Development of Bioinspired Mn₄O₄−Cubane Water Oxidation Catalysts: Lessons from Photosynthesis

Abstract

Hydrogen is the most promising fuel of the future owing to its carbon-free, high-energy content and potential to be efficiently converted into either electrical or thermal energy. The greatest technical barrier to accessing this renewable resource remains the inability to create inexpensive catalysts for the solar-driven oxidation of water. To date, the most efficient system that uses solar energy to oxidize water is the photosystem II water-oxidizing complex (PSII-WOC), which is found within naturally occurring photosynthetic organisms. The catalytic core of this enzyme is a CaMn(4)O(x) cluster, which is present in all known species of oxygenic phototrophs and has been conserved since the emergence of this type of photosynthesis about 2.5 billion years ago. The key features that facilitate the catalytic success of the PSII-WOC offer important lessons for the design of abiological water oxidation catalysts. In this Account, we examine the chemical principles that may govern the PSII-WOC by comparing the water oxidation capabilities of structurally related synthetic manganese-oxo complexes, particularly those with a cubical Mn(4)O(4) core ("cubanes"). We summarize this research, from the self-assembly of the first such clusters, through the elucidation of their mechanism of photoinduced rearrangement to release O(2), to recent advances highlighting their capability to catalyze sustained light-activated electrolysis of water. The [Mn(4)O(4)](6+) cubane core assembles spontaneously in solution from monomeric precursors or from [Mn(2)O(2)](3+) core complexes in the presence of metrically appropriate bidentate chelates, for example, diarylphosphinates (ligands of Ph(2)PO(2)(-) and 4-phenyl-substituted derivatives), which bridge pairs of Mn atoms on each cube face (Mn(4)O(4)L(6)). The [Mn(4)O(4)](6+) core is enlarged relative to the [Mn(2)O(2)](3+) core, resulting in considerably weaker Mn-O bonds. Cubanes are ferocious oxidizing agents, stronger than analogous complexes with the [Mn(2)O(2)](3+) core, as demonstrated both by the range of substrates they dehydrogenate or oxygenate (unactivated alkanes, for example) and the 25% larger O-H bond enthalpy of the resulting mu(3)-OH bridge. The cubane core topology is structurally suited to releasing O(2), and it does so in high yield upon removal of one phosphinate by photoexcitation in the gas phase or thermal excitation in the solid state. This is quite unlike other Mn-oxo complexes and can be attributed to the elongated Mn-O bond lengths and low-energy transition state to the mu-peroxo precursor. The photoproduct, [Mn(4)O(2)L(5)](+), an intact nonplanar butterfly core complex, is poised for oxidative regeneration of the cubane core upon binding of two water molecules and coupling to an anode. Catalytic evolution of O(2) and protons from water exceeding 1000 turnovers can be readily achieved by suspending the oxidized cubane, [Mn(4)O(4)L(6)](+), into a proton-conducting membrane (Nafion) preadsorbed onto a conducting electrode and electroxidizing the photoreduced butterfly...

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“…Indeed, even the oxygen evolving complex of photosystem II experiences up to a ten-fold reduction in catalytic activity in vivo as opposed to in vitro. 340,341 In addition, Bard et al 350 have noted that the heterogeneous electrochemical rate constants for reactants bound in a surface layer are generally 2-3 orders of magnitude smaller than those for the same reactants in the solution phase. In light of this, a direct comparison between surface based and solution based catalysts seems unreasonable.…”

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“…[18,19] This dichotomy between the two crystal structures made it possible to test the bioinspired hypothesis. [20] The higher calcination temperature and longer reaction times used to form the layered polymorph gives rise to increasing crystallite size. [16] To reduce this difference in particle sizes, the authors adapted a sol-gel method [21] that forms nano-particles (50-100 nm) and limited particle size differences to only two-fold.…”

“…[38] While this is of interest in relation to the photosensitisation of the manganese cubanes that ACES researchers and their collaborators have developed, [20] we have rather focussed our attention on the development of porphyrin-based catalytic systems for water oxidation. In 1994, Naruta et al reported for the first time oxygen evolution by a four electron oxidation of water using a rigid ortho linked manganese porphyrin dimer.…”

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