7-Membered chelate ring conformations

Structural Characterization. Compound 70a crystallized in the triclinic space group PI with two crystallographically independent molecules in the asymmetric unit, each with similar gross conformations, Fig. 12. The five-membered chelate ring on each molecule is essentially planar with the... 

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 a metal complex (102), five-membered chelate rings fonr d by 1,2-dia-mines exist in two puckered conformations, the enantiomeric X and 5 forms 36 and 37. For an achiral ligand in a bis or tris complex, steric interactions between rings are present, and the two conformations are no longer equivalent energetically. 

This result does not really contradict the structure which we previously proposed (51), for CPK-model study shows that both gauche-trans and gauche-gauche conformers between the methin and methylene protons can exist in the 5-membered chelate-ring constructed as follows ... 

In the square pyramidal complex [Ni(Me4cyclam)N3](C104) (379) (Me4cyclam is 1,4,8,11-tetramethylT,4,8,ll-tetraazacyclotetradecane) the coordinated azide anion is on the same side of the four methyl groups with the nickel atom 33 pm from the N4 plane. The five- and six-membered chelate rings are in the usual gauche and chair conformations respectively.2681 This complex is labile in comparison with the very inert dinuclear complex... 

Figure 29 Conformation of the six-membered chelate rings in the racemo forms (A) and meso form (B) of...

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... 

Figure 13 Preferred conformations of six-membered chelate rings containing an asymmetric carbon centre of (R) absolute configuration...

Rather more theoretical work has been done with respect to tris(ethylenediamine) complexes. By considering the two stable chiral (skew) conformations of the five-membered chelate rings (designated k and S as above), four unique conformers of M(en)"+ can be constructed for each absolute configuration (A or A) (12) kkk, kkS, kSS, and 886. In this representation, A(1U) is energetically equivalent to A(<5<5<5). 

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