Steric effects in coordination of 1,1,1-tris(dimethylaminomethyl)ethane and of 1,1,1-tris(monomethylaminomethyl)ethane—II. Zinc iodide and cadmium iodide complexes

Abstract

The coordination characteristics of the ligands 1,1,1-tris(monomethylaminomethyl)ethane (TSN) and 1,1,1-tris(dimethylaminomethyl)ethane (TTN) have been investigated by the preparation and characterization of their zinc iodide and cadmium iodide complexes. The structures of the complexes were established from 1H nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy and complementary physical-chemical studies. The zinc iodide complexes TSNZnI 2 and TTNZnI 2 are isomorphous with their chloro and bromo analogues, which indicates that the size of the halo group has no effect on the stereochemistry. The ligands in all of these zinc complexes are bidentate; the bound and free “legs” of the TSN ligands in these complexes exchange rapidly on the NMR time scale, while those of the TTN ligands do not. In contrast, cadmium iodide forms an ionic complex, [TSNCdI][CdI 3], and a covalent complex, TTNCdI 2, which differ stereochemically from the LZnX 2 complexes. In both cadmium complexes, the ligands are tridentate, due to a combination of the larger size of cadmium and the ability of these tripod systems to distort outwards about the C 3 ligand axis, thus lessening the steric interactions between the adjacent donor centres. Consequently, the cation [TSNCdI] + and the neutral complex TTNCdI 2 have bicyclic, cage-like structures featuring tetra- and pentacoordinated cadmium centres, respectively.