Mechanistic studies of hydrogen evolution in aqueous solution catalyzed by a tertpyridine-amine cobalt complex

The ability of cobalt-based transition metal complexes to catalyze electrochemical proton reduction to produce molecular hydrogen has resulted in a large number of mechanistic studies involving various cobalt complexes. While the basic mechanism of proton reduction promoted by cobalt species is well-understood, the reactivity of certain reaction intermediates, such as CoI and CoIII-H, is still relatively unknown owing to their transient nature, especially in aqueous media. In this work we investigate the properties of intermediates produced during catalytic proton reduction in aqueous solutions promoted by the [(DPA-Bpy)Co(OH2)]n+ (DPA-Bpy = N,N-bis(2-pyridinylmethyl)-2,20-bipyridine-6-methanamine) complex ([Co(L)(OH2)]n+ where L is the pentadentate DPA-Bpy ligand or [Co(OH2)]n+ as a shorthand). Experimental results based on transient pulse radiolysis and laser flash photolysis methods, together with electrochemical studies and supported by density functional theory (DFT) calculations indicate that, while the water ligand is strongly coordinated to the metal center in the oxidation state 3+, one-electron reduction of the complex to form a CoII species results in weakening the Co-O bond. The further reduction to a CoI species leads to the loss of the aqua ligand and the formation of [CoI-VS)]+ (VS = vacant site). Interestingly, DFT calculations also predict the existence of a [CoI(κ4-L)(OH2)]+ species at least transiently, and its formation is consistent with the experimental Pourbaix diagram. Both electrochemical and kinetics results indicate that the CoI species must undergo some structural change prior to accepting the proton, and this transformation represents the rate-determining step (RDS) in the overall formation of [CoIII-H]2+. We propose that this RDS may originate from the slow removal of a solvent ligand in the intermediate [CoI(κ4-L)(OH2)]+ in addition to the significant structural reorganization of the metal complex and surrounding solvent resulting in a high free energy of activation

Structural and Electronic Influences on Rates of Tertpyridine−Amine Co\u3csup\u3eIII\u3c/sup\u3e−H Formation During Catalytic H2 Evolution in an Aqueous Environment

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [CoII−Cl]+ species results in the formation of [CoII(κ4-L)(OH2)]2+. Further reduction produces [CoI(κ4-L)(OH2)]+, which undergoes a rate-limiting structural rearrangement to [CoI(κ5-L)]+ before being protonated to form [CoIII−H]2+. The rate of [CoIII−H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII−H]2+ estimated from PR experiments, we found that while the protonation of [CoIII−H]2+ is unfavorable, [CoII−H]+ reacts with protons to produce H2. The catalytic activity for H2 evolution tracks the hydricity of the [CoII−H]+ intermediate

Structural and Electronic Influences on Rates of Tertpyridine−Amine Co III

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [CoII−Cl]+ species results in the formation of [CoII(κ4-L)(OH2)]2+. Further reduction produces [CoI(κ4-L)(OH2)]+, which undergoes a rate-limiting structural rearrangement to [CoI(κ5-L)]+ before being protonated to form [CoIII−H]2+. The rate of [CoIII−H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII−H]2+ estimated from PR experiments, we found that while the protonation of [CoIII−H]2+ is unfavorable, [CoII−H]+ reacts with protons to produce H2. The catalytic activity for H2 evolution tracks the hydricity of the [CoII−H]+ intermediate

Role of Hydrogen Bonding in Photoinduced Electron–Proton Transfer from Phenols to a Polypyridine Ru Complex with a Proton-Accepting Ligand

Electron–proton transfer (EPT) from phenols to a triplet metal-to-ligand charge transfer (MLCT)-excited Ru polypyridine complex containing an uncoordinated nitrogen site, <b>1­(T)</b>, can be described by a kinetic model that accounts for the H-bonding of <b>1­(T)</b> to phenol, <b>1­(T)</b> to solvent, and phenol to solvent. The latter plays a major role in the kinetic solvent effect and commonly precludes simultaneous determination of the EPT rate constant and <b>1­(T)</b>-phenol H-bonding constant. A number of these quantities previously reported for similar systems are shown to be in error due to inconsistent data analysis. Control experiments replacing either <b>1­(T)</b> by its structural isomer with a sterically screened nitrogen site or phenol by its H-bonding surrogate, trifluoroethanol, and the observation of negative activation enthalpies for the overall reactions between <b>1­(T)</b> and phenols lend support to the proposed model and provide evidence for the formation of a precursor H-bonded complex between the reactants, which is a prerequisite for EPT

Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO₂ Electroreduction

Metallic silver (Ag) is known as an efficient electrocatalyst for the conversion of carbon dioxide (CO₂) to carbon monoxide (CO) in aqueous or nonaqueous electrolytes. However, polycrystalline silver electrocatalysts require significant overpotentials in order to achieve high selectivity toward CO₂ reduction, as compared to the side reaction of hydrogen evolution. Here we report a high-surface-area Ag nanocoral catalyst, fabricated by an oxidation–reduction method in the presence of chloride anions in an aqueous medium, for the electro-reduction of CO₂ to CO with a current efficiency of 95% at the low overpotential of 0.37 V and the current density of 2 mA cm^–2. A lower limit of TOF of 0.4 s^–1 and TON > 8.8 × 10⁴ (over 72 h) was estimated for the Ag nanocoral catalyst at an overpotential of 0.49 V. The Ag nanocoral catalyst demonstrated a 32-fold enhancement in surface-area-normalized activity, at an overpotential of 0.49 V, as compared to Ag foil. We found that, in addition to the effect on nanomorphology, the adsorbed chloride anions play a critical role in the observed enhanced activity and selectivity of the Ag nanocoral electrocatalyst toward CO₂ reduction. Synchrotron X-ray photoelectron spectroscopy (XPS) studies along with a series of control experiments suggest that the chloride anions, remaining adsorbed on the catalyst surface under electrocatalytic conditions, can effectively inhibit the side reaction of hydrogen evolution and enhance the catalytic performance for CO₂ reduction

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]⁺