A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes withC2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en route

Schmidt, Stefan; Heinemann, Frank W.; Grohmann, Andreas

doi:10.1002/1099-0682(200007)2000:7<1657::aid-ejic1657>3.0.co;2-3

Cited by 37 publications

(6 citation statements)

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“…In the context of O 2 reduction, cobalt(II) complexes bearing multidentate ligands have been widely studied because of the propensity of these d 7 centers to bind O 2 (usually reversibly) − to yield [LCo(III)O 2 ·] n superoxo monomers − that can be reactive intermediates themselves ,, or be trapped by a second [LCo(II)] to yield [LCo(III)–O–O–Co(III)L] 2 n peroxo dimers, − in which “end-on” Pauling-type binding is generally the norm (Scheme ). Another possibility involves (formal) trapping of the superoxo monomer with an [LCo(III)] center to give a superoxo dimer; these are thought to be key species in water-selective O 2 reduction reactions involving binuclear Co catalysts (see below) .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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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...

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Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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“…[20] Notably, the CoÀ O distance of 1.8930(17) Å as well as the OÀ O distance of 1.442(5) Å (Table S3) are in line with the ones reported for other pentaamine Co III À O 2 complexes. [26,73,74] Moreover, X-band EPR analysis in perpendicular mode of solid Co 2 O 2 {i-N 4 } 2 gave no signal as expected and the spectrum of the re-dissolved complex (1 mM) S20). These findings also suggest that Co 2 (O 2 ){i-N 4 } 2 is formed as a main species from the reaction of Co{i-N 4 } with O 2 and are in line with previous studies on the cyclam derivative Co{N 4 } which described that different CoÀ O 2 species are formed depending on the Co complex/O 2 ratio.…”

Section: Orr Capability and Reactivity Towards Omentioning

confidence: 64%

How Ligand Geometry Affects the Reactivity of Co(II) Cyclam Complexes

Iffland‐Mühlhaus,

Battistella,

Siegmund

et al. 2023

Eur J Inorg Chem

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Cobalt complexes are extensively studied as bioinspired models for non‐heme oxygenases as they facilitate both the stabilization and characterization of metal‐oxygen intermediates. As an analog to the well‐known Co(cyclam) complex Co{N4} (cyclam = 1,4,8,11‐tetraazacyclotetradecane), the CoII complex Co{i‑N4} with the isomeric isocyclam ligand (isocyclam = 1,4,7,11‐tetraazacyclotetradecane) was synthesized and characterized. Despite the identical N4 donor set of both complexes, Co{i‐N4} enables the 2e‾/2H+ reduction of O2 with a lower overpotential (ηeff of 385 mV vs. 540 mV for Co{N4}), albeit with a diminished turnover frequency. Characterization of the intermediates formed upon O2 activation of Co{i‐N4} reveals a structurally identified stable µ‐peroxo CoIII dimer as the main product. A superoxo CoIII species is also formed as a minor product, as indicated by EPR spectroscopy. In further reactivity studies, the electrophilicity of these in situ generated Co‐O2 species was demonstrated by the oxidation of the O‐H bond of TEMPO‐H (2,2,6,6‐tetramethylpiperidin‐1‐ol) via a H atom abstraction process. Unlike the known Co(cyclam), Co{i‐N4} can be employed in oxygen atom transfer reactions oxidizing triphenylphosphine to the corresponding phosphine oxide highlighting the impact of geometrical modifications of the ligand while preserving the ring size and donor atom set on the reactivity of biomimetic oxygen activating complexes.

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“…Each cobalt ion is surrounded by the N4 ligand and one carboxylate oxygen atom of MPA and a peroxide oxygen. In the peroxo moiety, the O–O bond distance is observed at 1.384(10) Å, which falls on the lower side of the range of O–O distances (1.34–1.53 Å) observed in the reported peroxo-bridged dicobalt(III) complexes but is higher than the superoxide bond length (average 1.33 Å). ,,,− The two cobalt centers in 5P1 are separated at a distance of 4.005 Å. The Co–N bond distances are found to be in the range 1.914–1.935 Å, although the Co1–N2 (2.013 Å) and Co2–N7 (2.005 Å) distances are significantly elongated due to the trans effect of the peroxide moiety .…”

Section: Results and Discussionmentioning

confidence: 81%

Dioxygen Activation and Mandelate Decarboxylation by Iron(II) Complexes of N4 Ligands: Evidence for Dioxygen-Derived Intermediates from Cobalt Analogues

et al. 2022

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The isolation, characterization, and dioxygen reactivity of monomeric [(TPA)MII(mandelate)]+ (M = Fe, 1; Co, 3) and dimeric [(BPMEN)2MII 2(μ-mandelate)2]2+ (M = Fe, 2; Co, 4) (TPA = tris(2-pyridylmethyl)amine and BPMEN = N1 ,N2 -dimethyl-N1 ,N2 -bis(pyridin-2-yl-methyl)ethane-1,2-diamine) complexes are reported. The iron(II)- and cobalt(II)-mandelate complexes react with dioxygen to afford benzaldehyde and benzoic acid in a 1:1 ratio. In the reactions, one oxygen atom from dioxygen is incorporated into benzoic acid, but benzaldehyde does not derive any oxygen atom from dioxygen. While no O2-derived intermediate is observed with the iron(II)-mandelate complexes, the analogous cobalt(II) complexes react with dioxygen at a low temperature (−80 °C) to generate the corresponding cobalt(III)-superoxo species (S), a key intermediate implicated in the initiation of mandelate decarboxylation. At −20 °C, the cobalt(II)-mandelate complexes bind dioxygen reversibly leading to the formation of μ-1,2-peroxo-dicobalt(III)-mandelate species (P). The geometric and electronic structures of the O2-derived intermediates (S and P) have been established by computational studies. The intermediates S and P upon treatment with a protic acid undergo decarboxylation to afford benzaldehyde (50%) with a concomitant formation of the corresponding μ-1,2-peroxo-μ-mandelate-dicobalt(III) (P1) species. The crystal structure of a peroxide species isolated from the cobalt(II)-carboxylate complex [(TPA)CoII(MPA)]+ (5) (MPA = 2-methoxyphenylacetate) supports the composition of P1. The observations of the dioxygen-derived intermediates from cobalt complexes and their electronic structure analyses not only provide information about the nature of active species involved in the decarboxylation of mandelate but also shed light on the mechanistic pathway of two-electron versus four-electron reduction of dioxygen.

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A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes withC2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en route

Cited by 37 publications

References 51 publications

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

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

How Ligand Geometry Affects the Reactivity of Co(II) Cyclam Complexes

Dioxygen Activation and Mandelate Decarboxylation by Iron(II) Complexes of N4 Ligands: Evidence for Dioxygen-Derived Intermediates from Cobalt Analogues

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A Structurally Characterised Pair of Dicobalt(III) Peroxo/Superoxo Complexes withC2-Symmetrical Tetrapodal Pentadentate Amine Ligands, and Some Reactivity en route

Cited by 37 publications

References 51 publications

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

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

How Ligand Geometry Affects the Reactivity of Co(II) Cyclam Complexes

Dioxygen Activation and Mandelate Decarboxylation by Iron(II) Complexes of N4 Ligands: Evidence for Dioxygen-Derived Intermediates from Cobalt Analogues

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

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