Mixed Sulfur-Nitrogen Macrocycles

Somewhat later, Travis and Busch reported that extraction of the residue from Eq. (6.8) with hot ethanol afforded the dimer of 10 in 25% yield . The latter, 1,4,8,11, 15,18,22,25-octathiooctacosane, was characterized by mass spectrometric analysis as well as the customary analytical methods. It was found that by dilution of reactants prior to mixing, the yield of [monomer] is greatly increased (50—60%) while the yield of [dimer] is lowered substantially . One might have expected the larger rather than the smaller ring to be more favored at higher dilution, but there is no further comment on this point. 

Ochrymowycz and his coworkers have also prepared a number of polysulfur macrocycles for use in biological or biological model systems . The synthetic methodology is essentially similar to that described above except that certain of the sulfur containing fragments were prepared by addition reactions to ethylene. Two examples of this approach, taken from ref. 59, are shown in Eq. (6.9). 

Although a large number of such compounds have been prepared, many of the mixed nitrogen-sulfur macrocycles are included in other tables, most notably in Chap. 4. Some have also been recorded in Chap. 8 as monocyclic precursors to bi- or polycyclic ligands (i.e., cryptands). The reader is directed to the tables of other chapters where the desired compound, if known, may be reported. 

Vbgtle and his coworkers have prepared a large number of compounds containing sulfur and nitrogen . An especially interesting type of compound is 11 which contains a pyridine subcyclic unit. This compound is prepared from one equivalent each of 2,2-dibromodiethyl ether and 2,6-dimercaptopyridine in a mixture of ethanol and 2-butanone. An excess (3 equivalents) of KOH served as base. Crystalline 11 (mp 168— 170°) was obtained in 9% yield. The approach is illustrated in Eq. (6.10). 

It is interesting to compare this approach with the method used by Newkome and his coworkers for the preparation of a similar material containing the pyrimidine sub-cyclic unit . In this case, 2,4-dichloropyrimidine was allowed to react with 1,2-eth- 

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Quite a number of mixed sulfur-nitrogen macrocycles have been prepared, but these have largely been by the methods outlined in Chaps. 4 and 5 for the respective heteroatoms. An alternative method, involves the formation of a Schiff base, followed by reduction to the fully saturated system, if desired. An interesting example of the Schiff base formation is found in the reaction formulated in (6.12). Dialdehyde 14 is added to ethylenediamine in a solution containing ferrous ions. Although fully characterized, the yield for the reaction is not recorded. To avoid confusion with the original literature, we note the claim that the dialdehyde [14] was readily prepared in good yield by reaction of the disodium salt of 3-thiapentane-l, 5-diol . The latter must be the dithiol rather than the diol. 

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). 

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. 

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-... 

Results from the correlation of stability constants in conjunction with redox data have led to insights regarding the coordination chemistry of thia macrocycles. For example, the electrochemical behavior of a number of copper(II)/(I) redox couples has been investigated, and redox potentials as well as protonation and stability constants of Cu species were determined for a number of tetradentate and pentadentate thia-derived macrocycles with thia- and mixed thia-aza rings with the basic backbones (51) and (52). Results of the examination of the stability constants in conjunction with the Cu redox potentials indicate that the stability constants for the Cu oxidation state are relatively constant regardless of the mixing in of nitrogen donor atoms. Hence, the dramatic increase in the Cu / redox potential which is observed in the presence of the sulfur macrocycles can be attributed to a destabilization of the Cu state rather than stabilization of the Cu state, contrary to popular belief from the hard-soft acid-base system. 

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