Flexible bifunctional monoethylphosphonate/carboxylates of Zn(ii) and Co(ii) reinforced with DABCO co-ligand: paradigmatic structural organization withpcutopology

Article abstract of DOI:10.1039/d0ce00275e

Two novel isostructural phosphonate-monoethylcarboxylate MOFs with the structural formula of [M₂(EtBCP)₂(DABCO)_0.5]·2DMA (M = Zn (2), Co (3); H₂EtBCP =O-ethyl-P-(4-carboxyphenyl)phosphonic acid, DABCO = 1,4-diazabicyclo[2.2.2]octane, DMA =N,N-dimethylacetamide) were synthesized and characterized. The frameworks of2and3are sustained by {Zn₂(PO₂(OEt))₂}_nmetal-phosphonate- and DABCO-extended {Zn₂(COO)₄(DABCO)}_npaddle-wheel carboxylate chain-SBUs. The chains providing connectivity in three, mutually orthogonal directions are running parallel and are combined in a framework, which could be interpreted as having apcutopology. The simple structure-organization principle, which suggests the possibility of the elongation of the bifunctional ligand with scaling in two directions, allows to view the structures of2and3as prototypes for an isoreticular series. The porosity of both compounds, based on a relatively short ligand, is low: no adsorption of N₂was registered, however, CO₂is adsorbed readily allowing to estimate the surface area at ～330 m²g^?1(～900-1060 m²g^?1geometric estimate). The compounds demonstrate a two-step CO₂adsorption isotherm both at 195 K (0-1 bar) and 298 K (0-20 bar). The adsorption isotherms are characterized by a gradual (type “F-I”), albeit still relatively steep onset of the second step, associated with structural flexibility/bistability. The estimated pore volumes at the start of the transformation (195 K) for2and3are ～0.11 (0.08P/P₀) and 0.12 cm³g^?1(0.12P/P₀) respectively, which corresponds considerably well to the geometrically calculated accessible volume of ～0.07 cm³g^?1for the experimental structure (3.3 ? probe diameter). The structural prerequisites of the observed flexibility of the framework, which might be associated with the non-planarity of the metal-phosphonate moieties, acting as ‘levers’ for propagating mechanical stress, are discussed.

Full text of DOI:10.1039/d0ce00275e

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Gil-Hernández, S. Millan, S. Gökpinar, C. Heering, I. Boldog and C. Janiak, CrystEngComm, 2020, DOI:
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DOI: 10.1039/D0CE00275E
ARTICLE
Flexible bifunctional monoethylphosphonate/carboxylates of
Zn(II) and Co(II) reinforced with DABCO co-ligand: paradigmatic
structural organization with pcu topology
Received 00th January 20xx,
Accepted 00th January 20xx
Anna Goldman,a Beatriz Gil-Hernández,b Simon Millan,a Serkan Gökpinar,a Christian Heering,a
Ishtvan Boldog*a and Christoph Janiak*a
DOI: 10.1039/x0xx00000x
Two novel isostructural phosphonate-monoethylcarboxylate MOFs with the structural formula of
[M2(EtBCP)2(DABCO)0.5] · 2DMA (M = Zn (2), Co (3); H2EtBCP = O-ethyl-P-(4-carboxyphenyl)phosphonic acid, DABCO =
1,4-diazabicyclo[2.2.2]octane, DMA = N,N-dimethylacetamide) were synthesized and characterized. The frameworks of 2
and 3 are sustained by {Zn2(PO2(OEt))2}n metal-phosphonate- and DABCO-extended {Zn2(COO)4(DABCO)}n paddle-wheel
carboxylate chain-SBUs. The chains, providing connectivity in three, mutually orthogonal directions are running parallel and
are combined in a framework, which could be interpreted as having a pcu topology. The simple structure-organization
principle, which suggests the possibility of the elongation of the bifunctional ligand with scaling in two directions, allows to
view the structures of 2 and 3 as prototypes for an isoreticular series. The porosity of both compounds, based on a relatively
short ligand, is low: no adsorption of N2 was registered, however, CO2 is adsorbed readily allowing to estimate the surface
area at ~ 330 m2 g–1 (~900-1060 m2 g–1 geometric estimate). The compounds demonstrate a two-step CO2 adsorption
isotherm both at 195 K (0-~1 bar) and 298 K (0-20 bar). The adsorption isotherms are characterized by a gradual (type “F-
I”), albeit still relatively steep onset of the second step, associated with structural flexibility/bistability. The estimated pore
volume at the start of the transformation (195 K) for 2 and 3 are ~0.11 (0.08 P/P0) and 0.12 cm3 g–1 (0.12 P/P0) respectively,
which corresponds considerably well to the geometrically calculated accessible volume of ~0.07 cm3 g–1 for the experimental
structure (3.3 Å probe diameter). The structural prerequisites of the observed flexibility of the framework, which might be
associated with the non-planarity of the metal-phosphonate moieties acting as ‘levers’ for propagating mechanical stress,
are discussed.
particularly the tridentate phosphonates of oxophilic9 metals
possess the thermodynamic prerequisites to be more stable
than carboxylates (even if the kinetic factors could often
Introduction
The field of metal-organic frameworks (MOFs) witnessed
explosive growth during the last two decades. The 'hybrid'
nature of the coordination polymers, consisting of both
inorganic and organic constituents, their crystallinity, and high
attainable surface areas are among the primary factors behind
the great expectations.
The structural versatility and functional tuneability1,2,3 of MOF
materials are indeed superior and a range of possible
applications are demonstrated, including gas storage, small
molecule separation, catalysis, sensing, etc.4,5,6,7 On the other
hand, the relatively low chemical/hydrolytic stability is a
hindrance to their real-world use. Nearly all of the most stable
MOFs are M(III, IV)= Cr, Fe, Al, Zr carboxylates and M(I, II) = Ag,
Cu, Zn, Co azolates. The bidentate phosphinates8 and
prevail). Porous organo-phosphonates are actively researched
for more than a decade, however the achievements were
modest until recently. There is a strong tendency towards
formation of dense layered structures,10 partly evident for
linear arylphosphonates (the stacking of the aryl-moiety
reinforces the dense parallel packing; the use of non-arylic
ligand platforms, namely (bi)piperidines, led to one of the most
successful early phosphonate MOFs, the isoreticular STA-12,
-1611,12). Discrete secondary building units (SBUs) tend to be less
frequent compared to infinite 1D or 2D cases, which favor dense
packing, due to relatively high denticity of the phosphonate
group.13
The analysis of the relatively rare phosphonates with high
porosity suggests a few strategies to avoid dense structures.
The use of large rigid molecules with non-collinear phosphonate
branches, e.g. trigonal shaped,14 or even better non-coplanar,
e.g. tetragonal shaped molecules,15 is one of the
approaches.16,17 The recent report of the ultrastable highly
porous zirconium-based SZ series (SBET up to ~600 m2 g–1 for
[N-Et-pyridinium]2[Zr3.5(LH)F9])15 with nearly fully deprotonated
tetrahedral adamantanetetraphosphonic acids (H8L) crowned
a. Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität
Düsseldorf, D-40204 Düsseldorf Germany. Email: janiak@hhu.de
b. Departamento de Química, Facultad de Ciencias de La Laguna, Sección Química,
Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
† Electronic supplementary information (ESI) available: Experimental details and
characterization data. CCDC 1985570 and 1985571. For ESI and crystallographic data
in CIF or other electronic format see DOI : xxx .
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the efforts of this type, and the use of ionic liquids as a medium
suggests the possibility of templating. The microporous CAU-14
copper phosphonate 14 with modest CO2 adsorption is also
worth mentioning.
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DOI: 10.1039/D0CE00275E
PO3H2
PO3H2
COOH
H3BCP
Alternatively, the introduction of synergistic co-ligands, e.g.
N-donor ligands, or the use of monoalkylphosphonates for the
decrease of the ligand's denticity were proven also efficient.
The use of N-donor co-ligands was shown to be partially
productive leading to 3D framework compounds incorporating
2,2´-bipyridine, 1,10-phenanthroline, 4,4´-bipyridine or
4,4´-trimethylenedipyridine.18,19,20,21 For most of the
compounds no permanent porosities were demonstrated,
however, extensive reports on properties including
luminescence,22,23,24,25 magnetism,26,27,28 catalysis,29 thermal
stability30,31 and water stability32,33 were published. It is
important to note that the combination of carboxylate and
N-donor ligands for divalent 3d metals is a viable strategy for
the increase of stability. An illustrative example are the
[M(BDC)2(DABCO)], M = Cu, Zn MOFs, which demonstrate
significantly higher hydrolytic stability than the carboxylate-
only peers.34,35 The role of the N-donor ligands could also be
interpreted as a means of decreasing the average local
connectivitiy of the networks. Other, more synergistic cases
could be also foreseen. Monoesters of phosphonic acids and
phosphinates are both bidentate and could function
analogously to carboxylate groups.36 The seminal works of
Shimizu et al.,36,37,38 eventually lead to the discovery of the
COOH
H3BPPA
O
O
HO
EtO
P
HO
P
OEt
O
P
EtO
HO
OH
P
OEt
HO
P
OH
O
O
OEt
OH
P
P
O
OEt
EtO
O
H2Et2BDP
H2Et2BPDP
H3Et3BTBP
EtO
O
P
O
P
EtO
OH
OH
O
OH
O
OH
H2EtBCP
H2EtBCEP
Fig. 1 The most relevant phosphonic and monoesterphosphonic acids reported in the
literature as building-blocks for MOFs.21,32,36,39,41
Our research was inspired by all the three mentioned strategies
aiming for robust phosphonate MOFs: the use of bifunctional
(monoalkyl)phosphonates, their potentially synergistic
combination with carboxylate functions in one ligand, and the
use of N-donor co-ligands. Herein, were report two novel
permanently
porous
copper
monoesterphosphonate
framework CALF-33-ET3 (CALF = Calgary Framework) with one
of the highest surface areas at 1000 m2 g–1 for a phosphonate-
based MOF.31,39 It is expected that the alkyl substituent could
tune the adsorption preference of the smaller CO2 molecules
over N2 and CH4.40,41
The use of phosphonate-monoesters as ligands for the synthesis
of MOFs with significant porosity is also known; the reported
materials are often characterized by high moisture and
water-vapor stability.42,43,44 Selected examples of the respective
monoesterphosphonic acids used for the syntheses of
coordination polymers/MOFs are summarized in Fig. 1.
isostructural
monoethylphosphonate-carboxylate
MOFs
obtained using that strategy: [Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2
and [Co2(EtBCP)2(DABCO)0.5] · 2DMA, 3, with H2EtBCP = O-ethyl-
P-(4-carboxyphenyl)phosphonic acid, the second only known
ligand of this class used for the syntheses of coordination
polymers. The implications, originating from the potentially
general importance of their special structures and the structural
flexibility observed from the CO2 adsorption data are discussed.
Results and discussion
Synthesis and structural characterization
The ligand, O-ethyl-P-(4-carboxyphenyl)phosphonic acid
(H2EtBCP), was synthesized in two steps. The intermediary
diethyl P-(4-carboxyphenyl)phosphonate was obtained by the
Ni(II)-catalyzed Michaelis-Arbuzov reaction between the methyl
4-iodobenzoate with triethylphosphite. Partial hydrolysis in 2
mol/L aqueous NaOH under mild conditions yielded the aimed
monoester, H2EtBCP (see the experimental section for details).
While H2EtBCP was reported before,45,46,47 it was never used for
the synthesis of coordination polymers. It is a rigid ligand, which
is the second only representative of the monoalkylphosphonic-
carboxylic acid class used in the synthesis of coordination
polymers/MOFs
after
the
flexible
O-ethyl-P-(4-
carboxyphenylmethyl)phosphonic acid (H2EtBCEP) used for the
preparation of non-porous luminescent cadmium based
compounds (Fig. 1).21 The initial screening for the best
2 | J. Name., 2012, 00, 1-3
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crystallization conditions was performed using the solutions of similarity of structures 2 and 3, as well as the phase purity of the
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DOI: 10.1039/D0CE00275E
Zn(NO3)2 · 6H2O and H2EtBCP in DMF. One of the experiments, two latter, could be observed from the comparison of the
(130 °C, 72 h see ESI† for further details) yielded a poorly experimental and simulated PXRD patterns (Fig. 3).
crystalline material, nonetheless containing single crystals with
a quality barely enough for a standard diffraction experiment. A
‘predecessor’ structure, [Zn2(EtBCP)2(Solv)] · Solv´, 1, was
identified (Fig. S14, S15; see ESI† for further details regarding
this ‘predecessor’ structure). Repeated attempts to improve the
quality of the measurement were only partially successful.
However, the motif of the 3D framework structure sustained by
{Zn2(RCOO)2} paddle-wheel and {Zn2(PO2(OEt))2}n 1D chain SBUs
was clearly established (for the latter see Fig. 5a). The nature of
the solvent molecule in 1 remains unclear and might be a minor
impurity, possibly formed in-situ. In this case, it might be a true
bidentate ligand, which cannot be removed after activation (the
possible role of formic acid/formate was checked in
comparative experiments with a negative result, see ESI†). The
observance of the paddle-wheel motif in 1 was an additional
Fig. 3 PXRD patterns of [Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2 (red) simulated from X-ray
crystal diffraction data and measured for the as-synthesized 2 (green) and for the as-
synthesized [Co2(EtBCP)2(DABCO)0.5] · 2DMA, 3 (blue).
incentive, which stimulated us to use spacer N-donor (LN) co-
ligands molecules, aiming stabilization of the structure via
additional ‘cross-linking’ through the formation of
{Zn2(RCOO)2(LN)2}, the stable N-capped paddle-wheel SBU.48
The isostructural compounds 2 and 3 crystallize in the I4/m
The approach seemed particularly suitable because the distance
space group, when the synthesis is done in DMA (interestingly,
between the paddle-wheel units in 1 was estimated to be 6.3 Å
no product was obtained from DMF, and vice versa, 1 was not
(Fig. S15), which corresponds well to the length of the DABCO
possible to obtain from DMA); the further description will be
molecule (Fig. 2).49
given on the example of 2. The asymmetric unit contains two
The DABCO ligand along with pyrazine or 4,4-bipyridine is
crystallographically independent metal ions, one deprotonated
frequently used for a pillaring design-strategy of joining planar
EtBCP2– ligand, one DABCO ligand (the two latter represented
topologies sustained by paddle-wheel units.35,50,51,52,53,54,55 The
by independent halves) and one uncoordinated DMA guest
typical distances between paddle-wheel units determined from
molecule (Fig. 4). The Zn1 atom has a tetrahedral environment
experimental structures are 6.7 Å for DABCO56, 7.1 Å for
consisting of four monoester phosphonate groups. Neighboring
pyrazine57 and 11.2 Å for 4,4-bipyridine58 (Fig. 2).
metal atoms are joined by the bridging monoester
phosphonates, forming a linear 1D {Zn2(PO2(OEt))2}n chain as an
O
O
infinite SBU (Fig. 5a). This arrangement is relatively typical, and
similar zinc and cobalt phosphonate chains are also known for
M
M
O
O
O
O
O
O
O
O
M
M
O
O
M
O
O
O
O
O
O
O
instance
in
[(enH2)2][Zn(PO4)2],
O
M
N
N
O
O
[C10N4H26][Zn5(H2O)4(HPO3)6] · 4H2O, [C6N2H18][Zn3(HPO3)4] and
[C5N2H14][Co(HPO4)2].59,60,61 The Zn2 atom is pentacoordinated,
adopting a {ZnO4N} square-pyramidal environment. Together
with the carboxylate and DABCO ligands, the Zn2 atoms
constitute the DABCO ‘capped’ {[Zn2(RCOO)2(DABCO)]}n paddle-
wheel unit, which is also well-known.35,50 The DABCO associates
the paddle-wheel units in a chain, so it is possible to interpret
the latter as a 1D SBU, instead of the interpretation on the level
of discrete paddle-wheel units.
O
O
N
N
~ 6.7 Å
~ 7.1 Å
~ 11.2 Å
N
O
O
O
O
M
N
O
O
O
M
M
O
O
O
O
O
O
M
O
M
M
O
O
O
O
O
O
O
O
O
O
Fig. 2 Distances between the paddle-wheel units bridged with pillaring ligands DABCO,
pyrazine and 4,4’-bipyridine, found in experimental crystal structures.56,57,58
The reaction of H2EtBCP, DABCO, and Zn(NO3)2 · 6H2O or
Co(NO3)2·6H2O
N,N-dimethylacetamide (DMA) at 100 °C lead to the formation
of two isostructural MOFs with composition of
(2:2:1
molar
ratio)
in
a
[M2(EtBCP)2(DABCO)0.5] · 2DMA (M= Zn, Co; compounds 2 and 3
respectively). Notably, no products of appreciable crystallinity
were obtained with the use of N,N-dimethylformamide (DMF).
The single crystal structure of 2, discussed in detail below,
showed the incorporation of the DABCO ligand precisely at the
allotted place, stitching the adjacent paddle-wheel units. The
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The idealized paddle-wheel unit represents 22 collinear
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directions (D4h local symmetry), which are mutually orthogonal.
The idealized prototype of the {[Zn2(PO2(OEt))2]}n SBU (Fig. 5a;
fragment in green brackets) features the same 22 mutually
orthogonal pairs, lying not within a single, but within two
parallel planes (D2d local symmetry of a translationally
independent part). A very simple and efficient synergism is
possible because both SBUs ensure mutually orthogonal
orientations of the connecting struts
The two types of 1D SBUs, each possessing four, pairwise
orthogonal, bonding directions run parallel in the structure of 2.
The resulting framework could be interpreted to have a
primitive cubic underlying net, pcu, if the 6-connected
{[Zn2(PO2(OEt))2]2} and extended paddle-wheel (i.e. including
the DABCO-connectivity) moieties are taken as nodes (Fig.
5a,b,d; Fig. S17, ESI†). Other interpretations are possible as
infinite SBUs could be fragmented to units of finite connectivity.
The interpretation above is the most ‘natural’, taking into
account the geometry of the fragments and the reduction of the
framework to a frequent, fundamental and easily recognizable
case. Under the simplest interpretation on the level of 4-c
{Zn2(PO2(OEt))2} and the 6-c paddle-wheel units the respective
net is a two nodal (4-c)2(6-c) net with a {55.6}2{58.64.83} point
symbol (Fig. 5d, Fig. S17g; see ESI† for additional details).62
In comparison, the structure of 1 without the bridge of still
unknown origin (equivalent to DABCO regarding the
connectivity) represents the well-known 4,4-c nbo net on the
level of the 4-c {Zn2(PO2(OEt))2} and 4-c {Zn2(COO)4} paddle-
wheel units (Figure S17f, ESI†).
Fig. 4 The extended asymmetric unit of [Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2 in a thermal
ellipsoid (50%) plot representation (with the exception of the disordered ethoxide group,
which is plotted in ball-and-stick model for clarity). The DMA guest molecule and the
disorder of the DABCO molecule is not given for clarity; see Fig. S16 for the disorder
modelling. Symmetry-related atoms are depicted semi-transparent. Symmetry
transformations: (i) −x+1, −y, z; (ii) −y+1/2, x−1/2, −z+3/2; (iii) y+1/2, −x+1/2, −z+3/2; (iv)
y, −x+1, z; (v) −y+1, x, z; (vi) −x+1, −y+1, z; (vii) −x+1,−y+1, −z+1; (viii) x, y, −z+1; (ix) x, y,
−z+2.
The structure of the framework is fully determined by the
geometries of the two SBUs, connected by linear organic joints
(Fig. 5c). There is a clear similarity between the two 1D SBUs.
Their ‘cruciform’ projections along the main axis are
indistinguishable in idealized high-symmetry representation.
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Fig. 5 The structural organization of 2 and 3: (a) the {Zn2(PO2(OEt))2}n phosphonate-monoester chain-SBU; the 4-c {Zn2(PO2(OEt))2} and the 6-c {[Zn2(PO2(OEt))2]2} topological node
interpretations are explained under the molecular drawing. (b) The {Zn2(COO)4(DABCO)}n extended paddle-wheel chain-SBU sustained by standard paddle-wheel units, joined axially
by the bridging DABCO molecules. (c) The view on the framework along the c-axis (no H-atoms are shown for clarity) with an underlay-image of the ball-and-stick model image (with
H-atoms shown) and a 1 Å grid, limited to the pores. (d) A topological representation of the framework, with the primary interpretation as a 6,6-c net (green and red nodes, pcu).
The alternative connections on the level of the 4,6-c net interpretation (blue and red balls, {55.6}2{58.64.83}) are given in thin yellow dashed lines. Additional structure images for 2
are given in Fig. S17 and S18, ESI†.
The experimental observation of 1, which is not reinforced by The structure organization principle of 2 (Fig. 6) defines the
DABCO, upholds the high resilience of the structural acceptable length of the N-donor ligand with only a minor
organization of 1-3. However, the impossibility to synthesize 1 tolerance. The DABCO molecule fits nearly ideally the interstice
in a large quantity and in a phase-pure form indicates that the of 6.0-7.0 Å (6.3 Å in 1 and 6.7 Å in 2) between the paddle-wheel
use of DABCO is practically essential. Due to the poor quality of units. Theoretically, there is a chance also to incorporate a two-
1, the nature of bridging ligands between the paddle-wheel SBU times longer ligand, like 4,4´-bipyridine (11.2 Å), when every
is not clear. We hypothesize that trace impurities, which also second metal atom in the chain is absent. Such a structure is
might form during the synthesis, could act as bidentate bridging potentially possible, if every second phosphonate will not
ligands, analogously to DABCO. It is potentially enough to have propagate
the
connectivity
in
the
a small amount of such molecules to produce seeds of crystals {[M2(R´O)(R)PO2][M2(R´´O)(RT)PO2]}n 1D chain-SBU. Half of the
with a structural type analogous to 2. The crystal structure of 1 chelating ligands of the latter should be blocking
suggests a site-sharing at the bridging ligand position, with a monofunctional monoalkylphosphonates (with terminal
seemingly dominant contribution by solvent molecules.
substituent designated as RT), or even monofunctional
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carboxylate terminal co-ligands (hence, they are potentially below 3.68 Å probe diameter corresponding to N2 is implied, if
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interesting modulators). The ‘terminated’ half of the rings not given otherwise). The channels would be significantly larger
would not propagate connectivity, which continues for those if not the protruding ethyl groups of the ligand. However, there
fragments only along the 1D phosphonate chain. As a result, a is a residual porosity, with a formal calculated surface area of
pcu topology would ensue again, but with longer edges, 904 - 1057 m2 g–1. The geometrically calculated potential
compared to the underlying net of 2.
solvent-accessible void volume is only 4.8% for N2, but quickly
grows to 7.3% for CO2 (3.30 Å probe), 15.7% for H2O (2.40 Å)
and 36.7% for a formal probe of 1.0 Å diameter (Table S4). The
fast growth of the void volume is due to the narrow and curved
pores, which accepts only a small amount of guest molecules
with sizes approaching the pore diameter. All the data in this
paragraph was calculated using the single crystal structure of 2
with removed solvent molecules and collapsed disorder
[Zn2(EtBCP)2(DABCO)0.5] (Table S4, see also the comments in the
ESI†).
It is important to note for the further discussion that the
structural organization of 2 and 3 allows expecting enhanced
flexibility due to the rotation of the phosphonate and
carboxylate moieties around the axis of the 1D rod-like SBUs.
Such type of flexibility is well known for carboxylate MOFs, e.g.
for MIL-53, where the channels, running along the rod-like SBUs
Fig. 6 A representative fragment of 2 showing the principles of structural organization.
The length scaling for phosphonate ligand is potentially allowed, but not for the N-donor
ligand-bridge.
On the other hand, the arbitrary length-scaling of the are already partially asymmetric (for example having rather a
phosphonate ligand in the structure type of 2 is in principle lozenge than a square shape; the latter case corresponds to a
allowed without any special conditions. The structure would formal state of mechanic equilibrium and is much more
scale in two out of the three directions. The retained scale along resistant to deformation). The (monoethyl)phosphonate differs
the c-axis should provide enough mechanical stability (MOFs from carboxylate by geometry (tetrahedral vs. trigonal). The
with ultra-large pores, like in IRMOF-74-XI representative of the {P(OEt)O2M2} arrangement constituting the chain SBU is out of
MOF-74 isoreticular series, dpore > 80 Å, are build according to the plane of the ligand, while the carboxylate based {CO2M2}, in
similar principles, i.e. geometric scaling in two out of three contrary, remains coplanar, or at least co-axial, with the phenyl
directions). Therefore, the structure type of 2 appears to be of group after coordination). Hence, the mechanical strain,
general importance, as a paradigmatic instance of a potential propagating along the ligand is displaced relatively to the axis of
isoreticular series.
the 1D rod-like SBU and acts as a lever with ~2.0 Å length (for
carboxylates the offset lever length is zero; the difference is
very clear from Fig. 5c), which is a structural level argument for
Thermogravimetric and gas adsorption analyses
Thermogravimetric analysis (TGA) of the as-synthesized increased flexibility.
[Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2 shows a weight loss of 23.9% Compounds 2 and 3 were washed after the synthesis with DMA
up to 150 °C (calc. 21.3% for two DMA molecules per formula of and activated at 160 °C under vacuum. The removal of the DMA
[Zn2(EtBCP)2(DABCO)0.5] · 2DMA
(Fig.
7a)
Chemical guest molecules in 2 or 3 leads to a phase transition and
decomposition of the compound takes place at 320 °C (Fig. 7b). formation of products 2´ or 3´ respectively, characterized by
quite different PXRD patterns. We were not able to elucidate
their structures, but the samples remain at least partially
crystalline. Moreover, the soaking of 2´ or 3´ in DMA reinstates
the initial structures of 2 and 3, as witnessed by PXRD (Fig. 8,
Fig. S20, ESI†). Complete reversibility of the solvent removal is
a supporting argument for retained integrity of the framework
in the degassed structures, as well as for its significant flexibility.
Fig. 7 TGA curves, 20 – 600 °C with a heating rate of 5 K min–1 under N2 atmosphere for
(a) the as-synthesized [Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2 (top curve) and the activated
[Zn2(EtBCP)2(DABCO)0.5], 2´ (bottom curve) (b) for the Co-analogues, 3 and 3´.
The structures of 2 and 3 feature roughly rectangular channels,
running along the c-axes, and forming a 3D pore network via
interconnections along other axes as well. The maximum
included sphere diameter is 5.9 Å, while the maximum passable
sphere diameter is 4.6 Å (as calculated by Zeo++63; here and
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decrease the pores further, rendering the compound to be
View Article Online
DOI: 10.1039/D0CE00275E
non-porous. In contrary, the uptake of the kinetically slightly
smaller CO2 was significant (115 cm3 g–1 for 2´ and 128 cm3 g–1
for 3´ at 1 bar, 195 K; Fig. 9), which allowed to make a crude
estimation of the Langmuir surface area (P/P0 = 0.004–0.04
range, 195 K). The surface area of 2´ and 3´ are estimated to be
330 m2 g–1 and 327 m2 g–1, respectively (see ESI† for details and
also for the even more imprecise BET estimation, which is,
nevertheless provides comparable values).
More interestingly, the CO2 adsorption isotherms at 195 K
demonstrate two distinctive steps. The second step, starting at
P/P0 = ~0.1 could be associated with pore opening from small to
larger pore size.64 It is rather a gradual process with a relatively
narrow ~0.1 P/P0 interval and could be classified as F-I type (the
sudden opening, i.e. a strong rise of adsorption in a small P/P0
is classified as F-II; the cases of 2´ and especially 3´, with a
slightly narrower interval, are approaching the latter type).64
The Gurvich rule was used to determine the pore volume from
the CO2 adsorption isotherms at 195 K (Fig. 9a,b).65 The
micropore volume corresponding to the first step is 0.11 and
0.12 cm3 g–1 for 2´ and 3´ (P/P0 < 0.08 and < 0.12 respectively).
The total values for the combined two steps are ~0.21 and
0.23 cm3 g–1 (maximum estimate, P/P0 = 1.0) for 2´ and 3´
respectively. Therefore, significant framework flexibility,
manifesting itself in a structural transformation, was found at a
low temperature (the flexible behavior was also observed for
carboxylate MOFs with DABCO as a co-ligand).35,49,66,67
Fig. 8 Comparison of the PXRD patterns: the simulated ones for SCXRD structure of 2; of
the as-synthesized bulk 2 and 3; of the activated 2´ and 3´; after the soaking of the
activated samples in DMA.
The gas adsorption behavior of the activated
[Zn2(EtBCP)2(DABCO)0.5], 2´, and [Co2(EtBCP)2(DABCO)0.5], 3´,
materials was studied for N2 (77 K) and Ar (87 K), CO2 (195 K,
273 K, 293 K) and CH4 (273 K, 293 K).
The adsorption of N2 is negligible, which is expected for
compounds with narrow pores (Fig. 9a,b). The maximum
thorough sphere pore radius of 4.6 Å, assessed for the solvated
structure of 2 (Table S4), is only ~1 Å larger than the kinetic
diameter of the N2 molecule. The respective adsorption kinetics
should be very slow, which is aggravated by a low accessible
pore volume, estimated at ~4.8% of the total volume (Table S4).
Furthermore, even slight flexibility of the structure could
The steep rise of the second step (tendency towards “F-II” type)
suggests rather an existence of bistability than a continuity of
states, which correlates well with the observed crystallinity of
the as-synthesized and degassed 2 (and 3), and yet with the
strong differences of the respective PXRD patterns.
(a)
(b)
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DOI: 10.1039/D0CE00275E
(c)
(d)
Fig. 9 Adsorption isotherms (the desorption branches are not shown for clarity in the cases of N2, H2 and Ar, see Fig. S23 for further details) for (a) [Zn2(EtBCP)2(DABCO)0.5], 2´; (b)
[Co2(EtBCP)2(DABCO)0.5], 3´; (c) 2´; (d) for 3´.
The hysteresis observed on the CO2 adsorption isotherms for 2 adsorption conditions (P, T) are approaching the supercritical
and 3 at low pressures is kinetically conditioned. It was checked state of CO2 and hence the use of the Gurvich rule for the
on the example of 3 that as the equilibration time increases, determination of the pore volume is highly questionable. Still,
the hysteresis width decreases. For the longest adsorption we have used it as an estimate: at 298 K the pore volume at the
experiment (~5+ times longer equilibration time per acquisition first step was calculated to be 0.11 cm3 g–1 for 2´ and 0.15 cm3 g–
point) the adsorption and desorption branches for the first step 1 for 3´ (12 bar or P/P0 = 0.19 in both cases) (Fig. S21, ESI†). It is
nearly coincide, while the remaining hysteresis is nearly solely presumed that after the pores are filled to a certain extent, not
limited to the second step (see Fig. S28, Table S5 and short very different for the low (195 K) or high temperatures (298 K),
corresponding discussion in the ESI†). Hence, the desorption a structural transformation occurs.
branch cannot be associated with a “metastable opened form”: The notable affinity to CO2 might partially explain another, not
longer equilibration times demonstrate full reversibility at least quite clear observation, namely the negligible adsorption of Ar,
at P/P0 < 0.1 pressures.
which is similar to N2. It is possible, that the increased
The heat of adsorption for CO2 from adsorption isotherms at adsorbent-adsorbate affinity is necessary for opening up the
273-293 K are calculated to be 33.0 and 21.0 kJ mol–1 for 2´ and initially closed pores of the flexible framework.
3´ at zero coverage (Fig. S22). The energy difference is
significantly higher than expected, and we believe that the
lower crystallinity of 3 should at least partially be held
responsible.
The sorption data for CO2 at 195 K, 273 K and 293 K are
summarized inTable 1.
Table 1 CO2 adsorption data for [Zn2(EtBCP)2(DABCO)0.5], 2´ and Co2(EtBCP)2(DABCO)0.5],
3´ at 1 bar.
Data
2´
3´
Temperature, K
195
273
32
293
20
195
128
273
33
293
23
Fig. 10 High-pressure CO2 adsorption isotherms at 298 K for 2´ (green) and 3´ (blue).
Quantity adsorbed, 115
cm3 g–1 (mmol g–1)
(5.1) (1.4) (0.9) (5.7) (1.5) (1.0)
It is interesting to compare the experimentally derived
micropore volumes at the transformation points with values
calculated for the structures of 2/3 (CO2 adsorption at 195 K).
The values are respectively 0.11/0.12 cm3 g–1 (P/P0 = ~0.1,
195K), 0.11/0.15 and cm3 g–1 (P/P0 = ~0.19, 195 K) 3´ vs. 0.065
cm3 g–1 calculated geometrically (Table S4, ESI†). The
transformations take place at an adsorption value, which
corresponds reasonably well to the calculated maximum
accessible pore volume for the structure of 2 with removed
solvent molecules (0.065 cm3 g–1). This observation supports
the viewpoint that the transformation is stimulated by
complete pore filling of the less porous variant of the structure.
CO2 adsorption at 298 K and 20 bar shows a larger uptake for 3´
with 147 mg g–1 compared to 2´ with 97 mg g–1 (Fig. 10). Both
isotherms demonstrate inflection points at approximately
12 bar indicating possible gradual structural transformations.
The dissimilarity of the transformation energetics is the only
evident explanation of such a strong difference. The pore-sizes
for 2´, 3´ are similar at low pressures at room temperature, but
start to diverge at elevated pressures (note that minor, but
evident dissimilarities between 2´ and 3´ are visible at low
pressures and low temperatures, as discussed above). The
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The low precision of the estimated value should not be {M2(PO2(OEt))2}n
ARTICLE
observed in
chains
already
View Article Online
surprising: the calculated accessible volume in the structure of phosphonate-monoester coordination DOpIo: 1ly0m.10e3r9s/D0aCnEd0027o5nE
2 quickly grows with the decrease of the size of the probe and DABCO-expanded archetypal paddle-wheel carboxylate units. A
reaches 37% for a probe of ~1.0 Å. While such probe size is potentially very important observation for structure-design
much smaller than a molecular size, the steep growth of the heuristics is the nearly equal length of the six-connected
void volume shows that there is potentially available space, {[Zn2(PO2(OEt))2]2}-phosphonate-monoester-
and
the
which could, possibly, be partially accessed upon minor {Zn2(COO)4(DABCO)} SBU-fragments (in this work only the
structural deformations. This is what, evidently, takes place.
simplest, pcu framework is demonstrated, but due to the
equivalence in dimensions a number of other framework
topologies could also be expected). The structural organization
shows high resilience, as the ‘parent’ compound, the
Moisture stability, water vapor adsorption, and liquid solvent
stability tests
Both the non-degassed 2 and 3, as well as the non-degassed 2´ [Zn2(EtBCP)2(Bridge/Solv)] · (Solv´), 1, was also observed as a
and 3´ demonstrated only a minor change of PXRD patterns minor product in a DABCO-free synthesis; the structural motif
upon two days of exposure to ambient air (Fig. S29). The for 1, similar to 2 and 3 was experimentally confirmed.
satisfactory stability allowed to measure the water-vapour Importantly, the compounds 2 and 3 represent a structural
adsorption at 293 K in the range of 0 – 0.95 P/P0 for the single type,
which
permits
the
elongation
of
the
case of 3´, representative enough for the case of isostructural monoalkylphosphonate ligand, leading to metrical scaling in
compounds. Relatively low affinity (IUPAC isotherm type III) and two directions in isoreticular analogues (see Fig. S19). Such
a maximum uptake of 48 mg g-1 was observed (Fig. S30), scaling is preferable, comparing to the scaling in all directions,
however the very well-defined hysteresis indicates that the for structural stability and avoiding interpenetration. The
binding of the adsorbed water associated with the second activated compounds 2´ and 3´ show two-step CO2 adsorption
coordination sphere of the metal ions is substantial. The at low-pressure (195 K) and an analogous behavior with an
relatively low affinity is in line with the hydrophobic ‘padding’ inflection point on the isotherm at high pressure (298 K). The
of the pores by the organic ligands and not the least by the ethyl effects could be explained by structural flexibility, which is
substituent of the monoethylphosphonate, protruding into the supported by the analysis of the structure (the ‘lever’ effect of
channels. The small maximum uptake correlates well with the the trigonal phosphonate, in contrary to trigonal-planar
low accessible pore volume. The crystallinity of the sample did carboxylate, remaining in-plane with the 1D rod-like SBU as well
not change significantly during the experiment as witnessed by as the particularly steep calculated growth of accessible volume
PXRD.
in the structure by decrease of the probe size, indicating the
The long-term solvent exchange of 2 with chloroform, ethanol, capacities for structural rearrangement).
acetone (~3 days each), and water (~1 day) was called to check
the stability of the framework against liquid solvents (no prior
degassing was carried out; the degassing itself is responsible for
Experimental section
some loss of crystallinity, associated with PXRD peak
broadening, which could mask finer effects, caused by the
action of moisture). After PXRD measurments, the samples
were exchanged again by DMA, in order to test the possibility
to return to the state of crystallinity of the as-synthesized 2 (Fig.
S31). The exchange with chloroform caused minor changes,
with ethanol moderate, and with acetone a transformation to
another phase. However, the subsequent exchange with DMA
reinstated the initial crystallinity in full, providing another
argument for flexibility of the framework of 2, this time upon
the action of solvent molecules (see ESI†). On the other hand,
water causes an irreversible transformation to a phase with an
appreciable crystallinity, and hence 2 should be regarded as
instable to the action of liquid water for prolonged time. It is
worth to stress that this instability is rather not an irreversible
hydrolysis into an amorphous phase as in the majority of other
MOFs, but a crystal-to-crystal transformation.
Materials and measurements
All the chemicals (purity equivalent to ACS grade or higher)
were obtained from commercial sources and used without
future purification.
The CHN elemental analysis was performed using Pekin
Elementar Vario EL III analyzer. IR-spectra were recorded on a
Bruker Tensor 37 IR spectrometer (Bruker Optics, Germany)
equipped with an ATR unit. ESI-MS spectra were recorded with
a Thermo-Quest Ion Trap API mass spectrometer Finnigan LCQ
Deca. Thermogravimetric analysis (TGA) was performed with a
Netzsch TG 209 F3 Tarsus using an aluminium crucible in the
20-600 °C range at 5 K min–1 rate under N2 atmosphere. The
powder X-ray diffraction patterns (PXRDs) were obtained on a
Bruker D2 Phaser diffractometer using Cu-Kα radiation (λ =
1.5418 Å; 30 kV, 10 mA source supply parameters) and a flat
low-background silicon sample holder.
The low pressure gas adsorption isotherms for CO2, CH4 and H2
were measured using a Micromeritics ASAP 2020 automatic gas
sorption The low pressure gas adsorption isotherms for CO2,
CH4 and H2 were measured using a Micromeritics ASAP 2020
automatic gas sorption analyzer equipped with one vacuum
pump for the sample preparation and one for the analysis
(~ 10– 6 Torr ultimate vacuum in the manifold).
Conclusions
Two new isostructural zinc and cobalt monoethylphosphonate-
carboxylate MOFs, [Zn2(EtBCP)2(DABCO)0.5] · 2DMA,
2 and
[Co2(EtBCP)2(DABCO)0.5] · 2DMA, including DABCO as
3
a
co-ligand were synthesized. The structures are built upon the
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The samples were degassed at the preparation port of the The procedure was adapted from Jaffrès et al.74 A stirred
View Article Online
analyzer until the pressure rising rate in the temporarily closed mixture of NiBr2 (0.87 g; 4 mmol), methylD4O-Ii:o1d0o.1b03e9n/zDo0aCtEe00(12375.0E
manifold was less than 2 μTorr min–1 at 160 °C. After degassing g, 50 mmol) and mesitylene (14 mL) were heated to 180 °C
the sample tube was weighted (seal frits were used for under nitrogen atmosphere. Triethylphosphite (12.8 mL,
permanent sealing) and transferred to the analysis port of the 75 mmol) was added dropwise for 4 h. During the addition of
sorption analyzer. Before the sorption experiments, the triethylphosphite, a color change of the solution from red to
compounds were subjected to a short, repeated degassing to violet and finally to yellow was observed. After the addition, the
remove remaining gas molecules. Helium gas was used for the solution was heated further at 180 °C for 2 h yielding a yellow
determination of the warm and cold free-space of the sample suspension. Then, the solvent was removed in vacuo. Water
tubes. The volumes of the adsorbed gases are given at standard (100 mL) was added to the residue and the mixture extracted
temperature and pressure, STP (293.15 K, 101.325 kPa). The with diethyl ether (250 mL). The organic phase was washed
adsorption isotherms were measured for CO2 at 195, 273, with brine (50 mL) and dried over MgSO4. The reaction mixture
293 K; for CH4 at 273, 293 K and for H2 at 77 K. The isosteric heat was concentrated in vacuo to produce an orange viscous oil that
of adsorption values for CO2 from the isotherms at 273.15 K and was purified by silica gel column chromatography using
293.0 K were calculated using Clausius-Clapeyron equation as (DCM/EtOH: 98/2) as eluent to give a colorless viscous oil of
implemented in the ASAP 2020 v3.05 software. The N2 and Ar methyl 4-(diethoxyphosphoryl)benzoate (7.1 g, 52%); 31P NMR
1
sorption isotherms were additionally measured on
a
(121 MHz, CDCl3): δ = 17.68; H NMR (300 MHz, DMSO-d6): δ
Quantachrome Autosorb iQ MP automatic gas sorption 8.09 (dd, 3J = 8.5, 4J =3.8 Hz, 2H), 7.86 (dd, 3J = 12.8, 3J = 8.5 Hz,
instrument, using parameters, similar to described for the 2H), 4.14 - 3.97 (m, 4H), 3.88 (s, 3H), 1.23 (t, J = 7.2 Hz, 6H);
measurement with the ASAP 2020. All used gases (He, N2, H2, 13C NMR (75 MHz, DMSO-d6) δ 165.5, 133.4, 132.9, 131.7,
CO2, Ar) were of ultra-high purity (UHP, grade 5.0, 99.999%).
The CO2 gas adsorption isotherms for 0–20 bar pressures at
298 K were measured using Rubotherm IsoSorb Static
instrument, which employs a gravimetric principle. The weight
129.2, 62.0, 52.5.
O-ethyl-P-(4-carboxyphenyl)phosphonic acid or
4-(ethoxyhydroxyphosphinyl)benzoic acid (H2EtBCP)
change was measured with a magnetic suspension balance The procedure is adapted from Lee et al.45 Methyl-
(resolution 0.01 ± 0.03 mg). 4-(diethoxyphosphoryl)benzoate (1.5 g, 5.5 mmol) was heated
The water vapor sorption isotherm was measured on a in a mixture of aqueous NaOH (2 mol/L, 33 mL) and EtOH (33
Quantachrome VSTAR vapor sorption analyser at 293 K. mL) at 80 °C during 3 h. The reaction mixture was then
The single crystal X-ray diffraction data were collected on a concentrated in vacuo to a third of the volume and acidified
Bruker Kappa APEX II DUO equipped with an APEX II CCD area with cooled 1N HCl to pH 1, followed by extraction with
detector, using Mo-Kα radiation (λ = 0.71073 Å) generated by a dichloromethane. The combined organic extracts were dried
microfocus sealed tube source. The data collection was over Na2SO4 and the solvent was removed under reduced
performed by ω scans and controlled by APEX2 software.68 The pressure, to afford the product as a white solid (0.71g, 56%); 31
P
data reduction was performed using SAINT68 and the NMR (121 MHz, DMSO-d6): δ = 14.61; 1H NMR (300 MHz,
experimental absorption correction using SADABS69. The DMSO-d6) δ 8.04 (dd, J = 8.4, J = 3.6 Hz, 2H), 7.81 (dd, J = 12.6, J
structure was solved by direct methods (SHELXS-2016), the = 8.4 Hz, 2H), 3.96 – 3.86 (m, 2H), 1.18 (t, J = 7.1 Hz, 3H);
refinement was done by full-matrix least squares on F2 using the 13C NMR (75 MHz, DMSO-d6) δ 166.79, 135.9, 133.5, 131.3,
SHELXL-2018 program suite,70 and the graphical user interface 129.2, 61.0, 16.3.
(GUI) ShelXle.71 The molecular graphics were prepared using
Diamond.72
MOF syntheses
Synthesis of [Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2;
[Co2(EtBCP)2(DABCO)0.5] · 2DMA, 3
Ligand synthesis
Methyl 4-iodobenzoate
The compounds 2 and 3 were synthesized according to the
The procedure was adapted from Zhao et al.73 Concentrated same procedure (for the synthesis of [Zn2(EtBCP)2(Solv)] · Solv´,
H2SO4 (5 mL) was carefully added to a solution of 4-iodobenzoic 1, see ESI†). Zn2(NO3)2 · 6H2O (26.77 mg, 0.09mmol) or
acid (10 g, 0.04 mol) in MeOH (200 mL). The mixture was Co2(NO3)2·6H2O (26.2 mg, 0.09 mmol), DABCO (5 mg,
refluxed for 14 h. After cooling to room temperature, the 0.045 mmol) and H2EtBCP (20 mg, 0.09 mmol) were dissolved in
solvent was removed in vacuo and the residue was dissolved in DMA (2 mL). The mixture was transferred in a thick-walled
dichloromethane (100 mL). The organic layer was washed with borosilicate glass tube, sealed with a screw-cap and heated up
saturated NaHCO3 solution (100 mL), brine (100 mL) and dried to 100 °C for 8 h. The temperature was held for 94 h, and then
over MgSO4. Methyl 4-iodobenzoate was obtained as a white decreased linearly to room temperature within 4 h. 34.6 mg
solid after removal of the solvent under reduced pressure (9.6 (60%) of 2 consisting of considerably large colorless single
g, 91%); 1H NMR (300 MHz, CDCl3): δ 7.80 (d, J = 8.6 Hz, 2H), 7.73 crystals and 37.2 mg (66%) of 3 consisting of very small dark-
(d, J = 8.6 Hz, 2H), 3.91 (s, 3H); 13C NMR (75 MHz, blue single crystals, too small for a routine SCXRD experiment,
CDCl3): δ= 166.7, 137.9, 131.2, 129.7, 100.9, 52.4.
were collected. The syntheses were reproduced six times.
Methyl 4-(diethoxyphosphoryl)benzoate
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Elemental
ARTICLE
Table 2 Crystal data and refinement details for [Zn2(EtBCP)2(DABCO)0.5] · 2DMA, 2 a.
2
analysis
[%]
calc.
for:
View Article Online
DOI: 10.1039/D0CE00275E
[Zn2(EtBCP)2(DABCO)0.5] · 2DMA, C29H42N3O12P2Zn2: C 42.61, H
5.18, N 5.14, found: C 42.3, H 5.17, N 5.01. IR (KBr) ṽmax [cm–1]:
3440, 2982, 2937, 1644, 1556, 1414, 1190, 1156, 1129, 1090,
1046, 1014, 965, 868, 848, 807, 776, 732, 703, 520, 484.
2´ (degassed 2). Elemental analysis [%] calc. for:
[Zn2(EtBCP)2(DABCO)0.5], C21H24NO10P2Zn2: C 39.22, H 3.76, N
2.18, found: C 38.48, H 3.68, N 2.14. IR (KBr) ṽmax [cm–1]: 3436,
3068, 2981, 2934, 2902, 1636, 1553, 1503, 1419, 1157, 1128,
1079, 1042, 948, 863, 771, 729, 702, 608, 476.
Empirical formula
M/g mol−1
Crystal dimensions /mm
T /K
C29H42N3O12P2Zn2
817.33
0.20 × 0.03 × 0.03
293
Tetragonal
I4/m
Crystal system
Space group
a, b, c /Å
19.912 (4), 19.912 (4), 9.5338 (17)
V /ų
3779.9 (16)
4
1.436
Z
/g cm−3
3
Elemental
analysis
[%]
calc.
for:
µ (Mo Kα) /mm−1
F(000)
1.41
1692
0.950, 0.958
0.81, −0.50
23360, 1947, 1145
[Co2(EtBCP)2(DABCO)0.5] · 2DMA, C29H42N3O12P2Co2: C 43.30, H
5.26, N 5.22, found: C 43.07, H 2.27, N 5.11. IR (KBr) ṽmax [cm–1]:
3435, 2981, 2936, 2902, 1649, 1553, 1450, 1413, 1264, 1189,
1155, 1129, 1088, 1046, 1014, 867, 846, 806, 505, 775, 732, 703,
591, 491.
3´ EA [%] calc. for: [Co2(EtBCP)2(DABCO)0.5], C21H24NO10P2Co2: C
40.02, H 3.84, N 2.22, found: C 39.28, H 3.91, N 2.15. IR (KBr)
ṽmax [cm–1]: 3433, 3065, 2980, 2935, 2900, 1620, 1551, 1503,
1413, 1287, 1157, 1128, 1076, 1043, 948, 863, 771, 729, 704,
608, 482.
Tmin, Tmax
Δρmax, Δρmin / e Å−3
No. of measured,
independent and
observed [I > 2σ(I)]
reflections
Rint
0.094
0.075, 0.223, 0.98
R[F2 > 2σ(F2)], wR(F2),b S
a
The CCDC reference number is 1985571. The crystallographic data in CIF
format can be downloaded free of charge at
Structure refinement details
https://www.ccdc.cam.ac.uk/structures/.
The DABCO molecule is located on the 4-fold axis special
position. The trigonal molecule has a lower symmetry than the
symmetry of the special position. The disorder was modeled
using a four-component model with equal weights. The ethyl
group of the ligand was refined using a two-component
disorder model with equally populated components related by
a mirror plane. The disordered DMA molecule was modeled
similarly. All atoms in the structure except the DMA guest
molecule were refined anisotropically. A part of the C-O, C-N, C-
C, and N-O bond lengths in the OEt moiety and the guest solvent
molecule were restrained. The hydrogen atoms of DABCO, DMA
and the ethyl group of the ligand were positioned geometrically
and refined using a riding model: d(C-H) = 0.96 Å (CH2), 0.97 Å
(CH3), 0.95 Å (CArH); Uiso(H) = 1.2*Ueq. The crystal data and
structure refinement details are given in Table 2.
b
w = 1/[σ2(Fo2) + (0.1387*P)2] where P = (Fo2 + 2Fc2)/3. Goodness-of-fit S =
[ [w(Fo2–Fc2)2] /(n–p)]1/2
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank one of our anonymous referee, who posed
interesting questions regarding the role of the in-situ formed
formic acid, as well as the hysteresis and kinetic effects for the
CO2 adsorption, which allowed to improve the manuscript. We
thank Dr. Gamall Makhloufi, then a doctoral student in the
group of CJ, for collecting preliminary data for the
“predecessor” structure of 1.
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