Synthesis, spectroscopy and electronic structure of the vinylidene and alkynyl complexes [W(CCHR)(dppe)(η-C7H7)]+ and [W(CCR)(dppe)(η-C7H7)]n+ (n = 0 or 1)

Abstract

The first examples of vinylidene complexes of the cycloheptatrienyl tungsten system [W(CCHR)(dppe)(η-C₇H₇)]⁺ (dppe = Ph₂PCH₂CH₂PPh₂; R = H, 3; Ph, 4; C₆H₄-4-Me, 5) have been synthesised by reaction of [WBr(dppe)(η-C₇H₇)], 1, with terminal alkynes HCCR; a one-pot synthesis of 1 from [WBr(CO)₂(η-C₇H₇)] facilitates its use as a precursor. The X-ray structure of 4[PF₆] reveals that the vinylidene ligand substituents lie in the pseudo mirror plane of the W(dppe)(η-C₇H₇) auxiliary (vertical orientation) with the phenyl group located syn to the cycloheptatrienyl ring. Variable temperature ¹H NMR investigations on [W(CCH₂)(dppe)(η-C₇H₇)][PF₆], 3, estimate the energy barrier to rotation about the WC_α bond as 62.5 ± 2 kJ mol⁻¹; approximately 10 kJ mol⁻¹ greater than for the molybdenum analogue. Deprotonation of 4 and 5 with KOBu^t yields the alkynyls [W(CCR)(dppe)(η-C₇H₇)] (R = Ph, 6; C₆H₄-4-Me, 7) which undergo a reversible one-electron oxidation at a glassy carbon electrode in CH₂Cl₂ with E_½ values approximately 0.12 V negative of Mo analogues. The 17-electron radicals [6]⁺ and [7]⁺ have been investigated by spectroelectrochemical IR, UV-visible and EPR methods. The electronic structures of representative vinylidene (3) and alkynyl (6) complexes have been investigated at the B3LYP/Def2-SVP level. In both cases, electronic structure is characterised by a frontier orbital with significant metal d_z²character and this dominates the structural and spectroscopic properties of the system.