La-Er except Pm[17,18]
[(NiL2)2Ln](NO3)N3O3O8139.9–143.8Eu, Gd, Tb, Dy[19,20]
[(NiL3)2Ln](NO3)N3O3O12178.4–178.7Gd[21]
[(NiL4)2Ln](NO3)N3O3O6/O7177.1–177.8Gd, Tb, Dy[22]
[(NiL4)2Ln](ClO4)N3O3O6174.6Dy[22]
[(NiL5)2Ln(solv)x](ClO4) aN3O3O6/O7/O8142.5–157.7La, Dy, Yb[23]
[(NiL6)2Ln(MeOH)](NO3)N4O2O7139.6–143.9La, Sm, Tb, Er, Lu[24,25]
[(NiL6)2Ln(MeOH)](ClO4)N4O2O7143.9Pr[24]
[(NiL7)2Ln](ClO4)2N4O2O8113.0, 113.3Gd, Dy[26]
[(NiL8)2Ln(MeCN)2](ClO4)N3O3N2O6142.4Gd[27]
[(NiL8)2Ln](ClO4)N3O3O6176.4Yb[27]
[(NiL9)2Ln(solv)x](ClO4) bN3O3O6, O7165.1–178.8Y, Ce-Lu except Pm, Yb[28]
[(NiL10)2Ln](ClO4)N3O3O6180.0Tb[29]
[(NiL11)2Ln](NO3)N3O3O12179.7–179.9La, Pr, Gd, Tb[30,31]
[{Ni(HL12)2}2Ln(NO3)](NO3)2N2O4O10169.2La[32]
[{Ni(H3L13)2}2Ln](NO3)3N2O4O8179.3–179.6Gd, Tb, Dy, Ho[33]
[{Ni(H3L14)2}2Ln(O2CMe)2](NO3)3N2O4O8180.0Gd, Tb[34]
[(NiL15)2Ln(NO3)2](NO3)N2O2O1267.8, 67.9La, Ce[35]
[(NiL16)2Ln(NO3)2](NO3)N2O2O1262.1Ce[36]
[(NiL17)2Ln(O2CMe)2(MeOH)2](NO3)N2O4O10180.0La, Nd, Ce, Pr[37,38]
[(NiL18)2Ln(NO3)3]N2O2O10122.5Ce[39]
[{Ni(L19)(H2O)}2Ln(H2O)](trif)3 cN2O3O9179.0, 177.2Gd, Eu[21,40]
[{Ni(H2L20)2(tren)2}2Ln](NO3)3 dN5OO893.0–95.3Gd, Dy, Er, Lu[41]
[{Ni(L21)3}2Ln(L21)]N3O3NO7129.6La[42]
[{Ni(L22)1.5}2Ln(OH)]N3O3O9180.0Eu, Gd, Tb[43]
(Me4N)[{Ni(L23)3}2Ln(L23)2]N6N2O8141.3–142.5La, Ce, Pr, Nd, Sm[44]
(Et4N)2[{Ni(L23)3}2Ln(dcnm)2](ClO4)2 eN6N2O8133.1, 133.2La, Ce[44]
[(NiL24)2Ln(NO3)2(MeOH)4](NO3)O6N2O8180.0La, Ce, Pr, Nd, Sm, Eu[45]
[(NiL24)2Ln(NO3)2(H2O)2(MeOH)2](NO3)O6N2O8177.6–177.8Sm, Eu, Gd[45]
[(NiL24)2Ln(NO3)3(MeOH)4]O6N2O8172.6–172.9Gd, Tb, Dy[45]
[(NiL24)2Ln(NO3)2(H2O)(MeOH)3](NO3)O6N2O8179.4–179.5Ho, Er, Tm, Yb, Lu[45]
[(NiL25)2Ln(O2CMe)3(MeOH)x] fO6N2O7,N2O8178.0–178.2Ce, Gd[46]
[(NiL25)2Ln(O2CPh)3(solv)x] gO6N2O8176.6Gd[46]
[(NiL26)2Ln(O2CMe)3(MeOH)2]O4S2N2O8176.9Pr[47]
[{Ni(piv)3(bpy)}2Ln(NO3)] hN2O4O8152.9, 153.3Sm, Gd[48]
[{Ni(piv)3(Hpiv)(MeCN)}2Ln(NO3)]NO5O8144.0Sm[48]
[{Ni(L27)3}2Ln](NO3)N3O3O6180.0Tb[10,11]
[(Ni(HL28)3}2Ln](NO3)N3O3O6180.0Tb[49]
[Ni2(L29’)3(L29’’)Ln(NO3)(H2O)](ClO4)2N3O3N2O654.2, 54.4Gd, Tb[50,51]
[Ni2(L29’)4Ln(NO3)(H2O)][Ln(NO3)5](ClO4)2N3O3N2O653.7Tb, Dy, Y[50,51]
[{{(NiL30(H2O))(N(CN)2)}2Ln}2(μ-NO3)](OH)N3O3O9, O10175.7Pr[52]
[{Ni2(L24)Ln(H2O)4}2(C2O4)3]O6N2O8178.6Gd[53]
[{Ni(L19)2}2Ln(H2O)Fe(CN)6]2N3O2O9179.1Dy[54]
[(Ni(L19)2}2Ln(H2O)Cr(CN)6]2N3O2O9177.6–178.5Y, Gd, Tb, Dy[55]
[(Ni(L19)2}2Ln(H2O)Fe(CN)6]2N3O2O9177.8–179.3Y, Gd, Tb[55]
[(Ni(L19)2}2Ln(H2O)Co(CN)6]2N3O2O9177.8Dy[55]
[(Ni(L19)2}2Ln(H2O)W(CN)8]2N3O2O9175.2/177.1 iTb, Dy, Y[56]
[(Ni(L19)2}2Ln(H2O)Co(CN)6]2N3O2O9177.6Tb[56]
[{(NiL31)2Ln(NO3)3}pyz]nN3O3O9149.6, 153.5Dy[57]
[{(NiL24)2Ln(H2O)4(oxy-bbz)}(NO3)]nO6N2O8178.8Dy[58]
[{[Ni(L32)1.5]2Ln(H2O)5}(CF3SO3)]nN6O796.7Tb[59]
[{[Ni(L32)1.5)]2Ln(H2O)4(NO3)]nN6O899.2, 99.3Gd, Tb[59]
[{[Ni(L33)1.5]2Ln(H2O)5}(NO3)]nN6O799.8Tb[60]
[{[Ni(L33)1.5]2Ln(H2O)6}(CF3SO3)]nN6O872.8, 73.1Tb, Dy[60]
[{(Ni(H2L34))2Ln(H2O)3}(ClO4)3]nN4O9179.2–179.4Pr, Sm, Gd[61]
[{(Ni(L35)1.5)2Ln(HL35)(dmf)4}(ClO4)4]nN4O2O8111.5, 111.9Gd, Tb[62]
[{(Ni(L35)2)2Ln(dmf)4}(ClO4)3]nN4O2O8115.1Dy[62]
[{Ni(L36)1.5}2Ln(dpds)2(H2O)4]nN2O4O9145.7Eu[63]
[{Ni(L37)2.5(N3)(H2O)}2Ln(H2O)]nN3O3O984.9–131.8Gd, Pr, Nd[64,65]
a x = 2 H2O/(MeOH)0.5/(EtOH)0.5 in Ni2La, x = 2 H2O/MeOH in Ni2Dy, x = 1, H2O in Ni2Yb; b x = 1 H2O in Ni2Sm/Ni2Eu/Ni2Tb x = 0 in all other complexes; c trif = CF3SO3−; d tren = tris(2-aminoethyl)amine; e dcnm = dicyanonitrosomethanide; f x = 2 or 3; g x = 2 MeOH in Ni2Gd, x = 2 MeOH/H2O in Ni2Ce; h Hpiv = pivalic acid, bpy = 2,2′-bipyridine; i average values for two crystallographically independent molecules.
Table
Table 2. Magnetic exchange parameters of the trinuclear Ni2Ln complexes.
Table 2. Magnetic exchange parameters of the trinuclear Ni2Ln complexes.
ComplexJNiLn (cm−1)JNiNi (cm−1)gNi/gLnD (cm−1)Ref
[(NiL1)2Gd](ClO4)+0.375 2.04 [17]
[(NiL2)2Gd(NO3)]+0.19 2.24+2.1[19,20]
[(NiL3)2Gd](NO3)0.91 1.984.5[21]
[(NiL4)2Gd](NO3) a0.64 2.04 [22]
[(NiL7)2Gd)](ClO4)1.02 2.01 [26]
[(NiL9)2Y(solv)x](ClO4) b −0.2942.091.933[28]
[(NiL9)2Lu(solv)x](ClO4) c −0.1292.182.838[28]
[(NiL9)2Gd(solv)x](ClO4) d−0.009−0.3772.102/1.974 [28]
[(NiL113)2Gd](NO3)+0.54 2.01/2.01 [30,31]
[(NiL113)2La](NO3) +0.462.245+4.91[30,31]
[{Ni(HL12)2}2La(NO3)](NO3)2 −0.9782.1773.133[32]
[{Ni(H3L13)2}2Gd](NO3)3+0.67 2.1174.92[33]
[{Ni(H3L14)2}2Sm(O2CMe)2](NO3)3 e −0.371.97 [34]
[{Ni(H3L14)2}2Gd(O2CMe)2](NO3)3+0.42 1.98/1.98+2.95[34]
[(NiL17)2La(O2CMe)2(MeOH)2](NO3) −0.752.18 [37,38]
[(NiL17)2Ce(O2CMe)2(MeOH)2](NO3) −1.12.23 [37,38]
[(NiL17)2Pr(O2CMe)2(MeOH)2](NO3) −1.32.15 [37,38]
[{NiL19(H2O)}2Gd(H2O)](trif)3 f4.8 2.030.03[21,40]
[{Ni(H2L20)(tren)2}2Gd](NO3)3−0.083 2.03 [41]
[{Ni(H2L20)(tren)2}2Lu](NO3)3 2.193.2[41]
[(NiL24)2La(NO3)3(H2O)4] −0.632.22 [45]
[(NiL24)2Lu(NO3)3(H2O)4] −0.652.17 [45]
[(NiL24)2Gd(NO3)3(H2O)4] g+0.79 2.20/2.02 [45]
[{Ni(piv)3(bpy)}2Gd(NO3)] h+0.105−0.702.015/2.0 [48]
[{Ni(piv)3(Hpiv)(MeCN)}2Gd(NO3)] i0.44−2.252.0/2.0 [48]
[{Ni(piv)3(Hpiv)(MeCN)}2La(NO3)] j −1.02.24 [48]
[Ni2(L29’)3(L29’’)Gd(NO3)(H2O)](ClO4)2 k+1.03 2.246 [50,51]
[Ni2(L29’)4Y(NO3)(H2O)][Ln(NO3)5](ClO4)2 l 02.15 [50,51]
azJ′ = 0.0009 cm−1; b E/D = 0.154 cm−1; c E/D = 0.468 cm−1; d TIP = 0.003 cm3Kmol−1; e TIP = 0.001 cm3mol−1; f jNiGd = 0.05(2) cm−1; g J′ = −0.64 cm−1; h tip = 0.0001; i including molar content of the S = 1 impurity is 5.5%; j zJ′ = +0.9(1) cm−1, tip = 2.4(9) × 10−4; k J′ = +0.9(2) cm−1; l J′ = +8.0(2) cm−1.
Table
Table 3. Complexes containing the NiII-LnIII-LnII moiety that behave as Single-Molecule Magnets (SMMs).
Table 3. Complexes containing the NiII-LnIII-LnII moiety that behave as Single-Molecule Magnets (SMMs).
ComplexU (K)τ0 (s)Ref.
[(NiL1)2Dy](ClO4) a10.82.3 × 10−5[17]
[(NiL4)2Dy](NO3) b14.171.09 × 10−6[22]
[(NiL4)2Dy](ClO4) b11.136.72 × 10−6[22]
[{Ni(L19)2}2Dy(H2O)Fe(CN)6]2 b25.01.6 × 10−7[54]
[(Ni(L19)2}2Tb(H2O)Cr(CN)6]2 b21.94.71 × 10−8[55]
[(Ni(L19)2}2Dy(H2O)Cr(CN)6]2 b38.94.89 × 10−9[55]
[(Ni(L19)2}2Dy(H2O)Cr(CN)6]2·2PPPO c37.26.44 × 10−9[55]
[(Ni(L19)2}2Tb(H2O)Fe(CN)6]2 b29.64.52 × 10−10[55]
[(Ni(L19)2}2Dy(H2O)Co(CN)6]2 b24.44.94 × 10−7[55]
[(Ni(L19)2}2Tb(H2O)W(CN)8]2 b23.02.57 × 10−7[56]
[(Ni(L19)2}2Dy(H2O)W(CN)8]2 b26.46.0 × 10−8[56]
a under 3500 Oe dc field; b under zero dc field; c PPPO = 4-(3-phenylpropyl)pyridine-1-oxide.
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Raptopoulou, C.P. Heterometallic Complexes Containing the NiII-LnIII-NiII Moiety—Structures and Magnetic Properties. Crystals 2020, 10, 1117. https://doi.org/10.3390/cryst10121117

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Raptopoulou CP. Heterometallic Complexes Containing the NiII-LnIII-NiII Moiety—Structures and Magnetic Properties. Crystals. 2020; 10(12):1117. https://doi.org/10.3390/cryst10121117

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Raptopoulou, Catherine P. 2020. "Heterometallic Complexes Containing the NiII-LnIII-NiII Moiety—Structures and Magnetic Properties" Crystals 10, no. 12: 1117. https://doi.org/10.3390/cryst10121117

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