Cyclam complexes conformations

Figure 5-32. The methylation of a nickel(n) cyclam complex to give one specific conformer of the N, N, N", yV"-tetramethylcyclam complex.

Cyclam, or 1,4,8,11-tetraazacyclotetradecane, was first synthesised, inadvertently, by van Alphen in 1937 through the reaction of 1,3 bis (2 aminoethyl)-aminopropane with 1,3 dibromopropane. The first major studies of the ligand by Bosnich et al. used X-ray crystallography to elucidate different conformations of the metal complexes of the macrocycle.27 Their synthesis was similar to van Alphen s procedure, but did not involve a template method and as a result yields were unimpressive. They showed that five out of the possible ten different metal-complex conformers exist in the solid-state (Figure 3.51). 

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

In addition to the charge control over the reaction discussed above, there is also a marked element of conformational control over alkylation reactions. This is seen clearly in the methylation of the nickel(n) complex of the tetraaza macrocyclic ligand, cyclam (Fig. 5-32). Reaction of the nickel complex with methylating agents allows the formation of a A, A V",A "-tetramethylcyclam complex. In this product, each of the four nitrogen atoms is four-co-ordinate and tetrahedral, and specific configurations are associated with each. Of the four methyl groups in the product, two are oriented above the square plane about the nickel, and two below it. 

Such hydrido intermediates have been detected in the reactions of Cr2 +, 120,121 Cu +, 122 Fe2 +,123 and Ti3 +.124 The rates of formation of the hydrido species conform to the usual reaction patterns for Id substitution kinetics. An example of H-atom addition to metal complexes is provided by the reaction of [Ni(cyclam)]2 + and a Co (II) tetraazamacrocycle.82,125... 

Typical examples of the rich coordination chemistry of saturated tetraaza macrocycles are the complexes given by 1,4,8,11-tetraazacyclotetradecane, also known as cyclam, with nickel(II) (Table 103). Isomerism is expected to occur in the Ni-cyclam system due principally to the different configurations about the asymmetric coordinated secondary amines which, in turn, influence the possible conformations which can be adopted by the chelate rings. In the tram octahedral complexes [NiX2(cyclam)] (373 X = Cl, and [NiI(cyclam)]I-H20 ... 

The NO2 complexes of Cr exist in the nitrito conformation and several nitrito complexes of cyclam derivatives (86) and amine derivatives (87) have been reported. An example is the Pd 5-[Cr(mbc)(ONO)2]BF4 (mbc = 5,7-dimethyl-6-benzylcyclam) compound whose structure is... 

A similar effect could not be achieved by increased alkylation of the donor nitrogens. For example there is a dramatic drop in the formation constant when cyclam is methylated to give tetramethylcyclam as indicated in Table VI (5). This is due to extensive steric repulsion between the methyl groups, which also serve to force the complex formed by cyclam with Cu(II) into a less stable trans-l conformation (the steric repulsions between the carbon groups in the more stable trems-m conformation would be even greater), discussed in the next section. 

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