Nickel complexes 2- aminoethanethiol

Two primary types of template reaction have been identified and these demonstrate equilibrium template (see Box 5.4) and kinetic template effects. An example of an equilibrium template effect is provided by the reaction between butane-2,3-dione and 2-aminoethanethiol which produces, as its major product, a heterocycle in equilibrium with smaller amounts of the acyclic imine (Scheme 5.3). However, in the presence of Ni " ions the acyclic product forms in high yield as its nickel complex. 

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-... 

Figure 5-78. The nickel(n) complex of 2-aminoethanethiol reacts smoothly with biacetyl at nitrogen to give a diimine ligand.

Unfortunately, this macrocycle cannot be prepared as a free ligand by this method. The starting diimine 6.10 could apparently be prepared from 2-aminoethanethiol and biacetyl. However, we saw in Fig. 5-79 that the direct reaction of 2-aminoethanethiol with 1,2-dicarbonyls leads to a range of cyclic and acyclic products, rather than to products such as 6.10. However, we also saw in Fig. 5-78 that the nickel(n) complex (6.12) of the 6.12 could be obtained if the reaction was conducted in the presence of an appropriate salt. 

Condensation of ketones or aldehydes with the methyl ester of dithiocarbazic acid produces hgands with S,N,0 or S,N donor sets. An example is (142), which acts as both a didentate and a tridentate to ReO. Nickel(II)-templated reactions of pentanedione or salicylaldehyde with aminoethanethiol produce N,0,S ligands such as (143) and (144) these form simple mononuclear complexes or else dinuclear species with thiolate bridging two Ni centers. Mixed donor Schiff-base hgands including other heteroatoms such as P (145) and Te (146) have appeared. 

The thermodynamic or equilibrium template effect is based on the fact that in the absence of the template the desired product is formed, but with one or more (undesired) compounds that are in equilibrium with each other [42], A marked tendency of the product to form chelates enables a template to displace the product equilibrium in the desired direction. The first example of an equilibrium template effect was the formation of a nickel(II) complex of the Schiff base 33 formed from -aminoethanethiol 31 and the a-diketone 32 (Scheme 11) [43]. 

Nickel(II) complexes with various sulfur-nitrogen chelating ligands are reported in Table 100. 2-Aminoethanethiol (HL) is the simplest representative sulfur-nitrogen ligand acting as an anionic chelate. It forms two types of nickel(II) complex, namely the bis chelate [N1L2I and the trinuclear Nial Ct (333). Both complexes are square planar. Heterometallic complexes similar to (333), [MCNil W", have been reported with different metals such as M = Cu, Cu", Pd +, and Most of the complexes with 2-aminoethanetbiol have been... 

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