From Transition Metals to Lanthanides to Actinides: Metal-MediatedTuning of Electronic Properties of Isostructural Metal−OrganicFrameworksTimur Islamoglu,*,†,∥ Debmalya Ray,‡,∥ Peng Li,† Marek B. Majewski,† Isil Akpinar,†

Xuan Zhang,† Christopher J. Cramer,‡ Laura Gagliardi,‡ and Omar K. Farha*,†,§

†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States‡Department of Chemistry, Chemical Theory Center and Minnesota Supercomputing Institute, University of Minnesota, 207Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States§Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

*S Supporting Information

ABSTRACT: Isostructural metal−organic frameworks (MOFs) have been prepared from a variety of metal−oxide clusters,including transition metals, lanthanides, and actinides. Experimental and calculated shifts in O−H stretching frequencies forhydroxyl groups associated with the metal−oxide nodes reveal varying electronic properties for these units, thereby offeringopportunities to tune support effects for other materials deposited onto these nodes.

■ INTRODUCTION

Self-assembled, atomically ordered porous materials composedof metal ions/clusters and organic linkers, known as metal−organic frameworks (MOFs), have attracted great attention inthe past two decades owing, in part, to their high surface areasand structural tunability.1,2 With these properties, MOFs havebeen identified as platform materials for many potential appli-cations such as catalysis,3,4 gas storage/separation,5−8 chemicalsensing,9,10 energy storage,11 and drug delivery.12,13 Of thesepotential applications, utilizing MOFs as catalysts or catalystsupports has garnered special attention. While the metal ions/clusters or organic linkers that form the structural backbone ofMOFs have been shown to behave as isolated catalytic sites,14−16

MOFs have also been employed as catalyst supports,17,18 andthe resulting hybrid catalytic materials have often been shownto exhibit superior performance in activity compared to anal-ogous catalysts on conventional supports such as silica, alu-mina, and zirconia, all of which present O−H groups at theirsurfaces.The long-range order intrinsic in MOFs permits installation

of catalysts with single-atom precision, something that isalmost impossible in the case of conventional supports due to

the limited synthetic control in the preparation of suchmaterials. This often leads to a broad distribution of aciditiesand densities of surface hydroxyl groups, and a concomitantdiversity of binding modes for deposited atoms and clusters.Additionally, it is well documented that the morphology andfacet orientation of the support, two aspects that are also chal-lenging to control in conventional porous bulk materials, canprofoundly affect the selectivity and conversion to the desiredproduct in a catalytic process.19 Ultimately, quantifying andcomparing the “support ef fect” among different materials whilekeeping consistent variables such as connectivity, facet orien-tation, and morphology becomes difficult. In this respect,MOFs, which permit isoreticular control over their structuralcomposition, offer a unique advantage over most conventionalsupport materials. More specifically, and as illustrated in thiswork, MOFs afford a platform where systematic changes canbe made to the core of the material (in this case the identityof the metal in the secondary building unit) while theenvironment surrounding any active site remains constant.

Received: June 24, 2018

Article

pubs.acs.org/ICCite This: Inorg. Chem. XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.8b01748Inorg. Chem. XXXX, XXX, XXX−XXX

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By comparison, such systematic variations can be verychallenging in most common support materials (such asmetal oxides) that are often composed of multiple phases thatmay interconvert (e.g., the thermally induced conversion of themetastable anatase or brookite phases of TiOx to the rutilephase).20 Furthermore, the atomically precise structure ofMOFs presents a distinct advantage with respect to the pro-spects for theoretical investigations to greatly assist in buildingan understanding of structural and reactivity details on anatomic scale, thereby accelerating progress toward solutions tochallenging problems from catalysis to gas separation.21−23

Among the many MOFs that have been employed as catalystsupports, zirconium(IV)-based MOFs have been extensivelyexplored due to their chemical and hydrothermal stability,these properties being crucial for practical catalytic applica-tions.24 Spatially oriented −OH and −H2O groups on themetal nodes of Zr-MOFs have been exploited to install well-defined catalytically active metal centers.25,26 Recently, we haveshown that installing metal centers on organic linkers viapendant −OH (e.g., catechol) groups yields a metal center thatexists in a measurably different chemical environment com-pared to metal centers installed on the node, resulting indecreased catalytic activity for oxidation of cyclohexane.27 Thisobservation afforded the possibility to compare the nature ofthe support effect on catalytic activity while demonstrating theimportance of employing the structural diversity of MOFs toobtain key information about structure−property relationshipsin hybrid catalytic materials. Importantly, the ability to keepthe connectivity, surface area, and pore size comparable whilechanging the identity of the active metal center in MOFs canelicit useful information for designing next generation catalysts.In this work, we demonstrate that the infrared stretching fre-quency of hydroxyl groups on the nodes, a key indicator oftheir electronic character when serving as catalyst binding sitesin the UiO-66 and MOF-808 MOF families, can be tuned bysubstituting the metal in the node (from zirconium to hafnium,thorium, or cerium). We expect this range of tuned MOFs willbe useful for future informed catalyst design.

■ RESULTS AND DISCUSSIONUnder carefully controlled reaction conditions, ideal (nearlydefect free) UiO-66 with each Zr6 node connected to sixbenzene-1,4-dicarboxylate (bdc) can be prepared.28 However,introducing defects (or missing linkers and/or nodes) in theMOF is of some interest for catalytic applications due to theBrønsted acidity of the protons on defect sites and the Lewisacidity of the exposed metal sites.24,29,30 We and others havedemonstrated that the UiO series of MOFs can be made inten-tionally defective by using a modulator method or changingthe reaction temperature.31−34 In these processes, defect sitescan be capped with a labile hydroxyl and water pair behaving ina charge compensating capacity. Residual modulators or for-mate anions formed from the decomposition of DMF duringsynthesis can also cap such defect sites.35 Known amounts ofopen sites in other Zr6-based MOFs can also be introduced bydesign, e.g., by controlling the connectivity around the nodethrough the targeting of specific topologies.36

Reactive capping groups on the node can be reacted withmetal-containing reagents (in vapor or condensed phase) toinitiate the formation of metal clusters on the node (or coor-dinatively unsaturated zirconium on the node can act as a Lewisacid to catalyze many important reactions).37 Thus, a growingamount of attention is being diverted to understanding the

nature of the secondary building unit (SBU) structure in Zr6-based MOFs. Importantly, Hf(IV), Th(IV), and Ce(IV) havealso been shown to form hexa-nuclear [M6O4(OH)4]

12+

clusters which can be employed to build fcu and spn topologyMOFs among others (Figure 1).38−42 In general, MOFs enable

the study of the intrinsic effects associated with the identity ofthe metal in metal-catalysts or catalyst supports as opposed tothe effect of crystallographic surfaces where different sites maybear different reactivity. To elucidate the potential role of themetal node in catalysis, we have prepared a series of isos-tructural MOFs and collected diffuse reflectance infrared Fou-rier transform spectroscopy (DRIFTS) spectra in order to com-pare the stretching of hydroxyl groups on the node. In thisstudy, we focus on the stretching frequencies of bridgingμ3-OH groups in the node core, as other OH vibrational bandscan show different intensities based on varying defect densitiesand/or the presence of other capping ligands on the node.UiO-66 and MOF-808, two Zr6-based MOFs with fcu and spntopologies, respectively, were prepared according to literatureprocedures, as were M-UiO-66 and M-MOF-808 [M = Zr(IV),Hf(IV), Ce(IV) and Th(IV)] analogues.32,38−43 Th-MOF-808 isreported here for the first time, and the details of the synthesisare provided in the Supporting Information. Volumetric nitro-gen uptakes for these series of MOFs showed comparable porosity(Figure 2), and powder X-ray diffraction (PXRD) patterns con-firmed the formation of both isostructural MOF series (Figure 3).The hydroxyl region of DRIFTS spectra for the M-UiO-66

and M-MOF-808 series is shown in Figure 4 and tabulated inTable 1 along with the μ3-OH stretches computed usingdensity functional theory (DFT). For the M-UiO-66 series, wetested different structural models (cluster44 and periodic45−48)and a variety of density functionals (M06-L,49 M06-L-D3,50

PBE,51 BLYP,52,53 and B3LYP53−56) as discussed in the Com-putational Details section in the Supporting Information. Boththe cluster and periodic models give consistent trends in pre-dicted IR frequencies for the M-UiO-66 series, as do all of thetested functionals. Thus, for the MOF-808 series, we usedexclusively M06-L cluster calculations to compute IR spectra.We note that, since MOF-808 has only a six-connected SBU,compared to the 12-connected UiO-66 node, every MOF-808node has six known “defect” sites (i.e., six nonstructural ligandsor ligand sets per node), ruling out variations in spectra, if any,associated with the number of defects as is the case in theM-UiO-66 series.

Figure 1. Representative structures of M-UiO-66 and M-MOF-808,where M = Zr, Hf, Ce, and Th. Hydrogens are omitted for clarity.MOF-808 is shown on the (110) direction.

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Notably, experiment observes and theory computes adistinct red shift in the μ3-OH hydroxyl stretching band acrossthe M-UiO-66 series when the metal is varied from Hf(IV) toZr(IV), Th(IV), and Ce(IV), with values measuring 3679,3674, 3653, and 3646 cm−1, respectively (Figure 4, Table 1).To further confirm that the observed sharp peaks belong tobridging μ3-OH groups on the nodesas opposed to beingcontributions from nonstructural capping ligandswe synthe-sized a nearly defect-free version of Zr-UiO-66 (Figure S4).The DRIFTS spectrum of the hydroxyl region shows a singlesharp peak centered at the same location as for the moretypically defective UiO-66 (Figure S10), securing the bandassignment. Table 1 also lists μ3-OH hydroxyl stretchingfrequencies predicted for pristine M-UiO-66 models, which

follow the same trend as that observed in the experimentalspectra.While defect free M-UiO-66 MOFs should nominally have a

single peak in the hydroxyl region corresponding to the μ3-OHgroup, new bands can appear near the μ3-OH signals upon theintroduction of −OH/−OH2 pairs capping missing linker sites,and indeed shoulders or distinct peaks appear in theexperimental spectra (Figure 4). We have performed DFTcalculations on defected UiO-66 cluster models to assess theeffects of increasing defects (missing linkers) in such spectra(Figures S23−S26). From our DFT calculations, we observethat terminal O−H stretching frequencies appear at higherfrequencies than μ3-OH frequencies in the pristine M-UiO-66(using the M06-L functional). However, the μ3-OH groupnearest the defect site has lower frequencies compared to otherμ3-OH groups in the M-UiO-66 nodes. This red shift in IRfrequency near the defect site is consistent with the red-sideshouldering observed in the experimental DRIFTS spectra(Figure 4). Notably, this spectral region for Th-UiO-66 doesnot show much shoulder on the μ3-OH stretch (Figure 4),suggesting a low defect formation during MOF synthesis,which is also evident from the single peak observed in the poresize distribution of Th-UiO-66 (Figure S3).

Figure 2. N2 isotherms of (A) M-UiO-66 and (B) M-MOF-808 seriesMOFs. M = Zr, Hf, Ce, and Th.

Figure 3. Experimental PXRD patterns of M-UiO-66 and M-MOF-808 series MOFs. M = Zr, Hf, Ce, and Th.

Figure 4. Experimental DRIFTS spectra M-UiO-66 (top) andM-MOF-808 (bottom) series of MOFs showing bridging μ3-OH stretch.

Table 1. Experimental and Calculated Bridging μ3-OHVibrational Modes in the M-UiO-66/MOF-808 Series ofMOFs

wavenumbers (cm1)

MOF experimental calculated

Hf-UiO-66/MOF-808 3679/3679 3661/3652−3669Zr-UiO-66/MOF-808 3674/3674 3656/3648−3665Th-UiO-66/MOF-808 3653/3650 3644/3640−3649Ce-UiO-66/MOF-808 3646/3645 3635/3636−3640

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As expected, analogous bands corresponding to bridgingμ3-OH groups and analogous red shifts with respect to metalidentity appear in the DRIFTS spectra of the M-MOF-808series, emphasizing that this observed shift in the MOF seriescorrelates to a change in the electronic structure of the nodeassociated with variations of the metal center. However, theMOF-808 spectra are complicated by other OH functionalitycapping open sites intrinsic to this MOF’s topology. Thus,because of the diversity of OH functionality in the M-MOF-808 series, qualitatively, we do not see perfect agreement whencomparing experiment to theory. However, from our DFTcalculations, we do observe that the μ3-OH IR stretchingfrequencies still follow the same red shifts observed in exper-imental DRIFTS spectra while going from Hf(IV) to Zr(IV),Th(IV), and Ce(IV) (Table 1 and Table S7). This suggests,despite the complexities arising from the capping of defect sitesby organic linkers or OH/H2O functionality, μ3-OH stretchingcan be used as a descriptor of changing electronic propertieswith changing metal for both the M-UiO66 and M-MOF-808series (Table 1, Figures S20 and S22).A similar trend in the shifting of bands associated with

bound hydroxyls, from Hf to Ce, has also been reported forthese groups on the surfaces of the bulk oxides HfO2, ZrO2,ThO2, and CeO2.

57 According to the Sanderson scale, elec-tronegativity values for Hf(IV), Zr(IV), and Ce(IV) are 0.96,0.90, and 0.41, respectively,58,59 suggesting that the observedshifts in hydroxyl stretching can be correlated with the elec-tronegativity of the metal center in the SBU of the MOF ratherthan textural/physical properties of the MOF. In general,bands for hydrogen-bonded hydroxyl groups are red-shiftedcompared to their non-hydrogen-bonded counterparts due to aweakening of the O−H bond attributed to charge transfer fromthe proton acceptor lone pair to the σ*OH of the protondonor.60 While our DRIFTS experiments showed bandsbetween 2716 and 2753 cm−1 which could be attributed to hydro-gen-bonded −H2O/−OH stretching bands (Figures S6−S10),the C−H bands from residual formate can also be observed inthe same region and therefore we will not focus on bands inthat region due to possible overlaps.35

In order to observe the effect of changing metal in the node,we have also calculated the energetic costs associated withdissociating the H2O ligand in the M-UiO-66 series so as to

obtain water binding strength in this isostructural M-UiO-66series. Our calculations suggest that removal of a terminal H2Omolecule from Hf-, Zr-, Th-, and Ce-UiO-66 nodes requires84.8, 79.7, 70.1, and 55.7 kJ mol−1 of energy, respectively.A correlation between the computed water binding affinitiesand μ3-OH stretching frequencies further confirms alteration ofthe electronic properties of the metal node according to theidentity of the metal (Figure 5). An energy difference ofca. 25 kJ mol−1 associated with water binding to a Ce(IV) nodecompared to a Zr(IV) node suggests higher water lability fromthe former. In this regard, we note that our recent finding59 ofenhanced catalytic activity of Ce-UiO-66 compared to Zr-UiO-66 for the hydrolysis of phosphate ester-based nerve agentsimulant, DMNP, may well be attributed to the difference inwater binding affinity (ca. ∼55.7 vs 79.7 kJ mol−1 for Ce(IV)and Zr(IV), respectively), as theory has computed that dehy-dration of Zr-based MOFs can significantly enhance thecatalytic hydrolysis rates.37,61−64

■ CONCLUSIONSComparisons of DRIFTS spectra corresponding to non-hydrogen-bonded hydroxyl groups located on the nodes oftwo series of MOFs, M-UiO-66, and M-MOF-808 [where M =Zr(IV), Hf(IV), Ce(IV), Th(IV)] have shown that modesassociated with these groups consistently shift to the red withdecreasing electronegativity of the metal center, and that thisshift also correlates with the binding affinity of H2O moleculescapping node metal atoms that are coordinatively unsaturatedfrom either accidental or designed missing-linker defects.These intrinsic MOF properties are associated exclusively withthe nature of the metal center in the node, as densities ofdefects, local environments, and morphology are effectivelyunchanged within individual series. Due to the structural var-iations inevitably found in common amorphous or nano-crystalline support materials, quantification of this “supportef fect” is most readily accomplished in materials such as MOFs,where regardless of the nature of the metal in the node localvariables such as the coordination environment can be held thesame. This has implications with respect to designing systemsfor applications in catalysis, where the reactivity of the activesite (in these MOFs, for example, the labile hydroxyl/waterpair and/or the exposed metal center) greatly influences cat-alytic activity, and the findings of this work may inform futuredesign of support materials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on the ACSPublications website at DOI: 10.1021/acs.inorgchem.8b01748.

Experimental details including N2 isotherms, PXRDpatterns, and full DRFITS spectra and calculation detailsincluding full DRIFTS spectra and tables for importantstretching bands and bond distances (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: timur.islamoglu@northwestern.edu.*E-mail: o-farha@northwestern.edu.ORCIDTimur Islamoglu: 0000-0003-3688-9158Peng Li: 0000-0002-4273-4577

Figure 5. Relationship between the binding affinity of water inM-UiO-66 series MOFs and the IR stretching frequency of hydroxyls.The red solid line is the linear fitting line (R2 = 0.91).

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Marek B. Majewski: 0000-0002-5190-7193Xuan Zhang: 0000-0001-8214-7265Christopher J. Cramer: 0000-0001-5048-1859Laura Gagliardi: 0000-0001-5227-1396Omar K. Farha: 0000-0002-9904-9845Author Contributions∥T.I., D.R.: These authors contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

O.K.F. acknowledges the support from the U.S. Department ofEnergy, National Nuclear Security Administration, underAward Number DE-NA0003763 and Northwestern Universityfor the experimental work. The theoretical work was supportedas part of the Inorganometallic Catalyst Design Center, anEFRC funded by the DOE, Office of Basic Energy Sciences(DE-SC0012702). The computations were performed at theMinnesota Supercomputing Institute.

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Inorganic Chemistry Article

DOI: 10.1021/acs.inorgchem.8b01748Inorg. Chem. XXXX, XXX, XXX−XXX

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