Stereochemically active lone pair on lead(II) nano 3D supramolecular network: Synthesis,structural characterization,thermal stability and DFT calculation of [Pb(p-2-minh) 2 ]

1 Introduction

Supramolecular chemistry is defined by Jean-Marie Lehn as “...the chemistry of the intermolecular bond, covering the structures and functions of the entities by association of two or more species [1]. The design and synthesis of extended network materials has been an area of intense research over the past decade. Specifically, the porous nature of many of these materials makes them attractive for numerous applications [2–11].

Discrete coordination compound (DCC) involves the synthesis of discrete, supramolecular entities that exist both in solution and the solid state [12]. One of the more prevalent and effective design strategies has been that of the directional-bonding approach [12]. In this building block-based methodology, complementary subunits are reacted in the appropriate ratio to give a final aggregate whose size and structure are dictated by the molecular information stored within the precursors themselves.The angles between the bonding sites of the subunits and the composition of the subunits preprogram the overall topology of the resultant product. Reactions of this sort occur in a single, high-yield step and often utilize readily-available building blocks. This process thus allows for a high degree of “synthetic economy” over more laborious covalent syntheses and has shown itself broadly applicable, in some cases even leading to self-assembled species that fall within the domain of small proteins in terms of molecular weight [2]. Additionally, due to the modularity of the approach, various functional groups can be incorporated into the design of any given assembly, further augmenting its versatility and the potential for future applications [13–17].

Lead(II) frameworks have additionally attracted great interest because of lead’s large ion radius, a variable coordination number, and the possible occurrence of a stereochemically active lone pair of 6s² outer electrons as well as novel network topologies [18]. According to the hard-soft acid-base theory, the intermediate coordination ability of Pb(II) means that it can flexibly coordinate small nitrogen or oxygen atoms as well as large sulfur atoms [19]. The investigation of “stereo-chemical activity” of valence shell electron lone pairs in polymeric and supramolecular compounds may be more interested [20]. Lead(II) has an electronic structure [Xe]4f¹⁴5d¹⁰6s². Due to relativistic effects the 6s orbital is contracted and stabilized. As mentioned above, this stabilized 6s pair reduces its participation in the chemistry of the element (becoming an “inert-pair”) and this explains why inorganic Pb forms compounds in a lower oxidation state (less by two) than would be expected from its group number [21]. The apparent reticence of the 6s electrons to play a role in the chemistry of the element may also affect the stereochemistry of Pb(II) complexes. This influence can be understood in terms of simple hybridization or valence shell electron-pair repulsion arguments [22]. It seems that the 6s orbital, in spite of its stabilization, can hybridize with the 6p orbitals to give a “stereochemically active” 6s electron pair occupying one position in the coordination sphere of the metal. Because the pair is not directly detectable, its presence is normally identified by a void in the distribution of the coordination bonds (symmetrical coordination (hemidirected), see Scheme 1). If hybridization does not occur and the pair has only s character, then it is “stereochemically inactive” and the complex does not show a gap or void in the bond distribution (asymmetrical coordination (holodirected), see Scheme 1) [14].

As a continuation of the previous study [23–31], in this paper we report the mew asymmetrical lead(II) coordination compound in the presence of 2-pyridinecarbaldehydebisonicotinoylhydrazone schiff-base ligand describe a simple synthetic sonochemical preparation of nano-structures of this coordination polymer and its use in the preparation of PbO nanoparticles.

2 Experimental

3 Results and discussion

The reaction between the “p-2-minh” ligand with Pb(CH₃COO)₂·3H₂O led to the formation of the new lead(II) coordination compound [Pb(p-2-minh)₂] (1). Nano structures of 1 were obtained by ultrasonic irradiation in a methanolic solution, and single crystalline material was obtained using a heat gradient applied to a solution of the reagents (the branched tube method [36]). Scheme 2 gives an overview of the methods used for the synthesis of 1 using the two different routes.

The elemental analysis and IR spectrum of the nano-structure and the single crystalline material are indistinguishable. The selected spectral data and the corresponding data obtained from DFT calculations are given in Table 1. The FT-IR spectrum of the nano-structures and the single crystalline materials show the characteristic absorption bands of the “p-2-minh” ligand. The relatively weak band around 3064 cm^–1 is attributed to the absorption of the aromatic CH hydrogen atoms. The absorption band in the frequency range 1499–1634 cm^–1 correspond to aromatic rings vibrations of “p-2-minh” ligand. The absorption bands with strong intensity in the 1460 cm^–1 correspond to C = N of imine group of the “p-2-minh” ligands [35].

Figure 1(a) shows the XRD pattern of 1 simulated from single crystal X-ray data. The experimental XRD pattern of 1, prepared by using the sonochemical process, is shown in Fig. 1(b). Acceptable matches, with slight differences in 2Θ, were observed between the simulated and experimental powder X-ray diffraction patterns (Fig. 1(b)). This indicates that the compound obtained is a single crystalline phase and that this phase is identical to that obtained by the sonochemical approach. The obtained value is D = 28 nm, as estimated by using the Scherrer formula (D = 0.891λ/βcos Θ, where D is the average grain size, λ is the X-ray wavelength (0.15405 nm), and Θ and β are the diffraction angle and full-width at half-maximum of an observed peak, respectively).

Figure 2 shows the nano sheet structures that were observed by SEM. The mechanism of formation of these structures needs to be further investigated. However, it may be a result of the crystal structure of a compound that is a supramolecular structure; i.e., the packing of the structure on a molecular level might have influenced the morphology of the nano structure of the compound [23–31, 44–47].

The X-ray structure of 1 revealed the composition and stereochemistry of a fundamental building block with a formula of [Pb(p-2-minh)₂]. A view of the molecular structure of 1, together with a selected atom numbering scheme, is shown in Fig. 3.

The lead(II) atom is coordinated by tow oxygen atoms of tow “p-2-minh” ligands with similar Pb–O distances of 2.359 (2) Å, and four nitrogen atoms of tow “p-2-minh” with similar Pb–N distances of 2.552 (3) and 2.552 (2) Å in a three-fashion with a PbN₄O₂ donor set. Thus, the coordination number of the lead(II) atom is six. Massive asymmetry can be seen in the coordination sphere of the Pb centers.

This arrangement suggests a hole in the coordination geometry around the metal ions that is occupied possibly by a “stereo-active” lone pair of electrons on lead(II) [48]. The observed shortening of the Pb–O bonds on the side of the Pb^2 + ion opposite to the putative lone pair (2.359 (2) Å compared with 2.872 (2) Å adjacent to the lone pair) supports this possibility [48]. In many lead(II) compounds, such an environment leaves space for close contact with another atom(s) and usually the researcher finds any potential donor center [48–51]. However, in this structure, we observed the strong interaction between N8(s) of “p-2-minh” ligand(s) and the Pb center (Pb···N8 = 3.214(7), Pb-N8ⁱ = 3.214(7), Pb-N8ⁱⁱ = 3.219(7), Pb-N8ⁱⁱⁱ = 3.219(7), see Fig. 4). The distances between Pb–N are close to covalent bond.

There are C–H···N interactions and C–H···H interactions amongst the weak non-covalent contacts belonging to fragments of adjacent, distances values of these interactions (see Table 3) that suggest relatively strong interactions within this class of weak non-covalent contacts [52].

There are two different types of noncovalent π - π stacking interactions between the parallel aromatic rings belonging to adjacent chains, as shown in Fig. 5 The interplanar distance of the aromatic rings (Fig. 5) are 3.551 and 3.586 Å, appreciably shorter than the normal π - π stacking [52].

Consequently, the labile interactions also allow the the discrete molecular architecture interacts with neighbors and the structure extended to 3D supramolecular architecture (Fig. 6).

Thus, these factors, lone pair activity, labile and π - π stacking interactions, may control the coordination sphere of lead(II) ions in this complex. The obvious question then is whether the lone pair activity has stretched coordinate bonds to result in ligand stacking or whether it is the stacking interaction. However, one could say that the cooperative effect of the π - π interactions and the presence of the lone pair give a closer packing of the solved structure.

To examine the thermal stability of 1, thermogravimetric (TG) analysis and differential thermal analysis (DTA) were carried out between 10 and 780°C under argon flow (Fig. 7). The compound melts at 133°C. Then, the simultaneous removal of both “p-2-minh” ligands is observed exothermically in the range of 200–590°C, leading to formation of PbO.

4 Conclusion

In this work, sonochemical preparation of a novel nano lead(II) coordination compound containing pyridin-2-ylmethylene-isonicotinohydrazide ligand is described for first time, with a description of its use in the preparation of PbO nano-particles. The morphology of 1 prepared by the sonochemical method is composed of nano-sheets with a thickness of about 30 nm. The crystal structure of the complex has been also determined indicating that it takes the form of a discrete coordination compound in the solid state. With several supramolecular interactions the discrete unite interacts with neighbours and the structure extended to 3D supramolecular coordination polymer. A novel theoretical study of the title coordination compound was undertaken to examine its electronic structure, which allowed computations of bond distances and angles and vibrational frequencies in good agreement with the experimental data. The nano-structures of this coordination compound used as precursor in the preparation of PbO nanoparticles

Supplementary material

Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-1402297 for [Pb(p-2-minh)₂] (1). Copies of the data can be obtained by application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033, e-Mail: de-posit@ccdc.cam.ac.uk).