Sulphide and Sulphoxide Complexes

Sulphide and sulphoxide complexes have been extensively studied since cis-RuC12(DMSO)4 (DMSO, dimethyl sulphoxide, (Me)2SO) was found to have anti-tumour properties and to be a precessor for radiosensitizing agents. Such complexes can act as catalysts for the oxidation of sulphides with molecular oxygen. 

The best characterized complexes are those of ruthenium(II) it is likely that several reports of ruthenium(III) complexes have, until very recently, 

Apart from DMSO complexes, others including those with tetramethylene sulphoxide have been increasingly examined, but the account here focuses on DMSO. 

When c -RuC12(DMSO)4 is stirred in methanol containing traces of water (to catalyse the formation of intermediate aqua species) Ru2C14(DMSO)5 is formed this has the unsymmetrical structure (DMSO)2ClRu(/i-Cl)3Ru(DMSO)3 based on face-sharing octahedra [110]. 

Again both S- and O-bonded sulphoxides are found. mer-[RuCl3(Ph2SO)3] has one S-bonded sulphoxide and two O-bonded sulphoxides (one trans to Cl, one tram to S) [112]. The imidazole-substituted complexes are being studied as possible radiosensitizers and for anti-tumour activity. 

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A determination of dimethyl sulphoxide by Dizdar and Idjakovic" is based on the fact that it can cause changes in the visible absorption spectra of some metal compounds, especially transition metals, in aqueous solution. In these solutions water and sulphoxide evidently compete for places in the coordination sphere of the metal ions. The authors found the effect to be largest with ammonium ferric sulphate, (NH4)2S04 Fe2(S04)3T2H20, in dilute acid and related the observed increase in absorption at 410 nm with the concentration of dimethyl sulphoxide. Neither sulphide nor sulphone interfered. Toma and coworkers described a method, which may bear a relation to this group displacement in a sphere of coordination. They reacted sulphoxides (also cyanides and carbon monoxide) with excess sodium aquapentacyanoferrate" (the corresponding amminopentacyanoferrate complex was used) with which a 1 1 complex is formed. In the sulphoxide determination they then titrated spectrophotometrically with methylpyrazinium iodide, the cation of which reacts with the unused ferrate" complex to give a deep blue ion combination product (absorption maximum at 658 nm). 

The first reductive kinetic resolution of racemic sulphoxides was reported by Balenovic and Bregant. They found that L-cysteine reacted with racemic sulphoxides to produce a mixture of L-cystine, sulphide and non-reduced optically active starting sulphoxide (equation 147). Mikojajczyk and Para reported that the reaction of optically active phosphonothioic acid 268 with racemic sulphoxides used in a 1 2 ratio gave the non-reduced optically active sulphoxides, however, with a low optical purity (equation 148). It is interesting to note that a clear relationship was found between the chirality of the reducing P-thioacid 268 and the recovered sulphoxide. Partial asymmetric reduction of racemic sulphoxides also occurs when a complex of LiAlH with chiral alcohols , as well as a mixture of formamidine sulphinic acid with chiral amines, are used as chiral reducing systems. ... 

An investigation of the electron-acceptor properties of TeCl4 with some donor molecules (sulphides, ethers, ketones, and sulphoxides) has been carried out. Enthalpies of formation and dipole moments of the complexes were measured and the heats of the reaction ... 

Tin.—Allyltin compounds, especially under rhodium-complex catalysis, react with acyl chlorides to give allyl ketones or with aryl halides to give allylarenes. Bridgehead alkyl bromides are reduced on photolysis with Bu"3SnH, whilst the system (Bu"3Sn)20-Br2 oxidizes sulphides to sulphoxides. )V-(Alkoxycar-bonyl)amino-acids are obtained upon treatment of cyclic anhydrides with Bu3SnN3 and heating in an alcohol. ... 

Calorimetric (enthalpy), i.r. (OH frequency shift), and n.m.r. (hydrogen-bond chemical shift) data have been reported for the interaction of 1,1,1,3,3,3-hexafluoropropan-2-ol with a series of Lewis bases (nitrogen and oxygen donors and the soft donor diethyl sulphide), and details of studies on the 1 1 and 2 1 complexes formed between this highly acidic alcohol [pJira(H20) 9 3] and dimethyl sulphoxide, tetramethylurea, tetramethylene sulphone, and 1,4-dioxan have become available. Hydrogen bonding between water vapour (the acceptor) and l,l,l,3,3,3-hexafluoropropan-2-ol has been detected by i.r. spectroscopy. ... 

The modified Sharpless reagent was also successfully applied288 for the asymmetric oxidation of a series of 1,3-dithiolanes 248 to their S-monooxides 249 (equation 134). It was observed that the optical induction on sulphur (e.e. from 68 to 83%) is not significantly affected by the substituents R1 and R2. Asymmetric oxidation of a few aryl methyl sulphides by organic hydroperoxides in the presence of a catalytic amount of the optically active Schiff base-oxovanadium(IV) complexes gave the corresponding sulphoxides with e.e. lower than 40%289. 

A more elegant, but expensive, approach22 has been the use of soluble iridium and rhodium catalysts which contain coordinated dimethyl sulphoxide (e.g. IrHCl2(Me2SO)3) which promote the oxidation of sulphoxides in aqueous media, equation (8). The ease of oxidation depends on the substituents and this decreases in the order Me > Ph > PhCH2. This reaction is especially useful since sulphides are not oxidized under the reaction conditions due to the formation of strong complexes with the catalyst. 

Although this mechanism could explain the inertness of di-t-butyl sulphide towards oxidation due to the absence of a-hydrogen atoms, it was later ruled out by Tezuka and coworkers They found that diphenyl sulphoxide was also formed when diphenyl sulphide was photolyzed in the presence of oxygen in methylene chloride or in benzene as a solvent. This implies that a-hydrogen is not necessary for the formation of the sulphoxide. It was proposed that a possible reactive intermediate arising from the excited complex 64 would be either a singlet oxygen, a pair of superoxide anion radical and the cation radical of sulphide 68 or zwitterionic and/or biradical species such as 69 or 70 (equation 35). 

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