Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
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Simple and Robust Immobilization of a Ruthenium Olefin Metathesis Catalyst Inside MOFs by Acid−Base Reaction Artur Chołuj, Robert Karczykowski, and Michał J. Chmielewski* Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, 02-089 Warszawa, Poland
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ABSTRACT: The first successful immobilization of a transition-metal catalyst inside MOFs by an acid−base reaction has been described. Simple absorption of a commercially available, amino-tagged, Hoveyda−Grubbs type ruthenium olefin metathesis catalyst inside an easy to make, Brønsted acidic MOF, (Cr)MIL-101-SO3H, yields a heterogeneous catalyst, which is stable even in very polar, “green” solvents, such as dimethyl carbonate and 2-propanol. The catalyst gives essentially ruthenium-free products upon simple filtration.
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simple physisorption from solution. Although in these two cases the catalysts were held inside the MOFs by only noncovalent forces, no leaching was observed even in relatively polar solvents, such as dichloromethane (DCM) and ethyl acetate (EA). Encouragingly, the physisorption was so robust that no Ru contamination could be found in the products by ICP-MS, and the materials could be used in a continuous-flow setup. Unfortunately, however, more environmentally benign solvents, such as isopropyl alcohol (i-PrOH) and methanol, rapidly wash out the physisorbed cationic Ru complexes from neutral MOFs such as (Al)MIL-101-NH2. Here we demonstrate that more robust immobilization of OM catalysts is possible by a simple acid−base reaction between commercially available amino-tagged catalysts and SO3H-functionalized MOFs. The heterogenized catalysts thus obtained can be used even in very polar “green” solvents, such as i-PrOH and dimethyl carbonate (DMC),9,10 without any appreciable leaching of Ru into the solution. Although this straightforward acid−base strategy has been used for the immobilization of various catalysts on conventional supports,11−15 to the best of our knowledge the only precedents for a similar approach in MOFs are limited to the two organocatalysts mentioned above.5,6 An alternative approach toward robust immobilization of OM catalysts for use in “green” solvents is presented in an accompanying paper by Grela and co-workers.16 Catalytic OM is one of the most important C−C bond forming reactions in organic chemistry and is currently on the verge of mass implementation in pharmaceutical and specialty-
mmobilization of catalysts inside nanoscopic cavities of metal−organic frameworks (MOFs) allows their facile separation and recycling and, in addition to that, provides excellent opportunities to control their size, shape, regio- and enantioselectivities. Moreover, the simultaneous encapsulation of different catalytic species in the same framework opens exciting prospects for tandem catalysis. However, robust immobilization of active catalysts inside MOFs is challenging. Thus far, the most popular strategies toward this goal have been the postsynthetic covalent attachment to the frameworks’ organic struts and the formation of coordination bonds with the frameworks’ metal nodes.1,2 Alternatively, catalysts based on salen, BINOL, and NHC ligands may be synthetically elaborated to become struts of MOFs.2 These approaches, however, require tedious tailoring of the catalyst’s structure, which not only is laborious but may also compromise its performance. Strongly coordinating groups, which are necessary to anchor the catalyst to the framework’s nodes, are likely to coordinate and deactivate the catalytic centers as well. Furthermore, only few catalysts can survive the harsh conditions of a typical solvothermal synthesis of a MOF. Thus, postsynthetic immobilization methods which avoid structural modification of the catalysts seem to be particularly promising, because they are mild and maintain the carefully optimized ligand systems of homogeneous catalysts. Surprisingly, however, there are very few examples of direct, noncovalent immobilization of discrete catalysts inside MOFs. Positively charged metal complexes were successfully incorporated into negatively charged frameworks by ion exchange.3,4 Basic organocatalysts were immobilized in acidic MOFs by acid−base reactions.5,6 Recently, we have also shown that cationic olefin metathesis (OM) catalysts can be easily immobilized inside (Al)MIL-101-NH27 and (Al)MIL-1018 by © XXXX American Chemical Society
Special Issue: Organometallic Chemistry within Metal-Organic Frameworks Received: April 29, 2019
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DOI: 10.1021/acs.organomet.9b00281 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
40% of ligands decorated with −NH2+(CH2)3SO3− groups. Its specific surface area of 1413 m2/g was much lower than that of the starting (Al)MIL-101-NH2 (2346 m2/g) but significantly higher than that of the chromium analogue obtained by Ko et al. (1020 m2/g at 58% functionalization).25 According to the literature, the sulfonic groups in (Cr)MIL101-SO3H exist in three different forms, each comprising roughly one-third of the total amount: acidic SO3H, neutral SO3Na, and anionic SO3− coordinated to the chromium nodes and balancing their charge.21,26 We assumed, therefore, that in both MOFs only approximately 30−40% of the linkers appear in their acidic form. However, since we wanted to accommodate no more than one Ru complex per meso cage (on average), there were still more than 10 acidic groups per Ru complex in both materials. Thus, an appropriate amount of each MOF was added to a concentrated (2.2 mM) solution of the catalyst 1 in DCM. The highly colored, green solutions of 1 turned completely colorless after a few minutes of stirring. The solid materials were washed three times with DCM, and the combined supernatants were examined by UV−vis spectroscopy. Very low concentrations of 1 were found in both solutions, suggesting that >99.95% and >99.4% of the catalyst were absorbed in (Cr)MIL-101-SO 3 H and (Al)MIL-101NH2+(CH2)3SO3−, respectively. To investigate the robustness of the catalyst immobilization in various solvents, challenging dynamic desorption tests were performed. Four increasingly polar solvents (50 mL each) were slowly passed through a thin layer of 1@MOF on a glass frit (ca. 30 mg of the material, ca. 1 mm in thickness). The amount of 1 in the filtrates was determined by UV−vis spectroscopy (Figure 1).
chemicals manufacturing.17 Accordingly, considerable efforts have been directed toward the immobilization of OM catalysts on various supports, motivated mainly by the prospect of catalyst recycling and facile separation of toxic heavy metals from the product.18 However, only very recently have we7,8 and others19,20 successfully supported OM catalysts inside MOFs. Inspired by the work of Kirschning, Grela, and co-workers,11 who immobilized an OM catalyst with an amino-tagged benzylidene moiety on a sulfonated polymeric substrate, we resolved to apply a similar strategy to MOFs. Unlike our predecessors, however, we decided to attach a catalyst through the imidazolidine ligand rather than through the benzylidene, because the latter is believed to dissociate during the catalytic cycle. Thus, we opted for the diamino-tagged catalyst 1, a direct analogue and precursor of 2, which was used in our previous studies (Chart 1). Chart 1. Ruthenium Catalysts Used in This (1) and in the Previous Study (2)7
One of the very few MOFs that combine the high chemical stability required to bear strongly acidic SO3H groups with the presence of cavities and windows large enough to accommodate bulky OM catalysts is (Cr)MIL-101-SO3H.21 This material can be easily obtained in a one-pot solvothermal synthesis from commercially available substrates and has two types of mesoporous cages (ca. 2.6 and 3.2 nm in diameter), connected through windows of 1.4 and 1.6 nm, respectively. Furthermore, the cages in MIL-101 form a 3D pore system, which facilitates rapid diffusion of substrates, products, and solvents. In order to investigate whether robust immobilization could be achieved inside a MOF with a much less acidic functional group, we also developed the synthesis of (Al)MIL-101NH2+(CH2)3SO3− and tested it as a mildly acidic alternative to (Cr)MIL-101-SO3H.22 (Cr)MIL-101-SO3H was obtained by direct solvothermal synthesis from sodium 2-sulfoterephthalate and chromium oxide.21 Nitrogen sorption analysis of this material gave a specific surface area value of 1818 m2/g (BET model), which agrees well with the literature value of 1842 m2/g. We have also tried a modified procedure proposed by Zhang, Zhu, and co-workers, which involved the postsynthetic treatment of (Cr)MIL-101-SO3H with large excess of aqueous NH4F.23 However, the material thus obtained contained large amounts of residual fluorides which poison the OM catalyst and was therefore abandoned after preliminary catalytic tests. In parallel, (Al)MIL-101-NH2 was treated with 1,3-propanesultone using the procedure developed by Britt et al. for IRMOF-324 and also utilized later by Ko et al.25 for the synthesis of (Cr)MIL-101-NH2+-R-SO3−. The material retained its crystallinity and was found to contain approximately
Figure 1. Leaching of 1 (basic complex) and 2 (cationic complex) from selected MOFs upon elution with various solvents.
The experiment shows that 1 is very firmly attached to both sulfo-MOFs: no detectable concentration of 1 was found in both toluene and DCM eluates, as opposed to what has been observed in the case of 1@(Al)MIL-101-NH2.7 EA washed out only 1−2% of the catalyst from (Cr)MIL-101-SO3H, while continuous leaching was observed in the case of (Al)MIL-101NH2+(CH2)3SO3−, eventually giving a total loss of 10%. Similar behavior was observed with i-PrOH, which washed out another 10% of 1 from the (Al)MOF and just 3% from the (Cr)MOF. Moreover, in the latter case all of it was washed out by the first portion of the solvent, and the remaining 45 mL of i-PrOH contained no catalyst at all. It seems likely, therefore, B
DOI: 10.1021/acs.organomet.9b00281 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics that this small fraction of 1 was relatively weakly adsorbed on the surface of crystals. Significantly more pronounced leaching from (Al)MIL-101NH2+(CH2)3SO3− is easy to rationalize in terms of the relatively low acidity of the zwitterionic ammonium salt and consequently more easily reversible protonation of 1: the pKa of −NH2+(CH2)3SO3− is ca. 10, while the pKa of −SO3H is ca. −2. Thus, the immobilization of basic compounds on (Al)MIL-101-NH2+(CH2)3SO3− is not sufficiently robust to withstand the most polar solvents, such as EA and i-PrOH, whereas the affinity of 1 to (Cr)MIL-101-SO3H is remarkably strong: only traces of the catalyst were lost despite extensive washing with i-PrOH. The catalytic activity of 1 supported on both acidic MOFs was initially investigated in a ring-closing olefin metathesis (RCM) reaction using diethyl diallylmalonate (DEDAM) as a model diene, toluene as solvent, and 1 mol % of the Ru catalyst. Significantly better results were obtained with the more robust (Cr)MOF (95% vs 80% yield); therefore, we focused our further studies on this material. 1@(Cr)MIL-101-SO3H was compared with a homogeneous solution of 1. Under homogeneous conditions, the catalytic activity of 1 was low (45% of conversion), likely due to selfdeactivation by its basic amino groups. However, in the presence of 10 equiv of p-toluenesulfonic acid (simulating the conditions inside the (Cr)MIL-101-SO 3H) its activity increased considerably, resulting in 95% conversion, identical with that of 1@(Cr)MIL-101-SO3H. Next, we investigated the catalytic activity of 1@(Cr)MIL101-SO3H in various polar solvents: DCM, which is one of the most popular solvents for homogeneous RCM reactions, and three other, more environmentally friendly solvents: i-PrOH, EA, and DMC (Table 1 and Figure 2).
Figure 2. Conversion vs time curves for RCM of DEDAM catalyzed by 1 mol % of 1@(Cr)MIL-101-SO3H in various solvents.
the conversion reaching 96% (Figure 2). Even after the amount of Ru was reduced to 0.1 mol %, 77−80% conversion was obtained in both EA and DMC (Table 1, entries 7 and 8), despite the solvents being neither dried nor purified. It is worth noting that in the latter case the conversion was higher than in the homogeneous solution (entry 9). To confirm the heterogeneity of our MOF-supported catalysts, we also performed split tests in various solvents (Table S4). After a specified time, each reaction mixture was divided into two portions. One portion was filtered through a 0.2 μm PTFE syringe filter and then both portions, heterogeneous and homogeneous, were stirred further. The conversion was determined in both portions at the time of the split and again after 24 h. We found that the only noticeable increase in conversion in the homogeneous portion occurred in EA (3.7%), suggesting minor leaching of active catalyst to the solution. The crude product from this reaction contained 74 ppm of Ru (by ICP-MS), which corresponds to 0.16% of the initial amount (Table S6). Gratifyingly, in other solvents the product contamination by toxic ruthenium was even lower, and in toluene, DMC and DCM did not exceed the 10 ppm level, which is acceptable for pharmaceutical use (
