Copper complexes bispidine ligands

THE TETRADENTATE BISPIDINE LIGAND DIMETHYL-(3,7-DIMETHYL-9-OXO-2,4-BIS(2-PYRIDYL)-3,7-DIAZABICYCLO[3.3.1]NONANE)-l,5-DICARBOXYLATE AND ITS COPPER(II) COMPLEX... 

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). 

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)... 

Figure 18-12. H-NMR spectra (200 MHz, CD3CN) of the isomeric copper(I) complexes of the tetraden-tate bispidine ligands L and (a) temperature dependent spectra of [Cu(L )(NCCH3)]+, (b) ambient temperature spectrum of the imsymmetrical four-coordinate form of [Cu(L XNCCH )]+ and (c) ambient temperature spectrum of [Cu(L )]+[39]...

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