pubs.acs.org/crystal Published on Web 03/22/2010 r 2010 American Chemical Society

DOI: 10.1021/cg901473d

2010, Vol. 101798–1807

Engineering Three-Dimensional Chains of Porous Nanoballs from a

1,2,4-Triazole-carboxylate Supramolecular Synthon

Anil D. Naik,† Marinela M. Dırtu,† Alexandre L�eonard,‡ Bernard Tinant,†

Jacqueline Marchand-Brynaert,† Bao-Lian Su,‡ and Yann Garcia*,†

†Institut de la Mati�ere Condens�ee et des Nanosciences, Universit�e Catholique de Louvain,Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium, and ‡Laboratoire de chimie des mat�eriauxinorganiques, D�epartement de Chimie, Facult�es Universitaires Notre-Dame de la Paix, Namur, Belgium

Received November 24, 2009; Revised Manuscript Received February 9, 2010

ABSTRACT: Glycine ethyl ester was recruited in an amine exchange process based on a transamination to afford ethyl4H-1,2,4-triazol-4-yl-acetate (L1). The acid hydrolysis of this molecule leads to quantitative isolation of 4H-1,2,4-triazol-4-ylacetic acid (L2). This versatile synthon crystallizes in a noncentrosymmetric orthorhombic space group (Fdd2)withZ=16.Thiscrystal structure is the first one for a 1,2,4-triazole ligand constructed from an amino acid derivative. The strong intermolecularhydrogen bonding O-H 3 3 3N (2.570(3) A) connects molecules into infinite one-dimensional chains running parallel to the baxis, and the structure is further extended by numerous butmoderate strength hydrogen bonds (C-H 3 3 3O). Prominent featuresof L2 are the presence of diverse potential coordinating groups such as carboxylic acid and triazole on the same framework aswell as the inherent flexibility of the ligand backbone. Reaction of L1 or L2 with aq. Cu(BF4)2 in aq. DMF gives dark bluecrystals which crystallize in a noncentrosymmetric, cubic space group (I43m) and which were formulated as [Cu3(μ3-O)(L2)3 3(H2O)3]BF4 3H2O (C1). The self-assembly of C3-symmetric, μ3-oxo bridged triangular tricopper secondary building blocks(SBB) formed an unique architecture which encompasses voluminous nanoball voids of 1 nm. The total solvent accessiblevolume is 4477.5 A3 which accounts for 48% of the cell volume. The crystal network stability was studied by thermogravimetricanalysis (TGA)-differential thermal analysis (DTA) and scanning electron microscopy (SEM) analyses. Sorption propertiesand gas storage capacities weremeasured byBET.C1 shows no preference forN2(g), but a reversibleH2(g) uptake of 21 cm

3/gwasobserved. Morphology analysis by SEM on single crystals of C1 shows “ultrawells” of square shape irregularly located on thesurface, whose origin is due to desolvation or crystal defects. Mercury porosimetry measurements reveal pore size distributionwith a diameter ranging from 350 nm to 2.3 μm.

1. Introduction

“Coordination crystal engineering” has become a success-ful design methodology to fabricate desired “functional ma-terials”, appropriately termed metal-organic frameworks(MOFs), wherein individual components can be guided toreact in a controlled way and allowed to decode their inherentcharacteristics or even to express hybrid startling assets infinal products.1-5 “Permutation and combination” of para-meters such as the choice of a wide range of metal ions andpredesigned organic ligands as interacting partners, theirmolar ratios, solvent medium, and experimental conditionsleads to isolation of materials of a wide range of architectureswith an unprecedented level of nano to meso porosities. Theintriguing but tunable topologies and porosities combinedwith robustness of these MOFs have been found to be usefulfor gas storage, catalysis, molecular switches, separation,purification, drugs loading, and other fields of industrialand fundamental interest.6-14 Because of their low density,high surface area, and porosities, MOFs have emerged as aprime competitor for zeolites,3-5,15 mesoporous materials,and carbon nanotubes16-18 in the field of storage and separa-tion sciences. Among all, particularly interesting is hydrogenstorage applications in energy conservative technologies andclean energy.3-9Mostof the celebratedMOFswerebuilt fromaromatic carboxylate based secondary building units (SBUs)

due to certain positive aspects.3-5 CombinationwithN-hetero-cyclic compounds (imidazole, pyrazole, triazole, tetrazole) wasalso conducted because of their various binding and bridgingmodes. In particular, μ3-oxo-bridged SBUs display a widearray of topologies and interesting properties.19-23

Dimension and tunability of porosities in MOFs are theprime focus for any possible applications. They can be usedfor instance as potential storage vessels or confined reactorsfor catalysis. A spectacular report on MOFs of isoreticularseries,4b in which open space represented up to 91.1% of thecrystal volume, revealed homogeneous periodic pores thatwere incremented form 3.8 to 28.8 A withmethane gas storageapplication. Gas adsorption and vapor adsorption behaviorof multifunctional porous cuprous triazolate frameworkwas also recently reported.19 The 3,5-diethyl substituted1,2,4-triazole forms a flexible framework that has interestingsorption behavior with cavities of 9 A diameter.23a Anotherinteresting family of oxo-bridged MOFs based on pyrazolesupported trinuclear triangular copper SBB which show sig-nificant catalytic activity for peroxidation of cyclohexane andcyclopentane.23b

This extensive investigation of MOFs with either carboxy-late or N-heterocycles building blocks slightly overshadowedan interesting class of biomolecules: amino acids. Compara-tively, less work has been reported so far on MOFs orcoordination networks constructed from these units,24-27

largely due to the lack of proper functionalization of amino-acids. Notable among them are metal-peptide frameworks

*To whom correspondence should be addressed. Fax: þ32-10-472330.E-mail: yann.garcia@uclouvain.be.

Article Crystal Growth & Design, Vol. 10, No. 4, 2010 1799

constructed from an oligovaline family,24a a chiral C3-sym-metric hexanuclear triangular-prismaticCu(II) cluster derivedfrom a highly modular dipeptidic N,N0-terephthaloyl-bis-(S-aminocarboxylato) ligand,24b a two-dimensional (2D) Cu(II)network constructed fromL-tryptophanato ligandandbidentate4,40-bipyridine.24cN-Benzesulfonyl-L-glutamic acid in the pre-sence of 2,20-bipyridine or 1,10-phenanthroline produced twonovel Pb(II) coordination polymers.24d Another example is acondensation product of glutamate moieties with xylene thatwas used to generate a chiralMOF.25 Building aN-heterocycleon an amino acid would be a benefit not only from a syntheticpoint of view but also in coordination and inorganicmedicinalchemistry.28 The advantage of such systems would be thediverse set of coordinating groups on the same ligand back-bone: a carboxylic group and a N-heterocycle. To do so, wehave recently reported a simplified transamination procedurefor converting the amine functional group of glycine into 1,2,4-triazole.29 Moderate yield and delicacy in synthesis led us torevisit the synthesis and to employ here an evenmore attractiveprecursor, an ester of glycine, affording a novel molecule (L1)which can be easily hydrolyzed to 4H-1,2,4-triazol-4-yl aceticacid (L2). The molecular framework of L2 is built from a 1,2,4-triazole head and a carboxylic tail, both of which areknown for their diverse coordinating ability, separated fromconformably flexible methylene group, which can form amixed coordination environment around metals (Chart 1).Another related molecule, 1,2-bis(1,2,4-triazol-4-yl)ethanebearing flexible ethylene spacer, was used in the constructionof MOFs thanks to its ability to bring metal centers closetogether with a 1,2-N,N-bridge.23,30 We have introducedfor the first time this potentially multidentate molecule forengineering a porous MOF of nanoball type (C1) whichencompasses a voluminous void and laid a foundation for anew family of porous MOFs. We have studied the networkstability and its adsorption and gas storage capacities that arediscussed herein.

2. Results

2.1. Synthesis and Characterization. Transamination is agood synthetic strategy to build 1,2,4-triazoles from primary

amines. Ever since the first report by Bartlett andHumphrey,31

there have been hardly any efforts to exploit this versatilemethod in synthetic chemistry. During our investigationon 4,40-bis-1,2,4-triazole synthesis, a celebrated precursorfor iron(II) spin crossover,32 we simplified the existingmethod.29 In that paper we also reported a one-pot synthesisof L2, but its yield was certainly not attractive. Thisprompted us to revise the synthetic route wherein we startedwith glycine ethyl ester (Scheme 1). This reaction not onlygave a new molecule (L1) which can be easily purified byflash column chromatography but also the hydrolysis of itgave almost quantitative yield of L2. This molecule is anappealing synthon bearing triazole and carboxylic group onthe same backbone.

Reaction of L2 with aq. Cu(BF4)2 in water afforded animmediate blue insoluble solid that precipitated from a darkgreen solution. After separation of this solid, slow evapora-tion of the green filtrate for a period of 2 weeks affordedsingle crystals of C1 in low yield. A higher yield (35%) wasobtained when L1 was reacted with aq. Cu(BF4)2 to afforddark blue cubic crystals of C1 by slow evaporation of themother solution. The idea was not only to have control overcrystal growth to form the thermodynamically favoredproduct by slowly hydrolyzing the ester moiety but also toencode the intrinsic tendency of -COOH over the estergroup to form supramolecular aggregation through itsunique coordination mode (Chart 1). Prolonged stirring ofthe above reactionmixture leads to isolation ofmicrocrystal-line powder ofC1. BothL1 andL2 are easily soluble inwater,alcoholic, and halogenated solvents, whereasC1 is soluble inDMF and ammonia.

Compound C1 [Cu3(μ3-O)(L2)3(H2O)3]BF4 3H2O wascharacterized by elemental analysis, IR, Raman, UV-vis,thermogravimetric analysis, scanning electron microscope-energy dispersiveX-ray (SEM-EDX) analysis, X-ray powderdiffraction, and by a X-ray crystal structure analysis.Two sharpRaman bands at 2980 and 2944 cm-1 are assignedto the va (CH2) and vs (CH2) modes, respectively. The strong,broadband around 1735 cm-1 identified for the -C=Ogroup by IR spectroscopy for L2 is shifted to 1650 cm-1 inC1 due to its coordination to copper. The asymmetric andsymmetric stretching bands for coordinated carboxylateanions with corresponding Δ[vasym(-COO-)-vsym(-COO-)]∼ 266 cm-1, agrees well with the presence of a carboxylatefunctional group coordinated in an unidentate fashion,33 afeature also confirmed by the X-ray crystal structure (videinfra). Very intense split bands around 1091 cm-1 areassigned to the BF4

- anion.33 The Raman shifts identifiedat 1583(w) and 1412(s) cm-1 in L2 are assigned to theasymmetric and symmetric vibrations of carboxylic groupthat almost vanish in C1, a weak signal being only observedat 1381 cm-1. The diffuse reflectance spectrum of C1

Chart 1. Possible Coordination Preferences of 1,2,4-Triazoleand Carboxylic Groups

Scheme 1. Synthetic Protocol Used for Ethyl 4H-1,2,4-Triazol-4-yl-acetate (L1) and 4H-1,2,4-Triazol-4-yl Acetic Acid (L2)

1800 Crystal Growth & Design, Vol. 10, No. 4, 2010 Naik et al.

displays a broad absorption band in the range 460-900 nmattributed to the d-d transition, in which three maxima at620 and 670 nm and a shoulder at ∼580 nm are observed(Figure S1, Supporting Information).23b A close look onthe morphology of the crystals of C1 by SEM reveals

cubic-shaped crystals with nearly square shaped openings,likely originating from solvent loss or crystals defects, whichare approximately 500 � 500 nm in dimension (Figure 1).Indeed, because SEM experiments are run in a vacuum,evacuation of lattice solvent molecules is expected. Note-worthy is that these crystals are not degraded under suchtreatment.

2.2. X-ray Crystal Structure of L2. The recrystallizationof ethyl-4H-1,2,4-triazol-4-yl acetic acid (L2) from metha-nol results in colorless single crystals. L2 crystallizes inthe Fdd2 polar space group with 16 molecules in the asym-metric unit. A perspective view of the molecule is shown inFigure 2.

Crystallographic data are given in Table 1, and bonddistances and bond angles are given in Table 2. Themoleculeis made of two planes, made by the triazole ring and by the-CH2-COOH group, which are mostly perpendicular toeach other. Indeed, the dihedral angle between the meanplanes through N1-C5 and N4-O9 is 83.7(1)�. The mole-cular units are not isolated; they are linked to each other byintermolecular hydrogen bonding interactions (Table 3),forming a pseudo-2D arrangement along the b axis(Figure 3).

The hydrogen bonding located between the nitrogenatoms of triazole ring and the hydroxy group from thecarboxylic part (O8-H8 3 3 3N1, 2.570(3) A) in b directionis strong, as evidenced by the angle of 171(2)�which tends tolinearity (Table 3). The other source of H-bonding is weakC-H 3 3 3O interactions between the two C-Hof the triazoleand the O8 and O9 of the carboxylic acid groups with anglesof 129(2)� and 130(2)� indicating moderate strength(Table 3). The origin of the noncentrosymmetry of the crystalstructure may be found in the electronic asymmetry34a of L2and in the packing of the molecular units.34b-e It is to benoted that the direction of 1D chains of H-bonding(O8-H8 3 3 3N1) are antiparallel (Figure 3) netted by theinterchain C-H 3 3 3O bonding paralyzing further the sym-metry.35 Such a non-centrosymmetric structure with electricpolarization may exhibits nonlinear optical (NLO) effectssuch as a second harmonic generation (SHG).35c

Figure 1. (a) SEM micrograph of polycrystalline sample of C1

revealing cubic crystals. (b) An enlarged view shows approximatelysquare shaped openings (500 � 500 nm) on the crystal surface.

Figure 2. Thermal ellipsoidal plot of L2 with atom labeling.

Table 1. Crystal Data for L2 and C1

parameters L2 C1

empirical formulaa C4H5N3O2 C2H2B0.25Cu0.5FN1.5O2.17

formula weight 127.11 135.19temperature, K 120(2) 120(2)wavelength 0.71069 A 0.71069 Acrystal system, space group orthorhombic, Fdd2 cubic, (I43m)unit cell dimensions a = 22.360 (8) A a = b = c = 21.037(6) A

b = 19.782 (8) Ac = 4.691 (2) A

volume (A 3) 2075 (1) 9310 (5)Z, calculated density (mg/m3) 16, 1.628 48, 1.16absorption coefficient (mm-1) 0.133 1.427F(000) 1056 3196crystal size (mm) 0.35 � 0.32 � 0.25 0.25 � 0.20 � 0.20θ range for data collection (�) 3.64 to 26.38 3.35 to 21.05number of reflections collected 3210 23138number of independent reflections (Rint) 1012(0.042) 967(0.068)completeness (%) 98 98.9goodness-of-fit on F2 1.267 1.065final R indices [I > 2σ(I)] R1 = 0.0341, [1008] R1 = 0.0626, [922]R indices (all data) R1 = 0.0342, wR2 = 0.0909 R1 = 0.0647, wR2 = 0.1717Flack parameter 0.5(17) not reliable 0.04(6)extinction coefficient 0.0012(4)largest diff peak and hole (e A-3) 0.34 and -0.36 0.62 and -0.59

aThe empirical formula for C1 includes solvent water inside the nanoball while other lattice solvent molecules were squeezed.

Article Crystal Growth & Design, Vol. 10, No. 4, 2010 1801

It has to be noted that crystal packing supported bymultiple secondary interactions has skipped the less favor-able dimer configuration through H-bonding between car-boxylic groups. The 1,2,4-triazole head and carbonyl ofcarboxylic acid group occupy a cis position with respect toeach other as in most of the glycine derivatives.36

The present ligand L2 is the premier member of thetriazole-carboxylate family whose crystal structure has beendetermined. Glycine and its derivatives have long been thefocus of biochemists since they are constituents of proteinsand enzymes and major inhibitory neurotransmitters inspinal cord and brain stem of vertebrate nervous system.36

Determination of the dihedral angles between the planecontainingmethylene spacer with 1,2,4-triazole ring and alsowith carboxylate plane was necessary to understand thespatial arrangement of this molecule not only for the presentfabrication ofMOFbut also possibly in drug chemistry.37 Itsability to create supramolecular interactions in uncomplexedform and possibly in MOF could be decisive for the betterunderstanding of the self-assembly process in MOF. Asimilar multifunctional ligand tetrazole-1-acetic acid with acarboxylate and tetrazolyl ring on the same framework wasalso recently reported.38

2.3. X-ray Crystal Structure of C1. [Cu3(μ3-O)(L2)3 3(H2O)3]BF4 3H2O (C1) crystallizes in a cubic space group(I43m) at 120K.Mixed coordination environments of N andO are seen around copper. Three copper atoms are lodged atthe vertices of an equilateral triangle around μ3-O keeping aCu 3 3 3Cu distance of 3.368 A (Figure 4). This value com-pares well with a similar triangular tricopper MOF builtfrom unsubstituted 1,2,4-triazole (Cu 3 3 3Cu = 3.439 A).38

The copper center is in “4þ 1” square pyramidal geometryand each copper binds three ligands: two ligands coordinatethrough nitrogen of 1,2,4-triazole in aN1,N2 bridging mode(Cu-N2 = 1.950(9) A), while one ligand coordinatesthrough a carboxylate group in an unidentate fashion(Cu-O9= 1.943(1) A). The Addison structural index para-meter τ, which is relevant for five-coordinate structuresas an index of the degree of trigonality,39 was evaluated as0.014 thus confirming a square pyramidal geometry. This

Table 2. Selected Bond Distances (A) and Bond Angles (�) in L2 and C1

L2 C1

N1-C5 1.311(3) Cu1-O9 1.944(11)N1-N2 1.388(2) Cu1-O10 1.946(2)N2-C3 1.312(3) Cu1-N2 1.950(9)N4-C5 1.348(3) Cu1-O100 2.32(2)N4-C3 1.360(2) N2-C3 1.315(15N4-C6 1.457(3) N2-N2 1.368(18)C6-C7 1.527(3) C3-N4 1.310(14)C7-O9 1.215(3) N4-C3 1.310(14)C7-O8 1.307(2) N4-C6 1.482(19)

C7-O8 1.19(2)C5-N1-N2 108.74(17) C7-O9 1.294(19)C3-N2-N1 105.37(17) B1-F2 1.308(13)C5-N4-C3 105.63(17)C5-N4-C6 126.18(17) O9-Cu1-O10 176.7(14)C3-N4-C6 127.58(17) O9-Cu1-N2 90.7(3)N2 -C3-N4 110.98(18) O10-Cu1-N2 89.2(3)N1-C5-N4 109.27(17) N2-Cu1-N2 175.8(6)N4-C6-C7 110.61(17) O9-Cu1-O100 94.7(7)O9-C7-O8 122.63(18) O10-Cu1-O100 88.7(14)O9-C7-C6 121.88(17) N2-Cu1-O100 91.9(3)O8-C7-C6 115.47(17) Cu1-O10-Cu1 119.85(18)

C3-N2-Cu1 133.0(8)N2-N2-Cu1 120.9(3)C7-O9-Cu1 116.2(11)O8-C7-O9 124.7(15)O8-C7-C6 121.5(15)O9-C7-C6 113.8(13)

Table 3. Overview of H-Bonding in L2 (Distances D 3 3 3A < 3.25 A)

D-H 3 3 3A d(D-H) d(H 3 3 3A) d(D 3 3 3A) —DHA symmetry to apply to A

O8-H8 N1 1.02(2) 1.56(2) 2.570(3) 171(2) x þ 1/y, 1/y - y, z - 3/4C3-H3 O9 1.01(2) 2.27(3) 3.007(3) 129(2) -x, 0.5 - y, z - 0.5C5-H5 O8 1.04(3) 2.45(3) 3.213(3) 130(2) 1/4 - x, y - 1/4, 3/4 þ z

Figure 3. View of the antiparallel chains of O-H 3 3 3N and othersubsidiary hydrogen bonding network as seen in the crystal packingof L2. Opposite 1D chains are marked with pink and cyan colors.The d(H 3 3 3A) distances are given in A.

Figure 4. View of the triangular tricopper μ3-oxo bridged SBUwithatom labeling. Symmetry code: (I) x, y, z; (II)-zþ 3, y,-xþ 3; (III)x, -z þ 3, -y þ 3.

1802 Crystal Growth & Design, Vol. 10, No. 4, 2010 Naik et al.

triangular, tricopper(II) unit (Figure 4) supported by -μ3-Obridging and six ligands forms the secondary building unit(SBU). The μ3-O atom is in a nearly planar triangularcoordination sphere with an average Cu 3 3 3 μ3-O distanceof 1.946(2) A. The Cu-O-Cu angle is of nearly 120�(119.8(1)�) which is indicative of a sp2 configuration ofoxygen atom.38 This SBB has a 3-fold axis of symmetrypassing through μ3-O. Thus, coordination around copper isformed by a CuN2O3 chromophore. Indeed, there is asquared basal plane of CuN2O2 which has a water moleculeloosely coordinated to copper (2.32(2) A). The basal plane isnot perfectly planar, the N-Cu-N and O-Cu-O anglesbeing 175.8(6)� and 176.7(1)�, respectively. Crystallographicdata are summarized in Table 1.

The self-assembly of this rigid SBU in 3D generates acagelike structure. The interlinking arrangement of eightsuch SBUs generates a molecular cage/porous ball whichcan be considered as tertiary building unit (TBU) or “super-cluster” (Figure 5).40 The axially bound 24 water moleculeson each of 24 copper atoms of TBU cage decorate a hydro-philic interior pointing toward the void. The total solventaccessible volume is 4477.5 A3 which accounts for 48%of thecell volume. The distance between opposite oxo-centers is18.219 A. Considering van der Waals radii, the void mayadmit a sphere of 1 nm of diameter (Figure 5). Each nanoballis the artifact of 96 molecules (24 Cu2þ ions, 24 watermolecules, and 48 ligands) self-assembling in three dimen-sions. Each nanoball is tunneled into six neighboring nano-balls through a roughly square window/aperture. Thesemutually perpendicular 3D channels represent a kind ofundulating passage (Figure 6).

Since each TBU is roughly spherical, their specific spatialarrangement in 3D obviously generates smaller voids/inter-stices as can be seen in the body centered cubic arrangementof spheres (Figure 6). The larger channels open into smallerhydrophobic ones which are roughly square shaped (6.328�6.328 A) and where the non-coordinated BF4

- anions arestranded. Indeed, anions are distributed in both peripheryof hydrophilic and center of hydrophobic channels. Watermolecules of crystallization are lying within the cavityanchored by H-bonding. The -CdO of carboxylic groupwith a length of 1.215(3) A in free ligand is lowered to1.19(2) A in the complex due to H-bonding with the watermolecules, whereas the C-O length in the ligand is lowered by

0.01 A, upon coordination to copper. The nonclassicalH-bonding between -C(H) of triazole and fluoride of BF4

- is3.182(12) A. C1 can also be viewed as chains of nanoballs(Figure 7).

The semirigid nature of the ligand framework was realizedafter closely studying the bond length and torsion anglechanges upon complexation. The crooked ligand with anobtuse angle of 110.6(2)�betweenN4-C6-C7has flexed outto 112.0(1)�, while that of C6-C7-O8with 115.5(2)� bent to122.2(1)� in C1. The dihedral angle between two planesinvolving N4-C6-C7-O9 in C1 is exactly 180� whichmeans that the two planes are stiffened to lie parallel asopposed to an angle of 176(1)� in the ligand. The flexibility isalso evident from the change in dihedral angle betweentriazole plane and plane containing the -CH2-O groupfrom 81�(1) in L2 to 87� in C1. These parameters may assistin redesigning the framework for specific applications suchas gas storage where a spacer length, bent angle and porevolume could be tuned.

The space group of C1 is noncentrosymmetric. The use ofan assymmetrical L2 ligand as well as the directional natureof the metal ions favor a supramolecular chirality. The bentCH2 spacer group may also offer prospect for lower sym-metry networks compared to those based upon “straight”spacers. The conformational flexibility of L2 may also be atthe origin of the chiral network developed by C1.34,35 Suchcharacteristic may also be appealing for NLO effects such asa SHG.

2.4. Thermal Analyses of C1. A crystalline sample washeated from 25 to 900 �C in air at a heating rate of 10 �C /min(Figure 8a). Three weight loss steps were observed. The firstweight loss that occurred between 25 to ∼150 �C is gradualand corresponds to the combined loss of coordinated anduncoordinated water molecules, which is an endothermicprocess, as seen by DTA. The relatively low temperature for

Figure 5. View of the self-assembly of the SBUs to a TBU, archi-tecturing a nanoball type cavity.

Figure 6. View of hydrophilic channels (larger voids) and hydro-phobic channels (with smaller voids, where BF4

- anions arestranded) formed by the 3D expansion of TBU (projection alongthe a axis).

Figure 7. View of the nanoballs along the c axis.

Article Crystal Growth & Design, Vol. 10, No. 4, 2010 1803

the loss of water molecules (below 60 �C) reveals their loosecoordination. Although desolvation is complete around100 �C, the compound continues to lose weight until theonset of a next decomposition step. So awell-defined plateauwas not reached and thereafter decomposes in two distinctsteps which are both exothermic as indicated by DTA. Thisbehavior corresponds to a slow collapse of the network uponguest removal.

A crystalline sample was also subjected to a TGA-DTAanalysis at 1 �C/min following the thermal cycle (25 �C f150 �C f 25 �C) (Figure 8b). After dehydration, a blue-turquoise compound was obtained and kept at room tem-perature and ambient air overnight. Surprisingly, althoughthere was no indication of moisture/water gain during theTGA cooling run, this sample regains its original dark bluecolor. We reran a TGA on this moisture absorbed sample toevaluate the amount ofwater uptake.Approximately 50%oftotal water lost in the first TGA run (Figure 8b) was taken upby the compound. When the desolvated compound wasresuspended in water, it immediately turned back to a darkblue color and followed the same thermal decompositionpattern as that of the fresh sample, thus indicating a completeresolvatation of C1 (Figure 8a).

2.5. Sorption Studies of C1. A gas sorption study for C1was carried out using N2(g) and H2(g). Before measurements,the crystalline sample was vacuumed either with a gradualincrease of temperature or rapid heating until 150 �C, thetemperature corresponding to the end of dehydrationaccording to TGA (Figure 8b). Within the relative pressurerange investigated, the N2(g) adsorption measurements didnot show any uptake of nitrogen, thusmaking impossible thedetermination of the specific surface area by routine BETcalculation. An adsorption was however observed for H2(g)

suggesting no preference for N2(g). This adsorption differ-ence between these two molecules could be related to theirsize, their kinetic diameter being 2.8 A (H2(g)) and 3.6 A(N2(g)).

41 The H2(g) adsorption of 1.8 and 2.7 cm3/g for thefast and slow outgassing, respectively, is irreversible, andseveral consecutive runs always lead to the same result. Theselow values could account for little surface adsorption asobserved for a related SBU system, for which a small N2(g)

sorption was recorded.21 These poor adsorption propertiescould also result from the breakdown of the network afterthermal treatment. Indeed, as seen from TGA, no stableplateau was observed after solvent loss at 150 �C (Figure 8a)and complete degassing was difficult to achieve.

In order to ascertain the framework rupture, we havestudied the X-ray powder diffraction pattern of the desol-vated compound fromTGA (which was heated slowly to 150at 1 �C/min) and of a sample which was rapidly heated to 150at 1 �C/s. It is compared with the diffractogram of air-driedfresh crystals. Interestingly, fast heating results in an amor-phization of the sample compared to slow heating and onlyvery weak signals are observed (Figure 9).

We then studied themorphology of these thermally heatedsamples by SEM (Figures 10 and 11). Deterioration ofcrystals is seen in slowly heated sample, while some crystalsare unaffected showing the effect of thermal gradient in thesample container (Figure 10).

More deterioration is observed for the rapidly heatedsample by SEM analysis (Figure 11). These results supportthe fact that crystallinity is affected with thermal pretreat-ment destabilizing the framework, although hardly anydifference is seen in the IR of these treated and fresh crystals(Figure S2, Supporting Information). This prompted us torevisit the gas adsorption measurement with mild pretreat-ment at ambient condition.

Figure 8. (a) TGA-DTA showing a complete thermal decomposition (10 �C/min) for C1. (b) TGA of desolvation profile for C1 (heated to150 �C with 1 �C/min, dotted line). The desolvated sampled was exposed to air for 12 h and remeasured by TGA (thick line).

Figure 9. Top: X-ray powder diffraction patterns of microcrystalsof C1 thermally treated compounds at 1 �C/min and 1 �C/s,respectively. Bottom: diffractogram of C1 as well as the calculatedone from its CIF file. Pronounced effect of thermal treatment onframework integrity is shown.

1804 Crystal Growth & Design, Vol. 10, No. 4, 2010 Naik et al.

The fresh crystals were evacuated overnight under avacuum without any warming treatment. Figure 12 showsan improved adsorption behavior of C1. H2(g) adsorptiongradually increases to 21 cm3/g with an increase of pressure.It not only bettered the earlier value but also shows narrowhysteretic reversibility. But again no preference for N2(g),which strengthens the hypothesis of size discrepancy dis-cussed above. Consequent runs do not follow the same trend.It reduces to ∼6-7 cm3/g in second and third runs. Thus,adsorption efficiency is not only reduced but also becomesirreversible. It is possible that, after the initial H2(g) sorptionwhere the increasing pressure flexes-out the network to openthe “gate” to accommodate the guest, it keeps the networkunder stress and the subsequent desorption probablypartially collapses the network while shrinking back. Theretention of almost half of the adsorbed gas (3 cm3/g) in thesecond or third run after depressurization at low tempera-ture, although less, is interesting and such retentions aresignificant in systems that show high sorption-desorptionproperty.42

Square -shaped openings discovered on the cubic crystalsurface of C1 stimulated us to investigate the “porositypartitioning” by Hg porosimetry.43 Intrusion curve andthe corresponding pore size distribution are displayed inFigure 13.

Figure 13a shows the volume of intruded Hg as a functionof applied pressure. We recall that pressure is inverselyproportional to the pore size according to Washburn equa-tion (high pressures being necessary to fill small pores by aninverted capillary effect).44 A fine analysis of Figure 13a

allows identification of several steps. Indeed, as the pressureis gradually increased and as soon as it corresponds to a poresize, mercury enters the pores and a new step is recorded.When considering the pore size distribution (Figure 13b) (byapplying theWashburn equation assuming constant contactangle and surface tension),44 the recorded steps can beunderstood as follows: the step between 1 and 5 psi corre-sponds to pores centered at∼50 μm, between 10 and 25 psi topores of about 6000 nm, between 30 and 80 psi to pores ofabout 2000 nm and the remaining uptake between 100 and500 psi reveals pores with sizes smaller than 350 nm. Afterthat, the curve reaches a plateau indicating that there are nosmaller pores present. The major contributions at 50 μm,6000, and 2000 nm would rather originate from interparti-cular porosity, whereas the pores of 350 nm could corre-spond to the voids identified by SEM (Figure 1). Theseresults illustrate the usefulness of Hg porosimetry for acomplete porosity characterization of C1.

3. Concluding Remarks

The endeavor to introduce transamination for triazolederivatization with aminoacid derivatives proceeds smoothly,yielding the target molecule in good yield. The crystal struc-ture of L2 reveals that the whole molecular packing issupported anddirected byH-bonding interactions. The amaz-ing nanoporous network constructed from this supramole-cular synthonprovedL2 to be an ideal scaffold for designing aMOF. Although it was understood from TGA that thethermal stability of C1 is not very high, SEM analysis was

Figure 10. SEM analysis of a TGA sample of C1 warmed at 1 �C/min. (a) View of damaged crystals. (b) Some unaffected C1 crystals (shownwith arrow mark) are also seen.

Figure 11. SEM analysis of a sample of C1 warmed up to 150 at1 �C/s. (a) General view. (b) Enlarged view. Deterioration is clearlyvisible. Figure 12. Adsorption anddesorption isotherms at 77KofH2(g) on

C1, degassed at room temperature.

Article Crystal Growth & Design, Vol. 10, No. 4, 2010 1805

very informative to clarify the texture alteration of thecrystals and to understand the poor adsorption properties.The “soft” skeleton of the ligand is not robust enough towithstand high temperature treatment unlike their unsubsti-tuted counterparts.23a

This report highlights a facile organic synthetic methodol-ogy and introduces a multifunctional “bioinspired” synthonin the booming field of MOFs. This opens up a new way ofheterocyclic derivatization in amino acid chemistry. This isparticularly intriguing because amino acid derivatives canoffer a virtually unlimited structural diversity. This tailoredsynthon undergoes stunning self-assembly to form a 3D arrayof nanoball architecture. The nanoporous void obtained inthe present system is the artifact of angular framework of theligand, thus hinting that fine-tuning of the void shape and sizeshould be possible with variation of themethylene ankle in theligand and even impose network interpenetration. Sinceaxially bound water molecules can be easily removed, notnecessarily thermally, it could possibly allow access to theLewis acid sites for catalytic applications as nanoreactors forcatalysis or anion/cation recognization. Another appealingperspective in the context ofmolecularmagnetismwouldbe todesign a switchable magnetic MOF since preliminary experi-ments have shown that L1 was appropriate to set a suitableligand field strength for FeII spin crossover behavior.45 Alongthis line, the investigation of the magnetic properties of C1would be a valuable addition potentially in the frame of spinfrustration.46

4. Experimental Section

Syntheses. All reagents and solvents were used as received fromcommercial sources: benzene (Fluka analytical), SOCl2 (Sigma-Aldrich), glycine ethyl ester hydrochloride (ACROS), and aq.Cu(BF4)2 from “Ventron”.

Synthesis of Ethyl 4H-1,2,4-Triazol-4-yl-acetate (L1). N,N-Di-methylformamide azine dihydrochloride (1) and its free base (2)were obtained following the reportedmethod.31,47 Compound 2wasrecrystallized twice from benzene with charcoal and used in trans-amination reactions (Scheme 1). The reaction was carried out in a dryround-bottomed flask fitted with a reflux condenser. To a suspen-sion of glycine ethyl ester hydrochloride (1.17 g, 8 mmol, 1.2 equiv)in benzene (30 mL) at approximately 60 �C was added solid 2 (1 g,7 mmol, 1 equiv) with stirring. The mixture was refluxed then for24 h with vigorous stirring. The reaction can be monitored byrunning thin layer chromatography at intervals of time. Finally,solvent was removed under a vacuum and chromatographic puri-fication (SiO2, CH2Cl2f 5% isopropanol in CH2Cl2) of the yellowoil gave a cream solid. Yield 700 mg (65%), m.p 93 �C. Rf 0.25 (5%isopropanol in CH2Cl2).

1H NMR (300 MHz, CDCl3, 298 K):δ = 8.50 (s, 2 H), 5.08 (s, 2 H), 4.20 (q, 2H, J = 7.11 Hz), 1.25

(t, 3H, J= 7.11). 13C NMR (300 MHz, CDCl3 298 K): δ = 169.4,146.2, 63.9, 47.6, 15.0. MS: m/z = 156.12 (M þ Hþ). FTIR (KBrdisk, in cm-1): 3112(s), 1743(vs), 1539(s), 1236(s), 1215(s), 1018(s),636(s). Anal. Calcd. for C6H9N3O2 (155.16): C, 46.45, H, 5.85, N,27.08; Found C, 46.34, H, 5.91, N, 26.88.

Synthesis of 4H-1,2,4-Triazol-4-yl Acetic Acid (L2). Acid hydro-lysis of ethyl 4H-1,2,4-triazol-4-yl-acetate (L1), refluxing with 6 NHCl for 6-8 h, and drying under reduced pressure gave a colorlessoil. This residue was coevaporated with water to give L2 as a whiteprecipitate. m.p. 161 �C (lit,29 161 �C). The compound was driedunder a vacuum. Yield 90%. It was recrystallized from MeOH toobtain single crystals suitable for X-ray diffraction. This approachhas substantially improved the yield of L2 compared to our earliermethod where compound analysis was fully described.29

Synthesis of [Cu3(μ3-O)(L2)3(H2O)3]BF4 3H2O (C1). L1 (0.1 g)was dissolved in 8 mL of water and heated to 60 �C and to this wasadded aq. Cu(BF4)2 (0.5 mL, excess) with stirring. The smallquantity of blue precipitate that appeared was redissolved by theaddition of 2 mL of DMF. To this blue solution, NaOH (1 equiv) inwater (5 mL) was added with vigorous stirring, and the dark bluemixture thus obtained was nearly boiled for 5 min and allowedto cool to room temperature. After 4-5 days, 35 mg dark bluecubes were obtained (C1). They were filtered, washed with water,and dried in air. A crystal from the mother solution was used forX-ray crystallographic analysis. Yield: 35%. Anal. Calcd forC2H2N1.5O2.17B0.25FCu0.5 3H2O (C1) C, 15.66; H, 1.3; N, 13.70;Cu, 20.20%. Found: C, 15.18; H, 3.54; N 12.92; Cu, 20.72%. FTIR(KBr disk): 3465 (br), 1650 (br,s), 1384 (s), 1306 (m), 1091 (m), 698(w), 636 (w), 406 (w) cm-1. Diffuse reflectance spectrum (λmax, vsPTFE): 667, 321, 249 nm. When the same reaction was carried outwith L2, a dark green solution with sky blue precipitate wasobtained which was insoluble in common solvents and no furtheranalysis on this precipitate was carried out as we decided to focusonly on crystal samples. The slow evaporation of the filtrate led toisolation of a small quantity (10 mg) of dark blue crystals (C1) in2 weeks that were filtered and air-dried. For unambiguous adsorp-tion measurements, it was essential to work with a pure compound,preferably a crystal sample in large amount. Multiple small scalesynthesis led us to isolate 300 mg of crystal sample, which was atedious task. SEManalysis fromvarious batches shows the presenceof microcrystals of cubic shape but of various sizes (Figure 1).Majority of the crystals are of roughly 50 μm3, while crystals of15 μm3 were also found. It was necessary to confirm the identityof these submicrocrystals to properly study the large batch. IR ofboth types of crystals is exactly the same. An energy dispersiveX-ray (EDX) coupled to a SEM performed on gold-coated 15 μm3

crystals shows a constituent pattern similar to that of large crystals(Figure S3, Supporting Information).

Physical Measurements.1H and 13CNMR spectra were recorded

at 300 MHz on a Bruker AC 300 instrument. IR spectra wererecorded on a Shimadzu Benelux FTIR-84005 spectrometer usingKBr discs at r.t. between 4000 and 400 cm-1. Raman spectra with1064 nm excitation were recorded between 2300 to 400 cm-1 with aBruker RFS 100/s FT-Raman spectrometer (I = 200 mW) at r.t.using a diode-pumped, air-cooled Nd:YAG laser as the excitation

Figure 13. Intrusion curve (a) and corresponding pore size distribution (b) with a logarithmic scale for C1 as observed by Hg porosimetry.

1806 Crystal Growth & Design, Vol. 10, No. 4, 2010 Naik et al.

source. Diffuse reflectance spectra were recorded with a CARY 5Espectrophotometer using Teflon as a reference. TGA-DTA ana-lyses were performed in air at a heating rate of either 10 K/min (298to 1173 K) or 1 K/min (298 to 423 K) using an Universal V2.5H TAInstruments. Powder X-ray diffraction patterns were recorded on aSiemensD5000 counter diffractometer workingwith aKR radiationat 293 K. CHN analyses were performed at the University CollegeLondon (UK). Melting points were determined with an oil bath3937-S Buchi device. SEM coupled with EDX were performed witha Gemini Digital Scanning Microscope 982 with 1 kV acceleratingvoltage using an aluminum sample holder. For EDXmeasurementssamples were coated with a thickness of 25 nm of gold to make itconductive using 208 HR Cressington Sputter coater. The gasadsorption study for C1 was carried out on a Micromeritics ASAP2010 porosimeter. Prior to analyses and in order to remove eventualsurface adsorbed/coated DMF after synthesis, fresh crystals of C1were soaked in water and filtered under a vacuum. Samples of60-100 mg were then outgassed under a vacuum at 150 �C or atroom temperature. Both N2(g) and H2(g) adsorption-desorptionisotherms were recorded at 77 K in a relative pressure rangefrom 0.0010 to 0.995 (with respect to N2(g)). The same sampleswere subjected to Hg porosimetry measurements using a Micro-meritics Autopore IV. A penetrometer with an appropriatestem volume was chosen in order to record significant Hg intrusion.Prior to analysis, the sample was evacuated at r.t. and Hg was thenforced into the sample under pressure up to 30000 psi (corres-ponding to aminimumpore size of 6 nm according to theWashburnequation44).

Single Crystal X-ray Analyses. The X-ray intensity data for L2and C1 were collected at 120 K with a MAR345 image plate usingMoKR (λ = 0.71069 A) radiation. The crystals were selected,mounted in inert oil, and transferred to the cold gas stream forflash cooling. The unit cell parameters were refined using all thecollected spots after the integration process. The data were notcorrected for absorption, but the data collection mode partiallytakes the absorption phenomena into account. The structures weresolved by direct methods and refined by full-matrix least-squares onF2 using SHELXL97.48a All the non-hydrogen atoms were refinedwith anisotropic temperature factors. For L2, all the hydrogenatoms were located by Fourier difference synthesis and includedin the refinement with a common isotropic temperature factor (B=0.025 A2). For C1, the H-atom (H3) of the triazole (H5 is symme-trical with H3) and those of the methylene group (H6A, H6B)was calculatedwithAFIX. TheHof the solventmolecules could notbe localized.At the end of the refinement theR valueswere relativelyhigh:R1= 0.090 for 922 observed reflections,R (all data)=0.0933,wR2 = 0.251. The option SQUEEZE of PLATON was used toobtain a better model giving quite lower R values (Table 3).

Acknowledgment. We acknowledge financial supportfrom IAP-VI (P6/17) INANOMAT, the Fonds National de laRecherche Scientifique (FNRS) (FRFC project no. 2.4508.08),from a Concerted Research Action of the “Communaut�eFranc-aise de Belgique” allotted by the Acad�emie UniversitaireLouvain.A.L�eonard thanks theFNRSBelgium forhis “Charg�ede Recherches” position. J. Marchand-Brynaert is a seniorresearch associate of the F.R.S-FNRS (Belgium).

Supporting Information Available: Diffuse reflectance spectrum,FT-IR, and EDX for C1. CIF files for L2 and C1. This material isavailable free of charge via the Internet at http://pubs.acs.org.

References

(1) (a)Desiraju, G. R.Angew. Chem., Int. Ed. 2007, 46, 8342. (b) Braga,D.; Maini, L.; Polito, M.; Scaccianoce, L.; Cojazzi, G.; Grepioni, F.Coord. Chem. Rev. 2001, 216-217, 225. (c) Braga, D.; Desiraju,G. R.; Miller, J. S.; Orpen, A. G.; Price, L. CrystEngComm 2002, 4,500. (d) Janiak, C. Dalton Trans 2003, 2781.

(2) (a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1989, 111, 5962.(b) Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213.

(3) (a) Li, H.; Eddaoudi, M.; O’Keeffe,M.; Yaghi, O.M.Nature 1999,402, 276. (b) Yaghi, O. M.; Li, Q. MRS Bull. 2009, 34, 682.

(4) (a) Chae, H.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.;Matzger, A.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523.(b) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe,M.; Yaghi, O. M. Science 2002, 295, 469.

(5) Kitagawa, S.; Kitaura, R.; Noro, S.-I.Angew. Chem., Int. Ed. 2004,43, 2334.

(6) (a) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem.Soc. 2006, 128, 3494. (b) Tranchemontagne, D. J.; Mendoza-Cort�es,J. L.; O'Keeffe, M.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1257.(c) Hayashi, H.; Cot�e, A. P.; Furukawa, H.; O'Keeffe, M.; Yaghi, O. M.Nat. Mater. 2007, 6, 501.

(7) (a) Murray, L. J.; Dinc�a,M.; Long, J. R.Chem. Soc. Rev. 2009, 38,1294. (b) James, S. L. Chem. Soc. Rev. 2003, 32, 276.

(8) Germain, J.; Fr�echet, J. M. J.; Svec, F. Small 2009, 5, 1098.(9) Xue, M.; Ma, S.; Jin, Z.; Schaffino, R. M.; Zhu, G. -Z.;

Lobkovsky, E. B.; Qiu, S. L.; Chen, B. Inorg. Chem. 2008, 47, 6825.(10) Wang, Z.; Kravtsov, V. Ch.;Walsh,R. B.; Zaworotko,M. J.Cryst.

Growth Des. 2007, 7, 1154.(11) (a) Saha, D.; Wie, J.; Deng, S. Int. J. Hydrogen Energy 2008, 33,

7479. (b) Saha,D.;Wei, J.; Deng, S.Sep. Purif. Technol. 2009, 64, 280.(12) Lee, J. Y.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.;

Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450.(13) Hwang, Y. K.; Hong, D. Y.; Chang, J. S.; Jhung, S. H.; Seo, Y. K.

Angew. Chem. 2008, 120, 4212.(14) Czaja, A. U.; Trukhan, N.; M€uller, U. Chem. Soc. Rev. 2009, 38,

1284.(15) Weitkamp, J.; Fritz,M.; Ernst, S. Int. J. Hydrogen Energy 1995, 20,

967.(16) Logar, N. Z.; Kau�ci�c, V. Acta Chim. Slov. 2006, 53, 117.(17) Yang, S. J.; Choi, J. Y.; Chae, H. K.; Cho, J. H.; Nahm, K. S.;

Park., C. R. Chem. Mater. 2009, 21, 1893.(18) Poirier, E.; Chahine, R.; B�enard, P.; Lafi, L.; Dorval-Douville, G.;

Chandonia, P. A. Langmuir 2006, 22, 8784.(19) Ding, B.; Yi, L.; Cheng, P.; Liao, D. Z.; Yan, S. P. Inorg. Chem.

2006, 45, 5799.(20) (a) Ouellette, W.; Hudson, B. S.; Zubieta, J. Inorg. Chem. 2007, 46,

4887. (b) Ouellette, W.; Li, H.; O'Connor, C. J.; Zubieta, J. Inorg.Chem. 2009, 48, 4655.

(21) Zhai, Q. G.; Lu, C. Z.; Chen, S. M.; Xu, X. J.; Yang, W. B. Cryst.Growth Des. 2006, 6, 1393.

(22) Seo, J. S.;Whang,D.; Lee,H.; Jun, S. I.; Oh, J.; Jeon,Y. J.;Kim,K.Nature 2002, 404, 982.

(23) (a) Zhang, J. P.; Chen, X. M. J. Am. Chem. Soc. 2008, 130, 6010.(b) Nicola, C. D.; Karabach, Y. Y.; Kirillov, A. M.; Monari, M.;Pandolfo, L.; Pettinari, C.; Pombeiro, A. J. L. Inorg. Chem. 2007, 46,221. (c) Casarin, M.; Corvaja, C.; Nicola, C. D.; Falcomer, D.; Franco,L.; Monari, M.; Pandolfo, L.; Pettinari, C.; Piccinelli, F. Inorg. Chem.2005, 44, 6265.

(24) (a) Mantion, A.; Massuger, L.; Rabu, P.; Palivan, C.; McCusker,L. B.; Taubert, A. J. Am. Chem. Soc. 2008, 130, 2517. (b)Wisser, B.;Chamayou, A. -C.; Miller, R.; Scherer, W.; Janiak, C.CrystEngComm2008, 10, 461. (c) Wisser, B; Lu, Y.; Janiak, C. Z. Anorg. Allg. Chem.2007, 633, 1189. (d) Ma, L. F.; Wang, L. Y.; Wang, J. G.; Wang, Y. F.;Feng, X. Z. Anorg. Allg. Chem. 2006, 632, 487.

(25) Rebilly, J. N.; Bacsa, J.; Rosseinsky, M. J. Chem. Asian J. 2009, 4,892.

(26) Ma,L.F.;Huo,X.K.;Wang,L.Y.;Wang, J.G.; Fan,Y.T. J. SolidState Chem. 2007, 180, 1648.

(27) Ingleson,M. J.; Bacsa, J.; Rosseinsky,M. J.Chem. Commun. 2007,3036.

(28) Storr, T.; Thompson, K. H.; Orvig, C. Chem. Soc. Rev. 2006, 35,534.

(29) Naik, A. D.; Marchand-Brynaert, J.; Garcia, Y. Synthesis 2008, 1,149.

(30) (a) Garcia, Y.; van Koningsbruggen, P. J.; Kooijman, H.; Spek,A. L.; Haasnoot, J. G.; Kahn., O. Eur. J. Inorg. Chem. 2000, 307.(b) Garcia, Y.; Bravic, G.; Gieck, C.; Chasseau, D.; Tremel,W.; G€utlich,P. Inorg. Chem. 2005, 44, 9723. (c) Habib, H. A.; Sanchiz, J.; Janiak,C.Dalton Trans. 2008, 1734. (d) Habib, H. A.; Hoffmann, A.; H€oppe,H. A.; Steinfeld, G.; Janiak, C. Inorg. Chem. 2009, 48, 2166.(e) Henninger, S. K.; Habib, H. A.; Janiak, C. J. Am. Chem. Soc.2009, 131, 2776. (f) Habib, H. A.; Sanchiz, J.; Janiak, C. Inorg. Chim.Acta 2009, 362, 2452.

(31) Bartlett, R. K.; Humphrey, I. R. J. Chem. Soc. C 1967, 1664.(32) Garcia, Y.; Kahn, O.; Rabardel, L.; Chansou, B.; Salmon, L.;

Tuchagues, J. -P. Inorg. Chem. 1999, 38, 4663.(33) Nakamoto, K. Infrared and Raman Spectra of Inorganic and

Coordination Compounds: Part B, 5th ed.; Wiley: New York, 1997.

Article Crystal Growth & Design, Vol. 10, No. 4, 2010 1807

(34) (a) Evans, O.R.; Lin,W.Acc. Chem.Res. 2002, 35, 511. (b) Holman,K. T.; Piovar, A. M.; Ward, M. D. Science 2001, 294, 1907. (c) Ahlers,R.; Ruschewitz, U. Solid State Sci. 2009, 11, 1058. (d) Craven, E.;Mutlu, E.; Lundberg, D.; Temizdemir, S.; Dechert, S.; Brombacher,H.; Janiak, C. Polyhedron 2002, 21, 553. (e) Seeber, G.; Tiedemann,B. E. F.; Raymond, K. N. Top. Curr. Chem. 2006, 265, 147.

(35) (a) Saha, B. K.; Nangia, A.; Nicoud, J.-F.Cryst. GrowthDes. 2006,6, 1278. (b) Desiraju, G. R. Crystal Engineering. The Design ofOrganic Solids; Elsevier: Amsterdam, 1989. (c) Toshiya, H.; Nuida,T.; Hashimoto, K.; Ohkoshi, S.-i.Cryst. Growth Design 2006, 6, 1736.

(36) Chawaleba,D.; Ilczyszyn,M.M.;Liczyszyn,M.;Ciunik, Z. J.Mol.Struct. 2007, 831, 119.

(37) Desiraju, G.; Steiner, T. The Weak Hydrogen Bond, In StructuralChemistry and Biology; Oxford University Press/International Unionof Crystallography: Chester, England, 2001.

(38) Yu, Q.; Zhang, X.; Bian, H.; Liang, H.; Zhao, B.; Yan, S.; Liao, D.Cryst. Growth Des. 2008, 8, 1140.

(39) Addison, A. W; Rao, T. N; Reedijk, J; Rijn, J. van; Verschoor,G. C. J. Chem. Soc., Dalton Trans. 1984, 1349.

(40) (a) Perry, J. J., IV; Perman, J.A.; Zaworotko,M. J.Chem.Soc.Rev.2009, 38, 1400. (b) Fletcher, A. J.; Thomas, M. K.; Rosseinsky, J. M.J. Solid State Chem. 2005, 178, 2491.

(41) Lee, H. J.; Cho, W.; Jung, S.; Oh, M. Adv. Mater. 2009, 21, 674.

(42) Zhao, X.; Xiao, B.; Fletcher, A. J.; Thomas, K. M.; Barshaw, D.;Rosseinsky, M. J. Science 2004, 306, 1012.

(43) (a) Giesche, H. Part. Part. Syst. Charact. 2006, 23, 9. (b) Giesche,H. In Handbook of Porous Solids; Sch€uth, F., Sing, K. S. W.,Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 2002, Vol. 1, 309.

(44) Washburn, E. W. Phys. Rev. 1921, 17, 273.(45) Dırtu, M. M.; Naik, A. D.; Marchand-Brynaert J.; Garcia, Y.

J. Phys. Conf. Ser. 2010, in press.(46) (a) Ferrer, S.; Lloret, F.; Bertomeu, I.; Alzuet, G.; Borras, J.;

Garcia-Granda, S.; Liu-Gonzalez, M.; Haasnoot, J. G. Inorg.Chem. 2002, 41, 5821. (b) Sarkar, B.; Ray, M. S.; Drew, M. G. B.;Figuerola, A.; Diaz, C.; Ghosh, A. Polyhedron 2006, 25, 3084. (c)Mukherjee, P.; Drew,M.G. B.; Estrader, M.; Diaz, C.; Ghosh, A. Inorg.Chim. Acta 2008, 361, 161. (d) Ray, M. S.; Chattopadhyay, S.; Drew,M. G. B.; Figuerola, A.; Ribas, J.; Diaz, C.; Ghosh, A. Eur. J. Inorg.Chem. 2005, 22, 4562.

(47) Fohlisch, B.; Braun, R.; Schultze, K. W. Angew. Chem., Int. Ed.Engl. 1967, 6, 361.

(48) (a) Sheldrick, G. M. SHELX97: Program for Crystal Struc-ture Refinement; University of G€ottingen: Germany, 1997. (b) Speck,A.L. PLATON, A Multipurpose Crystallographic Tool; Utrecht Uni-versity: Utrecht, The Netherlands, 2001, http://www.cryst.chem.uu.nl/platon/.