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 differ by the mutual arrangement of the H and Cl ligands. The complexes show dynamic equilibrium between the two most stable fac-isomers, which can be transformed into the meridional ones in the presence of coordinating additives (CH₃CN, DMSO, or CO, but not Et₃N). The process is reversible for CH₃CN but irreversible for DMSO and CO, in agreement with the Lewis basicity of these molecules.