The use of conductivity measurements in organic solvents for the characterisation of coordination compounds

The ferritins: molecular properties, iron storage function and cellular regulation☆

This critical review presents a comprehensive study of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) towards construction and synthesis of metal–organic frameworks (MOFs). We describe the geometries of 131 SBUs, their connectivity and composition. This contribution presents a comprehensive list of the wide variety of transition-metal carboxylate clusters which may serve as secondary building units (SBUs) in the construction and synthesis of metal–organic frameworks. The SBUs discussed here were obtained from a search of molecules and extended structures archived in the Cambridge Structure Database (CSD, version 5.28, January 2007) which included only crystals containing metal carboxylate linkages (241 references).

/pdf/secondary-building-units-nets-and-bonding-in-the-chemistry-o0m4ttzsgz.pdf

Secondary building units, nets and bonding in the chemistry of metal–organic frameworks

Molybdenum is the only second-row transition metal required by most living organisms, and is nearly universally distributed in biology. Enzymes containing molybdenum in their active sites have long been recognized,1 and at present over 50 molybdenum-containing enzymes have been purified and biochemically characterized; a great many more gene products have been annotated as putative molybdenum-containing proteins on the basis of genomic and bioinformatic analysis.2 In certain cases, our understanding of the relationship between enzyme structure and function is such that we can speak with confidence as to the detailed nature of the reaction mechanism and, with the availability of high-resolution X-ray crystal structures, the specific means by which transition states are stabilized and reaction rate is accelerated within the friendly confines of the active site. At the same time, our understanding of the biosynthesis of the organic cofactor that accompanies molybdenum (variously called molybdopterin or pyranopterin), the manner in which molybdenum is incorporated into it, and then further modified as necessary prior to insertion into apoprotein has also (in at least some cases) become increasingly well understood.

It is now well-established that all molybdenum-containing enzymes other than nitrogenase (in which molybdenum is incorporated into a [MoFe7S9] cluster of the active site) fall into three large and mutually exclusive families, as exemplified by the enzymes xanthine oxidase, sulfite oxidase, and DMSO reductase; these enzymes represent the focus of the present account.3 The structures of the three canonical molybdenum centers in their oxidized Mo(VI) states are shown in Figure 1, along with that for the pyranopterin cofactor. The active sites of members of the xanthine oxidase family have an LMoVIOS-(OH) structure with a square-pyramidal coordination geometry. The apical ligand is a Mo=O ligand, and the equatorial plane has two sulfurs from the enedithiolate side chain of the pyranopterin cofactor, a catalytically labile Mo–OH group, and most frequently a Mo=S. Nonfunctional forms of these enzymes exist in which the equatorial Mo=S is replaced with a second Mo=O; in at least one member of the family the Mo=S is replaced by a Mo=Se, and in others it is replaced by a more complex –S–Cu–S–Cys to give a binuclear center. Members of the sulfite oxidase family have a related LMoVIO2(S–Cys) active site, again square-pyramidal with an apical Mo=O and a bidentate enedithiolate Ligand (L) in the equatorial plane but with a second equatorial Mo=O (rather than Mo–OH) and a cysteine ligand contributed by the protein (rather than a Mo=S) completing the molybdenum coordination sphere. The final family is the most diverse structurally, although all members possess two (rather than just one) equiv of the pyranopterin cofactor and have an L2MoVIY(X) trigonal prismatic coordination geometry. DMSO reductase itself has a catalytically labile Mo=O as Y and a serinate ligand as X completing the metal coordination sphere of oxidized enzyme. Other family members have cysteine (the bacterial Nap periplasmic nitrate reductases), selenocysteine (formate dehydrogenase H), –OH (arsenite oxidase), or aspartate (the NarGHI dissimilatory nitrate reductases) in place of the serine. Some enzymes have S or even Se in place of the Mo=O group. Members of the DMSO reductase family exhibit a general structural homology to members of the aldehyde:ferredoxin oxidoreductase family of tungsten-containing enzymes;4 indeed, the first pyranopterin-containing enzyme to be crystallographically characterized was the tungsten-containing aldehyde:ferredoxin oxidoreductase from Pyrococcus furiosus,5 a fact accounting for why many workers in the field prefer “pyranopterin” (or, perhaps waggishly, “tungstopterin”) to “molybdopterin”. The term pyranopterin will generally be used in the present account.




Open in a separate window


Figure 1


Active site structures for the three families of mononuclear molybdenum enzymes. The structures shown are, from left to right, for xanthine oxidase, sulfite oxidase, and DMSO reductase. The structure of the pyranopterin cofactor common to all of these enzymes (as well as the tungsten-containing enzymes) is given at the bottom.

/pdf/the-mononuclear-molybdenum-enzymes-huemv59go8.pdf

The Mononuclear Molybdenum Enzymes

Mechanism of Molybdenum Nitrogenase.

Ligand effects and product distributions in molybdothiol catalyst systems

Identification of two reduction products of glyoxal bis(2-mercaptoanil)nickel. Characterization of the one-electron reduction product and the partially hydrogenated anion

Molybdoenzymes: The Role of Electrons, Protons, and Dihydrogen

The invention relates to lubricating oil compositions of a major amount of an oil of lubricating viscosity in admixture with an effective minor amount of an oil soluble or dispersible trinuclear molybdenum containing complex and of an antioxidant, preferably, copper containing antioxidants and diphenyl amines and effective diphenylamine derivatives to enhance the lubricating properties of the oil. Also included are additive concentrates containing these compositions. The invention further includes the method of making these compositions.

Molybdenum-antioxidant lube oil compositions

This invention relates to heterometallic thiocubane compositions containing the M 2 1 M 2 2 S 4 cluster core and methods of making them wherein M 1 is Re, V, Mo or W and M 2 is Co, Cr, Cu, Ni or Fe but preferably Co. More particularly, the invention relates to compositions of the formula M 2 2 M 2 2 S 4 L 2 1 L 2 2 L 2 3 wherein M 1 and M 2 are as above, L 1 is a bidentate sulfur or nitrogen bearing ligand (most preferably a dialkyldithiocarbamate), L 2 is optional but may be an O, N, P or S-containing monodentate donor ligand, and L 3 may be CO, a monodentate anion ligand such as a halide (preferably C1), mercaptide or alkoxide, or another O, N, P, or S containing monodentate donor ligand. The compositions are suitable for making active hydrotreating catalysts.

Edward I. Stiefel

Papers

Ligand effects and product distributions in molybdothiol catalyst systems

Identification of two reduction products of glyoxal bis(2-mercaptoanil)nickel. Characterization of the one-electron reduction product and the partially hydrogenated anion

Molybdoenzymes: The Role of Electrons, Protons, and Dihydrogen

Molybdenum-antioxidant lube oil compositions

Heterometallic thiocubanes (C-2044)