Structural Effects on Dioxygen Evolution from Ru(V)−Oxo Complexes

A series of ruthenium(V)−oxo compounds, LRu(V)O(n) [L=bipyridinedicarboxylate (BDA), alpha‐hydroxycarboxylate (AHA), porphyrin (POR), dimethylglyoximate (DMG), and nitrilotriacetate (NTA); n=+1,0, −1] are evaluated by Density Functional Theory for their ability to produce dioxygen through coupling of Ru(V)−oxo species, bimetallic peroxides (LRu(IV)‐O−O−Ru(IV)L), and dioxygen (LRu(IV)‐O2) complexes. Anionic Ru−oxo complexes (AHA)2RuO− (2) and (NTA)Ru(O)Cl− (5 e) have prohibitively large free energies of coupling, while neutral and monocationic species (1 b, 3–5 a–d) show small to moderate free energies of coupling. Transition states for O−O coupling were found for (NTA)RuO (5 a), (NTA)RuO(NH3) (5 c), (NTA)RuO(Pyr) (5 d), (DMG)2ClRu(O) (8) and (POR)RuO(Cl) (9), yielding moderate activation energies in the range of 18–22 kcal/mol. The overall oxygen evolution reaction (OER) free energies decrease in favourability as the coordination number of LRuO decreases, i. e. 7>6>5. The modest activation energies and free energies along the reaction coordinate for (NTA)(L)RuO and (POR)ClRu(O) suggest that these species would undergo kinetically and thermodynamically favorable oxygen evolution. DFT analysis of the energetics of oxygen evolution (OER) from ruthenium(V)−oxo species LRu(V)O(n) [L=BDA, AHA, POR, DMG, and NTA; n=+1,0,−1] via the oxo‐coupling (I2M) pathway shows favorable free energies for n=0,+1 with moderate coupling barriers. OER energies are decreasingly favorable as the LRuO coordination number decreases, i. e. 7>6>5. (NTA)(L)RuO and (POR)ClRu(O) appear to be most promising for OER catalysis.