RESUMO
The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a Sn6O6 stannoxane core are reported. Complex 1 acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e-/4H+ dioxygen-reduction (to water) to a 2e-/2H+ process (to hydrogen peroxide) only by increasing the temperature from -50 to 25 °C. A variety of spectroscopic methods (119Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O2 can be explained by the constraint provided by the stannoxane core that makes the O2-binding to 1 an entropically unfavorable process. This makes the end-on µ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of H2O2. In contrast, at -50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O-O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O2-to-2H2O catalytic conversion in an overall 4e-/4H+ step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.
RESUMO
The Siamese-twin porphyrin (2H4) is a unique pyrazole-expanded porphyrin providing two adjacent cavities each offering an {N4} binding motif. It was previously found to form stable dicopper(II) or dinickel(II) complexes where both metal ions are nested in a porphyrin-like environment. This work addresses the rich redox chemistry of the dicopper complex 2Cu2 that originates from the redox synergy of two proximate metal ions in combination with the potentially non-innocent expanded porphyrin ligand. Complementing previous X-ray crystallographic and SQUID data for solid material, the electronic structure of parent 2Cu2 in solution was now investigated by MCD and EPR spectroscopy. This allowed the assignment of UV-vis absorptions and confirmed the drastic twist of the molecule with ferromagnetically coupled copper(II) ions. 2Cu2 was found to exhibit multiple redox events in the potential range from -2.4 to +1.7 V versus Fc/Fc(+), and singly oxidized [2Cu2](+) as well as doubly oxidized [2Cu2](2+) were characterized in detail by various analytical and spectroscopic methods. [2Cu2](+) was found by EPR spectroscopy and DFT calculations to have an S = 1/2 ground state, while [2Cu2](2+) is diamagnetic. Single crystal X-ray crystallography of [2Cu2(acetone)2](BF4)2 revealed that the 2Cu2 core is structurally invariant upon two-fold oxidation, while XAS measurements at the Cu K-edge for 2Cu2 and [2Cu2(acetone)2](BF4)2 showed that the copper ions remain in the +2 oxidation state throughout. The combined experimental and computational evidence identified the Siamese-twin porphyrin as a multi-electron redox-active ligand with hidden non-innocence. Each ligand subunit upon oxidation forms a ligand-centered radical, though the spin vanishes because of covalency and strong antiferromagnetic coupling between the ligand radical and the proximate metal ion. Complexes of the Siamese-twin porphyrin may thus serve as a valuable bioinspired platform that combines both metal-ligand and two-metal-ion cooperativities for use in multi-electron processes.