Mixed Metal Phosphonates: Structure and Proton Conduction Manipulation through Various Alkaline Earth Metal Ions

Abstract

Three new layered mixed metal phosphonates [CoMg(notpH₂)(H₂O)₂]ClO₄·nH₂O (CoMg·nH₂O), [Co₂Sr₂(notpH₂)₂(H₂O)₅](ClO₄)₂·nH₂O (CoSr·nH₂O), and [CoBa(notpH₂)(H₂O)_1.5]ClO₄ (CoBa) were synthesized by reacting a tripodal metalloligand Co^III(notpH₃) [notpH₆ = C₉H₁₈N₃(PO₃H₂)₃] with alkaline earth metal ions. Along with an increase in the radius of the alkaline earth metal ions, the 6-coordinate {MgO₆}, 7-coordinate {SrO₇}, and 9-coordinate {BaO₉} geometries are the distorted octahedron, capped triangular prism, and tricapped triangular prism, respectively. Consequently, the metalloligand Co(notpH₂)⁻ adopts variable coordination modes to bind the alkaline earth metal nodes, forming diverse layer topologies in the three mixed metal phosphonates. The AC impedance measurements revealed that the proton conductivities at 25 °C and 95% relative humidity (RH) follow the sequence: CoMg·nH₂O > CoSr·nH₂O > CoBa. As expected, CoMg·nH₂O exhibits a 28-fold enhanced value for proton conductivity (4.36 × 10⁻⁴ S cm⁻¹) compared with the previously reported isostructural compound, CoCa·nH₂O, at 25 °C and 95% RH due to the greater Lewis acid strength of Mg(II) lowering the pKa of the coordinated water.

1. Introduction

Metal phosphonates (MPs) are a subclass of organic–inorganic hybrid materials, often combined with high water and thermal stability, showing potentially valuable properties such as ion exchange, sorption, catalysis, and proton conduction [1,2,3,4,5]. Incorporating mixed metal species can further expand the structural diversity of MPs and tune their properties through different metal components [6,7]. However, synthesizing mixed metal phosphonates is often challenging in terms of controlling, predicting, and crystallizing the final product. The coordination-driven self-assembly is highly influenced by numerous factors, such as temperature, pH value, solubility, coordination modes of the ligand, and other weak interactions [8]. Generally, there are three synthetic approaches to producing mixed metal phosphonates: (1) directly reacting different metal salts with ligands; (2) using ditopic or polytopic ligands that consist of two or more metal ion receptors; (3) using well-defined metal complexes as the metalloligands. The metalloligand approach is more controllable among the three approaches and has developed remarkably in the synthesis of mixed metal coordination polymers [9,10]. However, such a strategy is limited to mixed metal phosphonates due to rare phosphonate-based metalloligands compared to carboxylate-based, azole-based, and pyridine-based metalloligands [11].

In our previous work, we explored a neutral mononuclear complex Co(notpH₃) based on tripodal phosphonic acid [notpH₆ = C₉H₁₈N₃(PO₃H₂)₃] as the metalloligand, which can serve as a bi, tri, or tetradentate ligand to bind various metal cations such as Ag(I), Ca(II), Co(II), Ni(II), and Ln(III) [11,12,13]. In these Co(notpH_x)^3-x-based mixed metal phosphonates, the selected second metal ions can influence the structural dimensionality, the degree of protonation in phosphonate groups, and the amount of coordinated water. Such variable factors agree with the design considerations (proton sources and proton transfer pathways) of intrinsic proton-conductive materials and are ideal for systematically understanding the relationship between proton conductivity and structure [14,15]. Recently, isostructural [M^II₃Co^III₂(notp)₂(H₂O)₁₂]·2H₂O (M = Co or Ni) provided an example of the effect of hydrated metal ions on proton conduction, demonstrating the enhancement of proton conductivity as a result of the stronger Lewis acidity of Co(II) over Ni(II) [13]. As we know, the Lewis acid strengths of divalent alkaline earth metal ions follow the tendency: Be > Mg > Ca > Sr > Ba [16], and their unusual coordination geometries are often observed [17]. The reported [CoCa(notpH₂)(H₂O)₂]ClO₄·nH₂O (CoCa·nH₂O) shows a moderate proton conductivity of 1.55 × 10⁻⁵ S cm⁻¹ at 25 °C and 95% relative humidity (RH) [15]. It would be interesting to further study how the various alkaline earth metal nodes affect the topology and proton-conducive properties in related compounds. Herein, we use the metalloligand Co(notpH₃) to react with Mg(II), Sr(II), and Ba(II) ions to obtain three new Co-M phosphonates: [CoMg(notpH₂)(H₂O)₂]ClO₄·nH₂O (CoMg·nH₂O), [Co₂Sr₂(notpH₂)₂(H₂O)₅](ClO₄)₂·nH₂O (CoSr·nH₂O), and [CoBa(notpH₂)(H₂O)_1.5]ClO₄ (CoBa) (Figure 1). The synthesis, crystal structures, and proton-conductive properties of these compounds are reported.

2. Materials and Methods

The metalloligand Co(notpH₃)·3H₂O was synthesized using the literature procedure [12]. All other starting materials, reagents, and solvents were obtained from commercial suppliers and were used without further purification. The infrared spectra were recorded on a Bruker Tensor 27 spectrometer using KBr pellets, and the powder X-ray diffraction patterns were obtained with a Bruker D8 advance diffractometer using Cu-K_α radiation (λ = 1.5406 Å). Thermogravimetric analyses (TGA) were conducted with a Mettler Toledo TGA/DSC instrument from 25 to 500 °C, with a heating rate of 5 °C min⁻¹ under a nitrogen atmosphere. The conductivities of the sample pellets were obtained by AC impedance measurements, which were carried out under different environmental conditions by the conventional quasi-four-probe method with a Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface in the frequency range 1.0 MHz–0.1 Hz. The electrical contacts were prepared using the gold paste to attach the 50 μm-diameter gold wires to the 2.5 mm-diameter compressed pellets or selected single crystals. Exposure of the samples to a humid environment (40% to 95%) at different temperatures (15 to 45 °C) was performed using a GSJ-100 (Su-Ying Corp.) humidity-controlled oven.

2.1. Synthesis and Crystallization of [CoMg(notpH₂)(H₂O)₂]ClO₄·nH₂O (CoMg·nH₂O)

Mg(OH)₂ (0.60 mmol, 35 mg) was added to a solution of Co(notpH₃)·3H₂O (0.20 mmol, 104 mg) in water (10 mL). The suspension was stirred overnight at 100 °C and then filtered. A concentration of 1.0 mol/L HClO₄ adjusted the filtrate to pH 3.0; this was then left at room temperature for one week to afford the violet rectangular plate-like crystals of CoMg nH₂O at 34% (based on Co). IR (KBr, cm⁻¹): 3420(br), 2922(s), 2850(m), 2403(w), 1655(m), 1487(w), 1467(w), 1420(w), 1304(w), 1286(w), 1252(w), 1215(w), 1140(s), 1120(w), 1078(s), 1053(w), 1028(s), 1007(w), 968(w), 949(m), 864(w), 802(w), 781(m), 756(w), 625(m), 608(s), 526(w), 500(w), 488(w), 438(w).

2.2. Synthesis and Crystallization of [Co₂Sr₂(notpH₂)₂(H₂O)₅](ClO₄)₂·nH₂O (CoSr·nH₂O)

This compound was synthesized following a similar procedure to that of CoMg, except that Sr(OH)₂ (0.60 mmol, 73 mg) was used, and the filtrate was adjusted to pH 2.4. The violet tetragonal crystals of CoSr·nH₂O were precipitated from the filtrate and collected after two days. Yield: 70% (based on Co). IR (KBr, cm⁻¹): 3410(br), 2995(m), 2923(m), 2854(w), 1634(m), 1493(w), 1468(w), 1420(w), 1306(w), 1284(w), 1254(w), 1194(m), 1163(m), 1144(m), 1121(s), 1107(s), 1090(s), 1026(m), 995(s), 953(m), 924(w), 843(w), 804(w), 781(w), 758(w), 609(s), 586(m), 525(w), 492(w), 436(w).

2.3. Synthesis and Crystallization of [CoBa(notpH₂)(H₂O)₃]ClO₄ (CoBa)

This compound was synthesized following a similar procedure to that of CoMg, except that Ba(OH)₂ (0.60 mmol, 103 mg) was used, and the filtrate was adjusted to pH 2.5. The violet hexagonal plate-like crystals of CoBa were precipitated from the filtrate and collected after one day. Yield: 77% (based on Co). IR (KBr, cm⁻¹): 3435(br), 2976(w), 2929(w), 2858(w), 2414(w), 1637(m), 1473(w), 1445(w), 1306(w), 1249(w), 1211(m), 1188(w), 1153(w), 1121(s), 1107(s), 1065(s), 1020(m), 1002(s), 955(w), 926(m), 854(w), 804(w), 783(m), 756(w), 607(m), 586(w), 484(w), 449(w).

2.4. Structure Determinations

For CoMg·4H₂O and CoBa, single crystals with dimensions of 0.30 × 0.10 × 0.05 mm³ and 0.30 × 0.30 × 0.05 mm³ were respectively selected and sealed in the mother solution for data collection on a Bruker SMART APEX II diffractometer using graphite-monochromated Mo-K_α radiation (λ = 0.71073 Å) at room temperature (296 K). For CoSr·2H₂O, a single crystal with dimensions of 0.40 × 0.10 × 0.10 mm³ was used for data collection on a Bruker D8 diffractometer using graphite-monochromated Mo-K_α radiation (λ = 0.71073 Å) at 123 K. The numbers of collected and observed independent [I > 2σ(I)] reflections were 16662 and 5773 (R_int = 0.075) for CoMg·4H₂O, 16662 and 11247 (R_int = 0.054) for CoSr·2H₂O, and 10899 and 1384 (R_int = 0.049) for CoBa. The data were integrated using the Siemens SAINT program [18]. Adsorption corrections were applied. The structures were solved by direct methods and refined on F² by full-matrix least-squares using SHELXTL [19,20]. Anisotropic temperature factors were used to refine all atoms, excluding hydrogen. All hydrogen atoms bound to carbon were refined isotropically in the riding mode; hydrogen atoms of water molecules were detected in the experimental electron density and then refined isotropically with reasonable restriction of O-H bond distances and H-O-H angles. The crystallographic data are given in Table S1, and selected bond lengths and angles are in Tables S2–S4.

3. Results

3.1. Structural Describes

A single crystal of CoMg·4H₂O was sealed in the mother solutions to keep the saturated lattice water content, and these were used for the X-ray single-crystal structural determination at room temperature. Structural analysis reveals that CoMg·4H₂O is isostructural to CoCa·4H₂O [15]. It crystallizes in monoclinic system space groups P2₁/n (Table S1) and contains one [Co(notpH₂)]⁻ ligand, one Mg²⁺, one ClO₄⁻, and two coordinated and four lattice water molecules in the asymmetric unit (Figure 2a). The [Co(notpH₂)]⁻ links four Mg atoms via the phosphonate oxygen atoms O2, O5, O8, and O9 as a tetradentate metalloligand. The two phosphonate oxygen atoms (O3 and O6) of the [Co(notpH₂)]⁻ unit are protonated. Each Mg atom is six-coordinated, with four sites provided by four phosphonate oxygens and two water molecules. The average Mg-O bond length is 2.108(3) Å [2.031(3)–2.253(4) Å], which is shorter than the average Ca-O bond length of 2.353(3) Å [2.273(3)–2.477(3) Å]. The {CoN₃O₃} and {MgO₆} octahedra are each corner-shared with the {PO₃C} tetrahedra, forming a positively charged two-dimensional waved layer containing 8- and 16-member rings (Figure 2b). The positively charged layers are charge-balanced by ClO₄⁻ anions. The interlayer spaces are filled with the ClO₄⁻ anions and the lattice water molecules (Figure 2c). CoMg·4H₂O has the same layer topology and hydrogen bond networks as CoCa·4H₂O, including the protonated sites of the phosphonate oxygen atoms.

Unlike CoMg·4H₂O and CoCa·4H₂O, when the Co(notpH₃) ligand is assembled with larger Sr²⁺ and Ba²⁺ ions, the resulting products are crystallized with distinct coordination geometries and layered structures. The compound CoSr·2H₂O crystallized in the triclinic system space group P-1 (Table S1). It contains two [Co(notpH₂)]⁻, two Sr²⁺, two ClO₄⁻, and five coordinated and three lattice water molecules (occupancy: O6W 1.0, O7W 0.5, and O8W 0.5) in the asymmetric unit (Figure 3a). Each [Co(notpH₂)]⁻ unit has two protonated phosphonate oxygen atoms (O3 and O9 in Co1 unit; O12 and O19 in Co2 unit). The [Co1(notpH₂)]⁻ behaves as a tetradentate metalloligand and connects four Sr atoms by four phosphonate oxygen atoms, O1, O2, O4, and O8. In contrast, the [Co2(notpH₂)]⁻ behaves as a tridentate metalloligand and connects three Sr atoms by three phosphonate oxygen atoms, O10, O11, and O14. The Sr1 atom is seven-coordinated with two water molecules and five phosphonate oxygen atoms (O2, O4, O4A, O8B, and O10) from the three [Co1(notpH₂)]⁻ and single [Co2(notpH₂)]⁻ ligands. The Sr2 atom also adopts a seven-coordinated geometry but is surrounded by three water molecules and four phosphonate oxygen atoms (O1, O11, O14, and O14C) from one [Co1(notpH₂)]⁻ and two [Co2(notpH₂)]⁻ ligands. The average Sr-O bond length is 2.574(5) Å [2.453(5)–2.659(5) Å] for Sr1 and 2.591(5) Å [2.472(5)–2.693(5) Å] for Sr2. Each pair of seven-coordinated Sr1 or Sr2 atoms are bridged by two oxygen atoms (O4 and O4A, or O14 and O14C) to form a Sr1₂O₂ or Sr2₂O₂ dimeric unit, and such dimeric units are further alternately connected by a pair of O-P-O to form an inorganic chain. The {Sr1O₇} units within the adjacent inorganic chains are interconnected via a pair of O-Co1-O-P-O, resulting in a positively charged two-dimensional layer (Figure 3b). The ClO₄⁻ anions and the lattice water molecules fill the interlayer spaces (Figure 3c).

The crystal’s space group of CoBa is R-3c with the hexagonal unit cell of a = 8.5991(15) and c = 97.615(18) Å. The asymmetric unit contains 1/3 [Co(notpH₂)]⁻, 1/3 Ba²⁺, 1/3 ClO₄⁻, and a half-coordinated water molecule (Figure 4a). Each Co, Ba, and Cl atom is located at a three-fold rotation axis parallel to the c-axis. Furthermore, each coordinated water molecule is located at a two-fold rotation axis parallel to the a-axis. In the [Co(notpH₂)]⁻ unit, two of the three equivalent phosphonate groups should be monoprotonated according to the charge balance. Therefore, the occupancy of the H atom in the asymmetric -PO₃H group is set as 2/3. The [Co(notpH₂)]⁻ behaves as a hexadentate metalloligand, which chelates one Ba atom with O1, O1A, and O1B and connects another three Ba atoms via O2, O2A, and O2B. The Ba atom is coordinated to six phosphonate oxygen atoms (O1, O1A, O1B, O2C, O2D, and O2E) from four [Co(notpH₂)]⁻ ligands and three water molecules (O1W, O1WA, and O2WB), showing a distorted tricapped trigonal prismatic geometry. The average Ba-O bond length is 2.853(10) Å [2.724(13)–2.948(8) Å]. The [Ba(H₂O)_1.5]_n layer with a hexagonal grid extends in the ab plane (Figure 4b), where both sides are covered by the [Co(notpH₂)]⁻ units. The positive coordination layers [CoBa(notpH₂)(H₂O)_1.5] exhibit an ABCABC type of three-dimensional packing and are charge balanced by the lattice ClO₄⁻ anions (Figure 4c).

3.2. Thermal Stability

The thermogravimetric analyses were performed on CoMg·nH₂O, CoSr·nH₂O, and CoBa to compare their thermal stabilities. As shown in Figure 5, the CoMg·nH₂O stored in the air (ca. 50% RH) shows a two-step decomposition process from 25 to 500 °C. Dehydration occurs below 130 °C, and the weight loss of 11.2% agrees with releasing two lattice water and two coordinated water molecules (calc. 10.9% for CoMg·2H₂O). It indicates that the collected product of CoMg·nH₂O lost two lattice water molecules per formula unit in the air and formed the dihydrate phase, CoMg·2H₂O, which is confirmed by the Pawley fitting of the PXRD pattern (Figure S1). The fitted unit cell parameters of CoMg·2H₂O (P2₁/n, a = 13.14 Å, b = 9.93 Å, c = 18.58 Å, β = 109.2°, V = 2289.3 ų) are identical to those of the reported compound, CoCa·2H₂O. Following a weight-loss plateau between 130 and 270 °C, the rapid weight loss is attributed to the pyrolysis of the organic moieties. The water loss of CoSr·nH₂O occurs below 120 °C, and the weight loss of 8.4% agrees with the release of two lattice water and five coordinated water molecules (calc. 8.8%). A broad weight-loss plateau starts from 120 °C to 280 °C, followed by rapid weight loss due to the pyrolysis of organic moieties. CoBa shows high thermal stability without mass change up to 270 °C. Above 270 °C, the removal of coordinated water and the pyrolysis of organic moieties happens simultaneously.

3.3. Proton Conduction

The proton conductivities of CoMg·nH₂O, CoSr·nH₂O, and CoBa were evaluated by impedance measurements (Figures S5–S13) using pressed pellets (1.0 Gpa) with thicknesses of 0.76, 0.75, and 0.69 mm, respectively. As shown in Figure 6a, all three samples and the reported compound, CoCa·nH₂O, exhibit humidity-dependent proton-conductivities under 25 °C. At 40% RH, the proton conductivities of CoMg·nH₂O, CoCa·nH₂O, CoSr·nH₂O, and CoBa are 1.83 × 10⁻⁷, 1.08 × 10⁻⁹, 1.14 × 10⁻⁸, and 4.28 × 10⁻⁸ S cm⁻¹, respectively. All samples’ conductivities increased with relative humidity, reaching maximum values at 95% RH. Furthermore, the conductivity at 95 % RH and 25 °C followed the sequence: CoMg·nH₂O (4.36 × 10⁻⁴ S cm⁻¹) > CoCa·nH₂O (1.55 × 10⁻⁵ S cm⁻¹) > CoBa (1.31 × 10⁻⁵ S cm⁻¹) > CoSr·nH₂O (3.02 × 10⁻⁶ S cm⁻¹).

The temperature dependence of the conductivities was measured at 95% RH from 15 to 45 °C at 10 °C intervals. Figure 6b shows the ln(σT) plots vs. 1000/T for all samples. The activation energies, E_a, are estimated to be 0.80 eV for CoMg·nH₂O, 0.76 eV for CoSr·nH₂O, and 0.54 eV for CoBa. CoMg·4H₂O has a continuous hydrogen-bonding network involving ClO₄⁻ anions, -PO₃H groups, and water molecules. Therefore, the large proton conduction activation energy of CoMg·4H₂O can arise from the rotation and movement of the ClO₄⁻ anions, as is the case for CoCa·4H₂O. In CoSr·nH₂O and CoBa, the hydrogen bonds are isolated, and the proton might migrate through the medium via the vehicle-type mechanism, corresponding to the large activation energies.

4. Discussion

4.1. The Effects of Varying the Alkaline Earth Metal Nodes on the Structures

It could be interesting to compare the different coordination modes of tripodal metalloligand Co(notpH_x)^x−3 with Mg(II), Ca(II), Sr(II), and Ba(II), respectively (Figure 7). Steric factors commonly govern the coordination geometry of the alkaline earth metal cations. In CoSr·2H₂O, 7-coordinate {SrO₇} is the distorted capped triangular prism. The smaller Mg(II) and Ca(II) ions are 6-coordinated to form the distorted octahedron in CoMg·4H₂O and CoCa·4H₂O. In contrast to Sr(II), the larger Ba(II) ion is 9-coordinated to form the distorted tricapped triangular prism {BaO₉} in CoBa. Moreover, water molecules occupy three vertices on the prism’s three triangular faces. The coordination number increases with the radius of the alkaline earth metal ions; consequently, the tripodal metalloligand Co(notpH_x)^x−3 adopts variable bonding modes. In CoMg·4H₂O and CoCa·4H₂O, the Co(notpH₂)⁻ offers four oxygen atoms to bind four Mg(II) or Ca(II) ions, and the other two oxygen atoms are protonated. In CoSr·2H₂O, two coordination modes for Co(notpH₂)⁻ are observed, and two μ-O atoms bridge the two Sr(II) ions and adjacent Co(III) and Sr(II) ions, respectively. Interestingly, Co(notpH₂)⁻ chelates the Ba(II) ion in CoBa using three oxygen atoms, which are coordinated with the Co(III) ion. Furthermore, each equivalent phosphonate group of Co(notpH₃) is monoprotonated (occupancy of H is 2/3), and the other three oxygen atoms bind to three Ba(II) ions. Such a coordination mode differs from the reported Co(notpH_x)^x−3-based mixed metal phosphonates [11,12,13].

4.2. The Effects of Varying the Alkaline Earth Metal Nodes on Proton Conduction

Sufficient acidic proton concentration and continuous hydrogen-bonding networks play critical roles in efficient proton conduction. Coordination water, the acidic moieties of the frameworks, and acidic guest molecules can act as proton sources. For CoMg·nH₂O, CoCa·nH₂O, CoSr·nH₂O, and CoBa, the equal number of protonated phosphonate oxygen atoms is 2 per ligand, and the numbers of coordinated water are 2, 2, 2.5, and 1,5, respectively. However, the lack of continuous hydrogen bonds in CoSr·nH₂O and CoBa leads to poorer proton conduction (Figure 3d). It is worth noting that CoBa contains no lattice water but exhibits humidity-dependent proton conduction, which could be attributed to the effect of the grain boundary using a pellet for measurement [21]. In isostructural CoMg·nH₂O and CoCa·nH₂O, the proton sources and proton pathways are identical (Figure 8), but CoMg·nH₂O exhibits a 28-fold enhanced value for proton conductivity compared with CoCa·nH₂O at 95% RH and 25 °C. The only difference between the structures of both of the compounds is the different dihydrated metal centers: Mg(II) and Ca(II). The pK_a values of the aqueous metal ions Mg(H₂O)₆²⁺ and Ca(H₂O)₇²⁺ are 11.2 and 12.7 [22], respectively. This indicates that the water binding to the Mg(II) ion can provide more acidic protons, agreeing with the higher proton conductivity of CoMg·nH₂O.

5. Conclusions

We utilized the metalloligand Co(notpH₃) to obtain a series of layered Co(III)-M(II) phosphonates (M = Mg, Sr, and Ba). The layer topology of CoMg·4H₂O is identical to that of CoMg·4H₂O, whereas CoSr·2H₂O and CoBa exhibit diverse layer topologies. Especially in CoBa, where the Co(notpH₂)⁻ chelates the Ba(II) ion using three oxygen atoms, which are also coordinated with the Co(III) ion, specializing in the other reported Co(notpH_x)^x−3-based mixed metal phosphonates. The proton conductivities of all compounds were evaluated using compacted pellets and followed the sequence (at 25 °C and 95% RH): CoMg·nH₂O (4.36 × 10⁻⁴ S cm⁻¹) > CoCa·nH₂O (1.55 × 10⁻⁵ S cm⁻¹) > CoBa (1.31 × 10⁻⁵ S cm⁻¹) > CoSr·nH₂O (3.02 × 10⁻⁶ S cm⁻¹). There are no continuous hydrogen-bonding networks in CoSr·2H₂O and CoBa, resulting in poor proton conduction. CoMg·nH₂O and CoCa·nH₂O are isostructural except for the Mg(II) and Ca(II) centers and have identical hydrogen-bonding networks as a proton transfer pathway. However, the Mg(II) center shows greater Lewis acid strength and makes the coordinated water more acidic, leading to a 28-fold enhanced proton conductivity in CoMg·nH₂O at 25 °C and 95% RH.