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 the 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, respectively. Treatment of either 3 or 4 with Li[HAl(O^tBu)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 the modest activity. Solid-state structures of 6b and 7 were established by X-ray crystallography.

This work reports rational synthetic access to the Ir complexes of a bis(N-pyrrolyl)boryl/bis(phosphine) PBP pincer. This ligand scaffold demonstrates a preference for boryl-iridium complexes, enabling access of a high valent boryl-iridium polyhydride. Additionally, the sensitivity of the B−N connectivity to protolysis was examined.