Stable in Four Oxidation States: Exploring the Redox-Variability of Molybdenum and Tungsten Triazolylidene Complexes

The stability and (redox-)chemistry of molybdenum and tungsten triazolylidene complexes in four oxidation states is examined, starting from the oxo complexes L¹MO₂ and L¹ MoO(N^tBu) (1-M, M = Mo, W and 2-Mo, L¹= bisphenolate triazolylidene). Deoxygenative chlorination using either TMS-Cl or thionyl chloride gave access to the dichloro complexes L¹MOCl₂ 3-M (M = Mo, W) and L¹ M(NtBu)Cl₂ (4-Mo). The oxo-chlorido complexes 3-M can be deoxygenated or reduced, giving access to a plethora of halide complexes. Reduction with potassium graphite yielded the Mo(V) complexes L¹ MoOCl 6a-Mo, which can be further chlorinated using thionyl chloride to trichloro complex 8-Mo. Deoxygenation of the M(VI) complexes 3-M using trimethylphosphine forms the neutral M(IV) complexes L¹MCl₂(PMe₃) 9-M. Alternatively, starting from 3-M cationic M(IV) complexes [L¹MCl(PMe₃)₂][BArF₂₄] 11-M are accessible, by halide abstraction followed by deoxygenation. Cyclic voltammetry suggests a rich redox chemistry of these cations and L¹MCl(PMe₃)₂ 12-M can be obtained after one electron reduction. During all transformations, the M_carbene bond stays intact, proving that MIC ligands are valuable ligands for stabilizing tungsten and molybdenum complexes in a large variety of oxidation states with potential applications in catalysis and small molecule activation.