A structurally flexible triazolate-based metal-organic framework featuring coordinatively unsaturated copper(i) sites

Article abstract of DOI:10.1039/c6dt02672a

The novel metal-organic framework CFA-8 (Coordination Framework Augsburg University-8), [Cu₂(tqpt)], containing the organic linker H₂-tqpt {H₂-tqpt = 6,6,14,14-tetramethyl-6,14-dihydroquinoxalino[2,3-b]phenazinebis-triazol

Full text of DOI:10.1039/c6dt02672a

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A structurally flexible triazolate-based metal–
organic framework featuring coordinatively
Cite this: DOI: 10.1039/c6dt02672a
unsaturated copper(I) sites†
P. Schmieder,a D. Denysenko,a M. Grzywa,a O. Magdysyukb and D. Volkmer*a
The novel metal–organic framework CFA-8 (Coordination Framework Augsburg University-8), [Cu2(tqpt)],
containing the organic linker H2-tqpt {H2-tqpt = 6,6,14,14-tetramethyl-6,14-dihydroquinoxalino[2,3-b]
phenazinebis-triazole}, has been synthesized. Reaction of H2-tqpt and anhydrous CuCl2 in N,N-dimethyl-
acetamide (DMA) yields CFA-8 as orange crystals with lenticular shape. This framework shows a reversible
breathing effect and is robust upon solvent removal. It has been characterized by single-crystal and
powder X-ray diffraction, TGA, IR spectroscopy and gas sorption measurements. CO adsorption isotherms
show that Cu(I) sites in this framework are able to bind carbon monoxide forming a weak complex which
has been additionally characterized by IR spectroscopy and synchrotron powder X-ray diffraction
measurements.
Received 6th July 2016,
Accepted 1st August 2016
DOI: 10.1039/c6dt02672a
www.rsc.org/dalton
Introduction
With their patent in 1947 Procter and Gamble Ltd reported on
the corrosion inhibiting effect of 1H-benzo[d][1,2,3]triazole
(btaH) for copper metal.1 The same molecule was later men-
tioned in some additional patents describing corrosion inhibi-
tors.2,3 Driven by the economic impact of these inhibitors
various coordination modes between copper and 1,2,3-benzo-
triazoles have been proposed.4–13 Cotton and co-workers pub-
lished the first Cu(I)-bta structure and stated more decisively
that this structure contains Cu(I) ions.4 This coordination
mode and the monovalent oxidation state of copper was later
confirmed by Poling, but no crystal structure has been
reported for a Cu(I) complex derived from 1H-benzo[d][1,2,3]tri-
azole.13,14 The first crystal structure of a Cu(I) polymer incor-
porating the deprotonated ligand of 1H-benzo[d][1,2,3]triazole
was reported by Huang and co-workers.13 The connection
scheme of this 1D coordination polymer with the formula
[Cu2(bta)2] is shown in Fig. 1. The crystal structure is much
more complicated than the original model proposed by Cotton
et al., which suggested a linear polymeric structure. The asym-
Fig. 1 Connection scheme of the literature-known Cu(I) complex
[Cu2(bta)2].13
metric unit of this structure contains three differently co-
ordinated Cu(I) ions. One of the CuI ions is trigonally-planar
coordinated by nitrogen donor atoms of the triazolate ring, the
second CuI ion is two-fold linearly coordinated and the third
copper ion is tetrahedrally coordinated. The most interesting
feature of this compound is the presence of three different
coordination modes of the copper(I) ions. With a more focused
look at the structure it could be assumed that the two- and
three-fold coordinated Cu(I) ions should be able to bind
additional ligands by keeping their original connection
scheme, which would however require to engineer the dense
Cu(I)(bta) 1-D coordination polymer into a porous 3D frame-
work structure.
aAugsburg University, Institute of Physics, Chair of Solid State and Materials
Chemistry, Universitaetsstrasse 1, 86159 Augsburg, Germany.
E-mail: dirk.volkmer@physik.uni-augsburg.de
bDiamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot,
Oxfordshire, OX11 0DE, UK
To prove this assumption we have invented a novel copper(I)
bis-triazolate metal–organic framework (MOF) [Cu2(tqpt)]
(tqpt2− = 6,6,14,14-tetramethyl-6,14-dihydroquin-oxalino-[2,3-b]
phenazinebistriazolate), featuring the same Cu-azolate coordi-
†Electronic supplementary information (ESI) available: IR spectra, details of gas
sorption measurements, UV/vis spectrum and crystallographic data. CCDC
1483639–1483641. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c6dt02672a
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nation modes, which we refer to as CFA-8 (Coordination faces of the crystals stay intact, even after activation at 250 °C
Framework Augsburg University-8). This MOF represents a in vacuum. Different microscope pictures show that CFA-8
further member of Cu(I)-containing metal–organic frameworks forms crystals in the size range of 10–150 μm.
comprising open CuI metal sites, which might be amenable to
binding of additional ligands.15–17 To the best of our knowledge,
Crystal structure analysis
the crystal structure of the framework introduced here is the first
example for a crystalline and permanently porous Cu(I) network
incorporating a bistriazolate as organic bridging ligand.
A single crystal X-ray diffraction study reveals that CFA-8 crys-
tallizes in the monoclinic crystal system in the space group C2/c
(no. 15), which coincides with the crystal structure described
for Cu(I)(bta) which also crystallizes in the monoclinic crystal
system, in the same space group.13 The asymmetric unit of
CFA-8 consists of three copper, twenty-two carbon, sixteen
hydrogen and ten nitrogen atoms (Fig. 3).
Results and discussion
Syntheses and characterization
The three crystallographically independent copper ions
have different coordination geometries. The Cu3 atom (see
asymmetric unit Fig. 3) located on a site with crystallographi-
cally imposed twofold symmetry is linearly two-fold co-
ordinated by two nitrogen atoms (N10, N10a) of different
tqpt2− ligands. The distances between Cu3 and N10, N10a are
1.874(3) Å and the angle N10–Cu3–N10a is with 178.45(18)°
nearly straight (Fig. 4). The trigonally planar coordinated Cu1,
located on a general position, binds to three nitrogen atoms
(N1, N3 and N9). The Cu1–N1/3/9 bond lengths range from
1.931(3)–1.995(3) Å and the N–Cu1–N angles fall in between
The organic ligand H2-tqpt was synthesized in three steps.
2,2,5,5-Tetramethylcyclohexane-1,3-dione and 3,3,6,6-tetra-
methylcyclohexane-1,2,4,5-tetraone were synthesized by con-
ventional procedures.18 The condensation reaction of the latter
tetraone and two molecules of 1H-benzotriazole-5,6-
diamine19–21 leads to the organic bistriazolate ligand H2-tqpt,
which was previously used for the synthesis of the
CFA-7 metal–organic framework (Scheme 1).22
Metal–organic framework CFA-8 can be synthesized from
H2-tqpt and anhydrous CuCl2 in a good yield by using N,N-di-
methylacetamide (DMA) as a solvent at 140 °C (Scheme 2).
During the reaction, Cu(II) ions are reduced by DMA to Cu(I).
The network forms orange lenticular crystals (Fig. 2). Inspec-
tion of electron microscopic images reveals that the crystal
Fig. 3 Ball-and-stick model of the asymmetric unit of CFA-8 (grey: C-,
blue: N-, cyan: Cu-atoms, H-atoms are omitted for clarity; atoms dis-
played as 50% probability ellipsoids).
Scheme 1 Synthesis of the organic bistriazolate ligand H2-tqpt.
Scheme 2 Synthesis conditions to obtain CFA-8.
Fig. 4 Different coordination environments of Cu(I) ions in CFA-8
(Cu1a (−1 + x, −1 + y, −1 + z); N1a (−1 + x, −1 + y, −1 + z); N2a (1 − x, −1
+ y, 0.5 − z); N3a (−1 + x, −y, −0.5 + z); N8a (−0.5 + x, −0.5 + y, z); N9a
(−0.5 + x, −0.5 + y, −1 + z); N10a (−0.5 + x, −0.5 + y, −1 + z)).
Fig. 2 Optical microscopy image of as-synthesized CFA-8 crystals (left)
and SEM image of activated CFA-8 (right).
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113.28(11)° and 131.78(12)°. The third coordination geometry
of Cu(I) ions, located on a site with crystallographically
imposed twofold symmetry, is represented by the tetrahedrally
coordinated Cu2. The Cu2 ion is bound to four different nitro-
gen atoms (N2, N2a and N8, N8a). The bond length values are
2.008(3) Å for Cu2–N2/2a and 2.027(3) Å for Cu2–N8/8a. The
N–Cu2–N angles range from 107.21(11)–116.84(15)°. These
values are in very good agreement with those reported in
similar literature-known structures (Table 1) featuring Cu(I)
triazolate coordination units.13,23
The analysis of the data from single crystal measurement
reveals for the metal–organic framework CFA-8 a chemical
composition of [Cu2(tqpt)]. A schematic representation of the as-
synthesized and solvent-filled MOF, which we refer to as large-
pore phase CFA-8 network, is presented in Fig. 5. After activation
at 250 °C in vacuum all solvent molecules are evaporated out of
the pores. The monoclinic network exhibits channel-like pores
along the c-axis, which are illustrated as big yellow rods.
The maximum pore opening diameter of the channels cal-
culated with the Poreblazer software24 is around 7.8 Å (Fig. 6,
left). The plot on the right side in Fig. 6 along the b-axis shows
clearly that there are no other kinds of pores or channels
running in different crystallographic directions in the 3D
framework. The cyan dashed field illustrates the copper–nitro-
gen-strands which also run along the c-direction. The channels
in the as-synthesized large-pore-phase of CFA-8 are occupied
by disordered solvent molecules (DMA), the positions of which
were impossible to resolve and refine from the electron density
distribution. According to the crystallographic data there is an
electron count of 814 per unit cell (rest of the electron
density), which correspond to approximately 17 N,N-dimethyl-
acetamide molecules in the unit cell of CFA-8. Calculations
with the SQUEEZE program25 reveals that the initial solvent
Fig. 5 Schematic representation of the large-pore phase CFA-8
network showing the open channels in the crystal structure running
along the c-axis (red tetrahedra: fourfold coordinated Cu2 ions, green
triangles: trigonal planar coordinated Cu1 ions).
Fig. 6 Different plots of the large-pore phase of the copper MOF;
along c-axis (left) and tilted by 90° along the a-axis (right).
accessible void volume is 3358.4 Å3, which is about 47% of the
unit cell volume (7141.0 Å3).
Table 1 Selected crystallographic and geometric parameters for the 1D
Cu-coordination polymer [Cu(bta)] and the 3D CFA-8 [Cu2(tqpt)]
As mentioned before, CFA-8 exhibits permanent porosity.
In order to remove all solvent molecules, which are localized
in the channels, the as-synthesized sample was activated at
250 °C for at least twelve hours in high-vacuum. Powder XRD
measurements show that the unit cell of CFA-8 is changed
upon this activation. In the following the heated and solvent-
free CFA-8 network is referred to as activated phase of CFA-8.
The most obvious difference is the disappearance of the most
intensive peak of the XRD pattern around 4.9° in 2θ and the
formation of a new peak at 5.9° in 2θ (Fig. 7). The framework
remains crystalline after activation but has a smaller unit cell.
This process is fully reversible. Thus, treatment of a completely
activated sample of CFA-8 with fresh DMA for several hours
results in the regeneration of the original as-synthesized phase
(= large-pore phase of CFA-8), as shown by XRPD measure-
ments (Fig. 7). Such breathing effects have been repeatedly
reported for different MOF structures.26–29
Coordination polymer, [Cu(bta)]
CFA-8, [Cu2(tqpt)]
Wyckoff
position
Site
symmetry
Wyckoff
position
Site
symmetry
Cu1 8f
Cu2 4e
Cu3 4e
1
2
2
Cu1 8f
Cu2 4e
Cu3 4e
1
2
2
Selected geometric parameters
Cu1–N3
Cu1–N1i
Cu1–N5
Cu2–N2
Cu2–N2ii
Cu2–N6
Cu2–N6ii
Cu3–N4
Cu3–N4iii
1.942(3)
1.949(3)
2.003(3)
2.032(3)
2.032(3)
2.032(3)
2.032(3)
1.865(3)
1.865(3)
Cu1–N1
1.931(3)
Cu1–N3iv
Cu1–N9v
Cu2–N2
1.935(3)
1.995(3)
2.008(3)
2.027(3)
2.008(3)
2.027(3)
1.874(3)
1.874(3)
Cu2–N8v
Cu2–N2avi
Cu2–N8avii
Cu3–N10
Cu3–N10aviii
Symmetry codes: (i) x, −y, z −1/2; (ii) −x, y, −z + 1/2; (iii) –x, y, −z − 1/2,
(iv) x, −y + 1, z + 1/2; (v) x + 1/2, y + 1/2, z; (VI) #3 −x + 2, y, −z + 1/2;
(vii) −x + 3/2, y + 1/2, −z + 1/2; (viii) −x + 1, y, −z + 3/2.
Unfortunately, many attempts to collect a single crystal diffrac-
tion pattern of a completely dry single-crystal of CFA-8 were
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phase) single crystal are a = 27.49 Å, b = 23.66 Å, c = 11.00 Å, β
= 93.75° with the cell volume of 7141 Å3. In contrast to this
solution, the second crystal (narrow-pore phase) shows lattice
parameters of a = 32.04 Å, b = 16.79 Å, c = 10.77 Å, α, γ = 90°
and β = 92.36° with the unit cell volume of 5789 Å3. More
details of the narrow-pore phase are presented in the ESI
(Table S4; Fig. S9†).
Thermal analysis and VTXRPD studies
Structural and thermal stability of CFA-8 were investigated by
thermogravimetric analysis (TGA) and variable temperature
X-ray powder diffraction (VTXRPD) measurements. In the TGA
curve of a pre-dried CFA-8 sample (250 °C/vacuum/10 h) no
weight loss is detectable until a temperature of 300 °C is
reached (Fig. 8). The framework collapses under dry air (red
curve) at around 300 °C, because an exothermal reaction takes
place and after decomposition of the organic ligand only
copper(II) oxide is left. Under inert nitrogen atmosphere CFA-8
is stable up to 400 °C (blue curve). Above 400 °C CFA-8 is not
stable and the decomposition of the framework starts. These
results from TG analysis are in very good agreement with
VTXRPD measurements (Fig. 9), which were performed in a
quartz capillary under inert atmosphere. According to the
latter, the CFA-8 network is stable up to 350 °C. In the diffrac-
togram recorded at 400 °C, a new phase with peaks at 46° and
51° in 2θ could be detected, which corresponds to elemental
copper (PDF no. 4-836). No peaks of the CFA-8 phase can be
seen in the XRPD pattern at a temperature greater than 450 °C.
Fig. 7 Comparison of diverse XRPD patterns of CFA-8; the activated
sample (neon-green XRD graph) was treated at 250 °C in vacuum for
12 hours.
unsuccessful due to the fact that the crystals scattered very badly.
A synchrotron measurement of an activated powder sample of the
Cu-MOF made it possible to index the diffractogram and to deter-
mine at least the cell parameters of the dried phase of CFA-8.
These dimensions of the activated unit cell are a = 28.055(4) Å,
b = 19.212(1) Å, c = 9.996(1) Å, α, γ = 90.00°, β = 96.80(1)° and the
cell volume amounts to V = 5350.0(5) Å3. The proposed space
group is P21/c, which represents a subgroup of the space group
C2/c for the large-pore phase. A Le Bail fit of the XRPD pattern of
the activated CFA-8 phase is presented in Fig. S8.†
To get more details about the observed differences in the
powder X-ray diffractograms (Fig. 7), a second post-syntheti-
cally modified single crystal was measured. These crystals were
obtained by an experiment in order to check, if the phos-
phorus atom of trimethyl phosphite coordinates to the Cu1
atom as observed for CO gas molecules (see discussion below).
Therefore a sample of completely activated CFA-8 crystals was
treated with a diluted solution of trimethyl phosphite. After-
wards the modified CFA-8 crystals were dried under mild con-
ditions (vacuum/60 °C/4 h) and the single crystal diffraction
data were measured. In this case, the interactions between the
phosphorus atom of the remaining trimethyl phosphite mole-
cules and the Cu1 atom of the framework are not strong
enough to coordinate to each other. The observed distance
between the phosphorus atom and Cu1 is 2.8 Å. The distance
for similar compounds in the literature are between
2.122–2.161 Å.30–32 The single crystal structure of this crystal
phase, containing trimethyl phosphite guest molecules inside
the channels of the porous framework structure, will from now
on be referred to as “narrow-pore phase” of CFA-8. The refine-
ment of the data of the narrow-pore phase of CFA-8 leads to
the same crystal system and space group as for the large-pore
phase (C2/c (no. 15)). The difference of both crystal structures
arises from slight variations of the unit cell parameters. The
Gas sorption measurements
The adsorption/desorption isotherms for CFA-8 measured at
87.3 K for Ar and at 194.7 K for CO2 show in both cases a
strong hysteresis, which indicates a flexible character of the
MOF structure (Fig. 10 and 11). It should be noted, that Ar
isotherms show several steps at p/p0 > 0.05 which probably
corresponds to the structural transformation in crystals with
Fig. 8 Thermogravimetric analysis of pre-dried CFA-8 under nitrogen
unit cell parameters of the as-synthesized CFA-8 (large-pore (blue) and synthetic air (red).
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structural changes of CFA-8 are also shown by in situ XRPD
measurements under 80% humidity (Fig. S14†). Since argon
sorption isotherms seem to be not equilibrated and dependent
on the crystal size at p/p0 > 0.05 (probably due to the low
measurement temperature), the values of the BET surface
areas and pore volumes, obtained from CO2 sorption iso-
therms, seem to be more reliable. The adsorption branch of
the CO2 isotherm in the relative pressure range 0.06–0.1
reveals a BET surface area of 446 m2 g−1 for the activated and
totally dried CFA-8 phase. The adsorption branch of the Ar iso-
therm in the p/p0 range 0.007–0.05 reveals a BET surface area
of 518 m2 g−1 for the same phase. The values calculated with
the Poreblazer software24 (510 m2 g−1 for argon with a probe
diameter of 3.4 Å and 437 m2 g−1 for CO2 with a probe dia-
meter of 3.8 Å) fit very well. The desorption branch of the CO2
isotherm in the p/p0 range 0.06–0.08 reveals a value of
520 m2 g−1 for the BET surface area of the large-pore phase.
The calculated value from the Poreblazer software for the
large-pore phase of CFA-8 is 556 m2 g−1 for CO2. The pore
volume of the activated phase determined from the adsorption
branch of the CO2 isotherm at p/p0 = 0.6 is 0.15 cm3 g−1, the
volume of the large-pore phase determined from the adsorp-
Fig. 9 VTXRPD plot of a pre-dried (250 °C, vacuum, 12 h) powder
sample of CFA-8 in the temp.-range of 25–550 °C (* diffraction peaks
belong to copper, PDF no. 4-836).
tion branch of the CO2 isotherm at p/p0 = 0.95 is 0.20 cm3 g−1
.
Analysis of the argon adsorption isotherm at p/p0 < 0.05 using
the non-local density functional theory (NLDFT) reveals a quite
broad distribution of micropores with two maxima at 5.5 and
9.1 Å (Fig. 12).
The isosteric heat of CO adsorption in CFA-8 determined
from the adsorption isotherms in the temperature range
between 173 K and 193 K is relatively high at low loading
(37 kJ mol−1) and decreases to 18 kJ mol−1 at 0.9 mmol g−1
loading (Fig. 13). At higher loading, the isosteric heat of CO
adsorption reaches a nearly constant value of about 15–16
kJ mol−1, which corresponds to a typical physisorption value.
The value for the isosteric heat of H2 adsorption, in contrast,
remains constant at 6 kJ mol−1 over the whole measured
range. An enhanced heat of carbon monoxide adsorption at
loading below 0.9 mmol g−1 (16–37 kJ mol−1) indicates a weak
chemisorption and coordination of CO molecules to Cu(I)
sites. The binding of CO molecules in this case is much
weaker than reported previously for Cu(I)-MFU-4large, which
forms a very stable CO-complex (stable till 200 °C).33 Such
Fig. 10 Argon adsorption/desorption isotherm for CFA-8 at 87.3 K.
Fig. 11 CO2 adsorption/desorption isotherm for CFA-8 at 194.7 K.
different sizes. The CO2 adsorption isotherm shows, in con-
trast, only one step at p/p0 0.7–0.95 corresponding to the struc-
tural transformation. This behavior is confirmed by in situ
XRPD measurements under CO2 atmosphere which show
structural changes at p/p0 > 0.1 as well (Fig. S13†). Reversible
Fig. 12 Pore size distribution for CFA-8 calculated by fitting Ar adsorp-
tion data.
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Fig. 13 Loading-dependent isosteric heats of CO- and H2 adsorption
in CFA-8.
weak chemisorption could be advantageous for applications
such as gas separation or sensing, because CO binds reversibly
at low temperatures and can be easily removed.
As mentioned before, CFA-8 exhibits three different co-
ordinated Cu ions. The value of 0.9 mmol g−1 corresponds to
one copper(I) atom per formula unit.
Fig. 15 In situ DRIFT spectra of CFA-8 recorded under N2 and CO
atmosphere upon cooling and subsequent heating between 173 K–
323 K (black: activated CFA-8 sample before the CO experiment; other
curves: IR spectra after CO loading at different temperatures).
Tetrahedrally coordinated Cu2 ions in the CFA-8 framework
cannot interact with CO molecules because of their full coordi-
nation environment. Apart from binding stoichiometry the
synchrotron powder diffraction measurements of CFA-8 with
adsorbed CO molecules indicate formation of a CuI–CO
complex between Cu1 (previously trigonally coordinated) and
CO (Fig. 14).
The structure of CFA-8 with adsorbed CO molecules was
solved from synchrotron powder diffraction data at 100 K.
Under CO gas adsorption, the crystal structure of CFA-8 has
monoclinic space group C2/c with lattice parameters of a =
30.866(4) Å, b = 18.881(1) Å, c = 10.851(2) Å, α, γ = 90°, β =
93.29(2)° and a unit cell volume of 6313 Å3, thus having lattice
parameters which are between the cell parameters for large-
pore and narrow-pore phase of CFA-8. One CO molecule was
found to coordinate the Cu1 atom, and additional four mole-
cules were physisorbed in the center of the pores of CFA-8. The
originally planar trigonally coordinated CuI atom (Cu1) shows
a distorted tetrahedral coordination upon coordination of a
single CO molecule. During structure refinement from powder
diffraction data, the linear CO molecule was modeled by the
single sphere with a position corresponding to the center of
gravity of CO molecule. The comparison of structure solution
from powder diffraction data with a DFT-calculated model
(full lattice relaxation with CASTEP V8.0 PBE-DFT,34 plane-
wave norm-conserving pseudo-potentials, Ecut-off = 880 eV),
shows good agreement between experimentally determined
and theoretically calculated positions of CO molecules binding
to the corresponding CuI atoms.
The adsorption of CO in CFA-8 was further studied by
diffuse reflectance Fourier-transform IR spectroscopy (DRIFT).
The recorded IR spectra are shown in Fig. 15. The pre-dried
and activated CFA-8 was cooled down to 173 K and the IR spec-
trum was recorded under nitrogen atmosphere (black curve
Fig. 15). After this measurement the N2 atmosphere in the
reaction chamber was replaced by CO (5% CO in Ar) for one
minute and changed again to N2 atmosphere. The corres-
ponding FT-IR spectrum pictured in light green shows a
strong adsorption band at 2088 cm−1. The temperature was
gradually increased resulting in a decrease of the recorded
adsorption band. The additional bands arising at 2173 cm−1
and 2117 cm−1 belong to free CO molecules in the gas phase.
The recorded band at 2088 cm−1 corresponds to the stretch
mode of the CO molecule coordinatively bound at the CuI site.
The vibrational frequency is shifted to lower wavenumber as
compared to those of the free gas molecule and stays in good
agreement with similar literature-known systems.33,35–37
Preliminary examination for catalytic activity
Since the open coordination sits at the coordinatively unsatu-
rated Cu1/Cu3 center in the CFA-8 framework could be very
interesting for catalytic applications, a simple oxidation test
reaction in liquid phase was performed (Scheme 3). The oxi-
dation of ethylbenzene was previously employed for similar
purposes in our reports on 3d transition metal-containing
metal–organic frameworks MFU-1 and MFU-4large.15,38
Coordinatively unsaturated CuI ions in a porous framework
are promising active sites for catalytic applications. The test
Fig. 14 Illustration of the Cu(I) sites in CFA-8 by ball-and-stick models
before and after CO loading.
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we performed a hot-filtration experiment, since it was known
that even small amounts of leached metal ions in the solution
could be a single source of the observed catalytic activity.38
After seven hours, the CFA-8 particles were removed from the
reaction mixture by isothermal filtration. The recorded process
for the filtered reaction mixture showed no significant conver-
sion after removal of the CFA-8 catalyst.
Scheme 3 Liquid-phase oxidation of ethylbenzene catalyzed by CFA-8
(0.2 mol%) and NHPI (10 mol%) as a co-catalyst.
reaction reported here involves NHPI (N-hydroxyphthalimide)
as a co-catalyst and represents an effective strategy to activate
oxygen from air under mild conditions in the presence of tran-
sition metal ions. The so-formed PINO (phthalimide N-oxyl)
radical acts as initiator for the radical oxidation of C–H
bonds.15 The mechanism of this type of reaction has been
described by Ishii et al. in detail.39–43 An induction period at
the beginning of the reaction indicates the slow formation of
PINO radicals needed as an initiator. The reaction was per-
formed in acetonitrile under oxygen abstracted from the atmo-
sphere. The only detected product of the oxidation reaction of
ethylbenzene catalyzed by CFA-8 is acetophenone. The conver-
sion of ethylbenzene reached after 70 h reaction time is
around 60% and is nearly equal to the yield of acetophenone
determined by GC/MS analysis (Fig. 16). No 1-phenylethanol
as a by-product was detected. Very similar values for the con-
version of ethylbenzene and the total yield of acetophenone
indicate that no over-oxidation and formation of benzoic acid
takes place under these mild reaction conditions. The remain-
ing crystallinity of the recovered catalyst was checked after
each run (see Fig. S7†). Also the performance of the recycled
copper MOF was proven. At least for three runs the activity for
the oxidation reaction and also the crystallinity of the CFA-8
network remains the same. The value for BET surface of the
catalyst recovered after complete catalytic reaction is about
Conclusion
In conclusion, we have synthesized and structurally character-
ized a novel metal–organic framework CFA-8 constructed from
tqpt2− ligands and Cu(I) ions as metal nodes. The Coordi-
nation Framework Augsburg University-8 is to the best of our
knowledge the first example for a copper(I) MOF incorporating
a bistriazolate ligand. A special feature of the here reported
structure is its flexible character. The breathing effect, which is
common for several MOF structures, is completely reversible
as shown by the recorded powder XRD patterns.
Owing to the flexible nature of the framework its gas sorp-
tion properties are quite complex. Both adsorption/desorption
isotherms (Ar and CO2) exhibit a strong hysteresis, which is an
indication for the flexible character of the network. The calcu-
lated heat of adsorption for hydrogen is constant at 6 kJ mol−1
and typical for physisorbed hydrogen. In contrast the value for
reversible chemisorbed hydrogen in CuI-MFU-4large is 37 kJ
mol−1 33
A special focus for the CFA-8 is placed on the sorption
.
properties towards carbon monoxide. From low loading on up
to 0.9 mmol g−1 the loading-dependent heat of adsorption
values range from 37–16 kJ mol−1. From 0.9 mmol g−1 to
higher loading the value is nearly constant at 15–16 kJ mol−1
.
The weak chemisorption of CO molecules at the unsaturated
trigonally coordinated Cu(I) ions is very interesting for appli-
cations such as gas separation or sensing and was further con-
firmed by synchrotron powder diffraction and IR
measurements which revealed a characteristic absorption
band at 2088 cm−1 corresponding to the CuI–CO complex.
A last feature of this novel MOF is the catalytic activity of
the framework. A first preliminary examination for the catalytic
activity was performed with a simple liquid phase oxidation
reaction. The oxidation of ethylbenzene under mild conditions
catalyzed by CFA-8 in the presence of NHPI as a co-catalyst
leads to acetophenone as a single product. No byproduct, such
as 1-phenylethanol or benzoic acid, is detectable. The relatively
high conversion of 60% was achieved in this reaction. Further
investigations on the sorption properties and the catalytic
activity are the research focus in the next time.
503 m2 g−1 – close to the original value of 518 m2 −1. To
g
confirm the heterogeneous nature of the catalytic test reaction,
Experimental
Materials and methods
All starting materials were of reagent grade and used as
received from the commercial supplier. Thermogravimetric
analysis (TGA) was performed with a TGA Q500 analyzer in the
Fig. 16 Time-dependent formation of acetophenone in the CFA-8
catalyzed oxidation reaction of ethylbenzene with NHPI as a co-catalyst.
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temperature range of 25–800 °C in flowing nitrogen and syn-
N.B.: all trials to synthesize CFA-8 from Cu(I) salt (Cu(I)Cl,
thetic air at a heating rate of 10 K min−1. Ar, CO, CO2 and H2 Cu(I)Br or Cu(I)Ac) under similar reaction conditions have
sorption isotherms were measured with a BELSORP-max failed in our hands so far. Similarly, replacing DMA by other
instrument combined with a BELCryo system. Adsorbed gas organic solvents like N,N-dimethylformamide (DMF), N,N-
amounts are given in cm3 g−1 [STP], where STP = 101.3 kPa diethyl-formamide (DEF) or N-methylformamide (NMF) did
and 273.15 K. Prior to measurements, the sample was heated not yield phase pure CFA-8.
at 250 °C for 20 h in high vacuum in order to remove occluded
solvent molecules. Fourier transform infrared (FTIR) spectra
Synthesis of CFA-8 (narrow-pore phase)
were recorded with ATR in the range 4000–400 cm−1 on a An as-synthesized sample of CFA-8 was activated for 20 hours
Bruker Equinox 55 FT-IR spectrometer. Energy-dispersive X-ray in vacuum at 250 °C. Subsequently the powder sample (15 mg)
spectroscopy (EDX) was performed with a Philips XL 30 FEG was suspended in a methanolic solution of trimethyl phos-
scanning electron microscope equipped with an EDAX SiLi phite (20 mL, 0.3 mmol mL−1). The mixture was stirred for
detector. Elemental analyses were measured with a Vario EL III 30 min, filtered and washed with fresh MeOH. The still orange
instrument from Elementar Analysensysteme GmbH. GC/MS powder was dried for 4 hours at 60 °C per vacuum.
measurements were performed on a Hewlett Packard GC
Oxidation of ethylbenzene with CFA-8
System 6890 Series equipped with Rtx-5MS column and mass
selective detector MSD 5973. Powder X-ray diffraction data For preliminary examination for catalytic activity of the Cu(I)-
were collected in the 2θ range with 0.02° steps, with a time of MOF, an easy oxidation reaction was performed with CFA-8 as
3 s per step, by using a Seifert XRD 3003 TT diffractometer a catalyst and N-hydroxyphthalimide (NHPI) as a co-catalyst.
equipped with Meteor 1D detector. The diffuse reflectance The XRPD data under CO2 pressure and relative humidity were
Fourier-transform IR spectra (DRIFT) were collected between collected with an Empyrean (PANalytical) Diffractometer
3500–400 cm−1 on an Equinox 55 FT-IR spectrometer equipped equipped with Bragg–BrentanoHD mirror, PIXcel3D 2 × 2 detec-
with a Praying Mantis diffuse reflectance accessory and an tor and Cryo & humidity Chamber CHC plus+ (Anton Paar).
environmental chamber (Harrick Scientific Products) and
To a solution of ethylbenzene (1.06 g, 10 mmol), NHPI
referenced to KBr. The H2-tqpt ligand was synthesized in three (163 mg, 1.0 mmol) and n-decane (0.71 g, 5 mmol, used as a
steps from commercially available 5,5-dimethylcyclohexane- standard) in acetonitrile/water mixture (98 : 2, 40 mL) was
1,3-dione. The following methylation and oxidation reactions added an activated sample of CFA-8 (27 mg, 0.24 mmol). The
are documented in literature.18,44,45 The 1H-benzotriazole-5,6- suspension was stirred at 40 °C. To analyze the conversion of
diamine was synthesized as described in literature.19–21
the reaction 0.1 mL of the mixture was filtered through neutral
Al2O3 which was then eluted with Et2O/CH2Cl2/MeOH
(10 : 10 : 1) and GC/MS measurements were repeated three
times for each sample. Ethylbenzene and acetophenone were
Synthesis of 6,6,14,14-tetramethyl-6,14-dihydroquinoxalino-
[2,3-b]phenazinebistriazole, (H2-tqpt)
The H2-tqpt ligand is synthesized as previously described in used as standards for the determination of the conversion and
literature.22 Accordingly, a suspension of 3,3,6,6-tetramethyl- yield. To check the performance and the recyclability of the
cyclohexane-1,2,4,5-tetraone (1.00 g, 5.10 mmol) and 1H- CFA-8 MOF the catalysis experiment was repeated three times
benzotriazole-5,6-diamine (1.60 g, 10.70 mmol) in 200 mL of with the same CFA-8 sample recovered after the reaction. After
acetic acid was heated under good stirring to reflux for each experiment the XRPD pattern of the framework was
24 hours. The obtained dark red solution was cooled with an examined.
ice bath and diluted with 400 mL of water to precipitate a
solid. The suspension is neutralized with sodium carbonate.
Single-crystal X-ray diffraction
During this procedure the suspension turns to yellow. The For the single-crystal X-ray measurements, the crystals of
obtained solid was filtered off and washed with water and a [Cu2(tqpt)]·4.25DMA (large-pore phase) were taken from the
small amount of methanol.
Yield: 2.15 g (73%).
mother liquor, while the crystals of [Cu2(tqpt)]·0.94P(CH3O)3
(narrow-pore phase) were dried in vacuum (4 h/60 °C) prior to
the measurements. The refined solution of the crystals with
the composition [Cu2(tqpt)]·0.94P(CH3O)3 describes the crystal
Synthesis of CFA-8 (large-pore phase)
A mixture of copper(II)chloride (20.0 mg, 0.15 mmol) and H2- system for the narrow-pore phase of CFA-8. These crystals were
tqpt (5.0 mg, 0.01 mmol) was dissolved in 4 mL of N,N-di- obtained by an experiment in order to check, if the phos-
methylacetamide (DMA) and the solution was placed in a glass phorus atom of trimethyl phosphite coordinates to the Cu1
tube (10 mL). The tube was sealed and heated at 140 °C for 2 atom as observed for CO gas molecules. In this case, the inter-
days and then cooled to room temperature. The orange precipi- actions between the P donor atom and the Cu1 atom are not
tate was filtered, washed with DMA (3 × 1 mL) and MeOH strong enough, but the single crystal structure of the narrow-
(3 × 3 mL) and dried at 250 °C under vacuum.
pore phase of CFA-8 could be solved. The crystals were
Yield: 7.99 mg (73%). IR (ν (cm−1)): 2968, 1456, 1371, 1272, mounted on a MiTeGen MicroMount and tested on a Bruker
1211, 1146, 1105, 850, 806, 613, 426. Elemental analysis calcd diffractometer. X-ray diffraction data for the single crystal
(%): C 48.26, H 2.95, N 25.58; found: C 48.29, H 2.99, N 25.70.
structure analysis were collected on a Bruker D8 Venture dif
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Diamond (UK).50 Synchrotron X-ray powder diffraction
measurements of CO adsorption in CFA-8 were performed at
100 K, 100 kPa gas pressure. Sample was evacuated before gas
loading, cooled down to 100 K, and exposed to CO gas.
Indexing and structure solution by simulated annealing of
the crystal structure of CFA-8 with adsorbed CO molecules was
performed using TOPAS.51 Subsequent Rietveld refine-
ment52,53 confirmed the crystal structure.
Table 2 Crystal and experimental data for the large-pore phase of
CFA-8
CFA-8·4.25 DMA, large-pore phase
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C61H70.25Cu4N24.25O4.25
1465.32
100(2)
0.71073
Monoclinic
C2/c (no. 15)
a = 27.4916(12) Å
b = 23.6639(11) Å
c = 11.0002(5) Å
β = 93.7500(14)°
7141.0(6) Å3
4
1.363
1.237
3024
0.117 × 0.04 × 0.03
Unit cell dimensions (Å)
Acknowledgements
Volume
Financial support by the DFG (Priority Program SPP 1928
“COORNETs”) is gratefully acknowledged. We thank Diamond
Light Source for access to beamline I12-JEEP that contributed
to the results presented here.
Z
Density (calculated) (g cm−3
)
Absorption (mm−1
F(000)
)
Crystal size (mm3)
Theta range for data collection (°) 2.27 to 25.09
Refls. collected
Refls. unique
Data/restraints/parameters
GooF
38 185
6325 [R (int) = 0.0867]
6325/0/312
1.047
Notes and references
R1 [I > 2σ(I)]
0.0460
0.1171
0.734 and −0.829
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