Interplay between hydrophobicity and basicity toward the catalytic activity of isoreticular MOF organocatalysts

Article abstract of DOI:10.1039/c6nj00480f

Due to the structural diversity of metal-organic frameworks, MOFs, tailored engineering of these compounds is important for their use in catalytic processes. Among the MOFs tested as heterogeneous catalysts, there have been rare reports of size selective catalysts. In the present work, we could successfully indicate that subtle substrate selectivity can be induced in the catalytic system by designing a series of isoreticular MOFs with slight structural modifications. Four MOF catalysts possessing imine and/or amine basic N-donor pillars bearing phenyl or naphtyl cores showing different hydrophobic characters around the basic reaction center were prepared via a simple mechano-chemical synthesis. They were characterized thoroughly using TG, IR and PXRD analysis. For the first time, the aldol-type condensation reaction of malononitrile with ketone-functionalized carbonyl substrates was developed in the presence of the basic MOF organocatalysts. Moreover, it has been successfully shown that a subtle substrate selectivity can be addressed during the reaction of three slightly different α,β-unsaturated carbonyl compounds in contrast to the effect of size control barriers that commonly direct a heterogeneous reaction pathway.

Full text of DOI:10.1039/c6nj00480f

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Interplay Between Hydrophobicity and Basicity toward
the Catalytic Activity of Isoreticular Organocatalyst
MOFs
Sedigheh Abedi, Alireza Azhdari Tehrani, Hosein Ghasempour, and Ali Morsali*
Department of Chemistry, Faculty of sciences, Tarbiat Modares University, PO. BOX 14115-
4838, Tehran, Iran.
KEYWORDS: Metal-organic framework, Heterogeneous catalysis, Aldol condensation,
Hydrophobicity, Substrate selectivity
Abstract
The structural diversity of metal-organic frameworks, MOFs, tailored engineering of these
compounds is important for their use in catalytic processes. Among the MOFs tested as
heterogeneous catalysts, there have been rare reports of size selective catalysts. In the present
work, we could successfully indicate that subtle substrate selectivity can be induced in the
catalytic system by designing a series of isoreticular MOFs with slight structural modifications.
Four MOF catalysts possess imine and/or amine basic N-donor pillars bearing phenyl or naphtyl
cores owing different hydrophobic character around the basic reaction center were prepared via
1

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simple mechano-chemical synthesis. They were characterized thoroughly using TG, IR and
PXRD analysis. For the first time, aldol-type condensation reaction of malononitrile with ketone-
functionalized carbonyl substrates developed in the presence of the basic MOF organocatalysts.
Moreover, it has been successfully showed that a subtle substrate selectivity can be addressed
during the reaction of three slightly different α,β-unsaturated carbonyl compounds in contrast the
effect of size control barriers that commonly direct heterogeneous reaction pathway.
Introduction
High surface area, structural diversity and thermal stability of MOFs have attracted extreme
research interests considering their promising applications in a wide range of potential
applications such as catalysis,1 gas storage,2 3 air purification,4 chemical sensing,5 biomedicine,6
and drug delivery.7, 8 Designing heterogeneous MOF embedded with catalytic active sites is
generally performed depending on the final catalytic activity of interest. Two general
catalytically active groups can be divided as (I) MOFs containing catalytic metal Lewis acid
active sites, and (II) MOFs including catalytic active functional organic sites. In the former
subclasses of these catalysts that are the most popular cases, MOF structure has intrinsically
unsaturated metal nodes or employed as a support for stabilizing transition metal nanoparticles.1,
9
In the latter subclass of heterogeneous MOF catalysts, which is less developed, the actual
catalytic sites are the organic functional groups that are the organic linkers themselves used for
MOF construction or can be incorporated within the framework by post-synthesis
modifications.10, 11
Among common Lewis acid based MOF catalysts, those bearing basic functional groups provide
deserving heterogeneous catalysts for organic transformations. It has been indicated that NH2-
2

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funtionalized linkers, resulted in improved basicity and catalytic activity of these MOF
structures. Actually, overall catalysis reactions proceeded via synergic effect of the Lewis acid
metal nodes placed at neighbouring basic sites on the pore surface of MOF structure eventually
15 16, 17
make bifunctional acid-base heterogeneous catalysts.12-14
On the other side,
accommodation of the supramolecular organocatalysts especially Brønsted-acidic groups such as
urea, thioura and squaramide into MOF have attracted specific attentions. The improved
activities of MOFs containing these potential organocatalytic groups have been proved for
22
Friedel-Crafts,18-20 Henry,21 acetalization, Morita-Baylis-Hillman, reactions and ring opening
of epoxides.23
Application of MOFs as heterogeneous organocatalyst with pure Brønsted-basic moieties has
only been scarcely explored. Functionalization of the organic linkers, either by pre- or post-
synthetic modification within the MOF structures including saturated metal ion nodes can reveal
the role of these types of solid catalysts that are more important analogous for pharmaceutical
synthetic applications. The Knoevenagel condensation is a typical test reaction for the evaluation
of basic catalysts,24, 25 which is the most common used reaction for investigation of MOF
containing basic functionalized structures due to its amenable reaction conditions and analysis.26
Therefore, more competent catalytic systems with high efficiency for sluggish organic reactions
are desirable in this immature area. Structural tunability of MOF materials provides the
possibility of designing the framework with potential capability toward catalyzing organic
reactions. A feature that has rarely been taken into consideration for improving catalytic activity
of MOFs, is hydrophobicity character of the pore surface. Controlling the hydrophobicity
character has been usually performed to reduce the wettability of the pore surfaces to eventually
create moisture-stable MOF structures.27 In this regard, introduction of the organic moieties
3

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inside the pores of MOFs has been substantially developed upon post-synthetic modification.10,
11, 28
It has been known that this physical property of MOFs could improve through introduction
of hydrophobic alkyl chains. Accordingly, increasing the hydrophobic nature of these MOFs
can mainly change their capability for gas storage and water adsorption properties.3, 27, 29
Recently, Farrusseng and co-workers studied the effect of hydrophilic–hydrophobic balance
environment surrounding the catalytic sites using ZIF-type materials in Knoevenogel
condensation.30, 31
In this contribution, for the first time we demonstrated that tuning the hydrophobicity properties
of the MOF walls can adjust the permeation characteristics of substrates and a type of subtle
substrate selectivity can be directed toward their potential catalytic activities.
Experimental Section
Apparatus and Reagents
Zinc(II) nitrate hexahydrate, zinc(II) acetate dihydrate and 4,4'-oxybis(benzoic acid) (H2oba)
were purchased from Aldrich and Merck Company and used as received.
IR spectra were recorded using Thermo Nicolet IR 100 FT-IR. The thermal behaviour was
measured with a PL-STA 1500 apparatus with the rate of 10ºC.min-1 in a static atmosphere of
argon. X-ray powder diffraction (XRD) measurements were performed using a Philips X’pert
diffractometer with mono chromated Cu-Kα radiation. Gas chromatography (GC) runs were
performed on an Echrom GC A90 gas chromatograph. The identification was conducted by Mass
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1
spectrometry (Finnigan-MAT- 8430EI-MS mass spectrometer). H NMR measurement on a
Bruker 500-MHz spectrometer using CDCl3 as solvent and TMS as an internal standard.
Synthesis of the ligands
Synthesis of L1 and L2
The ligand L1 was prepared according to the previous report.32 The ligands N,N'-Bis-pyridin-4-
ylmethylene-benzene-1,5-diamine (L1) and N,N'-Bis-pyridin-4-ylmethylene-naphtalene-1,5-
diamine (L2) were synthesized according to previously reported methods.32, 33 To 4-pyridine
carboxaldehyde (1.88 ml, 20.0 mmol) in elthanol (50 ml) was added 1,5-diaminonaphtalene
(10.0 mmol) at room temperature. After the addition of dichloromethane (50 ml) and then two
drops of formic acid, the mixture was stirred at room temperature for 24 h and then filtered. The
resulting yellow crystalline solid was washed with ethanol (3 × 5 ml) and hexane (3 × 5 ml), and
crystallized from CH2Cl2-hexane to give ligands L2.
Synthesis of RL1 and RL2 ligands
The pillar ligands RL1 and RL2 were obtained using the similar procedure with slight
modification. Typically, to the L1 (0.57 g, 2.0 mmol) in ethanol (10 ml) was added NaBH4 (0.15
g, 4.0 mmol), portion wise, at room temperature. The reaction mixture was stirred for 4 h. The
reaction solvent was evaporated and the reduced ligand (RL1) was extracted using EtOAc/H2O
solvents. (yield: 96%)
For preparation of RL2, the L2 (0.67 g, 2.0 mmol) was added in ethanol:toluene 5:1 v/v and
the mixture was heated at 60 C until a clear solution was obtained. Then, NaBH4 (0.15 g, 4.0
mmol) was added portion-wise and the reaction was refluxed within 24 h. Subsequently, the
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reaction mixture was cooled to room temperature, the solvents were evaporated and the reduced
ligand (RL2) was extracted using EtOAc/H2O solvents. (Yield: 93%)
Mechanochemical synthesis of MOFs
TMU-6(L1) was synthesized according to our previous report. It was prepared by grinding of
Zn(OAc)2·2H2O (1 mmol), H2oba (1 mmol) and 4-bpmb (0.5 mmol) by hand for 30 minutes. The
resulting powder was washed with small amounts of DMF in order to remove any unreacted
starting materials. For activation, the product was heated at 120°C for 12 h after washing with
DMF (yield: 90%). Elemental analysis (%) calculated for [Zn(C14H8O5)(C18H14N4)0.5]: C: 59.5,
H: 3.2, N: 6.0; Found: C: 58.9, H: 3.5, N: 6.2
TMU-21(L2) was synthesized by grinding Zn(OAc)2·2H2O (1 mmol), H2oba (1 mmol) and L2
ligand (0.5 mmol) by hand for 30 minutes. The resulting powder was washed with small amounts
of DMF in order to remove any unreacted starting materials. For activation, the product was
heated at 120 °C for 12 h after washing with DMF (yield: 90%). Elemental analysis (%)
calculated for ZnC25H16N2O5 ([Zn(C14H8O5)(C11H8N2)]): C, 61.31; H, 3.29; N: 5.72, Found: C,
60.96; H, 3.24, N: 5.70.
TMU-6(RL1) and TMU-21(RL2) were obtained using the same mechanochemical reaction
conditions used for TMU-21(L2) but using RL1 and RL2 instead of L2. For activation purpose,
after washing with DMF, the products were isolated and then heated at 120 °C for 12 h. FT-IR
data KBr pellet, ν/cm-1): selected bands for TMU-6(RL1): 658(m),779(m), 876(m), 1021(m),
1063(m), 1159(s), 1238(vs), 1402(vs), 1502(m), 1603(vs) and 3411(w) and selected bands for
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TMU-21(RL2): 658(m),775(m), 874(m), 1019(m), 1160(s), 1237(vs), 1397(vs), 1542(s),
1602(vs) and 3401(w).
Aldol-type Condensation Reaction
15 mg (5 mol%) of solid MOF catalyst was added into the reaction vessel loaded with a mixture
of malononitrile (0.3 mmol) and carbonyl compound (0.2 mmol), in 3 ml MeOH. The resulting
mixture was stirred at 60 ºC for 24 h (or longer reaction times following the reaction progress).
The reaction progress was followed by GC analysis. For this, the reaction mixture was cooled to
room temperature, centrifuged and the solid catalyst was filtered off. Then the reaction mixture
passed through the 0.4 µ filter and subsequently analyzed with GC apparatus.
Catalyst Recycling
The reusability of two catalysts, more basic MOF, TMU-6(RL1) and less basic, more
hydrophobic structure, TMU-21((L2) were tested for the condensation reaction of 2-cyclopenten-
1-one and 4,4-dimethyl-2-cyclohexen-1-one, respectively in the presence of three-fold amounts
of the catalysts (5 mol%, 45 mg), substrates (0.6 mmol) and malononitrile (0.9 mmol). After 24
h, the heterogeneous mixture was allowed to settle completely followed by decanting the
supernatant liquid. The catalyst was washed with excess MeOH and the reaction vessel
containing the washed recycled catalyst was loaded with the same primary amount of the
substrates, namely malononitrile and MeOH. Further reaction runs were performed in the same
manner at 60 C.
Results and Discussion
Structural analysis and characterization
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Knowing the isoreticularity concept first disclosed by Yaghi and co-workers,34 four pillared
MOF structures with the same network topology, bearing different N-donor ligands were
prepared. The original preparation method and the crystallographic data of TMU-6(L1) and
TMU-21(L2), which respectively contain N1,N4-bis-((pyridin-4-yl)methylene)benzene-1,4-
diamine (L1) and N1,N5-bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine (L2) were recently
addressed by our research group.33, 35 It was indicated that apart basic azine-functionalized
MOFs, weak basic imine groups in TMU-6(L1) can poorly catalyse Knoevenogel reaction at
room temperature. On the other hand, the imine ligands, L1 and L2, can be easily synthesized
through one step reaction of the corresponding aldehyde (SI). For systematic investigation of the
hydrophobicity properties on catalytic activity, these two mentioned MOF were selected and
compared with their two analogues MOF structures containing the reduced imine functions (i.e.
amine-functionalized MOF structures). To prepare both amine ligands, the imine bonds of pillar
ligands, L1 and L2, were reduced using excess amount of NaBH4 in ethanol as solvent and the
new N-donor pillar ligands were encoded as RL1 and RL2, respectively (see SI for details). After
1
the full characterization of these two ligands with IR, H NMR and Mass spectrometry, they
were used to synthesize amine-functionalized MOFs, respectively named as TMU-6(RL1) and
TMU-21(RL2) (SI).
In order to achieve more facile preparation route of these MOFs, as we previously described in
the case of TMU-6(L1),35 all three other MOFs tried to be prepared via mechanochemical
grinding method. Actually, N-donor pillar ligands RL1, L2, and RL2 were individually mixed
with Zn(OAc)2.6(H2O), and the ditopic oxygen donor ligand, H2oba (4,4-oxybisbenzoic acid),
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mechanically grinded with each other in a simple mortar within 30 minutes and then washed
with N,N-dimethylformamide (DMF), (SI).
Interestingly, comparison of FT-IR spectrum and PXRD patterns of the whole new solid
compounds with the simulated XRD pattern of TMU-6(L1) not only indicates that the whole
MOFs can be synthesized with this simple method, but also it tightly confirms isoreticularity
among them (Figures S6-S12). Consequently, TMU-6(L1) and TMU-6(RL1) including phenyl
cores while TMU-21(L2) and TMU-21(RL2), those are containing naphthalene cores and the
whole four isoreticular structures possess imine or amine function, alternatively. Therefore, a
series of various basic imine and amine- functionalized MOFs were synthesized which are
different in terms of structural hydrophobicity (Scheme 1).
Scheme 1. Synthesis of a series of IRMOFs using four pillar ligands; according to the estimated
Pka, their basicity is in the order of RL1> L1> RL2 >L2. Images of the catalysts in the mixture of
H2O and PhCH3 right .
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As we previously reported, the calculated void spaces per unit cell for guest free TMU-6 and
TMU-21 frameworks are 34.2% (1852.4 Å3) and 32.9% (1864.1 Å3), respectively. Both TMU-6
and TMU-21 are nonporous to N2 at 77 K but are porous to CO2 at 195 K [120.66 and 118.0
cm3/g at at 1 bar; Brunauer-Emmett-Teller (BET) surface area of 456 and 490 m2/g,
respectively].33, 36 Since all the prepared MOFs built up from the same metal node as well 3D
network with honeycomb-like hexagonal channels, the zinc metal centers on their coordination
sphere are completely saturated and there are no Lewis acidic sites within MOF structures. It
means that the reaction progress can be more firmly attributed to the structural diversities fixed
in the constructed MOFs skeleton. Hence, the only places available and active for catalytic
reaction are the basic nitrogen centers emerged within the MOF structure walls.
Catalytic studies
Regarding to the weak Lewis basicity of these prepared MOF series, in addition to numerous
Knoevenogel reaction developed for investigation of basic MOFs, herein we chose aldol-type
coupling reaction of malononitrile with α,β-unsaturated carbonyl compounds. In contrast with
Knoevenogel reaction, replacing of aromatic aldehydes with less reactive ketone function has led
to more difficult coupling reaction in this case. While Knoevenogel reaction of malononitrile and
benzaldehyde in the absence of any catalyst produced 19% of the corresponding coupling
product at room temperature, no detectable product was formed during the reaction of
malononitrile and 1-cyclohexen-2-one in the same reaction conditions, even at elevated
temperature 60 C, in the absence of catalyst and in MeOH as solvent within 24 h).
At preliminary stage of the catalytic reaction investigations, during the optimization of the
reaction solvent, the reaction mixtures proceeded in water in the presence of all prepared MOFs
were subjected to toluene in order to extracting the organic phase and analysing the reaction
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progress with gas chromatography. The interaction of each of these immersed catalysts in water
after addition of non-polar solvent was very interesting. As illustrated in Scheme 1 (and Figure
S17, SI), while the relatively hydrophobic catalyst, TMU-21(RL2) lay on the solvents border,
more hydrophobic MOF, TMU-21(L2) is completely assembled and tends to move away from
the polar aqueous environment. In comparison, the phenyl-core containing catalysts, i.e. TMU-
6(L1) and particularly TMU-6(RL1) spread on the border of the solvents. The behaviour of four
MOFs was also checked with THF and DCM as extracting solvent. In the presence of THF, no
clear separated phase is detected. Further interesting results were observed in the presence of
DCM that is immiscible with water and places under that. As it Figure S17 shows MOFs
containing amine base pillars, TMU-6 (RL1) and TMU-21 (RL2) escape from the organic phase
while the imine based MOFs specially TMU-21 (L2) with naphtyl-core pillar tend to penetrate
into DCM phase.
Subsequently, during the reaction survey of 2-cyclohexen-1-one as electrophilic substrate, with
all the solid base MOF catalysts, best results were obtained in the presence of 5 mol of the
catalysts, and in MeOH as solvent at 60 C. As summarized in Table 1, after 24 h of reaction 60%
of the corresponding addition product was formed in the presence of TMU-21(L2) that can
catalyze the reaction better that other MOF compounds. To shed light on our prime purposes,
two other substrates i.e. 2-cyclopenten-1-one as less hydrophobic substrate and 4,4-dimethyl-2-
cyclohexen-1-one as more hydrophobic case were chosen and separately reacted in the presence
of the four prepared MOF catalysts in the same reaction conditions. All catalysts are arranged in
the order of their hydrophobicity character in Table 1. A quick look at these data surprisingly
indicates that in the case of less hydrophobic substrate, 2-cyclopenten-1-one, TMU-6(RL1)
which is the less hydrophobic and more basic catalyst proceeded the reaction with high
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efficiency (93.7% yield). Quite the opposite, the best conversion for the more hydrophobic
substrate, i.e. 4,4-dimethyl-2-cyclohexen-1-one was obtained in the presence of the more
hydrophobic MOF catalyst, TMU-21(L2). This impressive result encourages us to investigate
more precisely about our catalyst system.
Table 1. Coupling reaction of malononitrile and carbonyl compounds with the prepared MOFs within 24 h.
O
O
O
Catalyst
Yield (%)a
Yield (%)
Yield (%)
1
2
3
4
TMU-6(RL1)
TMU-6(L1)
93.7
89
40
45
36
60
48
53.4
31.3
72.7
TMU-21(RL2)
TMU-21(L2)
60
76.3
O
O
5
6
TMU-6(RL1)
TMU-21(L2)
73
---
---
98
a GC yield based on internal standard method, reaction conditions; substrates (0.2 mmol). Malononitrile (0.3 mmol), catalysts (5
mol , 15 mg , MeOH 3 ml , 60 C.
Very recently, Wang et al. systematically investigated the catalytic properties of Zn, Zr, Al and
Cr derived MOFs tagged with primary alkyl amino group via post-synthetic modifications.37 In
the size-selectivity studies performed using various sizes of electrophilic aldehydes; Cr-MIL-
101-NH-RNH2 showed higher catalytic reactivity because of its large pore and cage sizes in
comparison of the other selected MOFs. It should be noted that, according to single crystal data
reported for TMU-21(L2) and TMU-6(L1), within our MOFs series, the former containing
naphthalene cores possesses smaller channels and aperture size relative to the latter including
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phenyl core structures.33 Considering the undeniable effects of structural parameters such as
pore size and topology, the most and least catalytic activities of our system that observed in the
case of smallest and largest substrates, (2-cyclopenten-1-one and 4,4-dimethyl-2-cyclohexen-1-
one, respectively) logically related to their ability to get closer to the basic reaction center.
Although, decreasing the catalytic activity during the substrates series in the presence of more
basic amine containing catalysts, TMU-6 (RL1) and TMU-21(RL2), can be ascribed to
increasing the substrates size, however improving the catalytic activity in the cases of two larger
substrates in the presence of TMU-21(L2) indicates the subtle role of the catalyst structural
hydrophobicity. As Table 1 shows, the catalytic activity of less basic, more hydrophobic
structure, TMU-21(L2), is clearly more than its amine-functionalized analogous MOF, TMU-
21(RL2). More importantly, during the substrates series, (rows 3 and 4, Table 1), the mentioned
differences in catalytic activity become more obvious with the increase of the substrate
lipophilicity in order of 16, 24 and 41%.
For more precise investigation, the performance comparison of all catalysts for the less and more
hydrophilic substrates was respectively plotted in Figure 1a and b. These figures obviously show
that regardless of imine or amine functions of the MOFs, using phenyl-core including catalysts,
TMU-6(L1) and TMU-6(RL1), 2-cyclopenten-1-one reacts more effective than 4,4-Dimethyl-2-
cyclohexen-1-one, cause of its smaller size that facilitates its proximity process to the basic
reaction centers in the MOF structure. As Figure 1a shows, despite the increase in catalytic
activity in the case of 2-cyclopenten-1-one generally follow from the basic character of the
MOFs, TMU-21(L2) interrupt this process and indicates more reactivity than the more basic
analogous structure, TMU-21(RL2). On the other side, the reactivity of the catalysts changes
remarkably for 4,4-dimethyl-2-cyclohexen-1-one. Whereas, TMU-21(L2) is the most reactive
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catalyst, TMU-21(RL2) is the weaker one (Figure 1b). Taking into account that both of these
catalysts have naphthalene-core ligands as well as different basic centers, the results prove the
vital role of hydrophobic character of the former catalyst on the reaction process. It seems that
the mass transport limitations cause by pore diffusion has been fixed by increased
hydrophobicity nature around the reaction center. Moreover, decreasing the catalyst reactivity of
TMU-6(RL1) even less than its imine-containing analogous structure, TMU-6(L1), points out the
meaningful and prominent effect of this structural specificity that has not been reported yet. This
effect is more clearly illustrated in the Figure S19 (SI). As this figure shows, while TMU-6(RL1)
is the most reactive catalyst in the case of less hydrophobic substrate, cyclopenten-1-one,
however TMU-21(L2) is the most effective ones for less hydrophilic one, 4,4-dimethyl-2-
cyclohexen-1-one. In addition, these plots demonstrates that although TMU-21(RL2) contains
the amine-function, it is less reactive catalyst for all substrates especially those have larger size.
These data along with the moderate results of 2- cyclohexen-1-one (Table 1) explicitly
represent the close and break-even competition between the effective reaction parameters.
However, a clear change in the catalytic behaviour of TMU-21(L2) in the presence of the bulkier
substrate indicates that steric effect has been suppressed by the hydrophobicity induced within
the walls of the catalyst structure.
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Figure 1. Yield-versus-time profile of aldol-type condensation reaction of (a) 2-cyclopenten-1-one and (b) 4,4-dimethyl-2-
cyclohexen-1-one catalyzed by four MOF structures in the reaction conditions indicated in Table 1.
1
The product structure was examined by IR spectroscopy and H NMR analysis. The FT-IR data
(SI), clearly demonstrate that the CN group added in structure of the yield isolated from all the
reaction products. This is indicated by the absorption bands at 2204 cm-1, which correspond to
the symmetrical stretching vibrations of the cyanide and hydroxyl groups. In addition, the exact
structure of all the products obtained from the three selected substrates determined by 1H NMR,
indicated that they have been formed through 1,2-addition reaction into the carbonyl compounds
followed by removing a water molecule from the product, instead of 1,4-addition that was also
confirmed by disappearing of the carbonyl group vibration in the IR spectra. This kind of aldol-
type reaction of α,β-unsaturated carbonyl compounds catalysed by basic organocatalyst MOFs
has not yet reported before and originally observed during aldol reaction catalysed by heavy
transition metal complexes.38 These findings encourage us to design another experiment to
evaluate the elegant observed effect of the substrate selectivity. At this stage, cyclopentanone
and cyclohexanone were selected, which contain a little more hydrophobic skeletons in
comparison with their α,β-unsaturated analogous substrates. Under the similar reaction
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conditions mentioned in Table 1, in the presence of less hydrophobic catalyst, TMU-6(RL1), the
reaction of cyclopentanone proceeded 73% of the corresponding product within 24 h, which is
lower than its activity for the less hydrophobic 2-cyclopenten-1-one. On the other side, while the
reaction of 2-cyclohexen-1-one in the presence of more hydrophobic catalyst, TMU-21(L2),
gave 60% yield, surprisingly more than 98% of the corresponding product was formed after 24 h
of the cyclohexanone reaction. These data again show that very little change in hydrophobic
nature of the substrates clearly influences the catalyst activity of both TMU-6(RL1) and TMU-
21(L2) structures, and once again confirmed the subtle substrate selectivity derived from the
catalyst framework.
Further proof of this claim is the substrate selectivity test, which was implemented for the most
reactive catalysts, namely TMU-6(RL1) and TMU-21(L2). In two separated catalytic runs, the
solution mixture of two compounds, 2-cyclopenten-1-one and 4,4-dimethyl-2-cyclohexen-1-one
led to react in a reaction vessel in the presence of the equal amount of the catalysts and twice
amount of malononitrile under the same reaction conditions. As illustrated in Scheme 2, after 24
h of the reaction with TMU-6(RL1), 96% of the corresponding product was formed from the
former, whereas the latter substrate reacts moderately to produce 62% yield. Interestingly, during
the same reaction time with TMU-21(L2),
more
hindered substrate,4,4-dimethyl-2-
cyclohexen-1-one beats the competition to give 58% yield, while less hindered substrate, 2-
cyclopenten-1-one reacts more slowly and produces 45% yield.
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Scheme 2. Illustrative substrate selectivity observed within the catalytic aldol reaction of malononitrile in mixture solutions of
both substrates in the presence of more basis MOF left side and less basic, more hydrophobic MOF right side ; conditions:
substrates 0.2 mmol , malononitrile 0.6 mmol , catalysts 5 mol , MeOH 4 ml , 60 C.
Therefore the higher permeation capability of the more hydrophobic substrate into the pores of
this catalyst was approved in another way. Due to the small differences in the size of the selected
molecules to induce this subtle effect, it can be considered as a fingerprint remark for increasing
the substrate selectivity of the designed catalyst. As demonstrated in Figure 2, both TMU-6(RL1)
and TMU-21(L2) can be recycled and reuse for at least seven reaction cycles without significant
loss of their activity. No structural changes are evident from the XRD pattern of the used
catalysts after 7th cycle (Figures S15 and S16) which is almost identical to the fresh material,
especially in the case of more hydrophobic MOF, TMU-21(L2). The worth mentioning point is
that since water is formed during the condensation reaction, this solid catalyst shows less
reactivity fluctuations during the catalyst recycling test.
Heterogeneity of the catalyst system was investigated using ICP analysis of the filtrate reaction
mixture of 2-cyclohexen-1-one after 24 h. Our analyses reveal that less than 0.01% of zinc was
detected in the cases of whole MOFs. Furthermore, in a typical test, the reaction was stopped
after 12 h and the catalyst was filtered off and the yield was determined by GC. Consequently the
reaction was allowed to continue for another 12 h. No reaction was progressed after catalyst
removal during the test reactions of all four MOFs. These data in addition to remaining the
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integrity of MOF structures confirmed by PXRD, reveal that no metal and/or pillar leaching was
happened as well no reaction was performed through homogeneous processes.
Figure 2. Reusability of TMU-6(RL1) (gray) and TMU-21(L2) (orange) in aldol-type condensation reaction of 2-cyclopenten-1-
one and 4,4-dimethyl-2-cyclohexen-1-one, respectively. (Conditions: catalyst (10 mol%), malononitrile, MeOH, 60 °C, 24h)
Conclusion
In summary, we have synthesized a series of four isoreticular MOF structures containing Zn
metal node and ditopic oxygen donor ligand, with various N-donor pillar ligands L1, RL1, L2,
and RL2 through the simple mechanochemical method. TMU-6(RL1) and TMU-21(RL2)
bearing the reduced analogous pillar ligands of TMU-6(L1) and TMU-21(L2), respectively. The
structure-activity relationship, induced in this way, cause different hydrophobicity-polarity
character around the pore environment. Consequently, this subtle structural diversity developed a
heterogeneous catalyst system containing solid base organocatalyst MOFs that represented subtle
substrate selectivity through aldol-type condensation reaction. It has been indicated that a slight
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change in hydrophobicity environment around the basic reaction center in MOF structure could
interestingly change the preference of the substrates to be reacted while they have very little
structure diversity.
AUTHOR INFORMATION
Corresponding Author
*Phone:
+98-21-82884416.
Fax:+98-21-82884416.
E-mail:
Morsali_a@modares.ac.ir
Notes
The authors declare no competing financial interests.
Acknowledgements
This work was supported by Tarbiat Modares University.
Supporting Information
1
Electronic supplementary information ESI available: IR, H NMR, Mass spectroscopy, TGA,
PXRD patterns, profile of the catalytic reaction and recycling test.
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Table of Contents
Four MOF catalysts owing different hydrophobic character around the basic reaction center
showed subtle substrate selectivity in aldol-type condensation reaction.