Mercaptides coordinated

Rubredoxins do not have acid-labile sulfur as do ferredoxins, with the iron characterized by a mercaptide coordination. They are small proteins composed of some 50 amino acids. Both oxidized and reduced forms are high spin. 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

The platinum complex is square planar, while the palladium dimer also has planar 4-coordination (for other examples of mercaptide bridges see section 3.8.3) [116]. 

Coates, G.E., Kowala, C. and Swan, J.M. (1966) Coordination compounds of Group IB metals. I. Triethylphosphine complexes of Au(I) mercaptides. Australian Journal of Chemistry, 19, 539-545. 

Ewens and Gibson (29) reported an early observation on the reaction of the coordinated sulfur atom as a nucleophile (Equation 36), showing the conversion of the mercaptide ion to a thioether function within the coordination sphere. 

The bridge-forming proclivity of the neutral cobalt (III) complex, CoL3, has also been demonstrated (6). Two trinuclear compounds of the formula [M(CoL3)]Xn, where M is Ni(II) or Co (III), have been isolated in pure form. Detailed study of these substances indicates that the complex CoL3 forms three mercaptide bridges to the third metal ion, thus providing octahedral coordination of the six sulfur atoms about that ion. Data on these unusual compounds are also included in Tables I and II. 

Complexes of 2,2 -Dialkyl( ethanediylidenedinitrilo )diethanethiols. The previously unknown Schiff base derivatives of a-diketones with -mercaptoalkyl-amines should be ideally suited to form metal complexes in which the ligand is coordinated in a planar tetradentate manner. Further, the mercaptide functions should occupy cis positions in the coordination sphere of the metal ion. Structure V illustrates the structure expected for such substances. Three products might... 

Reaction of Coordinated Mercaptide Group with Alkyl Halides... 

The synthesis of a large number of metal complexes of mercaptoamines by Busch and coworkers (6, 15, 17, 24) has opened the way for detailed study of the reactions of the coordinated mercaptide function. Mercaptoamine complexes provide the first extensive series of complexes of known structure in which the effect of varying structural parameters on the reactivity of the mercaptide group can be subjected to investigation. 

The susceptibility of the mercaptide groups in these Schiff base complexes to ligand reaction was evaluated by treating these compounds with methyl iodide and benzyl bromide in chloroform solution. The pure compounds isolated from these reaction mixtures were of the composition NiL.2RX, where L represents the tetradendate Schiff base and RX represents the alkyl halide added. These reactions proceed smoothly and the stoichiometry of the products implies that both sufur atoms are reactive. The products are monomeric in dichloroethane. They exhibit magnetic moments consistent with octahedral structures, and they behave as di-univalent electrolytes in coordinating, polar solvents. 

In general, these free radical reactions bear some resemblance to the reactions found for the air-free bleaching of ferric mercaptoacetate (7) and ferric cysteinate (6) A bimolecular reaction occurs between two one-electron oxidants, one of which is an iron (III) ion with two mefcaptide groups coordinated to it in a two-electron reaction each of the oxidants is reduced and the mercaptides are oxidized to give a molecule of disulfide. 

The UV spectra of these complexes are very similar to those found in tetrahedral complexes of Co11 known to have a somewhat distorted geometry, suggesting a similar geometry in the cobalt enzyme.1383 Tetrahedral mercaptide complexes of the type [Co(SPh)4]2- were also shown to have similar absorption characteristics to those of [(LADH)Co2Co2j. This work is in complete agreement with the X-ray crystallographic studies of the native LADH, already mentioned, which shows distorted tetrahedral coordination of both the catalytic and non-catalytic zinc atoms of the enzyme. 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

The reaction of carbonyl sulfide with [M(02)(PPh3)2] (where M = Pd or Pt) has resulted in the first reported examples of transition metal complexes of the monothiocarbonate anion (47 R = O-). Bidentate S—O coordination was concluded from 3IP H NMR analyses of these compounds.184 A short structural review of metal complexes of monothiocarbamate ions (47 R = N(R )R") demonstrates their varied coordination chemistry.185 In complexes of the dialkyl forms the sulfur atom is seen to have considerable mercaptide character , whereas aromatic amine derivatives demonstrate C—S and M—S partial multiple bonding.185,251 A review on the coordination chemistry of these ligands has appeared.186 Additional detail is provided in Chapter 16.4 of this volume. 

Biologically, hydrocarbons are oxidized to alcohols or epoxides by molecular oxygen in the presence of cytochromes P-450, which contain Fe(PPIX) as a prosthetic group coordinated by a cysteinyl mercaptide (Scheme 29). 

Figure 4.89 Synthesis of a mercaptide functionalised NHC ligand in the coordination sphere of nickel (II).

The phosphane ligand can be removed in 18% aqueous hydrochloric acid and subsequently replaced by pyridine [281], The complex then features two mercaptide ligands on the paUadium(II) atom confirming that sulfur coordination on the transition metal is indeed preferred over nitrogen coordination. 

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