Bispidine ligands molecular structures

First transition metal coordination compounds with bidentate bispidine ligands were described in 1957 (30). The initial report with metal complexes of tetradentate bispidine ligands dates back to 1969 (31). Following these early reports, there have been a number of studies on the complexation properties of several bipidine derivatives (32-35). However, extensive, broad, and thorough studies of the bispidine coordination chemistry began only <10 years ago. These studies will be reviewed here. They include structural and theoretical work, spectroscopy, electron-transfer studies, metal ion selective complexation, and applications in biomimetic chemistry, catalysis, and molecular magnetism. 

The synthesis of the 2,4-bisphenol derivative of the standard tetradentate ligand 14 is quite tedious and so far has not been optimized (204). Apart from the fact that the noninnocent phenolate donors may lead to interesting ligand systems and metal complexes, it is the six-membered chelate rings involving the in-plane phenolate donors that lead to structural properties quite different from those of the other bispidine ligands [shown in Fig. 2(e) is a plot of a preliminary X-ray molecular structure of one of the possible conformers (meso form) of a copper(II) complex]. 

The rigidity of bispidine ligands has been analyzed on the basis of cavity size calculations with molecular mechanics [and density functional theory (DFT) — in order to check the accuracy and rehability of the force-field calculations] and a comparison of the computed and corresponding experimental structures and their analysis, based on the computed strain energy curves (42, 69, 189, 201). 

In a recent example, based on a copper(II/I) couple of a tetradentate bispidine ligand, ivith knoivn copper(II) and copper(I) structures, the molecular mechanics-based analysis, supported in this case by similar DFT calculations, using the above-described free energy perturbation-type approach, indicated that the inner-sphere reorganization energy must be rather small = 18.6kJ see... 

---