| Abstract
| - The macrobicyclic receptor L5,derived from 4,13-diaza-18-crown-6 incorporating a pyridinyl Schiff-base spacer, allows reversible complexation of lead(II), opening very interesting perspectives for the use of this receptor as a new lead(II) extracting agent.
- The macrobicyclic receptor L,5derived 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 theguest lead(II) ion may be modulated by external factors thanks to the optimal cavity size of L5 as well as the natureand distribution of its donor atoms. Both in solution and in solid state, the guest lead(II) is nearly centered into themacrobicyclic cavity of L5 when poorly coordinating groups such as perchlorate are present. The long Pb−donoratom distances found in the X-ray crystal structure of [Pb(L5)](ClO4)2·0.5H2O (1) reveal that weak interactionsbetween the lead(II) ion and the donor atoms of the receptor exist. 1H and 207Pb NMR spectroscopy studiesdemonstrate that monoprotonation of the receptor L5 moves the lead(II) ion to one end of the cavity, whereas itsdiprotonation causes the demetalation of the complex without receptor destruction. This demetalation process isreversible and very fast. All of this, together with the inertia of the receptor toward hydrolysis, opens very interestingperspectives for the use of receptor L5 as a new lead(II) extracting agent. The X-ray crystal structure of compound[Pb(HL5)(NO3)][Pb(NO3)4] (3) appears to be a good model for the monoprotonated intermediate of the demetalationprocess. In 3 the lead(II) ion is six-coordinate and clearly placed at one end of the macrobicyclic cavity, whichresults in a substantial shortening of the bond distances of the lead(II) coordination sphere.
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