Phosphonate monoesters as carboxylate-like linkers for metal organic frameworks

Article abstract of DOI:10.1021/ja207606u

Bidentate phosphonate monoesters are analogues of popular dicarboxylate linkers in MOFs, but with an alkoxy tether close to the coordinating site. Herein, we report 3-D MOF materials based upon phosphonate monoester linkers. Cu(1,4-benzenediphosphonate bis(monoalkyl ester), CuBDPR, with an ethyl tether is nonporous; however, the methyl tether generates an isomorphous framework that is porous and captures CO₂ with a high isosteric heat of adsorption of 45 kJ mol^-1. Computational modeling reveals that the CO ₂ uptake is extremely sensitive both to the flexing of the structure and to the orientation of the alkyl tether.

Full text of DOI:10.1021/ja207606u

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Phosphonate Monoesters as Carboxylate-like Linkers for Metal
Organic Frameworks
Simon S. Iremonger,† Junmei Liang,† Ramanathan Vaidhyanathan,† Isaac Martens,† George K. H. Shimizu,*,†
Thomas, D. Daff,‡ Mohammad Zein Aghaji,‡ Saeid Yeganegi,‡,§ and Tom K. Woo*,‡
†Department of Chemistry, University of Calgary, Calgary T2N 1N4, Canada
‡Centre for Catalysis Research and Innovation, Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
§Department of Physical Chemistry, Faculty of Chemistry, University of Mazandaran, Babolsar, Iran
S Supporting Information
b
Through a joint experimental and computational study, we
ABSTRACT: Bidentate phosphonate monoesters are ana-
logues of popular dicarboxylate linkers in MOFs, but with an
alkoxy tether close to the coordinating site. Herein, we
report 3-D MOF materials based upon phosphonate mono-
ester linkers. Cu(1,4-benzenediphosphonate bis(monoalkyl
ester), CuBDPR, with an ethyl tether is nonporous; however,
the methyl tether generates an isomorphous framework
that is porous and captures CO2 with a high isosteric heat
of adsorption of 45 kJ molÀ1. Computational modeling
reveals that the CO2 uptake is extremely sensitive both to
the flexing of the structure and to the orientation of the alkyl
tether.
demonstrate that the alkoxy tether can play a crucial role in
mediating gas sorption. More generally, we show that phospho-
nate monoesters, like carboxylates, can strike a balance between
robustness and crystallinity to form porous 3-D architectures.
Compound 2 shows preferential adsorption of CO2 over other
gases and an exceptionally high ΔHads (45 kJ/mol) for CO2.
The ligands, 1,4-benzenediphosphonate bis(mono ethyl and
methyl esters), BDPEt and BDPMe, respectively, were synthe-
sized from 1,4-dibromobenzene and the appropriate trialkylpho-
sphite via a MichaelisÀArbuzov reaction followed by con-
trolled hydrolysis to the bis(monoalkyl ester) as described
previously for the ethyl analogue.9 Compounds 1 and 2 were
synthesized (single crystal and bulk) by ethanol diffusion into
an aqueous solution of CuCl2 2H2O and the disodium salt of
3
the ligand.
The crystal structure of 1, depicted in Figure 1c, showed the
formation of a 3-D network with channels, in contrast to the Zn
structure. In1, one-dimensionalCu(RÀPO3Et) chainsare formed
down the c-axis (Figure 1b shows Me analogue), with the chains
being linked by fully deprotonated BDPEt dianions forming a
rhombohedral grid of 14.565(4) Â 7.790(2) Å (PÀP distance
and metalÀmetal distances as depicted by the arrows in Figure 1c).
The ethyl groups of the phosphonate esters protrude into the
pores restricting the aperture to ∼4.5 Å. The channel walls are
decorated with the ethyl groups of the ligand and the CuO4 metal
cores. The coordination geometry of Cu is 4-coordinate and best
described as heavily distorted tetrahedral tending to square
planar. Ligation is exclusively by O atoms excluding the alkylated
oxygen of the phosphorus. From the crystal structure, porosity
was not evident as the channels appeared largely filled by the
pendant ethyl groups. No solvent accessible pore space was able
to be calculated. The lack of porosity was corroborated by thermo-
gravimetry on solvated samples, which showed no mass loss from
RT to decomposition, and the fact that 1 showed no significant
adsorption of any gas, including CO2 at 298 K. The methyl ester
analogue, 2, was prepared with the intent that the BDPMe ligand
would form a framework of comparable dimensions but with
accessible pores owing to the smaller alkyl group. On the other
hand, the framework could also flex10 in response to the smaller R
group, thereby closing the pores.
etal Organic Frameworks (MOFs)/Porous Coordination
M
Polymers (PCPs) offer systematic and tunable routes to
the generation of new porous solids.1 This fact has led to these
materials being extensively pursued as new sorbents for gas sep-
aration and storage.2 The role of the organic linkers is foremost to
delineate the primary pore structure but these groups also define
the chemical potential of the pore surfaces3 and can possibly
kinetically gate sorption phenomena.4 The fact that these solids
can also demonstrate flexible architectures further extends their
scientific interest and offers potential for new applications tied to
their dynamic behavior.5
Many MOF/PCP materials reported incorporate carboxylate
anions as, in appropriate rigid and polyvalent forms,6 these
linkers provide sufficient thermodynamic stability to enable
permanent porosity but also sufficient kinetic lability that crystal-
line products are typically obtained. Phosphonate monoesters
(RP(dO)(ÀOH)(ÀOR) are a largely unexplored ligating func-
tionality in coordination polymer science.7,8 We recently re-
ported Zn 1,4-benzenediphosphonate bis(monoethyl ester),
Zn(BDPEt),9 a van der Waals compound with permanent poro-
sity despite having only two-dimensional connectivity. In this
compound, the monoanionic and bidentate coordination mode
of the phosphonate monoesters was reminiscent of the ligation
displayed by dicarboxylate linkers in MOFs. A fundamental
difference between a phosphonate monoester and a carboxylate
is obviously the presence of the alkoxy tether on the P atom.
Herein, we report two new 3-D MOFs, Cu(1,4-benzenediphos-
phonate bis(monoalkyl ester), CuBDPR, R = ethyl, 1; R = methyl, 2.
Received: August 12, 2011
Published: November 17, 2011
r
2011 American Chemical Society
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Figure 2. Comparison of experimental and simulated CO2 adsorption
isotherms at 273 K, where the geometry of the methoxy groups have
been altered from the X-ray structure in silico.
center. The enthalpy of adsorption (ΔHads) for CO2 was deter-
mined as a function of loading (Figures S13ÀS16). At zero
loading, indicative of the highest energy sites, the ΔHads was
calculated as 45 kJ/mol, a very high value for a physisorptive
process.11
The experimental determination of this ΔHads value merits
comment as the total CO2 uptake by 2 is modest and so relative
errors in extracting the ΔHads from experimental isotherms are
magnified. Using isotherms at 263 and 273 K, a zero-loading
ΔHads for CO2 of 52 kJ/mol was initially calculated for 2.
Recognizing the uncertainty in this value, new samples tubes
were prepared that decreased the dead volume of the analysis
by ∼70% (see Supporting Information). Isotherms were also
measured at four temperatures (263, 268, 273, 278 K) and the
linearity of all the van’t Hoff fits gave a high degree of confidence
in the reassessed value of 45 kJ/mol. This value was sustained
over the entire loading. Recent reports have shown a strong
correlation between high ΔHads and high selectivities for low
pressure CO2 capture.12 Often, high ΔHads can be connected to
the presence of coordinatively unsaturated metal centers (e.g.,
HKUST-1, 35 kJ/mol;13 MIL-101, 44 kJ/mol;14 Mg-MOF-74/
CPO-27-Mg, 47 kJ/mol;15 MIL-100, 63 kJ/mol.14 Considering
three pieces of data (high value for ΔHads; stoichiometry of
Figure 1. (a) The dianionic ligand, BDPR, where R = Et or Me.
(b) Single crystal structure of one column of CuBDPMe, 2 (1 is similar),
showing the bidentate coordination of the phosphonate monomethy-
lester and the 4-coordinate geometry of Cu. (c and d) Views down the
c-axis, of the frameworks showing (c) solvent inaccessible 1-D channels
in 1, and (d) 1-D pores in 2 with solvent accessible voids (1.2 Å probe in
gold). In c and d, one channel is shown in a space filling representation.
The single crystal X-ray analysis of 2 (Figure 1b,d) showed a
framework that could almost be superimposed (Figure S1) on
that of 1, excepting the position of the ester alkyl groups. One-
dimensional Cu(RÀPO3Me) chains and BDPMe linkers framed
one-dimensional channels (14.933(2) Â 7.024(2) Å, PÀP
distance and metalÀmetal) comprising ∼17% of the structure.
The methyl groups of the phosphonate esters still protrude into
the pores but leave an aperture of ∼6 Å. There is disordered water
in the pores located above the “axial” site of the 4-coordinate Cu
ion at 3.65(6) Å, a weak interaction at best (∑ vdW 2.92 Å).
This solvent inclusion was demonstrated in bulk samples as
TGA/DSC analysis (Figure S6) indicated a mass loss of ca.
5.0 wt% to 100 °C (calculated 5.2 wt% for 1 H2O). Between 100
and 350 °C, no significant mass loss was observed and the sample
decomposed at 350 °C. This stability was confirmed by PXRD
after heating the sample in air to 200 °C for 24 h (Figure S2).
During the desolvation process, the PXRD indicated a small
structural contraction. To elucidate the nature of this contrac-
tion, an in situ PXRD measurement was conducted on a desol-
vated sample under a nonadsorbing gas (helium). Refinement of
the resulting pattern (Figure S4) indicated a 1.8% reduction in
the cell volume in comparison to the as made crystal structure
(Table S1). This contraction did not collapse the pores and the
permanent porosity of 2 was confirmed by gas sorption analysis.
As with many small pore MOFs, the material showed no signi-
ficant adsorption of N2, H2, or CH4 at any temperature (Figure S8).
A modest uptake of CO2 at ambient temperatures was observed
(Figure 2), 30.9 cm3(STP)/g, 1.38 mmol/g, 6.06 wt% at 273 K
and 1200 mbar. This equates to a loading of 0.45 CO2 per Cu
0.45 CO2/Cu; transannular Cu Cu distance of 7.02 Å be-
3 3 3
tween pseudosquare planar Cu centers), it is tempting to hypo-
thesize that the CO2 molecules in this system are in a dual end-on
bridging mode between Cu centers. ΔHads values can also be
augmented by the presence of polar functional groups such as
amines lining the pores,16 although higher amination is not
always necessarily beneficial for CO2 uptake.17
Given the relevance to capture methodologies, molecular
insights to the nature of CO2 interactions with a porous sorbent
are valuable,18 especially one with a ΔHads of 45 kJ/mol as with 2.
To investigate the nature of the CO2 binding and sorption pro-
perties of 2, dispersion corrected periodic DFT and classical
grand canonical Monte Carlo (GCMC) simulations were em-
ployed.19 The dual end-on bridging interaction of CO2 with two
Cu centers was first examined with DFT as such a binding mode
might involve weak orbital interactions. All attempts to locate a
minimum energy structure corresponding to this mode of CO2
failed even when the framework and cell vectors were allowed to
fully relax. Instead, all structures optimized to geometries in
which the CO2 molecules were oriented roughly perpendicular
to the hypothesized end-on bridging mode in the direction of the
channels.
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2 suggests the potential of the alkoxy tethers in the phosphonate
monoester based MOFs to modulate the adsorption behavior.
The dynamic nature of the alkyl tether may also permit optimal
arrangement between CO2 molecules to maximize cooperative
interactions.23
A fundamental difference between a carboxylate and a phos-
phonate monoester is the presence of the alkoxy tether. Clearly,
between 1 and 2, the alteration from ethyl to methyl ester groups
greatly affects CO2 uptake but the orientation of the alkyl group
is also critical. Phosphonate monoesters as linkers in coordina-
tion polymer/MOF materials have not been nearly as studied as
carboxylate or phosphonate linkers. Some compounds were
formed from partial hydrolysis of phosphonate diesters in the
generation of target metal phosphonates.7 With respect to
targeted studies of phosphonate monoesters, it has been recog-
nized, by Kontturi et al., on studies of monoester derivatives of
clodronic acid,8 that the alkoxy tether is a structural variable
that can regulate dimensionality of the inorganic backbone in
coordination polymers (larger groups favoring lower dimen-
sional assemblies) but no gas sorption data on this family have
been reported. The present study shows that phosphonate
monoesters can balance crystallinity and permanent porosity
giving a structure (2) accessible to CO2 with a heat of adsorption
measured as 45 kJ/mol. Moreover, the alkoxy group represents a
mechanism for regulating porosity by both modifying the size/
shape of the pore but also by potentially acting in a dynamic
manner as a gate for guest diffusion. A final point of note is that
the alkyl tether must be in proximity to the MÀO bonds and may
kinetically enhance the hydrolytic stability of MOF materials.
Figure 3. Probability density plots of the CO2 oxygen atom in a unit cell
of 2 at 1 bar, 273 K from simulation. In panel a, probabilities along the
c-axis are projected onto the aÀb plane, while in panel b, probabilities
along the b-axis are projected onto the bÀc plane. A green CO2 molecule
is shown in panel ) for reference.
To further investigate the CO2 sorption in 2, GCMC simula-
tions were performed using the UFF parameters20 and REPEAT21
charges that were able to successfully reproduce the experimental
CO2 binding sites and isotherms of a Zn amino-triazole-oxalate
MOF.22 Figure 3 shows the CO2 oxygen atom probability dis-
tribution resulting from a GCMC simulation of 2 at 1 atm CO2
and 273 K. The plots show that there are four symmetrically
equivalent binding sites in the unit cell. The guest molecules align
along the channels with either the carbon or oxygen of the CO2 at
the midpoint between the methyl groups. Consistent with the
DFT calculations, no dual end-on bridging was seen with the
GCMC simulations. One key experimental observation support-
ing the lack of direct interaction of CO2 with the Cu sites is that,
in the crystal structure, although the Cu centers are 4-coordinate,
there is no ligated water. Typically, an open metal site requires
activation13À15 which is not the case with 2.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files, detailed physical
b
characterization of 1 and 2, and details of computational methods
including simulated HOA calculations. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
CO2 adsorption isotherms were also determined from the
GCMC simulations. The CO2 adsorption behavior of 2 was
found to be highly sensitive to the structure of the framework.
For example, when the solvent included X-ray structure of 2 is
used in the simulation, the CO2 uptake is found to be 15À20%
lower than that determined experimentally throughout the pre-
ssure range (Figure S19). However, if this structure is adjusted to
the lattice parameters determined in situ with 1.2 bar CO2, the
simulated isotherm (Figure 2, red) reproduces the experimental
uptake well throughout the pressure range examined. Addition-
ally, we found a strong CO2 uptake dependence on the methoxy
group orientation, which can be defined by the CmeÀOÀPÀO
dihedral angle, ϕ, as highlighted in Figure 3b. Shown in Figure 2
are the simulated adsorption isotherms with the ϕ angle of all
methoxy groups in the framework changed by Δϕ = À2 and +2
degrees from that in the original structure. Negative Δϕ’s corre-
spond to the channel closing, while positive values open the
channel. A 2° change in ϕ, which corresponds to a ∼0.08 Å
change in pore width, results in a dramatic increase or decrease in
the simulated gas adsorption. Therefore, the slight inflection in
the experimental isotherm of 2 at ∼0.6 bar can be attributed to a
subtle change in the framework geometry upon CO2 uptake.
While not explicitly demonstrated in this work, in general, the
high sensitivity of the uptake to the methoxy group orientation in
Corresponding Author
gshimizu@ucalgary.ca; twoo@uottawa.ca
’ ACKNOWLEDGMENT
The authors thank the Natural Sciences and Engineering
Research Council of Canada, the Institute for Sustainable Energy
Environment and Economy (ISEEE) at the University of
Calgary.
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