Transition metal dithiocarbamates

Similarly, nickel(ll) and copper(ll) transition metal dithiocarbamate ion-pair receptors 21, containing amide-and crown ether-recognition sites, bind alkali metal cations and various anions. The sandwich K+ complex of the nickel(ll) receptor cooperatively enhanced the binding of acetate anion, while the copper(ll) receptor electrochemi-cally can sense anions and cations via perturbation of the copper(n)/copper(m) dithiocarbamate redox couple <2002JSU89>. 

Scheme 1.5 Antioxidant mechanism of the transition metal dithiocarbamates.

The transition metal dithiocarbamates contain both a peroxide decomposer function and a latent prooxidant function. The latter is liberated by photolysis at the end of the induction period and in the case of the photoactive transition metal ions (notably iron), very rapid thermal and photooxidation of the polymer ensues. 

These are another category of materials to the filler and pigment screens mentioned previously. These materials are generally organic and include derivitives of thiophen, benzotriazole and transition metal dithiocarbamates, e.g., Ferro 101, nickel dithiocarbamate. These substances are effective at concentrations of 0.1 % or less. UV curable acrylate coatings and inks have been used in the manufacture of printed polymer containers. 

It is not clear when dithiocarbamates were first prepared, but certainly they have been known for at least 150 years, since as early as 1850 Debus reported the synthesis of dithiocarbamic acids (1). The first synthesis of a transition metal dithiocarbamate complex is also unclear, however, in a seminal paper in 1907, Delepine (2) reported on the synthesis of a range of aliphatic dithiocarbamates and also the salts of di-iTo-butyldithiocarbamate with transition metals including chromium, molybdenum, iron, manganese, cobalt, nickel, copper, zinc, platinum, cadmium, mercury, silver, and gold. He also noted that while dithiocarbamate salts of the alkali and alkali earth elements were water soluble, those of the transition metals and also the p-block metals and lanthanides were precipitated from water, to give salts soluble in ether and chloroform, and even in some cases, in benzene and carbon disulfide. 

It should be acknowledged that both before and after Coucouvanis s reviews (16, 17) others also reviewed aspects of transition metal dithiocarbamate chemistry (18-31). Notably, in 1984 Bond and Martin reviewed the electrochemical and redox behavior of transition metal dithiocarbamate complexes (20), and as such less emphasis is placed here on electrochemical properties. Further, as early as 1965, Thom and Ludwig produced an excellent book The Dithiocarbamates and Related Compounds (32), which is still well worth a read, especially for the newcomer to the area. 

This chapter is aimed at inorganic chemists and as such it focuses on the synthesis, properties, and reactivity of transition metal dithiocarbamate complexes. A flavor of the established and potential applications of each metal type is given in Section IV, but space restrictions nessetate that their applications in analytical chemistry and the agricultural industry (27), together with their widespread biological applications (45—48) are not fully developed. 

Transition metal dithiocarbamate complexes were probably first prepared in 1907 by Delepine (2) and over the following century, dithiocarbamate complexes of all the transition elements have been prepared and in a wide range of different oxidation states (Table 1). Perhaps most impressive is their ability to stabilize molybdenum in seven oxidation states ranging from - -6 to 0 (Fig. 24). 

While transition metal dithiocarbamate complexes can be prepared in a wide variety of ways, by far the most common is the direct ligand addition, which often, but not necessarily, results in loss of a coordinated anionic ligand, and sometimes a second neutral ligand also (Eq. 13). This route has few limitations and complexes of all the transition metals have been prepared using it. 

Casey and Vecchio (186,187) developed the oxidative-addition of thiuram disulfides to metals in order to produce a one-step preparation of a wide range of transition metal dithiocarbamate complexes. In this way, [Ni(S2CNMe2)2] and [Zn(S2CNMe2)2] can be prepared in near quantitative yields. With copper, only the copper(II) species are seen, and this is in keeping with Akerstrom s (188) earlier observation that copper(I) species react instantaneously with thiuram disulfides to generate the analogous copper(II) complexes. 

Perhaps the most characteristic feature of transition metal dithiocarbamate complexes are their IR spectra (16, 17, 511). Three regions can be identified (1) the backbone v(C—N) vibration at between 1450 and 1550 cm (2) the v(C—S) vibrations between 950 and 1050 cm (3) v(M—S) vibrations between 300 and 400 cm Variations in v(C—N) and v(C—S) as a function of the substituents are generally quite consistent within a particular class of complex. Some typical values for nickel bis(dithiocarbamate) complexes are given in Table IX. 

The thermochemistry of dithiocarbamate complexes is of considerable interest, primarily since they can be used as molecular precursors for the synthesis of a range of technologically important metal sulfides, especially those of copper and zinc (see Sections III.H.l.g.ii and III.I.Lh.i). The successful application of this approach relies on the volatility of the metal complexes and the strength of the metal-sulfur and metal-carbon bonds since the latter must be cleaved, while the former is retained (at least to some extent). Consequently, a large number of studies have focused on the thermochemical properties of transition metal dithiocarbamate complexes and HiU and co-workers (22, 562, 563) and others (23, 564) reviewed aspects of these. 

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