Schiff base macrocycles

By judicious choice of reaction conditions an acyclic Ni11 complex (784) could be isolated, which serves as a valuable starting material for the preparation of unsymmetrical and mixed metal complexes by subsequent reaction with various amines. Also, a symmetrical Schiff base macrocycle of larger size has been obtained as a minor byproduct upon condensation of (784) with 1,3-diaminopropane. The resulting Ni11 complex (785) is again bimetallic, although room to bind four metal ions is in principle available.1367... 

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. 

Figure 5 Some examples of silver complexes with Schiff base macrocycles.

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. 

Conjugated Schiff base macrocycles containing a 1,3,4-oxadiazole moiety were prepared by [1+1] cyclic condensation <2002SC3339>. 

Finally, Schiff base macrocycles incorporating telluroether functions are obtained by condensation of fe(2-formylphenyl)telluride with 1,2-diamino-ethane (analogues with Se incorporated have also been prepared).8,114... 

Three macrobicycles 40b (216), 40h (212), and 40k (211) were characterized by X-ray crystallography. Fig. 31 displays the structure of 40h. The compounds adopt a highly folded conformation, reminiscent of calixarenes (23) and related Schiff-base macrocycles (181). 

Manganese(II) has been commonly employed as a templating metal ion for the synthesis of a wide range of other mixed donor (oxygen/nitrogen) Schiff base macrocycles (and/or their imine-reduced derivatives). 

Dicobalt species have also been shown to result in cobalt s possessing characteristically unstable oxidation states. A binary cofacial dicobalt porphyrin species has been shown to undergo a pair of one-electron oxidation steps as well as a pair of one-electron reduction steps [49] the two cobalt centers behave essentially independently of one another, so that mixed-valence complexes can be generated. Brooker and coworkers [50] investigated a pyridazine Schiff-base macrocycle with two cobalt centers (4), each of which could undergo a separate one-electron oxidation or reduction to produce a species capable of existing in five different combinations of redox states. Most interesting about this complex is the formation of cobalt... 

Macrocyclic ligands of biological importance as thiophenolate-containing Schiff-base macrocycles and their amine analogs (see review [138]) and new helical complexes with bis(bidentate) SchifF-base ligand [139] were also described. 

Fenton, D. E. Kitchen, S. J. Spencer, . M. Tamburini, S. Vigato, P. A. Complexes of ligands providing endogenous bridges. Part 5. Solution studies on a novel 3 + 3 hexamine Schiff-base macrocyclic complex of lanthanum. J. Chem. Soc., Dalton Trans. 1988, 685-690. 

The crystal structures of alkaline earth metal complexes of several (1 + 1) and (2 + 2) Schiff base macrocycles have been reported. These macrocycles are formed by the metal template-controlled condensation of the required heterocyclic dicarbonyl derivative and a, co functional diamine in alcoholic solution. (1 +1) complexes arise from the condensation reaction of one dicarbonyl with one diamine and (2 + 2) complexes from the condensation of two dicarbonyls with two diamines. 

The synthetic routes may often involve template directed condensations, a widely used reaction being the (carbonyl + amine) to imine condensation that efficiently leads to a variety of Schiff-base macrocycles [2.58-2.60, A.7, A.14], macrobicyclic cryptands [2.61-2.63] and lacunar cyclidene ligands [2.60, 2.64]. 

A vast range of Schiff base macrocycles and macrobicycles exist and have been of great importance in macrocyclic coordination chemistry, particularly of the transition metals, from the very beginning of supramolecular chemistry. The key Schiff base condensation reaction involves simply the reaction of an amine with an aldehyde to eliminate (condense) water and give an imine. If desired, the product may be reduced (e.g. with NaBH4) to give an amine or a secondary amine-based macrocycle (Scheme 3.21). 

We have already looked at three key Schiff base macrocycles (Figure 3.56), which were amongst the first artificial metal macrocycle compounds to be synthesised. These compounds are generally formed by thermodynamic template effects because, unless water is removed during the course of the reaction, the condensation is reversible, allowing complexation to sequester the most stable metal-product... 

Schiff base macrocycles and phthalocyanines are readily prepared and pre-date crown ethers and cryptands but are more suitable for binding transition metals or softer main group ions. 

One of the earliest Schiff base macrocycles to exhibit a haemocyanine-like structure was the copper(II) perchlorate complex of 5.5 which binds readily to azide or hydroxide.8 The azide complex exhibits two square pyramidal copper binding domains with the basal plane occupied by one pyridyl nitrogen atom and two imine functionalities as well as a terminal azide ligand. The apices of the two pyramidal coordination polyhedra are linked by a single bridging azide anion. Continuing the biomimetic theme, manganese (II) cascade complexes of the unsymmetrical 5.6 have... 

Figure 33 Structure of the Mn Schiff base macrocyclic complex 76. (From Ref. 163a.)...

A further extension of m-xylyl dicopper complexation and xylyl hydroxy la-tion is seen in a Schiff-base macrocycle [175], When dicopper(I) complex 38 is reacted with 02, one of the two arene rings is hydroxylated, again producing a phenoxo-bridged dicopper(II) species a peroxo-dicopper(II) intermediate is suggested as the actual oxidant. 

Reinhoudt and co-workers (101-105) have reported a series of Schiff base macrocyclic polyether ligand complexes prepared via barium cation-templated Schiff base condensation of the appropriate polyether dialdehyde with a diamine, in the presence of a transition metal or uranyl acetate, followed by removal of the Ba2+ template cation on subsequent addition of guanidinium sulfate (Scheme 19). The copperdl) and nickeldl) complexes (62) and (63) exhibit reversible redox couples... 

The macrocyclic ligand in the Schiff base macrocyclic complexes Ln(L1)3+ slowly hydrolyzes in water. Decomposition products of the lantha-nide(III) complexes of L1 and L2 macrocycles as determined by use of... 

Schiff base macrocyclic complexes of lanthanides have been prepared by a metal-template condensation of a diamine and 2,6-diacetyl or 2,6-diformyl derivative of pyridine or p-cresol [74]. The yield of these complexes depends on the radius of the metal ion and the donor ability of the counterion [75]. The acetate anion gave high yields while chloride and perchlorate anions gave poor yields [76]. In general the template synthesis... 

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