Coordination Chemistry and Reactivity of New Zwitterionic Rhodium and Iridium Complexes Featuring the Tripodal Phosphine Ligand [PhB(CH2PiPr2)3]-. Activation of H−H, Si−H, and Ligand B−C Bonds

Abstract

The synthesis, characterization, and reactivity of zwitterionic rhodium and iridium complexes containing the tris(phosphino)borate ligand [PhB(CH2PiPr2)3]- ([PhBP3‘]-) are reported. The allyl complexes [PhBP3‘]IrH(η3-C8H13) (3) and [PhBP3‘]IrH(η3-C3H5) (4) were prepared by reaction of [PhBP3‘]Li(THF) (2) with the corresponding [(alkene)2IrCl]2 complex. Complex 3 reacted with secondary silanes (H2SiR2, with R = Et, Ph) to give silyl-capped trihydride complexes of the type [PhBP3‘]IrH3(SiHR2) (R = Et, 5; Ph, 6) with concomitant β-hydride elimination of 1,3-cyclooctadiene. Complex 5 underwent H/D exchange with D2 to incorporate deuterium into both the Ir−H and Si−H positions. The reaction of 5 with 1 equiv of PMe3 resulted in elimination of Et2SiH2 to form the corresponding dihydride complex, [PhBP3‘]Ir(H)2(PMe3) (7). Complexes of the type [PhBP3‘]Ir(H)2(L) (L = PMe3, 7; PH2Cy, 8; CO, 9) could also be prepared directly by the reaction of 3 with L. The observed reactivity of [PhBP3‘]Ir complexes is compared with that of the related [PhB(CH2PPh2)3]- ([PhBP3]-) species. The Rh(I) complexes [κ2-PhBP3‘]Rh(PMe3)2 (10) and [PhBP3‘]Rh(CO)2 (11) are also reported. Variable-temperature 1H and 31P NMR experiments did not reveal evidence for κ2−κ3 interconversion for 10 and 11. However at elevated temperatures 10 was found to engage in a dynamic equilibrium process involving dissociation of the PMe3 ligands and reversible migration of a −CH2 group in the ligand backbone from B to Rh. The product of this migration, (12), was prepared independently by the reaction of 2 with [RhCl(C2H4)2]2 and was structurally characterized by X-ray crystallography. Complex 10 reacted with H2 to give the oxidative addition product [PhBP3‘]Rh(H)2(PMe3) (13). The reaction of 10 with 1 equiv of Ph2SiH2 resulted in loss of a ligand arm to give the bis(phosphino)borane complex [PhB(CH2PiPr2)2]Rh(H)2(SiHPh2)(PMe3) (14). Complex 11 reacted with H2 in the presence of 1 equiv of Me3NO to give the oxidative addition product [PhBP3‘]Rh(H)2(CO) (15), with concomitant liberation of Me3N.