Sulfur-nitrogen macrocycles

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. 

Based on donor atom type, macrocyclic ligands can be considered to span two extreme types. First there are those systems which chiefly contain nitrogen, sulfur, phosphorus, and/or arsenic donors. These macrocycles tend to have considerable affinity for transition and other heavy metal ions they usually show much less tendency to form stable complexes with ions of the alkali and alkaline earth metals. The present discussion will be restricted to a consideration of a selection of such ligands and their complexes. 

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-... 

Two major areas of complexation have developed over the years with regard to synthehc macrocycles. Those with nitrogen, sulfur, phosphorus, and arsenic tend predominantly to form traditional covalent coordination complexes with transition metal ions. A notable exception to this tendency, however, is the rapidly expanding chemistry of the... 

The porphyrins constitute another class of nitrogen donor ligands which favor Ag(II) over Ag(I). Their macrocychc stmeture provides strong in-plane ligand fields, which facilitate oxidation of the metal. Phthalocyanine complexes of Ag(II) are very stable and can be oxidized electrochemically to Ag(III) further oxidation gives a Ag(III) phthalocyanine cation radical. The oxidation of Ag(I) complexes of sulfur macrocycles also leads reversibly to Ag(II) complexes. ... 

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. " " ... 

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. 

Azacrowns are those heteromacrocycles that contain one or more nitrogen atoms. Macrocycles containing only nitrogen or only nitrogen and sulfur are considered elsewhere in this volume so the present discussion focuses on compounds in which nitrogen replaces an oxygen atom in an otherwise standard crown ether. 

This section includes selected examples of electrochemistry of gold and silver complexes with other ligands and structural types that were not included in previous sections. We have also included a list of references divided into the following areas that can be consulted for additional information sulfur- and nitrogen-containing macrocycles , porphyrins " and clusters . 

Several other factors also play a role the presence of heteroatoms (oxygen, nitrogen, sulfur) in the chain or the presence of rigid groups (aromatic ring) usually favors the formation of macrocycles. 

Synthesis of macromonocycHc hydroxamic acids 13KGS1685. Synthesis of sulfur- and nitrogen-containing macrocyclic lactams and lactones 12MG212. 

Almost as soon as Pedersen announced his discovery of the crown ethers (see Chaps. 2 and 3) it was recognized by many that these species were similar to those prepared by Busch and coworkers for binding coinage and transition metals (see Sect. 2.1). The latter compounds contained all or a predominance of nitrogen and sulfur (see also Chap. 6) in accordance with their intended use. The crown ethers and the polyazamacrocycles represented two extremes in cation binding ability and preparation of the intermediate compounds quickly ensued. In the conceptual sense, monoazacrowns are the simplest variants of the macrocyclic polyethers and these will be discussed first. 

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