Bispidine ligands complex synthesis

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 piperidone precursor of 57 (see Chart 6) is a possible starting material for the synthesis of 2,6-substituted chiral, tetradentate bispidine ligands. The copper(II) complex of 57 is chiral [see Fig. 2(d) for a plot of the experimental structure] and has an interesting set of bond distances. Quite unexpectedly on the basis of the chelate ring sizes, the 2-pyridine substituent of the bispidine backbone is the axial donor (2.25 A), while N3 (2.08 A), N7 (2.05 A), the N7-pendant pyridine (2.01 A) and the chloride (2.28 A) are the in-plane donors (80). 

An interesting possible further extension is the functionalization of bispidine ligands with hydrophobic groups, for example, for metal ion selective extractions (69, 339). biopolymers for nuclear medicinal applications (340), solids for heterogeneous catalysis and sensors, or additional coordination sites for the synthesis of heterodinuclear complexes with applications in biomimetic chemistry, catalysis, and as luminescence sensors. There is a variety of possible sites for ligand modification. Of particular interest is the C9 position, which has been selectively and stereospecifically reduced to an alcohol (190), and the two hydrolyzed C1,C5 ester groups (167). 

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