What Factors Control O₂ Binding and Release Thermodynamics in Mononuclear Ruthenium Water Oxidation Catalysts? A Theoretical Exploration

Mononuclear Ru-based water oxidation catalysts (WOCs) constitute an important class of WOCs for water splitting. This work constitutes a systematic study of Ru–O₂ complexes of mononuclear ruthenium WOCs, with a focus on the thermodynamics of water-assisted O₂ release in various electronic states and conformations. Our extensive DFT study reveals several factors that affect the O₂ release thermodynamics: (1) steric effect from the ligand sphere of Ru; (2) trans effect of ligands trans to O₂; (3) oxygen cis coordinating effect; (4) carbon coordinating effect; and (5) Ru coordination strength. Some of these effects could selectively stabilize/destabilize some states/conformations of the Ru–O₂ complexes relative to Ru–OH₂ complexes, and affect thereby the O₂ release thermodynamics. The identification and rationalization of factors for O₂ release thermodynamics, as in this work, could be helpful toward a better understanding of this final step of the ruthenium-catalyzed water oxidation

Probing Ligand Effects on O–O Bond Formation of Ru-Catalyzed Water Oxidation: A Computational Survey

Ligand effects of some representative monomeric Ru-based water oxidation catalysts on the key O–O formation step are revealed in this work. Three effects, namely, cis-effect, net charge effect, and steric hindrance effect, are identified, which can exert sizable modulation on the O–O formation barriers for the two widely accepted O–O formation mechanisms of WNA (water nucleophilic attack) and I2M (direct coupling of two high-valent metal oxo units). The study demonstrates that, through the way of ligand design, there remains a large space for improving O–O bond formation reactivity

Spin–Orbit Coupling and Outer-Core Correlation Effects in Ir- and Pt-Catalyzed C–H Activation

The transition metal-dependent spin–orbit coupling (SOC) and outer-core (5s5p) correlation effects in Ir- and Pt-catalyzed C–H activation processes are studied here using high level ab initio computations. The catalysts involve complexes with oxidation states: Ir­(I), Ir­(III), Pt(0), and Pt­(II). It is demonstrated that for these heavy 5d transition metal-containing systems, the SOC effect and outer-core correlation effect on C–H activation are up to the order of ∼1 kcal/mol, and should be included if chemical accuracy is aimed. The interesting trends in our studied systems are: (1) the SOC effect consistently increases the C–H activation barriers and is apparently larger in higher oxidation states (Pt­(II) and Ir­(III)) than in low-oxidation states (Pt(0) and Ir­(I)); and (2) the magnitude of outer-core (5s5p) correlation effects is larger in less coordinate-saturated system. The effect of basis set on the outer-core correlation correction is significant; larger basis sets tend to increase the C–H activation barriers

Two-State Reactivity in Low-Valent Iron-Mediated C–H Activation and the Implications for Other First-Row Transition Metals

C–H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. <i>Our key mechanistic discovery for C–H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe­(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C–H bond cleavage</i>. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe­(II) to Fe­(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C–C coupling and finalizing the C–H functionalization. Regeneration of the Fe­(II) catalyst for the next round of C–H activation involves SET oxidation of the Fe­(I) species generated after the C–C bond coupling. <i>The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C–H activation</i>. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C–H activation. It is predicted that stabilization of low-spin Mn­(II) using an appropriate ligand sphere should produce another promising candidate for efficient C–H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C–H activation reactions