Highly Stable Five-Coordinated Mn(II) Polymer [Mn(Hbidc)]n (Hbidc)1H-Benzimidazole-5,6-dicarboxylate): Crystal Structure, Antiferromegnetic Property, and Strong Long-Lived Luminescence Yongqin Wei, Yunfang Yu, and Kechen Wu*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2087–2089
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on The Structure of Matter, Chinese Academy of Sciences, Fujian 350002, China ReceiVed April 1, 2008; ReVised Manuscript ReceiVed May 1, 2008
ABSTRACT: A novel five-coordinated Mn(II) polymer 1H-benzimidazole-5,6-dicarboxylato manganese (II) ([Mn(Hbidc)]n, 1) possessing abundant hydrogen bonds and π-π stacking interactions, synthesized by the hydrothermal reaction between MnSO4 and a multifunctional organic aromatic ligand H3bidc, displays antiferromagnetic character, strong long-lived red luminescence (λmax ) 726 nm and τ ) 0.3 ms) as well as high thermal stability (up to ca. 500 °C). The rational design and synthesis of metal-organic frameworks (MOFs) has undergone tremendous development owing to their fascinating structures and functional applications as electronic, magnetic, optical, and catalytic materials.1 In general, the structural motifs of these hybrid compounds are closely related to the geometries of metal centers and the number of coordination sites provided by multidentate ligands. On the other hand, the supramolecular interactions such as hydrogen-bonding, π-π stacking, and metallophilic interactions also play the key roles in the recognition process forming final three-dimensional architectures.2 Careful selection of a suitable organic ligand with certain features, such as versatile bonding modes and the ability to undergo supramolecular interaction, is helpful for constructing and tailoring metal-organic architectures with desirable properties.3 From the structural point of view, 1H-benzimidazole-5,6-dicarboxylic acid (H3bidc) molecule possesses three interesting characteristics: (1) as a multidentate and rigid ligand with multiproton acceptor-donor sites, the bidc anion possesses two nitrogen atoms of imidazole ring and four oxygen atoms of carboxylate groups, and might be utilized as versatile linker in constructing interesting coordination polymers with abundant hydrogen bonds and π-π stacking interactions; (2) the carboxylate group displays a variety of bonding geometries, such as monodentate, chelating, bidentate bridging, monodentate bridging and chelating bridging, and the collaboration of the two carboxylate groups in bidc anion might connect metal magnetic centers serving as mediators of magnetic exchange; (3) the relatively large π-conjugated system in the benzimidazole ring might contribute much to the desirable fluorescence property resulting from the interaction between bidc anion and metal ion. Guo et al. have used the family member of the N-heterocyclic carboxylic acid 1H-benzimidazole-5-carboxylatic acid (H2bic) to construct many novel metal-organic architectures.4 However, many studies about H3bidc were focused on the design of therapeutic agents,5 and few examples of H3bidc have been reported in coordination chemistry.6 Herein, we report the synthesis and crystal structure of a new five-coordinated Mn(II) polymer named [Mn(Hbidc)]n (1), which shows antiferromagnetic character, strong longlived red luminescence, as well as high thermal stability. Compound 1 was synthesized hydrothermally by treating MnSO4 and H3bidc at 160 °C.7 The thermogravimetric analysis (TGA) measurement showed that compound 1 has excellent thermal stability as no strictly clean weight loss step occurred below 500 °C. The high thermal stability of the title compound is comparable to that of the high-thermal-stability (up to 490 °C) diamondoidnetwork polymer [Mn(NCP)2]n (HNCP)2- (4-carboxyphenyl)imidazo(4,5-f)(1,10)phenanthroline), which we have reported recently.8 * Corresponding author. E-mail:
[email protected].
Figure 1. Coordination environment of Mn(II), bonding mode of Hbidc2- in coordination polymer [Mn(Hbidc)]n (1). Selected bond lengths (Å) and angles (deg): Mn-O1 2.221(2), Mn-O1#1 2.140(2), Mn-O3 2.136 (2), Mn-O4 2.087(2), Mn-N1 2.165(3), O3-Mn-O4 168.00(8), O1-Mn-O1#1 117.97(6), O1-Mn-N1 135.24(9), O1#1-Mn-N1 104.53(10), O3-Mn-O1 78.28(8), O4-Mn-O1 89.80(9). Symmetry codes: #1 1/2 - x, 1/2 + y, 3/2 - z; #2 1/2 - x, -1/2 + y, 3/2 - z; #3 -1/2 + x, 1/2 - y, 1/2 + z.
Scheme 1. Bonding Mode of Hbidc2- in Compound 1.
The polymeric structure of 1 was revealed by an X-ray singlecrystal diffraction investigation. As shown in Figure 1, each Mn(II) center is five-coordinated and the coordination geometry is a distorted trigonal bipyramid. Even if Mn(II) leads to a wide range of coordination numbers (2-8), the most common coordination number of structurally characterized Mn(II) compounds is 6.9 Five-coordinated Mn(II) compounds are still rarely described.10 In the coordination geometry, the apical positions are occupied by two oxygen atoms (O3 and O4) from two bidentate bridging carboxylate groups and the angle O3-Mn-O4 is 168.00(8)°. The equatorial plane is completed by one imidazole nitrogen atom (N1) and two syn oxygen atoms (O1 and O1#1, symmetry code: #1 1/2 - x, 1/2 + y, 3/2 - z) from two
10.1021/cg800333g CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
2088 Crystal Growth & Design, Vol. 8, No. 7, 2008
Communications
χchain )
Ng2β2S(S + 1) 1 + u 3kT 1-u
(1)
where
[ JS(SkT+ 1) ] - [ JS(SkT+ 1) ]
u ) coth
S ) 5/2
J is the parameter of exchange interaction between two Mn(II) ions bridged by the carboxylato-oxygen atoms. On the basis of the structural information, an additional coupling parameter, zj′, was added in eq 2 to take into account the magnetic behavior through hydrogen-bonding interaction between the Mn-carboxylate chains. The total magnetic susceptibility is
χm )
χchain 1 - (2zj ’ /Ng2β2)χchain
(2)
The least-squares fitting of magnetic susceptibilities data led to J ) -1.44 cm-1, g ) 1.98, zj′ ) -0.024 cm-1, and R ) 1 × 10-4 (the agreement factor defined as R ) Σ[(χm)obsd - (χm)calcd]2/ Σ[(χm)obsd]2. The J value is comparable to other Mn(II) species with similar carboxylate bridges12 and well-confirms the antiferromagnetic coupling between Mn(II) centers. This conclusion is consistent with the crystal structure of 1, where the exchange pathways between neighboring magnetic centers involve bidentate and
Figure 2. (a) 2D polymeric structure and (b) weak π-π attraction between phenyl rings.
monodentate bridging carboxylate groups. As shown in scheme 1, each Hbidc2- ligand link four Mn(II) centers, one imidazole nitrogen coordinating to Mn(II), and two carboxylate groups acting as µ3-bridging linker. Figure 2a shows polymeric structure of compound 1. The Mn(II) centers are linked via carboxylate bridges into a infinite ziplike Mn-carboxylate chain and adjacent MnsMn distance is 3.699(2)Å. The infinite Mn-carboxylate chains are linked though the coordination by two carboxylates and one imidazole nitrogen donor in Hbidc2-, making compound 1 a twodimensional polymer, in which the Mn-carboxylate chains array uniformly toward crystallographic b-axis. Another feature in the polymeric structure is that with a head-to-tail aggregation, two Hbidc2- link two Mn(II) centers into a [Mn(Hbidc)]2 ring, in which two phenyl rings are arranged in offset face-to-face mode with the parallel distance of 3.761Å (figure 2b), indicating the weak π-π stacking between phenyl rings. Figure 3 shows the threedimensional supramolecular network of compound 1. The twodimensional polymeric networks exhibit layered arrangement, in which the strong hydrogen-bonding between N-H groups in imidazole ring and uncoordinated carboxylato-oxygen atoms (N2sO2#4 2.802(3) Å; symmetry code: #4 -x, 1 - y, 1 - z) play a key role in final three-dimensional architectures. The magnetic property of 1 has been studied over the temperature range of 2-300 K. The temperature dependence of χmT is shown in Figure 4. The χmT value at room temperature is 3.91 cm3 K mol-1 lower than the spin-only value of 4.37 cm3 K mol-1 for one high-spin MnII ions (g ) 2, S ) 5/2) assuming no magnetic interaction, indicating antiferromagnetic coupling of compound 1, which is confirmed by the continuous decrease of χmT value to 0.11 cm3 K mol-1 at 2 K. We fitted the experimental susceptibility using the classical spin Heisenberg model for a one-dimensional chain (eq 1).11
Figure 3. H-bonding interactions (red dashed lines) in 3D supramolecular network of compound 1, viewed from the crystallographic b-axis.
Figure 4. Experimental and calculated temperature dependences of χmT (the solid line represents the theoretical fit).
Communications
Crystal Growth & Design, Vol. 8, No. 7, 2008 2089
Acknowledgment. We gratefully acknowledge the financial support of the Natural Science Foundation of China (20573114), the 973 Project (2007CB805307), and the Project of Fujian Province (2007F3115). Supporting Information Available: Figures of thermogravimetric analysis and powder X-ray patterns (PDF); crystallographic information for compound 1 (CIF). This material is available free of charge via the Internet at http: //pubs.acs.org.
References
Figure 5. (a) Excitation spectrum (left, black line, upon emission at 474nm) and emission spectrum (right, red line, upon excitation at 349 nm) of ligand H3bidc. (b) Excitation spectrum (left, black line, upon emission at 726 nm) and emission spectrum (right, red line, upon excitation at 288 nm) of compound 1.
monodentate carboxylate bridges and the angle of Mn-O-Mn relative to monodentate carboxylate bridge is 115.99(9)°. The results presented above conclude that the combination of two carboxylate groups in Hbidc2- favor magnetic exchange of metal centers. Another important feature of 1 is the strong long-lived red luminescence. Figure 5 shows the excitation and emission spectra of ligand H3bidc and compound 1 in the solid state. Irradiated by UV light at ambient temperature, compound 1 exhibits a strong red emission with broadband in the range of 625-850 nm. The strongest wavelength λmax is located at 726 nm, which is clearly red-shifted compared with the free ligand H3bidc (λmax ) 440nm). It is notable that the emission lifetime τ of compound 1 was measured to be 0.3 ms, which is obviously longer than that of free ligand H3bidc (τ ) 1.2 ns). The compound 1 represents a novel qualitative change of luminescence property resulted from the interaction between metal ion and ligand. The strong emission of 1 probably origins from metal-to-ligand charge transfer (MLCT) excited state, because the ligand Hbidc2- has relatively large π-conjugated system of benzimidazole ring and uses imidazole nitrogen and carboxylato-oxygen donors to coordinate to Mn(II) ions, which benefits the charge transfer between Mn(II) ion and Hbidc2-. The long-lived emission lifetime τ ) 0.3 ms indicates the existence of spin-orbital coupling between the excited states which have carried out intersystem crossing before emission. Considering the excellent luminescence properties and high stability against humid and thermal environments, the title compound possesses tremendous potential application as an efficient luminescence material, such as an emitter for electroluminescence devices and conversion phosphor for light-emitting diode devices.13 In summary, a novel neutral coordination polymer based on assembly of Mn(II) and Hbidc2- has been revealed. The condensed coordination polymer [Mn(Hbidc)]n (1) appears to be an excellent candidate for multifunctional material since, in addition to its high thermal stability, it is virtually insoluble in most solvents such as H2O, CH3OH, CH2Cl2, CH3COCH3, CH3CN, THF, DMF, and DMSO.
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CG800333G
