Synthesis of Ethylene Hydridotris(1-pyrazolyl)borate Triphenylphosphine Complexes of Rhodium and Iridium and Their Reactions with Hydrogen

Abstract

Solutions of TpM(C2H4)2 (M = Rh (1a) and Ir (1b)) react with 1 equiv of PPh3 to yield TpM(PPh3)(C2H4) (2a,b). The new complexes adopt trigonal-bipyramidal structures in solution with triphenylphosphine coordinated in the axial site and ethylene positioned in the equatorial plane. For 2a the axial and equatorial pyrazolyl arms of the Tp ligand exchange positions on the NMR time scale (ΔG⧧ = 14.3 kcal mol-1, 279 K); however, no exchange is observed in the case of 2b, even at 353 K (ΔG⧧ > 18.4 kcal mol-1). Complexes 2a,b react with molecular hydrogen to yield TpM(PPh3)H2 (3a,b) and free ethylene. Kinetic studies of the iridium system show that this reaction is first order in both 2b and H2 and is not inhibited by a 10-fold excess of ethylene or PPh3 (kH2/kD2 = 1.26 ± 0.18). These results indicate that the H2 addition reaction proceeds by rapid reversible dissociation of a pyrazolyl arm, through a square-planar (η2-Tp)Ir(PPh3)(C2H4) intermediate.