The role of the heteroatom (X = SiIV, PV, and SVI) on the reactivity of {γ-[(H2O)RuIII(μ-OH)2RuIII(H2O)][X n+W10O36]}(8-n)- with the O2 molecule

The mechanism of reaction of the di-Ru-substituted polyoxometalate, {gamma-[(H2O)Ru-III(mu-OH)(2)Ru-III(H2O)] [Xn+W10O36]}((8-n)-), I_X, with O-2, i.e. I_X + O-2 -> {gamma-[(O)Ru-IV(mu-OH)(2)Ru-IV(O-center dot)][Xn+W10O36]}((8-n)-) + 2H(2)O, (1), was studied at the B3LYP density functional and self-consistent reaction field IEF-PCM (in aqueous solution) levels of theory. The effect of the nature of heteroatom X (where X = Si, P and, S) on the calculated energies and mechanism of the reaction (1) was elucidated. It was shown that the nature of X only slightly affects the reactivity of I_X with O-2, which is a 4-electron oxidation process. The overall reaction (1): (a) proceeds with moderate energy barriers for all studied X's [the calculated rate-determining barriers are X = Si (18.7 kcal/mol) < S (20.6 kcal/mol) < P (27.2 kcal/mol) in water, and X = S (18.7 kcal/mol) < P (21.4 kcal/mol) < Si (23.1 kcal/mol) in the gas phase] and (b) is exothermic [by X = Si [28.7 (22.1) kcal/mol] > P [21.4 (9.8) kcal/mol] > S [12.3 (5.0) kcal/mol]. The resulting 1{gamma - [(O-center dot)Ru-IV(mu - OH)(2)Ru-IV(O-center dot) [Xn+W10O36]}((8-n)-) VI_X, complex was found to have two Ru-IV = O-center dot units, rather than Ru-V = O units. The "reverse'' reaction, i.e., water oxidation by VI_X is an endothermic process and unlikely to occur for X = Si and P, while it could occur for X = S under specific conditions. The lack of reactivity of VI_X biradical toward the water molecule leads to the formation of the stable [{(Ru4O4)-O-IV(OH)(2)(H2O)(4)}[(gamma-XW10O36](2)}(m-) dimer. This conclusion is consistent with our experimental findings; previously we prepared the [{(Ru4O4)-O-IV(OH)(2)(H2O)(4)}[(gamma - XW10O36](2)}(m-) dimers for X = Si (m = 10) [Geletii et al. in Angew Chem Int Ed 47:3896-3899, 2008 and J Am Chem Soc 131:17360-17370, 2009] and P (m = 8) [Besson et al. in Chem Comm 46:2784-2786, 2010] and showed them to be very stable and efficient catalysts for the oxidation of water to O-2.