Metal complexes of the new tetra-aza macrocyclic ligand 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane; carbon-13 nuclear magnetic resonance study of the complexes of Cd2+, Hg2+, and Pb2+ and X-ray crystal structure of the cadmium complex

Abstract

The new 16-membered tetra-aza macrocyclic ligand 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane (L⁴) has been prepared, and the metal complexes of Cd²⁺ and Hg²⁺ isolated and characterised. The Pb²⁺ complex was also investigated by ¹³C n.m.r. spectroscopy in nitromethane solution. Carbon-13 spectra of the Cd²⁺ and Hg²⁺ complexes revealed the presence of two isomers in methanol solution, including the complex with the unusual ‘three up and one down’ set of nitrogen configurations, together with the more usual ‘four up’ arrangement of the N-methyl groups. A crystal structure of the Cd²⁺ complex establishes the presence of these two structures in the solid phase. The crystals contain [CdL⁴(NO₃)]₂[Cd(NO₃)₄] and are monoclinic, space group C2/c, with a= 53.360(14), b= 10.248(6), c= 31.938(7)Å, β= 119.51 (2)°R= 0.063 for 4 920 diffractometer collected reflections with I/σ(I) 3.0. The cations have pseudo-trigonal-bipyramidal geometry, with bidentate nitrate groups occupying one equatorial position. They show disordered ligand conformations comprising a ‘three up and one down’ and two types of ‘all four up’ arrangements of the N-methyl groups. The anions are dodecahedral, with bidentate nitrate groups. Cd–N (axial) is 2.41 Å(average) and Cd–N(equatorial) is 2.37 Å(average), while Cd–O is the range 2.27–2.60 Å, with the anions showing greater asymmetry.