A porous metal-organic framework with helical chain building units exhibiting facile transition from micro- to meso-porosity

Article abstract of DOI:10.1039/c1cc16261f

A metal-organic framework (MOF) with helical channels has been constructed by bridging helical chain secondary building units with 2,6-di-p-carboxyphenyl- 4,4′-bipyridine ligands. The activated MOF shows permanent porosity and gas adsorption selectivity. Remarkably, the MOF exhibits a facile transition from micro- to meso-porosity.

Full text of DOI:10.1039/c1cc16261f

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COMMUNICATION
A porous metal–organic framework with helical chain building units
exhibiting facile transition from micro- to meso-porosityw
˜
Jinhee Park,a Jian-Rong Li,a E. Carolina Sanudo,b Daqiang Yuana and Hong-Cai Zhou*a
Received 8th October 2011, Accepted 10th November 2011
DOI: 10.1039/c1cc16261f
A metal–organic framework (MOF) with helical channels has
been constructed by bridging helical chain secondary building
units with 2,6-di-p-carboxyphenyl-4,40-bipyridine ligands. The
activated MOF shows permanent porosity and gas adsorption
selectivity. Remarkably, the MOF exhibits a facile transition
from micro- to meso-porosity.
Metal–organic frameworks (MOFs) have been attracting a
tremendous amount of interests due to their fascinating structural
topologies1 and potential applications2 such as gas storage,3 gas
separation,4 catalysis,5 and sensing.6 In particular, this new class
of porous materials have been considered as promising sorbents
for CO2 capture7,8 and H2 storage.9,10
MOFs prepared with ligands of high symmetry have been well-
studied because of synthetic and crystallographic considerations.
However, linkers with reduced symmetry containing mixed
carboxylate/pyridine coordinating groups are uncommon in MOF
studies.11 In this report, a ligand with one pyridyl and two
carboxylate groups has been used to react with a Co(II) salt under
solvothermal conditions to give a notable porous Co(II)-based
MOF with one-dimensional (1D) helical chains acting as secondary
building units.12,13 While an individual crystal is enantio-pure,
the bulk sample is racemic. The MOF also exhibits a facile
transition from micro- to meso-porosity, which may pave a
new way for the search of mesoporous MOFs.
Fig. 1 Views of (a) the asymmetric unit of PCN-121 (symmetry code:
(A) Y, ꢁX + Y, 0.83 + Y; (B) 1 ꢁ Y, X ꢁ Y, 1/3 + Z; (C) 1 ꢁ X + Y,
1 ꢁ X, Z ꢁ 1/3; (D) X ꢁ Y, X, 1/6 + Z; (E) Y, ꢁX + Y, Z ꢁ 1/6;
(F) X ꢁ Y, X, Z ꢁ 0.83); (b) the channels of PCN-121, viewed through
[0 0 1] direction; (c) the helical chains viewed through [1 1 0] direction.
four acetate O atoms, one carboxylate O atom of the dcbp2ꢁ
ligand, and one O atom from a m3-OH group (the identity of the
OH group was established through the pyramidal geometry of
the Co3O unit, the Co–OH distances and charge balance if the
Co atoms are divalent, which is further confirmed by magnetic
susceptibility measurements). Co3 is coordinated by one water
O atom, two carboxylate O atoms of the two different dcbp2ꢁ
molecules, one acetate O atom, one O atom from the m3-OH,
and one N atom of the dcbp2ꢁ ligand. Three Co(II) atoms are
thus connected to form a Co3(m3-OH) cluster, in which Co1 is
connected to Co2 through O3, C1 and O4 of one acetate ligand.
Co1 and Co2 share O8 of an acetate group. In addition, Co1 in
one cluster is connected to Co2 of another cluster through two
O atoms from two acetates. In this way, these Co3(m3-OH)
clusters link with each other to form a 1D chain along a 61 axis
(or 65 in its enantiomeric counterpart, Fig. 1c). Each organic
ligand coordinates three different m3-OH centered clusters to
form an overall three-dimensional (3D) structure with helical
channels (Fig. 1b). The helical structure of this MOF arises
from the non-planarity of the dcbp2ꢁ ligand.13
The MOF Co3(OH)(OH2)(OAc)3(dcbp) (PCN-121, PCN
represents porous coordination network) was synthesized by
a
reaction between 2,6-di-p-carboxyphenyl-4,40-bipyridine
(H2dcbp) and Co(NO3)2ꢀ6H2O in N,N0-dimethylacetamide
(DMA) at 85 1C. The structure of PCN-121 was determined
by single crystal X-ray diffraction. PCN-121 crystallizes in
space group P61 (or P65). As shown in Fig. 1a, there exist three
crystallographically independent Co(II) atoms, each of which
is octahedrally coordinated. Co1 and Co2 are coordinated by
a Department of Chemistry, Texas A&M University, College Station,
TX 77842-3012, USA. E-mail: zhou@chem.tamu.edu;
Fax: +1 979-845-1595
b
´
`
`
Departament de Quımica Inorganica i Institut de Nanociencia i
Nanotecnologia, Universitat de Barcelona, Diagonal 647,
08028 Barcelona, Spain
w Electronic supplementary information (ESI) available: Experimental
details, crystallographic data of PCN-121 and -122, TGA. IR and
additional gas adsorption isotherms. CCDC 813603 and 813719. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1cc16261f
Another interesting point is that using a different Co(II) salt
to react with this ligand resulted in a different MOF reported
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 883–885 883

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recently by Chen and co-workers.14 Octacobalt cluster based
(3,12)-connected frameworks were obtained by the reaction of
CoSO4 and H2dcbp under solvothermal conditions. Sulfate
participated in the formation of a Co8 cluster in this structure.
Furthermore, a porous Cd(II) coordination polymer with the
same ligand has also been reported recently.15 It is clear that
the formation of MOFs, even with the same ligand and metal
ions, is highly susceptible to reaction conditions such as pH,
temperature, solvents, and counterions.16,17 For instance, in this
work, although the PCN-121 structure contains an acetate as a
secondary ligand generated in situ, when acetic acid was added
intentionally to the reaction mixture in the synthesis, a new MOF
was obtained (PCN-122) and structurally characterized with
single crystal X-ray diffraction.z Interestingly, this new structure
does not contain an acetate group (Fig. S3, ESIw). The addition of
acetate to the reaction system, however, gave an uncharacterized
powder. These demonstrate the complexity of the dynamic
equilibriums among different components in a solvothermal
reaction mixture. Evidently, the acetate must be provided at a
suitable rate to ensure effective crystallization of PCN-121.
Thermal gravimetric analysis (TGA) revealed a loss of
uncoordinated solvates in as-synthesized PCN-121 around 100 1C
and the decomposition of the sample when the temperature is
higher than 300 1C (Fig. S3, ESIw). The relatively high thermal
stability of the sample and the 3D porous structure revealed by
X-ray diffraction studies prompted us to test the gas adsorption
performance of PCN-121.
This implies a transition of PCN-121 from microporosity to
mesoporosity and from crystalline to amorphous. When an
as-synthesized sample was pumped under reduced pressure for
10 days at 60 1C, a type IV isotherm was observed (Fig. 2),
albeit the total N2 uptake of the sample is similar to that of the
one with the type I isotherm. The PXRD pattern of the slowly
activated sample exhibiting a type IV isotherm is different
from those of the simulated based on single crystal data, the
as-synthesized sample, and the quickly activated sample showing
a type I isotherm (Fig. S1, ESIw). This implies a phase transition
of PCN-121 from a microporous form to a mesoporous one in
the solid state that maintains crystallinity. This type of transition
normally takes days to complete. The removal of the aqua
ligands may have promoted this transition. When activated
quickly, the crystal structure of the as-synthesized sample was
largely preserved due to the difficulty in aqua ligand removal
and the slow pace of structural rearrangements in a solid phase.
These rearrangements can be accelerated by solvent exchange.
After the solvent exchange with methanol, the water ligands
were most likely replaced by methanol ligands, which are easier
to eliminate than aqua ligands. The faster phase transition destroys
crystallinity and leads to an amorphous PCN-121. Experiments
using time-resolved PXRD techniques with synchrotron X-ray
sources are in progress collaborating with Argonne National Lab
and the results will be published elsewhere.
Mesoporous MOFs are a sort-after target due to their vast
potential in catalysis, sensing, and host–guest chemistry.19 The
discovery of the transition from micro- to meso-porosity by
constructing MOFs containing labile terminal ligands, followed
by slow removal of them to trigger the structural rearrangement,
may pave a new way for the preparation of mesoporous
MOFs. For gas storage and selective gas adsorption, however,
microporosity is more desirable,2 therefore the following
discussion will be focused on the microporous PCN-121.
CO2 and CH4 adsorption isotherms were measured at
different temperatures as shown in Fig. 3. The CO2 isotherm
at 195 K exhibited a rapid rise at low pressures to reach a
maximum uptake of 420 cm3 gꢁ1 at 1 bar. CH4 uptake at the
same temperature was 120 cm3 gꢁ1. Both gas uptakes were not
saturated even at 1 bar, and this unsaturation of CO2 uptake is
more pronounced at 273 and 295 K than that of CH4. Similar to
a lot of other MOFs, the selective adsorption of CO2 over CH4
of PCN-121 was observed at all three temperatures tested.
The most salient feature of the adsorption properties of
PCN-121 lies in its facile transition from microporosity to
mesoporosity depending on the methods of activation.
After evacuation of solvates from a fresh sample at 60 1C for
10 h under reduced pressure, the N2 adsorption isotherm at
77 K exhibits type I behavior revealing microporous nature of
the MOF (Fig. 2), consistent with the observed microporosity
from the crystal structure and the powder X-ray diffraction
(PXRD) pattern. Langmuir and Brunauer–Emmett–Teller
(BET) surface areas are 2618 m2 gꢁ1 and 1839 m2 gꢁ1
respectively, and the total pore volume is 0.93 cm3 gꢁ1
,
.
However, when activated at 60 1C under vacuum after soaking
in methanol and dichloromethane for three days, the activated
PCN-121 exhibits a type IV isotherm for N2 sorption at 77 K
and no diffraction peaks in its PXRD pattern were found.
Fig. 2 The N2 adsorption isotherms (77 K) of PCN-121 samples quickly
activated (green, 10 h), slowly activated (blue, 10 days), and activated
through solvent-exchange (red) (K: adsorption; J: desorption).
Fig. 3 The CO2 (blue) and CH4 (red) adsorption of PCN-121
(K: 195 K; ’: 273 K; m: 295 K. Filled markers for adsorption and
open ones for desorption.).
c
884 Chem. Commun., 2012, 48, 883–885
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Notes and references
z The space group of PCN-122 is I41cd (a = 23.394 A, c = 49.524 A,
V = 27102.9 A3). Co1 has an octahedral coordination geometry with
mixed functional groups, one pyridyl N atom, four carboxylate oxygen
atoms and one aqua ligand (Fig. S4, ESIw). Co1 is connected with Co1
in the other asymmetric unit through one aqua ligand and two
carboxylates from two different ligands.
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Fig. 4 The low pressure H2 uptake in PCN-121 at 77 K and 87 K.
This relatively high CO2 adsorption is a result of strong
interaction between CO2 (having a high quadruple moment)
and pore surface of the MOF. Presumably, the non-coordinated
N atoms (Lewis base site) of dcbp2ꢁ ligands in the wall may have
played a role in these interactions.18 The heats of adsorption of
CO2 and CH4 were 23.0 and 15.1 kJ molꢁ1, respectively.
H2 adsorption isotherms at 77 and 87 K were also measured
(Fig. 4). The uptakes of 1.4 wt% at 77 K and 1 wt% at 87 K
led to a heat of hydrogen adsorption of 6.63 kJ molꢁ1
consistent with literature values.9
,
To confirm the oxidation state of the Co atoms, magnetic
susceptibility data were collected for crystalline sample of
PCN-121. The data were not field dependent (Fig. S8, ESIw).
The wT product at 300 K has a value of 6.4 cm3 K molꢁ1, in
agreement with the expected for three Co(II) ions in a distorted
octahedral coordination environment with strong spin–orbit
coupling. In a magnetization vs. field plot at 2 K, each Co(II)
ion behaves as an effective S = 1/2. The magnetization does
not saturate, and it reaches a value of 2.5 at 5 T, indicating a
non-zero spin ground state for the tricobalt repeating unit of
PCN-121 with anisotropy and low lying excited states. This is
in agreement with the structural parameters of PCN-121.
PCN-121 consists of infinite chains of tricobalt units, where
the three Co(II) ions are bridged by a m3-OH and various
O atoms from the carboxylate groups.
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In summary, a helical-chain Co(II)-based MOF has been
synthesized. The structure of this MOF contains unusual
M–O–M clusters in its helical chain building units, which are
responsible for the observed magnetic properties. Selective
adsorption of CO2 over CH4 was demonstrated using a
quickly activated sample. Remarkably, PCN-121 exhibits a
facile transition from micro- to meso-porosity depending on the
methods of activation. Mechanistic studies using time-resolved
PXRD techniques are currently under way.
We acknowledge the U.S. Department of Energy (DOE DE-
SC0001015, DE-FC36-07GO17033, and DE-AR0000073), the
National Science Foundation (NSF CBET-0930079), and the
Welch Foundation (A-1725) for financial support to HCZ, as
well as Spanish Government (Grant CTQ2006/03949BQU and
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´
Ramon y Cajal contract) for financial support to ECS.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 883–885 885