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Published on 02 November 2012 on http://pubs.rsc.org | doi:10.1039/C2DT31842C
Cite this: Dalton Trans., 2013, 42, 54
Received 12th August 2012,
Accepted 12th October 2012
DOI: 10.1039/c2dt31842c
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A porous Sm(III) coordination nanotube with
hydrophobic and hydrophilic channels†
Nazario Lopez,a Hanhua Zhao,a Dan Zhao,b Hong-Cai Zhou,a Joseph P. Riebenspiesa
and Kim R. Dunbar*a
www.rsc.org/dalton
The π–π stacking interactions between tptz units from adjacent
Sm(tptz)(HCOO)3 coordination nanotubes leads to additional 1D
channels (tptz = 2,4,6-tris(2-pyridyl)-s-triazine). The present compound is a rare case of a tubular porous material with both hydrophobic and hydrophilic channels. Permanent porosity was
confirmed by N2 adsorption isotherms.
The discovery of carbon nanotubes was followed by studies of
other types of inorganic nanotubes,1 yet those based on
coordination compounds have just recently become a target of
interest.2 Coordination nanotubes offer certain advantages as
compared to other inorganic nanotubes including the fact that
(a) they are prepared at room temperature, (b) new interesting
physical phenomena can be observed which are difficult or
impossible to obtain with conventional nanotubes2o (c) they
are composed of building blocks that can be chemically
altered to tune the properties of the material and (d) pure crystalline single diameter coordination nanotubes are easily
obtained due to the self-assembly process. Herein we report a
single-walled coordination nanotube based on lanthanide
ions, tptz capping ligands and formate anions as bridging
ligands which is only the second report of a coordination
nanotube based on a lanthanide ion.3
Several years ago, our group reported a series of 3d–4f
cyano-bridged [Ln(tptz)(H2O)4Fe(CN)6] coordination polymers
that were obtained by layering a CH3OH–H2O solution of [Ln(tptz)(H2O)5–6]3+ over a CH3OH–H2O solution of K3Fe(CN)6.4
During the course of these studies, we unexpectedly isolated a
nanotube of formula {[Pr(tptz)(HCOO)3]·2.5H2O}∞ in low
yields from a DMF solution of [Pr(tptz)(H2O)6][Cl]3 that was
intended to be used for the formation of 3d–4f chains. In this
a
Department of Chemistry, Texas A&M University, College Station, TX 77842, USA.
E-mail: dunbar@chem.tamu.edu
b
Department of Chemical and Biomolecular Engineering, National University of
Singapore, Singapore, 117576
† Electronic supplementary information (ESI) available: Synthesis and characterization of {[Sm(tptz)(HCOO)3]·2.5H2O}∞, including thermogravimetric data and
Tables S1–2. CCDC 895842 and 895843. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2dt31842c
54 | Dalton Trans., 2013, 42, 54–57
case, the small amount of formate ions present in DMF, a fact
that has been documented in other studies,5 led to the formation of low yields of the nanotube. Subsequently, another
nanotube with Sm(III) was deliberately synthesized in high
yield by combining Sm(tptz)(H2O)3(triflate)3 with formate ions
in methanol. The crystal structure revealed that, in the solid
state, there are 1D internal channels for the tubes themselves
as well as additional 1D channels generated by π interactions
between tptz units from adjacent nanotubes. The packing of
the tubes produces these two channels of diameters 3.3 Å and
6.9 Å in a hexagonal honeycomb arrangement.
Single-crystal X-ray studies revealed that the isostructural
compounds {[Ln(tptz)(HCOO)3]·2.5H2O}∞ (Ln = Pr, Sm) crystallize in the hexagonal system in the space group P63/mcm,
hence the structural description will be provided only for the
Sm analogue for illustrative purposes (Table S1†). Compounds
{[Ln(tptz)(HCOO)3]·2.5H2O}∞ (Ln = Pr, Sm) consist of a nonacoordinate neutral building block [Ln(tptz)(HCOO)3] in which
the metal ion is in a distorted tricapped trigonal prismatic
coordination environment. The tptz ligand chelates through
three nitrogen atoms, with the Sm(III) coordination sphere
being completed by six formate ions (Fig. 1a). The Ln–O distances in the Sm analogue range from 2.35(1) Å to 2.448(9) Å
and the Sm–N distances are longer than the aforementioned
distances (Sm–N2 = 2.65(1) and Sm–N1 = 2.64(1) Å). The asymmetric unit consists of one-half of the [Ln(tptz)(HCOO)3] building block. The formate ions are bound to two different
lanthanide ions in the common anti–anti mode. The C–O
bond lengths (1.25(1) Å) and O–C–O angles (125(1)°) of the
formate ligand that is perpendicular to the plane of tptz
differs from the corresponding values at approximately 45°
with respect to the plane of tptz with C–O bond lengths of
1.23(1) Å and O–C–O angles of 117(2)°. The formate ion that is
perpendicular to the plane of tptz is bound to the Sm(tptz)
unit that resides directly on top of the former Sm(tptz) unit,
thus the interplanar separation of the tptz units (6.70 Å) is
dictated by the length of the formate ions and; such an interaction leads to the formation of a linear chain composed of
formate ions and Sm(tptz) units. The formate ligands that are
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Fig. 2 XRD powder patterns obtained from ground crystals after one week of
slow diffusion of Sm(tptz)(Otf )3 and [NH4][HCOO] in a 1 : 3 ratio in methanol.
Fresh sample (red), evacuated for 24 hours at 150 °C (blue); simulated XRD
powder pattern of [Sm(tptz)(HCOO)3]·2.5H2O}∞ (black).
Fig. 1 (a) A view of the [Sm(tptz)(HCOO)3] building block from the X-ray structure of {[Sm(tptz)(HCOO)3]·2.5H2O}∞, depicting the coordination environment
of the SmIII ion. (b) A top view of the nanotube depicting π–π interactions of
adjacent tptz ligands. (c) A side view of the nanotube depicting the available
space to intercalate with neighboring tubes engaging in π–π interactions of tptz
ligands. (d) Packing diagram along the c axis emphasizing the internal cavity of
the coordination tubes and the intercalation of nanotubes, which forms an
additional 1D channel. The hydrogen atoms are omitted for the sake of clarity.
Sm = pink, O = red, N = blue, C = gray.
at approximately 45° with respect to the plane of tptz are
bound to other Sm(tptz) units. Thus, the compound can be
described as linear Sm(tptz)(HCOO)2+ chains connected along
the sides to other chains by formate ligands resulting in a
tubular coordination polymer (Fig. 1b–c).
The pyridyl rings on the tptz ligand also contribute to the
stabilization of the nanotube; there are π–π intra-tube interactions between the pyridyl groups of adjacent tptz ligands
from the Sm(tptz) units that are connected on the sides by
formate ligands with an interplanar distance of 3.34 Å with a
slipped ring-over-ring conformation. The nanotubes have an
internal 1D channel with a diameter of 3.3 Å after subtraction
of van der Waals radii. The tptz ligands are located on the periphery of the nanotubes and the interplanar distance of the
tptz units (6.70 Å) within the linear Sm(tptz)(HCOO)2+ chain is
sufficient for π–π interactions to occur with aromatic molecules
(Fig. 1c). The nanotubes engage in intermolecular π–π interactions with neighboring nanotubes. The intercalation of
nanotubes leads to an additional 1D channel with a diameter
of 6.9 Å after subtraction of van der Waals radii (Fig. 1d).
The powder X-ray diffraction (XRD) patterns of products
from bulk and slow diffusion syntheses match the simulated
pattern of the {[Sm(tptz)(HCOO)3]·2.5H2O}∞ single crystal. The
{[Sm(tptz)(HCOO)3]·2.5H2O}∞ compound was subjected to
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thermal studies, and the stability of the framework was probed
by subsequent XRD studies. Powder XRD patterns of [Sm(tptz)(HCOO)3]·2.5H2O}∞ prepared by slow diffusion reactions
between Sm(tptz)(Otf )3 and [NH4][HCOO] in a 1 : 3 ratio in
methanol were collected first on a fresh sample. After taking
the first diffraction pattern, the sample was heated at 150 °C
under vacuum for 24 hours and the resulting diffraction
pattern matched the simulation (Fig. 2). Thus, the framework
is stable and retains its crystallinity, presumably due to the
extensive π–π interactions between the nanotubes. The stability
of the framework was tested under more rigorous conditions
by preparing [Sm(tptz)(HCOO)3]·2.5H2O}∞ in bulk by a reaction between Sm(tptz)(Otf )3 and [NH4][HCOO] in a 1 : 3 ratio in
methanol under reflux from the beginning of the reaction. The
course of the reaction was monitored periodically by collecting
data on the white powder that formed nearly instantaneously.
The diffraction pattern of a sample taken after refluxing for
15 minutes matched the simulated patterns of the [Sm(tptz)(HCOO)3]·2.5H2O}∞ crystal structure. The diffraction pattern of
a sample measured after refluxing for 90 minutes exhibited
very weak intensities that do not match the simulated patterns
for [Sm(tptz)(HCOO)3]·2.5H2O}∞ crystal and after refluxing for
2 days the powder diffraction data contains only two peaks,
one of which is very broad, a clear indication of degradation of
the original structural framework (Fig. 3). From the aforementioned data, it is concluded that the [Sm(tptz)(HCOO)3]·
2.5H2O}∞ material retains its crystallinity when heated under
vacuum in the solid state, but that the integrity is lost under
refluxing conditions. Thermogravimetric analysis of the freshly
prepared [Sm(tptz)(HCOO)3]·2.5H2O}∞ revealed the water
content to be ∼2.5 molecules per Sm ion which can be
removed with heating at 120 °C (Fig. S1†).
Permanent porosity of [Sm(tptz)(HCOO)3] was confirmed by
the N2 sorption isotherm at 77 K. Before the measurement, the
sample was degassed at 150 °C for 10 hours. As can be seen
from Fig. 4a, a hybrid isotherm between Type I and Type IV
was observed, indicating the microporosity and mesoporosity
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Fig. 3 Experimental XRD powder pattern obtained from the bulk reaction
between Sm(tptz)(Otf )3 and [NH4][HCOO] in methanol under refluxing conditions after 15 minutes (red), 90 minutes (blue) and two days ( pink); simulated
XRD powder pattern of [Sm(tptz)(HCOO)3]·2.5H2O}∞ (black).
Dalton Transactions
higher than typical MOFs (3.5–6.5 kJ mol−1),7 indicating a
stronger interaction between H2 and the tubular channel.
Unlike most MOFs, whose interaction with H2 decreases along
with the increased gas loading, the maximum heat of adsorption for H2 in our compound (9.29 kJ mol−1) appears at the H2
uptake of 1.6 mg g−1. This fact is most likely due to the confined environment within the tubular channel such that the
subsequent H2 loaded can interact not only with the channel
wall, but also with the adsorbed H2. The similar trend of heat
of adsorption was found in MOF-74, which also has a channellike structure.9 Normally, increased H2 interactions lead to
higher H2 uptake, but it is concluded that the limited porosity
and surface area leads to a moderate uptake of H2 in the
present compound.
In summary, coordination nanotubes of general formula
{[Ln(tptz)(HCOO)3]·2.5H2O}∞ (Ln = Pr, Sm) have been prepared. The present compounds are rare cases of porous
materials with mixed types of channels; one channel is formed
from the environment of the coordination bonds and the
other channel is based on supramolecular π–π interactions in
the solid state. These findings are interesting additions to the
relatively unexplored field of MOFs based on lanthanide ions
given that most of the porous framework studies to date have
focused on transition metals. In addition the results are interesting from the perspective of flexible porous tubular MOFs,
which are also scarce in the literature.
Acknowledgements
Fig. 4 (a) N2 sorption isotherm at 77 K (closed: adsorption; open: desorption).
(b) Pore size distribution from the DFT calculation. (c) H2 uptake capacities at
77 K (circle) and 87 K (triangle). (d) Isosteric heat of adsorption for H2.
of the compound,6 with a BET surface area of 220 m2 g−1.
Based on the crystal structure, the microporosity should stem
from the intra- and inner-void spaces of the nanotubes. Since
the particle size of the crystal varies from 0.2 μm to 15 μm
(Fig. S2†), the void spaces among crystal particles become prominent. Those inter-crystal spaces may contribute to the mesoporosity. Based on the pore size distribution data (Fig. 4b), the
micropores exhibit a very sharp distribution at 7.72 Å, which is
close to the channel size of 6.9 Å measured from the crystal
model.
Due to the presence of smaller pores capable of engaging in
stronger interactions with guest molecules, nanotubular compounds are potentially good candidates for hydrogen storage.7
The hydrogen uptake capacity of this compound was evaluated
at cryogenic conditions. At 77 K and 1 atm, 1 g of [Sm(tptz)(HCOO)3] reversibly adsorbs 4.2 mg of H2 (Fig. 4c). This value,
however, is substantially lower than MOFs with higher surface
areas.8 The isosteric heat of adsorption for H2 at low coverage,
however, can reach as high as 8.28 kJ mol−1 (Fig. 4d), which is
56 | Dalton Trans., 2013, 42, 54–57
K.R.D. gratefully acknowledges DOE (DE-FG02-02ER45999) for
support of this work and the Welch Foundation (A-1449) for
partial summer support of Nazario Lopez. The NSF generously
provided funds to purchase a CCD diffractometer
(NSF-9807975). We thank Prof. M. Dincă for helpful
discussions.
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