Self-assembly of metal-organic coordination polymers constructed from a bent dicarboxylate ligand: Diversity of coordination modes, structures, and gas adsorption

Article abstract of DOI:10.1021/ic901429u

We have synthesized five new metal-organic coordination polymers incorporating the bent ligand H₂hfipbb [4,4′- (hexafluoroisopropylidene)bls(benzoic acid)] with different transition metal ions and co-ligands via solvothermal reactions to give [

Full text of DOI:10.1021/ic901429u

Inorg. Chem. 2009, 48, 11067–11078 11067
DOI: 10.1021/ic901429u
Self-Assembly of Metal-Organic Coordination Polymers Constructed from a Bent
Dicarboxylate Ligand: Diversity of Coordination Modes, Structures, and Gas Adsorption
Wenbin Yang, Xiang Lin, Alexander J. Blake, Claire Wilson, Peter Hubberstey, Neil R. Champness,* and
€
Martin Schroder*
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.
Received July 22, 2009
We have synthesized five new metal-organic coordination polymers incorporating the bent ligand H2hfipbb
[4,40-(hexafluoroisopropylidene)bis(benzoic acid)] with different transition metal ions and co-ligands via solvothermal
reactions to give [Zn2(hfipbb)2(py)2] DMF (1), [Zn2(hfipbb)2(4,40-bipy)(H2O)] (2), [Zn2(hfipbb)2(bpdab)] 2DMF (3),
3
3
[Cd2(hfipbb)2(DMF)2] 2DMF (4), and [Co(hfipbb)(dpp)] MeOH (5) (py = pyridine, 4,40-bipy = 4,40-bipyridine, bpdab =
3
3
1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, dpp = 1,3-di(4-pyridyl)propane). Compound 1 displays a 2-fold 2Df2D
parallel interpenetrated layer network with one-dimensional (1D) helical channels, while 3 exhibits a three-dimensional
pillared helical-layer open framework of R-Po topology based upon binuclear paddlewheel units. In compounds 2 and 5,
binuclear motifs with double carboxylate bridges are linked by hfipbb2- ligands into a 1D ribbon, which are further
assembled into two-dimensional non-interpenetrated (4,4) layers via bipyridyl co-ligands. However, the different bridging
modes of hfipbb2- ligands and the different disposition of the coordinated co-ligands around metal ions result in subtle
differences in the final architecture. Thus, 2 is based on a binuclear cluster node, double-stranded hfipbb2- linkers, and
single-stranded 4,40-bipy linkers, while 5 is based on a binuclear cluster node and hfipbb2- and dpp linkers which are both
double-stranded. Among these compounds, the Cd(II) complex 4 is possibly the most interesting because it represents a
rare example in which metal centers are linked by carboxylate groups into infinite chains further joined together by hfipbb2-
spacers to form a 2D network with tubular helical channels. All these coordination polymers exhibit low solvent-accessible
volumes. Both 3 and 4 retain structural integrity and permanent microporosity upon evacuation of guest molecules, with
hydrogen uptakes of 0.57 and 0.78 wt %, respectively, at 20 bar and 77 K.
Introduction
not only for their structural diversity and intriguing mole-
cular topologies2,3 but also for their potential as a new class
of porous materials applied in the fields of separation,4
The design and synthesis of metal-organic coordination
polymers have drawn significant interest in recent years,1
*To whom correspondence should be addressed. E-mail: m.schroder@
nottingham.ac.uk (M.S.), neil.champness@nottingham.ac.uk (N.R.C.).
Phone: þ44 (0)115 9513490. Fax: þ44 (0)115 9513563.
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2009 American Chemical Society
Published on Web 10/30/2009
pubs.acs.org/IC

11068 Inorganic Chemistry, Vol. 48, No. 23, 2009
Yang et al.
heterogeneous catalysis,5 gas storage,6 and drug delivery.7
The structures of coordination polymers are dependent upon
a variety of factors such as the geometrical and electronic
properties of the metal ions used, the bulk and coordinative
abilities ofthe ligands, the ligand-to-metalstoichiometry, and
the use of different solvents, synthetic strategies, and methods
of crystal growth. In particular, variation of ligand confor-
mations often plays a key role in the self-assembly processes
to give metal-organic coordination polymers with different
structural topologies. By adjusting the shape, functionality,
flexibility, length, and symmetry of the spacers, a remarkable
range of materials containing various architectures and
functions can be prepared.1,8-11
carboxylate complexes a rich structural chemistry. In addi-
tion, the carboxylate unit can bind together with a phosphate
group or an N-containing heterocycle to give intriguing
hybrid structures.14 Thus, polycarboxylate ligands, in parti-
cular, aromatic di- and multibranched carboxylates (i.e.,
terephthalic, isophthalic, trimesic, and pyromellitic acid)
and various pyridine- and imidazole-dicarboxylates, have
been widely used in the rational design of metal-organic
coordination polymers.9-18 Recently, strategies have been
reported using combinations of appropriate N,N0-donor
bidentate ligands to introduce pillars to link well-defined
two-dimensional (2D) metal-carboxylate layers.19 In this way
the structures of a wide variety of coordination polymers can
be predicted, and the pore size and chemical functionality of
the resultant open frameworks readily controlled via a
modification of the pillars.20 One of the best-known exam-
ples is the series [M2(2,3-pyzdc)2Lpillar]n [M = Cu and Cd;
2,3-pyzdc2-=pyrazine-2,3-dicarboxylate; Lpillar=pyrazine,
4,40-bipyridine, 1,2-di(4-pyridyl)glycol, 1,4-diazabicyclo[222]-
octane, and py-NdN-py/py-CHdCH-py (py = pyridin-
4-yl)].20,21 These microporous materials incorporate pillar
ligand bridges between the neutral layers of [M2(2,3-
pyzdc)2]n. Other examples of porous pillar-layered architec-
tures are based upon binuclear paddlewheel building blocks
bridged by linear anionic dicarboxylates to form 2D square
grids further extended by various neutral N,N0-donor pillars
to give three-dimensional (3D) R-Po porous structures with
one-dimensional (1D) channels.19,22 Some of these layer-
pillared architectures display a degree of hydrogen storage
capacity.19,22a-c
Carboxylate O-donors have the ability to coordinate to
metal centers in a variety of modes, ranging from mono- or
bis-chelate fashion to μ3-η2:η, μ4-η2:η2, and μ5-η3:η3 bridging
modes.9-13 The wide variety of coordination modes of
carboxylates and their high affinity for metal ions give metal
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4,40-(hexafluoroisopropylidene)bis(benzoic acid) (H2hfipbb)
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The hfipbb2- ligand incorporates a C(sp3) fluorinated spacer
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Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11069
Scheme 1. Schematic Representation of the Diverse Structures Observed in 1-5a
a a-d coordination modes given in Scheme 2.
formation of helical and self-penetrating networks. This
semirigid molecule has been shown previously to bridge
different transition and rare-earth metal ions to give materi-
als with interesting sorption, catalytic, luminescent, or mag-
netic properties.4c,6f,23 Under appropriate synthetic con-
ditions and in the presence of auxiliary co-ligands, five
Experimental Section
General Procedures. All chemicals for synthesis were obtained
from commercial sources and used without further purification,
with the exception of 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
(bpdab), prepared according to the literature procedure.24 Ele-
mental analyses of C, H, and N were performed by the Ele-
mental Analysis Service of the School of Chemistry, University
of Nottingham. Infrared spectra were measured as KBr discs on
a Nicolet Avatar 360 FT-IR system over the 400-4000 cm-1
range. Thermal gravimetric (TG) measurements were per-
formed on a Rheometric Scientific STA 1500H thermal analyzer
at a heating rate of 1 °C/min-1 under nitrogen flow (generally
60 mL/min). X-ray powder diffraction data were collected on a
Philips X’pert powder diffractometer with Cu KR radiation from
samples mounted on the flat glass plate sample holder. Scans of
approximately 90 min were run for each sample over the range
5° e 2θ e 60° with a step size of 0.02° in 2θ. Simulated powder
patterns were generated from CIF data for the final single
crystal refinement model using MERCURY.25
new coordination polymers, [Zn2(hfipbb)2(py)2] DMF (1),
3
[Zn2(hfipbb)2(4,40-bipy)(H2O)] (2), [Zn2(hfipbb)2(bpdab)]
3
2DMF (3), [Cd2(hfipbb)2(DMF)2] 2DMF (4), and [Co-
(hfipbb)(dpp)] MeOH (5) (py = pyridine, 4,40-bipy = 4,40-
3
3
bipyridine, bpdab = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-buta-
diene, dpp=1,3-di(4-pyridyl)propane) have been successfully
prepared and characterized (Scheme 1). The coordination
modes of the ligands are discussed in detail herein, and, in
addition, the thermal stabilities and hydrogen adsorption
properties of 3 and 4 are also presented.
ꢁ
(23) (a) Monge, A.; Snejko, N.; Gutierrez-Puebla, E.; Medina, M.;
ꢁ
Synthesis of [Zn2(hfipbb)2(py)2] DMF (1). A mixture of
Zn(NO3) 6H2O (29.7 mg, 0.10 mmol), H2hfipbb (39.2 mg,
0.10 mmol), and pyridine (0.1 mL) was dissolved in
N,N-dimethylformamide (DMF; 8 mL), sealed in a 23 mL Parr
Cascales, C.; Ruiz-Valero, C.; Iglesias, M.; Gomez-Lor, B. Chem. Commun.
3
ꢁ
ꢁ
ꢁ
ꢁ
2005, 1291. (b) Gandara, F.; de Andres, A.; Gomez-Lor, B.; Gutierrez-Puebla, E.;
Iglesias, M.; Monge, M. A.; Proserpio, D. M.; Snejko, N. Cryst. Growth Des.
2008, 8, 378. (c) Zhou, Y.; Han, L; Pan, J.; Li, X.; Zheng, Y. Inorg. Chem.
Commun. 2008, 11, 1107. (d) Harbuzaru, B. V.; Corma, A.; Rey, F.; Atienzar, P.;
3
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Jorda, J. L.; García, H.; Ananias, D.; Carlos, L. D.; Rocha, J. Angew. Chem., Int.
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Ed. 2008, 47, 1080. (e) Gandara, F.; Gomez-Lor, B.; Gutierrez-Puebla, E.;
Iglesias, M.; Monge, M. A.; Proserpio, D. M.; Snejko, N. Chem. Mater. 2008,
20, 72. (f) Leea, J.; Li, J.; Jagiello, J. J. Solid State Chem. 2005, 178, 2527.
(g) Liu, Z.; Stern, C. L.; Lambert, J. B. Organometallics 2009, 28, 84.
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CURY, version 1.4.2; http://www.ccdc.cam.ac.uk/products/mercury.

11070 Inorganic Chemistry, Vol. 48, No. 23, 2009
Yang et al.
bomb, and heated at 120 °C for 3 days. This afforded colorless
crystals of 1, which were collected by filtration, washed with
ethanol, and dried in air. Yield 37.8 mg (66.2%). Elemental
analysis for C47H33F12N3O9Zn2 (calcd/found): C, 49.58/49.56;
H, 2.92/2.91; N, 3.69/3.71. Selected IR(KBr, νmax): 1659 (s),
of coordinated water molecules or by using a geometrical model
for methanol molecules. Further details for crystallographic
data and refinement conditions are summarized in Table 1,
and selected bond lengths and bond angles are given in Support-
ing Information, Table S1.
1601(s), 1528(m), 1384(m), 1212(m), 966(m) cm-1
.
Synthesis of [Zn2(hfipbb)2(bpy)(H2O)] (2). A mixture of Zn-
Hydrogen Adsorption Measurements. Hydrogen adsorp-
tion-desorption experiments were conducted at 77 K using an
IGA gravimetric adsorption apparatus equipped with a micro-
gram balance, 2 mbar, 100 mbar, and 20 bar pressure transdu-
cers, and a clean ultrahigh vacuum system. Ultrapure H2
(99.9995%) was purified further by using calcium aluminosili-
cate adsorbents to remove trace amounts of water and other
impurities before introduction into the IGA system. For mea-
surements at 77 K, a standard low-temperature liquid nitrogen
Dewar vessel was used. Before gas adsorption measurements,
the sample was activated at 150 °C (for 3) and 200 °C (for 4)
under ultrahigh vacuum (10-8 mbar) overnight. About 50 mg of
samples were loaded for gas adsorption, and the weight of each
sample was recorded before and after outgassing to confirm
complete removal of all guest molecules including the coordi-
nated DMF in 4. All isotherm data points were fitted by the
IGASwin system software v.1.03.143 (Hiden Isochema, 2004)
using a linear driving force model when >98% equilibration
had been reached. All changes in sample weight were corrected
for buoyancy effects.
(NO3) 6H2O (15.8 mg, 0.054 mmol), H2hfipbb (21 mg, 0.054
3
mmol), and 4,40-bipy (15.6 mg, 0.10 mmol) was suspended in
H2O (9 mL), sealed in a 23 mL Parr bomb, and heated at 125 °C
for 4 days. This afforded colorless crystals of 2, which were
collected by filtration, washed with ethanol, and dried in air.
Yield 10.3 mg (35.7%). Elemental analysis for C44H26F12N2-
O9Zn2 (calcd/found): C, 48.87/48.91; H, 2.42/2.46; N, 2.59/2.61.
Selected IR(KBr, νmax): 3428(br), 1659(s), 1601(s), 1212(m),
966(m), 857(m), 782(m), 458 (w) cm-1
.
Synthesis of [Zn2(hfipbb)2(bpdab)] 2DMF (3). A mixture of
Zn(NO3) 6H2O (18.0 mg, 0.061 mmol), H2hfipbb (25.3 mg,
3
3
0.065 mmol), and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
(bpdab, 10.5 mg, 0.050 mmol) was suspended in DMF (20
mL), sealed in a 25 mL reaction tube, and heated in an oil bath
at 100 °C for 60 h. This afforded yellow crystals of 3, which were
collected by filtration, washed with methanol, and dried in air.
Yield 24.5 mg (60.3%). Elemental analysis for C55H47F12-
N7O11Zn2 (calcd/found): C, 49.42/49.39; H, 3.54/3.53; N, 7.33/
7.31. Selected IR(KBr, νmax): 3075(s), 1685(m), 1635(s), 1563(s),
1399(m), 1214(m), 962(m), 841(m) cm-1
Synthesis of [Cd2(hfipbb)2(DMF)2] 2DMF (4). Cd(NO3)2
.
Results and Discussion
3
3
6H2O (19.6 mg. 0.06 mmol), H2hfipbb (20.8 mg, 0.05 mmol),
and 4,40-bipyridine (4,40-bipy, 7.8 mg, 0.050 mmol) were mixed
and stirred in DMF (9 mL) for several minutes, sealed in a 25 mL
reaction tube, and heated in an oil bath at 100 °C for 3 days. This
afforded colorless crystals of 4, which were collected by filtra-
tion, washed with DMF and ethanol, and dried in air. Yield 25.7
mg (65.5%). Elemental analysis for C46H44Cd2F12N4O12 (calcd/
found): C, 42.58/42.60; H, 3.42/3.46; N, 4.32/4.38; Selected
Synthesis. Hydro(solvo)thermal synthetic methods
were used for the synthesis and crystallization of metal
complexes. An interesting 2-fold parallel interpenetrated
layered network [Zn2(hfipbb)2(py)2] DMF (1) was ob-
tained by heating H2hfipbb, Zn(NO3)2 6H2O and pyr-
3
3
idine in DMF at 120 °C. The coordinated pyridine
molecules in 1 are nearly perpendicular to the layers
and protrude into the interlamellar regions. This suggests
that the coordinated pyridine molecules in 1 might be
replaced by neutral bridging N,N0-donors to form pillared
frameworks with potential stable porosity. The solvother-
IR(KBr, νmax): 2986(s), 1659(s), 1602(s) cm-1
.
Synthesis of [Co(hfipbb)(dpp)] MeOH (5). A mixture of Co-
(NO3) 6H2O (15.7 mg, 0.054 mmol), H2hfipbb (21.8 mg, 0.056
3
3
mmol), and 1,3-di(4-pyridyl)propane (dpp, 21 mg, 0.106 mmol)
was suspended in H2O (4 mL) and MeOH (4 mL), sealed in a 23
mL Parr bomb, and heated at 130 °C for 4 days. This afforded
purple crystals of 5, which were collected by filtration, washed
with methanol and dried in air. Yield 14.8 mg (40.4%). Ele-
mental analysis for C31H26CoF6N2O5 (calcd/found): C, 54.80/
54.77; H, 3.86/3.84; N, 4.12/4.17. Selected IR (KBr, νmax):
mal reaction of H2hfipbb and Zn(NO3)2 6H2O in the
3
presence of 4,40-bipy, however, afforded an alternative
layered coordination framework, [Zn2(hfipbb)2(4,40-
bipy)(H2O)] (2). When the spacer of the auxiliary ligands
is elongated to 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
(bpdab), the corresponding solvothermal reactions of
3468(br), 3079(s), 1654(s), 1597(s), 1210(m), 746(m) cm-1
.
Zn(NO3)2 6H2O with H2hfipbb gave the targeted 3D
2-fold interpenetrated framework, [Zn2(hfipbb)2(bpdab)]
X-ray Crystallography. Suitable crystals were glued to glass
fibers and mounted on a Bruker SMART APEX CCD area
detector diffractometer equipped with an Oxford Cryosystems
open-flow cryostat and graphite monochromated Mo KR radia-
3
3
2DMF (3), the structure of which is composed of layers of
{Zn2} paddlewheels and bpdab pillars. The solvothermal
˚
reaction of H2hfipbb, Cd(NO3)2 6H2O and 4,40-bipy in
DMF afforded a unique layered framework composed of
tion (λ = 0.71073 A). Data were collected at 150 K, reduced and
3
corrected for Lorentz and polarization effects using the
SAINT26 program and for absorption using the SADABS27
program. The structures were solved by direct methods using
SHELXS28 and refined with the full-matrix least-squares tech-
nique using SHELXTL.28 The non-H atoms, with the exception
of those in disordered solvent molecules, were modeled with
anisotropic displacement parameters, and refined by the full-
matrix least-squares on F2. Carbon-bound H atoms were placed
in calculated positions and refined using the riding model. In the
structures of 2 and 5 the starting positions for water (2) and
methanol (5) H atoms were located from the difference Fourier
syntheses, and then refined either with restraints for O-H bonds
helical tubes, [Cd2(hfipbb)2(DMF)2] 2DMF (4), that
3
surprisingly does not include 4,40bipy in the product.
With 1,3-di(4-pyridyl)propane (dpp), the corresponding
solvothermal reaction of H2hfipbb with Co(NO3)2 6H2O
3
in H2O-MeOH mixture afforded [Co(hfipbb)-
(dpp)] MeOH (5), a layered compound, structurally dif-
3
ferent from 1-4.
Description of Crystal Structures
Structure of [Zn2(hfipbb)2(py)2] DMF (1). Single-crys-
3
(26) SAINT, version 6.36a; Bruker AXS: Madison, WI, 2000.
tal X-ray analysis revealed that 1 displays a 2-fold parallel
2Df2D interpenetrated polymeric structure with 1D
helical channels. The asymmetric unit in 1 contains one
€
€
(27) Sheldrick G. M. SADABS; University of Gottingen: Gottingen,
Germany, 1996-2006.
(28) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11071
Table 1. Crystallographic Data and Structure Refinement Summary for 1-5
1
2
3
4
5
chemical formula
fw
C
47H33F12N3O9Zn2
C44H26F12N2O9Zn2
1085.41
0.44 ꢀ 0.19 ꢀ 0.07
P21/c
26.240(3)
11.9953(13)
13.383(2)
90
93.196(2)
90
4205.8(9)
1.714
2.28, 27.54
1.255
2176
18958
9812
5487
628
0.049
0.0432, 0.0627
0.982
C55H47F12N7O11Zn2
1340.74
0.37 ꢀ 0.20 ꢀ 0.02
Pca21
22.670(4)
16.086(3)
15.833(3)
90
C46H44Cd2F12N4O12
1297.65
0.45 ꢀ 0.20 ꢀ 0.09
P21/c
11.983(2)
15.990(2)
28.353(3)
90
104.736(5)
90
5254(2)
1.641
1.96, 27.56
0.914
2592
47588
12474
10912
694
0.044
0.0570, 0.131
1.109
C31H26CoF6N2O5
679.47
1142.50
0.44 ꢀ 0.35 ꢀ 0.12
C2/c
29.671(6)
7.8424(16)
23.240(5)
90
121.070(3)
90
4632(3)
1.638
2.02, 27.53
1.144
2304
21317
5686
4723
351
0.032
0.0321, 0.0900
1.13
cryst size (mm3)
space group
0.57 ꢀ 0.19 ꢀ 0.05
P1
˚
a (A)
8.8564(12)
12.527(2)
14.169(2)
81.870(2)
73.992(2)
80.204(2)
1481.6(4)
1.523
2.12, 25.0
0.661
694
9268
5069
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
90
90
3
˚
V (A )
F
5773.8(18)
1.542
2.02, 27.62
0.935
785
32111
12923
11875
785
calcd (g/cm3)
θ
min, θmax (deg)
μ (mm-1
F(000)
no. of reflns
)
no. of unique reflns
no. of obsd reflns [I > 2σ(I)]
no. of parameters
Rint
3624
410
0.048
0.0396, 0.0790
0.993
0.050
0.0740/0.151
1.199
Ra, Rw
b
GOFc
residuals (e A
-3
˚
)
0.45, -0.31
0.70, -0.64
0.71, -0.97
1.64, -1.99
0.45, -0.41
P
P
P
P
P
a R = ||Fo| - |Fc||/ |Fo|. b Rw = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
c GOF = { [w(Fo2 - Fc2)2]/(n - p)}1/2
.
a binuclear Zn(II)-tetracarboxylate paddlewheel cluster.
In this binuclear unit (Figure 1b), each Zn(II) ion resides
in a square pyramidal coordination geometry with the
apical position occupied by a coordinated pyridine
˚
molecule [Zn-N(apical) = 2.0300(15) A], while the basal
plane consists of four carboxylate O atoms [Zn-O =
˚
2.0295(14)-2.0593(12) A]. The intradimer Zn Zn se-
3 3 3
˚
paration is 2.9758(7) A, within the normal range found in
other reported bimetallic paddlewheel units of type
[M2(O2CR)4].29 In 1, each paddlewheel unit connects to
four neighboring paddlewheel clusters via bridging
hfipbb2- dianions, which adopt a bridging tetradentate
mode (Scheme 2a) to form a 2D undulating (4,4) net with
large rhomb-like windows (Figure 2a). The dimensions of
2
˚
the rhombic window are 14.02 ꢀ 14.02 A as measured
between the centroids of the binuclear Zn(II) subunits at
the corners of the window. As depicted in Figure 2b, the
skeleton of this 2D sheet can also be described as a unique
helical layer in which the left and right helical chains
appear alternatively by sharing the paddlewheel clusters.
˚
The pitch of helix is double the b parameter (15.685 A).
Most strikingly, the two diagonals of the rhombic
window are equal to unit cell parameters b and c, respec-
tively. Two closest identical helical layers shift by
b so that the paddlewheel units from one layer fall in
the center of the window of the other layer, and inter-
penetrate each other in a parallel 2Df2D fashion, result-
ing in an interwoven bilayer with 1D double helical
channels (Figures 2b and 2c). The hfipbb2- ligand
(29) (a) Eddaoudi, M.; Li, H.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122,
1391. (b) Eddaoudi, M.; Kim, J.; Wachter, J. B.; Chae, H. K.; O'Keeffe, M.; Yaghi,
O. M. J. Am. Chem. Soc. 2001, 123, 4368. (c) Wang, R.; Han, L.; Jiang, F.; Zhou,
Y.; Yuan, D.; Hong, M. Cryst. Growth Des. 2005, 5, 129. (d) Guo, X.; Zhu, G.;
Fang, Q.; Xue, M.; Tian, G.; Sun, J.; Li, X.; Qiu, S. Inorg. Chem. 2005, 44, 3850.
(e) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D.
Science 1999, 283, 1148. (f) Vos, T. E.; Liao, Y.; Shum, W. W.; Her, J.-H.;
Stephen, P. W.; Reiff, W. M.; Miller, J. S. J. Am. Chem. Soc. 2004, 126, 11630.
(g) Han, L.; Zhou, Y.; Zhao, W.-N.; Li, X.; Liang, Y.-X. Cryst. Growth Des. 2009,
9, 660.
Figure 1. (a) Asymmetric unit of 1, symmetry codes shown in Support-
ing Information, Table S1; (b) {Zn2} paddlewheel building unit observed
for 1.
Zn(II) ion, an hfipbb2- ligand, and one coordinated
pyridine molecule and a guest DMF solvent molecule
(Figure 1a) with the metal-based building unit comprising

11072 Inorganic Chemistry, Vol. 48, No. 23, 2009
Yang et al.
Scheme 2. Bridging Coordination Modes of hfipbb2- Ligands Observeda
a (a) Bridging tetradentate mode observed in 1, 3, and 4; (b) bridging chelated tetradentate mode observed in 4; (c) bridging tridentate mode observed
in 2; (d) bridging chelated tridentate mode observed in 2 and 5.
presents a dihedral angle of 72.7° between the two ben-
zene rings, and this configuration contributes to the
parallel 2Df2D interpenetration. Stacking of the inter-
woven bilayers along the a-axis reveals the coordinated
pyridyl molecules lying almost perpendicular to the
layers, but they are not long enough to create solvent-
accessible interlamellar space. The effective void volume
3
estimated by PLATON/SOLV30 is only 575.9 A corre-
˚
3
sponding to 12.4% of the unit cell volume of 4632(3) A .
˚
Disordered DMF molecules reside within the helical
channels. There are also weak interlayer C-H F hy-
3 3 3
drogen bonds formed between fluorine atoms and pyr-
idine C-H groups at the edges of adjacent interwoven
˚
bilayers [H F = 2.36-2.38 A, C F = 2.997-
3 3 3
3 3 3
˚
(3)-3.005(2) A]. A similar cobalt-hfipbb coordination
polymer has been reported recently.29g
Structure of [Zn2(hfipbb)2(bpy)(H2O)] (2). To extend
the parallel interpenetrating helical bilayers into 3D
frameworks, we employed the rod-like exobisdentate
4,40-bipy as a co-ligand to replace the monodendate
pyridine molecule in 1. However, the reaction of Zn-
(NO3)2 6H2O, H2hfipbb, and 4,40-bipy did not afford
3
the targeted 3D coordination polymer, but a different
layered compound, [Zn2(hfipbb)2(bpy)(H2O)] (2). Single
crystal X-ray diffraction reveals that 2 contains two
distinct Zn(II) ions in the asymmetric unit (Figure 3a).
Zn1 has a distorted octahedral coordination geometry,
provided by four oxygens [Zn-O = 2.007(2)-2.317(2)
˚
A] from three carboxylate ligands, one pyridyl nitrogen
˚
[Zn-N = 2.094(2) A] and one water molecule [Zn-O =
2.125(2) A]. Zn2 is tetrahedrally coordinated by three
˚
carboxylate oxygen atoms [Zn-O = 1.934(2)-1.965(2)
˚
˚
A] and one pyridyl nitrogen [Zn-N = 2.038(2) A]. The
two Zn(II) centers are bridged by a pair of syn-syn
hfipbb2- carboxylate groups into a binuclear unit with
˚
intradimer Zn Zn separation of 3.891 A. Significantly,
3 3 3
these binuclear units are doubly interlinked by the bent
hfipbb2- moieties to form an infinite ribbon where two
different bridging modes of the hfipbb2- ligands can be
distinguished (Scheme 2c and d).
Figure 2. (a) Undulating layers in 1 with (4,4) topology. (b) and (c)
schematic representations of 2-fold parallel 2Df2D interpenetration in 1
with 1D double helical channels (viewed along the a axis and close to the
[010] direction, respectively).
(30) (a) Spek, A. L. J. Appl. Crystallogr. 2003, 7. (b) Sluis, P.v.d.; Spek, A. L.
Acta Crystallogr., Sect. A 1990, 46, 194.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11073
Figure 3. (a) Viewofthe metal coordinationenvironmentin 2. Guestsolventmoleculesand hydrogenatomshavebeenomittedfor clarity;symmetry codes
are giveninSupportingInformation, Table S1. (b) 2D infinitenetworkswith large quasi-cuboid cavities within the lamella(viewedalongthe [001] direction).
A schematic representation of the quasi-cuboid cavities is also highlighted (C, dark gray, O, red, N, blue, F, green, Zn, cyan). Hydrogen atoms have been
omitted for clarity. (c) and (d) Schematic representations of interdigitation between layers in 2 (viewed close to the [010] direction).
The infinite ribbons formed by Zn(II) ions and
hfipbb2- ligands in 2 are further connected through
4,40-bipy ligands within a 2D network. If the central
carbon atom linked to two CF3 groups and two ben-
zene spacers of each hfipbb2- ligand is considered as
one of four corners of a cuboid and the Zn(II) binuclear
units occupy the other four corner sites of the cuboid,
then large quasi-cuboid cavities can be envisaged
(Figure 3b). However, these layers are tightly stacked in
pillars (Figure 4a). As expected, the helical layer is
identical to that of 1, and two identical helical layers
are interpenetrated in a parallel 2Df2D fashion. Upon
2-fold parallel interpenetration, 1D double helical chan-
nels (A) in the skeleton are formed (Supporting Infor-
mation, Figure S3b). However in 3, adjacent layers
are pillared by 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
(bpdab) ligands to generate a 3D network (Figure 4b) of
R-Po topology with additional interlamellar 1D channels
(B) (Figure 4c). Thus the interpenetrating helical layers
have been successfully extended by pillars to form tar-
geted 3D frameworks with the effective void volume30
drastically increasing from 12.4% (for 1) to 30.8% (for 3)
of the crystal volume. Disordered DMF molecules in the
channels A and B (Figure 4c) are anchored to the host
an
ABAB
fashion along the crystallographic
3 3 3
3 3 3
c axis. Furthermore, CF3 groups from adjacent layers
are embedded into the quasi-cuboid cavities of a layer
at both sides of the layer, resulting in no overall porosity
for 2 (Figure 3c and 3d). The PLATON/SOLV30 calcula-
tion shows no effective void volume (only 0.7% of the unit
cell volume), and hydrogen bonds between layers further
framework via weak C-H ODMF hydrogen bonds
3 3 3
˚
(H O = 2.30-2.52 A, C O = 3.215(11)-3.370(10)
lead to a tightly packed structure Ow-H Ocarboxylate
˚
3 3 3
3 3 3
3 3 3
˚
A, and <CHO = 162-170°).
Structure of [Cd2(hfipbb)2(DMF)2] 2DMF (4). Com-
[H O = 1.828(11)-2.241(16) A, O O = 2.668(3)-
3 3 3
3 3
˚
˚
3.014(3) A] and weak C-H F [H F = 2.46 A,
3
3 3 3 3 3 3
˚
plex 4 crystallizes in monoclinic space group P21/c, and
the asymmetric unit consists of two Cd(II) ions, two
hfipbb2- dianions, two coordinated DMF molecules,
and two lattice DMF molecules (Figure 5a). The two
crystallographically independent Cd(II) centers have dif-
ferent coordination geometries: Cd1 is coordinated by
five carboxylate oxygens from four hfipbb2- ligands
forming a distorted square pyramid, while the Cd2 center
is octahedrally coordinated by four carboxylate
oxygens in the equatorial plane [Cd-O = 2.193(4)-
C
2.54 A, C O = 3.468(4) A].
F = 3.405(4) A] or C-H Ocarboxylate [H O =
3 3 3
3 3 3 3 3 3
˚
˚
3 3 3
Structure of [Zn2(hfipbb)2(bpdab)] 2DMF (3). Com-
3
plex 3 crystallizes in the orthorhombic space group
Pca21, and the Zn(II) local coordination geometry is
similar to that in 1 with pairs of Zn(II) centers forming
a binuclear Zn(II)-tetracarboxylate paddlewheel cluster
node (cf., Figure 1b). These are bridged by hfipbb2-
ligands into a (4,4) helical layer with large rhombic pores,
and in contrast to 1, the two axial sites of each paddle-
wheel unit in 3 are occupied by N-donors from bpdab
˚
2.533(3) A] and by the O-donors from two DMF

11074 Inorganic Chemistry, Vol. 48, No. 23, 2009
Yang et al.
Figure 5. (a) Molecular structure of 4 showing the local coordination
geometry at Cd(II) ions. Guest solvent molecules and hydrogen atoms
have been omitted for clarity. Symmetry codes are given in Support-
ing Information, Table S1. (b) A view of the carboxylate-bridged
[Cd2(CO2)4]n chain formed by binuclear units via μ2-mode carboxylate
groups. (c) View of the layered structure of 4 with 1D double helical
tubular channels.
tubular channels running along the b axis are formed
within the layer with CF3 groups on their periphery
(Figure 5c), with accessible windows that can accommo-
˚
date a sphere of diameter ∼5.2 A. The 1D intralamellar
channels of 4 are similar to those found in the In(III)
Figure 4. (a) View of the metal coordination environment in 3. Guest
solvent molecules and hydrogen atoms have been omitted for clarity.
(b) The 3D pillar-layered framework in 3 (viewed along the c axis).
(c) Schematic representation of the 2-fold interpenetrating network in 3
(the parallel interpenetrated helical layers are colored with red and blue,
respectively, and pillars are shown as green lines).
material [In(OH)(hfipbb)] 0.5py, which consists of
3
{In(OH)}n chains linked together by hfipbb2- ligands.23e
The double helical tubular channels inside each layer
can alternatively be described as follows. Two crystal-
lographically independent hfipbb2- ligands (defined as
L1 and L2, respectively, in Figure 6) are used to construct
right-handed helical chains via coordination bonds be-
tween five-coordinated Cd(II) centers (Cd1) and L1 or
L2. As shown in Figure 6a, two helical chains are inter-
twined with a period equal to the unit cell parameter
˚
molecules [Cd-Oapical = 2.252(4)-2.275(4) A] in the
axial positions. In 4, the hfipbb2- ligands present two
different coordination modes: bridging tetradentate and
bridging chelated tetradentate (Scheme 2a and 2b, re-
spectively). Consequently, the Cd1 Cd2 pair is con-
3 3 3
˚
b [15.990(2) A] to form a double-stranded helical tube
nected through two carboxylate groups to form corner-
sharing binuclear building blocks, which are interlinked
via μ2-mode carboxylate groups to form a [Cd2(CO2)4]n
chain (Figure 5b). Linkages between chains via
(hexafluoroisopropylidene)bis(benzene) spacers afford
layers parallel to the crystallographic ab plane. Because
of the geometry of hfipbb2- ligands, double helical
running along a 21 screw axis. The tubes are further
aligned by sharing five-coordinated Cd1 centers to form
a layered network with small pores. Each pore is defined
by four carboxylate oxygen atoms from four hfipbb2-
ligands (Figure 6, panels b and c), and binds Cd2, which
has an octahedral coordination geometry completed

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11075
Figure 6. Views of the structure of 4. (a) A side view of the double helical tube. Two intertwined right-handed helical chains are shown in different colors:
black for the chain formed by Cd1 and L1, and blue for the chain formed by Cd1 and L2. (b) A space-filling view of the assembly of helical tubes into a 2D
layered network with small pores each capturing octahedral Cd(II) ions (color modes: five-coordinated Cd1 atoms, purple; octahedral Cd2 centers, cyan;
L1 ligands, black; L2ligands, blue; and one pore highlighted in red). (c) View of 1D tubular channels within each layer. All hydrogen atoms and coordinated
DMF molecules have been omitted for clarity.
by two axial oxygen atoms of two DMF molecules.
The coordinated DMF molecules are nearly perpendicu-
lar to the layers, pointing into the interlamellar regions,
revealing that the orientation of DMF is similar to that of
the coordinated pyridine molecules in 1. However, DMF
molecules are smaller and more flexible than pyridine
molecules and thus additional voids are created between
the undulating layers. A PLATON/SOLV30 calculation
gives a value of 26.8% for the void space in the unit
cell, and slightly more than half of this space is attributed
to the 1D intralamellar tubular channels. Disordered
DMF molecules situated in the intralamellar and inter-
lamellar voids are bound to the host framework via
in 5 are in syn positions [<N1-Co1-N2#3 = 92.74(8)o,
#3 = (x, 1 þ y, z)], and consequently, the dpp ligands in 5
only adopt the trans-trans (TT) conformation, and serve
as double-bridges connecting [Zn(hfipbb)]n ribbons into a
2D (4,4) non-interpenetrating layer with two kinds of
windows when viewed along the crystallographic a axis
(Figure 7c). Window A is made up of two binuclear
subunits and two hfipbb2- ligands along the ribbon
˚
(shortest diagonal Zn Zn distance = 12.49 A), while
3 3 3
window B consists of two binuclear subunits and two dpp
ligands perpendicular to ribbon chains (shortest diagonal
˚
Zn Zn distance=10.26 A). These layers stack in such a
3 3 3
way that lattice methanol molecules are anchored on
either side of window B via strong Omethanol-H
weak Caromatic-H ODMF or CDMF-H Ocarboxylate
O
3 3 3
3 3 3
3 3 3
˚
˚
˚
hydrogen bonds (C O = 3.12-3.41 A; <CHO =
hydrogen bonds [H O=1.89 A, O O = 2.767(3) A
3 3 3 3 3 3
3 3 3
137-157°).
Structure of [Co(hfipbb)(dpp)] MeOH (5). Single-crys-
tal X-ray analysis reveals that the Co(II) ion in 5 has a
slightly distorted octahedral coordination geometry, pro-
vided by three hfipbb2- and two dpp ligands [Co-O =
2.0325(18)-2.1828(19) A; Co-N=2.122(2)-2.165(2) A].
As shown in Figure 7b, each pair of Co(II) ions is bridged
by a pair of syn-syn hfipbb2- carboxylate groups to form
a binuclear subunit, and these are doubly interlinked by
the hfipbb2- ligands into an infinite ribbon, which is
similar to that observed for 2. However, only one co-
ordination mode (Scheme 2d) is observed for the hfipbb2-
ligands in 5. In principle, dpp ligands can exhibit two con-
formations, trans-trans (TT) and trans-gauche (TG),
with respect to the relative orientations of CH2 groups.29g
The two pyridyl nitrogens coordinated to each Co(II) ion
and <OHO =172°].
Structural Discussion. We have synthesized five co-
ordination polymers 1-5, as illustrated in Scheme 1,
that form 2D or 3D frameworks with diverse architec-
tures. In complexes 1-5, pairs of metal ions are bridged
by carboxylate groups to form bimetallic clusters
nodes which play an important role in determining the
resulting network structure. A total of four different
coordination modes of carboxylate groups are observed
3
˚
˚
b
in 1-5 (Scheme 3). In the μ2 -OCO coordination mode,
both carboxylate oxygen atoms are involved in coordina-
tion to two M(II) atoms in a monodentate fashion; in the
c
second coordination mode (μ1 -OCO), both oxygen
atoms of the carboxylate group are involved, but in
this case only coordinated to one metal atom in an
asymmetric chelating bidentate manner; in the third

11076 Inorganic Chemistry, Vol. 48, No. 23, 2009
Yang et al.
Scheme 3. Summary of Coordination Modes of Carboxylate Groups
Observed in 1-5
coordination mode (μ2-η2:η1-OCO) the carboxylate
group is coordinated to two metal sites in either mono-
dentate to one metal or asymmetric chelating bidentate
fashion to the other metal; the final coordination mode
t
that is observed is the μ1 -O(CdO), which has only one
carboxylate oxygen coordinated to a metal center. The
combination of the hfipbb2- spacer and these carboxylate
coordination modes can produce 10 possible bridging
modes of hfipbb2- ligands (Supporting Information,
Scheme S1),6f,23,29g four of which are observed in com-
pounds 1-5. The bridging tetradentate mode of hfipbb2-
(Scheme 2a) results in the formation of bimetallic tetra-
carboxylate paddlewheel clusters, as shown in 1 and 3 and
other previously reported compounds.,6f,23g,23f,29g where
the apical positions of the paddlewheel units can be either
occupied by terminal ligands (e.g., py in 1 and in [Co-
(hfipbb)(py)]29g) or neutral bridging pillars (e.g., bpdab
in 3 and H2hfipbb in ([Cu(hfipbb)(H2hfipbb)0.5]6f). The
hfipbb2- ligands in 2 exhibit two different bridging modes
(Scheme 2c and 2d) and link binuclear Zn(II) units to
form [Zn(hfipbb)]n ribbon-like chains, while the py moi-
eties of 4,40-bipy are coordinated to the Zn-carboxylate
units in a trans fashion, resulting in a (4,4) network based
on a binuclear cluster node, double-stranded hfipbb2-
linkers and single-stranded 4,40-bipy linkers. Although
5 contains a similar ribbon-like chain to that of 2, the
hfipbb2- ligands in 5 only take the bridging chelated
tridentate mode (Scheme 2d), and, thus, the py moieties
of dpp occupy two cis positions to fulfill the pseudo-
octahedral coordination at each Co(II) center. The re-
sultant structure is a non-interpenetrated (4,4) network
based on binuclear cluster nodes and double-stranded
organic linkers (Figure 7c). The Cd(II) complex 4 is the
only compound described here where two distinct brid-
ging modes of hfipbb2- are exploited to generate the layer
structure, with no additional bipyridyl spacers used.
Thus, Cd(II) centers with different coordination geome-
tries are linked by both μ2-η2:η1-OCO and μ2 -OCO
b
groups into [Cd2(O2C)4]n chains that are further linked
via hfipbb2- spacers to form a 2D network with tubular
helical channels inside the layer.
Figure 7. (a) View of the metal coordination environment in 5. Sym-
metry codes are given in Supporting Information, Table S1. Guest solvent
molecules and hydrogen atoms have been omitted for clarity. (b) View of
the infinite ribbon-like chain made up of binuclear Co(II) nodes.
(c) Space-filling representation of the 2D layer in 5. (d) Schematic
representation of the (4,4) network of 5 based on double-stranded organic
linkers (bent hfipbb2- and dpp ligands presented as red and blue lines,
respectively; the cyano-colored balls representing the center of binuclear
cobalt subunits as 4-connected nodes).
Thermal Stability, XRD Analysis, and H2 Adsorption.
Because of the very low porosity of 1, 2, and 5, as
confirmed by single-crystal diffraction analysis, only 3
and 4 were studied for porosity and gas adsorption.
Thermo gravimetric analysis (TGA) of 3 and 4 were
studied under N2 flow (60 mL/min) at a heating rate of
1 °C min-1. As shown in Figure 8a (curve I), all the DMF
in 3 is removed (weight loss, 16.4%, calcd 16.3%) by
250 °C. The desolvated form 3a is stable up to ∼360 °C
and then decomposes to an unidentified product. The
thermal stability of 4 is similar, with the weight loss of
20.3% completed at around 250 °C (curve IV in Figure 8a),
corresponding to the loss of all the guest and coordi-
nated DMF (calcd 22.5%). The weight loss beginning
at ∼350 °C corresponds to the decomposition of the
framework.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11077
Figure 8. (a) TGAanalysis of3and 4. Curve Iisthat ofthe as-prepared sampleof3. CurvesIIandIII areforsamplesobtained byimmersingthe desolvated
host crystals of 3a in DMF for 6 h and 1 day, respectively. Curve IV represents the as-synthesized sample of 4. (b) Powder XRD patterns of 3. I simulated
from single-crystal diffraction data, II for as-synthesized crystals, III for the sample 3a after reintroducing DMF, IV for the sample (3a) prepared by heating
3 at 260 °C under N2 flowfor 6 h or byevacuating3 at150 °C under ultrahigh vacuum(10-8 mbar) for overnight. (c) Powder XRD patterns of4. (I simulated
from single-crystal diffraction data, II for as-synthesized crystals, III for the partly desolvated phase at 125 °C, and IV for the fully desolvated solid (4a)
generated by heating 4 at 250 °C under N2 flow for 3 h or by evacuating 4 at 200 °C under ultrahigh vacuum (10-8 mbar) for 20 h).
The purity and thermal stability of 3 and desolvated
3a were confirmed by powder X-ray diffraction (PXRD)
analysis(Figure8b). ThePXRD patterns for the evacuated
sample obtained by heating 3 at 260 °C under N2 flow for
6 h to give 3a is similar to that of the as-synthesized
crystalline solid, suggesting that the guest-free phase of 3
remains intact on desolvation. After removal of DMF, the
host crystals were soaked in DMF and again subjected to
TGA. The results revealed the loss of DMF is completed at
the same temperature (250 °C). However, the quantity of
DMF in the channels is 6.9 or 5.2% less (see curves III and
II in Figure 8a) following 6 or 24 h immersion in DMF,
respectively, compared to the as-prepared crystals. This
indicates a reversible but gradual process of reintroducing
DMF into the framework pores.19a Although we could not
determine the single crystal structure after the reintroduc-
tion of DMF, its PXRD pattern (see III in Figure 8b) is
identical to that of the as-prepared crystals of 3 (pattern II
in Figure 8b).
A sample of 4 was heated to 125 °C under vacuum for
3 h to remove lattice DMF molecules. The partly desol-
vated phase [Cd2(hfipbb)2(DMF)2] shows a similar
PXRD pattern to the one simulated from single-crystal
diffraction of 4 (Figure 8c). However, after removal of
coordinated DMF at 250 °C, the PXRD pattern changes
slightly with a systematic shift to higher 2θ (lower
d-spacing) values indicating a compaction of the
stacked layers in the structure. The PXRD patterns,
however, retain sharp reflections and return to the
original on reintroduction of DMF, indicating that the
fully desolvated form remains crystalline and micropo-
rous (see below); such microporosity can be attributed to
the retention microtubes inside the layers after desolva-
tion.
To evaluate further the microporosity of these materials,
the adsorption of N2, CO2, and H2 were investigated for the
desolvated (guest-free) forms of 3 and 4 (i.e., 3a and 4a,
respectively). As shown in Figure9a, the CO2 adsorption of
3a and 4a at 195 K exhibits a reversible type-I isotherm
characteristic of microporous material. The uptake of CO2
for 3a and 4a at 1 bar corresponds to 2.67 mmol g-1 (or
60 cm3 g-1) and 1.83 mmol g-1 (or 41 cm3 g-1), respec-
tively, and the Brunauer-Emmett-Teller (BET; Lang-
muir) surface areas for 3a and 4a were calculated to be
201(305) and 125(224) m2 g-1, respectively. However, the
adsorption isothermfor 3a and 4arevealedvery low uptake
of N2 (77 K) (Figure 9a). The selectivity of CO2 over N2
˚
may be due to size exclusion (kinetic diameter: 3.3 A for
˚
CO2 vs3.64 A for N2)1p andthelow kinetic energy oftheN2
molecules at 77 K resulting in N2 molecules unable to
effectively enter small pores.
The isotherms for H2 adsorption/desorption were ob-
tained at 77 K using an IGA analyzer according to
previously described procedures.6g,h As shown in
Figure 9b, 3a reversibly adsorbs and desorbs H2 at 77
(31) Zhao, X.; Xiao, B.; Fletcher, A. J.; Thomas, K. M. J. Phys. Chem. B
2005, 109, 8880.

11078 Inorganic Chemistry, Vol. 48, No. 23, 2009
Yang et al.
both intralamellar tubular channels and interlamellar
voids (total free volume 26.3%), then the adsorbed
H2 density at 20 bar is half of the density of liquid H2
(0.0708 g cm-3).34
Conclusions
A new series of coordination polymers of different divalent
metal cations [Zn(II), Cd(II) and Co(II)] have been as-
sembled from the bent hfipbb2- ligand under solvothermal
conditions in the presence of auxiliary co-ligands. Complex 1
contains 2-fold 2Df2D parallel interpenetrated layers with
binuclear Zn(II)-tetracarboxylate paddlewheel motifs as pla-
nar nodes and 1D double helical channels. In 3, parallel
interpenetrating helical layers composed of Zn(II)-tetracar-
boxylate paddlewheels and hfipbb2- ligands are success-
fully extended by 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene
(bpdab) pillars into a 3D R-Po microporous framework.
However, the use of shorter 4,40-bipyridine (4,40-bipy) or
1,3-di(4-pyridyl)propane (dpp) as auxiliary ligands afforded
2D layered compounds 2 and 5, both consisting of metal-
hfipbb2- ribbon-like chains connected together through
bipyridyl ligands. Furthermore, the different bridging modes
of hfipbb2- ligands in 2 and 5 lead to different dispositions of
the coordinated co-ligands around metal ions, giving as final
products two significantly different (4,4) networks. The Cd-
(II) complex 4 represents a rare example in which metal
centers are linked by carboxylate groups into infinite chains
further joined together by double-stranded hfipbb2- linkers
to form a 2D network with tubular helical channels.
For the layered coordination polymers (1, 2, 4, and 5) low
accessible volumes (less than 27% of unit cell volume) were
observed, because of close-packing of 2D layers or because of
interpenetration of layers. Generally, these layered compounds
show low thermal stability even under ambient conditions
(e.g., 5), but in the case of 4 which features tubular channels
inside the layer, enhanced thermal stability of the guest-
free framework is observed and its guest-free phase reversibly
adsorbs and releases hydrogen at 77 K. The uptake of
hydrogen at 20 bar is 0.57 wt %, among the highest values
reported for layered frameworks. According to TGA, PXRD,
and hydrogen adsorption studies, both 3 and 4 retain
structural integrity and permanent microporosity upon evacua-
tion of guest molecules although their overall porosity is
relatively low.
Figure 9. Absorption isotherms (a) N2 (circles, 77 K) and CO2
(triangles, 195 K) for 3a (black) and 4a (red); (b) H2 absorption isotherm
(77 K) for 3a (black) and 4a (red) (solid symbols for adsorption, open
symbols for desorption).
K, a type-I adsorption behavior being observed. The H2
uptake at 20 bar is 0.78 wt %, which is comparable to the
values found for some carbon materials.31 The Langmuir
linear fitting of the H2 adsorption data affords a max-
imum uptake of 0.87 wt %. The adsorbed H2 density at
20 bar is 0.0326 g/cm3, with respect to the solvent-
accessible volume calculated from single-crystal data,
notably lower than those observed for other 3D layer-
pillared framework materials with R-Po topology
(Supporting Information, Table S2).6c,f,16,19a,22a-d,23f,32
Sorption studies show the activated 4a can adsorb
0.23 wt % H2 at 77 K and 1 bar, increasing to 0.57
wt % H2 at 20 bar (Figure 9b). These uptakes are com-
parable or superior to those observed for layered co-
ordination frameworks under similar conditions.23f,33
The absence of an absorption plateau before 20 bar
shows further uptake can be expected at higher pressure.
If H2 molecules are considered to be stored in
Acknowledgment. This work was supported by Engi-
neering and Physical Science Research Council (EPSRC),
the European Research Council (ERC), the University of
Nottingham, and the CVCP (ORS award to W.Y.).
M.S. gratefully acknowledges receipt of a Royal Society
Wolfson Merit Award.
Supporting Information Available: Additional structural fig-
ures and tables, and X-ray crystallographic information files
(CIF) for compounds 1-5. This material is available free of
charge via the Internet at http://pubs.acs.org.
(32) (a) Chun, H.; Moon, J. Inorg. Chem. 2007, 46, 4371. (b) Chen, B.; Ma,
S.; Hurtado, E. J.; Lobkovsky, E. B.; Zhou, H.-Z. Inorg. Chem. 2007, 46, 8490.
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(33) (a) Kongshaug, K. O.; Fjellvag, H. Inorg. Chem. 2006, 45, 2424. (b)
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Cabeza, A.; Gomez-Alntara, M. D. M.; Olivera-Pastor, P.; Sobrados, I.; Sanz, J.;
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