Remarkable Lewis acid catalytic performance of the scandium trimesate metal organic framework MIL-100(Sc) for C-C and C=N bond-forming reactions

Article abstract of DOI:10.1039/c2cy20577g

The porous metal organic frameworks scandium trimesate MIL-100(Sc), scandium terephthalates MIL-101(Sc), MIL-88B(Sc) and MIL-68(Sc), scandium 4,4′-biphenyl-dicarboxylate MIL-88D(Sc) and the scandium 3,3′,5,5-azobenzene-tetracarboxylate socMOF(Sc) have bee

Full text of DOI:10.1039/c2cy20577g

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Remarkable Lewis acid catalytic performance of the
scandium trimesate metal organic framework MIL-100(Sc)
Cite this: Catal. Sci. Technol., 2013,
3, 606
Q
for C–C and C N bond-forming reactions†
Laura Mitchell,a Berenice Gonzalez-Santiago,a John P. S. Mowat,a Mary E. Gunn,a
Patrick Williamson,a Nadia Acerbi,b Matthew L. Clarke*a and Paul A. Wright*a
The porous metal organic frameworks scandium trimesate MIL-100(Sc), scandium terephthalates
MIL-101(Sc), MIL-88B(Sc) and MIL-68(Sc), scandium 4,40-biphenyl-dicarboxylate MIL-88D(Sc) and the
scandium 3,30,5,5-azobenzene-tetracarboxylate socMOF(Sc) have been compared as Lewis acid catalysts
against Sc3+-exchanged zeolite Beta, MIL-100(Cr), MIL-101(Cr), MIL-100(Fe) and the divalent MOFs
HKUST-1(Cu), CPO-27(Ni) and STA-12(Ni), each of which can be prepared with coordinatively
unsaturated metal sites. The performance of these MOFs has been investigated in several Lewis acid-
catalysed reactions that are of importance in organic synthesis but have rarely been studied using MOF
catalysts. These reactions were (i) the intermolecular carbonyl ene reaction of nucleophilic alkenes and
electron-poor aldehydes, (ii) a Friedel–Crafts type Michael addition between electron-rich heterocycles
and electron-deficient alkenes and (iii) ketimine and aldimine formation. In each of these, MIL-100(Sc) is
both active and selective and significantly outperforms the other catalysts. Filtration and recycle tests
indicate that catalysis over MIL-100(Sc) is heterogeneous. The study of Michael addition reactions
carried out over scandium-bearing MOFs with different window sizes on indole-based substrates of
varying molecular dimensions indicates that most of the catalysis that involves molecules small enough
to enter the pores occurs within the internal pore space. These results indicate MIL-100(Sc) is an
exceptional Lewis acidic MOF catalyst, and suggest that MIL-100(Sc) and new derivatives of it could find
application as recyclable solid catalysts in synthetic chemistry.
Received 16th August 2012,
Accepted 6th October 2012
DOI: 10.1039/c2cy20577g
www.rsc.org/catalysis
pore openings, and therefore have potential in heterogeneous
catalysis with possible shape selectivity. Furthermore, MOFs
1. Introduction
In recent years there has been tremendous interest in the
synthesis and structural characterisation of porous metal
organic frameworks (MOFs).1 While much of this activity has
been driven by possible applications in gas storage and/or
separation2,3 and bio-medical applications4 there is an increas-
ing emphasis on research into the catalytic applications of
MOFs.5–7 Like zeolites, porous crystalline MOFs offer high
surface areas that can host active sites accessible via well-defined
have the advantages over zeolites of both greater chemical
variability and also porosity that can extend into the meso-
porous range, the latter allowing adsorption of larger molecules
than can be taken up by zeolites. However, MOFs have lower
thermal and chemical stability than zeolites, so they are better
suited to the catalytic preparation of fine chemicals rather than
bulk chemical synthesis, where zeolites play the leading role.
Both the framework structure and the local environments
around the metal cations in MOFs can be tuned by altering the
ligands that bridge between the metal centres, even after the
frameworks are synthesised.8 This versatility, reminiscent of
homogeneous organometallic catalysts, suggests that it should
be possible to control selectivity in the synthesis of fine
chemicals and target synthesis over a MOF-based catalyst.
Recyclable heterogeneous MOF catalysts with the desired
molecular features to exert control of regiochemistry, chemo-
selectivity, molecular selection from mixtures and enantioselectivity
a EaStCHEM School of Chemistry, University of St Andrews, Purdie Building,
North Haugh, St Andrews, Fife, KY16 9ST, UK. E-mail: mc28@st-andrews.ac.uk,
paw2@st-andrews.ac.uk; Fax: +44 (0)1334 463808
b Johnson Matthey Technology Centre, Blount’s Court Road, Sonning Common,
Reading, RG4 9NH, UK
† Electronic supplementary information (ESI) available: Full experimental details,
including diffraction and adsorption data on solids and NMR data on organic
compounds is given in the ESI. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cy20577g
c
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are therefore a realistic prospect. Furthermore, as true single family30,31 and the Ni(II) N,N0-piperazine-bismethylenephosphonate
site heterogeneous catalysts of the type described by Thomas,9 of the STA-12 family.32,33
MOFs might have better-defined active site environments
than heterogenised molecular catalysts supported on silica or MOFs and to compare their activity with that of the well-known
polymers. divalent and trivalent large pore Lewis acidic MOFs HKUST-
We therefore aimed to prepare porous scandium-bearing
One approach to using porous MOFs in catalysis is to 1(Cu), CPO-27(Ni), STA-12(Ni), MIL-100(Fe), MIL-100(Cr) and
functionalise a molecular catalyst (for example with carboxylate MIL-101(Cr) in archetypal Lewis acid-catalysed reactions. Here
groups) such that it can become incorporated as a linker within we show that MIL-100(Sc) significantly outperforms the other
the MOF.10 In this approach, the majority of metal ions in the heterogeneous Lewis acid catalysts in a range of synthetically
MOF are inert vertices and the catalytic activity originates from important Lewis acid-catalysed reactions.
the molecular catalyst that is cross-linking the MOF architec-
ture. An alternative approach is to design MOFs where the
metal cations that are integral to the framework can be made
2. Results and discussion
catalytically-active by removal of their solvent ligands to leave a Table 1 summarises the compositions and properties of the
vacant co-ordination site as a Lewis acid so long as the MOF MOFs examined as catalysts. Pore volumes are those measured
remains porous. One of the most widely studied MOFs for either by N2 adsorption at ꢀ196 1C or, for MIL-68(Sc), CO2
Lewis acid catalysis is HKUST-1 (Cu3BTC2, BTC = 1,3,5-benze- adsorption at ꢀ77 1C (ESI†). The window sizes are measured
netricarboxylate),11 which possesses framework Cu2+ cations assuming solvent O atoms are coordinated to the metal cations.
from which water can be removed to leave square planar If these were removed, the effective ring diameters would be
coordinated Cu2+. IR studies of small molecule adsorption increased.
identify these as Lewis acid sites.12 HKUST-1 has been widely
studied as a catalyst13 but few studies compare it with other
2.1 Synthesis and characterization of known scandium
carboxylates
Lewis-acidic MOFs under relevant catalytic reactions. Among
MOFs with coordinatively unsaturated metal sites active as The scandium terephthalates MIL-88B(Sc) and MIL-101(Sc)
Lewis acid catalysts, examples include copper, chromium, iron were prepared following our published syntheses23 or via their
and zirconium carboxylates14–18 and a cobalt phosphonate:19 slight modification. Both possess scandium ‘trimer’ units as
in some cases associated bifunctionality18,20 and/or enantio- the metal-based building units (Fig. 1 and 2). MIL-88B(Sc)
selectivity14 has been introduced.
(Fig. 1) exhibits marked ‘breathing’ behaviour, so that although
In the study reported here, our starting point was the it has little porosity for N2 when in the fully desolvated state, it
knowledge that homogeneous scandium triflate is a very power- adsorbs polar solvents such as methanol to give a greatly
ful Lewis acid catalyst, the discovery of which21 led to major expanded framework with a three dimensionally-connected
advances in the application of Lewis acid catalysis in organic pore system.23 In the case of the MIL-101 structure26 (Fig. 2)
synthesis, even in the presence of water. Although it was not the framework is rigid, and has the same tetrahedral connec-
obvious that scandium coordinated by carboxylate ligands in a tivity as the zeolite topology type MTN where super-tetrahedral
porous solid would be as powerful a Lewis acid as the triflate, building units are at the tetrahedral nodes. In MIL-101(Sc) the
previous studies have shown that non-porous scandium car- scandium trimers are at the vertices of these supertetrahedra
boxylate coordination polymers possess activity in relatively and terephthalate groups link along their edges (Fig. 2). The
facile Lewis acid catalysed reactions, via surface catalysis.22 MIL-101 structure contains two kinds of supercage, the larger of
Furthermore, we have previously reported23 the synthesis of a which at 34 Å, are themselves tetrahedrally-connected via
range of porous scandium carboxylate MOFs with structures windows of 16 Å in free diameter whereas the smaller ones
analogous to the structures MIL-88B,24 MIL-10025 and MIL- (29 Å) are connected to other supercages through windows of
10126 containing other trivalent metals (such as Cr and Fe) that 12 Å. This should lead to very high permanent porosities (a BET
have been described previously. MIL-88B(Sc) and related mate- surface areas of 2800 m2 gꢀ1 has been measured for
rials have also been reported elsewhere27 and a further report of MIL-101(Cr) following activation in vacuum34 and similar
MIL-100(Sc) and MIL-101(Sc) has appeared subsequently.28 The values reported for MIL-101(Sc) activated at low temperatures)28
scandium in [Sc3O(O2CR)6(OH)(OH2)2] trimers in these solids but the highest value we have obtained via N2 adsorption for
can lose coordinated water to leave five-fold coordinated scan- MIL-101(Sc) after evacuation under heating is ca. 640 m2 gꢀ1
,
dium cations that we reasoned would be strong Lewis acid even though the crystallinity is retained (ESI†). Although not
catalysts. Indeed, recent spectroscopic studies have shown that directly relevant to catalytic performance in solutions, this low
they interact strongly as Lewis acidic sites with molecules such surface area following evacuation suggests that the surface
as CO and even H2.29 The CQO stretching frequency of CO structure may recrystallize and result in pore blockage upon
chemisorbed on dehydrated MIL-100(Sc), for example, is 2182 cmꢀ1
,
thermal activation. Furthermore, MIL-101(Sc) was observed to
which is shifted to higher wavenumbers than when adsorbed lose crystallinity upon extended exposure to moist air or heat-
on the coordinatively unsaturated metal cation sites of other ing above 100 1C, as previously observed.28
large pore MOFs, including Cu2BTC3 (HKUST-1),12a the Ni(II)
The large pore scandium trimesate MIL-100(Sc) was pre-
1,4-dioxido-2,5-benzenedicarboxylate of the CPO-27/MOF-74 pared solvothermally using DMF at 150 1C. MIL-100 contains
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Table 1 Summary of composition and properties of MOFs used in catalytic tests
MOF [Formula]
Ligand
Structure
Pore volume/cm3 gꢀ1
Pore sizea/Å
Channels 10
STA-12(Ni) [Ni2L]
0.2
CPO-27(Ni)
[Ni2(DHTP)]
0.47
Channels 11
0.93 (Sc)
0.80 (Cr)
0.90 (Fe)
MIL-100 (Sc,Cr,Fe)
[M3O(OH)(BTC)2]
Cages 25 & 30
Windows 5 & 9
MIL-101 (Sc,Cr)
[M3O(OH)(BTC)2]
0.33 (Sc)
1.46 (Cr)
Cages 29 & 34
Windows 12 & 16
HKUST-1 (Cu)
[Cu3(BTC)2]
Cages 25
Windows 6
0.71
MIL-88B (Sc)
[Sc3O(OH)(BDC)3]
Cage 7
Channel o2
o0.02
MIL-88D (Sc)
[Sc3O(OH)(BPDC)3]
Cage 16
Channel 13
0.47
MIL-68(Sc)
[Sc(OH)(BDC)]
0.63
0.56
Channel 16
socMOF(Sc)
[Sc3O(ABTC)1.5NO3
3D network of 5 Å channels
ꢀ
]
a
Window sizes estimated for materials with solvent O atoms attached to metal site that can become coordinatively unsaturated.
two types of supercage, the larger of which (diameter 30 Å) are MIL-100(Fe) (BET surface area 1363 m2
themselves tetrahedrally-connected via windows of 9 Å whereas (BET surface area 3250 m2 gꢀ1) were also prepared.
the smaller ones (diameter 25 Å) are connected to other super-
Scandium 3,30,5,5-azobenzene-tetracarboxylate, socMOF(Sc),23,35
cages only through windows of ca. 5 Å (Fig. 2). The measured was also prepared for comparison. It also contains scandium
porosity of MIL-100(Sc) to N2 (1.45 cm3 ꢀ1) was close to its trimers, and is highly porous, but the internal pore space is
g
ꢀ1) and MIL-101(Cr)
g
theoretical maximum and MIL-100(Sc) presents the largest internal only accessible via narrow 5 Å channels, so that catalysis would
surface area of the large pore scandium MOFs reported here (typical only be expected to occur on the external surfaces. It therefore
preparations have BET surface areas of 1000–1400 m2
g
ꢀ1). acts as a control for surface activity of a scandium trimer-
For comparison, MIL-100(Cr) (BET surface area 1430 m2 gꢀ1), containing solid.
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Fig. 1 Views parallel to the c-axes (top), of cages (left, below) and approxi-
mately perpendicular to the c-axis of the isoreticular terephthalate MIL-88B (left)
and 4,40-biphenyldicarboxylate MIL-88D (right), with the dicarboxylate linkers
joining metal trimer units, where ScO6 units are represented as octahedra.
MIL-88B is shown in its closed state, adopted in apolar solvents, whereas the
framework MIL-88D is represented in the state of opening measured for
MIL-88D(Sc) in this work. In the material studied this work, it is likely that the
structure is interpenetrated, but the second interpenetrating framework is not
shown.
Fig. 3 The framework structure of as-prepared MIL-68(Sc), Sc(OH)(O2CC6H4CO2),
(left) showing the chains of corner-sharing ScO4(OH)2 octahedra and (right)
viewed down its channel axis, with the position of an extraframework DMF
molecule shown, as determined in this work by constrained Rietveld refinement
of PXRD data.
pore volume close to 0.59 cm3 gꢀ1, much higher than measured
via N2 adsorption. It is therefore likely that surface restructuring
resulting in pore blocking to N2 but not CO2 occurs during
activation at 473 K in vacuum. To check that larger molecules
can enter the pores of MIL-68(Sc) in solution, of relevance for
the catalytic studies reported here, the material was found to
adsorb N-2-hydroxyethyl-ethylenediamine from solution in
toluene (ESI†).
Another scandium dicarboxylate, initially assigned as the
scandium 4,40-biphenyldicarboxylate MIL-88D(Sc), was prepared
for the first time (MIL-88D(Fe) has been reported previously).24
MIL-88D is a larger pore, isoreticular version of MIL-88B, also
containing scandium trimers. Supporting evidence for the
structural assignment was obtained by comparison of the unit
cell symmetry and size determined from indexing the X-ray
pattern with that expected for the MIL-88D framework, and
also by comparing simulated and observed powder X-ray
patterns (see ESI†). Unlike MIL-88B(Sc), MIL-88D(Sc) shows
little structural flexibility upon evacuation or upon immersion
in solvents. The structural framework model consistent
with the unit cell determined from powder X-ray diffraction
Fig. 2 (Left) Connectivity of Sc3O(OH–, H2O)3(O2C–)6 trimers and terephthalate
and trimesate linkers to give supertetrahedra that give MIL-100 and MIL-101
structures, respectively, with the connectivity of the tetrahedra shown in the
stick diagram that represents the MTN topology. (Right, above) The 6- and
5-membered ring windows into smaller and larger cages in the MIL-100
structure. (Right, below) The tetrahedral connectivity of the larger supercages
(represented in light and dark blue) via windows that include six trimer units.
2.1.1 Synthesis of novel scandium MOFs. During exploration (Fig. 1) has 3-dimensional connectivity via large pores, but
of the synthesis of scandium terephthalates the Sc-analogue of N2 adsorption at 77 K indicates that the observed pore
the extra-large pore MIL-68 was prepared for the first time. volume (0.47 cm3 ꢀ1) is much less than that expected from
g
MIL-68 has a framework structure containing a hexagonal array the crystal structure. This fact, taken together with the frame-
of large pore channels approaching 20 Å in diameter, and has work rigidity of the structure, suggests that there could be
been reported previously for V, Fe, Ga, In and Al.36–39 Details of a high degree of interpenetration of the framework structure
the synthesis conditions and structural solution of as-prepared by a second framework. Indeed, recent reports of interpene-
MIL-68(Sc) are given in the ESI,† and the framework structure is trated (and therefore rigid and permanently porous) versions of
shown in Fig. 3. The Sc3+ cations are present in chains of MIL-88D(Fe)40 and MIL-88D(In)41 shows that this is able to
ScO4(OH)2 corner-sharing octahedra so that coordinatively occur, and our measured PXRD is consistent with that.
unsaturated metal sites are not present. The measured perma- Although the quality of the PXRD is insufficient to determine
nent porosity to N2 of MIL-68(Sc) is 0.1 cm3 gꢀ1, much lower the degree of interpenetration, this would explain the low pore
than expected from the crystal structure, and much lower than volume and framework inflexibility. As well as reducing
that reported from N2 adsorption for MIL-68(Ga), 0.46 cm3 gꢀ1
.
the pore volume, interpenetration will reduce the window size
Nevertheless, the uptake of CO2 at 196 K is very high, 14 mmol gꢀ1 and therefore the accessibility of the internal pore volume to
measured on the sample used for catalysis, corresponding to a organic molecules.
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Table 2 Intermolecular carbonyl ene reaction using various catalysts
Activation of the scandium-containing MOFs prior to use in
catalysis was carried out by stirring the material in methanol
for 24 h and drying under vacuum for 5 h at 150 1C, but simple
washing with methanol is also sufficient to activate MIL-100(Sc)
or remove water or products between catalytic reactions that
involve this solid.
For comparison, selected divalent MOFs that can be activated
to give coordinatively unsaturated metal sites were prepared.
These were Cu3BTC2 (HKUST-1) and CPO-27(Ni) and STA-12(Ni).
These MOFs cannot be activated by simple washing and were
therefore activated by heating (100–220 1C) under vacuum for
5 hours. In addition, MIL-100(Cr), MIL-100(Fe) and MIL-101(Cr)
were compared as catalysts with their scandium-bearing
counterparts. We also investigated the catalytic activity of
Sc3+-exchanged zeolite Beta, to examine whether extra frame-
work cations in zeolites could act as catalytically active Lewis
acid centres. The large pore (8 Å) zeolite Beta42 was chosen
because all extra-framework cations are accessible to adsorbed
species. Aqueous cation exchange using scandium nitrate
resulted in incorporation of Sc3+, with the Sc/Al ratio (0.6)
suggesting over-exchange, i.e. the scandium is included with
associated oxy,hydroxy-species. Sc-Beta was heated at 350 1C
prior to use in catalysis to remove physisorbed water.
Reactanta
(%)
Producta
(%)
Hydratea
(%)
Othera
(%)
Entry
Catalyst
1
2
3
4
5
6
7
8
No catalystc
HKUST-1(Cu)b
STA-12(Ni)
85
54
44
42
0
12
31
48
47
99
99
65
29
26
45
24
58
14
4
2
9
7
9
0
0
4
8
17
8
13
5
14
8
21
45
1
6
1
2
1
1
2
4
2
1
10
1
17
5
25
19
CPO-27(Ni)b
MIL-100(Sc)b
MIL-100(Sc)c
MIL-100(Cr)b
MIL-100(Fe)b
MIL-88B(Sc)
MIL-88D(Sc)b
MIL-101(Sc)
MIL-101(Cr)
MIL-68(Sc)
0
29
56
55
46
53
36
55
83
22
16
9
10
11
12
13
14
15
16
socMOF(Sc)
Zeolite H-b
Zeolite Sc-b
32
20
2.2 Lewis acid catalysis
a
b
Determined by 19F NMR. Reactions repeated using 1-fluoronaphtha-
c
2.2.1 Intermolecular carbonyl ene reaction. The catalytic
lene as an internal standard gave the same results. Reaction using
activity of the MOFs as well as the scandium exchanged zeolites as-prepared MIL-100(Sc) washed with methanol rather than activated by
heating under vacuum.
has been measured for a series of C–C bond forming reactions.
The ene reaction was chosen since it has great potential as a
100% atom-efficient method of C–C bond formation.43 Conse- and under the chosen conditions it gives 31% conversion to the
quently if the production of ene products can be carried out desired product. Slightly better performances (47%, 48%) are
with complete recovery of catalyst either by filtration or in flow obtained with the Ni2+ forms of the large pore carboxylate
mode, then pure organic products can be produced without the CPO-27 and the phosphonate STA-12, each of which has metal
need for an additional purification step. A few reports on the sites that are rendered 5-coordinate (and therefore Lewis acidic)
use of MOF catalysts for the intramolecular carbonyl ene by removal of water from the as-prepared materials.
reaction have appeared,13a,20 but the intermolecular reactions
By comparison with these divalent cation MOFs, the perfor-
have not been studied with MOF catalysts (although a hetero- mance of MIL-100(Sc) is very promising, because a single use of
genised molecular complex within a zeolite support has been the catalyst results in almost stoichiometric conversion of
studied44 and relevant computational studies on MOFs have reactants to product (99%), with only a very minor amount of
been performed45). Even in homogenous form, further research hydrate by-product and no isomerisation products. Even with-
is required to increase their effectiveness in synthesis. This out activation by heating under vacuum, full conversion is
class of reaction only takes place readily between a nucleophilic achieved (Table 2, entry 6). This very high activity and selectivity
alkene and an electron-deficient aldehyde or ketone. The reac- is not matched by the other scandium carboxylates or the
tion shown in Table 2 was chosen as a model reaction, because scandium-exchanged zeolites. In addition, conversions over
the presence of the trifluoromethyl group next to the carbonyl MIL-100(Cr) and MIL-101(Cr) while still high (65% and 58%
group enables it to proceed even with Lewis acid catalysts of product, respectively) are not close to that of the scandium
moderate strength,46 sometimes with the generation of isomer- form, and MIL-100(Fe) is significantly less active.
isation products. The catalysts were screened in toluene at a
catalyst loading of 2.5 mol% (metal cation/substrate ratio) and (as might be expected from the presence of the same scandium
the results show that the reaction and reaction conditions are trimer species in their framework as in MIL-100) their activities
Whereas the MIL-88B and ꢀ88D solids do give conversion
well-suited to differentiate the catalysts (Table 2).
are lower (Table 2, entries 9 and 10). Powder diffraction of
Cu3BTC2 (HKUST-1), which has been reported previously to MIL-88B(Sc) indicates that the structure remains in a ‘closed pore’
possess appreciable Lewis acidity upon dehydration because of configuration when immersed in toluene, so that only surface
the generation of coordinatively unsaturated Cu2+ cations, scandium trimer species would be involved. The moderate
might on the basis of the many reports of its catalytic activity activity of MIL-88D(Sc) (45% product) is consistent with
be regarded as a benchmark MOF catalyst. In this ene reaction reduced access of reactant molecules to active sites due to
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Table 3 Recycling of catalysts in carbonyl ene reaction between a-methyl
interpenetration of the structure. MIL-101(Sc) shows low activity,
probably associated with its low stability and the difficulties
experienced in its activation to give high porosity. For
MIL-68(Sc), the low activity can be explained both by pore
blocking (the porosity is much lower than expected from the
crystal structure) and also by the lack of coordinatively
unsaturated Sc3+sites, whereas for the scandium socMOF the
low activity, despite the high porosity of this material and the
presence of scandium trimers, results from the pores being too
small to admit substrate, and also from low surface activity.
Finally, the large pore zeolite Beta containing with Sc3+ was
also found to be an unselective catalyst, giving high yields of
unwanted products. It is likely this results from the presence of
Bronsted acidity associated with hydrolysis on the Sc3+ site,
since a similar pattern of reactivity is observed over the protonic
form. The local environment of Sc3+ within the carboxylate
MOF framework is therefore essential for its selective catalytic
performance.
styrene and trifluoropyruvate
Entry
Catalyst
Cycle
Producta (%)
1
2
3
4
5
6
7
8
STA-12(Ni)
STA-12(Ni)
STA-12(Ni)
MIL-100(Sc)b
MIL-100(Sc)b
MIL-100(Sc)
MIL-100(Sc)
MIL-100(Sc)
1
2
3
1
2
1
2
3
64
63
63
55
54
99
96
95
a-Methyl styrene (1.2 mmol) and ethyltrifluoropyruvate (1 mmol) added
to a suspension of activated MOF (2.5 mol%) in toluene (5 ml) and
stirred at 20 1C for 8 h (MIL-100(Sc)) or 16 h (STA-12(Ni)). The amount of
reactant used was adjusted in each recycle to account for small losses in
b
catalyst mass. a Determined by 19F NMR. Conversion of reaction after
3 h using 1 mol% catalyst, see ESI for graphs of conversion vs. time.
1-Fluoronaphthalene used as internal standard.
Table 4 Carbonyl ene reaction of ethylglyoxylate
The MIL-100(Sc) catalyst therefore possesses Lewis acid sites
accessible to reactant molecules (Sc3+ within the trimers) that
are able to activate the carbonyl-containing reactant to addition
and selectively catalyse the intermolecular ene reaction. The
ease of activation of MIL-100(Sc) to give a highly porous solid is
of significant practical importance in giving a useful catalyst,
and simply washing the hydrated solid with methanol is an
easier activation process than for the other catalysts examined.
Furthermore, the 30 Å supercages that are tetrahedrally-
connected via 9 Å windows (Fig. 2) are large enough to allow free
passage of reactant and product molecules. Three-dimensional
connectivity of supercages will greatly increase the ease of
molecular transport through the crystals. (There are other cages
within the MIL-100 structure that are large enough to host the
reactant molecules but these are only accessible via windows
5 Å in diameter, too small to take in reactant molecules.)
To confirm that the catalysts can be re-used, recycling
reactions were performed for both STA-12(Ni) and MIL-100(Sc),
each time slightly adjusting the amount of substrate and
solution according to the amount of catalyst recovered to keep
the ratio the same in each case. This was necessary since these
are small scale reactions. For STA-12(Ni) no change in either the
crystallinity (as measured by PXRD on samples recovered, see
ESI†) or the catalytic performance was measured over three
Entry Catalyst
Alkene
Product
Product (%)
0
1
No catalyst
MIL-100(Sc)
2
3
’’
’’
’’
84
6
HKUST-1(Cu) ’’
4
No catalyst
48
5
6
7
MIL-100(Sc)
’’
’’
’’
99
97
57
MIL-100(Sc)a ’’
HKUST-1(Cu) ’’
8
No catalyst
55
9
10
MIL-100(Sc)
HKUST-1(Cu) ’’
’’
’’
’’
99
75
Alkene (2.7 mmol) and enophile (2.7 mmol) added to a suspension of
activated MOF (5 mol%) in toluene (5 ml) and stirred at 90 1C for 8 h.
a
Conversion determined by 1H NMR. 2.5 mol% of catalyst/substrate.
catalytic tests (Table 3). Furthermore, if either catalyst is Depolymerised ethylglyoxylate (distilled at B150 1C) reacted
removed during the reaction, the conversion of substrate is with a-methylstyrene, methylene cyclohexane and methylene
stopped. The catalytic reaction was proved to be fully hetero- cyclopentane in the presence of MIL-100(Sc) to give conversions
geneous in this way. Further recycling tests were carried out of 97–99%, considerably higher than over HKUST-1 in each
using MIL-100(Sc) that also did not show any significant loss of case. For example, using the highly activated methylenecyclohexane
crystallinity. A reduced catalyst loading (1 mol%) was used to HKUST-1 gave 57% conversion, only 9% higher than the control
give conversions that could readily be followed by our experi- reaction.
mental method. Comparison of reaction over as-prepared
(methanol-washed) and once-used catalysts showed there was
no significant reduction in performance (see ESI† and Table 2, these studies we noticed that the hydrate by-product sometimes
entries 4 and 5). converted into the desired product. The ability to promote
Tandem deprotection carbonyl ene reactions. In the course of
The strong performance of the MIL-100(Sc) catalyst in this dehydration and other related processes is significant because
ene reaction of an activated carbonyl group encouraged us to all electron-poor aldehydes are either supplied in a protected
investigate it in reactions of other carbonyl compounds (Table 4). form (polymerised, hemiacetal, hydrate) or readily decompose
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loading of 10 mol%.48 At 10 mol% loading of MIL-100(Sc) the
same reaction was 91% complete after 1 hour. The results of the
reaction with 2-methylindole over a series of different Lewis
acidic MOF catalysts are given in Table 6 and the results of the
reactions of other indoles and pyrrole are given in Table 7. No
reaction occurs in the absence of a catalyst, but homogeneous
scandium triflate gives high conversion in a range of solvents.
Using dichloromethane as a solvent, MIL-100(Sc) is again
the most active of the MOF catalysts (99% conversion).
Conversions over MIL-100(Cr), MIL-100(Fe), MIL-101(Cr),
CPO-27(Ni), MIL-88D(Sc) and MIL-101(Sc) are lower but the
solids are still active catalysts. MIL-88B(Sc), which has similar
metal cation sites to MIL-100(Sc) but remains in the closed
form in CH2Cl2, socMOF(Sc), MIL-68(Sc) and STA-12(Ni) show
very low activity. The low activity of MIL-88B(Sc) and
socMOF(Sc) indicates (by comparison) that most of the Lewis
acidic scandium trimer sites responsible for the catalysis in
the active MIL-100(Sc), MIL-101(Sc) and (to a lesser extent)
MIL-88D(Sc) are in the nanopores and not on the surface (as
must be the case for MIL-88B(Sc) and socMOF(Sc)). The low
activity of MIL-68(Sc) indicates coordinatively unsaturated Sc3+
Table 5 Tandem deprotection carbonyl ene reactions
Entry Catalyst
Reactant
Product
Product (%)
1
2
No catalyst
MIL-100(Sc) ’’
0
’’
’’
99
3
4
No catalyst
0
MIL-100(Sc) ’’
99
5
6
No catalyst
0
MIL-100(Sc) ’’
’’
99
a-Methyl styrene (2.7 mmol) and enophile (2.7 mmol) were added to a
solution of activated MOF (5 mol%) in toluene (5 ml) and the solution
was stirred at 20 1C for 16 h (entries 1 and 2) or 90 1C for 8 h (entries 3–6).
Conversion determined by 1H NMR.
to a hydrate or polymer. We therefore investigated three different sites are required for activity, while the poor activity of
tandem processes (Table 5): (i) the dehydration-ene reaction STA-12(Ni) compared to CPO-27(Ni) could be related to the
of ethyl trifluoropyruvate hydrate, (ii) the depolymerisation- limited accessibility of its five-fold coordinate Ni2+ cations
carbonyl ene reaction of ethyl glyoxylate and (iii) the deprotection- compared with that of the sites in the more active CPO-27(Ni).
carbonyl ene reaction of trifluoroacetaldehyde methyl hemiacetal.
The conversion is found to be strongly solvent-dependent:
The MIL-100(Sc) catalyst is able to fully convert the hydrated for MIL-100(Sc) the reaction proceeds readily in the presence of
form of the ethyltrifluoropyruvate with >99% selectivity so it
can promote a tandem dehydration carbonyl ene reaction.
Since ethyltrifluoropyruvate forms its hydrate on exposure to
Table 6 Conjugate addition of indole to methylvinyl ketone
air, this is a useful feature. MIL-100(Cr) also performs well in
the reaction, but does not equal MIL-100(Sc). The Ni-containing
catalysts are not active.
The use of ethyl glyoxylate as a substrate in this reaction is
complicated by its occurrence as a polymer that needs to be
cracked at high temperature before use. To our knowledge,
there are only two reports on particularly active homogeneous
catalysts that use ethyl glyoxylate polymer directly.47 In the case
of MIL-100(Sc) pure product was obtained directly from the ene
reaction with using the polymeric form of ethyl glyoxylate, by
simple filtration and removal of solvent after the reaction
(Table 5, entry 4); in the case of this reaction without a catalyst
there is no conversion. Pure product is also obtained over
MIL-100(Sc) if methylenecyclohexane is used as a reactant.
The reaction was also carried out with trifluoroacetaldehyde
methylhemiacetal; in order to carry out this carbonyl ene
reaction the removal of methanol to form trifluoroacetaldehyde
is required before reaction with methylstyrene can occur.
MIL-100(Sc) catalysed the removal of methanol and subsequent
ene reaction with >99% selectivity (Table 5, entry 6).
Entry
Catalyst
Solvent
Producta (%)
1
2
3
4
5
6
7
8
No catalyst
Sc(OTf)3
Sc(OTf)3
CH2Cl2
CH2Cl2
CH3CN
Methanol
CH2Cl2
Toluene
Methanol
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
CH2Cl2
CH2Cl2
0
99(99)
89
90
99(99)
93
15
39
72
73
79(75)
40
10(9)
68(68)
11(10)
51(50)
56
12
66(65)
61
Sc(OTf)3
MIL-100(Sc)c
MIL-100(Sc)
MIL-100(Sc)
MIL-100(Sc)
MIL-100(Sc)b
MIL-100(Sc)b
MIL-100(Cr)c
MIL-100(Fe)
STA-12(Ni)
CPO-27(Ni)c
HKUST-1(Cu)c
MIL-101(Sc)
MIL-101(Cr)
MIL-88B(Sc)
MIL-88D(Sc)c
MIL-88D(Sc)
MIL-68
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2.2.2 Friedel–Crafts Michael addition reaction. The Friedel–
Crafts Michael addition of indoles and pyrrole to methyl vinyl
ketone was chosen as another synthetically important and fully
atom-efficient C–C bond-forming reaction. Indium(III) chloride is
perhaps the best homogeneous catalyst for this type of reaction,
delivering high yields of 92% after 2.5 hours using a catalyst
7
0
socMOF(Sc)c
a
b
From 1H NMR. Remaining mass balance is reactant. Conversion
after 2 h using 5 mol% catalyst, see ESI for graphs of conversion vs.
time. Reactions repeated using 1-methylnaphthalene used as internal
standard and gave same results.
c
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Table 7 Reaction of indoles with vinyl ketonea
heterogeneously as after catalyst removal from the reaction
no further conversion was observed (ESI†).
Entry
Catalyst
Indole
Ketone
Productb (%)
Other variations of the reaction were attempted (Table 7).
MIL-100(Sc) gave high conversions (ca. 90%) when N-methyl
indole and indole were used as substrates in reaction with
methyl vinyl ketone, and when 2-methylindole was reacted with
phenyl vinyl ketone.
Reaction of pyrrole with methyl vinyl ketone over the Lewis
acidic MIL-100(Sc), CPO-27(Ni) and HKUST-1 gave mixtures of
2-alkyl pyrrole and 2,5-dialkylpyrrole (Table 8), with the highest
conversion and a ratio of 6 : 1 in favour of the mono-substituted
pyrrole occurring over MIL-100(Sc).
1
No catalyst
0
2
3
4
MIL-100(Sc)c
MIL-100(Cr)
MIL-100(Fe)
’’
’’
88(88)
36(35)
25
5
Sc(OTf)3
’’
90(90)
Finally for this reaction, the two bulky indoles 5-(4-(tert-
butyl)phenyl)-1H-indole and 5-(4-phenoxyphenyl)-1H-indole were
prepared and used as substrates in the reaction with methyl
vinyl ketone. Successful reaction would result in the conver-
sion of 5-(4-tert-butyl phenyl)-1H-indole (ca. 15 Å ꢁ 7 Å) to
4-(5-(4-(tert-butyl)phenyl)-1H-indol-3-yl)butan-2-one (15 Å ꢁ 12 Å).
Both bulky indoles reacted readily in the presence of homo-
geneous scandium triflate, as did indole itself, but conversions
over MIL-100(Sc) were much lower (and lower still over
MIL-88B(Sc), socMOF(Sc) and CPO-27(Ni)) as seen in Tables 9
and 10. Additionally, no colour change was observed for the
solids after the experiment, in contrast to the adsorption of
products discussed previously, and N2 adsorption isotherms
6
7
MIL-100(Sc)
MIL-88B(Sc)
’’
’’
’’
’’
89(87)
15
8
MIL-100(Sc)
91(90)
a
Indole (0.84 mmol) and methyl vinyl ketone (0.84 mmol, 0.07 ml)
added to suspension of MOF in CH2Cl2 (5 ml) and was stirred for 6 h at
room temperature under N2. Determined by 1H NMR. Mass balance is
b
c
reactant. Reaction repeated using 1-methylnaphthalene used as inter-
nal standard and gave same result.
dichloromethane and toluene (Table 6, entries 5 and 6), but not measured before reaction, after reaction and after subsequent
in acetonitrile (39%) or methanol (15%) (entries 7 and 8). This methanol washing gave the same uptakes (see ESI†).
indicates that coordinating solvents compete effectively with
The products of these reactions are too large to be formed in
the substrate for the Sc3+ sites of MIL-100 when present in large the channels of CPO-27(Ni) (10 Å) or, if formed, to diffuse out of
excess. Notably, washing the catalyst with methanol does not the internal pore space of MIL-100(Sc) (the larger cages of
reduce catalytic performance, indicating the methanol can be which have a diameter of 30 Å but are connected by windows
displaced if not in an excess. After reaction, MIL-100(Sc) was only 9 Å across). The low conversions indicate that the surface
filtered off and found to have turned brown. N2 adsorption at activity is very low, even with scandium trimers. By corollary,
77 K showed a reduction in uptake from 25 mmol gꢀ1 to the high reaction rates of the smaller indoles must result from
20 mmol gꢀ1 indicating that some of the product and/or reaction within the pore space of MIL-100(Sc). It is possible that
reactants remained in the pore. Washing of the material with the conversion of 33% observed over MIL-100(Sc) could have
methanol reversed the colour change and the original porosity taken place within cages close to the surface.
was restored. NMR analysis of the material recovered in the
2.2.3 Imine synthesis. The formation of an imine from the
filtrate after methanol washing confirmed that 8% (0.016 g) of condensation of amines with carbonyl compounds is another
product had been retained in the catalyst. The MIL-100(Sc) synthetically useful reaction that can be catalysed by Lewis acids.
catalyst remains crystalline after reaction, and can be recycled While aldimine synthesis is trivial the more useful ketimines
successfully (ESI†). MIL-100(Sc) was also found to behave present greater challenges. Although most homogenously-catalysed
Table 8 Michael reaction of pyrroles with methyl vinyl ketone
Entry
Catalyst
Producta (%)
Ratio of di- to mono- product
1
2
3
4
No catalyst
MIL-100(Sc)
HKUST-1(Cu)
CPO-27(Ni)
35
89
39
41
1 : 3
1 : 6
1 : 3
1 : 6
a
Determined by 1H NMR.
c
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Table 9 Tert-butylphenylindole reaction with methyl vinyl ketone
Entry
Catalyst
Producta (%)
1
2
3
4
5
6
7
8
No catalyst
Sc(OTf)3
0
99
10
0
33
15
9
HKUST-1(Cu)
CPO-27(Ni)
MIL-100(Sc)
MIL-88B(Sc)
MIL-88D(Sc)
socMOF(Sc)
0
a
Determined by 1H NMR.
Table 10 Phenoxyphenylindole reaction with methyl vinyl ketone
Entry
Catalyst
Producta (%)
1
2
3
4
Sc(OTf)3
84
24
8
MIL-100(Sc)
MIL-88B(Sc)
MIL-88D(Sc)
6
a
Determined by 1H NMR.
reactions would benefit from a recyclable heterogeneous it was found that MIL-100(Sc) achieved 93% conversion after
alternative, this is especially true for ketimines which are 1 h and went to complete conversion after 3 h. However, even
generally prepared via catalysis using the Lewis acid ZnCl2 the uncatalysed reaction achieved 63% conversion after 1 h. For
followed by vacuum distillation if they are volatile enough the more challenging ketimine formation, the reaction was
(they can decompose on silica during purification).49 Complete carried out with p-anisidine, butylamine, chlorobenzylamine
conversion in these reactions with a recoverable catalyst would and (S)-1-phenylethanamine. The reaction of p-anisidine at
resolve this complicating issue for organic methodology using 90 1C gave complete conversion to product when an excess of
ketimines.
the amine was used – the pure product could be isolated using
Here we report the results of catalysis of the reaction of acid washing. The aliphatic butyl amine reaction was performed
40-fluoroacetophenone and benzylamine over MIL-100(Sc) and, at 70 1C, the boiling point of the amine and gave a conversion of
for comparison, HKUST-1 (Table 11). It was found that MIL-100(Sc) 70%. When using chlorobenzylamine the reaction temperature
outperformed other MOF materials, and by increasing the temp- could be lowered to 70 1C by changing the solvent to hexane and
erature of the reaction conversion could be increased significantly a high conversion was still attained. (S)-1-phenylethanamine
to 85% conversion at 100 1C, considerably higher than if no required the use of Dean Stark apparatus at high temperatures
catalyst is present. This conversion was also seen when for high conversion.
MIL-100(Sc) was recovered, washed with methanol, dried and used
2.2.4 Discussion. MIL-100(Sc) is an excellent, readily regen-
in another reaction. Simply increasing the concentration and erable and re-usable catalyst for a range of synthetically interest-
relative amounts of the amine in this equilibrium reaction enables ing C–C and CQN bond-forming reactions. It easily outperforms
complete conversion to imine to occur (Table 11, entry 11). The HKUST-1, widely reported as an active Lewis acidic MOF catalyst,
pure imine can be isolated by simple filtration and acid washing. and an array of other MOFs with metal sites that can become
In order to obtain a preliminary indication of the scope of coordinatively unsaturated upon loss of coordinating solvent,
the reaction, different carbonyl compounds and amines were including other scandium and chromium carboxylate MOFs.
reacted in the presence of MIL-100(Sc) (Table 12). For the
The active sites of MIL-100(Sc) are the Sc3+ cations that form
reaction of 4-fluorobenzaldehyde with benzylamine at 20 1C, part of Sc3O(OHꢀ,(OH2)2)(O2C–)6 trimeric units. Two out of
c
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Table 11 Imine formation reaction of benzyl amine
Entry
Catalyst
Producta (%)
1
2
3
4
No catalystb
MIL-100(Sc)b
MIL-100(Sc)c
No catalyst
3
49
60
8
5
6
7
8
9
10
11
HKUST-1(Cu)
MIL-100(Sc)
MIL-100(Sc)d
MIL-100(Cr)
HKUST-1(Cu)
MIL-100(Sc)e
MIL-100(Sc)f
20
85
85
20
11
92
>99
a
e
b
c
d
Determined by 1H NMR. Reaction carried out at room temperature. Reaction carried out at 60 1C. Recycled MIL-100(Sc) at 100 1C.
f
1.5 equivalents of benzylamine. Using 2 equiv. benzylamine and 1-methylnaphthalene as internal standard.
Table 12 Imine formation using various amines and MIL-100(Sc)
as methanol, the activity of MIL-100(Sc) for the conjugate
addition of electron deficient olefins to indoles is strongly
reduced – one explanation is that the solvent remains bound
to the Sc3+ active sites.
None of the other scandium MOFs containing trimers that
we have prepared are nearly as active as MIL-100(Sc). For
socMOF(Sc) and MIL-88B(Sc) in non-polar solvents this indicates
that the activity derives from the internal surface area rather
than from trimers at the external surface: low conversions in
experiments using bulky substrates confirm (by corollary) that
for the reaction of substrates smaller than the window size the
majority of the reaction comes from within the pores.
Entry
R1
R-NH2
Producta (%)
1b
2c
H
H
63
93
’’
It would not have been possible to predict the greatly
superior performance of MIL-100(Sc) compared to that of
MIL-101(Sc) or MIL-88D(Sc) from their idealized crystal struc-
tures. Instead, the practical advantages of MIL-100(Sc) such as
its ease of activation, porosity and stability against recrystalli-
zation are the critical factors. MIL-100(Sc) is readily prepared
with high surface areas, has a 3-dimensionally connected array
of mesoporous cages and is stable in moist air and under the
reaction conditions used. By contrast, MIL-101(Sc) is less
stable, recrystallizing to MIL-88B(Sc) under certain conditions,
and, at least in these experiments, could not be prepared with
surface areas close to those observed for its chromium analogue.
Its lower catalytic activity is likely to derive from these unsuitable
materials properties. The most likely reason for the lower than
expected activity of MIL-88D(Sc) is the occurrence of interpene-
tration, leading to reduction of pore volume and window size,
and it may be that modified preparations could give more
porous and active catalysts based on this solid. From the current
work, however, MIL-100(Sc) is the catalyst of choice.
3
CH3
82
4d
5
CH3
CH3
’’
>99
70
6e
7d
CH3
CH3
91
85
’’
’’
8f
CH3
96
9d
CH3
72
a
Determined by 1H NMR using 1-methylnaphthalene as internal
b
c
standard. Stirred at 20 1C, no catalyst. Stirred at 20 1C.
d 2 equivalents of amine. e 2 equivalents of amine used in hexane at 70 1C.
f
2 equivalents of amine used with Dean Stark apparatus at 120 1C.
three Sc3+ cations in each trimer are coordinated by water
molecules, which are expected to undergo ready exchange with
3. Conclusion
other coordinating solvents and possibly carbonyl O atoms of A series of MOFs were investigated for activity in different Lewis
the substrates. It is notable that in coordinating solvents such acid-catalysed reactions. MIL-100(Sc) was found to be very
c
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11 S. S. Y. Chui, S. M. F. Lo, J. P. H. Charmant, A. G. Orpen and
I. D. Williams, Science, 1999, 283, 1148–1150.
active in each of these reactions, and outperformed the known
and newly-synthesised MOFs tested in this work. In many cases
these catalytic reactions achieved full conversion of reactants
and are therefore synthetically useful. Not only were high
conversions attained but MIL-100(Sc) could be recycled without
significant loss in activity or crystallinity. The bulk of the
catalysed reactions take place in the pore and not on the
external surface of the MOF. These findings point towards
MIL-100(Sc) being one of the most useful MOFs for simple
Lewis acid catalysis. Further exploration of the activity of
MIL-100(Sc) in catalysis in batch and flow mode as-prepared
or modified with additional catalytic functionality is in progress.
12 (a) C. Prestipino, L. Regli, J. G. Vitillo, F. Bonino, A. Damin,
C. Lamberti, A. Zecchina, P. L. Solari, K. O. Kongshaug and
S. Bordiga, Chem. Mater., 2006, 18, 1337–1346;
(b) S. Bordiga, L. Regli, F. Bonino, E. Groppo, C. Lamberti,
B. Xiao, P. S. Wheatley, R. E. Morris and A. Zecchina, Phys.
Chem. Chem. Phys., 2007, 9, 2676–2685.
13 (a) L. Alaerts, E. Seguin, H. Poelman, F. Thibault-Starzyk,
P. A. Jacobs and D. E. De Vos, Chem.–Eur. J., 2006, 12,
7353–7363; (b) N. B. Pathan, A. M. Rahatgaonkar and
M. S. Chorghade, Catal. Commun., 2011, 12, 1170–1174.
14 K. S. Jeong, Y. B. Go, S. M. Shin, S. J. Lee, J. Kim, O. M. Yaghi
and N. Jeong, Chem. Sci., 2011, 2, 877–882.
Acknowledgements
15 A. Henschel, K. Gedrich, R. Kraehnert and S. Kaskel, Chem.
Commun., 2008, 4192–4194.
16 A. Dhakshinamoorthy, M. Alvaro, H. Chevreau, P. Horcajada,
T. Devic, C. Serre and H. Garcia, Catal. Sci. Techol., 2012, 2,
324–330.
17 L. Kurfirtova, Y.-K. Seo, Y. K. Hwang, J.-S. Chang and
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18 F. Vermoortele, R. Ameloot, A. Vimont. C. Serre and D. E.
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We gratefully acknowledge funding from the Engineering and
Physical Sciences Research Council (EPSRC) and Johnson-
Matthey PLC for a CASE award (LM), from CONACYT, Mexico
(BG-S) and Nuffield foundation for vacation scholarship (PW).
M. Borges Ordono (Johnson-Matthey) is thanked for providing a
sample of MIL-100(Fe). Professor A. M. Z. Slawin (Univ. St. Andrews)
is thanked for collection of single crystal diffraction data.
19 M. J. Beier, W. Kleist, M. T. Wharmby, R. Kissner,
B. Kimmerle, P. A. Wright, J.-D. Grunwaldt and A. Baiker,
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