Structures, in Situ Formation of Ligand, and Crystal-to-Crystal

ARTICLE pubs.acs.org/crystal

Microporous Metal
Organic Frameworks: Structures, in Situ Formation of Ligand, and Crystal-to-Crystal Transformations Zhen-Lan Fang, Rong-Min Yu, Xiao-Yuan Wu, Jing-Shun Huang, and Can-Zhong Lu* The State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian, 350002, P.R. China

bS Supporting Information ABSTRACT: Among these new microporous metal
organic frameworks (MOFs) {[CuI17(L1)12X2]X3(H2O)n}¥ (1, X = Cl, 1a, n = 6; 1b, 1c, n = 5, 1d, n = 3; 2, X = Br, 2a, n = 5; 2b, n = 3; L1 = 3,5-bis(4-aminophenyl)-1,2,4-triazolate), 1a and 2a were obtained through in situ copper-mediated ring conversion of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole into L1 under hydrothermal conditions, while the others were generated through interesting single-crystal-to-single-crystal transformations with an accompanying color change from red to black. In the process of synthesis, when CuII salt was used as starting reactant, the CuI complex 1a was obtained; so the redox reaction clearly took place under this condition. The fascinating features of these structures are the double-stranded helical chains and the largest metallamacrocycle based on triazolate ever reported. These isomorphous complexes adopt the rare binodal six-connected networks. In the solid state at low temperature (∼273 K), the red crystal sample 1b displays an emission band at ∼585 nm upon photoexcitation at 467 nm.

’ INTRODUCTION Topotactic reactions,1 which convert single crystals of starting compounds directly into single crystals of products, are elegant and potentially useful processes that have fascinated solid-state chemists for many years. Such reactions involve the cooperative movement of atoms in the solid state. In some cases, the difference between reactants and products is so small that three-dimensional order is preserved with relatively minor adjustments in the unit cell. Classical studies on the solid-state reactivity of different polymorphs of cinnamic acid2 paved the way for understanding topochemical reactions in molecular crystals. Recently, many examples of photochemical excitation resulting in transformations, such as polymerization,3 cyclization,4 dimerization,5 and racemization,6 have been reported; however, instances of thermally induced topotactic reactions are relatively rare.7 In this work, we report some interesting topotactic reactions that are thermally induced. Microporous metal
organic frameworks (MOFs) with different networks have potential functions in many areas, including gas storage8 and separation,9 nonlinear optics,10 molecular recognition,11 magnetism,12 and fluorescence.13 Metallamacrocycles play an important role in the development of molecular architecture and topology.14 The helical structures have potential applications in the separation of enantiomers,15 biological pharmacy, etc.16 Due to their potential as new, functional solid materials, interest in fabricating these aforementioned complexes has grown rapidly. However, microporous MOFs with helical structures have been scarce in contemporary literature, and r 2011 American Chemical Society

microporous MOFs with both helical structures and metallamacrocycles have been even more scarce. In situ metal/ligand reactions that occur under hydro(solvo)thermal conditions have been diffusely adopted in synthetic processes for the creation of new materials, especially those that cannot be prepared using the constituent ligands exclusively.17 Recently, several important hydro(solvo)thermal in situ metal/ligand reactions and their mechanisms have been extensively investigated. Such hydro(solvo)thermal reactions, including hydrolysis of carboxylate esters, organic nitriles, and aldehydes into the corresponding carboxylates,17a cleavage of acetonitrile/ethylene carbon
carbon bonds,18 cleavage and formation of disulfide bonds,19 substitution of aromatic groups,20 decarboxylation of aromatic carboxylates,21 and [2 þ 3] cycloaddition22 have been widely used in the synthesis of multifunctional materials. In the past few years, the heterocyclic 1,3,4oxadiazoles have been widely utilized as functional ligands, which not only can bridge metal ions to afford MOFs materials23 but also can have superexchange capacity due to the unusual magnetic properties of their complexes.24 Moreover, this kind of ligand can offer a range of charge-balance requirements, alternative linking modes, and different orientations of donor groups.25 Therefore, in this work, we have taken strides toward the design and construction of multifunctional MOFs with Received: March 10, 2011 Revised: April 19, 2011 Published: April 20, 2011 2546

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Crystal Growth & Design Scheme 1. The Ligands L1
L4 Involved in the Reactions

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KBr pellet, ν/cm
1): 3446(br), 3336(w), 3215(w), 2370(w), 1617(vs), 1536(w), 1455(vs), 1435(s), 1405(w), 1385(m), 1345(m), 1294(s), 1174(s), 1124(w), 1073(w), 1013(w), 923(w), 822(vs), 762(s), 701(w), 661(w), 641(w), 611(w), 581(m), 530(w), 490(w).

Synthesis of {[CuI17(L1)12Br2]Br3(H2O)5}¥ (2a) and {[CuI17(L1)12Br2]Br3(H2O)3}¥ (2b). Method A: A mixture contain-

fascinating architectures by utilizing copper cations and 2,5disubstituted-1,3,4-oxadiazole ligands as building blocks. Surprisingly, a series of new microporous metal
organic frameworks (MOFs) based on 3,5-bis(4-aminophenyl)-1,2,4-triazolate (L1) have been obtained through the in situ copper-mediated ring conversion of 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole (L3) under hydrothermal conditions (Scheme 1). It is very interesting that these complexes can undergo single-crystal-to-single-crystal transformations accompanied by a distinct color change from red to black.

’ EXPERIMENTAL SECTION Materials and General Methods. The original bridging ligands 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (L2) and L3 (Scheme 1) were synthesized according to previously reported literature procedure.26 Other chemicals were obtained from commercial sources and used without further purification. The IR spectra (KBr pellets) were recorded on a Magna 750 FT-IR spectrophotometer in the range 400
4000 cm
1. C, H, and N elemental analyses were determined on an EA1110 CHNS-0 CE element analyzer. Optical diffuse reflectance spectra were measured at room temperature with a PE Lambda 35 UV
vis spectrophotometer. The instrument was equipped with an integrating sphere and controlled by a personal computer. The samples were ground into fine powder and then pressed onto a thin glass slide holder. A BaSO4 plate was used as a standard (100% reflectance). The absorption spectra were calculated from reflectance spectra using the Kubelka
Munk function: R/S = (1
R)2/(2R),27 where R is the absorption coefficient, S is the scattering coefficient, which is practically wavelength independent when the particle size is larger than 5 μm, and R is the reflectance. Thermal stability studies, TGA, for 1b and 2a were carried out on a NETSCHZ STA 449C thermoanalyzer under N2 (30
1000 C range) at a heating rate of 10 C/min. Fluorescence spectra were measured on an Edinburgh Analytical Instrument FLS920. Synthesis of {[CuI17(L1)12Cl2]Cl3(H2O)6}¥ (1a) and {[CuI17(L1)12Cl2]Cl3(H2O)3}¥ (1d). A mixture containing CuCl2 3 2

H2O (200.0 mg, 1.17 mmol), L2 (30.0 mg, 0.13 mmol), L3 (30.0 mg, 0.12 mmol), benzene-1,2,4,5-tetracarboxylic acid (L4, 50 g, 0.2 mmol), aqueous ammonia (25%, 0.7
0.9 mL), and deionized water (10 mL) was placed in a Parr Teflon-lined stainless steel vessel (20 mL) under autogenous pressure and stirred at room temperature for 5 h. Then, the mixture was heated at 180 C for 72 h. This was followed by slow cooling to room temperature at a rate of 3 C h
1. After being washed with H2O and air-dried, the bright red block crystals, 1a, of the product were obtained in 49.7% yield based on L3. Anal. Calcd (%) for C168H156Cl5Cu17N60O6: C, 46.14; H, 3.57; N, 19.23. Found: C, 46.44; H, 3.48; N, 19.11. The red block crystals changed to black ones, 1d, when the red crystals were exposed to air at room temperature for several hours. Anal. Calcd (%) for C168H150Cl5Cu17N60O3: C, 46.72; H, 3.48; N, 19.47. Found: C, 46.24; H, 3.68; N, 19.41. FT-IR for 1d (solid

ing CuBr (200.0 mg, 1.39 mmol), L2 (30.0 mg, 0.13 mmol), L3 (30.0 mg, 0.12 mmol), aqueous ammonia (25%, 0.7
0.9 mL), and deionized water (10 mL) was placed in a Parr Teflon-lined stainless steel vessel (20 mL) under autogenous pressure, stirred at room temperature for 5 h, and then heated at 180 C for 72 h. This was followed by slow cooling to room temperature at a rate of 3 C h
1. After being washed with H2O and air-dried, the bright red block crystals of the product were obtained with a low yield of 14.3% based on L3. Method B: Methods B and A are similar, but a different mixture containing CuBr (200.0 mg, 1.39 mmol), L2 (30.0 mg, 0.13 mmol), L3 (30.0 mg, 0.12 mmol), L4 (0.05 g, 0.2 mmol), aqueous ammonia (25%, 0.7
0.9 mL), and deionized water (10 mL) was used in method B. The bright red block crystals, 2a, of the product were obtained (54.6% yield based on L3). Anal. Calcd (%) for C168H154 Br5Cu17N60O5: C, 44.08; H, 3.37; N, 18.37. Found: C, 44.34; H, 3.43; N, 18.23. They changed to black ones, 2b, when the red crystals were exposed to air at room temperature for several hours. Anal. Calcd (%) for C168H150Br5Cu17N60O3: C, 44.43; H, 3.31; N, 18.51. Found: C, 43.82; H, 3.56; N, 18.37. FT-IR for 2b (solid KBr pellet, ν/cm
1): 3446(br), 3336(w), 3215(w), 3025(w), 2370(w), 1617(vs), 1536(w), 1455(vs), 1435(s), 1405(w), 1385(m), 1345(m), 1294(s), 1174(s), 1124(w), 1073(w), 1013(w), 912(w), 953(w), 822(vs), 762(s), 701 (w), 661(w), 641(w), 611(w), 581(m), 530(w), 490(w).

Crystallographic Data Collection and Structural Determination. The single crystals of these complexes in the present work were mounted on a glass fiber for the X-ray diffraction analysis. Data sets were collected on a Rigaku AFC7R equipped with a graphite-monochromated Mo KR radiation (λ = 0.71073 Å) from a rotating anode generator at 88 K. Intensities were corrected for LP factors and empirical absorption using the ψ scan technique. The structure was solved by direct methods and refined on F2 with full-matrix least-squares techniques using Siemens SHELXTL, version 5, package of crystallographic software.28 All non-hydrogen atoms were refined anisotropically. Positions of the hydrogen atoms attached to carbon atoms were fixed at their ideal positions. The amino groups of the crystals are not coordinated to metal ions in the same manner as are the N atoms of 1,2,4-triazolate, inducing rotations and strong vibrations of the C
N single bond; therefore, the amino N atoms are disordered in the cif files of the crystals. Crystal data, as well as details of data collection and refinement, for 1a
2b are summarized in Table S1, Supporting Information. The selected interatomic distances and bond angles for 1a
2b are given in Table S2, Supporting Information.

’ RESULTS AND DISCUSSION Synthesis Strategy. 1,3,4-Oxadiazoles had been applied to construct microporous MOFs in this work; L3 was chosen as the first building block, and L2 as the second angular dipyridyl-like bridging linker, instead of using 4,40 -dipyridyl, which is usually preferred to coordinate to metal centers. Interestingly, in the presence of CuBr under hydrothermal conditions, a small amount of bright red block crystals of 2a based on L1 had been obtained. The result of elemental analyses indicates the obvious ring transformation from L3 to L1 in 2a. However, L2 decomposed to 4-pyridine carboxylate copper complex in the course of synthesis of 223a and does not exist in 2. In order to improve the yield of this reaction, we tried to control the reaction conditions, 2547

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Scheme 2. Possible Mechanism for the Ligand Transformationa

a

M1
M4 are proposed intermediates, and M5 is the final product.

such as the reaction temperature, time, original reactants, and solvent mediums. It is well-known that aromatic carboxylic acid is weak acid, which can be added into the basic reaction system to neutralize ammonia first, and then release ammonia molecules stepwise. As a result, it can slow the reaction speed and facilitate the growth of crystals. Inspired by this idea, L4 had been selected as aromatic carboxylic acid in our reaction system. As expected, the yield of 2 was obviously increased; meanwhile, another isomorphous complex 1 was obtained under similar reaction conditions when the starting reactant CuBr was substituted by CuCl2. However, up to now, it has not been successfully synthesized via replacing CuBr by CuCl. The reason for this result is unclear, which requires further study. Mechanism Study. To better understand the reaction process, a possible mechanism should be addressed here. It is a challenge to demonstrate this transformation mechanism as a series of more authentic, detailed reaction steps because multicomponent starting reagents were used in the reaction. When using 2,5-bis(4-aminophenyl)-1,3,4-oxadiazole as exclusive ligand, we cannot obtain the goal products; therefore, the other ligands in this reaction system may play an important role in the ring conversion. However, the mechanism (Scheme 2) can be proposed using guidance from the literature29 and possible intermediates, which were observed for a few years.29a,30 It should be noticed that in the presence of CuCl2, univalent copper complex 1 was obtained; therefore, the copper redox reaction took place during the process of the overall reaction. The first step (I) may be L3 coordinating with copper ions to form M1.29a,30a,31 As a result, the ring relaxes and is subject to the nucleophilic addition by the H2O and NH3 molecules. In the second step, there would be two paths to continue the reaction. Path a, the ammonolysis of M1, produces dinuclear diazabutadiene-1,4-diamine (M2, IIa),30b and path b, the hydrolysis of M1, results in the formation of diazabutadiene-1,4-hydroxy tautomers (M3, IIb).29a,30a In the third step, M2 can rapidly lose one NH3 (IIIa),31 whereas M3 suffers a nucleophilic substitution by NH3 molecules and then loses two H2O molecules (IIIb).32 Both of them generate the 4-H-3,5-disubstituted-1,2,4-triazole (M4). In the final step, M4 is deprotonated and further coordinates to metal cations using N atoms at the 4-position of the triazolate ring to yield the final complex M5 (IV). Description of the Structures. The stable black complexes 1d and 2b are isomorphous with quite similar cell parameters, so

we take the structure of 2b for example to discuss in detail, and only mention related points for 1d. The asymmetric unit of 1d and 2b consists of one μ4-halide anion, one and a half isolated halide anions, eight and a half Cu ions, six L1, and one and a half guest water molecules. To meet the charge balance requirement, each tridentate triazole must be deprotonated and have one negative charge; therefore, in their molecular formulas, there are 17 Cu(I) for both 1d and 2b. It is notable that the NH2 groups of L1 are uncoordinated, and all ligands adopt μ3-N,N0 ,N00 bridging fashion through the triazolate rings to connect with three different CuI centers. In the asymmetric unit, these Cu centers can be divided into three distinct types. The first type (Cu1, Cu4, Cu5, Cu8) is two-coordinated with two different triazolates; the second type (Cu2) is coordinated by three N atoms from three different triazolates; the last type (Cu3, Cu6, Cu7, Cu9) is threecoordinated by two N atoms from different triazolates and one shared μ4-Br anion. Almost all copper atoms with two- or threecoordinated modes are univalent according to the Cambridge Crystal Structural Database, further illustrating that all the copper atoms in 1d and 2b are CuI. It is worth noting that the simultaneous appearance of these two kinds coordination modes is very rare.33 The bonding angles of Cu
X
Cu are not the expected ones for tetrahedral geometry, and the halide anions are not located in a tetrahedral environment in those complexes. Perhaps these are because the halide anions would not act as bridging linker, and the four Cu
Br bonds would only act as four sticks to strengthen the construction of the MOFs (Figure 1a). One of the most striking features of 1d and 2b is that the various coordination environments of the copper atoms and the interesting arrangements of L1 result in the formation of unique double-stranded helical chains running along a crystallographic 21 axis in the b direction with a very long pitch of 43.0 Å, due to the steric repulsion of the substituents of these triazolates (Figure 1e). Like most double-helical complexes,29a,34 the two adjacent helical chains in them are entangled through the interchain Cu2
N bonds to form a single chirality of the double-stranded helical chain. In fact, among the contiguous double-stranded helical chains, each right-handed helical chain is connected to two left-handed helical chains, and vice versa. Consequently, the whole crystal exhibits no chirality. Another fascinating feature of 1d and 2b is the decanuclear (Figure 1b) and hexanuclear metallamacrocycles that result from the 2-fold 2548

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Figure 1. For 2b, (a) perspective view of coordination environments of Cu and L1 (all aminophenyl groups are omitted for clarity), (b) 2-fold symmetry related decanuclear copper metallamacrocycle defining cavities of 8.9  5.4 Å2 occupied by the water molecule and uncoordinated Br anions, (c) 3D microporous framework viewed along the c axis direction, (d) 1D hexagonal honeycomb channels viewed along c axis (all aminophenyl groups and guests in the channels are omitted for clarity) and (e) left-handed and right-handed double-stranded helical chains.

Figure 2. (a) The trigonal bipyramidal [Cu5(L1)6], in which the equatorial plane is defined by two Cu4 and one Cu1 atoms (purple polyhedron, namely SUB1), acts as a (412 3 63) light blue node; (b) 2-fold symmetry related hexanuclear metallamacrocycle (SUB2) acts as a (49 3 66) green node; (c) noninterpenetrating six-connected two-nodal 3D network of 2b.

symmetry (Figure 2b). In the decanuclear metallamacrocycle, 10 L1 ligands confer a bowl-shaped arrangement of 1- and 4-positions of the triazolate ring bridging bidentate binding sites on its framework along the c axis. The circular conformation of 10 copper centers has an external diameter of ∼20 Å and a central cavity with an effective dimension of approximately 8.9  5.4 Å2. The disordered water molecules and Br anions are located within the central cavity. The decagon array coordinates to other Cu

centers through the remaining 2-positions of the triazolate ring bridging binding sites and then extends to form a honeycomb lattice with pores running parallel to the c-axis (Figure 1c,d). Calculations using the program PLATON show that the effective volumes for solvent inclusion are 1629.2 and 1575.1 Å3 per unit cell, corresponding to 9.1%, and 8.7% of the whole crystal volume for 1d and 2b, respectively. All the amino groups on the benzene ring are projected into the channels and are pointing to 2549

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Figure 3. Photos of (a) fresh crystal sample of 1a, (b) crystal sample of 1b (obtained from 1a placed at 88 K for 5 h), (c) crystal sample of 1c (obtained from 1b placed at ∼273 K for 2 weeks), and (d) crystal sample of 1d (obtained from 1c placed at 298 K for 1 week) and The coordination environments of Cl1 in (e) 1a, (f) 1b and 1c, and (f) 1d.

the halide anions. Although successful strategies have been developed for the construction of metallamacrocycles with triangular and square shapes,35 molecular polygons with higher numbers of sides are scarce. Furthermore, no crystallographically characterized examples of decagonal ones containing triazolates have been documented so far.36 The [Cu5(L1)6] units (SBU1), analogous to the [Ag5(μ3-3,5-Ph2-tz)6
] units,37 consist of a trigonal bipyramidal arrangement of five Cu atoms (purple polyhedra in Figure 2a), where the axial (located on a 3-fold symmetry axis) and equatorial ones are three- and two-coordinated, respectively. This arrangement leaves six 4-positions of the triazolate ring bridging binding sites available to further coordinate to six copper atoms [(Cu5)2, (Cu7)2, (Cu8)2], forming a trigonal prism (green prism in Figure 2a). The trigonal prism then acts as a linker between adjacent [Cu6(L1)6] units extending in three dimensions. The [Cu5(L1)6] units and the hexanuclear metallamacrocycle [Cu6(L1)6] units (SBU2) act as the secondary building units for 2b and exhibit differences in the spatial arrangement of the six neighboring units. Therefore, the resulting framework is a binodal six-connected network with (412 3 63) (49 3 66) topology (Figure 2c),38 where SBU1 is a (412 3 63) node, the circular conformation of SBU2 is a (49 3 66) node, and each node only links nodes of the other type. The uninodal six-connected R-Po network is the overwhelmingly favored topology for six-connected networks; however, such binodal six-connected networks are uncommon.39 The L1 ligand can be considered as a new member of the five-membered heterocyclic bridging ligands according to the Cambridge Crystal Structural Database; therefore, 1d and 2b are the first examples of the coordination chemistry of L1 to date. Single-Crystal-to-Single-Crystal Transformation and Spectroscopic Properties. The colors of 1 and 2 can change from bright red to black when the fresh crystals are exposed to air for several hours at room temperature. The crystals can be kept red for quite a long time at low temperature (∼273 K), while the black crystals are stable at room temperature. All the copper atoms in the black crystals are univalent taking into account the coordination environment of copper atoms and the charge conservation. Therefore, crystal color change was not due to the oxidation of copper occurring in the whole crystal. These black crystals were crushed, and all the newly appearing crystal faces are still black, which excluded the oxidation of cuprous ions on the crystal surface. Consequently, we concluded that the irreversible color changes may be due to irreversible structural transformations resulting from the effect of guest water

molecules. Herein, we have investigated the structural transformations in detail. Taking Cl complex 1 for example, we have done X-ray diffraction analysis at low temperature (88 K) for the same single crystal of 1a (the fresh crystal sample, Figure 3a), 1b (formed after the data collection process of 1a finished, Figure 3b), 1c (obtained from 1b kept at ∼273 K for about two weeks, Figure 3c), 1d (generated from 1c placed at ∼298 K for one week, Figure 3d). It is very interesting that 1a contains one more active guest water molecule than 1b; that is to say, 1a quickly loses one active water molecule to form 1b. Therefore, the structure of 1a is dynamic and unstable, resulting in framework crystallographic disorder and very poor crystal data quality. For example, the chloride environment of 1a consists of four copper centers having irregular Cu
Cl distances (Cu7
Cl
) and very unusual Cu
Cl
Cu0 3.323 Å
and Cu9
Cl 3.493 Å angles (Figure 3e). The large bond distances of Cu
Cl reveal that Cu7 and Cu9 are basically not bonded to Cl1; in other word, Cl1 just acts as a bidentate linker in 1a. Interestingly, the above observed results are consistent with the idea that the coordination modes of Cl1 do not affect the whole construction of the MOFs and just strengthen the construction of the MOFs rather than acting as bridging linker. No distinctive difference between 1b and 1c was observed, indicating that 1b is stable at low temperature (∼ 273 K). After exposure to air at room temperature, 1b (1c) loses two unstable guest water molecules to convert to 1d, and all the bond distances for them are in the normal range (Figure 3f,g). For the Br complex, we can only catch two relatively stable 2a and 2b, which may be because the above-mentioned first step for the Br complex is too quick to catch. It is obviously to see 2a losing two unstable guest water molecules to convert to 2b, and it is worth noticing that all of the above crystal data and mentioned phenomena have good repeatability. All efforts failed when we have tried to make these black crystals turn to red ones by readsorption of water. One possible reason may be that the relatively high temperature and pressure of hydro(solvo)thermal conditions are usually able to stuff more guest molecules into the micropores of the fresh crystals.40 When the environment becomes mild, they would release some active guest molecules to form more stable complexes. Taking 1 for example, from the view of architectonics, 1d is in the ultimate stable state, resulting in the irreversibility of crystal transformation via the readsorption of water molecules The differences between the structures of 1a, 1b (1c), and 1d may result in different physical properties. Herein, we study the spectroscopic 2550

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that they have high thermal stability, which is one desirable quality of porous metal
organic frameworks for gas storage applications. However, N2 sorption experiments for 1d and 2b showed that only surface adsorption occurred, due to the amino groups on the benzene ring projecting into the channels and suppressing the sorption of N2.

Figure 4. Solid-state excitation (left) and emission (right) spectra of 1b at room temperature.

properties of 1b and 1d, because 1a is quite unstable. In the solid state at low temperature (∼273 K), the red crystal sample 1b displays an emission band at ∼585 nm upon photo excitation at 467 nm (Figure 4), which may be assigned to metal-to-ligand charge transfer triplet excited states (3[MLCT]) similar to other reported copper(I) 1,2,4-triazolates.41 The lifetime data of its emission was fitted by monoexponential decay with the value to be 0.12 μs; thus, 1b may be applied as a good potential candidate for hybrid inorganic
organic phosphorescent materials at low temperature. However, the black sample 1d is basically nonluminous. To further understand the nature of their different phosphorescent properties, we also investigated their optical diffuse reflectance spectra. The absorption bands of both 1b and 1d peaked at ∼273 nm may be assigned to intraligand π
π* transition. Complex 1b basically has no absorption band before 800 nm; however, 1d has a relatively intense absorption band in the whole vis
NIR spectral window (Figure S1, Supporting Information), which decays via ultrafast nonradiative pathways.42 Maybe this is the reason that the black sample is nonluminous. The optical absorption spectra of 1a and 1d reveal their optical gaps to be 2.76 and 1.15 eV (Figure S1, Supporting Information), respectively, which are consistent with the colors of the crystals and indicates that the crystal color deepens as band gap decreases. X-ray Powder Diffraction (XRPD), IR, and Thermogravimetric Analysis (TGA). Complexes 1b, 1d, 2a, and 2b were characterized via XRPD (Figures S2 and S3, Supporting Information), and the stable black samples 1d and 2b were characterized via IR spectra (Figure S4, Supporting Information). All the XRPD patterns measured for the as-synthesized original samples are in good agreement with the XRPD patterns simulated from the respective single-crystal X-ray data. Compared with the original crystals, the dehydrated solid, which was obtained by heating at 150 C under vacuum for 30 min, shows basically an identical XRPD pattern. This result indicates that the microporous MOF is robust even after the removal of the guest molecules. The IR and TG curves of 1b and 2a are analogous to each other (Figure S5, Supporting Information) as a result of their isomorphous architecture. The result of the TGA study shows that a small quantity of weight loss between 25 and 386 C for 1b and 2a corresponds to the loss of guest molecules. Then these two complexes decompose at a decomposition point T onset of 394 and 386 C, respectively, indicating

’ CONCLUSION In summary, we have successfully synthesized new Cu microporous MOFs 1a and 2a based on L1, which is a new ligand obtained through the in situ ring-to-ring conversion of L3. The interesting single-crystal-to-single-crystal transformations accompanied by a distinct color change have been investigated in detail. That structural differences lead to their different physical properties have also been investigated, which indicated that 1b is a good potential candidate for hybrid inorganic
organic phosphorescent material at low temperature (∼273 K). Moreover, this in situ metal/ligand reaction is important not only for enriching the study of in situ ligand synthesis but also for representing a promising new route for constructing diverse new complexes with fascinating structures and properties, which are unattainable through conventional methods. ’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray crystallographic files in CIF format and IR spectra, TG curves, and experimental and simulated X-ray powder diffraction patterns for complexes 1a
2b. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the 973 key program of the MOST (Grants 2007CB815304 and 2010CB933501), the National Natural Science Foundation of China (Grants 20873150, 20821061, 20973173, 50772113, and 91022008), the Chinese Academy of Sciences (Grants KJCX2-EW-H01 and KJCX2-YWM319), and the Natural Science Foundation of Fujian Province (Grants 2007HZ0001-1, 2009HZ0004-1, 2009HZ0006-1, and 2006L2005). We are also grateful to Professors Jie Zhang, Jian Zhang, and Qi-Sheng Zhang for helpful discussions. ’ REFERENCES (1) Thomas, J. M. Philos. Trans. R. Soc. London, Ser. A 1974, 277, 251. (2) (a) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1996. (b) Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. Soc. 1964, 2000. (3) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem., Int. Ed. 2000, 39, 2132. (4) Irie, M.; Kobatake, S.; Horichi, M. Science 2001, 291, 1769. (5) (a) Tanaka, K.; Toda, F.; Mochizuki, E.; Yasui, N.; Kai, Y.; Miyahara, I.; Hirotsu, K. Angew. Chem., Int. Ed. 1999, 38, 3523. (b) Papaefstathiou, G. S.; Zhong, Z. M.; Geng, L.; MacGillivray, L. R. J. Am. Chem. Soc. 2004, 126, 9158. (6) Ohashi, Y.; Yanagi, K.; Kurihara, T.; Sasada, Y.; Ohgo, Y. J. Am. Chem. Soc. 1982, 104, 6353. 2551

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