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

Abstract

The macrobicyclic receptor L,(5) 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 L(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 L(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(L(5))](ClO(4))(2).0.5H(2)O (1) reveal that weak interactions between the lead(II) ion and the donor atoms of the receptor exist. (1)H and (207)Pb NMR spectroscopy studies demonstrate that monoprotonation of the receptor L(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 L(5) as a new lead(II) extracting agent. The X-ray crystal structure of compound [Pb(HL(5))(NO(3))][Pb(NO(3))(4)] (3) appears to be a good model for the monoprotonated intermediate of the demetalation process. In 3 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.

Understanding Lead Chemistry from Topological Insights: The Transition between Holo‐ and Hemidirected Structures within the [Pb(CO)n]2+ Model Series

“…[8][9][10][11] The aqueous chemistry of lead has been investigated for many years in order to develop, among others, water or soil clean-up processes, probes, sensors or sequestering agents. [12][13][14][15][16][17][18] Recently, [19] a theoretical modelling of the solvation of Pb 2 + has been reported and has provided a description compatible with the sole experimental data known. [20,21] The gas-phase chemistry of Pb 2 + is less known although a number of species have been experimentally detected.…”

Effect of Protonation and Interaction with Anions on a Lead(II) Complex with a Lateral Macrobicycle Containing a Phenol Schiff‐Base Spacer

“…These results confirm that addition of acid to a solution of 1 causes the demetallation of the complex together with the formation of the diprotonated form of the ligand, as previously observed for [Pb- (L 5 )] 2+ . [14] However, in the latter case the demetallation process has been shown to occur in two steps, with the formation Figure 8 shows the structure of the cation in 2. Bond lengths and angles do not show significant deviations from the expected values.…”

“…[13] In particular, we are interested in the design of selective complexant agents for Pb II based on crown frameworks. In a recent work [14] we have found that the macrobicyclic receptor L 5 (see Scheme 1), derived from 4,13-diaza-18-crown-6, incorporating a pyridinyl Schiff-base spacer, allows reversible and fast complexation of Pb II . This, together with the inertia of the receptor towards hydrolysis, opens very interesting perspectives for the use of this receptor as a new lead(II) extracting agent.…”

Protonated Macrobicyclic Hosts Containing Pyridine Head Units for Anion Recognition

“…A similar situation has been observed for different Schiff-base lateral macrobicycles derived from crown ethers. [31] Addition of excess CF 3 COOH (8 equiv) causes further changes in the 1 H NMR spectrum ( Figure 2). In particular, the proton signals of the pyridine units undergo a downfield shift of 0.12-0.16 ppm, which suggests that the third process involves the protonation of the pyridine nitrogen atom.…”