A metal-organic framework containing unusual eight-connected Zr-oxo secondary building units and orthogonal carboxylic acids for ultra-sensitive metal detection

Article abstract of DOI:10.1002/chem.201405194

Two metal-organic frameworks (MOFs) with Zr-oxo secondary building units (SBUs) were prepared by using p,p′-terphenyldicarboxylate (TPDC) bridging ligands pre-functionalized with orthogonal succinic acid (MOF-1) and maleic acid groups (MOF-2). Single-crystal X-ray structure analysis of MOF-1 provides the first direct evidence for eight-connected SBUs in UiO-type MOFs. In contrast, MOF-2 contains twelve-connected SBUs as seen in the traditional UiO MOF topology. These structural assignments were confirmed by extended X-ray absorption fine structure (EXAFS) analysis. The highly porous MOF-1 is an excellent fluorescence sensor for metal ions with the detection limit of <0.5 ppb for Mn²⁺and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.

Full text of DOI:10.1002/chem.201405194

DOI: 10.1002/chem.201405194
Communication
&
Metal–Organic Frameworks
A Metal–Organic Framework Containing Unusual Eight-Connected
Zr–Oxo Secondary Building Units and Orthogonal Carboxylic
Acids for Ultra-sensitive Metal Detection
Michaꢀl Carboni, Zekai Lin, Carter W. Abney, Teng Zhang, and Wenbin Lin*[a]
First reported by Lillerud and co-workers, the UiO family of
Abstract: Two metal–organic frameworks (MOFs) with Zr–
oxo secondary building units (SBUs) were prepared by
using p,p’-terphenyldicarboxylate (TPDC) bridging ligands
pre-functionalized with orthogonal succinic acid (MOF-1)
and maleic acid groups (MOF-2). Single-crystal X-ray struc-
ture analysis of MOF-1 provides the first direct evidence
for eight-connected SBUs in UiO-type MOFs. In contrast,
MOF-2 contains twelve-connected SBUs as seen in the tra-
ditional UiO MOF topology. These structural assignments
were confirmed by extended X-ray absorption fine struc-
ture (EXAFS) analysis. The highly porous MOF-1 is an ex-
cellent fluorescence sensor for metal ions with the detec-
tion limit of <0.5 ppb for Mn2+and three to four orders of
magnitude greater sensitivity for metal ions than previous-
ly reported luminescent MOFs.
MOFs possess a robust Zr6O4(OH)4(RCO2)12 secondary building
unit (SBU) and are stable under a wide range of conditions.[10]
Using a p,p’-terphenyldicarboxylic (TPDC) ligand pre-functional-
ized with a phosphorylurea moiety, we recently demonstrated
this UiO platform was sufficiently stable for extracting UVI from
aqueous solutions.[11] While the high connectivity of the
Zr6O4(OH)4(RCO2)12 SBU helps to stabilize the MOF structure,
partial removal of ligands would result in a larger pore aper-
ture, enhancing the rate of diffusion into the MOF interior and
permitting larger molecules to be internalized. Recent publica-
tions reveal the addition of acid modulators during synthesis
results in UiO materials with increased surface area and larger
pore sizes.[12] It has been proposed that these effects are due
to formation of site defects at the MOF SBUs, resulting in
fewer than twelve dicarboxylate bridging ligands connecting
the Zr–oxo clusters. This postulation was supported through
various indirect analyses, such as thermogravimetric analysis
(TGA), 19F NMR, and IR spectroscopy,[13] as well as more ad-
vanced techniques such as PXRD/EXAFS[14] and high-resolution
neutron power diffraction,[15] but has thus far evaded single-
crystal X-ray diffraction analysis owing to the randomly distrib-
uted and stochastically oriented site defects in the previous
UiO MOF materials.
Metal–organic frameworks (MOFs) are a class of hybrid materi-
als built from metal ions or clusters bridged by organic link-
ers.[1] Due to their high porosity, stability, and tunability, these
materials are attractive for diverse applications.[2] Decorating
the bridging ligand with an orthogonal functionality represents
an effective strategy to functionalize MOF channels for poten-
tial applications. The incorporation of amine, carboxylic acid,
and other functional groups into MOFs has indeed resulted in
promising materials for CO2 sorption,[3] chemical sensing[4] and
catalysis.[2e,5] Direct synthesis of MOFs using pre-functionalized
ligands is challenging, as the orthogonal moieties may coordi-
nate metals during MOF preparation. While postsynthetic
modification (PSM)[6] and protection/deprotection[7] are effec-
tive strategies for introducing these functionalities, quantitative
functionalization is rarely achieved. As a result, few examples
of MOFs containing non-coordinated orthogonal functionalities
have been reported, either through use of pre-functionalized
ligands[8] or via PSM processes,[9] severely limiting the applica-
tions of many MOFs.
Herein, we report the preparation of two MOFs based on
Zr–oxo SBUs and TPDC ligands pre-functionalized with orthog-
onal moieties terminating with succinic acid (MOF-1) and
maleic acid (MOF-2). Single-crystal X-ray structure analyses re-
vealed eight-connected Zr6O8(OH2)8(RCO2)8 SBUs in MOF-1 and
twelve-connected Zr6O4(OH)4(RCO2)12 SBUs in MOF-2, which
was confirmed by fitting EXAFS data. MOF-1 provides the first
direct evidence for a UiO-type MOF possessing a SBU with
fewer than twelve bridging ligands. MOF-1 exhibits larger pore
apertures than MOF-2 and is highly sensitive for detecting
metal ions via fluorescence quenching.
The TPDC ligands with orthogonal succinic acid and maleic
acid functionalities, H2L1 and H2L2, were prepared by treating
2,5-bis(4-carboxyphenyl)aniline (TPDC-NH2) with succinic or
maleic anhydride in dry DMSO at room temperature for 12 h.
Following solvent removal, the solid was washed with water
and ethanol, then dried under vacuum to afford pure H2L1 and
H2L2 ligands. Crystals of MOF-1 and MOF-2 were obtained by
heating a solution of ZrCl4, H2L1 or H2L2, and trifluoroacetic
acid (TFA) in DMF at 1008C for three days (Scheme 1).
[a] Dr. M. Carboni,+ Z. Lin,+ C. W. Abney, T. Zhang, Prof. Dr. W. Lin
Department of Chemistry
University of Chicago
929 E 57th Street, Chicago, IL 60637 (USA)
E-mail: wenbinlin@uchicago.edu
[+] These authors contributed equally to this work.
A single-crystal X-ray diffraction study showed that MOF-
1 crystallizes in the tetragonal I4/mmm space group (Figure 1).
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405194.
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formula of Zr6O8(OH2)8(L1)4. Compared to a typical
Zr6O4(OH)4(L)6 cluster, the four bridging carboxylate
groups on the equatorial plane of the Zr6 octahedra
are missing, substituted by aqua coordination. The
resulting MOF possesses bcu network topology with
{424·64} connectivity. Although a similar SBU has been
reported in Zr MOFs constructed from tetracarboxy-
late ligands,[16] this is the first direct crystallographic
evidence for an eight-connected Zr SBU with a linear
bridging ligand, confirming the earlier proposal put
forward by Lillerud, Zhou, De Vos, and Hupp.[12–14]
¯
MOF-2 crystallizes in the cubic Fm3m space group.
The asymmetric unit consists of 1/48 of the
Zr6O4(OH)4(CO2)12 cluster and 1/8 of the bridging li-
gands. The SBU comprises six Zr cations, with each Zr
bridged by m3-O, m3-OH, and carboxylate groups.
Each SBU is connected to twelve neighboring SBUs
Scheme 1. Direct synthesis of MOFs 1 and 2 with pre-functionalized ligands possessing
orthogonal carboxylic acid moieties.
by twelve dicarboxylate linkers. Like other well-established UiO
MOFs, MOF-2 possesses fcu network topology.
Owing to the crystallographically imposed disorder for the
bridging ligands in UiO-type MOFs, the identities of the bridg-
ing ligands in MOFs 1 and 2 cannot be established by single-
crystal X-ray structure analyses. Instead, the identities of bridg-
ing ligands in MOFs 1 and 2 were determined by integration
1
of proton peaks in the H NMR spectra of digested samples of
1
MOFs 1 and 2 (in K3PO4/D2O/[D6]DMSO). H NMR spectroscopy
unambiguously revealed that the H2L1 ligand did not decom-
pose during the MOF synthesis; the 1H NMR spectrum ob-
tained for digested MOF-1 (Figure S7c) matches perfectly with
that of pure ligand H2L1 in a solution of K3PO4/D2O/[D6]DMSO
(Figure S7b). ESI-MS analysis (Figure S8) confirms the purity of
the digested MOF-1 with a unique peak at m/z 432.1 corre-
sponding to [H2L1-H]À. Unlike MOF-1, significant decomposi-
1
tion of the ligand is observed in the H NMR spectrum of di-
gested MOF-2 (Figure S9). Integration of the ethylene peaks
(d=5.75 and 6.25 ppm) indicate that approximately 25% of
the original L2 ligand remained intact after MOF synthesis. This
was further supported by ESI-MS analysis (Figure S10) exhibit-
ing signals at m/z 432.1 ([H2L2 +H]+), m/z 470.1 ([H2L2 +K]+),
and m/z 334.1 ([TPDC-NH2 +H]+). The decomposition of H2L2
can be reduced to 50% by decreasing the duration of MOF-2
synthesis to one day, while prolonged heating (7 days) results
in 91% degradation of L2 ligands (Figure S11). The difference
in topology can be attributed to the hydrolysis of the orthogo-
nal functionality in MOF-2, alleviating steric effects and accom-
modating formation of a twelve-connected SBU.
Figure 1. Comparison of eight- and twelve-connected MOF topologies from
the UiO family. The figures for eight-connected MOF-1 are displayed on the
left, and the figures for twelve-connected MOF-2 are shown on the right. a)
SBUs, b) octahedral cavities, and c) perspective views along the z axis. Chan-
nels of both MOFs show distinct differences as a result of different SBU con-
nectivity. Blue polyhedra and spheres are Zr atoms, with red and grey repre-
senting oxygen and carbon, respectively. Hydrogen atoms are omitted for
clarity, with the exception of those on equatorial water molecules and bridg-
ing m3-OH groups on the SBUs.
TGA results were consistent with the formulations of MOFs
1 and 2 as established by X-ray crystallography and NMR anal-
yses (Table S1). While the TGA profile obtained for MOF-1 is in
agreement with the presence of pure L1 ligands, the TGA data
for MOF-2 are consistent with partial decomposition of L2 li-
gands during MOF synthesis.
In spite of the significant structural difference between
MOFs 1 and 2, their PXRD patterns appear remarkably similar
(Figure 2c), thus offering no insight into the number of bridg-
ing ligands linking the SBUs in the bulk materials of MOFs
1 and 2. We have utilized X-ray absorption spectroscopy (XAS)
The asymmetric unit consists of 1/16 of the Zr6O8(OH2)8(CO2)8
cluster and 1/4 of the bridging ligands. Unlike traditional UiO
MOFs, the Zr SBU in MOF-1 is only eight-connected, with the
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68-NH2, as is expected from the
different SBUs obtained by
single-crystal
diffraction
(Figure 3). The spectral compo-
nent attributable to single scat-
tering off the bridging m3-oxo
group indicates a shorter path
length for MOF-1 than the other
two MOFs. This is expected, as
the twelve-connected UiO is
known to have alternating m3-
oxo/m3-hydroxo species to ac-
comodate charge balance in the
SBU, while the eight-connected
SBU would only possess m3-oxo
moieties. The single scattering
path off of the carboxylic carbon
is longer for MOF-2 than MOF-
1 or UiO-68. A minimal signal is
observed in this region due to
destructive interference from
multiple scattering paths,[14]
making definitive analysis chal-
lenging. Importantly, the inclu-
sion of a Zr–OH2 scattering path
was essential to obtain a good
fit for MOF-1, as the ZrÀO bond
Figure 2. a) TGA curves, b) N2 uptake isotherms, c) powder X-ray patterns, and d) pore size distributions for UiO-
68-NH2, MOF-1, and MOF-2.
to lend additional support for the structural assignments of
MOFs 1 and 2 and to prove the phase purity of these function-
alized UiO materials. MOF-1, MOF-2, and UiO-68-NH2 were in-
vestigated by XAS at the Zr K-edge, with data processed and
analyzed by using the Athena and Artemis programs of the
IFEFIT package based on FEFF 6.[17] A comparison of the X-ray
absorption near-edge (XANES) region reveals slight differences
between the three MOFs (Figure S22 and S23). MOF-1 has
slighly lower intensity in the XANES region when compared to
MOF-2 and UiO-68-NH2. As XAS measures the average spec-
trum for all absorbing elements, this difference in pre-edge in-
tensity can be attributed to the presence of two crystallo-
graphically inequivalent Zr centers in the SBUs: the axial Zr
centers are coordinated by four m3-oxo groups and four car-
boxylic oxygen, whereas the equatorial Zr centers are coordi-
nated by four m3-oxo groups, two carboxylic oxygen, and two
coordinating water molecules. For MOF-2 and UiO-68-NH2, all
Zr are crystallographically equivalent, and their XANES spectra
are identical in intensity.
lengths for the m3-oxo and carboxylic oxygen are 0.25 and
0.1 ꢂ shorter, respectively. The high quality of these EXAFS fits
confirm the phase purity and structure of the MOFs as deter-
mined by single-crystal analysis.
Permanent porosity of MOFs 1 and 2 were determined after
removal of solvent molecules by a previously published freeze-
drying process.[18] Nitrogen sorption at 77 K showed type I iso-
therms for both MOFs 1 and 2 as well as UiO-68-NH2, indica-
tive of their microporous structures. The BET surface areas for
MOFs 1 and 2 were slightly lower than that of UiO-68-NH2 due
to functionalization of the bridging ligand (2101, 3025, and
3750 m2 gÀ1 for MOF-1, MOF-2, and UiO-68-NH2, respectively).
The much lower surface area of MOF-1 suggests significant
framework distortion in the solvent-free MOF; such framework
breathing phenomena have been observed for many MOFs
with large open channels.[2e,19] The reduced surface area of
MOF-1 is thus consistent with the structural differences be-
tween the three MOFs, as the eightfold connectivity at the
SBU of MOF-1 is more susceptible to framework breathing
even following solvent removal under mild conditions. The
pore size distribution of MOF 1 and 2 was broader than that of
UiO-68-NH2, with pore apertures spanning 7–12 ꢂ.
Fits for the extended (EXAFS) region for all three MOFs were
also performed, using the corresponding crystal structures as
models. Fits yielded only minor changes in half path length,
reasonable values for Debye–Waller parameters, and final R-fac-
tors less than 0.02 in all instances, indicating <2% misfit. The
fits of MOF-1 with an eight-connected model and MOF-2 with
a twelve-connected model are shown in Figure 3, while the fit
of UiO-68-NH2 and fitting parameters are available in the Sup-
porting Information.
The orthogonal amide and carboxylic acid groups within the
MOF channel are expected to have high affinity for transition
metal cations. As MOF-1 displays a broad emission band at
390 nm upon excitation at 330 nm, chelating of transition
metal cations will likely lead to changes of the fluorescence in-
tensity. MOFs have been investigated for various sensing appli-
cations, often taking advantage of the intrinsic fluoresence of
the aromatic bridging ligands or luminescence of the metals in
Analysis of the EXAFS fits reveals some differences between
the Zr coordination environments in MOF-1, MOF-2, and UiO-
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Figure 3. Comparison between experimental EXAFS data (squares) and best fits (line) for MOF-1 (left) and MOF-2 (right). Top panels are displayed in R-space
containing both magnitude of Fourier Transform and real components. Bottom panels display k3-weighted c(k) data and fits. Final R-factors of 0.02 and <0.01
were obtained for MOFs 1 and 2, respectively.
the SBU.[4c,20] MOFs have also been applied to metal sensing
specifically, with emphasis on detection of Cu2+, Fe3+, Ag+,
Tb3+, and Eu3+ [4a,21]
Despite extensive research, the best
.
metal-sensing MOFs in the literature possess detection limits
of 20 ppb[21e] and Ksv values of 250 mmÀ1.[21c] We performed
fluorescence quenching studies of MOF-1 with different metal
cations (M2+ =Fe2+, Mn2+, Co2+, Cd2+, Cu2+, Ni2+, Zn2+, and
Mg2+) in MeOH solutions. The fluorescence signals of MOF
1 were measured in the presence of various metal ions in dif-
ferent concentrations (Figure 4) and the Stern–Vçlmer con-
stants are summarized in Table 1.
As shown in Table 1, transition metal ions with unpaired d-
electrons such as Mn2+, Cu2+, Fe2+, Co2+, and Ni2+ exhibited
Table 1. Stern–Vçlmer constants of MOF 1 and homogeneous controls
with different metal quenchers.
Metal ions
KSV [mmÀ1
(MOF-1)
]
KSV [mmÀ1
(Me2L1)
]
Preconcentration
factor
Fe2+
Mn2+
Co2+
Cd2+
Cu2+
Ni2+
264.2Æ7.6
906.5Æ41.2
207.8Æ6.2
2.2Æ0.1
4.7Æ0.7
202
300
76
À11.3Æ3.12
10.5Æ2.7
À1.2Æ0.1
À1.3Æ6
–
Figure 4. a) Stern–Vçlmer (SV) plots of the fluorescence emissions of MOF-
1 quenched by different metals in MeOH with 330 nm excitation (emission
decays were monitored at 390 nm). Inset: Magnification of SV plots in the
low concentration range. b) Fluorescence intensity of MOF-1 in 4.5 mm
MeOH solutions of different metals.
468.0Æ19.9
134.0Æ6.2
21.7Æ1.4
1.0Æ0.1
501
173
120
50
38.8Æ1.9
À29.4Æ2.1
À1.9Æ0.2
Zn2+
Mg2+
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significant quenching effects, while diamagnetic cations such
as Mg2+, Zn2+, and Cd2+ had negligible effects upon fluores-
cence. The proposed quenching mechanism is enhanced non-
radiative deactivation in the presence of nearby unpaired
spins. The fluorescence quenching follows the Stern–Vçlmer
(SV) equation: I0/I=1 + KSV[M], where I0 and I correspond to
the luminescence intensity for MOF-1 in absence and presence
of metal cations, respectively, [M] is the metal concentration,
and KSV is the Stern–Vçlmer constant. MOF-1 shows dramatic
fluorescence quenching at very low metal concentrations, with
remarkable sensitivity to Mn2+ and Cu2+ over other metal ions.
An exceptionally high KSV value of (0.91Æ0.04)ꢃ106 mÀ1 was
obtained for Mn2+, while KSV for Cu2+ was (0.47Æ0.02)ꢃ
106 mÀ1 (Table 1). The sensitivity of MOF-1 is three to four
orders of magnitude higher than that for previously reported
MOFs with Ksv values in the range of tens to hundreds
mÀ1.[4a,21b] Quenching studies were also conducted on the ho-
mogeneous compound Me2L1 under identical experimental
conditions, however, no fluorescence quenching was observed
for any of the metal cations except Ni2+, which exhibits
modest quenching of the Me2L1 fluorescence (Figure S13–S20).
Furthermore, investigation of MOF-1 by PXRD after soaking in
45 mm Mn2+ solution overnight reveals no loss of crystallinity
(Figure S21).
strating the utility of orthogonal functionalities for developing
highly sensitive MOF-based sensors.
Experimental Section
See Supporting Information for details. CCDC-1012331 (MOF-1),
and CCDC-1012332 (MOF-2) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by the DoE Office of Nuclear Energy’s
Nuclear Energy University Program (Sub-Contract - 20 #120427,
Project–#3151). XAS analysis was performed at GeoSoilEnviro-
CARS (Sector 13), Advanced Photon Source (APS), Argonne Na-
tional Laboratory. GeoSoilEnviroCARS is supported by the Na-
tional Science Foundation - Earth Sciences (EAR-1128799) and
Department of Energy (DOE)– GeoSciences (DE-FG02–
94ER14466). Single-crystal diffraction studies were performed
at ChemMatCARS (Sector 15), APS, Argonne National Laborato-
ry. ChemMatCARS is principally supported by the Divisions of
Chemistry (CHE) and Materials Research (DMR), National Sci-
ence Foundation, under grant number NSF/CHE-1346572. Use
of the APS, an Office of Science User Facility operated for the
U.S. DOE Office of Science by Argonne National Laboratory,
was supported by the U.S. DOE under Contract No. DE-AC02–
06CH11357. We thank the University of Chicago Physical Sci-
ence Division Mass Spectrometry and NMR facilities for instru-
mentation.
The enhancement of fluorescent quenching at low concen-
trations can be partly attributed to preconcentration of metal
ions within the MOF channels. To verify this, we soaked MOF-
1 in solutions of different metal cations (50 mm) and deter-
mined the distribution of metal cations in the supernatant and
MOF channels by inductively coupled plasma mass spectrome-
try (ICP-MS). ICP-MS analysis indicated that the concentrations
of 12.2 mm of Mn2+ and 7.6 mm of Cu2+ within the channels
of MOF-1 respectively, which were 300 and 501 times more
concentrated than that in the supernatant (Table 1).
Keywords: crystal structures
·
fluorescence sensing
·
To further corroborate the high sensitivity of MOF-1 com-
pared to other reported systems, we determined the detection
limit for Mn2+ first by the Taylor method,[22] then confirmed ex-
perimentally. The absolute standard deviations of the calibra-
tors were plotted against their respective concentrations to
give the background level S0 as the y-intercept of the plot. The
detection limit was estimated to be 3So, which equals 0.6 ppb.
Experimentally, the fluorescence intensity of the MOF solutions
was determined to be 347.10Æ3.94 and decreased to 321.71Æ
4.32 with the addition of 0.5 ppb Mn2+. The experiment indi-
cated that the detection limit for Mn2+ was below 0.5 ppb,
consistent with the estimation by the Taylor method.
functionalization · metal sensing · metal–organic frameworks
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Received: September 6, 2014
Published online on && &&, 0000
&
&
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
6
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
An excellent fluorescence sensor: A
metal-organic framework with an or-
thogonal carboxylic acid moiety adopts
a unique eight-connected Zr–oxo sec-
ondary building unit, and provides the
first direct single-crystal X-ray structure
evidence for the proposed “site-defect”
phenomenon. Fluorescence of this ma-
terial is extremely sensitive to the pres-
ence of metal cations with unpaired
electrons, affording limits of detection
below 0.5 ppb and surpassing other
MOF sensors by three to four orders of
magnitude.
&
Metal–Organic Frameworks
M. Carboni, Z. Lin, C. W. Abney, T. Zhang,
W. Lin*
&& – &&
A Metal–Organic Framework
Containing Unusual Eight-Connected
Zr–Oxo Secondary Building Units and
Orthogonal Carboxylic Acids for Ultra-
sensitive Metal Detection
Chem. Eur. J. 2014, 20, 1 – 7
www.chemeurj.org
7
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ