Muwei
Zhang
a,
Weigang
Lu
a,
Jian-Rong
Li
*ab,
Mathieu
Bosch
a,
Ying-Pin
Chen
ac,
Tian-Fu
Liu
a,
Yangyang
Liu
a and
Hong-Cai
Zhou
*ac
aDepartment of Chemistry, Texas A&M University, College Station, Texas 77842, USA. E-mail: zhou@mail.chem.tamu.edu; Fax: +1 979 845 1595; Tel: +1 979 845 4034
bCollege of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. E-mail: jrli@bjut.edu.cn
cDepartment of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, USA
First published on 13th January 2014
Two nucleobase-incorporated metal–organic materials were designed, synthesized and structurally characterized. PCN-530 is among the few examples of metal–organic frameworks that utilize adenine as a constructional unit, while TMOP-1 is the first existing example of a crystallographically characterized nucleobase-incorporated metal–organic polyhedron. This work also offers a general perspective for the design and synthesis of nucleobase-incorporated metal–organic materials.
Despite these advantages associated with nucleobase-incorporated MOMs, only a limited number of nucleobase-incorporated MOFs13–17 have been published to date, and no MOPs with nucleobase moieties have been reported yet. This limitation may have resulted from the lack of intrinsic symmetry of nucleobases and the difficulty of controlling their binding modes to metal ions. Highly symmetric units are typically more favored in MOF construction21,22 since they will significantly facilitate the packing of the repetitive units in the formation of crystalline materials. The incorporation of low-symmetry units, such as nucleobases, into MOFs is usually less favorable. Rosi and co-workers have published a few “bio-MOFs” constructed from highly symmetric zinc-adeninate secondary building units (SBUs).13,14 The presence of this SBU has increased the framework symmetry and eliminated undesired coordination modes between adenine and metal ions. Apart from their approach, herein we introduce two different strategies to synthesize nucleobase-incorporated MOMs. First, despite the low symmetry of nucleobase molecules, the introduction of a highly symmetric co-ligand may be an effective way to incorporate nucleobases into MOFs. Second, nucleobase-incorporated MOPs may be constructed by connecting the nucleobase molecules to a commonly seen moiety for MOP construction. The successful implementation of these two strategies has yielded two MOMs, PCN-530 (PCN represents porous coordination network) and TMOP-1 (TMOP represents thymine-incorporated metal–organic polyhedron). Both of them are novel MOM structures with the nucleobase moieties.
Colorless, blocky single crystals of PCN-530, Zn3[Zn2(μ2-H2O)]3(Ad)6(TATB)4(DMF), (Ad = adeninate, TATB = 4,4′,4′′-s-triazine-2,4,6-triyl-tribenzoate) were obtained via a solvothermal reaction between zinc acetate, adenine and H3TATB in N,N′-dimethylformamide (DMF) in the presence of water (see ESI‡). A single-crystal X-ray diffraction (XRD) study reveals that PCN-530 crystallizes in the triclinic space group P. Due to multiple metal-binding modes of adenine, two distinctive SBUs, denoted as “SBU 1” and “SBU 2” hereinafter, were identified in the structure (Fig. 1(a) and (b)). SBU 1 consists of a four-coordinate Zn(II) that links two adeninates via the 7-N atom on the imidazolate moiety and two carboxylates. SBU 2 consists of a Zn2(μ2-H2O) unit, where two adeninates bridge the dizinc center via 3-N and 9-N atoms at the two equatorial positions, while two carboxylates coordinate to the dizinc center at the two axial positions. It should be noted that in each adeninate, all the imino 7-N donors coordinate to SBU 1, and all the 3-N and 9-N atoms coordinate to SBU 2, leaving 1-N and the exocyclic 6-N atoms uncoordinated (Fig. 1(c)). SBU 1 and SBU 2 were connected by adeninate in a 1:1 ratio, forming one-dimensional zinc-adeninate chains (Fig. 1(d)).
Similar to many other MOFs constructed from the TATB ligand, the s-triazine ring located in the center allows it to adopt a nearly planar conformation (Fig. 2(a)).23–25 This conformation greatly facilitates the delocalization of the π electrons within a TATB ligand and strengthens the π⋯π interaction between two adjacent ones.24 In an s-triazine core, the N and C atoms are partially negatively and positively charged, respectively. The adjacent TATB ligands stagger themselves so that the N atoms in one ligand are aligned with the C atoms of the other to maximize the π⋯π interaction.25,26 The distance between adjacent s-triazine rings is 3.53 Å (Fig. 2(a)). The one-dimensional zinc-adeninate chain is connected by TATB ligands, generating a 3,4,4-connected framework denoted as (62·84)3(63)2. The solvent accessible volume of PCN-530 is 47.80% calculated by using the PLATON routine,27 indicating its porous nature. Indeed, PCN-530 possesses open channels of 7.4 × 11.9 Å along the a axis (Fig. 2(b)).
Fig. 2 (a) The graphical representation of the π⋯π stacking between two TATB ligands. The dashed lines indicate the interaction between two adjacent s-triazine rings. (b) Packing diagram of PCN-530 along the a axis. High-resolution figures of the crystal structure can be found in ESI.‡ |
Teal, blocky crystals of TMOP-1, Cu24(MDPI)24(DMA)4(H2O)20 (MDPI = 5-((5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)methyl)isophthalate, see Fig. 3(a)), were obtained by a direct reaction between copper acetate and H2MDPI at room temperature (see ESI‡). Isophthalates are one of the important categories of MOP constructional units,28–30 and many highly porous MOFs based on cuboctahedral cavities were also constructed from ligands with isophthalate moieties.31,32 A single-crystal XRD study shows that TMOP-1 crystallizes in the triclinic space group P. Unlike the idealized Oh symmetry encountered in many other isophthalate-based MOPs,28 due to various orientations and disorders of the terminal thymine moieties, TMOP-1 possesses Ci symmetry with an inversion center located at the geometric center (Fig. 3(b)). Two types of dicopper paddlewheel SBUs were found in this structure (Fig. 3(c)), which resulted from different coordinating solvents (DMA and H2O) at the axial positions of the dicopper paddlewheels on the exterior surface of the cage. It is probable that a coordinated DMA molecule may have facilitated the packing of TMOP-1 by forming a π⋯π interaction pair with a thymine moiety from an adjacent MOP (Fig. 3(d)). The distance between the coordinated DMA and the neighboring thymine is 3.65 Å. In addition, the hydrogen bonding between two adjacent thymine moieties from two different MOPs may have facilitated the packing of TMOP-1 as well (see ESI Fig. S8‡). It should be noted that DNA-coated molecular cages were reported by Fujita and co-workers, providing neither their single crystal structures nor evidence of their crystallinity, presumably due to the complexity of their system.33 However, simple MOPs are usually regarded as crystalline materials with well-defined structures.28–30 To the best of our knowledge, this is the first reported case of nucleobase-incorporated MOP that has been structurally characterized by single crystal XRD studies.
Footnotes |
† This work is dedicated to Prof. Dr Xiao-Zeng You on the occasion of his 80th birthday. |
‡ Electronic supplementary information (ESI) available: Detailed synthetic procedures of the ligands, the preparation of PCN-530 and TMOP-1, their structure solution and refinement, and high-resolution figures for their crystal structures can be found in the ESI. CCDC 962335 for PCN-530, 962336 for TMOP-1. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3qi00042g |
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