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Two novel supramolecular isomers based on 2,2′-biimidazole derivative and zinc ions: Syntheses, structures and luminescent properties
Inorganic Chemistry Communications 12 (2009) 169–172
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Two novel supramolecular isomers based on 2,20 -biimidazole derivative and zinc ions: Syntheses, structures and luminescent properties Yan-Hong Xu, Ya-Qian Lan, Ya-Hui Zhao, Dong-Ying Du, Guang-Juan Xu, Kui-Zhan Shao, Zhong-Min Su *, Yi Liao * Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China
a r t i c l e
i n f o
Article history: Received 3 November 2008 Accepted 28 November 2008 Available online 7 December 2008 Keywords: Supramolecular isomers Zn(II) 2,20 -Biimidazole Topology Luminescent property
a b s t r a c t Two Zn(II)-2,20 -biimidazole derivative supramolecular isomers 1 and 2 formulated as [Zn(L)] 2H2O (H2L = 4,40 -(1H,10 H-2,20 -biimidazole-1,10 -diylbis(methylene))dibenzoic acid) have been synthesized, and structurally characterized by elemental analysis, IR spectroscopy, single-crystal X-ray crystallography. Complex 1 exhibits a 2D four-connected network with (43.63) topology when the zinc center and the L2 ligand act as four-connected nodes (or a distorted (4,4) grid if the [Zn2(imidazole)4] units are considered as four-connected nodes). Complex 2 shows a novel four-connected twofold interpenetrated net with (42.62.82) topology when the zinc center and the L2 ligand act as four-connected nodes (or a twofold interpenetrated a-Po topology when [Zn2(imidazole)4] units are identified as six-connected nodes). Moreover, complexes 1 and 2 both exhibit strong luminescent properties at room temperature. Ó 2008 Published by Elsevier B.V.
The design and synthesis of metal–organic frameworks (MOFs) constructed from metal salts and bridging ligands have attracted great attention in recent years, not only because of their intriguing variety of architectures and topologies, such as rectangular grids, herringbones, boxes, ladders, brick walls, rings, diamondoids, and honeycombs, but also due to their potential applications in diverse areas such as electrical conductivity, magnetism, host-guest chemistry, molecular separation, gas storage, sensors and catalysis [1]. The rational design and controllable preparation of MOFs, which is highly influenced by several factors, for instance, the coordination geometry of the central atom, the structural characteristics of the ligand molecule, the counter anion, the pH values of the reaction solutions, symmetry the solvent system, the reaction conditions and so on [2,3]. From the viewpoint of crystal engineering, the most effective approach may be the appropriate choice of the well-designed organic bridging ligands containing modifiable backbones and connectivity information, together with the metal centers with various coordination preferences [4–7]. On the other hand, supramolecular isomerism, introduced by Zaworotko and coworkers [8], is the existence of more than one type of network superstructure for the same molecular building blocks and is therefore related to structural isomerism at the molecular level. It is one of the most important aspects of supramolecular chemistry, which has attracted great interest because of not only its structural diversity but also its potential application in recent years [9].
* Corresponding author. E-mail address: [email protected] (Z.-M. Su). 1387-7003/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.inoche.2008.11.030
2,20 -Biimidazole (H2biim) is an evergreen ligand that has attracted constant interest in supramolecular chemistry [10], biochemistry [11], cluster science [12], and antitumor drugs [13]. Some complexes based on H2biim or its derivatives have been reported because of their wide-ranging applications in enzymatic reactions and their intriguing structures [14,15]. On the other hand, it is well known that carboxylate ligands play important roles to construct novel MOFs in coordination chemistry and can adopt various binding modes such as terminal monodentate, chelating to one metal center, bridging bidentate in a syn–syn, syn– anti and anti–anti configuration to two metal centers, and bridging tridentate to two metal centers [16]. However, there are still only rare reports of ligands based on H2biim and carboxylate groups as building blocks for the construction of MOFs. According to the above considerations, we have recently designed and synthesized a novel ligand, namely, 4,40 -(1H,10 H-2,20 -biimidazole-1,10 -diylbis(methylene))dibenzoic acid (H2L) (see Supporting information, Scheme 1). The H2L ligand possesses the following interesting characteristics: (a) its flexibility due to the presence of a –CH2– spacer between the imidazole groups and phenyl groups, which allows subtle conformational adaptation of L2 for the self-assembly of unique supramolecular structures; (b) two rigid imidazole rings of L2 are connected by a rotatable C–C bond, which endows L2 ligand with subtle conformational adaptation. As well-known Zn(II)-containing coordination polymers have attracted extensive interest in recent years in that they not only exhibit appealing structures but also possess luminescent properties [17]. Here, we will describe the structures of the two Zn(II) supramolecular isomers with their formulas as [Zn(L)]
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2H2O. In addition, their luminescent properties were also investigated. Complex 1, which is synthesized through hydrothermal reaction [19], exhibits an interesting two-dimensional network structure. The single crystal X-ray analysis [20] shows that the asymmetrical unit of 1 contains a Zn(II) ion and a L2 ligand. Each zinc(II) center is coordinated by two nitrogen atoms (Zn1– N4C = 2.038(3) and Zn1–N1 = 2.051(3) Å) from two different L2 ligands and two carboxylic oxygen atoms (Zn1–O4B = 1.912(3) and Zn1–O1A = 1.958(3) Å) from two distinct L2 ligands in a distorted tetrahedral geometry ( Supporting information, Fig. S1) The bond distances are in the normal range of those observed in other zinc(II) complexes [21]. The L2 ligand bonds to four zinc ions through two carboxylic oxygen atoms and two nitrogen atoms in monodentate coordination fashion ( Supporting information, Scheme 2a). Two adjacent zinc atoms were linked by four nitrogen atoms from two H2biim rings of two L2 ligands to form a [Zn2(imidazole)4] unit with the Zn Zn distance of 4.286 Å (Fig. 1 left). Two imidazole rings of L2 ligand are not in the same plane with the dihedral angle of 70.47°. Because of the existence of –CH2– spacer between phenyl rings and H2biim unit, the L2 ligand exhibits its flexibility and two phenyl rings have the dihedral angle of 59.27°. A better insight into the nature of the framework can be achieved by the application of a topological approach. When the Zn(II) cation and the L2 ligand both act as a four-connected nodes, the structure can be identified as an unusual 2D four-connected framework with (43.63) topology (see Fig. 1, route I). Interestingly, when we take the [Zn2(imidazole)4] unit as a four-connected node and the L2 ligand as a linker, the structure can be described as a distorted 2D (4,4) grid (see Fig. 1, route II). When changing the pH value of the mixture under the same reaction conditions, complex 2 was obtained. The single crystal X-ray analysis [20] reveals complex 2 is an interesting twofold interpenetrated 3D network structure. Each Zn(II) atom is coordinated by three carboxylic oxygen atoms (Zn1–O2 = 1.970(2), Zn1–O3 = 2.029(3) and Zn1–O4 = 2.379(3) Å) from two different L2 ligands and two nitrogen atoms (Zn1–N1 = 2.052(3) and Zn1– N3 = 2.054(3) Å) from two distinct L2 ligands to furnish a slightly distorted trigonal bi-pyramidal geometry (Supporting information, Fig. S2). Compared with 1, the L2 ligand adopts another coordina-
tion mode in 2. The L2 ligand bonds to four zinc ions through one carboxylic oxygen atom and two nitrogen atoms in monodentate coordination fashion, and other two carboxylic oxygen atoms in chelate-bidentate bridging fashion (Supporting information, Scheme 2b). Similar to complex 1, two crystallographically equivalent Zn(II) atoms are bridged by four nitrogen atoms to generate a [Zn2(imidazole)4] fragment with the ZnZn separation of 4.366 Å (Fig. 2 left). Two imidazole rings of L2 ligand are not in the same plane, which are almost vertical to each other with the dihedral angle of 89.31°. Moreover, two phenyl rings of the L2 ligand have a dihedral angle of 13.29° which is larger than that of complex 1. When the Zn(II) atom and the L2 ligand both act as four-connected nodes, an unusual 3D four-connected twofold interpenetrated framework with (42.62.82) topology was obtained (see Fig. 2, route I). However, when the [Zn2(imidazole)4] fragment was regarded as a six-connected node and the L2 ligand acts as a linkage, the overall framework becomes a twofold interpenetrated a-Po topology (see Fig. 2, route II). Different linking modes between L ligand and [Zn2(imidazole)4] units lead to two isomers of the porous frameworks {[Zn(L)] 2H2O}n. Compound 1 crystallized in the monoclinic system and space group C2/c. Complex 1 has microporous viewed along the c-axes and all channels arrange in same direction, and calculations with PLATON show that the effective volume for the inclusion is 20.1% (935.4 Å3) of the crystal volume (4645.0 Å3) [22] (see Supporting information, Fig. S3a). Compound 2 also crystallized in the monoclinic system and space group C2/c. As illustrated in Fig. S3b, 2 has microporous viewed along the a-axes and all channels arrange in same direction, which account for 10.1% (414.9 Å3) of the crystal volume (4125.0 Å3) (see Supporting information, Fig. S3b). Supramolecular isomerism can be a consequence of the effect of the same molecular components generating different supramolecular synthons and could be synonymous with polymorphism. Complexes 1 and 2 are supramolecular isomers, the minor variance in the coordination sphere of Zn leads to their different properties. In addition, it is worth to note that the pH value of the mixture plays an important role in synthesis process of supramolecular isomers 1 and 2. The 2D framework of 1 was obtained at lower pH value, while 2 was constructed at a relatively high pH value showing
Fig. 1. The two simplified models applied in the topological analysis of 1. I: the metal and the L2 ligand as four-connected nodes, II: the [Zn2(imidazole)4] units as fourconnected nodes and all ligands as linear linkers.
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Fig. 2. The two simplified models applied in the topological analysis of 2. I: the metal and the L2 ligand as four-connected nodes, II: the [Zn2(imidazole)4] unit as a fourconnected node and the ligand as linear linker.
a 3D framework. Investigation on their structural difference reveals that the L2 ligand adopts different coordination mode to bridge metal atoms in the two complexes. In 1, all the coordination atoms of the L2 ligand adopt monodentate fashion, whereas, the coordination atoms of the L2 ligand adopt monodentate and chelate-bidentate coordination modes in 2. Interestingly, detailed structural comparison of two complexes indicates that the [Zn2(imidazole)4] unit induces the progressive increase in the connectivities of the ultimate nets: that is, the [Zn2(imidazole)4] unit also plays a significant role in tuning the connectivity of the specific network. The thermal stability of complexes 1 and 2 have been studied using thermal gravimetric analysis (TGA) from 50 to 800 °C under flowing nitrogen at a heating rate of 10 °C/min. Both TGA curves of 1 and 2 show a gradual weight loss from the beginning, and a loss about 8.06 wt% (1) at 127 °C and 8.28 wt% (2) at 148 °C (see Supporting information, Fig. S4), which can be attributed to the total water molecules in the crystals (calcd 7.17 wt% for 1 and 2). When the temperature is higher than 389 °C (1) and 426 °C (2), the organic ligands could be removed leading to the collapse of the framework. The residues are ZnO.
In addition, taking into account the excellent luminescent properties of d10 metal complexes, the luminescence of complexes 1 and 2 were investigated (Fig. 3). It can be observed that an emission occurs at 414 nm (kex = 324 nm) for 1 and 412 nm (kex = 324 nm) for 2, respectively. To understand the nature of emission band, the luminescent properties of free H2L ligand was analyzed. It was found that the intense emission could be observed for free H2L ligand at 408 nm (kex = 336 nm). By comparing the luminescence of two complexes with free H2L ligand, the emission of complexes 1 and 2 may be attributable to an intraligand transition (p ? p*), which is in reasonable agreement with reported examples on this class of zinc coordination polymers [23]. In summary, we have synthesized two supramolecular isomers 1 and 2 formed by the L2 ligand and divalent zinc ions under different pH value. Compound 1 shows a novel two dimensional network while 2 is three dimensional framework containing the twofold interpenetrated. More interestingly, the two complexes both exhibit two kinds of topologies when the zinc center and the L2 ligand act as four-connected nodes or the [Zn2(imidazole)4] units are considered as four-connected notes for 1 and six-connected nodes for 2, respectively. Moreover, H2L ligand, 1 and 2 display fluorescent properties indicating that they may have potential applications as optical materials and further endeavors for exploration of H2L complexes are underway in our workgroup. Acknowledgments We are thankful for financial support from the Program for Changjiang Scholars and Innovative Research Team in University, the National Natural Science Foundation of China (No. 20573016), and the Science Foundation for Young Teachers of Northeast Normal University (No. 20070309). Appendix A. Supplementary material
Fig. 3. Normalized fluorescent emission spectra of H2L ligand, complexes 1 and 2 in the solid state at room temperature.
CCDC 707247 and 707248 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2008.11.030.
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[15] (a) M.V. Yigit, K. Biyikli, B. Moulton, J.C. MacDonald, Cryst. Growth Des. 6 (2006) 63; (b) C.A. Hester, H.L. Collier, R.G. Baughman, Polyhedron 15 (1996) 4255; (c) C. Kirchner, B. Krebs, Inorg. Chem. 26 (1987) 3569. [16] (a) C. Policar, F. Lambert, M. Cesario, I. Morgenstern-Badarau, Eur. J. Inorg. Chem. (1999) 2201; (b) P.R. Levstein, R. Calvo, Inorg. Chem. 29 (1990) 1581; (c) J.M. Rueff, N. Masciocchi, P. Rabu, A. Sironi, A. Skoulios, Eur. J. Inorg. Chem. (2001) 2843. [17] (a) P.C. Ford, E. Cariati, J. Bourassa, Chem. Res. 99 (1999) 3625; (b) M.L. Tong, X.M. Chen, B.H. Ye, L.N. Ji, Angew. Chem., Int. Ed. 38 (1999) 2237; (c) V.W.W. Yam, K.K.W. Lo, Chem. Soc. Rev. 28 (1999) 323. [18] X. Meng, Y. Song, H. Hou, H. Han, B. Xiao, Y. Fan, Y. Zhu, Inorg. Chem. 43 (2004) 3528. [19] Experimental: the ligand of 2,20 -biimidazole was prepared by the procedure reported in the literature [18]. Preparation of 4,40 -(1H,10 H-2,20 -biimidazole1,10 -diylbis(methylene))dibenzoic acid (H2L). A solution containing 2,20 biimidzole (0.01 mol) and NaOH (0.02 mol) in DMF (25 mL) was heated at 60 °C for 30 min. Then, 4-(chloromethyl)benzonitrile (0.02 mol) was added directly, and the mixture was continuously stirred and heated for 10 h. After cooling, the mixture was precipitated with 200 ml of distilled water. The white deposit was obtained, then that was hydrolyzed in NaOH solution. [Zn(L)] 2H2O (1): A mixture of Zn(OAc)2 2H2O (0.10 mmol) and H2L (0.10 mmol) were added to 10 mL of distilled water at room temperature. When the pH value of the mixture was adjusted to about 7 with NaOH solution (1 M), the mixture was transferred into a 25 mL Teflon-lined stainless steel vessel and heated to 150 °C for 3 days, then cooled to room temperature at a rate of 5 °C/h. The colorless block crystals 1 were obtained in ca. 55% yield (based on Zn). Anal. Calcd for C22H20N4O6Zn (1): C, 52.65; H, 4.01; N, 11.16. Found: C, 52.89; H, 4.13; N, 11.24%. Selected IR data (KBr, cm1): 3147s, 1609s, 1559s, 1508s, 1471s, 1272s, 1017s, 948s, 855s, 746s, 622s, 547s, 488s, 414s. [Zn(L)] 2H2O (2): The preparation of 2 was similar to that of 1 except that the pH value of the mixture was adjusted to about 8.5. The colorless crystals 2 were obtained in ca. 78% yield (based on Zn). Anal. Calcd for C22H20N4O6Zn (2): C, 52.65; H, 4.01; N, 11.16. Found: C, 52.78; H, 4.11; N, 11.27%. Selected IR data (KBr, cm1): 3119s, 1613s, 1550s, 1468s, 1241s, 1099s, 950s, 812s, 762s, 658s, 525s, 484s, 416s. [20] Crystal data: for 1, Mr = C22H20N4O6Zn: monoclinic, space group C2/c, a = 19.413(2), b = 11.085(2), c = 22.644(4) Å, b = 107.576(3)°, V = 4645(10) Å3, Z = 8, l = 1.107 mm1, Dc = 1.429 Mg/m3. 11,115 reflections measured, 4097 unique (R(int) = 0.0613) which were used in all calculations. R1 = 0.0526 (I > 2r(I)), wR2 = 0.1956 (all data). For 2, Mr = C22H20N4O6Zn: monoclinic, space group C2/c, a = 11.309(1), b = 18.456(1), c = 20.075(2) Å, b = 100.107(1)°, V = 4125(6) Å3, Z = 8 l = 1.241 mm1, Dc = 1.603 Mg/m3. 10,298 reflections measured, 3666 unique (R(int) = 0.0375) which were used in all calculations. R1 = 0.0432 (I > 2r(I)), wR2 = 0.1095 (all data). Data were collected on a Bruker ApexII CCD diffractometer with graphitemonochromated Mo Ka radiation (k = 0.71073 Å). The structures were solved with direct methods and refined with full-matrix leastsquares (SHELX-97). [21] (a) R.Q. Fang, X.M. Zhang, Inorg. Chem. 45 (2006) 4801; (b) R.S. Rarig, J. Zubieta, J. Chem. Soc., Dalton Trans. (2001) 3446. [22] (a) P. van der Sluis, A.L. Spek, Acta Crystallogr. Sect. A 46 (1990) 194; (b) A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University, The Netherlands, 2001. [23] (a) Y.P. Tong, S.L. Zheng, X.M. Chen, Inorg. Chem. 44 (2005) 4270; (b) S.L. Zheng, J.H. Yang, X.L. Yu, X.M. Chen, W.T. Wong, Inorg. Chem. 43 (2004) 830.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Two novel supramolecular isomers based on 2,20 -biimidazole derivative and zinc ions: Syntheses, structures and luminescent properties Yan-Hong Xu, Ya-Qian Lan, Ya-Hui Zhao, Dong-Ying Du, Guang-Juan Xu, Kui-Zhan Shao, Zhong-Min Su *, Yi Liao * Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China
a r t i c l e
i n f o
Article history: Received 3 November 2008 Accepted 28 November 2008 Available online 7 December 2008 Keywords: Supramolecular isomers Zn(II) 2,20 -Biimidazole Topology Luminescent property
a b s t r a c t Two Zn(II)-2,20 -biimidazole derivative supramolecular isomers 1 and 2 formulated as [Zn(L)] 2H2O (H2L = 4,40 -(1H,10 H-2,20 -biimidazole-1,10 -diylbis(methylene))dibenzoic acid) have been synthesized, and structurally characterized by elemental analysis, IR spectroscopy, single-crystal X-ray crystallography. Complex 1 exhibits a 2D four-connected network with (43.63) topology when the zinc center and the L2 ligand act as four-connected nodes (or a distorted (4,4) grid if the [Zn2(imidazole)4] units are considered as four-connected nodes). Complex 2 shows a novel four-connected twofold interpenetrated net with (42.62.82) topology when the zinc center and the L2 ligand act as four-connected nodes (or a twofold interpenetrated a-Po topology when [Zn2(imidazole)4] units are identified as six-connected nodes). Moreover, complexes 1 and 2 both exhibit strong luminescent properties at room temperature. Ó 2008 Published by Elsevier B.V.
The design and synthesis of metal–organic frameworks (MOFs) constructed from metal salts and bridging ligands have attracted great attention in recent years, not only because of their intriguing variety of architectures and topologies, such as rectangular grids, herringbones, boxes, ladders, brick walls, rings, diamondoids, and honeycombs, but also due to their potential applications in diverse areas such as electrical conductivity, magnetism, host-guest chemistry, molecular separation, gas storage, sensors and catalysis [1]. The rational design and controllable preparation of MOFs, which is highly influenced by several factors, for instance, the coordination geometry of the central atom, the structural characteristics of the ligand molecule, the counter anion, the pH values of the reaction solutions, symmetry the solvent system, the reaction conditions and so on [2,3]. From the viewpoint of crystal engineering, the most effective approach may be the appropriate choice of the well-designed organic bridging ligands containing modifiable backbones and connectivity information, together with the metal centers with various coordination preferences [4–7]. On the other hand, supramolecular isomerism, introduced by Zaworotko and coworkers [8], is the existence of more than one type of network superstructure for the same molecular building blocks and is therefore related to structural isomerism at the molecular level. It is one of the most important aspects of supramolecular chemistry, which has attracted great interest because of not only its structural diversity but also its potential application in recent years [9].
* Corresponding author. E-mail address: [email protected] (Z.-M. Su). 1387-7003/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.inoche.2008.11.030
2,20 -Biimidazole (H2biim) is an evergreen ligand that has attracted constant interest in supramolecular chemistry [10], biochemistry [11], cluster science [12], and antitumor drugs [13]. Some complexes based on H2biim or its derivatives have been reported because of their wide-ranging applications in enzymatic reactions and their intriguing structures [14,15]. On the other hand, it is well known that carboxylate ligands play important roles to construct novel MOFs in coordination chemistry and can adopt various binding modes such as terminal monodentate, chelating to one metal center, bridging bidentate in a syn–syn, syn– anti and anti–anti configuration to two metal centers, and bridging tridentate to two metal centers [16]. However, there are still only rare reports of ligands based on H2biim and carboxylate groups as building blocks for the construction of MOFs. According to the above considerations, we have recently designed and synthesized a novel ligand, namely, 4,40 -(1H,10 H-2,20 -biimidazole-1,10 -diylbis(methylene))dibenzoic acid (H2L) (see Supporting information, Scheme 1). The H2L ligand possesses the following interesting characteristics: (a) its flexibility due to the presence of a –CH2– spacer between the imidazole groups and phenyl groups, which allows subtle conformational adaptation of L2 for the self-assembly of unique supramolecular structures; (b) two rigid imidazole rings of L2 are connected by a rotatable C–C bond, which endows L2 ligand with subtle conformational adaptation. As well-known Zn(II)-containing coordination polymers have attracted extensive interest in recent years in that they not only exhibit appealing structures but also possess luminescent properties [17]. Here, we will describe the structures of the two Zn(II) supramolecular isomers with their formulas as [Zn(L)]
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2H2O. In addition, their luminescent properties were also investigated. Complex 1, which is synthesized through hydrothermal reaction [19], exhibits an interesting two-dimensional network structure. The single crystal X-ray analysis [20] shows that the asymmetrical unit of 1 contains a Zn(II) ion and a L2 ligand. Each zinc(II) center is coordinated by two nitrogen atoms (Zn1– N4C = 2.038(3) and Zn1–N1 = 2.051(3) Å) from two different L2 ligands and two carboxylic oxygen atoms (Zn1–O4B = 1.912(3) and Zn1–O1A = 1.958(3) Å) from two distinct L2 ligands in a distorted tetrahedral geometry ( Supporting information, Fig. S1) The bond distances are in the normal range of those observed in other zinc(II) complexes [21]. The L2 ligand bonds to four zinc ions through two carboxylic oxygen atoms and two nitrogen atoms in monodentate coordination fashion ( Supporting information, Scheme 2a). Two adjacent zinc atoms were linked by four nitrogen atoms from two H2biim rings of two L2 ligands to form a [Zn2(imidazole)4] unit with the Zn Zn distance of 4.286 Å (Fig. 1 left). Two imidazole rings of L2 ligand are not in the same plane with the dihedral angle of 70.47°. Because of the existence of –CH2– spacer between phenyl rings and H2biim unit, the L2 ligand exhibits its flexibility and two phenyl rings have the dihedral angle of 59.27°. A better insight into the nature of the framework can be achieved by the application of a topological approach. When the Zn(II) cation and the L2 ligand both act as a four-connected nodes, the structure can be identified as an unusual 2D four-connected framework with (43.63) topology (see Fig. 1, route I). Interestingly, when we take the [Zn2(imidazole)4] unit as a four-connected node and the L2 ligand as a linker, the structure can be described as a distorted 2D (4,4) grid (see Fig. 1, route II). When changing the pH value of the mixture under the same reaction conditions, complex 2 was obtained. The single crystal X-ray analysis [20] reveals complex 2 is an interesting twofold interpenetrated 3D network structure. Each Zn(II) atom is coordinated by three carboxylic oxygen atoms (Zn1–O2 = 1.970(2), Zn1–O3 = 2.029(3) and Zn1–O4 = 2.379(3) Å) from two different L2 ligands and two nitrogen atoms (Zn1–N1 = 2.052(3) and Zn1– N3 = 2.054(3) Å) from two distinct L2 ligands to furnish a slightly distorted trigonal bi-pyramidal geometry (Supporting information, Fig. S2). Compared with 1, the L2 ligand adopts another coordina-
tion mode in 2. The L2 ligand bonds to four zinc ions through one carboxylic oxygen atom and two nitrogen atoms in monodentate coordination fashion, and other two carboxylic oxygen atoms in chelate-bidentate bridging fashion (Supporting information, Scheme 2b). Similar to complex 1, two crystallographically equivalent Zn(II) atoms are bridged by four nitrogen atoms to generate a [Zn2(imidazole)4] fragment with the ZnZn separation of 4.366 Å (Fig. 2 left). Two imidazole rings of L2 ligand are not in the same plane, which are almost vertical to each other with the dihedral angle of 89.31°. Moreover, two phenyl rings of the L2 ligand have a dihedral angle of 13.29° which is larger than that of complex 1. When the Zn(II) atom and the L2 ligand both act as four-connected nodes, an unusual 3D four-connected twofold interpenetrated framework with (42.62.82) topology was obtained (see Fig. 2, route I). However, when the [Zn2(imidazole)4] fragment was regarded as a six-connected node and the L2 ligand acts as a linkage, the overall framework becomes a twofold interpenetrated a-Po topology (see Fig. 2, route II). Different linking modes between L ligand and [Zn2(imidazole)4] units lead to two isomers of the porous frameworks {[Zn(L)] 2H2O}n. Compound 1 crystallized in the monoclinic system and space group C2/c. Complex 1 has microporous viewed along the c-axes and all channels arrange in same direction, and calculations with PLATON show that the effective volume for the inclusion is 20.1% (935.4 Å3) of the crystal volume (4645.0 Å3) [22] (see Supporting information, Fig. S3a). Compound 2 also crystallized in the monoclinic system and space group C2/c. As illustrated in Fig. S3b, 2 has microporous viewed along the a-axes and all channels arrange in same direction, which account for 10.1% (414.9 Å3) of the crystal volume (4125.0 Å3) (see Supporting information, Fig. S3b). Supramolecular isomerism can be a consequence of the effect of the same molecular components generating different supramolecular synthons and could be synonymous with polymorphism. Complexes 1 and 2 are supramolecular isomers, the minor variance in the coordination sphere of Zn leads to their different properties. In addition, it is worth to note that the pH value of the mixture plays an important role in synthesis process of supramolecular isomers 1 and 2. The 2D framework of 1 was obtained at lower pH value, while 2 was constructed at a relatively high pH value showing
Fig. 1. The two simplified models applied in the topological analysis of 1. I: the metal and the L2 ligand as four-connected nodes, II: the [Zn2(imidazole)4] units as fourconnected nodes and all ligands as linear linkers.
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Fig. 2. The two simplified models applied in the topological analysis of 2. I: the metal and the L2 ligand as four-connected nodes, II: the [Zn2(imidazole)4] unit as a fourconnected node and the ligand as linear linker.
a 3D framework. Investigation on their structural difference reveals that the L2 ligand adopts different coordination mode to bridge metal atoms in the two complexes. In 1, all the coordination atoms of the L2 ligand adopt monodentate fashion, whereas, the coordination atoms of the L2 ligand adopt monodentate and chelate-bidentate coordination modes in 2. Interestingly, detailed structural comparison of two complexes indicates that the [Zn2(imidazole)4] unit induces the progressive increase in the connectivities of the ultimate nets: that is, the [Zn2(imidazole)4] unit also plays a significant role in tuning the connectivity of the specific network. The thermal stability of complexes 1 and 2 have been studied using thermal gravimetric analysis (TGA) from 50 to 800 °C under flowing nitrogen at a heating rate of 10 °C/min. Both TGA curves of 1 and 2 show a gradual weight loss from the beginning, and a loss about 8.06 wt% (1) at 127 °C and 8.28 wt% (2) at 148 °C (see Supporting information, Fig. S4), which can be attributed to the total water molecules in the crystals (calcd 7.17 wt% for 1 and 2). When the temperature is higher than 389 °C (1) and 426 °C (2), the organic ligands could be removed leading to the collapse of the framework. The residues are ZnO.
In addition, taking into account the excellent luminescent properties of d10 metal complexes, the luminescence of complexes 1 and 2 were investigated (Fig. 3). It can be observed that an emission occurs at 414 nm (kex = 324 nm) for 1 and 412 nm (kex = 324 nm) for 2, respectively. To understand the nature of emission band, the luminescent properties of free H2L ligand was analyzed. It was found that the intense emission could be observed for free H2L ligand at 408 nm (kex = 336 nm). By comparing the luminescence of two complexes with free H2L ligand, the emission of complexes 1 and 2 may be attributable to an intraligand transition (p ? p*), which is in reasonable agreement with reported examples on this class of zinc coordination polymers [23]. In summary, we have synthesized two supramolecular isomers 1 and 2 formed by the L2 ligand and divalent zinc ions under different pH value. Compound 1 shows a novel two dimensional network while 2 is three dimensional framework containing the twofold interpenetrated. More interestingly, the two complexes both exhibit two kinds of topologies when the zinc center and the L2 ligand act as four-connected nodes or the [Zn2(imidazole)4] units are considered as four-connected notes for 1 and six-connected nodes for 2, respectively. Moreover, H2L ligand, 1 and 2 display fluorescent properties indicating that they may have potential applications as optical materials and further endeavors for exploration of H2L complexes are underway in our workgroup. Acknowledgments We are thankful for financial support from the Program for Changjiang Scholars and Innovative Research Team in University, the National Natural Science Foundation of China (No. 20573016), and the Science Foundation for Young Teachers of Northeast Normal University (No. 20070309). Appendix A. Supplementary material
Fig. 3. Normalized fluorescent emission spectra of H2L ligand, complexes 1 and 2 in the solid state at room temperature.
CCDC 707247 and 707248 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2008.11.030.
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