Computational studies of the geometry and electronic structure of an all-inorganic and homogeneous tetra-Ru-polyoxotungstate catalyst for water oxidation and its four subsequent one-electron oxidized forms

Abstract

Geometry and electronic structure of five species [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](10-) (1), [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](9-) (2), [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](8-) (3), [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](7-) (4), and [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)](6-) (5) with different oxidation states of Ru centers were studied at the density functional and COSMO levels of theory. These species are expected to be among the possible intermediates of the recently reported 1-catalyzed water oxidation (Geletii, Y. V.; Botar, B.; Kogerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. Angew. Chem. Int. Ed. 2008, 47, 3896-3899 and Sartorel, A.; Carraro, M.; Scorrano, G.; Zorzi, R. D.; Geremia, S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M. J. Am. Chem. Soc. 2008, 130, 5006-5007). It was shown that RI-BP86 correctly describes the geometry and energy of the low-lying electronic states of compound 1, whereas the widely used B3LYP approach overestimates the energy of its high-spin states. Including the solvent and/or countercation effects into calculations improves the agreement between the calculated and experimental data. It was found that the several HOMOs and LUMOs of the studied complexes are bonding and antibonding orbitals of the [Ru(4)O(4)(OH)(2)(H(2)O)(4)](6+) core, and four subsequent one-electron oxidations of 1, leading to formation of 2, 3, 4, and 5, respectively, involve only {Ru(4)} core orbitals. In other words, catalyst instability due to ligand oxidation in the widely studied Ru-blue dimer, [(bpy)(2)(O)Ru(V)-(mu-O)-Ru(V)(O)(bpy)(2)](4+), is not operable for 1: the latter all-inorganic catalyst is predicted to be stable under water oxidation turnover conditions. The calculated HOMOs and LUMOs of all the studied species are very close in energy and exhibit a "quasi-continuum" or "nanoparticle-type" electronic structure similar to that of nanosized transition metal clusters. This conclusion closely correlates with the experimentally reported oxidation and reduction features of 1 and explains the unusual linear dependence of oxidation potential versus charges for these compounds. The decrease in total negative charge of the system via 1 > 2 > 3 > 4 > 5, on average, decreases the {Ru(4)}-{SiW(10)} distance. It is predicted that at higher pH compound 1 will, initially, release protons from the mu-O(Ru) oxygen centers.