Rhodium Complexes Containing a Tridentate Bis(8-quinolyl)methylsilyl Ligand:  Synthesis and Reactivity

Synthesis of a series of rhodium complexes bearing a tridentate bis(8-quinolyl)methylsilyl (NSiN) ligand is reported. Bis(8-quinolyl)methylsilane (1) reacted with (PPh3)3RhCl via oxidative addition at the Si−H bond to afford (NSiN)Rh(H)Cl(PPh3) (2). Several coordinatively unsaturated, 16-electron complexes were synthesized, including the cationic complex [(NSiN)Rh(H)(PPh3)][B(C6F5)4] (3) and the neutral complex (NSiN)Rh(CH2Ph)2 (12). The X-ray structures of 3 and 12 reveal a square-pyramidal geometry about the Rh center, with the silyl group occupying an apical position. The dicationic complex [(NSiN)Rh(CH3CN)3][OTf]2 (11), synthesized by reaction of [(NSiN)RhCl2]2 (10) with 2 equiv of AgOTf in acetonitrile, exhibits moderate activity as a catalyst for alkene hydrogenation and for H/Cl exchange between Ph3SiH and CH2Cl2. Complex 2 reacts with 2 equiv of PhCH2MgCl in benzene to afford (NSiN)Rh(CH2Ph)Cl(PPh3) (6). Chlorination of 2 by DBU in CH2Cl2 afforded the solvent-activated Rh(III) product (NSiN)RhCl2(PPh3) (8). Similarly, with PMe3/CH2Cl2 as the chlorinating reagent, [(NSiN)Rh(PMe3)2Cl]Cl (9) was obtained. The low-valent complex (NSiN)Rh(COD) (14; COD = 1,5-cyclooctadiene) was synthesized via the reaction of 1 with (COD)Rh(η3-CH2Ph) in benzene. The COD ligand was replaced by the phosphine-based dppe ligand, resulting in the thermally more stable complex (NSiN)Rh(dppe) (18; dppe = Ph2PCH2CH2PPh2).