Five-Coordinate Cobalt(ii) Complexes of Macrocyclic Ligands — A New Reversible Oxygen Carrying System

Goedken, Virgil L.; Kildahl, Nicholas K.; Busch, Daryle H.

doi:10.1080/00958977708073045

Cited by 28 publications

(15 citation statements)

References 46 publications

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“…We note that the formation of bridging Co(III)–O 2 2– –Co(III) species cannot be excluded based on the observed shift to diamagnetic species in the EPR under O 2 saturation. However, the observation of EPR signals characteristic of Co(III)–O 2 •– species under low [O 2 ] exposures supports the intermediacy of monomeric species, − and we postulate that a dynamic off-cycle equilibrium exists between monomer and dimer. Stahl and co-workers have previously demonstrated that at the low [Co] used under catalytic conditions monomer speciation is generally >95% .…”

Section: Discussionsupporting

confidence: 57%

Pendent Relay Enhances H₂O₂Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N₂O₂Complexes

et al. 2021

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Generally, cobalt–N2O2 complexes show selectivity for hydrogen peroxide during electrochemical dioxygen (O2) reduction. We recently reported a Co(III)–N2O2 complex with a 2,2′-bipyridine-based ligand backbone which showed alternative selectivity: H2O was observed as the primary reduction product from O2 (71 ± 5%) with decamethylferrocene as a chemical reductant and acetic acid as a proton donor in methanol solution. We hypothesized that the key selectivity difference in this case arises in part from increased favorability of protonation at the distal O position of the key intermediate Co(III)–hydroperoxide species. To interrogate this hypothesis, we have prepared a new Co(III) compound that contains pendent −OMe groups poised to direct protonation toward the proximal O atom of this hydroperoxo intermediate. Mechanistic studies in acetonitrile (MeCN) solution reveal two regimes are possible in the catalytic response, dependent on added acid strength and the presence of the pendent proton donor relay. In the presence of stronger acids, the activity of the complex containing pendent relays becomes O2 dependent, implying a shift to Co(III)–superoxide protonation as the rate-determining step. Interestingly, the inclusion of the relay results in primarily H2O2 production in MeCN, despite minimal difference between the standard reduction potentials of the three complexes tested. EPR spectroscopic studies indicate the formation of Co(III)–superoxide species in the presence of exogenous base, with greater O2 reactivity observed in the presence of the pendent −OMe groups.

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Section: Discussionsupporting

confidence: 57%

Pendent Relay Enhances H₂O₂Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N₂O₂Complexes

et al. 2021

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“…While TDDFT correctly predicts the dramatic blue-shift of the Co 3d x 2 -y 2 → 3d z 2 transition accompanying a change in Co 2+ coordination number from five to four, it largely underestimates the energy of this transition for both forms of base-off Co 2+ Cbl, and to a lesser extent also for [Co 2+ (Me 6 [14]dieneN 4 )(H 2 O)] (Table 3). 69 In addition, TDDFT severely overestimates the Co 3d yz → 3d z 2 and Co 3d xz → 3d z 2 transition energies for [Co 2+ (Me 6 [14]dieneN 4 )(H 2 O)], suggesting that the energies of these transitions are also overestimated for base-off Co 2+ Cbl. Also, neither the Co 3d x 2 -y 2 → 3d z 2 transition, nor any other low-energy electronic transition in the TDDFT-computed Abs spectrum, is predicted to have the experimentally determined polarization of the 13600 cm −1 band of four-coordinate base-off Co 2+ Cbl (section 3.2).…”

Section: Resultsmentioning

confidence: 99%

Spectroscopic and Computational Characterization of the Base-off Forms of Cob(II)alamin

et al. 2009

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The one-electron reduced form of vitamin B12, cob(II)alamin (Co2+Cbl), is found in several essential human enzymes, including the cobalamin-dependent methionine synthase (MetH). In this work, experimentally validated electronic structure descriptions for two “base-off” Co2+Cbl species have been generated by using a combined spectroscopic and computational approach, so as to obtain definitive clues as to how these and related enzymes catalyze the thermodynamically challenging reduction of Co2+Cbl to cob(I)alamin (Co1+Cbl). Specifically, electron paramagnetic resonance (EPR), electronic absorption (Abs), and magnetic circular dichroism (MCD) spectroscopic techniques have been employed as complementary tools to characterize the two distinct forms of base-off Co2+Cbl that can be trapped in the H759G variant of MetH, one possessing a five-coordinate and the other a four-coordinate, square-planar Co2+ center. Accurate spin Hamiltonian parameters for these low-spin Co2+ centers have been determined by collecting EPR data using both X- and Q-Band microwave frequencies, while Abs and MCD spectroscopic techniques have been employed to probe the corrin-centered π → π* and Co-based d → d excitations, respectively. By using these spectroscopic data for evaluating electronic structure calculations, it was found that density functional theory provides a reasonable electronic structure description for the five-coordinate form of base-off Co2+Cbl. However, it was necessary to resort to a multireference ab initio treatment to generate a more realistic description of the electronic structure of the four-coordinate form. Consistent with this finding, our computational data indicate that in the five-coordinate Co2+Cbl species, the unpaired spin density is primarily localized in the Co 3dz2-based molecular orbital, as expected, whereas in the four-coordinate form, extensive Co 3d orbital mixing, configuration-interaction, and spin-orbit coupling cause the unpaired electron to delocalize over several Co 3d orbitals. These results provide important clues with regards to the mechanism of enzymatic Co2+Cbl → Co1+Cbl reduction.

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“…Materials. N - rac -[CoHMD(H 2 O)](ClO 4 ) 2 was prepared as previously described , and characterized by UV−vis and IR spectroscopies. Analyses for cobalt and anions of these complexes were satisfactory.…”

Section: Methodsmentioning

confidence: 99%

Effect of Pressure on the Reversible Binding of Acetonitrile to the “Co(I)−CO₂” Adduct To Form Cobalt(III) Carboxylate

Fujita

Eldik

1998

Inorg. Chem.

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The square-planar, low-spin d8 cobalt(I) macrocycles react with CO2 to yield five-coordinate CO2 adducts containing a bent OCO moiety bonded to cobalt through carbon. As the temperature is lowered, a solvent molecule binds to cobalt, and Co(III) carboxylates form. The pressure dependence of the equilibrium constant between five-coordinate [CoHMD(CO2)]+ and six-coordinate [CoHMD(CO2 2-)(CH3CN)]+ (HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) shows that increasing pressure shifts the equilibrium to the six-coordinate species with an overall reaction volume of ΔV° = −17.7 ± 1.0 mL mol-1 at 15 °C in CH3CN.

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Five-Coordinate Cobalt(ii) Complexes of Macrocyclic Ligands — A New Reversible Oxygen Carrying System

Cited by 28 publications

References 46 publications

Pendent Relay Enhances H₂O₂Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N₂O₂Complexes

Pendent Relay Enhances H₂O₂Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N₂O₂Complexes

Spectroscopic and Computational Characterization of the Base-off Forms of Cob(II)alamin

Effect of Pressure on the Reversible Binding of Acetonitrile to the “Co(I)−CO₂” Adduct To Form Cobalt(III) Carboxylate

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Five-Coordinate Cobalt(ii) Complexes of Macrocyclic Ligands — A New Reversible Oxygen Carrying System

Cited by 28 publications

References 46 publications

Pendent Relay Enhances H2O2Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N2O2Complexes

Pendent Relay Enhances H2O2Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N2O2Complexes

Spectroscopic and Computational Characterization of the Base-off Forms of Cob(II)alamin

Effect of Pressure on the Reversible Binding of Acetonitrile to the “Co(I)−CO2” Adduct To Form Cobalt(III) Carboxylate

Contact Info

Product

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Pendent Relay Enhances H₂O₂Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N₂O₂Complexes

Pendent Relay Enhances H₂O₂Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co–N₂O₂Complexes

Effect of Pressure on the Reversible Binding of Acetonitrile to the “Co(I)−CO₂” Adduct To Form Cobalt(III) Carboxylate