Confinement of Fe-Al-PMOF catalytic sites favours the formation of pyrazoline from ethyl diazoacetate with an unusual sharp increase of selectivity upon recycling

Article abstract of DOI:10.1039/C8CC06082G

The catalytic properties of a chemically stable iron porphyrin MOF were evaluated in a reaction with ethyl diazoacetate. In contrast to its homogeneous counterpart, an Fe-porphyrin-MOF features a different reaction pathway leading to the formation of pyrazoline due to the confinement of catalytic sites within the MOF network. Unexpectedly, a sharp increase of the selectivity from 35% (run 1) to 86% (run 5) occurs upon catalyst recycling.

Full text of DOI:10.1039/C8CC06082G

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Confinement of Fe-Al-PMOF catalytic sites favours the formation
of pyrazoline from ethyl diazoacetate with unusual sharp increase
of selectivity upon recycling
Received 00th January 20xx,
Accepted 00th January 20xx
Brian Abeykoona, Thomas Devicb, Jean-Marc Grenèchec, Alexandra Fateevaa* and Alexander B.
Sorokind*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Catalytic properties of a chemically stable iron porphyrin MOF Lewis acidity of FeIII and MnIII porphyrin sites in zwitterionic In-
were evaluated in a reaction with ethyl diazoacetate. In contrast MOF was demonstrated in cycloisomerization of enynes,
to its homogeneous counterpart, Fe-porphyrin-MOF features a cycloaddition of aziridines and alkenes, and hetero-Diels-Alder
different reaction pathway leading to the formation of pyrazoline cycloaddition of aldehydes with dienes.8 Cobalt porphyrin
due to confinement of catalytic sites within the MOF network. based porous covalent frameworks were highly efficient in
Unexpectedly, a sharp increase of the selectivity from 35 % (run electrochemical reduction of CO2 to CO.9 Photosensitizing
1) to 86 % (run 5) occurs upon catalyst recycling.
porphyrin was assembled with titanium-oxo cluster resulting in
highly crystalline TiO2-PMOF which showed photocatalytic
activity in alcohol oxidation.10 A Cu-PMOF constructed from
octatopic porphyrin mediated the first reported reaction of
CO2 with aziridines to form oxazolidinones.11 Finally, Fe and
Iron porphyrins are ubiquitous in enzymatic systems
performing essential life processes such as catalytic aerobic
oxidations, reduction and transport of O2, destruction of
peroxides, detoxification processes, etc. Their high efficiency in
many challenging reactions has stimulated an extensive
research devoted to the development of bio-inspired catalysts
based on porphyrin scaffold.1 In particular, the construction of
metal porphyrin based metal organic frameworks (M-PMOF)
arouses considerable interest due to their particular nature.2
The MOFs provide an interesting potential for heterogeneous
catalysis when considering their porous structure with
tuneable cavity sizes and polarities, high surface areas and
crystallinity which leads to a precise structuration of catalytic
sites3-5 allowing the preparation of single site catalysts.6
Catalytic reaction taking place inside the porous network can
potentially lead to a different output when compared to
homogeneous catalysis since positioning the catalytic sites in
an ordered and controlled manner may affect catalysis.
Numerous studies have reported on the catalytic activity of M-
PMOFs comparatively with homogeneous counterparts but
unusual reactivity brought about by the structure of the M-
PMOF has been far less documented.7 The enhancement of
Mn porphyrin sites isolated within
a Zr-MOF enabled
preparation and characterization of elusive superoxide and
peroxo complexes.12, 13
In our goal to study catalytic properties of coordination
polymers we focused on a highly porous chemically and
thermally stable Al-PMOF network ((AlOH)2H2TCPP) that
displays a simple arrangement of adjacent porphyrins.14 This
material prepared from free base porphyrins represents a
good platform for post synthesis metallation (PSM) with
different metal ions. This property has been explored and
16
catalytically active Al-PMOFs with Zn(II),14 Co(II),15, Pt(II),17
have been reported. In this paper we focus on the preparation
and investigation of the Fe(III) version of Al-PMOF that has not
been described before.
In all the previous studies, only divalent metal ions were
inserted by PSM. However, this strategy was not successful for
the Fe insertion. Upon the reaction of either FeCl2 or FeCl3 with
the free base H2-Al-PMOF, the framework collapse was
observed most probably due to the competitive binding of
Fe(III) and Al(III) ions by carboxylate groups. This phenomenon
was observed when MOFs were exposed to cations of similar
charge to those defining their frameworks.18 To prevent this
ion exchange, the stabilization of the Fe(II) precursor, that
should be harmless for the framework structure, is requested.
Indeed, using a ferrous thiocyanate precursor along with
ascorbic acid to prevent oxidation,19 the framework integrity
and crystallinity were preserved (Fig. 1a and 1c). However, the
a.Univ. Lyon, Université Claude Bernard Lyon 1, Laboratoire des Multimatériaux et
Interfaces (LMI), UMR CNRS 5615, F-69622 Villeurbanne, France.
E-mail: alexandra.fateeva@univ-lyon1.fr
b.Institut des Matériaux Jean Rouxel (IMN), UMR 6502, Université de Nantes, CNRS,
2 rue de la Houssinière, BP32229, 44322 Nantes cedex 3, France
c. Institut des Molécules et des Matériaux du Mans (IMMM), UMR CNRS 6283, Le
Mans Université, Avenue Olivier Messiaen, 72085 Le Mans cedex, France
d.Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON),
UMR 5256, Université Claude Bernard Lyon 1 - CNRS, , 2 av. A. Einstein, 69626
Villeurbanne, France. E-mail : alexander.sorokin@ircelyon.univ-lyon1.fr
Electronic Supplementary Information (ESI) available: Experimental details on
synthesis, characterization of Fe-Al-PMOF and catalytic study. See
DOI: 10.1039/x0xx00000x
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Figure 1: a) PSM route for Fe-Al-PMOF, b) direct synthesis route for Fe-Al-PMOF, c) PXRD patterns for the H2-Al-PMOF, Fe-Al-PMOF by PSM and Fe-Al-PMOF by direct
synthesis, d) solid state UV-vis absorption spectra for H2-Al-PMOF and Fe-Al-PMOF, e) Nitrogen sorption isotherms (77K) for Fe-Al-PMOF obtained by PSM and by direct
synthesis, f) TGA data in air, heating rate 10°C.min-1
metal insertion rate was uncomplete (ca. 70 %) as evidenced carbene complex can react with olefins and X-H bonds or with
by UV-vis, EDS and TG analyses (Fig. S6 and S7, Table S2).
EDA to form diethyl maleate (DEM, cis-isomer)/diethyl
To obtain the maximal amount of iron sites inside the fumarate (DEF, trans-isomer).
framework a direct synthesis using metallated FeTCPP was
explored (Fig. 1b), which required a re-optimization of the
N2
synthetic procedure, notably the solvent composition,
Fe
Fe
stoichiometry and temperature (Fig. S1 and S5). The lack of
iron leaching from the porphyrin core during the synthesis was
confirmed by UV-vis data (Fig. 1d and S5). Mössbauer
+ EDA
technique showed the high spin iron (III) state in the solid as
Fe-Al-PMOF: DEM:DEF = 29:71
attested by one doublet signal with
0.74 mm s-1 (Fig. S10).
δ ꢀEQ =
= 0.42 mm s-1 and
FeP:
DEM:DEF = 97:3
As seen from the PXRD patterns, the crystallinity of the
directly prepared compound is lower than that of PSM solid,
which is likely due to the greater solubility of the porphyrin in
DMF/water mixture instead of pure water leading to a much
faster reaction and precipitation of the polymeric material. The
size and shape of crystallites for the directly synthesised
material are indeed considerably smaller and less regular as
evidenced by SEM (Fig. S8 and S9). The Fe-Al-PMOF exhibits a
good thermal stability in air up to 300°C as shown by TGA (Fig.
1f and S5). After activation at 160°C under vacuum, this
material displayed a type I nitrogen sorption isotherm at 77K,
characteristic of a microporous character (Fig. 1c), with a
deduced BET surface area of ca. 1100 m2.g-1. Similar isotherm
heights for the directly synthesised and PSM samples indicate
that the adsorption capacity is comparable. The presence of a
positive slope instead of a horizontal plateau for the former
indicates the presence of larger pores that can originate either
from defects or interparticle nitrogen sorption, is in agreement
with the SEM analyses (Fig. S8 and S9).
DEF
DEM
Scheme 1. Formation of carbene complex from EDA and FeP/Fe-Al-PMOF and
its further reaction with EDA to form DEM and DEF. Conditions: 0.08 mol%
catalyst loading, MeCN, Ar, 60°C, 4h. Oval stands for the porphyrin macrocycle.
The influence of the porous structure on the activation of EDA
was evaluated using directly prepared Fe-Al-PMOF and soluble
iron tetrakis(4-methoxycarbonylphenyl)porphyrin (FeP). In a
typical reaction, 1.2 mmol of EDA in 1mL solvent containing 1
µmol of iron sites (catalyst:EDA = 1:1200) was stirred under Ar
until EDA was consumed. Initial experiments in MeCN showed
a very opposite selectivity of Fe-Al-PMOF and FeP in the EDA
dimerization with DEM/DEF ratio of 29:71 and 97:3,
respectively (Fig. S17 and S18). The strong influence of the
porosity of Fe-Al-PMOF on the activation of EDA prompted us
to study this reaction in more details (Table 1). The outcome of
the reaction of Fe-Al-PMOF with EDA was found to be solvent-
dependent. In toluene, a significant amount of a new product
was formed along with DEM and DEF. It was identified by GC-
MS and NMR as 3,4,5-triethyl-4,5-dihydro-1H-pyrazole-3,4,5-
Iron porphyrin complexes have emerged as remarkable
catalysts for carbene transfer reactions to olefins and to X-H (X
= N, O, C, S, Si) bonds20. Easy-to-handle commercially available
ethyl diazoacetate (EDA) is sustainable and atom-efficient
carbene precursor for these reactions, and is thus widely used
for the formation of active carbene complexes (Scheme 1). The
tricarboxylate (1) (Fig. S11 and S12). This compound can be
formed by the 1,3-dipolar cycloaddition of the diazo
compound via the nucleophilic addition of a carbon atom of
EDA to DEM or DEF double bonds. (Scheme 2). The best yield
2 | J. Name., 2012, 00, 1-3
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(35 %) was obtained in toluene. CH2Cl2 (25°C) and CHCl3 (50°C) Moreover, when this carefully washed Fe-Al-PMOF was re-
provided moderate yields of whereas only traces of were used, the further increase in the yield of from 42 % (run 3) to
1
1
1
obtained using MeCN at 60°C (Table 1).
57 % (run 4) was observed. Prior to run 5, recycled Fe-Al-PMOF
was incubated in toluene at 60°C for 1 h and the GC-MS
analysis of this solution showed no reaction products. After
Table 1. Reaction of Fe-Al-PMOF and FeP with EDA.a
Product yield,b
%
DEM/D
EF
EDA addition, further increase of the yield of
observed in run 5. Consequently, the eventual leaching of the
adsorbed from recycled material does not contribute to
1 to 86 % was
EDA conversion
(Reaction time)
Entry
Solvent (T, °C)
DEM DEF
1
1
Homogeneous FeP
1c
2
MeCN (60)
PhMe (60)
100% (4 h)
97% (4 h)
nd
91
nd
6
0
97:3
94:6
remarkable increase in the reaction selectivity. However, the
reaction rate was lower in run 5 and 7 % of EDA was still left in
the reaction mixture even after 3 h.
traces
Heterogeneous Fe-Al-PMOF
PXRD studies evidenced that the structure of the Fe-Al-
PMOF framework remains intact (Fig. S13). The drop of the
reaction rate can therefore be related to the inactivation of
the iron porphyrin sites. The carbene complex formed by iron
porphyrin in cytochrome P-450 was reported to undergo an
intramolecular attack to the pyrrolic nitrogen to form inactive
N-alkylprotoporphyrin derivatives.22 Similar structures with a
loss of one metal–pyrrolic nitrogen bond and the carbene
carbon bonding to the metal and nitrogen were detected by
mass spectrometry.23 To check this possibility, we dissolved
the recycled Fe-Al-PMOF in a 0.1 M NaOH solution. ESI-MS
analysis of this solution showed the signals compatible with
the addition of carbene to iron porphyrin moiety (Fig. S16).
3c
4
5
6
7
8
MeCN (60)
CH2Cl2 (25)
CHCl3 (50)
PhMe (40)
PhMe (50)
PhMe (60)
100% (2 h)
nd
nd
traces
29:71
96% (4 h)
100% (4 h)
100% (6 h)
100% (4 h)
95% (0.5 h)
99% (1 h)
100 (2 h)
19
12
29
26
19
20
20
20
13
12
11
4
73
69
44
46
46
47
45
45
51
46
31
3
4
21:79
15:85
39:61
36:64
30:70
30:70
30:70
31:69
21:79
21:79
26:74
54:46
28:72
58:42
19
27
28
30
32
35
35
36
42
57
86
57
87
9d run 1
run 2
run 3
run 4
run 5
10e
PhMe (60)
100% (1 h)
100% (1 h)
100% (1 h)
99% (2 h)
93% (3 h)
100% (1 h)
90% (6 h)
Therefore, it appears that the selectivity to product
1 rises
PhMe (60)
PhMe (60)
12
2
31
1
upon decrease of the amounts of catalytic sites. When the
reaction with EDA was performed with a half amount of Fe-Al-
11f
a
Conditions: catalyst (1 µmol), EDA (1.2 mmol), solvent (1 mL), (0.08 mol%
catalyst), Ar. b Yields are based on EDA. DEM/DEF and
EDA molecules, respectively. Experiments in the presence of 1 mmol styrene.
Recycling study with reagent amounts increased by factor 3 (0.08 mol% catalyst).
With 0.5 µmol Fe-Al-PMOF (0.04 mol% catalyst). With 0.1 µmol Fe-Al-PMOF
PMOF (1:2400 catalyst/EDA ratio, entry 10),
1 was obtained
1 are formed from 2 and 3
d
c
with a 57 % yield. Further decrease of the catalyst loading by
factor 10 (1:12000 catalyst/EDA ratio, entry 11) resulted in 87
% yield of 1 (Fig. S22). However, a smaller catalyst loading led
to a slower reaction rate and EDA was not completely
consumed even after 6 h. Nevertheless, each Fe site of Fe-Al-
PMOF performed 10800 catalytic turnovers.
e
f
(0.008 mol% catalyst).
COOEt
COOEt
COOEt
COOEt
COOEt
EtOOC
We propose that confined Fe sites efficiently catalyze the
formation of DEF and DEM in the first step followed by their
reaction with EDA to provide a new reaction pathway. The
constrained environment of the Fe-Al-PMOF cages leads to the
change in the selectivity of the EDA dimerization in favour of
the trans-isomer (DEF) instead of cis-isomer (DEM) obtained
+
N
N
-
H
N
N
EtOOC
EtOOC
N
N
H
COOEt
1
Scheme 2. Cycloaddition of EDA to diethyl maleate and diethyl fumarate.
Previously, 1 was obtained by treating EDA by [RuCl(cod)(Cp)]
with 17.5 % yield.21 Remarkably, when homogeneous FeP
reacted with EDA, DEM and DEF were obtained in a 94:6 ratio
using homogeneous FeP. A 87 % yield of
1 obtained with 0.008
mol% catalyst loading confirms that EDA can react with both
DEM and DEF. Cage confinement of the Fe porphyrin might
activate DEF and DEM and favour their reaction with EDA to
with only traces of 1 (on the GC-MS detection limit) (Fig. S20).
To evaluate stability of Fe-Al-PMOF under reaction
conditions, the recycling study was performed (Table 1, entry
9). After each run, the solid was recovered by centrifugation,
washed with toluene 5 times and re-used in the next run.
form pyrazoline. Since
1 was not formed with FeP, one can
suggest that a non-catalytic reaction between DEM/DEF and
EDA should not be efficient. Indeed, when EDA was incubated
with DEM and DEF at 60°C for 1 h, only 2 % DEM and 20 % DEF
Surprisingly, the yields of
1 progressively increased upon
recycling from 35 % in run 1 to reach 86 % in run 5 (Table 1
and S21). Such a sharp increase of the reaction selectivity upon
catalyst recycling is quite unusual. In general, recycled
catalysts retain their activity in several runs or their
performance decreases owing to deactivation. A possible
were converted to 1. Importantly, DEF is more active in non-
catalytic reaction with EDA than DEM. In contrast to FeP which
provides DEM and DEF in 94:6 ratio, Fe-Al-PMOF furnishes
mainly DEF (DEM/DEF = 29:71). Thus, preferential formation of
more active trans-isomer in the confined environment of Fe-
Al-PMOF results in its faster reaction with EDA and efficient
adsorption of
1 by Fe-Al-PMOF followed by its release in the
next runs could contribute to its amout in the solution. To
check this, the solid recovered in run 3 was washed with 1 mL
formation of 1. The activation of DEF and DEM at confined iron
sites and their reaction with EDA without escape to bulk
solution cannot be excluded. We believe that Fe-Al-PMOF
of toluene 3 times but only traces of 1 were found in washings.
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