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

• Published in: Organometallics 2004-06, Vol.23 (13), p.3153-3163
• Main Authors: Liu, Xiang-Yang, Venkatesan, Koushik, Schmalle, Helmut W, Berke, Heinz
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om034196y

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(PMe3)2(S)][BArF] (4(PhCl) and 4(THF)) and [mer-Re(CO)2(PMe3)3(S)][BArF] (5(PhCl) and 5(THF)) (BArF = [B(3,5-(CF3)2C6H3)4]-; S = PhCl)) were obtained from their corresponding hydride complexes 1 and 2 after treatment with [Ph3C][BArF] in chlorobenzene. The five-coordinated cationic complex [Re(CO)(PMe3)4][BArF] (6) (BArF = [B(3,5-(CF3)2C6H3)4]-) was obtained by the reaction of ReH(CO)(PMe3)4 (3) with 1 equiv of [Ph3C][BArF] in chlorobenzene. Hydride abstraction also occurred except for 1 from 2 and 3 with B(C6F5)3, producing [Re(CO)2(PMe3)3(S)][BH(C6F5)3] and [Re(CO)(PMe3)4][BH(C6F5)] (S = PhCl, THF). Treatment of ReH(CO)3(PMe3)2 (1) and ReH(CO)2(PMe3)3 (2) with 1 equiv of [isopropylisopropylideneiminium][BArF] in chlorobenzene at room temperature produced a mixture of 4(PhCl) and [Re(CO)3(PMe3)2(HNiPr2)][BArF] (8) or in the case of 2 a mixture of 5(PhCl) and [Re(CO)2(PMe3)3(HNiPr2)][BArF] (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 H2 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).