Replacement Reactions Involving Macrocycles

The study of the complexing of macrocycle ligands should be considered for its intrinsic importance rather than for its value in illuminating the mechanism of substitution. Kinetic (but much more thermodynamic °) data are available for the reactions of the different macrocycle ligand types, shown in Fig. 4.5, including azamacrocycles,crown ethers and cryp-tands, and porphyrins.  

Conventional, flow, temperature-jump, ultrasonic absorption, electric-field jump and nmr line broadening have all been used to measure the rates. UV-vis spectrophotometry and conductivity are the monitoring methods of choice. A variety of solvents have been used. The focus has been often on the dissociation since the dissociation rate constant appears in general to be the main controller of the overall stability. 

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Ligand replacements for which kinetic data have been reported include [Ni(dgen)] + plus edta (parallel dissociative and associative paths), [Pb(edta)] plus R-( —)-pdta, polyether complexes of lead(n) plus nitrogen macrocycles, metal(ii)-oxine complexes plus edta, and a variety of analogous reactions involving lanthanide and actinide complexes (see Chapter 10). Kinetic data are also available for ligand exchange between trien and tetren complexes of cad-mium(ii) and the edta complex of copper(ii). ... 

Examples of template procedures in which the ring-closing condensation involves reaction at a centre other than a donor atom have been documented. In an early synthesis of this type, the reaction of bis-(dimethylglyoximato)nickel(n) (62) with boron trifluoride was demonstrated to yield the corresponding complex of the N4-donor macrocycle (63) (Schrauzer, 1962 Umland Thierig, 1962). In this reaction the proton of each bridging oxime linkage is replaced by a BF2+ moiety. X-ray... 

Differences between results with edta and cydta are attributed to steric factors. Both steric and proton-transfer barriers lead to slow reaction of [Cu trigly)] with the macrocyclic ligand 1,4,7,10-tetra-azacyclododecane (4). A mixed-ligand intermediate is involved in the replacement of par (5) by cydta in [Cu -(par)]. Reaction rates are modified in the presence of such anions as acetate. 

The N2S4 macrocycle (50) forms a deep blue low-spin iron(II) complex, characterized by X-ray diffraction. One hopes that kinetic studies will follow. Kinetic studies of N4 macrocycle complexes [Fe(N4)LL ]" appear regularly, usually involving replacement of one or both of L, L. Reaction of phthalocyaninatoiron(II) with imidazole, in dimethyl sulfoxide solution, proceeds by consecutive first-order processes, the second some thousand times slower than the first. The N4 macrocycles (51) link this section with the previous section kinetics of reactions of [Fe(51)(MeCN)]2 with such ligands as 1-methylimidazole, pyridine, tributyl phosphite, tributyl phosphine, and carbon monoxide have been investigated. A series of complexes [Fe(51)L2], this time only for (51) with R = Me, has been studied with a... 

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