Selective host-guest interactions in metal-organic frameworks: Via multiple hydrogen bond donor-acceptor recognition sites

Article abstract of DOI:10.1039/c8ta12190g

Targeted recognition of medium sized molecules with mixed hydrogen bond units is essential for using porous materials for molecular separation, sensing and drug delivery. One promising way to achieve selectivity, is to make use of the key-and-lock principle guiding the design of hydrogen bond receptors matching the chemical signature of the target molecules. Among the class of porous materials metal-organic frameworks are particularly well suited for this purpose, as they allow for functionalizing the inner surfaces with various pending groups. Here we report the successful incorporation of 2-pyridyl urea (URPy) side groups with hydrogen bond donor-donor-acceptor (DDA) patterns into the framework MIL-101 with pores in the mesoporous range. Their influence on the sorption properties was investigated by competitive adsorption of 2-aminopyridine (2-AP) and 3-aminopyridine (3-AP) on MIL-101-URPy (Al, Cr) derivatives and comparison to the behaviour of a single donor function within MIL-101-NH₂ (Al, Cr) derivatives. Grafting the coordinatively unsaturated sites at the inorganic building units (IBUs) with diethylamine, additionally allowed the adsorption at these sites to be suppressed and thus to focus on the hydrogen bond receptors. Compared to the single D sites the selectivity of 2-AP over 3-AP is enhanced by a factor of five for the DDA pending groups. Based on ¹⁵N NMR spectroscopy and DFT calculations this observation is explained by forming double hydrogen bonds between the pyridyl urea groups and the 2-AP molecules, while 3-AP exhibits a single hydrogen bond only. At the D site of MIL-101-NH₂ both 2-AP and 3-AP form single hydrogen bonds only.

Full text of DOI:10.1039/c8ta12190g

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B.L. Tschense, L. Zappe, C. Koschnick, R. Siegel, R. Staeglich, B. V. Lotsch and J. Senker, J. Mater. Chem.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C8TA12190G
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ARTICLE
Selective host-guest interactions in metal-organic frameworks via
multiple hydrogen bond donor-acceptor recognition sites
T. Wittmann,a C. B. L. Tschense,a L. Zappe,a C. Koschnick,b R. Siegel,a R. Stäglich,a B. V. Lotsch,b J.
Received 00th January 20xx,
Accepted 00th January 20xx
Senker*a
DOI: 10.1039/x0xx00000x
Targeted recognition of medium sized molecules with mixed hydrogen bond units is essential for using porous materials for
molecular separation, sensing and drug delivery. One promising way to achieve selectivity, is to make use of the key-and-
lock principle guiding the design of hydrogen bond receptors matching the chemical signature of the target molecules.
Among the class of porous materials metal-organic frameworks are particularly well suited for this purpose, as they allow
for functionalizing the inner surfaces with various pending groups. Here we report the successful incorporation of 2-pyridyl
urea (URPy) side groups with hydrogen bond donor-donor-acceptor (DDA) patterns into the framework MIL-101 with pores
in the mesoporous range. Their influence on the sorption properties was investigated by competitive adsorption of 2-
aminopyridine (2-AP) and 3-aminopyridine (3-AP) on MIL-101-URPy (Al, Cr) derivatives and comparison to the behaviour of
a single donor function within MIL-101-NH2 (Al, Cr) derivatives. Grafting the coordinatively unsaturated sites at the inorganic
building units (IBUs) with diethylamine, additionally allowed the adsorption at these sites to be suppressed and thus to focus
on the hydrogen bond receptors. Compared to the single D sites the selectivity of 2-AP over 3-AP is enhanced by a factor of
five for the DDA pending groups. Based on 15N NMR spectroscopy and DFT calculations this observation is explained by
forming double hydrogen bonds between the pyridyl urea groups and the 2-AP molecules, while 3-AP exhibits a single
hydrogen bond only. At the D site of MIL-101-NH2 both 2-AP and 3-AP form single hydrogen bonds only.
www.rsc.org/
For example, subtle differences within the van der Waals interactions
by introducing polar framework sites were exploited to increase the
affinity for adsorbing carbon dioxide preferentially over nitrogen and
Introduction
Porous materials have potential for purification of gasses1–3 and
liquids,4,5 drug delivery6–8 and sensing.9,10 The desired selectivity for
host-guest interactions might be achieved by matching size, shape
and binding preferences of guest molecules with the framework
topology, respectively.11–13 For example, kinetic separation with
respect to size and shape is exploited for molecular sieving with
zeolites, which requires rigid and periodic structures with uniform
pores and windows.1 Thermodynamic separation based on preferred
binding sites, is achieved by providing chemical functionalities
adapted in type and strength to targeted guest molecules. This
includes van-der-Waals interactions, single and multiple hydrogen
bond pattern, -interactions and reversible bond formation for
Lewis acid/base pairs.1–3 This type of selectivity is widely spread for
metal-organic frameworks (MOFs) due to their modular constitution
and hybrid character. In particular, the organic linkers can be
functionalized in many different ways by direct synthesis and
postsynthetic modification (PSM), respectively.14–18
methane.2 Single hydrogen bonds accounted for the enhanced
binding strength towards acetone compared to ethanol and water in
MIL-53(Al)-NHCHO.19 The selectivity for ortho-xylene over the meta
and para analogues for MIL-53 (Al, Fe)20,21 was explained by a
simultaneous interaction of the two methyl units with two
carboxylate groups of the terephthalate linkers, which is possible
only for o-xylene due to the four-sided channel-type pores.20 In
contrast, the pronounced selectivity for p-xylene in MIL-47(V), MIL-
125(Ti), MIL-125(Ti)-NH2 and CAU-1(Al) solely arises from the shape
of their cavities promoting a more efficient molecular π-π stacking of
the respective isomer.22,23
Coordinatively unsaturated metal sites (CUS) lead to even higher
sorption affinities and selectivities by adjusting coordination bonds
via the Lewis acidity of the metal ions. For instance, CUSs with hard
Lewis acidic metals as in MIL-100 (Al, Fe, Cr, V)24 and MIL-101 (Cr)20–
23
proved to be effective for adsorption of hard Lewis basic N-
heterocyclic compounds, whereas CUSs with softer Lewis acidic
metals like HKUST (Cu)29 and CPO-27 (Ni, Co)25,30 preferred softer
Lewis basic sulphur compounds.
However, controlled loading and release of more complex molecules
bearing several functional groups like substituted heterocycles
cannot be realized by molecular sieving and unspecific functional
groups (e.g. -NH2, -OH); it requires specific interactions. These might
be achieved by supramolecular binding motifs, which are common
for the formation of biological and artificial self-assembly systems.31–
a. University of Bayreuth, Inorganic Chemistry III and Northern Bavarian NMR Centre
(NBNC), Universitätsstraße 30, 95440 Bayreuth, Germany. E-mail:
juergen.senker@uni-bayreuth.de
b. Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart,
Germany and Department of Chemistry, University of Munich (LMU),
Butenandtstr. 5-13, 81377 Munich, Germany Address here.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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34 They strongly rely on non-covalent -interactions and multiple Cr- and Al-MIL-101-URPy was obtained as follows. Referring to a
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hydrogen bond networks35 where geometric and electrostatic synthesis route from Holt et al.,51 triethylamDinOeI:(110.3.1803m9L/C, 8VTWA1R2)19w0aGs
complementarity between host and guest drives their association.36 added to a solution of 2-picolinic acid (1.23 g, Sigma Aldrich) in
This opens up the design of host-guest adducts that respond to dimethylformamide (15 mL, VWR) and stirred for 30 min at RT. Then,
certain external physical and chemical stimuli.32,36,37 The reversibility a solution of diphenyl phosphoryl azide (2.15 mL, Sigma Aldrich) in
of such binding patterns mainly depends on the strength of the dimethylformamide (2.5 mL, VWR) was added within 10 min under
multiple H-bond pattern and competing solvent-guest interactions. stirring. Having stirred for 3 h at RT, we poured the mixture over a
The former can be controlled by size and sequence of donor-acceptor water/dichloromethane solution. The organic phase was washed
units providing favorable or unfavorable binding sites.35,38
with a saturated solution of sodium hydrogen carbonate (VWR) in
Although, the incorporation of supramolecular building blocks into water followed by two times washing with water. After removing
MOFs has been shown in several studies,39 often crystallinity, dichloromethane under reduced pressure, white solid 2-pyridyl acyl
permanent porosity and chemical stability suffer.39 This makes azide (1.12 g, 76%) was obtained in high purity as proven by solution
detailed studies of their efficiency with respect to targeted 1H, 13C NMR and ATR-IR spectroscopy (Supporting Information). 2-
recognition challenging. Yaghi et al. attached macrocyclic polyethers pyridyl acyl azide was dissolved in dichloromethane (40 mL) and
to the linkers of MOF-1001 in order to stereospecifically poured over dried Cr- and Al-MIL-101-NH2 (300 mg). After refluxing
accommodate paraquat dications via ion-dipole interactions.40 More for three days, we cooled the suspension to RT, centrifuged and
recently, Li et al. integrated the nucleobase adenine as a co-ligand washed two times in fresh dichloromethane. The resulting solid was
into a ZnBTC-MOF and established complementary double hydrogen dried at 100 °C for 2 h.
bonding to thymine.41
Here we present the successful incorporation of pyridine substituted Liquid-phase adsorption
urea groups (URPy) pending at the organic linkers of MIL-101 by
Prior to the sorption experiments, the commercially obtained 2-
postsynthetic modification without affecting the long-range order of
aminopyridine and 3-aminopyridine (Sigma Aldrich) were purified by
the framework. The URPy units provide a donor-donor-acceptor
sublimation, dried in a desiccator and stored in argon atmosphere.
sequence (DDA) as recognition pattern, which is also effective in
Anhydrous 1,4-dioxane (Sigma Aldrich) was used to prepare the
nature42,43 and obeys the key-and-lock principle.36,37 We consider
solutions for the sorption experiments. The adsorbents were dried at
MIL-101 (Fig. S1) as an appropriate model framework due to its high
90 °C for 2 h under reduced pressure to remove physisorbed species.
chemical stability, large surface areas and pores in the lower
The liquid-phase adsorption and desorption isotherms were
mesoporous regime.44,45 The pore dimensions meet the requirement
obtained based on the batch circulation method using a piston pump
for accommodating space demanding anchor groups and guest
(Ismatec® IP 65), where the MOFs were successively equilibrated
molecules at the same time.
with the solutions of increasing and decreasing concentrations,
respectively. The MIL-101 derivatives (100 mg) were mixed with
We studied the sorption properties of the URPy units by using the
structural isomers 2-aminopyridine (2-AP) and 3-aminopyridine (3-
dried glass beads (250 mg, Ø 30 - 50 µm, Polysciences, Inc.) and
AP) as probe molecules. Out of these two only 2-AP presents a
packed into a glass pipette under argon atmosphere. This generated
counterpart to the recognition pattern of the URPy units. To be able
a permeable bed which prevented densification of the MIL-101
to separate the adsorption at the pending groups from the one at the
powder to guarantee a constant flow of the solutions (8 mL) through
CUS, the latter were also grafted with diethyl amine (DEA).28,46 All
the MIL-101 derivatives with a rate of 0.5 mL min-1 at room
samples were then characterized by combining multinuclear solid-
temperature. The quasi-equilibrium state was monitored every
state NMR spectroscopic measurements and DFT chemical shift
60 min and circulation was continued until the quasi-equilibrium
calculations to derive structural models of preferred host-guest
state was reached. The uptakes qeq were determined from gas
chromatographic data (Agilent 69890N) by using the differences
arrangements.
between the initial and quasi-equilibrium concentrations as shown in
equation (1)
Experimental
Synthetic procedures
(
)
풄ퟎ ― 풄풆풒 ∙ 푽 푴풇풓풂풎풆풘풐풓풌
풒풆풒
=
∙
(1)
풎
풔
where c0 is the initial concentration of the guest molecules in the
solution, ceq corresponds to the concentration after reaching the
quasi-equilibrium state, V is the volume of solution, m is the mass of
the adsorbent, Mframework is the molar mass of the frameworks as
given in Table 1 and s represents the number of strong adsorption
sites. The latter amounts to five, due to two positions at the CUS and
three pending groups. For the GC analyses dodecane was used as the
internal standard. The selectivity coefficients α were calculated
Al- and Cr-MIL-101-NH2 were synthesized according to a slightly
modified procedure of Hartmann et al.47 which we previously
described in ref. 48 using AlCl3 ∙ 6 H2O and CrCl3 as metal sources.49
The synthesis of the 15N enriched amino-terephthalic acid was
performed according to the protocol of Morris et al..50 Diethyl nitro
terephthalate was synthesized from reaction of potassium nitrate
(15N) with diethyl terephthalate. Afterwards, diethyl amino
terephthalate was obtained by treating diethyl nitro terephthalate
with hydrogen (5 bar) for 24 h on a Pd/charcoal. Finally, amino
terephthalic acid was obtained by reaction of diethyl amino
terephthalate (15N) with sodium hydroxide in a methanol/water
solution.
according to equation (2)
풒ퟐ ― 푨푷 풄ퟑ ― 푨푷
휶 =
∙
(2)
풒ퟑ ― 푨푷 풄ퟐ ― 푨푷
where q2-AP and q3-AP are the adsorbed amounts of 2-AP and 3-AP,
respectively, and c2-AP and c3-AP represent the bulk equilibrium
concentrations of the respective target molecules.
2 | J. Name., 2012, 00, 1-3
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Methods
6 µs for directly excited 15N. The recycle delays for 13CVCiePwaAnrtdicle15ONnlCinPe
of the frameworks were 2 s, 1800 s for 15N CP of the crystalline guest
DOI: 10.1039/C8TA12190G
Powder X-ray diffraction (PXRD) experiments were carried out in
Bragg-Brentano geometry on a Panalytical X'pert Pro diffractometer
equipped with a X'Celerator Scientific RTMS detector using Ni filtered
CuKα radiation (λ = 1.54187 Å, 40 kV, 40 mA). Measurements were
performed in the range of 2-30° (2θ) with a step size of 0.017°. The
simulated PXRD pattern was obtained by replacing Cr by Al in a
published structure model of Cr-MIL-101.52 The cell parameters of
the face centered cubic cell were refined to 87.7 Å while maintaining
molecules and 3600 s for the single pulse 15N NMR measurement.
1
1
The CP mixing times were set to 5 ms for H-13C and H-15N each.
Broadband proton decoupling was achieved by applying the
SPINAL6454 sequence with of nutation frequency of around 80 kHz.
The signal assignments of the NMR spectra were based upon
simulation of the chemical shifts using the ACD/Labs software
package;55 the IR vibration bands were identified according to
literature data.48,49
ퟑ
the space group F d m (No. 227).
Nitrogen sorption measurements were performed on
a
Attenuated total reflectance IR spectra were recorded in the range
400 - 4000 cm-1 with a resolution of 4 cm-1 on a Jasco FT/IR-6100
spectrometer with a PIKEGladiATR accessory.
Quantachrome NOVA 2000e at 77 K after evacuating the adsorbents
at 393 K for 14 h under reduced pressure. Brunauer-Emmet-Teller
(BET) equivalent surface areas were determined in the relative
pressure range between 0.06 and 0.15 to accommodate for
microporous materials.56 Specific total pore volumes were
determined at a relative pressure of 0.98 according to the Gurvich
rule.57 The pore-size distributions were derived using the argon
NLDFT model at 77 K for the adsorption branch assuming a
silica/zeolite surface and a spherical/cylindrical pore shape which is
implemented in the Quantachrome ASiQ v3.0 software package.
The samples for chemical analyses were prepared in a glove box
under argon atmosphere after evacuating at 90 °C for 2 h. Carbon,
hydrogen and nitrogen contents (wt%) were obtained on a Vario
elementar EL III. Aluminum and chromium contents (wt%) were
determined by atomic absorption spectroscopy (AAS) in extinction
mode on a Varian AA100 using a N2O/acetylene flame upon chemical
digestion in a mixture of hydrochloric acid, nitric acid and sulfuric
acid. EDX spectroscopy was carried out on a Joel JSM 6400 scanning
electron microscope equipped with a Noran energy dispersive X-ray
analyzer and using a beam voltage of 20 kV.
Solution 1H and 13C NMR spectra were obtained using a Bruker DRX
500 spectrometer operating at a proton frequency of 500.13 MHz.
Chemical shifts are given in parts per million using the DMSO-d6 peak
as an internal standard according to literature values (2.54 ppm for
1H and 40.45 ppm for 13C).53
All solid-state NMR spectra were acquired under magic-angle
spinning using ZrO2 rotors. The chemical shifts were referenced to
TMS for 13C, to nitromethane for 15N and to an aqueous solution of
AlCl3 ∙ 6 H2O for 27Al. 13C CP MAS measurements were collected on a
Bruker Avance III HD spectrometer at 9.4 T using a 4 mm double
resonance probe at a spinning frequency of 10 kHz. 15N CP MAS
spectra were recorded on a Bruker Avance II 300 spectrometer at a
spinning rate of 5 kHz using a 7 mm triple resonance probe. 13C CP
MAS and 15N CP MAS spectra were obtained after 90° pulse
Results and discussion
Synthesis and characterisation
Al- and Cr-MIL-101-NH2 were prepared as previously reported.48,49
Al- and Cr-MIL-101-URPy were synthesized after suspending Al- and
Cr-MIL-101-NH2 in a solution of 2-pyridyl acyl azide and subsequent
Curtius rearrangement to the reactive 2-pyridyl isocyanate (Scheme
1). Attempts to isolate and then use 2-pyridyl isocyanate for the PSM
reaction failed because of dimerization of the reactive isocyanates.51
Therefore, we isolated 2-pyridyl acyl azide after synthesis from 2-
picolinic acid (Figs. S2-S4, ESI) and loaded the azide species over the
internal surface of MIL-101-NH2. Thus, the spatially separated 2-
pyridyl isocyanate reacts more likely with the closer amino groups
than to undergo a dimerization (black bend arrow within Scheme 1).
1
excitation of 3.0 µs and 3.5 µs on the H channel, respectively, and
Scheme 1 Conventional and in situ routes for the synthesis of Al/Cr-MIL-101-URPy. Both routes are based on the preparation of 2-pyridyl
acyl azide. While conventionally the Curtius rearrangement to 2-pyridyl isocyanate is carried separately, 2-pyridyl isocyanate is created in
situ within the pore space of MIL-101-NH2 (represented by the scaffold) after loading the frameworks with 2-pyridyl acyl azide.
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DOI: 10.1039/C8TA12190G
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The inorganic building units of Cr-MIL-101-NH2 and Cr-MIL-101-URPy
were grafted with diethylamine (DEA) following the protocol of
Hwang et al.46 The activation of the accessible terminal metal sites
(CUS) was carried out at 120 °C for all frameworks to avoid
degradation of the URPy pending group at higher temperatures.
The PXRD patterns of all products fully coincide with those of the
respective parent MOFs and the simulated powder patterns. This
confirms successful MOF syntheses and retention of the structure
after PSM without formation of crystalline side products (Fig. S5).
The slight intensity variations are attributed to disorder of the
pending pyridyl urea and diethyl amine groups.
The PSM of the organic linkers was monitored by 13C and 15N NMR
spectroscopy of the diamagnetic Al-MIL-101 derivatives. The most
characteristic changes within the 13C MAS NMR spectra were
observed for the signal of the amino bearing carbon at 151.5 ppm
(Fig. 1a, black line), which shifts to 137.4 ppm upon PSM
Fig. 2 13C MAS NMR spectra of DEA-Cr-MIL-101-H (black line), DEA-
Cr-MIL-101-NH2 (green line) and DEA-Cr-MIL-101-URPy (blue line).
accompanied by the emergence of a signal for the urea carbon at
152.1 ppm (Fig. 1a, red line). Upon PSM the 15N NMR amino signal of
Al-MIL-101-NH2 at -316 ppm vanishes (Fig. 1b, black line), whereas
the ones for the pyridyl urea group appear at -272.7 ppm, -
263.7 ppm and -96.5 ppm (Fig. 1b, red line). Together with intensity
changes for the characteristic vibrational bands of the amino and
pyridyl urea groups within the IR spectra (Fig. S8), we expect an
almost quantitative PSM for the Al- and Cr-MIL-101 derivatives.
For the Cr-MIL-101 derivatives grafted with DEA, the 13C pNMR
spectra (Fig. 2) exhibit both hyperfine shifted and diamagnetic
resonances. This is typical for DEA coordinated to the CUS of the IBU
(hyperfine shifted signals) and DEA physisorbed in a second
coordination sphere (diamagnetic shifts).28 For the DEA@MIL-101-
URPy, we observed additional resonances between 140 and 160
ppm, originating from the URPy groups. The incorporation of DEA is
also supported by the small blueshifts of 9 cm-1 and 7 cm-1 for the
aliphatic methylene C-H and C-N stretching vibration bands of
coordinated DEA and free ligand (Fig. S8b). These shifts are in line
with the results for other alkyl amines adsorbed at metal
centres.46,58,59
Based on these results, EDX, AAS, elemental analysis and 1H solution
NMR spectroscopy (after digesting the products in basic solutions)
allowed to determine the chemical composition of the frameworks
and the yields of grafting with DEA as well as of the PSM (URPy).
Details of the corresponding analyses are given in the supporting
information (Figs. S9 and S10 and Table S1). The resulting formula
units are summarized in Table 1. In total, the PSM of the amino into
the pyridine urea pending groups was achieved with a yield of about
90% for both the chromium and aluminum frameworks. For the
exchange of water with DEA molecules at the CUS of the Cr-MIL-101-
X derivatives a conversion degree larger than 60% was reached.
Fig. 1 a) 13C CP MAS NMR spectra of Al-MIL-101-NH2 (black line)
and Al-MIL-101-URPy (red line) and b) 15N CP MAS NMR spectra of
Al-MIL-101-NH2 (black line) and Al-MIL-101-URPy (red line)
including assignment of all resonances. The weak signal at -247
ppm within the 15N MAS spectrum of Al-MIL-101-NH2 (black line)
was attributed to a formamide moiety, which might form by a side
reaction of DMF and the amino groups. Its total amount is less than
5%. For more information refer to Fig. S6.
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Table 1 Experimental formula units for the synthesized MIL-101
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derivatives.
DOI: 10.1039/C8TA12190G
Formula unit
Al-MIL-101-NH2
Al-MIL-101-URPy
Al3O(H2O)2Cl(bdc)3
Al3O(H2O)2Cl(bdc-NH2)0.3(bdc-URPy)2.7
H2O-Cr-MIL-101-NH2 Cr3O(H2O)2.1Cl0.9(bdc-NH2)3
DEA-Cr-MIL-101-NH2 Cr3O(H2O)0.85(DEA)1.5Cl0.95(bdc-NH2)3
H2O-Cr-MIL-101-
URPy
Cr3O(H2O)2.04Cl0.96(bdc-NH2)0.3(bdc-
URPy)2.7
DEA-Cr-MIL-101-
URPy
Cr3O(H2O)1.05(DEA)1.2Cl0.97(bdc-
NH2)0.3(bdc-URPy)2.7
Nitrogen sorption isotherms reveal lower porosities after PSM for Al-
MIL-101-NH2 and Cr-MIL-101-NH2 (Fig. S11 a,c). The bulkier URPy
groups reduced the apparent BET surface areas and total pore
volumes by nearly 50 % from 2340 m2 g-1 and 1650 m2 g-1 for Al-MIL-
101-NH2 and Cr-MIL-101-NH2 to 1325 m2 g-1 and to 779 m2 g-1 for the
URPy derivatives (Table S2). The DEA molecules reduced the latter
quantities again by about 30 % for Cr-MIL-101-NH2 and about 20 %
for Cr-MIL-101-URPy (Table S2). This is in line with the pore size
distribution analyses. While for the amino derivatives cavity sizes of
2.5 nm and 3.0 nm were obtained, the URPy functionalization
reduced the pores to 2.0 nm and 2.7 nm. Grafting the CUS of Cr-MIL-
101-X with DEA both pores are reduced in size again by about 0.3 to
0.4 nm (Table S2 and Fig. S11 b, d).
Sorption experiments
Fig. 3 Competitive adsorption isotherms of 2-AP (solid squares) and
3-AP (empty circles) on a) Al-MIL-101-NH2 (black curves) and DEA-
Cr-MIL-101-NH2 (green curves); b) Al-MIL-101-URPy (red curves)
and DEA-Cr-MIL-101-URPy (blue curves).
To characterize the binding preferences and sorption properties of
the pending pyridyl urea groups, which could not be incorporated
into the pore space of MOFs before, we determined adsorption
isotherms for H2O@Al-MIL-101-X, H2O@Cr-MIL-101-X and DEA@Cr-
MIL-101-X with X equal to amino and pyridyl urea groups. As probe
molecules, 2-AP and 3-AP were used in single and competitive
experiments. For H2O@Al-MIL-101-X additionally, desorption
isotherms were collected. The sorption isotherms are depicted in
Figs. 3 and S12 - S14 and are expressed as adsorbed molecules per
binding formula unit (FU) as function of the equilibrium
concentration of the bulk liquid phase. The calculation of the
molecular weight per formula unit is based on the chemical
compositions of the frameworks (Table 1).
The single adsorption isotherms (Fig. S12) show almost identical
uptakes for the pairs H2O@Al-MIL-101-NH2 / H2O@Cr-MIL-101-NH2
and H2O@Al-MIL-101-URPy / H2O@Cr-MIL-101-URPy. This was
observed for 2-AP and 3-AP, respectively. Furthermore, some of the
desorption branches depicted in Fig. S13 show a hysteresis, which
indicates that some guest molecules, probably the ones attached to
the CUSs at the IBU, are harder to remove than the ones interacting
with the side chains. For the Cr-MIL-101 framework it is known, that
both probe molecules interact strongly with the CUS at the IBUs.28
Due to the equivalence of the single adsorption isotherms for the
above-mentioned pairs we expect a similar behavior for the Al-MIL-
101 derivatives. Indeed, the 27Al MAS NMR spectra of 2-AP@Al-MIL-
101-NH2 and 3-AP@Al-MIL-101-NH2 show a significantly more
asymmetric line shape compared to H2O@Al-MIL-101-NH2 (Fig. S15).
We attribute this to a larger second-order quadrupolar interaction,
due to an increasingly asymmetric charge distribution after attaching
2-AP and 3-AP, respectively.
By grafting the CUS with DEA prior to the adsorption experiments,
we strive to minimize the influence of the CUS on the isotherm to
single out the selective adsorption at the pending groups X. Since Al-
MIL-101 does not withstand the grafting conditions, these
experiments were carried out for the Cr-MIL-101-X derivatives only.
The reduced uptake for both 2-AP and 3-AP for the single isotherms
of DEA@Cr-MIL-101-X (Fig. S12) shows, that the blocking of the CUS
is effective. As such, we take differences in the competitive
adsorption isotherms (mixtures of 2-AP and 3-AP) of H2O@Al-MIL-
101-X and DEA@Cr-MIL-101-X and X being amino (Fig. 3a) and pyridyl
urea (Fig. 3b) groups as a measure for the selective adsorption at the
pending groups.
The competitive adsorption isotherms follow Langmuir type
isotherms (Fig. 3). The amino bearing derivatives (Fig. 3a) take up
significantly more 3-AP from the mixture compared to 2-AP. For the
highest uptakes this amounts to a factor of two and four for H2O@Al-
MIL-101-NH2 and DEA@Cr-MIL-101-NH2, respectively. While the
uptake of 2-AP is only slightly reduced for both frameworks, blocking
the CUS reduces the uptake of 3-AP by 50%. This follows the binding
affinity derived before28 and demonstrates that 3-AP is preferentially
attached to the CUS, while 2-AP is adsorbed in an unspecific fashion.
For the pyridyl urea derivatives the uptake of 2-AP is strongly
enhanced. Compared to the amino MIL-101 this results in a factor of
five. Simultaneously, the uptake of 3-AP is slightly reduced compared
to the amino derivatives. Grafting the CUS is apparently less effective
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DOI: 10.1039/C8TA12190G
Fig. 4 Selectivity values α on Al-MIL-101-NH2 (black curve), Al-MIL-
101-URPy (red curve) and DEA-Cr-MIL-101-URPy (blue curve).
as the uptake of DEA@Cr-MIL-101-URPy is only slightly lower than
the one of H2O@Al-MIL-101-URPy. This trend is explained by two
effects. On the one hand, the grafting reaction is less efficient for the
pyridyl urea derivatives leaving roughly 40% of the CUS accessible.
On the other hand, the interactions of 3-AP and 2-AP with the pyridyl
urea units will be stronger, providing a stronger competition to the
CUS. This effect is also observed for the 27Al MAS NMR spectra of 2-
AP@Al-MIL-101-URPy and 3-AP@Al-MIL-101-URPy, which are both
similar to the one of H2O@Al-MIL-101-URPy indicating that
coordination to the CUS is less relevant. For 2-AP the affinity to the
pending groups seems to be significantly more pronounced as shown
in the increasing uptake and the expected better match with the DDA
recognition pattern.
As a consequence, we observe a clear trend for the selectivity of 2-
AP over 3-AP (Fig. 4) represented in the selectivity coefficient α2-AP vs.
3-AP (equation 2). While amino functionalized derivatives with active
CUS exhibit a selectivity of about 0.4 at higher uptakes (Fig. 4 black
curve), introducing the pyridyl urea groups increases the selectivity
by roughly a factor of three. By additionally blocking the CUS, the
selectivity raises again by roughly 50 %. In total, the selectivities of
the amino and pyridyl urea groups differ by a factor of  4,
demonstrating that the difference for the bonding affinity between
a single donor (NH2) and a donor-acceptor pattern (URPy) is large
enough to separate 2-AP and 3-AP in solutions. Both PXRD data (Fig.
S16) and TEM images (Figs. S17 – S18) demonstrate that neither the
crystallinity nor the morphology of Al/Cr-MIL-101-X changed
significantly upon loading with 2-AP, 3-AP and mixtures thereof. As
such, the increase of the selectivity is attributed to the host-guest
interactions.
Fig. 5: a) 15N NMR CP MAS spectra from top to bottom: 0.25 M 3-
AP@Al-MIL-101-NH2, 3-AP, 0.25 M 2-AP@Al-MIL-101-NH2, 2-AP
and Al-MIL-101-NH2; b) Enlarged section of the 15N NMR (98% 15
N
labeled) spectra from top to bottom: 0.25 M 3-AP@Al-MIL-101-
15NH2, 0.05 M 3-AP@Al-MIL-101-15NH2, 0.25 M 2-AP@Al-MIL-101-
15NH2, 0.05 M 2-AP@Al-MIL-101-15NH2 and Al-MIL-101-15NH2. The
vertical lines in a) and b) represent calculated iso values for the
model with the best match to the experimental data. The
corresponding structure is depicted on the left (3-AP) and right (2-
AP). The asterisk denotes a small impurity of NHCHO units (Fig. S6).
depicted in Figs. 5 – 7 and the simulated iso values for the most likely
adducts are plotted in colored vertical lines.
The spectrum of 3-AP@Al-MIL-101-NH2 shows three signals at -
109 ppm (N2), -303 pm (N3) and -325 ppm (N1) (Fig. 5a, olive line).
They match 3-AP@amino adducts best, where the pyridine nitrogen
(N2) of 3-AP is hydrogen bonded to one of the amino protons (N3-H3).
This causes a downfield shift for N3 and an upfield shift for N2
compared to pure 3-AP and H2O@MIL-101-NH2 (Fig. 5a, orange and
black lines). The signal for the amino groups of 3-AP (N1), which do
not participate to the adduct formation, is not affected. In spite of
the broadening of the resonances within the 27Al MAS spectra of 2-
AP/3-AP@MIL-101-NH2 (Fig. S15), which hints towards a competing
interaction of the guest molecules with the CUS, no resonances for
the corresponding adducts could be singled out within the 15N MAS
NMR spectra (Fig. 5a). Characteristic resonances for adducts should
cause upfield shifts for the pyridine nitrogen (N2) by approximately
Host guest interactions
In order to develop a better understanding for preferred binding sites
at the amino and pyridyl urea groups we collected 15N MAS spectra
for all Al-MIL-101 derivatives as function of loading with 2-AP/3-AP.
These data were compared to results of quantum chemical
calculations on DFT level providing energetically favored spatial
arrangements for the adducts of guests and pending groups as well
as simulated isotropic chemical shifts for the nitrogen sites within
these adducts. The corresponding 15N MAS NMR spectra are
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40-50 ppm according to DFT calculations and literature data.60 We The 15N MAS NMR spectra of 2-AP@Al-MIL-101-URPy exhibit
View Article Online
DOI: 10.1039/C8TA12190G
thus conclude that at RT the equilibrium between adsorption at the resonances typical for the urea pyridyl group and 2-AP molecules
CUS and the pending groups is dynamic with exchange rates in the (Table S3). With respect to the single component phases 2-AP and Al-
fast motion regime. Thus, only one averaged resonance at -109 ppm MIL-101-URPy the resonances for the amino group N1, the outer
(N2, 3-AP) is visible within the 15N MAS NMR spectra.
amido fragment N4 and the pyridyl nitrogen N2 of 2-AP are
A similar trend is observed for the 15N MAS NMR spectrum (Fig. 5a, downfield-shifted by roughly 9, 3 and 7 ppm. Simultaneously, the
magenta line) of 2-AP@Al-MIL-101-NH2, which exhibits resonances pyridyl nitrogen of the URPy unit (N5) shift upfield by 5 ppm. These
at about -142 ppm (N2) and -306 ppm (N3-H3, N1). The heavy overlap trends become more pronounced with higher loading and lower
of the signals for N3 and N1, however, does not allow to determine temperatures as the hydrogen bonds between the pending groups
the downfield shift quantitatively and thus prevents to judge the and the guests become stronger (Fig. 6). These findings are matched
binding preference of 2-AP and 3-AP at the amino function of the best with the prediction of isotropic chemical shifts at the DFT level
framework. The resonance assigned to N2 matches closely the (vertical lines within Fig. 6 and Table S3) for a scenario where 2-AP
simulated iso for the fragment 2-AP@bcd-NH2, which hints to a and the pending URPy groups form the complementary hydrogen
preferential adsorption of 2-AP at the amino function. This in line bond pattern DAAD. According to the DFT calculations this scenario
with the weaker binding strength of 2-AP compared to 3-AP at the it energetically favored by roughly 80 kJ/mol with respect to the
CUS.28
single component phases. Additionally, for higher loadings also 2-AP
By labeling Al-MIL-101-NH2 with 15N at the amino group we could adsorbed in an unspecific fashion was observed as demonstrated by
single out the characteristic resonances for the free amino units as the 15N NMR signal at around -307.0 ppm, which is typical for 2-AP in
well as for the 2-AP/3-AP@amino adducts (Fig. 5b). With increasing its crystalline form (Fig. 6, green line).
guest loading the downfield shift rises for both guests reaching 4.4 In contrast, the DFT calculations suggested at least two possible
ppm for 2-AP and 7.5 ppm for 3-AP after equilibrating with solutions arrangements for 3-AP within Al-MIL-101-URPy. Both are
of 0.25 M for each guest. This suggests, that 3-AP exhibits a higher depicted in Fig. 7. For the first one, 3-AP forms weak H- bonds
binding affinity to the amino function compared to 2-AP, which is in between its pyridine nitrogen N2 and the N4-H4 unit of the urea
line with an energy lower by 6 kJ/mol for 3-AP as derived from the fragment and between the aromatic C-H bond in ortho position
DFT calculations.
to N2 and the pyridine nitrogen atom N5 of URPy. This induces
In case of Al-MIL-101-URPy the DFT calculations suggest that the urea significant high- and lowfield shifts for the resonances of N2 and
fragment close to the linker N3-H3 forms a strong hydrogen bond with N4, while the one for N5 remains essentially unchanged. The
the oxygen atoms of neighboring carboxylate groups of the second scenario is stabilized by a single H bond between the
terephthalates. Therefore, the N3-H3 fragment is not available for amino hydrogen atom N1-H1 of 3-AP and the pyridine nitrogen
hydrogen bonding to the guest molecules 2-AP/3-AP and reduces the N5 of the pending urea pyridyl groups. Again, the resonance for
DDA to a DA pattern. This is experimentally supported as the 15N the amino function (N1) is shifted downfield while the signal for
NMR shift of N3 at -273.5 ppm is essentially independent of the guest N5 shifts highfield. While the double H-bond scenario is favored
loading (Fig. 6 and 7).
by 50 kJ/mol with respect to the single component phases
according to the DFT calculations the single H-bond scenario is
Fig. 6 15N CP MAS NMR spectra of Al-MIL-101-URPy (black line), 2-
AP (green line), 0.05 M 2-AP@Al-MIL-101-URPy (orange line),
0.25 M 2-AP@Al-MIL-101-URPy (blue line) and 0.25 M 2-AP@Al-
MIL-101-URPy at -25 °C (magenta line). Red vertical lines represent
the chemical shifts derived from preferred arrangements of 2-AP
within Al-MIL-101-URPy. The black vertical line corresponds to the
chemical shift of N2 for protonated 2-AP (2-APH+).61
Fig. 7 15N CP MAS NMR spectra of Al-MIL-101-URPy (black line), 3-
AP (green line), 0.05 M 3-AP@Al-MIL-101-URPy (orange line),
0.25 M 3-AP@Al-MIL-101-URPy (olive line) and 0.25 M 3-AP@Al-
MIL-101-URPy at -25 °C (magenta line). The green and blue vertical
lines indicate the chemical shifts derived from most preferred
arrangements. The black vertical line corresponds to the chemical
shift of N2 for protonated 3-AP (3-APH+).61
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stabilized by roughly 30 kJ/mol. The respective 15N chemical resolved 15N MAS NMR spectra. We then compared the
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DOI: 10.1039/C8TA12190G
shifts are given in Table S3.
observed chemical shifts with the ones calculated for the DFT
The observed 15N NMR spectra of 3-AP@Al-MIL-101-URPy model systems to probe preferred adsorption sites at the side
exhibit resonances typical for of both arrangements (Fig. 7 and groups.
Table S3) for all loadings and temperatures. In spite of the Both H2O@Al-MIL-101-NH2 and H2O@Cr-MIL-101-NH2 showed
stronger stabilization for the double H-bond scenario no clear enhanced uptakes and selectivities for 3-AP. By blocking the
trend for the intensities assigned to both scenarios is visible. CUS with DEA the uptake for both 2-Ap and 3-AP is reduced with
This indicates that both scenarios exhibit similar interaction a stronger effect for 3-AP. This suggests, that 3-AP exhibits a
strengths experimentally. We attribute this to secondary higher affinity to the CUS compared to 2-AP. At the same time,
interactions with further guest and solvent molecules which both guest molecules are attached to the pending amino
might lead to additional stabilizing contributions. The latter functions by single hydrogen bonds between the pyridine
could not be considered for the DFT calculations.
nitrogen and the NH unit of the amino functions as indicated by
Both the 15N NMR spectra for 2-AP@Al-MIL-101-URPy (Fig. 6) the 15N MAS NMR spectra as function of the guest loading.
and 3-AP@Al-MIL-101-URPy (Fig. 7) exhibit an additional After introducing the pending URPy groups the selectivity for 2-
resonance at -220 ppm (2-AP@Al-MIL-101-URPy) and –176 ppm AP is enhanced by a factor of three and blocking the CUS for
(3-AP@Al-MIL-101-URPy). Both signals are typical, for guest DEA@Cr-MIL-101-URPy increases the selectivity by another
molecules protonated at the pyridyl nitrogen atoms.61 We 50%. The latter effect is mainly caused by a reduction of the
attribute this to an increased acidity of residual water in the uptake of 3-AP. The 15N MAS NMR spectra collected for 2-AP/3-
vicinity of the pending URPy groups, which partially protonate AP@Al-MIL-101-URPy demonstrated that 2-AP is indeed
the guest molecules.
adsorbed at the pending URPy groups according to the key-and-
lock principle via complementary double hydrogen bonds, while
for 3-AP two scenarios seem to be equally attractive. Both of
these arrangements feature one single hydrogen bond only and
are thus less favored by roughly 30 kJ/mol compared to the
double hydrogen bond scenario realized with 2-AP.
Our results suggest that introducing selectivity into the sorption
processes of MOFs can be established by introducing side
groups with multiple hydrogen bonding motifs, which are
selected to bind targeted guest molecules via the key-and-lock
principle of supramolecular chemistry. The concept is realized
straightforwardly and can thus be applied not only to MOFs but
to a broad variety of porous materials. This offers potential for
targeted recognition of larger and complex molecules, which is
necessary for applications like specific separation, sensors and
drug delivery.
Conclusions
Here we report on the successful incorporation of a 2-pyridyl
urea side group with an apparent DDA hydrogen bond donor-
acceptor pattern into the framework topology MIL-101 with
pore cavities in the mesoporous range. For this, the parent
material MIL-101-NH2 was modified postsynthetically by
loading it with 2-pyridyl acyl azide solutions. The reactive
intermediate 2-pyridyl isocyanate, formed in situ in the pore
space, then reacts preferentially with the amino functions of the
framework to the desired product with a yield of about 90 %
instead of undergoing a dimerisation.
The influence of the pending URPy groups on the sorption
properties of MIL-101-URPy was studied by using a combination
of solution phase sorption experiments, multinuclear MAS NMR
spectroscopy and DFT calculations for various model systems.
All energetically favored structure models feature a strong intra
framework hydrogen bond between the inner urea N-H units
and the adjacent carboxylate groups of the terephthalate
linkers. We thus expect the pyridyl urea groups to act as a
double hydrogen bond donor-acceptor (DA) within the
framework instead of the DDA pattern typical for free URPy
groups. As probe molecules for the sorption preferences, we
thus loaded MIL-101-URPy frameworks with organic solutions
of 2-AP and 3-AP. While 2-AP matches both function and
geometry of the accessible hydrogen bond donor-acceptor
sequence of the pending URPy groups, 3-AP does not.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
We gratefully acknowledge financial support by the DFG (SE
1417/7-1 and SE 1417/8-1 as well as LO 1801/4-1 within the
SPP1928). We thank Beate Bojer for her steady support with the
NMR measurements.
Notes and references
By analyzing the response of two pairs of MIL-101 derivatives –
Al-MIL-101-NH2 and Cr-MIL-101-NH2 as well as Al-MIL-101-
URPy and Cr-MIL-101-URPy on single component and
competitive adsorption isotherms, we were able to separate
the influence of the pending URPy groups from CUSs at the
metal centers of the IBUs. While the higher chemical stability of
Cr-MIL-101-X enabled us to block the CUS with diethyl amine
and thus to reduce their influence on the adsorption isotherms
markedly, the diamagnetic Al-MIL-101-X derivatives lead to well
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