Three-Dimensional Supramolecular Network Directed by Intermolecular Interactions in [ Pb 2 ( dmp ) 2 ( hfacac ) 4 ]

To investigate the interactions between noncovalent bond donor and acceptor giving rise to three dimensional networks, compound [Pb2(dmp)2(hfacac)4] (1) (dmp = 2,9-dimethyl-1,10-phenanthroline and Hhfacac = hexafluoroacetylacetonate) has been prepared and characterized by elemental analysis, IR, H NMR, and C NMR spectroscopy and its crystal structures was investigated. The single crystal structure show the coordination number of Pb(II) to be eight with two N-donor atoms from a ‘‘dmp’’ ligand and six O-donors from the anionic ligands. The supramolecular structure of 1 is realized by weak directional C–H···O–C, C–F···F–C and π–π stacking interactions.


Introduction
Control of the supramolecular solid state organization of individual molecules is one of the most important ways to develop novel functional materials.In this field the use of molecular self-organization processes for the development of self-assembled and self-organized structures has attracted attention both in biology and in materials chemistry 1 .Understanding of charge assisted non-covalent interactions such as hydrogen bonding, π-π stacking, dipole-dipole, hydrophobic interactions, etc., collectively constitute a promising field of persistent research interest 2 .Molecular organization into one-, two-, or three-dimensional networks is mainly driven by noncovalent interactions, of which hydrogen bonding (of both the strong and the weak variations) plays an important role 3,4 .Weak hydrogen bonding can play a role in enzymatic reactions because it is capable of modulating the bonding situation in catalytic sites to a significant degree, thus influencing catalytic properties 3b .The construction of metallo-supramolecular systems based on these types of intermolecular interactions is becoming of wide interest nowadays.Recently in an effort to explore weak interactions in complexes, the lead(II) complexes with β-diketonate and neutral diimine chelating ligands have been synthesized and determined by X-ray crystal structure [5][6][7][8] .In this paper we report the synthesis and crystal structures of [Pb 2 (dmp) 2 (hfacac) 4 ] 1 (dmp and hfacac are the abbreviations of 2,9-dimethyl-1,10-phenanthroline and hexafluoroacetylacetonate ligands, respectively, which have the potential to generate intermolecular interactions).

Experimental
IR spectra were recorded as nujol mulls using Perkin-Elmer 597 and Nicolet 510P spectrophotometers.Microanalyses were carried out using a Heraeus CHN-O-Rapid analyzer.Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected. 1H NMR spectra were measured with a BRUKER DRX-500 AVANCE spectrometer at 500 MHz.

Crystallography
The structure was solved by direct methods and refined by full matrix least squares using SHELXL-97 11,12 .SHELX operations were automated using ORTEX, which was also used to obtain the drawings 13 .Data were corrected for Lorentz and polarization effects but not for absorption.Hydrogen atoms were included in calculated positions with thermal parameters 30% larger than the atom to which they were attached.The non hydrogen atoms were refined anisotropically.All calculations were performed on a Pentium PC.

Results and Discussion
Determination of the structure of [Pb 2 (dmp) 2 (hfacac) 4 ] by X-ray crystallography (Table 1) showed the complex to be dinuclear in the solid state.The crystallographic unit of [Pb 2 (dmp) 2 (hfacac) 4 ] is illustrated in the ORTEP-3 plot Figure 1 and it consists of two lead(II) ions coordinated to two dmp molecules and four "hfacac" ligands.The coordination number of Lead(II) ions in [Pb 2 (dmp) 2 (hfacac) 4 ] is eight (two of bidentate "dmp", two of bidentate "hfacac -" anions and four of two bridging "hfacac -" ligands).Distances of Pb-O bonds with terminal "hfacac -" ligands are less than bridging "hfacac -" ligands Table 2.It is possible, nonetheless, to view the Pb atoms of asymmetric units as linked via Pb 2 O 2 rhombs, structurally a feature of many Pb(II) complex structures 6,8b, [14][15][16] , involving these four hfacac-O atoms and which may be indicative of very weak Pb•••Pb bonding 17 aided by the O-donoratom bridging.that suggest strong interactions within this class of weak non-covalent contacts. 20,21There are short F•••F interactions with the distances of 2.773(5), 2.936(3) and 2.937(3) Å which are less 22 than the sum of the van der Waals radii for fluorine at 2.94 Å (Figure 2).
The packing diagram of this complex exhibits fascinating self-assembled structural topologies via π-π stacking (face-to-face with distance of 3.43 and 3.52 Å) (Figure 2), appreciably shorter than the normal π-π stacking 23,24 .Consequently, the π-π stacking and F•••F and O•••HC interactions allow dinuclear compound to form a hybrid three dimensional network.The molecular unit of 1 is both dimentionally and structurally different from that [Pb(phen)(hfacac) 2 ] 2 and [Pb(bipy)(hfacac) 2 ] 3 8a . 1 is a dinuclear wheres 2 and 3 are joined into polymeric chains.However 1 is similare to [Pb(bipy) 2 (hfacac) 2 ] 4 8a and the similarity of 1 and 4 further suggests that no significant "lone pair effect" can be operative at the Pb atoms.The two structures may be regarded as having two; approximately opposed faces occupied either by aromatic (A) or by fluorinated (F) entities and this may explain the segregated nature of its lattice (Figure 2).A possible analysis of this lattice is to regard it as

Figure 2 .
Figure 2. A view, down a, fragment of the 3D network of 1 via intermolecular interactionsThe molecular unit of 1 is both dimentionally and structurally different from that [Pb(phen)(hfacac) 2 ] 2 and [Pb(bipy)(hfacac) 2 ] 3 8a . 1 is a dinuclear wheres 2 and 3 are joined into polymeric chains.However 1 is similare to [Pb(bipy) 2 (hfacac) 2 ] 4 8a and the similarity of 1 and 4 further suggests that no significant "lone pair effect" can be operative at the Pb atoms.The two structures may be regarded as having two; approximately opposed faces occupied either by aromatic (A) or by fluorinated (F) entities and this may explain the segregated nature of its lattice (Figure2).A possible analysis of this lattice is to regard it as