Macrocyclic Schiff base ligands

Complexation of macrocyclic ligands to lanthanide cations has been studied extensively [207,208], One main reason for the current interest in those macro-cyclic complexes are their intrinsic paramagnetic and luminescent properties. There is also the steadily increasing number of tailor-made macrocyclic ligands [209], This section will focus on complexes which contain macrocycles as discrete counterions and in particular on the coordination chemistry of phthalocyanine (Pc) and porphyrin (Por) ligands. Schiff base ligands which display another source of amine functionalities are usually not deprotonated under the prevailing reaction conditions [210]. 

Dioxygen binding in a dinuclear copper(I)-complex, based on a macrocyclic tetra Schiff base ligand 11, was reported by Martell and co-workers (scheme 5). 

Macrocyclic Schiff base compartmental ligands (750) (Robson-type ligands) derived from the [2 + 2] condensation of a 2,6-diformyl- or 2,6-diketo-substituted phenol and a diamine are very prominent in dinuclear Ni coordination chemistry.1901-1903 Particular interest lies in magnetic exchange interactions between the adjacent metal ions as well as in bioinorganic chemistry, where such dinuclear complexes have been proposed as synthetic analogues for bimetallosites. 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Similarly, by Schiff-base condensation reactions have been used to generate free cryptands from triamines and dicarbonyls in [2+3] condensation mode. These ligands react with silver(I) compounds to give dinuclear or trinuclear macrocyclic compounds where Ag Ag interactions may be present. Thus, with a small azacryptand a dinuclear complex with a short Ag- Ag distance (55) is found.498 With bigger azacryptand ligands also dinuclear complexes as (56) are achieved but without silver-silver interaction. 65,499-501 A heterobinuclear Ag1—Cu1 cryptate has also been... 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

The third class of metal catalysts includes nickel and cobalt complexes of Schiff bases and nitrogen macrocyclic ligands, which can form on electroreduction cobalt(I) and nickel(I) reactive intermediates for the activation of organic halides. 

The ligand reaction step may occur either with the template metal still intact or may take place after removal of the metal ion from the ring. As already mentioned, many of the Schiff-base macrocycles are unstable in the absence of a coordinated metal ion. However, for such systems, it has often been possible to hydrogenate the coordinated imine functions directly. The resulting saturated ligands will not be subject to the hydrolytic degradation which occurs for the imine precursors in the absence of their metal ion. 

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