Stable porphyrin Zr and Hf metal–organic frameworks featuring 2.5 nm cages: high surface areas, SCSC transformations and catalyses

Two isostructural porphyrin Zr and Hf metal–organic frameworks (FJI-H6 and FJI-H7) are rationally synthesized, and are constructed from 2.5 nm cubic cages.

Owing to their high surface area, permanent porosity and tunable pores, metal-organic frameworks (MOFs) have been used for catalysis, 1 gas separation and storage, 2 and drug delivery. 3 Nonetheless, one big obstacle to the practical applications of MOFs is their stability, which includes water and chemical stability. To target stable MOFs, one effective method is the use of high-valence metal ions such as Fe 3+ , Al 3+ , Cr 3+ , Zr 4+ and Hf 4+ ions as the metal nodes. 4 Compared with the traditional Cu 2+ , Zn 2+ and Co 2+ ions, 5 the aforementioned trivalent or tetravalent metal ions will form stronger bonds with carboxylate groups according to the theory of hard and so acids and bases. Therefore, the stability of the obtained frameworks will be strengthened. Furthermore, these highvalence metal ions tend to form highly connected inorganic clusters via the OH À and/or O 2À bridges, which also signicantly contributes to the stability of the frameworks. As for the Zr 4+ ion, it prefers to form the classical 12-connected Zr 6 O 4 (OH) 4 node. When assembled with linear carboxylate ligands, 3D fcu frameworks with ordered cubic cages (known as the UiO series) can be obtained. 6 These kinds of Zr-based MOF oen have high water and chemical stability, and can even serve as water adsorbents. 7 Up to now, extensive investigation has been carried out to tune the porosity of the UiO series of Zr-MOFs by selectively removing organic linkers 8 or by functionalizing the ligands (by pre-modication or post-synthetic methods). 9 However, there are several examples based on the assembly of Zr 6 O 4 (OH) 4 clusters with polycarboxylic ligands such as planar tetracarboxylic acids. 7b,10 In addition, compared with Zr-based MOFs, Hf-based MOFs are also rare. 6f,11 Herein, we present two ultra-stable metal-organic frameworks ([Zr 6 O 4 (OH) 4 (H 2 TBPP) 3 ] n $(solvent) x ) (FJI-H6) and ([Hf 6 O 4 (OH) 4 -(H 2 TBPP) 3 ] n $(solvent) x ) (FJI-H7), which are isostructural and both constructed from M 6 O 4 (OH) 4 (CO 2 ) 12 nodes (M ¼ Zr, Hf) and porphyrin tetracarboxylic ligands (H 6 TBPP ¼ 4 0 ,4 000 ,4 00000 ,4 0000000 -(porphyrin-5,10,15,20-tetrayl)tetrakis([1,1 0 -biphenyl]-4-carboxylic acid)). As expected, both FJI-H6 and FJI-H7 have high water and chemical stability and can undergo a single-crystal-to-singlecrystal (SCSC) transformation to embed Cu 2+ ions into the open porphyrin rings. Interestingly, they both feature 2.5 nm cages. Notably, FJI-H6 has a high BET surface area of 5033 m 2 g À1 .

Syntheses and structures of porphyrin Zr and Hf MOFs
Reaction of H 6 TBPP with ZrCl 4 or HfCl 4 modulated by benzoic acid gives rise to dark red crystals of FJI-H6 or FJI-H7. Single crystal X-ray structural analysis shows that FJI-H6 and FJI-H7 are isostructural. 12 Therefore, we chose FJI-H6 as the example in the following discussion. FJI-H6 crystallizes in the high symmetry space group Pm 3m. In the Zr 6 O 4 (OH) 4 cluster, six equivalent Zr 4+ ions are in a square-antiprismatic O 8 coordination environment and form a regular octahedron. In this Zr 6 octahedron, the eight triangular faces are alternatively capped by four m 3 -OH À and four m 3 -O 2À groups. Additionally, the twelve edges of the Zr 6 octahedron are bridged by twelve carboxylate groups from twelve unique H 2 TBPP ligands. At the same time, each H 2 TBPP ligand, in which the peripheral four phenyl rings are coplanar with the inner porphyrin rings, links four independent Zr 6 O 4 (OH) 4 clusters. Thus, a rarely seen (4,12)-connected w framework can be acquired. 10a,11b,13 FJI-H6 has two kinds of polyhedral cages, i.e. a small octahedral cage and a large cubic cage. As seen in Fig. 1, the octahedral cage is constructed from two Zr 6 O 4 (OH) 4 clusters and four H 2 TBPP ligands, with a cavity diameter of ca. 1.5 nm. However, the cubic cage consists of eight Zr 6 O 4 (OH) 4 clusters as the vertices and six H 2 TBPP ligands as the sides. Signicantly, the diameter of the cubic cage is approximately 2.5 nm, which is larger than that in PCN-221 (2.0 nm). 11b Accordingly, the available volume is 15 000Å 3 . Additionally, the window of the cubic cage is 1.2 nm Â 2.0 nm, which allows large organic molecules to freely get in and out.

Gas adsorption experiments and stability tests
Calculation by PLATON soware 14 reveals that in FJI-H6 the free volume is up to 78.6%. For FJI-H6 the permanent porosity is conrmed by an N 2 adsorption isotherm measured at 77 K. The sample exchanged with acetone exhibits a reversible type I isotherm and has a saturated uptake of 1346 cm 3 g À1 at 1 atm ( Fig. 2). When pre-treated with 8 M HCl, the value of the N 2 adsorption slightly increases to 1393 cm 3 g À1 , which indicates that FJI-H6 is stable with respect to the acid. From the above data, the calculated BET surface area of the sample exchanged with acetone is up to 5007 m 2 g À1 (5033 m 2 g À1 for the sample pre-treated with 8 M HCl), which is much larger than those of PCN-222(Fe) (2200 m 2 g À1 ), 4b NU-1000 (2320 m 2 g À1 ), 15 PCN-223(Fe) (1600 m 2 g À1 ), 10c PCN-94 (3377 m 2 g À1 ), 10b NU-1100 (4020 m 2 g À1 ) 10a and PCN-229 (4619 m 2 g À1 ), 13 but less than those of the just reported NU-1103 (5646 m 2 g À1 ) and NU-1104 (5290 m 2 g À1 ). 10e In addition, FJI-H6 also has a high total pore volume of 2.16 cm 3 g À1 . The experimental BET surface area and pore volume are consistent with theoretical values calculated by Poreblazer 16 (accessible surface area: 4695 m 2 g À1 ; pore volume: 2.06 cm 3 g À1 ), 14 which demonstrates that the sample is fully activated. Additionally, FJI-H6 also shows good capacity for H 2 storage. The H 2 uptake reaches 172 cm 3 g À1 (1.54 wt%) at 1 atm and 77 K, and 108 cm 3 g À1 (0.94 wt%) at 87 K and 1 atm. Moreover, the adsorption heat of H 2 calculated by the Clausius-Clapeyron equation is 6.54 kJ mol À1 at zero coverage and decreases slowly with increasing H 2 loading. These values are comparable to those of famous MOF materials, such as HKUST-1 (6.6 kJ mol À1 ), 17 MOF-5 (5.2 kJ mol À1 ), 17 and NOTT-122 (6.0 kJ mol À1 ). 18 As for FJI-H7 the sample exchanged with acetone also exhibits a reversible type I isotherm and has a saturated uptake of 1029 cm 3 g À1 at 1 atm. From the above data, the calculated BET surface area of FJI-H7 is up to 3831 m 2 g À1 , which is among the highest reported for Hf-based MOFs. 11 Since the Zr 6 O 4 (OH) 4 and Hf 6 O 4 (OH) 4 clusters are both highly connected with twelve carboxylate groups, FJI-H6 and FJI-H7 are expected to have a high stability. To test their stabilities, we immersed a microcrystalline sample of FJI-H6 or FJI-H7 into water with various pH values for 24 h (see Fig. S2 †). The PXRD patterns of the resulting samples match well with the simulated ones, which suggests that FJI-H6 and FJI-H7 retain their crystallinity. In particular, FJI-H6 has high stability in acid. As seen in Fig. 2, the curves and the adsorption values for the sample treated with 8 M HCl do not deviate much from those of the untreated sample.

Incorporating Cu 2+ ions into the open porphyrin rings via SCSC transformations
Considering that in both FJI-H6 and FJI-H7 two nitrogen atoms of the porphyrin ring are not deprotonated, we attempted to introduce a second kind of metal ion into the framework. Immersing single-crystals of FJI-H6 or FJI-H7 into a solution of 0.5 M Cu(NO 3 ) 2 in N,N-dimethyl formamide (DMF) at 85 C for Fig. 1 (a) The large cubic cage constructed from six porphyrin ligands and eight Zr 6 O 4 (OH) 4 clusters. (b) The small octahedral cage constructed from four porphyrin ligands and two Zr 6 O 4 (OH) 4 clusters. (c) Packing of the two kinds of cages. 72 h results in metallated FJI-H6(Cu) or FJI-H7(Cu). As anticipated, single crystal X-ray structural analysis denitively shows that in both FJI-H6 and FJI-H7 the Cu 2+ ions have been embedded in the porphyrin rings. The Cu 2+ ion is in a square planar N4 coordination environment with two axial sites exposed, which is typical for divalent metal ions in metalporphyrin complexes. 19 We believe that, although there are several examples of exchanging metal ions in porphyrin MOFs, this is the rst observation of incorporating metal ions into open porphyrin rings via SCSC transformations in porphyrin MOFs. N 2 adsorption measurements at 77 K for FJI-H6(Cu) also show a type I isotherm. At 1 atm, FJI-H6(Cu) has the maximum N 2 adsorption of 1128 cm 3 g À1 , which is smaller than that of FJI-H6. Accordingly, the BET surface area of FJI-H6(Cu) is reduced to 3731 m 2 g À1 . Similarly, the maximum N 2 adsorption and BET surface area of FJI-H7(Cu) (918 cm 3 g À1 and 3195 m 2 g À1 , respectively) are also lower than those of FJI-H7. The reason may be ascribed to the introduction of Cu 2+ ions, which can slightly diminish the surface area.

Cycloaddition reactions of CO 2 with epoxides
Recently, owing to global warming, efficient CO 2 capture and storage is urgently needed to reduce CO 2 emissions before scientists nd a practical clean energy. If we can convert this abundant inorganic waste into usable organic chemicals utilizing reasonable reactions at ambient conditions, the above problem can be perfectly solved. One practical method is the synthesis of cyclic carbonates from CO 2 and epoxides, which have extensive applications as degreasers, polar aprotic solvents and electrolytes in lithium ion batteries. Though many catalysts have been explored for the above reaction, metalloporphyrins show relatively high catalytic activity. 20 Hence, we have evaluated FJI-H6, FJI-H6(Cu), FJI-H7 and FJI-H7(Cu) as heterogeneous catalysts for the cycloaddition reaction of CO 2 with epoxides (Scheme 1). Typically, 25.5 mmol 3-chloropropylene oxide, 0.51 mmol (2.0 mol%) tetrabutylammonium bromide and 0.051 mmol (0.2 mol%) catalyst were added to a thickwalled glass tube with a stirring bar. The tube was placed under vacuum and then purged with CO 2 . The above cycle was repeated three times. Finally, the pressure of CO 2 was set as 1 atm. The mixture was stirred at 25 C for 60 hours. Analysis of the resulting solution by gas chromatography indicated that 52.6%, 61.8%, 64% and 66.5% of the epoxide was converted into the cyclic carbonate for FJI-H6, FJI-H6(Cu), FJI-H7 and FJI-H7(Cu), respectively. Though the yields are not very high compared with homogeneous catalysts, it is nevertheless promising considering the low temperature and pressure.
Compared with FJI-H6 or FJI-H7, FJI-H6(Cu) or FJI-H7(Cu) has a higher catalytic ability. The reason may be that, as a Lewis catalytic site, the embedded Cu(II) ion in the porphyrin ring contributes to some extent. At the same time, the Hf-based MOFs FJI-H7 and FJI-H7(Cu) have higher catalytic abilities than the corresponding Zr-based MOFs FJI-H6 and FJI-H6(Cu), respectively since the Hf ion is more oxophilic than the Zr ion and acts as a stronger Lewis acid. Additionally, the PXRD patterns of FJI-H6(Cu), FJI-H7 and FJI-H7(Cu) aer catalyses are in good agreement with the simulated ones (see Fig. S2 †), which further demonstrates that they all retain their framework. However, it is a pity that FJI-H6 lost its crystallinity during the catalytic process.

Conclusions
In conclusion, we report the design and synthesis of two ultrastable MOFs FJI-H6 and FJI-H7, which both feature 2.5 nm cages. In particular, FJI-H6 has a high BET surface area of 5033 m 2 g À1 . Due to the high connectivity of the M 6 O 4 (OH) 4 clusters (M ¼ Zr and Hf), FJI-H6 and FJI-H7 are stable in water with pH values ranging from 0 to 10. Interestingly, they can undergo a single-crystal to single-crystal transformation to embed Cu 2+ ions into the porphyrin rings, which also indicates their high chemical stability. Additionally, preliminary catalysis evaluation shows that FJI-H6(Cu), FJI-H7 and FJI-H7(Cu) exhibit promising catalytic capacity for converting CO 2 and epoxides into cyclic carbonates at low temperature and pressure. Consequently, FJI-H6, FJI-H7 and their derivatives may be applied in catalysis due to their high surface area, ultra-high stability and easy post-modication. Further research is ongoing.