Ligand‐Controlled Acid–Base and Redox Behavior of Ru(II)‐Ammine Complexes Toward Multiproton‐Coupled Electron Transfer

Abstract

Ruthenium–ammine complexes have long been studied in electron transfer processes relevant to catalysis, but their ammine ligands also make them promising platforms for exploring proton-coupled electron transfer (PCET) reactivity. This work examines how electronic variation in π-accepting and σ-donor ligands—specifically (poly)pyridines and ammines—affects the redox and acid–base behavior of several Ru intermediates. Ligands modulate electron density at the metal center, influencing oxidation state accessibility and reactivity toward PCET and multi-PCET transformations. A comparative analysis is performed using [Ru(L)6–m(NH3)m]2+ complexes, where L denotes the number of pyridine-type ligands and m the number of ammines (from 1 to 6). Spectroscopic (UV–vis, IR) and electrochemical (cyclic voltammetry) methods are used to characterize their structural and electronic properties. The results show clear trends in redox potentials, metal–ligand charge distribution, and acidity, which are rationalized through density functinal theory (DFT) calculations. Both ligand nature and coordination geometry govern the formation of molecular orbitals in oxidized and deprotonated species, enabling stabilization of products via concerted two-PCET are demonstrated. These findings provide mechanistic insights and design principles for tailoring multiredox coordination compounds in PCET-driven reactivity and for stabilizing high-valent species under mild conditions.