Reactions of co-ordinated ligands .60. Synthesis and reactivity of eta(2)(4e)-alkyne and eta(2)(3e)-vinyl complexes of rhenium

Reaction of cis-/trans-[ReBr2(CO)(2)(eta-C2H5)] with PhC(2)Ph and MeC(2)Ph in refluxing toluene afforded good yields of the eta(2)(4e)-donor alkyne complexes [ReBr2(eta(2)-PhC(2)Ph)(eta-C5H5)] 1 and [ReBr2(eta(2)-MeC(2)Ph)(eta-C5H5)] 2, respectively. Treatment of 1 and 2 with either AgBF4 or TIPF6 in the presence of PPh(3), PMePh(2) or P(OMe)(3) (L) gave monocations [ReBr{eta(2)(4e)-alkyne}L(eta-C5H5)](+), whereas a similar reaction with 2 equivalents of AgBF4 and 1 equivalent of Ph(2)PCH(2)CH(2)PPh(2) (dppe) afforded dications [Re(eta(2)-PhC(2)Ph)(dppe)(eta-C5H5)][BF4](2) and [Re(eta(2)-MeC(2)Ph)(dppe)(eta-C5H5)][BF4](2). The structural identity of [ReBr(eta(2)-PhC(2)Ph)(PMePh(2))(eta-C-5-H-5)][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) Angstrom] is relatively short. Treatment of [ReBr(eta(2)-PhC(2)Ph)(PPh(3))(eta-C5H5)][BF4] and [ReBr(eta(2)-PhC(2)Ph)(PMePh(2))-(eta-C5H5)][PF6] with K[BHBu(3)(5)] in dichloromethane at -78 degrees C led to neutral eta(2)(3e)-vinyl complexes [Re{=C(Ph)CHPh}Br(PPh(3))(eta-C5H5)] and [Re{=C(Ph)CHPh}Br(PMePh(2))(eta-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-Hiickel molecular orbital calculations. Reaction (-78 degrees C) of [Re(eta(2)-PhC(2)Ph)(dppe)(eta-C5H5)](2) with 1 equivalent of K[BHBu(3)(5)] in tetrahydrofuran afforded the X-ray crystallographically identified monocationic eta(2)(3e)-vinyl complex [Re{=C(Ph)CHPh}(dppe)(eta-C5H5)][BF4], which reacted at room temperature with a further equivalent of K[BHBu(3)(s)] to give the cis-stilbene-substituted complex [Re{eta(2)-Z-PhCH=CHPh}(dppe)(eta-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(5)(s)] and [Re(eta(2)-MeC(2)Ph)(dppe)(eta-C5H5][BF4)](2) gave initially the eta(2)(3e)-vinyl complex [Re{=C(Me)CHPh}(dppe)(eta-C5H5)][BF4]; a further equivalent of [(eta-C5H5)] led to deprotonation and formation of the eta(2)-allene complex [Re{eta(2)-CH(Ph)=C=-CH2}(dppe)-(eta-C5H5)], in which the substituted allenic bond is co-ordinated to the rhenium. The dinuclear complex [Re2Br2(PPh(3))(2)(mu-O)(eta-C5H5)(2)][BF4](2) was also prepared and shown crystallographically to possess a single rhenium-rhenium bond [2.731(5) Angstrom].