A crystallographic study of a red and a blue complex of copper(II) formed with different geometric isomers of a reinforced macrocycle

Abstract

Two complexes of copper(II), one red and one blue, with the macrocyclic ligand L³, have been prepared (L³ occurs as both the syn and anti form of 6-methyl-6-nitro-1,4,8,11-tetraazabicyclo-[9,3,2]hexadecane). A structural study of the two complexes shows the blue one to be [CuL³(NO₃)]ClO₄ and the red one to be [CuL³(H₂O)][ClO₄]₂. In the blue complex the nitro group on the ligand is syn to the ethylene part of the double bridge, while in the red complex it is anti. The structural study shows, further, that the co-ordination geometry around the copper is different in the two complexes: mean Cu–N distance (blue) 2.00 Å; (red) 1.99 Å; Cu–O to the axial oxygen 2.20 (blue), 2.34 Å(red); the copper atom lies out of the plane of the four N-donors by 0.34 (blue), 0.25 Å(red). The difference in colour is related to the difference in co-ordination geometry. The crystals of both complexes were orthorhombic, space group pnma, with Z= 4. Other details: (red)a= 18.683(2), b= 8.897(1), c= 13.558(2)Å and final conventional R factor of 0.0531; (blue)a= 20.417(5), b= 9.033(4), c= 10.870(2)Å and R 0.0941.