Synthesis and reactivity of η²(4e)-alkyne and η²(3e)-vinyl complexes of rhenium

Abstract

Reaction of cis-/trans-[ReBr2(CO)2(η-C5H5)] with PhC2Ph and MeC2Ph in refluxing toluene afforded good yields of the η2(4e)-donor alkyne complexes [ReBr2(η2-PhC2Ph)(η-C5H5)]1 and [ReBr2(η2-MeC2Ph)(η-C5H5)]2, respectively. Treatment of 1 and 2 with either AgBF4 or TlPF6 in the presence of PPh3, PMePh2 or P(OMe)3(L) gave monocations [ReBr{η2(4e)-alkyne}L(η-C5H5)]+, whereas a similar reaction with 2 equivalents of AgBF4 and 1 equivalent of Ph2PCH2CH2PPh2(dppe) afforded dications [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2. The structural identity of [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] was confirmed by single-crystal X-ray crystallography. The alkyne C–C vector lies parallel to the Re–Br bond and the alkyne C–C bond length [C(1)–C(2) 1.26(4)Å] is relatively short. Treatment of [ReBr(η2-PhC2Ph)(PPh3)(η-C5H5)][BF4] and [ReBr(η2-PhC2Ph)(PMePh2)(η-C5H5)][PF6] with K[BHBus3] in dichloromethane at –78 °C led to neutral η2(3e)-vinyl complexes [[graphic omitted]HPh}Br(PPh3)(η-C5H5)] and [[graphic omitted]HPh}Br(PMePh2)(η-C5H5)]. The crystal structure of the latter showed that the C–C vector of the vinyl moiety lies almost parallel to the Re–Br bond. The stereochemistry of these reactions is discussed in the light of extended-Hückel molecular orbital calculations. Reaction (–78 °C) of [Re(η2-PhC2Ph)(dppe)(η-C5H5)][BF4]2 with 1 equivalent of K[BHBus3] in tetrahydrofuran afforded the X-ray crystallographically identified monocationic η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4], which reacted at room temperature with a further equivalent of K[BHBus3] to give the cis-stilbene-substituted complex [Re(η2-Z-PhCHCHPh)(dppe)(η-C5H5)]. The crystal structure of the latter showed that the alkene phenyl substituents are orientated towards the cyclopentadienyl ring. In contrast, a similar reaction between K[BHBus3] and [Re(η2-MeC2Ph)(dppe)(η-C5H5)][BF4]2 gave initially the η2(3e)-vinyl complex [[graphic omitted]HPh}(dppe)(η-C5H5)][BF4]; a further equivalent of K[BHBus3] led to deprotonation and formation of the η2-allene complex [Re{η2-CH(Ph)CCH2}(dppe)(η-C5H5)], in which the substituted allenic bond is co-ordinated to the rhenium. The dinuclear complex [Re2Br2(PPh3)2(µ-O)(η-C5H5)2][BF4]2 was also prepared and shown crystallographically to possess a single rhenium–rhenium bond [2.731(5)Å].