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

Abstract

The complex of the macrocycle 1,5,9,13-tetraazacyclohexadecane ([16]aneN₄) with Zn^II, [Zn([16]aneN₄)][ClO₄]₂, has been synthesized and its crystal structure determined [monoclinic, space group P2₁/n, a= 8.558(3), b= 13.997(4), c= 16.738(3)Å, β= 92.48(2)°, 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)° within the chelate rings of the complex, and 122.8(5) and 123.4(4)° 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)Å. A molecular mechanics analysis of [16]aneN₄ complexes shows that three types of conformers occurs over the range of M–N bond lengths. At M–N bond lengths less than about 2.15 Å 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]aneN₄ 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 Å fit best, becomes the most stable.