Arene Incorporation and Ring Expansion in the Thermal Decomposition of a Divanadium(III) Complex With Four Bridging Methyl Groups

Open-shell vanadium organometallic species with unprecedented structures and reactivity patterns are reported. Divanadium(III) complexes with four symmetric pyramidal bridging methyl or parent amido ligands, [{VCp*(µ-X)₂}₂] (Cp* = η⁵-C₅Me₅, X = CH₃, NH₂), exhibit an open-shell singlet ground state with a single V─V bond and a relatively small singlet‒triplet energy gap, according to theoretical calculations and magnetic measurements. The thermal decomposition of the tetramethyl species in hexane at 160°C cleanly affords the tetranuclear cube-type methylidene complex [{VCp*(µ₃-CH₂)}₄] with the release of methane. In contrast, the analogous thermolysis in benzene or toluene allows the isolation of trovacene derivatives with η⁷-cycloheptatrienyl ligands [VCp*(η⁷-C₇H₆R)] (R = H, Me), via a formal insertion of a CH fragment into the six-membered aromatic ring of the solvent. A similar ring expansion is observed in the thermolysis of the tetramethyl compound in naphthalene (C₁₀H₈) solution, yielding a trovacene derivative [VCp*(η⁷-C₁₁H₉)] with the slipped triple-decker sandwich vanadium(I) species [(VCp*)₂(µ-η⁶:η⁶-C₁₀H₈)] as an isolated byproduct. Overall, the thermal decomposition of [{VCp*(µ-CH₃)₂}₂] in benzenic solvents leads to variable mixtures of methylidene [{VCp*(µ₃-CH₂)}₄] and trovacene complexes depending on arene substituents.