Spectroscopic studies on copper(II) complexes of chiral cyclens: [CuN4Cl] chromophores varying from square pyramidal to trigonal bipyramidal stereochemistry

A series of five copper(II) complexes with four geometrically isomeric ligands of 1,4,7,10-tetrabenzyl-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane and with (2R,5R,8R,11R)-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane has been investigated by means of ESR, CD and MCD in solution. X-ray studies reported separately had shown that the [CuN4Cl] chromophores were distorted to varying extents from square pyramidal to trigonal bipyramidal geometry. The stability and rigidity of the molecular structure have enabled us to carry out a complementary study of the crystalline state and solution chemistry. The ground state Kramers doublets varied from 3dx2−y2 to 3dz2 type with increasing distortion around the copper ion. The orbital sequence of the 3d1 positive hole, dx2−y2