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).

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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.




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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.

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The Mononuclear Molybdenum Enzymes

Mechanism of Molybdenum Nitrogenase.

The invention relates to lube oil compositions containing a major amount of a lube oil and a minor amount of at least one compound containing a heterometallic tetranuclear preferably cubane, more preferably thiocubane core having 1 to 3 molybdenum atoms and the remainder Co, Cr, Cu, Ni, W, Mn, and Fe. Preferred is Cu. Thiocubane cores are also preferred and these typically have the formula M4-y Moy S4 Ln Qz, wherein y is 1 to 3, n is 2 to 6 and z is 0 to 4. Additive concentrates of the compounds as well as the method of making the compositions are disclosed.

Heterometallic lube oil additives

A novel sulfur-rich rhenium(III) diamagnetic, mononuclear complex, Re(S2CC6H5)(S3CC6H5)2, has been isolated from the induced internal redox reaction of tetraethylammonium tetrathioperrhenate(VII) with dithiobenzoate disulfide. The green six-coordinate octahedral complex displays two reversible one-electron oxidations and undergoes reversible sulfur abstraction with triphenylphosphine or tetraethylammonium cyanide to give purple seven-coordinate [ReL3L‘]n- complexes where L‘ = P(C6H5)3 (n = 0) or CN- (n = 1).

A New Sulfur-Rich Rhenium(III) Complex, Re(S2CC6H5)(S3CC6H5)2, and Seven-Coordinate Complexes Formed by Reversible Sulfur Abstraction Reactions, Re(S2CC6H5)3(PPh3) and [Re(S2CC6H5)3(CN)]-

Single-crystal electron paramagnetic resonance study of a trigonal vanadium tris(dithiolate) complex

Oil-soluble or oil-dispersible trinuclear molybdenum-sulfur compounds are made by reacting, in a polar medium, a reactant molybdenum compound containing an anion that possesses a trinuclear molybdenum core, and a dithiocarbamate that is produced in situ.

Manufacture of lubricant additives

X-ray absorption spectroscopy was used to examine the ligation of the heme of Azobacter vinelandii bacterioferritin scrupulously cleaned of non-heme iron. We find that the iron of the protoporphyrin IX of the protein has two axial sulfur ligands at 2.35 A, with four nitrogen ligands at 1.97 A. This result confirms the previous suggestion of Cheesman et al. (Nature 1990, 346, 771-773; Biochem. J. 1992, 786, 361-367.), on the basis of less direct spectroscopic techniques, that the heme of bacterioferritin is ligated by a pair of methionine ligands. To date, this mode of coordination is unknown in any other heme protein

https://www.slac.stanford.edu/pubs/slacpubs/6000/slac-pub-6248.pdf

Edward I. Stiefel

Papers

Heterometallic lube oil additives

A New Sulfur-Rich Rhenium(III) Complex, Re(S2CC6H5)(S3CC6H5)2, and Seven-Coordinate Complexes Formed by Reversible Sulfur Abstraction Reactions, Re(S2CC6H5)3(PPh3) and [Re(S2CC6H5)3(CN)]-

Single-crystal electron paramagnetic resonance study of a trigonal vanadium tris(dithiolate) complex

Manufacture of lubricant additives

Direct observation of bis-sulfur ligation to the heme of bacterioferritin