A model compound sheds light on the puzzling role of calcium in the metal cluster that oxidizes water during photosynthesis.

Redox-inactive metals are found in biological and heterogeneous water oxidation catalysts, but their roles in catalysis are currently not well understood. A series of high oxidation state tetranuclear-dioxido clusters comprised of three manganese centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Mn3M(μ4-O)(μ2-O) core that remains intact upon changing M or the manganese oxidation state. Electrochemical studies reveal that the reduction potentials span a window of 700 mV, dependent upon the Lewis acidity of the second metal. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between reduction potential and acidity with a slope of ca. 100 mV per pKa unit. The Sr2+ and Ca2+ compounds show similar potentials, an observation that correlates with the behavior of the OEC, which is active only in the presence of one of these two metals.

Understanding the effect of redox-inactive metals on the properties of biological and heterogeneous water oxidation catalysts is important both fundamentally and for improvement of future catalyst designs.

Golden opportunity: A terminal mononuclear gold(I) hydride complex has been stabilized by an N‐heterocyclic carbene ligand (see picture: Au orange, N blue, C gray). The complex is stable to a wider range of conditions than other transition metal hydride compounds and reacts to form a number of different gold(I) species.

Mesitaldehyde reacts cleanly with (IPr)CuB(pin) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); pin = 2,3-dimethyl-2,3-butanediolate] to afford the product complex 1, the first well-defined product of carbonyl group insertion into a metal-boron bond. Analysis of 1 by NMR spectroscopy and single-crystal X-ray diffraction indicates the formation of a copper-carbon and a boron-oxygen bond. A copper(I) precatalyst supported by the less sterically demanding ligand ICy (1,3-dicyclohexylimidazol-2-ylidene) achieves the efficient 1,2-diboration of aryl-, heteroaryl-, and alkyl-substituted aldehydes at room temperature.

The oxygen-evolving complex (OEC) of photosystem II contains a Mn 4 CaO n catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn 3 MO n complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ 3 -oxido moieties. We found that Mn IV 3 CaO 4 and Mn IV 3 ScO 4 were unreactive toward trimethylphosphine (PMe 3 ). In contrast, our Mn III 2 Mn IV 2 O 4 cubane reacts with this phosphine within minutes to generate a novel Mn III 4 O 3 partial cubane plus Me 3 PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn III 2 Mn IV 2 O 4 and Mn IV 3 CaO 4 . We found that the most favorable reaction path leads to partial detachment of the CH 3 COO − ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn III 2 Mn IV 2 O 4 . These results indicate that even with a strong oxygen atom acceptor, such as PMe 3 , the oxygen atom transfer chemistry from Mn 3 MO 4 cubanes is controlled by ligand lability, with the Mn IV 3 CaO 4 OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, Mn III 4 O 3 , was observed experimentally upon treatment with water, base, and oxidizing equivalents. 18 O-labeling experiments provided mechanistic insight into the position of incorporation in the partial cubane structure, consistent with mechanisms involving migration of oxide moieties within the cluster but not consistent with selective incorporation at the site available in the starting species. These results support recent proposals for the mechanism of the OEC, involving oxido migration between distinct positions within the cluster.

Understanding the structural and compositional origins of midgap states in semiconductor nanocrystals is a longstanding challenge in nanoscience. Here, we report a broad variety of reagents useful for photochemical reduction of colloidal CdSe quantum dots, and we establish that these reactions proceed via a dark surface prereduction step prior to photoexcitation. Mechanistic studies relying on the specific properties of various reductants lead to the proposal that this surface prereduction occurs at oxidized surface selenium sites. These results demonstrate the use of small-molecule inorganic chemistries to control the physical properties of colloidal QDs and provide microscopic insights into the identities and reactivities of their localized surface species.

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O2 remains a significant challenge in this context. Mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex (OEC) of photosystem II (PSII). These structural motifs are also related to heterogeneous mixed metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low oxidation state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure-reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pKa of the redox-inactive metal-aqua complex as a measure of Lewis acidity, structurally analogous clusters display a linear dependence between reduction potential and acidity; each pKa unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed.