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

Abstract

The first examples of vinylidene complexes of the cycloheptatrienyl tungsten system [W(C=CHR)(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 HC[triple bond, length as m-dash]CR; 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(C[double bond, length as m-dash]CH2)(dppe)(η-C7H7)][PF6], 3, estimate the energy barrier to rotation about the W[double bond, length as m-dash]Cα 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 KOBut yields the alkynyls [W(C≡CR)(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 E½ 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 dz2character and this dominates the structural and spectroscopic properties of the system.