Supramolecular assembled of hexameric water clusters into a 1D chain containing (H2O)6 and [(H2O)4O2] stabilized by hydrogen bonding in a copper complex

Background Various water clusters including hexamers, heptamers, octamers, decamers and 1D or 2D infinite water chains in a number of organic and inorganic-organic hybrid hosts, have been reported. Results {[Cu(pydc)(amp)].3H2O}n has been hydrothermally synthesized and characterized by elemental analysis and by IR spectroscopy. A wide range of hydrogen bonds (of the O-H...O, N-H...O and N-H...N type) are present in the crystal structure. Hydrogen bond interactions between the co-crystallized water molecules led to formation of six-membered rings with chair conformation. Conclusion In {[Cu(pydc)(amp)].3H2O}n, there are three uncoordinated water molecules. Thermal methods confirm number of co-crystallized water molecules in polymer. Hydrogen bond interactions between the co-crystallized water molecules led to the formation of a six-membered ring with the chair conformation. These rings are part of a 1D chain containing six-membered O6 rings, which are alternately made from (H2O)6 and [(H2O)4O2] rings. [(H2O)4O2] rings are also in chair conformation.


Background
Water is nature's solvent and plays a fundamental role in biological and chemical processes. It is the most abundant and cheapest solvent available for the development of new reagents and the study of chemical processes [1]. Understanding of the behavior of water clusters are important and structural information of small water clusters is the first step towards the understanding of the behavior of bulk water [2]. An extensive investigation of small and medium sized water cluster (H 2 O) n (n = 2-100) structures have been reported in recent years [3,4]. Various water clusters including pentamers [5,6], hexamers [6][7][8], heptamers [9][10][11], octamers [6,8,12,13], decamers [6,[14][15][16] and 1D or 2D infinite water chains [2,17,18] in a number of organic and inorganic-organic hybrid hosts, have been structurally characterized. Recently, we also reported a tetrameric water cluster rings in the crystal structure of a new proton transfer system derived from pyridine-2,6-dicarboxylic acid and 2-amino-4-methylpyridine [19]. Several different conformations for the water hexamer, such as allboat [20], all chairs or an intermediate between chair and boat [3,21] have been characterized by x-ray crystallography. The search for an understanding of small water cluster structures is important for an improvement of our knowledge about structures of liquid water or ice. The water hexamer is the building block of ice I h [6] and it is known that cyclic hexameric water clusters are present in liquid water [22,23].
Recently, Synthesis of supramolecular compounds has attached great interest due to their interesting topologies and potential applications as functional materials. The reasonable design of supramolecular structure necessarily depends on the concepts of selfassembly and utilizes non-covalent forces as varied as follows: (1) coordination bonds, (2) hydrogen bonding, including both strong hydrogen bonding (e.g., O-H···O) and weak hydrogen bonding (e.g., C-H···O and C-H···N), (3) electrostatic and charge-transfer attractions, and (4) aromatic π stacking interactions [24]. Hydrothermal synthesis has been successful for the preparation of some supramolecular compounds. In continuation of our earlier work on the synthesis of metal complexes with polycarboxylate ligands in the presence of 2-aminopyrimidine and under hydrothermal condition [25,26], the reaction of Cu(NO 3 ) 2 ·3H 2 O with Pyridine-2,6-dicarboxylic acid and 2-aminopyrimidine was investigated. Crystal structure of complex moiety of the copper complex, {[Cu(pydc) (amp)].3H 2 O} n , (pydc = pyridine-2,6-dicarboxylate ion, amp = 2-aminopyrimidine), was reported by Altin et al. in 2004 [27]. Here we report the formation of hexameric water clusters with chair configuration in the crystal structure of above mentioned compound. Crystals of {[Cu(pydc)(amp)].3H 2 O} n were obtained from the reaction of pyridine-2,6-dicarboxylic acid, 2-aminopyrimidine and Cu(NO 3 ) 2 ·3H 2 O under hydrothermal condition.

Crystal structure
The crystal is built up of a complex Cu(pydc)(amp) and three co-crystallized water molecules (Scheme 1). The one-dimentional polymeric chains showing the connectivity and extending along the [010] direction are shown in Figure 1. Each Cu II atom is coordinated by O,N,O-tridentate dipicolinate ligand (bound via pyridine N and two carboxylate O atoms) and one heterocyclic nitrogen atom of 2-aminopyrimidine ligand. Each metal ion is weakly connected to two neighboring ones, through two carboxylate bridging groups of dipicolinate and aminonitrogen of NH 2  The hydrogen bonding parameters are outlined in Table 1. As it was shown in Figure 3, uncoordinated water molecules are linked together and to carboxylate group of complex via hydrogen bonds. Hydrogen bond interactions between the co-crystallized water molecules (O-H···O hydrogen bonds, range from 2.74 -2.79 Å) led to formation of a six-membered ring water cluster with chair conformation (Figure 4). These rings are part of a 1D chain ( Figure 5) containing six-membered O6 rings, which are alternately made from (H 2 O) 6 4 O 2 ] are also in chair conformation ( Figure 6) and the rings are linked as in cis-decalin. Hexameric water clusters with all-boat conformation are similar to ice I h [20] and structures with all-chair conformation (all-cis) are different from ice I h (all-trans) [3].
In addition, the crystal structure of complex is stabi-

IR Spectra
The FTIR spectrum of the compound (Figure 7) shows broad strong bands at the region 3275-3520 cm -1 , which could be related to the existence of O-H···O hydrogen bonds between water molecules. It must have been coupled by other indicative peaks such as N-H and O-H stretching frequencies and the stretching frequencies due to the aromatic rings, which originally fall within this region [28][29][30].

Thermal Analyses
The thermogravimetric analysis curve ( Figure 8) for compound shows that the weight loss from 200°C to 250°C, corresponds to the loss of three molecules of The further exothermic decomposition began at 400°C and finished at 500°C indicating the complete removal of the organic part of the complex. The main product was CuO with a residual value of 20.0% (theoretical residual value, 21.1%).

Materials and physical measurements
All purchased chemicals were of reagent grade and used without further purification. IR spectra were recorded using FTIR Spectra Bruker Tensor 27 spectrometer (KBr pellets, 4000-400 cm -1 ). TGA/DTA measurements were performed at heating rate of 10°C min -1 in the temperature range of 25-800°C, under nitrogen flow of 20 mL min -1 on instrument Shimadzu DTG-50H. Elemental analyses were performed using a Costech ECS 4010 CHNS analyzer.