Vinylidene complexes of the molybdenum auxiliary Mo(Ph2PCH2CH2PPh2)(C 7H7). Structural and spectroscopic investigations on vinylidene ligand orientation

Roy L. Beddoes, Caroline Bitcon, Richard W. Grime, Allen Ricalton, Mark W. Whiteley

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    Abstract

    The new vinylidene complexes [Mo{C=C(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, RC≡CH, either with [Mo(Me2CO)-(L-L)(η-C7H7)]+ (L-L = dppe or dmpe) {formed in situ from [Mo(η6-C6H5Me)(η-C7H 7)]+ 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{C=C(H)But}(dmpe) (η-C7H7)]+ is accompanied by conversion of the excess of alkyne into polymeric ButC≡CH with respective Mn and Mw values of 29 × 103 and 63 × 103 as determined by gel permeation chromatography. Deprotonation of [Mo{C=C(H)Bun}(dppe) (η-C7H7)]+ with KOBut yields the alkynyl [Mo(C≡CBun) (dppe) (η-C7H7)]. The disubstituted vinylidenes [Mo{C=C(Me)R}(dppe)(η-C7H7)]+ (R = But or Bun) were obtained by Cβ methylation of [Mo(C≡CR)(dppe)(η-C7H7)] with Mel. One-electron oxidation of the alkynyls [Mo(C≡CR)(dppe)(η-C7H7)] (R = Bun or Ph) with [Fe(η-C5H5)2]+ yields the 17-electron radicals [Mo(C≡CR)(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{C=C(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(C=CH2)(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{C=C(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.
    Original languageEnglish
    Pages (from-to)2873-2883
    Number of pages10
    JournalJournal of the Chemical Society, Dalton Transactions
    Issue number17
    DOIs
    Publication statusPublished - 1995

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