Tuning Energetics of 2 e−/2H+ PCET Properties with Model Ru‐bisamido Complexes

Abstract

AbstractMost 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)RuII(en*)2](PF6)2 and [(X2bpy)RuIV(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, pKa’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, ΔpKa>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.