Two-Electron Mixed Valency in a Heterotrimetallic Nickel–Vanadium–Nickel Complex

Mixed-valence complexes represent an enticing class of coordination compounds to interrogate electron transfer confined within a molecular framework. The diamagnetic heterotrimetallic anion, [V(SNS)2{Ni(dppe)}2]−, was prepared by reducing (dppe)NiCl2 in the presence of the chelating metalloligand [V(SNS)2]− [dppe = bis(diphenylphosphino)ethane; (SNS)3– = bis(2-thiolato-4-methylphenyl)amide]. Vanadium–nickel bonds span the heterotrimetallic core in the structure of [V(SNS)2{Ni(dppe)}2]−, with V–Ni bond lengths of 2.78 and 2.79 Å. One-electron oxidation of monoanionic [V(SNS)2{Ni(dppe)}2]− yielded neutral, paramagnetic V(SNS)2{Ni(dppe)}2. The solid-state structure of V(SNS)2{Ni(dppe)}2 revealed that the two nickel ions occupy unique coordination environments: one nickel is in a square-planar S2P2 coordination environment (τ4 = 0.19), with a long Ni···V distance of 3.45 Å; the other nickel is in a tetrahedral S2P2 coordination environment (τ4 = 0.84) with a short Ni–V distance of 2.60 Å, consistent with a formal metal–metal bond. Continuous-wave X-band electron paramagnetic resonance spectroscopy, electrochemical investigations, and density functional theory computations indicated that the unpaired electron in the neutral V(SNS)2{Ni(dppe)}2 cluster is localized on the bridging [V(SNS)2] metalloligand, and as a result, V(SNS)2{Ni(dppe)}2 is best described as a two-electron mixed-valence complex. These results demonstrate the important role that metal–metal interactions and flexible coordination geometries play in enabling multiple, reversible electron transfer processes in small cluster complexes