From Zn(II)-Carboxylate to Double-Walled Zn(II)-Carboxylato Phosphate MOF: Change in the Framework Topology, Capture and Conversion of CO2, and Catalysis of Strecker Reaction

Article abstract of DOI:10.1021/acs.inorgchem.7b02443

The ligand H₂L has been built by linking an imidazole moiety to the 5-position of isophthalic acid. It forms two types of porous frameworks, {[Zn(L)]·2DMF·2H₂O}_n (1) and {[(CH₃)₂NH₂][Zn

Full text of DOI:10.1021/acs.inorgchem.7b02443

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
From Zn(II)-Carboxylate to Double-Walled Zn(II)-Carboxylato
Phosphate MOF: Change in the Framework Topology, Capture and
Conversion of CO2, and Catalysis of Strecker Reaction
Mayank Gupta, Dinesh De, Kapil Tomar, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
S
* Supporting Information
ABSTRACT: The ligand H2L has been built by linking an
imidazole moiety to the 5-position of isophthalic acid. It forms
two types of porous frameworks, {[Zn(L)]·2DMF·2H2O}n (1)
and {[(CH3)2NH2][Zn2(L)(H2O)PO4]·2DMF}n (2). 1 is a
porous neutral framework and has rtl rutile 3,6-conn topology,
while 2 is an organo-metallophosphate anionic porous frame-
work with double-walled hexagonal channels. Framework 1′
(desolvated) exhibits moderate CO2 adsorption (58 cc g−1 at
273 K, 1 bar), whereas 2′ (desolvated) shows a microporous
nature with a high adsorption of CO2 (111.7 cc g−1 or 22 wt %
at 273 K, 1 bar). Interestingly, this adsorbed CO2 could be
converted very efficiently to cyclic carbonates under mild con-
ditions using 2′ as the catalyst in the presence of tetrabutyl-
ammonium bromide as the cocatalyst. The presence of open metal sites in 2′ makes it an efficient heterogeneous catalyst for
solvent-free three-component Strecker reaction using various aldehydes/ketones together with amines and trimethylsilyl cyanide
in high yields at room temperature. The straightforward experimental and product isolation procedure along with easy recovery
and reusability of the catalyst provided an attractive route for the synthesis of α-amino nitriles.
isophthalic acid15 H2L (Scheme 1) was chosen to construct a
phosphate based metal organic framework.
Herein, we have successfully achieved the synthesis of two
INTRODUCTION
Syntheses of nanoporous materials1 with extended network
■
topologies have been emerging as very powerful platforms for
categories of porous frameworks (Scheme 1) from the same
various applications such as gas storage,2 separation,3 sensing,4
ligand, L2− (hereafter L), {[Zn(L)]·2DMF·2H2O}n (1), a metal−
catalysis,5 and so on. These porous materials can largely be divided
organic framework, and {[(CH3)2NH2][Zn2(L)(H2O)PO4]·
into two major categories, one with only inorganic frameworks,6
2DMF}n (2), an organo-metallophosphate anionic framework.
such as zeolites, germinates, and phosphate based metal oxides;
Compound 2 is the first OMPO which is synthesized without
any external templating molecule which generally obstructs the
porosity of the framework.
and the other one is metal−organic hybrid frameworks or the
porous coordination polymers.7 In this regard, porosity in
phosphate based metal oxides (MPOs) has been thoroughly
investigated.8 Recently, multidentate organic ligands, like
oxalate9 and 4,4′-bpy10 type moieties, have been incorporated
into MPOs to obtain structural diversity in the resultant organo-
metallophosphate frameworks (OMPOs). Unfortunately, very
few OMPOs have shown a porous nature which could be utilized
for application purposes.11
Aromatic carboxylates have been found to be very effective
building blocks for obtaining porous metal−organic frameworks
(MOFs).12 Incorporation of the carboxylates into MPOs to
obtain porous hybrid materials is very low in number and has
been achieved only by Wang and co-workers in two separate
reports.13,14 These materials, like other MPOs, have been syn-
thesized with the help of 4,4′-bipyridine templates which remain
in the pores of frameworks as cationic species and were difficult
to remove to obtain porous frameworks. We thought to combine
these moieties into one ligand, and the imidazole-substituted
Both the frameworks can be activated to obtain porous struc-
tures, 1′ and 2′, without breakdown of the overall architecture.
Gas adsorption and heterogeneous catalytic activities of the
frameworks afforded interesting results. Compared to 1′, the
framework 2′ afforded greater CO2 adsorption and its conversion
to cyclic carbonates that are valuable as precursors for having
polycarbonates, aprotic polar solvents, pharmaceutical/fine
chemical intermediates, and in many biomedical applications.16
The framewotrk 2′, having open metal sites, exhibited efficient
heterogeneous catalytic activity for solvent-free three-compo-
nent Strecker reaction using various aldehydes/ketones together
with amines and trimethylsilyl cyanide (TMSCN) to afford the
corresponding α-amino nitriles in high yields at room temper-
ature. The Strecker reaction is one of the most efficient and
Received: September 23, 2017
© XXXX American Chemical Society
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Scheme 1. Structure of the Ligand H2L and Synthetic Scheme of Porous Frameworks, 1 and 2
straightforward methods for the synthesis of α-amino nitriles,17
which are very useful precursors for the synthesis of α-amino
acids and various nitrogen-containing heterocycles such as
imidazoles and thiadiazoles, etc.18
where the catalyst 2′ (10 wt %) has been previously introduced. The
mixture (in solvent-free conditions) was stirred at 25 °C between 0.5
and 18 h depending on the substrate, under a N2 atmosphere. When the
reaction was completed, 10 mL of DCM was added to the mixture in
order to dissolve the Strecker−aminonitrile. The mixture was then
filtered and washed to afford the crude product. It was further purified
with a small pad of silica using hexane/ethyl acetate as an eluent.
EXPERIMENTAL SECTION
■
Materials and Measurements. All chemicals were reagent grade
quality obtained from either Aldrich or TCI and used without further
purification. All solvents were procured from S. D. Fine Chemicals,
India. These solvents were purified following standard methods prior to
use. The details of spectroscopic techniques and X-ray structural studies
are provided in the Supporting Information.
RESULTS AND DISCUSSION
■
The solvothermal reaction of Zn(ClO4)2 at 90 °C with the ligand
H2L in DMF afforded 1, while the solvothermal reaction with
Zn(NO3)2 at 120 °C in the presence of phosphoric acid gave an
anionic framework 2 with a double-walled rosette structure and
large open hexagonal pores (Scheme 1).
Synthesis of 5-(1H-imidazol-1-yl)isophthalic acid (H2L). The
ligand was synthesized according to a procedure reported in the
literature.15
Single crystal X-ray analysis revealed that 1 crystallized in the
monoclinic space group P21/c. The asymmetric unit contains one
Zn(II) ion, one ligand L2−, two DMF, and two water molecules
in the lattice. As shown in Figure 1a, Zn1 center showed penta-
coordinated geometry with ligation from four O atoms of two
COO− groups of one ligand and one N atom of the imidazole
moiety of another ligand to form a paddle-wheel SBU,
Zn2(COO)4N2. The distance between the two Zn···Zn ions
was found to be 3.032 Å. The resulting coordination arrange-
ment of Zn(II)ion and the ligand gave rise to a 3D porous
framework with 1D open channels along the crystallographic
a-axis with channel dimensions of ∼12 × 7 Å2 (Figure 1b). The
channels in the framework are filled with DMF and water solvent
molecules. The total potential solvent accessible void volume
of 1 was calculated to be 51.8% (956.1 Å3/1847.3 Å3) when DMF
and water solvent molecules were removed from the lattice.
Topological analysis (Figure 1c) with TOPOS revealed that the
Zn2(COO)4N2 paddle-wheel SBU could be considered as a six
connected node and the ligand L as a three connected node. This
made the framework a 3,6-c binodal net having point symbol
{4.62}2{42.610.83}, assignable to the topological type, rtl rutile
3,6-conn (topos&RCSR.ttd).
Synthesis of {[Zn(L)]·2DMF·2H2O}n (1). H2L (30 mg, 0.13 mmol)
and Zn(ClO4)·6H2O (60 mg, 0.16 mmol) were mixed with 3 mL of
DMF in a 5 mL Teflon vessel and heated at 90 °C for 72 h. After cooling
to room temperature, the resulting colorless block-shaped crystals were
harvested and washed repeatedly with DMF, followed by acetone, and
air-dried to give a high yield of 80% (based on the ligand). Anal. Calcd
for C17H24N4O8Zn: C, 42.74; H, 5.06; N, 11.73%. Found: C, 42.98; H,
5.21; N, 11.69%. IR (cm−1): 3424 (broad), 3138 (s), 1628 (s), 1584 (s),
1511 (s), 1434(s), 1382 (m), 1320 (s), 1269 (m), 1140 (w), 1118 (m),
1071 (s), 1004 (w), 945 (m).
Synthesis of {[(CH3)2NH2][Zn2(L)(H2O)PO4]·2DMF}n (2). H2L
(30 mg, 0.13 mmol), Zn(NO3)2·6H2O (60 mg, 0.20 mmol), and H3PO4
(85%, 20 μL) were mixed with 3 mL of DMF in a 5 mL Teflon vessel and
heated at 120 °C for 72 h. After cooling to room temperature, the
resulting colorless block-shaped crystals were harvested and washed
repeatedly with DMF, followed by acetone, and air-dried to give a high
yield of 83% (based on the ligand). Anal. Calcd for C19H28N5O11PZn2:
C, 34.36; H, 4.25; N, 10.54%. Found: C, 34.53; H, 4.39; N, 10.61%. IR
(cm−1): 3434 (broad), 3111 (m), 2449 (bw), 2041 (bw), 1669 (s),
1623(s), 1593 (w), 1402 (s), 1334 (m), 1242 (m), 1166 (w), 1109 (w),
1064 (s), 1026 (w).
General Procedure for the Coupling of Epoxides with CO2.
Catalyst (2) was activated by heating at 150 °C for a period of 8 h after
soaking 5 days in the methanol, which was changed two times per day
with fresh methanol. The activation temperature was chosen from TG
data in order to remove all traces of solvent inside the pore. Reactions
were carried out in a Schlenk tube with stirring at 30 or 70 °C depending
on the substrate, and CO2 (99.999%) bubbling. Epoxide (20 mmol),
catalyst 2′ (10 wt %), and cocatalyst TBAB (1 mmol) were added. Upon
completion, the reaction mixture and catalyst were separated by
filtration to obtain a crude compound. This crude mixture was given for
1H NMR (in CDCl3) for calculating the yield.
On the other hand, the phosphate based anionic framework 2
crystallized in the rhombohedral space group R3, and the
̅
3−
asymmetric unit consisted of two Zn(II) ions, one L, one PO4
anion, one coordinated water and disordered solvent molecules,
DMF and (CH3)2NH2 (DMA) cations. Two carbon atoms of
the imidazole moiety were found to be disordered over two posi-
tions with assigned occupancies. The solvent and DMA cations
are too disordered to be modeled and hence were removed using
SQUEEZE. Structural analysis revealed that both Zn1 and
Zn2 acquire five-coordinate geometry (Figure 2a) with different
coordination environments. Zn1 was coordinated from two O
General Procedure for the Strecker Reactions. Catalyst (2) was
activated as described above. Then, the mixture of ketone/aldehyde,
amine, and trimethylsilyl cyanide (TMSCN) was added to a Schlenk
tube (1 mmol of ketone, 1 mmol of amine, and 1.2 mmol of TMSCN),
B
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Figure 1. (a) Cooridnation environment of Zn(II) in 1 and coordination modes of L, (b) 3D view of 1 showing the open channels, (c) 3,6-c net of 1 with
rtl rutile 3,6-conn topology.
atoms of one COO− group, two O atoms of the PO43− anion, and
one N atom of the imidazole group, while Zn2 showed ligation
from two O atoms of one COO− group, two O atoms of the PO43−
anion, and one from a coordinated water molecule. Charge
balancing calculations revealed that a total of 4+ charge came
from two Zn(II) ions while a total of 5− charge provided by two
COO− and one PO43−. Finally, one more positive charge to
balance came from a DMA cation originating from the decom-
position of DMF solvent during the solvothermal reaction.19 The
presence of DMA cations in the channels was further confirmed
by TGA, IR, and elemental analysis studies (see the Supporting
Information).
activated framework, 2′, with open metal sites as a potential
candidate for gas adsorption and heterogeneous catalysis.
TGA measurements for 1 and 2 were carried out to examine
the thermal stability of the networks. The TGA curve of the
as-synthesized sample of 1 showed a weight loss of 38% (calcu-
lated 38.1%) between 50 and 350 °C corresponding to the loss of
all solvent molecules in the cavities. Thereafter, a sharp weight
loss was observed that corresponded to the decomposition of the
framework (Figure S2). For 2, the sample showed a weight loss of
32% (calculated 31.7%) between 50 and 275 °C corresponding
to the loss of all solvent molecules in the cavities along with the
loss of coordinated water molecules. Thereafter, a sharp weight
loss indicated collapsing of the framework. Phase purity of the
synthesized frameworks was analyzed by PXRD measurements
of the as-synthesized samples of 1 and 2. The experimental
PXRD patterns for the as-synthesized compounds were in good
agreement with the simulated patterns of 1 and 2. The methanol
exchanged materials were heated under high vacuum at 150 °C
for 8 h to generate the activated compounds, 1′ and 2′. The
crystalline nature of the evacuated material was also confirmed by
powder XRD (Figures S3 and S4) measurements in which no
significant shift or broadening of peaks were observed compared
to the as-synthesized materials, 1 and 2.
3−
Structural analysis of 2 revealed that Zn(II) and PO4 ions
formed a continuous 1D chain along the crystallographic c-axis
(Figure 2b) connected by the ligand L to form double-walled
hexagonal channels (Figure 2c,d) with the window size of ∼15 ×
16 Å2 and, furthermore, formed an interesting 3D framework
with a rosette structure. The large hexagonal channels were filled
up with DMF molecules and DMA cations. The total poten-
tial solvent accessible void volume of 2 was found to be 42.5%
(4477 Å3/10532 Å3). The framework contained coordinated
water molecules inside the walls of the channels. Upon heating,
the coordinated water molecules could be removed to obtain
C
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Figure 2. (a) Coordination environment of Zn(II) in 2, (b) continuous 1D chain formed by Zn(II) ion and PO43− anion along the c-axis, (c, d) views of 2
showing the double-walled hexagonal channels along the c-axis.
Figure 3. (a) N2 sorption curve of 2′ at 77 K, (b) CO2 sorption curves of 1′ and 2′ at 273 K.
Gas Adsorption Studies. The considerable porosity and
thermally robust nature of 1 and 2 along with the presence of
open metal sites prompted us to study the gas adsorption studies.
Activation of 1 and 2 was achieved by keeping the sample in
anhydrous methanol for 5 days, followed by heating at 150 °C
under high vacuum for 8 h, to produce guest-free 1′ and 2′. The
guest-free frameworks were first subjected to N2 sorption studies
at 77 K. For 1′, N2 uptake (Figure S5) was negligible (3.8 cc g−1),
whereas 2′ showed a comparably higher uptake with a maximum
uptake of 214.7 cc g−1 (Figure 3a). The low N2 uptake in 1′ can be
due to the large kinetic diameter of N2 (3.6 Å) than CO2 (3.3 Å).
Furthermore, the low kinetic energy of the N2 molecules at 77 K
does not allow entering of the small windows of 1′. Therefore,
the BET surface area for 1′ was calculated based on the CO2
sorption isotherm at 273 K. The Brunauer−Emmett−Teller
(BET) surface areas for 1′ and 2′ were estimated to be 215 m2 g−1
D
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(from CO2 adsorption isotherm) and 750 m2 g−1 (from N2
adsorption isotherms), respectively.
Table 1. Cycloaddition of CO2 and Various Epoxides
Catalyzed by 2′ in the Presence of TBAB
a
Further, the CO2 adsorption studies of 1′ and 2′ revealed
that the isotherms were quite similar and showed complete
reversibility and no hysteresis with a type I adsorption curve
(Figure 3b). Sample 1′ showed adsorption of CO2 gas at 273 K
with a maximum uptake of 58 cc g−1at 1 bar pressure, whereas the
uptake value for 2′ was 111.7 cc g−1 at 1 bar pressure. Higher
adsorption of N2 and CO2 for compound 2′ could be attributed to
the open metal sites and larger open channels in the framework.
Catalytic Studies. Cycloaddition of CO2 to Epoxides for
the Synthesis of Cyclic Organic Carbonates. The cycloaddition
reaction between CO2 and epoxides comprises one of the most
efficient examples of artificial CO2 fixation as this reaction is
a 100% atom-economic. This fixation of CO2 is advantageous
compared to conventional syntheses using the highly toxic and
corrosive phosgene.20,21 Because of these advantages, this
reaction has been studied extensively.22 In general, for the CO2
fixation reaction using common catalysts such as metal−salen
complexes, metal oxides, and zeolites, high pressure and/or
temperature is needed.23 Recently, few MOFs have been used as
promising and efficient catalyts for the conversion of captured
CO2 to cyclic carbonates.24
Given the significant CO2 uptake capacity and highly robust
nature of 2′, we performed the cycloaddition reaction of CO2 to
epoxides using the coordinatively unsaturated Zn(II) Lewis
acidic sites. 2′ in the presence of tetra-n-tert-butylammonium
bromide (TBAB) as a cocatalyst was found to effectively convert
a variety of substituted terminal epoxides to the corresponding
cyclic carbonates in excellent yields (Table 1). This included
epoxides containing aliphatic, aromatic, electron-withdrawing,
and electron-donating substituents. In the case of the internal
epoxide (Table 1, entry 7), the yield was relatively lower (68%
after 22 h). Meanwhile, in a control reaction in the absence of the
cocatalyst TBAB, the cycloaddition reaction of CO2 to epichlo-
rohydrin catalyzed by 2′ yielded only 4% cyclic carbonate after
32 h (Table 1, entry 8). Furthermore, in the absence of 2′, the
same reaction catalyzed by the cocatalyst TBAB alone gave a 9%
yield (Table 1, entry 9). Yet, when the catalyst 2′ and cocatalyst
TBAB were combined together, a facile reaction occurred (Table 1,
entry 1).
To confirm the heterogeneous nature, the hot filtration method
was applied.25 The reaction using epichlorohydrin as substrate was
stopped by hot filtration of the catalyst at partial conversion
(∼56%) after dilution with dichloromethane. No further signif-
icant reaction occurred if the filtrate was allowed to continue at
the same reaction condition (Figure S6). To probe the reusability
of the catalyst 2′, a series of catalytic cycles were examined using
epichlorohydrin as substrate. In each cycle, the catalyst 2′ was
recovered by simple filtration, washed with methanol, dried
under vacuum, and then subjected to a second run of the reaction
using the same substrates. The yields in 4 runs were almost the
same (Figure S7). The recyclability of the catalyst makes the
reaction economically and practically relevant for commercial
applications. Notably, the framework integrity of catalyst 2
was sustained after each cycle, as shown by PXRD (Figure S4).
Formation of the desired cyclic carbonates was confirmed by the
1H NMR (Figures S8−S14).
a
Reaction conditions: epoxide (20 mmol), catalyst (10 wt %, 1.85 mol %
b
for epichlorohydrin as substrate), TBAB (1 mmol). Conversion was
evaluated from the H NMR spectra by integration of epoxide versus
cyclic carbonate peaks (see the Supporting Information). Catalyzed
by 2′ only. Catalyzed by the cocatalyst TBAB alone.
1
c
d
the Scheme S1, it was suggested that the epoxide first coordinates
on a Zn center. Subsequently, the Br− produced from nBu4NBr
attacks the less-hindered carbon atom of the epoxide to open the
three-membered epoxy ring. Finally, CO2 inserts to form an alkyl
carbonate anion, which subsequently undergoes ring-closing to
form the cyclic carbonate product with the regeneration of the
catalyst.
One-Pot Synthesis of α-Amino Nitriles by Three-Compo-
nent Strecker Reaction. Further, we found that 2′ was a highly
powerful and recyclable catalyst for the three-component Strecker
reaction of ketones, in good to excellent yields under solvent-free
conditions. This reaction is a direct and viable method for
synthesis of α-amino nitriles which are versatile building blocks
for synthesis of α-amino acids and their derivatives.
We started by testing the catalytic activity of 2′ in the three-
component reaction between acetophenone, trimethylsilyl
cyanide (TMSCN), and aniline. A catalytic amount of 2′ was
placed in a Schlenk tube, followed by addition of three reactants.
The reaction was performed in a solvent-free state at room tem-
perature. The results of the reaction are summarized in Table 2.
The increment of the reaction was monitored by TLC analysis.
The catalyst was separated by filtration, washed with chloroform
and methanol, followed by drying under vacuum at 100 °C for
5 h, to regenerate the active catalyst. The integrity of the frame-
work was maintained even after four cycles of the reaction, as
confirmed by the PXRD of the recovered catalyst (Figure S15),
while the catalytic activity was almost unchanged (Figure S16).
Formation of the desired product, α-amino nitrile, was confirmed
The proposed mechanism of the cycloaddition of CO2 to
epoxides in the presence of Lewis acidic transition metal catalysts
and quaternary ammonium salts as cocatalysts has been exten-
sively explored.26 The MOF presents a high activity on CO2
conversion due to its exposed Lewis acid Zn sites. As shown in
by the H NMR and 13C NMR data (Figures S17−S38). The
results of the catalytic activity of 2′ (Table 2) revealed that both
electron-withdrawing as well as electron-donating groups attached
to ketone molecules gave good yield.
1
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a
Table 2. Results Obtained for the Strecker Reaction Catalyzed by 2′
a
Reaction conditions: ketone (1 mmol), amine (1 mmol), TMSCN (1.2 mmol), and cat. 2′ (10 wt %, 2.4 mol % for acetophenone as substrate) at
b
25 °C. Isolated yields.
The proposed mechanism (Scheme S2) for one-pot Strecker
reaction is similar as that described by Monge et al.27 The reaction
involves the formation of an imine intermediate between the
activated carbonyl and the amine derivative, followed by attack of
the cyano group to the imine carbon atom. The heterogeneous
nature of 2′ was ensured by the hot filtration method. After
allowing the reaction to go for 3 h taking acetophenone and aniline
as substrates, the solid 2′ was separated from the reaction mixture
by filtration and the mother liquor was transferred to an empty
Schlenk flask. The reaction did not progress (Figure S39) at all,
proving that the catalyst 2′ was necessary and there was no leaching.
while the other was an organo-metallophosphate anionic frame-
work, 2. The activated framework 2′ exhibited superior N2 and
CO2 adsorption properties compared to 1′. In additiuon to this,
2′, having Lewis acidic open metal sites located in the channel
walls, was an effective catalyst for the cycloaddition of CO2 to
epoxides and one-pot synthesis of α-amino nitriles from a var-
iety of aldehydes/ketones together with amines and trimethylsilyl
cyanide. This catalyst 2′ was in particular highly stable and could be
reused in several successive runs with no considerable loss of activity
and with no significant structural change.
ASSOCIATED CONTENT
■
CONCLUSIONS
S
* Supporting Information
■
In summary, we showed that the same ligand L formed two
categories of porous frameworks: one was a neutral framework, 1,
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b02443.
F
DOI: 10.1021/acs.inorgchem.7b02443
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Materials and methods, X-ray structural studies, IR
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Accession Codes
CCDC 1575928 and 1575929 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in.
■
ORCID
Parimal K. Bharadwaj: 0000-0003-3347-8791
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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Metal-adeninate vertices for the construction of an exceptionally porous
metal-organic framework. Nat. Commun. 2012, 3, 604−609.
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We gratefully acknowledge the financial support received from
the DST and MNRE, New Delhi, India (to P.K.B.), and SRF
from the University Grants Commission (UGC), New Delhi,
India, to M.G. D.D. thanks IIT Kanpur for a Research Associate
fellowship. YSF to K.T. from SERB (YSS/2015/001088/CS),
New Delhi, India.
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DOI: 10.1021/acs.inorgchem.7b02443
Inorg. Chem. XXXX, XXX, XXX−XXX