Hydrogen sulfide metal complexes

Hydrogen ligands, 689-711 Hydrogen selenide metal complexes, 663 Hydrogen sulfide metal complexes, 516 Hydrogen telluride metal complexes, 670 Hydroporphyrins, 814-856 basicity, 853 dehydrogenation, 853 demetallation, 854 deuteration, 853 mass spectra, 852 metallation, 854 NMR, 852 non-aromatic, 855 photochemistry, 854 redox chemistry, 855 synthesis, 852... 

CHAPTER 5. ACTU ATION OF HYDROGEN ON METAL COMPLEXES WITH SULFIDE LIGANDS AND RELATED COORDINATION CHEMISTRY OFH2S... 

GC/FPD has been used to measure hydrogen sulfide, free disulfide, and dissolved metal sulfide complexes in water (Radford-Knoery and Cutter 1993). Hydrogen sulfide was measured in the headspace of the sample (100 mL) with a detection limit of 0.6 pmol/L. A detection limit of 0.2 pmol/L was obtained for total dissolved sulfide. This method allows for the determination of the concentration of free sulfide that is in equilibrium with hydrogen sulfide. Complexed sulfide can be estimated from the difference between total dissolved sulfide and free sulfide. 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

A still more complicated reaction is the chemiluminescent oxidation of sodium hydrogen sulfide, cysteine, and gluthathione by oxygen in the presence of heavy metal catalysts, especially copper ions 60>. When copper is used in the form of the tetrammin complex Cu(NH3) +, the chemiluminescence is due to excited-singlet oxygen when the catalyst is copper flavin mononucleotide (Cu—FMN), additional emission occurs from excited flavin mononucleotide. From absorption spectroscopic measurements J. Stauff and F. Nimmerfall60> concluded that the first reaction step consists in the addition of oxygen to the copper complex ... 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

Chiu prepared monodisperse crystalline particles of metal sulfides, such as lead sulfide (PbS cubes 100 A) (I), cupric sulfide (CuS hexagonal bipyramids 200 A) (2), and zinc sulfide (ZnS multifaceted spheres 0.1-0.4 p,m) (3) by introducing hydrogen sulfide gas into dilute acidic solutions of the ethylenediamine tetraacetic acid (EDTA) complexes of the corresponding metal ions (10 4-10-1 mol dm-3) for several minutes at room temperature. 

In the presence of hydrogen sulfide produced by anaerobic bacterial activity, particularly sulfate reducers, conditions are created whereby sulfides of copper and zinc could be formed. The partition of these metals between the sulfide phase and the organic phase depends on the relation between the stability constants of the complexes and the solubility product of the sulfides of these metals. Elements with small solubility products of their sulfides and low stability constants of their chelates would be expected to go into the sulfide phase when hydrogen sulfide is present. Copper is typical of such elements. Chalcocite has a solubility product of about 10" ° and covellite about 10"44, whereas the most stable chelates of copper have stability constants of about 10" Consequently, copper could be expected to be accumulated as the sulfide. Zinc sulfide has a much larger solubility product however, the stability of its chelates is lower. From the fact that zinc appears to be completely associated with the inorganic fraction of coal, it can be assumed that the relation between the solubility product of any of its sulfides and its chelates favors formation of the sulfide. Iron could be expected to follow a similar pattern. 

The development of chiral peptide-based metal catalysts has also been studied. The group of Gilbertson has synthesized several phosphine-modified amino adds and incorporated two of them into short peptide sequences.[45J,71 They demonstrated the formation of several metal complexes, in particular Rh complexes, and reported their structure as well as their ability to catalyze enantioselectively certain hydrogenation reactions.[481 While the enantioselectivities observed are modest so far, optimization through combinatorial synthesis will probably lead to useful catalysts. The synthesis of the sulfide protected form of both Fmoc- and Boc-dicyclohexylphosphinoserine 49 and -diphenylphosphinoserine 50 has been reported, in addition to diphenylphosphino-L-proline 51 (Scheme 14).[49 To show their compatibility with solid-phase peptide synthesis, they were incorporated into hydrophobic peptides, such as dodecapeptide 53, using the standard Fmoc protocol (Scheme 15).[451 For better results, the phosphine-modified amino acid 50 was coupled as a Fmoc-protected dipeptide 56, rather than the usual Fmoc derivative 52.[471 As an illustrative example, the synthesis of diphe-nylphosphinoserine 52 is depicted in Scheme 16J45 ... 

Thiohydrates were regarded for a long time as the only H2S complexes which could be isolated. They are formed by addition of H2S to Lewis acids in liquid hydrogen sulfide (for example A1C13 H2S and TiCl4 H2S, n = 1, 2) and decompose thermally below room temperature. To stabilize complexes with the neutral ligands H2S and RSH, metal ions in low oxidation states, preferably with inert and bulky units, are required. 

The direct connection of rings A and D at C-l cannot be achieved by enamine or sulfide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D-secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-l is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 a-electron intermediate occurs. Only the 1,19-rrons-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

However, the dithioketones may be readily prepared by reaction of the metal complexes of the monothio derivatives, 3.11, with hydrogen sulfide (Fig. 3-21). In this case, the metal probably plays a dual role, both in polarising the ligand and stabilising the product. Clearly, the use of a soft metal ion such as lead(it) or platinum(u) is to be preferred in a reaction of this type. 

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