Abstract:
A new mechanism involving the formation of a dihydrogen intermediate is described using the TPSS level of the DFT method for the generation of hydrogen from water catalyzed by the PNN Ru(II) pincer complex 4 (Science 2009, 324, 74). The Mulliken charge of the hydride ligand in 4 is highly negative (-1.574) and facilitates a dihydrogen bonding with one of the solvated water molecules to yield 4 center dot center dot center dot H(2)O. In the next step, water coordinates to the metal center by forcing a conformational change in the orientations of the hydride and CO ligands to form a transient intermediate, 4'(H(2)O). The activation free energy, G(act) for this step is 14.92 kcal/mol. Subsequently, the intermediate liberates the dihydrogen with a G(act) of 10.47 kcal/mol. With respect to (4 + H(2)O) as reference, the effective G(act) of the entire mechanism is calculated to be 32.56 kcal/mol. Unlike the previously reported mechanism, the new mechanism operates without the cooperation of the aromatization - dearomatization processes of the pincer ligand and bypasses a highly reversible pathway involving the water-mediated 4 to 1 conversion. Further, a direct pathway for the formation of the, cis-dihydroxo intermediate is possible with the new mechanism.
