Iridium Pincer Complexes with an Olefin Backbone

Abstract

Among the large variety of pincer complexes, those with carbon–carbon double bonds in the backbone have received little attention. Here, we report the reactions of complex (PC═CP)IrPh (3) and its derivatives with small molecules. Compound 3 readily adds CO to give the 18e adduct (PC═CP)IrPhCO (5a), which upon heating undergoes isomerization into the thermodynamically more stable isomer (PC═CP)IrCO(Ph) (5b), via reversible loss of CO. Reaction of 5 with hydrogen leads to the formation of saturated carbonyl compounds (PC–CP)IrCO (9) and (PC–CP)IrH(CO)H (10). In contrast to the hydrides (PC═CP)IrH3 (6) and (PC–CP)IrH4 (7), which are in tautomeric equilibrium via insertion of one of the hydrides into the olefin moiety, the former compounds do not isomerize into the olefin form. Protonation of 5 with CF3COOH gives a complex with an agostic methylene group, 11, which undergoes a rare transformation for Ir pincers, which is insertion of CO into the Ir–Ph bond with subsequent formation of (PC═CP)IrOCOCF3 (12) and Ph–CHO. The trihydride 6 reacts with CO to give 9, which can add a second molecule of CO to reversibly form the dicarbonyl 13. Exposure of 6 to CO2 leads to the formate (PC–CP)Ir(H)OC(O)H (14). Complex 3 can take up a molecule of dioxygen to give peroxide (PC═CP)IrPhO2 (8); a similar reaction is observed for the saturated complex 9, with formation of (PC–CP)Ir(CO)O2 (15). XRD structures as well as reactivity point to a higher degree of O–O bond activation in 15.