In the last ten years, a number of advances have been made in the study of the oxygen-evolving complex (OEC) of photosystem II (PSII). Along with this new understanding of the natural system has come rapid advance in chemical models of this system. The advance of PSII model chemistry is seen most strikingly in the area of functional models where the few known systems available when this topic was last reviewed has grown into two families of model systems. In concert with this work, numerous mechanistic proposals for photosynthetic water oxidation have been proposed. Here, we review the recent efforts in functional model chemistry of the oxygen-evolving complex of photosystem II.

A novel class of derivatized acetylacetonate (acac) linkers for robust functionalization of TiO2 nanoparticles (NPs) under aqueous and oxidative conditions is reported. The resulting surface adsorbate anchors are particularly relevant to engineering photocatalytic and photovoltaic devices since they can be applied to attach a broad range of photosensitizers and photocatalytic complexes and are not affected by humidity. Acac is easily modified by CuI-mediated coupling reactions to provide a variety of scaffolds, including substituted terpy complexes (terpy = 2,2':6,2''-terpyridine), assembled with ligands coordinated to transition-metal ions. Since Mn-terpy complexes are known to be effective catalysts for oxidation chemistry, functionalization with Mn(II) is examined. This permits visible-light sensitization of TiO2 nanoparticles due to interfacial electron transfer, as evidenced by UV-vis spectroscopy of colloidal thin films and aqueous suspensions. The underlying ultrafast interfacial electron injection is complete on a subpicosecond time scale, as monitored by optical pump-terahertz probe transient measurements and computer simulations. Time-resolved measurements of the Mn(II) EPR signal at 6 K show that interfacial electron injection induces Mn(II) --> Mn(III) photooxidation, with a half-time for regeneration of the Mn(II) complex of ca. 23 s.

This paper reports visible-light sensitization of TiO 2 nanoparticles by surface modification with Mn(II)-terpyridine complexes, as evidenced by UV-vis spectroscopy of colloidal thin films and aqueous suspensions. Photoexcitation of the [Mn II (H 2 O) 3 (catechol-terpy)] 2+ /TiO 2 (terpy ) 2,2′:6,2′′-terpyridine) complex, attached to the TiO 2 surface, leads to interfacial electron transfer within 300 fs as indicated by ultrafast optical pumpterahertz probe transient measurements and computational simulations. Photoinduced interfacial electron transfer is accompanied by Mn(II) f Mn(III) photooxidation. The half-time for regeneration of the Mn(II) complex is ca. 23 s (at 6 K), as monitored by time-resolved measurements of the Mn(II) EPR signal.

HAT trick: [MnIV(OH)2(H,MePytacn)]2+ (A) and [MnIV(O)(OH)(H,MePytacn)]+ (B) differ in their reactions with CH bonds: compound A engages in typical single‐step hydrogen atom transfer (HAT) reactions, whereas B first forms a substrate–B encounter complex (C; see scheme). This equilibrium alters the relative CH reactivity from that expected from CH bond dissociation energies.

Photosynthetic water oxidation occurs naturally at a tetranuclear manganese center in the photosystem II protein complex. Synthetically mimicking this tetramanganese center, known as the oxygen-evolving complex (OEC), has been an ongoing challenge of bioinorganic chemistry. Most past efforts have centered on water-oxidation catalysis using chemical oxidants. However, solar energy applications have drawn attention to electrochemical methods. In this paper, we examine the electrochemical behavior of the biomimetic water-oxidation catalyst [(H 2 O) (terpy)Mn(μ-O) 2 Mn(terpy)(H 2 O)](NO 3 ) 3 [terpy = 2,2′:6′,2″-terpyridine] (1) in water under a variety of pH and buffered conditions and in the presence of acetate that binds to 1 in place of one of the terminal water ligands. These experiments will show that 1 not only exhibits proton-coupled electron-transfer reactivity analogous to the OEC, but also may be capable of electrochemical oxidation of water to oxygen. IntroductionIn the past ten years, a number of manganese-based water oxidation catalysts have been identified. 1-4 All of these new manganese catalysts are dimeric and all use two-electron oxygen-donor oxidants for catalytic turnover. [5][6][7] The necessity of such oxidants as well as the need for multiple manganese atoms have been issues of interest due to the tendency of manganese complexes to disproportionate peroxides to give O 2 via a catalase pathway rather than by water oxidation, 8 and due to the complexity inherent in multi-metal systems. To firmly establish the ability of manganese to oxidize water to oxygen, catalysis must be carried out using either a nonoxygen-donor oxidant or electrochemically, and a greater understanding of the necessary design elements must be reached.The oxidation of water to oxygen using nonoxygen-donor oxidants such as Ce(IV) has been well established in the ruthenium literature. [9][10][11] Due to the lack of any oxygen atom in the oxidant, O 2 must be formed from water. Unfortunately, in the history of manganese-based water oxidation, Ce(IV), and other similar oxidants, have generally failed to give catalytic O 2 . The most recent attempts to use Ce(IV) as an oxidant have at best yielded no more than a single turnover of oxygen. 6,12,13 It appears that oxidants such as Ce(IV) are often too acidic to be compatible with the basic oxo bridges of manganese dimers. 12 Figure 1) in aqueous solution. Previous electrochemical studies of water-oxidizing manganese complexes have been hindered by both the scarcity of such complexes and the ubiquitous background oxidation of water to oxygen at the electrode. Past efforts to study 1 by electrochemical methods have characterized the main redox features of 1. 14 By studying the previouslycharacterized redox behavior of 1 as a function of pH and with different ligands bound in place of the terminal water(s), we aim to gain a better understanding of the factors that influence water oxidation using manganese. These electrochemical studies also provide insight into the differing...

Transition metal oxides containing cubic B4O4 subcores are noted for their catalytic activity in water oxidation (OER). We synthesized a series of ternary spinel oxides, AB2O4, derived from LiMn2O4 by either replacement at the tetrahedral A site or Co substitution at the octahedral B site and measured their electrocatalytic OER activity. Atomic emission and powder X-ray diffraction confirmed spinel structure type and purity. Weak activation of the OER occurs upon A-site substitution: Zn2+ > Mg2+ > A-vacancy > Li+ = 0. Zn and Mg substitution is accompanied by (1) B-site conversion of Mn­(IV) to Mn­(III), resulting in expansion and higher symmetry of the [Mn4O4]4+ core relative to LiMn2O4 (inducing greater flexibility of the core and lower reorganization barrier to distortions), and (2) the electrochemical oxidation potential for Mn­(III)/IV) increases by 0.15–0.2 V, producing a stronger driving force for water oxidation. Progressive replacement of Mn­(III/IV) by Co­(III) at the B site (LiMn2–x Co x O4, 0 ≤ x ≤ 1.5) both symmetrizes the [Mn4–x Co x O4] core and increases the oxidation potential for Co­(III/IV), resulting in the highest OER activity within the spinel structure type. These observations point to two predictors of OER catalysis: (1) Among AMn2O4 spinels, those starting with Mn­(III) in the resting lattice (prior to oxidation) result in longer, weaker Mn–O bonds for this eg 1 antibonding electronic configuration, yielding greater core flexibility and a higher oxidation potential to Mn­(IV), and (2) a linear free energy relationship exists between the electrocatalytic rate and the binding affinity of the substrate oxygen (*OH and *OOH) to the B site.

A series of diiron(II) complexes of the dinucleating ligand HPTP (N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diaminopropane) with one or two supporting carboxylate bridges has been synthesized and characterized. The crystal structure of one member of each subset has been obtained to reveal for subset A a (micro-alkoxo)(micro-carboxylato)diiron(II) center with one five- and one six-coordinate metal ion and for subset B a coordinatively saturated (micro-alkoxo)bis(micro-carboxylato)diiron(II) center. These complexes react with O(2) in second-order processes to form adducts characterized as (micro-1,2-peroxo)diiron(III) complexes. Stopped-flow kinetic studies show that the oxygenation step is sensitive to the availability of an O(2) binding site on the diiron(II) center, as subset B reacts more slowly by an order of magnitude. The lifetimes of the O(2) adducts are also distinct and can be modulated by the addition of oxygen donor ligands. The O(2) adduct of a monocarboxylate complex decays by a fast second-order process that must be monitored by stopped-flow methods, but becomes stabilized in CH(2)Cl(2)/DMSO (9:1 v/v) and decomposes by a much slower first-order process. The O(2) adduct of a dicarboxylate complex is even more stable in pure CH(2)Cl(2) and decays by a first-order process. These differences in adduct stability are reflected in the observation that only the O(2) adducts of monocarboxylate complexes can oxidize substrates, and only those substrates that can bind to the diiron center. Thus, the much greater stability of the O(2) adducts of dicarboxylate complexes can be rationalized by the formation of a (micro-alkoxo)(micro-1,2-peroxo)diiron(III) complex wherein the carboxylate bridges in the diiron(II) complex become terminal ligands in the O(2) adduct, occupy the remaining coordination sites on the diiron center, and prevent binding of potential substrates. Implications for the oxidation mechanisms of nonheme diiron enzymes are discussed.

The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(2)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.