Reactivity toward Dioxygen of Dicobalt Face-to-Face Diporphyrins in Aprotic Media. Experimental and Theoretical Aspects. Possible Mechanistic Implication in the Reduction of Dioxygen

Abstract

The reactivity toward dioxygen of two series of dicobalt cofacial diporphyrins in solution in an aprotic solvent is described. Some of these compounds are efficient electrocatalysts for the four-electron reduction of dioxygen when adsorbed on a graphite electrode immersed in aqueous acid. Their electrochemical and spectroscopic (UV−vis, EPR) behavior in solution shows that, contrary to what is observed with cobalt monomers, the neutral [PCoII CoIIP] (1) (P stands for a porphyrin ring) form does not react with dioxygen. Uniquely the one- and two-electron-oxidized forms of the dimer, [PCoII·CoIIP]+ (1 + ) and [PCoII---CoIIP]2+ (1 2+ ), respectively, reversibly bind dioxygen, giving two complexes, 2 and 3, at room temperature and in the absence of a good axial ligand. The stability constants of the two O2 complexes have been measured spectrophotometrically and/or electrochemically, and prove to be remarkably high. As a whole, the present O2 binding processes appear unprecedented as basically different in many respects from the process classically described in the case of cobalt monomers. Extended Hückel molecular orbital (EHMO) calculations, based on the crystal structure of the Co2FTF4 dimer in its uncomplexed form (Co−Co distance 3.42 Å), show that, in the absence of very important deformations of its structure, the only possible geometry for the O2 complex of the two-electron-oxidized derivative [PCo−O2−CoP]2+ (3) is the μ-η2:η2-peroxo structure. The calculated corresponding electronic diagram affords a rationale for most of the experimentally observed properties. Specifically, the O2 complex of the one-electron-oxidized form [PCo−O2 •−CoP]+ (2), the reduced form of complex 3, should be considered as a species in which the O2 moiety is further reduced, at least partially, as compared to its peroxo state in 3, i.e., consequently in an oxidation state intermediate between peroxo (−1) and oxo (−2). Preliminary results indicate that this species reacts with one proton, while the two-electron-oxidized O2 complex 3 is resistant to protonation. The possible implications of these specific properties of the dicobalt dimers in the four-electron reduction mechanism of O2 are discussed, and structural and mechanistic similarities with bioinorganic dinuclear sites appear significant.

Dioxygen Reduction at Dicobalt Complexes of a Schiff Base Calixpyrrole Ligand

“…However, quantitative calibration by double integration of the EPR signal of 2 + to a [Cu(acac) 2 ] standard reveals that this former complex is only a minor component of the mixture (< 10 %); presumably, 2 + is formed by initial oxidation of 1 to 1 + followed by dioxygen uptake. [5] No significant changes are observed in the 1 H NMR spectrum of 1 in CDCl 3 upon exposure to O 2 . However, the subsequent addition of [D 5 ]pyridine results in the observation of an NMR spectrum of the diamagnetic peroxido complex 2-py 2 .…”

“…[4,10,11] Plausible mechanistic pathways for both two-and four-electron reductions of dioxygen have also been elucidated. [1,2,5] [1,5] Significantly, neither of these modes has been corroborated by solidstate structural data for any dioxygen-containing cobalt diporphyrin complexes or porphyrinic analogues.…”

Proton Pump for O2 Reduction Catalyzed by 5,10,15,20‐Tetraphenylporphyrinatocobalt(II)

“…[13][14][15] In most cases, monomeric cobalt porphyrins catalyze the electroreduction of O 2 to hydrogen peroxide (H 2 O 2 ), whereas dimeric cofacial cobalt porphyrins demonstrate the catalysis of four-electron reduction of O 2 to water. [3,7,8,10,12,15,16,[24][25][26][27] Employing ferrocene derivatives as electron donors, O 2 reduction catalyzed by various cobalt porphyrins has been investigated by Fukuzumi et al in organic media in the presence of HClO 4 . [13][14][15] In the reaction scheme, the steps of electron coordination to form a superoxide adduct and its reduction by ferrocene derivatives to produce H 2 O 2 /H 2 O and Co III are fast, and that of reducing Co III by ferrocene derivatives is slow and rate limiting.…”

Oxygen reduction at negatively charged iron porphyrins heat-treated and bridged by alkaline-earth metal ions

“…A new oxygen reduction mechanism has been proposed in the face-to-face cobalt porphyrin [8], in which the formation of a !-cation radical is a key factor in the coordination of oxygen to the metal site. Recently, a study reported that oxygen coordinates Fe(III) rather than Fe(II) to form an oxo dimer of Fe(IV) [9], although this study was conducted with a non-heme complex.…”