The Origin of Catalytic Benzylic C?H Oxidation over a Redox-Active Metal–Organic Framework

Article abstract of DOI:10.1002/anie.202102313

Selective oxidation of benzylic C?H compounds to ketones is important for the production of a wide range of fine chemicals, and is often achieved using toxic or precious metal catalysts. Herein, we report the efficient oxidation of benzylic C?H groups in a broad range of substrates under mild conditions over a robust metal–organic framework material, MFM-170, incorporating redox-active [Cu₂^II(O₂CR)₄] paddlewheel nodes. A comprehensive investigation employing electron paramagnetic resonance (EPR) spectroscopy and synchrotron X-ray diffraction has identified the critical role of the paddlewheel moiety in activating the oxidant ^tBuOOH (tert-butyl hydroperoxide) via partial reduction to [Cu^IICu^I(O₂CR)₄] species.

Full text of DOI:10.1002/anie.202102313

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Accepted Article
Title: The origin of catalytic benzylic C-H oxidation over a redox-active
metal-organic framework
Authors: Martin Schröder, Louis Kimberley, Alena M. Sheveleva,
Jiangnan Li, Joseph H. Carter, Xinchen Kang, Gemma L.
Smith, Xue Han, Sarah J. Day, Chiu C. Tang, Floriana Tuna,
Eric J. L. McInnes, and Sihai Yang
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The origin of catalytic benzylic C-H oxidation over a redox-active
metal-organic framework
Louis Kimberley[a], Alena M. Sheveleva[a], Jiangnan Li[a], Joseph H. Carter[a,c], Xinchen Kang[a], Gemma
L. Smith[a], Xue Han[a], Sarah J. Day[c], Chiu C. Tang[c], Floriana Tuna[a,b]*, Eric J. L. McInnes[a,b]*, Sihai
Yang[a]* and Martin Schröder[a]*
[a]
L. Kimberly, A. M. Sheveleva, J. Li, J. H. Carter, X. C. Kang, G. L. Smith, X. Han, S. J. Day, C. C. Tang, F. Tuna, E. J. L. McInnes, S. H. Yang, M. Schröder
Department of Chemistry, University of Manchester, Manchester, M13 9PL, UK
sihai.yang@manchester.ac.uk; m.schroder@manchester.ac.uk
[b]
[c]
F. Tuna, E. J. L. McInnes
Photon Science Institute, University of Manchester, Manchester, M13 9PL, UK
S. J. Day, C. C. Tang
Diamond Light Source, Harwell Science Campus, Oxfordshire, OX11 0DE, UK
Abstract: Selective oxidation of benzylic C-H compounds to
ketones is important for the production of a wide range of fine
chemicals, and is often achieved using toxic or precious metals
catalysts. Here, we report the efficient oxidation of benzylic C-H
groups in a broad range of substrates under mild conditions over
MFM-170, [Cu2(C33H17NO8)(H2O)]·3(DMF), was synthesised via
our previously reported method.[17] SEM images indicate a Bilinski
dodecahedron morphology with a wide crystal size distribution
centred at 1-65 µm (Figure S1). MFM-170 forms a self-
interpenetrated (3,36)-connected network with [Cu2(O2CR)4]
paddlewheels connected through bound carboxylate groups in
the equatorial plane. The axial positions of the paddlewheel are
occupied by a pyridyl N-donor on one Cu(II) and a water molecule
on the other. The bound H2O can be removed by heating under
vacuum to generate a coordinatively unsaturated Cu(II) site.[17]
The phase purity of the bulk material was confirmed by powder X-
ray diffraction (PXRD) and TGA (Figures S2 and S3). The fully
desolvated MOF shows an apparent surface area of 2325 m2 g-1
as determined from the N2 sorption isotherm (Figure S4).
a
robust metal-organic framework material, MFM-170,
II
incorporating redox-active [Cu2 (O2CR)4] paddlewheel nodes. A
comprehensive investigation employing electron paramagnetic
resonance (EPR) spectroscopy and synchrotron X-ray
diffraction has identified the critical role of the paddlewheel
moiety in activating the oxidant tBuOOH (t-butyl hydroperoxide)
via partial reduction to [CuIICuI(O2CR)4] species.
The selective oxidation of benzylic compounds is widely
employed in chemical industry for the production of ketones and
numerous fine chemicals.[1] Conventional homogeneous
processes use stoichiometric amounts of transition metal
complexes [e.g., Cr(VI) and Mn(VII)], which have significant
environmental implications and result in costly procedures for
waste treatment.[2] In contrast, heterogeneous catalysts typically
consisting of precious metal nanoparticles supported on porous
materials (e.g., zeolites, silica, titania) have been used
extensively for a wide variety of large scale catalytic reactions.[3]
However, the nature of the dispersion within these supported
materials makes them prone to deactivation via aggregation and
leaching of the active metal.[4] Therefore, the development of new
efficient heterogeneous catalysts based upon porous materials
with immobilised, uniformally- and atomically-dispersed active
metal sites remains an important and challenging target.
The oxidation of indane was studied to assess the catalytic activity
of MFM-170 towards the oxidation of the benzylic C-H group.
While negligible oxidation of indane was observed using H2O2 or
O2 alone, MFM-170 displayed remarkable activity in the presence
t
of BuOOH. A series of tests were conducted to optimise the
reaction conditions: a yield of 94% of 1-indanone was achieved
t
o
with MFM-170 (10 mol%), BuOOH (3 eq.) at 65 C for 1 day
(Table 1, Entry 5). In the absence of tBuOOH, negligible
conversion of indane was observed (Table 1, Entry 1), and without
MFM-170, the oxidant is thermally activated and offers a
moderate conversion of indane to a mixture of the alcohol and
ketone, demonstrating the critical role of MFM-170 in activating
tBuOOH in a selective manner (Table 1, Entry 2). The time-
conversion plot confirms that there is no induction period and a
rapid conversion of indane is observed within the first 5 h with a
diminished instantaneous rate as the reaction proceeds (Figure
1a). The activity of homogenous catalysts based upon simple
Cu(II)-salts including CuCl2, Cu(NO3)2 and Cu(OAc)2 was also
studied under optimised conditions, and afforded 1-indanone in
yields of 34, 58 and 63%, respectively, lower than that (94%) over
MFM-170 (Figure S5). This result demonstrates that immobilising
the Cu(II) ions into the paddlewheel moiety of the MOF drastically
enhances the catalytic stability and activity towards benzylic
oxidations by preventing their aggregation and deactivation. The
reusability of MFM-170 was studied (Figure S6) over 5 cycles
(yields of 1-indanone in the range of 87-94%). The variation in
intensities of some Bragg peaks of recycled catalysts is likely due
to the inclusion of guest molecules in the pore[17] and/or some
minor local structural change after reaction (Figure S7); the latter
is consistent with a small reduction of the apparent surface area
(2076 m2 g-1; Figure S4). Elemental analytical data suggest the
leaching of Cu sites into solution is within the detection limit of
0.5% (Table S1). Removal of the catalyst by filtration at ~30 %
conversion led to a further production of 1-indanone of ~20%,
which is likely owing to the thermal activation of the oxidant as
confirmed by the blank test (Figures 1b, S8).
Metal-organic framework (MOF) materials consist of single-site
metal nodes bridged by organic linkers, and can effectively hinder
the aggregation and leaching of metal sites. Thus, they have
shown great promise in catalysis.[5] In recent years, MOFs and
their composites have proven to be active catalysts in various
organic
Knoevenagel condensations[8]-[10], cross coupling reactions[7],[11]
cyanosilylations[12] reactions[12]
Friedel-Crafts
transformations
including
hydrogenations[6],[7]
,
,
,
and
oxidations[7],[14]-[16]. However, the determination of reaction
intermediates in these MOF-catalysed reactions is a major
challenge not least because binding of substrates often
undergoes complex, dynamic processes with short-lived
intermediates, particularly for radicals. Here, we report, a study
on the use of MFM-170, constructed from redox-active
II
[Cu2 (O2CR)4] paddlewheels linked by a pyridyl functionalised
tetracarboxylate ligand[17], in combination with an external oxidant,
as a highly active catalyst for oxidation of benzylic C-H functions.
II
The redox behaviour of [Cu2 (O2CR)4] paddlewheels and the
detection of intermediate radicals has been investigated by EPR
t
spectroscopy. The selective activation of the oxidant BuOOH (t-
butylhydroperoxide) on the paddlewheel has been interrogated by
synchrotron X-ray powder diffraction (SPXRD).
1
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reaction solution after 24 h consistent with the proposed
mechanism (Scheme 1).
Table 1. Yields of indane oxidation catalysed by MFM-170.[a]
Entry
Catalyst
loading
(mol %)
tBuOOH
(eq)
T (°C)
Yield
(%)
TOF[b]
1
10
-
-
65
65
65
65
65
65
50
60
70
80
65
65
2
8
2
3
3
3
3
3
3
3
3
3
1.5
6
31
82
87
94
92
87
93
90
94
53
98
n/a
3
1
3416
725
392
192
363
388
375
392
221
408
4
5
5
10
20
10
10
10
10
10
10
6
7
8
9
10
11
12
Scheme 1. Proposed mechanism for the catalytic cycle.
[a]Reaction conditions unless noted otherwise: indane (0.25 mmol), MFM-170
(0.025 mmol), tBuOOH (0.75 mmol), MeCN (4 mL) 65 oC, 24 h.
[b]TOF = 1000 x (molproduct molactive Cu-1 h-1).
a)
b)
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Indane
1-indanone
Optimised Conditions
Hot Filtration Test
Figure 2. Views of (a) the crystal structure of MFM-170; (b) the [Cu2(O2CR)4]
paddlewheel in MFM-170 with the pyridyl-N donor shown in blue; (c) tBuOOH-
loaded MFM-170.
Catalyst
Removed
The use of spin traps, such as N-tert-butyl-α-phenylnitrone
(PBN)[21], facilitates detection by EPR spectroscopy of short-lived
radicals that form during the catalytic process which would
otherwise be undetectable. Reaction of MFM-170 and tBuOOH in
the presence of PBN in MeCN at 65 ⁰C for 2 min gave an intense
EPR signal at g = 2.006, characteristic of PBN-spin adducts.
Thermally-activated tBuOOH gives only a very weak signal
(Figures S10 and S11). This confirms that MFM-170 activates
tBuOOH to generate intermediate radicals. In situ EPR
measurements at 65 ºC in the presence of PBN yielded narrower
0
5
10
15
20
25
0
5
10
15
20
25
Time / h
Time / h
Figure 1. (a) Plot of time vs conversion for the oxidation of indane catalysed by
MFM-170 showing the conversion of indane (black) and the percentage of 1-
indanone formed (red) over time. (b) Reaction profiles for the optimised
conditions (red) and hot filtration, leach test (black). Reaction conditions: indane
t
(0.25 mmol), MFM-170 (0.025 mmol, 0.05 mmol Cu), BuOOH (0.75 mmol),
MeCN (4 mL) 65 oC, 24 h.
The likely mechanism of catalysis involves the generation of
tBuO˙ and BuOO˙ radicals by reactions of Cu centers within the
t
[Cu2(O2CR)4] paddlewheel.[1],[18-20] The reactive tert-butoxy and
tert-peroxy derived radicals abstract a hydrogen atom from the
benzylic position of the substrate, and the resultant C radical is
then converted to the product via O-transfer. Rietveld refinement
of the SPXRD data of tBuOOH-loaded MFM-170 confirmed
binding of tBuOOH to the vacant metal site within the
[Cu2(O2CR)4] paddlewheel (Figure 2, Figure S9) with Cu···O of
2.840(2) Å. The geometry of one of the Cu centers in the
paddlewheel is clearly distorted with the O-Cu-O bond angles
decreasing from 167.9(8)° to 161.1(3)°. This is accompanied by
an increase in the Cu-O(carboxylate) bond length from 1.955(2)
to 2.070(8) Å. Bond valence sum calculations suggests an
oxidation state change at this Cu center from 1.86 to 0.99 (Table
S2), suggesting the formation of a mixed Cu(II)Cu(I) [Cu2(O2CR)4]
paddlewheel species (Figure 2). This is consistent with the
proposed formation of Cu(I) centers via activation of peroxide
t
spectra that enabled unambiguous identification of BuO· and
tBuOO· radicals (Figure 3a, Table S3).[22],[23]
Binuclear Cu(II) paddlewheel [Cu2(O2CR)4] moieties are generally
EPR silent at low temperatures due to strong antiferromagnetic
exchange between Cu(II) centers (3d9, s = ½) ions. This leads to
a non-magnetic S = 0 spin ground state.[24],[25] However , if one of
the Cu ions is reduced to 3d10 Cu(I), as suggested by the
structural analysis above, the EPR signal due to the remaining,
uncoupled Cu(II) S = ½ ion will be observable. Continuous-wave
(CW) and pulse echo-detected (ED) EPR spectra at 5 K of MFM-
170 give only very weak signals due to adventitious Cu(II)
impurities that are ubiquitous in polynuclear Cu(II) species. On
addition of tBuOOH an intense EPR spectrum is observed
characteristic of isolated Cu(II) (Figures 3c and S12). Modelling
the spectra[26] gives gx,y,z = 2.006, 2.078, 2.353 and 63/65Cu nuclear
hyperfine constants Ax,y,z = 50.4, 28, 481.6 MHz (Figure S13)
consistent with the square pyramidal geometry of the N-bound
Cu(II) ion remote from the tBuOOH binding site. The local
reagents by Cu(II).[18]-[20]
A
significant increase in the
concentration of BuOH was also detected by GC-MS in the
t
2
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environment of this Cu(II) site was further investigated by
electron-nuclear double resonance (ENDOR) spectroscopy,[27]
which revealed hyperfine interactions with surrounding H nuclei
MFM-170 to leading systems places MFM-170 as a top
performing catalyst for benzylic C-H oxidation (Table S5).
1
(Figure 3d). The spectra were successfully modelled with a point
dipole model taking into account interaction of Cu(II) with the
nearest 1H nuclei of the aromatic ligand, that is the two –CH group
protons of the pyridine ring (Hpy) and four –CH group protons of
1
the phenyl ring (Hph) (Figure 3b). Each H hyperfine interaction
matrix (Adip) is defined in its own frame with the z-axis directed
along the Cu(II)–H vector and calculated in the high-field
approximation as Adip = [-T;-T; 2T]. Here, 푇 = 휇0푔푒푔푛휇푒휇푛/4휋푟3,
where µ0 is the vacuum permeability, µn is the nuclear magneton,
gn is the nuclear g-factor and
r
is the Cu…H distance.
Transformation of the Adip tensor into the molecular frame defined
by the g-tensor frame of the Cu(II) site (X||gxx, Y||gyy, Z||gzz) used
the Euler angles which we fixed from the SPXRD structure
refinement.[28] The r values as determined from the orientation
t
selective ENDOR spectra of MFM-170 treated with BuOOH
agree well with the structural data. For example, the obtained
Cu(II)···Hpy and Cu(II)···HPh distances of 3.0 Å and 4.0 Å are in
good agreement with the 3.146(2) Å and 4.358(9) Å derived from
SPXRD data (Table S4). To characterise the interaction of Cu with
14N nuclei, the Larmor frequency of which is much smaller than
1
that of H, we used hyperfine sublevel correlation (HYSCORE)
spectroscopy. The low frequency part of the HYSCORE spectrum
of MFM-170 treated by tBuOOH is dominated by cross-peaks that
are assigned to double-quantum correlation peaks from the 14N
Figure 3. Views of (a) X-band (9.4 GHz) EPR spectrum of PBN spin trap with
tBuOOH and MFM-170 in MeCN at 338 K (black), and simulation (red) of the
combined spectrum of PBN-tBuO· radical (aH = 2.25 G; aN = 14.25 G) and PBN-
tBuOO· radical (aH = 1.4 G; aN = 13.47 G) (Figure S13 and Table S3 for details);
(b) structure of MFM-170- tBuOOH from SPXRD refinement, with definition of x,
y, z molecular axis and crystallographic Cu···(Ar)H distances; (c) X-band
electrically-detected (ED) EPR spectra (9.724 GHz) at 5 K of MFM-170 (blue)
nucleus (Figure S14). The magnitude of the 14N hyperfine (Aiso
=
-0.7 MHz) and quadrupole interactions are similar to those
reported for pyridines coordinated at the axial position of Cu(II),
e.g. in bis-diketonate complexes.[29],[30] Thus, ENDOR and
HYSCORE data provide clear evidence that the paramagnetic
signal observed is due to a Cu(II) site that is incorporated within
the MOF framework. Overall, the experimental data provide good
evidence for the proposed reaction cycle and compliments the
computational study recently reported for a Cu/POM system.[31]
t
and BuOOH-loaded MFM-170 (black); (d) X-band (9.724 GHz) 1H ENDOR
spectra of tBuOOH-loaded MFM-170 at 290 and 338 mT (black) and their
simulation using the point dipole model. Simulated ENDOR spectra (red) were
obtained by summing simulations for the two –CH protons of the pyridine ring
(Hpy; green) and the four –CH protons of the phenyl rings (Hph;blue) (see text for
details).
The catalytic performance of MFM-170 was tested with a broad
range of substrates using the optimised conditions (Table 2). A
comparable yield of α-tetralone (92 %) was obtained from the
oxidation of the chemically similar tetralin. When replacing the α-
carbon to the benzylic position with an oxygen (phthalan), a
greater than 99 % yield of phthalide was obtained within 4 h,
attributed to the enhanced stabilisation of generated radicals by
Table 2. Catalytic oxidation of benzylic C-H compounds to the corresponding
ketone using MFM-170 in the presence of tBuOOH.[a]
the
adjacent
heteroatoms.
The
oxidation
of
p-
methoxyethylbenzene was more rapid than for ethylbenzene and
its halogenated derivatives due to the additional electron donating
effect of the methoxy group. Substrates with oxidatively sensitive
groups such as -OH and –CHO groups were also tested. While
the -OH group can be retained, -CHO undergoes oxidation to –
COOH species. The reaction proved more active towards
diphenylmethane and its halogenated derivatives compared to
ethylbenzene and its halogenated derivatives. This was attributed
to the increased radical stabilisation offered by increased
aromaticity. 9,10-Dihydroanthracene is more readily oxidised than
diphenylmethane due to better resonance stabilisation of the
intermediate radical. This arises from the more coplanar orbital
overlap resulting in significant over-oxidation to the diketone
(conversion is 98%, of which 50 % is the mono-ketone and 48 %
the di-ketone). With xanthene, an extremely rapid conversion to
xanthone with full oxidation observed over 3 h consistent with a
combination of radical stabilisation effects. While some reported
MOFs can catalyse oxidations of small substrates with
comparable activity to MFM-170, larger substrates have been
shown to suffer typically from reduced yields due to steric
constraints arising from narrow pore diameters.[15] The high
porosity of MFM-170 (cage size up to 22.2 x 16.3 Å[17]) allows
more sterically encumbered and complex substrates, such as
t
Reaction conditions: substrate (0.25 mmol), MFM-170 (0.025 mmol), BuOOH
(0.75 mmol), tBuOOH (0.75 mmol), MeCN (4 ml) 65 °C, 24 h (unless otherwise
stated). TOF = 1000 x (molproduct molactive Cu-1 h-1).
Deactivation of transition metal catalysts often arises via
degradation to metal oxide particles.[36] We have demonstrated
xanthene (9.18 Å)[32]
, ,
9,10-dihydroanthracene (9.12 Å)[33]
t
fluorene (8.42 Å)[34] and 2,7-di-tert-butylfluorene (13.03 Å)[35] to be
oxidised catalytically in high yields. Comparison of the activity of
the utilisation of MFM-170 and BuOOH in MeCN as an efficient
and stable catalyst for benzylic carbon oxidations to give ketones
3
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showed excellent stability over multiple cycles.
A viable
mechanism for this reaction has been proposed with experimental
evidence demonstrating pore-based catalysis. An in-depth EPR
spectroscopic study, in combination with SPXRD, has provided
evidence for the crucial redox behaviour of the [Cu2(O2CR)4]
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Acknowledgements
We thank EPSRC (EP/I011870), the Royal Society and University
of Manchester for funding, EPSRC for funding of the EPSRC
National EPR Facility at Manchester and D. J. Procter for helpful
discussions. This project has received funding from the European
Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement No
742401, NANOCHEM). We are grateful to Diamond Light Source
for access to the Beamline I11. AMS and XK are supported by
Royal Society Newton International Fellowships.
Associated content
[36] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, J. Catal. 2009, 267, 1–4.
Supplementary Information is available in the online version of
the paper. Synthetic procedures, characterisation, catalysis
testing, and additional analysis of crystal structure. Crystal data
of MFM-170- tBuOOH is deposited at Cambridge
Crystallographic Data Center (CCDC) 2017619 and can be
obtained free of charge.
Conflict of interest
The authors declare no competing interests.
Keywords: metal-organic framework; catalysis; benzylic
oxidation; electron paramagnetic resonance; copper
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10.1002/anie.202102313
Angewandte Chemie International Edition
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The oxidation of benzylic C-H groups in a broad range of substrates under mild conditions has been achieved using a robust metal-
II
organic framework material, MFM-170, incorporating redox-active [Cu2 (O2CR)4] paddlewheel nodes. Electron paramagnetic
resonance (EPR) spectroscopy and synchrotron X-ray diffraction has identified the critical role of the paddlewheel moiety in activating
the oxidant tBuOOH (t-butyl hydroperoxide) via partial reduction to [CuIICuI(O2CR)4] species.
5
This article is protected by copyright. All rights reserved.