Ligand ring conformation

Young DA, Freedman TB, Lipp ED et al (1986) Vibrational circular-dichroism in transition-metal complexes. 2. Ion association, ring conformation, and ring currents of ethylenediamine ligands. J Am Chem Soc 108 7255-7263... 

The relatively low potencies of analgesics such as methadone and dex-tromoramide, also with dual aromatic features, are considered due to the fact that they cannot assume the critical conformation of rings A and F proposed for the potent derivatives. Conversely, however, it is difficult to account for the low orders of potencies (on the scale of the compounds of Fig. 13.9) of the 4-phenylpiperidines 35 that are capable of conversion to conformations appropriate to high activity (provided energy barriers may be overcome and/or compensated for) the derivative 36 presents little difficulty in this respect. There are clearly hazards in attempting the correlation of aromatic molecular features in ligands of disparate structure. 

This tendency of superphthalocyanine to contract to phthalocyanine may be attributed to the internal ring strain, the impaired macrocyclic n system, the inadequately large centre hole and the unstable coordination geometry. In (45a) the ligand ring cannot take a planar conformation but is buckled severely, and the uranium atom is centred in a distorted pentagonal bipyramidal structure with an average U—Ngp, distance of 2.52 A. 

The interpretation of the observed proton and C PCS led to the conclusion that the two structural isomers are enantiomeric pairs of diastereoisomers differing in the octadentate ligand conformation, namely, capped square antiprismatic (SA or M) and twisted capped square antipris-matic (TSA or m) geometries, respectively (Figure 5). " The Ln + coordination polyhedral of SA and TSA have two opposite parallel faces occupied by the ligand ring nitrogen... 

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