Hydrophobic Shielding of Outer Surface: Enhancing the Chemical Stability of Metal–Organic Polyhedra

Article abstract of DOI:10.1002/anie.201811037

Metal–organic polyhedra (MOP) are a promising class of crystalline porous materials with multifarious potential applications. Although MOPs and metal–organic frameworks (MOFs) have similar potential in terms of their intrinsic porosities and physicochemical properties, the exploitation of carboxylate MOPs is still rudimentary because of the lack of systematic development addressing their chemical stability. Herein we describe the fabrication of chemically robust carboxylate MOPs via outer-surface functionalization as an a priori methodology, to stabilize those MOPs system where metal–ligand bond is not so strong. Fine-tuning of hydrophobic shielding is key to attaining chemical inertness with retention of the framework integrity over a wide range of pH values, in strong acidic conditions, and in oxidizing and reducing media. These results are further corroborated by molecular modelling studies. Owing to the unprecedented transition from instability to a chemically ultra-stable regime using a rapid ambient-temperature gram-scale synthesis (within seconds), a prototype strategy towards chemically stable MOPs is reported.

Full text of DOI:10.1002/anie.201811037

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Accepted Article
Title: Outer Surface Hydrophobic Shielding Strategy to Enhance the
Chemical Stability of Metal-Organic Polyhedra
Authors: Samraj Mollick, Soumya Mukherjee, Dongwook Kim, Qiao
Zhiwei, Aamod V. Desai, Rajat Saha, Yogeshwar D. More,
Jianwen Jiang, Myoung Soo Lah, and Sujit K. Ghosh
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811037
Angew. Chem. 10.1002/ange.201811037
Link to VoR: http://dx.doi.org/10.1002/anie.201811037
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10.1002/anie.201811037
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Outer Surface Hydrophobic Shielding Strategy to Enhance the
Chemical Stability of Metal-Organic Polyhedra
Samraj Mollick,[a] Soumya Mukherjee,[a] Dongwook Kim,[b] Qiao Zhiwei,[c] Aamod V. Desai,[a] Rajat
Saha,[d] Yogeshwar D. More,[a] Jianwen Jiang,[c] Myoung Soo Lah,[b] Sujit K. Ghosh*[a],[e]
Abstract: Metal-organic polyhedra (MOPs) are a promising class of
crystalline porous materials with multifarious potential applications.
Although MOPs and Metal-organic frameworks (MOFs) have similar
potentials in terms of their intrinsic porosity and physicochemical
properties, the exploitation of carboxylate MOPs is still rudimentary
due to lack of systematic development addressing their chemical
stability. Here we describe the fabrication of chemically robust
carboxylate MOPs via outer surface functionalization as an a priori
methodology which could be applied to stabilize of those MOPs
system also where metal-ligand bond is not so strong. Fine-tuning of
hydrophobic shielding transpire as a key facet to attain chemical
inertness with retention of framework integrity under wide ranges of
pH, strong acidic conditions, oxidizing and reducing media among
these materials and these results are further corroborated by
molecular modelling insights. Owing to the unprecedented transition
from instability to chemically ultra-stable regime accompanied by
ambient temperature gram-scale rapid synthesis (within seconds), a
prototype strategy towards engineered construction of chemically
stable MOPs is reported.
application studies relative to MOFs/COFs. Broadly, there are
few strategies proposed for synthesis of porous MOPs[17-20] but
majority of carboxylate MOPs reported in the literature have
been found to be hydrolytically unstable. The solid state
applications of such MOPs are largely impeded owing to their
instability and loss of crystallinity as an outcome of unavoidable
aggregation-driven blockage of active sites on guest
removal.[21,22] Although some discrete reports have been
proposed to improve stability of MOPs by enhancing the metal-
[23-25]
[26]
ligand bond strength
or appending long alkyl chains
there has been scarcity of a broad design approach to attain
chemically inert carboxylate MOPs. In principle, MOPs offer
similar potential applications as MOFs including additional
advantages of solution processability [27] for MOPs in common
organic solvents which is highly sought for easy fabricating
membranes, thin film devices, preparation of electrolytic
solutions, various active additives in polymers, ion channel
including various biochemical applications etc.[28,29]
A generalized strategy for synthesizing chemically inert MOPs
can contribute significantly to develop of structure-function
relationship derived crystal engineering and boost up the growth
of MOPs in view of their miscellaneous applications. Herein, we
develop a prototypal synthesis strategy which allows us to
prepare a series of hydrolytically stable carboxylate MOPs using
grafting of hydrophobic exterior to basic building blocks and in
principle this versatile approach can be applied to stabilize any
MOP structures, irrespective of metal ions and nature of
coordinating ligands used in the synthesis. Amino acids are
Over the last few decades, there has been an astonishing
evolution of crystalline porous solid materials and have been
centre of great attention to researchers across multiple
disciplines.[1,2] Among the various crystalline porous solids,
metal-organic frameworks (MOFs), covalent-organic frameworks
(COFs) have emerged as a distinguished research area and on
account of high surface area these materials have been
explored for several applications such as selective gas storage
and separation ability, sensing, catalysis, energy storage and
conversion capacity etc.[3-8] Carboxylate metal-organic polyhedra
(MOPs) form a subclass of crystalline porous materials which
were proposed almost at the same time as MOFs.[9,10] Over the
chosen as building blocks as their functional groups could be
[30]
smoothly modulated
where as electron-deficient anhydride
decorated polar pore surfaces have often imparted excellent
guest-responsive characteristic features of diimide linkers. While
among transition metal ions, binuclear copper paddle wheel is
well recognized for easily affording carboxylate MOPs, owing to
its 4-connected secondary building units.[31] Water stability being
the key targeted criterion, deliberately enhanced steric crowding
surrounding the metal coordination sites results into an ultra-
stable and first superhydrophobic MOP.[32] More importantly,
consequent of increased hydrophobicity, a distinct enhancement
of chemical stability could be achieved by rationally tuning the
alkyl functionality of the substituted diimide linkers, which was
further upheld by molecular simulation studies. We hereby
present the multi-gram scale quick synthesis (within seconds) of
a chemically ultra-stable (wide ranges of pH, oxidizing and
last few years realization of rational design strategies has
[11-16]
yielded several hydrolytically stable MOFs and COFs
and
actuated their development towards practical implementation,
but lack of systematic approaches to prepare stable MOPs has
severely hindered the progress of carboxylate MOPs for
[a]
S. Mollick, Dr. S. Mukherjee, Dr. A. V. Desai, Y. D. More, Dr. S.
K. Ghosh.
Department of Chemistry, Indian Institute of Science Education
and Research (IISER) Pune.
E-mail: sghosh@iiserpune.ac.in
[b]
[c]
[d]
[e]
D. Kim, Prof. M. S. Lah
reducing media) MOP employing
a
systematic surface
Department of Chemistry, Ulsan National Institute of Science and
Technology (UNIST), Ulsan, 44918, Korea.
Q. Zhiwei, Prof. J. Jiang
Department of Chemical and Biomolecular Engineering, National
University of Singapore, 117576, Singapore.
Dr. R. Saha
Department of Chemistry, Kazi Nazrul University, Kalla, Asansol,
713340, India.
Centre for Energy Science, IISER Pune, Pune, 411008, India.
engineering aided hydrophobicity increment principle.
Constructed from naphthalene diimide (NDI) core based
homologous amino acids (Scheme S1-S5, Figure S1-S8),[33-37]
the syntheses and single crystal structures of four Cu(II)-based
MOPs (coined as IPMOP-n; n denotes the carboxylate amino
acid variant) have been described herein (Figure 1). When
amino acids are alanine (A), valine (V), isoleucine (IL) and
phenyl alanine (PA), derived MOPs are IPMOP-A, IPMOP-V,
IPMOP-IL and IPMOP-PA respectively.
Supporting information for this article is given via a link at the end of the
document.
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Figure 1. Schematic synthetic strategy for MOPs and their corresponding crystal structures following a systematic increment order of their chemical stabilities (as
illustrated from bottom to top).
In the current report, we describe their gradual increment of
chemical stability attributes to finally attain ultra-high stability
under acidic/basic and different oxidizing and reducing
conditions. Solvothermal syntheses of the MOPs (A, V, IL and
PA) led to green crystals (cube and diamond shaped, depending
on specific MOPs system; Scheme S6-S9). Optimization of
reaction conditions guided us to obtain IPMOP at room
temperature (multi-gram scale) within seconds (Figure S9 and
S10), in presence of 2,6-lutidine as a base (Video S1). Crystal
structure of IPMOP-A was determined by single-crystal X-ray
diffraction studies while those for IPMOP-V, IPMOP-IL and
IPMOP-PA were determined using synchrotron beamline
radiation. Each MOP contains twelve ligand molecules and
twelve water molecules to form neutral cages (Figure S11) of
general formula: [Cu12(Lx)12(H2O)12] [LX: corresponding ligands
system LA, LV, LIL and LPA]. Each cage molecule contains twenty-
four alkyl groups which exohedraly decorate the entire outer
surface of the spherical cages (Figure S12-S31). All the cages
have eight triangular windows with average aperture size of ~2 Å
as well as average internal cavity diameter ~15 Å. Each
bimetallic paddle-wheel Cu(II) unit is coordinated to two terminal
water molecules: one pointed to the internal cavity of the
polyhedron and another one, pointed outside the polyhedron
surface.
removed by conventional activation protocol (heating under
vacuum). CO2 gas adsorption isotherms were recorded for the
MOPs at 195 K,[38] (Figure S38 and S39) which reflected similar
saturation capacities for IPMOP-V (4.6 mmol), IPMOP-IL (4.3
mmol) and IPMOP-PA (4.5 mmol). This was an outcome of their
isoreticular nature.[39] Substantially lower saturation capacity for
IPMOP-A (1.3 mmol) can be attributed to the solvent loss
induced aggregation of individual cages, as often observed for
MOPs.[38] Ambient temperature CO2 sorption profiles also
exhibited similar CO2 saturation capacities for IPMOP-V,
IPMOP-IL and IPMOP-PA (Figure S40-S43). Although PXRD
patterns of IPMOP-V indicates weak crystallinity (Figure S44),
similar saturation capacity for all three compounds (IPMOP-PA,
IPMOP-IL and IPMOP-V) suggests gas sorption occurs inside
the intrinsic cavity of the cage molecules rather than the extrinsic
intercage porosity. Due to flexible nature of the pore window,
gas sorption happens inside the cage cavity even though
aperture size of MOPs is smaller than the kinetic diameter of
CO2.
Chemical stability aspect is examined by treating the crystals of
IPMOP-PA in water, steam, solutions of widely varying pH,
including harsh acidic solutions, buffer solutions, as well as
subjecting to diverse oxidizing and reducing media (Figure 2,
Figure S45 and S46).[40] For assessment of water stability,
IPMOP-PA was soaked in water for several months; unaltered
PXRD patterns (Figure S47 and S48) point out the retention of
both crystallinity and structural integrity in water. Intended for
further checking water stability under more harsh conditions, the
sample was placed under steam for 5 days; similar PXRD
patterns afterwards indicate structural robustness of the
Thermogravimetric analysis (TGA) shows loss of guest
molecules until ~250 °C (Figure S32-S37) for all the four
compounds. The slow loss of guest molecules can be ascribed
to the presence of polar sites in the host cage having limited
aperture size, which interact strongly with guest solvent
molecules. Most of the guest solvent molecules could be
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Figure 2. Characterization of stability confirmation for IPMOP-PA. (a) PXRD patterns under different conditions: pH~1 (5 days), pH~11(5 days), 10N HCl (6 h),
buffer solution [pH~9] (7 days), reducing media (FeSO4 solution; 30 days), oxidizing media (fenton solution; 30 days). (b) post-treatment SC-XRD data, under
different conditions.(c) Bar diagram representing the reproducibility of IPMOP-PA miscellaneous harsh treatments, navy blue: before treatment of crystal, and dark
yellow: after crystal treatment. (d) CO2 sorption isotherms recorded at 195 K after treating the adsorbent MOP under different harsh conditions: pH~1(5 days),
pH~11(5 days), 10N HCl (6 h), buffer solution [pH~9] (7 days), reducing media (FeSO4 solution; 30 days), oxidizing media (fenton solution; 30 days). (e) FE-SEM
images after immersing IPMOP-PA crystals under different conditions: pH~1(5 days), pH~11(5 days),10NHCl (6 h), buffer solution [pH~9] (7 days), reducing
media (FeSO4 solution; 30 days), and oxidizing media (fenton solution; 30 days).
compound. Crystals of IPMOP-PA were kept immersed in a wide
range of solutions with varying pH (pH~1 to pH~11) over 5 days,
ensuing similar PXRD patterns (Figure S49 and S50) confirmed
its exceptional stability over a wide pH-range. In addition to this,
we studied TGA, gas sorption and FT-IR for different pH
immersed (pH~1 and pH~11) crystals and their similar profiles
demonstrate retention of the framework. Crystals of IPMOP-PA
were also examined under various harsh acidic conditions, and
found to exhibit super-stability (Figure S51-S53). For further
introspection concerning framework stability, we dipped the
crystals of IPMOP-PA in a number of oxidizing and reducing
solutions over several weeks. Precise matching of all the main
peaks in the experimental PXRD patterns (Figure S54-S55)
implied that IPMOP-PA is exceptionally chemical stable. In
addition to PXRD, we surveyed TGA, gas sorption, FT-IR, and
FESEM data-sets. In all these measurements, uniform patterns
reflected inertness of the framework (Figure S56-S60). Along
with, inductively coupled plasma (ICP) analysis of supernatants
from diversely treated chemical environments (pH=1 and 11, 10
N HCl and equivalent mixture of THF/water) were recorded. A
negligible amount of copper leaching (Table S1) confirms the
stability of IPMOP-PA in such chemical environments. For
solution phase stability of cage molecules, we solubilized
IPMOP-PA in THF media and treated with diverse chemical
environment before performing UV-Vis measurement and ICP
analysis. The UV-Vis spectra (Figure S61) indicates that
IPMOP-PA is also quite stable in such chemical environments
under solution phase.
IPMOP-IL and IPMOP-V are also chemically stable (Figure
S62-S78) but not to the extent of IPMOP-PA. IPMOP-A is
neither water-stable (Figure S79 and S80), nor chemically stable.
In IPMOP-A, among the twenty-four methyl groups only twelve
outer methyl groups interact with oxygen centers of NDI
moieties from their neighboring cages. These interactions are
collectively insufficient to maintain the overall channel continuity
in the guest-removed packed architecture. Besides, methyl
group is sp3 hybridized, hence anticipated to assume weak
interactions with NDI core. While in IPMOP-PA all benzyl groups
interact with neighboring cages, sp2 hybridized phenyl hydrogens
strongly interact with NDI moieties from their neighboring cages
leading to maintaining their long range order in bulk phase.
Therefore, such electrostatic interactions seem more prominent
in IPMOP-PA, relative to IPMOP-A which is further corroborated
by molecular modelling results (Figure 3).
To present microscopic insights into the stability aspect of
IPMOPs, binding energies among the cages (most stable and
least stable compounds) were estimated. For each IPMOP,
energies for a single unit cell (Euc) and a single cage (Ecage) was
separately calculated, followed by estimation of binding energy
(E) i.e. E= (Euc – nEcage)/n, where n is the number of cages in
a unit cell. For IPMOP-A and IPMOP-PA, the binding energies
are -94.06, and -150.74 kcal mol-1, respectively. This energy
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Figure 3. Structural insight and solution processability study of IPMOPs. (a) Contact areas between cages: (I) IPMOP-A; (II) IPMOP-PA. (b) 195 K CO2 sorption
isotherms for phase-I (as-synthesized), and phase-III (reprecipitation after dissolving in THF) of IPMOP-PA. Phase-II represents THF-dissolved solution phase for
IPMOP-PA. (c) Water droplet cast on the surface of the pelletized IPMOPs: WCA values for (I) IPMOP-A: ~74°; (II) IPMOP-V: ~118°; (III) IPMOP-IL: ~130°; (IV)
IPMOP-PA: ~155°.
increases in the order of IPMOP-A <IPMOP-PA, as attributed to
Acknowledgements
the increasing size of alkyl functional group of the substituted
diimide linkers. Figure 3a shows structures of the two IPMOPs,
green circles highlight the contact areas between neighbouring
cages. It is obvious that the contact area increases with
increasing size of alkyl functional groups involved. In Figure 3a,
the -CH3 groups of IPMOP-A offer only a small contact area
while in IPMOP-PA, the aromatic groups are interpenetrate
between the neighbouring cages i.e. leading to maximal contact
area. With increasing contact area, increasing binding energy
leads to stronger interactions, corresponding to the maintenance
of long range order in solid state, which is consistent with our
experimental observations.
S. M. is thankful to UGC for research fellowship. A.V. D. and
Y.D.M. are thankful to IISER-Pune for research fellowship.
M.S.L. acknowledges the National Research Foundation of
Korea (2016R1A5A1009405) for financial support. The authors
also acknowledge PAL for beam line use (2016-1st-2D-005) and
2016-3rd-2D-026).We are grateful to IISER-Pune for research
facilities. INSA (Project No: IISER-P/GAP-142/7/2014), SERB
(Project No. EMR/2016/000410) and DST-FIST (SR/FST/CSII-
023/2012;
for
micro-focus
SCXRD
instrument)
are
acknowledged for generous funding. The authors thank to G. R.
Tripathy and his student (IISER-Pune) for helping ICP analysis
measurements.
Unlike extended frameworks, solution-processability of MOPs
offer multifarious advantages over other porous materials.[41]
Encouraged by our obtained results, solution-processability for
IPMOP-PA was assessed and found to be solution-processable
in tetrahydrofuran (THF); as confirmed by gas sorption
measurement of Phase-I (as-synthesized) and Phase-III
(reprecipitation after dissolving in THF) (Figure 3b and Figure
S81-S86, Table S2). Among this IPMOP series, we observed
that the solubility increases with increasing size of the alkyl
functional groups upto a certain limit, beyond which the MOP
solubility decreases with higher alkyl groups. Among IPMOPs,
IPMOP-A is insoluble in common solvents, while rest are soluble
in THF (Figure S87-S92 and Table S3), and thus solubility was
found to increase from IPMOP-V to IPMOP-PA to IPMOP-IL. For
hydrophobicity, the surface hydrophobicity was confirmed from
water contact angle (WCA) measurements of all IPMOPs (Figure
3c) and found that WCA also tuned from hydrophilic (WCA~74º)
to superhydrophobic region (WCA~154º) (Table S4) for the first
time for MOPs system.
Keywords: metal-organic polyhedra • carboxylate MOPs •
chemically stable MOPs • copper paddlewheel based MOPs •
hydrophobicity.
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Layout 2:
COMMUNICATION
Samraj Mollick, Soumya
Mukherjee, Dongwook Kim,
Qiao Zhiwei, Aamod V. Desai,
Rajat Saha, Yogeshwar D.
More, Jianwen Jiang, Myoung
Soo Lah, Sujit K. Ghosh*
Schematic diagram for correlation between hydrophobicity and chemical stability of
MOPs. Enhancement of chemical stability consequent of their increment in
hydrophobicity although the internal core of each cage is similar. Outer green surface
indicate the hydrophobicity shielding while inner core represent the same internal
cage molecule.
Page No. – Page No.
Outer Surface Hydrophobic Shielding
Strategy to Enhance the Chemical
Stability of Metal-Organic Polyhedra
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