Axial bonds in copper(II) compounds

Structures and spectroscopic properties of copper(II) compounds of the two cyclam derivatives L1 and L2 are reported. L1 has four 4-cyanobenzyl N-substituents and L2 has a fused barbiturate ring at one of the two propylene bridges. The copper(II) complexes of L1 ([Cu(L1)(NCCH3)]2+ and [Cu(L1)(OH2)]2+) have trans-I configuration and are five-coordinate with extremely short axial bonds. The reaction of [(Cu(L2)(OH2)2]2+ (purple, six-coordinate, trans-III configuration) with urea and NCS− leads to a blue five-coordinate, and with urea alone to a red four-coordinate species. The CuN4 chromophores of the three copper(II) compounds of L2 are close to identical, and the color changes are analyzed to be due to the differences in axial interactions.