Metal-ligand cooperation in the trans addition of dihydrogen to a pincer Ir(I) complex: a DFT study.

DFT calculations on the hydrogenation of a (PNP)Ir(I) complex, to give the trans--rather then the cis--dihydride isomer, show that the reaction proceeds via a deprotonation/protonation of the ligand arm with concomitant dearomatization/aromatization of the pyridine core. Thus, the actual H(2) activation step occurs by an Ir(III) complex and not by the Ir(I) starting complex, as supported by experimental observations. This ligand participation allows for products that would otherwise be inaccessible. In addition, trace amounts of water, which are likely to be present in the solvent, facilitate proton transfer reaction steps.