Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

We report the synthesis of vanadium­(V) oxo complex 1 with a pincer-type dianionic mesoionic carbene (MIC) ligand L 1 and the general formula [VOCl­(L1)]. A comparison of the structural (SC-XRD), electronic (UV–vis), and electrochemical (cyclic voltammetry) properties of 1 with the benzimidazolinylidene congener 2 (general formula [VOCl­(L2)]) shows that the MIC is a stronger donor also for early transition metals with low d-electron population. Since electrochemical studies revealed both complexes to be reversibly reduced, the stronger donor character of MICs was not only demonstrated for the vanadium­(V) but also for the vanadium­(IV) oxidation state by isolating the reduced vanadium­(IV) complexes [Co­(Cp*) 2 ]­[1] and [Co­(Cp*) 2 ]­[2] ([Co­(Cp*)2] = decamethylcobaltocenium). The electronic structures of the compounds were investigated by computational methods. Complex 1 was found to be a moderate precursor for salt metathesis reactions, showing selective reactivity toward phenolates or secondary amides, but not toward primary amides and phosphides, thiophenols, or aryls/alkyls donors. Deoxygenation with electron-rich phosphines failed to give the desired vanadium­(III) complex. However, treatment of the deprotonated ligand precursor with vanadium­(III) trichloride resulted in the clean formation of the corresponding MIC vanadium­(III) complex 6, which undergoes a clean two-electron oxidation with organic azides yielding the corresponding imido complexes. The reaction with TMS-N3 did not afford a nitrido complex, but instead the imido complex 10. This study reveals that, contrary to popular belief, MICs are capable of supporting early transition-metal complexes in a variety of oxidation states, thus making them promising candidates for the activation of small molecules and redox catalysis.