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

CO2 Activation with Manganese Tricarbonyl Complexes through an H-Atom Responsive Benzimidazole Ligand

Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI (HL)(CO)3 Br (1 a-Br) and MnI (MeL)(CO)3 Br (1 b-Br) (where HL=2-(2'-pyridyl)benzimidazole; MeL=1-methyl-2-(2'-pyridy)benzimidazole) and assayed their electrocatalytic properties for CO2 reduction. A redox-active pyridine benzimidazole ancillary ligand in complex 1 a-Br displayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO2 conversion at a markedly lower reduction potential than that observed for 1 b-Br. Notably, a one-electron reduction of 1 a-Br yields a structurally characterized H-bonded binuclear Mn(I) adduct (2 a') rather than the typically observed Mn(0)-Mn(0) dimer, suggesting a novel method for CO2 activation. Combining advanced electrochemical, spectroscopic, and single crystal X-ray diffraction techniques, we demonstrate the use of an H-atom responsive ligand may reveal an alternative, low-energy pathway for CO2 activation by an earth-abundant metal complex catalyst.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with Mn^II present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

Merging of a Perylene Moiety Enables a RuII Photosensitizer with Long-Lived Excited States and the Efficient Production of Singlet Oxygen

Multichromophoric systems based on a Ru^II polypyridine moiety containing an additional organic chromophore are of increasing interest with respect to different light‐driven applications. Here, we present the synthesis and detailed characterization of a novel Ru^II photosensitizer, namely [(tbbpy)₂Ru((2‐(perylen‐3‐yl)‐1H‐imidazo[4,5‐f][1,10]‐phenanthrolline))](PF₆)₂RuipPer, that includes a merged perylene dye in the back of the ip ligand. This complex features two emissive excited states as well as a long‐lived (8 μs) dark state in acetonitrile solution. Compared to prototype [(bpy)₃Ru]²⁺‐like complexes, a strongly altered absorption (ϵ=50.3×10³ M⁻¹ cm⁻¹ at 467 nm) and emission behavior caused by the introduction of the perylene unit is found. A combination of spectro‐electrochemistry and time‐resolved spectroscopy was used to elucidate the nature of the excited states. Finally, this photosensitizer was successfully used for the efficient formation of reactive singlet oxygen.

Bioinspired Heterobimetallic Photocatalyst () for Visible-Light-Driven C-H Oxidation of Organic Substrates via Dioxygen Activation

We report a bioinspired heterobimetallic photocatalyst RuIIchrom-FeIIIcat and its relevant applications toward visible-light-driven C-H bond oxidation of a series of hydrocarbons using O2 as the O-atom source. The RuII center absorbs visible light near 460 nm and triggers a cascade of electrons to FeIII to afford a catalytically active high-valent FeIV═O species. The in situ formed FeIV═O has been employed for several high-impact oxidation reactions in the presence of triethanolamine (TEOA) as the sacrificial electron donor.

Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands

Ruthenium(II) complexes, like [(tbbpy)₂Ru(dppz)]²⁺ (Ru-dppz; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there has been ongoing interest in the design of π-extended Ru-dppz systems with red-shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, we explore the photophysical properties of protonation isomers of the linearly π-extended [(tbbpy)₂Ru(L)]²⁺-type complexes bearing a dppz ligand with directly fused imidazole (im) and methyl-imidazole units (mim) as L. Steady-state UV-vis absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that Ru-im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two long-lived excited states in the ³ILCT and ³MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-)imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-off.