Sulfur donor macrocycles

An electrospray mass spectrometric study of the interaction of manganese(II) salts with the tetrathia analogue of cyclam showed the utility of this technique for detecting complex formation for both manganese and a series of other metal cations. 

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Mixed nitrogen-sulfur donor macrocycles show relatively low affinities for lead(II) despite assumptions based upon hard-soft arguments. The ligands (187) and (188) complex lead with moderate efficiency. " " ... 

Reviews of synthetic procedures can be found for tridentate and pentadentate macrocyclic ligands with nitrogen donors, mixed nitrogen donors, and sulfur donor macrocycles, the techniques of which can be expanded to other ring sizes. The general procedures will be summarized below. 

Other donor systems. Although not as common as all-nitrogen donor rings, macrocycles incorporating sulfur donor atoms have been widely reported. Structures (42)-(45) illustrate four representative examples of this type. 

Ligand (119) yields nickel complexes of type [Ni2L](BF4)4 (low spin) and [Ni2L(NCS)4].2H20 (high spin). For each of these complexes, physical studies indicate that the macrocycle circumscribes the two nickel ions such that each ion is surrounded by four sulfur donors in a planar array. 

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

It has been recognized that sulfur donors aid the stabilization of Cu(i) in aqueous solution (Patterson Holm, 1975). In a substantial study, the Cu(ii)/Cu(i) potentials and self-exchange electron transfer rate constants have been investigated for a number of copper complexes of cyclic poly-thioether ligands (Rorabacher et al., 1983). In all cases, these macrocycles produced the expected stabilization of the Cu(i) ion in aqueous solution. For a range of macrocyclic S4-donor complexes of type... 

Complexes with mixed-donor macrocycles and with all-phosphorus and all-sulfur macrocycles 257... 

In Table 108 significant examples of nickel(II) complexes with mixed-donor macrocycles of various denticity are reported. Apart from the nitrogen atoms, the heteroatoms in the macrocyclic rings are usually either O or S, or in a few cases P. Few examples of nickel complexes with macrocycles containing all-sulfur or all-phosphorus donor atoms have been reported to date they are also included in Table 108. In nickel(II) complexes formed by mixed-donor penta- and hexa-dentate ligands the oxygen atoms of the macrocycle are often only weakly coordinated or are not coordinated at all. 

Classification of macromonocycles (Figure 2) can be made on the basis of donor atom, i.e. nitrogen, oxygen, sulfur, phosphorus, arsenic and mixed donor systems, as well as topicity, i.e. mono- versus di- or poly-topic. The planar nitrogen donor macrocycles and other tetraaza macro-... 

To further exemplify this methodology, let us take a typical example of the application of a template reaction as seen in the synthesis of a mixed N2S2 donor macrocyclic ligand 6.11. This compound is of interest to the co-ordination chemist as it possesses a potentially square-planar array of soft (sulfur) and harder (nitrogen) donor atoms. What sort of co-ordination chemistry is it likely to exhibit Will the hard or the soft characteristics dominate The most obvious route for the synthesis of 6.11 would involve the reaction of the dithiol 6.10 with l,2-bis(bromomethyl)benzene (Fig. 6-7). 

Macrocycles with 2 nitrogen and 3 sulfur donors have been prepared (14) by a template synthesis in which the dialdehyde (XCI) is condensed with primary diamines, e.g., ethylenediamine gives XCII in boiling acetonitrile containing Fe(II) perchlorate. The reaction is typical of template condensation between carbonyl compounds and primary amines. An unusual monanionic macrocyclic ligand was produced (2) when formaldehyde was condensed with the hydrazine (XCIII) instead of a primary amine in the presence of Ni(II) salts, and complexes XCIV have been characterized. 

Examples of the use of synthetic mixed donor macrocycles in heavy metal ion separations are found in the discrimination of silver from lead. A number of studies indicate that the inclusion of sulfur in macrocyclic sequestering agents shifts the discrimination to silver. An example of this is seen with (57) and (58). For the aza-oxa macrocycle (57) the log K is 5.9 for both silver and lead ions, while the thia-incorporated ligand (58) complexes silver more efficiently (log K = 9.9) compared to lead (log K = 5.7). ... 

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