paddle-wheel zinc.pdf

Paddle-Wheel ZincCarboxylate Clusters asBuilding Units for Metal-Organic FrameworksSergei I. Vagin, Anna K. Ott, and Bernhard Rieger*

Dedicated to Professor Dr.-Ing. J. Weitkamp on the occasionof his 65th birthday

Growth of interest to a field of materials chemistry such as the metal-organic frameworks

(MOFs) demands the comprehensible surveying of new results, and the reviews systematiz-

ing and highlighting one or another trend in the MOF research appear periodically. Struc-

tural peculiarities of coordination polymers constructed by zinc paddle-wheel clusters and

the investigation of their microporosity are reviewed in detail to emphasize those features of

the materials, which are interesting both for the pure and applied chemists.

Keywords: Clusters, Metal-Organic Frameworks (MOFs), Microporosity

Received: April 10, 2007; Accepted: April 18, 2007

1 Introduction

Within the last decades, the interest towardscoordination polymers as microporous materi-als, often called metal-organic frameworks(MOFs), has immensely grown, as can be seenfrom a number of publications appearing an-nually and devoted to this field [1]. Such a highconcernment is mostly caused by a colossal po-tential of these materials for numerous appli-cations in modern science and technology. Theoverwhelming majority of these applicationsare, however, based on the ability of MOFs tobehave as hosts for certain molecules. MOFsand the related organic-inorganic hybrid mate-rials have already been tested as microporousmaterials for the storage of gases [2], as cata-lysts, sometimes showing the enantioselectiv-ity [3], as sensors for special classes of mole-cules [4], as active materials for non-linearoptics [5], as organic magnets [6], as materialsfor selective sorption from the gaseous and li-quid mixtures [7], etc. The versatility of attrac-tive features manifested by this class of materi-als is mostly due to the unlimited possibilitiesfor modification and fine-tuning their struc-tures and properties. In general, MOFs andother coordination polymers can be consideredas materials assembled with high order fromthe so called “secondary building units“

(SBUs) containing metal ions or their clusters[8], which are linked by polytopic or polyden-tate organic fragments (ligands or linkers) tobuild multidimensional nets or frameworks.

The process of MOF’s self-assembling is dri-ven by the formation of metal ion to ligand co-ordination bonds as well as weaker hydrogenbonds and Van-der-Waals interactions betweennon-metallic components. It is obvious, thatvariation of only the SBU’s nature should leadto a high number of different coordinationpolymers with diverse properties, while the de-sign of organic linkers allows the chemists tomultiply their number and functionality nearlyto infinite. In support of the said above, alreadyseveral hundreds structures of homoleptic andheteroleptic coordination polymers based onunsubstituted terephthalic acid were reportedin CSD up to now, and the number of allMOFs described to date is incommensurablyhigher. It is clear, that nowadays a comprehen-sive review covering all the reported MOFswould be nearly impossible to accomplish, andthere is more sense in doing a detailed surveydevoted to a narrow field of this type of chem-istry, which would be useful both for the spe-cialists and for those who begins the work inthis topic or is just interested in. The aim ofthe present survey was inspired by recent pub-lications [9], and is to summarize the achieve-

Within the last dec-ades, the interesttowards coordina-tion polymers as mi-croporous materials,often called metal-organic frameworks(MOFs), has immen-sely grown.

The process ofMOF’s self-assem-bling is driven bythe formation ofmetal ion to ligandcoordination bondsas well as weakerhydrogen bondsand Van-der-Waalsinteractions be-tween non-metalliccomponents.

Metal-Organic Frameworks (MOFs) 767Chemie Ingenieur Technik 2007, 79, No. 6

© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.de

DOI: 10.1002/cite.200700062

ments in coordination chemistry of polymericzinc carboxylates, in particularly constructedby binuclear Zn clusters with paddle-wheelgeometry.

2 Paddle-Wheel Clusters

Usually, binuclear metal clusters in which twometal ions are bridged by four carboxylicgroups in syn-syn mode are called in the litera-ture as “paddle-wheel“. A typical example ofpaddle-wheel complex is copper(II) acetate hy-drate [10], in which four acetate-anions areeach coordinated as dimonodentate ligands totwo copper(II) ions forming the D4h-symmetri-cal structure having the appearance of a pad-dle-wheel with four blades, in axial positions ofwhich the molecules of water are coordinated(see Fig. 1). The similar constructions employ-ing different bridging carboxylates are knownfor nearly all metal ions in periodic system,starting from magnesium(II) in its N,N-diphe-nylcarbamato hexamethylphosphortriamidecomplex [Mg2(Ph2NCO2)4(HMPA)2] [11] andfinishing with bismuth(II) in its trifluoroace-tates (see Tab. 1 for chemical structures of li-gands referred to in this review) [12].

Alteration of the size and geometry of coor-dination sphere from one metal in the periodicsystem to another results in some structuralvariations observed for their paddle-wheel clus-ters, such as absence of the axial ligands orpresence of the extra ones, formation of themultiple metal-metal bonds, etc. This is oftenrealized for the metals of 5-th and 6-th periods,among which, perhaps, the complexes of mo-lybdenum, rhodium and ruthenium are inves-tigated to the utmost. For these metals, thepaddle-wheel structures with lowered symme-try are also known. The symmetry lowering toC2v or D2h can occur, for example, when twodifferent bridging ligands are used for the pad-

Figure 1. Structural model of copper(II) acetatemonohydrate paddle-wheel complex.

Abbreviation Name Structural formula

DMF N,N′-dimethyl-formamide N

O

DMSO Dimethylsulfoxide

S

O

HMPA Hexamethylpho-sphor-triamide

P

O

NN

N

1,2-Dimethox-yethane

OO

DABCO Triethylenediamine{1,4-diazabicy-

clo[2,2,2]octane}N N

Py PyridineN

4-MeOPy 4-MethoxypyridineN O

Lut 3,4-Lutidine

N

iQ Isoquinoline

N

Q Quinoline

N

BIPY 4,4′-BipyridineN N

BPE Trans-1,2-bis-(4-pyridyl)-

ethylene N

N

1,2-Bis-(4-pyridyl)-ethane

N

N

DPNI N,N′-bis(4-pyridyl)-1,4,5,8-naphthalene-tetra-carboxydiimide

N N

O

O

O

O

N N

1,4-Bis(1,2,4-triazol-1-yl)-butane

N N

NNN

N

OAc Acetate O

O

-

Trifluoroacetate O

O F

F

F

-

Table 1. Structure and name abbreviation of ligands referred to in the text.

768 Chemie Ingenieur Technik 2007, 79, No. 6Übersichtsbeiträge

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dle-wheel construction [13], whereas C4v-syme-trical cluster will arise if the axial ligands aloneor together with the oxidation states of metalions are non-equivalent [14]. For the metalswith high coordination numbers such aslanthanides even higher decrease of the sym-metry can take place due to the coordination ofadditional axial ligands and distortion of thepaddle-wheel structure (see for example [15]).

Paddle-wheel structural motifs can be foundboth in the discrete (molecular) coordinationcompounds and in the infinite coordinationpolymers such as 1D chains, 2D nets, and 3Dframeworks. 2D and 3D MOFs based on thecarboxylate paddle-wheel SBUs are known, forexample, for cobalt ([Co2(2,6-NDC)2(BPE)]n�nC6H6�nH2O, where 2,6-NDC denotes 2,6-naphthalenedicarboxylate and BPE denotestrans-1,2-bis(4-pyridyl)ethylene) [16], copper([Cu3(BTC)2(H2O)3]n, where BTC denotes ben-zene-1,3,5-tricarboxylate) [17], cadmium([Cd2(3,3′-BPDC)2(DPE)]n, where 3,3′-BPDCdenotes biphenyl-3,3′-dicarboxylate) [18],lanthanides [19] and other metals besides zinc.The latter, among few other ions, which alsoexhibit the coordination number ≤ 6, is consid-ered to have a flexible coordination sphere.Zinc can easily adopt the coordination geome-try as well as form clusters to fit the minimumenergy according to the requirements of li-gands and solvent surrounding in addition toother conditions. This becomes apparent fromthe number of different zinc carboxylate clus-ters observed in MOFs, for example. Alreadyterephthalic acid along or in combination withother ligands gives a variety of MOFs based onsingle zinc ions [20], different binuclear Zn2

clusters including paddle-wheel structures[21], diverse trinuclear- [22], tetranuclear- [23],pentanuclear- [24], and clusters with higher Znion number, finishing with the infinite l-(OCO) and l-O bent bridged Zn-rods [25]. Aswas already said above, we will try here to sum-marize the peculiarities of MOFs utilizing Zn-carboxylate paddle-wheel clusters as SBUs.MOFs constructed by binuclear Zn2 clusters inwhich less then three carboxylate groups arebridging two metal ions in syn-syn mode {e.g.,(l-OCO)2(l-O)Zn2} will not be consideredfurther as being only distantly related to pad-dle-wheel structures.


Crotonate O

O

-

Tiglate O

O

-

Benzoate O

O

-

2-Chlorobenzoate

O

O

Cl

-

Fumarate O

O O

O

--

BDC 1,4-Benezenedi-carboxylate

(terephthalate)

O

O

O

O

- -

ABDC 2-Amino-terephthalate

O

O

O

O

NH2

- -

TBDC Tetramethyl-terephthalate O

O

O

O

- -

F4BDC Tetrafluoro-terephthalate O

O

O

O

F F

F F

- -

CB-BDC Dihydro-cyclobuta[1,2-b]-

terephthalateO

O

O

O

- -

1,3-BDC 1,3-Benezene-dicarboxylate(isophthalate)

O

O O

O- -

BTC Benzene-1,3,5-tri-carboxylate(trimesitate)

O

O O

O

O O

- -

-

2,6-NDC 2,6-Naphthalene-dicarboxylate

O

O

O

O

-

-

Table 1. Continued.



3 Coordination Polymers with ZincPaddle-Wheel SBUs

3.1 “Three-Blade“ Paddle-Wheels:Structure

Two types of binuclear zinc clusters will be re-ferred further as paddle-wheels (PW), namelythose symmetrically bridged by three (three-blade PW-3) and by four (four-blade PW-4) car-boxylates in syn-syn mode.

Clusters of the first type (see Fig. 2) are rela-tively often observed in coordination polymersbased on monocarboxylic acids, both aliphatic(crotonic, tiglic, etc. [26]) and aromatic (ben-zoic, 2-chlorobenzoic [27]), and more rare asmolecular units [28]. Extension of such clus-ters into 1D polymeric chain occurs via axialcoordination either with a hydroxide (l-OH,[27b]) or with a carboxylate group in syn-antibridging mode, which makes such polymericchains structurally related to infinite zinc rodclusters. All zinc ions have tetrahedral coordi-nation sphere in these polymers. Anti-anti car-boxylate bridging mode can also take place, asfor example in case of catena-(tris-(l2-trifluor-oacetato)-l2′-trifluoroacetato-bis-(dimethox-yethane)-dizinc(II)) [29], where it is probablypromoted by the presence of additional axial li-gands (dimethoxyethane) at the PW cluster. Allzinc ions have pseudo-octahedral coordinationsphere in this polymer, with Zn-Zn distance inthe cluster being the highest among all re-ported PW-3 Zn compounds. Dense packing ofsuch 1D polymers in the bulk of the crystals re-sults in no noticeable porosity, and no proper-ties related to host-guest interactions are re-ported for these materials.

Similar Zn2 cluster structures were observedin several 3D coordination polymers, whichwere formed from polycarboxylic acids such astrimesic (H2BTC) [30], adamantanetetracar-boxylic (H2ATC) [31], tetramethylterephthalic(H2TBDC) [32a], and 2-aminoterephthalic(H2ABDC) [32b]. In the case of a 3D coordina-tion polymer [Zn2(TBDC)2(H2O)1.5(DMF)0.5�(DMF)(H2O)]n, (DMF = N,N-dimethylforma-mide) known also as MOF-47 [32a], the three-blade PW-3 Zn2 SBU is formed by four acidmolecules and one additional neutral axial li-gand, e.g., water. Three carboxylate groups ofthree different acid molecules are bridging twozinc ions in a dimonodentate fashion, and thefourth TBDC is axially coordinated to one ofthe zinc ions in a monodentate anti mode. Ex-tension of the structure based on this SBU toform the infinite polymer reveals two types ofconnectivity of TBDC: one unit exhibits a bis-dimonodentate coordination to zinc centers,whereas the other TBDC unit coordinates in


1,4-NDC 1,4-Naphthalene-dicarboxylate

O

O

O

O

- -

BPDC Biphenyl-4,4′-dicarboxylate

O

O

O

O

- -

3,3′-BPDC Biphenyl-3,3′-dicarboxylate

O

O

O

O

-

-

DPA Biphenyl-2,2′-dicarboxylate(diphenate)

O

O

O

O

-

-

CDC 1,4-Cubane-dicarboxylate

O

O

O

O

- -

ATC Adamantanetetra-carboxylate

O

O

O

O

OO

O O

- -

-

-

MTB 4,4′,4′′,4′′′-Tetra-phenyl-methane-tetracarboxylate

O

O

O O

O

O

OO

- -

-

-

Ph2NCO2 N,N-diphenyl-carbamate

O O

N

-

Table 1. Continued.



dimonodentate-monodentate fashion. Thisleads to the double layer structural motif of thepolymer with the tetrahedral Zn2-based SBU(see Fig. 3) [32a].

If axial ligands are not considered, the sym-metry of such cluster would ideally be D3h withZn-O-C-O-Zn bound atoms lying nearly inone plane. This is not the rule, however, andthe symmetry of such clusters can be oftenlowered due to the noticeable out-of-planetorsion and in-plane tilting of bridging car-boxylic groups. Additionally, depending onthe axial ligands, different connectivity ofPW-3 Zn2 SBUs can be observed. Thus,in [Zn2(BTC)(NO3)�(H2O)(C2H5OH)5]n [30a],called also MOF-4, the axial positions in Zn2

cluster are occupied by non-bridging nitrateand ethanol ligands, resulting in the triangularconnectivity of this SBU via the BTC units (seeFig. 4). Combined with the triangular connec-tivity of the BTC linkers, this leads to a 3D por-ous network, guest binding properties ofwhich will be discussed in the subsequentchapter.

Coordination polymer [Zn4(ABDC)3(NO3)2

(H2O)2]n (PNMOF-3) [32b] shows zinc PW-3cluster structure similar to MOF-4 with axiallycoordinated nitrate and aqua ligands. The zincions in the cluster are syn-syn bridged by threeABDC units (paddle-wheel blades), and theclusters are linked through linear ABDC strutsinto infinite hexagonal grid. The staking ofgrids generates large channels (14.9 Å) ortho-normal to the layer, which are filled with sol-vent.

The structure of the PW-3 cluster in MOF-35([Zn2(ATC)�(C2H5OH)2(H2O)2]n) [31] is similarto that of MOF-47, with the only differencethat ATC unit in axial position coordinates viaa carboxylic group in monodentate syn mode,and the opposite axial position is occupied byethanol instead of water, resulting in a dis-torted tetrahedral shape of such SBU. Assem-bly of such SBUs via tetrahedral four-connect-ing ATC linker gave a 3D-network with

Figure 2. Structural model of a typical zinc three-blade paddle-wheel cluster ex-tended into 1D polymeric chain via the axial bridging carboxylate in syn-anti coordi-nation mode (left), and the possible dimonodentate (non-chelating) coordinationmodes of acetate (right). Pink is zinc, green is any metal, red is oxygen, cyan is carbon.

Figure 3. Structure of zinc PW-3 SBU in MOF-47, its representation as a tetrahedronand extension into a 3D structure [32a]. Reproduced by permission of The Royal So-ciety of Chemistry.

Figure 4. PW-3 (a) and its representation with triangular building units (b) in MOF-4.Reprinted in part with permission from [8]. Copyright 2001 American Chemical So-ciety.

Figure 5. PW-3 in MOF-35(left), its representation withpolygons (middle) and theassembly into a frameworkwith hexagonal channels(right). Reprinted in partwith permission from [31].Copyright 2001 AmericanChemical Society.



internal cavities of 5 Å filled with ethanol andwater guest molecules (see Fig. 5).

Another interesting network USF-4([Zn6(BTC)4(iQ)4(MeOH)2�(MeOH)8(C6H5Cl)]n,where iQ denotes isoquinoline) was recentlyreported [30b], formed by a combination oftriangular, square and pseudo-tetrahedralbuilding blocks (ternary 3,4-connected net)in a ratio 4:1:2, respectively. The square andtetrahedral units were represented by PW-4and PW-3 zinc clusters correspondingly, andthe triangular unit was BTC. The structureof the PW-3 SBU in this coordination poly-mer differs from that in MOF-35 only bythe nature of neutral ligands at one of theaxial positions.

3.2 Guest Binding Properties of the“Three-Blade“ PW-Based MOFs

The tendency of nature to avoid empty volumein the course of self-organization in condensedmatter, e.g., in the crystal growth processes, isquite understandable as it corresponds to thelowering of total energy in the system. For thisreason all potentially porous MOFs are formedas networks filled with guest molecules, andthis is also true for the described above MOF-4,MOF-35, MOF-47, PNMOF-3, and USF-4.Among these five, MOF-4 is the only studiedin detail for the guest removal and binding.Although MOF-35 and PNMOF-3 were foundto have internal cavities with guest molecules,no further investigation on the guest removaland related processes in these materials wasundertaken. For MOF-47 only the thermogravi-metric analysis (TGA) was carried out, show-ing a decrease of weight (21 %) in the range 50to 90 °C and corresponding to the loss of oneand half DMF and one water molecule perstructural unit [32a]. Coordination polymerUSF-4 is reported to possesses 32 % free vo-lume filled with guest molecules such asmethanol and chlorobenzene. TGA and tem-perature dependent PXRD (powder X-ray dif-fraction) spectra were measured for this mate-rial, with the latter indicating a substantial lossof crystallinity already at 130 °C.

As-synthesized MOF-4 ([Zn2(BTC)(NO3)�(H2O)(C2H5OH)5]n) has channels filled withunbound ethanol and water molecules (one ofeach per formula unit), removal of whichwould lead to open pores with approximately9 Å diameter (see Fig. 6).

Further removal of bound ethanol moleculeswould extend the channels to 14 Å in diameterand lead to coordinative unsaturated zinc moi-eties, which would be highly desirable forsome applications, e.g., catalysis and selective

binding of small molecules. However, the poreconstriction is likely to take place upon re-moval of guest molecules from this material,as no N2, Ar, or CO2 sorption isotherms couldbe measured for it [33]. On the other hand, anuptake of ethanol expressed by Type V iso-therm was found for this material, related tocoordinative sorption mechanism followed bypores filling (see Fig. 7).

Such behavior can be referred, according toclassification of Kitagawa [34], to flexible mi-croporous coordination polymer of third gen-eration with healing or breathing type pores,although the structure of the guest-freeMOF-4 material was not investigated. Partiallydesolvated MOF-4 with a compositionZn2(BTC)(NO3)�(H2O)0.5(C2H5OH) seems tomaintain the network structure of as-synthe-sized material, since its PXRD pattern remainsthe same with only slight broadening of thelines [30a].

Figure 6. View along the diagonal axis in the cubiccell of MOF-4 [8], showing the open channelsthrough several elementary cells. Hydrogen atomsand unbound solvent molecules are omitted forclarity, disorder of the nitrate ligand is maintained.Created from the data supplied by the CambridgeCrystallographic Data Centre.

Figure 7. Ethanol sorption isotherm of MOF-4after complete removal of guest and bound sol-vent molecules. Reprinted in part with permissionfrom [33]. Copyright 2000 American ChemicalSociety.

The tendency ofnature to avoidempty volume inthe course of self-organization in con-densed matter, e.g.,in the crystalgrowth processes, isquite understand-able as it corre-sponds to the low-ering of total en-ergy in the system.



As-synthesized MOF-4 exhibits specificguest exchange upon immersing into multi-component toluene solutions. Only low mole-cular weight alcohols (homologues) and DMFcan participate in such exchange processes incontrast to many other tested compounds, aswas confirmed by solid state 13C NMR of MOFsamples and by GC analysis of the solutionsbefore and after interaction with MOF-4.

Host-guest interactions in MOFs based onfour-blade PW-4 zinc clusters are not less inter-esting, but first let us consider some structuralaspects of such coordination polymers.

3.3 Structural Features of MOFs with“Four Blade“ Paddle-Wheel SBUs

The paddle-wheel PW-4 cluster of zinc in mole-cular coordination compounds is observed re-latively often and was reported for the firsttime already more than 20 years ago for te-trakis-(l-crotonato)-bis-(quinoline)-dizinc [35],with the structure similar to that of copper(II)acetate monohydrate. The average distance be-tween two zinc ions in PW-4 Zn2 cluster is no-ticeably shorter than in the three-blade PW-3clusters (approx. 2.9 to 3.0 Å versus 3.2 to 4.0Å) and each zinc ion has square-pyramidal co-ordination sphere corresponding to a coordina-tion number five.

It is not surprising that this structural unitbecame a relatively common building block incoordination polymers, as it can act as two-con-necting (bipolar SBU), four-connecting (squareand pseudo-square SBU) or six-connecting (oc-tahedral and pseudo-octahedral SBU) structur-

al fragment (see Fig. 8). Two-connectivity isachieved if monocarboxylic acids are bridgingtwo zinc ions in the cluster, while bidentateneutral molecules, e.g., 1,2-bis-(4-pyridyl)-ethane or 1,2-bis-(4-pyridyl)-ethylene, are act-ing as linkers at axial positions, thus leading to1D polymeric chains [36].

Four-connecting Zn2 PW-4 are usually con-structed by polycarboxylic acids and non-brid-ging (typically monodentate) axial ligands.Combination of such building units with line-ar ditopic connectivity of corresponding dicar-boxylic acids results in square grid 2D-planarframeworks with 44 regular tiling topology[37], represented, for example, by MOF-2([Zn2(BDC)2(H2O)2�(DMF)2]n, where H2BDCdenotes terephthalic acid [38]) and its closeanalogues [Zn2(BDC)2(DMSO)2�(DMSO)5]n[39] (where DMSO denotes dimethylsulfoxide)and [Zn2(BDC)2(DMF)2]n [40]. The nature ofaxial ligands and of inclusion molecules inthese three parent MOFs appeared to be essen-tial in defining their bulk structure, e.g., bycontrolling via hydrogen bonding and Van-der-Waals interaction the degree of rectangular dis-tortion of 2D layers and the deviation of PW-4symmetry from ideal (D4h), as well as deter-mining the offsets of neighboring layers andthe distances between them [40].

Other examples of 2D 44 tiling nets based onzinc paddle-wheel PW-4 clusters are:[Zn2(ABDC)2(DMF)2�(C6H5Cl)0.5]n (MOF-46,where H2ABDC denotes 2-amino-terephthalicacid) [32a], [Zn2(2,6-NDC)2(Lut)2]n (where 2,6-H2NDC denotes 2,6-naphthalenedicarboxylicacid, Lut denotes 3,4-lutidine) [41] and its ana-logues [Zn2(2,6-NDC)2(DMF)2�(C6H5Cl)]n

Figure 8. Connectivity of zincpaddle-wheel clusters and thesimplest framework topolo-gies based on them.

The paddle-wheelPW-4 cluster of zincin molecular coordi-nation compoundsis observed rela-tively often andwas reported forthe first time al-ready more than20 years ago.



(MOF-105) [42], [Zn2(2,6-NDC)2(DMF)2�(solv)x]n (where solv denotes DMF, C6H6, to-luene or p-xylene) [43], as well as[Zn2(CDC)2(DMF)2�(DMF)2(C6H5Cl)]n (MOF-104, where H2CDC denotes 1,4-cubanedicar-boxylic acid) [42], [Zn2(CB-BDC)2(H2O)2�(H2O)3(DMF)1.8]n (MOF-103, where H2CB-BDC denotes dihydrocyclobuta[1,2-b]terephtha-lic acid) [42] and [Zn2(BPDC)2(DMSO)2�(DM-SO)4]n (where H2BPDC denotes 4,4′-biphenyl-dicarboxylic acid) [44].

The latter shows especially big grid cavities(approx. 15×15 Å), which create upon staggeredstacking of 2D sheets the rectangular channels(approx. 7.5×15 Å) filled with DMSO molecules.In case of MOF-103 and -105 a noticeable corru-gation of 2D-layers was observed, caused by thelowered symmetry of organic linkers [42].

Coordination polymers constructed fromzinc paddle-wheels and non-linear dicarboxylicacids, such as isophthalic (1,3-H2BDC) with120° linking angle, can give several net topolo-gies different from planar 44-grid motif. Thus,strongly ruffled or undulating 2D 44-connectednets can be formed in course of assemblingzinc paddle-wheel PW-4 clusters by isophtha-late, as represented, for example, by a series ofparent metal-organic frameworks [Zn2(1,3-BDC)2(L)2�(solv)x]n, where L can be pyridine orsome of its substituted derivatives as well asisoquinoline, and solv denotes benzene, nitro-benzene or o-dichlorobenzene [45]. However,besides the difference in axial ligands andguest molecules, some structural variation of2D-layers was observed in these coordinationpolymers. The latter can be easily understoodwhen considering the tetragonal supra-SBUs(nSBUs) constructed by four paddle-wheelclusters, the shapes of which resemble to ca-lix[4]arenes and for which four atropoisomerswith cone-, partial cone-, 1,2-alternate- and 1,3-alternate-form can be drawn (see Fig. 9). Threetypes of 2D-layers with different kind of undu-lation were observed in the mentioned aboveMOFs, composed either from pure 1,2-alter-nate shaped nSBUs, pure partial cone shapednSBUs, or from combination of cone and 1,3-alternate shaped nSBUs. Upon staking thelayers, this becomes at least a partial reason fordifferent potentially solvent-accessible areascalculated for these polymers [45b].

Combination of isophthalate and Zn2 PW-4can also lead to an undulated trigonal 2D net-work (Kagome lattice, see Fig. 10) with 3262 to-pology, as observed for [Zn2(1,3-BDC)2(4-MeO-Py)2�(guest)x]n (where 4-MeOPy denotes 4-methoxypyridine and the nature of guest is notspecified). This network can be represented asconstructed by triangular nSBUs consisting ofthree paddle-wheel clusters connected by 1,3-BDC. Staking of such 2D-layers generates hex-agonal channels with effective diameter 9.3 Å,and potential free volume of this MOF is esti-mated to be 46.3 % upon removal of all guestmolecules [45b]. Three-dimensional coordina-tion polymer with complex 65.8 net topologyconstructed from 1,3-BDC and Zn2 PW-4,namely [Zn2(1,3-BDC)2(Q)2�(C6H5NO2)]n(where Q denotes quinoline), was also reported[45b, 46]. Although the compound does notpossess notable pores, the potential solvent-ac-cessible area was calculated to be 17.3 % uponremoval of guest nitrobenzene molecules.

Diphenic acid (H2DPA) belongs to non-line-ar ditopic linkers as well. So far, only one coor-dination polymer was reported to be based onZn2 paddle-wheel clusters formed by DPA andtriethylenediamine (DABCO) as additional li-

Figure 9. Representation of nSBUs in [Zn2(1,3-BDC)2(L)2�(solv)x]n with cone (a),partial cone (b), 1,2-alternate (c) and 1,3-alternate (d) shapes [45b]. Axial ligandsare omitted for clarity. Created from the data supplied by the Cambridge Crystallo-graphic Data Centre.

Figure 10. 2D-trigonal net-work constructed from zincisophthalate paddle-wheelclusters shown as rectangles.Reproduced with permissionfrom [45b].

Figure 11. Structure of a 2D-layer in [Zn2(DPA)2(DABCO)]n with disordered DABCOligands [47]. Created from the data supplied by the Cambridge Crystallographic DataCentre.

Coordination poly-mers constructedfrom zinc paddle-wheels and non-linear dicarboxylicacids can give sev-eral net topologiesdifferent from pla-nar 44-grid motif.



gand, namely [Zn2(DPA)2(DABCO)]n.[47] Thestructure of the compound is described ascomposed of two-dimensional layers. Theseare formed by zinc PW-4 clusters recumbent inone plane and linearly bridged by DABCO viaaxial positions into 1D chains, which are inter-linked by DPA above and below the plane. For-mally, paddle-wheel zinc clusters in this MOFcan be considered as capable for six-connectiv-ity, but their assembling leads to a 2D net with44 topology due to specific geometry and flex-ibility of the diphenate linker (see Fig. 11).

Zinc paddle-wheel PW-4 MOFs based on thecarboxylic acids with the connectivity higherthan two are also already known. Four-connect-ing tetrahedral 4,4′,4′′,4′′′-tetraphenylmethane-tetracarboxylic acid (H2MTB) gave a 3D-frame-work (see Fig. 12) MOF-36 ([Zn2(MTB)(H2O)2�(DMF)6(H2O)5]n) upon assembling withsquare-connecting paddle-wheel zinc clustersterminated axially with aqua ligands [31].

Three-connecting trimesic acid as a triangularSBU led to complex 3D coordination frameworkUSF-3 (see Fig. 13) composed from triangular,square and tetrahedral SBUs in ratio 4:2:1, re-spectively, where squares are represented byZn2 PW-4 clusters with axially coordinated iso-quinoline. The tetrahedra are bis-(l-carboxylate)-l-oxo bridged dizinc clusters with additional iso-quinoline or methanol ligands and chelatingcarboxylate group at each zinc ion [30b].

MOFs, in which zinc PW-4 clusters imple-ment the function of six-connecting SBUs andlead to three-dimensional frameworks, havebeing described only relatively recently, withthe first example being [Zn2(BDC)2(DAB-CO)�(DMF)4(H2O)0.5]n.[48]. This framework isformed by distorted square grid 2D-layers ofZn2 clusters bridged by unusually bent ter-ephthalate dianions, and the clusters are pil-lared by DABCO via the axial positions to builta compressed primitive cubic 3D structurewith interconnecting channels. Removal ofguest DMF molecules results in a relaxation ofthe strained 2D layers into perfect square grid,whereas inclusion of benzene leads to theshrinkage of the pores and formation of 2Drhombic grid (see Fig. 14). Such behavior ofthis coordination polymer, observed by X-raycrystal analysis and supported by PXRD mea-surements, fits with Kitagawa classification forthird generation MOFs with guest-exchangedeformation type pores or induced-fit typepores.

Additionally, a series of coordination poly-mers filled with guest molecules and congene-ric with the described above [Zn2(BDC)2(DAB-CO)�(DMF)4(H2O)0.5]n was reported, in whichthe nature of bridging dicarboxylate as well asof pillaring ligand was varied. Using DABCO

as “pillars“ allowed to prepare Zn2 PW-4 3DMOFs based on tetramethyl- (H2TBDC) or tet-rafluoroterephthalic acid (H2F4BDC), 1,4-naphthalenedicarboxylic acid (1,4-H2NDC), aswell as 1:1 combination of terephthalic and tet-ramethylterephthalic acids [9a]. Applying 4,4′-bipyridine (BIPY) as a pillaring ligand led tonew zinc paddle-wheel frameworks assembledvia terephthalic, tetramethylterephthalic, fu-maric and 2,6-naphthalenedicarboxylic acid [9,21b]. MOFs with larger pillaring molecules,e.g., N,N′-bis(4-pyridyl)-1,4,5,8-naphthalenete-tracarboxydiimide (DPNI), and terephthalic or4,4′-biphenyldicarboxylic acid as Zn2 PW-4forming struts are also described [9b]. One ofthe important features of the above MOFs istheir tendency to form interpenetrated struc-

Figure 12. Simplified presentation of MOF-36 structure, in which PW-4 clusters areshown as squares. Right – view along the c-axis. Reprinted in part with permissionfrom [31]. Copyright 2000 American Chemical Society.

Figure 13. Structural fragment of USF-3 and its representation with polyhedra andpolygons, axial ligands are omitted for clarity. Reproduced with permission from[30b].

Figure 14. Structures of a 3D-framework in the as-synthesized MOF [Zn2(BDC)2

(DABCO)�(DMF)4(H2O)0.5]n (a), its guest free (b) and benzene-filled (c) forms withdisordered DABCO pillars [48]. Guest molecules and hydrogen atoms are omittedfor clarity. Created from the data supplied by the Cambridge Crystallographic DataCentre.



tures upon increasing the length of struts (seeFig. 15). For example, BIPY pillared MOFs areall interpenetrated except the one formed byH2TBDC. Because of the bulkiness of TBDC(high rotational volume, disordering in MOFcrystal), the latter MOF has too small windowsin the zinc carboxylate 2D square grid to allowinterpenetration. Interestingly, [Zn2(2,6-NDC)2(BIPY)]n was reported to have boththree-fold [9a] and two-fold interpenetratedstructures [9b]. MOFs formed with DPNI pil-lars are double-interpenetrated [9b]. Three-foldinterpenetrated structure without guest mole-cules was also reported, namely a MOF formedby zinc terephthalate paddle-wheel PW-4 and1,4-bis(1,2,4-triazol-1-yl)butane as pillaring li-gand [49].

Structural investigations for all these metal-organic frameworks by either single crystal X-ray analysis or by PXRD reveal that most ofthem retain a 3D network upon removal ofguest solvent molecules, forming open inter-connected channels. The sizes of the channelscross-sections are dependent on the geometryof struts and the degree of interpenetration,and thus can be tuned in a reasonable scope.For example, the channels can be varied fromnearly absent in triply-interpenetrated[Zn2(2,6-NDC)2(BIPY)]n up to 10 to 11 Å in in-ternal diameter and 7.5 Å in openings for[Zn2(BDC)2(DABCO)]n [9a].

Comparison of the properties concerningthe porosity and guest-host interactions in de-scribed above MOFs will be given in the follow-ing chapter.

3.4 Sorption Pproperties of ZincPaddle-Wheel PW-4 CoordinationFrameworks

Many of the up to date reported coordinationpolymers based on zinc “four blade“ paddle-wheel SBUs were described exclusively respect-ing their single crystal structures and net topolo-gies with no further investigation on their guestremoval-binding behavior. In a better case, theirpotential free volumes were calculated and theirTGA and temperature variable PXRD character-istics were measured and discussed. It is worthto mention that the highest reported tempera-tures, at which some zinc paddle-wheel MOFsstill maintain microcrystallinity, are in the rangeof 300 to 400 °C, above which the decarboxy-lation processes usually take place.

Among paddle-wheel MOFs studied in detailwith the object to guest removal-binding re-sponse is MOF-2 (see above), as-synthesizedcomposition of which is expressed by the for-mula [Zn2(BDC)2(H2O)2�(DMF)2]n. Removal ofwater and DMF subsequently from this materi-al results in the crystalline phase formulatedas [Zn2(BDC)2]n, which is stable in the tem-perature range 190 to 315 °C and is differentfrom the parent material according to PXRD.It was proposed that removal of axially coordi-nated H2O and included DMF molecules,which provide a hydrogen bonding frameworkand connect the layers in MOF-2, results in acloser contact between Zn2 clusters of neigh-boring layers with formation of coordinationbonds between the carboxylate group in onelayer and Zn ion in another adjacent layer (seeFig. 16) [8, 38].

Such linking should lead to a quasi 3D-net-work with open channels, and the authors re-veal a Type I isotherms for N2 and CO2 sorp-tion at 77 K and 195 K, respectively, with theLangmuir apparent surface calculated to be270 m2/g and 310 m2/g, respectively. Besides,the material can also absorb dichloromethane,chloroform, benzene and cyclohexane, withthe latter showing a noticeable absorption-des-orption hysteresis [33]. Resolvation of[Zn2(BDC)2]n by 1:1 mixture H2O:DMF re-sulted in appearing of only several PXRD linesbelonging to the parent solid. Irreversible reac-tion of [Zn2(BDC)2]n with water to yield a non-porous phase was also reported [33]. Surpris-ingly, practically the same material[Zn2(BDC)2]n prepared by another groupshowed no Type I sorption of N2 at 77 K, butits subsequent resolvation by moist air andDMF resulted in full restoration of the originalstructure equivalent to MOF-2 [50].

[Zn2(BDC)2(DABCO)�(DMF)4(H2O)0.5]n [48]is another example of MOF with well-studied

Figure 15. Double (left) and triple (right) interpenetrated structures of[Zn2(BDC)2(BIPY)]n and [Zn2(2,6-NDC)2(BIPY)]n, respectively. Reproducedwith permission from [9a].

Figure 16. Possible structural changes upon solvent removal fromMOF-2. Reprinted in part with permission from [38]. Copyright 1998American Chemical Society.

Many of the upto date reportedcoordination poly-mers based on zinc“four blade“ pad-dle-wheel SBUswere described ex-clusively respectingtheir single crystalstructures and nettopologies with nofurther investiga-tion on their guestremoval-bindingbehavior.



sorption properties. The guest-free frameworkshows permanent porosity, as confirmed by N2

and H2 sorption measurements at 77 K. Thesorption of N2 follows Langmuir (Type I) iso-therm with a BET surface area of 1450 m2/gand Langmuir surface area of 2090 m2/g [9a].The hydrogen sorption at 1 bar pressurereaches 225 mL/g or 2.0 % by weight, whichcorrespond to 5.7 H2 molecules per formulaunit, and the sorption curve still indicated un-saturation under these conditions (see Fig. 17).

Guest-free MOFs isoreticular to [Zn2

(BDC)2(DABCO)]n with compositions [Zn2

(BDC)(TBDC)(DABCO)]n, [Zn2(F4BDC)2(DAB-CO)]n, [Zn2(1,4-NDC)2(DABCO)]n, [Zn2

(TBDC)2(DABCO)]n and [Zn2(TBDC)2(BIPY)]nshow comparable Langmuir/(BET) surfaceareas ranging from 1740/(1120) m2/g to 1400/(920) m2/g and comparable hydrogen uptakeunder standard temperature and pressure (1.7to 2.1 % by weight) [9a]. However, at low rela-tive pressures of H2 the difference in hydrogenabsorption by these isoreticular MOFs be-comes more distinct, which was related to thediscrepancy in the shape and size of frame-work’s channels rather than in the chemicalnature of organic linkers [9a].

Doubly interpenetrated MOF [Zn2

(BDC)2(BIPY)�(guests)x]n, called also MOF-508a when as-synthesized or MOF-508b whenbeing guest-free [21b], shows mutual displace-ment of interpenetrating frameworks upon re-moval of guest molecules. Together with dis-tortion of paddle-wheel clusters, this leads to areduction of the calculated potential free vo-lume from MOF-508a to MOF-508b by 16.7 %(dense form of MOF). However, these changesappeared to be reversible, since MOF-508b canbe resolvated to give exactly the same PXRDpattern as MOF-508a, and the drying-resolvat-ing procedure can be repeated many timeswithout loss of crystallinity. The sorption iso-therms for N2 and H2 at 77 K or CO2 at 195 Kmeasured on MOF-508b samples all show anoticeable hysteresis, which was related to re-

versible open-dense framework transforma-tions (see Fig. 18). Apparent Langmuir surfacearea of the guest-free compound was found tobe approximately 950 m2/g (BET surface areaof 660 m2/g was measured by another groupfor the identical compound under the sameconditions [9b]), and the hydrogen uptake atstandard temperature and pressure was 90mL/g or 0.8 % by weight [21b]. Additionally,the compound was tested as a sorbent in GCexperiments on separation of natural gas, pre-senting good preliminary results. It also exhib-ited efficient GC separation of some branchedand linear alkanes, showing the higher reten-tion times for the latter because of the better fitof linear molecules to the apertures of thechannels (approx. 4×4 Å in MOF-508a).

Another framework isomorphic to MOF-508also exhibits permanent porosity upon removalof guest molecules. As found from N2 sorptionmeasurements, doubly interpenetrated guest-free [Zn2(2,6-NDC)2(DPNI)]n [9b] has BET sur-face area of 420 m2/g.

Besides the studies of guest sorption proper-ties, practically no other specific behavior ofzinc paddle-wheel based MOFs was reported.

Figure 17. Gas sorption isotherms of guest free [Zn2(BDC)2(DABCO)]n.under standard temperature and pressure. Reproduced with permis-sion from [48].

Figure 18. Left – representation of open (a) and dense (b) phases of MOF-508 (guest molecules are omitted).Right – N2 (triangles), CO2 (circles) and H2 (squares) adsorption (filled symbols) and desorption (open symbols)isotherms measured on MOF-508b. Reproduced with permission from [21b].



One can mention only the examination of re-dox activity of [Zn2(2,6-NDC)2(DPNI)]n causedby the nature of pillaring ligand [9b].

3.5 Paddle-Wheels Based MOFs:Some Aspects of Synthesis

The manner in which the assembling of zinccarboxylate coordination polymer occurs de-pends strongly on the applied reaction condi-tions, such as the solvent and co-solventnature, reaction time and temperature, con-centrations, ratios and nature of reactants andbasic additives if used, etc. The reaction condi-tions can influence both the structure of SBUsand the type of network adopted in the processof polymer assembling.

In general, the exact structure of coordina-tion polymer can not be predicted unambigu-ously. In some particular cases application ofthe same reaction conditions to a series of or-ganic linkers results in maintaining the SBUand framework topologies in formed MOFs in-cluding those based on zinc paddle-wheel [2a,9a], whereas in other cases minor variations ofa co-ligand structure results in the same type(PW-4) of zinc SBU but leads to drastically dif-ferent net topologies [45b]. On the other hand,different co-solvents alone can already lead to achange in the SBU structure and consequentlyin the framework topology [30b, 36b]. The in-fluence of a co-solvent is often connected withits ability to modulate and stabilize the frame-work structure by means of the fitted inclusioninto the pores as guest upon MOF assembling(a template effect) [30b, 43], participating inthe network of hydrogen bonds and Van-der-Waals interactions.

By the example of terephthalic acid andits substituted derivatives, the bulkiness ofthe dicarboxylate was shown to influence thepreferential assembling of zinc paddle-wheels,leading either to PW-4 or to PW-3 type of clus-ters [32a]. Basicity of the reaction medium, aswell as the nature (coordination ability andstrength) of applied base was shown to affectthe structures of zinc carboxylate SBUs and ofresulting coordination frameworks [49, 30a].The lower pH led to MOF assembled via singlezinc ions, whereas higher pH value or pre-sence of stronger base with lower coordinationability, e.g., triethylamine instead of pyridine,resulted in formation of PW-4 zinc clusters.

One should mention that depending on theadditives, reaction time or ratio of the reagents,also the mixtures of coordination polymersshowing different structures, including thosewith zinc paddle-wheel SBUs, can be simulta-neously formed during the synthesis [32b, 36b,

47, 50]. Possible reasons are the comparablethermodynamics of formation or existence of akinetic control for the reaction products. It is,therefore, essential to always verify the unifor-mity of the bulk product by comparing itsPXRD pattern with the one calculated from thedata on single crystals picked up for the X-rayanalysis. Additionally, one should keep inmind the relative flexibility of zinc-based coor-dination polymers, which becomes apparentfrom a number of reported solid-state intercon-versions of MOFs at ambient temperature, in-volving even the SBU rearrangement [40, 50].The interconversion can take place both in thereaction solution and upon exposure to the at-mosphere, for example, because of the absorp-tion of the moisture or loss of the inclusionsolvents. Interconversion of zinc MOF in sol-vent at increased temperature to form zincpaddle-wheel based coordination polymer isalso reported, which is promoted by the addi-tion of a complementary ligand DABCO [47].

Systematization of the reported proceduresfor the preparation of Zn PW MOFs indicatesthat several very general synthetic methodolo-gies for coordination polymers can lead to theframeworks of interest. One of the methods in-volves refluxing the corresponding carboxylicacid with freshly prepared zinc hydroxide in apolar solvent such as water or alcohol, or mix-ture of solvents, followed by slow cooling ofthe reaction mixture and, when necessary, re-duction of the solvent volume to form crystalsof coordination polymer. The method was typi-cally applied to prepare 1D polymeric chainsconstructed by zinc monocarboxylates three-blade paddle-wheel clusters [26, 27a]. Slightlymodified procedure in which zinc nitrate isused as Zn source, and therefore addition ofbase such as pyridine is required, was appliedto prepare 2D polymer based on zinc isophtha-late PW-4 [45a]. Unfortunately, no yields werereported for the above preparations, and the ef-fectiveness of the method from synthetic pointof view can not be estimated. A variation of theabove method is the hydrothermal synthesis,where water heated above 100 °C in autoclaveis applied as a reaction medium [27b, 41, 49].

Solvothermal synthesis is more often usedmethod, in which substituted formamides areutilized as solvent. The reactants are usuallyzinc nitrate and carboxylic acid, often in com-bination with other ligands. The reaction iscarried out at the elevated temperatures result-ing in slow thermal decomposition of forma-mides to give amines, which serve as base toslowly generate the corresponding MOF-build-ing carboxylates. The yields of zinc PW-4 basedMOFs in solvothermal synthesis are typicallyhigh [9a, 21b].

Besides the studiesof guest sorptionproperties, practi-cally no other speci-fic behavior of zincpaddle-wheel basedMOFs was reported.

In general, the exactstructure of coordi-nation polymer cannot be predicted un-ambiguously.



The most common method for Zn PW-4MOF preparation is based on the diffusioncontrolled basification of the reactants solu-tion, typically carried out at ambient tempera-ture. One approach involves the absorption ofvolatile amines by the reaction solution fromthe vapor phase, and another is based on thelayering technique. The yields in this methodvary from very low to moderate [30b, 31, 45b].

4 Conclusions

Whereas the binuclear paddle-wheel clusters ofdifferent metals as building blocks for coordi-nation polymers have been already widely em-ployed, the zinc analogs have received higherattention only relatively recently. The grown in-terest towards MOFs constructed from zincpaddle-wheel clusters is to a great extent con-nected with the recently applied “pillaring“ ap-proach to prepare new 3D architectures withhigh microporosity, good sorption properties,and thermal stability. Such coordination poly-mers can be prepared in very high yield, theycan be respectively functionalized and theirstructure can be flexibly tuned to fit the desiredapplication by means of altering the nature ofcarboxylate and pillaring ligands. This can beconsidered as an additional advantage of suchMOFs over many other coordination frame-works. However, the contribution of earlierworks on zinc paddle-wheel coordination com-pounds is not less valuable, as they lead to un-derstanding the regularities in clusters andnetworks formation.

Prof. B. Rieger([email protected]),Dr. S. Vagin,A. K. Ott,Waker-Lehrstuhl für Makromolekulare Chemie,Technische Universität München,Lichtenbergstraße 4, D-85748 Garching,Germany

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