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 given the fleeting nature of these species. Here we utilize a novel dianionic pentadentate ligand system that enables detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, known intermediates in oxygen reduction catalysis to hydrogen peroxide. It is 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 is 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 O 2 with two equivalents of Co(II) to H 2 O, and suggests a new pathway for selective reduction of O 2 to water via Co(III)-O-O-Co(III) peroxo intermediates. tic and computational studies on the diprotonation of the resulting peroxo product. An ensemble of spectroscopic, chemical trapping, isotope labeling studies and density functional theory calculations suggest that a Co(IV) oxyl cation is a likely intermediate generated in this protonation reaction. This is a novel mode of reactivity for a diprotonated Co(III)-O-O-Co(III) species that is directed by the nature of the ligand system used and provides insight into both O-O bond cleavage and formation paths in Co(II) mediated transformations of O 2. RESULTS AND DISCUSSION Synthesis, characterization and reaction chemistry of Co complexes. Our previous reports utilizing the B 2 Pz 4 Py ligand featured a phenyl-substituted borate group; 49 to enhance solubility, the para-tolyl substituted derivative was employed for the chemistry described here. This ligand was prepared analogously to what was described for the Ph derivative and details of its synthesis and characterization can be found in the Supplementary Information and Figure S1. The pentacoordinate, paramagnetic complex 1 can be prepared analytically Scheme 3. Synthesis of B 2 Pz 4 PyCo Compounds 1-THF, 1 and 3-Br pure in 73% yield (Scheme 3) and is essentially NMR silent, exhibiting a HS, S = 3/2 configuration (µ eff = 3.94 m B , Evans' method). Its structure was confirmed by X-ray crystallography (Figure S2). In THF, a 19-electron THF adduct is formed whose structure was also confirmed by X-ray crystallography (Figure S3) and gives rise to assignable NMR spectra (Figures S4 and S5). However, the THF ligand is quite labile and can be removed by heating under vacuum. Compounds 1-TH...

Neutral cobalt(II) complexes of the dianionic tetrapodal pentadentate ligand BPzPy, in which borate linkers supply the anionic charges, are reported. Both the six-coordinate THF adduct 1-THF and the five-coordinate THF-free complex 1 are in a high-spin S = 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 1 or 1-THF 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 BPzPy ligand framework render the imido ligand formed highly reactive.

Synopsis: a highly reactive Fe(iii)–NH2 complex is generated via activation of ammonia or hydrazine in reactions of relevance to fundamental steps in ammonia oxidation processes mediated by an abundant, first row transition metal.

Diprotonation of a remarkably stable, toluene soluble cobalt peroxo complex supported by a neutral, dianionic pentadentate ligand leads to facile O-O bond cleavage and production of a highly reactive Co(IV) oxyl cation intermediate that dimerizes and releases O₂. These processes are relevant to both O₂ reduction and O₂ evolution and the mechanism was probed in detail both experimentally and computationally.

A variety of neutral alkyl-cobalt(III) complexes bearing a dianionic tetrapodal pentadentate ligand B2Pz4Py are reported. Compounds 2-R (R = CH3, CH2SiMe3, CH2SiMe2Ph, i Bu, CH2(c-C5H9) and (CH2)4CH=CH2) are synthesized in 58-90% yield. These diamagnetic, octahedral complexes are thermally stable up to 110˚C and are also remarkably stable to ambient atmosphere. They were fully characterized by spectroscopic techniques, and in three cases, X-ray crystallography. Evidence for reversible homolytic cleavage of the Co-C bonds was found in their reactions with the hydrogen atom donor 1,4-cyclohexadiene and the radical trap TEMPO, as well as the observed cyclization of the 5-hexenyl group to the methylcyclopentyl derivative over the course of several hours. Despite these observations, it can be concluded that the diborate B2Pz4Py ligand provides a very stable platform for these Co(III) alkyls. Reduction by one electron to a Co(II) alkyl can accelerate bond homolysis, but in this instance, using cobaltocene as the reducing agent leads to ejection of an alkide anion through bond heterolysis, an unusual reaction for Co(III) alkyls. Finally, protonation of compound 2-Me with the strong acid HNTf2 leads to divergent reactivity in which the major protonation site is the pyridyl nitrogen of the ligand as opposed to protonation of the methyl group. The produce of protonation at nitrogen is the dimeric species 4 which was prepared via separate synthesis and characterized by X-ray crystallography. Introduction. In Nature, two types of cobalamins, also known as Vitamin B12, exhibit a Co-C bond, methylcobalamin (MeCbl) 1 and adenosylcobalamin (AdoCbl), 2 featuring-CH3 and-CH2R groups in the axial position, respectively (Figure 1). In both stable forms of Vitamin B12, the cobalt center bears a +III oxidation state. During catalysis, however, the Co-C bond of MeCbl, can undergo heterolytic cleavage to give Co(I) and CH3 + , while for AdoCbl the bond can cleave homolytically to provide a source of Co(II) and organic radical. 3 Thus, the alkyl groups in these Co(III) alkyls exhibit divergent reactivity in the in vivo function of Vitamin B12. Although both MeCbl and AdoCbl bear the same functionalized corrin L3X supporting

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Diprotonation of a remarkably stable, toluene soluble cobalt peroxo complex supported by a neutral, dianionic pentadentate ligand leads to facile O-O bond cleavage and production of a highly reactive Co(IV) oxyl cation intermediate that dimerizes and releases O₂. These processes are relevant to both O₂ reduction and O₂ evolution and the mechanism was probed in detail both experimentally and computationally.

We report the use of electron rich iron complexes supported by a dianionic diborate pentadentate ligand system, B2Pz4Py, for the coordination and activation of ammonia (NH3) and hydrazine (NH2NH2). For ammonia, coordination to neutral (B2Pz4Py)Fe(II) or cationic [(B2Pz4Py)Fe(III)]+ platforms leads to well characterized ammine complexes from which hydrogen atoms or protons can be removed to generate, fleetingly, a proposed (B2Pz4Py)Fe(III)- NH2 complex (3Ar-NH2). DFT computations suggest a high degree of spin density on the amido ligand, giving it significant aminyl radical character. It rapidly traps the H atom abstracting agent 2,4,6-tri-tert-butylphenoxy radical (ArO•) to form a C-N bond in a fully characterized product (2Ar), or scavenges hydrogen atoms to return to the ammonia complex (B2Pz4Py)Fe(II)-NH3 (1ArNH3). Interestingly, when (B2Pz4Py)Fe(II) is reacted with NH2NH2, a fully characterized bridging diazene complex, 4Ar, is formed along with ammonia adduct 1Ar-NH3 as the spectroscopically observed (-78˚C) (B2Pz4Py)Fe(II)-NH2NH2-Fe(II)( B2Pz4Py) dimer (1Ar)2-NH2NH2 is allowed to warm to room temperature. Experimental and computational evidence is presented to suggest that (B2Pz4Py)Fe(II) induces reductive cleavage of the N-N bond in hydrazine to produce the Fe(III)-NH2 complex 3Ar-NH2, which abstracts H• atoms from (1Ar)2-NH2NH2 to generate the observed products. All of these transformations are relevant to proposed steps in the ammonia oxidation reaction, an important process for the use of nitrogen-based fuels enabled by abundant first row transition metals. <br>