Square-planar nickel macrocyclic complexes synthesis

The impressive sulfur-based reactivity of square planar nickel complexes containing tetradentate N2S2 ligands has been known for many years. Interest has recently resurfaced because of the discovery of similar donor sites in metalloproteins that bind nickel, iron, and cobalt.The iV,iV -bis(mercaptoethyl)-l,5-diazacyclooc-tane ligand H2(BME-D ACO) and its nickel complex have been particularly useful in establishing the scope of S-based reactivity with electrophiles as displayed in the reaction summary shown in Scheme 1." The fundamental features of this reactivity include templated macrocycle production, S-oxygenation as contrasted to oxidation, Lewis acid/base adduct formation, metal-ion capture, and the synthesis of heterodi- and polymetallic complexes. ... 

The nickel ) complex of 92 cannot be prepared directly via the template method, but can be prepared by a transmetallation procedure. Synthesis of the macrocycle in the presence of one of the metal ions known to be effective as a template is followed by a metal exchange process in solution to insert the nickel ) ion. This cation exhibits a strong preference for the square planar, square pyramidal, and octahedral geometries 79). Thus the failure of the nickel ) cation to behave as a template ion in the synthesis of 92 is probably due to the disinclination of the metal to accommodate the pentagonal array of donor nitrogen atoms necessary for reaction to occur. 

Thus, in examples of synthesis of tetraazamacrocyclic compounds on nickel ions (mainly) and on copper(II), square-planar system assembling is realised [38,47-49]. Guanidium ion promotes building of 27-crown-9-ether [60]. o-Aminobenzaldehyde self-condensation on the copper ion as matrix results exclusively in forming a complex with TAAB [67]. When using nickel(II) or cobalt(II) as templates under the same conditions, two types of macrocyclic azomethine systems - TAAB and TRI - may be synthesised [67-71]. Macrocychsation of phthalonitrile on the anisotropic matrix 0=U=0 ends with obtaining the so-called superphthalocyanine product [U02(L29)] [72, 73], rather than with the isolation of the corresponding complex with the phthalocyanine (Pc), as observed for other metal ions (Eq. 1.18) [11,74]. 

---