Dual-fixations of europium cations and TEMPO species on metal-organic frameworks for the aerobic oxidation of alcohols

Article abstract of DOI:10.1039/d0dt01324b

The efficient and selective aerobic oxidation of alcohols has been investigated with judicious combinations of europium-incorporated and/or TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)-functionalized zirconium-based porous metal-organic frameworks (MOFs). Although MOFs are well-known catalytic platforms for the aerobic oxidation with radical-functionalities and metal nanoparticles, these systematic approaches involving metal cations and/or radical species introduce numerous interesting aspects for cooperation between metals and TEMPO for the aerobic oxidation of alcohols. The role of TEMPO as the oxidant in the heterogeneous catalytic aerobic oxidation of alcohols was revealed through a series of comparisons between metal-anchored, TEMPO-anchored, and metal and TEMPO-anchored MOF catalysis. The fine tunability of the MOF allowed the homogeneously and doubly functionalized catalysts to undergo organic reactions in the heterogeneous media. In addition, the well-defined and carefully designed heterogeneous molecular catalysts displayed reusability along with better catalytic performance than the homogeneous systems using identical coordinating ligands. The role of metal-cation fixation should be carefully revised to control their coordination and maximize their catalytic activity. Lastly, the metal cation-fixed MOF displayed better substrate tolerance and reaction efficiencies than the TEMPO-anchored MOF or mixture MOF systems.

Full text of DOI:10.1039/d0dt01324b

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Dual-fixations of europium cation and TEMPO species on metal-
organic frameworks for aerobic oxidation of alcohols†
Seongwoo Kim,a Jooyeon Lee,†a Sungeun Jeoung,†b Hoi Ri Moon,b,* and Min Kima,*
Received 00th January 20xx,
Accepted 00th January 20xx
The efficient and selective aerobic oxidation of alcohols has been investigated with judicious combinations of europium-
incorporated and/or TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)-functionalized zirconium-based porous metal-organic
frameworks (MOFs). Although MOFs are well-known catalytic platform for the aerobic oxidation with radical-functionalities
and metal nanoparticles, these systematic approaches for metal cation and/or radical species allows many interesting
aspects for cooperation between metal and TEMPO for the aerobic oxidation of alcohols. The role of TEMPO as the oxidant
in the heterogeneous catalytic aerobic oxidation of alcohols was revealed through a series of comparisons between metal-
anchored, TEMPO-anchored, and metal and TEMPO-anchored MOF catalysis. The fine tunability of MOF allowed the
homogeneously and doubly functionalized catalysts for organic reaction in the heterogeneous media. In addition, the well-
defined and carefully designed heterogeneous molecular catalysts displayed reusability along with better catalytic
performance than the homogeneous systems within identical coordinating ligands. The role of metal-cation fixation should
be carefully revised to control their coordination and maximize the catalytic activity. Lastly, metal cation-fixed MOF displayed
better substrate tolerances and reaction efficiency than TEMPO-anchored MOF or mixture MOF systems.
DOI: 10.1039/x0xx00000x
Introduction
The selective oxidation of alcohols to aldehydes and ketones is
one of the most important transformations in organic chemistry
that is also widely used in pharmaceuticals, agrochemicals, and
fine chemicals.1 Typically, while traditional oxidations (e.g.,
Jones oxidation, Dess-Martin oxidation, and Swern oxidation)
require strong oxidants and generates more than an equivalent
of byproducts under the harsh conditions. Compared with
traditional oxidants, aerobic oxidation is a clean, low cost, atom
economic, and sustainable methods because it uses molecular
oxygen as a final oxidant.2 Therefore, various metal-catalyzed
aerobic oxidations have been developed, and typically
transition metal-catalyzed aerobic oxidations of alcohols also
require organic oxidants such as TEMPO ((2,2,6,6-
tetramethylpiperidin-1-yl)oxyl) to efficiently turn over the
catalytic cycle.3
Metal-organic frameworks (MOFs), porous inorganic/organic
hybrid materials, have attracted increasing interest in catalytic
applications over the last two decades.4 First, the construction
of metal clusters or the incorporation of metal cations into the
secondary building units (SBUs) could be considered for
Scheme 1 Preparation of Eu-based MOFs in nodes and Eu-anchored MOFs in
ligands.
catalysts such as Lewis acid catalysis.5 In addition, one of the
major advantages of MOFs, the tunability of the ligands (i.e., the
functionalization of the ligands), could allow the introduction of
coordinating functionalities and external metal catalysts onto
the frameworks. Pore engineering and metal derivatization of
MOFs allow to be used in various heterogeneous catalysis
applications.6 Cu- and Pd-anchored or palladium nanoparticle-
embedded MOFs were reported as efficient catalysts for
a. Department of Chemistry and BK21Plus Research Team, Chungbuk National
University, Cheongju 28644, Republic of Korea
E-mail: minkim@chungbuk.ac.kr
b. Department of Chemistry Ulsan National Institute of Science and Technology
Ulsan 44919, Republic of Korea
E-mail: hoirimoon@unist.ac.kr
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Journal Name
aerobic oxidation.7 Moreover, materials functionalized with the
radical oxidant TEMPO have widely been reported and utilized
for the selective aerobic oxidation of alcohols to aldehydes.8
In this study, we have additionally installed recently developed
europium-based aerobic oxidation catalysts into TEMPO-
functionalized MOFs to maximize the tunability and control of
single catalytic sites deep in the MOF pores. Although europium
is an underexplored, lanthanide metal species relative to
transition metals for redox catalysis and oxidative organic
transformations, both europium-based MOFs (Eu in the SBU)
and europium-anchored MOFs (Eu in the ligand) are well-known
systems due to their unique coordination environments and
photophysical properties.9 Since Eu-SBU-based MOFs show
superior stabilities and Eu3+ can be installed in various common
chelating groups, such as bipyridyl, europium catalysis is a good
target for MOF-based heterogeneous catalysis toward aerobic
oxidation of alcohols based on molecular level design and
modifications. Therefore, in the present work, four classes of
MOF-based catalysts for aerobic oxidation could be prepared
along with their mixtures: Eu-based MOFs, Eu-anchored MOFs,
TEMPO-functionalized MOFs (known system), Eu and TEMPO-
anchored MOFs (dual functionalization of a single MOF), and
applied to the aerobic oxidation of alcohols. In addition, the
catalytic efficiencies of europium cations in MOFs have been
extensively studied by comparing with homogeneous system,
for the first time. Lastly, useful mechanistic information could
be acquired by comparing these four unique, finely tuned
heterogeneous catalyst systems.
Table 1 Eu-catalyzed aerobic oxidation with various catalytic speciesa
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DOI: 10.1039/D0DT01324B
O
Eu (1 mol%)
TEMPO (1 mol%)
OH
H
NaNO3 (1 mol%)
toluene-AcOH, 60 o
under air
1
2
C
Entry
Europium catalyst
―
A1; Eu-MOF-1 with NDC
A2; Eu-MOF-2 with DOBDC
B0; UiO-67-bpy
B; UiO-67-bpy-Eu
C; Eu(Me-BPY)2(NO3)3
B; UiO-67-bpy-Eu2nd
B; UiO-67-bpy-Eu3rd
Conv. (%)b
1
2
3
4
5
6
7
8
<1
<1
<1
<1
98c
56
99c
99c
a
All reactions were performed on a 0.25-mmol scale: benzyl alcohol (1a, 0.25
mmol), Eu catalyst (1 mol%), TEMPO (0.5 mg), NaNO3 (0.2 mg), toluene (1 mL), and
acetic acid (0.1 mL). b GC conversion with >99% selectivity. c Isolated yield.
As Eu-based fcu MOFs, two isostructural fcu MOFs were
successfully prepared following reported procedures by simply
employing two ligands from derivatizations (A1: Eu-MOF-1,
known fcu MOF with NDC ligand; and A2: Eu-MOF-2 with
DOBDC, DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid).
In addition, to anchor Eu cations, bpy-functionalized UiO-67
(B0: UiO-67-bpy) was synthesized with BPYDC (2,2’-bipyridyl-
5,5’-dicarboxylic acid) as described in Scheme 1.14 Subsequent
metalation with Eu(NO3)3 afforded UiO-67-bpy-Eu (B). The bulk
crystallinities of A1, A2, B0, and B after metalation were
confirmed by powder X-ray diffraction (PXRD), and Eu-
metalation did not impact the frameworks of UiO-67 (Fig. S1 in
the ESI†). The PL spectra confirmed the 3+ oxidation state of the
Eu in A1, A2 and B (Fig. S2†).10
The homogeneous system was modified to allow a direct
comparison between heterogeneous and homogeneous Eu
catalysis. Since the previous homogeneous Eu-catalyzed aerobic
oxidation was performed within simple Eu(NO3)3,10 which lacks
external chelating ligands, we synthesized Eu(Me-BPY)2(NO3)3
(C, Me-BPY = 5,5’-dimethyl-2,2’-bipyridyl) by the coordination
of Me-BPY ligands to Eu(NO3)3, allowing confirmation of effect
of the bipyridyl on the catalysis. The structure of the Eu-Me-BPY
complex was determined by single-crystal X-ray diffraction
(Table S1 and Fig. S3,† and Cambridge Crystallographic Data
Centre Deposit # 1962612), and it was utilized for the direct
comparison of the aerobic oxidation.
Results and discussion
Heterogenization of Eu cation for the aerobic oxidation of alcohols
The recently developed, Eu-catalyzed aerobic oxidation of
alcohols to the corresponding aldehydes was selected as a
model reaction since Eu catalysis has two factors that make it
well suited for heterogenization with MOFs.10 First, both Eu-
based MOFs (i.e., the SBU of the MOF consists of Eu(III)) and Eu-
anchored MOFs are well established in the literature.11 To
achieve Eu loading in the ligand, Zr-based UiO-67-bpy (UiO =
University of Oslo, bpy = 2,2’-bipyridyl) was prepared due to its
superior chemical and physical stabilities. MOFs in the UiO
series are based on the Zr6O4(OH4) cluster as the SBU, and these
clusters are connected to 12 carboxylate ligands.12 In addition,
Eu-based fcu MOFs reported by the Eddaoudi group generally
have very similar frameworks to the Zr-based UiO MOFs. Eu-
based fcu MOFs also have 12 BDC ligands derivatives (BDC = be
nzene-1,4-dicarboxylic acid; NDC
=
naphthalene-1,4-
dicarboxylic acid was used for MOF preparation) connecting the
with Eu6 SBUs.9a The second advantage of Eu catalysis is the
photoluminescence (PL) properties of Eu. Interestingly, Eu2+ and
Eu3+ are easily distinguished by their PL spectra. While Eu3+ has
strong PL signals at approximately 550 – 700 nm, Eu2+ does not
have intense peaks in the same range of wavelength.10,13
Therefore, the oxidation state of Eu (e.g., Eu2+ or Eu3+) after
incorporation into the MOFs can be easily confirmed.
Next, conditions for the catalytic reaction were screened using
several Eu-MOFs and Eu species. In the absence of catalyst, the
aerobic oxidation did not proceed under mild heating (60 °C,
entry 1 in Table 1). Both Eu-based MOFs, A1: Eu-MOF-1 with
NDC and A2: Eu-MOF-2 with DOBDC, were ineffective in the
aerobic oxidation (entries 2 and 3) even though both had Eu(III)
2 | J. Name., 2012, 00, 1-3
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cations in their SBU, which are confirmed by the strong signals (2d-2f for chloro group and 2h-2j for methoxy group) with good
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for Eu(III) in their solid-state PL spectra (Fig. S2†). While pristine conversions (>99% conversions for 6DOexI:a1m0.1p0l3e9s/)D.0DFTo0r13t2h4Be
UiO-67-bpy (B0) also showed no catalytic activity in this aerobic heteroaromatic alcohols, both 2-thiophene and 3-thiophene
oxidation (entry 4), the benzyl alcohol substrate (1a) was fully showed excellent conversions (2k, and 2l). Lastly, aliphatic
converted to benzaldehyde (2a) with Eu-anchored UiO-67-bpy- alcohols were subjected to the optimized conditions, both
Eu (B, >98% conversion, entry 5). In the direct comparison hexanal (2m) and 2-heptanone (2n) were successfully
between the homogeneous and heterogeneous systems with synthesized in good conversion from primary alcohol and
external bipyridyl ligands, the MOF-based catalysts were much secondary alcohol.
more efficient than homogenous catalysis with Eu(Me- The best catalyst, Eu-anchored B was characterized in detail for
BPY)2(NO3)3 (C, entry 6). We attributed this difference to the physical properties. The full N2 isotherms (at 77 K) were
nature of the framework. Since in the MOFs, the bipyridyl acquired to determine the porosity and surface area of Eu-
functionality is fixed on the ligand (i.e., in the strut of the anchored MOFs. The Brunauer-Emmett-Teller (BET) surface
framework), each Eu3+ cation could only coordinate with one areas of B0 and B were generally decreased from 1828 m2/g to
bipyridyl group (i.e., two coordination sites), in which accessible 671 m2/g and pore volume was also decreased from 0.96 cm3/g
sites to substances remained. However, in the homogeneous to 0.36 cm3/g upon europium metalation (Fig. S4 and Table
-
system, an additional bipyridyl ligand could bind with the Eu3+ S2†). The existence of NO3 in B was confirmed by FT-IR (Fig.
cation since the coordination number of Eu3+ is greater than 6.15 S5†), and thermogravimetric analysis (TGA) also indicated the
Therefore, the nature of the framework system (i.e., the MOF) multispecies in the frameworks (Fig. S6†).
prevents the formation of the coordinatively saturated metal Last, the recyclability, the major advantage of heterogeneous
species and produces a single active catalytic site with labile catalysis of B was studied. The Eu-anchored B could be recycled
-
chelating groups such as NO3 .
by simple centrifugation without structural changes or changes
in the catalytic activity (entries 7, 8 in Table 1 and Fig. S7†).
Indeed, the hot filtration test was performed during the
reaction (at 3 h), and the conversion of benzyl alcohol (1a) to
benzaldehyde (2a) was halted at around 30%, which displayed
the heterogeneous characteristic of Eu-catalysis in this
condition (Fig. S8†).
Strategical Combination with TEMPO-functionalized MOFs for the
aerobic oxidation of alcohols.
TEMPO, the organic oxidant is a key molecule for the aerobic
oxidation. TEMPO can catalyze the aerobic oxidation with
radical reagents or cooperate redox cycle of metal (Scheme
S1†).16 The heterogeneous, MOF-based catalysis for aerobic
oxidation was initiated with TEMPO moiety. Due to the
tunability of MOFs, the organic oxidant can be covalently bound
into the MOF pores to perform the aerobic oxidation, for
example, Cu-catalyzed click chemistry.8c,8d TEMPO-
functionalized MOFs have recently been achieved with Zr-based
MOFs, UiO-67-TEMPO (D), and successfully applied to aerobic
oxidation. In addition, the combination of homogeneous
Eu(NO3)3 salt and TEMPO-functionalized MOF D displayed good
catalytic performance for the aerobic oxidation of alcohols
under mild condition.8c Since the same MOF platform, UiO-67,
was employed to prepare the analogous system with Eu-
anchored MOFs (Scheme 3), the pores of Zr-based MOFs were
further tuned by the fixation of metals and oxidants, together.
MOFs are good heterogeneous platform for multi-
functionizations and the tandem catalytic reactions.17 More
than two different, sequential catalytic sites could be installed
together in a single MOF’s pore by ligand-mixing strategy.18
Therefore, both BPYDC and TEMPO-functionalized ligands were
mixed and incorporated in to the UiO-67 frameworks, and Eu-
metalation followed for combining two catalytic cycles (the
redox cycle of Eu and the redox cycle of TEMPO) in a single MOF
pore (Scheme 3).
Scheme 2 Substrate scope for various alcohols using UiO-67-bpy-Eu (Condition I).
Then, Eu-anchored UiO-67-bpy-Eu, B was employed for the
aerobic oxidation of alcohols under the optimized reaction
conditions and generally showed good reactivity for various
primary and secondary alcohols (Scheme 2). The oxidation
reactions were finished for full conversion (>99% by GC) and
>99% selectivity to aldehyde (toward carboxylic acid, by 1H
NMR) within 8-18 h. A series of alcohol substrates with electron-
withdrawing groups (2b-2f in Scheme 2) and electron-donating
groups (2g-2j in Scheme 2) were successfully converted to
corresponding aldehydes. Indeed, all ortho-, meta-, and para-
substituted primary alcohols with chloro and methoxy groups
were successfully converted to the corresponding aldehydes
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J. Name., 2013, 00, 1-3 | 3
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Table 2 Eu-loaded, TEMPO-functionalized UiO-67s versus ligand mixture in the framework versus crystal mixtures in solution for the aerobic oxidation of alcoholsa
Condition III:
Condition IV:
Condition I:
Condition II:
Eu(NO3)3 (1 mol%)
; UiO-67-TEMPO (1 mol%)
OH
R2
O
B
; UiO-67-bpy-Eu (1 mol%)
E
or
B
; UiO-67-bpy-Eu&TEMPO
(2 mol%)
; UiO-67-bpy-Eu (1 mol%)
TEMPO (1 mol%)
or
or
R1
R1
R2
D
; UiO-67-TEMPO (1 mol%)
D
1
2
NaNO3 (1 mol%), toluene-AcOH, air, 60 o
C
Entry
Substrate
Product
Condition I
Yieldb / Time
Condition II
Yieldb / Time
Condition III
Yieldb / Time
Condition IV
Yieldb / Time
1
2
3
4
99 / 9
99 / 9
99 / 9
99 / 9
68 / 18
49 / 18
42 / 18
30c / 18
70 /18
48 / 18
45 / 18
28c / 18
99 / 18
85c / 18
68 / 18
60c / 18
a All reactions were performed on a 0.25-mmol scale: 1 (0.25 mmol), UiO-67-bpy-Eu (2 mg), TEMPO (0.4 mg), and NaNO3 (0.2 mg) for Conditions I; 1 (0.25 mmol), UiO-
67-TEMPO (3 mg), Eu(NO3)3 (1 mg), and NaNO3 (0.2 mg) for Conditions II; UiO-67-(bpy-Eu&TEMPO) (6 mg), and NaNO3 (0.2 mg) for Conditions III; 1 (0.25 mmol), UiO-67-
bpy-Eu (2 mg), UiO-67-TEMPO (3 mg), and NaNO3 (0.2 mg) for Conditions IV in the solvent mixture (toluene 1 mL + AcOH 0.1 mL) at 60 °C. b Based on the GC conversion
with >99% selectivity. c Isolation yield.
analogues were investigated. It was widely reported that Zr(IV)-
Since the detail characterizations of UiO-67-TEMPO were based UiO MOFs generally good chemical and physical
previously reported,8c the additional characterizations for stabilities than other MOFs due to the highly oxophilic character
mixed UiO-67-(bpy&TEMPO), E0 were followed. The porosity of of Zr(IV).12,19 Not surprisingly, UiO-67-bpy-Eu, UiO-67-TEMPO,
UiO-67-(bpy-Eu&TEMPO), E was confirmed by N2 full isotherm UiO-67-(bpy&TEMPO), and UiO-67-(bpy-Eu&TEMPO) were
at 77K (609 m2/g, Fig. S4†), and thermal stability was analyzed maintained their frameworks in various organic solvents such as
(Fig. S9†). The 1H NMR spectrum after acid digestion confirmed acetone, dichloromethane, pyridine, and water. The main
the ratio of the BPYDC and BPDC-TEMPO ligands (3:2 molar frameworks were decomposed under 1M HCl aqueous
ratio, Fig. S10†). In addition, the metalation ratio of UiO-67-bpy- condition, which is evidenced by PXRD (Fig. S11†).
Eu (B) and UiO-67-(bpy-Eu&TEMPO) (E) was determined by As summarized in Table 2, UiO-67-bpy-Eu (B, Eu-fixed, Condition
inductively coupled plasma optical emission spectrometry (ICP- I in Scheme 2), UiO-67-TEMPO (D, TEMPO-fixed, Condition II)
OES), and generally, more than 70% of the bipyridyl sites on and UiO-67-(bpy-Eu&TEMPO) (E, Eu and TEMPO-fixed together,
UiO-67-bpy MOFs were decorated with Eu3+ cations (Table S3†). Condition III) were employed for the aerobic oxidation of
Finally, the chemical stability of newly synthesized UiO-67 alcohols and compared to the oxidation of alcohols under the
optimized reaction conditions. At the same time, the mixtures
of crystalline UiO-67-bpy-Eu and UiO-67-TEMPO were also
investigated to compare their reactivities for the aerobic
oxidation of alcohols (Conditions IV in Table 2). UiO-67-bpy-Eu
and UiO-67-TEMPO (1 mol%) were both employed for the
aerobic oxidation of alcohols and compared to the oxidation of
alcohols with external TEMPO (1 mol%) or Eu(NO3)3 (1 mol%).
Both Eu-anchored and TEMPO-functionalized UiO-67s
(Conditions I & Conditions II in Table 2) generally showed good
reactivities for benzylalcohol (1a) and diphenyl methanol (1o).
Scheme 3 Preparation of TEMPO-functionalized and Eu&TEMPO-functoinalized
MOFs.
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In the case of bulky substrates such as pyrenyl methanol and system, the coordination sites of Eu(III) are occupied by two
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DOI: 10.1039/D0DT01324B
phenyl pyrenyl methanol (1p and 1q), relatively higher bipyridyl ligands and three nitrates, and coordination of the
conversions were observed with UiO-67-bpy-Eu / TEMPO starting material can only occur in the MOF system.
(Conditions I, 99% and 85%) than UiO-67-TEMPO / Eu Not only Eu-anchored MOF-based catalysts but also TEMPO-
(Conditions II, 68% and 60%), respectively. Neither fixed system functionalized MOFs have been utilized in aerobic oxidations.
(e.g., ligand mixture in the framework, Conditions III) nor Both systems provided good catalytic conversions of alcohols to
physical mixture of crystals in solution (Conditions IV) efficiently the corresponding aldehydes or ketones within 8-18 h. The high
catalyzed the aerobic oxidation since the three redox cycles tunabilities of MOFs allowed the preparation of the dual-
should be incorporated together and placed nearby.
functionalized UiO-67-(bpy-Eu&TEMPO) system, and the
A comparison of Conditions I-IV for the conversion of benzyl physical mixtures of UiO-67-bpy-Eu and UiO-67-TEMPO crystals
alcohol (1a) to benzaldehyde (2a) demonstrated that Eu cation were investigated. A comparison of their catalytic activities
in Eu-BPY should be quite labile and the mobility of Eu could be revealed the crucial role of TEMPO, and TEMPO is considered as
critical for alcohol oxidation since TEMPO was fully fixed in the first oxidant to substrate alcohol for cooperative
Conditions II, III and IV (entry 1 in Table 2). If the coordination metal/organic oxidant/O2 redox cycles. We believe that the
of Eu3+ in Eu-BPY complex is not in equilibrium, the catalytic mobility of the Eu3+ cation is responsible for the reactivities for
conversion using Conditions IV should be restricted to only the TEMPO-fixed heterogeneous systems.
surface-to-surface collisions for cooperative redox cycles. Neither fixed system nor mixture of crystals in solution
Otherwise, the mobilities of NO3- or O2 species in the solutions efficiently catalyzed the aerobic oxidation since the two redox
should be carefully considered, and the sequence of redox cycles should be incorporated together and placed nearby. In
cycles in the proposed general mechanism of aerobic oxidation additions, since the first oxidant to substrate is TEMPO species,
(Scheme S1†) needs to be revisited since Eu-anchored B the TEMPO-fixed system has been found to reduce in the
displayed good reusability in Table 1 (Fig. S7†). In addition, the reactivity for bulky sized substrates due to mismatches between
similar conversions achieved with Conditions III and IV for all pore size of the frameworks and substrate size. Finally, Eu-
substrates indicate the presence of mobile oxidants in reaction anchored into bipyridyl functionalities on UiO-67-bpy and
mixtures.
molecular TEMPO additive system showed good catalytic
To understand the catalytic performance depending on the size activity for various sized alcohols and reusability (Table 1 and
of substances, the molecular sizes of starting alcohols were Fig. S7†).8c
simply calculated through B3LYP/6-311G (Fig. S12†) and
compared with NLDFT pore size distribution of MOF catalysts
(Table S2†).8c It reveals the diffusion difficulty of pyrenyl
methanol (1p) and phenyl pyrenyl methanol (1q) to MOF pores
(Fig. S4†). While Condition III requires the substrate to enter the
pores to interact with both the metal cation and organic oxidant
(TEMPO); in the case of Condition IV, two species must enter
the pores (i.e., the substrate and oxidant) to perform aerobic
oxidation. However, similar conversions were observed with
substrates of various sizes, and size discrimination of the
substrate was not achieved in the present system due to the
mobility of Eu(III) (Table 2).
Experimental
Eu-MOF-1 (A1) or Eu-MOF-2 (A2).9a 1,4-NDC (10.8 mg, 0.05
mmol for Eu-MOF-1) or 1,4-DOBDC (9.9 mg, 0.05 mmol for Eu-
MOF-2) were mixed with Eu(NO3)3·5H2O (21.4 mg, 0.05 mmol),
2-fluorobenzoic acid (56 mg, 0.4 mmol), DMF (10 mL), H2O (2
mL) and 60% HNO3 (80 μL). The solution was placed in a 20-mL
scintillation vial and heated at 115 °C for 60 h. The colorless
polyhedral crystals were isolated by centrifugation (3600 rpm)
and washed 3 times with DMF (30 mL) and then 3 times with
CH3CN (30 mL). Heating at 60 °C under vacuum for 12 h yielded
activated-Eu-MOF-1 and 2.
UiO-67-bpy (B0).14a BPYDC (122 mg, 0.5 mmol) and DMF (40 mL)
were placed in a 250-mL round-bottom flask equipped with a
Conclusions
The europium-catalyzed aerobic oxidation of alcohols with the magnetic stir bar, and the mixture was heated until the solid
molecular oxidant TEMPO has been successfully achieved with was fully dissolved. ZrCl4 (117 mg, 0.5 mmol), benzoic acid (1.83
MOF-based catalysts. Since the catalytic applicability of the g, 15 mmol), and H2O (18 μL, 1.0 mmol) were added to the
redox cycle of europium (Eu2+/Eu3+) was recently revealed, both BPYDC solution, and the resulting clear solution was heated
Eu-based MOFs (Eu-MOF-1 with NDC and Eu-MOF-2 with with magnetic stirring for 5 days at 130 °C under a nitrogen
DOBDC) and Eu-anchored Zr-based MOFs have been utilized for atmosphere. The colorless microcrystalline powder was
the heterogeneous aerobic oxidation of alcohols in the isolated by centrifugation (3600 rpm) and washed 5 times with
presence of TEMPO as an oxidant. While Eu-based MOF-1 and DMF (150 mL) and then 5 times with CH3CN (150 mL). Heating
Eu-MOF-2 were completely inert in the aerobic oxidation since at 100 °C under vacuum for 12 h yielded activated-UiO-67-bpy.
all the europium(III) was fixed in the SBUs and fully coordinated, UiO-67-bpy-Eu (B). The microcrystalline powder of UiO-67-bpy
Eu anchored into bipyridyl functionalities on UiO-67-bpy (Zr6(O)4(OH)4(BPYDC)6 (100 mg, 0.28 mmol), Eu(NO3)3·5H2O
showed good catalytic activity, even higher than the (120 mg, 0.28 mmol) and CH3CN (10 mL) were placed in a 20-mL
homogeneous Eu(Me-BPY2(NO3)3 species. We attributed the scintillation vial and heated at 60 °C for 24 h. The colorless
different reactivities to the different coordination microcrystalline powder was isolated by centrifugation (3600
environments and numbers of ligands. In the homogeneous rpm) and washed 5 times with hot CH3CN (150 mL). Then,
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Journal Name
3
(a) Z. Shi, C. Zhang, C. Tang and N. Jiao, Chem. Soc. Rev., 2012,
heating at 100 °C under vacuum for 12 h yielded activated-UiO-
67-bpy-Eu.
View Article Online
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UiO-67-TEMPO (D).8c,8d MOF D was synthesized according to
the reported literature.
UiO-67-(bpy&TEMPO) (E0): BPYDC (24.4 mg, 0.1 mmol), BPDC-
TEMPO (49.4 mg, 0.1 mmol) and DMF (8 mL) were placed in 20
mL scintillation vial and heated until fully dissolved. ZrCl4 (46.6
mg, 0.2 mmol) and conc. HCl (0.25 mL) were added then
o
resulting clear solution was incubated at 85 C for 24 h. The
colorless microcrystalline powder was isolated by
centrifugation (3600 rpm) and washed 5 times with DMF (150
mL) and washed 5 times with CH3CN (150 mL). Then heating at
4
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o
50 C under vacuum for 12 h yielded the activated E0. This
procedure was prepared by a reported method with minor
modifications.8c,8d
UiO-67-(bpy-Eu&TEMPO) (E): This mixed MOF E could be
obtained by following B procedure using E0 (130 mg, 0.28
mmol) and Eu(NO3)3·5H2O (72.8 mg, 0.17 mmol) instead of B0.
Europium loading was determined by ICP-OES based on the
molar ratio of the inserted metal relative to zirconium in the
framework.
General procedure for aerobic oxidation
Condition I: the alcohol (1, 0.25 mmol), UiO-67-bpy-Eu (2 mg,
0.0025 mmol), TEMPO (0.4 mg, 0.0025 mmol), NaNO3 (0.2 mg,
0.0025 mmol) and toluene (1 mL) were added to a scintillation
vial. Acetic acid (0.1 mL) was added to the solution, and then
the mixture was stirred at 60 °C for 8 h under air. The resulting
mixture was filtered through a syringe filter, and the conversion
ratio was determined by GC or the desired product was isolated
by silica gel column chromatography.
5
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Conflicts of interest
There are no conflicts to declare.
7
Acknowledgements
We thank Mr. Chan Hee Ryu and Prof. Dr. Kang Mun Lee
(Kangwon National University, Korea) for experimental help and
discussions. This work was supported by the Basic Science
Research Program (2019R1A2C4070584) and the Science
Research Center (2016R1A5A1009405) through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Science and ICT. S. Kim and J. Lee were supported by NRF Global
Ph.D. Fellowship program (2018H1A2A1062013 for S. Kim;
2019H1A2A1076014 for J. Lee) funded by the Ministry of
Education.
8
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TOC Entry
The systematic approaches for Eu and/or TEMPO-functionalized MOFs for aerobic oxidation
of alcohols have been performed.