Iridium Pincer Complexes with an Olefin Backbone

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.