PREDICTION AND OBSERVATION OF THE STRUCTURE OF THE ZINC(II) COMPLEX OF 1,5,9,13-TETRAAZACYCLOHEXADECANE - A CRYSTALLOGRAPHIC AND MOLECULAR MECHANICS STUDY OF THE CONFORMATIONS ASSUMED BY THE MACROCYCLE WHEN COMPLEXED WITH METAL-IONS

The complex of the macrocycle 1,5,9,13-tetraazacyclohexadecane ([16]aneN(4)) with Zn-II, [Zn([16]aneN(4))][ClO4](2), has been synthesized and its crystal structure determined [monoclinic, space group P2(1)/n, a = 8.558(3), b = 13.997(4), c = 16.738(3) Angstrom. beta = 92.48(2)degrees, Z = 4; R = 0.0826]. The Zn-II ion shows a somewhat distorted tetrahedral co-ordination geometry with N-Zn-N angles ranging from 101.9(5) to 105.4(5)degrees within the chelate rings of the complex, and 122.8(5) and 123.4(4)degrees for the N-Zn-N angles outside of the chelate rings. The Zn-N bond lengths ranged from 1.97(1) to 2.03(1) Angstrom. A molecular mechanics analysis of [16]aneN, complexes shows that three types of conformers occur over the range of M-N bond lengths. At M-N bond lengths less than about 2.15 Angstrom a novel previously predicted conformer is of low energy with small tetrahedral metal ions, and is the predicted conformer observed here for the Zn-II ion. At longer M-N bond lengths, and for metal ions which do not readily assume tetrahedral co-ordination geometry, no low energy conformations exist, accounting for the fact that [16]aneN(4), complexes are generally of low thermodynamic stability. At intermediate M-N bond lengths a variety of conformers containing twist-boat conformation chelate rings are the most stable, while at longer M-N bond lengths the conformer actually observed in the Cd-II. Hg-II and Pb-II structures, which has a large cavity such that large metal ions with M-N bond lengths of 2.4 Angstrom fit best, becomes the most stable.