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 *

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-PhCH[double bond, length half m-dash]C(Ph)C(Ph)[double bond, length half m-dash]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{[double bond, length half m-dash]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[BHBu^s₃] led to the selective delivery of ‘H^–’ to the Re[double bond, length half m-dash]C_α carbon of the η⁴(5e)-butadienyl ligand and formation of the crystallographically identified d⁶ η⁴-1,3-diene complex [Re{η⁴-CH(Ph)[double bond, length half m-dash]C(Ph)CH[double bond, length half m-dash]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.

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