CRYSTAL GROWTH & DESIGN
Anion Coordination in Metal-Organic Frameworks Functionalized with Urea Hydrogen-Bonding Groups Radu
Custelcean,*,†
Bruce A.
Moyer,†
Vyacheslav S.
Bryantsev,‡
and Benjamin P.
2006 VOL. 6, NO. 2 555-563
Hay‡
Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6119, and Chemical Sciences DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed September 28, 2005; ReVised Manuscript ReceiVed NoVember 9, 2005
ABSTRACT: A series of metal-organic frameworks (MOFs) functionalized with urea hydrogen-bonding groups has been synthesized and structurally analyzed by single-crystal X-ray diffraction to evaluate the efficacy of anion coordination by urea within the structural constraints of the MOFs. We found that urea-based functionalities may be used for anion binding within metal-organic frameworks when the tendency for urea‚‚‚urea self-association is decreased by strengthening the intramolecular CH‚‚‚O hydrogen bonding of N-phenyl substituents to the carbonyl oxygen atom. Theoretical calculations indicate that N,N′-bis(m-pyridyl)urea (BPU) and N,N′bis(m-cyanophenyl)urea (BCPU) should have enhanced hydrogen-bonding donor abilities toward anions and decreased tendencies to self-associate into hydrogen-bonded tapes compared to other disubstituted ureas. Accordingly, BPU and BCPU were incorporated in MOFs as linkers through coordination of various Zn, Cu, and Ag transition metal salts, including Zn(ClO4)2, ZnSO4, Cu(NO3)2, Cu(CF3SO3)2, AgNO3, and AgSO3CH3. Structural analysis by single-crystal X-ray diffraction showed that these linkers are versatile anion binders, capable of chelate hydrogen bonding to all of the oxoanions explored. Anion coordination by the urea functionalities was found to successfully compete with urea self-association in all cases except for that of charge-diffuse perchlorate. Introduction Metal-organic frameworks (MOFs), or coordination polymers, have recently emerged as a promising new class of materials that combine the characteristics of inorganic networks, such as stability and crystallinity, with versatility and structural diversity of organic structures.1 These materials are easily accessible by self-assembly of transition metal cations with various organic linkers and are often amenable to detailed structural characterization by X-ray crystallography, which facilitates their rational design. When neutral linkers are employed, the resulting frameworks are cationic and intrinsically include anions for charge balance. Accordingly, some MOFs have been reported to function as anion exchangers.2 The anion exchange process can be solvent-mediated, involving dissolution of the initial framework and recrystallization of the final one, as recently demonstrated in a one-dimensional coordination polymer.3 In our quest for new receptors for applications to anion complexation and separation, we have become interested in MOFs as potential anion-binding hosts. Our approach toward this end is to functionalize these materials with hydrogenbonding groups that would act as specific coordination sites for the included anion. We are particularly attracted by urea groups, as they are known to form chelate hydrogen bonding to oxoanions,4 which can provide enhanced binding strength and recognition abilities. However, ureas are also known to persistently self-associate into hydrogen-bonded tapes,5 which if formed inside MOFs would render them unusable for anion binding and recognition (Figure 1). Two previous structural studies on urea-functionalized MOFs containing linkers L1 and L2 (Chart 1) showed either urea selfassociation or the absence of the chelate hydrogen bonding to the anion (nitrate), which was apparently disrupted by hydrogen bonding to the included water solvent.7 More recently, Steed et * To whom correspondence should be addressed. E-mail: custelceanr@ ornl.gov. † Oak Ridge National Laboratory. ‡ Pacific Northwest National Laboratory.
Figure 1. (a) Chelate hydrogen bonding of a generic oxoanion by the urea group. (b) Self-association of urea groups in the solid state, as exemplified by the crystal structure of N,N′-diphenylurea.6
Chart 1
al. demonstrated chelate hydrogen bonding of nitrate in a coordination polymer made from linker L3 with one pyridine group directly attached to urea.8 Our recent work involving L4 as an MOF linker resulted in unprecedented sulfate binding by six urea groups in a total of 12 hydrogen bonds, which represents the highest coordination number observed for sulfate in a natural or synthetic host.9 However, our attempts to bind other oxoanions with lower charge density than sulfate, such as AcO-, NO3-, CH3SO3-, or BF4-, using L4, resulted only in ligand
10.1021/cg0505057 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005
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Custelcean et al.
Table 1. Hydrogen-Bonding Parameters for the Chelate Binding of Nitrate by Various R-NH-C(dO)-NH-R Ureasa R
d(H‚‚‚O) (Å)
∆E (kcal/mol)
Me Ph m-NO2-Ph m-pyridyl m-CN-Ph
1.888 1.817 1.779 1.801 1.781
-30.8 -43.4 -56.6 -49.8 -55.7
aCalculated
with DFT at the B3LYP/6-31G* level.
self-association by urea‚‚‚urea hydrogen bonds. The need to design urea-containing linkers with stronger hydrogen-bonding donor abilities and lower tendency for self-association became thus apparent. Herein we report the design, synthesis, and structural characterization of MOFs containing the N,N′-bis(mpyridyl)urea (BPU)10 and N,N′-bis(m-cyanophenyl)urea (BCPU) linkers, which show great versatility in binding various anions including SO42-, NO3-, CH3SO3-, CF3SO3-, and ClO4-. Results and Discusion Ligand Design. Table 1 shows the interaction energies (∆E) and hydrogen-bonding distances, d(H‚‚‚O), for nitrate binding by various symmetrically disubstituted ureas, calculated with DFT at the B3LYP/6-31G* level. As expected, aryl-substituted ureas are significantly stronger hydrogen-bonding donors compared to analogous alkylsubstituted derivatives, most likely as a result of the electron withdrawing character of the aryl rings through a combination of inductive and conjugation effects.11 We therefore reasoned that the incorporation of N,N′-diarylurea groups into our linkers should greatly enhance the anion-binding properties of the resulting MOFs. Such groups have been successfully utilized for the design of discrete coordination complexes that showed binding of anions with relatively low charge density like NO3or PF6-,12 and recently a dipyridylurea-MOF that binds ClO4was reported.10 The self-association of ureas remains competitive though, especially when the less basic anions are targeted, or when anion binding by multiple urea groups is intended, as complexation by each urea incrementally decreases the charge density on the anion. In our search for a way to inhibit urea‚‚‚urea hydrogen bonding, we found inspiration in the seminal study of Etter et al., who showed that N,N′-bis(m-nitrophenyl)urea has a decreased tendency to self-associate in the solid state, which corresponded with its increased ability to cocrystallize with various weak hydrogen-bonding acceptor guest molecules.13 Etter rationalized these findings based on the enhanced acidity of the H atom ortho to the nitro group, which translated into stronger intramolecular C-H‚‚‚O hydrogen bonding to the urea’s carbonyl and accordingly a decreased ability of this group to engage in intermolecular hydrogen bonding. This hypothesis was supported by the observation that while most other diarylureas displayed dihedral angles between their aromatic rings and urea groups ranging between 26° and 46° to facilitate the formation of urea‚‚‚urea hydrogen bonding, the bis(mnitrophenyl)urea had a nearly planar conformation, consistent with the presence of C-H‚‚‚O intramolecular hydrogen bonds.14 Etter’s structural evidence is consistent with our high-level quantum mechanical calculations (MP2/aug-cc-pVDZ) showing that the nitro group increases the rotation barrier around the N-aryl bond from 2.4 kcal/mol in phenylurea to 3.2 kcal/mol in m-nitrophenylurea. Additionally, we found that the nitro group significantly enhances the hydrogen-bonding donating ability of urea resulting in a 13.2 kcal/mol increase in nitrate-binding energy relative to diphenylurea (Table 1).
Figure 2. (a) The three possible low-energy conformers of BPU, as found by DFT at the B3LYP/6-31G* level. (b) Crystal structure of BPU, showing the observed intramolecular C-H‚‚‚O hydrogen bonds. (c) Crystal structure of BPU‚(H2O)2, showing the C-H‚‚‚O and urea‚‚‚water hydrogen bonding (only one of the two water molecules is shown).
By analogy with bis(m-nitrophenyl)urea, we reasoned that replacing the Ph group with pyridine or m-CN-Ph, which also have good electron withdrawing characteristics, would have comparable effects on the hydrogen-bonding abilities of their corresponding urea groups. Our calculations indeed found N-aryl rotation barriers for pyridylurea (2.9 kcal/mol) and m-cyanophenylurea (3.2 kcal/mol) that are similar with the analogous value calculated for m-nitrophenylurea.15 We also found a significant increase in nitrate binding by BPU and BCPU of 6.4 and 12.3 kcal/mol, respectively, relative to diphenylurea (Table 1). Finally, it was expected that metal complexation by pyridine or -CN upon incorporation of BPU or BCPU into MOFs would further improve the electron-withdrawing character of these groups, thereby enhancing the anion-binding abilities of ureas in these linkers. Structural Analysis of BPU and BCPU. A conformational analysis of the BPU (B3LYP/6-31G*) found three low-energy conformers that differ with respect to the dihedral angle between the pyridine rings and the urea group. While all three conformers display perfectly flat geometries, each pyridine ring may assume two possible orientations relative to the urea group, with the N atom either syn or anti relative to the CdO group (Figure 2a). The anti-anti isomer was found computationally to be the most stable conformation, with the syn-anti and syn-syn being 1.6 and 3.2 kcal/mol higher in energy, respectively.15 BPU crystallized from acetone/toluene in the less stable syn-syn conformation, as found by single crystal X-ray crystallography (Figure 2b). The C2-symmetrical BPU molecule is nearly flat, with the pyridine rings forming a dihedral angle of 4.1° relative to urea. This allows intramolecular C-H‚‚‚O hydrogen-bond formation, with an H‚‚‚O distance of 2.160 Å. Accordingly, as in the analogous bis(m-nitrophenyl)urea, no urea‚‚‚urea hydrogen bonds were observed in this structure. BPU crystallized as a dihydrate from EtOH/H2O, and the X-ray structural analysis revealed the presence of the most stable anti-anti conformer in this case (Figure 2c). The BPU assumes again a nearly planar geometry with pyridine-urea dihedral angles of 1.8 and 4.7°, which favors the formation of intramolecular C-H‚‚‚O hydrogen bonds with observed H‚‚‚O distances of 2.29 and 2.26 Å, respectively. As in the other structure, no urea‚‚‚urea hydrogen bonds were formed; instead, the urea interacts with the weaker hydrogen-bond acceptor, water. Thus, as designed, both isolated crystalline forms of BPU showed the absence of urea self-association,
Anion Coordination in Metal-Organic Frameworks
Crystal Growth & Design, Vol. 6, No. 2, 2006 557
Figure 3. (a) The three possible low-energy conformers of BCPU, as found by DFT at the B3LYP/6-31G* level. (b) Crystal structure of BCPU showing the self-association of urea by hydrogen bonding. (c) Crystal structure of BCPU‚H2O, showing the C-H‚‚‚O and urea‚‚‚ water hydrogen bonding.
which was correlated with the presence of intramolecular C-H‚‚‚O interactions. Like BPU, BCPU has three planar conformations with the anti-anti one being the most stable and the syn-anti and synsyn being 0.4 and 1.0 kcal/mol higher in energy, respectively (B3LYP/6-31G*).15 BCPU crystallized from acetone/EtOH in the less stable syn-syn conformation, with the two CNPh rings, however, being significantly twisted relative to urea by 27.3 and 30.8°, which allowed the urea groups to self-associate by hydrogen bonding (Figure 3). However, only one of the two NH donors participates to these interactions, with the remaining NH donor hydrogen bonding to a CN group from a neighboring chain. Thus, although the urea‚‚‚urea hydrogen bonds were present in this case, they were significantly perturbed by the CN groups. Crystallization of BCPU from acetone/water resulted in the formation of a monohydrate, with the BCPU assuming the most stable anti-anti conformation. With a dihedral angle of 5.6°, one of the CNPh rings is nearly coplanar with urea and forms an intramolecular C-H‚‚‚O interaction with an observed H‚‚‚O distance of 2.29 Å. The second CNPh group is, however, significantly twisted relative to the urea group by 21.0°, which resulted in longer C-H‚‚‚O interactions of 2.35 Å. Nevertheless, no urea‚‚‚urea hydrogen bonds were present in this case, with the urea group hydrogen bonding a water molecule instead (Figure 3). BPU as an Anion-Binding MOF Linker: (a) SO42Coordination. Slow diffusion of an ethanol solution of BPU into an aqueous solution of ZnSO4 yielded a one-dimensional (1D) coordination polymer with the composition [Zn(SO4)(BPU)(H2O)3](EtOH) (1). Each zinc is coordinated by two pyridine groups, three water molecules, and a unidentate sulfate, arranged in an octahedral geometry. The BPU linker adopts the syn-anti conformation and is connected by the Zn nodes into a zigzag chain (Figure 4). One of the pyridine rings is nearly coplanar with urea (dihedral angle of 3.8°), which results in an intramolecular C-H‚‚‚O hydrogen bond with an H‚‚‚O contact distance of 2.27 Å. The second pyridine is twisted relative to urea by 19.5°. The urea group coordinates the sulfate through chelate hydrogen bonding (H‚‚‚O ) 2.01 Å, 2.09 Å;
