Joseph L. Templeton

Mechanisms of Water Oxidation from the Blue Dimer to Photosystem II

The blue dimer, cis, cis-[(bpy)2(H2O)Ru(III)ORu(III)(H2O)(bpy)2](4+), is the first designed, well-defined molecule known to function as a catalyst for water oxidation. It meets the stoichiometric requirements for water oxidation, 2H2O --> -4e(-), -4H(+) O-O, by utilizing proton-coupled electron-transfer (PCET) reactions in which both electrons and protons are transferred. This avoids charge buildup, allowing for the accumulation of multiple oxidative equivalents at the Ru-O-Ru core. PCET and pathways involving coupled electron-proton transfer (EPT) are also used to avoid high-energy intermediates. Application of density functional theory calculations to molecular and electronic structure supports the proposal of strong electronic coupling across the micro-oxo bridge. The results of this analysis provide explanations for important details of the descriptive chemistry. Stepwise e(-)/H(+) loss leads to the higher oxidation states [(bpy)2(O)Ru(V)ORu(IV)(O)(bpy)2] (3+) (Ru(V)ORu(IV)) and [(bpy)2(O)Ru(V)ORu(V)(O)(bpy)2](4+) (Ru(V)ORu(V)). Both oxidize water, Ru(V)ORu(IV) stoichiometrically and Ru(V)ORu(V) catalytically. In strongly acidic solutions (HNO3, HClO4, and HSO3CF3) with excess Ce(IV), the catalytic mechanism involves O---O coupling following oxidation to Ru(V)ORu(V), which does not build up as a detectable intermediate. Direct evidence has been found for intervention of a peroxidic intermediate. Oxidation of water by Ru(V)ORu(IV) is far slower. It plays a role late in the catalytic cycle when Ce(IV) is depleted and is one origin of anated intermediates such as [(bpy)2(HO)Ru(IV)ORu(IV)(NO3)(bpy)2](4+), which are deleterious in tying up active components in the catalytic cycle. These intermediates slowly return to [(bpy)2(H2O)Ru(IV)ORu(III)(OH2)(bpy)2](5+) with anion release followed by water oxidation. The results of a recent analysis of water oxidation in the oxygen-evolving complex (OEC) of photosystem II reveal similarities in the mechanism with the blue dimer and significant differences. The OEC resides in the thylakoid membrane in the chloroplasts of green plants, and careful attention is paid in the structure to PCET, EPT, and long-range proton transfer by sequential local proton transfers. The active site for water oxidation is a CaMn 4 cluster, which includes an appended Mn site, Mn(4), where O---O coupling is thought to occur. Photochemical electron transfer results in oxidation of tyrosine Y Z to Y Z (.), which is approximately 7 A from Mn(4). It subsequently oxidizes the OEC through the stepwise stages of the Kok cycle. O---O coupling appears to occur through an initial peroxidic intermediate formed by redox nucleophilic attack of coordinated OH(-) in Ca-OH(-) on Mn (IV)=O.

Catalytic Water Oxidation by Single-Site Ruthenium Catalysts

A series of monomeric ruthenium polypyridyl complexes have been synthesized and characterized, and their performance as water oxidation catalysts has been evaluated. The diversity of ligand environments and how they influence rates and reaction thermodynamics create a platform for catalyst design with controllable reactivity based on ligand variations.

Molecular Chromophore–Catalyst Assemblies for Solar Fuel Applications

Applications Dennis L. Ashford,† Melissa K. Gish,† Aaron K. Vannucci,‡ M. Kyle Brennaman,† Joseph L. Templeton,† John M. Papanikolas,† and Thomas J. Meyer*,† †Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599, United States ‡Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

Four-electron alkyne ligands in molybdenum(II) and tungsten(II) complexes

Publisher Summary This chapter explores that the term “four-electron donor,” which is used to describe the alkyne ligand in circumstances where alkyne π ⊥ donation supplements classic metal–olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance is relied on the “four-electron donor” terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors are typical of broad classification schemes in chemistry—they are often conceptually helpful but seldom specifically correct. Criteria for recognizing four-electron alkyne donation encompass stoichiometry, structure, spectra, and reactivity. The chapter reviews that the chemistry that has been developed for molybdenum (II) and tungsten (II) alkyne monomers encompasses syntheses, structures, spectra, molecular orbital descriptions, and reactions. The Mo (II) and W (II) complexes addressed in the chapter are not unique in terms of alkyne π ⊥ donation. Related alkyne chemistry is appearing for d4 metals other than molybdenum and tungsten, as well as for d 2 complexes in general. The chapter also examines that chromium alkyne chemistry and reflects the importance of π ⊥ donation, but the stoichiometries differ from those of heavier Group VI monomers.

The Patterning of Sub-500 nm Inorganic Oxide Structures†

Elastomeric perfluoropolyether molds are applied to pattern arrays of sub-500 nm inorganic oxide features. This versatile soft-lithography technique can be used to pattern a wide range of materials; in this work inorganic oxides including TiO2 , SnO2 , ZnO, ITO, and BaTiO3 are patterned on a variety of substrates with different aspect ratios. An example of TiO2 posts is shown in the figure.

Mediator-assisted water oxidation by the ruthenium “blue dimer” cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+

Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuel-forming reactions, such as water splitting into hydrogen and oxygen (2 H2O + 4 hν → O2 + 2 H2) or water reduction of CO2 to methanol (2 H2O + CO2 + 6 hν → CH3OH + 3/2 O2), or hydrocarbons, which could provide clean, renewable energy. The “blue ruthenium dimer,” cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+, was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to ≈30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.

Water oxidation by an electropolymerized catalyst on derivatized mesoporous metal oxide electrodes.

A general electropolymerization/electro-oligomerization strategy is described for preparing spatially controlled, multicomponent films and surface assemblies having both light harvesting chromophores and water oxidation catalysts on metal oxide electrodes for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The chromophore/catalyst ratio is controlled by the number of reductive electrochemical cycles. Catalytic rate constants for water oxidation by the polymer films are similar to those for the phosphonated molecular catalyst on metal oxide electrodes, indicating that the physical properties of the catalysts are not significantly altered in the polymer films. Controlled potential electrolysis shows sustained water oxidation over multiple hours with no decrease in the catalytic current.

Low-overpotential water oxidation by a surface-bound ruthenium-chromophore- ruthenium-catalyst assembly

When anchored to nanoITO (indium tin oxide), the ruthenium chromophore-catalyst assembly shown acts as an electrocatalyst for water oxidation, with O2 evolution occurring at an overpotential of 230 mV in 0.1 M HClO4 . The potential response of the electrode points to 3 e(-) /2 H(+) oxidized [Rua (III) Rub (IV) O](5+) as the active form of the assembly.

Transition metal η2-vinyl complexes

Abstract Transition metal η 2 -vinyl complexes, L n M(η 2 -CRCR 2 ), adopt structures in which both alkenyl carbons are bound to the metal. The M(CRCR 2 ) unit is not planar in these 1-metallocyclopropene complexes. Instead, the CR 2 entity is twisted by ca. 90° away from planarity with the three-membered ring formed by M, C α , and C β . The ring carbons of η 2 -vinyl ligands are bound asymmetrically to the metal with one short metal–carbon bond and one long metal–carbon bond. These geometrical features reflect double bond character between C α and the metal at the expense of the carbon–carbon double bond in the classic organic vinyl moiety. Coordination of both carbons of the four-electron donating η 2 -vinyl ligand increases the electron count at the metal by two electrons relative to an η 1 -bound vinyl ligand. This review summarizes synthetic methods which yield η 2 -vinyl complexes, describes bonding and structural characteristics of η 2 -vinyl ligands, tabulates NMR spectroscopic signatures of η 2 -vinyl ligands, and outlines reactions of this growing class of compounds.

Direct patterning of CdSe quantum dots into sub-100 nm structures

Ordered, two-dimensional cadmium selenide (CdSe) arrays have been fabricated on indium-doped tin oxide (ITO) electrodes using the pattern replication in nonwetting templates (PRINT) process. CdSe quantum dots (QDs) with an average diameter of 2.7 nm and a pyridine surface ligand were used for patterning. The PRINT technique utilizes a perfluoropolyether (PFPE) elastomeric mold that is tolerant of most organic solvents, thus allowing solutions of CdSe QDs in 4-picoline to be used for patterning without significant deformation of the mold. Nanometer-scale diffraction gratings have been successfully replicated with CdSe QDs.