Chelate rings isomerization

If one end of a chelate ring on an octahedral complex is detached from the metal, the five-coordinate transition state can be considered as a fluxional molecule in which there is some interchange of positions. When the chelate ring reforms, it may be with a different orientation that could lead to racemization. If the chelate ring is not symmetrical (such as 1,2-diaminopropane rather than ethyl-enediamine), isomerization may also result. For reactions carried out in solvents that coordinate well, a solvent molecule may attach to the metal where one end of the chelating agent vacated. Reactions of this type are similar to those in which dissociation and substitution occur. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

In six-membered chelate rings such as those formed by 1,3-propanediamine (tn), at least two conformational energy minima, which correspond to an achiral chair structure (23a) and the chiral skew- or twist-boat forms (25b) and (25c),166-168 occur and both have been observed in crystal structures.169 176 The possible conformational isomerism in tn complexes is therefore even more involved than in en complexes.177 In (OC-6-12)-[CoCl2(tn)2]+ (tratns-[CoCl2(tn)2]+), for example, there are two achiral bis(chair) forms (26a and 26b), two enantiomeric (chair, skew-boat) forms (26c and 26d), one achiral form (26e) and two enantiomeric bis (skew-boat) forms (26f and 26g). Several of these have been isolated in conformationally locked systems.178 180... 

Evidence for the existence of such equilibria comes from the eight-line spectrum obtained for the parent compound in 4-cyano-4 - -pentylbiphenyl, an inert liquid crystal. Bailey and Yesinowski35 found the only tenable rationalization for the fewer than expected 12-line spectrum was a rapid intramolecular rotation of one chelate ring with respect to the other, amounting to a degenerate cis-trans isomerization. 

Condensation of a monocarbonyl compound with a dihydrazone initially yields a macrocycle with a tetraaza six-membered chelate ring, e.g. (21 Scheme 8) or (23 Scheme 9), but this can isomerize to give a triaza five-membered chelate ring, as for (27 Scheme 10), where cyclization is by a reaction subsequent to the hydrazone/carbonyl condensation,21 or for the isomeric pair of compounds (25) and (26) of Scheme 9. Compounds with triaza (28) and tetraaza (22) seven-membered chelate rings have also been prepared.22 Tetradentate and pentadentate aza macrocycles are formed by condensations of 2,6-diacetylpyridine with hydrazine (29) or with dihydrazines (30).21... 

The drastic drop in the rate of epimerization of ortho-substituted derivatives compared to the unsubstituted or para-substituted complexes is ascribed to the steric effect of the ortho substituents. In an intramolecular isomerization, steric hindrance is expected to increase with the size of the ortho substituents (//, 92). For a dissociation reaction, or for chelate ring opening, a steric acceleration should be observed if the ortho substituents are bulkier. These substituent effects therefore support an intramolecular formulation of the epimerization reaction in Scheme 16. 

On the other hand, it can be expected that on the respective potential energy surfaces, there should exist a variety of the isomeric structures without the M-O bond. Two examples (14a, 14b) are shown in Figure 4-28. Thus, in order to understand the details of the mechanism, one has to consider the one-step chelate opening reaction by the monomer insertion, as well as the two-step process in which the M-0 bond is broken (chelating ring is opened) prior to the monomer insertion, and the insertion starts from the higher energy complex without a M-O bond. 

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