Metal-free dithiocarbamates

Depending on their carbon skeleton, DTCs can be categorized into three subclasses, that is,dimethyldithio-carbamates (DMDs), ethylenebis(dithiocarbamates) (EBDs), and propylenebis(dithiocarbamates) (PBDs) (Table 26.1).They are mainly produced as complexes of transition metals, preferably manganese and zinc. In the case of EBD and PBD, it is unclear if the complexes are monomer or polymer in type. Therefore, the stmcture formulae presented in Table 26.1 only provide composition. The two metal-free fungicides in use are thiram and dazomet. Dazomet is mainly used as soil sterilant (precursor of methyl isothiocyanate), whereas DTCs are typically applied to aerial parts of plants. 

Ammonium dithiocarbamates can be prepared through reaction (11). With HC1 or H2S04 at lower temperatures the free acids are formed. Only one transition metal complex (37) is known, with a pseudo-octahedral coordination of Ni four sulfur atoms of the dithiocarbamate in the equatorial plane and two pyridine solvent molecules at the apical positions. Sulfides can be eliminated from monosubstituted dithiocarbamates to give isothiocyanates.53... 

Many free-radical scavengers (including dithiols and dithiocarbamates) have potential therapeutic usefulness as radioprotective agents. Copper complexes are known to be scavenging agents for the superoxide radical, which is believed to play a role in the induction of radiation damage. The toxic effects of superoxide are believed to lie in its ability to reduce metal ions, for example Cu(II) to Cu(I),... 

Transition metal atoms coordinated by dithiocarbamate ligands through four sulfur atoms have both electronic (free d-orbitals) and steric (free octahedral sites) propensity to form adducts with nitrogen bases, since nitrogen atoms have one sp -hybrid orbital, which is occupied by the lone pair of stereochem-ically active electrons. To characterize newly prepared adducts of dimethyl-, diethyl- and morpholinedithiocarbamate zinc(II) complexes with cyclic N-donor bases, such as pyridine, piperidine, hexamethyleneimine and morpholine, compounds 28-35 have been prepared and studied by and CP/MAS... 

Metal deactivators, which form inactive chelates or insoluble reaction products with transition metals originally present in a form that promotes the decomposition of peroxides to free radicals. Examples are ethylenediaminetetraacetic acid, salicylaldehyde-diamine condensation products or metalalkyl dithiocarbamates such as of nickel or zinc. 

Phosphites can react with ROOH to give alcohols and phosphates, avoiding the decomposition of the hydroperoxide to free radicals. They include the tristearyl, diphenyl or tri-isodecyl phosphite, and di-isodecyl pentaerythritol diphosphite. Besides phosphites, sulphur compounds can be used, such as thioesters, thioethers, thiodipropionates (including the dilauryl and distearyl thiodipropionates) and the metal dithiolates, e.g., iron dithiocarbamate. 

Vibrational spectroscopy, as well as proving a useful characterization tool, has also been widely used in the study of the structure and reactivity of dithiocarbamate complexes. For example, Ramadevi and co-workers used IR spectroscopy to follow isotopic exchange for both [Cu(S2CNEt2)2] (519) and [Cr(S2CNEt2)3] (520) and their free metal ions in solution. As expected, increasing both the temperature and concentration of the metal complexes results in an uwreased reaction rate. 

Stability constants have been measured for a wide range of dithiocarbamate complexes from ultraviolet (UV)-visible spectroscopy (316,392,596,597), pH (598), and potentiometric (599,600) titrations, electron spin resonance (ESR) spectroscopy (601), and HPLC (602-605). They are determined typically from exchange studies between a free metal salt and its dithiocarbamate complex (Eq. 47). Sachinidis and Grant (597) identified two pathways for ligand transfer the first involves dissociation of a dithiocarbamate followed by substimtion at the metal ion, while the second results from direct electrophilic attack by the metal ion on the dithiocarbamate complex. 

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