Bis(N-xylyl-imino)phenyl “NCN” iridium pincer complexes. Thermodynamics of ligand binding and C[sbnd]C bond cleavage

Abstract

Iridium dibromide complexes of the phenyldiimine ligand 2,6-bis(1-((2,6-dimethylphenyl)imino)ethyl)phenyl, trans-(XyPhDI)IrBr2L, have been synthesized, and relative Ir-L BDFEs have been experimentally determined for a wide range of corresponding adducts of ligands L. An estimate of the absolute enthalpy of Ir-L binding has been obtained from dynamic NMR measurements. The results of DFT calculations are in very good agreement with the relative and absolute experimental values. Computational studies were extended to the formation of adducts of (XyPhDI)IrH2 and (XyPhDI)IrI, as well as other (pincer)IrI fragments, (Phebox)IrI and (PCP)IrI, to enable a comparison of electronic and steric effects with these archetypal pincer ligands. Attempts to reduce (XyPhDI)IrBr2(MeCN) to a hydride or an IrI complex yielded a dinuclear CN-bridged complex with a methyl ligand on the cyanide-C-bound Ir center (characterized by scXRD), indicating that C-CN bond cleavage took place at that Ir center. DFT calculations indicate that the C-CN bond cleavage occurs at one Ir center with strong assistance by coordination of the CN nitrogen to the other Ir center.