Short communicationA urea-containing metal-organic framework as a multifunctional heterogeneous hydrogen bond-donating catalyst
Graphical abstract
Introduction
Hydrogen bond-donating (HBD) catalysis has been recognized as an effective approach for the synthesis of various highly valuable intermediates for pharmaceutical and agricultural industries because HBD catalysts are capable of binding selectively and activating reactive substrates during the reaction process through hydrogen bonding or other noncovalent interactions [[1], [2], [3], [4]]. In the past decades, numerous HBD catalysts with different structures and functionalities have been designed for specific reactions including Friedel–Crafts (FC) reactions, Mannich reaction, Strecker reaction, etc. [[5], [6], [7], [8]]. Among these catalysts, urea has been widely utilized as a catalytic moiety owing to its chelating hydrogen bonding through acidic NH sites [9,10]. However, the intrinsic self-assembly of urea molecules caused by the intermolecular interactions of hydrogen bonding always leads to a potential deterioration in catalytic reactivity [11,12]. Thus, it is necessary to introduce enhanced catalyst loading or elongated reaction time or external toxic additives to achieve high yield in a certain homogeneous catalytic system [13,14]. In contrast to the conventional approach of enhancing the reactivity of urea-based catalysts, various supports have been exploited to break the self-association of urea moieties in catalysis. For instance, the Lin group demonstrated that mesoporous silica nanosphere materials could serve as support for a urea-based catalyst to achieve superior catalytic behavior in some important chemical transformations compared with their corresponding precursors [15,16]. The Portnoy group has developed a urea-based organocatalyst supported by a porous organic polymer. This organocatalyst has been established as an efficient catalyst for promoting nitro-Michael addition [17]. Additionally, metal-organic frameworks (MOFs), emerging as new support materials for heterogeneous catalysis, are gaining increasing interest owing to their unique features of high porosity, structural tunability, excellent stability, and well-defined catalytic sites [[18], [19], [20], [21], [22]]. For example, Roberts and co-workers first synthesized one MOF-based catalyst bearing a urea moiety, which is highly active in the FC reactions between pyrroles and nitroalkenes. Subsequently, a few MOF-based HBD catalysts derived from urea-, thiourea-, or squaramide-functionalized molecules were prepared through direct synthesis or post-synthetic modification. These MOF catalysts also displayed high efficiency in catalyzing FC reactions [[23], [24], [25], [26], [27], [28], [29], [30]]. In spite of a growing interest in such solid catalysts, studies on multifunctional MOF-based HBD catalysts remain limited. In this work, a urea-containing MOF was synthesized and utilized as a highly efficient HBD catalyst to promote the cyanosilylation reaction of aldehydes, the Henry reaction of aldehydes, and the methanolysis reaction of epoxides. Additionally, this urea-based MOF catalyst exhibits a broad substrate scope and excellent recyclability.
Section snippets
Synthetic chemistry
In this work, the urea-containing MOF (1) was synthesized from a V-shaped dicarboxylate ligand (L1−H2) with urea moiety according to a previously reported procedure (Scheme 1) [31]. For the comparison of catalytic activity, MOF 2 was readily obtained from a solvothermal reaction of copper nitrate and ligand L2-H2. MOF 1 and 2 both possessed a similar two-dimensional framework. The structures of both MOFs were confirmed by the results of single-crystal X-ray diffraction analysis (Scheme 1 and
Results and discussion
To the best of our knowledge, the intrinsic unwanted self-association behavior of the urea moieties is quite common in homogeneous catalysis. This phenomenon usually hampers their catalytic performance. It can be clearly seen that the self-association of the urea-containing ligand L1-H2 (Scheme 2a) was successfully restricted when it was immobilized on the MOF structure. Therefore, MOF 1 can serve as a potential HBD catalyst. Cyanosilylation of various aldehydes using trimethylsilyl cyanide is
Conclusions
In summary, a urea-containing MOF was synthesized from a V-shaped urea-functionalized ligand. The MOF possesses catalytically active dual NH groups that allow for the activation of the carbonyl compound. This HBD MOF catalyst demonstrated excellent activity in the cyanosilylation reaction, the Henry reaction of aldehydes, and the methanolysis of epoxides. Further, the HBD MOF catalyst can be easily recovered and reused without any apparent loss of catalytic activity. Our study, therefore,
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21401037 and 21672049), Anhui Provincial Natural Science Foundation (No. 1508085QB26), and Fundamental Research Funds for the Central Universities of China (No. PA2019GDPK0058 and PA2019GDPK0084). We also thank the staff from BL17B beamline of National Facility for Protein Science in Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility for assistance during crystal data collection.
Declaration of Competing Interest
None.
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