Electrocatalytic H₂ Evolution by Supramolecular Ru^II–Rh^III–Ru^II Complexes: Importance of Ligands as Electron Reservoirs and Speciation upon Reduction

The supramolecular water reduction photocatalysts [{(Ph₂phen)₂Ru­(dpp)}₂RhX₂]­(PF₆)₅ (Ph₂phen = 4,7-diphenyl-1,10-phenanthroline, dpp =2,3-bis­(2-pyridyl)­pyrazine X = Cl, Br) are efficient electrocatalysts for the reduction of CF₃SO₃H, CF₃CO₂H, and CH₃CO₂H to H₂ in DMF or DMF/H₂O mixtures. The onset of catalytic current occurs at −0.82 V versus Ag/AgCl for CF₃SO₃H, −0.90 V for CF₃CO₂H, and −1.1 V for CH₃CO₂H with overpotentials of 0.61, 0.45, and 0.10 V, respectively. In each case, catalysis is triggered by the first dpp ligand reduction implicating the dpp as an electron reservoir in catalysis. A new species with <i>E</i>_pc ∼ −0.75 V was observed in the presence of stoichiometric amounts of strong acid, and its identity is proposed as the Rh­(H)^III/II redox couple. H₂ was produced in 72–85% Faradaic yields and 95–116 turnovers after 2 h and 435 turnovers after 10 h of bulk electrolysis. The identities of Rh­(I) species upon reduction have been studied. In contrast to the expected dissociation of halides in the Rh­(I) state, the halide loss depends on solvent and water content. In dry CH₃CN, in which Cl^– is poorly solvated, a [Ru] complex dissociates and [(Ph₂phen)₂Ru­(dpp)­Rh^ICl₂]⁺ and [(Ph₂phen)₂Ru­(dpp)]²⁺ are formed. In contrast, for X = Br^–, the major product of reduction is the intact trimetallic Rh­(I) complex [{(Ph₂phen)₂Ru­(dpp)}₂Rh^I]⁵⁺. Chloride loss in CH₃CN is facilitated by addition of 3 M H₂O. In DMF, the reduced species is [{(Ph₂phen)₂Ru­(dpp)}₂Rh^I]⁵⁺ regardless of X = Cl^– or Br^–

Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation

The tetradentate ligand, 2-(pyrid-2′-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp² center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl₃, a μ-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl₃. Both structures are verified by X-ray analysis. While the Fe^III(dpa) complex shows two reversible one-electron oxidation waves, the Fe^III(ppq) complex shows a clear two-electron oxidation associated with the process H₂O–Fe^IIIFe^III → H₂O–Fe^IVFe^IV → OFe^VFe^III. Subsequent disproportionation to an FeO species is suggested. When the Fe^III(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h^–1. Under the same conditions the mononuclear Fe^III(dpa) complex also evolves oxygen with TOF = 842 h⁻¹

Tuning the Photophysical Properties of Ru(II) Monometallic and Ru(II),Rh(III) Bimetallic Supramolecular Complexes by Selective Ligand Deuteration

A series of three new complexes of the design [(TL)₂Ru­(BL)]²⁺, two new complexes of the design [(TL)₂Ru­(BL)­Ru­(TL)₂]⁴⁺, and three new complexes of the design [(TL)₂Ru­(BL)­RhCl₂(TL)]³⁺ (TL = bpy or <i>d</i>₈-bpy; BL = dpp or <i>d</i>₁₀-dpp; TL = terminal ligand; BL = bridging ligand; bpy = 2,2′-bipyridine; dpp = 2,3-bis­(2-pyridyl)­pyrazine) were synthesized and the ¹H NMR spectroscopy, electrochemistry, electronic absorbance spectroscopy, and photophysical properties studied. Incorporation of deuterated ligands into the molecular architecture simplifies the ¹H NMR spectra, allowing for complete ¹H assignment of [(<i>d</i>₈-bpy)₂Ru­(dpp)]­(PF₆)₂ and partial assignment of [(bpy)₂Ru­(<i>d</i>₁₀-dpp)]­(PF₆)₂. The electrochemistry for the deuterated and nondeuterated species showed nearly identical redox properties. Electronic absorption spectroscopy of the deuterated and nondeuterated complexes are superimposable with the lowest energy transition being Ru­(dπ) → BL­(π*) charge transfer in nature (BL = dpp or <i>d</i>₁₀-dpp). Ligand deuteration impacts the excited-state properties with an observed increase in the quantum yield of emission (Φ^em) and excited-state lifetime (τ) of the Ru­(dπ) → <i>d</i>₁₀-dpp­(π*) triplet metal-to-ligand charge transfer (³MLCT) excited state when dpp is deuterated, and a decrease in the rate constant for nonradiative decay (<i>k</i>_nr). Choice of ligand deuteration between bpy and dpp strongly impacts the observed photophysical properties with BL = <i>d</i>₁₀-dpp complexes showing an enhanced Φ^em and τ, providing further support that the lowest electronic excited state populated via UV or visible excitation is the photoactive Ru­(dπ) → dpp­(π*) CT excited state. The Ru­(II),Rh­(III) complex incorporating the deuterated BL shows increased hydrogen production compared to the variants incorporating the protiated BL, while demonstrating identical dynamic quenching behaviors in the presence of sacrificial electron donor