Conformational Flexibility of Dibenzobarrelene-Based PC(sp3)P Pincer Iridium Hydride Complexes: The Role of Hemilabile Functional Groups and External Coordinating Solvents

Bifunctional dibenzobarrelene-based PC(sp3)P pincer iridium complex 1 is known as an efficient catalyst in acceptorless dehydrogenation of alcohols and hydrogenation/hydroformylation of alkenes. In order to shed light on the mechanism of the hydrogen formation/activation, we performed variable-temperature IR and NMR (1H, 31P) analysis of intra- and intermolecular interactions involving a hydride ligand and hydroxymethyl cooperating group in 1 and its analogues. The results of the spectroscopic measurements in different media (dichloromethane, toluene, DMSO, and mixed solvents) were compared with the quantum chemical (DFT/M06 and B3PW91) calculations. The obtained data imply flexibility of the dibenzobarrelene-based scaffold, unprecedented for conventional pincer ligands. Both the CH2OH-substituted complex 1 and its COOEt analogue 3 prefer facial configuration of the PCP ligand with a P–Ir–P angle of ca. 100°. Such geometries are stabilized by Ir···O interaction with the dangling functional group and diffe...