Interaction of environmentally important metal ions with nitrogen–sulfur-donor macrocycles. The crystal structures of seven-co-ordinate lead(II) and mercury(II) complexes of an 18-membered N4S2 macrocycle

Abstract

Metal-ion-templated cyclocondensations of two acyclic diamines containing nitrogen and sulfur heteroatoms with pyridine-2,6-dicarbaldehyde in the presence of the ions Pb^II, Hg^II, Cd^II and Ag^I afforded 1 : 1 complexes of the appropriate metal ion with the corresponding 18-membered, potentially sexidentate (N₄S₂), diimine macrocycle L¹ or L². The soluble lead, mercury and cadmium complexes show strong metal–imine interactions in the NMR spectra: they are rare examples of co-ordination compounds of these ions that exhibit satellites due to proton and carbon-13 coupling to the naturally abundant metal nuclei with spin I=½. The crystal structures of the complexes [PbL¹(MeOH)(H₂O)][ClO₄]₂1a and [HgL¹(SCN)₂]·MeOH 2 have been determined. In both complexes the macrocycle adopts a stepped conformation with respect to the two pyridyl units; this conformation facilitates endo-dentate configurations for the sulfur donors which interact through long bonds to the metal which, in turn, is located towards the trimethine part of the macrocycle. The combination of the asymmetrical location of the metal ion within the macrocyclic cavity and the long bonds to the sulfurs leaves the second pyridine nitrogen remote and unco-ordinated. The metal geometry in 1a is best described as a very distorted nido-hexagonal bipyramid in which the macrocycle donors occupy five of the six ‘quatorial’ sites and solvent molecules define severely bent ‘axial’ positions, implicating a stereochemically active role for the 6s² lone-pair electrons on Pb^II. The structure of 2 is similar to that of 1a but the axial positions are linear and occupied by thiocyanate ions; in contrast to 1a, the mercury atom is displaced well out of the plane of the trimethine co-ordinating fragment. Both are rare examples of structural characterisations of the interaction of mixed nitrogen–sulfur-donor macrocycles with these toxic heavy-metal ions.