Thiolate transition metal complexes

Thiolates as Ligands in Transition Metal Complexes (J. R. Dilworth)... 

The aim of this chapter is to review the chemistry of chalcogenolates in the last 10 years. The more recent reviews in this field included chalcogenolates of the s-block elements,13,14 early transition metal thiolates,15 metal complexes with selenolate and tellurolate ligands,16 copper(I), lithium and magnesium thiolates,17 functionalized thiolate complexes,18 19 pentafluorobenzenethiolate platinum group compounds,20 tellurium derivatives,21 luminescent gold compounds,22 and complexes with lanthanide or actinide.23... 

As mentioned earlier, the bulky terphenyl thiolate ligand -SC6H3-2,6-Mes2 has been shown to stabilize nominal two-coordination in transition metal complexes.53,54 Figure 30 represents the X-ray crystal structure of the iron derivative, Fe(SC6H3-2,6-Mes2)2.53... 

Transition metal complexes of sterically hindered thiolate ligands have been reviewed. " These ligands give rise to unusual geometries and oxidation states, and low coordination numbers in their complexes. 

The best enantioselectivity (35% ee) was observed in the reaction of l-(l-naphthyl)-2-propyn-l-ol with acetone in the presence of a complex bearing a 1-naphthylethylthio-lato moiety as a chiral ligand. Although the enantioselectivity is not yet satisfactory, this was the first example of an enantioselective propargylic substitution reaction catalyzed by transition metal complexes [27]. It is noteworthy that the chiral thiolate-bridged ligands work to control the chiral environment around the diruthenium site. 

With establishment of the crystal structure, three major features concerning the electronic structure of the blue copper site can be addressed. These features are 1) the nature of the thiolate and thioether bonds, 2) the nature of the ground state wavefunction and 3) the extent of covalency. We have also become strongly involved in using photoelectron spectroscopy as a powerful approach toward determining covalency in transition metal complexes. These will be discussed in turn. 

In light of these results, hypothetical mechanisms for the oxidation of Ni thiolate complexes by 02 may be discussed. The only well-characterized mechanism for the oxidation of thiolates to sulfinates in transition metal complexes involves 0 as an oxidant and proceeds via the stepwise formation of sulfenates (Scheme 1) (90, 91). 

Scheme5-39. Ferrocenyl thiolate and l,l -ferrocenylene dithiolate derivatives of half-sandwich transition metal complexes.

Organotin thiolate complexes such as the title compound are useful reagents for the synthesis of transition metal complexes with RS ligands. Examples of this use are given in the procedures that follow this synthesis. 

A large fraction of the binuclear mixed chalcogen/carbonyl transition metal complexes of iron and manganese contain the M2E (CO)6 core with the butterfly-type structure (n = 2) or substructure (n — 3, see Section 1.10.3). As an example of a complex with thiolate ligands, the structure of [Fe2(SC3H7)2(CO)6] is shown in... 

Equation (7) has been studied in several articles and a summary is available in the literature [50,70], The mercury-cystine system is, however, more complex than what is implied in Eq. (7) and can, depending on the electrode potential, include adsorbed species such as (RS)2Hg and (RS)2Hg2 [70], The system has been studied and explored in detail [70], Cystine can also be reduced at carbon electrodes modified with conducting polymers containing fixed metal thiolate sites [50,71], different macrocyclic transition metal complexes (often containing cobalt and phtalocyanines or porphyrins) [50,55,57,72,73], or vitamin Bj2 [56], which lower the overpotential necessary for reduction. 

One is the ligand-exchange reaction between high-valent transition metal complexes and thiols to give the complexes bearing only thiolate ligands... 

RS-M" L c-i)- The other is the oxidative addition of thiols to low-valent transition metals (M"L ) to give the corresponding transition metal complexes bearing both hydride and thiolate ligands (RS-M" L c 2-H). The reaction of the former complexes (RS-M" L c-i) with carbon-carbon unsaturated compounds such as alkynes may proceed via thiometallation, in which relatively more bulky is bonded at the terminal carbon of alkynes. On the other hand, in the reaction of the latter complexes (RS-M" L c-2 H) with alkynes both hydrometallation and thiometallation processes are possible. These processes proceed via sy -addition. Alternative pathway for the addition of thiols to alkynes involves coordination of alkynes to transition metals and then nucleophilic addition of thiols (or thiolate anions) to the alkynes. These processes take place via a f -addition. By controlling these pathways, regio- and stereoselective hydrothiolation of alkynes is expected to be attained successfully. 

However, the reaction requires only a general acid catalyst rather than the specific acid catalyst H+, and the corresponding reactions of the soft thioether may be better mediated by softer Lewis acids such as Cu+, Ag+, Hg2+, Pd2+, Pt2+ or Au3+. In many cases the aqua-ted metal ion is the most convenient Lewis acid, but in the case of some metals, particularly the second and third row transition metal ions, the aqua ions are not isolable and other complexes (particularly those with chloride ligands) are equally effective. The role of these softer metal ions as Lewis acids is two-fold. Firstly, the sulfur is co-ordinated to the metal, which increases the polarisation of the C-S bond and enhances the electrophilic character of the carbon, and, secondly, the thiol (or thiolate) leaving group is stabilised by co-ordination (Fig. 4-39). 

---