Synthesis and redox chemistry of cycloheptatrienyl molybdenum carbon-chain complexes featuring diimine support ligands: [Mo{(Ctriple bond; length of mdashC)nCtriple bond; length of mdashCR}(R′Ndouble bond; length as m-dashCH–CHdouble bond; length as m-das

The alkynyl complexes [Mo(Ctriple bond; length of mdashCR)(R′-dab)(η-C7H7)] (dab = diazabutadiene; R′ = tBu, R = Ph, 2a; C6H4–4-Me, 3a; tBu, 4a; R′ = C6H4–4-Me, R = Ph, 2b) have been synthesised by reaction of [MoBr(R′-dab)(η-C7H7)] with LiCtriple bond; length of mdashCR in thf. Crystallographic characterisation of [Mo(Ctriple bond; length of mdashCC6H4–4-Me)(tBu-dab)(η-C7H7)], 3a determines a Mo-Cα bond distance of 2.154(3) Å. Cyclic voltammetric investigations on the alkynyl complexes 2a, 2b, 3a, 4a and the extended chain butadiynyl derivative [Mo(Ctriple bond; length of mdashCCtriple bond; length of mdashCSiMe3)(tBu-dab)(η-C7H7)], 5a, reveal that each complex exhibits a chemically and electrochemically reversible one-electron oxidation to give the corresponding 17-electron radical cation. Characterisation of the radicals [2a]+, [2b]+, [3a]+, and [5a]+ by IR and EPR spectroscopic techniques indicates that the redox process is strongly metal centred. Spectroscopic and synthetic investigations establish that 17-electron radicals of the type [Mo{(Ctriple bond; length of mdashC)nCtriple bond; length of mdashCR)}(R′-dab)(η-C7H7)]+ (n = 0 or 1) have relatively low thermodynamic stability.