Supramolecular architectures and structural diversity in a series of lead (II) Chelates involving 5-Chloro/Bromo thiophene-2-carboxylate and N,N’-donor ligands

Background Lead is a heavy toxic metal element in biological systems and is one of the major pollutants as a result of its widespread use in industries. In spite of its negative roles the coordination chemistry of Pb(II) complexes is a matter of interest. The N,N’-bidentate aromatic bases such as BPY,4-BPY and PHEN (BPY = 2,2′bipyridine, 4-BPY = 4,4′-dimethyl-2,2′-bipyridine, PHEN = 1,10-Phenanthroline) are widely used to build supramolecular architectures because of their excellent coordinating ability and large conjugated system that can easily form π-π interactions among their aromatic moieties. A series of novel Pb(II) complexes in concert with 5-CTPC, 5-BTPC (5-CTPC = 5-chlorothiophen-2-carboxylate, 5-BTPC = 5-bromothiophen-2-carboxylate) and corresponding bidentate chelating N.N′ ligands have been synthesized and characterized. Results Five new Pb (II) complexes [Pb(BPY)(5-CTPC)2] (1), [Pb(4-BPY)(5-CTPC)2] (2), [Pb2(PHEN)2(5-CTPC)4] (3), [Pb(4-BPY)(5-BTPC)2] (4) and [Pb2(PHEN)2(5-BTPC)2(ACE)2] (5) have been synthesized. Even though in all these complexes the molar ratio of Pb, carboxylate, N,N-chelating ligand are the same (1:2:1), there is a significant structural diversity. These complexes have been characterised and investigated by elemental analysis, IR, 1H-NMR,13C-NMR, TGA, and photoluminescence studies. Single crystal X-ray diffraction studies reveal that complexes (1, 2) and (4) are mononuclear while (3 and 5) are dinuclear in nature which may result from the chelating nature of the ligands, various coordination modes of the carboxylates, and the coordination geometry of the Pb(II) ions. Conclusions The observation of structures 2,4 and 3,5 show the structural changes made just chloro/bromo substituent of the thiophene ring. A detailed packing analysis has been undertaken to delineate the role of valuable non covalent interactions like X…π, H…X, (X = Cl/Br). A quadruple hydrogen bond linking the monomeric units and generating a supramolecular architecture is observed in (1). The metal bite unit comprised of PbN2C2 (i.e. Pb-N-C-C-N-Pb) is the repeating unit in all the five complexes and they have almost same geometrical parameters. This metal bite has been identified as the self assembly unit in complexes.


Background
The molecular self assembly with discrete supramolecular units directed by metal ligand coordination and weak interactions are of considerable interest [1][2][3][4][5][6]. The interplay of coordination motifs and supramolecular synthons are of current interest to generate novel materials (metal organic frameworks (MOF's), coordination polymers, inorganic hybrid materials, etc.). When a sensible choice of the central metal ion and a ligand whose favourite coordination modes are known one can design an anticipated variety of supramolecular motifs [6][7][8][9][10].
Lead is a heavy toxic metal element in biological systems and is one of the major pollutants as a result of its widespread use in industries. In spite of its negative roles the coordination chemistry of Pb(II) complexes is a matter of interest, which display interesting structural features as a consequence of the large radius, adoption of different coordination numbers from 2 to 10 and especially based on the extent to which the lone pair is stereo chemically active [11][12][13][14][15]. A common strategy followed in designing of the Pb(II) complexes is the presence of carboxylate and bipyridne systems in the coordination sphere [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Lead(II) carboxylate systems containing mono, di and polycarboxylate have been extensively used in the construction of metal organic networks [23][24][25][26][27][28][29][30][31][32][33]. The N,N'bidentate aromatic bases such as BPY,4-BPY and PHEN are widely used to build supramolecular architectures because of their excellent coordinating ability and large conjugated system that can easily form π-π interactions among their aromatic moieties, which are either intra or intermolecular [16][17][18][19][20][21][22]. The thiophene carboxylates possess various coordination modes such as monodentate, bidentate chelating, bidentate bridging when coordinated to the metal ion (Scheme 1). The rigid heterocyclic five member thiophene ring possesses distinct physical and chemical properties [34,35]. Also in this heterocyclic thiophene ring the relatively bigger size of the sulphur atom makes the possibility for delocalisation of its lone pair of electrons, which results in good charge transfer ability of the ligand [35].
Our strategy is to investigate the chelating behaviour of these ligands towards lead (II) along 5-CTPC and 5-BTPC. In all the five complexes BPY, 4-BPY and PHEN act as a nitrogen donor ligand forming 3D supramolecular architectures in the solid state. Although there are some reports of diverse coordination polymers involving multicarboxylate ligands containing S atom and bidentate chelating ligands like PHEN, 2-BPY as well as bridging ligand such as 4-4′-bipyridine as coligands [36][37][38][39][40][41], there are only a few reports involving the halo substituted thiophene carboxylate ligand for the construction of organic-inorganic hybrids [42]. 5-CTPC not only shows versatile coordination modes but also exhibits non covalent interactions like Cl…π and C-H…Cl [42][43][44][45]. In our present investigation of five Pb(II) complexes, we have observed the different coordination number, and different coordination modes of the carboxylate ligand which gives the idea of constructing different supramolecular architectures. The coordination modes of the carboxylate group, observed in these

Results and discussion
Geometry around lead The lone pair of electrons has a great influence on the structure of the complex [11,47,48]. In the coordination chemistry of the Pb(II) ion, the terms holo and hemi directed are used to describe the geometries around the central Pb atom. Pb(II) complexes where the bonds to ligand atoms are placed throughout the surface of the encompassing globe are said to be holo directed, while hemidirected refers to those cases in which the bonds to ligand atoms are directed throughout only part of an encompassing globe [49]. The coordination numbers of lead in complexes 1 and 2 are six, and they exhibit a pentagonal pyramidal geometry. While the coordination numbers of 4 and 5 are five and seven respectively, and they show distorted square pyramidal and distorted pentagonal bipyramidal geometry respectively. Comparison of coordination modes of the carboxylates are listed in (Table 1). The structure 3 shows an unusual capped square pyramidal geometry. In complex 1, the axial position of the pentagonal pyramid is occupied by the N1 of the BPY by which it differs from 2 where this position is occupied by O4 of the 5-CTPC ( Figure 1). In 4 the axial position is occupied by O3 of the 5-BTPC. The coordination geometry of complexes 1-3,4 as well as the Pb-O and the Pb-N bond directions show a gap around the Pb (II) ion, occupied possibly by a stereoactive lone pair of electrons on lead(II). The coordination around the lead atoms is hemidirected with a significant gap trans to chelating BPY, 4-BPY and PHEN ligands ( Figure 1). In 5 PHEN is a space demanding ligand upon coordination with the Pb(II) ion it forms a seven coordinated holodirected distorted pentagonal bipyramidal complex instead a hemidirected one with less coordination number. The hemidirected geometry is the most preferred for intermediate coordination numbers between 6-8 [50].
There is a π-π stacking interaction, the parallel aromatic rings belonging to adjacent chains in complexes 1, 2 and 3 that may help to increase the 'gap' in the coordination geometry around the Pb(II) ion.
Although the coordination geometry of lead (II) ion is same for both complexes 1 and 2, they differ in the atoms   Figure 6). Thus these interactions forms I shaped dimers ( Figure 6). This I shaped dimer is nailed to the two other I shaped dimer on its adjacent sides by two sets of π -π stacking interactions (Cg1 → Cg3 iv on one side and Cg3 → Cg4 iii , Cg4 → Cg3 iii on the other side). Another pi-pi stacking is found between the two six member rings of the two adjacent 4-BPY ligands.

Crystal structure description of [Pb(4-BPY)(5-BTPC) 2 ] (4)
Complexes 2 and 4 differ only by the chloro/bromo substitution in the thiophene ring. Hence they are near isomorphous with similar cell parameters (Table 3). Regarding the coordination geometry, in 4 one carboxylate is bidentate chelating while the other is in monodentate fashion. Whereas in 2 both are in bidentate fashion. Thus in 4 the Pb(II) ion is pentacoordinated, with two N atoms of the (4-BPY), one O atom of the bidentate chelating carboxylate and one O of the monodentate carboxylate in the basal plane and one O from the bidentate chelating carboxylate completing the distorted square-pyramidal geometry ( Figure 10).
Although the first order coordination of 4 is different from that of 2, the same kind of C-H…O and π-π stacking interactions are found in 4. The monomeric units are linked to each other by the π-π stacking interactions in between the two five membered thiophene rings Cg1 → The C-H…O interactions found inbetween C9-H9…O2 i [symmetry code i: 2-X,1-Y,1-Z]. This leads to the formation of I shaped dimer as in 2. This dimeric unit is further linked to each other to form the tetrameric unit by a pair of C-H…Br interactions ( Figure 11) while these kind of interactions were absent in 2.
Thus they form a chain. Each of these chains are further linked by the the π-π stacking interactions in between the five membered thiophene rings and a N containing heterocyclic ring of the 4-BPY Cg3 → Cg1 iv (Cg3 = N1, C11-C15 and Cg1 = S1,C2-C5) ( Figure 12). The most unusual feature of 4 is the monodentate coordination of the carboxylate group leading to a variety of changes in the 3Dstructure. The reason for the monodentate coordination of the 5-BTPC may be attributed to the strong C-H…Br and C-H…O interactions involving the 5-BTPC atoms ( Figure 11).
Crystal structure of description of [Pb 2 (PHEN) 2 (5-BTPC) 2 (ACE) 2 ] (5) As anticipated the usage of PHEN ligand produced a dinuclear structure as 3 ( Figure 13). The central Pb(II) ion not only differs in primary coordination from 3 but it is    Two of these adjacent chains are interlinked by C-H… Cg interactions inbetween the H of PHEN ring of a chain and five membered thiophene ring of the adjacent chain [C10-H10…Cg1 ii symmetry code ii: 2-X,1/2 + Y,3/2-Z] ( Figure 15).

Involvement of X (X = Cl, Br) in interactions
Amid other non covalent interactions involving hetero atoms the role of Cl•••π, C-H •••π, & Br•••π have been given importance due to their role in lattice stabilization and their contribution in determining the modes of packing in a molecular packing and in solid state it can never be over looked [52]. The non covalent interactions involving  halogen fall in four types as given in (Figure 16a-d). The geometry of this R-H…X interaction play a major role in predictability of these interactions. In this backdrop it is worth in mentioning that we have studied various non covalent interactions involving the halogen atom in complexes 1-5 and the results are given in (Table 4). Various θ1 values have been observed with the highest C-Cl•••π for complex 1. Previous reports state that an interaction of the type H…X-C the preferred angle would be in the range of 90-130°rather than higher angles [53,54]. The observed θ2 values agree well with the above survey.

Repeating metal bite unit comprised of PbC 2 N 2 in (1-5)
The metal bite (PbN 2 C 2 ) unit is the repeating common unit of 1-5. This metal bite in 1 is formed with two coordinating N atoms, two carbon atoms of the BPY and the central lead atom, while in 2,4 it is formed by two coordinating N atoms, two carbon atoms of the 4-BPY and the central lead atom. While in 3,5 it is formed with two coordinating N atoms, two carbon atoms of the PHEN and the central lead atom.
The metal bite is a monomeric unit in 1, 2 and 4 while it is dimeric in 3,5. In 3 the metal bites are separated by a   (Figure 17a,b,d) and (Figure 17c,e) respectively. On seeing the packing arrangement of these metal bite units, they are self assembled irrespective of whether they are monomeric in 1, 2 and 4 or dimeric in 3, 5. A view of the metal bites is shown in (Figure 18a-e). The dihedral angles between the two pyridine rings in 1, 2 and 4 are 4.69(15), 1.23(12)°and 1.43 (15)°respectively. This shows that the planarity of BPY, 4-BPY and PHEN is maintained.

Thermal analysis
Thermogravimetric analysis (TGA) experiments of the complexes 1-5 were conducted under a static atmosphere of nitrogen at temperatures ranging from RT (room temperature) to 1000°C in order to determine the thermal stabilities (Additional file 1). Due to similarities of 1, 2, 4 and 3, 5 they show similar decomposition patterns ( Figure 19). The complexes 1-5 started to melt well above 150°C showing a very small thermal effect. Complexes 1, 2, 4 showed two steps of thermal decomposition at (148-430°C, 150-540°C, 168-662°C) which probably due to two 5-TPC ligands. The decomposition of the  bipyridine ligands took place at (435-957°C, 545-745°C, 667-842°C) in 1, 2 and 4 respectively. Complexes 3, 5 exhibit their first weight loss at temperature ranges of (145-405°C and 212-497°C). This is due to loss of four 5-TPC molecules in 3 as well as two 5-TPC molecules and two ACE molecules in 5. The second stage of decomposition at (416-848°C, 501-733°C) in 3 and 5 respectively are due to the decomposition of PHEN ligands.

Photoluminiscent properties
Previous studies show that Pb(II) complexes constructed with conjugated organic ligands can act as potential candidates for photoactive materials with appealing photoluminescence properties. Therefore, solidstate photo luminescence properties of Pb(II) complexes 1-5, with that of N,N' chelating ligand, were investigated at room temperature based on their UV-vis spectra Table 4 Bond metrics for X (X = Cl,Br) involved non covalent interactions in complexes 1-5

Materials and methods
Commercial starting materials were used without further purification. 2,2′bipyridine (Aldrich), 4,4′-dimethyl-2,2′-bipyridine (Aldrich), 5-Chlorothiophen-2carboxylic acid (Hoechst Aktiengesellschaft), methanol (Qualigens, India), 5-Bromothiophene-2-carboxylic acid (Aldrich), Pb(CH 3 COO) 2 .3H 2 O were used. IR spectra of the complex in region 400-4000 cm -1 were recorded as pressed disks (1% by weight in KBr) on a Shimadzu FT IR spectrophotometer (Additional file 1). 1 H-NMR and 13 C-NMR spectra were recorded with a Bruker spectrometer at 400 MHz in [D6] DMSO (Additional file 1). CHNS analysis was carried out using Elementar Vario EL III in solid state ( Table 5). The fluorescent properties were studied in solid state on a HITACHI spectrofluorimeter in solid state at room temperature. Both the excitation slit and emission slit were 5 nm. Thermal stability studies were carried out on a STA 409 PL Luxx thermal analyzer at a heating rate of 10°C/min under nitrogen atmosphere.

Synthesis of [Pb(BPY)(5-CTPC) 2 ] (1)
A solution of Pb(CH 3 COO) 2 .3H 2 O (0.098 g) in 10 ml of (1:1) CH 3 OH/H 2 O mixture was stirred over a hot plate magnetic stirrer for half an hour and 5-CTPC (0.0833 g) dissolved in 10 ml of CH 3 OH was added to it. The mixture was stirred for an additional of 2 hours. A yellow coloured solution was formed. About (0.0442 g) of (2-2′-bipyridine) was dissolved in 10 ml of hot water and added to the reaction mixture; to this solution about (5 ml) of glacial acetic acid was added. The mixture was stirred for 3 hours (Scheme 2). The dirty white precipitate was filtered off and the resulting pale yellow solution was kept for slow evaporation. Crystals were deposited at room temperature from the saturated solution. After 3 days, pale yellow coloured crystals suitable for X-ray diffraction were obtained. The crystals were filtered and washed with small portions of methanol and were dried in air (yield 75% based on Pb). IR selected bands (cm

Conclusions
A series of novel Pb(II) complexes in concert with 5-CTPC, 5-BTPC and corresponding bidentate chelating N.N' ligands have been synthesized and characterized.
There is significant structural diversity even though, in all the complexes the molar ratio of Pb, carboxylate, N, N-chelating ligand (ie, 1:2:1) is the same. The complexes 1, 2, 4 are mononuclear and are structurally diverse compared with 3, 5 which are dinuclear in nature. The observation of structures 2, 4 and 3, 5 show the structural changes made just chloro/bromo substituent of the thiophene ring. In addition to noncovalent interaction like C-H…O, which is the reason for assembly of primary motifs; various other interactions like X…π, H…X, (X = Cl/Br) add additional support in organizing these supermolecules in to extended architectures. It is noteworthy that the organic ligands not only serve as a space filling component but they are also involved in structural propagation either by π-π stacking or C-H… π stacking interactions. Complexes 1-5 exhibit strong emissions and may be potential materials for emitting diode devices. However this work not only shows the influence of replacement of different ligands on the structure but also it illustrates the effects of substituent's on the ligands on the crystal structure and geometry of the central metal ion, which also provides valuable instruction in design of coordination complexes with desired supramolecular architectures.