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Recently, N,N-transRe(O)(LN–O)2X (LN–O= monoanionic N–O chelates; X = Cl or Br prior to being replaced by solvents or alkoxides) complexes have been found to be superior to the corresponding N,N-cisisomers in the catalytic reduction of perchlorate via oxygen atom transfer. However, reported methods for Re(O)(LN–O)2X synthesis often yield only the N,N-ciscomplex or a mixture of transand cisisomers. This study reports a geometry-inspired ligand design rationale that selectively yields N,N-transRe(O)(LN–O)2Cl complexes. Analysis of the crystal structures revealed that the dihedral angles (DAs) between the two LN–Oligands of N,N-cisRe(O)(LN–O)2Cl complexes are less than 90°, whereas the DAs in most N,N-transcomplexes are greater than 90°. Variably sized alkyl groups (−Me, −CH2Ph, and −CH2Cy) were then introduced to the 2-(2′-hydroxyphenyl)-2-oxazoline (Hhoz) ligand to increase steric hindrance in the N,N-cisstructure, and it was found that substituents as small as −Me completely eliminate the formation of N,N-cisisomers. The generality of the relationship between N,N-trans/cisisomerism and DAs is further established from a literature survey of 56 crystal structures of Re(O)(LN–O)2X, Re(O)(LO–N–N–O)X, and Tc(O)(LN–O)2X congeners. Density functional theory calculations support the general strategy of introducing ligand steric hindrance to favor synthesis of N,N-transRe(O)(LN–O)2X and Tc(O)(LN–O)2X complexes. This study demonstrates the promise of applying rational ligand design for isomeric control of metal complex structures, providing a path forward for innovations in a number of catalytic, environmental, and biomedical applications.

A set of bimetallic Pd−Cu/PVP (PVP = poly(N-vinylpyrrolidone)) colloids, with copper proportions ranging from 0 to 50 atom %, has been examined as catalysts in a batch reactor with flowing hydrogen for the reduction of aqueous nitrate and/or nitrite. The encapsulated Pd−Cu nanoparticles were characterized by powder XRD, TEM, EDX, and IR of adsorbed CO. A significant decrease in average particle diameter and changes in the Pd−Cu crystallinity occurred above ca. 30% copper content, and this transition corresponded with a significant increase in observed nitrate reduction rates. The strong dependence on composition suggests that specific Cunensembles on the surface of the Pd−Cu nanoparticles are needed for effective nitrate-to-nitrite conversion. In contrast, nitrite reduction rates were only minimally enhanced by the presence of copper. Increasing pH had little effect on the nitrate reduction rates, but it strongly inhibited the rate of nitrite reduction. The requisite protonation of a palladium−nitrite surface intermediate is proposed.

Hydrogenation of Os3(CO)9(μ3-CPh)(μ3-COMe) (1) at one atmosphere results in alkylidyne-alkylidyne coupling, forming the alkyne complex (μ-H)2Os3(CO)9(μ3, η2-C2(OMe)Ph) (3). Reduction of 1by two equiv. of sodium benzophenone ketyl, followed by protonation with tetrafluoroboric acid, yields the phenylacetylide complex(μ-H)OS3(CO)9(μ3,η2-CCHPh) (4). Sequential reduction/protonation involving (μ-H)OS3(CO)10(μ3-CPh) (2) also generates 4, apparently via benzylidyne-carbonyl coupling.

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