Solid state structural transformation of bromide coordination polymer to chloride by anion replacement; new precursors for preparation of PbBr₂ and PbCl₂ nanoparticles

Abstract

A reversible anion-exchange of 2D lead(II) coordination polymers with the ligand 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb), from 2D [Pb(4-bpdb)Br₂]_n (1) to 2D [Pb(4-bpdb)Cl₂]_n (2) coordination polymer by solid state anion-replacement processes under mechanochemical reactions, has been studied. The reversible solid state structural transformations of compound 1 to compound 2 by anion-replacement have been verified by PXRD and IR spectroscopy. PbBr₂ and PbCl₂ nanoparticles were obtained by thermal decomposition of compounds 1 and 2 in oleic acid as surfactant at 180 °C under an air atmosphere. These nanoparticles were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM).

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During the last two decades, the rational design and synthesis of novel coordination polymers has made considerable progress initiating a remarkable advance in the fields of supramolecular chemistry and crystal engineering.^1–7 The importance of coordination polymers is based not only on their intriguing structural motifs, but they also exhibit a range of potentially useful applications in catalysis, molecular adsorption, magnetism, nonlinear optics, luminescence, and molecular sensing. Studies on transformations involving anion replacement in coordination polymers are more recent.^8–20 It is well known that the anions may have a major influence on constructing novel network geometries.²¹ Solid state reactions by manual or mechanical grinding solid reactants together with either no added solvent or only nominal amounts for molecular synthesis have triggered lots of attention.^22,23 Mechanochemical synthesis, a burgeoning field in coordination polymers, has been utilized to synthesize various coordination polymers from the reactants without solvents or using liquid or ionic liquid assisted grinding (ILAG).^24,25 To develop further our understanding of the supramolecular architecture, it is challenging to continue the investigations on the transformations involving anion-replacement using mechanochemical manner.²⁶ During the course of the syntheses of the coordination polymers from ligand 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb), we observed the solid state structural transformations of 2D lead(II) coordination polymers by solid-state reversible anion-replacement, [Pb(4-bpdh)Br₂]_n (1) to [Pb(4-bpdh)Cl₂]_n (2). The ligand 4-bpdb was prepared by reported method.²⁷ Single crystals of compounds 1 and 2 were prepared by a branched tube method²⁸ from reaction between 4-bpdb and lead(II) nitrate with ratio (1 : 2) of KBr and KCl, respectively. The compounds 1 and 2 could be prepared by grinding of 4-bpdb and lead(II) nitrate with ratio (1 : 2) of KBr and KCl too. In mechanochemical manner compound 1 could be synthesized from grinding of row materials for 20 minutes in an agate mortar. Compound 2 could be synthesized from grinding of 1 mmol of compound 1 with 2 mmol of KCl, respectively, and these processes could be reversible by using of 2 mmol KBr for converting compound 2 to 1. For purification of coordination polymers with mechanochemical manner after each stage washing with water, three times, have been done until extra KBr, KCl or KNO₃ removed. Determination of the structures of compounds 1 and 2 by X-ray crystallography (Table S1 and Fig. S1 and S2†) shows interesting substantial structural changes on anion-replacement between compound 1 and 2. Compounds 1 and 2 are 2D coordination polymers and the lead(II) atoms are linked by two nitrogen atoms of 4-bpdb ligands and two Br anions in compound 1 and two nitrogens of 4-bpdb ligands and two Cl anions in compound 2 (Fig. S1 and S2†).

Crystals of 1 upon grinding with 2 mmol of solid KCl for 20 minutes in an agate mortar lead to formation of compound 2. These processes being accompanied without a color changes and reversible with grinding with 2 mmol of solid KBr (Fig. 1). Compound 1 and compound 2 both, crystallizes in the triclinic P1̄ space group and providing us with one of the examples of solid state structural transformations along with anion-replacement.

Fig. 1 A schematic diagram illustrating the structural conversions from 2D coordination polymer 1 (up) to 2D coordination polymers 2 (bottom) by solid state reversible anion-replacement.

The structural conversions from 2D coordination polymer 1 (up) to 2D coordination polymer 2 (bottom) by solid state reversible anion-replacement are shown in Fig. 1. In two polymers, each Pb^II ion is in the holo-directed geometry²⁹ and coordination numbers are same. The environment of lead(II) atoms is PbN₂Br₄ in compound 1, and PbN₂Cl₄ in compounds 2.

Reversible solid state structural transformations with anion-replacement from compound 1 to 2 were confirmed by powder X-ray diffraction patterns. The structures of the bulk materials for the compounds were confirmed by matching their powder X-ray diffraction patterns with those generated from the corresponding single-crystal structures (Fig. S3†). In the case of conversion 2 to 1, acceptable match was observed between the patterns simulated from single-crystal X-ray data (Fig. S3a†) and that measured by powder X-ray diffraction for the bulk crystalline sample as obtained from the synthesis of compound 2 with 2 mmol KBr (Fig. S3b†).

The same procedure was observed for transformation of 1 to 2. Those powder X-ray diffraction patterns shown in Fig. S3c and d.† These two reactions are reversible and polymers 1 is converted back to 2 by solid state grinding with 2 mmol KCl (Fig. S3d†) that have acceptable match with the patterns simulated from single-crystal X-ray data for compound 2.

To further confirm the reversible anion-replacement from compound 1 to 2 and 2 to 1, IR spectra were recorded (Fig. S4†). The IR spectra of compounds 1 and 2 are similar and the Cl⁻ anions in compound 2 were completely exchanged by Br⁻ anions (Fig. S4b†). Fig. S4d† shows that compound 1 could be converted to compound 2 with 2 mmol KCl grinding and Br⁻ anions in compound 1 were completely exchanged by Cl⁻ anions. Fig. S5† shows a schematic diagram for these solid state structural transformations.

To study the sufficiency of coordination polymers as suitable precursors for the syntheses of metal nanostructures materials,³⁰ coordination polymers, [Pb(4-bpdh)(Br)₂]_n (1) and the same samples after grinding with 2 mmol KCl, [Pb(4-bpdh)(Cl)₂]_n (2) used as precursors to preparation of lead(II) bromide and lead(II) chloride nanostructures by thermal decomposition in oleic acid as a surfactant, respectively. Fig. S6† provides the XRD patterns of the residues obtained from thermal decomposition of coordination polymers 1 and 2 in oleic acid at 180 °C under air atmosphere for 2 h. The obtained patterns match with the standard patterns of PbBr₂ and PbCl₂ which are the same as the reported values, JCPDS card numbers 31-067 and 26-1150, respectively. Fig. 2 shows the SEM images of PbBr₂ and PbCl₂ nanoparticles obtained by thermolysis of compounds 1 and 2 in oleic acid, respectively. This method for preparation of nano-scale materials may have some advantages such as: it takes place in shorter reaction times, produces better yields and also it may do not need special conditions like high temperature, long reaction times and pressure control.

Fig. 2 The SEM images of (a) PbBr₂ and (b) PbCl₂ nanoparticles prepared by thermolysis of compounds 1, 2 in oleic acid at 180 °C under air atmosphere for 2 h, respectively.

In summary, a 2D lead(II) coordination polymer [Pb(4-bpdb)(Br)₂]_n (1) polymerize on grinding the solid with 2 mmol KCl to form the 2D coordination polymers, [Pb(4-bpdb)(Cl)₂]_n (2) and this process could be reversible with grinding of compound 2 with 2 mmol KBr to produce compound 1. PbBr₂ and PbCl₂ nanoparticles were obtained by thermolysis of compounds 1, 2 in oleic acid as surfactant at 180 °C under air atmosphere, respectively. This work is one of the reports about conversion of a two-dimensional bromide coordination polymer to two-dimensional chloride polymer and shows one of the series for preparation of 2D coordination polymers by solid state reaction.

Acknowledgements

Support of this investigation by Tarbiat Modares University and the Iran National Science Foundationis (INSF) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, XRD patterns, IR spectra. CCDC 983875 and 983876. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra01579g

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