A Titanium-Organic Framework as an Exemplar of Combining the Chemistry of Metal- and Covalent-Organic Frameworks

Article abstract of DOI:10.1021/jacs.6b01233

A crystalline material with a two-dimensional structure, termed metal-organic framework-901 (MOF-901), was prepared using a strategy that combines the chemistry of MOFs and covalent-organic frameworks (COFs). This strategy involves in situ generation of an amine-functionalized titanium oxo cluster, Ti₆O₆(OCH₃)₆(AB)₆ (AB = 4-aminobenzoate), which was linked with benzene-1,4-dialdehyde using imine condensation reactions, typical of COFs. The crystal structure of MOF-901 is composed of hexagonal porous layers that are likely stacked in staggered conformation (hxl topology). This MOF represents the first example of combining metal cluster chemistry with dynamic organic covalent bond formation to give a new crystalline, extended framework of titanium metal, which is rarely used in MOFs. The incorporation of Ti(IV) units made MOF-901 useful in the photocatalyzed polymerization of methyl methacrylate (MMA). The resulting polyMMA product was obtained with a high-number-average molar mass (26 850 g mol^-1) and low polydispersity index (1.6), which in many respects are better than those achieved by the commercially available photocatalyst (P-25 TiO₂). Additionally, the catalyst can be isolated, reused, and recycled with no loss in performance.

Full text of DOI:10.1021/jacs.6b01233

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Communication
A Titanium-Organic Framework as an Exemplar of Combining
the Chemistry of Metal- and Covalent-Organic Frameworks
Ha L. Nguyen, Felipe Gándara, Hiroyasu Furukawa, Tan L. H. Doan, Kyle E. Cordova, and Omar M. Yaghi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01233 • Publication Date (Web): 21 Mar 2016
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A Titanium-Organic Framework as an Exemplar of Combining the
Chemistry of Metal- and Covalent-Organic Frameworks
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Ha L. Nguyen,†,‡ Felipe Gándara,# Hiroyasu Furukawa,†,§ Tan L. H. Doan,¶ Kyle E. Cordova,†,§
and Omar M. Yaghi†,§,*
†Department of Chemistry, University of California-Berkeley; Materials Sciences Division, Lawrence Berkeley Na-
tional Laboratory; Kavli Energy NanoSciences Institute at Berkeley; and Berkeley Global Science Institute, Berkeley,
California 94720, United States
‡Vietnam National University-Ho Chi Minh City (VNU-HCM), Ho Chi Minh City 721337, Vietnam
#Department of New Architectures in Materials Chemistry, Materials Science Institute of Madrid, Consejo Superior
de Investigaciones Científicas, Madrid 28049, Spain
§King Fahd University of Petroleum and Minerals, Dhahran 34464, Saudi Arabia
¶Faculty of Chemistry, University of Science, VNU-HCM, Ho Chi Minh City 721337, Vietnam
Supporting Information Placeholder
this contribution, we articulate a strategy for making discrete
A crystalline material with a two-dimensional structure,
metal clusters in situ that are appropriately functionalized to
affect imine-condensation reactions, commonly used in the
chemistry of covalent-organic frameworks (COFs).5 We find
that the chemistry of cluster formation and COFs, when car-
ried out in sequence, overcome the challenge of synthetic
incompatibility.
termed metal-organic framework-901 (MOF-901), was pre-
pared using a strategy that combines the chemistry of MOFs
and covalent-organic frameworks (COFs). This strategy in-
volves in situ generation of an amine-functionalized titanium
oxo cluster, Ti6O6(OCH3)6(AB)6 (AB = 4-aminobenzoate),
which was linked with benzene-1,4-dialdehyde using imine
condensation reactions; typical of COFs. The crystal struc-
ture of MOF-901 is composed of hexagonal porous layers that
are likely stacked in staggered conformation (hxl topology).
This MOF represents the first example of combining metal
cluster chemistry with dynamic organic covalent bond for-
mation to give a new crystalline, extended framework of tita-
nium metal, which is rarely used in MOFs. The incorporation
of Ti(IV) units made MOF-901 useful in the photocatalyzed
polymerization of methyl methacrylate (MMA). The result-
ing polyMMA product was obtained with a high number
average molar mass (26,850 g mol-1) and low polydispersity
index (1.6), which in many respects are better than those
achieved by the commercially available photocatalyst (P-25
TiO2). Additionally, the catalyst can be isolated, reused, and
recycled with no loss in performance.
Scheme 1. Synthetic scheme depicting the generalized formation of
a discrete hexameric titanium cluster, which can be appropriately
functionalized with amine groups to affect imine condensation reac-
tions. Atom colors: Ti, blue; C, black; O, red; R groups, pink; H at-
oms and capping isopropoxide units are omitted for clarity.
Inspired by the chemistry of hexameric titanium oxo
clusters,6 we reasoned that it is possible to functionalize a
known cluster with amine functionalized carboxyl ligands
(Scheme 1). Indeed, this allows the resulting cluster to be
linked together through imine condensation reactions. Initial
synthetic attempts were performed in a sequential, step-wise
In the chemistry of carboxylate metal-organic frame-
works (MOFs), the chelation of the carboxyl organic linker to
metal ions gives metal-carboxyl clusters, secondary building
units (SBUs), which act as anchors ensuring the overall archi-
tectural stability of the MOF.1 Although many of these SBUs
are known as discrete clusters, it has been difficult to directly
use them as starting building blocks for MOFs.2 The main
reason is the sensitivity of cluster formation to reaction con-
ditions and, in many cases, the incompatibility of such condi-
tions with those required for MOF synthesis and crystalliza-
tion.3 This limitation has prevented access to the vast, di-
verse and well-developed cluster chemistry,4 and the poten-
tial richness of properties they would provide to MOFs. In
manner, in which
a discrete, isolated Ti(IV) cluster,
[Ti6O6(OiPr)6(AB)6] (AB = 4-aminobenzoate; OiPr = iso-
propoxide), was first synthesized according to previous re-
ports.6a Based on reticular chemistry, this hexagonal building
block has six points-of-extension and when linked with a
linear linker would lead to a hxl layered topology.7 Thus,
titanium(IV) isopropoxide [Ti(OiPr)4] and 4-aminobenzoic
acid (H-AB) were reacted under solvothermal conditions in
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isopropanol (see Supporting Information, Section S2). After
cm-1) and C-C=N-C (vC-C=N-C = 1199.7 cm-1) stretching vibra-
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successful synthesis and full characterization of the cluster,
exhaustive efforts were undertaken to form the correspond-
ing MOF upon reactions with benzene-1,4-dialdehyde (BDA).
However, due to the low solubility of the cluster in a variety
of solvents, only poorly crystalline powders or amorphous
solids were obtained. Accordingly, a one-pot synthetic ap-
proach, with BDA present, was performed given the fact that
the synthetic conditions to form the hexameric
[Ti6O6(OiPr)6(AB)6] were found to be robust (Figure 1).
A one-pot mixture containing Ti(OiPr)4, H-AB, and BDA
in methanol was solvothermally reacted in a sealed tube (N2
atmosphere) at 125 °C for 3 days to yield a yellow microcrys-
talline MOF-901 (Figure 1). The microcrystalline MOF-901
was subsequently washed with N,N-dimethylacetamide prior
to any further analysis in order to remove unreacted starting
material. It is noted that the formation of metal oxide (TiO2)
impurities is a real possibility when synthesizing Ti-based
MOFs, thus a density separation was performed using bro-
moform to ensure that the as-synthesized MOF-901 was
pure.8 The resulting purified material was then activated at
130 °C for 24 h, under dynamic vacuum (1 mTorr), to remove
guest molecules residing within the pores (2.9 mg of activat-
ed product collected, 33.9% yield based on Ti(OiPr)4; SI, Sec-
tion S3).
tion frequencies were observed whereas these frequencies
were absent in the FT-IR spectra of the isolated hexameric
cluster and starting materials. Furthermore, there was a no-
table absence of the aldehyde-originating stretching band
(vC=O = 1685 cm-1) that would result from a partially reacted
1
BDA. H NMR spectroscopy on an acid-digested sample re-
vealed a singlet resonance at 3.5 ppm with an integrated val-
ue of 3 protons, which was attributed to the presence of
methoxide functionalities. Interestingly, there was no reso-
nance observed for isopropoxide leading to the conclusion
that methoxide moieties capped the hexameric cluster.
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Structural elucidation of MOF-901 was carried out by
powder X-ray diffraction analysis (PXRD) using a synchro-
tron radiation source (SI, Section S6). The experimental dif-
fraction patterns exhibited low-angle diffraction peaks with
no evidence of the starting materials or discrete clusters
(Figure 2). The experimental pattern was then indexed in a
primitive hexagonal system (P63, No. 173) with unit cell pa-
rameters, a = 27.4037 and c = 8.9403 Å. Given the connectivi-
ty of the SBUs and linkers and to understand the lattice
packing of MOF-901, structural models based on the hxl
topology were generated using Materials Studio 6.0 software.
The proposed 2D structure of MOF-901 was first modeled
with two extreme possibilities of stacking modes of the lay-
ers: (i) A stacking sequence with both layers fully eclipsed
(modeled in the trigonal space group P-3, No. 147). A lower
symmetry space group was utilized for this model in order to
avoid overlapping of the asymmetric units. (ii) A staggered
stacking sequence (modeled in the hexagonal space group
P63, No. 173). This model was generated by translating every
We initially examined the underlying structural features
of MOF-901 using Fourier transform infrared analysis (FT-IR)
to prove the imine formation and H nuclear magnetic reso-
1
nance (NMR) spectroscopy to gain a better understanding of
the cluster composition (SI, Sections S4 and S5). The for-
mation of the imine functionalities was clearly revealed in
the FT-IR spectrum of activated MOF-901. Specifically, the
presence of characteristic imine-derived C=N (vC=N = 1625.9
second layer along the (xy) plane by ꢀ 3 Å (where a = 27.2
Å, the distance of two hexameric titanium clusters on the
same layer). The translation allowed the hexameric titanium
cluster of the second layer to be located in the center of the
triangular window generated from the first layer (Figure 1).
After a geometrical energy minimization of the structural
models, diffraction patterns were simulated and compared
with the experimental diffraction patterns obtained for
MOF-901; agreement was observed with the staggered con-
formational model (Figure 2). A full profile Pawley fitting was
performed in order to achieve the final unit cell parameters
(a = 27.2039(6) and c = 8.7473(7) Å) leading to satisfactory
agreement factors and acceptable profile differences. We
note that the disappearance of some experimental peaks
(most notably, [2 -1 0] and [2 0 1]) was detected in compari-
son with the simulated pattern from the staggered model.
This is anticipated, as MOF-901 is a layered structure, in
which there is a real possibility of an imperfect stacking se-
quence (inclined or turbostratic disorder). Ultimately, the
low data resolution precluded the use of Rietveld refinement
to fully assign the stacking sequence. It is noted that this is
often the case for the related class of materials, COFs.5
The geometry of the proposed SBU in MOF-901 is com-
posed of a trigonal prismatic Ti6O6 inner core, in which the
octahedral Ti atoms are divided evenly over two separate
triangular planes and bound to one another, in corner- shar-
ing fashion, through 6 µ3-O atoms (Scheme 1). The inner core
is surrounded by six equatorial, planar, and chelating termi-
nal AB ligands. This arrangement affords the SBU with six
points-of-extension in the shape of a hexagon. Finally, cap-
ping the top and bottom of the inner core are six methoxide
moieties. When linked together through imine condensation
Figure 1. MOF-901 was synthesized by exploiting the robust condi-
tions used to form the discrete hexameric Ti(IV) oxo cluster. In
forming MOF-901, 4-aminobenzoic acid was judiciously chosen to
generate in situ amine-functionalized hexameric clusters, which
were, in turn, stitched together through imine-condensation reac-
tions involving benzene-1,4-dialdehyde (a). The crystal structure of
MOF-901 is projected along c-axis (b) and a-axis (c). The discrete
hexameric cluster, now incorporated into an extended, crystalline
structure, is presented in the inset of (b). Atom colors: Ti, blue; C,
black; O, red; N, green; H, pink; and second layer, orange. Capping
methoxide moieties are removed for clarity.
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NH2 (2.6 eV).9 The charge transfer process, and thus, the
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demonstration of successful generation and separation of
electron-hole pairs, was analyzed by photoluminescence
spectroscopy. Under laser irradiation at 365 nm, MOF-901
exhibited a broad luminescent band centered at 484 nm.
Mott-Schottky measurements were then performed to un-
derstand the conductivity and flat band potential of MOF-
901. The results indicated a negative slope revealing the fact
that MOF-901 can be described as a p-type semiconductor
with a flat band potential of 0.57 V vs. Ag/AgNO3.
9
Encouraged by these findings, we sought to demonstrate
the photocatalytic activity of MOF-901. We used MOF-901 as
a photocatalyst in the visible light mediated radical polymer-
ization of methyl methacrylate (MMA) with a co-initiator,
ethyl α-bromophenylacetate (SI, Section S11). Initially, we
focused on optimizing the catalyst loading, co-initiator quan-
tity, and reaction time. As shown in Table 1, a catalyst and
co-initiator loading of 0.034 mol% and 0.61 mol% (with re-
spect to MMA), respectively, and a reaction time of 18 h was
found to be ideal. The optimized conditions resulted in
polymerization with a high molecular weight of polyMMA
(26,850 g mol-1), high yield (87%), and reasonable distribu-
tion, as indicated by a low polydispersity index (PDI) of 1.6.
Control experiments were subsequently performed in order
to assess the photocatalytic activity of MOF-901. As expected,
there was no observable product when the discrete
[Ti6O6(OiPr)6(AB)6] cluster was used, nor in the absence of
catalyst (i.e. blank sample) or visible light.
MOF-901 clearly outperforms Degussa P-25 TiO2 in pro-
ducing high molecular weight (Mn) polyMMA product (Table
1). Here, MOF-901 affords a polyMMA product whose Mn
(26,850 g mol-1) was 1.5 times higher than the commercially
available Degussa P-25 TiO2 (17,000 g mol-1). Additionally,
the gel permeation chromatogram (GPC) of the polyMMA
product is more uniform when MOF-901 is used, owing to a
higher degree of control over the polymerization process. It
is also noted that the activity of MOF-901 is higher than pre-
viously reported photocatalysts employed in this reaction,
especially with respect to the larger molecular weight of the
resulting polyMMA and lower PDI value achieved.10 To fur-
ther demonstrate MOF-901’s high performance, comparative
experiments were performed using the photocatalysts MIL-
125-NH2,9 VNU-1,11 and UiO-66-NH2,12 who all possess similar
band gap energies as MOF-901. Interestingly, there was no
polyMMA product detected when VNU-1 and MIL-125-NH2
were employed as the photocatalyst (Table 1). On the contra-
ry, UiO-66-NH2 was demonstrated to promote the polymeri-
zation of MMA, albeit the polyMMA product was achieved in
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Figure 2. PXRD analysis of activated MOF-901 displaying the exper-
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imental pattern (black), refined Pawley fitting (red), and calculated
pattern from the staggered structural model (blue). The difference
plot (green) and Bragg positions (pink) are also provided.
reactions with BDA, the resulting extended structure con-
tains triangular windows, 14 Å in width, and an interlayer
spacing of ca. 3.47 Å (Figure 1). With respect to the topology,
MOF-901 has one vertex, represented by the Ti6O6(-CO2)6
SBU, and one edge (linker) leading to a 36 faced hxl topology.
To further support and formulate the character of MOF-
901, scanning electron microscopy (SEM), elemental microa-
nalysis (EA), thermogravimetric analysis (TGA), and N2 ad-
sorption measurements were performed on an activated
sample of MOF-901 (SI, Sections S7-S9). Accordingly, SEM
analysis demonstrated a small particle size whose crystal
morphology was ambiguous due to aggregation of smaller
crystallites, but nevertheless appeared homogeneous in na-
ture (SI, Section S9). In terms of chemical formulation of
MOF-901, EA data revealed the chemical formula of MOF-901
to be Ti6O6(OCH3)6L3 (where L = C20H14N2(CO2)2), Calcd: C,
51.46; H, 3.60; N, 5.00% and found: C, 51.10; H, 3.28; N, 5.23%,
which is in line with the formula derived from the proposed
structure. TGA supported the derived chemical formula as
determined by EA as well as the high thermal stability of the
0
material (200 C). Specifically, the TiO2 residue (32.3 wt%)
was found to be close to the theoretical TiO2 value (28.6
wt%) that was calculated from the modeled crystal structure.
Furthermore, the weight loss percentage (67.7 wt%) originat-
0
ing at 450 C or was attributed to, and consistent with, the
imine-formed organic linker and the capping methoxide
moieties (71.4 wt% based on the modeled structure). Finally,
the permanent porosity of MOF-901 was examined by N2
adsorption measurements at 77 K. The resulting Brunauer-
Emmett-Teller surface area of MOF-901 was calculated to be
550 m2 g-1. It is noted that this value is in agreement with the
theoretical specific surface area calculated for the staggered
model of MOF-901 (650 m2 g-1; SI, Section S8).
Given the high likelihood of the new material’s photo-
responsiveness, based on the integration of Ti atoms within
the framework, we sought to analyze the photoactivity of
MOF-901 (SI, Section S10). Accordingly, UV-Vis diffuse re-
flectance spectroscopy was performed, in which the absorp-
tion range extended from 340 to 550 nm with absorption
maximum located at 360 nm. As expected, from the yellow
color of MOF-901, this material absorbs light in the visible
region. The calculated band gap, based on the Tauc plot, was
found to be 2.65 eV. This band gap is slightly larger than that
calculated for PCN-22 (1.93 eV)3a and similar to Ti-MIL-125-
significantly lower yield (13%) and molecular weight (Mn
=
18,500 g mol-1 and 26,850 g mol-1 for UiO-66-NH2 and MOF-
901, respectively) than MOF-901 (Table 1).
The heterogeneous nature of MOF-901 was validated
through recycling experiments. Upon conclusion of the reac-
tion, MOF-901 was isolated by centrifugation and washed
several times with dichloromethane before regeneration un-
der vacuum (1 mTorr) (SI, Section S11). PXRD analysis of this
regenerated material revealed that the structure of MOF-901
was maintained. The photoinitiated polymerization reaction
was then performed again under identical conditions. The
activity of MOF-901 remained, without any loss of perfor-
mance, over at least three consecutive cycles. Indeed, this
recyclability further demonstrates the outperformance of
MOF-901 versus other photocatalysts for this purpose.13
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5421. (c) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O.
M. Science 2013, 341, 1230444.
Table 1. Photocatalyzed polymerization of methyl methacry-
late (MMA) under visible light irradiation.
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Entry
Catalyst
[mol%]
Yield
[%]
Mn
[g mol-1]
Mw/Mn
9
MOF-901a
0.034
0.034
0.034
0.034
0
87
52
26,850
17,000
n.a.
1.6
1.6
10
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a
P-25 TiO2
[Ti6O6(OiPr)6(AB)6]a
MOF-901b
n.d.
n.d.
n.d.
n.d.
n.d.
13
n.a.
n.a.
n.a.
n.a.
n.a.
1.6
n.a.
Blankc
n.a.
(4) Guillerm, V.; Gross, S.; Serre, C.; Devic, T.; Bauer, M.; Férey,
G. Chem. Commun. 2010, 46, 767.
a
MIL-125-NH2
0.034
0.034
0.034
n.a.
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ChemPhysChem, 2012, 13, 3651. (c) Hendon, C. H.; Tiana, D.;
Fontecave, M.; Sanchez, C.; D’arras, L.; Sassoye, C.; Rozes, L.;
Mellot-Draznieks, C.; Walsh, A. J. Am. Chem. Soc. 2013, 135,
10942.
(10) Although MOF-901 outperforms the photocatalysts reported
in the following references, especially with regard to the
polyMMA product’s yield, larger molecular weight, and lower
PDI value, the varying conditions reported for these photo-
catalytic polymerizations make direct comparisons challeng-
ing. (a) Wang, Z. J.; Landfester, K.; Zhang, K. A. I. Polym.
Chem. 2014, 5, 3559. (b) Uemura, T.; Ono, Y.; Kitagawa, K.;
Kitagawa, S. Macromolecules 2008, 41, 87. (c) Kiskan, B.;
Zhang, J.; Wang, X.; Antonietti, M.; Yagci, Y. ACS Macro Lett.
2012, 1, 546. (d) Fors, B. P.; Hawker, C. J. Angew. Chem. Int.
Ed. 2012, 51, 8850.
(11) Doan, T. L. H.; Nguyen, H. L.; Pham, H. Q.; Pham-Tran, N.-
N.; Le, T. N.; Cordova, K. E. Chem. Asian J. 2015, 10, 2660.
(12) Long, J.; Wang, S.; Ding, Z.; Wang, S.; Zhou, Y.; Huang, L.;
Wang, X. Chem. Commun. 2012, 48, 11656.
(13) (a) Uemura, T.; Kitagawa, K.; Horike, S.; Kawamura, T.; Kita-
gawa, S.; Mizuno, M.; Endo, K. Chem. Commun. 2005, 5968.
(b) Vitorino, M. J.; Devic, T.; Tromp, M.; Férey, G.; Visseaux,
M. Macromol. Chem. Phys. 2009, 210, 1923. (c) Inokuma, Y.;
Nishiguchi, S.; Ikemoto, K.; Fujita, M. Chem. Commun. 2011,
47, 12113. (d) Uemura, T.; Mochizuki, S.; Kitagawa, S. ACS
Macro Lett. 2015, 4, 788.
VNU-1a
n.a.
a
UiO-66-NH2
18,500
aReaction conditions: MMA (2.0 M in N,N-dimethylformamide), catalyst
(0.034 mol%), and ethyl α-bromophenylacetate (0.61 mol%) at room
temperature with visible light from a compact fluorescent lamp (4U, 55
c
W) for 18 h. bReaction performed in the absence of light irradiation. Re-
action performed without catalyst. n.d. = not detected; n.a. = not applica-
ble; Mn = number average molar mass; Mw = mass average molar mass.
ASSOCIATED CONTENT
Supporting Information
Synthesis and characterization of MOF-901, PXRD analysis
data, including computational modeling, and photocatalysis
measurements details and data. This material is available free
of charge via the Internet at http://pubs.acs.org.
Corresponding Author
*yaghi@berkeley.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
BASF SE (Ludwigshafen, Germany) supported the synthesis
of MOF-901. The photocatalysis work was funded by VNU-
HCM (A2015-50-01-HĐ-KHCN and MANAR-CS-2015-04) and
U.S. Office of Naval Research Global: Naval International
Cooperative Opportunities in Science and Technology
(N62909-15-1N056). PXRD data was collected at the Ad-
vanced Photon Source (beamline 11-BM-B), a U.S. Depart-
ment of Energy (DOE) Office of Science User Facility operat-
ed by Argonne National Laboratory (DE-AC02-06CH11357). F.
G. acknowledges funding by the Spanish Ministry of Econo-
my and Competitiveness (MAT2013-45460-R) and Fundacion
General CSIC (Programa ComFuturo). We thank Messrs. T.
N. Tu and V. Q. Nguyen (VNU-HCM) as well as Messrs. J.
Jiang and Y. Zhao (UC Berkeley) for their helpful discussions.
REFERENCES
(1) (a) Luebke, R.; Belmabkhout, Y.; Weselinski, L. J.; Cairns, A.
J.; Alkordi, M.; Norton, G.; Wojtas, L.; Adil, K.; Eddaoudi, M.
Chem. Sci. 2015, 6, 4095. (b) Alezi, D.; Peedikakkal, A. M. P.;
Weseliński, Ł. J.; Guillerm, V.; Belmabkhout, Y.; Cairns, A. J.;
Chen, Z.; Wojtas, Ł.; Eddaoudi, M. J. Am. Chem. Soc. 2015, 137,
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