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 = But, Bun or H; L–L = Ph2PCH2PPh2(dppm), R = But; L–L = Me2PCH2CH2PMe2(dmpe), R = But] 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)But}(dmpe)(η-C7H7)]+ is accompanied by conversion of the excess of alkyne into polymeric ButCCH with respective Mn and Mw values of 29 × 103 and 63 × 103 as determined by gel permeation chromatography. Deprotonation of [Mo{CC(H)Bun}(dppe)(η-C7H7)]+ with KOBut yields the alkynyl [Mo(CCBun)(dppe)(η-C7H7)]. The disubstituted vinylidenes [Mo{CC(Me)R}(dppe)(η-C7H7)]+(R = But or Bun) were obtained by Cβ 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 Cβ 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.