Design and synthesis of squaramide-based MOFs as eﬃcient MOF-supported hydrogen-bonding organocatalysts †

Herein, we utilize a new, squaramide-based ligand, combined with a postsynthetic exchange (PSE) synthetic approach to prepare a series of Cu( II )–squaramideMOFs that are activecatalysts for the Friedel–Crafts reaction.

Among various organocatalysts, those based on the thiourea core are broadly utilized in the field of H-bond donor catalysis. 1 Recently, bifunctional squaramide moieties have emerged as powerful hydrogen-bonding groups for Lewis acid catalysis. [2][3][4][5][6][7][8] Squaramides are four-membered ring systems, which are derived from squaric acid and possess hydrogen-bond accepting and donating functionality through their carbonyl and N-H groups, respectively. The squaramide functionality possesses features such as ditopic binding, structural rigidity, high N-H acidity, and ease of preparation. 9 Squaramide compounds have been shown to be competent for biomimetic transport, 5,10,11 molecular recognition, 12 ion sensing, 13,14 and organocatalysis. 15,16 The high propensity for hydrogen-bonding is driven via a concomitant increase in aromaticity on the squaramide ring. 17 Although the increased acidity of squaramides can be useful in hydrogen-bond donating organocatalysis, the strong hydrogen-bonding also drives self-association/ aggregation of squaramides that impedes catalysis. 18 To prevent self-association and enhance the catalytic performance of squaramides, one strategy is to immobilize these groups within porous materials such as metal-organic frameworks (MOFs). 19,20 MOFs are a class of porous, crystalline materials composed of inorganic nodes and organic linkers. They have uniform 3-dimensional structures of high surface area, large pore sizes, low density, and have potential applications in gas sorption, 21,22 molecular recognition, 23 proton conductivity, 24 and organocatalysis. [25][26][27][28][29] Along with their modular synthesis and tunable porosity, MOFs constitute attractive candidates as platforms for heterogeneous catalysis.
The H 4 dbda ligand was synthesized using a modified literature procedure. 35 4 ] SBUs to form a 2-dimensional Kagomé lattice that are connected via the squaramide core to form an overall 3-dimensional framework with nbo topology ( Fig. S3 and S4, ESI †). There are large channels with approximate dimensions of 13 Â 5 Å along the crystallographic c-axis. N 2 sorption of Zn(dbda) showed essentially no uptake (Fig. 3), which is attributed to a loss of Zn(dbda) crystallinity upon thermal activation (60 1C, 10 h) as evidenced by powder X-ray diffraction (PXRD, Fig. S5, ESI †). Zn(dbda) was found to be stable in common organic solvents such as CHCl 3 for 24 h at room temperature, but not stable in water ( Fig. S6 and S7, ESI †). Despite these stability limitations, the catalytic performance of Zn(dbda) in the Friedel-Crafts reaction was examined. Indole (0.15 mmol) and b-nitrostyrene (0.1 mmol) were chosen as test substrates, and the reaction was performed at 50 1C in CHCl 3 with 5 mol% loading of the Zn(dbda) catalyst (based on an empirical formula of Zn 4 (dbda) 2 (H 2 O) 4 ); however, no products were observed after 24 h (Fig. S8, ESI †). PXRD of Zn(dbda) after 24 h revealed that the MOF had lost crystallinity under the reaction conditions (Fig. S9, ESI †).
Literature reports suggest that [Zn 2 (COO) 4 ] SBUs may possess low chemical stability. 36 (Table S3, ESI †) due to the shorter Cu-O bonds. For Zn(dbda), the average distance of Zn-O (carboxylate) bonds is 2.036 Å, while for Cu(dbda), the average Cu-O (carboxylate) distance is 1.947 Å. Moreover, the metalmetal distance in the paddlewheel SBUs decreased from 2.999 Å (Zn-Zn) to 2.629 Å (Cu-Cu). The permanent porosity of Cu(dbda) was measured by N 2 absorption at 77 K, which showed a typical type I isotherm, 38 giving a BET surface area of 1516 AE 66 m 2 g À1 and a total pore volume of 0.662 cm 3 g À1 (Fig. 3). The PXRD pattern of Cu(dbda) remains intact upon immersion in organic solvents for 90 h, suggesting improved stability consistent with the gas sorption data; however, Cu(dbda) was unstable in water ( Fig. S12 and S13, ESI †).
An advantage of heterogeneous catalysts is reusability, which was also investigated for Cu(dbda). Only a small decrease in activity (from 499% to B96%) was observed after 5 runs (Fig. S17, ESI †). Characterization of Cu(dbda) after the 5th run showed the catalyst retained its crystallinity, with the PXRD pattern of the recycled Cu(dbda) in good agreement with the calculated patterns (Fig. S18, ESI †). A test was performed to confirm the heterogeneity of the catalysts (and rule out soluble species) by removing the MOF catalyst by filtration after 30 min (at which time the yield was B22%). The filtrate was then re-analyzed after a total of 24 h showing no formation of new product and indicating that the catalyst is heterogeneous and there is no leaching of a catalytic species into solution (Fig. S16, ESI †). 20 The substrate scope for various substituted-nitrostyrene derivatives was also examined to assess the utility of Cu(dbda) (Fig. S19, ESI †). Good to excellent yields, ranging from B61% for 4-nitro-b-nitrostyrene (poor solubility in CHCl 3 ) to 97% for 4-chloro-b-nitrostyrene, were obtained (Table 1). Overall, these results show that Cu(dbda) is an efficient, recyclable, heterogeneous catalyst for the Friedel-Crafts reaction.
Taken together, Cu(dbda) has features that compliment and distinguish it from the previously reported UiO-67-Squar/bpdc catalyst. 20 Cu(dbda) has a different MOF structure type from UiO-67-Squar/bpdc, wherein the catalytic group is part of the ligand 'backbone' rather than a dangling component. Depending on catalyst design, one can envision scenarios where one or the other functional group arrangement might be preferable. Also, in contrast to UiO-67-Squar/bpdc, Cu(dbda) allows for 100% functional ligand incorporation without loss of surface area. Indeed, using H 4 tptc as a complimentary, unfunctionalized ligand the Cu(dbda) system is not limited with respect to the amount of squaramide ligand that can be included while maintaining a high degree of porosity and activity. Both Cu(dbda) x (tptc) 1Àx and UiO-67-Squar/bpdc catalysts show good activity under similar conditions, and hence each helps advance the ability of utilizing squaramides in MOF-based catalytic systems.
In summary, we prepared a squaramide tetracarboxylate ligand, and developed catalytic squaramide MOFs prepared via metal PSE from Zn(II) precursor MOFs. The Cu(II) MOFs were more stable and hence showed good catalytic performance when compared to the unstable parent Zn(II) MOF, in a Friedel-Crafts reaction of indole with b-nitrostyrenes. A series of isostructural MOFs Cu(dbda) x (tptc) 1Àx (x = 0.75, 0.49, 0.18, 0) showed that the catalytic performance of these MOFs increased with increasing amounts of the squaramide ligand, demonstrating that squaramide MOFs are promising MOF-supported heterogeneous organocatalysts. Our ongoing progress with squaramide MOFs are focusing on their applications in molecular recognition and ion sensing.
This work was financially supported by a grant from the Division of Chemistry of the National Science Foundation (CHE-1359906) and China Scholarship Council.