Sulfur as a ligand

The S-S bond can also be formed by a direct coupling reaction, e.g.  

At least 8 modes of coordination are known (Table 15.9) - they are all based on either side-on S2 or bridging -S-S- with possible further ligation via one or two lone-pairs as shown schematically below  

Moil xV, a. MGxler, a. Neuman, W. Rittner and G. M. SfiELDRiCK, Inorf . Chem. 19, 2066-9 (1980). 

KUlumer. E. Roitinger and H, VahrEiNKamp, J. Chem. Sac., Chem. Commun.. 782-3 (1977). 

Complexes with chelating polysulfidc ligands can be made either by reacting complex metal halides with. solutions of polysulfides or by reacting hydrido complexes with elemental 

S can be regarded as a 6e Utinijr in Ihc iiniqae liricir cutri dex f(CjHj)tCO)j ] 

Some examples of the S atom as a bridging ligand are given in Fig. 15.12. In the bridging mode S is usually regarded as a 

2-electron donor, though in the linear bridge [ (C5H5XCO 2Cr 2S] it is probably best regarded 

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Albracht, S. P. J., Kroger, A., Van der Zwaan, J. W., Unden, G., Bocher, R., Mell, H. and Fontijn, R. D. (1986) Direct evidence for sulfur as a ligand to nickel in hydrogenase An EPR study of the enzyme from Wolinella succinogenes enriched in O2. Biochim. Biophys. Acta, 874, 116-27. 

In 1999, these authors reported the synthesis of the first family of sugar-derivative dithioethers containing sulfur as a unique donor atom. These chiral Ci-symmetric dithioether ligands were tested in the iridium-catalysed... 

A palladium-catalyzed protocol for carbon-sulfur bond formation between an aryl triflate and para-methoxybenzylthiol was introduced by Macmillan and Anderson (Scheme 6.66) [138], Using palladium(II) acetate as a palladium source and 2,2 -bis(diphenylphosphino)-l,l -binaphthyl (BINAP) as a ligand, microwave heating of the two starting materials in N,N-dimethylformamide at 150 °C for 20 min in the presence of triethylamine base led to the formation of the desired sulfide in 85% yield. 

Sulfur-bound L-Met, as opposed to S,N-chelated L-Met, is more reactive as a ligand on Pt(II) and can be slowly replaced by N7 of G (95, 96). Transfer of Pt onto DNA via Met-containing peptides or proteins may therefore be possible. Monofunctional adducts of the type [Pt(en)(G)(L-Met-S)] appear to be very stable (97) and so methionine may play a role in trapping these adducts. Also, the high trans influence of S as a Pt(II) ligand can lead to the facile labilization of trans-am(m)ine ligands and this allows cisplatin to react with GMP faster in the presence of L-Met then in its absence (98), which introduces another route to DNA platination. 

NAH is composed of four subunits (SDS-PAGE) and contains a molybdenum cofactor (Dilworth 1983). Analysis of the electron paramagnetic resonance (EPR) spectra of the molybdenum center of NAH revealed a coordination of molybdenum to selenium (Gladyshev et al. 1994b). Apparently NAH is much like other selenium-dependent molybdenum hydroxylases such as XDH from C. barkeri and other purinolytic Clostridia. Whether or not the selenium is present as a ligand of molybdenum or is coordinated to molybdenum while being bound to another molecule (e.g., sulfur of cysteine) is still not known. The nature of the selenium cofactor and the mechanism of its incorporation into NAH are most likely similar to XDH and thus also require more study. 

Covalency of the fag iron orbitals with sulfur implies that the Moss-bauer spectra of these proteins will be sensitive to ligand changes between the members of the plant-type ferredoxins. That is, the substitution of tyrosine or histidine for cysteine as a ligand is certain to cause a change in isomer shift which is not observed for these proteins. 

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