A crystalline porous coordination polymer decorated with nitroxyl radicals catalyzes aerobic oxidation of alcohols

Article abstract of DOI:10.1021/ja5019095

A porous coordination polymer (PCP) has been synthesized employing an organic ligand in which a stable free radical, isoindoline nitroxide, is incorporated. The crystalline PCP possesses one-dimensional channels decorated with the nitroxyl catalytic sites. When O₂ gas or air was used as the oxidant, this PCP was verified to be an efficient, recyclable, and widely applicable catalyst for selective oxidation of various alcohols to the corresponding aldehydes or ketones.

Full text of DOI:10.1021/ja5019095

Communication
pubs.acs.org/JACS
A Crystalline Porous Coordination Polymer Decorated with Nitroxyl
Radicals Catalyzes Aerobic Oxidation of Alcohols
†
†
,†
‡
†
†
†
,†,§
†
Liangchun Li, Ryotaro Matsuda,* Iku Tanaka, Hiroshi Sato, Prakash Kanoo, Hyung Joon Jeon,
‡
Maw Lin Foo, Atsushi Wakamiya, Yasujiro Murata, and Susumu Kitagawa*
†
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Institute for Chemical Research, Kyoto University, Kyoto 611-0011, Japan
‡
§
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura,
Nishikyo-ku, Kyoto 615-8510, Japan
*
S Supporting Information
catalysts while alleviating their limitations. Several PCPs with
unique catalytic properties, including asymmetric catalysis, have
been reported to date. Compared with the homogeneous
ABSTRACT: A porous coordination polymer (PCP) has
been synthesized employing an organic ligand in which a
stable free radical, isoindoline nitroxide, is incorporated.
The crystalline PCP possesses one-dimensional channels
5
catalysts, the remarkable features of heterogeneous catalysts are
that they can be easily isolated by filtration to simplify the workup
procedures and their robustness usually enables recyclability.
Moreover, in contrast with other immobilized catalytic systems,
PCPs bearing large surface areas, uniform pores, and extremely
dense active sites generally exhibit good efficiency and selectivity.
Their highly ordered crystalline structures can be solved by
single-crystal X-ray diffraction (SXRD) and well-characterized,
which will benefit further rational designs. Herein we report a
new free-radical-decorated PCP (FRPCP) with the formula
decorated with the nitroxyl catalytic sites. When O gas or
2
air was used as the oxidant, this PCP was verified to be an
efficient, recyclable, and widely applicable catalyst for
selective oxidation of various alcohols to the corresponding
aldehydes or ketones.
orous coordination polymers (PCPs), assembled from
P
metal ions and organic linkers, have been successfully
created as intriguing platforms united with chemistry, physics,
and material sciences. The modular synthesis of PCPs with a
variety of metal nodes and tunable organic ligands provides their
vastly diverse porous structures with desired pore sizes and
[
Cu(DPIO) (SiF )], where DPIO is 4,7-bis(4-pyridyl)-1,1,3,3-
2 6
tetramethylisoindolin-2-yloxyl. The elegant DPIO ligand pos-
sesses pyrroline nitroxide in the central moiety as a stable free
radical with two 4-pyridyl groups symmetrically placed on two
sides to act as linear linkers (Figure 1a). The DPIO ligands are
thus periodically integrated into a porous framework and
decorate the pore surface with free radicals. The resulting
FRPCP featuring large 1D channels catalyzes an efficient,
selective oxidation of a variety of alcohols to their corresponding
1
functionalities directed to various applications such as chemical
separation, gas storage, sensing, and catalysis. On the other
hand, stable free radicals constantly attract great interest because
of their manifestation of polymerization, spin labeling, and
organic magnets. It is expected that the combination of PCPs
2
3
4
5
6
7
8
aldehydes or ketones under an O or air atmosphere and mild
2
and stable free radicals will afford novel materials with multiple
conditions.
functionalities and extraordinary properties. However, very few
PCPs decorated with radicals have been reported because of
The DPIO pillar ligand was designed and successfully
synthesized in five steps starting from the reported 3,6-
dibromophthalic anhydride and finally employing Suzuki−
Miyaura coupling of a key intermediate, 4,7-dibromo-1,1,3,3-
tetramethylisoindolin-2-yloxyl (DBIO), with 4-pyridinylboronic
acid [see the Supporting Information (SI)]. Although the total
yield was only 9.3%, we were able to obtain DPIO on a gram
scale. DPIO was characterized by high-resolution mass
spectrometry and SXRD. The length of DPIO calculated from
the distance between the two pyridyl nitrogen atoms is 11.4 Å
9
synthetic challenges, such as the instability of the radicals and the
bulky structure of the ligands. To our delight, nitroxyl radicals,
which possess the whole radical character with additional
catalytic oxidation properties, are very stable and relatively
designable. Great efforts have been continuously devoted to the
development of nitroxyl radical compounds as homogeneous
catalysts for the oxidation of alcohols to their corresponding
carbonyl compounds, which is a fundamental and pivotal
10
(
Figure 1b). Moreover, the radical character of DPIO was probed
transformation in organic chemistry. Although impressive
advances have been made, there is still plenty of room to develop
the ideal catalysts.
by electron paramagnetic resonance (EPR) spectroscopy and
exhibited a typical three-line signal centered at g = 2.006 in
CH Cl solution (see the SI).
As the ultimate goal of catalysis research, the ideal catalysts
would be highly active, efficient, recyclable, and environmentally
benign with good selectivity and simple workup procedures. This
goal is readily realized by rationally designed PCPs incorporated
with catalytic sites on the pore surfaces because of their ability to
engage the advantages of both homogeneous and heterogeneous
2
2
With DPIO in hand, we then used it to build an analogue of the
11
well-known PCPs [M(4,4′-bipyridine) (SiF )] (M = Zn, Cu),
2
6
n
Received: February 23, 2014
Published: May 9, 2014
©
2014 American Chemical Society
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Journal of the American Chemical Society
Communication
under high vacuum (Figure S2). Thermogravimetric analysis
(
TGA) revealed that the guest molecules were completely
removed and that the frameworks are stable up to 200 °C (Figure
S3).
To investigate the porosity of the activated FRPCP, gas
sorption studies of N , O , and CO were conducted at different
2
2
2
temperatures (Figures 2 and S4). All of the isotherms at low
Figure 2. Sorption isotherms of CO (195 K), N (77 K), and O (90 K)
2
2
2
on the FRPCP. STP and P denote standard temperature and pressure
0
and saturated vapor pressure, respectively.
temperature are type I, and little hysteresis was observed,
suggesting a robust structure and no strong interactions between
the nitroxyl radicals and gas molecules. The Brunauer−Emmett−
2
Teller (BET) surface area was calculated to be 822 m /g from the
N gas sorption at 77 K.
2
Figure 1. (a) Chemical structure of the DPIO ligand. (b) Crystal
structure of DPIO. (c) View of the FRPCP along the c axis (red balls
represent O atoms). (d) View of the FRPCP along the a or b axis. (e)
View of the 3D porous structure of the FRPCP along the c axis. Atoms:
Cu (green), Si (blue), F (yellow), N (purple), O (red), C (gray). H
atoms and guest water molecules have been omitted for clarity.
The proper structural features, good thermal stability, and
extremely high radical density (∼1.8 mol/L) of the FRPCP
encouraged us to employ it for catalytic reactions. Among
catalytic oxidation systems for the transformations of alcohols to
the corresponding carbonyl compounds, the recently reported
TEMPO/tert-butyl nitrite (TBN) system is prominent because
the reaction can be performed under neutral conditions, avoiding
1
0d
since they usually offer robustness and large 1D channels that are
general acid or base reagents.
Using similar reaction
quite appealing for catalytic reactions. Dissolving DPIO in EtOH
conditions, we first examined the oxidation of benzyl alcohol.
The reactions were carried out in NMR tubes with 3.5 mol %
FRPCP (7.0 mol % loading of DPIO ligand) or 7.0 mol % of
other catalysts, 20 mol % TBN, and 0.75 mL C D Cl as the
and layering this solution onto an aqueous CuSiF solution
6
afforded the FRPCP as violet single crystals. The crystallographic
structure clearly reveals a non-interpenetrated three-dimensional
2
2
4
(
3D) framework with octahedral coordination of the Cu(II)
solvent under air or O at 80 °C. As shown in Table 1,
2
centers in which each Cu(II) is coordinated by four N atoms of
DPIO ligands to generate [Cu(DPIO) ] squares in the ab plane
comparable to TEMPO, our ligand intermediate DBIO also gave
a high yield of 96%, indicating the high activity of isoindoline
nitroxide (entries 1 and 2). Although using the FRPCP under air
afforded a lower yield than DBIO, the yield is still quite
impressive (79%) considering that the reaction is heterogeneous
(entry 3). Improvement in the yield was achieved by utilizing O2
gas instead of air (entry 4). It is also noteworthy that the FRPCP
gave much higher yields than another heterogeneous catalyst,
siliacat@TEMPO, under the same conditions (entries 5 and 6).
To study whether the oxidation occurs inside the pores or only
on the outer surface, ground and unground FRPCP samples were
used in the catalytic reactions. Typically, if the reaction occurs
only on the surface of the crystals, the ground sample should
show a much higher yield than the unground sample because the
grinding process decreases the particle size, resulting in an
increased outer surface area. However, the unground sample with
a particle size of ∼1 mm afforded an 79% yield and the ground
sample with much smaller particle size showed a comparable
2
n
2−
and the squares are axially linked by SiF6 groups to provide
channels (Figure 1c,d). The channel dimensions calculated from
the neighboring Cu atoms linked by DPIO are 15.3 Å × 15.3 Å.
As the isoindoline moieties form a dihedral angle of 43.7° with
respect to the channel walls, the narrowest cross-section size and
its diagonal length considering the van der Waals radii are
calculated to be 7.6 Å × 12.4 Å and 14.4 Å, respectively (Figure
1
e and Figure S1 in the SI). Thus, with the radical species
appended regularly in the pores, the FRPCP possesses 51.5%
12
void space based on PLATON calculations. Interestingly, all of
the nitroxyl moieties in this crystal adopt the same orientation
along the c axis (Figure 1d). Along the a and b axes, the windows
are almost closed by the isoindoline nitroxide groups (Figure
S1). The simulated powder X-ray diffraction (PXRD) pattern
from SXRD of the FRPCP is in good agreement with that of the
freshly obtained crystals and samples activated at 60 °C for 24 h
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Journal of the American Chemical Society
Communication
a
Table 1. Catalytic Oxidation of Benzyl Alcohol under Various
Reaction Conditions
Table 2. FRPCP-Catalyzed Oxidation of Various Alcohols
a
a
The reactions were performed in an NMR tube with 0.016 mmol of
b
1
c
alcohol. Determined by H NMR spectroscopy. The reaction time
was 12 h. The recycled catalyst gave yields of 86% and 72% in the
d
second and third runs, respectively.
yield of 80%, suggesting that the oxidation reaction primarily
occurs in the pores, although partial degradation of the sample
upon grinding should be considered (Table 1).
The confinement effect of the FRPCP for catalytic reactions
was examined by experiments using different catalyst/substrate
(
C/S) ratios. At C/S = 1/14, both the FRPCP and DBIO gave
almost quantitative conversions (94% vs 96%). When the C/S
ratio was decreased from 1/14 to 1/5000, the conversion almost
vanished (decreasing from 96% to 0.3%) with DBIO as the
catalyst. On the other hand, with FRPCP as the catalyst, the yield
decreased, but a 4% yield of product was still obtained (see the
SI). The >13 times higher conversion for FRPCP compared with
DBIO at the lower C/S ratio suggested that the confinement of
the reactants in the nanospace of PCP renders its high reactivity
for the catalytic oxidation. Moreover, the catalyst was found to be
recyclable. After the reaction was completed, the catalyst was
recovered by filtration, and this solid was used for the next run.
The three runs gave yields of 94%, 86%, and 72%, respectively.
The preserved PXRD pattern of the recovered PCP further
confirmed its recyclability (Figure S2). Conversion to the
product completely ceased after removal of the FRPCP crystals,
indicating that the reactions are catalyzed heterogeneously
(
Figure S5).
A wide range of alcohols were successfully oxidized to the
corresponding carbonyl compounds under the following
conditions: 0.016 mmol of alcohol, 3.5 mol % activated
FRPCP crystals, 20 mol % TBN, ∼1 atm of O (gas balloon),
2
a
The reactions were performed in an NMR tube with 0.016 mmol of
0
.5 mL of C D Cl , 80 °C (Table 2). Substituted benzyl alcohols
b
1
c
2
2
4
alcohol. Determined by H NMR spectroscopy. The reactant was a
were easily oxidized to the corresponding aldehydes in almost
quantitive yields (entries 2−6). Electron-donating substituents
on the phenyl group (i.e., methoxy and tert-butyl) accelerated the
reaction, while reactions of alcohols with electron-withdrawing
groups (e.g., nitro) required longer reaction times. Some other
aromatic substrates such as 1g and 1h were readily oxidized in
excellent yields (entries 7−9). Oxidation of secondary alcohols
mixture of 0.008 mmol of 1q and 0.008 mmol of 1r.
The specificity of PCPs compared with other solid materials is
that they host well-defined pores that facilitate size or shape
13
selectivity for the substrates. By the use of PCPs constructed
from ligands with catalytic sites, this special selectivity will be
grafted onto the reactions. Indeed, higher branched alcohols led
to lower yields in the same reaction time catalyzed by our FRPCP
(entries 13−15), and the linear benzylic alcohol 1q is easier to
1
j−l gave yields of 99%, 90%, and 63%, respectively. However,
the oxidation of aliphatic alcohols in 4 days afforded lower yields
entries 13−16).
(
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Journal of the American Chemical Society
Communication
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mixture of 1q and 1r as the reactant (entries 17−19). To evaluate
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2
2
4
to afford the pure product in 93% isolated yield (see the SI for
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In summary, using the newly designed free-radical ligand as the
organic linker, we synthesized a novel PCP decorated with
nitroxyl radicals. The full investigation of this PCP and its
application to catalytic reactions proved that this PCP could be a
mild, efficient, and recyclable catalyst for selective oxidation of a
variety of alcohols to aldehydes or ketones. We envision that
these results will provide inspiration for the further design and
synthesis of PCPs as ideal catalysts.
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ASSOCIATED CONTENT
Supporting Information
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*
S
Experimental details, characterization data, crystal data (CIF),
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material is available free of charge via the Internet at http://pubs.
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AUTHOR INFORMATION
Corresponding Authors
rmatsuda@icems.kyoto-u.ac.jp
kitagawa@icems.kyoto-u.ac.jp
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Japan Society for the Promotion
of Science and the “ERATO Kitagawa Integrated Pores Project”
and “ACCEL Project” from the Japan Science and Technology
Agency.
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