Reactions of Neutral Cobalt(II) Complexes of a Dianionic Tetrapodal Pentadentate Ligand: Cobalt(III) Amides from Imido Radicals

Neutral cobalt­(II) complexes of the dianionic tetrapodal pentadentate ligand B₂Pz₄Py, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct <b>1-THF</b> and the five-coordinate THF-free complex <b>1</b> are in a high-spin <i>S</i> = 3/2 configuration in the ground state and have been structurally characterized by X-ray crystallography. These two Co­(II) starting materials react rapidly with aryl azides of moderate steric bulk. The thermodynamic products of these reactions are low-spin, diamagnetic, Co­(III) amido complexes that are either monomeric, when an external hydrogen atom source such as 1,4-cyclohexadiene is present, or dimeric products formed via C–C coupling of the azide aryl group and internal transfer of H^• to the nitrogen. These products are fully characterized and are rare examples of octahedral Co amido compounds; structural determinations reveal significant pyramidalization of the amido nitrogens due to π–π repulsion wherein the amido ligand is primarily a σ donor. The amido products arise from highly reactive Co­(III) imido radical intermediates that are the kinetic products of the reactions of <b>1</b> or <b>1-THF</b> with the azide reagents. The imido radicals can be detected by X-band EPR spectroscopy and have been probed by density functional theory computations, which indicate that this doublet species is characterized by a high degree of spin localization on the imido ligand, accounting for the reactivity with hydrogen atom sources and dimerization chemistry observed. The high coordination number and the electron-rich nature of the dianionic B₂Pz₄Py ligand framework render the imido ligand formed highly reactive

Oxygen–Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O–O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt­(III)–cobalt­(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O–O bond cleavage step that generates a Co­(III) aquo complex and a highly reactive Co­(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O–O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O–O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co­(II) and suggests a new pathway for selective reduction of O2 to water via Co­(III)–O–O–Co­(III) peroxo intermediates

Oxygen–Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O–O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt­(III)–cobalt­(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O–O bond cleavage step that generates a Co­(III) aquo complex and a highly reactive Co­(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O–O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O–O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co­(II) and suggests a new pathway for selective reduction of O2 to water via Co­(III)–O–O–Co­(III) peroxo intermediates

Oxygen–Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O–O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt­(III)–cobalt­(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O–O bond cleavage step that generates a Co­(III) aquo complex and a highly reactive Co­(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O–O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O–O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co­(II) and suggests a new pathway for selective reduction of O2 to water via Co­(III)–O–O–Co­(III) peroxo intermediates

Oxygen–Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O–O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt­(III)–cobalt­(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O–O bond cleavage step that generates a Co­(III) aquo complex and a highly reactive Co­(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O–O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O–O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co­(II) and suggests a new pathway for selective reduction of O2 to water via Co­(III)–O–O–Co­(III) peroxo intermediates

Oxygen–Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O₂ Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O–O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt­(III)–cobalt­(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O–O bond cleavage step that generates a Co­(III) aquo complex and a highly reactive Co­(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O–O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O–O bond cleavage and formation in the stoichiometric reduction of O2 to H2O with 2 equiv of Co­(II) and suggests a new pathway for selective reduction of O2 to water via Co­(III)–O–O–Co­(III) peroxo intermediates