Electrochemistry of Ru-bpm reveals the proton-coupled redox activity of the Ru-center in the anodic region and of the bpm ligand in the cathodic region. This behavior is illustrated in cyclic voltammograms (CV) of Ru-bpm in aqueous medium, across a large pH region (Figures and). Representative CVs in acidic, neutral and basic solutions (Figure ) show the presence of a single reduction feature in the cathodic region (0 to -1 V vs. NHE, light blue traces) and a single oxidation feature in the anodic region (0 to +1.5 V vs NHE, dark blue traces). The cathodic feature is chemically reversible and exhibits pronounced pH-dependence. Similar behavior was observed previously for bipyrimidine-based nanographenes, and is assigned to a two-electron, two-proton reduction of the bpm moiety to the corresponding dihydro-bipyrimidine (bpmH2) analog (Scheme 1). The bpm reduction feature exhibits a modest current enhancement in the cathodic scan for CVs collected at pH<2.0 (observable in the pH=2 CV, Figure ). This current increase indicates the presence of a catalytic process. The possibly of catalytic hydrogen-evolution was explored, but the headspace analysis of the electrolysis solution did not result in the detection of molecular hydrogen. The origin of this current enhancement is currently not understood. The experimental CVs were used to construct the Pourbaix diagram for the bpm-based reduction feature (Figure ). Experimental data were fitted using a single Nernst equation for PCET across the entire pH range, yielding a slope of -0.061 V/pH unit. This slope implies that the equal number of electrons and protons are transferred in the reduction process and is consistent with the twoelectron, two-proton transfer proposed in Scheme 1. Scheme 1. Scheme summarizing PCET processes in Ru-bpm. Ligand (bpm/bpmH2) reduction involves a two-electron, two-proton process in all pH regions. The proton stoichiometry associated with Ru-oxidation depends on the pH: a two-electron, two-proton (Ru=O/Ru(OH2)) oxidation occurs in the pH range 1-10, while a two-electron, one proton (Ru=O/Ru(OH)) oxidation occurs in the pH=10-14 range. The peaks in the anodic region (0 to +1.5 V vs NHE) were assigned to Ru-based oxidations. Unlike many similar polypyridine-ligated Ru complexes, where two separate Ru III /Ru II and Ru IV /Ru III features are observed, the Ru-bpm features only a single oxidation peak in the anodic region across all pH values (Figure , dark blue traces). This oxidation feature is attributed to the two-electron Ru IV/II couple of Ru-bpm. The merge of two one-electron processes into one oxidation feature are the result of the bpm ligation. The increased π-backbonding and σ-donation of bpm ligand selectively stabilizes the Ru II and Ru IV states relative to Ru III . This consequence of this stabilization effect is the lower potential of the Ru IV/III couple relative to Ru III/II , leading to the presence of a single two-electron oxidation feature. Similar behavior was observed previously for other Rubased complexes with similar ligand structure. Plotting E1/2 for Ru IV/II against pH and fitting to the Nernst equation for PCET reveals two different regions (Figure ). The transition between two regions occurs at pH=9.9 where the deprotonation of the aqua ligand is expected to occur. At pH<10, the data were fit to a Nernst equation with the slope of -0.054 V/pH unit, which is consistent with the two-proton two-electron conversion from [Ru II -OH2] 2+ to [Ru IV =O] 2+ (Scheme 1). At pH>10, the slope is reduced to -0.026 V/pH unit, corresponding to the one-proton twoelectron conversion from [Ru II -OH] + to [Ru IV =O] 2+ (Scheme 1).