MACROCYCLIC LIGAND DESIGN - EFFECT OF DONOR-SET AND RING-SIZE VARIATION ON SILVER(I) LEAD(II) DISCRIMINATION WITHIN AN EXTENDED SERIES OF DIBENZO SUBSTITUTED RINGS

The effect of systematic variation of the donor-atom set within dibenz o-substituted, 17- to 20-membered macrocycles on their ability to disc riminate between silver(I) and lead(II) has been investigated. The log K values for the respective complexes. determined potentiometrically in 95% methanol (l = 0.1 mol dm(-3), NEt(4)ClO(4)), show that successi ve replacement of either nitrogen or oxygen heteroatoms by thioether d onors leads to increasing affinity for silver(I) relative to lead(II), as does the incorporation of an 'aliphatic' NON or NSN donor sequence in the structure. For the 17-membered system incorporating a S2N2S-do nor set, exceptional discrimination (of the order of 10(9)) was observ ed. The characterisation of selected complexes has included the crysta l structure of the sliver perchlorate complex of the 17-membered ON2S2 macrocycle (L(17)). The silver ion in [AgL(17)]ClO4 has a very distor ted trigonal-bipyramidal geometry with all metal to donor bond lengths appearing normal. The complex crystallises in the monoclinic space gr oup P2(1)/n with lattice parameters a = 19.410(4), b = 9.602(2), c = 1 2.747(3) Angstrom, beta = 103.67(2)degrees and Z = 4.