Redox Reactions of Nickel, Copper, and Cobalt Complexes with “Noninnocent” Dithiolate Ligands: Combined in Situ Spectroelectrochemical and Theoretical Study

Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site

Bimetallic transition metal complexes with site-specific redox properties offer a versatile platform for understanding electron polarization, intramolecular electron transfer processes, and customizing electronic and magnetic properties that might impact reactivity and catalyst design. Inspired by the dissymmetric nickel sites in the Acetyl CoA Synthase (ACS) Active Site, three new bimetallic Ni(N2S2)-Ni(S2C2R2) complexes based on Ni(N2S2) metalloligand donor synthons, Nid, in mimicry of the nickel site distal to the redox-active iron sulfur cluster of ACS, and nickel dithiolene receiver units, designated as Nip, the nickel proximal to the 4Fe4S cluster, were combined to explore the influence of ligand environment on electronic structure and redox properties of each unit. The combination of synthons gave a matrix of three S-bridged dinickel complexes, characterized by X-ray crystallography, and appropriate spectroscopies. Computational modeling is connected to the electronic characteristics of the nickel donor and receiver units. This study demonstrated the intricacies of identifying sites of electrochemical redox processes, within multi-metallic systems containing non-innocent ligands.

Nickel(II) Complexes with Redox Noninnocent Octaazamacrocycles as Catalysts in Oxidation Reactions

Nickel(II) complexes with 15-membered (1-5) and 14-membered (6) octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide were prepared by template synthesis. The compounds were characterized by elemental analysis, electrospray ionization mass spectrometry, IR, UV-vis, ¹H NMR spectroscopies, and X-ray diffraction. The complexes contain a low-spin nickel(II) ion in a square-planar coordination environment. The electrochemical behavior of 1-6 was investigated in detail, and the electronic structure of 1e-oxidized and 1e-reduced species was studied by electron paramagnetic resonance, UV-vis-near-IR spectroelectrochemistry, and density functional theory calculations indicating redox noninnocent behavior of the ligands. Compounds 1-6 were tested in the microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce the industrially significant mixture of cyclohexanol and cyclohexanone (i.e., A/K oil). The results showed that the catalytic activity was affected by several factors, namely, reaction time and temperature or amount and type of catalyst. The best values for A/K oil yield (23%, turnover number of 1.1 × 10²) were obtained with compound 6 after 2 h of microwave irradiation at 100 °C.

Metal-ligand covalency enables room temperature molecular qubit candidates

Metal–ligand covalency enables observation of coherent spin dynamics to room temperature in a series of vanadium(iv) and copper(ii) catechol complexes.

Harnessing synthetic chemistry to design electronic spin-based qubits, the smallest unit of a quantum information system, enables us to probe fundamental questions regarding spin relaxation dynamics. We sought to probe the influence of metal–ligand covalency on spin–lattice relaxation, which comprises the upper limit of coherence time. Specifically, we studied the impact of the first coordination sphere on spin–lattice relaxation through a series of four molecules featuring V–S, V–Se, Cu–S, and Cu–Se bonds, the Ph₄P⁺ salts of the complexes [V(C₆H₄S₂)₃]^2– (1), [Cu(C₆H₄S₂)₂]^2– (2), [V(C₆H₄Se₂)₃]^2– (3), and [Cu(C₆H₄Se₂)₂]^2– (4). The combined results of pulse electron paramagnetic resonance spectroscopy and ac magnetic susceptibility studies demonstrate the influence of greater M–L covalency, and consequently spin-delocalization onto the ligand, on elongating spin–lattice relaxation times. Notably, we observe the longest spin–lattice relaxation times in 2, and spin echos that survive until room temperature in both copper complexes (2 and 4).

Tuning the Mesomorphism and Redox Response of Anionic-Ligand-Based Mixed-Valent Nickel(II) Complexes by Alkyl-Substituted Quaternary Ammonium Cations

The combination of the redox-active mesogenic anion [Ni^II (Bdt)(BdtSQ)]^- (Bdt=1,2-benzenedithiolato; BdtSQ=1,2-dithia-semi-benzoquinonato) with alkyl-substituted ammonium cations afforded a series of redox-active ionic complexes of the type [NR₄ ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NC16₄ Ni) and C8,10 (NC8,10₄ Ni); C8,10=6-octylhexadecyl] or [NMe₂ R₂ ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NMe₂ C16₂ Ni) and C8,10 (NMe₂ C8,10₂ Ni)]. X-ray crystallographic analyses of NMe₂ C16₂ Ni and NC16₄ Ni revealed the formation of cation-dependent integrated ionic layers separated by interdigittated alkyl chains. Complexes NMe₂ C16₂ Ni and NC16₄ Ni commonly form crystalline phases at room temperature, whereas complexes NMe₂ C8,10₂ Ni and NC8,10₄ Ni, which contain branched alkyl chains, form a metastable mesophase and an amorphous phase at the same temperature, respectively. Furthermore, complexes NMe₂ C16₂ Ni, NMe₂ C8,10₂ Ni, and NC16₄ Ni commonly form a smectic A phase (SmA) at 375, 317, and 342 K, respectively. For the four complexes, well-defined cyclic voltammetry responses, derived from ligand-based oxidation and reduction, were observed in solution and the condensed phases, that is, upon casting these complexes on an indium-doped tin oxide working electrode. The present study demonstrates the tunability of the mesomorphism of ionic molecular assemblies composed of alkyl-substituted quaternary ammonium cations, while maintaining the well-defined redox responses of the anions even in the condensed phases.

Molecular engineering of organic electroactive materials for redox flow batteries

With high scalability and independent control over energy and power, redox flow batteries (RFBs) stand out as an important large-scale energy storage system.

Strategic design of thiophene-fused nickel dithiolene derivatives for efficient NLO response

The zwitterionic donor–acceptor group significantly reduces the HOMO–LUMO energy gap resulting in an enormous increase in the first hyperpolarizability values.

Group 10 Metal Benzene-1,2-dithiolate Derivatives in the Synthesis of Coordination Polymers Containing Potassium Countercations

The use of theoretical calculations has allowed us to predict the coordination behavior of dithiolene [M(SC₆H₄S)₂]^2- (M = Ni, Pd, Pt) entities, giving rise to the first organometallic polymers {[K₂(μ-H₂O)₂][Ni(SC₆H₄S)₂]}_n and {[K₂(μ-H₂O)₂(thf)]₂[K₂(μ-H₂O)₂(thf)₂][Pd₃(SC₆H₄S)₆]}_n by one-pot reactions of the corresponding d¹⁰ metal salts, 1,2-benzenedithiolene, and KOH. The polymers are based on σ,π interactions between potassium atoms and [M(SC₆H₄S)₂]^2- (M = Ni, Pd) entities. In contrast, only σ interactions are observed when the analogous platinum derivative is used instead, yielding the coordination polymer {[K₂(μ-thf)₂][Pt(SC₆H₄S)₂]}_n.

Identification of an Electrode-Adsorbed Intermediate in the Catalytic Hydrogen Evolution Mechanism of a Cobalt Dithiolene Complex

Analysis of a cobalt bis(dithiolate) complex reported to mediate hydrogen evolution under electrocatalytic conditions in acetonitrile revealed that the cobalt complex transforms into an electrode-adsorbed film upon addition of acid prior to application of a potential. Subsequent application of a reducing potential to the film results in desorption of the film and regeneration of the molecular cobalt complex in solution, suggesting that the adsorbed species is an intermediate in catalytic H₂ evolution. The electroanalytical techniques used to examine the pathway by which H₂ is generated, as well as the methods used to probe the electrode-adsorbed species, are discussed. Tentative mechanisms for catalytic H₂ evolution via an electrode-adsorbed intermediate are proposed.

H2 evolution by a cobalt selenolate electrocatalyst and related mechanistic studies

[Co(bds)₂][nBu₄N] (where bds = 1,2-benzenediselenolate) was identified as an electrocatalyst for H₂ evolution. Mechanistic studies indicate that with acid a protonated oligomeric {[Co(bds)₂(H)_x]^x−1}_m is formed, which was found to reenter the catalytic cycle and generate H₂.

Unprecedented layered coordination polymers of dithiolene group 10 metals: magnetic and electrical properties

One-pot reactions between Ni(ii), Pd(ii) or Pt(ii) salts and 3,6-dichloro-1,2-benzenedithiol (HSC6H2Cl2SH) in KOH medium under argon lead to a series of bis-dithiolene coordination polymers. X-ray analysis shows the presence of a common square planar complex [M(SC6H2Cl2S)2](2-) linked to potassium cations forming either a two-dimensional coordination polymer network for {[K2(μ-H2O)2(μ-thf)(thf)2][M(SC6H2Cl2S)2]}n [M = Ni () and Pd ()] or a one-dimensional coordination polymer for {[K2(μ-H2O)2(thf)6][Pt(SC6H2Cl2S)2]}n (). In the coordination environment of the potassium ions may slightly change leading to the two-dimensional coordination polymer {[K2(μ-H2O)(μ-thf)2][Pt(SC6H2Cl2S)2]}n () that crystallizes together with . The physical characterization of compounds show similar trends, they are diamagnetic and behave as semiconductors.

New isostructural transition metal complexes with a non-innocent dithiolate ligand

Three new complexes with 3,6-dichlorobenzene-1,2-dithiol (bdtCl2), namely methyltriphenylphosphonium bis(3,6-dichlorobenzene-1,2-dithiolato-κ(2)S,S')cobaltate(1-), (C19H18P)[Co(C6H2Cl2S2)2], (I), bis(methyltriphenylphosphonium) bis(3,6-dichlorobenzene-1,2-dithiolato-κ(2)S,S')cuprate(2-) dimethyl sulfoxide disolvate, (C19H18P)2[Cu(C6H2Cl2S2)2]·2C2H6OS, (II), and methyltriphenylphosphonium bis(3,6-dichlorobenzene-1,2-dithiolato-κ(2)S,S')cuprate(1-), (C19H18P)[Cu(C6H2Cl2S2)2], (III), have been synthesized and characterized by single-crystal X-ray diffraction. The X-ray structure analyses of all three complexes confirm that the four donor S atoms form a slightly distorted square-planar coordination arrangement around the central metal atom. An interesting finding for both the Cu(II) and Cu(III) complexes, i.e. (II) and (III), respectively, is that the coordination polyhedra are principally the same and differ only slightly with respect to the interatomic distances.

Photocatalytic hydrogen evolution from a cobalt/nickel complex with dithiolene ligands under irradiation with visible light

Novel cobalt/nickel dithiolene complexes displayed efficient hydrogen evolution in a noble-metal-free photocatalytic system under visible light.