Characterization of a Dinuclear Mn^VO Complex and Its Efficient Evolution of O₂ in the Presence of Water

Abstract

The oxidation of water in the process of dioxygen evolution is catalyzed by an oxygen-evolving complex (OEC) in photosynthesis, which is one of the most important and fundamental chemical processes in nature. [1][2][3] The active site in a photosynthetic OEC protein contains a tetramanganese cluster, [4] which adopts a Y-shaped geometry as revealed by recent X-ray structure analysis of photosystem II.[5] Though the mechanism of dioxygen evolution has not been determined, the stage of oxygen evolution would involve either a high valent terminal oxo manganese species [3,4,6] or the coupling of bridging oxo units. [2,3] Manganese complexes have been extensively studied as artificial OEC models in structural and functional investigations to understand the mechanism of oxygen evolution from water in photosynthetic OEC.[3] However, only a few Mn complexes that can catalyze homogeneous water oxidation have been reported. [7,8] We have previously reported dimanganese complexes of dimeric tetraarylporphyrins linked by 1,2-phenylene bridge (Scheme 1).[8] The anodic oxidation of an aqueous solution of acetonitrile (5 % v/v H 2 O in CH 3 CN) with the dimanganese tetraarylporphyrin dimer evolved oxygen in the potential range > 1.20 V versus Ag/Ag + . The catalyst can also oxidize olefins such as cyclooctene to form epoxide with stoichiometric amounts of m-chloroperbenzoic acid (mCPBA).[9] We proposed that the oxidation of a dimanganese(iii) tetraarylporphyrin dimer could give the corresponding high valent Mn=O complex, which is the active species in these oxidation. However, the mechanisms of oxygen evolution and epoxidation, especially the formation of a high-valent Mn=O intermediate have not been fully confirmed. Herein, we report on the oxidation of the dimanganese porphyrin dimer by employing mCPBA as an oxidant, and the characterization of the resulting Mn (Figure 1). When one molar equivalent of 1,1-diphenyl-2-picrylhydrazine (DPPHa one-electron reductant for each Mn ion) was added to 2, a Mn IV 2 species 3 was rapidly formed, which has a Soret band centered at 415 nm. Complex 3 was also prepared when one molar equivalent of mCPBA for each Mn ion was added to 1 in CH 2 Cl 2 /CH 3 CN (1:4, v/v) solution in the presence of five equivalents of Bu 4 NOH, confirmed by UV/Vis spectrum. When the solution of 2 was left stand at room temperature, the Soret band at 423 nm gradually disappeared and a 468 nm band accordingly appeared with isosbestic points indicating (1) and a reaction pathway for O 2 formation.

Functional models for the oxygen-evolving complex of photosystem II

“…Top: bis-porphyrin functional model for oxygen evolution (figure was reproduced from ref. [80], with permission of the copyright holders). Bottom: a proposed mechanism for biological oxygen evolution that closely resembles the proposed reactions of the bis-porphyrin complex (figure was reproduced from ref.…”

“…However, ligand lability of Mn complexes can lead to instability of the catalyst under catalytic conditions, although Nature has overcome this problem of ligand lability by encapsulating the Mn ions in the OEC within the proteins of PSII. While instability is an issue for manganese model complexes, a number of successes have been reported [40,[79][80][81][82][83]. In this section, the manganese-based synthetic water-oxidation catalysts will be examined and their proposed mechanisms compared to the mechanisms proposed for the OEC.…”

“…The bis-porphyrin model compound reported by Naruta et al [80,84] (Figure 4, top) uses the known stability of manganese porphyrin complexes to circumvent the ligand lability issues of Mn mentioned above. By coupling two stable manganese-oxo-forming units together, a bis Mn(V)=O intermediate can be isolated after addition of m-chloroperbenzoic acid.…”

“…The Naruta et al [80] complex is oxidized by the full four electrons to form a stable bis-oxo species which is then destabilized to initiate O-O bond formation, while the Hoganson and Babcock mechanism [44] proposes the formation of the O-O bond simultaneously with the removal of the fourth electron and proton. This difference is minor if you consider that the bis-oxo species isolated in the Naruta et al system [80] can be viewed as an intermediate species formed during the final oxidation step in the Hoganson and Babcock mechanism [44]. [40,85,86].…”

Molecular water-oxidation catalysts for photoelectrochemical cells

“…41 Gao et al 46 very recently reported that a [Mn III -corrole] + complex can be converted into a Mn V -oxo complex which undergoes reductive elimination of oxygen on attack by hydroxide ion. An earlier study by Shimazaki et al 47 found that a dimanganese(III)-porphyrin dimer is oxidised by mchloroperbenzoic acid (mCPBA) to a dinuclear Mn V =O complex, which releases oxygen almost stoichiometrically on addition of acid. However, catalytic turnover was not demonstrated in these studies.…”