Single-walled polytetrazolate metal-organic channels with high density of open nitrogen-donor sites and gas uptake

Article abstract of DOI:10.1021/ja2092882

The self-assembly between zinc dimer and 1,3,5-tris(2H-tetrazol-5-yl) benzene (H₃BTT), promoted by a urea derivative, leads to a highly porous 3D framework with a large percentage (67%) of N-donor sites unused for bonding with metals. The material exhibits high gas storage capacity (ca. 1.89 wt % H₂ at 77 K and 1 atm; 98 cm³/g CO₂ at 273 K and 1 atm), even in the absence of open metal sites. The high percentage of open N-donor sites, coupled with the low framework density resulting from single-walled channels, is believed to contribute to the high uptake capacity.

Full text of DOI:10.1021/ja2092882

Communication
pubs.acs.org/JACS
Single-Walled Polytetrazolate Metal−Organic Channels with High
Density of Open Nitrogen-Donor Sites and Gas Uptake
Qipu Lin,† Tao Wu,† Shou-Tian Zheng,‡ Xianhui Bu,*,‡ and Pingyun Feng*,†
†Department of Chemistry, University of California, Riverside, California 92521, United States
‡Department of Chemistry and Biochemistry, California State University, 1250 Bellflower Boulevard, Long Beach, California 90840,
United States
S
* Supporting Information
framework compositions and the nature of gas molecules.7 Of
ABSTRACT: The self-assembly between zinc dimer and
1,3,5-tris(2H-tetrazol-5-yl)benzene (H3BTT), promoted
by a urea derivative, leads to a highly porous 3D frame-
work with a large percentage (67%) of N-donor sites
unused for bonding with metals. The material exhibits high
gas storage capacity (ca. 1.89 wt % H2 at 77 K and 1 atm;
98 cm3/g CO2 at 273 K and 1 atm), even in the absence of
open metal sites. The high percentage of open N-donor
sites, coupled with the low framework density resulting
from single-walled channels, is believed to contribute to
the high uptake capacity.
particular interest are functional groups such as aromatic
−N(H)− donors found in metal azolate frameworks.4 Despite
the fact that the use of triazoles and tetrazoles has led to many
MOF materials, it is still an ongoing challenge to create porous
frameworks in which the largest possible number of N-donor
sites are left uncoordinated to metals (called open donor sites,
in analogy with open metal sites). To increase the percentage of
open donor sites, individual triazole or tetrazole ligands are of
limited utility, because at least two N-donor sites will be needed
for the framework connectivity, and the maximum percentage
of open donor sites would be 33% for a triazole and 50% for a
tetrazole framework. For this reason, we are especially
interested in ligands containing multiple triazole and tetrazole
groups (i.e., polytriazole or polytetrazole such as 1,3,5-tris(2H-
tetrazol-5-yl)benzene or H3BTT in short), as such ligands can
achieve high connectivity even if they only use one N-donor
site (per triazole or tetrazole group) for bonding with metals.
The highest percentage of open donor sites would be 67% for a
triazole and 75% for a tetrazole, assuming each ligand uses just
one N-donor site for the framework formation. Moreover, as in
the case of H3BTT, steric hindrance could reduce the likelihood
for N1 and N4 sites to bind with metals.8
H3BTT is one of the most interesting polytetrazole ligands
and was successfully employed by Long et al. in their design of
Cu-BTT or Mn-BTT MOFs with open metal sites and
enhanced H2 uptakes.2a,b However, despite this great potential
for the MOF construction, relatively few BTT MOFs have been
made since the initial success by Long’s group.9 The challenge
with the synthesis of BTT-MOFs was highlighted by earlier
work showing that many attempts with the 3d metals (Fe−Zn)
and various solvents (or solvent mixtures) led to insoluble,
amorphous solids.2b
orous metal−organic frameworks (MOFs) are a promising
P
class of sorbents for various applications such as on-board
H2 storage and CO2 capture.1 Continuing efforts are being
devoted to the optimization of various chemical and structural
factors such as surface area, pore geometry, and functional sites
in MOFs, in hope of increasing storage capacity and separation
efficiency, particularly under ambient conditions. Synthetic
strategies employed include creation of unsaturated metal
sites,2 introduction of cross-linking ligands with additional
functional groups such as −NH2,3 lowering of the connectivity
of multidonor ligands (e.g., triazoles and tetrazoles) to leave
behind unused donor sites,4 and design of pore architecture to
fit the size of the gas molecule (e.g., through catenation and
cage-within-cage configurations).5
To increase gravimetric storage capacity, the use of
lightweight elements such as Li, B, Mg, and Al has attracted
wide attention.6 However, the coordination chemistry of such
lightweight elements is generally less diverse than that of 3d
metals such as Zn. To take advantage of the rich coordination
chemistry of 3d metals and to simultaneously achieve the low
framework density, an alternative method is to construct
porous frameworks that have the highest level of “atom
economy” (a term which here denotes the percentage of
framework atoms exposed toward the pore space and thus
accessible for interactions with gas molecules). Recent studies
have shown that gas molecules can interact with both metal
sites and ligands of the porous frameworks.7 Thus, having
available the largest number of exposed framework atoms (both
metals and nonmetals) would likely contribute to enhanced
gravimetric storage capacity.
In this work, by using a urea derivative as the cosolvent in a
procedure reminiscent of urothermal synthesis method
developed recently,10 an interesting porous framework
(denoted CPF-6, where CPF = crystalline porous frameworks)
with 1D square single-atom-walled channel system has been
prepared, further demonstrating the great synthetic utility of
urea-derivative-containing solvent system.2b
Also of interest is that even though CPF-6 does not contain
open metal sites, its H2 uptake capacity comes very close to that
Clearly, the binding affinity between the framework atoms
Received: October 2, 2011
and gas molecules can vary significantly, depending on both
Published: December 19, 2011
© 2011 American Chemical Society
784
dx.doi.org/10.1021/ja2092882 | J. Am. Chem.Soc. 2012, 134, 784−787

Journal of the American Chemical Society
Communication
of the related Mn-BTT MOF that does contain a significant
number of open metal sites. We attribute this property to the
large number of open donor sites and the single-walled
channels that maximize the “atom economy”. CPF-6 is parti-
cularly unusual, because two-thirds of its tetrazolate groups use
only one N-donor site (out of four N-donor sites per tetrazole
group) for binding with metals, with an overall of 66.6% of
N-donor sites left unbonded with metals. To our knowledge,
this is the highest percentage of N-donor sites left
uncoordinated to metals in azolate-based MOFs.8,9
Ligand H3BTT was synthesized by following a modified
literature procedure (see Supporting Information).2a,11 Sol-
vothermal reaction between H3BTT and Zn(NO3)2·6H2O in a
dma:dmpu (3:2 v/v, dma = N,N-dimethylacetamide, dmpu =
1,3-dimethylpropyleneurea) mixed solvent with one drop of
concentrated HCl afforded block-shaped crystals of Zn(HBTT)
(1, CPF-6).12
Single-crystal X-ray diffraction studies reveal that CPF-6
crystallizes in the tetragonal space group P42/mnm and consists
of 1D square single-walled channels (Figure 1). The asym-
metrical unit of the host framework includes one-fourth of the
Zn2+ ion and one-fourth of the HBTT2− ligand. Two
tetrahedrally coordinated Zn2+ centers are bridged by two
tetrazole groups to form a dimer core. Each Zn2+ site is further
connected to two N2 sites from two other tetrazolate rings.
Thus, each zinc dimer is connected to a total of six HBTT2−
ligands and serves as the 6-connected node. In turn, each
HBTT2− spacer is bound to three zinc dimers and serves as the
3-connected node. The overall framework adopts a (3,6)-rtl
(rutile) topology.
In comparison with Mn-BTT (or Cu-BTT) sodalite-type
frameworks which exhibit one type of ligand−metal binding
mode,2a,b three tetrazole groups in CPF-6 show two types of
binding modes (one bidentate and two unidentate fashions). It
is worth noting that in MOFs it is quite unusual for a tetrazole
group to bind to metals using only one N-donor site, as shown
in CPF-6. Such an unusual mode makes it possible to increase
the density of uncoordinated N-donor sites. In addition, the
coexistence of such two types of coordination modes by
three tetrazole groups in the same tristetrazolate HBTT2−
allows the synthesis of a rare 3D network based on single-
walled channels with a diagonal separation of about 1.5 nm.
Additionally, as depicted in Supporting Information, nanosized
channels are interconnected to each other through triangular
apertures, forming a continuous 3D pore system, which may
contribute to the enhanced gas sorption properties. The anionic
charge of the framework is balanced by a proton (correspond-
ing to the N−H stretching peak at 3391 cm−1 in the infrared
spectrum of CPF-6), whose position could not be located
crystallographically.
The total solvent-accessible volume of CPF-6 was estimated
to be 69.7% by PLATON. Thermogravimetric analysis (TGA)
of solvated CPF-6 under N2 atmosphere showed a sharp weight
loss of 22% below 160 °C, suggesting that dma and/or dmpu
molecules were evacuated at this temperature. No steep weight
loss between 160 and 210 °C (see Supporting Information)
were observed. Thermal powder X-ray diffraction (PXRD)
analysis indicated the sample CPF-6 retains its crystallinity after
being heated to ca. 180 °C. To evaluate the porosity, N2, H2,
and CO2 isotherms on the guest-free sample were measured.
Prior to the measurement, as-synthesized sample of CPF-6 was
immersed in low-boiling-point acetone. During the exchange,
the solvent was refreshed multiple times. The resulting
Figure 1. (a) View of the zinc dimer bridged by two tetrazolate groups
(in bidentate fashion) and completed by four other tetrazolate groups
(in unidentate fashion) (symmetry codes: a = y, x, −z; b = 1 − x, 1 −
y, z; c = 1 − y, 1 − x, −z; d = 0.5 − x, −0.5 + y, 0.5 − z; e = −0.5 + y,
0.5 − x, −0.5 + z; f = 0.5 + x, 1.5 − y, 0.5 − z; g = 1.5 − y, 0.5 + x,
−0.5 + z; h = 1 − y, 1 − x, 1 − z). (b) Ball-and-stick and (c) space-
filling representations of the extended network with single-atomic-
walled channels.
exchanged sample was then evacuated (10−3Torr) successively
at room temperature and 80 °C.
CPF-6 exhibits typical type I reversible sorption isotherms
and takes up N2 to 220 cm3/g at 77 K and 1 atm,
corresponding to a Langmuir and BET surface areas of 883
785
dx.doi.org/10.1021/ja2092882 | J. Am. Chem.Soc. 2012, 134, 784−787

Journal of the American Chemical Society
Communication
and 599 m2/g, respectively. The H2 isotherm collected at 77 K
was completely reversible, and the material adsorbs a maximum
of 1.85 wt % at 77 K and 1 atm (Figure 2a), which is close to
dramatic enhancement of gas sorption if it is possible to
integrate high concentrations of open metal sites together with
open donor sites and single walls.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, additional structural figures, powder X-ray
diffraction, thermal analysis. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
pingyun.feng@ucr.edu; xbu@csulb.edu
■
ACKNOWLEDGMENTS
■
This work was supported by the Department of Energy Office
of Basic Energy Sciences under Contract No. DE-SC0002235
(P.F.) and by NSF (X.B., DMR-0846958).
REFERENCES
(1) (a) Murray, L. J.; Dinca,
■
̆
M.; Long, J. R. Chem. Soc. Rev. 2009, 38,
1294−1314. (b) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev.
2009, 38, 1477−1504. (c) Wang, X.-S.; Ma, S.-Q.; Forster, P. M.;
́
Yuan, D.-Q.; Eckert, J.; Lopez, J. J.; Murphy, B. J.; Parise, J. B.; Zhou,
H.-C. Angew. Chem., Int. Ed. 2008, 47, 7263−7266. (d) Rowsell, J. L.
C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670−4679.
(e) Wang, F.; Liu, Z.; Yang, H.; Tan, Y.; Zhang, J. Angew. Chem., Int.
Ed. 2011, 50, 450−453. (f) Huang, S.; Lin, C.; Wu, W.; Wang, S.-L.
Angew. Chem., Int. Ed. 2009, 48, 6124−6127. (g) Liao, Y.; Liao, F.;
Chang, W.; Wang, S.-L. J. Am. Chem. Soc. 2004, 126, 1320−1321.
(h) Fei, H.; Paw, U, L.; Rogow, D. L.; Bresler, M. R.; Abdollahian, Y.
A.; Oliver, S. R. J. Chem. Mater. 2010, 22, 2027−2032. (i) Fei, H.;
Rogow, D. L.; Oliver, S. R. J. J. Am. Chem. Soc. 2010, 132, 7202−7209.
(j) Rogow, D. L.; Zapeda, G.; Swanson, C. H.; Fan, X.; Campana, C.
F.; Oliver, A. G.; Oliver, S. R. J. Chem. Mater. 2007, 19, 4658−4662.
(k) Oliver, S. R. J. Chem. Soc. Rev. 2009, 38, 1868−1881.
(2) (a) Dinca,
̆
M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.;
M.;
Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876−16883. (b) Dinca,
̆
Han, W. S.; Liu, Y.; Dailly, A.; Brown, C. M.; Long, J. R. Angew. Chem.,
Int. Ed. 2007, 46, 1419−1422. (c) Caskey, S. R.; Wong-Foy, A. G.;
Matzger, A. J. J. Am. Chem. Soc. 2008, 130, 10870−10871. (d) Lee,
Y.-G.; Moon, H. R.; Cheon, Y. E.; Suh, M. P. Angew. Chem., Int. Ed.
2008, 47, 7741−7745. (e) Bloch, E. D.; Britt, D.; Lee, C.; Doonan,
C. J.; Uribe-Romo, F. J.; Furukawa, H.; Long, J. R.; Yaghi, O. M. J. Am.
Chem. Soc. 2010, 132, 14382−14384.
Figure 2. (a) N2 (blue circles) and H2 (green squares) sorption
isotherms and Horvath−Kawazoe pore distribution at a highest pore
width of 4.7 Å (inset) at 77 K. (b) CO2 (magenta squares) and N2
(blue diamonds) sorption isotherms at 273 K, showing selective
adsorption of CO2 over N2. STP = standard temperature and pressure
(solid symbols, adsorption; open symbols, desorption).
(3) (a) Vaidhyanathan, R.; Iremonger, S. S.; Dawson, K. W.; Shimizu,
G. K. H. Chem. Commun. 2009, 5230−5232. (b) Panda, T.; Pachfule,
P.; Chen, Y.; Jiang, J.; Banerjee, R. Chem. Commun. 2011, 47, 2011−
2013. (c) An, J.; Geib, S. J.; Rosi, N. L. J. Am. Chem. Soc. 2010, 132,
38−39. (d) Demessence, A.; D’Alessandro, D. M.; Foo, M. L.; Long, J.
R. J. Am. Chem. Soc. 2009, 131, 8784−8786.
(4) (a) Lin, J.-B.; Zhang, J.-P.; Chen, X.-M. J. Am. Chem. Soc. 2010,
132, 6654−6656. (b) Zhang, J.-P.; Zhu, A.-X.; Lin, R.-B.; Qi, X.-L.;
Chen, X.-M. Adv. Mater. 2011, 23, 1268−1271.
(5) (a) Farha, O. K; Malliakas, C. D.; Kanatzidis, M. G.; Hupp, J. T.
J. Am. Chem. Soc. 2010, 132, 950−952. (b) Zheng, S.-T.; Bu, J. T.; Li,
Y.; Wu, T.; Zuo, F.; Feng, P.; Bu, X. J. Am. Chem. Soc. 2010, 132,
17062−17064. (c) Guo, Z.; Wu, H.; Srinivas, G.; Zhou, Y.; Xiang, S.;
Chen, Z.; Yang, Y.; Zhou, W.; O’keeffe, M.; Chen, B. Angew. Chem.,
Int. Ed. 2011, 50, 3178−3181.
(6) (a) Zhang, J.; Wu, T.; Zhou, C.; Chen, S.; Feng, P.; Bu, X. Angew.
Chem., Int. Ed. 2009, 48, 2542−2545. (b) Wu, T.; Zhang, J.; Zhou, C.;
Wang, L.; Bu, X.; Feng, P. J. Am. Chem. Soc. 2009, 131, 6111−6113.
(c) Zhao, X.; Wu, T.; Zheng, S.-T.; Wang, L.; Bu, X.; Feng, P. Chem.
Commun. 2011, 47, 5536−5538. (d) Volkringer, C.; Popov, D.;
́
Loiseau, T.; Guillou, N.; Ferey, G.; Haouas, M.; Taulelle, F.; Mellot-
Draznieks, C.; Burghammer, M.; Riekel, C. Nat. Mater. 2007, 6, 760−
the observation for Mn-BTT.2a A recent theoretical study
suggests that nitrogen atoms on aromatic rings can enhance the
adsorption energy of H2.13 Since such a structural feature is also
expected to enhance interactions with CO2, the CO2 isotherm
at 273 K was also measured, which shows an uptake of 98 cm3/g
at 1 atm (Figure 2b). For comparison, the N2 uptake at 273 K is
only 4.7 cm3/g at 1 atm, which leads to the calculated CO2/N2
selectivity of 20.8:1 at 273 K. The H2 and CO2 uptake capacity
of CPF-6 ranks among the high end for MOF materials. So far,
the number of materials and structure types with the CO2
uptake of more than 100 cm3/g (273 K and 1 atm) and H2
uptake of 2.0 wt % is still very limited.14
In conclusion, a single-walled 3D framework CPF-6,
containing a high concentration of tetrazolate nitrogen sites
uncoordinated to metal sites, has been synthesized. The high
percentage of open N-donor sites, together with low framework
density (due to the single wall) leads to the high uptake
capacity for H2 and CO2, even in the absence of any open metal
sites. This work points to an intriguing possibility for the
786
dx.doi.org/10.1021/ja2092882 | J. Am. Chem.Soc. 2012, 134, 784−787

Journal of the American Chemical Society
Communication
764. (e) Zhang, H.; Wang, F.; Yang, H.; Tan, Y.; Zhang, J.; Bu, X.
J. Am. Chem. Soc. 2011, 133, 11884−11887.
(7) (a) Duren, T.; Bae, Y.-S.; Snurr, R. Q. Chem. Soc. Rev. 2009, 38,
̈
́
1237−1247. (b) Han, S. S.; Mendoza-Cortes, J. L.; Goddard, W. A. III
Chem. Soc. Rev. 2009, 38, 1460−1476. (c) Chen, B.; Zhao, X.;
Putkham, A.; Hong, K.; Lobkovsky, E.; Hurtado, E.; Fletcher, A.;
Thomas, K. J. Am. Chem. Soc. 2008, 130, 6411−6423. (d) Xiang, S.;
Zhou, W.; Gallegos, J. M.; Liu, Y.; Chen, B. J. Am. Chem. Soc. 2009,
131, 12415−12419. (e) Chen, B.; Xiang, S.; Qian, G. Acc. Chem. Res.
2010, 43, 1115−1124. (f) Xiang, S.; Zhou, W.; Zhang, Z.; Green, M.
A.; Liu, Y.; Chen, B. Angew. Chem., Int. Ed. 2010, 49, 4615−4618.
(g) He, Y.; Xiang, S.; Chen, B. J. Am. Chem. Soc. 2011, 133, 14570−
14573.
(8) (a) Ouellette, W.; Prosvirin, A. V.; Whitenack, K.; Dunbar, K. R.;
̆
Zubieta, J. Angew. Chem., Int. Ed. 2009, 48, 2140−2143. (b) Dinca, M.;
Dailly, A.; Long, J. R. Chem.Eur. J. 2008, 14, 10280−10285.
(9) (a) Liao, J.-H.; Chen, W.-T.; Tsai, C.-S.; Yang, C.-C.; Wang,
C.-C. CrystEngComm 2010, 12, 3033−3037. (b) Li, M.-N.; Yang,
G.-S.; Li, S.-L.; Zang, H.-Y.; Xu, G.-J. Inorg. Chem. Commun. 2010, 13,
1203−1206. (c) Li, M.-N.; Du, D.-Y.; Yang, G.-S.; Li, S.-L.; Lan, Y.-Q.;
Shao, K.-Z.; Qin, J.-S.; Su, Z.-M. Cryst. Growth Des. 2011, 11, 2510−
2514.
(10) (a) Zhang, J.; Bu, J.; Chen, S.; Wu, T.; Zheng, S.; Chen, Y.;
Nieto, R.; Feng, P.; Bu, X. Angew. Chem., Int. Ed. 2010, 49, 8876−
8879. (b) Kang, Y.; Chen, S.; Wang, F.; Zhang, J. Chem. Commun.
2011, 47, 4950−4952. (c) Luo, H.; Baker, G. A.; Lee, J. S.; Pagni, R.
M.; Dai, S. J. Phys. Chem. B 2009, 113, 4181−4183. (d) Bell, J. R.; Luo,
H.; Dai, S. Tetrahedron Lett. 2011, 52, 3723−3725.
(11) Hill, M.; Mahon, M. F.; Molloy, K. C. J. Chem. Soc., Dalton
Trans. 1996, 1857−1865.
(12) Crystal data for [Zn(HBTT)]·x(solv) (1, CPF-6), C9H4N12Zn,
M = 345.63, tetragonal, P42/mnm (No. 136), a = 15.4962(7) Å, c =
13.7523(12) Å, V = 3302.4(4) Å3, Z = 4, Bruker APEX II, Mo Kα
radiation, λ = 0.71073, T = 150(2) K, 10 126 reflections collected,
2137 unique (Rint = 0.0394). R1 = 0.0372 (I > 2σ(I)). wR (F2) =
0.0816 (I > 2σ(I)). GOF = 1.062.
(13) Samanta, A.; Furuta, T.; Li, J. J. Chem. Phys. 2006, 125, 084714.
(14) (a) Sculley, J.; Yuan, D.-Q.; Zhou, H.-C. Energy Environ. Sci.
2011, 4, 2721−2735. (b) Mason, J. A.; Sumida, K.; Herm, Z. R.;
Krishna, R.; Long, J. R. Energy Environ. Sci. 2011, 4, 3030−3040.
(c) Wang, C.; Luo, H.; Jiang, D.; Li, H.; Dai, S. Angew. Chem., Int. Ed.
2010, 49, 5978−5981. (d) Wang, C.; Luo, H.; Luo, X.; Li, H.; Dai, S.
Green Chem. 2010, 12, 2019−2023. (e) Wang, C.; Mahurin, S. M.;
Luo, H.; Baker, G. A.; Li, H.; Dai, S. Green Chem. 2010, 12, 870−874.
787
dx.doi.org/10.1021/ja2092882 | J. Am. Chem.Soc. 2012, 134, 784−787