Showing papers by "Aleksey E. Kuznetsov published in 2010"

A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals

Abstract: Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions. We report that the complex [Co4(H2O)2(PW9O34)2]10-, comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)3]3+ (bpy is 2,2'-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 production > or = 5 s(-1) at pH = 8. The rate's pH sensitivity reflects the pH dependence of the four-electron O2-H2O couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that [Co4(H2O)2(PW9O34)2]10- is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.

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 ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 10- (1), ({Ru4O4(OH)2(H2- O)4}(γ-SiW10O36)2) 9- (2), ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 8- (3), ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2) 7- (4), and ({Ru4O4(OH)2(H2O)4}(γ-SiW10O36)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 (Ru4O4(OH)2(H2O)4) 6+ core, and four subsequent one-electron oxidations of 1, leading to formation of 2, 3, 4, and 5, respectively, involve only {Ru4} core orbitals. In other words, catalyst instability due to ligand oxidation in the widely studied Ru-blue dimer, ((bpy)2(O)Ru V -(µ-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 {Ru4}-{SiW10} distance. It is predicted that at higher pH compound 1 will, initially, release protons from the µ-ORu oxygen centers.

Insights into the Mechanism of O2 Formation and Release from the Mn4O4L6 "Cubane" Cluster

Abstract: To probe photoinduced water oxidation catalyzed by the Mn4O4L6 cubane clusters, we have computationally studied the mechanism and controlling factors of the O2 formation from the [Mn4O4L6] catalyst, 6. It was demonstrated that dissociation of an L = H2PO2− ligand from 6 facilitates the direct O−O bond formation that proceeds with a 28.3 (33.4) kcal/mol rate-determining energy barrier at the transition state TS1. This step (the O−O single bond formation) of the reaction is a two-electron oxidation/reduction process, during which two oxo ligands are transformed into to μ2:η2−O22− unit, and two (“distal”) Mn centers are reduced from the 4+ to the 3+ oxidation state. Next two-electron oxidation/reduction occurs by “dancing” of the resulted O22− fragment between the Mn1 and Mn2/Mn2′-centers, keeping its strong coordination to the Mn1′-center. As a result of this four-electron oxidation/reduction process Mn centers of the Mn4-core of I transform from {Mn1(III)-Mn1′(III)-Mn2(IV)-Mn2′(IV)} to {Mn1(II)-Mn1′(II)-Mn...

On the Aromaticity of Square Planar Ga42‐ and In42‐ in Gaseous NaGa4‐ and NaIn4‐ Clusters.

Abstract: We investigated the electronic structure and chemical bonding of two bimetallic clusters NaGa 4 and NaIn4. Photoelectron spectra of the anions were obtained and compared with ab initio calculations. We found that the ground state of the two anions contains a square planar dianion interacting with a Na + cation. The Ga4 and In4 dianions both possess two delocalized π electrons and are considered to be aromatic, similar to that recently found in Al 4. Using calculations for a model compound, we showed that a recently synthesized Ga 4-organometallic compound also contains an aromatic -Ga4unit, analogous to the gaseous clusters.

A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals.

Abstract: Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions. We report that the complex [Co4(H2O)2(PW9O34)2]10-, comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)3]3+ (bpy is 2,2'-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 production > or = 5 s(-1) at pH = 8. The rate's pH sensitivity reflects the pH dependence of the four-electron O2-H2O couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that [Co4(H2O)2(PW9O34)2]10- is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.

Does the MgO(100)-support facilitate the reaction of nitrogen and hydrogen molecules catalyzed by Zr2Pd2 clusters? A computational study.

Abstract: Reactions of the “naked” and MgO(100) supported Zr2Pd2 cluster with nitrogen and four hydrogen molecules were studied at the density functional level using the periodic slab approach (VASP). It was shown that adsorption of the Zr2Pd2 cluster on the MgO(100) surface does not change its gas-phase geometry and electronic structure significantly. In spite of this the N2 coordination to the MgO(100)-supported Zr2Pd2 cluster, I/MgO, is found to be almost 30 kcal/mol less favorable than for the “naked” one. The addition of the first H2 molecule to the resulting II/MgO, that is, II/MgO + H2 → IV/MgO reaction, proceeds with a relatively small, 9.0 kcal/mol, barrier and is exothermic by 8.3 kcal/mol. The same reaction for the “naked” Zr2Pd2 cluster requires a slightly larger barrier (10.1 kcal/mol) and is highly exothermic (by 23.3 kcal/mol). The interaction of the H2 molecule with the intermediate IV/MgO (i.e., the second H2 molecule addition to II/MgO) requires larger energy barrier, 23.3 kcal/mol vs 8.8 kcal/mol...