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[BHBu^s₃] 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[BHBu^s₃] 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[BHBu^s₃] 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[BHBu^s₃] 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[BHBu^s₃] 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].