Kuznetsov Aleksey E, Geletii Yurii V, Hill Craig L, Morokuma Keiji, Musaev Djamaladdin G
Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, USA.
J Am Chem Soc. 2009 May 20;131(19):6844-54. doi: 10.1021/ja900017g.
Dioxygen and water activation on multi-Ru-substituted polyoxometalates were studied using the B3LYP density functional method. It was shown that the reaction of the Ru(2)-substituted gamma-Keggin polyoxotungstate {gamma-[(H(2)O)Ru(III)-(mu-OH)(2)-Ru(III)(H(2)O)][SiW(10)O(36)]}(4-), I(H(2)O), with O(2) is a 4-electron highly exothermic [DeltaE(gas) = 62.5 (DeltaE(gas) + DeltaG(solv(water)) = 24.6) kcal/mol] process and leads to formation of (H(2)O){gamma-(O)Ru-(mu-OH)(2)-Ru(O)[SiW(10)O(36)]}(4-), IV(H(2)O). Both the stepwise (or dissociative) and the concerted (or associative) pathways of this reaction occurring with and without water dissociation, respectively, were examined, and the latter has been found to be kinetically more favorable. It was shown that the first 1e-oxidation is achieved by the H(2)O-to-O(2) substitution, which might occur with a maximum of 23.1 (10.5) kcal/mol barrier and leads to the formation of {gamma-(OO)Ru-(mu-OH)(2)-Ru(H(2)O)[SiW(10)O(36)]}(4-), II(H(2)O). The second 1e-oxidation is initiated by the proton transfer from the coordinated water molecule to the superoxide (OO(-)) ligand in II(H(2)O) and is completed upon formation of hydroperoxo-hydroxo intermediate {gamma-(OOH)Ru-(mu-OH)(2)-Ru(OH)[SiW(10)O(36)]}(4-), III-1(H(2)O). The final 2e-oxidation occurs upon the proton transfer from the terminal OH-ligand to the Ru-coordinated OOH fragment and is completed at the formation of (H(2)O)...{gamma-(O)Ru-(mu-OH)(2)-Ru(O)[SiW(10)O(36)]}(4-), IV(H(2)O), with two Ru=O bonds. Each step in the associative pathway is exothermic and occurs with small energy barriers. During the process, the oxidation state of Ru centers increases from +3 to +4. The resulting IV(H(2)O) with a {Ru(O)-(mu-OH)(2)-Ru(O)} core should be formulated to have the Ru(IV)=O(*) units, rather than the Ru(V)=O groups. The reverse reaction, water oxidation by IV(H(2)O), is found to be highly endothermic and cannot occur; this finding is different from that reported for the "blue-dimer" intermediate, {(bpy)(2)(O())Ru-(mu-O)-Ru(O())(2)}(4+), which readily oxidized an incoming water molecule to produce O(2). The main reason for this difference in reactivity of IV(H(2)O) (i.e., Ru(2)-POM) and the "blue-dimer" intermediates toward the water molecule is found to be a high stability of IV(H(2)O) as compared to the analogous "blue-dimer" intermediate relative to O(2) formation. This, in turn, derives from the electron-rich nature of SiW(10)O(36) as compared to bpy ligands.
采用B3LYP密度泛函方法研究了多Ru取代的多金属氧酸盐上的双氧和水活化。结果表明，Ru(2)取代的γ-Keggin型聚氧钨酸盐{γ-[(H₂O)Ru(III)-(μ-OH)₂-Ru(III)(H₂O)][SiW₁₀O₃₆]}(4-)，I(H₂O)与O₂的反应是一个4电子的高度放热过程[ΔE(gas)=62.5(ΔE(gas)+ΔG(solv(water))=24.6)kcal/mol]，并导致(H₂O){γ-(O)Ru-(μ-OH)₂-Ru(O)[SiW₁₀O₃₆]}(4-)，IV(H₂O)的形成。分别研究了该反应在有水解离和无水解离情况下发生的逐步（或解离）途径和协同（或缔合）途径，发现后者在动力学上更有利。结果表明，第一次1e-氧化是通过H₂O到O₂的取代实现的，其最大势垒可能为23.1(10.5)kcal/mol，并导致{γ-(OO)Ru-(μ-OH)₂-Ru(H₂O)[SiW₁₀O₃₆]}(4-)，II(H₂O)的形成。第二次1e-氧化是由配位水分子中的质子转移到II(H₂O)中的超氧(OO⁻)配体引发的，并在形成氢过氧-氢氧中间体{γ-(OOH)Ru-(μ-OH)₂-Ru(OH)[SiW₁₀O₃₆]}(4-)，III-1(H₂O)时完成。最后的2e-氧化发生在质子从末端OH配体转移到Ru配位的OOH片段上，并在形成具有两个Ru=O键的(H₂O)...{γ-(O)Ru-(μ-OH)₂-Ru(O)[SiW₁₀O₃₆]}(4-)，IV(H₂O)时完成。缔合途径中的每一步都是放热的，并且以较小的能垒发生。在此过程中，Ru中心的氧化态从+3增加到+4。所得具有{Ru(O)-(μ-OH)₂-Ru(O)}核的IV(H₂O)应被表述为具有Ru(IV)=O(*)单元，而不是Ru(V)=O基团。发现IV(H₂O)的逆反应，即IV(H₂O)氧化水，是高度吸热的，不能发生；这一发现与报道的“蓝色二聚体”中间体{(bpy)₂(O*)Ru-(μ-O)-Ru(O*)₂}(4+)不同，后者很容易氧化进入的水分子以产生O₂。发现IV(H₂O)（即Ru(2)-POM）和“蓝色二聚体”中间体对水分子反应性差异的主要原因是与类似的“蓝色二聚体”中间体相比，IV(H₂O)相对于O₂形成具有高稳定性。反过来，这源于SiW₁₀O₃₆与bpy配体相比的富电子性质。
