Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol Complex: Stepwise Oxidation and Concerted Reduction

Proton-coupled electron transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII)-inspired [Ru­(bpy)₂(phen-imidazole-Ph­(OH)­(^<i>t</i>Bu)₂)]²⁺, in which Ru­(III) generated by a flash–quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen-bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the Ru^III/II couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru­(III) is energetically favorable by both stepwise and concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with <i>k</i>_ET = 2.7 × 10⁷ s^–1 and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λ_DA = 1.16 eV and 280 < <i>H</i>_DA < 540 cm^–1 consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen-evolving complex. In our PSII-inspired complex, the recombination reaction activation energy is <2 kcal mol^–1. The reaction is nonadiabatic (<i>V</i>_PCET ≈ 22 cm^–1 (H) and 49 cm^–1 (D)) and concerted, and it exhibits an unexpected inverse KIE = 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels

Tetra- and Heptametallic Ru(II),Rh(III) Supramolecular Hydrogen Production Photocatalysts

Supramolecular mixed metal complexes combining the trimetallic chromophore [{(bpy)₂Ru­(dpp)}₂Ru­(dpp)]⁶⁺ (<b>Ru</b>_<b>3</b>) with [Rh­(bpy)­Cl₂]⁺ or [RhCl₂]⁺ catalytic fragments to form [{(bpy)₂Ru­(dpp)}₂Ru­(dpp)­RhCl₂(bpy)]­(PF₆)₇ (<b>Ru</b>_<b>3</b><b>Rh</b>) or [{(bpy)₂Ru­(dpp)}₂Ru­(dpp)]₂RhCl₂(PF₆)₁₃ (<b>Ru</b>_<b>3</b><b>RhRu</b>_<b>3</b>) (bpy = 2,2′-bipyridine and dpp = 2,3-bis­(2-pyridyl)­pyrazine) catalyze the photochemical reduction of protons to H₂. This first example of a heptametallic Ru,Rh photocatalyst produces over 300 turnovers of H₂ upon photolysis of a solution of acetonitrile, water, triflic acid, and <i>N</i>,<i>N</i>-dimethylaniline as an electron donor. In contrast, the tetrametallic <b>Ru</b>_<b>3</b><b>Rh</b> produces only 40 turnovers of H₂ due to differences in the excited state properties and nature of the catalysts upon reduction as ascertained from electrochemical data, transient absorption spectroscopy, and flash-quench experiments. While the lowest unoccupied molecular orbital of <b>Ru</b>_<b>3</b><b>Rh</b> is localized on a bridging ligand, it is Rh-centered in <b>Ru</b>_<b>3</b><b>RhRu</b>_<b>3</b> facilitating electron collection at Rh in the excited state and reductively quenched state. The Ru → Rh charge separated state of <b>Ru</b>_<b>3</b><b>RhRu</b>_<b>3</b> is endergonic with respect to the emissive Ru → dpp ³MLCT excited and cannot be formed by static electron transfer quenching of the ³MLCT state. Instead, a mechanism of subnanosecond charge separation from high lying states is proposed. Multiple reductions of <b>Ru</b>_<b>3</b> and <b>Ru</b>_<b>3</b><b>Rh</b> using sodium amalgam were carried out to compare UV–vis absorption spectra of reduced species and to evaluate the stability of highly reduced complexes. The <b>Ru₃</b> and <b>Ru₃Rh</b> can be reduced by 10 and 13 electrons, respectively, to final states with all bridging ligands doubly reduced and all bpy ligands singly reduced

Push or Pull? Proton Responsive Ligand Effects in Rhenium Tricarbonyl CO₂ Reduction Catalysts

Proton responsive ligands offer control of catalytic reactions through modulation of pH-dependent properties, second coordination sphere stabilization of transition states, or by providing a local proton source for multiproton, multielectron reactions. Two <i>fac</i>-[Re^I(α-diimine)­(CO)₃Cl] complexes with α-diimine = 4,4′- (or 6,6′-) dihydroxy-2,2′-bipyridine (4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts for the reduction of carbon dioxide. Consecutive electrochemical reduction of these complexes yields species identical to those obtained by chemical deprotonation. An energetically feasible mechanism for reductive deprotonation is proposed in which the bpy anion is doubly protonated followed by loss of H₂ and 2H⁺. Cyclic voltammetry reveals a two-electron, three-wave system owing to competing EEC and ECE pathways. The chemical step of the ECE pathway might be attributed to the reductive deprotonation but cannot be distinguished from chloride dissociation. The rate obtained by digital simulation is approximately 8 s^–1. Under CO₂, these competing reactions generate a two-slope catalytic waveform with onset potential of −1.65 V vs Ag/AgCl. Reduction of CO₂ to CO by the [Re^I(4DHBP–2H⁺)­(CO)₃]⁻ suggests the interaction of CO₂ with the deprotonated species or a third reduction followed by catalysis. Conversely, the reduced form of [Re­(6DHBP)­(CO)₃Cl] converts CO₂ to CO with a single turnover

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^–

Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO₂ Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH₃)]²⁺ (6DHBP = 6,6′-(OH)₂bpy)

Ruthenium complexes with proton-responsive ligands [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]­(CF₃SO₃)₂ (tpy = 2,2′:6′,2″-terpyridine; <i>n</i>DHBP = <i>n</i>,<i>n</i>′-dihydroxy-2,2′-bipyridine, <i>n</i> = 4 or 6) were examined for reductive chemistry and as catalysts for CO₂ reduction. Electrochemical reduction of [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]²⁺ generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru­(tpy)­(<i>n</i>DHBP–2H⁺)]⁰) triggers catalysis of CO₂ reduction; however, the catalytic efficiency is strikingly lower than that of unsubstituted [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ (bpy = 2,2′-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO₂ at both the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP–2H⁺ with CO₂

Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO₂ Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH₃)]²⁺ (6DHBP = 6,6′-(OH)₂bpy)

Ruthenium complexes with proton-responsive ligands [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]­(CF₃SO₃)₂ (tpy = 2,2′:6′,2″-terpyridine; <i>n</i>DHBP = <i>n</i>,<i>n</i>′-dihydroxy-2,2′-bipyridine, <i>n</i> = 4 or 6) were examined for reductive chemistry and as catalysts for CO₂ reduction. Electrochemical reduction of [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]²⁺ generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru­(tpy)­(<i>n</i>DHBP–2H⁺)]⁰) triggers catalysis of CO₂ reduction; however, the catalytic efficiency is strikingly lower than that of unsubstituted [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ (bpy = 2,2′-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO₂ at both the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP–2H⁺ with CO₂

Photocatalytic CO₂ Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO₂, CO, or Proton

The cobalt complexes Co^IIL1­(PF₆)₂ (<b>1</b>; L1 = 2,6-bis­[2-(2,2′-bipyridin-6′-yl)­ethyl]­pyridine) and Co^IIL2­(PF₆)₂ (<b>2</b>; L2 = 2,6-bis­[2-(4-methoxy-2,2′-bipyridin-6′-yl)­ethyl]­pyridine) were synthesized and used for photocatalytic CO₂ reduction in acetonitrile. X-ray structures of complexes <b>1</b> and <b>2</b> reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co­(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO₂ reduction was observed near the Co­(I/0) redox couple for both complexes <b>1</b> and <b>2</b> at <i>E</i>_1/2 = −1.77 and −1.85 V versus Ag/AgNO₃ (or −1.86 and −1.94 V vs Fc^+/0), respectively. Under photochemical conditions with <b>2</b> as the catalyst, [Ru­(bpy)₃]²⁺ as a photosensitizer, tri-<i>p</i>-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H₂ were produced under visible-light irradiation, despite the endergonic reduction of Co­(I) to Co(0) by the photogenerated [Ru­(bpy)₃]⁺. However, bulk electrolysis in a wet CH₃CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co–H]^<i>n</i>+ (<i>n</i> = 1 and 0) under electrochemical conditions. The one-electron-reduced complex <b>2</b> reacts with CO to produce [Co⁰L2­(CO)] with ν_CO = 1894 cm^–1 together with [Co^IIL2]²⁺ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV–vis spectroscopy indicate a slow CO binding rate with the [Co^IL2]⁺ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO₂ is much slower than the photochemical formation of [Ru­(bpy)₃]⁺ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru­(bpy)₃]⁺