Rhodium Complex Capable of Electron Storage Activated by Hydrogen Molecules: A Computational Study

Density functional theory (DFT) calculations were performed to elucidate the mechanistic factors governing the two-electron reduction of a Rh center by H₂ functioning in an aqueous solution. The proposed mechanism proceeds via Rh-H₂ → Rh^III-H + H⁺ → Rh^I + 2H⁺, involving oxidative cleavage of the H-H bond to form a hydride complex (Rh^III-H), followed by heterolytic cleavage and proton abstraction by water. The formation of Rh-H₂ is facilitated by back-donation from Rh d orbitals to the H₂ σ* orbital, while the subsequent formation of low-valent Rh (Rh^I) requires an activation energy of 10.2 kcal/mol. Intrinsic bond orbital analysis revealed that Rh acts as a Brønsted acid with an acidity exceeding that of formic and acetic acids, enabling efficient proton transfer. Comparative calculations for group 9 elements indicated that Ir favors formation of the hydride complex, whereas Co and Rh are more efficient in forming low-valent complexes, highlighting a trade-off between the two steps. Rh uniquely combines the advantages of both processes, offering superior overall reactivity. These insights provide design principles for developing aqueous-phase complexes that enable two-electron reductions using water as the electron source with potential applications in subsequent catalytic transformations.