Supramolecular architectures constructed with the skeletons of zinc(II) 2,2′-bipyridine and barbiturate anion: Synthesis and characterization
Graphical abstract
A barbiturate derivative complexed with Zn(bpy)(NO3)2·2H2O and Zn(NO3)2·H2O provides fluorescent supramolecular architectures which are characterized by their single crystal X-ray crystallographic studies.
Introduction
Non-covalent interactions between molecular components play important roles in the binding and conformation of various biological molecules as well as in the functioning of enzymes, antibodies, membranes, carriers and channels [1], [2]. The molecular assembly through H-bonding, π–π stacking and CH–π interactions has emerged as an interesting field of crystal engineering [3], [4], [5], [6], [7]. Supramolecular interactions provide a path to understand the packing of coordination polymers in crystal lattice. In this context, nitrogen-containing ligands are prominent in coordination polymers and supramolecular coordination chemistry [8], [9], [10], [11]. However, barbiturate ring systems besides their pharmaceutical importance [12], [13], [14], [15] possess thymine like hydrogen bonding functionalities and are considered good precursor for the construction of H-bonded networks. Additionally, water clusters are found important in crystal engineering in view of several reports on 1D, 2D and 3D water polymers [16], [17], [18], [19], [20]. These water clusters are usually incorporated in the hollow space of the metal–organic framework and provide a path for supramolecular interactions between the metal and the ligand. The presence of water molecules in the structure can play an important role in stabilizing some supramolecular species as the number of hydrogen bond donors and acceptors can differ significantly from those of the anhydrous compounds [21]. Krygowski et al. have already described the role of water as a ‘glueing factor’ in organic crystals because of its readiness to deform from ideal H-bonded geometry [22]. Despite some success, it is still extremely difficult to construct water networks artificially because the structural constraints required in stabilizing these chains are not fully understood. On one hand, there have been a few studies in constructing artificial hosts for water clusters [23], [24], [25]. On the other hand, in recent papers, the role of water clusters in the crystallization process has also been investigated by introducing other solvent molecules such as methanol and DMSO [26]. Zn(bpy)(NO3)2·2H2O was selected as metal precursor and the 2,2′-bipyridine ring as a ligand to facilitate π-interactions. The barbiturate 1 possesses multiple hydrogen donor and acceptor functionalities which could be suitable for H-bonding as exploited by Hamilton and co-workers [27].
The synthesis of Zn(II) bipyridyl barbiturate has been accomplished to study the X-ray single crystal structure and the photo physical and thermal properties.
Section snippets
Materials and methods
Barbituric acid, 2,2′-bipyridine, salicylaldehyde and zinc(II) nitrate dihydrate were purchased from Sigma–Aldrich and were used as received without further purification. All the solvents were purchased from E. Merk and were freshly distilled prior to use. Ligand 1 and Zn(bpy)(NO3)2·2H2O were prepared and characterized using reported procedures [28], [29].
Physical measurements
The IR spectra (KBr disc, 400–4000 cm−1) were recorded on a Varian FTIR 3100 spectrometer; NMR (300 MHz, DMSO, Me4Si) spectra were recorded on
Synthesis and characterization
Compound 1 and Zn(bpy)(NO3)2·2H2O are prepared and characterized using reported procedures [28], [29]. However, compound [Zn(bpy)2(H2O)2]2+·()2·7H2O (2) is synthesized by the reaction of LH4 with Zn(bpy)(NO3)2·2H2O in a 1:1 molar ratio in DMF and methanol followed by addition of excess water. Complex 3 is isolated as crystals upon leaving a solution of 2 in DMSO along with an aqueous solution of 4-methylamino-6-oxopyrimidine for one month at room temp [23]. Attempts were made to crystallize
Conclusion
In summary, complex (2) consists of an octahedral Zn(II) cation and a derivative of a barbiturate anion and is synthesized and characterized by its single crystal X-ray study. The assembled structure constitutes an example of channel type organization giving an appearance of helical chains encapsulating several water molecules. 2 incorporates DMSO and stabilizes the resulting architecture through H-bonding interactions from co-crystallized water molecules. However, H-bonding interactions have
Acknowledgements
Financial support from DAE-BRNS (Grant No. 2005/37/33/BRNS/1807 to L. M.), Mumbai, single crystal X-ray data from IITB, Powai, Mumbai, India and Kyushu University, Japan (Professor Teruo Shinmyozu) are gratefully acknowledged.
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