▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

Life on earth is almost entirely solar-powered. We can get some idea of the enormous quantity of energy received from the sun by noting that during daylight hours, the sun provides several thousand times more power to the surface of the U.S.A. than is produced by all of the nation’s electrical power stations. 1,2 Around 50% of the radiation that reaches the earth’s surface, roughly the visible region, is of a frequency useful to photosynthetic organisms. Oxygenic photosynthetic organisms convert this radiation into chemical energy, in the form of carbohydrate and dioxygen, at an optimal efficiency of something like 25%. 3 These products together sustain the rest of aerobic life, with carbohydrate acting as a source of high-energy electrons and dioxygen providing a lower-energy destination for these electrons. The overall equation of oxygenic photosynthesis is given in eq 1, where (CH2O) represents carbohydrate:

Water-splitting chemistry of photosystem II.

Artificial photosynthesis: molecular systems for catalytic water oxidation.

6.2. Photosystem II as a Source of Inspiration 2364 6.3. Ruthenium Complexes 2364 6.4. Manganese Complexes 2365 6.5. Concluding Remarks 2366 7. Reduction of Carbon Dioxide 2366 7.

Molecular Catalysis of Electrochemical Reactions. Mechanistic Aspects

Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO2 are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO2 supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.

https://science.sciencemag.org/content/sci/331/6014/195.full.pdf

Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles

The oxidation of a variety of substrates (thioethers, hydrocarbons, alkenes, benyzl alcohol and benzaldehyde) by t BuOOH catalyzed by Mn 4 O 4 (O 2 PPh 2 ) 6 ( 1 ) and Mn 4 O 4 (O 2 P( p -MePh) 2 ) 6 ( 2 ) is reported. These reactions illustrate the first examples of oxidative catalysis using a manganese-oxo complex with a Mn 4 O 4 cubane core. These uncharged complexes contain Mn ions in a mixed valence oxidation state, formally Mn 4 (2III, 2IV), and are bridged by bulky diphenylphosphinate chelates across each of the six faces of the cube. Using this system, methyl phenyl sulfide is selectively mono-oxygenated to methyl phenyl sulfoxide with high catalytic efficiency, and no evidence for further oxidation to the thermodynamically preferred sulfone. Toluene is oxidized to a mixture of benzyl alcohol, benzaldehyde, and benzoic acid with high catalytic efficiencies. Lower catalytic efficiencies are observed in the oxidation of styrene to a mixture of styrene oxide and benzaldehyde, of cyclohexene to a mixture of cyclohexene oxide, 2-cyclohexen-1-ol, and 2-cyclohexen-1-one, and of cyclohexane to a mixture of cyclohexanol and cyclohexanone. The observed product distribution from the oxidation of hydrocarbons has the characteristics of a free radical-based oxidation mechanism. However, the sulfoxidation and epoxidation activity of the 1 / t BuOOH system, as well as the observed steric preferences for less congested substrates, suggest that a metal-oxo centered oxidation mechanism is active in the reactions studied here. An intermediate species, characterized by a UV–VIS band centered at 610 nm is observed in all reaction mixtures, and forms upon reaction of 1 or 2 with t BuOOH . Preliminary evidence suggests this reactive intermediate may correspond to a Mn(V)O species. Kinetic studies suggest two pathways for oxidation: one involving an oxygen atom transfer (two-electron branch), and the other involving a hydrogen atom abstraction (one-electron branch).

Oxidative catalysis by Mn4O46+ cubane complexes

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.

Tuning the Photoinduced O2-Evolving Reactivity of Mn4O47+, Mn4O46+, and Mn4O3(OH)6+ Manganese−Oxo Cubane Complexes

Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.

Transition from hydrogen atom to hydride abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O-H bond dissociation energies and the formation of Mn4O3(OH)(O2PPh2)6.

The dependence of rate, entropy of activation, and (1H/2H) 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.

Thomas G. Carrell

Papers

Oxidative catalysis by Mn4O46+ cubane complexes

Tuning the Photoinduced O2-Evolving Reactivity of Mn4O47+, Mn4O46+, and Mn4O3(OH)6+ Manganese−Oxo Cubane Complexes

Transition from hydrogen atom to hydride abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O-H bond dissociation energies and the formation of Mn4O3(OH)(O2PPh2)6.

Entropy and enthalpy contributions to the kinetics of proton coupled electron transfer to the Mn4O4(O2PPh2)6 cubane