Water-based synthesis and characterisation of a new Zr-MOF with a unique inorganic building unit †

A new, microporous Zr-MOF was obtained using 2,5-pyrazinedi-carboxylic acid (H 2 PzDC). The linker leads to the formation of a new 1D inorganic building unit composed of l -OH bridged {Zr 6 O 4 (OH 4 )} clusters which are arranged in a hexagonal array and connected by the PzDC 2 (cid:2) ions. The structure was determined from powder X-ray diﬀraction data.

Metal organic frameworks (MOFs) have been intensively investigated for various applications like gas storage, 1 gas separation, 2 catalysis, 3 heat transformation 4 or drug delivery 5 due to their high specific surface areas and the diversity of their structures. 6 In particular Zr-MOFs are studied for possible applications, due to their thermal and chemical stability. 7,8 An outstanding structural feature in carboxylate-based Zr-MOFs is the presence of hexanuclear clusters of composition {Zr 6 O 4 (OH 4 )} which are also well known from molecular Zr(IV) complexes ( Fig. 1, bottom, left). 9 The clusters are readily formed under various reactions conditions and dominate the structural chemistry of Zr-MOFs. The first example that contains this inorganic building unit (IBU) is UiO-66 ([Zr 6 O 4 (OH) 4 (BDC) 6 ], BDC 2À = benzenedicarboxylate, UiO = University of Oslo), which was reported in 2008. In the ideal structure the IBUs are twelvefold connected and form a fcu network topology. 10 Isoreticular structures have been reported as well with linkers of different sizes, e.g., DUT-52 (DUT = Dresden University of Technology) 11 or UiO-67, 10 Zr-Fum, 12 ZrSQU 13 or for example with functionalized terephthalic acids. 14,15 Other topologies have been observed as well, when the hexanuclear IBU is ten (DUT-69 16 ), eight (DUT-67, 16 DUT-51, 17 6 } cluster has been reported once and the structure refinement lead only to rather high R 1 /wR 2 values (0.1924/0.4359 for Zr-PCN-221(Cu)). In the phosphonate-and phenolate-based MOFs UPG-1 23 (UPG = University of Perugia) and MIL-163, 24 ZrO 6polyhedra ( Fig. 1, top, left) and edge-sharing ZrO 8 -polyhedra IBUs ( Fig. 1, middle, right) have been observed, respectively (Table S1.3, ESI †).
Zr-MOFs have been mainly synthesized under autogenous pressure using DMF as the solvent, 8,10,16,18 however, recently green synthesis routes and reaction upscaling have been reported. [25][26][27][28][29][30] For green syntheses the choice of the metal source and the solvent employed are crucial, and reactions in water, ethanol, acetic or formic acids are usually preferred.
The linker H 2 PzDC was synthesized as previously reported 31 (see S2, ESI †) and to the best of our knowledge it has only been successfully used in the synthesis of a series of lanthanide-MOFs. 32,33 The discovery and synthesis optimization of the title compound was carried out in 2 ml Teflon reactors using highthroughput methods as established in our group, 34 and setting the reaction time and temperature to 24 h and 120 1C. Initially the influence of the Zr salt on the product formation was evaluated. Using Zr(SO 4 ) 2 Á4H 2 O and ZrO(NO 3 ) 2 ÁxH 2 O in a solvent mixture of water and formic acid yielded exclusively X-ray amorphous products, while the use of ZrCl 4 and ZrOCl 2 Á8H 2 O resulted in crystalline products. Systematic optimization of the solvent composition (formic acid to H 2 O ratio) led to a highly crystalline product (Fig. 2). Synthesis details and PXRD patterns are given in the supporting information (Table S3. CAU-22 was only obtained as a microcrystalline product and hence the structure had to be determined from PXRD data. Indexing was carried out with Topas 35 and the structure was solved by direct methods using the program suit Expo. 36 The initial structure model was completed using Material Studio 37 and could be successfully refined by Rietveld methods (Fig. 3, see S4, ESI † for details).
CAU-22 (Fig. 4) 10 In this cluster a square antiprismatic coordination of each Zr 4+ ion by eight oxygen atoms from O 2À , OH À and -CO 2 À groups is observed.
In contrast to all other known Zr-MOFs, in CAU-22 these clusters are condensed into {[Zr 6 O 4 (OH) 4 (m-OH) 2 ]} chains by edge-bridging OH À ions (Fig. 4, see also ESI †). Each resulting 1D IBUs is connected to six others by PzDC 2À linker molecules (Fig. 4) and 1D triangular pores with a diameter of ca. 3 Å are formed, taking the van der Waals radii of the atoms lining the pores into account. These pores are occupied by water molecules. Every hexanuclear subunit is coordinated by six PzDC 2À linker molecules and furthermore two formate as well as H 2 O/OH À molecules capping residual coordination sites (Fig. S4.1-S4.10, ESI †).
The latter has been proposed for many other Zr-MOFs where a connectivity o12 of the clusters is observed. 7 Although H 2 O, OH À and O 2À groups cannot be distinguished from the PXRD data, based on the available data and the structure of the hexanuclear clusters, we propose the following sum formula [Zr 6 (m 3 -O) 4 (m 3 -OH) 4 -(m-OH) 2 (OH) 2 (H 2 O) 2 (HCO 2 ) 2 (PzDC) 3 ] for CAU-22, which is also corroborated by additional characterization data.
In addition to the crystal structure analysis, CAU-22 was thoroughly characterised by various methods to confirm the composition and to evaluate the thermal and chemical stabilities as well as the porosity. Temperature dependent powder X-ray diffraction shows a thermal stability for CAU-22 up to approximately 270 1C (Fig. S5.1, ESI †). The thermogravimetric Fig. 2 Results of the synthesis optimization by systematically varying the ratio formic acid to H 2 O from 0% (1,13) to 100% (11,23) formic acid.  To determine the porosity of CAU-22 a sample was activated at 120 1C for 16 h under reduced pressure. The nitrogen sorption measurement was carried out at À196 1C. The isotherm shows a Type I behaviour, typical for microporous compounds (Fig. 5), with a small hysteresis (H4-type). The latter is typical for microporous aggregated samples ( Fig. S3.6, ESI †). 38 The specific surface area is 276 m 2 g À1 and the micropore volume is 0.12 cm 3 g À1 (determined at p/p 0 = 0.5). It is slightly smaller than the theoretical micropore volume of 0.17 cm 3 g À1 calculated with the program Platon. 39 CAU-22 is highly hydrophilic, as demonstrated by water vapour sorption measurements carried out at 25 1C (Fig. 5).
The water isotherm shows a steep increase at low relative humidity values and an absolute uptake of about 0.16 g g À1 CAU-22, which is in good agreement with the anticipated pore volume. Compared to UiO-66 (0.30 g g À1 ) the absolute water uptake is decreased but the steep rise in the isotherm starts at much lower p/p 0 values, which can be associated with the more hydrophilic properties of the linker compared to terephthalate ions in UiO-66. After the sorption measurements, the high crystallinity of the samples was demonstrated by powder X-ray diffraction ( Fig. S9.2, ESI †).
In conclusion, we synthesised a new Zr-MOF denoted CAU-22 by a water-based synthesis route employing a solvent mixture of water and formic acid. The structure of CAU-22 contains a unique 1D IBU which is formed by edge-sharing of the well-known hexanuclear {[Zr 6 (m 3 -O) 4 (m 3 -OH) 4 ]} clusters. It is permanently porous towards nitrogen and water and In contrast to UiO-66 CAU-22 is very hydrophilic which is due to the incorporation of pyrazinedicarboxylate ions as the linker. We anticipate that other linker molecules with similar functionalities will lead to the same IBU.
We thank Daria Smazna from the technical faculty of the CAU Kiel for taking the SEM micrographs.