Thiolate ligands metal complex formation

Simple a-diimines are hydrolytically unstable, but can be stabilized as metal complexes by virtue of the formation of stable five-membered chelate rings.68 69 a-Diketones and glyoxal undergo metal template reactions with amines to yield complexes of multidentate ligands such as (34),70 (35)71 and (36).72>73 In the last case, the metal exerts its stabilizing influence on the a-diimine partner in an equilibrium process (Scheme 5). The same phenomenon occurs with amino alcohols74 75 in addition to amino thiols. The thiolate complexes (37) can be converted to macrocyclic complexes by alkylation in a kinetic template reaction (Scheme 5).76 77... 

One of the simplest and widely used methods of forming C-S bonds involves nucleophilic attack of a thiolate on a suitable C-centred electrophile such as an alkyl halide (Fig. 5-74). Co-ordinated thiolate ligands behave as nucleophiles in exactly the same manner, and the method has been extensively used for the preparation of thioethers and their metal complexes. The method has been particularly commonly utilised in the formation of macrocyclic ligands in templated syntheses (see Chapter 6). 

As mentioned above, reactions of this type have been widely used in the synthesis of macrocyclic ligands. Indeed, some of the earliest examples of templated ligand synthesis involve thiolate alkylations. Many of the most important uses of metal thiolate complexes in these syntheses utilise the reduced nucleophilicity of a co-ordinated thiolate ligand. The lower reactivity results in increased selectivity and more controllable reactions. This is exemplified in the formation of an A -donor ligand by the condensation of biacetyl with the nickel(n) complex of 2-aminoethanethiol (Fig. 5-78). The electrophilic carbonyl reacts specifically with the co-ordinated amine, to give a complex of a new diimine ligand. The beauty of this reaction is that the free ligand cannot be prepared in a metal-free reac-... 

Yet another possibility involves H2 activation at thiolate ligands. This possibility is suggested by the chemistry of an Fe11 tetrathiolate complex (Scheme 4) (110), which evolves H2 when H+ is added to the system. Intermediates proposed for this reaction include thiol complexes derived from the protonation of the thiolate ligands. A role for a metal cluster in the catalysis is also suggested by the mechanism, which involves the formation of dimeric species in order to provide the two electrons necessary for the production of H2. 

Metal thiolate complexes will reduce elemental sulfur or red selenium via the oxidative elimination of RSSR. In a similar manner, metal selenolate complexes ean be used to reduce elemental selenium. The resulting E ligands favor the formation of polynuclear cluster complexes, due to the greater tendency of E (vs. RE ) to form bridging interactions between metal centers. Originally developed in the synthesis of [Fe4Se4(SPh)4], this method has been well utilized in the synthesis of a number of iron thiolate/sulfide clusters, as well as their selenide counterparts (Equation (5)). More recently, sulfur- and selenium-containing lanthanide clusters (see Section 7.2.5.5) have been synthesized by the displacement of ER from Ln(ER)3 ... 

Other Applications. TBST is widely used as a thiolate ligand for the s)mthesis of various metal thiolates as exemplified by the formation of Zn(II) thiolate 31 (eq 13) and silver(II) complex 32 (eq 14). ... 

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