Modular Chemistry:Secondary Building Units as a Basis for the Design of Highly Porous and Robust Metal-Organic Carboxylate Frameworks MOHAMED EDDAOUDI,†DAVID B.MOLER,†HAILIAN LI,†BANGLIN CHEN,†
THERESA M.REINEKE,†
MICHAEL O’KEEFFE,‡AND OMAR M.YAGHI*,†Materials Design and Discovery Group,Department of Chemistry,University of Michigan,930North University Avenue,Ann Arbor,Michigan48109-1055,and Department
of Chemistry and Biochemistry,Arizona State University,
Box871604,Tempe,Arizona85287-1604
Received November13,2000
ABSTRACT
Secondary building units(SBUs)are molecular complexes and cluster entities in which ligand coordination modes and metal coordination environments can be utilized in the transformation of the
fragments into extended porous networks using polytopic linkers(1,4-benzenedicarboxylate,1,3,5,7-adamantanetetracarboxyl-ate,etc.).Consideration of the geometric and chemical attributes of the SBUs and linkers leads to prediction of the framework topology,and in turn to the design and synthesis of a new class of porous materials with robust structures and high porosity.
Bridging Molecular and Solid-State Chemistry The construction of extended solids from molecular building blocks is now of great interest due to the advantages it offers for the design of materials.The u of discrete molecular units in the asmbly of extended networks is an attractive synthetic approach since it permits reactions to take place at or near room temper-ature,where the structural integrity of the building units can be maintained throughout the reaction s an aspect that allows for their u as modules in the asmbly of extended structures.1-18Molecular modules can be de-signed to direct the formation of target structures as well as to impart desired physical properties to solid-state materials.In an effort to understand and fully develop this area of chemistry,often termed modular chemistry,a large number of synthetic approaches involving solution as-mbly of molecules have been pursued to achieve functional mesoscopic phas,modified surfaces,and designed crystals.1
We have focud on the study of crystalline materials since the attainment of well-defined structures is inti-mately linked to an understanding of the design,synthesis, and properties of materials.In particular,we have been interested in the construction of porous materials due to their immen impact on the global economy and the fascinating prospects that open networks offer for building complexity into molecular voids in which highly lective inclusion and chemical transformations can be effected. In an earlier Account,6we showed how extended metal-organic frameworks(MOFs)can be designed,crystallized, and fully characterized.Here,we prent the next stage in the development of MOFs as a new class of porous materials:the recent progress in using the concept of condary building units(SBUs)for understanding and predicting topologies of structures,and as synthetic modules for the construction of robust frameworks with permanent porosity.We also outline strategies for incor-porating unprecedented arrays of coordinatively unsatur-ated open-metal(OM)sites in porous materials and for the synthesis of modular interpenetrating and non-interpenetrating networks with optimal porosity.
*To whom correspondence should be addresd.E-mail:oyaghi@ umich.edu.
†University of Michigan.
‡Arizona State University.
Mohamed Eddaoudi was born in Agadir,Morocco(1969).He received his B.S. (1991)from the University of Ibnou Zohar with Honors,and M.S.(1992)and Ph.D. (1996)from the University Denis Diderot Paris7with Tres Honorable avec Felicitations du Jury.He has been a Faculty Rearch Associate with Professor Yaghi since August1997.His rearch focus is on the synthesis,characterization, and inclusion/sorption chemistry of organic and inorganic porous materials. David B.Moler was born in Ann Arbor,MI(1979).He is a junior student in the Department of Chemical Engineering at the University of Michigan.His interests in chemistry include molecular shapes,topology,and patterns.
Hailian Li was born in Jiangsu,China,in1964.He received his B.S.(1987)and M.S.(1990)in chemistry from Nanjing University,China.He rved as a faculty rearch associate with Professor Yaghi while at Arizona State University.At prent,he is a doctorate candidate at the Department of Chemistry of the University of Michigan.His primary focus is on the synthesis of porous crystalline materials with novel linkages.
什么是团员Banglin Chen was born in Zhejiang,China(1965).He received his B.S.(1985) and M.S.(1988)from Zhejiang University,and his Ph.D.(1999)from the National University of Singapore.He has been a postdoctoral fellow in Professor Yaghi’s group since August1999.He has invested his efforts in the us
e of expanded and decorated links for the synthesis of porous materials,in particular interwoven structures.Theresa M.Reineke was born in St.Paul,MN,in1972.She received her B.S. degree from the University of Wisconsin-Eau Clair(1995),her M.S.from Arizona State University(1998),and her Ph.D.in inorganic chemistry(2000)from the University of Michigan,Ann Arbor,with Professor O.M.Yaghi.She is currently a Postdoctoral Scholar at the California Institute of Technology with Professor Mark Davis,studying nonviral gene delivery polymers.
Michael O’Keeffe was born in Bury St Edmunds,England(1934).He received his B.Sc.(1954),Ph.D.(1958),and D.Sc(1976)from the University of Bristol.He is Regents’Professor of Chemistry at Arizona State University,where he has been since1963.His current rearch is particularly focud on studying beautiful patterns found in chemistry and elwhere.
Omar M.Yaghi was born in Amman,Jordan(1965).He received his B.S.in chemistry from the State University of New York-Albany(1985)and his Ph.D. from the University of Illinois-Urbana(1990)with Professor Walter G.Klemperer. From1990to1992,he was an NSF Postdoctoral Fellow at Harvard University with Professor Richard H.Holm.He joined the faculty at Arizona State University in1992.He was awarded the ACS-Exxon Solid-State Chemistry Award in1998. In June1999,he moved to the University of Michigan as a Professor of Chemistry, establishing veral rearch programs dealing
with molecular and solid-state chemistry,in particular the transformation of molecular organic and inorganic building blocks to functional extended frameworks.
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10.1021/ar000034b CCC:$20.00©2001American Chemical Society VOL.34,NO.4,2001/ACCOUNTS OF CHEMICAL RESEARCH319 Published on Web02/17/2001
导致白发的原因Strategies for Construction of Rigid Porous Frameworks
Although synthesis of open frameworks by asmbly of metal ions with di-,tri-,and poly-topic N-bound organic linkers such as 4,4′-bipyridine (BPY)has produced many cationic framework structures (Figure 1a),attempts to evacuate/exchange guests within the pores almost invari-ably,with few exceptions,12,15results in collap of the host framework.We have found that multidentate linkers such as carboxylates allow for the formation of more rigid frameworks due to their ability to aggregate metal ions into M -O -C clusters that we refer to as condary building units (SBUs)(Figur
e 1b).The SBUs are sufficiently rigid becau the metal ions are locked into their positions by the carboxylates;thus,instead of having one metal ion
at a network vertex (as is the ca in M-BPY compounds),now the SBUs rve as large rigid vertices that can be joined by rigid organic links to produce extended frame-works of high structural stability.The frameworks are also neutral ,obviating the need for counterions in their cavities.
To appreciate the impact of SBUs on pore size and porosity of frameworks,it is instructive to compare two strategies developed for the construction of highly porous frameworks.(a)The u of long links that increa the spacing between vertices in a net yields void space proportional to the length of linker (Figure 1a).This means that a bond is replaced by a quence of bonds s a process we call expansion .Although in principle such expanded structures provide for large pores,in practice they
are
FIGURE 1.Asmbly of metal -organic frameworks (MOFs)by the copolymerization of metal ions with organic linkers to give (a)flexible metal -bipyridine structures with expanded diamond topology and (b)rigid metal -carboxylate clusters that can be linked by benzene “struts”to form rigid extended frameworks in which the M -O -C core (SBU)of each cluster acts as a large octahedron decorating a 6-connected vertex in a cube.All hydrogen atoms have been omitted for clarity.(In (a),M,orange;C,gray,N,blue;in (b),M,purple;O,red;C,gray.Structures were drawn using single-crystal X-ray diffraction data.)
320ACCOUNTS OF CHEMICAL RESEARCH /VOL.34,NO.4,2001
often found to be highly interpenetrated and to have low porosity.(b)In contrast,replacement of a vertex of a framework net by a group of vertices,a process termed decoration,results in open structures with high rigidity and without a tendency to interpenetrate(Figure1b).In fact,when such networks are interpenetrating,optimal pore volume may be achieved as described below.(The replacement of the vertices of an N-connected net by a group of N vertices is a special ca of decoration which we refer to as augmentation.24)
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An increasing number of asmbled frameworks have as components polytopic groupings of linkers such as 1,3,5-benzenetribenzoate(BTB),which in themlves may act to decorate a vertex in an asmbly.Polytopic links can thus be employed both to decorate and to expand a
net.14
It is worth noting that the sizes of rings(or pore size) and voids in nets can be significantly incread with decoration,augmentation,or a combination thereof.For example,4-rings in a simple cubic structure can be augmented by octahedra(carboxylate carbon atoms in Figure1b,right)to produce a5-connected structure with 8-ring pores and void space significantly larger than that of the original structure.
From Molecular SBUs to Open Framework Solids
Identification and U of SBUs.Discrete di-,tri-,and tetranuclear metal carboxylate clusters19-23such as the paddle-wheel copper acetate(D4h)and the basic zinc acetate(T d)motifs were targeted as symmetrical modules suited for polymerization reactions involving multidentate carboxylate linkers.Such clusters can rve as SBUs in that they have three components relevant to their polymeri-zation into modular porous metal-carboxylate networks (Figure1b).(a)The M-O-C core struc
ture is an SBU who shape is defined by tho atoms reprenting points of extension to other SBUs,and which are generally parated only by links.Thus,such atoms define the underlying geometry of the SBU;they are relevant to predicting the overall topology of the modular network (Figure1b).(b)Potentially,each monocarboxylate ligand in the molecular complex can be substituted with a di-, tri-,or multicarboxylate in order to polymerize the SBU into an extended network.However,the coordination mode of each carboxylate ligand in the molecular complex provides important geometric and conformational infor-mation that is critical to predicting the topology of the resulting network.(c)In some clusters where terminal ligands are prent,their coordination sites may be removed to allow the study of metal site reactivity:by using weak ligands(such as methanol or ethanol)as solvents in the synthesis,it is possible to produce extended structures in which the ligands point toward the center of the voids,making them susceptible to dissociation and evacuation from the pores,thereby producing periodically arranged OM sites.
Solution Synthesis and Crystallization of Modular Porous Solids.Generally,when3-D extended solids linked by strong chemical bonds are asmbled under mild temperature conditions,it is difficult to effect their crystallization,and in general this remains a significant obstacle to synthesis.However,we have developed path-ways for synthesizing macrocrystalline modular solids.For example,we have sh
own that copolymerization of SBUs with the polytopic linkers1,4-benzenedicarboxylate(BDC), 4,4′-azodibenzoate(ADB),1,3,5-benzenetricarboxylate (BTC),and1,3,5,7-adamantanetetracarboxylate(ATC)(Fig-ure2)is an ideal pathway for obtaining crystalline products.Typically,a solution of the acid form of a linker and the appropriate simple metal salt(nitrate)is prepared in the desired stoichiometry.The key step to obtaining crystals is to slowly diffu into the reaction mixture an organic amine that deprotonates the acid and initiates the asmbly.Nucleation rates and the number of nucleation sites can be controlled by controlling the rate of amine diffusion,solvent polarity,and concentration gradients. When the acid is poorly soluble,reactions are performed above room temperature,up to190°C.In the cas, inorganic hydroxides such as NaOH may be ud(if needed)instead of the organic amines to deprotonate the acid.
Predicting Framework Topologies.A large number of crystalline materials constructed from the SBUs and linkers have been prepared and reported by our group. Illustrative examples involving each SBU mentioned above will be prented.In consideration of possible structures that would form by the polymerization of the SBUs,we rely on our thesis:in general only a small number of simple,high-symmetry structures will be of overriding general importance,and they would be expecte
d to form most commonly.24We know of no systematic survey of the occurrence of structural topologies,and when we say that a particular topology is very common,we really mean that it occurs very frequently in structures that we are likely to have examined and to have analyzed their geometry. Elaboration of this thesis has appeared in a recent tutorial article.24For the purpos of this prentation,a partial list of structures that are most likely to form in the asmbly of certain symmetrical geometric shapes is shown in Table1and reprented in Figure3.25
The FIGURE2.Reprentative polytopic organic linkers.(Same coloring scheme as in Figure1.)
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esnce of this thesis is captured by discussion of the topologies of extended porous structures prepared from the SBUs discusd above.Unlike other MOFs,the SBUs produce unprecedented porosity due to their rigid struc-ture.A summary of porosity data obtained from gas sorption isotherms is shown in Table 2and discusd further below.
Design of Periodic Molecular Imprints for Sensing
Decoration and Expansion of Primitive Cubic Net.The trinuclear SBU s known in a molecular cluster 21s has been polymerized with BDC to form a 3-D porous network,Zn 3(BDC)3‚6CH 3OH (MOF-3)(Figure 4).26The central zinc atom is octahedrally bound to carboxylate oxygens,and each of the other zinc centers is coordinated to three such oxygens in addition to two oxygens from terminal metha-nol ligands,where one methanol is bound more weakly to zinc than the other (Figure 4a).In the crystal,two additional methanol molecules are hydrogen bonded to the methanol ligands,thus acting as guests to fill a 3-D channel system of 8Åcross ction.
To determine the topology of MOF-3bad on the guidelines prented above,each trinuclear unit can be considered as an octahedral SBU (Figure 4b).Linking the units with BDC produces the pri
mitive cubic net arrangement (Table 1,Figure 3i):the octahedral SBU decorates the vertices that are then spaced (expanded)from other such units by 2-connected benzene units (in
Table 1.Partial List of Basic Nets with One or Two
Kinds of Vertex Figures a
coordination
coordination figures net
3triangle triangle SrSi 23triangle triangle ThSi 2
3triangle triangle 63honeycomb 3,4triangle square Pt 3O 44square
square
NbO
4tetrahedron tetrahedron diamond (C)4,4square tetrahedron cooperite (PtS)4square square 44square lattice 6octahedron octahedron primitive cubic
8
cube
cube
body-centered cubic
a
For a more complete list of basic structures,e refs 24and
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25.
FIGURE 3.Common nets (e Table 1)for one and two kinds of links.A gment of each net has been highlighted in orange for clarity.(Structures were drawn using single-crystal X-ray diffraction data.)
Table 2.Comparison of Porosity in MOFs a
MOF -2MOF -3MOF -4MOF -5MOF -6MOF -9MOF -11
红玫瑰蜘蛛pore diameter (Å)
781412
487surface area (m 2/g)2701402900127
560pore volume (cm 3/g)
0.0940.0380.612 1.040.0990.035
0.20
a
All parameters were obtained from gas sorption isotherm data.Surface areas for MOF-4and -6are not included since the frameworks did not adsorb
gas.
FIGURE 4.(a)Building unit in the crystal structure of Zn 3(BDC)3‚6CH 3OH (MOF-3),in which each carboxylate carbon of four BDC units and an oxygen from each of the remaining BDC links form.(b)Octahedral (Oh)SBU,that asmbles into (c)a primitive cubic-like decorated diamond net topology.(Same coloring scheme as in Figure 1;structures were drawn using single-crystal X-ray diffraction data.)
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the ca of BDC having one carboxylate oxygen as part of the SBU,the expansion is done with a benzyl unit)(Figure 4c).
We found that all methanol,including the ligands,can be removed from the voids to give a porous network (Table 2)having open zinc sites.It is interesting to note that MOF-3maintains its framework integrity,even in the abnce of methanol guests or ligands s an aspect that is relevant to nsing and catalysis.27In the following c-tions,we give examples that illustrate how the OM sites can act as molecular imprints.
中秋节的诗
A Chiral Framework with Imprinted Crevices.A cubic network with larger pores and with OM sites can be achieved using the common dinuclear cluster D 3h -Zn 2-(CO 2)3.20Copolymerization of BTC with zinc(II)yields a building unit with a trigonal SBU geometry (Figure 5a)in which the carbon atoms of the Zn -O -C cluster core are the vertices of a trigonal SBU and three carbon atoms of the benzene rings of BTC are the vertices of another trigonal SBU (Figure 5b).This arrangement yields Zn 2-(BTC)(NO 3)‚(C 2H 5OH)5(H 2O)(MOF-4),28which adopts one of the most symmetric and frequently obrved 3-con-nected topologies belonging to the t summarized in Table 1,namely the chiral Si net of SrSi 2(Figure 3a).24,25Here,each SBU and benzene unit alternately decorates the Si vertices (Figure 5c).
Each building unit has a nitrate ion bound strongly to one of the Zn centers and three ethanol molecules bound weakly to the other Zn center.The crystal structure of MOF-4is cubic and contains 3-D channels of 14Ådiameter filled with five ethanol (three act as ligands to
Zn and two are free guests filling the pores)and one water molecule per formula unit.Using liquid and vapor sorp-tion,it was shown that all ethanol and water can be evacuated from the pores to produce pyramidal Zn sites that are found to be highly lective to alcohol uptake.29Guest competition experiments using 13C CPMAS NMR,GC,and liquid isotherms showed that when evacuated MOF-4i
s expod to an equimolar mixture of CH 3CN and CH 3OH,only CH 3OH is allowed into the pores.Similar experiments involving larger molecules produced similar affinity of the pores to alcohols.This has led us to conclude that the expod zinc sites are part of a crevice left behind by the liberated ethanol,where a geometric and electronic imprint (specific host -guest hydrogen-bonding interactions)for alcohols has been registered at the binding sites (Figure 5d).29
Design and Crystal Structure of Materials with Open Metal Sites
Decorated Square Grids.The paddle-wheel cluster is known in many complexes;19it has a square geometry (Figure 6a),which when linked with benzene,a linear ditopic “strut”,produces Zn(BDC)(H 2O)‚(DMF)(MOF-2);DMF )N ,N ′-dimethylformamide)having a layered struc-ture.32Here,Zn 2(CO 2)4square SBUs decorate the interc-tion of a 44planar grid,and the benzene units expand the links between vertices to produce the 4.82tiling pattern (Figure 6b).The axial positions on Zn are occupied by water ligands which rve to hold together the sheets by mutual hydrogen bonding to the carboxylate oxygen
on
FIGURE 5.(a)Building unit in the crystal structure of Zn 2(BTC)(NO 3)‚(C 2H 5OH)5(H 2O)(MOF-4)having terminal ethanol and nitrate ligands bound to opposite zinc centers.The carboxylate carbon atoms within each M -O -C cluster core and the 1,3,5-carbon atoms of each BTC reprent (b)two triangle SBUs (respectively shown in orange and gray),which asmble (c)into decorated Si net of SrSi 2,to form (d)an extended porous framework with open metal sites and large pores.(Same coloring scheme as in Figure 1;structures were drawn using single-crystal X-ray diffraction data.)
VOL.34,NO.4,2001/ACCOUNTS OF CHEMICAL RESEARCH 323
an adjacent sheet.The sheets stack in registry to allow the formation of 1-D channels where DMF guests reside (Figure 6c).The DMF and water can be thermally removed to allow each metal to bond to a carboxylate oxygen of an adjacent sheet,thereby linking the sheets in the third dimension and leading to a quasi 3-D network with open channels.The porosity of this material was examined and found to exhibit zeolite-like type I gas sorption isotherms (Table 2).32
Porous Crystals with Open Metal Sites.It is well-known that when the axial ligands in the paddle-wheel structure have been dissociated,as in MOF-2,the metal sites thus produced bind to a Lewis ba
atom on a neighboring unit to give polymeric structures having no OM sites.33Our strategy to prevent coupling of the units and to achieve an MOF with accessible OM sites relies on using the square SBU with a tetrahedral linker.Using the organic adamantane cluster as a cond SBU,we expected that the combination of a square (Figure 7a,b)and a tetrahedron would polymerize to form the PtS topology,as indicated in Table 1(Figure 3g).Here,each square Pt atom and tetrahedral S atom in PtS would be replaced by an inorganic square and an organic tetrahe-dral SBU,respectively,to give a decorated form of PtS (Figure 7c).Indeed,the success of this approach has been
demonstrated by performing reactions that give the cop-per carboxylate square motif:addition of Cu(NO 3)2‚2.5H 2O to 1,3,5,7-adamantanetetracarboxylic acid (H 4ATC)in basic aqueous solution at 190°C yields green crystals of Cu 2(ATC)‚6H 2O (MOF-11)having the desired geometry in which lf-aggregation of SBUs is prevented since the labile ligands point toward the centers of voids (Figure 7d).34Each square has two axial water ligands (Cu -OH 2)2.148(3)Å),one bound to each of its copper(II)centers.The point away from the Cu -O -C framework into a 3-D channel system of 6.0-6.5Ådiameter,where four additional water guests per square unit reside.Estimates bad on van der Waals radii showed that water ligands and guests occupy approximately 50%of the crystal volume.Al
l water guests and ligands were liberated to give an anhydrous framework with a thermal stability range 120-260°C.Furthermore,the rigid and porous nature of the anhydrous framework was evident upon measurement of its gas sorption isotherms,where fully reversible type I behavior was obrved for N 2(g)and Ar (g)s unequivocally confirming the existence of permanent porosity in anhy-drous MOF-11(Table 2).皇天不负苦心人
The crystal structure of the anhydrous material (Figure 7d)showed that the originally monoclinic framework relaxes to the tetragonal symmetry of the ideal PtS net with negligible change in volume (Table 3).The abnce of water ligands on copper is evident from the shorter distance obrved for Cu -OCO and a significant shorten-ing of the Cu -Cu distance upon liberation of water s the latter (2.490Å)being the shortest distance known for copper(II)carboxylate compounds.Magnetic susceptibility data obtained for the as-synthesized MOF-11follow the expected behavior typically obrved for antiferromagnetic coupling in the copper(II)acetate dimer (2J )-280cm -1);however,the anhydrous material showed incread cou-pling (2J )-444cm -1),as expected for the obrved shortening of the Cu -O distance found in the X-ray crystal diffraction analysis.
The full characterization of the OM sites in MOF-11paves the way for their u in nsors,in catalysis,and as functionalization sites for mounting molecular moieties into the pores.An example of
the potential u of OM sites in nsing is prented in the ction that follows.Luminescent Porous Frameworks.To examine the viability of MOFs with OM sites in nsing small mol-ecules,we initiated a project aimed at exploring the chemistry of frameworks with lanthanide metal ions.It was shown that the compound Tb 2(BDC)3‚(H 2O)4(MOF-6),which has an extended nonporous structure,can be converted to a porous network,Tb 2(BDC)3,by thermally liberating the water ligands (Table 2).30The dehydrated form has extended 1-D channels and the same framework structure as that of the as-synthesized solid,as shown by X-ray powder diffraction.Water sorption isotherm data proved that Tb 2(BDC)3has permanent porosity and ac-cessible Tb(III)sites.Luminescence lifetime measure-ments confirmed that when water is resorbed or ammonia is sorbed into the pores,they bind to Tb,giving distinctly different decay luminescence lifetime constants.
Further
FIGURE 6.(a)Paddle-wheel cluster,a square SBU,which is the building unit in the crystal structure of Zn(BDC)(H 2O)‚(DMF)(MOF-2),which forms (b)a decorated square grid and (c)an open framework with DMF occupying the pores.(Zn,yellow;O,red;N,green;C,gray.Structures were drawn using single-crystal X-ray diffraction data.)
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