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

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.

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.

Synthetic model compounds have been targeted to benchmark and better understand the electronic structure, geometry, spectroscopy, and reactivity of the oxygen-evolving complex (OEC) of photosystem II, a low-symmetry Mn4CaOn cluster. Herein, low-symmetry MnIV3GdO4 and MnIV3CaO4 cubanes are synthesized in a rational, stepwise fashion through desymmetrization by ligand substitution, causing significant cubane distortions. As a result of increased electron richness and desymmetrization, a specific μ3-oxo moiety of the Mn3CaO4 unit becomes more basic allowing for selective protonation. Coordination of a fifth metal ion, Ag+, to the same site gives a Mn3CaAgO4 cluster that models the topology of the OEC by displaying both a cubane motif and a “dangler” transition metal. The present synthetic strategy provides a rational roadmap for accessing more accurate models of the biological catalyst.

Early-late intermetallic phases have garnered increased attention recently for their catalytic properties. To achieve the high surface areas needed for industrially relevant applications, these phases must be synthesized as nanoparticles in a scalable fashion. Herein, Pt 3 Y-targeted as a prototypical example of an early-late intermetallic-has been synthesized as nanoparticles approximately 5-20 nm in diameter in a solution process and characterized by XRD, TEM, EDS and XPS. The key development is the use of a molten borohydride (MEt 3 BH, M= Na, K) as both the reducing agent and reaction medium. Readily available halide precursors of each metal are used. Accordingly, no organic ligands/surfactants are necessary as the resulting halide salt byproduct prevents sintering, which further permits dispersion of the nanoscale intermetallic onto a support. The versatility of this approach was validated by synthesis of other intermetallic phases such as Pt