296 Natl Sci Rev, 2017, Vol. 4, No. 3 PERSPECTIVES

MATERIALS SCIENCE

Multivariate metal-organic frameworksAasif Helal1,∗, Zain H. Yamani1, Kyle E. Cordova1,2 and Omar M. Yaghi1,2,∗

Reticular chemistry is founded upon theprinciples of linking simple organic andinorganic building blocks through strongbonds to form extended, porous andcrystalline solids, such as metal-organicframeworks (MOFs) [1]. This field hasdeveloped and advanced by making newstructures and studying their propertiesfor applications covering gas storage, sep-arations and catalysis, to name a few [2].It is striking that the large majority ofMOFs being studied thus far have rela-tively ‘simple’ structures in which a smallnumber of building units is repeatedthroughout the crystal. In considering thefuture of MOFs, it is useful to take in-spiration from biological systems and thefact that these are more complex becauseof the large number of building units andfunctionality (i.e. heterogeneity) makingup their structure. We believe that, in

order to achieve a new level of utilityfor MOFs, it is important to developstrategies that use the precision withwhich MOFs can be made towards mak-ing them more complex. In this context,the challenge becomes how to intro-duce heterogeneity and still maintainordered materials as well as the handleon characterizing them [3]. This contri-bution focuses on how the multivariateapproach to making MOFs is a viablestrategy for introducing heterogeneitywhile maintaining order, and how thesequalities lead to a higher degree offunctionality.

Although MOFs with increasedcomplexity (i.e. more than one linker)have been reported as early as 2002[4], the first step in truly realizing thepotential of introducing heterogeneitywithin MOF structures came with the

discovery of multivariate (MTV) MOFsin 2010 [5]. In this report, the incor-poration of up to eight distinct linkerfunctionalities within one pure phase ofMOF-5 was successfully demonstrated(Fig. 1) [5]. The MTV discovery hada profound implication: the resultingMTV-MOFs possessed properties thatdid not arise from linear combinationsof the pure constituents. This was ev-idenced by one MTV-MOF-5 materialexhibiting a 400% enhancement in se-lectivity for carbon dioxide over carbonmonoxide when compared to the parentMOF-5 analogues containing only pure-link components. Following this, the spa-tial apportionment of the different func-tionalities (small clusters, random or al-ternating) in MTV-MOF-5 systems wasdetermined in the context of predictingthe adsorptive properties of such systems

PERSPECTIVES Helal et al. 297

MTV-MOF-5Mixed metal

MTV-MOF-74 MTV-MOF-177

2010 2014 2015

2013 2015 2016

Programmed pores in MUF-7

Sequential linker installation in PCN-700

Spatially arranged MTV-MOF:

(M3O)2(TCPP-M)3

Figure 1. Representative discoveries of the multivariate approach for introducing heterogene-ity within ordered MOFs. Color code: black, C; red, O; green, installed functional group; andvarious colored polyhedra, metal (M) atoms. H atoms are omitted for clarity. TCPP, tetrakis(4-carboxyphenyl)porphyrin.

[6]. The MTV concept was later appliedto MOF-177, resulting in a 25% highervolumetricH2 uptake in theMTV-MOF-177 material, and was extended to the in-organic secondary building units (SBUs)ofMOF-74with the introduction ofmul-tiple metals (up to 10) within the sameSBU (Fig. 1) [7,8]. It is noted that,in these systems, the MTV-MOF back-bone is ordered and the heterogeneity(mixed organic functionalities or metalions) is covalently bound at well-defineddistances and ratios. Yet, even with thissuccess, there was only limited controlin placing different functionalized linkers(ormetals) in exact positions throughouttheMTV-MOF crystal.

In 2013, a strategy was reported to ad-dress this issue: linkers of varying sym-metry and connectivity were judiciouslychosen in order to obtain a MOF, MUF-7a, in which each linker was placed inpre-determinedpositions throughout thestructure [9]. Accordingly, MUF-7a wasconstructed from three distinct linkersthat were, in turn, functionalized to pro-duce seven MTV-like MUF-7 analogues(Fig. 1). The beauty of this strategywas proven through the understandingthat each of the three linkers occupiedspecific crystallographic site symmetries,

which meant that, upon functionaliza-tion, no crystallographic disorder was tobeobserved. Introducing complexity intoMUF-7a led to the exact placement of theintroduced functionalities, control overthe pore composition and up to 94% en-hancement in CO2 adsorption capacity.An additionally unique strategy for con-trolling the exact positions of functionali-ties was reported in 2015 (Fig. 1) [10]. Inthis case, terminal ligands (OH−/H2O)of a Zr-based SBU were targeted for re-moval, thus allowing for the creation ofappropriately sized pockets (16.4 and7.0 A).These pockets provided the requi-site space needed for new linkers of vary-ing sizes, andwith varying functionalities,to be installed.

Beyond applying the MTV conceptto the organic linker, the SBU also rep-resents a viable target for achieving het-erogeneity. Accordingly, it was shownthat three distinct SBU geometries (Cu-based triangles, Zn-based square pyra-mids and Zn-based octahedra) could becombined with only one linker to con-struct a new mesoporous MOF, FDM-3 [11]. The endowed complexity led toprecise arrangements of cages and poreapertures, thus limiting guest moleculetransport to specific pathways. Recently,

the MTV-SBU approach was applied to36 porphyrin-based MOFs, in which thetrigonal SBUs were composed of mixed-metal ions (Fig. 1) [12]. Remarkably,the spatial arrangement of the multi-variate metals within the SBUs of theseMOFs were found to be well mixed ordistributed throughout the MTV-MOFcrystals in domains. In cases of the for-mer arrangement, the MTV-MOFs wereproven as kinetically more robust cat-alysts for the photo-oxidation of 1,5-dihydroxynapthalene in comparison tothe parent MOF structures with SBUscontaining only one metal ion.

Although this is but a small sample ofthe progress reported for accomplishinga greater degree of heterogeneity withinotherwise ordered structures, one can beassured that there is vast space left toexplore. For example, performing frame-work chemistry, through post-syntheticmodification, carries the potential of re-alizing pore environments with ‘enzyme-like complexity’ [13]. Indeed, this wasrecently achieved through seven success-ful post-synthetic reactions within thepores of an isoreticular MOF-74 sys-tem to install tripeptide functionalities.This modification afforded the resultingMTV-MOF with high catalytic selectiv-ity resembling that of the enzyme TEVendopeptidase [13]. Significant consid-eration has also turned to the introduc-tion, control and exploitation of defectsand flexibility within MOFs—conceptsthat most certainly fall within the in-troduction of heterogeneity realm [14].Furthermore, the strategy of designingdynamic, yet robust, frameworks thatcomprise different counting, coverageand sorting domains for guest moleculesleads toMOFswith complex sets of phys-ical properties [15]. Suffice it to say thatintroducing heterogeneity may be thefirst step in rectifying the disconnect as-sociated with the functional capabilitiesof the synthetic world when comparedto those observed in nature. Indeed, itis generally true that synthetic materialslargely operate serially, whereas natureoperates in parallel to perform a muchwider array of functions at the same time.Therefore, it is a natural extension for us,as synthetic chemists, to speculate andexplore how to bridge this functionality

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divide. In our opinion, the answer to thislies in achievingmaterials whose parts aremany, heterogeneous and systematicallyvaried [2].

ACKNOWLEDGEMENTSWe acknowledge Saudi Aramco (ORCP2390) fortheir continued collaboration and support of thisresearch.

Aasif Helal1,∗, Zain H. Yamani1, KyleE. Cordova1,2 and Omar M. Yaghi1,2,∗1Center for Research Excellence inNanotechnology (CENT), King Fahd University ofPetroleum and Minerals, Saudi Arabia2Department of Chemistry, University ofCalifornia-Berkeley; Materials Sciences Division,Lawrence Berkeley National Laboratory; KavliEnergy NanoSciences Institute at Berkeley; andBerkeley Global Science Institute, USA

∗Corresponding authors.E-mails: yaghi@berkeley.edu;aasifh@kfupm.edu.sa

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National Science Review4: 296–298, 2017doi: 10.1093/nsr/nwx013Advance access publication 24 February 2017