Synthesis, structure and redox chemistry of the aminoallenylidene complex [Mo{C[dbnd]C[dbnd]C(Me)NEt₂}(dppe)(η-C₇H₇)][BPh₄]

The reaction of [MoBr(dppe)(η-C₇H₇)] (dppe = Ph₂PCH₂CH₂PPh₂) with HC[tbnd]CC[tbnd]CSiMe₃ and Na[BPh₄] in 1:1 NHEt₂/THF as solvent yields the aminoallenylidene complex [Mo{C[dbnd]C[dbnd]C(Me)NEt₂}(dppe)(η-C₇H₇)][BPh₄], [1][BPh₄]. The reaction likely proceeds via nucleophilic addition of NHEt₂ at C_γ of a butatrienylidene intermediate. Structural and spectroscopic characterisation of [1][BPh₄] indicate a significant contribution of an iminium alkynyl resonance form to the overall structure of the heteroatom stabilised allenylidene ligand. The X-ray structural study of [1][BPh₄] determines a Mo[sbnd]C_α bond length of 2.077(3) Å, intermediate between that of the cumulenic diphenylallenylidene analogue [Mo(C[dbnd]C[dbnd]CPh₂)(dppe)(η-C₇H₇)][PF₆] (1.994(3) Å) and the alkynyl compound [Mo(C[tbnd]CPh)(dppe)(η-C₇H₇)] (2.138(5) Å). Complex [1][BPh₄] undergoes a reversible one-electron oxidation with E_½ = −0.19 V with respect to the FeCp₂/FeCp₂⁺ couple and the stable 17-electron radical dication [1]²⁺ is readily observed by spectroelectrochemical methods. IR spectroelectrochemistry in CH₂Cl₂ demonstrates that the ν(CCC) stretch, characteristic of the allenylidene ligand, shifts to higher wavenumber (from 1959 to 2032 cm⁻¹) as a result of oxidation of [1]⁺ to [1]²⁺, consistent with a strongly metal-centred redox process and an enhancement in the alkynyl character of the allenylidene ligand following one-electron oxidation.