CrystEngComm

PAPER

Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-

Luther-King-Platz 6, 20146 Hamburg, Germany. E-mail: froeba@chemie.uni-

hamburg.de; Fax: +49-40-42838-6348; Tel: +49-40-42838-3100

† Electronic supplementary information (ESI) available: (1) Synthesis schemeof the linker, (2) thermal analysis of UHM-7, (3) BET plot of the N2

physisorption measurement, (4) hydrogen adsorption isotherms and isostericheat of adsorption, (5) further structural description, (6) linker-network corre-lation matrix. CCDC 931921. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c3ce40594j

CrystEngComm,This journal is © The Royal Society of Chemistry 2013

Cite this: CrystEngComm, 2013, 15,9429

Received 5th April 2013,Accepted 11th May 2013

DOI: 10.1039/c3ce40594j

www.rsc.org/crystengcomm

Linker extensions in metal–organic frameworks: a wayto isoreticular networks or new topologies?†

Daniela Frahm, Frank Hoffmann and Michael Fröba*

The synthesis of a new microporous metal–organic framework (UHM-7) under solvothermal conditions and its

characterisation by single-crystal X-ray analysis and several physisorption measurements is presented. The MOF

structure is composed of copper ions and a novel non-linear silicon containing tetracarboxylate linker and exhibits

an unprecedented topology (chs-1). The surface area and the micropore volume were determined by nitrogen

physisorption measurements to be 1290 m2 g−1 and 0.48 cm3 g−1, respectively. Furthermore, UHM-7 shows gas

storage capacities of 1.5 wt% H2 at 77 K, 0.5 mmol g−1 CH4 at 298 K and 2.3 mmol g−1 CO2 at 298 K.

Introduction

Metal–organic frameworks (MOFs) are crystalline, organic–inorganic hybrid, porous solids that have recently attractedgreat interest for a wide range of applications like gas storageand separation, catalysis, sensing or drug delivery.1–11

In comparison to other porous materials MOFs are a spe-cial class of compounds because of their modular construc-tion principle in combination with a great variety of theirunderlying components (linkers and connectors), allowing –

in principle – a target-oriented design, for instance in termsof chemical functionalities or pore sizes. Probably, the mostfamous example of pore size tailoring is constituted throughthe isoreticular MOF (IRMOF) series (all based on lineardicarboxylates and tetrahedral Zn4O clusters) by Yaghi et al.12

Cu3(btc)2 (HKUST-1) (linker: 1,3,5-benzenetricarboxylate),PCN-20 (linker: triphenylene-2,6,10-tricarboxylate), PCN-6′(4,4′,4′′-(1,3,5-s-triazine-2,4,6-triyl)tribenzoate) and MOF-399constitute another illustrative example of four isoreticularMOFs with a tbo net, all constructed by a symmetric tritopiclinker and the square-planar copper paddle wheel motifs,whose pore diameters increase from 8.0 to 43.2 Å.13–16 How-ever, the isoreticular approach is limited for at least two rea-sons: (i) if the linker (still of the same type) exceeds a certaindimension catenation may occur; (ii) in particular concerninglinkers with a certain degree of (conformational or other)

flexibility it is observed in some cases that linkers of thesame connectivity (tritopic) but different length (compared tothe above cited examples), as benzene-1,3,5-tribenzoate inMOF-14,17 or chemical composition, as 4,4′,4′′-(triazine-2,4,6-triyl-tris(benzene-4,1-diyl))tribenzoate in MOF-38816 formcompletely different networks; both MOFs form nets with ptoinstead of tbo topology.

Even if identical linkers and metal sources are used in aMOF synthesis the linker to metal ratio as well as the solventcan play a decisive role for the resulting product leadingsometimes to solvent-dependent polymorphs of a particularMOF.18–21 While all these circumstances contribute to theplethora of possible MOF structures the question arises whatfactors determine finally the structure of a MOF? Hitherto,this question can only be partly answered as the relationshipbetween linker, connector, solvent, temperature and othersynthesis conditions are too complex and still prevent theprediction of MOF structures. While the secondary buildingunit (SBU) and topology concept,22–29 i.e. the determinationand classification of the underlying net of a MOF, is very use-ful unravelling the ‘mystery’ of the diversity of MOF struc-tures is still an explorative field, partly assessing by high-throughput methods.30

Concerning different structures and topologies an interest-ing group of linkers is given by the non-linear tetracarboxylatesbased on isophtalic units (the respective copper-based MOFscontaining linear tetracarboxylate linkers were invented byChen et al.31 and Lin et al.32). In 2008, Zhou and coworkersreported on PCN-12 – with 3.05 wt% hydrogen storage capacityat 77 K and 1 bar being for a long time the world record holder,which is based on the linker mdip (5,5′-methylene-di-isophthalate), exhibiting a bridging methylene unit.33 PCN-12builds a zhc net. While the modification of this linker bysubstitution of the central methylene unit with propane-2,2-diyl, dimethylsilanediyl or dimethylgermanediyl resulted in

2013, 15, 9429–9436 | 9429

Fig. 1 Constitutional formula of the new organosilicon linker

5,5′-[4,4′-(dimethylsilanediyl)bis(1,4-phenyl)bis(ethyne-1,2-diyl)]diisophthalic acid.

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isoreticular structures (UHM-2, UHM-3 and UHM-4)34,35

the extension of the linker by two phenylene rings resultedin a completely different (2-fold interpenetrated) structure(UHM-6)36 with zmj topology, already known from the twoMOFs [Cu24(L)12(H2O)16(DMSO)8]n

37 (H4L = 1,3-bis(5-methoxy-1,3-benzene dicarboxylic acid)benzene) and [Cu24(L)12(H2O)16(DMF)8]n (PMOF-3,38 H4L = 1,3-bis(3,5-dicarboxylphenylethynyl)benzene), whose linkers are also members of the non-lineartetracarboxylates and have the same dimensions as the linkerin UHM-6.39

In order to get a deeper understanding of the determiningfactors of intended designed networks knowledge of possibleresulting MOF topologies in dependence on the linker andmetal sources has to be accumulated. Here, we expand ourseries of MOFs built of non-linear silicon-containingtetracarboxylates and copper ions. The new linker (Fig. 1)5,5′-[4,4′-(dimethylsilanediyl)bis(1,4-phenyl)bis(ethyne-1,2-diyl)]diisophthalate (dmsbphed) comprises two additional C–Ctriple bonds and is an extended version of the recently pub-lished linkers 5,5′-(dimethylsilanediyl)diisophthalate (dmsdip)of UHM-334 and 4′,4′′-(dimethylsilanediyl)bis(biphenyl-3,5-dicarboxylate) (sbbip) of UHM-6.36 The new MOF UHM-7 isstructurally characterized by single-crystal analysis and pow-der X-ray diffraction (PXRD). Special focus is placed upon theinvestigation of the linker conformation and network analysis.Furthermore, thermal analysis as well as different low pres-sure physisorption measurements (N2, H2, CH4 and CO2) werecarried out.

ExperimentalChemicals

1,4-Dibromobenzene (Sigma-Aldrich, 98%), n-butyl lithium(Sigma-Aldrich, 1.6 M in n-hexane), dichlorodimethylsilane(Merck, ≥98.0%), trimethylsilylacetylene (ABCR, 98%),dichlorobis(triphenylphosphine)palladium (ABCR, 99%), cop-per iodide (Sigma-Aldrich, ≥99.5%), triphenylphosphine(Sigma-Aldrich, ≥95.0%), dimethyl 5-aminoisophthalate(Sigma-Aldrich), sodium nitrite (Merck, extra pure) and potas-sium iodide (Applichem, 99.0%) were used without furtherpurification. Diethylether (Biesterfeld, technical) andtriethylamine (Grüssing, 99%) were used after drying; metha-nol (Honeywell, for HPLC), tetrahydrofuran (THF) (Grüssing,99.5%), N,N-dimethylformamide (DMF) (Sigma-Aldrich,

9430 | CrystEngComm, 2013, 15, 9429–9436

≥99.8%) and ethanol (Merck, absolute) were used asobtained.

Methods

NMR spectra were acquired using a Bruker Fourier 300 NMRspectrometer. Infrared spectra were recorded with a BrukerVertex 70 FT-IR spectrometer. The single-crystals were mea-sured on an Oxford Diffraction (Agilent Technologies) Super-Nova diffractometer at 100(2) K with Cu Kα radiation (λ =1.54184 Å) by ω scanning mode. PXRD patterns wereobtained at room temperature using a STOE STADI P trans-mission powder diffractometer with Cu Kα radiation (40 kV,30 mA, counting time 20 s, steps: 0.1° (2θ)). Thermal analysis(TG (thermogravimetry)/MS (mass spectrometry)) was carriedout under O2/Ar (20/80) flow (20 mL min−1) with a NETZSCHSTA 449 F3 Jupiter coupled by capillary with a Aëolos QMS403 mass spectrometer. The heating rate was 5 K min−1

from room temperature to 700 °C. The N2 physisorption mea-surements were conducted with a Quantachrome QuadrasorbSI-MP at 77 K. Using the Brunauer–Emmett–Teller (BET)method, the specific surface area was calculated from theadsorption branch in the relative pressure interval from 0.01to 0.05. The micropore volume was estimated from the quan-tity of gas adsorbed at a relative pressure of 0.2. Low pressurevolumetric hydrogen physisorption data were recorded on aQuantachrome Autosorb 1-C (purity of helium and hydrogen:99.999%). Low pressure carbon dioxide and methanephysisorption data were recorded on a QuantachromeAutosorb iQ (purity of CO2: 99.5%, purity of CH4: 99.9995%).

Synthesis of the linker 5,5′-[4,4′-(dimethylsilanediyl)bis(1,4-phenyl)bis(ethyne-1,2-diyl)]diisophthalic acid

The constitutional formula of the new organosilicon linker isshown in Fig. 1. The whole synthesis scheme is presented inthe ESI.†

Bis(4-bromophenyl)dimethylsilane (1). The synthesis of (1)was carried out under N2 atmosphere using dried solvents.At 0 °C 50.5 mL (80.8 mmol) of a n-butyl lithium solution(1.6 M in n-hexane) was added dropwise to 19.1 g (81.0 mmol)of 1,4-dibromobenzene dissolved in 300 mL of diethyl ether.After stirring at 0 °C for 2 h 4.90 mL (40.6 mmol) ofdichlorodimethylsilane in 10 mL of diethyl ether were addeddropwise. Afterward, the mixture was heated to roomtemperature and stirred for 16 h. The reaction mixture wasseparated from the resulting precipitate by filtration, washedtwice with water, and dried over anhydrous sodium sulfate.Finally the solvent was evaporated in vacuum andrecrystallized from ethyl acetate to give 14.3 g (38.6 mmol,yield: 96%) of a colorless powder.

1H NMR (CDCl3, 300 MHz): δ [ppm]: 7.48 (d, 4H), 7.33(d, 4H), 0.52 (s, 6H); 13C NMR (CDCl3, 300 MHz): δ [ppm]:136.4, 135.6, 131.0, 124.2, −2.6; IR [cm−1]: 3068, 3031, 3012,2961, 1647, 1567, 1477, 1376, 798, 723.

Bis(4-ethynylphenyl)dimethylsilane (2). Under nitrogenatmosphere 10.0 g (27.0 mmol) of (1), 1.02 g (5.36 mmol)

This journal is © The Royal Society of Chemistry 2013

‡ Crystal data for UHM-7: C228H120Cu12N8O61Si6, Mw = 4878.47, tetragonal, a =b = 28.7527(1), c = 35.4316(2) Å, V = 29 291.9(2) Å

3, T = 100.01(10) K, space

group I4/m, Z = 4, 209 392 measured reflections, 12 710 independentreflections (Rint = 0.095). The final R1 value was 0.116 (I > 2σ(I)). The finalwR(F

2) value was 0.388 (I > 2σ(I)). The goodness of fit on F

2was 1.790. The

structure was solved by direct methods and refined on F2by full-matrix, least-

squares methods using the SHELXL-97 program package. The structure con-tains structurally disordered guest molecules in the voids, which could not becompletely identified. Except for the two coordinated DMA molecules, whichare disordered, all non-hydrogen atoms were refined anisotropically. Hydrogenatoms were calculated in idealized positions using a riding model with isotro-pic temperature factors. CCDC entry number 931921 contains the supplemen-tary crystallographic data for this paper.

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CuI, 1.42 g (5.41 mmol) triphenylphosphine and 0.95 g(1.35 mmol) dichlorobis(triphenylphosphine) palladium weredissolved in 200 mL of dried triethylamine. After heating to90 °C 9.4 mL (67.5 mmol) of trimethylsilylacetylene wereadded dropwise and the reaction mixture was stirred at 90 °Cfor 15 h. After separating the insoluble residue the organicsolvent was evaporated in vacuum. The raw product waspurified by column chromatography with n-hexane to give ayellow-orange powder. Afterward, this product was dissolvedin 200 mL MeOH–THF (1 : 1) and treated with 30 mL NaOH(5 wt% in water) for 2 hours at room temperature. Theorganic phase was removed and the water phase wasextracted with DCM. After drying the DCM over sodium sul-fate, the solvent was evaporated in vacuum to give 5.24 g(20.1 mmol, yield: 74%) of an orange powder.

1H NMR (CDCl3, 300 MHz): δ [ppm]: 7.46 (s, 8H), 3.1 (s, 2H),0.54 (s, 6H); 13C NMR (CDCl3, 300 MHz): δ [ppm]: 139.1, 134.2,131.6, 123.1, 83.8, 78.1, −2.4; IR [cm−1]: 3270, 3067, 2957, 2104,1593, 1488, 1381, 781.

Dimethyl 5-iodoisophthalate (3). 26.0 g (124 mmol) ofdimethyl 5-aminoisophthalate were suspended in 100 mL aq.hydrochloric acid (6 M) and cooled to −10 °C before a solu-tion of 8.63 g (125 mmol) sodium nitrate in 50 mL water wasadded over a period of 1 hour. Afterward, 200 mL of toluol aswell as a solution of 42.0 g (253 mmol) potassium iodide in50 mL water were added. The reaction mixture was stirred for18 hours at room temperature and then heated under refluxfor one hour. After cooling and phase separation the organicphase was extracted with aq. Na2S2O3 solution and HCl (1 M)and dried over sodium sulfate. After evaporating the solventin vacuum the raw product was obtained, which was purifiedby recrystallisation from methanol to give 24.3 g (75.9 mmol,yield: 61%) of a pale yellow powder.

1H NMR (CDCl3, 300 MHz): δ [ppm]: 8.63 (t, 1H), 8.55(d, 2H), 3.95 (s, 6H); 13C NMR (CDCl3, 300 MHz): δ [ppm]:164. 9, 142.6, 132.3, 130.0, 93.6, 52.8; IR [cm−1]: 3023, 3023,1709, 1568, 1432, 1291, 1235, 1099, 990, 964, 746, 714.

Tetramethyl 5,5′-[4,4′-(dimethylsilanediyl)bis(1,4-phenyl)-bis(ethyn-1,2-diyl)]diisophthalate (4). Under nitrogen atmosphere12.3 g (38.4 mmol) of (3), 0.59 g (3.10 mmol) CuI, 0.81 g(3.09 mmol) triphenylphosphine and 1.08 g (1.54 mmol)dichlorobis(triphenylphosphine) palladium were dissolved in200 mL of dried triethylamine. After heating to 90 °C 4.0 g(15.4 mmol) of (2) were added dropwise and the reactionmixture was stirred at 90 °C for 15 h. After separating theinsoluble residue the organic solvent was evaporated invacuum. The raw product was purified by columnchromatography with DCM to give 7.40 g (11.5 mmol, yield:75%) of a yellow-orange powder.

1H NMR (CDCl3, 300 MHz): δ [ppm]: 8.60 (t, 2H), 8.34(d, 4H), 7.51 (d, 8H), 3.95 (s, 12H), 0.57 (s, 6H); 13C NMR(CDCl3, 300 MHz): δ [ppm]: 165.8, 139.2, 136.7, 134.3, 131.2,130.3, 124.6, 123.5, 91.4, 88.3, 52.7, −2.4; IR [cm−1]: 3068,3001, 2951, 2844, 2215, 1730, 1598, 1439, 1244, 1102, 813.

5,5′-[4,4′-(Dimethylsilanediyl)bis(1,4-phenyl)bis(ethyne-1,2-diyl)]diisophthalic acid (5). 7.4 g (11.5 mmol) of (4) was

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dissolved in 200 mL of MeOH–THF (1 : 1). After addition of50 mL of aq. KOH (3 M) the reaction mixture was heatedunder reflux for 4 hours. Afterward, the solvent wasevaporated in vacuum and the aq. solution was acidified withHCl (6 M) to give a precipitate, which was filtrated and driedunder vacuum. 6.18 g (10.5 mmol, yield: 91%) of a yellow-orange product was obtained.

1H NMR (DMSO-d6, 300 MHz): δ [ppm]: 13.54 (s, 4H),8.44 (t, 2H), 8.24 (d, 4H), 7.60 (q, 8H), 0.58 (s, 6H); 13C NMR(DMSO-d6, 300 MHz): δ [ppm]: 166.3, 139.6, 136.0, 134.6,132.5, 131.3, 130.3, 123.7, 122.9, 91.3, 88.6, −2.6; IR [cm−1]:3440, 2642, 3068, 2955, 2212, 1699, 1598, 1444, 1250, 1102,812.

Synthesis of the MOF

In a typical synthesis 100 mg (0.17 mmol) of the organosiliconlinker (Fig. 1) were dissolved in 10 mL of DMF–EtOH–H2O(3 : 3 : 2) including 5 drops of HCl (37%). Afterward, 200 mg(0.83 mmol) of copper(II) nitrate trihydrate dissolved in 5 mLof the same solvent mixture were added and the reaction mix-ture was heated to 50 °C for 48 hours. The resulting blue MOFsingle crystals were filtrated and washed twice with DMF anddried in vacuum. For activation the MOF was suspended inethanol for 5 days. During that time the solvents werereplaced by fresh solvents after every 24 h. After the solventexchange the MOF was thermally activated in vacuum for 24 hat 120 °C.

Results and discussionsStructural description

The structure was resolved by single-crystal X-ray analysis.UHM-7 crystallises in the space group I4/m, no. 87, belonging tothe tetragonal crystal system (a = b = 28.7527 Å, c = 35.4316 Å,α = β = γ = 90°, V = 29291.9 Å3)‡. Fig. 2 shows a single unit cellof the structure displayed along the a-axis (left) as well as a2 × 2 × 1 super cell along the c-direction (right).

As the majority of copper MOFs UHM-7 too is based onthe well-known copper paddle wheel motif as SBU. Investiga-tion of the construction principle reveals two different typesof pores: the first pore (Fig. 3, left) with a diameter of approx.12 Å (van-der-Waals radii were taken into account, the dis-tance from copper atom to copper atom amounts to 15 Å) iscuboctahedral, where one copper paddle wheel SBU occupies

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Fig. 2 Left: the unit cell of UHM-7 displayed along the a-axis; right: a 2 × 2 × 1

super cell displayed along the c-axis visualising straight channels in the c direction;

black: carbon, red: oxygen, blue: copper, yellow: silicon, hydrogen atoms are omit-

ted for clarity.

Fig. 4 Visualisation of the pore arrangement within the framework displayed with

a slightly oblique view, mainly along the a-axis (left) and along the c-axis (right) and.Dark blue: pore type I, orange: pore type II.

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every corner of a cuboctahedron in a way, that the openmetal sites point directly into the pore while the isophtalicunits of the linkers span the edges; the second pore (Fig. 3,right) is bordered by four paddle wheel units and four linkersgiving rise to a truncated quadratic bipyramide with a diame-ter (the diagonal of the basal plane) of about 17 Å (the dis-tance from silicon to silicon atom amounts to 20 Å). Thepore arrangement is depicted in Fig. 4 using polyhedrons:the cuboctahedra are located at the corners (0 0 0) and thecenter (1/2 1/2 1/2) of the unit cell. Along the c-axis thecuboctahedra are linked via pore type II by sharing four cop-per paddle wheel units, meaning the pore type II is located atthe positions (0 0 1/2) and (1/2 1/2 0). This pore combinationgives rise to a channel-like structure along the c-direction[0 0 1] (see Fig. 2, right). The cuboctahedra are furtherdirectly connected to each other via linkers as follows: everycuboctahedral cage is connected to the eight nearest neigh-bour cuboctahedral cages by 16 linkers, that means that acentral cuboctahedron is connected by a pair of two linkersto every neighbouring cuboctahedron. This connectionscheme is more clearly illustrated in Fig. 5 and in the ESI.†

Taking a closer look at the linker reveals that two differentconformers of the linker can be distinguished (see Fig. 6).

Fig. 3 Two types of pores within the structure of UHM-7. The cuboctahedral pore

constructed by 12 copper paddle wheel SBUs and 24 isophthalic units (I, left) and

the truncated quadratic bipyramide bordered by four linkers and 8 copper paddle

wheel SBUs (II, right); black: carbon, red: oxygen, blue: copper, yellow: silicon,

hydrogen atoms are omitted for clarity.

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The first conformer (a) has CS-symmetry and is characterisedby a twist of 70° between the two phenylene rings in both ofthe arms of one linker. The second conformer (b) has onlyC1-symmetry: the twist of the two phenylene rings are differ-ent for both arms and dissimilar to the amount in conformer(a), i.e. they are twisted against each other by 16° on the onearm and by 28° on the other arm of the same linker. Thesetwo conformers take on different roles within the UHM-7structure: conformer (a) can be found exclusively as bound-aries of pore type II, while the connection between thecuboctahedra (pore type I) at the corners (0 0 0) and the cen-ter (1/2 1/2 1/2) of the unit cell is accomplished only withconformer (b). At first glance, the linker constitutes a quiterigid molecule, but interestingly, these two conformersexhibit also different overall shapes concerning the angle anddistance between the two pairs of the copper paddle-wheelsthey are connected to (see Fig. 6): while the carbon–silicon–carbon angle in both conformers is almost identical(∼106.5°) and a little bit smaller than the ideal value for atetrahedron the angle between the two pairs of copperpaddle-wheels at the isophthalate units is about 120° for con-former (a) while conformer (b) shows a considerably moreacute angle of 97°. Consequently, conformer (b) spans only adistance of ∼18 Å between two pairs of paddle-wheels while

Fig. 5 Combination of the two different pore types within one unit cell of the

framework of UHM-7 displayed along the a-axis (left) and c-axis (right). The different

pores are illustrated by the dark blue (cuboctahedral pore I) and the orange (trun-

cated quadratic bipyramidal pore II) spheres. Black: carbon, red: oxygen, blue: cop-

per, yellow: silicon, hydrogen atoms are omitted for clarity.

This journal is © The Royal Society of Chemistry 2013

Fig. 6 The two conformations and the overall geometries of the linker in the

structure of UHM-7; (left) the CS-symmetric conformer (a), which builds the bound-

aries of pore type II; (right) the asymmetric (C1) conformer (b), which connects the

cuboctahedral pore type I with each other.

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this value is ∼22 Å for conformer (a). The implications of thisflexibility on the topology will be discussed below.

Topological analysis

The topology of UHM-7 was analysed using TOPOS 4.0.40 Thecopper paddle-wheel motif was considered as a 4-connectingnode (blue points in Fig. 7). Between the two general possibili-ties for the simplification of the linker we decided to define itas a twofold 3-connected node (red points in Fig. 7) and notas one 4-connecting node. The latter would be an oversimpli-fication, which does not reflect the coordination geometryand connection scheme properly. This often leads to nets hav-ing higher symmetry than the underlying structures, whichcannot be the case. The outcome of the topological analysis ofUHM-7 revealed that the underlying net of the structure isquite complex and that it was hitherto unknown. Becausethere are two topologically different copper paddle-wheels itconstitutes a 4-nodal (3,3,4,4)-connected net. In the meantimeit is deposited as chs-1 as a new entry in the personal TOPOSdatabase. Note, that it is not possible to compute a ‘naturaltiling’ representation of the simplified net as chs-1 possessesso-called ‘crossing edges’.

Fig. 7 The chs-1 net describing the topology of UHM-7: red points represent

threefold coordinated cluster (two neighbored points mean one linker) and blue

points represent the copper paddle wheel motifs. Left: view along [0, 1, 0], right:

view along [0, 0, 1].

This journal is © The Royal Society of Chemistry 2013

Apparently, the relationship between linker shapes, linkerconformations, linker lengths on the one hand and thelikeliness of interpenetration (there are only a few publicationswhich claim to accomplished ‘control’ over catenation/inter-penetration or at least the degree of the shift of theinterpenetrated nets)41–44 and the resulting topological net onthe other hand are rather complicated. In some instances sub-tle modifications of the used linker (still belonging to the samegeneral type: V-shaped, one sp3-hybridised centre and two rela-tively rigid ‘arms’, tetratopic) can lead to unexpected resultsconcerning the realised structure and topology of the MOF: inPCN-12′ the mdip linker shows only one conformation (C2v),no interpenetration occurred and gives rise to a ssa or 3,4T36net, if the linker is regarded as a twofold 3-connected node.In PCN-12 the same linker possesses two conformations(C2v and Cs) resulting in a non-interpenetrated structure toobut with zhc topology.33 Exchange of the central methyleneunit of the linker with a dimethyl silyl moiety leads to acompletely analogue structure as realised in UHM-3.34 Exten-sion of the UHM-3 linker by one phenyl ring on both ‘arms’resulted in a 2-fold interpenetrated zmj net of UHM-636 (analo-gously to PMOF-3), in which the linker has the two conforma-tions CS and C2. And finally, further extension of this linkerwith an additional C–C triple bond between the two phenylrings of the side ‘arms’ (this work) leads surprisingly not to astructure being isoreticular to UHM-6 but to a non-interpenetrated structure with chs topology, in which the linkerhas CS and C1 symmetry. (A synoptical correlation matrix canbe found in the ESI† section). These different conformationsare essential prerequisites for building the chs-1 net and it isprobably impossible to realise this net with a shorter linker.

Taking altogether, these observations have also generalimplications on the (iso)reticular concept of framework com-pounds. The validity of this concept, at first sight intriguingsimple and comprehensive, is based upon a relatively highnumber of prerequisites that have to be fulfilled. One absolutelynecessary prerequisite is the employment of strictly non-flexibleand extreme rigid linkers. Once conformational flexible linkersare used or linkers, which lengths exceed a certain thresholdand can therefore adopt also (symmetric and asymmetric) bentshapes (see Fig. 6) the resulting structure is almostunpredictable. This implicates also that the energy hypersurfaceof a certain SBU/linker combination which leads to a distinctMOF/network is often very flat and that other multiple localminima exists, which are separated only by very low energy bar-riers. In not so few cases even the solvent used in the synthesisplay a crucial role for the resulting (isomeric or polymorphic)structure.45–49 The key question – which cannot be answeredyet, not even rudimentary – is, which factor is finally determin-ing the resulting structure? This still remains to be elucidatedand is probably also dependent on the specific system at hand.

Powder X-ray diffraction

Fig. 8 shows the powder X-ray diffraction (PXRD) pattern ofthe as synthesised material in comparison with the simulated

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Fig. 8 PXRDs of UHM-7 as-synthesised and after the activation in comparison with

the simulated XRD obtained from the single-crystal X-ray analysis data.

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one obtained from the single-crystal X-ray analysis as well asthe pattern of the material after solvent exchange with etha-nol and thermal activation in vacuum. The only significantdifference between these patterns is the intensity of thereflection at 4.3° 2θ which is much stronger in the activatedmaterial in comparison to the as-synthesised material, thepositions of the reflections are almost identical in all threediffractograms, confirming the structurally identity of themicrocrystalline and single-crystal phase.

Fig. 10 Gas sorption isotherms of activated UHM-7, top: hydrogen physisorption

isotherms measured at 77 K, down: methane and carbon dioxide physisorption

isotherms measured at 298 K.

Gas sorption studies

To test the permanent porosity of the structure nitrogenphysisorption isotherms of the activated form of the MOFwere measured at 77 K (see Fig. 9). The analysis of theobtained type I isotherm, typical of microporous materials,reveals a specific BET surface area of 1290 m2 g−1 (calculatedat p/p0 = 0.01–0.05, with a linearity of r = 0.999999, see ESI†section) and a micropore volume of 0.47 cm3 g−1 (calculatedat p/p0 = 0.2). These values are in good agreement with thetheoretically determined ones estimated with the ‘Atom Vol-umes and Surfaces’ tool of Accelrys' Materials Studio packagev5.5,50 assuming a probe molecule radius of 1.84 Å (thekinetic radius of nitrogen): the theoretical surface areaamounts to 1333 m2 g−1 and the theoretical pore volume to0.45 cm3 g−1.

Fig. 9 Nitrogen physisorption isotherms of activated UHM-7 measured at 77 K.

9434 | CrystEngComm, 2013, 15, 9429–9436

Furthermore, the gas storage capacity of the materialconcerning different gases was studied. Therefore low pres-sure sorption measurements with hydrogen, methane andcarbon dioxide at different temperatures were recorded. Whilethe hydrogen storage capacity at 77 K is 1.5 wt%, the methaneand carbon dioxide uptake at 298 K amounts to 0.5 mmol g−1

and 2.3 mmol g−1, respectively (Fig. 10). Compared to othergas storage materials these values are moderate.51–53 With thehelp of sorption measurements at three different tempera-tures (77 K, 87 K, 97 K, see ESI†) the isosteric heat of adsorp-tion for low hydrogen loadings (0.13 wt%) was found to be7.3 kJ mol−1. This value is in the typical range of other MOFswith open copper sites (e.g. CuBTC: 6.6 kJ mol−1, UHM-3:7.9 kJ mol−1)54,34 but far below the highest value reported sofar which amounts to 13.5 kJ mol−1.55

Conclusions

In summary, by employing a new extended silicon-containingnon-linear tetracarboxylate linker in a MOF synthesis –

expecting a framework being isoreticular to UHM-3 (zhc net)or UHM-6 (zmj net) – again a framework with a different andhitherto unknown topology (chs net) was obtained. Thisexemplifies again the only limited validity of the reticularconcept, which is bound to the fulfilment of quite a highnumber of preconditions, which are seldom simultaneously

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CrystEngComm Paper

met. The new MOF UHM-7 comprises copper paddle-wheelsmotifs and constitutes a 4-nodal (3,3,4,4)-connected net. Thegas storage capacity of UHM-7 for hydrogen, methane andcarbon dioxide at different temperatures was studied in thelow pressure regime, revealing only moderate performancefor H2, CH4 and CO2. However, as UHM-7 possess an unprec-edented topology, high pressure measurements (very recentlyanother MOF with an unprecedented sty-a net was published,which already meets the U.S. DOE target concerning methanestorage at room temperature and 35 bar)56 and, in particulardue to the very narrow cage windows, the gas separationpotential for several binary gas mixtures like CO2/N2 orCO2/CH4 should be elucidated. These investigations areunderway.

Acknowledgements

We thank V. Blatov and D. M. Proserpio for helpful discus-sions concerning the topology of UHM-7. DF thanks theGraduate School for Resource and Energy Management ‘C1-REM’ (Excellence Initiative of the Federal State Hamburg) forfinancial support.

Notes and references

1 S. Ma, Pure Appl. Chem., 2009, 81, 2235.

2 J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112,

869.3 M. Yoon, R. Srirambalaji and K. Kim, Chem. Rev., 2012, 112,

1196.4 D. Farrusseng, S. Aguado and C. Pinel, Angew. Chem., Int.

Ed., 2009, 121, 7502.5 V. I. Isaeva and L. M. Kustov, Pet. Chem., 2010, 50, 167.

6 J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev.,

2009, 38, 1477.7 L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van

Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105.8 A. U. Czaja, N. Trukhan and U. Müller, Chem. Soc. Rev.,

2009, 38, 1284.9 L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van

Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105.10 J. Della Rocca, D. Liu and W. Lin, Acc. Chem. Res., 2011, 44,

957.11 P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin,

P. Couvreur, G. Férey, R. E. Morris and C. Serre, Chem. Rev.,2012, 112, 1232.

12 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter,

M. O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469.

13 S. S. Y. Chui, S. M. F. Lo, J. P. H. Charmant, A. G. Orpen and

I. D. Williams, Science, 1999, 283, 1148.

14 X.-S. Wang, S. Ma, D. Yuan, J. W. Yoon, Y. K. Hwang,

J.-S. Chang, X. Wang, M. R. Jørgensen, Y.-S. Chen andH.-C. Zhou, Inorg. Chem., 2009, 48, 7519.

15 S. Ma, D. Sun, M. Ambrogio, J. A. Fillinger, S. Parkin and

H.-C. Zhou, J. Am. Chem. Soc., 2007, 129, 1858.

This journal is © The Royal Society of Chemistry 2013

16 H. Furukawa, Y. B. Go, N. Ko, Y. K. Park, F. J. Uribe-Romo,

J. Kim, M. O'Keeffe and O. M. Yaghi, Inorg. Chem., 2011, 50,9147.

17 B. Chen, M. Eddaoudi, S. T. Hyde, M. O'Keeffe and

O. M. Yaghi, Science, 2001, 291, 1021.

18 D. Banerjee, J. Finkelstein, A. Smirnov, P. M. Forster,

L. Borkowski, J. B. Parise and S. J. Teat, Cryst. Growth Des.,2011, 11, 2572.

19 C.-P. Li and M. Du, Chem. Commun., 2011, 47, 5958.

20 B. Liu, L.-Y. Pang, L. Hou, Y.-Y. Wang, Y. Zhang and

Q.-Z. Shi, CrystEngComm, 2012, 14, 6246.21 T. Birsa Čelič, M. Rangus, K. Lázár, V. Kaučič and

N. Zabukovec Logar, Angew. Chem., Int. Ed., 2012, 51,12490.

22 M. O'Keeffe, M. Eddaoudi, H. Li, T. M. Reineke and

O. M. Yaghi, J. Solid State Chem., 2000, 152, 3.

23 M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke,

M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34,319.

24 N. W. Ockwig, O. Delgado-Friedrichs, M. O'Keeffe and

O. M. Yaghi, Acc. Chem. Res., 2005, 38, 176.

25 S. Natarajan and P. Mahata, Chem. Soc. Rev., 2009, 38, 2304.

26 O. Delgado-Friedrichs, M. O'Keeffe and O. M. Yaghi, Phys.

Chem. Chem. Phys., 2007, 9, 1035.27 M. O'Keeffe, M. A. Peskov, S. J. Ramsden and O. M. Yaghi,

Acc. Chem. Res., 2008, 41, 1782.28 J. J. Perry IV, J. A. Perman and M. J. Zaworotko, Chem. Soc.

Rev., 2009, 38, 1400.29 M. O'Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112, 675.

30 N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933. 31 B. Chen, N. W. Ockwig, A. R. Millward, D. S. Contreras and

O. M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4745.32 X. Lin, J. Jia, X. Zhao, K. M. Thomas, A. J. Blake,

G. S. Walker, N. R. Champness, P. Hubberstey andM. Schröder, Angew. Chem., Int. Ed., 2006, 45, 7358.

33 X.-S. Wang, S. Ma, P. M. Forster, D. Yuan, J. Eckert,

J. J. López, B. J. Murphy, J. B. Parise and H.-C. Zhou, Angew.Chem., 2008, 120, 7373.

34 S. E. Wenzel, M. Fischer, F. Hoffmann and M. Fröba, Inorg.

Chem., 2009, 48, 6559.

35 S. E. Wenzel, F. Hoffmann and M. Fröba, J. Mater. Chem.,

2012, 22, 10294.

36 D. Frahm, M. Fischer, F. Hoffmann and M. Fröba, Inorg.

Chem., 2011, 50, 11055.

37 J. J. Perry, V. Ch. Kravtsov, G. J. McManus and

M. J. Zaworotko, J. Am. Chem. Soc., 2007, 129, 10076.

38 X. Liu, M. Park, S. Hong, M. Oh, J. W. Yoon, J.-S. Chang and

M. S. Lah, Inorg. Chem., 2009, 48, 11507.

39 One might argue that this is not surprising since the central

methyl or silyl group of the mentioned linkers can easilybent or rotate and therefore easily adopt differentconformations. However, it turned out that this is not thecase. The conformation of the central group of the linkers isin all instances almost identical, i.e. possesses a synclinal(eclipsed) conformation or show, analogously, twocongruent linker arms in meta position as in PMOF-3.

CrystEngComm, 2013, 15, 9429–9436 | 9435

CrystEngCommPaper

40 V. A. Blatov, Struct. Chem., 2012, 23, 955.

41 D. Rankine, A. Avellaneda, M. R. Hill, C. J. Doonan and

C. J. Sumby, Chem. Commun., 2012, 48, 10328.42 J. M. Roberts, O. K. Farha, A. A. Sarjeant, J. T. Hupp and

K. A. Scheidt, Cryst. Growth Des., 2011, 11, 4747.43 R. K. Deshpande, G. I. N. Waterhouse, G. B. Jameson and

S. G. Telfer, Chem. Commun., 2012, 48, 1574.44 J. Duan, J. Bai, B. Zheng, Y. Li and W. Ren, Chem. Commun.,

2011, 47, 2556.45 C.-P. Li and M. Du, Chem. Commun., 2011, 47, 5958.

46 B. Liu, H. Miao, L.-Y. Pang, L. Hou, Y.-Y. Wang and

Q.-Z. Shi, CrystEngComm, 2012, 14, 2954.47 Q. Yang, X. Chen, Z. Chen, Y. Hao, Y. Li, Q. Lu and

H. Zheng, Chem. Commun., 2012, 48, 10016.48 T. Birsa Čelič, M. Rangus, K. Lázár, V. Kaučič and

N. Zabukovec Logar, Angew. Chem., Int. Ed., 2012, 51,12490.

9436 | CrystEngComm, 2013, 15, 9429–9436

49 M. du Plessis and L. J. Barbour, Dalton Trans., 2012, 41,

3895.

50 Materials Studio, v5.5, Accelrys Inc., San Diego, CA, 2010.

51 M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem.

Rev., 2012, 112, 782.52 Q. M. Wang, D. Shen, M. Bulow, M. L. Lau, S. Deng,

F. R. Fitch, N. O. Lemcoff and J. Semanscin, MicroporousMesoporous Mater., 2002, 55, 217.

53 J. An and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 5578.

54 J. L. C. Rowsell and O. M. Yaghi, J. Am. Chem. Soc.,

2006, 128, 1304.55 J. G. Vitillo, L. Regli, S. Chavan, G. Ricchiardi, G. Spoto,

P. D. C. Dietzel, S. Bordiga and A. J. Zecchina, J. Am. Chem.Soc., 2008, 130, 8386.

56 X. Duan, J. Yu, J. Cai, Y. He, C. Wu, W. Zhou, T. Yildirim,

Z. Zhang, S. Xiang, M. O'Keeffe, B. Chen and G. Qian, Chem.Commun., 2013, 49, 2043.

This journal is © The Royal Society of Chemistry 2013