Electrocatalytic Proton Reduction by Dimeric Nickel Complex of a Sterically Demanding Pincer-type NS2 Aminobis(thiophenolate) Ligand

Thiolate Coordination vs C-S Bond Cleavage of Thiolates in Dinickel(II) Complexes

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, ⁿBu, ^tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni₂(BPMP)(μ-SH)(DMF)₂]²⁺ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni₂(BPMP)(μ-SEt)(ClO₄)]¹⁺ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni₂(BPMP)(Cl)₂(MeOH)₂]¹⁺ (3). While the reaction of thiolates with anhydrous Ni(OTf)₂ in dry conditions could only yield [Ni₂(BPMP)(OTf)₂]¹⁺ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni₂(BPMP)(SCOPh)₂]¹⁺ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni₂(BPMP)(SPh)(DMF)(MeOH)(H₂O)]²⁺ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni₂(BPMP)(MeOH)₂(H₂O)₂]³⁺(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni₂(BPMP)(μ-SPh)₂]¹⁺ (8).

Synthesis, structure, and biological activity of bis(benzimidazole)amino thio- and selenoether nickel complexes

Four new nickel (II) complexes with bis(benzimidazole)thio- and selenoether-based ligands have been synthesized and characterized in the solid state by elemental analysis, IR, magnetic susceptibility and X-ray crystallography, and in solution by FAB⁺ mass spectrometry, UV-vis spectroscopy and cyclic voltammetry. Single-crystal X-ray diffraction analysis of the compounds revealed octahedral geometries for all nickel centers. Three of the four complexes are dimers with chloride bridges between the two Ni(II) ions. However, in solution all complexes have a monomeric formulation, based on mass spectrometry and osmometry measurements. The complexes were also screened for their cytotoxic activity on human cell lines (HeLa, SK-LU-1 and HEK-293), and compared with a related Cu(II) complex.

A Mixed-Valence Tetra-Nuclear Nickel Dithiolene Complex: Synthesis, Crystal Structure, and the Lability of Its Nickel Sulfur Bonds

In this study, by employing a common synthetic protocol, an unusual and unexpected tetra-nuclear nickel dithiolene complex was obtained. The synthesis of the [Ni4(ecpdt)6]2− dianion (ecpdt = (Z)-3-ethoxy-3-oxo-1-phenylprop-1-ene-1,2-bis-thiolate) with two K+ as counter ions was then intentionally reproduced. The formation of this specific complex is attributed to the distinct dithiolene precursor used and the combination with the then coordinated counter ion in the molecular solid-state structure, as determined by X-ray diffraction. K2[Ni4(ecpdt)6] was further characterized by ESI-MS, FT-IR, UV-Vis, and cyclic voltammetry. The tetra-nuclear complex was found to have an uncommon geometry arising from the combination of four nickel centers and six dithiolene ligands. In the center of the arrangement, suspiciously long Ni–S distances were found, suggesting that the tetrameric structure can be easily split into two identical dimeric fragments or two distinct groups of monomeric fragments, for instance, upon dissolving. A proposed variable magnetism in the solid-state and in solution due to the postulated dissociation was confirmed. The Ni–S bonds of the “inner” and “outer” nickel centers differed concurrently with their coordination geometries. This observation also correlates with the fact that the complex bears two anionic charges requiring the four nickel centers to be present in two distinct oxidation states (2 × +2 and 2 × +3), i.e., to be hetero-valent. The different coordination geometries observed, together with the magnetic investigation, allowed the square planar “outer” geometry to be assigned to d8 centers, i.e., Ni2+, while the Ni3+ centers (d7) were in a square pyramidal geometry with longer Ni–S distances due to the increased number of donor atoms and interactions.

A bioinspired thiolate-bridged dinickel complex with a pendant amine: synthesis, structure and electrocatalytic properties

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-1SSS':2SS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-1SSN:2SS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-1SSX:2SS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.

Conformational Effects of [Ni2 (μ-ArS)2 ] Cores on Their Electrocatalytic Activity

Two nickel complexes supported by tridentate NS₂ ligands, [Ni₂ (κ-N,S,S,S'-N^Ph {CH₂ (MeC₆ H₂ R')S}₂ )₂ ] (1; R'=3,5-(CF₃ )₂ C₆ H₃ ) and [Ni₂ (κ-N,S,S,S'-N^iBu {CH₂ C₆ H₄ S}₂ )₂ ] (2), were prepared as bioinspired models of the active site of [NiFe] hydrogenases. The solid-state structure of 1 reveals that the [Ni₂ (μ-ArS)₂ ] core is bent, with the planes of the nickel centers at a hinge angle of 81.3(5)°, whereas 2 shows a coplanar arrangement between both nickel(II) ions in the dimeric structure. Complex 1 electrocatalyzes proton reduction from CF₃ COOH at -1.93 (overpotential of 1.04 V, with i_cat /i_p ≈21.8) and -1.47 V (overpotential of 580 mV, with i_cat /i_p ≈5.9) versus the ferrocene/ferrocenium redox couple. The electrochemical behavior of 1 relative to that of 2 may be related to the bent [Ni₂ (μ-ArS)₂ ] core, which allows proximity of the two Ni⋅⋅⋅Ni centers at 2.730(8) Å; thus possibly favoring H⁺ reduction. In contrast, the planar [Ni₂ (μ-ArS)₂ ] core of 2 results in a Ni⋅⋅⋅Ni distance of 3.364(4) Å and is unstable in the presence of acid.

Nickel(ii) PE1CE2P pincer complexes (E = O, S) for electrocatalytic proton reduction

The catalytic activity of a series of nickel complexes with different phosphinite/thiophosphinite ligands for electrocatalytic proton reduction strongly depends on the nature of the pincer ligands.

Easily reduced bis-pincer (NS2)2molybdenum(iv) to (NHS2)2Mo(ii) by alcohols vs. redox-inert (NS2)(NHS2)iron(iii) complexes

Iron and molybdenum complexes supported by a pincer-type dianionic [NS₂]²⁻ donor were prepared to compare their structural, spectroscopic, and electrochemical properties.

Reactant or reagent? Oxidation of H2 at electronically distinct nickel-thiolate sites [Ni2(μ-SR)2]+ and [Ni–SR]+

The chemical bond between a Lewis-acidic metal and a Brønsted/Lewis-basic sulphur donor provides M–S structures with functional properties that are relevant for a variety of processes such as the heterolytic cleavage of H₂.

Dinuclear Nickel Complexes of Thiolate-Functionalized Carbene Ligands and Their Electrochemical Properties

Four dimeric nickel(II) complexes [Ni₂Cl₂(BnC₂S)₂] [1], [Ni₂Cl₂(BnC₃S)₂] [2], [Ni₂(PyC₂S)₂]Br₂ [3]Br₂, and [Ni₂(PyC₃S)₂]Br₂ [4]Br₂ of four different thiolate-functionalized N-heterocyclic carbene (NHC) ligands were synthesized, and their structures have been determined by single-crystal X-ray crystallography. The four ligands differ by the alkyl chain length between the thiolate group and the benzimidazole nitrogen (two −C₂– or three −C₃– carbon atoms) and the second functionality at the NHC being a benzyl (Bn) or a pyridylmethyl (Py) group. The nickel(II) ions are coordinated to the NHC carbon atom and the pendent thiolate group, which bridges to the second nickel(II) ion creating the dinuclear structure. Additionally, in compounds [1] and [2], the fourth coordination position of the square-planar Ni(II) centers is occupied by the halide ions, whereas in [3]²⁺ and [4]²⁺, the additional pendant pyridylmethyl groups complete the coordination spheres of the nickel ions. The electrochemical properties of the four complexes were studied using cyclic voltammetry and controlled-potential coulometry methods. The thiolate-functionalized carbene complexes [1] and [2] appear to be poor electrocatalysts for the hydrogen evolution reaction; the complexes [3]Br₂ and [4]Br₂, bearing an extra pyridylmethyl group, show higher catalytic activity in proton reduction, indicating that the pyridine group plays an important role in the catalytic cycle.

Redox flexibility of iron complexes supported by sulfur-based tris(o-methylenethiophenolato)amine relative to its tripodal oxygen-based congener

Tripodal ligands designed to generate a local C₃ symmetry have resulted in novel types of metal complexes that feature unusual bonding and electronic properties.

Binuclear complexes of Ni(i) from 4-terphenyldithiophenol

The preparation and characterisation of binuclear Ni(i) complexes from a 4-terphenyldithiophenol ligand is described.