Influence of Solvent-Dependent Carbene–Carbanion Equilibrium for Tris(2-pyridylthio)methanido Ligand on the Dioxygen Reactivity of Bioinspired Iron–Benzilate Complexes

Starting from the iron(II) complex, [(L1)FeII(CH3CN)2]+ (1), of the tris(2-pyridylthio)methanido (L1) ligand, an iron(II)-benzilate complex, [(L2)FeII(PyS)(benzilate)] (2), (L2 = bis(2-pyridylthio)carbene and PyS = pyridine-2-thiolate) was isolated and characterized. Single crystal X-ray diffraction studies of 2 reveal that the iron(II) center is coordinated by the carbene ligand (L2) in a tridentate N2C mode, along with a bidentate PyS formed upon C–S bond cleavage of L1. A solvent-dependent carbene-carbanion equilibrium for L1 is favored toward carbene in the presence of anionic benzilate in dichloromethane, leading to the formation of 2. Complex 2, upon reaction with dioxygen, undergoes a rearrangement of the carbene (L2) to carbanion (L1), ultimately affording an iron(III)-benzilate complex, [(L1)FeIII(benzilate)] (3). Intriguingly, the iron(III)-benzilate complex in the presence of complex 1 reacts with dioxygen, leading to complete decarboxylation of the coordinated benzilate. This observation suggested possible cooperative involvement of a dimeric intermediate in the reaction pathway. Consistent with this hypothesis, a benzilate-bridged mixed-valent diiron, [(L1)FeIII(benzilate)(L2)FeII(PyS)]+ (4), was isolated independently, and its dioxygen reactivity was evaluated. Expectedly, complex 4 undergoes quantitative decarboxylation upon reaction with dioxygen. These findings highlight the importance of solvent-dependent rearrangement capability of the carbanionic ligand in directing dioxygen activation and benzilate decarboxylation at the iron center.