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(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane ((R,R)-trop(2)dach) and (S,S)-N,N'-bis(5H-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 = CF3SO3-). 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 (pK(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 degrees = -0.34 V (vs Fc/Fc(+)) to the radical cation (R,R)-[Rh(trop(2)dach-H)], The pK(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:KOtBu: cat = 1:0.1:0.01, T 80 degrees C, reaction time 1 h).