Driving Multi-electron Reactions with Photons: Dinuclear Ruthenium Complexes Capable of Stepwise and Concerted Multi-electron Reduction

Abstract

Using biological precedents, it is expected that concerted, multi-electron reduction processes will play a significant role in the development of efficient artificial photosynthetic systems. We have found that the dinuclear ruthenium complexes [(phen)(2)Ru(tatpp)Ru(phen)(2)](4+) (P) and [(phen)(2)Ru(tatpq) Ru(phen)(2)](4+) (Q) undergo photodriven 2- and 4-electron reductions, respectively, in the presence of a sacrificial reductant. Importantly, these processes are completely reversible upon exposure to air, and consequently, these complexes have the potential to be used catalytically in multi-electron transfer reactions. A localized molecular orbital description of the ligands and complexes is used to explain both the function and spectroscopy of these complexes. In both complexes, the reducing equivalents are stored in the pi* orbitals of the bridging ligands and depending on the solution pH, various protonation states of the reduced species of P and Q are obtained. Under basic conditions, the photochemical pathway favors sequential single-electron reductions, while neutral or slightly acidic conditions give rise to proton-coupled multi-electron transfer. In fact, at sufficiently acidic pH, only a coupled two-electron, 2-proton process is seen. Few molecular photocatalysts are capable of proton-coupled multi-electron transfer, which is believed to be a fundamental component of light-activated energy storage in nature.

The NAD+/NADH Redox Couple—Insights from the Perspective of Electrochemical Energy Transformation and Biomimetic Chemistry

“…Tri-nuclear [{(bpy)₂Ru(dpb)}₂IrCl₂]⁵ + (bpy = 2,2'-bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline) reported by Molnar and coworkers in 1994, and the tetranuclear [{(bpy)₂Ru(dmbbbpy)}₃Ru]⁸ + (dmbbbpy =2,2'-bis(N-methylbenzimidazole-2-yl)-4,4'-bipyri-dine) reported by Ali and coworkers in 1998, have been shown to have the ability in storing more than one photoexcited electron in the π* orbitals of the ligands using a sacrificial donor of electron. Furthermore, dinuclear [(phen)₂Ru(tatpq) Ru(phen)₂]⁴ + (phen = 1,10-phenanthroline, tatpq = 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3",2"l:2''',3'''-n]pentacene-10,21-quinone) (Wouters et al, 2006) has been shown to undergo a photoinduced two-and four-electron reduction of the bridging ligand in the presence of sacrificial electron donor such as triethylamine (TEA) or triethanolamine (TEOA).…”

Excited State Interaction of Ruthenium (II) Imidazole Phenanthroline Complex [Ru(bpy)2ipH]2+ with 1,4‐Benzoquinone: Simple Electron Transfer or Proton‐Coupled Electron Transfer?

“…Proton‐coupled electron transfer (PCET) reactions are ubiquitous in nature, especially those involving conversion of feedstock molecules to fuels . Majority of the work has been carried out on ground state PCET, also referred as thermal‐PCET . Thermal PCET can occur either by stepwise electron transfer followed by proton transfer or vice‐versa (ET‐PT or PT‐ET) or in a concerted single step electron and proton transfer (CEPT) pathway .…”

Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

“…For artificial photosynthesis, multi‐electron redox chemistry and the accumulation of oxidative and reductive equivalents are highly relevant. [1] Light‐driven charge accumulation in molecular compounds is readily possible with sacrificial electron donors or acceptors,[2] but ultimately the use of such energy‐rich reagents is undesirable . Consequently, it seems important to explore the fundamentals of light‐driven charge accumulation.…”