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Reactivity of Metal Complexes with Redox-Active Ligands

A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Inorganic Chemistry".

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 17632

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Guest Editor
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3 Akad. Lavrentiev Ave., 630090 Novosibirsk, Russia
Interests: chemistry of coordination and cluster compounds of transition metals; chalcogenide clusters of early transition metals; transition metal complexes with redox-active ligands
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Special Issue Information

Dear Colleagues,

Redox-active ligands are capable of being in several stable oxidation states, between which reversible redox changes are possible. They were introduced in the late 1960s and are now widely exploited by synthetic chemists to obtain coordination compounds of metals with unusual chemical properties.

The classical redox-active ligands include α-diimines, dithiolenes, and o-quinones. A unique property of these ligands lies in their ability to reversibly transform into radical anion or dianionic forms while maintaining a bond with a metal atom. Acenaphthene-1,2-diimine (bian) ligands are of particular note, as they are capable of accepting up to four electrons due to the reduction of both diimine and naphthalene components.

Transition metal complexes in combination with redox-active ligands are often used to stimulate redox-based catalytic reactions and to activate small molecules. Redox-active ligands are part of many metalloenzymes in living systems, participating in electron transfer processes. For non-transition metals, unlike transition metals, a change in the oxidation state is not characteristic. However, the combination of non-transition metals and a redox-active ligand induces oxidative addition and reductive elimination reactions. This property makes such complexes similar to catalytically active compounds of transition metals and opens up the possibility of their use in catalysis.

Dr. Artem L. Gushchin
Guest Editor

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Keywords

  • Redox-active ligands
  • Transition metals
  • Non-transition metals
  • Reactivity
  • Synthesis
  • Structure
  • Catalysis
  • Activation of small molecules
  • Electrochemistry

Published Papers (8 papers)

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Research

14 pages, 2108 KiB  
Article
Insights into Triazolylidene Ligands Behaviour at a Di-Iron Site Related to [FeFe]-Hydrogenases
by Andrea Mele, Federica Arrigoni, Catherine Elleouet, François Y. Pétillon, Philippe Schollhammer and Giuseppe Zampella
Molecules 2022, 27(15), 4700; https://doi.org/10.3390/molecules27154700 - 22 Jul 2022
Cited by 2 | Viewed by 1289
Abstract
The behaviour of triazolylidene ligands coordinated at a {Fe2(CO)5(µ-dithiolate)} core related to the active site of [FeFe]-hydrogenases have been considered to determine whether such carbenes may act as redox electron-reservoirs, with innocent or non-innocent properties. A novel complex featuring [...] Read more.
The behaviour of triazolylidene ligands coordinated at a {Fe2(CO)5(µ-dithiolate)} core related to the active site of [FeFe]-hydrogenases have been considered to determine whether such carbenes may act as redox electron-reservoirs, with innocent or non-innocent properties. A novel complex featuring a mesoionic carbene (MIC) [Fe2(CO)5(Pmpt)(µ-pdt)] (1; Pmpt = 1-phenyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene; pdt = propanedithiolate) was synthesized and characterized by IR, 1H, 13C{1H} NMR spectroscopies, elemental analyses, X-ray diffraction, and cyclic voltammetry. Comparison with the spectroscopic characteristics of its analogue [Fe2(CO)5(Pmbt)(µ-pdt)] (2; Pmbt = 1-phenyl-3-methyl-4-butyl-1,2,3-triazol-5-ylidene) showed the effect of the replacement of a n-butyl by a phenyl group in the 1,2,3-triazole heterocycle. A DFT study was performed to rationalize the electronic behaviour of 1, 2 upon the transfer of two electrons and showed that such carbenes do not behave as redox ligands. With highly perfluorinated carbenes, electronic communication between the di-iron site and the triazole cycle is still limited, suggesting low redox properties of MIC ligands used in this study. Finally, although the catalytic performances of 2 towards proton reduction are weak, the protonation process after a two-electron reduction of 2 was examined by DFT and revealed that the protonation process is favoured by S-protonation but the stabilized diprotonated intermediate featuring a {Fe-H⋯H-S} interaction does not facilitate the release of H2 and may explain low efficiency towards HER (Hydrogen Evolution Reaction). Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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17 pages, 1792 KiB  
Article
Redox-Switchable Behavior of Transition-Metal Complexes Supported by Amino-Decorated N-Heterocyclic Carbenes
by Mirko Ruamps, Stéphanie Bastin, Lionel Rechignat, Alix Sournia-Saquet, Laure Vendier, Noël Lugan, Jean-Marie Mouesca, Dmitry A. Valyaev, Vincent Maurel and Vincent César
Molecules 2022, 27(12), 3776; https://doi.org/10.3390/molecules27123776 - 11 Jun 2022
Cited by 2 | Viewed by 1591
Abstract
The coordination chemistry of the N-heterocyclic carbene ligand IMes(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d6 [Mn(I), Fe(II)], d8 [Rh(I)], and d10 [Cu(I)] transition-metal centers. [...] Read more.
The coordination chemistry of the N-heterocyclic carbene ligand IMes(NMe2)2, derived from the well-known IMes ligand by substitution of the carbenic heterocycle with two dimethylamino groups, was investigated with d6 [Mn(I), Fe(II)], d8 [Rh(I)], and d10 [Cu(I)] transition-metal centers. The redox behavior of the resulting organometallic complexes was studied through a combined experimental/theoretical study, involving electrochemistry, EPR spectroscopy, and DFT calculations. While the complexes [CuCl(IMes(NMe2)2)], [RhCl(COD)(IMes(NMe2)2)], and [FeCp(CO)2 (IMes(NMe2)2)](BF4) exhibit two oxidation waves, the first oxidation wave is fully reversible but only for the first complex the second oxidation wave is reversible. The mono-oxidation event for these complexes occurs on the NHC ligand, with a spin density mainly located on the diaminoethylene NHC-backbone, and has a dramatic effect on the donating properties of the NHC ligand. Conversely, as the Mn(I) center in the complex [MnCp(CO)2 ((IMes(NMe2)2)] is easily oxidizable, the latter complex is first oxidized on the metal center to form the corresponding cationic Mn(II) complex, and the NHC ligand is oxidized in a second reversible oxidation wave. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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9 pages, 2476 KiB  
Communication
Oxazolidine Nitroxide Transformation in a Coordination Sphere of the Ln3+ Ions
by Philippe Rey, Anton I. Smolentsev and Kira E. Vostrikova
Molecules 2022, 27(5), 1626; https://doi.org/10.3390/molecules27051626 - 01 Mar 2022
Cited by 2 | Viewed by 1395
Abstract
Upon the interaction of the hydrated lanthanide(III) salts found in acetonitrile solution with a tripodal paramagnetic compound, 4,4-dimethyl-2,2-bis(pyridin-2-yl)-1,3-oxazolidine-3-oxyl (Rad), functionalized by two pyridyl groups, three neutral, structurally characterized complexes with diamagnetic polydentate ligands—[Dy(RadH)(hbpm)Cl2], [Yb2(ipapm)2(NO3)4 [...] Read more.
Upon the interaction of the hydrated lanthanide(III) salts found in acetonitrile solution with a tripodal paramagnetic compound, 4,4-dimethyl-2,2-bis(pyridin-2-yl)-1,3-oxazolidine-3-oxyl (Rad), functionalized by two pyridyl groups, three neutral, structurally characterized complexes with diamagnetic polydentate ligands—[Dy(RadH)(hbpm)Cl2], [Yb2(ipapm)2(NO3)4], and [Ce2(ipapm)2(NO3)4(EtOAc)2]—were obtained. These coordination compounds are minor uncolored crystalline products, which were formed in a reaction mixture due to the Rad transformation in a lanthanide coordination sphere, wherein the processes of its simultaneous disproportionation, hydrolysis, and condensation proceed differently than in the absence of Ln ions. The latter fact was confirmed by the formation of the structurally characterized product of the oxazolidine nitroxide transformation during its crystallization in toluene solution. Such a conversion in the presence of 4f elements ions is unique since no similar phenomenon was observed during the synthesis of the 3d-metal complexes with Rad. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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26 pages, 6227 KiB  
Article
Novel Oxidovanadium Complexes with Redox-Active R-Mian and R-Bian Ligands: Synthesis, Structure, Redox and Catalytic Properties
by Anton N. Lukoyanov, Iakov S. Fomenko, Marko I. Gongola, Lidia S. Shul’pina, Nikolay S. Ikonnikov, Georgiy B. Shul’pin, Sergey Y. Ketkov, Georgy K. Fukin, Roman V. Rumyantcev, Alexander S. Novikov, Vladimir A. Nadolinny, Maxim N. Sokolov and Artem L. Gushchin
Molecules 2021, 26(18), 5706; https://doi.org/10.3390/molecules26185706 - 21 Sep 2021
Cited by 26 | Viewed by 2723
Abstract
A new monoiminoacenaphthenone 3,5-(CF3)2C6H3-mian (complex 2) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl2(dpp-mian)(CH3CN)] (3) and [VOCl(3,5-(CF3) [...] Read more.
A new monoiminoacenaphthenone 3,5-(CF3)2C6H3-mian (complex 2) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl2(dpp-mian)(CH3CN)] (3) and [VOCl(3,5-(CF3)2C6H3-bian)(H2O)][VOCl3(3,5-(CF3)2C6H3-bian)]·2.85DME (4) from [VOCl2(CH3CN)2(H2O)] (1) or [VCl3(THF)3]. The structure of all compounds was determined using X-ray structural analysis. The vanadium atom in these structures has an octahedral coordination environment. Complex 4 has an unexpected structure. Firstly, it contains 3,5-(CF3)2C6H3-bian instead of 3,5-(CF3)2C6H3-mian. Secondly, it has a binuclear structure, in contrast to 3, in which two oxovanadium parts are linked to each other through V=O···V interaction. This interaction is non-covalent in origin, according to DFT calculations. In structures 2 and 3, non-covalent π-π staking interactions between acenaphthene moieties of the neighboring molecules (distances are 3.36–3.40 Å) with an estimated energy of 3 kcal/mol were also found. The redox properties of the obtained compounds were studied using cyclic voltammetry in solution. In all cases, the reduction processes initiated by the redox-active nature of the mian or bian ligand were identified. The paramagnetic nature of complexes 3 and 4 has been proven by EPR spectroscopy. Complexes 3 and 4 exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The yields of products of cyclohexane oxidation were 43% (complex 3) and 27% (complex 4). Based on the data regarding the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play the most crucial role in the reaction. The initial products in the reactions with alkanes are alkyl hydroperoxides, which are easily reduced to their corresponding alcohols by the action of triphenylphosphine (PPh3). According to the DFT calculations, the difference in the catalytic activity of 3 and 4 is most likely associated with a different mechanism for the generation of OH radicals. For complex 4 with electron-withdrawing CF3 substituents at the diimine ligand, an alternative mechanism, different from Fenton’s and involving a redox-active ligand, is assumed. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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16 pages, 3202 KiB  
Article
Selective Binding and Redox-Activity on Parallel G-Quadruplexes by Pegylated Naphthalene Diimide-Copper Complexes
by Valentina Pirota, Enrico Lunghi, Alessandra Benassi, Emmanuele Crespan, Mauro Freccero and Filippo Doria
Molecules 2021, 26(16), 5025; https://doi.org/10.3390/molecules26165025 - 19 Aug 2021
Cited by 2 | Viewed by 2270
Abstract
G-quadruplexes (G4s) are higher-order supramolecular structures, biologically important in the regulation of many key processes. Among all, the recent discoveries relating to RNA-G4s, including their potential involvement as antiviral targets against COVID-19, have triggered the ever-increasing need to develop selective molecules able to [...] Read more.
G-quadruplexes (G4s) are higher-order supramolecular structures, biologically important in the regulation of many key processes. Among all, the recent discoveries relating to RNA-G4s, including their potential involvement as antiviral targets against COVID-19, have triggered the ever-increasing need to develop selective molecules able to interact with parallel G4s. Naphthalene diimides (NDIs) are widely exploited as G4 ligands, being able to induce and strongly stabilize these structures. Sometimes, a reversible NDI-G4 interaction is also associated with an irreversible one, due to the cleavage and/or modification of G4s by functional-NDIs. This is the case of NDI-Cu-DETA, a copper(II) complex able to cleave G4s in the closest proximity to the target binding site. Herein, we present two original Cu(II)-NDI complexes, inspired by NDI-Cu-DETA, differently functionalized with 2-(2-aminoethoxy)ethanol side-chains, to selectively drive redox-catalyzed activity towards parallel G4s. The selective interaction toward parallel G4 topology, controlled by the presence of 2-(2-aminoethoxy)ethanol side chains, was already firmly demonstrated by us using core-extended NDIs. In the present study, the presence of protonable moieties and the copper(II) cavity, increases the binding affinity and specificity of these two NDIs for a telomeric RNA-G4. Once defined the copper coordination relationship and binding constants by competition titrations, ability in G4 stabilization, and ROS-induced cleavage were analyzed. The propensity in the stabilization of parallel topology was highlighted for both of the new compounds HP2Cu and PE2Cu. The results obtained are particularly promising, paving the way for the development of new selective functional ligands for binding and destructuring parallel G4s. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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18 pages, 6410 KiB  
Article
Square-Planar Heteroleptic Complexes of α-Diimine-NiII-Catecholate Type: Intramolecular Ligand-to-Ligand Charge Transfer
by Kira I. Pashanova, Vladlena O. Bitkina, Ilya A. Yakushev, Maxim V. Arsenyev and Alexandr V. Piskunov
Molecules 2021, 26(15), 4622; https://doi.org/10.3390/molecules26154622 - 30 Jul 2021
Cited by 10 | Viewed by 2109
Abstract
Two heteroleptic NiII complexes combined the redox-active catecholate and 2,2′- bipyridine ligand platforms were synthesized to observe a photoinduced intramolecular ligand-to-ligand charge transfer (LL’CT, HOMOcatecholate → LUMOα-diimine). A molecular design of compound [NiII(3,6-Cat)(bipy)]∙CH3CN (1 [...] Read more.
Two heteroleptic NiII complexes combined the redox-active catecholate and 2,2′- bipyridine ligand platforms were synthesized to observe a photoinduced intramolecular ligand-to-ligand charge transfer (LL’CT, HOMOcatecholate → LUMOα-diimine). A molecular design of compound [NiII(3,6-Cat)(bipy)]∙CH3CN (1) on the base of bulky 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) was an annelation of the ligand with an electron donor glycol fragment, producing derivative [NiII(3,6-Catgly)(bipy)]∙CH2Cl2 (2), in order to influence the energy of LL’CT transition. A substantial longwave shift of the absorption peak was observed in the UV-Vis-NIR spectra of 2 compared with those in 1. In addition, the studied NiII derivatives demonstrated a pronounced negative solvatochromism, which was established using a broad set of solvents. The molecular geometry of both compounds can be ascribed as an insignificantly distorted square-planar type, and the π–π intermolecular stacking of the neighboring α-diimines is realized in a crystal packing. There is a lamellar crystal structure for complex 1, whereas the perpendicular T-motifs with the inter-stacks attractive π–π interactions form the packing of complex 2. The redox-active nature of ligand systems was clearly shown through the electrochemical study: a quasi-reversible one-electron reduction of 2,2′-bipyridine and two reversible successive one-electron oxidative conversations (“catecholate dianion—o-benzosemiquinonato radical anion—neutral o-benzoquinone”) were detected. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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13 pages, 2789 KiB  
Article
Spin Crossover in Nickel(II) Tetraphenylporphyrinate via Forced Axial Coordination at the Air/Water Interface
by Alexander V. Shokurov, Daria S. Kutsybala, Andrey P. Kroitor, Alexander A. Dmitrienko, Alexander G. Martynov, Yulia Yu. Enakieva, Aslan Yu. Tsivadze, Sofiya L. Selektor and Yulia G. Gorbunova
Molecules 2021, 26(14), 4155; https://doi.org/10.3390/molecules26144155 - 08 Jul 2021
Cited by 7 | Viewed by 2622
Abstract
Coordination-induced spin crossover (CISCO) in nickel(II) porphyrinates is an intriguing phenomenon that is interesting from both fundamental and practical standpoints. However, in most cases, realization of this effect requires extensive synthetic protocols or extreme concentrations of extra-ligands. Herein we show that CISCO effect [...] Read more.
Coordination-induced spin crossover (CISCO) in nickel(II) porphyrinates is an intriguing phenomenon that is interesting from both fundamental and practical standpoints. However, in most cases, realization of this effect requires extensive synthetic protocols or extreme concentrations of extra-ligands. Herein we show that CISCO effect can be prompted for the commonly available nickel(II) tetraphenylporphyrinate, NiTPP, upon deposition of this complex at the air/water interface together with a ruthenium(II) phthalocyaninate, CRPcRu(pyz)2, bearing two axial pyrazine ligands. The latter was used as a molecular guiderail to align Ni···Ru···Ni metal centers for pyrazine coordination upon lateral compression of the system, which helps bring the two macrocycles closer together and forces the formation of Ni–pyz bonds. The fact of Ni(II) porphyrinate switching from low- to high-spin state upon acquiring additional ligands can be conveniently observed in situ via reflection-absorption UV-vis spectroscopy. The reversible nature of this interaction allows for dissociation of Ni–pyz bonds, and thus, change of nickel cation spin state, upon expansion of the monolayer. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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12 pages, 1701 KiB  
Article
Synthesis, Structure and Electrochemical Properties of Acetamide- and Caprolactam-Containing Silicon Catecholates
by Eugenia P. Kramarova, Alexander D. Volodin, Vadim V. Negrebetsky, Anastasia D. Shagina, Teimur M. Aliev, Pavel V. Dorovatovskii, Roman A. Novikov, Anna V. Vologzhanina and Alexander A. Korlyukov
Molecules 2021, 26(12), 3548; https://doi.org/10.3390/molecules26123548 - 10 Jun 2021
Cited by 5 | Viewed by 2326
Abstract
Hexacoordinated heteroligand silicon catecholates, although being prospective as easily soluble compounds with high hydrolytic stability and diverse redox properties, have been insufficiently studied. The transesterification of 1-(trimethoxysilylmethyl)-2-oxohexahydroaze or N-methyl-N-(trimethoxysilylmethyl)acetamide by two equivalents of catechol derivatives in the presence of dicyclohexylamine [...] Read more.
Hexacoordinated heteroligand silicon catecholates, although being prospective as easily soluble compounds with high hydrolytic stability and diverse redox properties, have been insufficiently studied. The transesterification of 1-(trimethoxysilylmethyl)-2-oxohexahydroaze or N-methyl-N-(trimethoxysilylmethyl)acetamide by two equivalents of catechol derivatives in the presence of dicyclohexylamine afforded a series of target compounds in good yield. The complexes were characterized using elemental analysis, FTIR, 1H, 13C and 29Si NMR spectra, X-ray crystallography and cyclic voltammetry. X-ray diffraction confirmed that the silicon atom possesses the octahedral geometry of the SiCO5 polyhedron that remains unchanged in solution as it follows from 29Si NMR data. The compounds demonstrated up to three oxidation waves; and the reduction profile strongly depended on the nature of the substituents on a catecholate anion. Full article
(This article belongs to the Special Issue Reactivity of Metal Complexes with Redox-Active Ligands)
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