Ligands rigidity

Best overall selectivity for all M /Mf couples is shown by compound 30, [2.2.2], However a given selectivity may be increased by structural modifications. For instance the K+/Rb+ selectivity increases markedly from 30 to 36. Indeed the introduction of the benzo ring is expected to decrease the cavity size (shorter O. .. O distance) and to increase ligand rigidity (rotation about the central C—C bond in the benzo bridge is frozen). Both effects hinder the cavity dilatation required for Rb+ inclusion on the other hand, they also favour the smaller Na+ cation with respect to K+, thus diminishing the K+/Na+ selectivity of 36 as compared with 30. 

The chemical nature of the link between two 2-pyridinecarboxamide parts of several iV,iV -bis(2-pyridinecarboxarnide) complexes of Mn and Fe has been systematically changed from ethyl (bpen, 20), to propyl (bppn, 21), cyclohexyl (bpch, 22), and phenyl (bpb, 23) to examine effects, such as influence of ligand rigidity upon degree of distortion and catalyst behaviour of the complexes encapsulated in the FAU topology.1135,1361... 

Ligand Rigidity and Elasticity of the Coordination Sphere / 648 Properties of Bispidine Complexes / 650... 

The copper coordination chemistry of bispidine ligands has been studied extensively, and this has been particularly rewarding (70, 71, 81, 82, 168, 169, 192, 194, 196, 199, 201, 213, 214). The main reasons are that (1.) the bispidine backbone is complementary with respect to copper(ll) and, therefore, complex stabilities may be relatively high, comparable to those with macrocyclic ligands (69, 201), and that modifications of the ligand backbone can be used to tune the stabilities and redox potentials (199) (2.) due to the ligand rigidity, the copperfll)... 

If liquid chromatographic techniques are to remain the premier analytical methods for the analysis of samples containing multiple lanthanides, what improvements could be made The obvious suggestion is the development of reagents or processes that interact with greater sensitivity to the lanthanide cation radius than hiba. We have shown above some examples of the effect of ligand rigidity on the selectivity of a... 

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