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Microporous and Mesoporous Materials xxx (2015) 1e9

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate/micromeso

Functional group tolerance in BTB-based metaleorganic frameworks(BTB e benzene-1,3,5-tribenzoate)

Stella Helten a, Basudev Sahoo b, Philipp Müller a, Daniel Janßen-Müller b, Nicole Klein a, 1,Ronny Grünker a, 2, Volodymyr Bon a, Frank Glorius b, Stefan Kaskel a, Irena Senkovska a, *

a Technische Universit€at Dresden, Inorganic Chemistry, Bergstr. 66, 01062 Dresden, Germanyb Westf€alische Wilhems-Universit€at Münster, NRW Graduate School of Chemistry, Organisch-Chemisches Institut, Corrensstr. 40, 48149 Münster, Germany

a r t i c l e i n f o

Article history:Received 7 December 2014Received in revised form15 February 2015Accepted 24 February 2015Available online xxx

Keywords:Metaleorganic frameworksCrystallizationFunctional BTB derivativesIRMOFs

* Corresponding author.E-mail address: irena.senkovska@chemie.tu-dresd

1 Present address: Fraunhofer-Institut für WerkstoOberfl€achen-und Reaktionstechnik, Winterbergstr. 28

2 Present address: Technische Universit€at Dresdenrials, Mommsenstr. 4, 01069 Dresden, Germany.

http://dx.doi.org/10.1016/j.micromeso.2015.02.0551387-1811/© 2015 Elsevier Inc. All rights reserved.

Please cite this article in press as: S. Helten,tribenzoate), Microporous and Mesoporous

a b s t r a c t

The effect of different substituents on the structural tolerance of benzene-1,3,5-tribenzoate (BTB) basedMOFs was studied. For this purpose, three new BTB derivatives ((NH2)3-BTB, tri((tert-butoxycarbonyl)-amino)-BTB; tri((S)-2-acetoxy-2-phenylacetamido)-BTB) were synthesized and utilized in MOF synthesisusing synthetic conditions established for eight known MOFs. Three new isoreticular compounds (NH2-DUT-6, NHBoc-DUT-25, mand-DUT-25) could be identified. Furthermore, two compounds with networktopologies differing from the parent compound comprising NHBoc-BTB and Cu-paddlewheels as nodeswere found: Cu2(H2O)2(H(NHBoc)-BTB)2 (1), a dense structure containing layers with sql topology, andCu3(H2O)(NHBoc-BTB)2(bipy) (bipy e 4,40-bipyridine, DUT-109), with crosslinked iab-network. Theporosity of DUT-109 was proven by liquid phase adsorption of ethyl cinnamate from n-heptane. Ingeneral, a significant influence of the BTB substituent on the framework formation could bedemonstrated.

© 2015 Elsevier Inc. All rights reserved.

1. Introduction

Metal-organic frameworks (MOFs) offer an intriguing conceptfor the modular design of functional porous materials [1]. Byvarying the interconnection of inorganic nodes (metal atoms ormetal-oxo-clusters) and organic linkers (typically poly-carboxylates) a plethora of 3D frameworks with various topol-ogies and textural properties, pore accessibility as well aschemical properties of their inner surface could be formed [2].Therefore, a lot of applications can be envisioned with specificallytailored adsorbents, including gas storage [3,4], gas capture [5,6],separation applications [7], catalysis [8,9], and sensing [10].However, for specific applications, additional functionalization ofthe framework is often required in order to achieve the targetedproperties. Realizing such specific functionalizations of MOF innersurface remains a challenge. In this regard, several approaches for

en.de (I. Senkovska).ff-und Strahltechnik, Chem., 01277 Dresden, Germany., Molecular Functional Mate-

et al., Functional group toleraMaterials (2015), http://dx.do

MOF functionalization were developed so far. The first andsimplest one, namely isoreticular synthesis, allows for variation ofthe chemical functionality of the organic linker while maintainingthe desired framework topology. It is a useful concept to preciselytune the chemical properties of a given coordination polymer.Initially introduced by Yaghi and co-workers with the IRMOFseries [11], it has been successfully applied several times, result-ing in intriguing compounds combining the topology and porosityof the parent MOF with an adjustable inner surface functionality.With the abundance of MOFs and functional groups at hand, thevariation and manipulation of chemical properties of MOFs bymeans of isoreticular synthesis remains fascinating. However, this“model-kit” concept for introducing functional groups into knownMOF structures has some limitations: (i) A number of usefulfunctional groups are not stable under the typically applied sol-vothermal synthetic conditions of MOFs [12]; (ii) Due to stericaldemand, frameworks with different topologies can be formed[13]; or (iii) side reactions [12], proceeding in the synthesismixture, can perturb the formation of an isoreticular product. Sofar, such effects are generally not completely predictable [14];therefore the compatibility of a given functionalized linker with aspecific MOF synthesis requires additional experimentalinvestigations.

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S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e92

Another powerful approach to functionalize MOFs is the post-synthetic introduction of functional groups using the originalframework bearing an anchoring group [15]. Many postsyntheticmodifications (PSM) were performed with amino functionalizedlinkers as they have been shown to tolerate various synthesisconditions and enabled isoreticular synthesis in many cases.

Several modifications were performed on MOFs containingcommercially available amino-terephthalate (NH2-BDC), such asMOF-5 [11], UMCM-1 and DMOF-1 [16], MIL-101(Fe) [17], and UiO-66 [18]. In order to incorporate amino-modified biphenyldi-carboxylate (NH2-BPDC) into the corresponding IRMOF-9, theamino function had to be protected with a tert-butyloxycarbonyl(Boc) protecting group [19]. Amino-functionalized benzene tricar-boxylic acid (H3(NH2-BTC)) was synthesized by Fr€oba and co-workers with further successful utilization in the synthesis of anHKUST-1 analog [20].

However, for most of the MOFs composed of the linear NH2-BDClinkers, the pore sizes and their accessibility are the critical factorsthat usually restrict the applicability of the whole system to smallsubstrate molecules. This problem could be partially overcome byapplying a co-polymerization approach using an additional ligandin the synthesis of the mesoporous UMCM-1, where a functional-ized BDC linker is copolymerized with the Zn4O6þ cluster andbenzene-1,3,5-tribenzoate (BTB). However, because of the topo-logical peculiarity of the framework, all functional groups arelocated in the micropore, which hinders the interaction with largesubstrate molecules.

H3BTB is probably one of the most widely used trigonal li-gands in MOF chemistry. The CSD substructure search using theBTB fragment results in 411 crystal structures, containing BTB inpure or substituted form [21]. This could be explained by a va-riety of possible topologies involving binodal nets containing atrigonal node [22]. Usually, MOFs comprising the BTB ligand havelarge pores sizes, high pore volumes, and highly accessible poresystems. These features render MOFs with functionalized BTBligands especially interesting for catalytic and separation appli-cations, wherein the size of the substrate molecules plays acrucial role.

Up to now, investigations on isoreticular synthesis of MOFs withfunctionalized BTB linkers are limited. This is probably due to therequired effort in organic synthesis as functionalized BTB linkersrequire multiple step synthetic protocols. In the group of Cohen,BTB derivatives with methoxy and hydroxyl groups in one of theortho positions to the carboxylate donor groups were tested forisoreticular MOF-177 synthesis. While an analog was obtained withmethoxy substituted BTB, the hydroxyl substituents lead to acompound with a different topology [23]. Our group reported Zn-based MOFs synthesized with BTB linkers derivatized with chiralauxiliaries [24] at the same position. Depending on the auxiliaryand metal used, frameworks with different network topologieswere obtained [25]. Zhang and co-workers reported the synthesisof Cd(II) MOFs with H3BTB and 4,40-bipyridine as ligands. Thesystem's tolerance to functional groups was studied using modifiedBTB linker versions with a hydroxyl or amino group attached to thecentral phenyl ring. As a result, an isoreticular compound wasformed with the hydroxylated version of the linker, while theamino functionalization resulted in a framework with a differenttopology [26]. Interestingly, utilization of aniline-2,4,6-tribenzoate(ATB), which is the BTB linker with one amino substituents in thecentral phenyl ring, in the synthesis of MOF-177 results in theisoreticular structure [27]. R. Schmid and co-workers conductedtheoretical investigations suggesting that bromine substitution ofthe two ortho positions in BTB should lead to an alteration of thenetwork topology for the corresponding Cu-based paddle-wheelcontaining MOFs [13].

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In this work, we have studied the structural tolerance of selectedBTB-containing MOFs (Table 1) by the introduction of differentsubstituents in the ortho position to the carboxylic group. Eighthighly porous MOF materials with open, non-interpenetratedframeworks and high pore volumes were selected for in-vestigations (Table 1). Three different BTB substituted ligands(Fig. 1) were used in this screening: 1) NH2-BTB was chosen havingfurther postsynthetic modification in mind; 2) NHBoc-BTB wasused in order to compare the influence of protected and unpro-tected amino groups on the formation of the desired MOF. More-over, the Boc group could be easily removed by thermal treatmentto form the amino group for postsynthetic modification; 3) mand-BTB was used to study the structural tolerance towards a bulkysubstituent in the ortho position to the carboxylate groups thatcould provide spacial proximity to the metal node, which can bebeneficial for catalytic applications.

2. Experimental

2.1. Materials and methods

Metal salts, co-ligands (terephthalic acid, 4,40-bipyridine) andsolvents for MOF synthesis, were obtained from commercial ven-dors and used as received. H4BenzTB was synthesized according tothe previously published procedure [28]. H3BTB was synthesizedaccording to the known literature procedures [29,30]. The synthesisdetails for the linker molecules 3,300-diammonio-50-(3-ammonio-4-carboxylatophenyl)-[1,10:30,100-terphenyl]-4,400-dicarboxylate(H3(NH2-BTB)), 3,300-bis((tert-butoxycarbonyl)amino)-50-(3-((tert-butoxycarbonyl)amino)-4-carboxyphenyl)-[1,10:30,100-terphenyl]-4,400-dicarboxylic acid (H3(NHBoc-BTB)) and3,300-bis((S)-2-acetoxy-2-phenylacetamido)-50-(3-((S)-2-acetoxy-2-phenylacetamido)-4-carboxyphenyl)-[1,10:30,100-terphenyl]-4,400-dicarboxylic acid(H3mand-BTB) are given in Supplementary data S1.

Powder X-ray diffraction measurements were carried out on aSTADI P diffractometer (STOE) with Cu-Ka1 radiation (l¼ 1.5405Å)at room temperature. Thermogravimetric analysis was carried outon an STA 409 (Netzsch) with synthetic air as carrier gas with a flowof 100 mL min�1 and a heating rate of 5 K min�1. Nitrogen phys-isorption measurements were conducted on a BELSORP Max (BEL,Japan) at 77 K up to 1 bar. GC/MS measurements were carried outon a GCMS QP5000 (Shimadzu) equipped with a non-polar BPX5column (SGE). 1H NMR spectroscopy of dissolved MOFs was carriedout on an AC 300-P (Bruker).

2.2. Single crystal X-ray diffraction

Single crystals of Cu2(H2O)2(H(NHBoc-BTB))2 andCu3(H2O)(NHBoc-BTB)2(bipy) were transferred in a glass capillary(0.3 mm) with a small amount of solvent. The capillaries weresealed with melted wax. The datasets were collected at BESSY onMX BL14.2 beamline of Helmholtz Zentrum Berlin für Materialienund Energie [31]. All diffraction experiments were performed atroom temperature using the radiation with energy of 14 keV(l ¼ 0.88561 Å). The 4-scans with oscillation range of 1� were usedfor data collection. The datasets were processed using CCP4 soft-ware [32]. Both crystal structures were solved by direct methodsand refined by full matrix least-squares on F2 using SHELXTL pro-gram package [33]. All non hydrogen atoms were refined inanisotropic approximation. Hydrogen atoms were refined ingeometrically calculated positions using the “riding model” withUiso(H) ¼ 1.2Uiso(C). In the crystal structure of Cu3(H2O)((NHBoc-BTB)2(bipy)), the 4,40-bipyridine is disordered over 3 positions, twoof which are generated by the mirror plane. Because of the rota-tional disorder of the Boc group in Cu3(H2O)(NHBoc-BTB)2(bipy),

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Table 1Selected MOF materials for investigation of the structural tolerance.

Common name, Composition,Crystal structure

Pore volume (cm3 g�1),Max pore size (Å),Pore limiting diameter (Å)

Co-ligand Topology Ref.

2.0, 10.7, 8.8 e [33]

2.2, 17.4, 15.4 [34]

2.2, 22.0, 7.7 [29]

2.0, 19.3, 7.1 [37]

2.0, 19.8, 6.9 [36]

2.4, 19.1, 7.0 e [36]

(continued on next page)

S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e9 3

Please cite this article in press as: S. Helten, et al., Functional group tolerance in BTB-based metaleorganic frameworks (BTB e benzene-1,3,5-tribenzoate), Microporous and Mesoporous Materials (2015), http://dx.doi.org/10.1016/j.micromeso.2015.02.055

Table 1 (continued )

Common name, Composition,Crystal structure

Pore volume (cm3 g�1),Max pore size (Å),Pore limiting diameter (Å)

Co-ligand Topology Ref.

2.4, 22.6, 14.4 e [35]

S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e94

only nitrogen atoms in the ortho-positions to the carboxylategroups in BTB could be localized unambiguously. The lattice solventmolecules could not be localized because of the disorder in the highsymmetry space group. The SQUEEZE routine was not applied tothe collected data. CCDC-1034130 and CCDC-1034131 contain thesupplementary crystallographic data for Cu3(H(NHBoc-BTB))2(H2O)4(DMF)4 and Cu3(NHBoc-BTB)2(bipy). This data can beobtained free of charge from the Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif.

2.3. MOF synthesis screening

The synthesis screening with BTB derivatives was conductedaccording to synthetic procedures previously published for MOFswith non-substituted BTB as ligand. Some of the syntheses weredownscaled. All of the downscaled syntheses were previously

Fig. 1. Linker molecules used for MOF synthesis: H3BTB (a), H3

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tested with unmodified H3BTB to confirm feasibility. MOF synthe-ses were run in capped glass Pyrex tubes (10 � 120 mm).

As a general strategy for the screening, the synthetic conditionspublished for the synthesis of the unmodified MOFs were adopted.Optimization of synthetic conditions followed up, when the prod-uct resulting from a screening trial had the potential for furtherimprovement.

Synthesis trials analog to MOF-177were conducted according tothe procedure published in Ref. [34]. Synthesis trials of UMCM-1analog were conducted according to the procedure published inRef. [35], but downscaled to one sixth of the original batch size.Synthesis trials analog to DUT-6 were conducted according to theprocedure published in Ref. [36], but downscaled to one half of theoriginal batch size. Synthesis trials analog to DUT-9were conductedaccording to the published procedure [37], but downscaled to onefourth of the original batch size. Synthesis trials analog to DUT-

(NH2-BTB) (b), H3(NHBoc-BTB) (c) and H3(mand-BTB) (d).

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S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e9 5

23(Cu), DUT-23(Zn) and DUT-34 were conducted according to thepublished procedure [38], but downscaled to one half, one quarterand one fifth of the original batch size, respectively. Synthesis trialsanalog to DUT-25 were conducted according to the procedurepublished in Ref. [39]. For synthesis of mand-DUT-25, the reactionwas scaled up to 150% of the original batch size and reaction timewas prolonged to 120 h.

3. Results and discussion

For the crystallization of amino-functionalized highly porousMOFs as starting materials for variable postsynthetic functionali-zations, H3(NH2-BTB) bearing three amino groups (Fig. 1) wassynthesized. Unfortunately, synthesis screening applying H3(NH2-BTB) in the established synthesis procedures for the targeted MOFstabulated in Table 1, resulted in amorphous reaction products inmost of the cases. Only in the case of DUT-6, a few single crystalscould be found in the reaction product and the structure could beconfirmed by single X-ray diffraction analysis. However, the bulk ofthe reaction product was amorphous. All attempts to optimize thesynthesis conditions by varying reaction parameters such as thetype of Zn2þ-source or adding modulators such as acetic acid orbenzoic acid to the reaction mixture did not afford highly crystal-line and phase pure products. Similar optimization of syntheticconditions was attempted for MOF-177 analog as well, includingutilization of basic zinc acetate as Zn source (following controlled

Fig. 2. Linker molecule involved, crystal structure, and topological repres

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SBU approach). Those experiments have also resulted in amor-phous reaction products. It has to be noted that Telfer et al. reportedsimilar problems while applying amino substituted 4,40-biphenyldicarboxylic acid (H2(NH2-BPDC)) in an IRMOF synthesis [19].

In the case of the targeted copper containing analogs of DUT-23and DUT-34 (Table 1), utilization of NH2-BTB leads to a rapid pre-cipitation of amorphous material. Modification of the reactionconditions, such as addition of different acids as modulator orvariation of the solvent mixture did not improve the crystallinity ofthe products. This observation could point to an undesired inter-action between the copper ions and the amino groups.

According to the 1H NMR spectrum of H3(NH2-BTB) in DMSO-d6,the solvated linker exists as a zwitterion: the carboxylic acidfunctions are deprotonated while the amino functions are proton-ated. It should be mentioned that typically, the acid functions aregradually deprotonated in the course of the solvothermal reaction.This gradual deprotonation allows the crystallization process toproceed at a rate that allows for healing of defects [40]. If the linkerinvolved in the reaction is present in its fully deprotonated state,the reaction rate may be very high, preventing healing of defectsand resulting in amorphous reaction products. According to Telferet al., for a successful synthesis of the amino functionalized IRMOFstructure, the amino functionality can be protected with a Bocprotecting group. Therefore, the Boc protected amino-BTB linker(H3(NHBoc-BTB)) was synthesized and utilized in further syntheticattempts.

entation of DUT-34 (top) and Cu2(H2O)2(H(NHBoc-BTB))2 (bottom).

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Fig. 3. a) Crystal structure of DUT-109 (view along [100] direction). b) Representationof cross-linked iab-net.

S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e96

The protection of the amino group was performed starting withthe ester form of the amino functionalized BTB derivative. Directsingle protection of the amino groups was not possible syntheti-cally. Therefore, two Boc protecting groups were first introducedper amino group and afterward one of these groups was removedselectively, using the method developed by Martin et al. [41] (formore details see Supplementary data S1).

Unfortunately, the utilization of H3(NHBoc-BTB) in combinationwith the Zn4O6þ cluster again did not lead to the formation ofcrystalline products. Only in the case of copolymerization withH4BenzTB, the isoreticular analog of DUT-25 could be obtained asisolated single crystals (The isostructurality was verified by deter-mining the corresponding cell parameters by single crystal X-raydiffraction.). However, the powder XRD pattern indicates the pre-dominant formation of amorphous reaction product (Fig. S3).

Using copper nitrate as metal source for the SBU under reactionconditions typical for DUT-23(Cu) and DUT-34, crystalline productscould be obtained. However, single crystal X-ray diffraction studyreveals the formation of topologically different frameworks in bothcases.

If the reaction conditions for DUT-34 were applied, a coordi-nation network crystallizing in the monoclinic space group P21/n isformed (Table 2). The asymmetric unit contains one copper atom,one monoprotonated NHBoc-BTB linker molecule, two water andtwo DMF (DMF e N,N-dimethylformamide) molecules. The overallcomposition of the compound is Cu2(H2O)2(H(NHBoc-BTB))2$2H2O$4DMF (1). The structure is based on Cu paddle-wheelunits interconnected by only two carboxylic groups from theNHBoc-BTB ligand resulting in a two dimensional 44-sql net (Fig. 2).The third carboxylic group of NHBoc-BTB remains protonated anddoes not contribute to the network connectivity, but formshydrogen bonds with the lattice DMF molecules (Table S2). Theresulting layers run parallel to the (101) plane.

The analysis of crystallographic porosity, performed usingPLATON software [42], shows no solvent accessible void in thecrystal structure.

If synthetic conditions of DUT-23(Cu) were applied for thesynthesis using H3(NHBoc-BTB) instead of H3BTB, a product, whichcrystallizes in the tetragonal space group I41/amd with theframework composition Cu3(H2O)((NHBoc-BTB)2(bipy)) (DUT-109,DUT e Dresden University of Technology) is formed (Table 2). Theasymmetric unit contains two Cu2þ atoms, located in the 16h (.m.)and 8e (2mm.) positions of the unit cell which belong to thedifferent, symmetrically independent paddle-wheel units. Theseare interconnected by NHBoc-BTB linkers into the 3D framework.Remarkably, symmetry dependent paddle-wheel clustersinvolving Cu atoms in 16h positions are additionally inter-connected by 4,40-bipyridine ligands in axial direction, increasingthe connectivity of the node from 4 (square-planar node) to 6(octahedral node). The interconnection of the paddle-wheelsalong [100] and [010] directions should considerably improvethe robustness of the framework. At the same time, paddle-wheelsinvolving 8e Cu atoms are coordinated by terminal water/DMF li-gands and therefore, remain square planar nodes. The simplifica-tion of the crystal structure leads to the 3-nodal 3,4,6-connectednetwork. Although this network could not be classified by TOPOS,it can be considered as a cross linked iab topology [22] (alsosqc5580 net [38]) with interconnected 8d vertexes (Fig. 3). Thepossibility of the formation of this type of framework involvingpaddle-wheel as cluster and trigonal linker, where only 75% of thepaddle wheels are connected by a linear cross-linker, was dis-cussed earlier by us [38] and DUT-109 is the first example of suchMOF achieved synthetically.

The structure contains channels running along the crystallo-graphic a- and b-directions measuring 12 Å in diameter. The pore

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limiting diameter is 6.5 Å. The pore volume of the desolvatedframework calculated theoretically using the Poreblazer 3.0 soft-ware [43] is 0.93 cm3 g�1.

The analysis of 1H NMR spectra of dissolved material revealedthe preservation of the Boc-protecting groups during synthesis.Unfortunately, none of the attempts to activate the compound forgas physisorption measurements were successful. Neither applyingdynamic vacuum to the ethanol exchanged sample nor supercriti-cal CO2 activation could remove the solvent molecules from thepores without framework collapse. In each case, an amorphous,non porous compound was obtained after activation.

Therefore, liquid phase adsorption of ethyl cinnamate from n-heptane at 25 �C was performed in order to verify the framework'sporosity and accessibility. The uptake was compared with otherMOFs, which were also measured using the method described byHenschel et al. [44]. DUT-109 adsorbs 514 mg g�1 of ethyl cinna-mate, clearly demonstrating the framework accessibility in liquidphase. Furthermore, the uptake of ethyl cinnamate with respect tothe pore volume is significantly larger compared to compoundswithout open metal sites or polar side groups (Fig. 4). In our view,this result is in good accordance with the presence of accessible Cu-paddlewheels not coordinated by 4,40-bipyridine as well as thepolar NHBoc-substituents.

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Table 2Single crystal data for Cu2(H(NHBoc-BTB))2(H2O)4(DMF)4 (1) and Cu3(H2O)((NHBocBTB)2(bipy)) (DUT-109).

Cu2(H2O)2(H(NHBoc-BTB))2$2H2O$4DMF (1) Cu3(H2O)(NHBoc-BTB)2(bipy) (DUT-109)

Empirical formula C96H114Cu2N10O32 C64H30Cu3N8O13

Formula weight, g/mol 2047.04 1309.58Crystal system, space group Monoclinic, P21/n Tetragonal, I41/amdUnit cell dimensions, Å, deg. a ¼ 9.6800(19)

b ¼ 24.270(5)c ¼ 23.410(5)b ¼ 90.07(3)

a ¼ 27.860(4)c ¼ 53.540(11)

Volume, Å3 5500(2) 41557(14)Z, Calc. density, g/cm3 2, 1.236 8, 0.419m, mm�1 0.833 0.587F(000) 2148 3438Limiting indices �11 � h � 8

�29 � k � 29�20 � l � 28

�31 � h � 33�33 � k � 21�63 � l � 59

Reflections collected/unique 23,111/9274 (Rint ¼ 0.0176) 62,742/9523 (Rint ¼ 0.0515)Data/parameters 9274/656 62742/133GooF on F2 1.080 1.598R [I > 2s(I)]a 0.0503 0.1186wR (all data)a 0.1440 0.3927Largest diff. peak/hole, e/Å3 0.480/�0.474 0.168/�0.302

a R1 ¼ PðjjFoj � jFcjjÞ=P jFoj; wR ¼ ½PwðF2o � F2c Þ2=

PwðF2o Þ2�1=2.

Fig. 4. Liquid phase adsorption of 0.1 M ethyl cinnamate in n-heptane at 25 �C on DUT-109 in comparison to selected MOFs.

S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e9 7

In order to make the substituents of DUT-109 susceptible topost-synthetic modifications, the amino function should bedeprotected. In general, the Boc protecting group can be removedthermally or by acid treatment. Acid treatment could not be appliedin this particular case because of the instability of the compoundagainst acids. Telfer et al. succeeded in deprotecting the modified

Fig. 5. Pawley refinement plot for as synthesized mand-DUT-25.

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IRMOF-9 by heating activated samples [19] up to 150 �C. Due to theframework flexibility and as a consequence the framework collapseduring activation, this procedure could not be applied for DUT-109.Therefore, the deprotection experiments were carried out usingDUT-109 suspended in DMF or mesitylene. The suspension washeated for 24 h to 130 �C or 150 �C, respectively. The thermallytreated samples were characterized by PXRD (Fig. S2Supplementary data) and 1H NMR to monitor the crystallinityand the chemical composition of the linker. It was found, that byheating to 130 �C only partial deprotection could be achieved inboth solvents used. Heating to 150 �C causes deprotection of ca. 90%of the linker molecules, but is accompanied by a significant loss incrystallinity of the samples. This loss of crystallinity is more pro-nounced for the samples treated in DMF.

Since startingMOFmaterials for post synthetic functionalizationcould not be synthesized in good yield and under full preservationof crystallinity, the pre-synthetic functionalization route was alsoapplied to selected structures as well.

Starting fromH3(NH2-BTB), H3(mand-BTB) could be synthesizedby one synthetic step (for synthesis details see Supplementary dataS1). With this linker, the structural analog to DUT-25 (mand-DUT-25) could be obtained according to powder X-ray diffraction mea-surements (Fig. 5, Fig. S4, Table S1). The presence of the side groupsin the mand-DUT-25 was proven by 1H NMR after mild

Fig. 6. Nitrogen physisorption isotherm of mand-DUT-25 at 77 K.

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Table 3Summary of the results from MOF synthesis screening experiments.

NH2-BTB NHBoc-BTB mand-BTB

MOF-177 � � �UMCM-1 � MOF-5 �DUT-6 NH2-DUT-6a � �DUT-25 � NHBoc-DUT-25a mand-DUT-25DUT-23(Zn) � � �DUT-23(Cu) � New structure: Cu3(H2O)(NHBoc-BTB)2(bipy) (DUT-109) �DUT-34 � New structure: Cu2(H2O)2(H(NHBoc-BTB))2$2H2O$4DMF (1) �DUT-9 � � �a Phase purity could not be achieved.

S. Helten et al. / Microporous and Mesoporous Materials xxx (2015) 1e98

decomposition of the supercritically dried sample in DMSO anddeuterium chloride (Fig. S5).

To desolvate mand-DUT-25 for adsorption experiments, super-critical CO2 drying was applied. Nitrogen physisorption at 77 Kshowed a saturation uptake of 100 cm3 g�1 resulting in a specificpore volume of 0.16 cm3 g�1. The BET surface area is 400 m2 g�1

(Fig. 6). However, simulation of the crystal structure of mand-DUT-25 usingMaterials Studio by Accelryswith further calculation of thetextural properties using Poreblazer software [43], results in a muchhigher value for the expected pore volume of 1.23 cm3 g�1 andgeometrically calculated accessible surface area of ca. 3300 m2 g�1.The presence of the bulky substituents hence seems to preventcomplete and nondestructive framework desolvation in this case.All trials to synthesize the remaining targeted MOFs, such as DUT-23/34, DUT-6, DUT-9 or MOF-177 analogs failed.

The overall results of the MOF synthesis screening are summa-rized in Table 3. Most of the substituted BTB derivatives show arather limited compatibility with the original BTB-containing MOFsystems, providing valuable information concerning the futuredesign of functionalized MOFs. While the formation of the SBU canbe considered as a critical step in MOF synthesis [40], the results ofthis work once again clearly demonstrate the crucial role of sub-stituents in ligand molecules and their effect on the formation ofselected framework topologies, particularly in the case of Cu-paddlewheel based MOFs DUT-23(Cu) and DUT-34. Interestingly,while four different Zn4O-based MOFs were screened for compati-bility with the BTB derivatives, successful synthesis of structuralanalogs was observed in the case of DUT-25 only. This possiblypoints to the fact that the framework of DUT-25 (nbo-b topology) ismuchmore tolerant towards sterically demanding substituents thanother investigated frameworks, involving the Zn4O SBU as a node.

4. Conclusions

Three new derivatives of H3BTB, namely e H3(NH2-BTB),H3(NHBoc-BTB) and H3(mand-BTB) were used in the crystallizationof eight selected BTB containing highly porous non-interpenetratedMOFs. In summary, five frameworks, namely NH2-DUT-6, NHBoc-DUT-25, mand-DUT-25, Cu2(H2O)2(H(NHBoc-BTB))2 (1) andCu3(H2O)((NHBoc-BTB)3(bipy)) (DUT-109) could be identified. NH2-DUT-6 and NHBoc-DUT-25 could not be synthesized as pure phase.Crystallization experiments show empirically that the DUT-25structure has the highest tolerance towards bulky substituents inthe BTB ligand. In the case of DUT-23(Cu) and DUT-34, coordinationpolymers with different structural types were obtained. At thesame time, all Zn-based MOFs, except DUT-6 and DUT-25, could notbe crystallized by using substituted ligands despite their apparentsuitability in terms of pore size to accommodate larger sub-stituents. DUT-9 shows no tolerance to the selected ligands as well.The textural properties of mand-DUT-25 and DUT-109 were eval-uated experimentally. mand-DUT-25 could be activated applyingsupercritical CO2 drying and reached a specific surface area of

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400 m2 g�1 and a specific pore volume of 0.16 cm3 g�1. While theporosity of DUT-109, synthesized using established DUT-23(Cu)synthesis procedure, could not be investigated by gas phys-isorption, a high adsorption capacity of 514 mg g�1 in liquid phaseadsorption of ethyl cinnamate in n-heptane was achieved. Thermaldeprotection of the amino groups was possible at 150 �C in DMF ormesitylene, but coincided with some loss of crystallinity.

Acknowledgements

The authors are grateful for the financial support provided bythe DFG (SPP 1362) and the HZB. We thank Dr. Andreas Notzon forhelpful discussions. B. S. thanks the NRW Graduate School ofChemistry for their support.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.micromeso.2015.02.055.

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