INTERACTION OF ENVIRONMENTALLY IMPORTANT METAL-IONS WITH NITROGEN SULFUR-DONOR MACROCYCLES - THE CRYSTAL-STRUCTURES OF 7-CO-ORDINATE LEAD(II) AND MERCURY(II) COMPLEXES OF AN 18-MEMBERED N4S2 MACROCYCLE

Metal-ion-templated cyclocondensations of two acyclic diamines contain ing nitrogen and sulfur heteroatoms with pyridine-2,6-dicarbaldehyde i n 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-imi ne interactions in the NMR spectra: they are rare examples of co-ordin ation compounds of these ions that exhibit satellites due to proton an d carbon-13 coupling to the naturally abundant metal nuclei with spin I = 1/2. The crystal structures of the complexes [PbL1(MeOH)(H2O)][ClO 4]2 1a and [HgL1(SCN)2]-MeOH2 have been determined. In both complexes the macrocycle adopts a stepped conformation with respect to the two p yridyl units; this conformation facilitates endo-dentate configuration s for the sulfur donors which interact through long bonds to the metal which, in turn, is located towards the trimethine part of the macrocy cle. The combination of the asymmetrical location of the metal ion wit hin the macrocyclic cavity and the long bonds to the sulfurs leaves th e second pyridine nitrogen remote and unco-ordinated. The metal geomet ry in 1 a is best described as a very distorted nido-hexagonal bipyram id in which the macrocycle donors occupy five of the six 'equatorial' sites and solvent molecules define severely bent 'axial' positions, im plicating a stereochemically active role for the 6s2 lone-pair electro ns on Pb(II). The structure of 2 is similar to that of 1a but the axia l positions are linear and occupied by thiocyanate ions; in contrast t o 1a, the mercury atom is displaced well out of the plane of the trime thine co-ordinating fragment. Both are rare examples of structural cha racterisations of the interaction of mixed nitrogen-sulfur-donor macro cycles with these toxic heavy-metal ions.