Size-Selective Acid Catalysis 

Framework pores control substrate access to exposed metal ions                                                               

Steve Ritter                                                                                       
Metal-organic frameworks (MOFs), once synonymousonly with hydrogen gas storage, are starting to show their potential asselective catalysts. In the latest development on this front, Jeffrey R. Longand coworkers at the University of California, Berkeley, havedemonstrated how to take advantage of a MOF's pore size to preciselyselect substrates for heterogeneous catalytic reactions (J. Am. Chem. Soc., DOI: 10.1021/ja800669j).

                                                                                                                        J. Am. Chem. Soc.                                                                        View Enlarged Image

                                                               
                                
MOFs are porous crystalline materials typically made up ofmultimetal clusters interconnected by organic linkers. The researchersbegan their study by probing the catalytic activity of a manganesebenzenetristetrazolate MOF (Mn-BTT), a material Long's group firstsynthesized in 2006. This MOF started out as a record-setting H2 storage material, with part of its success owing to two different types of unsaturated Mn2+ ions left exposed within the pore structure, Long says.
Besides being good at snagging H2 molecules, the Mn2+ions are Lewis acids and are well positioned to interact with guestmolecules that are small enough to enter the framework pores, he adds.The team reasoned that Mn-BTT could function as a stand-in forzeolites, an industrially relevant class of microporous heterogeneouscatalysts.
To prove the idea, Long and research group members Satoshi Horike, Mircea Dinc,and Kentaro Tamaki carried out two sets of reactions that rely on Lewisacid catalysts. One reaction was the cyanosilylation of aromaticaldehydes or ketones to form cyanohydrins, while the other was theMukaiyama aldol reaction that couples aldehydes with silyl enol ethersto form β-hydroxyketones.
The MOFs turned out to function just as the researchers imaginedthey would. For example, cyanosilylation of an aryl aldehyde containinga small aryl group, such as a phenyl, went essentially to completion—asignificant improvement in yield over other MOF catalysts. On the otherhand, when the larger biphenyl group was used with an aryl methylketone substrate, the yield dropped to nearly zero, presumably becausethe substrate was too large to enter the MOF's pores and come incontact with the metal sites.
"Developing new catalysts is one of the most important challenges inthe chemical industry, and the studies carried out by Long andcoworkers illustrate well the power of being able to tailor activesites within MOFs," comments UCLA's Omar M. Yaghi, a leading authorityon MOFs. "Their findings are along the lines of what researchers haveenvisioned to be possible by designing MOFs."
Using MOFs for catalysis has enormous potential, Long says. Comparedwith zeolites, their surface area is much higher, leading to greatercatalytic efficiency on a weight basis. And simple modification of thebridging ligands and the ability to readily exchange the metal ionsprovide a tunability of properties that is lacking in zeolitechemistry, he says.
Zeolites are mostly made from silicon, aluminum, and oxygen—inessence they are nanoporous rocks and are very robust, explainsNorthwestern University's Joseph T. Hupp, another MOF researcher. That's why they are used as catalysts in many high-temperature, gas-phase refinery processes, he says.
Where MOFs really shine is in permitting chemists to retain themicroporous character of zeolites while creating more versatilematerials for condensed-phase catalysis, Hupp adds. And that makes MOFspromising for high-value transformations under milder conditions, suchas enantioselective reactions for synthesizing pharmaceuticalintermediates and other fine chemicals, he says.
"Given the broad utility of zeolites in catalysis, I believe thepossible applications for MOFs in this area have really only just begunto be explored," Long says.

 

 

Size-Selective Lewis Acid Catalysis in a Microporous Metal-Organic Framework with Exposed Mn²⁺ Coordination Sites

Satoshi Horike, Mircea Dinca?, Kentaro Tamaki, and Jeffrey R. Long^*

Department of Chemistry, University of California, Berkeley, California 94720

jrlong@berkeley.edu

Received January 27, 2008

 

 

Abstract:

Treatment of selected aldehydes and ketones with cyanotrimethylsilane in the presence of the microporous metal-organic framework Mn₃[(Mn₄Cl)₃BTT₈(CH₃OH)₁₀]₂ (1, H₃BTT = 1,3,5-benzenetristetrazol-5-yl) leads to rapid conversion to the corresponding cyanosilylated products. The transformation is catalyzed by coordinatively unsaturated Mn²⁺ ions that serve as Lewis acids and lead to conversion yields of 98 and 90% for benzaldehyde and 1-naphthaldehyde, the highest thus far for a metal-organic framework. Larger carbonyl substrates cannot diffuse through the pores of 1, and conversion yields are much lower for these, attesting to the heterogeneity of the reaction and its dependence on guest size. The Mukaiyama−aldol reaction, known to require much more active Lewis catalysts, is also catalyzed in the presence of 1, representing the first such example for a metal-organic framework. Conversion yields obtained for the reaction of selected aldehydes with silyl enolates reach 63%, on par with those obtained with zeolites. Size selectivity is demonstrated for the first time with this reaction through the use of larger silyl enolate substrates.

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