RESUMO

The dependence of rate, entropy of activation, and ((1)H/(2)H) kinetic isotope effect for H-atom transfer from a series of p-substituted phenols to cubane Mn4O4L6 (L = O2PPh2) (1) reveals the activation energy to form the transition state is proportional to the phenolic O-H bond dissociation energy. New implications for water oxidation and charge recombination in photosystem II are described.

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Herein we describe the molecular Co(4)O(4) cubane complex Co(4)O(4)(OAc)(4)(py)(4) (1), which catalyzes efficient water oxidizing activity when powered by a standard photochemical oxidation source or electrochemical oxidation. The pH dependence of catalysis, the turnover frequency, and in situ monitoring of catalytic species have revealed the intrinsic capabilities of this core type. The catalytic activity of complex 1 and analogous Mn(4)O(4) cubane complexes is attributed to the cubical core topology, which is analogous to that of nature's water oxidation catalyst, a cubical CaMn(4)O(5) cluster.

RESUMO

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 complexes by the application of an external bias. Catalytic water oxidation can be achieved using sunlight as the only source of energy by replacing the external electrical bias with redox coupling to a photoanode incorporating a Ru(bipyridyl) dye.

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RESUMO

High valence states in manganese clusters are a key feature of the function of one of the most important catalysts found in nature, the water-oxidizing complex of photosystem II. We describe a detailed electrochemical investigation of two bio-inspired manganese-oxo complexes, [Mn(4)O(4)L(6)] (L = diphenylphosphinate (1) and bis(p-methoxyphenyl)phosphinate (2)), in solution, attached to an electrode surface and suspended within a Nafion film. These complexes contain a cubic [Mn(4)O(4)](6+) core stabilized by phosphinate ligands. They have previously been shown to be active and durable photocatalysts for the oxidation of water to dioxygen. A comparison of catalytic photocurrent generated by films deposited by two methods of electrode immobilization reveals that doping of the catalyst in Nafion results in higher photocurrent than was observed for a solid layer of cubane on an electrode surface. In dichloromethane solution, and under conditions of cyclic voltammetry, the one-electron oxidation processes 1/1(+) and 2/2(+) were found to be reversible and quasi-reversible, respectively. Some decomposition of 1(+) and 2(+) was detected on the longer timescale of bulk electrolysis. Both compounds also undergo a two-electron, chemically irreversible reduction in dichloromethane, with a mechanism that is dependent on scan rate and influenced by the presence of a proton donor. When immersed in aqueous electrolyte, the reduction process exhibits a limited level of chemical reversibility. These data provide insights into the catalytic operation of these molecules during photo-assisted electrolysis of water and highlight the importance of the strongly electron-donating ligand environment about the manganese ions in the ability of the cubanes to photocatalyze water oxidation at low overpotentials.

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RESUMO

The manganese-oxo "cubane" core complex Mn(4)O(4)L(1)(6) (1, L(1) = Ph(2)PO(2-)), a partial model of the photosynthetic water oxidation site, was shown previously to undergo photodissociation in the gas phase by releasing one phosphinate anion, an O(2) molecule, and the intact butterfly core cation (Mn(4)O(2)L(1)(5+)). Herein, we investigate the photochemistry and electronic structure of a series of manganese-oxo cubane complexes: [Mn(4)O(4)L(2)(6)] (2), 1(+)(ClO(4-)), 2(+)(ClO(4-)), and Mn(4)O(3)(OH)L(1)(6) (1H). We report the atomic structure of [Mn(4)O(4)L(2)(6)](ClO(4)), 2(+)(ClO(4-)) [L(2) = (4-MeOPh)(2)PO(2-)]. UV photoexcitation of a charge-transfer band dissociates one phosphinate, two core oxygen atoms, and the Mn(4)O(2)L(5)(+) butterfly as the dominant (or exclusive) photoreaction of all cubane derivatives in the gas phase, with relative yields: 1H >> 2 > 1 > 2(+) > 1(+). The photodissociation yield increases upon (1) reducing the core oxidation state by hydrogenation of a corner oxo (1H), (2) increasing the electron donation from the phosphinate ligand (L(2)), and (3) reducing the net charge from +1 to 0. The experimental Mn-O bond lengths and Mn-O bond strengths and the calculated ligand binding energy explain these trends in terms of weaker binding of phosphinate L(2) versus L(1) by 14.7 kcal/mol and stronger Mn-(mu(3)-O)(core) bonds in the oxidized complexes 2(+) and 1(+) versus 2 and 1. The calculated electronic structure accounts for these trends in terms of the binding energy and antibonding Mn-O(core) and Mn-O'(ligand) character of the degenerate highest occupied molecular orbital (HOMO), including (1) energetic destabilization of the HOMO of 2 relative to 1 by 0.75 eV and (2) depopulation of the antibonding HOMO and increased ionic binding in 1(+) and 2(+) versus 1 and 2.

Assuntos

RESUMO

A new member of the Mn-oxo cubane core complex family [Mn2III,2IV4O4L6] (1), where L = (p-MeOPh)2PO2-, has been synthesized and characterized. Compound 1 possesses structurally inequivalent MnIII and MnIV with clear valence electron localization in the crystal phase, quite unlike the structurally equivalent sites, tetrahedral core symmetry, and delocalized valence of its analogue where L = Ph2PO2-. Compound 1 exhibits appreciable positive shifts (0.1-0.3 V) of both the oxidation and reduction electrochemical potentials, attributable to the remote electron donating p-MeO groups. Reduction of 1 by methanol yields a novel core complex [MnIII4O2(OMe)2(MeOH)[(p-MeOPh)2PO2]6] (2). Each MnIII of 2 is tetragonally distorted due to the Jahn-Teller effect with elongated Mn-O bonds that are directed at the two micro3-MeO bridges and neither of the two micro3-oxos. These electronically driven distortions provide a structural rationale for the greater basicity of the former sites and explain why 2 of the 4 corner oxos are preferentially reduced to water molecules.

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The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn(4)O(4)(O(2)PPh(2))(6), 1, and [Mn(4)O(4)(O(2)PPh(2))(6)](+), 1(+), with phenothiazine (pzH). Both pcet reactions form 1H, by H transfer to 1 and by hydride transfer to 1(+). Surprisingly, the rate constants differ by only 25% despite large differences in the formal charges and driving force. The driving force is proportional to the difference in the bond-dissociation energies (BDE >94 kcalmol for homolytic, 1H --> H + 1, vs. approximately 127 kcalmol for heterolytic, 1H --> H(-) + 1(+), dissociation of the OH bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (deltaH = -1.2 kcalmol and deltaS= -32 calmol.K; 25-6.7 degrees C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no HD kinetic isotope effect (k(H)k(D) = 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H + pzH --> 1H(2) + pz(*)). These lines of evidence indicate that formation of a reactive precursor complex before atom transfer is rate-limiting (conformational gating), and that little or no NH bond cleavage occurs in the transition state. H-atom transfer from pzH to alkyl, alkoxyl, and peroxyl radicals reveals that BDEs are not a good predictor of the rates of this reaction. Hydride transfer to 1(+) provides a concrete example of two-electron pcet that is hypothesized for the OH bond cleavage step during catalysis of photosynthetic water oxidation.

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Cores for thought! A new [Mn4 O4 ]6+ "cubane" core complex (L6 Mn4 O4 ) with six facially bridging phosphinate chelate ligands (L- =(MePh)2 PO2- ) was synthesized. Photo-excitation releases molecular O2 by intramolecular coupling of two core oxygen atoms and selective rearrangement to a [Mn4 O2 ]6+ "butterfly" core ([L5 Mn4 O2 ]+ ; see scheme). Thus the Mn4 O4 cubane core exhibits unique reactivity in O2 evolution which may account for its presence in the photosynthetic enzyme.