Macrocyclic ligand design. Effect of donor-set and ringsize variation on silver(I)/lead(II) discrimination within an extended series of dibenzo substituted rings

Abstract

The effect of systematic variation of the donor-atom set within dibenzo-substituted, 17- to 20-membered macrocycles on their ability to discriminate 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, NEt₄ClO₄), show that successive replacement of either nitrogen or oxygen heteroatoms by thioether donors 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-donor set, exceptional discrimination (of the order of 10⁹)was observed. The characterisation of selected complexes has included the crystal structure of the silver perchlorate complex of the 17-membered ON₂S₂macrocycle (L¹⁷). The silver ion in [AgL¹⁷]ClO₄ has a very distorted trigonal-bipyramidal geometry with all metal to donor bond lengths appearing normal. The complex crystallises in the monoclinic space group P2₁/n with lattice parameters a= 19.410(4), b= 9.602(2), c= 12.747(3)Å, β= 103.67(2)° and Z= 4.