Macrocyclic effect and conformational analysis of the mixed-donor macrocycle 12-ane N<inf>2</inf>O<inf>2</inf>by means of quantum chemical methods

Abstract

Minimal GLO basis set ab initio and CNDO/2 calculations have been performed for four possible conformations of 1,7-dioxa-4,10-diazacyclododecane (12-ane N2O2) and their Li(I) complexes, using experimental bond lengths and angles. The special complex stability enhancement of cyclic ligands, known as the macrocyclic effect, has also been investigated by comparison with open chain analogues. The results indicate a good agreement between CNDO/2 and ab initio methods in the prediction of the lowest energy conformation of the free ligand, which is the alternate from with torsional angles of 68° and 66°, respectively. For the Li(I) complex, the CNDO/2 method predicts as expected a wrong conformation. The 12-ane N2O2/Li(I) complex is more flexible than either tetraaza or tetraoxa analogues. In addition, the lesser flexibility of the ring of the free 12-ane N2O2ligand compared with that of macrocycles with only one sort of donor atom, reflects a higher energy requirement in arranging the donor atoms in a way suitable for complexation, leading to a lower net macrocyclic effect of the 'mixed-donor' macrocyclic ligand. © 1992.