Synthesis and characterization of iridium hydride complexes with meso-Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2 (meso-dpmppm) as an unsymmetric pincer ligand

Abstract

Reaction of [IrCl(CO)(PPh3)2] with meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (meso-dpmppm) afforded a mononuclear IrI complex, [Ir(meso-dpmppm−κ3)(CO)2]Cl (1), which showed excellent reactivity towards HX, H2, HCOOH, and R3SiH to yield a series of IrIII hydride complexes, [IrH(meso-dpmppm−κ3)(CO)2]X2 (X = Cl (2), PF6 (2*)), [Ir(H)2(meso-dpmppm−κ3)(CO)]Cl (4), and [IrH(SiR3)(meso-dpmppm−κ3)(CO)]Cl (R3 = Me2Ph (5a), Ph2H (5b)). The hydride IrIII complexes with isocyanides, [IrH(meso-dpmppm−κ3)(RNC)2](PF6)2 (R = Xyl (2,6-xylyl) (3a), Mes (2,4,6-mesityl) (3b), Cy (cyclohexyl) (3c), tBu (tert-butyl) (3d)), were also prepared by reacting [IrCl(cod)]2 with meso-dpmppm and RNC in the presence of NH4PF6. Complexes 2–5 were characterized by 1H and 31P NMR and ESI−MS spectroscopies and X-ray diffraction analyses (3a, 4, 5a,b) to have distorted octahedral IrIII structures supported by a meso-dpmppm in meridional mode as an unsymmetrical PPP−κ3 pincer ligand, coordinating with two outer and one inner phosphorus atoms to form fused six- and four-membered chelate rings and bearing an uncoordinate inner phosphine unit. The terminal hydride occupied the axial open site surrounded by the equatorially oriented phenyl groups of meso-dpmppm and is trans to the carbonyl (2, 4) and isocyanide ligands (3) nested in the closed site with respect to the {Ir(meso-dpmppm−κ3)} pincer plane. The remaining equatorial site is coordinated by CO (2), RNC (3), hydride (4), and silyl (5) ligand. These structural features demonstrated that oxidative additions of H+, H2, and R3SiH occurred at the axial open site of 1. The uncoordinate inner phosphine of 1 is readily reacted with [Cp*IrCl2]2 to give [Ir(μ−meso-dpmppm−κ3)(η5−Cp*IrCl2)(CO)2]Cl (1∙Cp*IrCl2, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl), which further transformed by oxidative addition of H2 and HCl to [Ir(H)2(μ−meso-dpmppm−κ3)(η5−Cp*IrCl2)(CO)]Cl (4∙Cp*IrCl2) and [IrH(μ−meso-dpmppm−κ3) (η5−Cp*IrCl2)(CO)2]Cl2 (2∙Cp*IrCl2), respectively, and however addition of bulky Me2PhSiH resulted in a disproportionation mixture of [IrH(Me2PhSi)(μ−meso-dpmppm−κ3)(η5-Cp*IrCl2)(CO)]Cl (5a∙Cp*IrCl2) as well as 4∙Cp*IrCl2, indicating an allosteric influence by attaching Cp*IrCl2 unit on the uncoordinate phosphine of the {Ir(meso-dpmppm−κ3)} unsymmetric pincer unit in 1.