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).