Electro- and Photocatalytic Generation of H2 Using a Distinctive CoII "PN3 P" Pincer Supported Complex with Water or Saturated Saline as a Hydrogen Source

Electrocatalytic Reduction of CO2 and H2O with Zn(II) Complexes Through Metal-Ligand Cooperation

Air and water-stable zinc (II) complexes of neutral pincer bis(diphenylphosphino)-2,6-di(amino)pyridine ("PN3P") ligands are reported. These compounds, [Zn(κ2-2,6-{Ph2PNR}2(NC5H3))Br2] (R=Me, 1; R=H, 2), were shown to be capable of electrocatalytic reduction of CO2 at -2.3 V vs. Fc+/0 to selectively yield CO in mixed water/acetonitrile solutions. These complexes also electrocatalytically generate H2 from water in acetonitrile solutions, at the same potential, with Faradaic efficiencies of up to 90 %. DFT computations support a proposed mechanism involving the first reduction of 1 or 2 occurring at the PN3P ligand. Furthermore, computational analysis suggested a mechanism involving metal-ligand cooperation of a Lewis acidic Zn(II) and a basic ligand.

Elucidating Two Distinct Pathways for Electrocatalytic Hydrogen Production Using CoII Pincer Complexes

Hydrogen gas is a sustainable energy source with water as the sole combustion product. As a result, efforts to catalyze H₂ production are pertinent and widespread. The electrocatalytic H₂ generating capabilities of two Co^II complexes, [Co(κ³ -2,6-{Ph₂ PNR}₂ (NC₅ H₃ ))Br₂ ] with R=H (I) or R=Me (II), were presented for a variety of proton sources including trifluoroacetic acid (TFA), acetic acid (AA), and trifluoroethanol (TFE). Cyclic voltammetry and controlled potential coulometry demonstrated that electrocatalysis from I and II occurred at two different potentials and are associated with different reduction processes. Density functional theory analysis provided insight into the identities of the catalyst and supported two distinct reaction pathways for electrocatalytic proton reduction. Specifically, stronger acids (e. g., AA, TFA) proceeded at -1.31 to -1.45 V through a M^I /M^III pathway while sources with higher pK_a values (e. g., TFE, H₂ O) generated hydrogen at -2.4 V via M⁰ /M^II ligand-assisted metal-centered reduction.

Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO₂). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO₂, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO₂ in their reduced states. The Ni(0) d¹⁰ species associates with CO₂ to form a putative Aresta-type Ni-η²-CO₂ complex, where electron transfer to CO₂ through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d⁹ intermediate binding CO₂ can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d⁷-to-d⁹ configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO₂ activation for complexes based on redox-innocent ligands.

A series of 3d transition-metal complexes (M = Mn, Fe, Co, Ni, Cu, and Zn) coordinated by a new redox-innocent PNP pincer ligand system were synthesized and structurally as well as electrochemically analyzed to illuminate the role of the metal center in molecular electrochemical carbon dioxide (CO₂) activation.

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in "PN3 P" Pincer-Supported NiII Complexes

Water is the most sustainable source for H₂ production, and the efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ³ -2,6-{Ph₂ PNR}₂ (NC₅ H₃ )Br₂ ] (R=H I, Me II) is reported. Hydrogen generation from H₂ O/CH₃ CN solutions is initiated at -2 V against Fc^+/0 , and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s^-1 . A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H₂ formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni^0I species bonded to an anionic PN³ P ligand ("L^- /Ni^0I ") and a Ni^I -hydride ("Ni-H"). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pK_a .