Vinylidene complexes of the molybdenum auxiliary Mo(Ph2PCH2CH2PPh2)(C7H7). Structural and spectroscopic investigations on vinylidene ligand orientation

Abstract

The new vinylidene complexes [Mo{CC(H)R}(L–L)(η-C₇H₇)]⁺[L–L = Ph₂PCH₂CH₂PPh₂(dppe), R = Bu^t, Buⁿ or H; L–L = Ph₂PCH₂PPh₂(dppm), R = Bu^t; L–L = Me₂PCH₂CH₂PMe₂(dmpe), R = Bu^t] have been synthesised by reaction of an excess of alk-1-yne, RCCH, either with [Mo(Me₂CO)-(L–L)(η-C₇H₇)]⁺(L–L = dppe or dmpe){formed in situ from [Mo(η⁶-C₆H₅Me)(η-C₇H₇)]⁺ and dppe or dmpe in acetone} or with [MoCl(L–L)(η-C₇H₇)](L–L = dppe or dppm) in methanol in the presence of [NH₄][PF₆]. The formation of [Mo{CC(H)Bu^t}(dmpe)(η-C₇H₇)]⁺ is accompanied by conversion of the excess of alkyne into polymeric Bu^tCCH with respective M_n and M_w values of 29 × 10³ and 63 × 10³ as determined by gel permeation chromatography. Deprotonation of [Mo{CC(H)Buⁿ}(dppe)(η-C₇H₇)]⁺ with KOBu^t yields the alkynyl [Mo(CCBuⁿ)(dppe)(η-C₇H₇)]. The disubstituted vinylidenes [Mo{CC(Me)R}(dppe)(η-C₇H₇)]⁺(R = Bu^t or Buⁿ) were obtained by C_β methylation of [Mo(CCR)(dppe)(η-C₇H₇)] with Mel. One-electron oxidation of the alkynyls [Mo(CCR)(dppe)(η-C₇H₇)](R = Buⁿ or Ph) with [Fe(η-C₅H₅)₂]⁺ yields the 17-electron radicals [Mo(CCR)(dppe)(η-C₇H₇)]⁺(R = Buⁿ or Ph) which undergo coupling at C_β to yield dimeric, divinylidene-bridged [Mo₂(dppe)₂(η-C₇H₇)₂(η-C₄R₂)]²⁺(R = Buⁿ or Ph). The crystal structures of [Mo{CC(H)Ph}(dppe)(η-C₇H₇)][BF₄] and [Mo₂(dppe)₂(η-C₇H₇)₂(µ-C₄Ph₂)][PF₆]₂, have been determined. In the former the vinylidene substituents lie approximately in the pseudo-mirror plane of the Mo(dppe)(η-C₇H₇) moiety (a vertical orientation) with the phenyl substituent directed ‘up’ towards the cycloheptatrienyl ring. In the latter centrosymmetric dimer the vinylidene ligand is rotated by 19.1° away from an exact vertical orientation and the phenyl substituents are located ‘down’ into a pocket enclosed by two phenyl groups of the dppe ligand. Variable-temperature ¹H NMR investigations on [Mo(CCH₂)(dppe)(η-C₇H₇)]⁺ are consistent with a preferred vertical orientation of the vinylidene ligand in solution at low temperature and the barrier to vinylidene rotation has been estimated as 51.9 ± 1 kJ mol^–1. The ¹H and ³¹P NMR solution spectra of [Mo{CC(R′)Buⁿ}(dppe)(η-C₇H₇)]⁺(R′= H or Me) are also consistent with a vertically orientated vinylidene ligand and reveal the existence of two isomeric forms of each complex which differ in the location of the n-butyl substituent in the ‘up’ or ‘down’ position.