Bibrachial ligands from sterically protected tetra-azamacrocycles. Synthesis of bis(N-2-cyanoethyl)tet-a and the crystal structure of its copper(II) complex

The preparation of the ligand bis(N-2-cyanoethyl)-C-meso-5,7,7,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecan (L) is described. The ligand can be reduced with H-2/Raney nickel in the presence of NaBH4 to give the bis(N-3-aminopropyl) derivative. The red copper(II) complex [CuL](ClO4). 0.5 MeOH has been characterised by X-ray crystallography. The complex has v(C=N) at 2225 cm(-1) identical to that found in the free ligand, indicating that there is no interaction between the central metal ion and the pendant cyanoethyl groups. The crystal structure confirms that the copper(II) is four-coordinate and planar with the four ring nitrogens acting as donors. The Cu-N bond lengths are 2.068(6) and 1.980(5) Angstrom. The macrocyclic ligand has the centrosymmetric trans-III configuration with chair six-membered chelate rings and gauche five-membered rings. The methyl groups on C(3) and C(5) are equatorial.