Structure and reactivity of δ4-cyclobutadiene and cisoid-δ4(5e)-butadienyl-substituted rhenium complexes formed by reaction of [ReBr2{δ2(4e)-alkyne}(δ-C5H5)] with alkynes or o-diphenylphosphinostyrene
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Abstract
When cis-/trans-[ReBr2(CO)2(η-C5H5)] and PhC2Ph were heated together under reflux in toluene solution for 24 h the η4-cyclobutadiene-substituted complex [ReBr2(η4-C4Ph4)(η-C5H5)] 1 was formed in good yield via the intermediate [ReBr2{η2(4e)-PhC2Ph}(η-C5H5)]. A single-crystal X-ray diffraction study confirmed an overall pseudo-tetrahedral structure for 1, establishing a Br–Re–Br angle of 83.9°. Treatment of 1 with PPh3 or PMe3 (L) in the presence of AgBF4 afforded the cations [ReBr(L)(η4-C4Ph4)(η-C5H5)][BF4] 2 (L = PPh3) and 3 (L = PMe3). Reaction of 1 with an excess of Li[AlH4] gave the dihydride [ReH2(η4-C4Ph4)(η-C5H5)] 4 characterised by X-ray crystallography, whereas, 1 equivalent of Li[AlH4] afforded [ReH(Br)(η4-C4Ph4)(η-C5H5)] 5. In contrast with predictions from the Davis–Green–Mingos rules, reaction of 2 with Li[BHEt3] afforded 5 (major) and the minor product [ReH(PPh3)(η4-C4Ph4)(η-C5H5)]Br 6. Extended Hückel molecular orbital calculations suggested that protonation of 4 should give the cationic trihydride [ReH3(η4-C4Ph4)(η-C5H5)]+, however a novel ring-opening reaction occurred with CF3CO2H to give the crystallographically characterised η4-1,3-diene complex [ReH{OC(O)CF3}{η2,η2-Z,Z-PhCHC(Ph)C(Ph)
C(Ph)H}(η-C5H5)] 7. When [ReBr2{η2(4e)-PhC2R}(η-C5H5)] (R = Me or Ph) was treated with AgBF4 (2 equivalents) and o-diphenylphosphinostyrene (dpps) a carbon–carbon coupling reaction between the co-ordinated alkyne and alkene part of the dpps ligand took place followed by a deprotonation reaction to give the cisoid-η4(5e)-butadienyl-substituted complexes [Re{
C(Ph)-η3-C(R)CHCHC6H4PPh2-o}(η-C5H5)][BF4] 8 (R = Me) and 9 (R = Ph); the structure of 8 being confirmed by single-crystal X-ray crystallography. Treatment of 9 with K[BHBus3] led to the selective delivery of ‘H–’ to the Re
Cα carbon of the η4(5e)-butadienyl ligand and formation of the crystallographically identified d6 η4-1,3-diene complex [Re{η4-CH(Ph)
C(Ph)CH
CHC6H4PPh2-o}(η-C5H5)] 10. Interestingly, reaction of 10 with [Ph3C][BF4] led to regeneration of the parent cisoid-η4(5e)-butadienyl complex 9 confirming the relationship between η4(5e)-butadienyl and η4-1,3-diene ligands.