STA-27, a porous Lewis acidic scandium MOF with an unexpected topology type prepared with 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine

Article abstract of DOI:10.1039/c8ta10610j

A porous scandium MOF denoted STA-27 (St Andrews Porous Material-27) has been synthesised solvothermally using the 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine anion (TCPP^4-) as the tetratopic carboxylate linker. STA-27 possesses a unique scandium-based 1D rod secondary building unit (SBU) comprising corner-sharing scandium Sc₂O₁₁ dimers connected via carboxylate groups from the linker. After activation under mild conditions STA-27 is an active Lewis acidic catalyst, while heating at elevated temperatures results in rupturing of the Sc-O-Sc linkages and a phase transition to a different topological type. Under similar synthesis conditions the smaller Al³⁺ and Ga³⁺ cations give isostructural MOFs with a different, previously reported, topology type based on chains of corner-sharing MO₄(OH)₂ octahedra: the Al-form possesses attractive properties for CO₂ adsorption.

Full text of DOI:10.1039/c8ta10610j

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Seidner, D. B. Cordes, M. Lozinska, D. Dawson, M. Thompson, T. Düren, K. Chakarova, M. Mihaylov, K.
Hadjiivanov, F. Hoffmann, A. Slawin, S. E. Ashbrook, M. Clarke and P. Wright, J. Mater. Chem. A, 2019,
DOI: 10.1039/C8TA10610J.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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STA-27, a porous Lewis acidic scandium MOF with an unexpectedDtOoI: 1p0.o10l3o9/gC8yTA10610J
type prepared with 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine
Ram R. R. Prasad,a Sarah E. Seidner,a David B. Cordes,a Magdalena M. Lozinska,a Daniel M.
Dawson,a Megan J. Thompson,b Tina Düren,b Kristina K. Chakarova,c Mihail Y. Mihaylov,c
Konstantin I. Hadjiivanov,c Frank Hoffmann,d Alexandra M. Z. Slawin,a Sharon. E.
Ashbrook,a Matthew L. Clarke a and Paul A. Wrighta*
a
EaStCHEM School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St
Andrews, KY16 9ST, UK.
b Centre for Advanced Separations Engineering, Department of Chemical Engineering, University of
Bath, Bath, BA2 7AY, UK.
c Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria.
d
Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg,
Martin-Luther-King-Platz 6, 20146 Hamburg, Germany.
*E-mail : paw2@st-andrews.ac.uk
Abstract: A porous scandium MOF denoted STA-27 (St Andrews Porous Material-27) has
been synthesised solvothermally using the 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine anion
(TCPP4-) as the tetratopic carboxylate linker. STA-27 possesses a unique scandium-based 1D
rod secondary building unit (SBU) comprising corner-sharing scandium Sc2O11 dimers
connected via carboxylate groups from the linker. After activation under mild conditions
STA-27 is an active Lewis acidic catalyst, while heating at elevated temperatures results in
rupturing of the Sc-O-Sc linkages and a phase transition to a different topological type.
Under similar synthesis conditions the smaller Al3+ and Ga3+ cations give isostructural MOFs
with a different, previously reported, topology type based on chains of corner-sharing
MO4(OH)2 octahedra: the Al-form possesses attractive properties for CO2 adsorption.
Introduction
Reticular chemistry, the underlying principle of which is the designed assembly of metal
cation-based nodes and polytopic organic linkers, has been used to great effect in the
preparation of numerous families of porous metal organic frameworks (MOFs).1-5 Among
these, the carboxylates MIL-53,6 -88,7 -100,8 -101,9 soc-MOF10 and MFM-30011 of the
trivalent metal cations Al3+, Cr3+, Fe3+ and Ga3+ are some of the most intensely studied MOFs
for potential applications in adsorption, separation, drug delivery and catalysis. In these
1

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materials, the metal cation-based inorganic nodes take the form of corner-sharing chains or
DOI: 10.1039/C8TA10610J
trimers of MO6 octahedra, as shown in Figure 1.
(A)
(B)
(C)
Figure 1. Observed SBUs in M3+-based MOFs: (A) trimer-based SBUs of MIL-88, -100, -101 and
socMOF, (B) 1D rod SBU of MIL-53 family and (C) 1D rod SBU of MFM-300.
It is possible to prepare Sc3+ analogues of these carboxylate MOFs, where the slightly larger
Sc3+ cations (ionic radius 0.745 Å vs 0.675 Å (Al3+)) adopt octahedral coordination in the
chain or trimer building units, modifying the breathing behaviour of MIL-53 and imparting
highly effective Lewis acidic catalytic sites to MIL-100, for example.12 Additionally, it is
possible to prepare porous MOFs with Sc3+ within the metal nodes that are structurally
different from those prepared with the other cations, due to its larger radius.13-15 These
include Sc2BDC3, a highly stable MOF, not been prepared for metals other than scandium,
which displays a unique set of properties for the adsorption of molecules such as CO2 and
light alkanes.14 Despite the attractive properties for adsorption and catalysis reported
above, the area of Sc MOFs remains relatively little studied, and so the investigation of
tetracarboxylate Sc-MOFs was deemed timely, as only three had been reported, NOTT-
40116a and the scandium analogues of In-soc-MOF12a,c and MFM-300.16b 2,3,5,6-Tetrakis(4-
carboxyphenyl)pyrazine (H4TCPP, Scheme 1) is readily-prepared and its use had only been
reported rarely in MOF synthesis.17 We anticipated that the resulting linker TCPP4-
(hereafter TCPP) would give tetracarboxylate scandium MOFs with interesting adsorptive
and catalytic properties.
2

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DOI: 10.1039/C8TA10610J
Scheme 1 Structure of the acid forms of the linkers TCPP (left) and TCPB (right).
Here we describe the results of syntheses of TCPP-based MOFs with Sc3+, as well as those
carried out with the trivalent metal cations Al3+ and Ga3+ for comparison. The latter two
cations give structural analogues of the reported Al-MOF, [Al2(OH)2(TCPB)]18 prepared from
the similar acid 2,3,5,6-tetrakis(4-carboxyphenyl)benzene (H4TCPB, Scheme 1), as might
have been predicted by reticular chemistry. By conrast, the use of Sc3+ results in STA-27, a
microporous 3D-connected scandium MOF with an unprecedented infinite rod building unit.
We describe its adsorptive and Lewis acidic catalytic properties and report that it undergoes
a structural phase transition at elevated temperature.
Experimental
Synthesis of H4TCPP
H4TCPP was synthesised in two steps by modifying the reported procedure.17 The synthesis
procedure and details of its characterization are given in the ESI.
Synthesis of Al2(OH)2TCPP
Al2(OH)2TCPP was synthesised in a mixed solvent system of N, N-dimethylformamide (DMF)
and nitric acid (HNO3). 190.6 mg Al(NO3)3.9H2O or 216.1 mg AlCl3.6H2O (0.895 mmol) and
41.2 mg H4TCPP (0.074 mmol) were weighed out into a Teflon-liner (total volume ~ 30 mL)
equipped with a magnetic stirrer bar. 7.5 mL of DMF followed by 7.5 mL of 3.5 M aq. HNO3
were added to the mixture using either a syringe or measuring cylinder. The solution was
stirred at room temperature until it turned pale yellow (usually 45-50 min). The stirrer bar
was removed, and the Teflon-liner was placed inside a stainless-steel autoclave before
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heating in a pre-heated oven at 150 °C for 30 h. After cooling to room temperature at a rate
DOI: 10.1039/C8TA10610J
of 5 °C min-1, the resulting white powder was filtered and washed multiple times with DMF,
followed by methanol and kept in methanol overnight before filtering and drying at 80 °C
overnight. Calc. C, 47.93; H, 2.98; N, 4.55 wt%. Meas. C, 48.01; H, 2.90; N, 4.45 wt%.
Synthesis of Ga2(OH)2TCPP
Ga2(OH)2TCPP was synthesised in DMF and HNO3 similarly to Al2(OH)2TCPP: 228.8 mg
Ga(NO3)3.6H2O (0.628 mmol) and 41.2 mg H4TCPP (0.074 mmol) were weighed into a Teflon-
liner (total volume ~30 mL) equipped with a magnetic stirrer bar. 7.5 mL of DMF followed by
the same volume of3.5 M aq. HNO3 was added to the mixture using either a syringe or
measuring cylinder. The solution was stirred at room temperature until the colour of the
solution changed to pale yellow (usually 45-50 min). The stirrer bar was removed, and the
Teflon-liner was placed inside a stainless-steel autoclave before heating in a pre-heated
oven at 150 °C for 40 h. After cooling to room temperature at a rate of 5 °C min-1, the single
crystals of Ga-TCPP obtained were filtered and washed multiple times with DMF followed by
methanol and kept in methanol overnight before filtering and drying at 80 °C overnight.
Synthesis of STA-27, Sc2(O)(H2O)2TCPP
STA-27 was synthesised using a mixed solvent system of DMF, aqeous 3.5 M HNO3 and
acetonitrile with ScCl3.6H2O and H4TCPP as the precursors. 232.4 mg ScCl3.6H2O (0.895
mmol) and 41.2 mg TCPP (0.074 mmol) were weighed out into a Teflon-liner (total volume
~30 mL) equipped with a magnetic stirrer bar. 7.5 mL of DMF followed by 7.5 mL of 3.5 M
aq. HNO3 and 7.5 mL acetonitrile were added to the mixture using either a syringe or
measuring cylinder. The solution was stirred at room temperature until the colour of the
solution changed to pale yellow (usually 45-50 min). The stirrer bar was removed, and the
Teflon-liner was placed inside a stainless-steel autoclave before heating in a pre-heated
oven at 150 °C for 30 h. After cooling to room temperature at a rate of 5 °C min-1, lozenge
shaped single-crystals of STA-27 were obtained. The crystals were washed multiple times
with DMF and methanol and kept in methanol overnight before filtering and drying at 80 °C
overnight. CHN analysis : Calc. C, 49.33; H, 3.42; N, 5.49 wt%. Meas. C, 49.37; H, 3.33; N,
5.49 wt%.
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DOI: 10.1039/C8TA10610J
Catalysis
3.1 Conjugate addition of 2-methylindole to methyl vinyl ketone
1 mmol of 2-methylindole, 1 mmol of methyl vinyl ketone, 0.5 mmol of 1-
methylnaphthalene and 1.5 mol % of STA-27 (or MIL-100(Sc)) were added to 5 mL DCM and
stirred at room temperature for 8 h under an inert atmosphere. After the reaction, the
reaction mixture was filtered and washed multiple times with a minimum amount of water.
The product was extracted using DCM and dried over MgSO4. After the removal of DCM
under vacuum, the crude product obtained was purified by flash chromatography using
hexane: ethyl acetate (4:1) eluent. The pure product, 4-(2-methyl-1H-indol-3-yl)butan-2-one
was obtained as a yellow liquid which yields a white solid when kept undisturbed. Isolated
yield = 66 % ( 0.06 mmol). 1H NMR (400MHz, CDCl3): 7.81 (s, 1H), 7.52 (m, 1H), 7.29 (m, 1H),
7.13 (m, 2H), 3.0 (t, 2H, J = 7.8 Hz), 2.80 (t, 2H, J = 7.7 Hz), 2.41 (s, 3H), 2.14 (s, 3H). 13C NMR
(75 MHz, CDCl3): 209.1 (C-O), 135.3 (Ar-C), 131.2 (Ar-C), 128.3 (Ar-C), 121.0 (Ar-CH), 119.2
(Ar-CH), 117.8 (Ar-CH), 110.5 (Ar-CH), 110.3 (ArCH-CH2), 44.3 (CH2), 30.3(CH3), 18. 4(CH2),
11.7 (CH3). HRMS (EI+) Calculated: 202.122; Obtained: 202.126. Data in accordance with the
literature.12e
3.2 Imine condensation between 4’-fluoroacetophenone and benzyl amine
1 mmol of 4’-fluoroacetophenone, 1 mmol of benzylamine, 0.5 mmol of 1-
methylnaphthalene and 1.5 mol % of STA-27 (or MIL-100(Sc)) were added to 5 mL toluene
and heated at 90 °C for 24 h under an inert atmosphere. After the reaction, the reaction
mixture was diluted using minimum amount of toluene and filtered through Na2SO4. The
filtrate was washed with 0.1 M HCl solution and concentrated under vacuum to obtain the
pure product as a yellowish-brown oil. Isolated yield = 75 % (0.48 mmol). 1H NMR (400MHz,
CDCl3): 7.90 (ddd, 2H, J = 8.4, J = 5.2, J= 2.6), 7.39 (m, 6H), 7.10 (m, 2H), 4.76 (s, 2H), 2.35 (s,
13
3H). C NMR (75 MHz, CDCl3): 163.8 (ArC-F, J = 249 Hz), 164.7 (C=N), 140.4 (ArC), 137.1
(ArC, J = 3.0 Hz), 128.7 (2xArCH, J = 8.3 Hz), 128.45 (2xArCH), 127.7 (2xArCH), 126.5 (ArCH),
115.0 (2×ArCH, J = 21 Hz), 55.7 (CH2), 15.8 (CH3). HRMS (EI+) Calculated: 228.118; Obtained:
228.122. Data in accordance with the literature.12e
3.3 Carbonyl ene reaction between ethyl trifluoropyruvate and α-methylstyrene
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2.25 mmol of ethyl trifluoropyruvate, 2.7 mmol of α-methylstyrene, 0.5 mmol of 1-
DOI: 10.1039/C8TA10610J
methylnaphthalene and 1.5 mol% of STA-27 (or MIL-100(Sc)) were added to 5 mL toluene
and stirred at room temperature for 6 h under inert atmosphere. After the reaction, the
reaction mixture was filtered and concentrated under vacuum, the crude product obtained
was purified by flash chromatography using hexane: ethyl acetate (6:1) eluent. The pure
product, ethyl-2-hydroxy-4-phenyl-2-trifluoromethyl)pent-4-enoate, was obtained as a pale
liquid. Isolated yield = 60 % (1.3 mmol). 1H NMR (400MHz, CDCl3): 7.34 (m, 5H), 5.42 (s, 1H),
5.32 (s, 1H), 4.06 (dq, 1H, J = 10.6 Hz, 7.0Hz), 3.65 (dq, 1H, J = 10.6 Hz, 7.0Hz), 3.33(d, 1H, J =
19
13
14.1 Hz), 3.08 (d, 1H, J = 14.1Hz), 1.14 (t, 3H, 7.0Hz). F NMR (282 MHz, CDCl3): -78.4. C
NMR (75 MHz, CDCl3): 168.7 (C=O), 140.8 (C=CH2), 128.0 (ArC), 127.5 (2xArCH), 126.6
(ArCH), 124.3 (2xArCH), 122.3 (CH2=C), 119.3 (C-OH), 63.3 (CH2), 36.8 (CH2), 13.3 (CH3).
HRMS (EI+) Calculated: 289.273; Obtained: 289.276. Data in accordance with the
literature.12e
Characterisation
Powder X-ray diffraction (PXRD) patterns on finely ground MOF samples were collected in
Debye-Scherrer geometry on Stoe STADI P diffractometers with primary monochromation
(Cu Kα1, λ = 1.54056 Å), using either 0.5- or 0.7-mm glass capillaries. Variable temperature
PXRD (VT-PXRD) measurements for STA-27 were performed in open quartz capillaries in a
flow of dry nitrogen flow with a ramp rate of 5 °C min-1 and held at the desired temperature
for 10 min prior to measurements. Data were collected from 25 to 225 °C and after cooling
down to 25 °C using a Cobra Plus non-liquid-nitrogen cryostream (Oxford Cryosystems).
Single crystal X-ray diffraction data for Ga2(OH)2TCPP were collected at 93 K using a Rigaku
FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with XtaLAB P200
diffractometer [Mo Kα radiation (λ = 0.71075 Å)]. Intensity data were collected using ω
steps accumulating area detector images spanning at least a hemisphere of reciprocal
space. Data for STA-27 and STA-27-C were collected at 173 K using a Rigaku MM-007HF High
Brilliance RA generator/confocal optics with XtaLAB P100 diffractometer [Cu Kα radiation (λ
= 1.54187 Å)]. Intensity data were collected using both ω and φ steps accumulating area
detector images spanning at least a hemisphere of reciprocal space. Data for all compounds
analysed were collected and processed (including correction for Lorentz, polarization and
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absorption) using CrystalClear19 or CrysAlisPro.20 Structures were solved by DdOuI: a10l.-1s0p39a/Cc8eTA10610J
methods (SHELXT),21 and refined by full-matrix least-squares against F2 (SHELXL-2018/2).22
Non-hydrogen atoms were refined anisotropically, and aryl hydrogen atoms were refined
using a riding model. Hydroxyl hydrogen atoms in Ga2(OH)2TCPP were located from the
difference Fourier map and refined isotropically, subject to a distance restraint. Terminal
oxygen atoms in STA-27 were identified as water molecules, although hydrogen atoms could
not be located. STA-27-C showed extremely weak diffraction at higher angles and
broadening of some diffraction spots. This led to the high values of Rint and other metrics
but did not prevent the unambiguous determination of the structure of the MOF apart from
–
the bridging species, which we tentatively assign as HCO3 . The data for STA-27 showed non-
merohedral twinning (twin law 1 0 0.313 0 -1 0 0 0 -1, twin fraction 47 %), which was
accounted for in the refinement. All three structures structure showed high proportions of
void space (Ga2(OH)2TCPP: 856 Å3, STA-27: 1069 Å3, STA-27-C: 817 Å3) and the SQUEEZE23
routine implemented in PLATON24 was used to remove the contribution of the disordered
electron density in the void spaces. All calculations except SQUEEZE were performed using
the CrystalStructure25 interface. Selected crystallographic data are presented in Table 1.
CCDC 1875994-1875996 contains the supplementary crystallographic data for this paper.
The data can be obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/structures. The topological analysis was carried out with the
package ToposPro.26
Solid-state NMR (SS-NMR) spectra were recorded using a Bruker Avance III spectrometer
1
equipped with a 9.4 T superconducting magnet (Larmor frequencies of 400.1 MHz for H,
13
45
100.6 MHz for C and 97.2 MHz for Sc). Samples were packed into standard ZrO2 rotors
with outer diameters of 4 mm and rotated at the magic angle at a rate of 12.5 kHz (13C) or
1
14 kHz (45Sc, 27Al). The 13C NMR spectrum was recorded with cross polarisation (CP) from H
1
with a contact pulse (ramped for H) of 2 ms. Signal averaging was carried out for 4096
transients with a recycle interval of 3 s. Two-pulse phase modulation (TPPM) decoupling of
1H (n1  100 kHz) was carried out during acquisition. For the 27Al NMR spectra, signal
averaging was carried out for 4096 transients with a recycle interval of 0.25 s. A rotor-
synchronised x-2x spin echo pulse sequence was used with an echo delay of 71.43 μs and
pulses of 1 and 2 μs (inherent flip angles of  36 and 72°). Signal averaging was carried out
for 120 transients for each of 128 t1 increments of 71.43 μs with a recycle interval of 0.25 s.
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For the 45Sc NMR spectra, signal averaging was carried out for 1024 transients wiDthOI:a10r.e10c3y9/cCl8eTA10610J
27
45
interval of 0.25 s. Both Al and Sc multiple-quantum (MQ) magic angle spinning (MAS)
spectrum was recorded using a z-filtered pulse sequence and then sheared and referenced
45
according to Pike et al.27 For Sc NMR spectra, signal averaging was carried out for 5664
transients for each of 20 t1 increments of 35.7 ms with a recycle interval of 0.25 s. Chemical
shifts are reported in ppm relative to TMS (13C) or 0.06 M Sc(NO3)3 in D2O (45Sc), using L-
alanine (CH3 δ = 20.5 ppm) and LaScO3 (δ= 162 ppm) as secondary solid references.
Thermogravimetric analysis (TGA) of all MOF samples was carried out on a Netzsch TGA 760
insrument for a temperature range of 20 - 800 °C at a heating rate of 5 °C min-1 in a
continuous air flow. N2 adsorption isotherms for all the MOFs were measured volumetrically
on a Micrometrics Tristar at -196 °C. The CO2 adsorptions isotherms were measured
gravimetrically on Hiden IGA porosimeter at room temperature. All samples were activated
at 150 °C prior to measurements.
The FTIR spectra were recorded with Nicolet Avatar 360 and Nicolet 6700 FTIR
spectrometers accumulating 64 scans at a spectral resolution of 2 cm-1 and accuracy of 0.01
cm−1. The IR measurements of STA-27 were made with (i) MOF samples spread on the
surface of preliminary pressed KBr discs and (ii) self-supporting pellets. The pellets were
treated directly in a specially designed IR cell allowing measurements at ambient and low
(100 K) temperatures. The cell was directly connected to a vacuum-adsorption apparatus
with a residual pressure lower than 10-3 Pa. The IR experiments were made in situ with as-
prepared (air exposed) samples and samples activated by outgassing under high vacuum
(~ 10-6 mbar) at ambient and elevated temperatures.
Grand canonical Monte Carlo simulations were carried out using the general-purpose
molecular simulation code MuSiC.28 The Lennard-Jones (LJ) parameters for the framework
atoms were taken from the Dreiding forcefield,29 with the exception of scandium, the
parameters for which were taken from the Universal forcefield.30 Dinitrogen LJ parameters
were taken from the Transferable Potentials for Phase Equilibria (TraPPE) forcefield.31
Lorentz-Berthelot mixing rules were applied for LJ interactions between different atom
types, and interactions between atoms separated by a distance greater than the cut-off
radius of 18 Å were truncated. GCMC moves implemented on the N2 molecules (insertion,
deletion, translation and rotation) were carried out with equal weighting. For each pressure
point, 10 million iterations were used, 40 % of which were used for equilibration.
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Table 1. Crystallographic data for Ga2(OH)2TCPP, STA-27 and STA-27-C
DOI: 10.1039/C8TA10610J
Parameter
Ga2(OH)2TCPP
C32H18N2O10Ga2
695.91
STA-27
C32H20N2O11Sc2
698.42
173
STA-27-C
Formula
C34H18N2O14Sc2
768.43
173
Formula weight/g mol-1
Temperature/K
93
Crystal System
Orthorhombic
Cmmm
6.68115(16)
21.2861(4)
15.9030(3)
-
Monoclinic
P2/c
Orthorhombic
Pmna
Space group
a/Å
15.504(7)
9.443(4)
15.975(7)
99.277(14)
2308.2(18)
2
32.741(4)
7.797(3)
8.763(3)
-
b/Å
c/Å
β/°
V/Å
2261.66(8)
2
2237.0(12)
2
Z
ρ(calcd) g cm-3
Radiation type
μ mm-1
1.072
1.005
1.141
Mo Kα
Cu Kα
Cu Kα
1.233
2.910
3.090
F(000)
732
712
780
Rint
0.0219
0.1396
1.560
0.5507
0.968
GOF
1.120
Final R1 values [I>2σ(I)]a
Final wR2 values (all data)b
0.0316
0.1409
0.4218
0.1479
0.4332
0.0958
Results and Discussion
Inspired by the synthesis conditions employed for isoreticular Al-soc-MOFs by Alezi et al.10d
using a tetracarboxylic acid, the optimised solvothermal crystallisations (Table 2) in DMF and
3.5 M aq. HNO3 using H4TCPP and either Al(NO3)3.9H2O (AlCl3.6H2O) or Ga(NO3)3.6H2O as
metal sources gave products containing crystalline Al2(OH)2TCPP or Ga2(OH)2TCPP,
respectively (Figures S1, S2).
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Table 2: Optimising synthesis conditions for Al2(OH)2TCPP and Ga2(OH)2TCPP. In each caseD0O.I0: 1704.10m39m/Co8lTA10610J
of H4TCPP was used, plus either 0.895 mmol of Al precursor (M:L = 12:1) or 0.628 mmol of Ga
precursor (M:L = 8:1), respectively.
DMF
(mL)
HNO3 CH3CN Temperature Time
Metal Source
(3.5 M)
(mL)
(mL)
(°C)
(h)
Phase
Al(NO3)3.9H2O
Al(NO3)3.9H2O
Al(NO3)3.9H2O
Al(NO3)3.9H2O
AlCl3.6H2O
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
150
150
150
150
150
150
150
150
150
150
150
150
30
30
30
30
30
16
30
35
40
48
72
96
Unidentified phase
Amorphous
-
-
-
7.5
Poorly crystalline
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
-
-
-
-
-
-
-
-
-
Al2(OH)2TCPP
Al2(OH)2TCPP
Ga(NO3)3.6H2O
Ga(NO3)3.6H2O
Ga(NO3)3.6H2O
Ga(NO3)3.6H2O
Ga(NO3)3.6H2O
Ga(NO3)3.6H2O
Ga(NO3)3.6H2O
Unidentified phase (I)
Unidentified phase (I)
Ga2(OH)2TCPP + impurity
Ga2(OH)2TCPP + minor imp.
Ga2(OH)2TCPP + impurity
Ga2(OH)2TCPP + Impurity
Unknown phase (II)
PXRD indicates the aluminium and gallium MOFs prepared using H4TCPP are isostructural
(Figure 2A). The aluminium phase was prepared as a microcrystalline powder, while the
gallium phase was prepared as single crystals (with a minor amount of impurities) that were
amenable to structure solution by single crystal diffraction.
(A)
(B)
Figure 2 (A) Simulated pattern of (a) Ga2(OH)2TCPP compared with the PXRD patterns of (b)as-
synthesised Al2(OH)2TCPP and (c) as-synthesised Ga2(OH)2TCPP and (B) SEM image of
Al2(OH)2TCPP.
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The structure solution of Ga2(OH)2TCPP is described in the experimental section and Table 1
DOI: 10.1039/C8TA10610J
and the structure is illustrated in Figure 3. The Al and Ga TCPP MOFs are isostructural with
Al2(OH)2TCPB reported by Krüger et al., the structure of which was solved from PXRD and
computational modelling.18 The difference between the TCPP and TCPB linkers is that in the
former the CH groups of the central benzene ring in TCPB are replaced by N atoms (Scheme
1). In all these structures, the metal-based inorganic node is the infinite MO4(OH)2 chain of
corner-sharing MO6 octahedra linked by µ2OH groups, similar to that found in the MIL-53
structure.6 27Al MAS NMR and MQ (multiple-quantum) MASNMR of as-synthesised
Al2(OH)2TCPP reveals a single resonance with a typical quadrapolar line shape (Figure 4A,
Figure S3) similar to that observed for MIL-53(Al).32
(B)
(A)
(C)
(D)
Figure 3 Crystal structure of Ga2(OH)2TCPP. A) 1D rod SBU comprising corner-sharing GaO6
octahedra; Ga2(OH)2TCPP viewed along (B) the z axis; (C) the x axis and D) the y axis.. Hydrogen
atoms are omitted for clarity. Colour codes: Ga, green; N, light blue; C, black; O, red.
(Al2(OH)2TCPP is isostructural with Ga2(OH)2TCPP)
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Although the MO4(OH)2 chain is the same type as that in MIL-53, the Al and Ga-DTOCIP: 1P0.1M039O/CF8sTA10610J
here are rigid. Each chain is linked to eight others by the TCPP linkers, the carboxylate
groups of which bridge adjacent metal cations in each chain. The four phenyl rings attached
to the central pyrazine group are rotated at 90° to the plane of the pyrazine, planar with
their carboxylate groups. Solid-state 13C CP MAS NMR gives only a single resonance for each
of the chemically distinct carbon atoms of the TCPP linker as expected from the crystal
symmetry of the structure (Figure 4B).
3,4
5
1
2
6
Figure 4 (A) 27 Al MAS NMR and (B) 13C CP MAS NMR of Al2(OH)2TCPP. Peaks marked with asterisk
are spinning side-bands. Assignment of 13C peaks in B is shown in scheme (right).
Comparison of the experimental PXRDs of our materials with those simulated from the
single crystal structure of Ga2(OH)2TCPP indicates that the Al phase has been prepared pure
while the Ga phase contains a small amount of an unidentified impurity (Figure 2A). SEM
images of Al2(OH)2TCPP (Figure 2B) showed lenticular crystals while the gallium sample
(Figure S4) showed crystallites of different shapes confirming the presence of impurity. We
concentrated on measuring properties of the pure Al phase. Notably, the previously-
reported Al2(OH)2TCPB preparations contained some pseudo-boehmite impurity in all cases
as detected by 27Al MAS NMR, which are not present in Al2(OH)2TCPP (Figure 4A, Figure S3).
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TGA of as-synthesised samples of Al2(OH)2TCPP carried out in air indicated an initial weight
DOI: 10.1039/C8TA10610J
loss of 26 % attributed to the loss of adsorbed solvent molecules from the pores. The
framework was found to be stable up to 500 °C before linker degradation leads to structural
disintegration (Figure 5A). N2 adsorption at -196 °C gave a type I isotherm (Figure 5B), as
predicted from the structure, with an uptake at p/po = 0.1 of 12.4 mmol g-1 (corresponding
to a pore volume of 0.43 cm3 g-1) and a BET surface area of 1136 m2 g-1. This compares well
with the 1118 m2 g-1 (pore volume of 0.43 cm3 g-1) corrected specific surface area for the
Al2(OH)2TCPB phase by Krüger et al. after taking the boehmite impurity in that preparation
into account.18
(A)
(C)
Figure 5 (A) TGA, (B) N2 adsorption isotherm at -196 °C and (C) CO2 adsorption isotherms (black,
5 °C; red, 15 °C; green, 25°C; blue, 35°C) on Al2(OH)2TCPP.
The material was also investigated for its CO2 uptake, because the availability and low
toxicity of Al, the ease of synthesis of the linker and the presence of accessible hydroxyl and
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amine groups of the MOF suggested potential application as a practical adsorbent. Variable-
DOI: 10.1039/C8TA10610J
temperature CO2 uptake studies of Al2(OH)2TCPP showed CO2 uptake at 5 bar of 8.1 mmol g-
1, 7.2 mmol g-1 , 6.4 mmol g-1 and 5.8 mmol g-1 at 5 °C , 15 °C, 25 °C and 35 °C, respectively
(Figure 5C). All of them showed full reversibility without hysteresis upon desorption. The
uptake of CO2 at 25 °C reached 3 mmol g-1 at 1 bar which is at the upper end of specific
uptakes for MOFs without coordinatively unsaturated metal sites.10d,33 (No CO2 adsorption
has been reported for Al2(OH)2TCPB)). Al2(OH)2TCPP was found to retain its crystallinity after
N2 and CO2 adsorption by PXRD (Figure S5).
When moving to the Sc-TCPP synthesis system, employing the same metal:linker ratio,
changing the metal source to ScCl3.6H2O and varying the solvent mix did not give a
scandium analogue of the Al2(OH)2(TCPP) and Ga2(OH)2(TCPP) phases reported above, but
instead gave the distinct STA-27 phase under a relatively narrow set of conditions (Table
ST1). Using ScCl3.6H2O and H4TCPP as precursors, with metal:linker ratio of 12:1 and a mixed
solvent system of DMF, CH3CN and 3.5 aq. HNO3 (molar ratio Sc: H4TCPP:DMF:CH3CN:HNO3
= 12:1:1311:1946:1608), heating at 150 °C gave phase-pure microcrystalline STA-27 after 16
h. Extending the heating time to 30 h gave lozenge-shaped single crystals of STA-27 suitable
for structure solution by laboratory SCXRD (Figure 6A). The experimental PXRD pattern for
the bulk sample of STA-27 showed good agreement with the pattern simulated from the
solved structure, confirming its phase purity (Figure 6B).
(B)
(A)
Figure 6 (A) SEM image of lozenge-shaped single crystals of STA-27. (B) (a) Simulated PXRD
pattern of STA-27 compared with (b) the experimental pattern of as-synthesised STA-27.
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STA-27 displays monoclinic crystal symmetry (P2/c, a =15.504(7) Å, b = 9.44D3O(4I: )10.Å10,39c/C8=TA10610J
15.975(7) Å, β = 99.277°(14)) with infinite rods containing octahedrally-coordinated
scandium cations running along the z axis, bridged by TCPP, giving rise to two different
diamond-shaped channels parallel to the z axis (Figure 7). The rods are made up of Sc2O11
dimers linked to one another via two carboxylate groups at each end (Figure 7). This
definition of an infinite rod SBU containing carboxylate as well as O atoms linking metal
cations is as described elsewhere.34 Each Sc2O11 dimer comprises two corner-sharing ScO6
octahedra related by symmetry, the six O atoms of each of which include four from the
TCPP linkers (Sc-O distances of 2.041 Å, 2.071 Å, 2.076 Å and 2.080 Å), one bridging µ2O
atom (Sc-O = 2.039 Å) and one O atoms of a terminal ligand (Sc-O = 2.097 Å). Notably, all
these Sc-O distances are longer than those observed in Ga2(OH)2TCPP ((Ga-O bond distance
of 1.983 Å and Ga-OH bond distance of 1.898 Å).
Charge balance requirements of the Sc2O11 dimer suggest that if the µ2O is unprotonated,
the two terminal ligands are water molecules, giving an empirical formula of
Sc2O(H2O)2TCPP. Sc2O11 units have been reported previously in the scandium succinate by
Perles et al.,35 where the Sc in each octahedron was coordinated to five carboxylate O
atoms and a shared µ2OH group, but in that case Sc cations in adjacent dimers are also
connected by two succinate linkers. A similar rod-like SBU to that of STA-27 was reported
for an actinide MOF34,36 but not to our knowledge in transition or main group metal-based
MOFs.
Each TCPP linker in STA-27 is bound to four different rod SBUs, when one set of diagonally
opposite carboxylate groups of the linker connects the ScO6 octahedra from different Sc2O11
dimers to form two parallel rods, while the other set connects the dimers of different
parallel rod building units. As a consequence, the benzoate groups of TCPP are rotated at
50° to its central planar pyrazine core. The resulting diamond-shaped channels of the
structure are of two types, 1 and 2, of Figure 7. In type 1, the narrower dimension of its
opening (3.9 Å free diameter) of the channel (viewed down z) separates dimers of two rods,
while the wider dimension (10.8 Å free diameter) is between two pyrazine rings, while in the
type 2 channels, the shorter distance (4.3 Å free diameter) is between the N atoms of
pyrazine rings while the longer (8.2 Å free diameter) is between the Sc-based rods.
Measured from the crystal structure, channels 1 and 2 are connected via narrow openings
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of 2.5 Å while the openings between adjacent channels of type 2 are only 2 Å, but rotation
DOI: 10.1039/C8TA10610J
of the relevant benzoate group phenyl rings could increase this. The channels are therefore
1D for most molecules.
(A)
(C)
(B)
(D)
(1)
(2)
(E)
Figure 7 Crystal structure of STA-27. (A) Sc2O11 dimer of STA-27, (B) 1D rod SBU of STA-27 made
up of connected Sc2O11 dimers, (C) two distinct diamond shaped pore channels of STA-27 viewed
along the z axis, (D) STA-27 connected via TCPP viewed along x axis, (E) rod SBUs of STA-27
connected via TCPP viewed along y axis. Hydrogen atoms are omitted for clarity. Colour codes:
Sc, lavender; N, light blue; C, black; O, red.
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The determination of the topology of the underlying net in STA-27 is not as straightforward
DOI: 10.1039/C8TA10610J
as it is for other MOFs with rod-like SBUs because there are several ways to deconstruct
STA-27. According to the recommendations given by Schoedel et al. in a recent account on
topology of the MOFs with rod SBUs,34 the carboxylate carbon atoms should be considered
as point of extensions to define the shape of the metal SBU. In STA-27, these nodes (all are
symmetry-related to other) have three links within the rod SBU and one additional to the
tetratopic 4-c linker which altogether give rise to a binodal (4,4)-c net, which can be derived
from the binodal (4,5)-c sqc847 net of the EPINET project37 via splitting the 5-c vertex into
two 4-c vertices and modifying the bond sets accordingly which is accompanied with a
symmetry reduction from P4/mmm and to Pmma. The deconstruction of STA-27 and the
resulting net in its maximum symmetry embedding is visualized in Figure 8. Interestingly and
in contrast to MIL-47, MIL-53 or MIL-71 the rod SBU in STA-27 does not form a zig-zag
ladder but rather a sinoidal double-strand, highlighted in green in Figure 8D. STA-27 is the
first MOF with this particular topology; to the best of our knowledge there is only one other
framework compound, a ZMOF (Zeolitic Metal–Organic Framework), with an underlying
topology that can also be derived from the sqc847 net, which builds also a binodal (4,4)-c
net, however with a different set of edges and a different symmetry;38 the net of this ZMOF
can be found under the entry zec in the RCSR database.39
Figure 8: (A) Carboxylate carbon atoms (shown as larger black) spheres are taking as the points of
extension, (B) which are the connected along the rod SBU, (C) giving rise to the binodal (4,4)-c
net derived from the sqc847 net, (D) in the maximum symmetry-embedding the points of
extension form a sinoidal double-strand, highlighted in green.
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An alternative way of simplifying the structure of STA-27 into the underlying netDiOsI:g10iv.1e03n9/Cb8yTA10610J
the straight-rod representation: In STA-27 there is only a single Sc atom all of which are
bridged by carboxylate groups and each two of the bridging carboxylate carbon atoms point
to the centre of an imaginary point lying almost exactly between two Sc atoms along the
rods (Figure 9A). Connecting these midpoints leads to the binodal (4,4)-c net sqc969. Its
representation with augmented linkers is shown in Figure 9B. From a rigorously
mathematical perspective one could also argue that objects with infinite 1-periodic rods
should be represented by a plane net, i.e. a rod packing. In this case the topology can be
simply described with the 2-periodic binary 4-c net sql (sql-b): The rods have connections to
four linkers in the plane and the linker links 4 rods (Figure 7C).
Figure 9: (A) Straight-rod representation of STA-27: Dummy atoms are placed between each two
Sc atoms and (B) the final binodal (4,4)-c net sqc969 comprising of vertices along straight rods
(green) and the linker which is represented in its augmented version (blue).
45
45Sc is a 100% abundant nucleus (I=7/2) and so Sc MOFs can be analysed by Sc MAS NMR
45
to understand the Sc coordination enviornment.12,14,15 The Sc MAS NMR gives a single
resonance with an isotropic chemical shift of 39 ppm (Figure 10) as determined by a
combination of MQ MASNMR and peak shape matching, which is intermediate between the
values of ScOH chains (δ = 55-62 ppm) and Sc3O trimers (δ = 61-65 ppm) on the one hand
and isolated ScO6 octahedra (Sc2BDC3, δ = 4 ppm) on the other. A plot of the average
quadrupolar product against isotropic chemical shift (Figure 10) distinguishes the local Sc
environments according to their NMR characteristics (Figure S6).
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DOI: 10.1039/C8TA10610J
Figure 10 (A) 45Sc MAS NMR and MQ MAS NMR spectra of STA-27 (asterisk denotes spinning
side-band) and (B) Plot of mean 45Sc isotropic shift <δiso> and quadrupolar product <PQ> for a
series of Sc-containing MOFs with different Sc coordination motifs.40 Red diamond = isolated ScO6
octahedra (Sc2BDC3 and derivatives), blue squares = chains of ScO6 octahedra (MIL-53 and
derivatives) and black triangles = trimers of ScO6 octahedra (MIL-88, MIL-100 and Sc-ABTC, ABTC
= 3,3´,5,5´-azobenzenetetracarboxylate). Points for STA-27 are shown in green before () and
after (+) heating. Reference points were obtained from multiple-quantum (MQ) MAS
experiments carried out at 20.0 T, whereas spectra for STA-27 were recorded at 9.4 T.
TGA of as-synthesised STA-27 showed an initial mass loss of 8% attributed to the
simultaneous evaporation of solvent molecules residing in the pores and water
molecules attached to the clusters. Another mass loss of 7% was observed around
150 °C to 200 °C corresponding to the removal of DMF from the pores and possibly
water from the ScO6 octahedra41 (Figure 11).
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DOI: 10.1039/C8TA10610J
Figure 11 TGA of as-synthesised STA-27.
The response of STA-27 to heating was also studied by VT-PXRD and IR spectroscopy.
VT-PXRD showed that STA-27 remained stable until 150 °C, above which a
transformation to a second phase, displaying broader diffraction peaks, was observed
(Figure 12).
(A)
(B)
Figure 12 (A) VT-PXRD of STA-27 from 25 °C to 225 °C and cooled back to 25 °C. (B) Expanded
low angle view of the PXRD patterns showing the change in peak positions after 175 °C.
IR measurements were performed heating the sample in vacuum up to 300 °C, at which
temperature the STA-27 will have changed phase. Heating at 150 °C results in the removal
of physically-bound water (broad absorbance centred around 3100 cm-1) and the
appearance of a band at 3669 cm-1, which is attributed to the hydroxyl groups or isolated
water molecules bound at Sc3+ sites (Figure 13A). Heating above this temperature results in
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a decrease of the intensity of this band, as chemisorbed species are removed from the Sc3+
DOI: 10.1039/C8TA10610J
coordination sphere. Two probe molecules, CO (at -173 °C) and CD3CN (at room
temperature) were used to monitor the adsorption sites in STA-27 heated at different
temperatures. For CO, a band at 2143 cm-1 that increases for sample heated to 150 °C and
then decreases after heating at 200 °C is attributed to CO bound at hydroxyl groups (Figure
13B). These hydroxyls are of very low acidity as ν(OH) is shifted by -40 cm-1 upon CO
adsorption (see the inset in Fig. 13B). For CD3CN, bands at 2298 and 2277 cm-1 are assigned
to CD3CN binding at Sc3+ sites and a shoulder around 2270 cm-1 evidences interaction with
OH groups. The fact that no Lewis acid sites were detected by CO indicates that CD3CN
displaces water molecules from the coordination sphere of scandium. Thus, IR spectroscopic
studies showed the presence of Lewis acid sites and weak Brönsted sites upon heating the
sample in vacuum at 150 °C, where the original STA-27 material is still stable (Figure 12).
Evacuated at:
RT
...
OH NCCD3
OH−CO
(A)
0.2
(B)
(C)
0.2
0.1
3502
3629
50oC
100oC
150oC
200oC
250oC
300oC
3669
free OH
3669
free OH
0.1
0.3
Phys.
CO
Phys.
CD3CN
3700 3600 3500
3600 3400 3200
Evacuated at:
RT
OH
3669
100oC
150oC
200oC
OH−CO
Sc3+ NCCD3
...
...
OH NCCD3
2139
2120
Wavenumber, cm-1
4000
3500
3000
2200
2160
2080
2320 2300 2280 2260 2240 2220
Wavenumber, cm-1
Wavenumber, cm-1
Figure 13 (A) IR spectra of STA-27 upon heating in vacuum at elevated temperatures. (B) IR
spectra of CO adsorbed on the STA-27 sample pre-heated in vacuum at different temperatures.
(C) IR spectra of STA-27 pre-heated in vacuum at different temperatures upon adsorption of
CD3CN showing the presence of coordinatively unsaturated metal sites.
N2 and CO2 gas adsorption was performed on a sample of STA-27 activated under vacuum at
150 °C to determine its permanent porosity. Moderate uptakes were achieved in each case
(N2, -196 °C, 0.1 bar, 8.2 mmol g-1; CO2, 25 °C, 1.6 mmol g-1 at 1 bar and 3.1 mmol g-1 at 5
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bar; Figures 14A and 14B). The observed N2 uptake of STA-27 was less than that predicted by
DOI: 10.1039/C8TA10610J
Grand Canonical Monte Carlo (GCMC) simulation (10.5 mmol g-1 at 0.1 bar), suggesting
some loss of accessible porosity upon heating and evacuation.
(A)
(B)
Figure 14 (A) N2 adsorption isotherm at -196 °C for STA-27 (black) with the GCMC simulated
isotherm from the crystal structure (red). (B) CO2 adsorption isotherm at 25 °C for STA-27.
Single crystals obtained after heating STA-27 at 225 °C in N2 and then being left to stand in
laboratory air for 4 days showed uniform birefringence under crossed polars in the optical
microscope (Figure 15A) and so were examined by SCXRD in spite of the broad peaks
observed by PXRD (Figure 15B). Remarkably, despite the relatively low quality of the data, it
was possible to obtain a crystal structure of this heated material, denoted STA-27-C (C for
calcined), in the orthorhombic space group Pmna (Table 1, Figure 16). Projection of this
structure down the z-axis shows the same arrangement of ScO6 octahedra and TCPP linkers
as the parent material, but there are important differences that are most clearly illustrated
by views along the y-axis (Figure 16C). The ScO6 octahedra in STA-27-C are isolated, linked
along the z-axis by bridging carboxylate groups from the TCPP linkers. Each Sc3+ atom is still
coordinated by four carboxylate O atoms and a terminal O ligand but the linking Sc-O-Sc
atom is now replaced by two O atoms and the difference Fourier analysis reveals additional
scattering between them. The exact nature of this species is unclear, but our model has a
carbonate or bicarbonate species bridging adjacent Sc3+ cations, which we speculate forms
upon exposure of the heated sample to air. The thermal decomposition of the scandium
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dimer through breaking of the Sc-O-Sc linkage is unambiguous, however, and results in a
DOI: 10.1039/C8TA10610J
phase change that is likely to be topotactic.
(A)
(B)
Figure 15 (A) Optical micrograph of crystals of STA-27-C. (B) (a) Simulated PXRD pattern of STA-
27-C from single crystal structure compared with (b) pattern of STA-27-C after VT-PXRD.
(A)
(B)
(1)
(2)
(D)
(C)
Figure 16 Crystal structure of STA-27-C. (A) 1D rod SBU of STA-27 (right) compared with the
same of STA-27-C (left) the difference between the two SBUs are highlighted in the green sphere.
(B) two distinct diamond shaped pore channels of STA-27-C viewed along the z-axis, (C) STA-27-C
viewed along the y-axis and (D) STA-27-C viewed along the x-axis. Hydrogen atoms are omitted
for clarity. Colour codes: Sc, lavender; N, light blue; C, black; O, red.
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Further information on the transformation comes from 45Sc MAS NMR (Figure 17). The
DOI: 10.1039/C8TA10610J
heated sample shows two resonances, an intense sharp peak (δiso -50 ppm) together with a
broad resonance of much lower intensity. Plotting the mean 45Sc isotropic shift <δiso> and
quadrupolar product <PQ> for STA-27 before and after heating along with those from a
series of Sc MOFs with different Sc coordination (Figure 10), it was found that the value
observed for the major peak of heated STA-27 was near to that of the isolated octahedra, in
agreement with the structure indicated by SCXRD (Figure 16). N2 adsorption of STA-27-C at -
196 °C showed considerable reduction in porosity, which we attribute to the loss of
crystallinity resulting from the phase transition (Figure S7). In addition to the characteristic
13
peaks of TCPP, C CP MAS NMR of STA-27-C shows a signal at 179 ppm, which might be
assigned to carbonate or bicarbonate species similar to those reported previously at shifts
of 168 – 171 ppm.42
(C)
2, 3, 4
5
6
1
*
Figure 17 45Sc MAS NMR of (A) STA-27, (B) STA-27-C and (C) 13C CP MAS NMR Of STA-27-C.
Numbers on (C) corresponds to assignments to C atoms of TCPP as shown in Figure 4.
STA-27 pre-heated at 120 °C was found to be a highly active Lewis acidic catalyst for Friedel-
Crafts Michael addition reactions, imine condensation and carbonyl ene reaction. Its
performance is comparable with that of MIL-100(Sc) (Tables 3-5).12d STA-27 recovered after
catalysis retained its crystallinity (Figure 18), and also retained its crystalline nature over five
catalytic cycles when tested for imine condensation, with slight decrease in the catalytic
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activity in each cycle (Figure S8). This loss in catalytic activity can be due to mass loss in
DOI: 10.1039/C8TA10610J
repeated recycling and activation or due to substrate species occupying the pores.
Table 3: Conjugate addition of 2-methylindole to methyl vinyl
ketonea
Entry
Catalyst
Productb(%)
1
2
3
No Catalyst
STA-27
MIL-100(Sc)
2
70
74
a
Reaction conditions: 1 mmol of 2-methylindole, 1 mmol of methyl
vinyl ketone, 0.5 mmol of 1-methylnaphthalene and 1.5 mol% catalyst
was added to 5 mL DCM and stirred at room temperature for 8 h. The
mol% is calculated according to the molecular weight based on a
b
formula unit containing one Sc-O cluster.
Conversion was
1
determined by H NMR using 1-methylnaphthalene as the internal
standard.
Table 4: Imine condensation between 4’-fluoroacetophenone
and benzylaminea
Entry
Catalyst
Productb(%)
1
2
3
No Catalyst
STA-27
MIL-100(Sc)
2
83
80
aReaction conditions: 1 mmol of 4’-fluoroacetophenone, 1.3 mmol of
benzylamine, 0.5 mmol of 1-methylnaphthalene and 1.5 mol % catalyst
b
was added to 5 mL toluene and heated at 90 °C for 24 h. Conversion
determined by 1H NMR using 1-methylnaphthalene as internal
standard.
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Table 5: Carbonyl ene reaction between ethyl trifluoropyruvate
and α-methylstyrenea
View Article Online
DOI: 10.1039/C8TA10610J
Entry
Catalyst
Productb(%)
1
2
3
No Catalyst
STA-27
MIL-100(Sc)
2
68
75
a Reaction conditions: 2.25 mmol of ethyl trifluoropyruvate, 2.7 mmol
of α-methylstyrene, 0.5 mmol of 1-methylnaphthalene and 1.5 mol %
catalyst was added to 5 mL toluene and stirred at room temperature
b
for 6 h.
Conversion determined by 1H NMR using 1-methyl-
naphthalene as the internal standard.
Figure 18 (a) Simulated pattern of STA-27 with the PXRD patterns of (b) STA-27 before catalysis and
(c) STA-27 recovered after imine condensation reaction. STA-27 was filtered, washed multiple times
with toluene followed by methanol before drying at 100 °C overnight prior to PXRD measurements.
To prove the heterogeneous nature of the catalyst, the MOF was filtered off 5 h into the
imine condensation reaction of Table 4 while the solution was still hot, and the conversion
measured (Figure S9, 41.5%,). The conversion in the solution was re-measured after a
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further 7 h of heating (42.1%). That the conversion stays is so similar after the MOF was
DOI: 10.1039/C8TA10610J
removed confirms the reaction is catalysed heterogeneously. Furthermore, after completion
of the carbonyl-ene reaction between ethyl trifluoropyruvate and -methylstyrene, STA-27
was recovered by removal of the reaction solution via syringe and N2 adsorption and TGA
were measured on the dried MOF. N2 adsorption showed much lower uptake than for
activated STA-27 (ca. 50%) and the TGA of the MOF showed the loss of residual organics
19
above 100 °C (Figure S10). F{1H} NMR of dissolved MOF after N2 porosimetry showed the
characteristic peak of the product, ethyl-2-hydroxy-4-phenyl-2-trifluoromethyl)pent-4-
enoate, at -78.4 ppm (Figure S11). The MOF was then filtered after being immersed in
acetone overnight, washed multiple times with acetone, and dried. N2 adsorption and TGA
then show similar uptake and TGA trace to the original activated STA-27 (Figure S10). Taken
together, these observations indicate that product is formed catalytically within, and can
leave, the pores of the MOF.
In contrast to the high activity of STA-27, STA-27-C was found to have low activity for the
Lewis acid catalysed reactions. For example, while STA-27 showed up to 83 % conversion in
the imine condensation between 4’-fluoroacetophenone and benzylamine, STA-27-C
showed only 36 %. The loss of crystallinity resulting from the phase transition presumably
reduces the accessibility to potential Lewis acidic Sc3+ sites in the framework for substrate
molecules, and hence decreases the catalytic activity of the MOF.
Conclusions
A series of MOFs has been prepared using trivalent metal cations of different ionic radii
(Al3+, Ga3+ and Sc3+) and the tetracarboxylic acid H4TCPP, which is readily formed via a two-
step synthesis using inexpensive reagents. Al and Ga give M2(OH)2TCPP, which is
isostructural to a previously reported Al2(OH)2TCPB, as might have been expected from the
similarity of the linker. Investigation of the Al form shows that it is a thermally stable MOF
which possesses a promising CO2 adsorption performance.
Using ScCl3.6H2O and H4TCPP in a mixed solvent system of DMF, CH3CN and 3.5 M aq. HNO3
gives a new Sc MOF, STA-27. Unlike the Al and Ga forms, which possess a metal-based 1D
rod SBU of infinite corner-sharing octahedra, STA-27 possesses a 1D rod SBU not observed
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Journal of Materials Chemistry A
Page 28 of 34
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previously in trivalent metal MOFs. In this rod, corner-sharing dimers of two ScO6 octahedra
DOI: 10.1039/C8TA10610J
(formula Sc2O11) are linked to give a MOF that possesses two distinct types of parallel
diamond-shaped channels. The material is an active and reusable catalyst for organic C-C
and C=N forming reactions: Sc3+ cations that have had water molecules displaced from their
octahedral coordination shell are thought to be the active Lewis acid sites. The activity is
comparable to that reported for the active Lewis acid catalyst MIL-100(Sc). Upon heating at
200 °C and above, STA-27 transforms to STA-27-C, in which the dimeric Sc2O11 units present
in the rods rearrange to isolated ScO6 octahedra, as shown by X-ray diffraction and solid-
state NMR. The topotactic transformation results in some loss of crystallinity and a
reduction in pore volume and catalytic activity.
Acknowledgements
RRRP, MLC and PAW would like to thank the Engineering and Physical Sciences Research
Council (EPSRC) and CRITICAT Centre for Doctoral Training for Financial Support [Ph.D.
studentship to RRR; EP/L016419/1]. MML and PAW would also like to thank EPSRC for the
FLEXICCS grant (Versatile Adsorption Process for the Capture of Carbon Dioxide from
Industrial Sources; EP/N024613/1). KKC, MYM and KIH are grateful to the Bulgarian National
Science Fund for the financial support (Contract no: DFNI T02/20). We thank Vladislav
Blatov, Davide Proserpio, and Alexandrov Eugeny for helpful discussions regarding the
topological analysis.
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