Macrocyclic complexes group

Table 1 Hsts a number of chelating agents, grouped according to recognized stmctural classes. Because systematic nomenclature of chelating agents is frequently cumbersome, chelants are commonly referred to by common names and abbreviations. For the macrocyclic complexing agents, special systems of abbreviated nomenclature have been devised and are widely used. Some of the donor atoms involved ia chelation and the many forms ia which they can occur have been reviewed (5).

Reaction of the Ru macrocyclic complex [RuLCy (L= 1,5,9,13-tetramethyl-l,5,9,13-tetraaza-cyclohexadecane) with N02 results in a disproportionation of the initial [Ru °LCl(N02)], the final products being traTO-[Ru L(0)Cl]" " and [Ru L(OH)(NO)] " "." The reaction between [Ru(OEP)Me] (H2OEP = octaethylporphyrin) and 2,2,6,6,-tetramethylpiperidine-l-oxyl (TEMPO) produces [Ru(OEP)CO]. There is clear evidence that the CO ligand is derived from the axially bound CH3 group, making this reaction an important example of CH3 to CO transformation." ... 

Currently, Ni(I) macrocyclic complexes have attracted much attention. This is because Ni(II) tetraaza macrocyclic complexes catalyze the electrochemical reduction of C02 and alkyl halides, and it is proposed that the Ni(I) species are involved in such reactions (1,2, 76-79, 82, 124-126). Furthermore, F430, a Ni(II) hydrocorphinoid complex, is a prosthetic group of methyl coenzyme M reductase that catalyzes the reductive cleavage of S-methyl coenzyme M to methane in the final stage of C02 reduction to methane (127-130). An EPR signal detected in whole cells of Methanobacterium thermoautotrophicum has been attributed to an Ni(I) form of F430 in intact active enzyme (131,132). 

Ni(DMG)2 reacts with BF3 etherate to substitute the hydrogen atoms of oxime groups by BF2 moieties forming a macrocyclic complex. Two such molecules are held together in the... 

The situation with pendant phenol groups is much less ambiguous. The crystal structure of the nickel(II) complex [Ni H 1[14]ane-N4C6H5OH]+ (Fig. 5) shows it to be a square-pyramidal, high-spin species and the reduction potential of the nickel(III) complex is 0.15 V lower than that of the parent saturated 14-membered macrocyclic complex (100). Strong coordination by phenolate oxygen is even more... 

A variety of macrocyclic complexes which have adjacent nitrogen atoms (cyclic hydrazines, hydrazones or diazines) are formed by condensations of hydrazine, substituted hydrazines or hydrazones with carbonyl compounds. The reactions parallel in diversity those of amines, but are often more facile since the reacting NH2 groups is generally not coordinated and the electrophile is thus not in competition with the metal ion. The resulting macrocycles may be capable of coordination isomerism, since either of the adjacent nitrogen atoms can act as donor atom. 

Metal template syntheses of complexes incorporating the p-amino imine fragment have been introduced by Curtis as a result of his discovery that tris(l,2-diaminoethane)nickel(II) perchlorate reacted slowly with acetone to yield the macrocyclic complexes (40) and (41) (equation 8).81-83 In this macrocyclic structure the bridging group is diacetone amine imine, arising from the aldol condensation of two acetone molecules. This reaction is widely general, in the same way that the aldol reaction is, and can be applied to many types of amine complexes. The subject has been reviewed in detail with respect to macrocyclic complexes by Curtis.84... 

The metal ion does, however, introduce a new subtlety into these reductions. The reduction of the two imine groups in the nickel(n) complex 4.10 is readily achieved with Na[BH4], The free tetraamine ligand would be expected to exhibit a facile pyramidal inversion at each nitrogen atom, whereas in the nickel(n) complex this inversion is not possible without significant weakening (or breaking) of the Ni-N bonds. In macrocyclic complexes it is very often found that the complex obtained by the reduction of a co-ordinated imine does not possess the same stereochemistry as that obtained by the direct reaction of the free amine with metal ion. 

Figure 5-83. The formation of a macrocyclic complex by the formation of S-S disulfide bonds. The co-ordination of the halide leaving groups to the metal is common in reactions of this type.

Let us start by considering the reaction of the copper(n) complex 6.49 with formaldehyde. Initially we might expect the diimine 6.50 to be formed, but this ignores the nature of the intermediates. As we saw earlier, the reaction of an amine with an aldehyde initially produces an aminol. Consider the addition of the second molecule of formaldehyde to 6.49. The product will be 6.51, which contains an imine and an aminol (Fig. 6-43). The imine is co-ordinated to a metal ion, and the polarisation effect is likely to increase the electrophilic character of the carbon. The hydroxy group of the aminol is nucleophilic and it is correctly oriented for an intramolecular attack upon the co-ordinated imine. The result is the formation of the copper(n) macrocyclic complex 6.52. 

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