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Coordination Chemistry Reviews 257 (2013) 2232– 2249

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews

jo ur nal home p age: www.elsev ier .com/ locate /ccr

eview

nterpenetration control in metal–organic frameworks forunctional applications

ai-Long Jianga,b, Trevor A. Makala, Hong-Cai Zhoua,∗

Department of Chemistry, Texas A&M University, College Station, TX 77843, USADivision of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, University of Sciencend Technology of China, Hefei, Anhui 230026, PR China

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22322. Structural Interpenetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2233

2.1. Polycatenation, polythreading and polyrotaxane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22332.2. Polyknotting or self-penetrating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22342.3. Interpenetration based on 1D chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22362.4. Interpenetration based on 2D layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22362.5. Interpenetration based on 3D networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22372.6. Interpenetration of networks with different dimensionalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2238

3. Interpenetration control and related functional applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22383.1. Reaction temperature and concentration control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22393.2. Template-directed control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22403.3. Ligand design/modification-induced control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22433.4. Coordinated or uncoordinated solvent removal/addition-triggered control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22453.5. Layer-by-layer assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22463.6. Relationship between structural interpenetration and functional applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2246

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2247

a r t i c l e i n f o

rticle history:eceived 8 November 2012ccepted 13 March 2013

a b s t r a c t

Interpenetration in metal–organic frameworks (MOFs) is an intriguing phenomenon with significantimpacts on the structure, porous nature, and functional applications of MOFs. In this review, we provide anoverview of interpenetration involved in MOFs or coordination polymers with different dimensionalities

eywords:nterpenetration

etal–organic frameworkoordination polymerurface area

and property changes (especially gas uptake capabilities and catalysis) caused by framework interpen-etration. Successful approaches for control of interpenetration in MOFs have also been introduced andsummarized.

Published by Elsevier B.V.

ydrogen uptake

. Introduction

Metal–organic frameworks (MOFs) are porous organic–

norganic hybrid materials, often regarded as a subclass of coor-ination polymers, constructed from metal ions or clusters andrganic ligands linked via coordination bonds to form infinite

∗ Corresponding author. Tel.: +1 979 845 4034; fax: +1 979 845 4719.E-mail address: zhou@mail.chem.tamu.edu (H.-C. Zhou).

010-8545/$ – see front matter. Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.ccr.2013.03.017

systems [1]. MOFs are becoming one of the most rapidly develop-ing fields in chemical and materials sciences, not only due to theintriguing structural topologies but also because of their potentialas functional materials in structure-dependent applications, suchas gas storage and separation, sensing, catalysis, and drug delivery,as well as various proof-of-concept demonstrations [2–7]. MOFs

are generally constructed by inorganic vertices (metal ions orclusters) and organic linkers via metal–oxygen or metal–nitrogencoordination bonds. The most attractive features of MOFs aretheir crystalline nature, high and permanent porosity, uniform

istry Reviews 257 (2013) 2232– 2249 2233

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H.-L. Jiang et al. / Coordination Chem

ore sizes in the nanoscale range (from several angstroms up to0 nm), as well as high surface area (typically, BET surface area of000–3000 m2/g, and the Langmuir surface area record is morehan 10,000 m2/g) [8]. Additionally, the chemical versatility andtructural tailorability provide a significant level of tunabilityo the physical and chemical properties of MOFs. The judiciousombination of metal ions and predesigned organic ligands underuitable reaction conditions afford various kinds of MOF structuresith desired functionalities.

Interpenetration, sometimes also referred to as catenation, cane expressed as polymeric analogs of catenanes and rotaxanes,nd is the most common form of entanglement [9,10]. The occur-ence of structural interpenetration is currently becoming veryommon with significant increases in the number of reportedOFs. While there are no chemical bonds between the interpen-

trated networks, they cannot be separated without the breakagef bonds. Largely, it seems that the formation of structurally inter-enetrated systems is hardly anticipated and is discovered rathererendipidously. Generally speaking, porous materials minimizehe systematic energy through optimal filling of void space, andhus structural interpenetration may occur only if the pore spacef an individual net is sufficiently large to accommodate an addi-ional net. When two or more networks (less guest species) areenerated from the same combination of ligand and metal clus-ers, varying only in the degree of interpenetration, they may beescribed as interpenetration isomers, a specific type of framework

somer [11]. Interpenetrated motifs that minimize the empty spaceould significantly enhance the stability of frameworks, not onlyhrough filling of void space but also in the formation of repul-ive forces that serve to prevent one net from collapsing in ontself. Therefore, the use of elongated organic linkers in attempts toynthesize MOFs with expected large pores is generally a very chal-enging task, as the formation of interpenetrated frameworks maye preferred to increase the stability of the framework. Undoubt-dly, structural interpenetration, which is closely associated withore character (such as size, environment, etc.), plays a crucial role

n the functional applications of MOFs.It should be noted here that a great number of interpenetrated

rameworks have been reported in coordination polymers with 1Dr 2D networks due to their remarkable structural flexibility andiversity [9]. Generally, the term “coordination polymer” consti-utes extended structures based on metal ions and organic ligandshat link into an 1D chain, 2D sheet, or 3D architecture, and theerm “MOF” is widely used to describe 3D porous coordinationetworks, but is seldomly used to describe 1D or 2D structures.owever, the boundary between coordination polymer and MOF

s still confusing, as neither term has been rigidly used in previouseports [12]. In this contribution, we maintain that both terms maye used interchangeably, preferring the term MOF, no matter theimension of the structures. In the first section, we briefly review

nterpenetration involved in MOFs with different dimensions. Theifferences in the concepts of interpenetration, polycatenation,olythreading, polyrotaxanes and polyknotting (self-penetrating)hat are frequently employed to describe structures (especially forexible coordination polymers) have also been discussed. In theecond section, successful strategies for interpenetration controln the syntheses of interpenetration isomers and other noninter-enetrated frameworks have been introduced, highlighting theifferences in functional applications (esp. gas adsorption andatalysis) resulting from structural interpenetration in these MOFs.

. Structural Interpenetration

As the most investigated type of entanglement, interpenetrations very common in coordination polymers. Along with increas-ng number of coordination polymers with very flexible structures

trated framework involving the polycatenation of 0D cages to 3D architectures thatare interpenetrated.

Adapted with permission from Ref. [13a]. Copyright 2010, Nature Publishing Group.

reported, interpenetration readily accompanies new and morecomplex types of entanglement being recognized, such as, poly-catenation, polythreading and polyknotting, which are reminiscentof molecular catenanes, rotaxanes, and knots, respectively, and willbe discussed based on their respective features. Following that, wepresent an introduction of structural interpenetration based on thedimension of single interpenetrating units (0D, 1D chain, 2D layeror 3D network), as quite a few reviews examining interpenetra-tion in flexible coordination polymers have been published [9,10].Interpenetration among 0D structures (also termed as catenane,borromean, etc.) only occurs in very flexible coordination polymers[9a,d] and will not be discussed in detail.

2.1. Polycatenation, polythreading and polyrotaxane

It is almost unavoidable to mention polycatenation whendescribing interpenetration because of their close relation,especially in flexible structures. In fact, the strict/detailed classifi-cation of entanglements indicates polycatenation has significantlydifferent features from interpenetration [9d]. Generally, for poly-catenation: (1) the motifs could be the same or different types in0D, 1D or 2D nets that contain closed loops to be interlocked; (2)the number of entangled motifs can be finite or infinite; (3) all theconstituent motifs have lower dimensionality than that of resultantarchitectures; (4) each individual motif is catenated only with thesurrounding ones but not with all the others, etc. In contrast, forinterpenetration systems: (1) all individual motifs with an iden-tical topology are usually extended 2D or 3D networks; (2) thenumber of interpenetrated motifs is finite; (3) the single networkand final structure have the same dimensionalities; (4) each sin-gle network is interlaced with all the other ones to afford the finalstructure. To date, polycatenation has been reported in many flexi-ble coordination polymers [13]. One vivid example is a recent 2-foldinterpenetrated MOF exhibiting two identical 3D polycatenatedarchitectures, each of which is achieved by mechanical interlockingof 0D adamantane-like molecular cages as building blocks in threedirections (Fig. 1) [13a]. The first polycatenated array of 1D nano-tubes has been obtained by interlocking of each nanotube alignedin parallel with the two nearest neighboring ones, giving rise tothe final 2D polycatenated layer [13b]. Recently, a coordinationpolymer consisting of 2-fold interpenetrated layers was reported,

in which the interpenetrated layers are further catenated to thetwo adjacent such sheets in parallel fashion to afford an overallunique (2D → 3D) polycatenated framework, further revealing thecommon co-existence of interpenetration and polycatenation in a

2234 H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249

F endenfi

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ig. 2. (a) Perspective view of the polycatenated structure and (b) temperature deptting.

dapted with permission from Ref. [13i]. Copyright 2011, The Royal Society of Chem

tructure [13e]. It is noticeable that, just like MOF and coordinationolymer, the terms polycatenation and interpenetration are some-imes not rigidly used as the strict definitions above. The terms ashey appeared in the original works are maintained in this review.

Some entangled frameworks with polycatenation charac-er show interesting magnetic properties. Two polycatenatedomplexes have been assembled by appropriately combiningo(II) with long, linear bidentate N-donor ligands and the anti-

nflammatory drug olsalazine, which display a new 2D → 3Darallel polycatenation of undulating (4,4) layers and an unusualD → 3D inclined polycatenation of (6,3) layers, respectively. Theagnetic properties of both compounds have been studied by mea-

uring their magnetic susceptibility in the temperature range of–300 K. For the compound with parallel polycatenated structure,he �mT remains roughly constant from 300 to 50 K, and thenecreases upon further cooling, which is attributed to the zero-fieldplitting (ZFS). The Curie–Weiss fitting of 1/�m in the tempera-ure range of 2–300 K gives a good result with C = 2.33 cm3 K/molnd � = −1.57 K. For the compound with inclined polycatenatedtructure, the 1/�mT vs. T plot in the range of 21–300 K followshe Curie–Weiss law with C = 6.67 cm3 K/mol and � = −18.64 K. Theeviation of the fitted curve from the experimental data at loweremperature suggests the ZFS effect still plays a role [13h]. Annteresting MOF with 2D + 2D → 3D inclined polycatenation basedn a mixed N-donor and carboxylate ligands has been assembledFig. 2a). Cobalt(II) ions are first bridged by carboxylate ligands toorm Co3 clusters, which are further linked by the N-donor lig-nds to afford 2D 44-square lattice (sql) layers with large rhomboidrids. Magnetic investigation indicates that paramagnetism andanted antiferromagnetism coexist with Tc of 48 K. The recipro-al molar magnetic susceptibility data obey the Curie–Weiss lawn the high temperature region of 58–300 K with a Curie constantf C = 3.53 cm3 K/mol and a Weiss constant of � = −14.76 K, whichndicates that dominant interactions between the Co(II) ions arentiferromagnetic. When the temperature is lowered the �mT valuencreases abruptly to a maximum of 5.41 cm3 K/mol at 48 K, andhen drops quickly to a minimum of 3.29 cm3 K/mol at 5 K (Fig. 2b).his behavior indicates the occurrence of a ferromagnetic couplingelow 64 K, which may be attributed to spin-canting in a nonlinearntiferromagnet or a ferrimagnetic transition [13i].

Polythreaded architectures are polymeric analogs of molecu-ar rotaxanes and pseudorotaxanes, where numerous fascinating

opologies have been observed [9d,10f]. Both polyrotaxanes andolypseudorotaxanes contain closed 2-membered rings/loops andod/string elements that thread through the loops. The only dif-erence between them is that both ends of the rod/string have

ce of the magnetic susceptibility curves. The solid red line shows the Curie–Weiss

.

capping groups (similar to a dumbbell) in the former system,making the motifs inseparable without breaking links, whereasthe rod/string may slip off and disentangle due to the absenceof capping groups in the latter. The constituent units could have0D, 1D or higher dimensionalities and the resultant network canpresent the same or an increased dimensionality compared tothat of the polythreaded motifs. Quite a few interesting coordi-nation polymers exhibiting polythreading network architectureshave been reported so far [14], in which polythreading withfinite components is relatively rare. Some reported compoundsinvolve polythreaded 0D rings with side arms that afford 1D or2D arrays [15], and molecular ladders with dangling arms, leadingto 1D → 2D or 1D → 3D [16] polythreaded arrays. The 3D poly-threading coordination polymers based on 2D motifs was firstreported in Zn(Hbtc)(4,4′-bpy) (btc = 1,2,4-benzenetricarboxylate;4,4′-bpy = 4,4′-bipyridyl, Scheme 1), in which 2D motifs with sidearms lead to polythreaded network exhibiting a 2-fold interpen-etrated 3D array, when H-bonds are taken into account (Fig. 3)[17]. Since then, several polythreading networks assembled from2D motifs were constructed [18,13b].

In addition, there are also some entangled networks withcoexistence of polycatenation, polythreading and polyrotaxanecharacters [19]. A novel entangled framework incorporatingfunctional nanosized polyoxoanions in the presence of bothpolycatenane and polyrotaxane has been reported, in which loop-containing 2D layers are catenated to a 3D network of primitivecubic (pcu) topology [19a]. Subsequently, a Cu-based MOF based onin situ generated ligand involves chemically and structurally differ-ent 2D square grids and irregular layers in a unique 3D frameworkthat presents polycatenation and polythreading features [19c].Recently, the long rigid bisimidazole ligand, 4,4′-bis(imidazol-1-yl)biphenyl, and long flexible pimelic acid have been used forconstructing a 3D nickel complex with T-shaped bilayer units.The structure represents an unprecedented 2D → 3D network con-taining both polycatenation and polythreading [19d], representinganother unprecedented polythreaded framework featuring 3Dentangled motif constructed from 2D polyrotaxane layers. It is thefirst 2D → 3D polythreaded framework with both polyrotaxane andpolypseudorotaxane character [19g].

2.2. Polyknotting or self-penetrating

Besides polycatenation and polythreading mentioned above, aswell as the extensively studied interpenetration, self-penetratingnetworks (also called self-catenating, self-entangling or polyknot-ting), are single nets that have the special feature that the smallest

H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249 2235

Scheme 1. The organic ligands/linkers/precursors and related abbreviations mentioned in this review.

Fig. 3. The structure of a single 2D motif with dangling arms (left) and schematic illustration showing the mutual polythreading of the 2D sheets (middle) and twointerpenetrating motifs with rutile topology (right).

Reproduced with permission from Ref. [17]. Copyright 2004, The Royal Society of Chemistry.

2236 H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249

Fig. 4. (a) Parallel interlacing of the chains in a polyrotaxane layer of[Zn2(bix)3(SO4)2]·8H2O (top) and a schematic illustration for the chain entangle-ment (bottom). (b) Schematic view of two inclined polyrotaxane motifs involving1

A2

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Fig. 5. (a) Schematic representation of the 2-fold parallel interpenetration between2D layered structures. (b) Side-on view of the 3-fold parallel interpenetration from2D to 3D. (c) View of the 3D array fabricated by interpenetration of the three sets of

D infinite interlaced polymer that results in a 2D polythreaded layer.

dapted with permission from Ref. [22]. Copyright American Chemical Society,005.

opological rings are passed through (catenated) by other compo-ents of the same network [9c,20]. To build binodal high-connectedelf-penetrating networks, a strategy involving the constructionf two distinct metal clusters with suitable coordination geome-ries with the help of appropriate ligands has been investigated.

ith this strategy, the first (6,8)-connected self-penetrating MOFas been obtained using an asymmetric neutral ligand, dinu-lear zinc clusters as six-connected nodes and trinuclear zinclusters as eight-connected nodes [20a]. A rare ten-connected self-enetrating MOF on the basis of tetranuclear cobalt clusters is theighest connected uninodal network topology in self-penetratingystems so far [20b]. The first binodal (5,12)-connected 3D self-enetrating MOF, constructed by dinuclear [Ba2(�2-OH2)2] cores 12-connected node and flexible 4,4′-sulfonyldibenzoate ligandScheme 1) as 5-connected node, represents the highest connectedelf-penetrating topology in MOFs [20c]. Complicated structuresometimes involve both interpenetration and self-penetratingharacter. With the combination of long and rigid dicarboxylateinkers with a flexible V-shaped pyridylamide derivative, two Co-ased MOFs built by amide derivative and organodicarboxylateo-ligands, display 3-fold interpenetration of 65·8-mok nets whichre 4-connected self-penetrating nets described theoretically in thearly nineties [20e].

.3. Interpenetration based on 1D chains

The prerequisite of interpenetration between 1D chains is thathe individual chains must contain rings. In addition, variouseak supramolecular forces (H-bonding, �· · ·� aromatic stack-

ng interactions, and van der Waals forces) are believed to playmportant roles in the formation of interpenetrated structures.nterpenetration between 1D chains containing both rings andods generally occurs in a manner similar to that of catenanes orotaxanes. Usually, simple 1D chains are interpenetrated/entangledn parallel to give new 1D structures. A Zn complex with 1Dhain structure is constructed by alternating rings and rods, inhich the rods pass through the center of the rings and are

ormed via Br Br interactions [21]. Whether the interpenetratedD chains are in parallel or inclined (not colinear), either formf entanglement could lead to higher dimensional frameworks.inc(II) sulfates have been reacted with the flexible ligand 1,4-is(imidazol-1-ylmethyl)benzene (bix, Scheme 1) to afford theovel coordination network Zn2(bix)3(SO4)2, containing 1D poly-

eric motifs of alternating rings and rods. This framework shows

xtended rotaxane-like mechanical links generating 2D sheets viaarallel interlacing modes of the chains (Fig. 4a) [22]. The com-ound [Ag2(bix)3](NO3)2 has been reported in which inclined 1D

layers with different colors down the a-axis.

Adapted with permissions from Refs. [26,29b]. Copyrights 2009, The Royal Societyof Chemistry and 2002, American Chemical Society, respectively.

chains are interpenetrated to give a 2D square sheet [23]. Asschematically illustrated in Fig. 4b, only rotaxane-like interactionsare involved in such network, with each ring of each net containinga rod from another net. In addition, the complicated interpene-tration of 1D chains or ladders has even been demonstrated to beable to afford 3D networks [24]. Most of the known 1D → 3D trans-formations occur through 2-fold inclined catenation, with eachsquare interlocked by the other two ladders [24a–c]. An unusual 1Dladder-like silver(I) complex exhibits interesting 3D plywood-likestacking arrays. When Ag· · ·O interactions are considered, a novel5-fold interpenetrated framework is observed with a unique [3+2]catenation [24d].

2.4. Interpenetration based on 2D layers

Similar to that of 1D chains mentioned above, interpen-etration between 2D layers also has two types: parallel andinclined. The majority of interpenetrated 2D frameworks areprimarily based on either (4,4) or (6,3) topological nets. Par-allel interpenetration gives either a new 2D layered or a 3Dstructure, while a dimensionality increase from 2D layers toan overall 3D network inevitably occurs in the case of inclinedinterpenetration. A layered MOF, Cu(hfipbb)(H2O) (H2hfipbb = 4,4′-(hexa-fluoroisopropylidene)bis(benzoic acid), Scheme 1) features2-fold parallel interpenetration of 2D layers (Fig. 5a). Each 2Dlayer has an undulating net with a rhombic window, in whicheach dinuclear Cu(II) paddlewheel secondary building unit (SBU)connects with four neighboring SBUs by the bent hfipbb ligands[25]. CoSO4·7H2O has been assembled with two long V-shapedligands of 1,3-bis(4-carboxy-phenoxy)propane (pcp, Scheme 1)and 1,3-bis(4-pyridyl)propane (bpp, Scheme 1) to provide 3-foldparallel interpenetrating networks showing 2D to 3D motifs[26]. The mean planes in the interpenetrating layers are parallel;however, these mean planes are offset but not coincident (Fig. 5b),and thus generate an overall 3D entanglement structure (3-fold 2D

to 3D parallel interpenetration). To present such interpenetrationmode, each sheet rotates roughly 90◦ relative to its two adjacentlayers, as displayed in Fig. 5b. It is interesting to note that thelength of ligand and/or the distance between two metal centers in

istry Reviews 257 (2013) 2232– 2249 2237

apibism

Mbiinph(H4lic[espiisatdciacst

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2

twtrrf[1iitevsh

Fig. 6. Low-pressure CO2 isotherms for Zn-MOF at temperatures from 263 K to298 K. Closed symbols, adsorption; open symbols, desorption. Lines between data

H.-L. Jiang et al. / Coordination Chem

structure could have intrinsic influence on the final number ofarallel interpenetration; the longer length of a ligand may result

n a larger number of identical interpenetrating nets, which haseen well demonstrated. For example, a 3-fold 2D → 2D parallel

nterpenetrated framework with (4,4) topology was increased to atructure with 5-fold interpenetration when the distance betweenetal centers was increased [27].In addition to parallel interpenetration commonly reported in

OFs [28], inclined interpenetration between 2D nets has alsoeen reported to produce 3D interlocked structures. In inclined

nterpenetration, there are two stacks of layers, with one stacknclined compared to the other. Any particular 2D layer has an infi-ite number of inclined layers passing through it. In a dicopperaddlewheel-based MOF, the Cu(II) ion has an elongated octa-edral environment with four nitrogen atoms of 4,4′-bipyridine4,4′-bpy) ligands in the equatorial plane and two oxygen atoms of2O molecules in the axial sites. The Cu(II) centers are bridged by,4′-bpy ligands to give a 2D sheet with square grids. The 2D sheets

ying in the (a–b)c and (b–a)c planes present an inclined 2-foldnterpenetration way to afford a 3D framework with microporoushannels along the c-axis that are filled by free SiF6

2− dianions29a]. The structure of [Cu(bpp)2Cl]Cl·1.5H2O consists of (4,4) lay-rs on the basis of five-coordinated copper ions in a geometry ofquare-pyramid. The three similar, but crystallographically inde-endent sets of layers (shown in red, green, and blue in Fig. 5c), lie

n the ac (red) and ab (green and blue) planes and present inclinednterpenetration to give an overall 3D architecture [29b]. In thetructure of In(bdc)1.5(2,2′-bipy) (H2bdc = 1,4-benzendicarboxyliccid, 2,2′-bipy = 2,2′-bipyridyl, Scheme 1), the bdc connector playswo different roles. One bdc, located in the general position, coor-inates two In(III) ions to afford 1D [In(bipy)(bdc)]+ infinite zigzaghains along the b-axis. The other bdc ligand, the centroid of whichs located on an inversion center, interconnects with these chainslong the a-axis to result in a (6,3) hexagonal layer. Two identi-al sets of parallel hexagonal layers, extending from two differenttacking directions, are interlocked in an inclined way, giving riseo an overall 3D entangled network [29c].

It should be pointed out that polyrotaxane network entan-lement, which was described above as the extended periodictructure of rotaxane motifs, plays a very important role in suchD interpenetration. Since Robson and coworkers reported the firstnusual 2D to 2D polyrotaxane network [Zn(bix)2(NO3)2]·4.5H2O,

n which the independent 2D polymeric layer contains bix rodsnd Zn2(bix)2 loops [30a], there have been many MOFs exhibit-ng parallel interpenetration of 2D layers to 2D or 3D polyrotaxanerchitectures [19b,30], and also inclined interpenetration of 2D lay-rs to 3D polyrotaxane networks [31].

.5. Interpenetration based on 3D networks

Compared to MOFs with lower dimensionality, interpenetra-ion in 3D MOFs is more common. Generally, MOFs constructedith longer ligands usually have larger voids, which make

hem unstable. Thus, interpenetration reasonably occurs toeduce pore space in order to meet the systematic stabilityequirement in MOFs. Different degrees of interpenetration,or example, 2-fold [32], 3-fold [32b,33], 4-fold [33d,34], 5-fold18e,35], 6-fold [34b,36], 7-fold [37], 8-fold [38], 9-fold to beyond0-fold [39], and even recently reported the highest 54-fold

nterpenetration [40], have been widely investigated. Among thenterpenetrated structures, MOFs with diamond-type structuralopology, a 4-connected network with tetrahedral nodes and lin-

ar linkers are the most commonly observed structures and haveery high propensity for the formation of highly interpenetratedtructures [39c–e]. A 2-fold interpenetrated MOF as a luminescentost has been developed for molecular decoding by embedding

points are intended to guide the eye.

Reproduced with permission from Ref. [32c]. Copyrights 2010, Wiley-VCH VerlagGmbH & Co. KGaA.

naphthalenediimide into the entangled MOF framework thatexhibits flexible structural dynamics. After encapsulation of aclass of aromatic compounds, an intense turn-on emission can beobserved and the chemical substituent of the aromatic compoundsstrongly affects the observed luminescent color. It is proposedthat an enhanced naphthalenediimide–aromatic guest interac-tion contributes to the unprecedented chemoresponsive andmulticolor luminescence, according to the observed induced-fitstructural transformation of the entangled framework [32d]. Inaddition, quite a few MOFs with interpenetrated frameworksexhibit unique gas sorption properties [41]. A microporous doublyinterpenetrated MOF that features a primitive cubic net hasbeen rationally designed with small pore cavities of around 3.6 Ainterconnected by pore openings of 2.0 A × 3.2 A, exhibiting highlyselective sorption behavior toward certain gases [32a].

A 2-fold interpenetrated MOF with pillared paddlewheel frame-work based on Zn(II) coordinated to tetratopic carboxylate ligandsand linear dipyridyl ligands has been developed. Interestingly, thisMOF exhibits dramatic steps in the adsorption and hysteresis in thedesorption of CO2 (Fig. 6). This type of behavior has been observedpreviously in MOF materials, but is generally only observed athigher pressures (generally above 5–10 bar). Careful characteriza-tion shows that structural changes possibly occur in the MOF, withinterpenetrated frameworks moving with respect to each otherupon CO2 sorption. In contrast to most other MOFs with dynamicframework behavior, the interconversions here are robust, with thematerial retaining essentially all its porosity even after more than adozen cycles, which could be attributed to the absence of torque orother strain on individual chemical bonds in this Zn-MOF [32c].A MOF with a 4-fold interpenetrating diamondoid network hasbeen developed in which the MOF shows selective gas adsorptionbehavior upon activation and potential applications in gas sepa-ration technologies for H2, CO2, and O2 over N2 and CH4 [34a].Recently, a flexible 6-fold interpenetrated MOF with hydrophobicpores has been fabricated (Fig. 7a and b). The MOF is able to adsorba wide variety of common gases, while water molecules cannot beadsorbed even under 100% humidity at room temperature. More-over, the framework is flexible enough to transform in response

to adsorption of O2, N2, and CO2 to exhibit stepwise adsorptionisotherms (Fig. 7c) [36].

Most recently, a unique MOF, NOTT-202, displaying partially2-fold interpenetrated structure in which the dominant network

2238 H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249

Fig. 7. (a) Topological view of the 6-fold interpenetrated diamondoid network. (b) A perspective view showing the channels running along the c-axis involved in the 6-foldinterpenetrated structure. Cu: turquoise, C: grey, H: white, N: blue, O: red. (c) Adsorption isotherms for N2 (77 K), O2 (77 K), CO2 (195 K), and H2O (298 K). Filled and opens (P0) e2

A & Co

isrfdNisiMass[

2

whmiecbai[blfo22i3ichtiR

hapes represent adsorption and desorption, respectively. The saturation pressure3.57 Torr for H2O at 298 K.

dapted with permission from Ref. [36]. Copyrights 2011, Wiley-VCH Verlag GmbH

s fully present whereas the secondary partially formed networkhows an occupancy of only 0.75, has been reported. Such MOF rep-esents a new class of dynamic material that undergoes pronouncedramework phase transition upon desolvation. The temperature-ependent adsorption/desorption hysteresis in desolvated form,OTT-202a, responds selectively to CO2. The CO2 isotherm shows

nteresting three steps in the adsorption profile at 195 K, andtepwise filling of pores generated within the observed partiallynterpenetrated structure has been modeled by grand canonical

onte Carlo (GCMC) simulations. Adsorption of N2, CH4, O2, Arnd H2 exhibit reversible isotherms without hysteresis under theame conditions, allowing capture of gases at high pressure, butelectively leaving CO2 trapped in the nanopores at low pressure41g].

.6. Interpenetration of networks with different dimensionalities

As mentioned above, the interpenetration between networksith chemically and topologically identical structure, termedomo-interpenetrating nets, is quite common because the sameolecular fragments favor the same periodicity. In contrast, the

nterpenetration of chemically and/or crystallographically differ-nt structures, termed hetero-interpenetrating nets, is not veryommon, especially those with different dimensionalities. Systemsased upon interpenetration of networks with different dimension-lities, such as 0D + 1D, 1D + 2D, 1D + 3D, and 2D + 3D, are still rare,n spite of the intrinsic attraction of these topologies for chemists9d,42]. An interpenetrated MOF has been fabricated by mixeddc and 4,4′,4′′-benzene-2,4,6-triyltribenzoate (btb, Scheme 1)

igands and low-nuclear metal–carboxylate SBUs. Two differentrameworks of 2D 63 bilayer and rare (3,5)-connected 3D hexag-nal mesoporous silica (hms) net are involved in the interestingD + 3D framework, as a rare example of interpenetration amongD bilayers and a 3D hms net [42e]. An unprecedented 2D + 3D

nterpenetrated MOF has also been constructed, that features aD pcu net entangled by (4,4)-connected 2D layers, by introduc-

ng a large flat anthracene group in the organic ligand as structureontrolling unit to direct the whole structure [42f]. In addition to

etero-interpenetrating structures with different dimensionalities,here are also very few hetero-interpenetrating nets contain-ng different chemical compositions or topologies reported [43].ecently, an unprecedented 3D + 3D hetero-interpenetrating MOF

quals 760 Torr for N2 at 77 K and for CO2 at 195 K, 155.73 Torr for O2 at 77 K, and

. KGaA.

with different chemical compositions and topologies has beendeveloped. An exceptional acentric Cd-based MOF material with2-fold hetero-interpenetrated nets consisting of a 3D diamond net-work and a 3D CsCl framework, which present two different nodes(4- and 8-connected), different chemical compositions [mononu-clear Cd(CO2)4 node and trinuclear Cd3(CO2)8 node], and differenttopologies of networks (66 and 424·64) and three 6-, 7-, and 8-coordinated Cd2+ ions, has been constructed (Fig. 8). The resultantMOF with acentric structure displays a high thermal stability (upto 420 ◦C) and weak SHG activity (ca. 0.8 times that of urea) [43e].

3. Interpenetration control and related functionalapplications

As discussed above, the use of long linkers for the design offrameworks often affords interpenetrated MOFs with smaller pores[44]. Although interpenetration usually makes MOFs robust, it neg-atively affects the porosity of open frameworks by reducing thesize of open pores. Highly interpenetrated frameworks typicallyhave low porosity (<20%) and surface area, and high density, neg-atively affecting the potential applications of such materials sincehigh surface area and porosity are generally most desired in porousmaterials. Taking MOF-5 as an example, it has been subjectedto numerous studies and reports in the past several years andreported to exhibit adsorption properties with significant differ-ences (specific surface areas in the range from 570 to 3800 m2/gand H2 uptakes from 1.3 to 7.1 wt.%). The Long group has found thetime period of MOF-5 exposure to air/water is a key point to itsphase purity and gas sorption capabilities [45a]. Around the sametime, the Lillerud group has employed single-crystal XRD to inves-tigate structural differences between the MOF-5 with low and highsurface areas. The MOF-5 sample with low surface area includestwo different types of crystals. One of the phases has lower gasadsorption capacity and surface area than the anticipated valuesbecause it is composed by 2-fold interpenetrated MOF-5 networks.In contrast, the cavities in the other phase are partially occupied byZn(OH)2 species, which inevitably makes the hosting cavity, andpossibly also adjacent cavities, inaccessible, and thus reduces the

pore volume of the material [45b].

Therefore, it is necessary to develop strategies to suppress theinterpenetration in order to construct highly porous MOFs withhigh surface area. The Yaghi group has introduced the use of

H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249 2239

Fig. 8. (a) The 2-fold 3D/3D hetero-interpenetrated networks in a Cd-MOF (blue, green, and purple polyhedra represent 8-, 7- and 6-coordinated Cd2+, respectively; the MeOgroups and H atoms are omitted for clarity); (b) the Cd3(CO2)8 cluster-based CsCl-like framework; (c) the Cd(CO2)4 unit-based diamond net; (d) a simplified CsCl unit fromthe 8-connected node; (e) a simplified diamond unit from the 4-connected node; (f) simplified 2-fold 3D/3D hetero-interpenetrated nets in the MOF (blue: diamond unit;r

A emistr

ibZtctbtSrmtspstfp

3

hfodMnr1os

tb2Tt48sa3

tration, the channels almost disappear, with free volume of only3.4%. By only decreasing the reaction temperature from 160 to105 ◦C, microporous 4, with a noninterpenetrated structure, wasobtained. It has a framework structure similar to the single network

Table 1Summary of the products obtained at different temperatures and concentrations byreaction combination of Cd(NO3)2·4H2O, 4,4′-bpy, and H2bdc [49].

0.2 M 0.1 M 0.05 M 0.025 M 0.0125 M 0.00625 M

85 ◦C Unknown 1 1 1 1 Unknown

ed: CsCl unit).

dapted with permission from Ref. [43e]. Copyrights 2012, The Royal Society of Ch

nfinite SBUs to address this issue [46]. They have constructed 4,4′-iphenyldicarboxylate (bpdc, Scheme 1)-based Zn-MOFs bearingn O C columnar units running along one direction. The struc-ural topology is the same as that of the Al net in SrAl2. However,ompared to original SrAl2, all these MOFs have an open struc-ure in which Zn O links within the SBUs and C6H4 C6H4 linksetween the SBUs expand the Al net. The intrinsic arrangement ofhese rods in the structure has effectively avoided interpenetration.ubsequently, they demonstrated the usefulness of the concept ofod SBUs in the design and synthesis of a series of MOFs [47]. Suchethodology may be applicable to longer linkers than bpdc with

he same width (one benzene ring), which could be used in identicalyntheses to afford the same framework but with accordingly largerores. However, the controllable construct of rod SBUs is relativelyubtle and many factors are associated with the final MOF struc-ures. Many other groups have also devoted work to controllingramework interpenetration in MOFs by regulating the syntheticarameters or approaches in the last decade [48–68].

.1. Reaction temperature and concentration control

Reaction parameters, such as, temperature and concentration,ave been found to be important in the determination of the

ramework interpenetration of MOFs. It has been found that longerrganic linkers readily lead to 2-fold interpenetrating structuresuring their synthesis of the IRMOF series (IRMOF = isoreticularOF). However, more dilute reaction solutions are prone to afford

oninterpenetrating MOFs with larger pores. Along with thisesearch strategy, pairs of interpenetration isomers for IRMOF-0, -12, -14 and -16 have been obtained, each pair composed ofne noninterpenetrated and the other with 2-fold interpenetratedtructure [48].

The variation of temperature and concentration for con-rollable syntheses of a noninterpenetrated form of [Cd(4,4′-py)(bdc)]·3DMF·H2O (compound 1) and its previously reported-fold interpenetrated form, [Cd(4,4′-bpy)(bdc)] (compound 2,able 1) have been systematically investigated [49]. The nonin-erpenetrated 1 has been obtained by reacting Cd(NO3)2·4H2O,,4′-bpy, and H2bdc in a 1:1:1 molar ratio in DMF/DEF (2:1, v/v) at

5 ◦C. Compound 1 features a 3D network bearing a pillared-layertructure, in which the layers, constructed by Cd2N4O8 clustersnd bdc ligand, are linked by 4,4′-bpy as pillars. The resultantD framework has 8.1 A × 11.7 A square channels along the a-axis

y.

and 12.5 A × 12.5 A channels in rhombic shape along the c-axiswhich are occupied by disordered DMF and H2O solvents. Thenoninterpenetrated structure of 1 is surprisingly similar to thepreviously reported single net of the doubly interpenetrated com-pound 2. To rationalize the influence of reaction conditions forthe framework interpenetration, the authors carefully adjusted thetemperature and concentration parameters and the results clearlyshowed that both variables subtly affected the interpenetration andproduct purity (Table 1). Based on these studies, they concludedthat the interpenetrated isomer was preferentially produced at ele-vated temperature, whereas lowering the concentration of startingmaterials in a certain range reduced the possibility of forming asublattice in the voids of noninterpenetrated structures.

Recently, on the basis of the same reaction starting mate-rials, nonporous to microporous MOFs have been prepared byinterpenetration control through decreasing reaction tempera-ture, and micropores have been further enlarged to mesoporesby simply decreasing reactant concentrations and reducing reac-tion time [50]. As displayed in Fig. 9, solvothermal reactions ofthe same amounts of Cd(NO3)2·4H2O, 4,4′-bpy, and 2-amino-1,4-benzenedicarboxylic acid (H2abdc, Scheme 1) in DMF yieldedCd(abdc)(4,4′-bpy) (3), Cd(abdc)(4,4′-bpy)·4H2O·2.5DMF (4), andCd(abdc)(4,4′-bpy)·4.5H2O·3DMF (5) with the same framework for-mula but different guest solvents. All these MOFs are frameworkisomers with hierarchical pores based on the same dicadmium(II)SBU and mixed ligands. The nonporous 3 has a 3D 2-fold inter-penetrated network, in which each network has a pillared-layerstructure with 4,4′-bpy as pillars and the planar channel size of13 A × 17 A composed by Cd(II) and abdc ligands. After interpene-

95 ◦C Unknown 1 1 1 1 Unknown105 ◦C Unknown 1 + 2 1 + 2 1 1 Unknown115 ◦C Unknown 2 2 1 + 2 1 Unknown125 ◦C Unknown 2 2 1 + 2 Unknown Unknown

2240 H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249

F poresm ructu

A ty.

ot4b4(ll4lamatamataroti

3wfcta3Peewuridsto(

ig. 9. Schematic illustration for the synthesis of MOF isomers 3–5 with hierarchicalode for clearly showing the pore space. The 2-fold interpenetrated networks in st

dapted with permission from Ref. [50]. Copyrights 2010, American Chemical Socie

f 3, but slight distortions inevitably occur in order to meet the sys-ematic stability. The resultant pillared-layer framework of 4 with,4′-bpy as pillars has planar channel size of 11 A × 19 A surroundedy Cd(II) and abdc ligands. Upon controlling the interpenetration,

possesses a free volume of 61.2%, much higher than that of 33.4%). Strikingly, in contrast to 4, pillared-layer mesoporous 5 witharger hexagonal channel sizes of 18 A × 23 A can be prepared withower concentrations of reactants. Similar to that in 3 and 4, the,4′-bpy linker still acts as a pillar in 5 to connect Cd-abdc layers,

eading to a 3D open-framework with enormous open channelsnd a very high free volume of 68.2%. The above results, in agree-ent with Zaworotko’s proposition, suggest that high temperature

nd concentration favor interpenetrated frameworks, while lowemperature and concentration prefer noninterpenetrated phasesnd even frameworks with very large mesopores. For the possibleechanism, multiple independent frameworks are self-assembled

t the same time to give rise to interpenetrated structures dueo faster crystal growth during MOF synthesis at higher temper-tures. On the other hand, lowering the temperature reduces theate at which deprotonation of the carboxylic acids and nucleationccurs. As a result, fewer independent frameworks would form,hus substantially decreasing the possibility of the formation ofnterpenetrated MOFs.

Under the excitation wavelength of �ex = 362 nm, compound exhibits weak photoluminescence (PL) at ∼435 and ∼525 nm,hich could be assigned to the ligand-to-metal charge trans-

er (LMCT) and intraligand fluorescent emission, respectively. Inomparison, 4 displays strong emission at ∼435 nm attributedo LMCT. However, after desolvation, the PL of compound 4 islmost quenched and shows similar emission bands as those in, possibly due to the distortion of framework. Interestingly, theL spectra of desolvated 4 in different solvent emulsions exhibitxcellent fluorescence sensing for small molecules (Fig. 9). Themission intensity is strongly dependent on the solvent molecule:hen dispersed in acetonitrile, the fluorescence intensity grad-ally increases with the addition of H2O. It is assumed that theestoration from the distorted framework of 4 in different solventss responsible for the fluorescence enhancement. The result revealsesolvated 4 could be a promising luminescent probe for detecting

mall molecules. In addition, 5 has been demonstrated to be effec-ive for size-selective dominant liquid chromatographic separationf Rhodamine 6G (smaller than pores of 5) and Brilliant Blue R-250larger than pores of 5) dyes.

and related applications for 4 and 5. The structures are all presented in space-fillingre 3 are in blue and purple, respectively.

3.2. Template-directed control

The use of hard templates has been widely employed to con-struct porous materials. It is reasonable to introduce a template inorder to construct noninterpenetrated MOFs with larger pores. It isexpected that the MOF will grow around the surface of the templateand thus prevent the interpenetration of multiple nets.

The Zhou research group first conducted template-directedinterpenetration control for MOFs and, most significantly, theyhave been able to make the direct comparison of hydrogen uptakebetween the interpenetrated and noninterpenetrated MOF coun-terparts and found that structural interpenetration can effectivelyenhance the hydrogen uptake capacity [51]. Starting with a planarligand 4,4′,4′′-s-triazine-2,4,6-triyltribenzoate (tatb, Scheme 1) andCu(NO3)2·2.5H2O, a 2-fold interpenetrated MOF, Cu3(tatb)2(H2O)3(PCN-6), was firstly synthesized. PCN-6 involves dicopper tetracar-boxylate paddlewheel SBUs with aqua ligands that are linked bytatb bridges in the equatorial plane to afford a network, in whichthe distance between opposite corners of the involved cuboctahe-dral cage is as large as 38 A (Fig. 10a). The overall structure containstwo identical interpenetrated nets, with the second net generatedby a translation of the first by c/5 in the [001] direction (Fig. 10b).By introducing oxalate as a template, the noninterpenetrated MOFisomer, Cu6(H2O)6(tatb)4·DMA·12H2O (PCN-6′), was successfullysynthesized. As shown in Fig. 10b and c, the structure of PCN-6 canbe generated by two identical interpenetrated nets of PCN-6′, andthus PCN-6 and 6′ are the first pair of interpenetration isomers. Theauthors have attempted to modulate the synthetic parameters suchas temperature and solvent, while controlled experiment resultsrevealed that none of these can determine the final phase but onlythe template can account for the presence or absence of interpen-etration. To demonstrate the generality of the template strategy,another large trigonal-planar ligand htb (s-heptazine tribenzoate)was also employed to react with Cu(NO3)2·2.5H2O under reactionconditions similar to those of PCN-6 and PCN-6′, and the resultsfurther confirmed that the addition of oxalic acid as template cancontrol the framework interpenetration.

The Langmuir surface area of PCN-6′ (2700 m2/g, pore volume1.045 mL/g) is lower than that of PCN-6 (3800 m2/g, pore volume

1.453 mL/g), indicating a 41% enhancement in Langmuir surfacearea upon interpenetration (Fig. 10d), although PCN-6′ has a highersolvent-accessible volume (86%) than PCN-6 (74%). Such counter-intuitive surface area increase could be attributed to the generation

H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249 2241

Fig. 10. (a) Cuboctahedral cage with the inner void highlighted with large red sphere. (b) Space-filling model of the interpenetrated nets in PCN-6 and (c) noninterpenetratedn n at 52 bols, d

A Chem

otpgcaHttt

ititmAuPwcsPcCsoamsistgfb[

et in PCN-6′ . (d) N2 sorption isotherms of PCN-6 and PCN-6′ at 77 K upon activatio98 K (black): circles, PCN-6; squares, PCN-6′; solid symbols, adsorption; open sym

dapted with permissions from Refs. [51b,c]. Copyrights 2007 and 2008, American

f new adsorption sites and some small pores upon interpenetra-ion. In addition to increasing the surface area, the presence of smallores is beneficial to strengthen the overall interaction betweenas molecules and the pore walls. It should be noted that the openhannels of PCN-6 with such small pores are still large enough toccommodate gas molecules, which guarantees high surface area.owever, other examples of interpenetration isomers have shown

hat the overall surface area may drop significantly in the case thathe open channels are blocked as a result of interpenetration, as inhe MOF-5 example provided above.

Hydrogen uptake has been carefully studied for the pair ofnterpenetration isomers [51b]. To clearly differentiate the con-ributions by coordinatively unsaturated metal sites (CUSs) andnterpenetration to hydrogen uptake, the samples were subjectedo activation at 50 ◦C and 150 ◦C, related to removal of guest

olecules and axial aqua ligands on the Cu centers, respectively.s shown in Table 2, PCN-6 exhibits 133% enhancement in vol-metric and 29% in gravimetric hydrogen uptake compared toCN-6′ due to the structural interpenetration when both samplesere activated at 50 ◦C. When activated at 150 ◦C, hydrogen uptake

apacities of both MOFs are improved, attributed to CUSs. Themaller improvement of hydrogen uptake upon CUS activation inCN-6 than that in PCN-6′ suggests that some of the CUSs in PCN-6ould be blocked due to the structural interpenetration whereas theUSs remain accessible in PCN-6′. Inelastic neutron scattering (INS)tudies showed that the adsorbed H2 molecules first occupy thepen Cu centers of the paddlewheel units with comparable inter-ction energies in the two isomers, while the H2 molecules adsorbainly on or around the organic linkers at high H2 loadings. Under-

tandably, the interaction between H2 molecule and frameworkn interpenetrated PCN-6 with smaller pores is found to be sub-tantially stronger than that in noninterpenetrated PCN-6′, leadingo much higher H2 uptake in the interpenetrated isomer. Hydro-

en sorption measurements at high pressures up to 50 bar haveurther demonstrated that framework interpenetration in MOFs iseneficial to the enhancement of hydrogen adsorption (Table 2)51c].

0 ◦C. (e) Excess hydrogen sorption isotherms of PCN-6 and PCN-6′ at 77 K (red) andesorption.

ical Society.

Furthermore, there is additional evidence supporting thatinterpenetration increases hydrogen uptake, especially at lowtemperature [52]. In a series of Zn4O carboxylates studied, theinterpenetrated IRMOF-11 material showed the greatest hydrogenuptake at 77 K [52a]. The influence of framework interpenetrationfor hydrogen storage in MOFs has also been examined with GCMCsimulations [52c]. The investigations have shown that interpene-trated frameworks, with more metal-corner sites per unit volumeand increased heats of adsorption due to smaller pore size, exhibithigher hydrogen uptake at low pressure regime and low tempera-tures, while noninterpenetrated MOFs, with larger free volume foradsorbed molecules, generally present greater hydrogen storagecapacities at higher temperatures and pressures.

Most recently, PCN-6 and PCN-6′ framework isomers havebeen reproduced by a sonochemical approach and demonstratedthat interpenetrated PCN-6 has higher CO2 adsorption capacity(189 mg/g) than that of PCN-6′ (156 mg/g) and excellent selectiv-ity over N2 (>20:1) at 298 K and 1 atm. The adsorption capacity ofPCN-6 can be well retained over 5 adsorption–desorption cyclesover the course of 800 min with high purity CO2 in a flow sys-tem, and can be regenerated at 75 ◦C under He flow. The workhas also determined that PCN-6 has high CO2 adsorption capac-ity (1200 mgCO2

/gadsorbent) under high-pressure of 30 bar at 298 K[53].

The Lin group has reported that solvents, especially DEF andDMF with different molecular sizes, can be employed as moleculartemplates for effectively controlling framework interpenetration[54]. They have firstly realized framework interpenetration controlin homochiral MOFs. The solvothermal reaction of Cu(NO3)2 andracemic tetratopic carboxylic acid L in DMF/H2O mixed solventsat 80 ◦C afforded a 3D MOF, meso-[LCu2(H2O)2]·(DMF)8·(H2O)4,with doubly interpenetrated networks. It crystallizes in the cen-trosymmetric space group I 41/a and features two interpenetrating

nets of opposite chirality. Although large pores exist in the sin-gle network, the MOF exhibits relatively small open channels withthe largest dimension of ∼14 A, due to the interpenetration ofthe two enantiomeric networks. Interestingly, the interpenetration

2242 H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249

Table 2Hydrogen uptake data of PCN-6 and PCN-6′ .

Activated at 50 ◦C(77 K, 1 bar)

Activated at 150 ◦C(77 K, 1 bar)

Excess (mg/g,298 K, 50 bar)

Total (mg/g,298 K, 50 bar)

Enthalpy(kJ/mol)

PCN-6 1.74 wt.% 1.9 wt.% 9.3 15 6.2–4.5PCN-6′ 1.35 wt.% 1.62 wt.% 4.0 8.1 6.0–3.9

Excess (mg/g,77 K, 50 bar)

Excess (g/L,77 K, 50 bar)

Total (mg/g,77 K, 50 bar)

Total (g/L, 77 K,50 bar)

Deliverable(77 K,1.5–50 bar)

ieD[isgoDss

btrDtwpobtCMipCstr2IdiCalc

snr[HDtawccttt

of 6 and 7, respectively (Fig. 11). More importantly, the reduction ofthe Ru(III) centers in compound 7 turned on the catalytic activity,making 7R highly active for the asymmetric cyclopropanation

PCN-6 72 40.2

PCN-6′ 42 11.8

somerism can be controlled by solvent molecules with differ-nt sizes as templates. Reaction of Cu(NO3)2 and racemic L inEF/H2O mixed solvents at 80 ◦C gave noninterpenetrating rac-

LCu2(H2O)2]·(DEF)12·(H2O)16, which is isostructural to one of thenterpenetrated networks of the meso-structure. The DMF and DEFolvents are proposed to play the roles of templates during the MOFrowth by coordinating to the Cu centers that are on the surfacef growing single crystals. In this case, with larger molecular size,EF solvent does not favor the formation of interpenetrating MOF

ince the formation of smaller pores is not permitted with the largerolvent [54a].

Subsequently, on the basis of systematically elongated dicar-oxylate struts derived from chiral Mn-salen catalytic subunits,he Lin group has also synthesized a family of isoreticular chi-al MOFs (CMOFs 1–5) with Zn4O SBUs [54b]. Similarly, DMF andEF as reaction solvents have successfully controlled the struc-

ural interpenetration of CMOFs 1 and 2 and CMOFs 3 and 4 pairsith the same Zn4O SBUs, in which CMOFs 1 and 3 present inter-enetrated while 2 and 4 have noninterpenetrated structures. Thepen channel sizes in these CMOFs have been systematically tunedy using different lengths of organic linkers and combination ofhe framework interpenetration control with the solvent template.MOF-5 features a 3-fold interpenetrated structure with very longn-salen-derived dicarboxylate strut. Considering that pore size

n MOFs plays important roles in transportation of substrates androducts to facilitate asymmetric catalysis, catalytic activities ofMOFs 1–5 and the dependence of reaction rates on the open poreizes were examined for the asymmetric epoxidation of unfunc-ionalized olefins. The results have shown that the conversionate decreases in the order of CMOF-1 > CMOF-5 > CMOF-3 > CMOF-

> CMOF-4, in agreement with the decrease of open channel sizes.t is proposed that the reaction rates are highly dependent oniffusion of the bulky alkene and oxidant reagents and the epox-

de product when the CMOF channels are small. However, ForMOFs-2 and -4 with large open channels, the catalytic activitiesre dominated by the intrinsic reactivity of the catalytic molecu-ar building blocks and are comparable to homogeneous controlatalyst.

Recently, following a similar research line to generalize such atrategy, they have again obtained a pair of interpenetrated andoninterpenetrated chiral MOFs featuring pcu topology based onedox active Ru(III)/salen-based bridging ligands and Zn4O SBUs54c]. As shown in Fig. 11, Zn(NO3)2·6H2O reacted with [Ru(L-2)(py)2]Cl in DBF/DEF/EtOH (DBF = N,N-dibutylformamide) or inMF/DEF/EtOH at 80 ◦C afforded compounds 6 and 7, respec-

ively. Compound 6 crystallizes in the R32 space group and forms 2-fold interpenetrated 3D network with 54.5% void space, inhich the largest cavities are 8 A in diameter and are inter-

onnected by 4 A × 3 A windows. In comparison, compound 7

rystallizes in the R3 space group with a noninterpenetrated struc-ure. The structures of 6 and 7 have the same asymmetric unit,he same metal–ligand connectivity and network topology, whilehe framework 7 has only the single net of 6. Therefore, the

95 53.0 75 mg/g (41.9 g/L)58 13.2 42 mg/g (11.8 g/L)

noninterpenetrated 3D structure of 7 has 78.8% void space andmuch larger open channel sizes of 14 A × 10 A and a cavity diameterof 17 A

Remarkably, compounds 6 and 7 can be reduced in a single-crystal to single-crystal fashion by the treatment with strongreducing agents of LiBEt3H or NaB(OMe)3H. The reduction con-verts the Ru(III) to Ru(II) in the MOFs, along with a color changefrom dark green to dark red. The resultant MOFs 6R and 7R main-tain the same space groups and similar cell parameters as those of 6and 7, revealing their identical framework topology. Interestingly,the reduced frameworks can be oxidized back in air to yield crystals

Fig. 11. Synthesis and single-crystal to single-crystal reduction/oxidation of com-pounds 6 and 7. Typical colors and morphologies of 7 (green) and 7R (red) are shownin the right-side photographs.

Adapted with permission from Ref. [54c]. Copyrights 2011, Wiley-VCH Verlag GmbH& Co. KGaA.

H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249 2243

Fig. 12. (a) Schematic illustration for the control of degree of interpenetration and structural flexibility in 8 and 9 with benzene as a template. C gray, N blue, O red, S yellow,H light blue. (b) CO2 sorption of 8 at 195 K. (c) CO2 sorption of 9 at 195 K. A: amount adsorbed at STP (standard temperature and pressure). P/P0 is relative pressure; P0 of CO2

at 194.7 K is 101.3 kPa.

A & Co

olsr

Ms2LsDl3s

doeTtdistsutamfAbpaaih

dapted with permission from Ref. [56]. Copyrights 2011, Wiley-VCH Verlag GmbH

f substituted alkenes with very high diastereo- and enantiose-ectivities. In contrast, compounds 6 and 6R are interpenetratedtructures with small channels that are unable to transport theeaction substrates, thus resulting in the inactivity of the catalyst.

Similarly, Guo and Sun have synthesized two 3D Zn-basedOFs with the same framework formula of Zn3(L1)2(L2) (guest

olvents are not considered, L1 = 4-[3-(4-carboxyphenoxy)--[(4-carboxyphenoxy)methyl]-2-methyl-propoxy]benzoate,2 = 1,4-bis(1-imidazolyl)benzene, Scheme 1) under almost theame reaction conditions except for different solvents used [55].ue to the flexible coordination environments of L2 and extended

engths of both ligands, the resultant two MOFs feature 2- and-fold interpenetrated structures in which DMF and acetonitrileerve as templates, respectively.

A reaction between Zn(NO3)2·6H2O, 2,2′-bithiophene-5,5′-icarboxylic acid (H2btdc, Scheme 1), and 4,4′-bpy with solventf DMF only or mixed DMF/benzene (1:1) yielded 3-fold interpen-trated 8 and 2-fold interpenetrated 9, respectively (Fig. 12a) [56].he benzene molecule with larger size in the reaction could play aemplating role, which prevents the dense packing of frameworksuring the synthesis process, leading to the resultant 9 with a 2-fold

nterpenetrated structure. By replacing benzene with other similarolvents, such as toluene or cyclohexane, the degree of interpene-ration can also be controlled. However, introduction of much largerolvent molecules, such as naphthalene or mesitylene, provednsuccessful to control the structural interpenetration. The struc-ural analyses showed that the compositions of both frameworksre the same, indicating that they are a pair of interpenetration iso-ers. Due to the different degrees of framework interpenetration, 8

eatures a rigid framework whereas 9 has a rather flexible structure.s a result, both compounds display entirely different CO2 sorptionehaviors: the CO2 sorption in 8 indicates that its rigid 3-fold inter-enetrated framework remains intact during the CO2 molecule

ccommodation (Fig. 12b), while 9 shows much higher CO2 uptakend dynamic features involving framework transformations (slid-ng motions and/or shrinkage/expansion), further verifying theigh flexibility of the framework (Fig. 12c).

. KGaA.

The interpenetration modulation has also been investigated byassembling a long rigid ligand, 2,5-bis(4′-(imidazol-1-yl)benzyl)-3,4-diaza-2,4-hexadiene (ImBNN, Scheme 1), and M(CF3SO3)2(M = Cd or Mn) in the absence or presence of aromatic molecules[57]. Without aromatic guest molecules, the 3-fold interpenetratednetworks were fabricated with closely packed layered structures;when aromatic guest molecules, such as, o-xylene, naphthalene,phenanthrene, benzene, p-xylene, and pyrene, are introduced, the2-fold interpenetrated networks were obtained with the inclu-sion of the aromatic molecules. The results have clearly revealedthat the aromatic molecules act as templates that control thedegree of structural interpenetration, and the 2-fold interpen-etrated networks display strong preference for aromatic guestinclusion likely due to �–� interactions between ligand and aro-matic guest, but less selectivity toward shape and size differencesof these guest molecules.

3.3. Ligand design/modification-induced control

Similarly to the template effect, the presence of stericly bulkygroups on the organic ligand could be an alternative approachto effectively prohibit framework interpenetration. Therefore, thegoal could be achieved by selective modification of a ligandwith pendant groups. For example, the hydrothermal reactionof dicarboxylate ligand bdc with uranyl cation (UO2

2+) led to alayered structure with a doubly interpenetrated (6,3) net. Thestructural interpenetration can be effectively prevented by thepre-introduction of bulky substituents on the bdc ligand, but theframework structure is fixed to be (6,3) net topology [58].

Interpenetration has been suppressed through simple liganddesign and succeeded in preparation of a series of interpenetrated-noninterpenetrated framework isomers [59]. As shown in Fig. 13,Zn-L5 or Zn-L6 form 2D sheets within the xy-plane and are pil-

lared by a variety of dipyridyl ligands to afford similar pillared-layerMOFs, although the sizes of dipyridyl ligands are widely tailoredfrom L1–L4 [59a]. Remarkably, all MOFs constructed from L6 arenoninterpenetrated whereas 2-fold interpenetrated structures are

2244 H.-L. Jiang et al. / Coordination Chemistry

Fig. 13. (Top) Organic linkers involved in the syntheses of MOFs. (Bottom) Singlecrystal X-ray structures: 2-fold interpenetrated structures (10, 12, 14, and 16) withbuilding block H4L5 and noninterpenetrated structures (11, 13, 15, and 17) withbuilding block H4L6.

AS

ofmhtoasitnnM

wMet(s

aIbpbdot

b4y

Compared to MOF 18, chelating phen ligand has been employed

dapted with permission from Ref. [59a]. Copyrights 2010, American Chemicalociety.

btained with L5 under identical reaction conditions. The only dif-erence between L5 and L6 is the change from aryl-H to aryl-Br

oieties furnished on the tetra-topic carboxylate ligand, whichas been demonstrated to be efficient for interpenetration con-rol. The CO2 sorption experiments demonstrate the differencesf porosity and sorption properties between interpenetrated 10nd noninterpenetrated 11. Furthermore, The potential dynamictructural behavior in interpenetrated 16 gives rise to the stepsn the isotherm at P/P0 ≈ 0.022 upon activation and guest adsorp-ion, while the behavior disappears in 17 with a noninterpenetratedetwork. All these results illustrate that interpenetration plays sig-ificant roles in structural dynamics and gas sorption properties inOFs.Subsequently, with standard biphenyl dicarboxylate linkers

ith one or two pendant azolium moieties, two cubic Zn-basedOFs have been obtained with interpenetrated and noninterpen-

trated structures, respectively. Results have clearly indicated thathe introduction of different numbers of bulky azolium moietiesone vs. two) onto the linear dicarboxylate linkers is able to controltructural interpenetration in MOFs [59b].

Reactions of bdc, 4,4′-bpy, and cobalt(II) salt in DMF gave nonporous, doubly interpenetrated MOF, Co2(bdc)2(4,4′-bpy)2.n contrast, similar reaction conditions using 2-amino-1,4-enzene dicarboxylate realized the formation of a porous doublyillared-layer MOF without interpenetration, [Co2(abdc)2(4,4′-py)2]·8DMF, with permanent porosity. The results have furtheremonstrated that introduction of bulky substituents on therganic linkers could be a general method for interpenetration con-rol [60].

Four structurally similar rod-like ligands (Scheme 1), 1,4-

is(benzoimidazol-1-yl)-phenyl, 1,4-bis(imidazol-1-yl)-benzene,,4′-bis(imidazol-1-yl)-biphenyl, and 4,4′-bis(benzoimidazol-1-l)biphenyl have been employed to react with Co(II) or Cd(II) salts

Reviews 257 (2013) 2232– 2249

to produce 3D MOFs with different degrees of interpenetrationunder similar conditions [61]. Structural analyses confirmed thatboth coordination mode and spacer length play significant roles inthe determination of the degree of interpenetration of MOFs, whichalso could be tunable by ligand modification, such as varying theligand spacer or terminal group.

The successful syntheses of noninterpenetrated and inter-penetrated Cu-MOFs with expanded sodalite-type network byrespectively employing benzene- and triazine-centered versionsof an elongated triangular N-donor ligand have been reported[62]. It is concluded that the reason for the interpenetrationmanipulation for the structures is not only because of the threedifferent atoms (C or N) involved in the two ligands, but alsomainly due to the presence of some degree of torsion betweenthe central and outer phenyl rings in the benzene-centered ligand,whereas such strain does not exist in the triazine-centered one.It is found that the noninterpenetrated structure, although itaffords larger pore space, is prone to collapse upon desolvation,while interpenetration can stabilize the other framework, leadingto increases in both surface area and hydrogen storage capac-ity. This result shows that interpenetration control could be animportant factor in the synthesis of hydrogen storage materials.Most recently, interpenetration control in MOFs with similar nettopologies can be manipulated via a simple change in the ligandby replacing a C C bond with a C C bond [63]. The reactionsof Zn(NO3)2·6H2O and 4-(2-carboxyvinyl)benzoic acid (H2cvb) or4-(2-carboxyethyl)benzoic acid (H2ceb), respectively afford thenoninterpenetrated MOF [Zn4O(CVB)3]·13DEF·2H2O and the 2-foldinterpenetrated MOF [Zn4O(CEB)3]·6DEF·H2O under almost identi-cal reaction conditions (Fig. 14). Gas sorption experiments for thetwo MOFs indicate both structure- and pressure-dependent gassorption properties and the results are almost in agreement withprevious reports mentioned above [51]. Under higher pressures,the MOF with noninterpenetrated structure and larger pore volume(pore size: ca. 9.0 × 9.0 A) can accumulate more gas molecules andhas much higher gas adsorption capacities regardless of the tem-perature. At pressures lower than 1 atm, the interpenetrated MOFwith smaller pores (ca. 2.5 × 2.5 A) has higher H2 and CH4 sorp-tion capacities, possibly due to stronger interactions between gasmolecules and the framework, while it adsorbs less N2 at 77 K andCO2 at 195 K. The interpenetration control for both MOFs could alsobe attributed to the ligand conformations that are generally differ-ent in the structures involving C C and C C bonds. Based on theabove examples, it can be envisioned that not only ligand modifica-tion with bulky groups but also the conformation of organic linkersaffects structural interpenetration in MOFs.

The first interpenetration control for lanthanide-basedMOFs has also been realized [64]. Isostructural MOFs ofLn(bdc)1.5(DMF)(H2O) (Ln = Er, 18; Tm, 19) have been synthe-sized under the solvothermal reactions between H2bdc andErCl3·6H2O or Tm(NO3)3·3H2O in DMF/EtOH/H2O. The isostruc-tural MOFs feature 2-fold interpenetrated 3D frameworks withlimited porosity. Careful analysis of the structure of 18 has revealedthat the coordinated H2O and DMF molecules locate at adjacentpositions with OH2O Er ODMF angle of 78.56◦. Therefore, inorder to suppress interpenetration and improve the porosity, thefollowing two strategies have been identified to achieve greatsuccess: (a) use of other chelating ligands such as phen to replacethe coordinated H2O and DMF, (b) modified bdc ligand with ahindrance group. Using both of these strategies independently,and then together, generated three other MOFs, 20–22, thatpossess the same topology as 19, but remain noninterpenetrated.

to replace DMF and H2O molecules for the synthesis of MOF 20, asthe first strategy. MOF 21 is constructed by tbdc (H2tbdc = 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid, Scheme 1), instead of

H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249 2245

F nterper

A H & Co

b2sacamlcis3

tbpbsrtpftibbtitclblTfaetbrtfpt

3r

ip

desorption in a very slow stream of dry air or by heating at 375 Kfor 3 h to give partially desovated product [Ag6Cl(atz)4]·OH·xH2O(x ≤ 2) at 293 K. Compared to the original MOF, the framework of the

Fig. 15. (a) Schematic showing the incorporation of a bulky NHBoc group in theorganic linkers anticipated to suppress the framework interpenetration in MOFs.The post-synthetic cleavage of the Boc group by thermolysis expands the voidspace within the framework and also exposes reactive amino functional groups. (b)MOF synthesis with ligand H21 affords a noninterpenetrated cubic MOF, [Zn4O(1)3],

ig. 14. (a) Ligands used for the MOF synthesis and crystal structures of (b) noniepresented in red and blue in (c).

dapted with permissions from Ref. [63]. Copyrights 2012, Wiley-VCH Verlag Gmb

dc, but the coordinated solvates are unchanged, based on thend strategy. Finally, the two strategies have been combined toynthesize MOF 22, in which both bdc and coordinated solventsre replaced by tbdc and chelating phen ligands, respectively. Theoordination modes of carboxylate groups of tbdc remain the sames those of bdc in MOFs 20 and 21 and the steric hindrance of theethyl groups in tbdc ligand as well as the large terminal phen

igand are proposed to be responsible for the interpenetrationontrol. Gas sorption examinations for MOF 22 have shown thatt selectively adsorbs CO2 and H2 over N2 and Ar based on theize-exclusive selectivity due to the limited pore opening sizes of.3–3.54 A

The Telfer group has succeeded in interpenetration control byhe pre-modification of a tert-butylcarbamate (NHBoc) group oniphenyldicarboxylate ligand before MOF synthesis. After incor-oration of the modified ligand in the MOF, the amino group cane unmasked by complete removal of the protective group via aimple thermolytic reaction that does not contain any externaleagents and results in no non-volatile species (Fig. 15a). Due tohe steric hindrance of the large NHBoc group, framework inter-enetration has been suppressed and the postsynthetic processurther expands the cavities/pores in the MOF [65a]. Followinghis research line, they have incorporated organocatalytic moietiesnto MOFs by introducing thermolabile protecting groups that cane released to create accessible catalytic sites within the poresy simple thermal treatment process after MOF synthesis. Dueo the interpenetration control induced by the bulky group bind-ng to the ligand, the removal of thermolabile protecting groupo expose organocatalytic moieties makes the highly porous MOFatalytically active for asymmetric aldol reactions with relativelyarge substrates [65b]. Most recently, they reported that photola-ile bulky groups can be grafted onto the biphenyldicarboxylate

igand and subsequently introduced into a cubic Zn-based MOF.he framework interpenetration in the MOF has been success-ully prevented in the presence of a 2-nitrobenzyl ether groupnd such group can be quantitatively cleaved by photolysis toxpose a hydroxyl group (Fig. 15b) [65c]. It should be noted thathe resultant Zn-MOF bearing unmasked hydroxyl group cannote directly synthesized because the hydroxyl group is reactive andeadily coordinates to metal centers during the solvothermal reac-ion process. Therefore, the work not only affords a new strategyor suppressing framework interpenetration but also opens up newerspectives to produce tailored pore surfaces with desired func-ional groups in MOFs for possible applications.

.4. Coordinated or uncoordinated solventemoval/addition-triggered control

Solvent molecules have been demonstrated to be able tonfluence the interpenetration not only during the syntheticrocess as described above, but also after the formation of

netrated and (c) 2-fold interpenetrated MOFs. The interpenetrated networks are

. KGaA.

MOFs. The guest solvent desorption/absorption have been foundto cause the reversible rearrangement between 5- and 6-foldinterpenetrated 3D networks [66]. Slow evaporation of an ammo-nia solution of 3-amino-1,2,4-triazole (Hatz, Scheme 1) affords[Ag6Cl(atz)4]OH·6H2O, which features parallel 5-fold interpene-trated networks possessing square 1D channels with diameters ofca. 8.5 A running along the c-axis. This MOF undergoes guest water

followed by the photolytic deprotection of the hydroxyl group to produce non-interpenetrated [Zn4O(2)3]. Ligands H22 or H23 in the MOF synthesis give theinterpenetrated frameworks.

Reproduced with permissions from Refs. [65a,c]. Copyrights 2010, Wiley-VCH VerlagGmbH & Co. KGaA and 2012, The Royal Society of Chemistry, respectively.

2246 H.-L. Jiang et al. / Coordination Chemistry Reviews 257 (2013) 2232– 2249

Fig. 16. (a) MOF-123 is converted into the interpenetrated crystals of MOF-246 by the removal of coordinated DMF (large pink spheres). C black, N blue, O red, Zn bluepolyhedra. The interpenetrated framework is shown in green. (b) N2 adsorption isotherms measured at 77 K for the samples, MOF-123, intermediates (a–e), and MOF-246.T ◦C, (d

A & Co

db(adp

atTnyticbamDnffbotuMmisattu

he samples have been prepared by heating MOF-123 at (a) 70 ◦C, (b) 90 ◦C, (c) 140

dapted with permission from Ref. [67]. Copyrights 2012, Wiley-VCH Verlag GmbH

esolvated one is 6-fold interpenetrated and drastically distorted,ut still retains the same topology with elliptical 1D channels4.3 A × 10.4 A). Significantly, the conversion between the originalnd desolvated structures is reversible, as confirmed by both pow-er and single crystal XRD analyses of samples of the desolvatedroduct after exposure to saturated water vapor for 1 day.

Similar to that for uncoordinated solvents, the removal andddition of coordinated solvent molecules in MOFs can also triggerhe transformation between interpenetrated and single nets [67].he solvothermal reactions of zinc(II) nitrate and H2nbd (nbd = 2-itrobenzene-1,4-dicarboxylate, Scheme 1) in DMF/methanolielded MOF-123 with formula Zn7O2(NBD)5(DMF)2, which fea-ures a noninterpenetrated 3D porous structure with DMF residingn the pores as terminal ligands coordinated to Zn centers. Theoordinated DMF ligands can be completely removed below 270 ◦Cy simple heating. Remarkably, single-crystal X-ray diffractionnalysis for the heated MOF-123 indicated that the coordinationode of Zn(II) ions changed upon removal of the coordinatedMF molecules and a doubly interpenetrated structure, desig-ated as MOF-246, with the same backbone as MOF-123 was

ormed (Fig. 16a). It is interesting and quite rare that the trans-ormation between MOF-123 and MOF-246 has been proven toe reversible and even can be recycled many times by additionf DMF and simple heating.Dinitrogen sorption was employed torack the changes in porosity of the system as it underwent grad-al transition from MOF-123 to MOF-246. As shown in Fig. 16b,OF-123 shows typical Type I isotherm with BET and Lang-uir surface areas of 1200 and 1340 m2/g, respectively. The two

ntermediates a and b exhibit similar maximum N2 uptake andurface areas as that of MOF-123, which could be understand-

ble since partial interpenetration offsets the space created byhe removal of DMF. The pore space gradually decreases whenhe interpenetration proceeds to MOF-246, which shows no N2ptake.

) 180 ◦C, (e) 220 ◦C, and 260 ◦C under vacuum.

. KGaA.

3.5. Layer-by-layer assembly

Framework interpenetration suppression has also been ele-gantly demonstrated by using liquid-phase epitaxy on an organicmonolayer modified surface followed by a layer-by-layer growthapproach [68]. It has been shown that the pillared-layer MOF-508can be fabricated in a noninterpenetrated form via this syntheticroute. In contrast, conventional solvothermal synthesis always pro-duced 2-fold interpenetrated networks [69]. Such unconventionalapproach to generate noninterpenetrated MOF-508 takes advan-tage of the separate ethanolic solutions of the components. Zincacetate and a mixture of the organic components (H2bdc and4,4′-bpy) were separately stayed in two beakers, then a properlyfunctionalized organic surface was alternately immersed in the twosolutions with intermittent rinsing. The organic surface was basedon an Au substrate, on which a self-assembled monolayer (SAM)of 4,4-pyridyl-benzenemethanethiol (PBMT) was fabricated. It isassumed that the second, interpenetrating network in the surface-mounted MOF (SURMOF) cannot match the pyridine-terminatedorganic surface that acts as a nucleation template (Fig. 17), andtherefore such second lattice cannot nucleate at the surface andthe interpenetration can be successfully suppressed.

3.6. Relationship between structural interpenetration andfunctional applications

As previously mentioned, interpenetration in MOFs is nownumerous and extensively studied and originates from the pres-ence of large free voids in a single network. The most prominentinfluence of interpenetration should be the decrease of pore size

and pore volume, which are directly associated with gas sorptionproperties and selective catalysis, as summarized above. Gener-ally, for the small pore size and volume in an interpenetratedstructure, the interaction between framework and gas molecules

H.-L. Jiang et al. / Coordination Chemistry

Fig. 17. Schematic showing that two identical interpenetrated networks (coloredred and green) are usually formed by conventional synthesis, while using liquid-phase epitaxy, the equivalence of these two networks is lifted by the presence ofthe substrate (dotted line in the schematic diagram on the right) and the formationo

RG

(istcpciosipasstsHctifssta

4

lpllmbsTobo

f interpenetrated networks is suppressed, generating SURMOFs with only one net.

eproduced with permission from Ref. [68]. Copyright 2009, Nature Publishingroup.

H2, CO2, etc.) could be strong, which makes gas sorption capac-ty considerable, at low pressure. In contrast, for the large poreize and volume in a non-interpenetrated MOF, the gas sorp-ion capacity at low pressure could be lower whereas the storageapacity should be higher at high pressure than that for an inter-enetrated MOF because the latter has smaller pore volume, aslearly demonstrated in previous pairs of MOFs [51–53,63], typ-cally for PCN-6 and -6′. The results reveal the respective meritsf interpenetration and non-interpenetration in gas sorption andtorage applications. Meanwhile, the pore size limitation in annterpenetrated MOF will not allow large catalytic substrates androducts to go through, which, of course, is not definitely a dis-dvantage because this point can be reasonably employed forelective catalytic reactions in some cases [54b]. Various catalyticubstrates and products with different sizes may readily accesshe pore space in a non-interpenetrated structure with large poreizes, which is desirable for many catalytic applications [54c,65b].ence, both interpenetrated and non-interpenetrated frameworksan find their special advantages in catalytic applications. In addi-ion to the main applications in gas sorption and catalysis, variationn interpenetration may also affect other functional applications,or example, interpenetrated structures with better stability allowtudies for molecular sensing and recognition [50], interpenetratedtructures could potentially induce stronger magnetic interac-ions due to closer distances between magnetic metal centers,nd so on.

. Conclusions

Framework interpenetration and interpenetration control haveong been topics of interest in MOF research. Though we haverimarily focused on interpenetration in systems built upon rigid

inkers, it is also frequently observed in MOFs with flexible organicigands and presents diverse and fantastic structures [9d]. Further-

ore, we also wish to note that in this review no distinction haseen made between interpenetration and interweaving, althoughome distinctions have previously been identified between them.

he term of interweaving tends to be used when the interlockingf multiple networks enhances the stability of a single network,ut blocks potential adsorptive sites. Interpenetration is moreften used when the pore size is decreased without blockage of

Reviews 257 (2013) 2232– 2249 2247

adsorptive sites, thus maximizing the exposed surfaces of eachnetwork.

We have summarized several routes reported to realizeinterpenetration control in MOFs: reaction temperature or con-centration control, template-directed control, ligand design/modification-induced control, coordinated or uncoordinated sol-vent removal/addition-triggered control as well as layer-by-layerassembly. So far, it is still hard to conclude some general rules tocontrol framework interpenetration because there are too manyfactors that can influence the formation of the final structures ofMOFs. While the first three strategies have been demonstratedto work well by different research groups or in various MOFs, nomethod has been identified that may universally apply, though itseems the layer-by-layer approach should be able to serve for inter-penetration control in quite a variety of cases, though it wouldbe relevant only for small-scale MOF fabrication. These reportedroutes and strategies may be used or modified for future effortsto control framework interpenetration. Generally speaking, inter-penetration can be used for practical advantage if one desires aMOF with small pore sizes, which could be beneficial to H2 andCO2 uptakes [51–53]. In addition, the interaction of interpenetratednetworks can lead to interesting dynamical behavior and the guest-induced displacement of networks with respect to each other cangive rise to hysteretic sorption, which could also be beneficial to gasstorage or separation [32c,56,70]. In contrast, noninterpenetratedframeworks with lower densities offer larger pore sizes and porevolumes, which could be favorable for gravimetric gas uptake andsurface area, as well as gas storage capacities under high pressure.The large pores involved in MOFs could allow large substrates to beaccessible to catalytically active sites in catalytic reactions. There-fore, interpenetration and noninterpenetration have their ownadvantages and the existence of interpenetration is not necessar-ily negative. With continued efforts of scientists and developmentof the MOF field, we expect to see more rational approaches tomanipulate interpenetration, and thus to serve for enhanced per-formances of targeted MOFs.

Acknowledgements

The authors thank Prof. A.B.P. Lever and the reviewers for theirvaluable comments and suggestions. This work was supported aspart of the Center for Gas Separations Relevant to Clean EnergyTechnologies, an Energy Frontier Research Center funded by theU.S. Department of Energy (DOE), Office of Science, Office of BasicEnergy Sciences under Award Number DE-SC0001015. H.-L.J. wassupported by U.S. Department of Energy (DE-FC36-07GO17033)and University of Science and Technology of China. T.A.M. wassupported by the Welch Foundation (A-1725). H.-C.Z. gratefullyacknowledges support from the U.S. Department of Energy (DE-AR0000073).

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