Understanding Factors that Control the Structural (Dis)Assembly of Sulphur-Bridged Bimetallic Sites

Abstract

Bimetallic structures of the general type [M2(µ-S)2] are omnipresent in nature, for biological function [M2(µ-S)2] sites interconvert between electronically distinct, but isostructural, forms. Different from structure-function relationships, the current understanding of the mechanism of formation and persistence of [M2(µ-S)2] sites is poorly developed. This work reports on bimetallic model compounds of nickel that interconvert between functional structures [Ni2(µ-S)2]+/2+ and isomeric congeners [2{κ-S–Ni}]2+/+, S = Aryl-S−, in which the nickel ions are geometrically independent. Interconversion of the two sets of structures was studied quantitatively by UV–VIS absorption spectroscopy and cyclic voltammetry. Assembly of the [Ni2(µ-S)2]+ core from [2{κ-S–Ni}]+ is thermodynamically and kinetically highly preferred over the disassembly of [Ni2(µ-S)2]2+ into [2{κ-S–Ni}]2+. Labile Ni-η2/3-bonding to aromatic π-systems of the primary thiophenol ligand is critical for modeling (dis)assembly processes. A phosphine coligand mimics the role of anionic donors present in natural sites that saturate metal coordination. Three parameters have been identified as critical for structure formation and persistence. These are, first, the stereoelectronic properties of the metals ions, second, the steric demand of the coligand, and, third, the properties of the dative bond between nickel and coligand. The energies of transition states connecting functional and precursor forms have been found to depend on these parameters.

Modulating Effect of Ligand Charge on the Electronic Properties of 2Ni–2S Structures and Implications for Biological 2M–2S Sites

“…The spatial trans alignment of two arylthiolates across the central π-system renders the 1,4-terphenyldithiolate a suitable platform for detailed studies of the electronic properties of 2Ni−2S structures as it can bind the metals in different formal oxidation states, coordination numbers, and geometries, 18,21,22 which had been beneficial to mechanistic work on the skeletal (dis)assembly of 2Ni−2S cores. 23,24 Preserving the principal η 2 :η 2 :μ-coordination mode while converting the arene into cyclohexadienide minimizes structural effects that may bias the suspected effect of donor strength on the electronic properties of the 2Ni−2S core. DFT computational analysis relates the thermodynamic preference of arene over cyclohexadienide coordination to [2Ni 1.5+ -2S] + inferred from electrochemical and solid-state structure data to the reduced electronic coupling of the Ni atoms in 2.…”