Chiral Rhodium(I) and Iridium(I) Amino−Olefin Complexes: pKa, N−H Bond Dissociation Energy, and Catalytic Transfer Hydrogenation

Abstract

The chiral tetrachelating amino−olefins (R,R)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane ((R,R)-trop2dach) and (S,S)-N,N‘-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diphenyl-1,2-ethylenediamine ((S,S)-trop2dpen) were prepared and used as ligands in the complexes (R,R)-[Rh(trop2dach)]OTf and (S,S)-[Rh(trop2dpen)]OTf (OTf- = CF3SO3-). Quasi-reversible reductions, d8-[RhI(trop2diamine)]+ + e- → d9-[Rh0(trop2diamine)] and d9-[Rh0(trop2diamine)] + e- → d10-[Rh-I(trop2diamine)]-, at rather negative potentials (trop2diamine = trop2dach, E1/21 = −1.83 V, E1/22 = −2.27 V; trop2diamine = trop2dpen, E1/21 = −1.78 V, E1/22 = −2.24 V; vs Fc+/Fc) indicate the donor capacity of the amine functions. One NH group in (R,R)-[Rh(trop2dach)]OTf (pKa = 15.7(2), NH bond dissociation energy (BDE) 317(2) kJ mol-1) is easily deprotonated to give the neutral amide (R,R)-[Rh(trop2dach-H)], which is quasi-reversibly oxidized at E° = −0.34 V (vs Fc/Fc+) to the radical cation (R,R)-[Rh(trop2dach-H)]•+. The pKa and BDE values of the NH group in these 16-electron complexes are lower than in related pentacoordinated 18-electron complexes. X-ray diffraction analyses show that the rhodium complexes have distorted-square-planar structures. The Rh−N bond shortens by about 7% upon deprotonation. The rhodium complexes are inactive as catalysts for transfer and direct hydrogenation of ketones. However, the distorted-trigonal-bipyramidal iridium complex (S,S)-[IrCl(CO)(trop2dpen)], where the amino−olefin ligand serves as a tridentate ligand, serves as a chiral precursor to an active phosphane-free catalyst in the transfer hydrogenation of acetophenone with 2-propanol, and the R isomer of 1-phenylethanol was obtained in 82% ee (>98% conversion when acetophenone:KOtBu:cat = 1:0.1:0.01, T = 80 °C, reaction time 1 h).