Theoretical Study of O–O Single Bond Formation in the Oxidation of Water by the Ruthenium Blue Dimer

Abstract

The first key step in the oxidation of water to O(2) by the oxidized species [(bpy)(2)(O)Ru(V)ORu(V)(O)(bpy)(2)](4+) of the Ru blue dimer is studied using density functional theory (DFT) and an explicit solvent treatment. In the model reaction system [L(2)(O)Ru(V)ORu(V)(O)L(2)](4+)·(H(2)O)(4)·W(76), the surrounding water solvent molecules W are described classically while the inner core reaction system is described quantum mechanically using smaller model ligands (L). The reaction path found for the O--O single bond formation involves a proton relay chain: direct participation of two water molecules in two proton transfers to yield the product [L(2)(HOO)Ru(IV)ORu(IV)(OH)L(2)](4+)·(H(2)O)(3)·W(76). The calculated ∼3 kcal/mol reaction free energy and ∼15 kcal/mol activation free energy barrier at 298 K are consistent with experiment. Structural changes and charge flow along the intrinsic reaction coordinate, the solvent's role in the reaction barrier, and their significance for water oxidation catalysis are examined in detail.

Reduction of CO2 to Methanol Catalyzed by a Biomimetic Organo-Hydride Produced from Pyridine

“…As will be seen, in the formic acid and formaldehyde reductions, the solvating water molecule(s) play an additional, more active role; they act as a proton relay, for which this mixed explicit/implicit solvation approach 59,60,134 is especially important for an accurate description. 47,[54][55][56][57] For the DHT-1H2O and DHT-2H2O models, we obtain the barriers of ∆G ‡ HT = 17.1 and 14.3 kcal/mol for the CO2 reduction to HCOO -, ~6 and 9 kcal/mol lower than for the DHT model, reflecting the importance of quantum mechanically described water polarization (see Table 1). a All free energies and enthalpies, referenced to separated reactants, are reported in kcal/mol at 298K and 1 atm.…”

“…In addition to stabilizing the TS, these water molecules also intimately participate in the reaction by acting as a proton relay chain during the proton transfer event. 47,[54][55][56][57][58][59][60][61][62][63][64][65][66][67] We calculate vibrational force constants at the M06/6-31+G** level of theory to: 1) verify that the reactant and product structures have only positive vibrational modes, 2) confirm that each TS has only one imaginary mode and that it connects the desired reactant and product structures via Intrinsic Reaction Coordinate (IRC) calculations, and 3) compute entropies, zero-point energies (ZPE) and thermal corrections included in the reported free energies at 298K.…”

“…Only a very modest activation entropy effect is anticipated here because in the coupled HTPT process, the PT step(s) is (are) considerably delayed relative to the HT such that any entropic penalties due to PT contribute to the free energies of structures well past the TS. This view is also supported by the prior configuration of the water molecules in the aqueous solution solvating the reactant complex and the widespread occurrence of proton relays in other processes, [55][56][57]59,60,[64][65][66][67] including water oxidation 54,58 and enzymatic reactions. [61][62][63] In any event, the ∆G ‡ HT's reported in Table 1 show that the reaction is viable even without involvement of any proton relay chain.…”

“…99 In particular, using a hybrid explicit/implicit solvent model, we calculated low enthalpic barriers with respect to the complexed reactants of 13.6-18.5 kcal/mol (depending on the number of solvating waters) for PyCOOH 0 formation via a proton relay mechanism; the importance of proton relays have been extensively described in assorted chemical reactions. [54][55][56][57][58][59][60][61][62][63][64][65] Charge analysis on CO2 and PyH 0 along the reaction coordinate reveals that PyH 0 's propensity to recover its aromaticity drives the sequence of ET to CO2 followed by PT (mediated by a proton relay) to ultimately form PyCOOH 0 . 47,100 While this particular reaction is not of direct interest in the present work, 101 we will see that the themes of aromaticity recovery and proton relay mechanisms also prove to be important for our three HTPT step reduction of CO2 to CH3OH.…”

Molecules in Motion: Chemical Reaction and Allied Dynamics in Solution and Elsewhere

“…Our efforts on CO 2 reduction with Boulder colleague Charles Musgrave and his students are still ongoing (148), so we confine our discussion to our other energy research wing. In our first endeavor in this business, postdoc Bianco, ex-Los Alamos and now Colorado resident Jeff Hay, and I undertook a quantum chemical investigation of the mechanism of water oxidation 2H 2 O → O 2 + 4H + + 4e − by Tom Meyer's famous blue dimer biruthenium (Ru) transition metal complex, a very successful homogeneous catalyst for this daunting and most difficult part of water splitting (149,150). The quantum complex's ligands were modeled in a reasonable fashion, and the complex was surrounded by a number of quantum waters and large number of classical waters.…”