Spectroelectrochemical studies of a ruthenium complex containing the pH sensitive 4,4′-dihydroxy-2,2′-bipyridine ligand

Abstract

Attaining high oxidation states at the metal center of transition metal complexes is a key design principle for many catalytic processes. One way to support high oxidation state chemistry is to utilize ligands that are electron-donating in nature. Understanding the structural and electronic changes of metal complexes as higher oxidation states are reached is critical towards designing more robust catalysts that are able to turn over at high rates without decomposing. To this end, we report herein the changes in structural and electronic properties as [Ru(bpy)(44'bpy(OH))] is oxidized to [Ru(bpy)(44'bpy(OH))] (bpy = 2,2'-bipyridine; 44'bpy(OH) = 4,4'-dihydroxy-2,2'-bipyridine). The 44'bpy(OH) ligand is a pH-dependent ligand where deprotonation of the hydroxyl groups leads to significant electronic donation to the metal center. A Pourbaix Diagram of the complex reveals a pH independent reduction potential below pH = 2.0 for the Ru process at 0.91 V vs. Ag/AgCl. Above pH = 2.0, pH dependence is observed with a decrease in reduction potential until pH = 6.8 where the complex is completely deprotonated, resulting in a reduction potential of 0.62 V vs. Ag/AgCl. Spectroelectrochemical studies as a function of pH reveal the disappearance of the Metal to Ligand Charge Transfer (MLCT) or Mixed Metal-Ligand to Charge Transfer bands upon oxidation and the appearance of a new low energy band. DFT calculations for this low energy band were carried out using both B3LYP and M06-L functionals for all protonation states and suggest that numerous new transition types occur upon oxidation to Ru.

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations

“…have used the value of 1210 mV vs Fc +/0 for HMB as a reference for their redox ladder, we have chosen [Ru­(bpy-(OMe) 2 ) 3 ] 2+ with a reversible couple at a less positive potential of 557 mV vs Fc +/0 as our reference. We believe that using reference couples with less anodic potentials provides better accuracy for determining true E °′ due to the stability of oxidized species as demonstrated for [Ru­(bpy-(OMe) 2 ) 3 ] 3+ using cyclic voltammetry and UV–vis spectroelectrochemistry . The redox scale measured by PR is consistent with the potential of BP determined by Guldi et al using AC voltammetry .…”

“…We believe that using reference couples with less anodic potentials provides better accuracy for determining true E°′ due to the stability of oxidized species as demonstrated for [Ru(bpy-(OMe) 2 ) 3 ] 3+ using cyclic voltammetry 28 and UV−vis spectroelectrochemistry. 42 The redox scale measured by PR is consistent with the potential of BP determined by Guldi et al using AC voltammetry. 13 Regardless of the choice of a reference potential, the comparison of data in Table 2 to values determined photochemically and those obtained by AC voltammetry demonstrates the applicability of the PR equilibrium technique for determining the potentials of biphenyl and benzene derivatives.…”

Relevance of the electron transfer pathway in photodynamic activity of Ru(ii) polypyridyl complexes containing 4,7-diphenyl-1,10-phenanthroline ligands under normoxic and hypoxic conditions

“…Theoretical calculations: assignment of the lowest excited state of the complexes Theoretical calculations have been successfully used to reveal the photophysics of Ru(II) polypyridyl complexes. [55][56][57][58][59] We carried out DFT/TD-DFT calculations for the studied complexes in ethanol (Table 1). We found that for Ru1 and Ru2 the lowest singlet (S 1 ) and triplet states (T 1 ) are the MLCT states.…”

Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes

“…In work similar to the original Meyer paper, we reported a system that utilized a Ru II diimine complex bearing a hydroxylated bipyridine ligand and MQ + . In this system, we explored the thermodynamic landscape of reactions. − We discovered that despite the favorability of a concerted PCET reaction from the excited-state potential energy surface, outer sphere electron transfer was observed. We speculated that because of a combination of electrostatic repulsion from the same charge species and a small driving force for proton transfer, and therefore also weak hydrogen bonding, the otherwise favorable concerted excited-state PCET (PCET*) reaction was not observed.…”