A charge-separated diamondoid metal-organic framework

Article abstract of DOI:10.1039/C8CC07098A

We report the synthesis, characterization, and gas adsorption analyses of a new charge-separated metal-organic framework (MOF), UNM-1 (C₅₂H₁₆BCuF₁₆N₄), possessing diamondoid structures, assembled from an anionic tetrahedral borate ligand and cationic Cu(i) metal ion. The resulting MOF structure displays four-fold interpenetration, resulting in high environmental stability, and at the same time possesses relatively large surface area (SA_BET = 621 m² g^?1) due to the absence of free ions. Gas adsorption measurements revealed temperature-dependent CO₂ adsorption/desorption hysteresis and large CO₂/N₂ ideal selectivities up to ca. 99 at 313 K and 1 bar, suggesting potential applications of this type of charge-separated MOFs in flue gas treatment and CO₂ sequestration.

Full text of DOI:10.1039/C8CC07098A

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A charge-separated diamondoid metal–organic
framework†
Cite this: Chem. Commun., 2018,
54, 12654
a
b
a
Sheela Thapa,
Eshani Hettiarachchi,
Diane A. Dickie,
*
‡
Gayan Rubasinghege *b and Yang Qin
a
Received 31st August 2018,
Accepted 18th October 2018
DOI: 10.1039/c8cc07098a
rsc.li/chemcomm
We report the synthesis, characterization, and gas adsorption the MOF frameworks. On the other hand, a sub-class of MOFs
analyses of a new charge-separated metal–organic framework known as ionic MOFs have emerged and attracted significant
(MOF), UNM-1 (C52H16BCuF16N4), possessing diamondoid structures, research attention.5,6 These ionic MOFs contain isolated
assembled from an anionic tetrahedral borate ligand and cationic Cu(I) charged centers, either positive or negative, at the metal nodes
metal ion. The resulting MOF structure displays four-fold interpene- or on the organic ligands. The local electric fields, electrostatic
tration, resulting in high environmental stability, and at the same time interactions, and/or coordinating effects generated by these
possesses relatively large surface area (SABET = 621 m2 gÀ1) due to the isolated charges can exert stronger interactions with polar
absence of free ions. Gas adsorption measurements revealed substrates and molecules with high polarizability, than simple
temperature-dependent CO2 adsorption/desorption hysteresis and van der Waals interactions typically found in non-ionic MOF
large CO2/N2 ideal selectivities up to ca. 99 at 313 K and 1 bar, materials.7,8 Most of the ionic MOFs reported to date contain a
suggesting potential applications of this type of charge-separated single type of charge, either positive or negative, decorated
MOFs in flue gas treatment and CO2 sequestration.
covalently onto the framework, which leaves the free charge-
balancing counter-ions within the pores. These counter-ions,
Metal–organic frameworks (MOFs) have become an emerging depending on their sizes, unavoidably reduce accessible pore
field of intensive scientific research over the past few decades.1 volumes of the MOF and can potentially occlude pore openings
From limitless combinations of metal centers and organic and inhibit adsorption of guest molecules. In this regard, it is
ligands, a vast number of MOF materials have been developed appealing to incorporate both positive and negative charges
and studied, leading to microporous scaffolds with precisely at fixed distances and precisely controlled locations into one
tunable surface areas, pore volumes, pore sizes and shapes, and framework structure, forming the so-called charge-separated,
surface functionalities.2–4 The majority of the MOF structures or zwitterionic MOFs that can possess both the favorable
studied to date contain charge-neutral metal clusters as nodes, interaction properties of ionic MOFs and free pore spaces as
or secondary building units (SBUs), connected by organic struts in non-ionic MOFs.
that are also non-charged, leading to an overall charge-neutral
The existing examples of charge-separated MOFs are relatively
framework. In these structures, the positive charges on metal scarce and mainly utilize zwitterionic organic ligands containing
atoms are immediately balanced by surrounding anionic both cationic and anionic species, e.g., ligands containing both
ligands, e.g., carboxylates, and as a consequence, there are no carboxylate and pyridinium,9–15 imidazolium,16,17 or metallo-
exposed ionic species or accessible local electric fields within porphyrin moieties,18 which upon complexation with cationic
metal ions lead to charge-separated MOFs with or without the
need for free charge-compensating counter-ions. Ziegler and
coworkers have systematically explored the application of anionic
tetrakis(imidazolyl)borate ligand in constructing charge-separated
MOF structures through coordination with different metals.19–22
a Department of Chemistry & Chemical Biology, University of New Mexico,
MSC03-2060, 1 UNM, Albuquerque, NM 87131, USA. E-mail: yangqin@unm.edu
b Department of Chemistry, New Mexico Institute of Mining and Technology, 801,
Leroy Place, Socorro, NM 87801, USA. E-mail: gayan.rubasinghege@nmt.edu
† Electronic supplementary information (ESI) available: Synthetic details, NMR When the metal ions possess oxidation state of +1 and coordina-
spectra, single crystal data, TGA histogram, gas adsorption isotherms, IAST
tion number of 4, three dimensional charge-separated MOFs are
calculation and results, QST calculation and results. CCDC 1865216. For ESI
formed with an overall charge neutrality and the absence of free
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
ions.23 Although the four B–N bonds are arranged in tetrahedral
c8cc07098a
geometry at the boron center in these borate ligands, due to the
‡ Current address: Department of Chemistry, P. O. Box 400319, University of Virginia,
Charlottesville, VA, 22904, USA.
off-set angle between the B–N and N–metal bonds at ca. 1451,
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Scheme 1 Synthesis of ligands and schematic representation of the
structure of UNM-1.
rotation around these single bonds can lead to a variety of
conformations that make construction of ordered three dimen-
sional structures difficult and hard to predict.24 In this regard,
borate ligands that possess colinear boron-organic-metal arms are
preferred. In this report, we describe the synthesis of such a
borate ligand containing tetrafluorophenylethynyl pyridine
arms (4) and its coordination with Cu(I) ions for the formation
of a charge-separated MOF, UNM-1. The synthesis of compounds
1–3 leading to ligand 4 was adapted from previously published
procedures25 and is summarized in Scheme 1. Sonogashira
coupling of 3 with 4-bromopyridine led to the borate ligand 4
without the loss of tetrabutylammonium counter-ions. Compound 4
was fully characterized by multi-nuclear NMR spectroscopy (ESI†).
Briefly, a single sharp 11B NMR signal at À16.3 ppm and two
19F signals at À129.9 and À139.7 ppm were observed, consistent
with literature reported values for 3.25 The 13C NMR signals
were not reported in the original synthesis of 3, so our signal
assignments were based on an analogous compound, lithium
tetrakis(4-bromotetrafluorophenyl)borate.26 Two sets of doublets
1
at 148.2 and 145.7 ppm having JCF coupling constants of 241
and 255 Hz are assigned to the F4-phenyl carbon atoms ortho-
and meta- to the boron center, respectively. The ipso-carbon
appears as a broad signal ranging from 132 to 134 ppm caused
by the splitting effects from 1J boron and 2J fluorine atoms. The
F4-phenyl carbon para- to boron center is a triplet at 98.9 ppm
2
with a JCF coupling constant of 14 Hz. The two triple bond
carbon signals appear at 96.0 and 80.5 ppm, while the signals
from the pyridine rings are relatively enhanced and located at
149.9, 130.5 and 125.5 ppm, respectively. Integration of the
1H NMR signal confirms that there is one tetrabutylammonium
cation per borate anion.
UNM-1 was synthesized through an interfacial diffusion
method, in which an CH3CN solution of (CH3CN)4CuBF4 was
laid on top of a CH2Cl2 solution of borate 4. Orange needle-
shaped crystals of UNM-1 were obtained after three days with
ca. 74% yield based on 4. The single crystal X-ray diffraction
analysis is summarized in Fig. 1, Fig. S5 and Table S1 (ESI†).
Fig. 1 (A) Single crystal X-ray structure of UNM-1. (B) Space-filling model
of a 2 Â 2 Â 2 unit cell viewed from the Y-axis; and (C) space-filling model
of a 2 Â 2 Â 2 unit cell viewed from the Z-axis.
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UNM-1 crystalizes in the tetragonal crystal system with space
%
group I4 and unit cell dimension of a = b = 23.5586(7) Å and
c = 24.6516(9) Å. The coordination environment around the
boron and copper atoms is close to tetrahedral, with dihedral
angles around boron centers ranging from 1011 to 1161 and
those around copper centers from 1001 to 1211, leading to an
overall diamondoid-like net as shown in Fig. 1A and Fig. S5
(ESI†). One unique feature of the structure of UNM-1 is the
four-fold interpenetration as color-coded in Fig. 1(B and C),
which is likely caused by the relatively long arms in borate
ligand 4, leading to a boron–copper separation distance of
ca. 13.3 Å. From the space-filling model of a 2 Â 2 Â 2 unit
cell viewed from the X- or Y-axis (Fig. 1B) and from the Z-axis
(Fig. 1C), the crystal structure is porous with straight channels
having two different pore sizes (yellow spheres labeled 1 and 2
for visual assistance). The larger channels are approximately
circular in shape and ca. 7.4 Å in diameter. The smaller
channels have alternating square and octahedral shapes, both
of which possess pore size of ca. 2.7 Å.
UNM-1 is stable under ambient conditions as the powder
X-ray diffraction (PXRD) pattern of UNM-1 after drying under
high vacuum at room temperature for 24 h and being stored in
air for a week (Fig. S6, red trace, ESI†) closely matches that of
the simulated pattern from single crystal X-ray data (Fig. S6,
black trace, ESI†). The water stability of UNM-1 was tested by
soaking a few MOF crystals in water for 48 h and subjected to
PXRD measurements after drying under high vacuum for 24 h.
Again, no significant changes in diffraction pattern were
observed (Fig. S6, ESI†). Changes in the XRD patterns of
UNM-1 could be observed after soaking the crystals in pH 4
and pH 10 aqueous solutions for 48 h, but the major scattering
peaks remained, indicating certain stability of UNM-1 under
mildly acidic and basic conditions. The crystals disintegrated
under pH 1 and pH 13 conditions, as shown by the complete
disappearance of scattering signals in XRD profiles (Fig. S6,
ESI†). Even after repeated gas adsorption trials with different
gases at various temperatures (vide infra), the main features of
PXRD pattern still remain, except becoming broader (Fig. S6,
green trace, ESI†) indicating retention of the basic crystal
structure but loss of long-range order. Furthermore, thermo-
Fig. 2 (A) Adsorption/desorption isotherms of CO2 and N2 on UNM-1 at
various temperatures; inset: hysteresis percentages of CO2 adsorption/
desorption isotherms; (B) ideal CO2/N2 selectivities at different tempera-
tures and pressures.
gravimetric (TGA) analysis of UNM-1 (Fig. S7, ESI†) under N2 SALangmuir of ca. 915 m2 gÀ1. Thus, the surface area of UNM-1 is
shows ca. 2–3% weight loss up to 150 1C, likely due to loss of among the highest in MOFs with four-fold interpenetrated
trapped solvent and water molecules, and no decomposition up structures,27,28 which is likely resulted from the rigid borate
to ca. 300 1C with a total 50% weight loss at 600 1C. We ascribe arms and absence of charge compensating free ions. The pore-
the observed stability of UNM-1 to its four-fold interpenetration size distribution was estimated by fitting the N2 adsorption
geometry that interlocks each layer and prevents dislocation.
isotherm at 77 K using non-local density functional theory
The surface area of UNM-1 was estimated by multi-point (NLDFT) as shown in Fig. S10 (ESI†). A monomodal size-
Brunauer–Emmett–Teller (BET) formalism through N2 adsorp- distribution with pore-diameter of ca. 6.14 Å was obtained,
tion measurements at 77 K and the isotherm is shown in Fig. S8 which is consistent with the microporous structure revealed by
(ESI†). The isotherm shows Type-I adsorption behaviour that single crystal X-ray analysis.
confirms the microporous nature of UNM-1. Linear fit (Fig. S9,
The N2 and CO2 adsorption isotherms of UNM-1 at 273 K,
ESI†) between partial pressures (P/P0) 0.05 and 0.30 gives the 298 K, 303 K, 313 K and 323 K are shown in Fig. 2A. These
average BET surface area (SABET) of ca. 621 m2 gÀ1. Based on temperatures are relevant in real-world applications including
the Type-I shape of N2 adsorption isotherm and assuming the CO2 capture from industrial flue gases.29,30 It is clearly seen
absence of meso- and macro-pores, we also fit the isotherm that the adsorption of CO2 by UNM-1 is much enhanced over
with Langmuir method (Fig. S9, ESI†) and obtained the average that of N2 at all temperatures tested, up to ca. 27 cc per g CO2 at
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Y. Q. would like to acknowledge National Science Foundation
(DMR-1453083) and United States Department of Agriculture
(NIFA 2015-38422-24059) for financial support for this research.
Conflicts of interest
There are no conflicts to declare.
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