Tuning Energetics of 2 e^-/2H⁺ PCET Properties with Model Ru-bisamido Complexes

Most redox processes that break/form bonds involve net 2e^- changes, and many are coupled to protons. Yet most proton-coupled electron transfer (PCET) studies focus on 1e^-/1H⁺ reactions. Reported here is a family of molecular models that undergo tunable 2e^-/2H⁺ redox changes. Complexes [(X₂bpy)Ru^II(en*)₂](PF₆)₂ and [(X₂bpy)Ru^IV(en*-H)₂](PF₆)₂ have been synthesized with bpy=2,2'-bipyridine with 4,4'-subtitutions X=-NMe₂, -OMe, -Me, -H, -CF₃; and en*=2,3-dimethyl-2,3-butanediamine. They have been characterized by IR, UV-vis, and NMR spectroscopies, XRD, electrochemistry, mass spectrometry, DFT and (TD)DFT computations. The introduction of electron-withdrawing and donating groups at the 4,4'-position of the bpy ligand affects the complexes' redox potentials, pK_a's, and Bond Dissociation Free Energies (BDFEs) of the N-H bonds in the en* ligands. The average BDFEs for the overall 2e^-/2H⁺ PCET span over 5 kcal/mol. Notably, these complexes all show marked potential inversion over an extended range, ΔpK_a>25 units and ΔE⁰>1.4 V. Potential inversion remains despite the electronic influence of bpy's substitutions which regulate N-H properties several bonds apart by trans-effect over dπ-molecular orbitals at the Ru center. The experimental and computational results presented in this work support the presence of strong coupling between electrons and protons, for modelling insights of 2e^-/2H⁺ transfer reactivity.