Synthesis and redox chemistry of cycloheptatrienyl molybdenum carbon-chain complexes featuring diimine support ligands: [Mo{(CC)nCCR}(R′NCH–CHNR′)(η-C7H7)], (n = 0 or 1)

•A new class of alkynyl complexes supported by diimine ligands has been synthesised.•Structural characterisation of [Mo(CCC6H4–4-Me)(tBu-dab)(η-C7H7)] is described.•IR and EPR spectroscopy of the radicals [Mo(CCR)(R′-dab)(η-C7H7)]+ is reported.•The redox chemistry of [Mo(CCR)L2(η-C7H7)] (L2 = P- or N-donor ligand) is compared.

The alkynyl complexes [Mo(CCR)(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 LiCCR in thf. Crystallographic characterisation of [Mo(CCC6H4–4-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(CCCCSiMe3)(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{(CC)nCCR)}(R′-dab)(η-C7H7)]+ (n = 0 or 1) have relatively low thermodynamic stability.

Graphical abstractA series of alkynyl complexes featuring a cycloheptatrienyl molybdenum diimine support group have been synthesised. One-electron oxidation gives the corresponding 17-electron radical cations which have been characterised by IR and EPR spectroscopy.Figure optionsDownload full-size imageDownload as PowerPoint slide