Solvent Stabilization and Hydrogenation Catalysis of Trimethylphosphine-Substituted Carbonyl Rhenium Cations

A series of cationic complexes were designed as catalysts for imine hydrogenation processes, and it was anticipated that for this purpose naked 16e - cations or relatively labile solvent-coordinated ones possessing noncoordinating counterions would suffice. Solvento complexes [Re(CO) 3 (PMe 3 ) 2 (S)][BAr F ] ( 4(PhCl) and 4(THF) ) and [ mer -Re(CO) 2 (PMe 3 ) 3 (S)][BAr F ] ( 5(PhCl) and 5(THF) ) (BAr F = [B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] -; S = PhCl)) were obtained from their corresponding hydride complexes 1 and 2 after treatment with [Ph 3 C][BAr F ] in chlorobenzene. The five-coordinated cationic complex [Re(CO)(PMe 3 ) 4 ][BAr F ] ( 6 ) (BAr F = [B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] - ) was obtained by the reaction of ReH(CO)(PMe 3 ) 4 ( 3 ) with 1 equiv of [Ph 3 C][BAr F ] in chlorobenzene. Hydride abstraction also occurred except for 1 from 2 and 3 with B(C 6 F 5 ) 3, producing [Re(CO) 2 (PMe 3 ) 3 (S)][BH(C 6 F 5 ) 3 ] and [Re(CO)(PMe 3 ) 4 ][BH(C 6 F 5 )] (S = PhCl, THF). Treatment of ReH(CO) 3 (PMe 3 ) 2 ( 1 ) and ReH(CO) 2 (PMe 3 ) 3 ( 2 ) with 1 equiv of [isopropylisopropylideneiminium][BAr F ] in chlorobenzene at room temperature produced a mixture of 4(PhCl) and [Re(CO) 3 (PMe 3 ) 2 (HN i Pr 2 )][BAr F ] ( 8 ) or in the case of 2 a mixture of 5(PhCl) and [Re(CO) 2 (PMe 3 ) 3 (HN i Pr 2 )][BAr F ] ( 9 ) within a few minutes. After 4 h both mixtures were completely converted to 8 and 9, respectively. 8 and 9 could also be obtained reacting 4(PhCl) and 5(PhCl) with excess diisopropylamine. Under mild conditions several imines underwent hydrogenation with H 2 in the presence of 4(PhCl) and 5(PhCl) as catalysts. 6, however, showed only poor catalysis. Further studies revealed details of the mechanism of the catalytic process. X-ray diffraction studies were carried out on the molecular structures of 4(PhCl), 5(PhCl), 6, and 5(THF) .