Lateral Macrobicyclic Architectures:  Toward New Lead(II) Sequestering Agents

The macrobicyclic receptor <b>L</b>,<b></b>derived from 4,13-diaza-18-crown-6 incorporating a pyridinyl Schiff-base spacer, forms stable complexes with lead(II) in the presence of different counterions. The coordination environment of the guest lead(II) ion may be modulated by external factors thanks to the optimal cavity size of <b>L</b>⁵ as well as the nature and distribution of its donor atoms. Both in solution and in solid state, the guest lead(II) is nearly centered into the macrobicyclic cavity of <b>L</b>⁵ when poorly coordinating groups such as perchlorate are present. The long Pb−donor atom distances found in the X-ray crystal structure of [Pb(L⁵)](ClO₄)₂·0.5H₂O (<b>1</b>) reveal that weak interactions between the lead(II) ion and the donor atoms of the receptor exist. ¹H and ²⁰⁷Pb NMR spectroscopy studies demonstrate that monoprotonation of the receptor <b>L</b>⁵ moves the lead(II) ion to one end of the cavity, whereas its diprotonation causes the demetalation of the complex without receptor destruction. This demetalation process is reversible and very fast. All of this, together with the inertia of the receptor toward hydrolysis, opens very interesting perspectives for the use of receptor <b>L</b>⁵ as a new lead(II) extracting agent. The X-ray crystal structure of compound [Pb(HL⁵)(NO₃)][Pb(NO₃)₄] (<b>3</b>) appears to be a good model for the monoprotonated intermediate of the demetalation process. In <b>3</b> the lead(II) ion is six-coordinate and clearly placed at one end of the macrobicyclic cavity, which results in a substantial shortening of the bond distances of the lead(II) coordination sphere.