Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and Mn3MO(OH) Clusters.

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn3 and Fe3 cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn3MO4], [Mn3MO(OH)], and [Fe3MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics.

Molecular Mixed-Metal Manganese Oxido Cubanes as Precursors to Heterogeneous Oxygen Evolution Catalysts.

Well-defined mixed-metal [CoMn3 O4 ] and [NiMn3 O4 ] cubane complexes were synthesized and used as precursors for heterogeneous oxygen evolution reaction (OER) electrocatalysts. The discrete clusters were dropcasted onto glassy carbon (GC) and indium tin oxide (ITO) electrodes, and the OER activities of the resulting films were evaluated. The catalytic surfaces were analyzed by various techniques to gain insight into the structure-function relationships of the electrocatalysts' heterometallic composition. Depending on preparation conditions, the Co-Mn oxide was found to change metal composition during catalysis, while the Ni-Mn oxides maintained the NiMn3 ratio. XAS studies provided structural insights indicating that the electrocatalysts are different from the molecular precursors, but that the original NiMn3 O4 cubane-like geometry was maintained in the absence of thermal treatment (2-Ni). In contrast, the thermally generated 3-Ni develops an oxide-like extended structure. Both 2-Ni and 3-Ni undergo structural changes upon electrolysis, but they do not convert into the same material. The observed structural motifs in these heterogeneous electrocatalysts are reminiscent of the biological oxygen-evolving complex in Photosystem II, including the MMn3 O4 cubane moiety. The reported studies demonstrate the use of discrete heterometallic oxide clusters as precursors for heterogeneous water oxidation catalysts of novel composition and the distinct behavior of two sets of mixed metal oxides.

Redox Processes Involving Oxygen: The Surprising Influence of Redox-Inactive Lewis Acids.

Metalloenzymes with heteromultimetallic active sites perform chemical reactions that control several biogeochemical cycles. Transformations catalyzed by such enzymes include dioxygen generation and reduction, dinitrogen reduction, and carbon dioxide reduction-instrumental transformations for progress in the context of artificial photosynthesis and sustainable fertilizer production. While the roles of the respective metals are of interest in all these enzymatic transformations, they share a common factor in the transfer of one or multiple redox equivalents. In light of this feature, it is surprising to find that incorporation of redox-inactive metals into the active site of such an enzyme is critical to its function. To illustrate, the presence of a redox-inactive Ca2+ center is crucial in the Oxygen Evolving Complex, and yet particularly intriguing given that the transformation catalyzed by this cluster is a redox process involving four electrons. Therefore, the effects of redox inactive metals on redox processes-electron transfer, oxygen- and hydrogen-atom transfer, and O-O bond cleavage and formation reactions-mediated by transition metals have been studied extensively. Significant effects of redox inactive metals have been observed on these redox transformations; linear free energy correlations between Lewis acidity and the redox properties of synthetic model complexes are observed for several reactions. In this Perspective, these effects and their relevance to multielectron processes will be discussed.

NO formation by a catalytically self-sufficient bacterial nitric oxide synthase from Sorangium cellulosum.

The role of nitric oxide (NO) in the host response to infection and in cellular signaling is well established. Enzymatic synthesis of NO is catalyzed by the nitric oxide synthases (NOSs), which convert Arg into NO and citrulline using co-substrates O2 and NADPH. Mammalian NOS contains a flavin reductase domain (FAD and FMN) and a catalytic heme oxygenase domain (P450-type heme and tetrahydrobiopterin). Bacterial NOSs, while much less studied, were previously identified as only containing the heme oxygenase domain of the more complex mammalian NOSs. We report here on the characterization of a NOS from Sorangium cellulosum (both full-length, scNOS, and oxygenase domain, scNOSox). scNOS contains a catalytic, oxygenase domain similar to those found in the mammalian NOS and in other bacteria. Unlike the other bacterial NOSs reported to date, however, this protein contains a fused reductase domain. The scNOS reductase domain is unique for the entire NOS family because it utilizes a 2Fe2S cluster for electron transfer. scNOS catalytically produces NO and citrulline in the presence of either tetrahydrobiopterin or tetrahydrofolate. These results establish a bacterial electron transfer pathway used for biological NO synthesis as well as a unique flexibility in using different tetrahydropterin cofactors for this reaction.

Vinyl quinones as Diels-Alder dienes: concise synthesis of (-)-halenaquinone.

A concise asymmetric synthesis of (-)-halenaquinone is described. The synthesis features a diastereoselective Heck cyclization to set a quaternary center as well as a novel intramolecular inverse-electron-demand Diels-Alder reaction involving a vinyl quinone. The synthesis is highly convergent and features a minimal amount of protecting group manipulations.

Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers.

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.

Intramolecular Arene C-H to C-P Functionalization Mediated by Ni(II) and Pd(II).

A trisphosphine ligand with a triarylbenzene backbone was employed to support mono-nickel(II) and palladium(II) complexes. Two phosphine arms coordinated to the metal center, while the third phosphine was found to form a C-P bond with dearomatization of the central arene. Deprotonation effected the rearomatization of the central ring and metal reduction from M(II) to M(0). The overall conversion corresponds to a functionalization of an unactivated arene C-H bond to a C-P bond. This transformation represents a rare type of mechanism of C-H functionalization, facilitated by the interactions of the group 10 metal with the arene π-system. This conversion is reminiscent of and expands the scope of recently reported intramolecular rearrangements of biaryl phosphine ligands common in group 10 catalysis.

Trinuclear Nickel Complexes with Metal-Arene Interactions Supported by Tris- and Bis(phosphinoaryl)benzene Frameworks.

Triphosphine and diphosphine ligands with backbones designed to facilitate metal-arene interactions were employed to support multinuclear Ni complexes. Di- and trinuclear metal complexes supported by a triphosphine containing a triarylbenzene linker display diverse metal-arene binding modes. Multinuclear Ni halide complexes were isolated with strongly interacting metal centers bound to opposite faces of the coordinated arene. Upon reaction of the trinickel diiodide complex, 2, with disodium tetracarbonylferrate, a cofacial triangulo nickel(0) complex, 4, was isolated. The Ni03 cluster motif can also be supported by a para-terphenyldiphosphine, where a terminal carbon monoxide ligand replaces the third phosphine donor. All multinuclear complexes feature strong metal-arene interactions, demonstrating the use of an arene as a versatile ligand design element for small clusters.