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Two new octamolybdate-based metal–organic polymers: Structures, semiconducting and photoluminescent properties
Journal of Molecular Structure 935 (2009) 69–74
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Two new octamolybdate-based metal–organic polymers: Structures, semiconducting and photoluminescent properties Hong-Ying Zang, Ya-Qian Lan, Guang-Sheng Yang, Zhong-Min Su *, Xin-Long Wang, Kui-Zhan Shao, Li-Kai Yan Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 28 April 2009 Received in revised form 23 June 2009 Accepted 27 June 2009 Available online 2 July 2009 Keywords: Inorganic–organic hybrid compounds Octamolybdate Semiconductor Nickel Copper
a b s t r a c t Two new octamolybdate-based inorganic–organic hybrid compounds NiII(HL)2(H2O)2(b-Mo8O26) (1) and CuI4L4(b-Mo8O26) (2) (L = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine) have been hydrothermally synthesized and structurally characterized. Compound 1 is built up of octamolybdate anions [b-Mo8O26]4 covalently linked by [Ni(HL)2(H2O)2]4+ cations into 1D infinite chains and was further connected by hydrogen bonds into a 3D supramolecular structure. Compound 2 is constructed from [b-Mo8O26]4 building blocks and [Cu2L2]2n4n+ strands to form a 3-connected 2D network. Furthermore, the diffuse reflectance spectra reveal that these two inorganic–organic hybrid compounds are potential wide gap semiconductor materials. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Currently, inorganic–organic hybrid compounds, as a new generation of solid-state materials, have widely aroused chemists’ interest owing to their academic significance and various potential applications in different areas such as heterogeneous catalysis, sorption, electrical conductivity, magnetism, and nonlinear optics [1]. Meanwhile, the research on polyoxometalates (POMs), which is an immense class of well-defined inorganic metal oxide clusters, is driven by their remarkable structural and electronic properties and also realized applications in catalysis [2,3]. As a consequence, in most of the polyoxoanion-based hybrid compounds, chemically robust polyoxoanions are usually utilized as inorganic components, while organic components are generally composed of organic amines, profoundly influencing the structure of the composite materials [4]. One remarkable strategy for designing such inorganic–organic hybrid materials relies on linking molecular cluster subunits POMs with secondary transition-metal complexes (TMCs) through bridging oxygen groups to achieve one-, two-, or three-dimensional hybrid compounds, there being an important advance in this realm [5], nevertheless, an ongoing research project is necessary to enrich and develop this field. Since Schwing-Weill and Arnaud-Neu presented the first clear evidence for isomers of octamolybdate in an infrared (IR) study in 1970 [6], the research on octamolybdate has * Corresponding author. E-mail address: [email protected] (Z.-M. Su). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.045
sprung up which is an interesting cluster with a variety of structural isomers, especially preparation of octamolybdate-based hybrid materials, such as in Zubieta and Lu’s groups [7,8]. As reported previously, most of researches are based on rigid ligands such as o-phenanthroline and 4,40 -bipyridine, however, flexible ones have more advantages in that their flexibility and conformational freedom allow them to conform to the coordination environment of the transition-metal ions and POMs [1a,4a,5b]. Newly, a large number of intriguing hybrid compounds based on b-octamolybdate anions have been reported, for example, K[{Cu(imi)(1,10-phen)}2Cl][Mo8O26] [9a], [Cu2(C8H6N2)2(C7H6N2)]2[Mo8O26] [8a], (nBu4N)2n[Ag2Mo8O26(dmso)2]n [9b], and so on. In this paper, we took advantage of asymmetrical 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine (L) as a flexible organic ligand and synthesized two inorganic–organic hybrid materials based on [b-Mo8O26]4 anions at different pH values under hydrothermal conditions, that is, NiII(HL)2(H2O)2(b-Mo8O26) (1) and CuI4L4(b-Mo8O26) (2). The crystal structures of two compounds and the IR, thermogravimetric analysis, as well as semiconducting and photoluminescent properties will be represented and discussed. 2. Experimental 2.1. Materials and instruments All chemicals purchased were of reagent grade and used without further purification. Elemental analyses (C, H, and N) were
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performed on a Perkin-Elmer 240C elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets in the range 4000–400 cm1 on a Mattson Alpha-Centauri spectrometer. Solid-state fluorescence spectra were measured on a Cary Eclipse spectrofluorometer (Varian) equipped with a xenon lamp and quartz carrier at room temperature. Room-temperature UV–Vis/near IR diffuse reflectance spectra were collected on a finely ground sample with a Cary 500 spectrophotometer equipped with a 110 mm diameter integrating sphere and computer control using the ‘‘Scan” software. Diffuse reflectance was measured from 200 to 900 nm using barium sulfate (BaSO4) as a standard with 100% reflectance. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 40 to 700 °C with a heating rate of 5 °C min1 under nitrogen. 2.2. Synthesis 2.2.1. Synthesis of 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine (L) 3-(Chloromethyl) pyridine hydrochloride and 1H-1,2,4-triazole reacted as the method described in the literature [10], and the ligand L was achieved. IR (cm1): 3114 (s), 2916 (m), 1656 (m), 1588 (sh), 1510 (w), 1480 (m), 1429 (m), 966 (w), 740 (s). 2.2.2. Synthesis of [NiII(HL)2(H2O)2(Mo8O26)] (1) A mixture of (NH4)6Mo7O24?4H2O (0.2472 g, 0.2 mmol), NiCl26H2O (0.0237 g, 0.1 mmol), L (0.5 mL) and H2O (10 mL) was stirred at ambient temperature for 30 min, and the pH was adjusted to 3.0 with 1 M HCl solution. The resulting suspension was transferred to a Teflon-lined stainless autoclave (25 mL) and kept at 150 °C for 72 h. Then it was cooled to 100 °C at a rate of 5 °C h1, held for 8 h, and followed by further cooling to 30 °C at a rate of 3 °C h1. Green crystals of 1 were collected in 65% yield based on NiCl26H2O. Elemental Anal. Found: C, 12.04; H, 1.42; N, 6.90. Anal. Calcd for C16H22N8O28NiMo8 (1600.65): C, 12.01; H, 1.39; N, 7.00%. IR (cm1): 3314 (w), 3179 (w), 1604 (m), 1535 (m), 1465 (m), 1247 (m), 1131 (m), 949 (s), 914 (sh), 838 (s), 722 (s), 671 (s), 555 (s), 519 (s). 2.2.3. Synthesis of [CuI4L4(Mo8O26)] (2) A mixture of (NH4)6Mo7O24?4H2O (0.2472 g, 0.2 mmol), CuCl (0.198 g, 0.2 mmol), L (0.5 mL) and H2O (10 mL) was stirred at ambient temperature for 30 min, and the pH was adjusted to 5.0 with 1 M HCl and 1 M NaOH solution. The resulting suspension was transferred to a Teflon-lined stainless autoclave (25 mL) and kept at 150 °C for 72 h. Then it was cooled to 100 °C at a rate of 5 °C h1, held for 8 h, and followed by further cooling to 30 °C at a rate of 3 °C h1. Red crystals of 2 were collected in 66% yield based on CuCl. Elemental Anal. Found: C, 18.42; H, 1.51; N, 10.82. Anal. Calcd for C32H32N16O26Cu4Mo8 (2078.42): C, 18.49; H, 1.55; N, 10.78%. IR (cm1): 3421 (w), 3111 (w), 1605 (m), 1529 (m), 1480 (m), 1434 (m), 1283 (m), 1128 (s), 941 (s), 898 (s), 830 (s), 662 (s), 557 (s), 518 (s). 2.3. Single-crystal X-ray diffraction Single-crystal X-ray diffraction data collection of the compounds was performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71069 Å) at room temperature. All absorption corrections were performed by using the SADABS program. The two crystal structures were solved by the Direct Method of SHELXS-97 [11] and refined with full-matrix least-squares techniques (SHELXL-97) [12] within WINGX [13]. Anisotropic thermal parameters were employed to refine all nonhydrogen atoms. The hydrogen atoms of organic ligands were fixed at idealized positions as rigid groups. Protonated hydrogen atoms
attached to nitrogen atoms and H atoms of water molecules were positioned from the difference Fourier map. The detailed crystallographic data and structure refinement parameters were listed in Table 1. 3. Results and discussion 3.1. Crystal structure descriptions 3.1.1. [NiII(HL)2(H2O)2(Mo8O26)] (1) X-ray crystallographic analysis reveals that compound 1 crystal and the asymmetric unit contains izes in the triclinic space group P1, one nickel(II), one 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine whose nitrogen atom from pyridine is protonated, half a [Mo8O26]4 anion and one coordinated water molecule. The six-coordinated NiII ion is surrounded by two nitrogen atoms from two ligands (Ni(1)– N(4) = 2.050(3), Ni(1)–N(4#2) = 2.050(3) Å), two Ot (terminal oxygen) atoms from octamolybdate anions (Ni(1)–O(1) = 2.055(2), Ni(1)–O(1#2) = 2.055(2) Å) and two coordinated water molecules (Ni(1)–O1W = 2.078(3), Ni(1)-O1W(#2) = 2.078(3) Å), showing a distorted octahedral configuration (Fig. 1a and Table 2). The [Mo8O26]4 anion exhibits the well-known b isomer consisting of eight distorted corner- and/or edge-sharing {MoO6} octahedra and containing four kinds of O atoms: l5-O, l3-O, l2-O and the terminal atoms (Ot) [4b]. Intriguingly, in compound 1 this organonitrogen molecule, of which N atom in pyridine ring is protonated, serves as a ligand simultaneously as a charge-compensating unit. As illustrated in Fig. 1b, each [Mo8O26]4 polyoxoanion bridges two adjacent [Ni(HL)2(H2O)2]4+ cations with Ni–O distances of 2.055(2) Å, giving rise to a 1D extended infinite chain. Additionally, hydrogen bonds provided by coordinated water molecules and protonated N atoms are of great importance in stabilizing the crystal structure of compound 1. As shown in Fig. 1b, there exist intramolecular and intermolecular hydrogen-bonding interactions (Table 3), namely, coordinated water molecule O1W and N atom from pyridine ring of ligand donating H-bonds to terminal oxygen atoms of two distinct [b-Mo8O26]4 anions (O1W–H1A O13#1, O1W–H1B O8#4, N1– H1N O10 and N1–H1N O2#3), respectively, which contributes to the formation and stability of the 3D supramolecular structure. 3.1.2. [CuI4L4(Mo8O26)] (2) When substituting CuCl for NiCl26H2O, and adjusting pH value to 5.0, compound 2 was obtained. The structure of hydrothermally Table 1 Crystal data and structure refinement for compounds 1 and 2.
Formula Molecular weight Crystal system Space group a (Å) b (Å) c (Å) a (deg) b (deg) c (deg) V (Å3) Z Dcalcd (g cm3) F (0 0 0) Reflns collected/unique R (int) GOF on F2 R1a [I > 2r (I)] wR2b a b
1
2
C16H22Mo8N8NiO28 1600.65 Triclinic P1
C32H32Cu4Mo8N16O26 2078.42 Triclinic P1
9.7340(6) 10.4050(6) 10.5830(6) 108.6610(7) 91.0500(7) 116.4910(7) 892.35(10) 1 2.979 762 4494/3093 0.0120 1.112 0.0220 0.0615
10.717(3) 11.645(3) 12.116(3) 93.429(4) 116.208(3) 98.566(3) 1328.0(6) 1 2.599 996 6657/4594 0.0260 1.029 0.0490 0.1287
R1 = R||Fo| |Fc||/R|Fo|. wR2 = |Rw(|Fo|2 |Fc|2)|/R|w(Fo2)2|1/2.
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Fig. 1. (a) Coordination environment of the NiII center in compound 1 (Most of the hydrogen atoms are omitted for clarity except for the protonated ones). (b) View of the supramolecular structure through hydrogen-bonding interactions along c axis. [Symmetry codes: (#1) x + 1, y + 1, z + 1; (#2) x + 2, y + 2, z + 2; (#3) x + 2, y + 2, z + 1; (#4) x, y, z + 1].
Table 2 Selected bond lengths (Å) and angles (°) for compound 1. Ni(1)–N(4) Ni(1)–O(1W) N(4)–Ni(1)–O(1)#2 N(4)–Ni(1)–O(1) N(4)#2–Ni(1)–O(1w) O(1)#2–Ni(1)–O(1W)
2.050(3) 2.078(3) 90.60(12) 89.40(12) 87.49(13) 92.93(11)
Ni(1)–O(1)
2.055(2)
N(4)–Ni(1)–O(1W) N(4)#2–Ni(1)–O(1) O(1)–Ni(1)–O(1W)
92.51(13) 90.62(12) 87.07(11)
Symmetry transformations used to generate equivalent atoms: #2 x + 2, y + 2, z + 2.
Table 3 Hydrogen bonds length (Å) and angles (°) for compound 1. D–H A
d(D–H)
d(H A)
d(D A)
<(DHA)
N(1)–H(1N) O(2)#3 N(1)–H(1N) O(10) O(1W)–H(1A) O(13)#1 O(1W)–H(1B) O(8)#4
0.849(10) 0.849(10) 0.847(10) 0.844(10)
2.21(4) 2.34(4) 1.906(13) 1.983(17)
2.885(4) 3.021(5) 2.742(4) 2.807(4)
137(5) 137(5) 169(4) 165(4)
Symmetry transformations used to generate equivalent atoms: #1 x + 1, y + 1, z + 1 #3 x + 2, y + 2, z + 1 #4 x, y, z + 1.
synthesized 2 was determined by X-ray crystallographic study and showed to be constructed from [b-Mo8O26]4 building blocks and [Cu2L2]2n4n+ strands. According to charge conversation and bond
valence sum calculations [14], Cu atoms are proven to be in +I oxidation state, as well as confirmed by the particular coordination environments of CuI [15]. It is noteworthy that CuI is metastable and prone to be oxidized to CuII, however, in this system the CuI compound has been successfully obtained probably because organonitrogen species function not only as ligands but also as reducing reagent under hydrothermal conditions [5a,7a,16]. There are two distinct CuI environments: CuI (1) is surrounded by two nitrogen atoms from two distinct L ligands and an oxygen atom from terminal oxygen atoms of an octamolybdate anion displaying T-type geometry, while CuI (2) is coordinated by three nitrogen atoms from different L ligands exhibiting Y-shaped geometry (Fig. 2a and Table 4). In this compound, the ligand, 3-((1H-1,2,4triazol-1-yl)methyl)pyridine, which is not protonated, performs as a bridging linker, showing a diverse coordination mode. For one thing, it acts as a bidentate linkage, bridging two CuI ions through apical N atoms; for the other, as a tridentate linkage connecting three CuI ions, meanwhile each CuI (1) ion links two L ligands and each CuI (2) ion binds to three L ligands to form a railroad motif (Fig. 2b and Fig. S1). The most striking structural feature of compound 2 is that these cationic strands are paired in an inclined way and parallel with each other, and further connected by the [b-Mo8O26]4 anions through Cu–O bonds with the distance of 2.394(6) Å. Meanwhile, [b-Mo8O26]4 anions are sandwiched be-
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Fig. 2. (a) Coordination environment of CuI center in compound 2 (all of the hydrogen atoms are omitted for clarity). (b) The metal–organic structure of compound 2 shows a railroad motif and the corresponding schematic representation of a 3-connected 2D network. [Symmetry codes: (#1) x, y + 1, z 1; (#2) x + 1, y + 1, z; (#3) x + 1, y, z].
Table 4 Selected bond lengths (Å) and angles (°) for compound 2. Cu(1)–N(4) Cu(1)–N(5) Cu(1)–O(10)#3 N(5)–Cu(1)–N(4) N(5)–Cu(1)–O(10)#3 N(4)–Cu(1)–O(10)#3
1.904(7) 1.894(7) 2.394(6) 172.8(3) 89.0(3) 96.3(3)
Cu(2)–N(1) Cu(2)–N(8)#2 Cu(2)–N(3)#1 N(1)–Cu(2)–N(8)#2 N(1)–Cu(2)–N(3)#1 N(8)#2–Cu(2)–N(3)#1
3.2. Thermogravimetric analysis 1.942(7) 1.977(8) 2.008(7) 128.0(3) 126.7(3) 104.8(3)
Symmetry transformations used to generate equivalent atoms: #1 x, y + 1, z 1 #2 x + 1, y + 1, z #3 x + 1, y, z.
tween adjacent strands (Fig. 2b). Furthermore, CuI ions as well as the ligands can be regarded as 3-connected nodes respectively, so a 3-connected 2D network can be achieved. Two new compounds with distinct structural characteristics have been attained, by using the same ligand L, polyoxoanions and different metal ions. The L ligands coordinate to NiII or CuI ions in different modes, respectively, contributing to different metal– organic units: in 1 at pH 3.0, the nitrogen atom of pyridine ring from the ligand is protonated, so it is unbound to NiII ion, acting as a counterion, while with the increase of the pH value to 5.0, no protonation occurs to the nitrogen atom of the pyridine ring, and the nitrogen atom connects to CuI ion, further extending to a railroad strand. Furthermore, NiII and CuI ions exhibit different coordination mode: in 1, each NiII ion links two [b-Mo8O26]4 anions through terminal oxygen atoms, two water molecules and two protonated ligands, resulting in a 1D chain. Different from 1, in 2 CuI ions are surrounded by nitrogen atoms, extending to a railroad strand, meanwhile part of them are bridged by [b-Mo8O26]4 anions through terminal oxygen atoms, so as to pillar adjacent strands to a 2D network.
The thermal stabilities of the two compounds are shown in Fig. S2. The TG curve for 1 (Fig. S2a) indicates that the first weight loss in the range 219 to around 230 °C corresponds to the departure of coordination water molecules (exptl, 2.55%; calcd, 2.25%), and the second sharp weight loss occurred at around 379 °C, corresponding to the decomposition of organic ligands (exptl, 19.38%; calcd, 21.14%). Compound 2 was stable up to 289 °C (Fig. S2b), whereupon one sharp stage of weight loss occurs (exptl, 28.52%; calcd, 29.29%). Thermogravimetric experimental results indicate that 1 and 2 have promising thermal stabilities. 3.3. Optical energy gap Several POM-based inorganic–organic hybrid compounds have been reported to be promising semiconductors, such asCo2(bpy)6(W6O19)2 (Eg = 2.2 eV) [17a], [Cd(BPE)(a-Mo8O26)][Cd(BPE)(DMF)4]2DMF (Eg = 3.45 eV) [17b], (n-Bu4N)2[Mo6O17–(„NAr)2] (Ar = o-CH3OC6H4) (Eg = 2.25 eV) [17c]. In order to explore the conductivity of the two compounds, the measurements of diffuse reflectance for powder samples were done to achieve their band gap (Eg). The reflectivity data were converted by a Kubelka–Munk tansformation to better locate the corresponding adsorption bands and adsorption threshold, and the band gap Eg was determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in a plot of Kubelka–Munk function F against E [18]. As shown in Fig. 3, the corresponding well-defined optical absorption associated with band gaps (Eg) can be assessed at 3.40 eV for 1 and 2.25 eV for 2, which indicates that these hybrid compounds are potential wide gap semiconductor materials. The 1D
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photoluminescent property. Further research is ongoing to prepare new compounds with outstanding luminescent and semiconducting properties. Acknowledgments
Fig. 3. The diffuse reflectance UV–Vis spectra of K–M functions versus energy (eV) of compound 1 and 2.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project Nos. 20573016 and 20703008), Chang Jiang Scholars Program (2006), Program for Changjiang Scholars and Innovative Research Team in University (IRT0714), the National High-tech Research and Development Program (863 Program 2007AA03Z354), Department of Science and Technology of Jilin Province (20082103), the Science Foundation for Young Teachers of NENU (No. 20090407) and the Training Fund of NENU’s Scientific Innovation Project (NENUSTC07017 and -STC08019). Appendix A. Supplementary data
[NiII(b-Mo8O26)]n2n chain and extended [CuI2(b-Mo8O26)]n2n framework in hybrid structures appear to be responsible for their optical band gap respectively [8a,17b]. In our viewpoint, given the different Eg values of the two compounds, we assume that optical band gap can be influenced by metal ions, however, more experimental evidences are needed to confirm that. 3.4. Photoluminescent property Currently, inorganic–organic hybrid compounds containing CuI cations have been investigated on fluorescent properties for their potential applications as luminescent materials [1a,4b,19]. The photoluminescent properties of free neutral ligand L and compound 2 were investigated at room temperature (Fig. S3). The character of the solid-state emission spectrum of ligand may be attributed to excimer or trimer/multimer due to the intense p–p interaction between ligands. It’s well-known that the emission of excimer exhibits a broad and structureless band [20]. However, after the formation of complex, the extent of face-to-face coplanarity between the flexible ligands become weaken arising from the restriction of Cu(I), and thus the intense of p–p interaction may significantly decreased or even disappear. This observation is consistent with the geometry of the compound by means of density functional calculation. Compared with the maximum emission wavelength for the L ligand at 394 nm (kex = 245 nm), compound 2 exhibits an intense peak photoluminescence emission at 370 nm (kex = 298 nm), showing an obvious blue shift. Based on the theoretical calculation TDDFT/ B3LYP/6-31+G(d) (Cu with LANL2DZ), the origin of the emission of compound 2 may be tentatively assigned to metal-to-ligand charge transfer (MLCT) [21], and the high-energy emission may originate from metal-centered 3d ? (4s,4p) triple excited state [22]. In fact, several theories have been published to explain the electronic transition mechanism of the CuI compounds, but the photoluminescence of these compounds is not well understood so far [23]. 4. Conclusion In summary, two new octamolybdate-based inorganic–organic hybrid compounds, NiII(HL)2(H2O)2(b-Mo8O26) and CuI4L4(bMo8O26) have been successfully synthesized under hydrothermal condition, by using NiII or CuI ions, (NH4)6Mo7O244H2O and the organonitrogen molecule 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine which can act as ligand simultaneously as counterion. The diffuse reflectivity spectrum measurements reveal that these two inorganic–organic hybrid compounds with Eg 3.40 eV and 2.25 eV, respectively, are potential wide gap semiconductor materials. Additionally, the solid-state fluorescence spectra indicate that compound 2 has
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Two new octamolybdate-based metal–organic polymers: Structures, semiconducting and photoluminescent properties Hong-Ying Zang, Ya-Qian Lan, Guang-Sheng Yang, Zhong-Min Su *, Xin-Long Wang, Kui-Zhan Shao, Li-Kai Yan Institute of Functional Material Chemistry, Key Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 28 April 2009 Received in revised form 23 June 2009 Accepted 27 June 2009 Available online 2 July 2009 Keywords: Inorganic–organic hybrid compounds Octamolybdate Semiconductor Nickel Copper
a b s t r a c t Two new octamolybdate-based inorganic–organic hybrid compounds NiII(HL)2(H2O)2(b-Mo8O26) (1) and CuI4L4(b-Mo8O26) (2) (L = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine) have been hydrothermally synthesized and structurally characterized. Compound 1 is built up of octamolybdate anions [b-Mo8O26]4 covalently linked by [Ni(HL)2(H2O)2]4+ cations into 1D infinite chains and was further connected by hydrogen bonds into a 3D supramolecular structure. Compound 2 is constructed from [b-Mo8O26]4 building blocks and [Cu2L2]2n4n+ strands to form a 3-connected 2D network. Furthermore, the diffuse reflectance spectra reveal that these two inorganic–organic hybrid compounds are potential wide gap semiconductor materials. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Currently, inorganic–organic hybrid compounds, as a new generation of solid-state materials, have widely aroused chemists’ interest owing to their academic significance and various potential applications in different areas such as heterogeneous catalysis, sorption, electrical conductivity, magnetism, and nonlinear optics [1]. Meanwhile, the research on polyoxometalates (POMs), which is an immense class of well-defined inorganic metal oxide clusters, is driven by their remarkable structural and electronic properties and also realized applications in catalysis [2,3]. As a consequence, in most of the polyoxoanion-based hybrid compounds, chemically robust polyoxoanions are usually utilized as inorganic components, while organic components are generally composed of organic amines, profoundly influencing the structure of the composite materials [4]. One remarkable strategy for designing such inorganic–organic hybrid materials relies on linking molecular cluster subunits POMs with secondary transition-metal complexes (TMCs) through bridging oxygen groups to achieve one-, two-, or three-dimensional hybrid compounds, there being an important advance in this realm [5], nevertheless, an ongoing research project is necessary to enrich and develop this field. Since Schwing-Weill and Arnaud-Neu presented the first clear evidence for isomers of octamolybdate in an infrared (IR) study in 1970 [6], the research on octamolybdate has * Corresponding author. E-mail address: [email protected] (Z.-M. Su). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.045
sprung up which is an interesting cluster with a variety of structural isomers, especially preparation of octamolybdate-based hybrid materials, such as in Zubieta and Lu’s groups [7,8]. As reported previously, most of researches are based on rigid ligands such as o-phenanthroline and 4,40 -bipyridine, however, flexible ones have more advantages in that their flexibility and conformational freedom allow them to conform to the coordination environment of the transition-metal ions and POMs [1a,4a,5b]. Newly, a large number of intriguing hybrid compounds based on b-octamolybdate anions have been reported, for example, K[{Cu(imi)(1,10-phen)}2Cl][Mo8O26] [9a], [Cu2(C8H6N2)2(C7H6N2)]2[Mo8O26] [8a], (nBu4N)2n[Ag2Mo8O26(dmso)2]n [9b], and so on. In this paper, we took advantage of asymmetrical 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine (L) as a flexible organic ligand and synthesized two inorganic–organic hybrid materials based on [b-Mo8O26]4 anions at different pH values under hydrothermal conditions, that is, NiII(HL)2(H2O)2(b-Mo8O26) (1) and CuI4L4(b-Mo8O26) (2). The crystal structures of two compounds and the IR, thermogravimetric analysis, as well as semiconducting and photoluminescent properties will be represented and discussed. 2. Experimental 2.1. Materials and instruments All chemicals purchased were of reagent grade and used without further purification. Elemental analyses (C, H, and N) were
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performed on a Perkin-Elmer 240C elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets in the range 4000–400 cm1 on a Mattson Alpha-Centauri spectrometer. Solid-state fluorescence spectra were measured on a Cary Eclipse spectrofluorometer (Varian) equipped with a xenon lamp and quartz carrier at room temperature. Room-temperature UV–Vis/near IR diffuse reflectance spectra were collected on a finely ground sample with a Cary 500 spectrophotometer equipped with a 110 mm diameter integrating sphere and computer control using the ‘‘Scan” software. Diffuse reflectance was measured from 200 to 900 nm using barium sulfate (BaSO4) as a standard with 100% reflectance. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 40 to 700 °C with a heating rate of 5 °C min1 under nitrogen. 2.2. Synthesis 2.2.1. Synthesis of 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine (L) 3-(Chloromethyl) pyridine hydrochloride and 1H-1,2,4-triazole reacted as the method described in the literature [10], and the ligand L was achieved. IR (cm1): 3114 (s), 2916 (m), 1656 (m), 1588 (sh), 1510 (w), 1480 (m), 1429 (m), 966 (w), 740 (s). 2.2.2. Synthesis of [NiII(HL)2(H2O)2(Mo8O26)] (1) A mixture of (NH4)6Mo7O24?4H2O (0.2472 g, 0.2 mmol), NiCl26H2O (0.0237 g, 0.1 mmol), L (0.5 mL) and H2O (10 mL) was stirred at ambient temperature for 30 min, and the pH was adjusted to 3.0 with 1 M HCl solution. The resulting suspension was transferred to a Teflon-lined stainless autoclave (25 mL) and kept at 150 °C for 72 h. Then it was cooled to 100 °C at a rate of 5 °C h1, held for 8 h, and followed by further cooling to 30 °C at a rate of 3 °C h1. Green crystals of 1 were collected in 65% yield based on NiCl26H2O. Elemental Anal. Found: C, 12.04; H, 1.42; N, 6.90. Anal. Calcd for C16H22N8O28NiMo8 (1600.65): C, 12.01; H, 1.39; N, 7.00%. IR (cm1): 3314 (w), 3179 (w), 1604 (m), 1535 (m), 1465 (m), 1247 (m), 1131 (m), 949 (s), 914 (sh), 838 (s), 722 (s), 671 (s), 555 (s), 519 (s). 2.2.3. Synthesis of [CuI4L4(Mo8O26)] (2) A mixture of (NH4)6Mo7O24?4H2O (0.2472 g, 0.2 mmol), CuCl (0.198 g, 0.2 mmol), L (0.5 mL) and H2O (10 mL) was stirred at ambient temperature for 30 min, and the pH was adjusted to 5.0 with 1 M HCl and 1 M NaOH solution. The resulting suspension was transferred to a Teflon-lined stainless autoclave (25 mL) and kept at 150 °C for 72 h. Then it was cooled to 100 °C at a rate of 5 °C h1, held for 8 h, and followed by further cooling to 30 °C at a rate of 3 °C h1. Red crystals of 2 were collected in 66% yield based on CuCl. Elemental Anal. Found: C, 18.42; H, 1.51; N, 10.82. Anal. Calcd for C32H32N16O26Cu4Mo8 (2078.42): C, 18.49; H, 1.55; N, 10.78%. IR (cm1): 3421 (w), 3111 (w), 1605 (m), 1529 (m), 1480 (m), 1434 (m), 1283 (m), 1128 (s), 941 (s), 898 (s), 830 (s), 662 (s), 557 (s), 518 (s). 2.3. Single-crystal X-ray diffraction Single-crystal X-ray diffraction data collection of the compounds was performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Ka radiation (k = 0.71069 Å) at room temperature. All absorption corrections were performed by using the SADABS program. The two crystal structures were solved by the Direct Method of SHELXS-97 [11] and refined with full-matrix least-squares techniques (SHELXL-97) [12] within WINGX [13]. Anisotropic thermal parameters were employed to refine all nonhydrogen atoms. The hydrogen atoms of organic ligands were fixed at idealized positions as rigid groups. Protonated hydrogen atoms
attached to nitrogen atoms and H atoms of water molecules were positioned from the difference Fourier map. The detailed crystallographic data and structure refinement parameters were listed in Table 1. 3. Results and discussion 3.1. Crystal structure descriptions 3.1.1. [NiII(HL)2(H2O)2(Mo8O26)] (1) X-ray crystallographic analysis reveals that compound 1 crystal and the asymmetric unit contains izes in the triclinic space group P1, one nickel(II), one 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine whose nitrogen atom from pyridine is protonated, half a [Mo8O26]4 anion and one coordinated water molecule. The six-coordinated NiII ion is surrounded by two nitrogen atoms from two ligands (Ni(1)– N(4) = 2.050(3), Ni(1)–N(4#2) = 2.050(3) Å), two Ot (terminal oxygen) atoms from octamolybdate anions (Ni(1)–O(1) = 2.055(2), Ni(1)–O(1#2) = 2.055(2) Å) and two coordinated water molecules (Ni(1)–O1W = 2.078(3), Ni(1)-O1W(#2) = 2.078(3) Å), showing a distorted octahedral configuration (Fig. 1a and Table 2). The [Mo8O26]4 anion exhibits the well-known b isomer consisting of eight distorted corner- and/or edge-sharing {MoO6} octahedra and containing four kinds of O atoms: l5-O, l3-O, l2-O and the terminal atoms (Ot) [4b]. Intriguingly, in compound 1 this organonitrogen molecule, of which N atom in pyridine ring is protonated, serves as a ligand simultaneously as a charge-compensating unit. As illustrated in Fig. 1b, each [Mo8O26]4 polyoxoanion bridges two adjacent [Ni(HL)2(H2O)2]4+ cations with Ni–O distances of 2.055(2) Å, giving rise to a 1D extended infinite chain. Additionally, hydrogen bonds provided by coordinated water molecules and protonated N atoms are of great importance in stabilizing the crystal structure of compound 1. As shown in Fig. 1b, there exist intramolecular and intermolecular hydrogen-bonding interactions (Table 3), namely, coordinated water molecule O1W and N atom from pyridine ring of ligand donating H-bonds to terminal oxygen atoms of two distinct [b-Mo8O26]4 anions (O1W–H1A O13#1, O1W–H1B O8#4, N1– H1N O10 and N1–H1N O2#3), respectively, which contributes to the formation and stability of the 3D supramolecular structure. 3.1.2. [CuI4L4(Mo8O26)] (2) When substituting CuCl for NiCl26H2O, and adjusting pH value to 5.0, compound 2 was obtained. The structure of hydrothermally Table 1 Crystal data and structure refinement for compounds 1 and 2.
Formula Molecular weight Crystal system Space group a (Å) b (Å) c (Å) a (deg) b (deg) c (deg) V (Å3) Z Dcalcd (g cm3) F (0 0 0) Reflns collected/unique R (int) GOF on F2 R1a [I > 2r (I)] wR2b a b
1
2
C16H22Mo8N8NiO28 1600.65 Triclinic P1
C32H32Cu4Mo8N16O26 2078.42 Triclinic P1
9.7340(6) 10.4050(6) 10.5830(6) 108.6610(7) 91.0500(7) 116.4910(7) 892.35(10) 1 2.979 762 4494/3093 0.0120 1.112 0.0220 0.0615
10.717(3) 11.645(3) 12.116(3) 93.429(4) 116.208(3) 98.566(3) 1328.0(6) 1 2.599 996 6657/4594 0.0260 1.029 0.0490 0.1287
R1 = R||Fo| |Fc||/R|Fo|. wR2 = |Rw(|Fo|2 |Fc|2)|/R|w(Fo2)2|1/2.
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Fig. 1. (a) Coordination environment of the NiII center in compound 1 (Most of the hydrogen atoms are omitted for clarity except for the protonated ones). (b) View of the supramolecular structure through hydrogen-bonding interactions along c axis. [Symmetry codes: (#1) x + 1, y + 1, z + 1; (#2) x + 2, y + 2, z + 2; (#3) x + 2, y + 2, z + 1; (#4) x, y, z + 1].
Table 2 Selected bond lengths (Å) and angles (°) for compound 1. Ni(1)–N(4) Ni(1)–O(1W) N(4)–Ni(1)–O(1)#2 N(4)–Ni(1)–O(1) N(4)#2–Ni(1)–O(1w) O(1)#2–Ni(1)–O(1W)
2.050(3) 2.078(3) 90.60(12) 89.40(12) 87.49(13) 92.93(11)
Ni(1)–O(1)
2.055(2)
N(4)–Ni(1)–O(1W) N(4)#2–Ni(1)–O(1) O(1)–Ni(1)–O(1W)
92.51(13) 90.62(12) 87.07(11)
Symmetry transformations used to generate equivalent atoms: #2 x + 2, y + 2, z + 2.
Table 3 Hydrogen bonds length (Å) and angles (°) for compound 1. D–H A
d(D–H)
d(H A)
d(D A)
<(DHA)
N(1)–H(1N) O(2)#3 N(1)–H(1N) O(10) O(1W)–H(1A) O(13)#1 O(1W)–H(1B) O(8)#4
0.849(10) 0.849(10) 0.847(10) 0.844(10)
2.21(4) 2.34(4) 1.906(13) 1.983(17)
2.885(4) 3.021(5) 2.742(4) 2.807(4)
137(5) 137(5) 169(4) 165(4)
Symmetry transformations used to generate equivalent atoms: #1 x + 1, y + 1, z + 1 #3 x + 2, y + 2, z + 1 #4 x, y, z + 1.
synthesized 2 was determined by X-ray crystallographic study and showed to be constructed from [b-Mo8O26]4 building blocks and [Cu2L2]2n4n+ strands. According to charge conversation and bond
valence sum calculations [14], Cu atoms are proven to be in +I oxidation state, as well as confirmed by the particular coordination environments of CuI [15]. It is noteworthy that CuI is metastable and prone to be oxidized to CuII, however, in this system the CuI compound has been successfully obtained probably because organonitrogen species function not only as ligands but also as reducing reagent under hydrothermal conditions [5a,7a,16]. There are two distinct CuI environments: CuI (1) is surrounded by two nitrogen atoms from two distinct L ligands and an oxygen atom from terminal oxygen atoms of an octamolybdate anion displaying T-type geometry, while CuI (2) is coordinated by three nitrogen atoms from different L ligands exhibiting Y-shaped geometry (Fig. 2a and Table 4). In this compound, the ligand, 3-((1H-1,2,4triazol-1-yl)methyl)pyridine, which is not protonated, performs as a bridging linker, showing a diverse coordination mode. For one thing, it acts as a bidentate linkage, bridging two CuI ions through apical N atoms; for the other, as a tridentate linkage connecting three CuI ions, meanwhile each CuI (1) ion links two L ligands and each CuI (2) ion binds to three L ligands to form a railroad motif (Fig. 2b and Fig. S1). The most striking structural feature of compound 2 is that these cationic strands are paired in an inclined way and parallel with each other, and further connected by the [b-Mo8O26]4 anions through Cu–O bonds with the distance of 2.394(6) Å. Meanwhile, [b-Mo8O26]4 anions are sandwiched be-
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Fig. 2. (a) Coordination environment of CuI center in compound 2 (all of the hydrogen atoms are omitted for clarity). (b) The metal–organic structure of compound 2 shows a railroad motif and the corresponding schematic representation of a 3-connected 2D network. [Symmetry codes: (#1) x, y + 1, z 1; (#2) x + 1, y + 1, z; (#3) x + 1, y, z].
Table 4 Selected bond lengths (Å) and angles (°) for compound 2. Cu(1)–N(4) Cu(1)–N(5) Cu(1)–O(10)#3 N(5)–Cu(1)–N(4) N(5)–Cu(1)–O(10)#3 N(4)–Cu(1)–O(10)#3
1.904(7) 1.894(7) 2.394(6) 172.8(3) 89.0(3) 96.3(3)
Cu(2)–N(1) Cu(2)–N(8)#2 Cu(2)–N(3)#1 N(1)–Cu(2)–N(8)#2 N(1)–Cu(2)–N(3)#1 N(8)#2–Cu(2)–N(3)#1
3.2. Thermogravimetric analysis 1.942(7) 1.977(8) 2.008(7) 128.0(3) 126.7(3) 104.8(3)
Symmetry transformations used to generate equivalent atoms: #1 x, y + 1, z 1 #2 x + 1, y + 1, z #3 x + 1, y, z.
tween adjacent strands (Fig. 2b). Furthermore, CuI ions as well as the ligands can be regarded as 3-connected nodes respectively, so a 3-connected 2D network can be achieved. Two new compounds with distinct structural characteristics have been attained, by using the same ligand L, polyoxoanions and different metal ions. The L ligands coordinate to NiII or CuI ions in different modes, respectively, contributing to different metal– organic units: in 1 at pH 3.0, the nitrogen atom of pyridine ring from the ligand is protonated, so it is unbound to NiII ion, acting as a counterion, while with the increase of the pH value to 5.0, no protonation occurs to the nitrogen atom of the pyridine ring, and the nitrogen atom connects to CuI ion, further extending to a railroad strand. Furthermore, NiII and CuI ions exhibit different coordination mode: in 1, each NiII ion links two [b-Mo8O26]4 anions through terminal oxygen atoms, two water molecules and two protonated ligands, resulting in a 1D chain. Different from 1, in 2 CuI ions are surrounded by nitrogen atoms, extending to a railroad strand, meanwhile part of them are bridged by [b-Mo8O26]4 anions through terminal oxygen atoms, so as to pillar adjacent strands to a 2D network.
The thermal stabilities of the two compounds are shown in Fig. S2. The TG curve for 1 (Fig. S2a) indicates that the first weight loss in the range 219 to around 230 °C corresponds to the departure of coordination water molecules (exptl, 2.55%; calcd, 2.25%), and the second sharp weight loss occurred at around 379 °C, corresponding to the decomposition of organic ligands (exptl, 19.38%; calcd, 21.14%). Compound 2 was stable up to 289 °C (Fig. S2b), whereupon one sharp stage of weight loss occurs (exptl, 28.52%; calcd, 29.29%). Thermogravimetric experimental results indicate that 1 and 2 have promising thermal stabilities. 3.3. Optical energy gap Several POM-based inorganic–organic hybrid compounds have been reported to be promising semiconductors, such asCo2(bpy)6(W6O19)2 (Eg = 2.2 eV) [17a], [Cd(BPE)(a-Mo8O26)][Cd(BPE)(DMF)4]2DMF (Eg = 3.45 eV) [17b], (n-Bu4N)2[Mo6O17–(„NAr)2] (Ar = o-CH3OC6H4) (Eg = 2.25 eV) [17c]. In order to explore the conductivity of the two compounds, the measurements of diffuse reflectance for powder samples were done to achieve their band gap (Eg). The reflectivity data were converted by a Kubelka–Munk tansformation to better locate the corresponding adsorption bands and adsorption threshold, and the band gap Eg was determined as the intersection point between the energy axis and the line extrapolated from the linear portion of the adsorption edge in a plot of Kubelka–Munk function F against E [18]. As shown in Fig. 3, the corresponding well-defined optical absorption associated with band gaps (Eg) can be assessed at 3.40 eV for 1 and 2.25 eV for 2, which indicates that these hybrid compounds are potential wide gap semiconductor materials. The 1D
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photoluminescent property. Further research is ongoing to prepare new compounds with outstanding luminescent and semiconducting properties. Acknowledgments
Fig. 3. The diffuse reflectance UV–Vis spectra of K–M functions versus energy (eV) of compound 1 and 2.
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project Nos. 20573016 and 20703008), Chang Jiang Scholars Program (2006), Program for Changjiang Scholars and Innovative Research Team in University (IRT0714), the National High-tech Research and Development Program (863 Program 2007AA03Z354), Department of Science and Technology of Jilin Province (20082103), the Science Foundation for Young Teachers of NENU (No. 20090407) and the Training Fund of NENU’s Scientific Innovation Project (NENUSTC07017 and -STC08019). Appendix A. Supplementary data
[NiII(b-Mo8O26)]n2n chain and extended [CuI2(b-Mo8O26)]n2n framework in hybrid structures appear to be responsible for their optical band gap respectively [8a,17b]. In our viewpoint, given the different Eg values of the two compounds, we assume that optical band gap can be influenced by metal ions, however, more experimental evidences are needed to confirm that. 3.4. Photoluminescent property Currently, inorganic–organic hybrid compounds containing CuI cations have been investigated on fluorescent properties for their potential applications as luminescent materials [1a,4b,19]. The photoluminescent properties of free neutral ligand L and compound 2 were investigated at room temperature (Fig. S3). The character of the solid-state emission spectrum of ligand may be attributed to excimer or trimer/multimer due to the intense p–p interaction between ligands. It’s well-known that the emission of excimer exhibits a broad and structureless band [20]. However, after the formation of complex, the extent of face-to-face coplanarity between the flexible ligands become weaken arising from the restriction of Cu(I), and thus the intense of p–p interaction may significantly decreased or even disappear. This observation is consistent with the geometry of the compound by means of density functional calculation. Compared with the maximum emission wavelength for the L ligand at 394 nm (kex = 245 nm), compound 2 exhibits an intense peak photoluminescence emission at 370 nm (kex = 298 nm), showing an obvious blue shift. Based on the theoretical calculation TDDFT/ B3LYP/6-31+G(d) (Cu with LANL2DZ), the origin of the emission of compound 2 may be tentatively assigned to metal-to-ligand charge transfer (MLCT) [21], and the high-energy emission may originate from metal-centered 3d ? (4s,4p) triple excited state [22]. In fact, several theories have been published to explain the electronic transition mechanism of the CuI compounds, but the photoluminescence of these compounds is not well understood so far [23]. 4. Conclusion In summary, two new octamolybdate-based inorganic–organic hybrid compounds, NiII(HL)2(H2O)2(b-Mo8O26) and CuI4L4(bMo8O26) have been successfully synthesized under hydrothermal condition, by using NiII or CuI ions, (NH4)6Mo7O244H2O and the organonitrogen molecule 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine which can act as ligand simultaneously as counterion. The diffuse reflectivity spectrum measurements reveal that these two inorganic–organic hybrid compounds with Eg 3.40 eV and 2.25 eV, respectively, are potential wide gap semiconductor materials. Additionally, the solid-state fluorescence spectra indicate that compound 2 has
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