Recently much attention has been paid to design and synthesis of metal-organic hybrid materials with fascinating network topologies1 and potential applications as functional materials.2
Isonicotinic acid (HIN) have been widely used in synthesizing metal organic frameworks as asymmetrical rigid ligand. With regard to isonicotinic acid complex, hydrogen bonds (O-H…O) can be commonly found between uncoordinated carboxylic group and water in the complex and at the same time, π-π stackings also exist in the coordination. As a consequence, a lot of isonicotinic acid complexes can form infinite extended three-dimensional network supramolecular compounds.3 In the view of development of synthetic strategies and functional materials, the combination of carboxylate-containing ligands and neutral pyridylcontaining ligands can afford much more assembly process with metal ions through changing one of two types of organic ligands.4
During studies aimed at constructing cavity containing rectangular two- or three-dimensional networks using dicarboxylate and 4-pyridylcarbonitrile as spacer, we have isolated 3D-coordination polymers, [Zn(IN)2(H2O)4] (1) and [Cd(IN)4(H2O)4] (2), respectively, using diphenic acid and 4-pyridylcarbonitrile as spacer. It is interesting that 4-pyridylcarbonitrile turned to isonicotinic acid by the acid hydrolysis in the process of reaction.5 We report here the synthesis and crystal structures of the complex 1 and 2.
All chemicals are commercially available and were used as received without further purification. Elemental analyses (C, H, N) were performed on a Carlo Erba EA-1106 Elemental Analyzer. Infrared spectra were recorded in the range from 4000 to 400 cm−1 on a Mattson Polaris FT-IR Spectrophotometer using KBr pellets. Thermogravimetric analysis (TG) was performed on a TA Discovery TGA instrument with a heating rate of 10 ℃·min−1. All reactions were carried out in 23 ml Teflon-lined stainless-steel autoclave. The vessels were filled approximately to 40% capacity. The initial and final pH of the reaction was measured using Sentron 1001 pH meter.
Preparation of [Zn(IN)2(H2O)4] (1)
A mixture of Zn(OAc)2·2H2O (0.111 g, 0.5 mmol), diphenic acid (0.121 g, 0.5 mmol), 4-pyridinecarbonitrile (0.052 g, 0.5 mmol), and H2O (10 mL) in the mole ratio of 1.0:1.0:1.0:1111 was placed in a 23 mL Teflon-lined Parr acid digestion bomb and heated for 3 d at 180 ℃ under autogenous pressure. After the mixture was removed from the oven and allowed to cool under ambient conditions for 3d., colorless needles of 1 suitable for X-ray diffraction were isolated in 57% (0.110 g) yield based on zinc. Initial pH, 4.0; final pH, 4.0. Anal. Calcd. for C12H16N2O8Zn: C, 37.77; H, 4.22; N, 7.34. Found: C, 38.35; H, 4.25; N,7.22%. IR (KBr pellet, cm−1): 3283(m), 2972(s), 1614(m), 1579(s), 1536(s), 1444(m), 1406(s), 1054(m), 1033(m), 755(m), 710(m).
Preparation of [Cd(IN)2(H2O)4] (2)
A mixture of Cd(NO3)2·4H2O (0.153 g, 0.5 mmol), diphenic acid (0.121 g, 0.5 mmol), 4-pyridinecarbonitrile (0.052 g, 0.5 mmol), and water (7 mL) was placed in a 23 ml Teflon-lined Parr acid digestion bomb and heated for 3 d at 165 ℃ under autogenous pressure. After the mixture was removed from the oven and allowed to cool under ambient conditions for 3d, colorless crystals (block) of 2 suitable for X-ray diffraction were isolated in 40.0% (0.083 g) yield based on Cd. Initial pH, 5.0; final pH, 5.0. Anal. Calcd. for C12 H16N2O8Cd: C, 33.62; H, 3.76; N, 6.54. Found: C, 34.23; H, 3.58; N, 6.42%. IR (KBr pellet, cm−1): 3309(s), 2975(w), 1586(s), 1541(s), 1417(s), 1384(s), 1229(m), 1058(m), 1021(m), 820(w), 770(m), 686(s).
X-ray Structure Determination
Single crystals of 1 and 2 were obtained by the method described in the above procedures. Structural measurement for the complexes were performed on a Bruker SMART APEX CCD diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at the Korea Basic Science Institute. The structures were solved by direct method and refined on F2 by full-matrix least-squares procedures using the SHELXTL programs.6 All non-hydrogen atoms were refined using anisotropic thermal parameters. The hydrogen atoms were included in the structure factor calculation at idealized positions by using a riding model, but not refined. Images were created with the DIAMOND program.7 The crystallographic data for complex 1 and 2 is listed in Table 1.
Table 1.Crystal data and structure refinement for complexes 1 and 2
RESULTS AND DISCUSSION
The compounds were isolated from the reaction mixture of Zn(OAc)2·2H2O (1)/Cd(NO3)2·4H2O (2), diphenic acid, 4-pyridylcarbonitrile, and H2O in the mole ratio of 1.0:1.0: 1.0:1111(1)/778(2) by the hydrothermal technique. The diphenic acid present in the initial reaction mixture was not found in the crystalline product and 4-pyridylcarbonitrile was changed to isonicotinic acid by the acid hydrolysis in the hydrothermal processes. However, the compound 1 was reported previously by the reaction of Zn(NO3)2 with isonicotinic acid.8
Our first aim in this work was to obtain the 2D or 3D network MOF complexes which metal centers are bridged by the dicarboxylate anion and N-donor system. Unfortunately, attempts to obtain such materials by varying stoichiometry, temperature, and other reaction parameters proved to be generally unsuccessful.
Complex 1 and 2 show essentially the same structure(Fig. 1 and 2). Selected bond lengths and angles for both complexes are listed in Table 2. Each metal(II) ion in both complexes is coordinated in a slightly distorted octahedral N2O4 environment by two isonicotinic N-atoms and four O atoms of four coordinated water molecules. Each metal(II) ion lies in the equatorial plane (O2wO1wO1wAO2wA mean deviation, 0.000(1) Å). The M(II)-O (Zn-O; 2.096(1), 2.157(1) and Cd-O: 2.288(1), 2.328(2) Å) and M(II)-N distances (Zn-N; 2.135(1), Cd-N; 2.311(1) Å) are similar to those of [Zn(C6H4NO2)2(H2O)2] 0.5C3H7NO9 and [Cd(IN)2 (H2O)]·DMF,10 respectively. The bond angles around the metal(II) ion are as followings: trans-L-M-L = 180(8) and cis-L-M-L = 86.61(5)−91.86(5) for Zn(II) and trans-L-M-L = 180(8) and cis-L-M-L = 86.16(5)−93.84(5)° for Cd(II) (Table 2). Due to delocalization of electron density in the carboxylate anion the C-O bond distances are the same within an experimental uncertainties (Table 2). The carboxyl groups in isonicotinic acid are deprotonated and the dihedral angle of pyridyl rings are 3.758(41)° for 1 and 0.000(85)° for 2, respectively.
Figure 1.Coordination environment in 1 (Symmetry codes: A, -x, -y, -z).
Figure 2.(a) Coordination environment in 2. (b) 1D chain in 2. (c) 2D layer in 2. (d) 3D supramolecular framework formed by O-H…O hydrogen bond in 2. Symmetry codes: (a) A,-x, -y, -z; (b)~(d) A, x-1, y-1, z-1; B, -x+1, -y, -z+1; C, x-1, y, z-1; D, x, y, z-1.
Table 2.Symmetry transformations used to generate equivalent atoms: A) -x, -y, -z.
The crystal packing is directed by hydrogen bond interactions with the participation of water H-donor atoms and carboxylic group O atoms acting as acceptors (Table 3). In a typical complex 2, the hydrogen bonds (O2w–H2…O4D) form 1D chains along c-axis (Fig. 2(b)). The chains are interlinked by hydrogen bonds (O1w–H2…O4B, O2w-H1…O3C) to form 2D network along a-axis (Fig. 2(C)). In addition, hydrogen bond (O1w-H1…O3A) between the water molecule and carboxyl oxygen atom of another adjacent acplane contribute significantly to the assembly of these 2D networks into supramolecular 3D network (Fig. 2(d)).
Table 3.Symmetry transformations used to generate equivalent atoms: A, x-1, y-1, z-1; B, -x+1, -y, -z+1; C, x-1, y, z-1; D, x, y, z-1.
The IR spectra show a typical the antisymmetic and symmetric stretching bands of carboxylate groups at 1579 and 1406 cm−1 for 1 and at 1541 and 1384 cm−1 for 2, respectively.11 For the correlation of the infrared spectra with the structures of metal carboxylates the difference between the asymmetric and symmetric carboxylate stretches is often used.12 The separation (Δ) between νasym(CO2) and νsym (CO2) is 173 cm−1 for 1 and 157 cm−1 for 2, respectively. This value is similar to the observed values (170±10 cm−1) for the compounds with the ionic COO− group.11 On the other hand, the Δ values calculated from the structural data of the complex are 165.29 cm−1 for 1 and 170.25 cm−1 for 2, respectively. (Δ = 1818.1 δr + 16.47 (θOCO – 120) + 66.8, where δr is difference between the two CO bond lengths (Å) and θOCO is the OCO angle (°))12
The ν(O-H) stretch was observed at ~3300 cm−1 for both complex. The peaks at 1614−1536 cm−1 are attributable to the ν(C=O), ν(C=N) or ν(C=C) of aromatic group.13 In addition, the peaks between 820 and 686 cm−1 can be assigned to the ν (C-H) of pyridine ring.14
TG analysis of compound 1 and 2 was performed to observe their thermal behaviors. As shown in Fig. 3, the TG curve of complex 1 shows the first weight loss of 16.00% (calcd. 18.88%) from 50 ℃ to 120 ℃ in accord with the release of four water molecules based on the chemical formula. Upon subsequent heating, the compound remains intact to 365 ℃, followed by a rapid loss of 45.56% (calcd. 63.99%), corresponding to the loss of two IN molecules in the temperature range of 365−600 ℃. The total weight loss of 61.63% is less than the calculated value of 78.76% if the final product is assumed to be ZnO, which indicate that the decomposing process is not complete due to the use of nitrogen protection. For 2, The first weight loss of 16.43% from 63 to 132 ℃ corresponds to the release of four water molecules (calcd. 16.81%). The second weight loss of 56.52% (calcd. 56.97%) from 275 to 600 ℃ corresponds to the release of two IN ligands. The final product is also assumed to be CdO (obsd. 26.4%, calcd. 29.95%).
Figure 3.Thermogravimetric analysis of (a) 1 and (b) 2.
In conclusion, two new polymeric complexes, [M(IN)2 (H2O)4]n (M = Zn(1) and Cd(2)) were successfully isolated by the hydrothermal technique, respectively. In the complex, monomeric metal (II) complex is interconnected by hydrogen bond between water H-donor atoms and carboxylic group O atoms to give 3D network. The isonicotinate ligand in the crystalline product was formed from 4-pyridylcarbonitrile as initial reactant by the acid hydrolysis in the process of reaction.