Syntheses and characterization of two novel tetranuclear lead(II) clusters self-assembled by hydrogen bonded interactions

Background The usage of polynuclear metal clusters as secondary building units (SBU’s) in designing of metal organic frameworks (MOF’s) is a field of current interest. These metal clusters have attracted a great deal of attention not only due to their interesting structural topologies but also due to promising physical and chemical properties. In this regard various d,f block (transition and lanthanide) metal clusters have been widely investigated so far. Less attention is paid to construction of heavy p-block Pb(II) clusters. Results Two mixed ligand Pb(II) clusters have been synthesized with bipy(2,2’-Bipyridine), phen(1,10-Phenanthroline), quin (8-Hydroxy quinolinate) and 5-tpc (5-chloro thiophene 2-carboxylate). They have been characterized by elemental analysis, IR, TGA and X-ray crystallography. X-ray diffraction analysis reveals that the complexes [Pb4(quin)4(bipy)2(5-tpc)4] (1) and [Pb4(quin) 4(phen) 2(5-tpc)4] (2) are tetranuclear. The complexes show a slight variation in unit cell parameters, due to the replacement of bipy and the phen ligands. Both complexes contain two types of Pb(II) ions which differ in the coordination geometry around the Pb(II) ion. Conclusions In both complexes the four lead ions Pb1,Pb2, Pb1i and Pb2i lie on the same plane bridged by the 5-tpc anions. Pb1 and Pb2 of both complexes contain a 5-tpc and quin coordinated in a bidentate chelating bridging fashion. In addition the Pb2 and Pb2i ions alone contain a bipy and phen in a bidentate chelating fashion in (1) and (2) respectively. An additional notable feature in both of these complexes are the bridging ability of the quin oxygen which forms a network of coordination bonds in between the four Pb(II) ions. In both complexes the individual units are self-assembled by C-H---Cl/C-H---S hydrogen bonding interactions to generate 2-D aggregates.


Background
Although metal clusters show a lot of superiorities, the synthesis of unique metal clusters is a real challenge. Recent reports show that there are two strategies which can be employed in the design of metal clusters. They are usage of multidentate carboxylates and usage of mixed ligands [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. In the mixed ligand approach a small multidentate ligand as well as a long bridging ligand is used, where the small multidentate ligand connects the metal ions to form the polynuclear cluster and the long bridging ligand can bridge these clusters.

Materials and methods
Commercial starting materials were used without further purification. 2,2'bipyridine (Aldrich),5-Chloro thiophene 2-carboxylic acid (Hoechst Aktiengesellschaft), methanol (Qualigens, India), 8-hydroxyquinoline (Loba Chemie), Pb (CH 3 COO) 2 .3H 2 O (Reidel) 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. 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. 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.

Preparation of [Pb 4 (quin) 4 (bipy) 2 (5-tpc) 4 ] (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-Chloro thiophene 2-Carboxylic acid (0.0833 g) dissolved in 10 ml of CH 3 OH was added to it. A hot methanol solution of 8hydroxyquinolin (0.0543 g) was added to it. The mixture was stirred for an additional of 2 hours. A yellow colored 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. The dirty white precipitate was filtered off and the resulting pale yellow solution was kept for slow evaporation. After 3 days, pale yellow colored 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 The structure of complex (1) inspired us to design the preparation of complex (2) with same chelating mode using the 1,10-Phenanthroline ligand. The procedure of preparation of (2) is similar to (1). Instead of 2-2'-bipyridine, 1,10-Phenanthroline was used (yield 62% based on Pb). IR selected bands (cm

Characterization of the complex IR spectra
On the basis of literature evidences assignment of selected characteristic IR bands (4000-400 cm −1 ) of the two Pb(II) complexes has been carried out. The spectra of the lead(II) complexes (1) and (2) were essentially similar and clearly show the carboxyl stretching vibrations of the 5-tpc. The asymmetric and symmetric stretches ν as (COO − ) and ν s (COO − ) were observed at 1564 and 1367 cm −1 , 1560 and 1367 cm −1 for the complexes (1) and (2) (1) and (2) respectively. These observed values are only a slightly higher than those expected for ionic carboxylate group in acetates (170 cm −1 ) [23].

Crystal structure determination
Intensity data sets were collected at room temperature, on a BRUKER SMART APEXII CCD [24] area-detector diffractometer equipped with graphite monochromated   (17) O5-Pb2-N4 129.58 (15) Mo Kα radiation (λ = 0.71073 Å). The data were reduced by using the program SAINT [24] and empirical absorption corrections were done by using the SADABS [24]. The structures were solved by direct methods using SHELXS-97 [25] and refined anisotropically by full-matrix least-squares method using SHELXL-97 [25] within the WINGX suite of software, based on F 2 with all reflections. All carbon hydrogens were positioned geometrically and refined by a riding model with U iso 1.2 times that of attached atoms. All non H atoms were refined anisotropically. The molecular structures were drawn using the ORTEP-III [26] and POV-ray [27]. Crystal data and the selected parameters are summarized in (Tables 1 and 2) respectively. The crystals remained stable throughout the data collection. The CIF files of complexes 1 and 2 ate provided as Additional files 1 and 2 respectively.

Geometry around lead
It has been well studied that the lone pair of electrons has a great influence on the structure of the Pb(II) complexes [28][29][30][31]. 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 [29]. Pb(II) complexes in which 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 [32]. The hemidirected geometry is the most preferred for intermediate coordination numbers between 6-8 [31]. The coordination geometry of both the complexes (1,2) as well as the Pb-O and the Pb-N bond directions show a gap around the Pb(II) ion which is also well depicted from (Figure 1). In both the complexes, the O-Pb-O angle suggests that there is a big gap in the coordination sphere due to lone pair-bond pair repulsion (Table 2). This gap is occupied possibly by a stereoactive lone pair of electrons on the lead(II) ion. The coordination around the Pb(II) ions in both (1,2) are hemidirected indicating that the stereochemical lone-pair electrons of them are active. The presence of a lone pair of the Pb(II) ions in both Pb1 and Pb2 in the direction opposite to that of the quinolate bridging is apparently the reason that the bridging ligand can come closer to the next Pb(II) ion.

Crystal structure description of [Pb 4 (quin) 4 (bipy) 2 (5-tpc) 4 ] (1) and [Pb 4 (quin) 4 (phen) 2 (5-tpc) 4 ] (2)
The two crystals (1) and (2) are nearly isostructural and have similar unit cell parameters due to the similarity of the chelating ligands used (bipy, phen). In both the complexes the tetranuclear units has an inversion centre, coinciding with the crystallographic inversion centre. Pb1 is seven coordinated in both (1) and (2) but the Pb2 is eight coordinated in (1) and seven coordinated in (2) (Figure 1). Thus each crystal has two type unique Pb centres (Pb1 and Pb2) while the other two are generated by inversion (Figure 2). Both the complexes (1) and ( (17) O1-C1-O2 125.4 (6) O5-C34-O6 124.8 (7) In Complex 1[symmetry code i = 1-x,1-y,1-z, in complex 2 [symmetry code i = 1-x,1-y,-z]. [29,30]. It is interesting that there is a weak Pb…Pb interaction between Pb1 and Pb2 in both (1) and (2). The extent of direct Pb…Pb interactions has been rarely reported and there are a few reports of these interactions in between adjacent Pb atoms with distance range of 3.44-4.09 Å in the clusters [33][34][35][36]. The short Pb…Pb distances in both the complexes which are smaller than the sum of van der Waals radii of two Pb(II) atoms, suggest a possibly weak metallophilic Pb…Pb interaction [35,36]. In both the complexes the quin ligands acts as   (2). Further each of these individual chains is linked to each other by weak C-H…S interactions in between the S of the thiophene ring of one chain and H of quin ring of another chain, which leads to formation a 2D layer (Figure 3).

Luminescent properties
Luminescent properties of complexes 1 and 2 have been investigated in solid state at room temperature. It can be observed from the emission spectrum that complexes 1 and 2 exhibits intense and broad emission band with an emission maximum at ca. 543 and 552 nm upon excitation at 459, 460 nm ( Figure 5). It is also observed that the emission spectra of 1 and 2 are similar. This emission band could be assigned to the emission of ligand-to-metal charge transfer (LMCT). This observation indicates that complexes 1 and 2 may be used as potential candidates for a new class of photoactive materials.