Spontaneous dehydrocoupling in peri-substituted phosphine–borane adducts

Abstract

Bis(borane) adducts Acenap(PiPr₂·BH₃)(PRH·BH₃) (Acenap = acenaphthene-5,6-diyl; 4a, R = Ph; 4b, R = ferrocenyl, Fc; 4c, R = H) were synthesised by the reaction of excess H₃B·SMe₂ with either phosphino-phosphonium salts [Acenap(PiPr₂)(PR)]⁺Cl⁻ (1a, R = Ph; 1b, R = Fc), or bis(phosphine) Acenap(PiPr₂)(PH₂) (3). Bis(borane) adducts 4a–c were found to undergo dihydrogen elimination at room temperature, this spontaneous catalyst-free phosphine-borane dehydrocoupling yields BH₂ bridged species Acenap(PiPr₂)(μ-BH₂)(PR·BH₃) (5a, R = Ph; 5b, R = Fc; 5c, R = H). Thermolysis of 5c results in loss of the terminal borane moiety to afford Acenap(PiPr₂)(μ-BH₂)(PH) (14). Single crystal X-ray structures of 3, 4b and 5a–c are reported.