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

The chiral tetrachelating amino−olefins ( R, R )- N, N ‘-bis(5 H -dibenzo[ a, d ]cyclohepten-5-yl)-1,2-diaminocyclohexane (( R, R )-trop 2 dach) and ( S, S )- N, N ‘-bis(5 H -dibenzo[ a, d ]cyclohepten-5-yl)-1,2-diphenyl-1,2-ethylenediamine (( S, S )-trop 2 dpen) were prepared and used as ligands in the complexes ( R, R )-[Rh(trop 2 dach)]OTf and ( S, S )-[Rh(trop 2 dpen)]OTf (OTf - = CF 3 SO 3 - ). Quasi-reversible reductions, d 8 -[Rh I (trop 2 diamine)] + + e - → d 9 -[Rh 0 (trop 2 diamine)] and d 9 -[Rh 0 (trop 2 diamine)] + e - → d 10 -[Rh - I (trop 2 diamine)] -, at rather negative potentials (trop 2 diamine = trop 2 dach, E 1/2 1 = −1.83 V, E 1/2 2 = −2.27 V; trop 2 diamine = trop 2 dpen, E 1/2 1 = −1.78 V, E 1/2 2 = −2.24 V; vs Fc + /Fc) indicate the donor capacity of the amine functions. One NH group in ( R, R )-[Rh(trop 2 dach)]OTf (p K a = 15.7(2), NH bond dissociation energy (BDE) 317(2) kJ mol - 1 ) is easily deprotonated to give the neutral amide ( R, R )-[Rh(trop 2 dach-H)], which is quasi-reversibly oxidized at E ° = −0.34 V (vs Fc/Fc + ) to the radical cation ( R, R )-[Rh(trop 2 dach-H)] • + . The p K a 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)(trop 2 dpen)], 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:KO t Bu:cat = 1:0.1:0.01, T = 80 °C, reaction time 1 h).