Novel Water Clusters in Two Complexes of Pyridine-2,3,5,6

Ai-Hong Yang , Ji-Yong Zou , Wen-Min Wang , Xue-Ying Shi , Hong-Ling ... Ai-Hong Yang , Li-Hua Zhao , Ying-Ping Quan , Hong-Ling Gao , Jian-Zhong Cui ...
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CRYSTAL GROWTH & DESIGN

Novel Water Clusters in Two Complexes of Pyridine-2,3,5,6-tetracarboxylate

2008 VOL. 8, NO. 9 3354–3359

Ai-Hong Yang, Hong Zhang, Hong-Ling Gao, Wen-Qin Zhang, Lei He, and Jian-Zhong Cui* Department of Chemistry, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed March 19, 2008; ReVised Manuscript ReceiVed May 18, 2008

ABSTRACT: Two complexes, [Zn4(pdtc)2(phen)4(H2O)6] · 20H2O (1) and [Ni(H2pdtc)(H2O)3] · 3H2O (2), containing the new ligand pyridine-2,3,5,6-tetracarboxylate (pdtc) have been synthesized and characterized by X-ray single-crystal diffraction, elemental analysis, IR, UV-vis, and fluorescence spectroscopies, and thermal gravimetric (TG) analysis. Complex 1 is a tetranuclear zinc(II) complex bridged by two pdtc and terminated with four phen molecules, whereas 2 is a mononuclear nickel(II) complex. A novel eightcentered (pentafurcate) hydrogen bond has been observed in the huge (H2O)24 cluster in the crystal packing voids of 1, and a novel L6(4)6(6)10(8) water layer is present in 2. Introduction The design and investigation of hydrogen-bonded water clusters or networks continue to be of considerable interest not only because of the diversity of water patterns that exist in nature but also because of the fundamental importance of water for human life and in biological and chemical processes.1 During the past decade, investigations have included both theoretical and experimental scrutiny of a number of different water clusters,2–4 such as trimers, tetramers, pentamers, hexamers, octamers, and decamers.3 However, studies of these clusters linking themselves to form larger clusters especially 2D networks are very rare. Only recently, have several water/ice layers containing large 12-, 18-, or 45-membered water rings5 been observed in the solid state. These show the existence of new ice phases with chain segments, which provide novel structural aspects of water and new insights into water with implications for biological environments. In this contribution, we present both a large (H2O)24 cluster and one 2D water morphology observed in the complexes [Zn4(pdtc)2(phen)4(H2O)6] · 20H2O (1) and [Ni(H2pdtc)(H2O)3] · 3H2O (2), respectively (pdtc ) pyridine-2,3,5,6-tetracarboxylate; phen ) 1,10-phenanthroline). The (H2O)24 cluster in 1 reveals an interesting aspect of the structure. It contains an eightcentered (pentafurcate) hydrogen bond, which is quite uncommon and has never before been reported. Theoretical and experimental reports on rich hydrogen-bond modes such as multifurcate hydrogen bonds are scarce. A five-centered (tetrafurcate) hydrogen bond6 and a four-centered (trifurcate) hydrogen bond7 have been reported. The five-centered (tetrafurcate) hydrogen bond contains Cl · · · H-C and Cl · · · H-N interactions between adjacent molecules, which may be formed easier because of the larger size of the chloride anion. In another report,7 the four-centered (trifurcate) N-H · · · O/N-H · · · N hydrogen bond is mentioned but not described in detail. But a pure pentafurcate O-H · · · O hydrogen bond in a large water cluster has never been reported. The design and synthesis of complexes with pyridine carboxylic acids such as pyridine-dicarboxylic acid8 and pyridinetricarboxylic acid is of intense interest.9 Complexes with these organic multicarboxylate ligands have potential application in molecular sieves, optoelectronics, magnetism, and chemical sensors. Pyridine-tetracarboxylic acids have more coordination sites than pyridine-di- or tricarboxylic acids. Thus they should

produce some novel structures and coordination modes, but to the best of our knowledge, they have never been incorporated into synthetic complexes. The two complexes reported in this paper are the first examples with pyridine-2,3,5,6- tetracarboxylate. Experimental Section Materials and Measurements. Chemicals purchased were reagent grade and used without further purification. Elemental analysis (C, H, and N) was performed on a Perkin-Elmer 240 CHN elemental analyzer. IR spectra were recorded in the range of 400-4000 cm-1 with a Bruker TENOR 27 spectrophotometer using a KBr pellet. Thermogravimetric analysis (TGA) experiments were performed on a NETZSCH TG 209 instrument with a heating rate of 10 °C min-1. Photoluminescence spectra were measured by a Cary Eclipse EL06063917 fluorescence spectrophotometer with a xenon arc lamp as the light source. The synthesis of potassium pyridine-2,3,5,6-tetracarboxylate (K4pdtc) is described in the Supporting Information. Preparation of [Zn4(pdtc)2(phen)4(H2O)6] · 20H2O (1). Complex 1 was synthesized from a reaction mixture of Zn(NO3)2 · 6H2O (0.3 mmol, 0.0892 g), K4pdtc (0.2 mmol, 0.0815 g), phen (0.2 mmol, 0.0396 g), and H2O (12 mL) in a 25 mL Teflon high-temperature kettle, under autogenous pressure at 120 °C for 3 days and then cooled to room temperature at a rate of 1.5 °C h-1. Colorless crystals of 1 were obtained in a yield of 52% based on Zn. Elemental Analysis, found C, 42.58; H, 3.85; N, 8.49%; calcd. for C33H43N5O21Zn2 (fw ) 976.46) C, 42.42; H, 3.78; N, 8.54%. IR (KBr): ν ) 3427.07s, 1613.9vs, 1517.9m, 1427.5s, 1360.1s, 857.3s, 727.5s cm-1. Preparation of [Ni(H2pdtc)(H2O)3] · 3H2O (2). A mixture of Ni(NO3)2 · 6H2O (0.2 mmol, 0.0582 g), K4pdtc (0.2 mmol, 0.0815 g), H2O (15 mL), and HNO3 (1.5 mL, 0.1 mol · L-1) was refluxed for 4 h. The green crystals of 2 were obtained after the filtrate was allowed to stand at room temperature for four months. Yield 68% based on Ni. Elemental Analysis, found C, 25.75; H, 3.50; N, 3.23%; calcd. for C9H15NO14Ni (fw ) 419.93) C, 25.72; H, 3.57; N, 3.33%. IR (KBr): ν ) 3390.8vs, 1720.8m, 1541.5s, 1358.2s, 750.7s cm-1. Crystal Structure Determination. Diffraction intensities of 1 were collected on a computer-controlled Rigaku Saturn and those of 2 with a SCX min diffractometer equipped with graphite-monochromated Mo KR radiation with a radiation wavelength of 0.71073 Å using the ω-scan technique. Lorentz polarization and absorption corrections were applied. The structures were solved by direct methods and refined with a fullmatrix least-squares technique based on F2 using the SHELXS-97 and SHELXL-97 programs.10 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically; the hydrogen atoms of the water molecules were located from difference maps and refined with isotropic temperature factors. Analytical expressions of neutral-atom scattering factors were employed,

10.1021/cg800291p CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

Novel Water Clusters in Two Complexes of pdtc

Crystal Growth & Design, Vol. 8, No. 9, 2008 3355

Table 1. Crystal Data and Structure Refinement Information for Complexes 1 and 2

formula fw temp (K) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z F(000) Dc (Mg · m-3) abs coeff (mm-1) GOF on F2 params θ range (deg) reflns collected/unique reflns [>2σ(I)] final R [I > 2σ(I)] R indices (all data) Largest diff. peak and hole (e. Å-3) a

1

2

C33H43N5O21Zn2 976.46 113(2) triclinic P1j 13.056(3) 13.458(3) 13.602(3) 108.60(3) 113.69(3) 97.68(3) 1977.7(7) 2 1008 1.640 1.305 1.063 559 1.79-25.02 12123/6897 [R(int) ) 0.0512] 5285 R1a ) 0.0666, wR2a ) 0.1429 R1a ) 0.0902, wR2a ) 0.1584 1.213 and -0.693

C9H15NO14Ni 419.93 293(2) monoclinic P21/n 12.431(3) 6.8239(14) 19.112(4) 90 107.16(3) 90 1549.1(5) 4 864 1.801 1.332 1.028 275 3.19-27.48 15745/3551 [R(int) ) 0.0545] 3035 R1a ) 0.0351, wR2a ) 0.0831 R1a ) 0.0444, wR2a ) 0.0880 0.343 and -0.324

R1 ) ∑||Fo| - |Fc||/|Fo|; wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

Figure 1. ORTEP representation of 1 showing the molecular structure with 35% probability of thermal ellipsoids. The hydrogen atoms are not shown for clarity. Symmetry code: A, 1 - x, 1 - y, 1 - z. and anomalous dispersion corrections were incorporated. The crystallographic data for 1 and 2 are listed in Table 1.

Results and Discussion Structural Analysis of [Zn4(pdtc)2(phen)4(H2O)6] · 20H2O (1). Complex 1 contains a tetranuclear Zn(II) coordination framework. The four Zn(II) cations are coplanar, and all are octahedrally coordinated. They are bridged by two pdtc. The atoms labeled with “A” are centrosymmetrically related to the original atoms (Figure 1). The equatorial positions of Zn1 are coordinated by a phen (N2, N3), a carboxylate oxygen atom (O1), and a water molecule (O9), and the axial sites are occupied by two water molecules (O10, O11). As for Zn2, the equatorial positions are coordinated by a phen (N4, N5), a nitrogen atom (N1) from pdtc4-, and an oxygen atom (O3) from the other

Figure 2. Crystal structure of 1 showing π-π interactions (dashed lines). The hydrogen atoms are not shown for clarity. Zinc, cyan; carbon, gray; oxygen, red; nitrogen, blue.

pdtc4-. The axial sites are occupied by two carboxylate oxygen atoms (O6, O7). Pdtc4- is a tridentate chelate. Each pdtc4- chelates the Zn2 cation and bridges the Zn1 and Zn2A cations via the remaining carboxylate groups. This coordination mode leads to a discrete tetrameric entity in complex 1, where the phen molecules act as blocking ligands to avoid further polymerization. In the crystal (Figure 2), the complexes hold together to form staircase-like layers parallel to the ac plane by means of face-to-face π-π interactions, and the distance between adjacent phen molecules is 3.343 Å, which is in accordance with an earlier report.11 The water molecules are in the interlayer space and link to each other by hydrogen bonds to form (H2O)24 clusters. In addition to the (H2O)24 cluster, there are two disordered water molecules (W21, W22) with uncertain positions. Figure 3a shows the 12 water molecules (O9-O20) and their symmetric equivalents that form the (H2O)24 cluster. At the center of the cluster is a four-membered ring (two O19 and two O20), which is connected on each side to a five-membered ring (O12, O13, O14, O16, O17) through O18 or O18a. Every fivemembered ring is further hydrogen bonded to a water trimer (O15, O10, O9) and a water monomer (O11). The tetramer ring sits in the symmetrical center. Both theoretical calculations and vibration rotation tunneling spectroscopy of water tetramers in the gas phase indicate a quasiplanar structure, in which each water molecule forms two hydrogen bonds, one as a donor and the other as an acceptor. The average O-H · · · O bond length in this tetramer is estimated to be 2.79 Å.12 Interestingly, the hydrogen-bonding motif of the water tetramer presented herein, in which each water molecule is involved in the formation of three hydrogen bonds, is actually two dimers connected to each other and is very different from the theoretically predicted or experimentally observed structure in the gas phase. What is even more interesting is the novel eight-centered (pentafurcate) hydrogen bond in the (H2O)24 cluster (Figure 3b). The eight-centered (pentafurcate) hydrogen bond forms around W16 (W ) water). In each of the two O16 donor hydrogen bonds, the electron density attracted toward O16 allows for the formation of the three acceptor hydrogen bonds (O16 · · · H15B, O16 · · · H17A, O16 · · · H18A). The formation of the five hydrogen bonds overcomes the steric hindrance factors. In addition, another sort of tetramer comprised of W16 and W18 also exists in the large water cluster. Nonbonded O · · · O distances and O-H · · · O angles for the cluster are presented in Table 2. The O · · · O distance ranges from 2.685 to 2.897 Å, resulting in an average O · · · O distance of 2.768 Å, which is very close to the corresponding value of 2.759 Å found in ice Ih.5c The average O-O-O angle is 108.39°. We believe that this type of water cluster structure has not even been speculated previously.

3356 Crystal Growth & Design, Vol. 8, No. 9, 2008

Yang et al.

Figure 3. (a) The structure and hydrogen-bond connectivity of (H2O)24 and (b) the eight-centered (pentafurcate) hydrogen bond in 1. Hydrogen bonds are drawn as dotted lines. Table 2. Nonbonded Distances [Å] and Angles [deg] for the Water Cluster in 1 O9 · · · O15i O10 · · · O15ii O11 · · · O12iii O12 · · · O13iV O12 · · · O17V O13 · · · O14Vi O15 · · · O16i O16 · · · O14i O16 · · · O18i O17 · · · O16i O18 · · · O19V O19 · · · O20V O19 · · · O20aVii

2.685 2.753 2.709 2.808 2.821 2.816 2.703 2.795 2.736 2.767 2.897 2.776 2.717

O9-H9B · · · O15i O10-H10B · · · O15ii O11-H11B · · · O12iii O12-H12A · · · O13iV O12-H12B · · · O17V O13-H13A · · · O14Vi O15-H15B · · · O16i O16-H16A · · · O14i O16-H16B · · · O18i O17-H17A · · · O16i O18-H18A · · · O16i O18-H18B · · · O19V O19-H19B · · · O20aVii O19-H19B · · · O20V O20-H20B · · · O19Viii

173.18 158.67 173.12 167.37 154.59 129.11 161.97 170.90 178.03 153.98 113.72 156.85 117.14 120.27 130.62

Symmetry code: i x, y, z; ii 1 - x, -y, 1 - z; iii 1 - x, 1 - y, -z; x, y, -1 + z; V 1 - x, 1 - y, 1 - z; Vi 1 - x, 1 - y, 2 - z; Vii 1 + x, y, z; Viii -1 + x, y, z.

iV

Figure 5. Packing diagram of the sheet-like 2-D layer of (H2O)24 clusters with zinc complexes acting as connectors. The phen, pdtc4-, and hydrogen atoms are omitted for clarity. Zinc, cyan; carbon, gray; oxygen, red; nitrogen, blue.

Figure 4. View showing the interactions between the water clusters and the complexes. Hydrogen atoms are omitted for clarity. Zinc, cyan; carbon, gray; oxygen, red; nitrogen, blue.

These water clusters are coordinated to Zn1 through O9, O10, and O11 and then form 3D arrays as shown in Figures 4 and 5. Structural Analysis of [Ni(H2pdtc)(H2O)3] · 3H2O (2). In complex 2, Ni(II) adopts an octahedral coordination geometry and is coordinated by the tridentate H2pdtc2- (ONO) and three water molecules (Ow2, Ow5 and Ow6). Ni(II), H2pdtc2-, and Ow6 are approximately in the equatorial plane, and Ow2 and Ow5 are at the axial positions (Figure 6). The very startling feature in complex 2 is the 2D hydrogenbonding water network. The water molecules form chairlike tenmembered and chair six-membered cyclic rings, in which three water molecules (Ow1, Ow3, Ow4) are shared to form an

Figure 6. ORTEP representation of 2 showing the molecular structure with 35% probability of thermal ellipsoids.

extended tape. This can be defined as a T6(3)10(3) water tape13 (as illustrated in Figure 7). In addition, in the T6(3)10(3) water tape, every decamer ring has hydrogen-bonding interactions with two water molecules (Ow5 and Ow5′) through the Ow3 and Ow3′ positions, respectively. In the cyclic decamer, the O · · · O distances range from 2.678 to 2.912 Å, resulting in an average O · · · O distance of 2.78 Å, whereas in the structure of ice, Ic,14

Novel Water Clusters in Two Complexes of pdtc

Crystal Growth & Design, Vol. 8, No. 9, 2008 3357

Figure 7. Substructure within the layers, the T6(3)10(3) water tape in 2. Table 3. Nonbonded Distances [Å] and Angles [deg] Involving the Water Cluster in 2 O1W · · · O4Wi O2W · · · O6Wii O2W · · · O1W O3W · · · O1Wiii O4W · · · O3WiV O5W · · · O3WV O6W · · · O4WV O6W · · · O2W

2.900 2.782 2.732 2.912 2.798 2.831 2.678 2.749

O1W-H1WB · · · O4W O2W-H2WA · · · O6W O2W-H2WB · · · O1W O3W-H3WA · · · O1W O4W-H4WA · · · O3W O5W-H5WB · · · O3W O6W-H6WA · · · O4W O6W-H6WB · · · O2W

162.64 164.53 174.27 159.26 178.20 174.21 172.81 93.05

Figure 9. View showing the interactions of the water layer in 2. Hydrogen atoms are omitted for clarity. Nickel, cyan; carbon, gray; oxygen, red; nitrogen, blue.

Symmetry code: i x - 1/2, -y + 3/2, z + 1/2; ii -x + 1/2, y - 1/2, -z + 5/2; iii x + 1/2, -y + 1/2, z - 1/2; iV -x + 2, -y, -z + 2; V -x + 1, -y + 1, -z + 2.

Figure 10. Emission and excitation spectra of 1.

Figure 8. Packing diagram of the sheet-like L6(4)6(6)10(8) 2-D water layer in the bc plane. Adjacent tapes are present in green and black, alternately. Hydrogen atoms are omitted for clarity.

determined at -130 °C, the O · · · O distance is 2.75 Å. The corrugated geometry of the decamer rings in a 2D water layer may be attributed to the different modes of connectivity with the surrounding water molecules. The hydrogen bond data are shown in Table 3. In one water tape, the two water molecules Ow2 and Ow6 hydrogen bond to Ow6′ and Ow2′ of an adjacent tape. These four water molecules, together with Ow1 and Ow4, form another type of hexamer. This hexamer further fuses with the water tapes to assemble into a water layer in the bc plane (Figure 8). The O · · · O distances range from 2.678 to 2.912 Å, resulting in an average O · · · O distance of 2.805 Å, which is comparable to those for the ice II phase (2.77-2.84 Å).15 Furthermore, Ow2 and Ow6′ both coordinate to the same Ni(II) atom, so the coordinated bonds also have a structurestabilizing effect for the water layer. This observation indicates that the water network is stabilized not only by hydrogen bonds but also by coordination interactions (Ni-Ow2, Ni-Ow6). Interestingly, in addition to the interactions with the surrounding cyclic rings, the Hw6′ also has a hydrogen-bonding interaction with one of the carboxylate oxygen atoms of H2pdtc2- (O3). This results in further assembly of the water layers through

hydrogen bonds between water molecules and carboxylate oxygen atoms into a 3D network (Figure 9). IR Results. The IR spectra have been employed to distinguish the coordination modes of the carboxylate groups in 1 and 2. In the IR spectrum of 1, lattice water absorption bands appear at 3427 cm-1 and may be attributed to asymmetric and symmetric O-H stretching modes. The δC-H (out-of-plane bending) bands of the phen ligands are at 857 and 727 cm-1. The stretching vibration band of CdN appears at 1563 cm-1 for the free phen and is shifted to 1518 cm-1 for 1, which indicates that the nitrogen atoms coordinate to the metal ions. The asymmetric and symmetric stretching vibrations of the carboxylate groups are observed at 1613, 1427, and 1360 cm-1 for 1. For 2, the lattice water absorption bands appear at 3390 cm-1. The carboxyl group absorption band appears at 1720 cm-1, which indicates the partial protonation of the pdtc ligand in acid solution. The absorption band of the carboxylate groups are observed at 1541 and 1358 cm-1 for 2. These observations are all in agreement with the X-ray results. Fluorescent Properties. The emission and excitation spectra of 1 in aqueous solution at room temperature are shown in Figure 10. There is a strong emission with λmax ) 368 nm and excitation at 270 nm. The fluorescent properties of the pdtc and phen ligands were also measured. For the free pdtc ligand, only a very weak emission is found at λmax ) 381 nm. The free phen spectra are analogous to 1 but with obviously weaker fluorescence emission signals at λem ) 364 nm. These observations

3358 Crystal Growth & Design, Vol. 8, No. 9, 2008

Figure 11. UV/vis spectra for 1 in aqueous solution.

clearly indicate that the emission signals of 1 are mainly related to the phen-centered fluorescent emission, which is also observed in the UV/vis spectra (see below). The enhancement of fluorescence in 1 may be attributed to the phen chelation to the metal center, which effectively increases the rigidity of the ligand and reduces energy loss by radiation decay. UV/Vis Spectra. The UV/vis spectra of 1 are shown in Figure 11. The absorption peaks can be assigned to ligand-centered (LC) transitions. The assignments are confirmed by a comparison of the UV/vis absorption spectra of free pdtc, phen, and 1. As shown in Figure 11, phen and 1 show four similar absorption bands (200, 224, 265, and 288 nm for phen and 200, 224, 271, and 291 nm for 1). These absorptions are due to the E2-, K-, B-, and R-bands. The E2-, K-, and B- most likely correspond to π-π* transitions, while the R-band is due to an n-π* transition. Three separate absorption peaks at 192, 245, and 282 nm occur in free pdtc and are assigned to the E2-, B-, and R-bands, which correspond to π-π* transitions and an n-π* transition. The B-band of complex 1 is red-shifted from 245 or 265 to 271 nm, and the R-band is also red-shifted from 282 or 288 to 291 nm relative to pdtc or phen. This may be caused by the partial overlapping of the π electrons in phen and pdtc4- in complex 1, which intensifies the n-π* transitions. This not only results in the red shift but also enhances the absorbance. TGA. TGA was carried out for polycrystalline samples of 1 and 2 in the temperature range of 22-800 °C (Figure 12). The weight loss of complex 1 from 22 to 135 °C is 23.5%, corresponding to the loss of 20 uncoordinated and 6 coordinated water molecules (calcd 24.0%). From 135to 285 °C, 1 has almost no mass change. The second weight loss, 59.1%, occurred from 285 to 535 °C, which was due to the loss of pdtc4- and phen ligands (calcd 59.3%). Above 535 °C, ZnO was obtained representing the remaining mass, 17.4% (calcd 16.7%). For 2, the first weight loss takes place between 95 and 200 °C. The total weight loss in this temperature range is 25.7%, corresponding to the loss of three uncoordinated and three coordinated water molecules (calcd 25.7%). The second weight loss, 44.4%, occurred from 200 to 418 °C, which was due to the loss of the H2pdtc2- ligand (calcd 46.0%), and NiCO3 was obtained. Above 418 °C, NiCO3 continued to decompose. Conclusion In summary, two different novel water cluster patterns not in the Cambridge Structural Database (CSD)16 have been

Yang et al.

Figure 12. TGA curves for 1 and 2.

characterized here. In complex 1, a large (H2O)24 cluster composed of a tetramer, pentamers, and dimers contains multiform hydrogen bonds. Specifically, W16 acts as a donor and an acceptor overcoming steric hindrance factors so that an eight-centered (pentafurcate) hydrogen bond can form with the surrounding water molecules. In complex 2, there is an association of cyclic ten-membered water rings, six-membered rings, and monomer waters. This structure has not been predicted theoretically nor previously reported experimentally. Utilization of novel ligands to form novel water clusters is very important for providing insight into hydrogen-bonding motifs and understanding the property of water clusters of different sizes and shapes in diverse environments. Acknowledgment. This work was supported financially by the Natural Science Foundation of Tianjin (Grant No. 08JCZDJC22400) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Education Ministry of China. Supporting Information Available: The synthesis of potassium pyridine-2,3,5,6- tetracarboxylate (K4pdtc) and selected bond lengths (Å) and angles (deg) for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for 1 and 2 are in CIF files, CCDC (666544 and 664596 for 1 and 2), which can be obtained free of charge via www.ccdc.can.ac.uk/conts/ retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax (+44) 1223-336-033 or e-mail [email protected]).

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Novel Water Clusters in Two Complexes of pdtc

(4)

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