A Metal-organic Framework with Paddlewheel Zn 2 ( CO 2 ) 4 Secondary Building Units and Cubane-1 , 4-dicarboxylic Acid Linkers

A new crystal structure of catena-(bis((μ4-cubane-1,4-dicarboxylato)-(N-methyl-2-pyrrolidone)-zinc(II)) N-methyl-2-pyrrolidone solvate) (1) was prepared by solvothermal method. The crystal structure of the compound was analyzed by single-crystal X-ray diffraction. It has a P1 space group, with lattice parameters a = 10.7190(4) Å, b = 10.8245(5) Å, c = 10.8403(8) Å, α = 80.291(5)°, β = 70.0015(5)°, γ = 77.531(4)°, V = 1147.97(12) Å3. The secondary building units of 1 consist of 2 central Zn ions, coordinated by 4 carboxylate groups in a bis-monodentate way, forming a square planar configuration of Zn2(CO2)4, known as paddle-wheel units. The paddle-wheels are connected by cubane-1,4-dicarboxylic acid linkers at the edges, resulting in a two-dimensional coordination polymer with a square lattice (sql) underlying network topology. The axial sites of the zinc atoms are occupied by N-methyl-2-pyrrolidone molecules. In this new crystal structure the two-dimensional polymer planes are interstacked by weak dispersion bonds. The axial N-methyl2-pyrrolidone solvent molecules determine the distances of planar polymer planes. The thermal properties of this new structure were studied by thermogravimetry/mass spectrometry in inert atmosphere. It was found, that the organic linkers in the framework structure do not decompose below 200 °C. The stochiometry of the activated compound is Zn2[C8H6(COO)2]2(C5H9NO)2, as determined by thermogravimetry in oxidative atmosphere.


Introduction
Metal-organic frameworks are porous coordination polymers with crosslinks [1].Multifunctional linkers, for example carboxylates can form strong covalent-like multidentate bonding with the metal centers, resulting in robust coordination polymer structures with high thermal stability [2,3].In the crystalline MOF structures, the rigid, metal-containing clusters, the so-called secondary building units (SBUs) at the vertices are interconnected by organic linkers.The most important metal-organic framework is MOF-5 [2], which builds up from Zn 4 O(CO 2 ) 6 SBUs at the nodes and terephthalate linkers at the edges.Since the discovery of MOF-5 more than 20000 MOF structures were reported [3].The combination of hundreds of SBUs and thousands of organic linkers led to discover of thousands of new MOF structures each year.Metal-organicframeworks are typically microporous compounds with exceptional high porosity and surface area enabling many potential industrial applications, for example gas-storage, heterogeneous catalysis, molecular separation, molecular sensing or drug delivery.For a common description of the family of MOFs, Yaghi and coworkers introduced the principle of reticular chemistry based on the design of topology of framework structures [4].For example, MOF-5 has Zn 4 O(CO 2 ) 6 SBUs with six points of extension (carboxylate groups) and ditopic terephtalate linkers, so MOF-5 has a 6-c primitive cubic (pcu) underlying topology with only one kind of edges (links).MOFs are highly modular structures: the organic linkers or the metallic centers can be changed without changing the underlying network topology, so the pore size, surface area and chemical reactivity of the materials can be fine-tuned.The so-called isoreticular structures are frameworks with the same underlying network topologies.The most important isoreticular series is the IRMOF-n (n=1-16) family [5], which is an isoreticular series to MOF-5 with pcu underlying network topology.The inorganic SBUs and the organic linkers determine the underlying topology of the periodic networks.Inorganic SBUs can be discrete objects or infinitive chains.Organic linkers (e.g.tritopic linkers) also can act as nodes (organic SBUs).
Paddle-wheel-like binuclear metal (Cu, Mo, Rh, Cr…) carboxylate complexes are well known for decades [6][7][8].In these structures two metallic centers are connected by four bis-monodentate carboxylate bridges, forming a rigid, binuclear, paddle-wheel-like cluster structure.The axial coordination sites of the metals can be free or can be occupied by other ligands, for example solvents (water, DMF, THF…).Discrete monomer molecules, one-, two-, or three-dimensional MOF structures were also prepared with paddle-wheel M 2 (CO 2 ) 4 units [9].In the CSD database [10] 2567 hits were found with M 2 (CO 2 ) 4 units 1 .Among these structures 722 polymer structures and 1845 discrete molecules were found.One of the earliest carboxylate-based metal-organic framework structures, MOF-2 [11] was the first metal-organic framework which was studied for gas-adsorption.MOF-2 is a two-dimensional, porous coordination polymer with paddle-wheel Zn 2 (CO 2 ) 4 SBUs at the nodes and terephthalates at the edges.The axial sites of the two zinc atoms are occupied by water molecules.The adjacent polymer layers are interstacked by hydrogen-bonding of axial water molecules and the carboxylate oxygen atoms of the adjacent layers.MOF-2 has a two-dimensional square-lattice (sql) underlying network topology.The two-dimensional polymer planes can be bonded by replacing the axial terminating ligands (for example water) to bridge-ligands, for example 1,4-diaza-bicyclo[2.2.2]octane or 4,4'-bipyridine, forming three-dimensional framework structures [12].
The combination of paddle-wheel SBUs with organic SBUs led to the discovery of many important MOF structures with complicated multi-nodal topologies, for example 1 ConQuest Version 1.20 (CSD Ver.5.39) was used for structural search.The O-M-O bond angles in the M 2 (CO 2 ) 4 units were limited to the range between 85° and 95°.
In this paper, we describe the preparation, structure and thermal stability of 1, a new solvate of MOF-104 [16] with the same underlying topology and linkers, but different structure and different coordination of the solvent.
The solvothermal synthesis was performed in a PARR Teflon-lined high pressure vessel.Heat treatment was carried out in a Memmert UR30 electric oven.

Solvothermal synthesis
231.1 mg (0.779 mmol) zinc nitrate hexahydrate and 50.3 mg (0.262 mmol) cubane-1,4-dicarboxylic acid were stirred in 6.9 ml of N-methyl-2-pyrrolidone for 1 hour at room temperature under an argon atmosphere.After all the starting materials were dissolved, the reaction mixture was filtered by a 0.2 µm PTFE filter and the solution was annealed in a PTFE lined high pressure vessel at 105 °C for 30 hours.Irregular shaped single-crystals were formed with hundreds of microns edge lengths and the solution became yellowish.After sampling some crystals together with the mother liquor for single-crystal X-ray diffraction, the crystals were settled and washed by 2 x 10 ml NMP and 3 x 3 ml acetone and the sample was dried in vacuum.The yield of the dried sample was 65 milligrams (70 % of theoretical yield).

Single-crystal X-ray diffraction
The single-crystal X-ray diffraction measurements were carried out at room temperature by an Agilent Supernova diffractometer, equipped with dual microfocal source, Kappa goniometer and CCD detector.
The deconstruction of the crystal structure and the topology analysis of the underlying network were carried out with ToposPRO [19].

Thermogravimetry and thermogravimetry/mass spectrometry (TG/MS)
The TG/MS measurements were performed with a modified Perkin Elmer TGS-2 thermobalance coupled to a Hiden HAL 301/PIC2 quadrupole mass spectrometer.The TG/MS measurement for determining the heat stability was acomplished in an inert argon atmosphere using about 4 mg sample size and 10 °C min -1 .The thermogravimetry measurement for stoichiometry determination was carried out in dry synthetic air atmosphere using about 1 mg sample and 5 °C min -1 heating rate.Before the experiments, the thermobalance was purged by the carrier gas for 45 minutes.19) Å (Zn1-O1), 2.0526( 19) Å (Zn1-O2), 2.032(2) Å (Zn1-O3) and 2.039( 2) Å (Zn1-O4).1 is composed of two-dimensional polymer sheets with paddle-wheel Zn 2 (CO 2 ) 4 SBUs at the vertices and cubane-1,4-dicarboxylate linkers at the edges (Fig. 1, and also see the supplementary cif file).Oxygen atoms of N-methyl-2-pyrrolidone (NMP) molecules are coordinated to the axial sites of pentacoordinated zinc atoms with 1.9804(19) Å Zn1-O5 distances, so the coordination of solvents is relatively strong.The planar polymer layers are interstacked by dispersion bonds (Fig. 2), the distance of adjacent layers is 9.91 Å. Ordered and disordered solvent molecules (NMP) are also stacked in the voids of MOF structure by weak dispersion bonds.The disordered solvent content was modeled with a superposition of 2 NMP molecules that were also doubled by inversion symmetry.
The comparison of the three isoreticular structures 1, MOF-2 and MOF-104 is summarized in Table 1

TG and TG/MS measurements
The heating of the sample in inert atmosphere results in mass loss in two distinct steps.In the first step between 100 °C and 200 °C the release of solvent (NMP), while in the second step between 200 °C and 500 °C the products of cubane-1,4-dicarboxylic acid decomposition (benzene, carbon-dioxide) were detected by mass-spectrometry (Fig. 3), so the framework structure is stable up to 200 °C.The TG/MS measurement in argon atmosphere results in the formation of zinc oxide and amorphous carbon as end products, so the stoichiometry of the compound can not be determined this way.
A TG measurement was carried out in an oxidative, synthetic air atmosphere (Fig. 4) to burn out the amorphous carbon and leave only zinc oxide as end product.This way the stoichiometry of the compound can be determined from the mass loss caused by the release of solvent and from the mass of ZnO residue.According to the oxidative TG measurement, the stoichiometry of 1 is Zn 2 (C 8 H 6 (COO) 2 ) 2 (C 5 H 9 NO) 2 , so only the two axial NMP molecules were present.

Fig. 1
Fig. 1 Crystal structure of 1, view down crystallographic a axis.H atoms are omitted for clarity.Blue: Zn, red: O, green:N, cyan: C.

Fig. 2
Fig. 2 Intermolecular stacking of adjacent polymer layers.The weakly bonded solvents in the voids are omitted for clarity.View down crystallographic c axis.Blue: Zn, red: O, green:N, cyan: C, white:H.

4
Conclusion A new solvate of MOF-104 was prepared by solvothermal method.In the new crystal structure (1), binuclear Zn 2 (CO 2 ) 4 paddle-wheel secondary building units are interconnected by cubane-1,4-dicarboxylate linkers, resulting in a 2-dimensional coordination polymer structure with square lattice (sql) underlying topology.The axial sites of zinc atoms are occupied by NMP molecules.The adjacent polymer layers are interstacked by dispersion bonds and the distances of the layers are directed by the axial solvent molecules.According to TG/MS measurement, the MOF is stable up to 200 °C.The activated material has a stoichiometry of Zn 2 [C 8 H 6 (COO 2 )] 2 (C 5 H 9 NO) 2 , as determined by thermogravimetry.Acknowledgement The research was supported by the National Research, Development and Innovation Office (NKFIH) through projects FK-125063, OTKA NK105691 and VEKOP-2.3.3-15-2016-00001.