Synthesis, structure and redox chemistry of the aminoallenylidene complex [Mo{C C C(Me)NEt 2 }(dppe)(η-C 7 H 7 )][BPh 4 ]

The reaction of [MoBr(dppe)(η-C7H7)] (dppe = Ph2PCH2CH2PPh2) with HCCCCSiMe3 and Na[BPh4] in 1:1 NHEt2/THF as solvent yields the aminoallenylidene complex [Mo{CCC(Me)NEt2}(dppe)(η-C7H7)][BPh4], [1][BPh4]. The reaction likely proceeds via nucleophilic addition of NHEt2 at Cγ 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 MoCα bond length of 2.077(3) Å, intermediate between that of the cumulenic diphenylallenylidene analogue [Mo(CCCPh2)(dppe)(η-C7H7)][PF6] (1.994(3) Å) and the alkynyl compound [Mo(CCPh)(dppe)(η-C7H7)] (2.138(5) Å). Complex [1][BPh4] undergoes a reversible one-electron oxidation with E½ = −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.