Supramolecular Interactions Involved in the Solid State Structure of N , N '-[ bis ( pyridin-2-yl ) formylidene ] ethane-1 , 2-diamine

The structure of the symmetrical Schiff base, N,N'-[bis(pyridin-2-yl)formylidene]ethane-1,2diamine (bpfd) has been characterized by single crystal X-ray diffraction. The non-covalent supramolecular chemistry involved in the crystal structure of this ligand has been carefully investigated. The structure adopted different motifs of nitrogen-hydrogen interactions that led to the formation of centrosymmetric dimers. In addition, edge-edge and face-face nitrogen-nitrogen interactions were observed and reported. The Schiff base (bpfd) ligand crystallizes in a monoclinic space group C12/c1 with a = 19.128(2) Å; b = 5.8776(6) Å; c = 13.1403(15) Å; α = 90; β = 121.970(4); γ = 90 and z = 4. This structure is an example of compounds with many symmetry-independent molecules in the asymmetric unit cell (Z > 2).


INTRODUCTION
Recently, there has been some interest in the preparation of various Schiff base ligands 1 due to their preparative accessibilities, structural varieties and varied denticities.Metal-chelate Schiff base complexes have played an important role in developing stereochemical models in main group and transition metal coordination chemistry, mainly due to their stability, ease of preparation, and structural variability. 2−23 As a part of our ongoing research into the structure and utility of Schiff base ligands, we report the crystal structure of N,N'-[bis(pyridin-2yl)formylidene]ethane-1,2-diamine (bpfd) ligand, and study the supramolecular chemistry is based on noncovalent interactions involved in the crystal structure of (bpfd) ligand.

EXPERIMENTAL General Experimental
All chemicals were purchased from Aldrich chemical company and used as received.The solvents used were AR grade. 1 H and 13 C NMR spectra were recorded in CDCl 3 on a Brucker 400 MHz at Jordan University of Science and Technology.Chemical shifts were referenced to tetramethylsilane (TMS).The infrared spectra were recorded on a Maltson 5000 FTIR spectrophotometer.Electronic absorption spectra were recorded on a Perkin Elmer Lambda 25 UV/Vis Spectrometer in dimethylsulfoxide (DMSO) using 1 x 10 −6 M solution.The symmetrical Schiff base, N,N'-[bis(pyridin-2yl)formylidene]ethane-1,2-diamine (bpfd) was prepared by refluxing ethane-1,2-diamine and pyridine-2carboxaldehyde in 1:2 mole ratio in absolute ethanol (Figure 1). 24The (bpfd) ligand was characterized by FTIR, UV-vis, 1 H and 13 C NMR spectroscopy.It was recrystallised from diethyl ether to give colourless crystals with X-ray quality.

Solution and Refinement of the Crystal Structure
Single-crystal X-ray diffraction experiment was carried out on a Rigaku Saturn CCD area detector with graphite monochromate Mo-Kα radiation.The data were collected at a temperature of −135 ± 1 °C to a maximum 2θ value of 80.6°.Data were collected and processed using Crystal Clear. 25The linear absorption coefficient, μ, for Mo-Kα radiation is 0.79 cm −1 .A numerical absorption correction was applied which resulted in transmission factors ranging from 0.9394 to 0.9408.The data were corrected for Lorentz and polarization effects. 26The structure was solved by direct methods 27 and expanded using Fourier techniques. 28The non-hydrogen atoms were refined anisotropically.Hydrogen atoms were refined using the riding model.The final cycle of full-matrix least-squares refinement 29  Neutral atom scattering factors were taken from Cromer and Waber. 30Anomalous dispersion effects were included in F calc ; 31 the values for Δf′ and Δf″ were those of Creagh and McAuley. 32The values for the mass attenuation coefficients are those of Creagh and Hubbell. 33All calculations were performed using the CrystalStructure 25,34 crystallographic software package except for refinement, which was performed using SHELXL-97. 29

IR and UV/vis Spectroscopy
The Schiff base (bpfd) ligand has a strong absorbance at 1647 cm −1 in the infrared region, confirming the presence of the imine moiety.The stretching vibration of C=N and C=C of the pyridine ring are observed at 1586 and 1427 cm −1 , respectively.Additionally, the UV/vis spectrum has strong absorptions near 272 and 282 nm.These bands are assigned to the ligand centered (LC) or -* transitions.The IR data are consistent with the reported value. 35

NMR Spectroscopy
The 1 H NMR spectrum for the (bpfd) ligand give signals corresponding to the aromatic protons in the range 7.25-8.59ppm.The 1 H NMR signals arising from ethylene protons appear as singlet at 4.02 ppm. 13C NMR spectrum supports the formation of the Schiff base (bpfd) ligand.The 13 C NMR signals corresponding to the aromatic carbons are visible in the region 117-166 ppm.Signal corresponding to the aliphatic and vinylic carbons are found at 61 and 154 ppm, respectively.

Supramolecular Interactions Involved in the Solid State Structure of (bpfd) Ligand
A solvent free sample of the N,N'-[bis(pyridine-2-yl)formylidene] ethane-1,2-diamine ligand (bpfd) (Figure 1) was obtained after crystallization from diethyl ether to give material that was good enough to be examined by the single crystal X-ray technique.The (bpfd) ligand crystallizes in a monoclinic space group C12/c1 with a = 19.128(2)Å; b = 5.8776(6) Å; c = 13.1403(15)Å; α = 90°; β = 121.970°(4);γ = 90° and z = 4. Numerical details of the crystal structure are presented in experimental section.ORTEP drawing of (bpfd) molecular structure is shown in Figure 2. The bond distances and angles for (bpfd) ligand (Tables 1, 2) are as expected. 36,37The crystal structure of (bpfd) is an example of compounds with many symmetry-independent mole-cules in the asymmetric unit cell (Z > 2).This phenomenon was extensively analyzed by Bernstein et al. 38 and accordingly reported that high-Z structures have been described as ''snapshot pictures'' and also as ''fossil relics'' of early stages in crystallization.Such description has a certain charm.However, as we are almost totally ignorant of the nucleation process, the early stages in crystallization, these picturesque descriptions have the disadvantage that they are almost impossible to confirm or to refute.Detailed analysis of (bpfd) crystal structure showed the presence of edge-edge C6−H5…N1 centrosymmetric dimer interactions.This centrosymme-tric C−H…N dimer is composed of C−H…N and C−H…N interactions with distances of 2.7 and 3.55 Å, respec-    tively and C6−H5…N1 bond angle of 145.68° as shown in Figure 3.
In addition, detailed analysis of (bpfd) crystal structure showed the presence of a layer arrangement between the aromatic ring and aliphatic methyl hydrogen to produce the bifurcated centrosymmetric C−H…N and C−H…C interactions with distances of 2.73 and 2.84 Å, respectively as shown in (Figure 4).In addition, face-face nitrogen-nitrogen interaction is observed in the crystal structure of (bpfd) through C1−N1…N2 to form centrosymmetric dimer.The centrosymmetric C1−N1…N2 dimer had C−N…N interactions with a distance of 3.92 Å and bond angle of 63.92° as shown in Figure 5.This centrosymmetric dimer is stabilized by mean of face-face -stacking interactions between the pyridine rings and the C1−N1 of each dimer with distances of 3.92 Å with offset angle of 63.92° (Figure 5).Such interactions will stabilize the formation of the dimer.Furthermore, edge-edge nitrogen-nitrogen interaction is observed between two molecules of (bpfd) through C1−N1…N1−C1 interaction with a bond distance of 3.89 Å and C1−N1…N1 bond angle of 68.79° as shown in Figure 6.

CONCLUSION
The structure of the Schiff base, N,N'-[bis(pyridine-2-yl) formylidene]ethane-1,2-diamine (bpfd) has been characterized by NMR spectroscopy and a single-crystal X-ray diffraction.The non-covalent supramolecular chemistry involved in the crystal structure of this ligand has been carefully investigated.The analysis of bpfd ligand crystal structure demonstrates the presence of different supramolecular synthons such as edge-edge (aliph-H…N1 and C1−N1…N1−C1) and face-face C1−N1…N2 centrosymmetric dimer interactions.The crystal structure of (bpfd) is found to be a new example of compounds with many symmetry-independent molecules in the asymmetric unit cell (Z > 2).Different motifs of non-covalent interactions were observed and reported in terms of crystal engineering and supramolecular chemistry.

Figure 5 .
Figure 5. Two molecules of (bpfd) ligand interact together through their nitrogen atoms to form centrosymmetric dimer with a distances of 3.92 Å.

Figure 6 .
Figure 6.Edge-edge nitrogen-nitrogen interaction between two molecules of (bpfd) ligand with a distance of 3.89 Å.
on F 2 was based on 1420 observed reflections and 84 variable parameters and converged.Crystallographic data for the structural analysis reported in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, Number (721559).This data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033; email: deposit@ccdc.cam.ac.uk].