Received 1 April 2012
A new potential NLO compound with a supramolecular layered structure: aqua(hexamethylenetetramine-N)(iminodiacetato-3O,N,O')copper(II)
aCollege of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, People's Republic of China, and bDepartment of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of the Education Ministry, Zhejiang Science and Technology University, Hangzhou 310018, People's Republic of China
In the noncentrosymmetric title compound, [Cu(C4H5NO4)(C6H12N4)(H2O)] or [Cu(IDA)(HMTA)(H2O)], where IDA is iminodiacetate and HMTA is hexamethylenetetramine, the asymmetric unit consists of a whole mononuclear neutral molecule, where the CuII cation is coordinated by two carboxylate O atoms and one N atom from the IDA ligand, by one N atom from the HMTA ligand and by the O atom of the coordinated water molecule, giving rise to a CuN2O3 distorted square-pyramidal coordination geometry. The IDA and HTMA ligands adopt terminal tri- and monocoordinated modes, respectively. All adjacent molecules within the ac plane are connected to each other via two pairs of O-HO and one N-HO hydrogen bond, forming a (4,4) supramolecular two-dimensional network. In the unit cell, these layers stack alternately in an ...ABABAB... sequence along the b axis. The optical absorption properties of this compound have been studied on powder samples, which had previously been examined by powder X-ray diffraction.
Second-order nonlinear optical (NLO) compounds are usually materials with advanced functionality, which must crystallize in a noncentrosymmetric space group (Chai et al., 2010a; Bella, 2001; Kanis et al., 1994). Due to the rapid development of laser techniques in recent decades, these materials have attracted more and more attention for their applications in second-harmonic generation (SHG), wave-mixing effects, optical parametric oscillator (OPO) processes, and so on. There are currently two types of inorganic second-order NLO crystal materials which have been developed into commercial products (Shen, 2002; Boyd, 2008). The first corresponds to oxide-type crystals, for instance potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), barium metaborate (-BBO), lithium triborate (LBO) etc. These types of crystal are suitable for working in the visible and near-IR regions. The second type of inorganic second-order NLO materials corresponds to semiconductor crystals, for example tellurium, reddish silver (Ag3AsS3) etc. This type of material is suitable for working in the far-IR region.
However, these inorganic second-order NLO crystals can still not meet the various needs of optical signal processing. Thus, new second-order NLO materials of coordinated compounds have attracted widespread attention and great developments have been made in the last 20 years (Nalwa & Miyata, 1997). For this type of second-order NLO material, intramolecular charge transfer, i.e. metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT) or intra-ligand charge transfer (ILCT), is a key factor (Frazier et al., 1986; Qin et al., 1994), while the noncentrosymmetric character of the space group in which the coordinated compound crystallizes is an inescapable prerequisite for the possible existence of its second-order NLO properties. The controlled synthesis of noncentrosymmetric crystals is still a significant challenge, although many noncentrosymmetric coordinated compounds of this sort have been obtained so far (Bella, 2001; Kanis et al., 1994). According to reports in the available literature, the cage-like hexamethylenetetramine ligand (HMTA) may easily form noncentrosymmetric coordinated crystal structures with metal cations (Chen et al., 2005; Banerjee et al., 2010; Guo et al., 2010; Sun et al., 2011). In addition, the iminodiacetate anion (IDA) is a good tridentate chelating ligand for coordinating to copper(II) to form a [Cu(IDA)(H2O)2]n polymeric structure (Roman-Alpiste et al., 1999), which would allow interruption by N-heterocyclic donors (the auxiliary ligand) to form an asymmetric mononuclear molecule. In this work, we utilized HMTA and IDA to assemble with a copper(II) salt, and successfully obtained the title mixed-ligand coordinated compound, (I), which crystallizes in the noncentrosymmetric space group Pca21.
Compound (I) consists of a mononuclear neutral molecule (Fig. 1), made up of one N-coordinated HMTA ligand, one O,N,O'-coordinated IDA ligand, one O-coordinated water molecule and one CuII cation, which exhibits a distorted square-pyramidal coordination geometry with the water O atom in the apical position. Within this CuO3N2 square pyramid, the bond lengths (Table 1) are similar to those in analogous structures (Roman-Alpiste et al., 1999; Kundu et al., 2005; Chen et al., 1990; Zhang et al., 2008; Yang et al., 2011). The IDA ligand adopts a three-coordinated terminal binding mode, and the fact that it does not exhibit the bridging mode observed in some previously reported polymeric structures containing IDA (Roman-Alpiste et al., 1999; Podder et al., 1979) could be attributed to the steric blocking effect of the large HTMA terminal ligand.
The mononuclear molecules of (I) are connected to each other to form a two-dimensional supramolecular layered structure, mainly by virtue of two pairs of O-HO hydrogen bonds and one N-HO hydrogen bond (Table 2). As shown in Fig. 2, adjacent molecules are connected (entries 1-3 in Table 2) to form supramolecular layers. From a topological point of view, the molecular centres can be regarded as supramolecular 4-connected nodes and the hydrogen-bonding interactions as linkages, thus defining a (4,4) supramolecular network structure. These layers stack with their c-glide-related neighbours in an alternating ...ABABA... fashion along the b axis (Fig. 3).
The UV-vis diffuse-reflectance spectrum of (I) was measured on a powder sample previously examined by powder X-ray diffraction, and is plotted in Fig. 4 as F2(R) versus wavelength, according to the Kubelka-Munk function (Chai et al., 2007a,b). As can be seen, there are two obvious absorption bands in the spectrum. One strong band, located in the region of about 200-350 nm, is due to the transition of ligands between different energy levels. The other, very weak, band at about 550-800 nm is due either to a d-d transition of the copper(II) centre or to an LMCT mechanism. That is to say, (I) absorbs very few photons in the visible region, so this region is a suitable window for SHG. Thus, (I) may be investigated as a potential second-order NLO material.
| || Figure 1 |
The molecular structure of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.
| || Figure 2 |
The supramocular layer network structure of (I), viewed along the b-axis direction. The structure is constructed by hydrogen-bonding interactions, shown as dashed lines (red in the electronic version of the journal), and the HMTA ligands are shown as small balls (blue). [Symmetry codes: (i) -x - , y, z + ; (ii) -x + , y, z + .]
| || Figure 3 |
A layered packing diagram for (I), viewed along the a-axis direction. H atoms have been omitted for clarity.
| || Figure 4 |
A plot of F2(R) versus wavelength for (I), showing the lack of absorption in the visible region.
The title compound, (I), was synthesized by the solution reaction of Cu2(OH)2(CO3) (23 mg, 0.1 mmol), H2IDA (27 mg, 0.2 mmol) and HTMA (29 mg, 0.2 mmol) in water (15 ml) at room temperature. The subsequent solution was filtered and left to evaporate. After several days, blue crystals of (I) were obtained in a yield of 93% (65.6 mg). Samples suitable for single-crystal X-ray diffraction were selected directly from the obtained crop.
All H atoms bonded to C and N atoms were added at calculated positions, with C-H = 0.99 Å and C-N = 0.93 Å, and refined using a riding model, with Uiso(H) = 1.2Ueq(C,N). Water H atoms were located from difference Fourier peaks and refined isotropically, with a restrained O-H distance of 0.82 (2) Å.
Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO; data reduction: CrystalStructure (Rigaku/MSC, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: BG3151 ). Services for accessing these data are described at the back of the journal.
The authors are grateful for financial support from the National Natural Science Foundation of China (project Nos. 11075147 and 51002147) and the Natural Science Foundation of Zhejiang Province (project Nos. Y4100610 and LY12E02010).
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