Iridium Complexes of a Bis(N-pyrrolyl)boryl/Bis(phosphine) PBP Pincer Ligand

This work reports the synthesis of a bis(pyrrolylphosphino)phenyl borane (PBP)Ph (2) and its incorporation of Ir by metal insertion into B–Ph to afford dipyrrolylboryl/bis(phosphine) pincer complex (PBP)Ir(Ph)Cl (3). Hydrogenolysis of 3 afforded (PBP)Ir(H)Cl (4). Compound 4 was converted into (PBP)IrCl2 (5a) via reaction with N-chlorosuccinimide, and exposure of 5a to CO produced (PBP)IrCl2(CO) (6a). Compounds 5a and 6a were converted into their analogs (PBP)IrI2 (5b) and (PBP)IrI2(CO) (6b) via metathesis with Me3SiI. Treatment of either 3 or 4 with Li[HAl(OBut)3] under H2 resulted in the formation of (PBP)IrH4 (7), with traces of 4 as a persistent impurity. Attempts to access 7 via the reaction of 4 with NaBH4 in isopropanol led to the loss of boron from the pincer and isolation of L2IrH5 (8, L = 2-diisopropylphosphinopyrrole). Compounds 4, 7, and 8 were examined as catalysts for alkane transfer dehydrogenation but displayed only modest activity. Solid-state structures of 6b and 7 were established by X-ray crystallography.