"Click" post-functionalization of a metal-organic framework for engineering active single-site heterogeneous Ru(III) catalysts

Article abstract of DOI:10.1039/c5cc02741a

We present a general strategy for incorporating π-conjugated NNN-chelators (e.g., terpyridyl moiety) into a porous metal-organic framework (MOF) MIL-101(Cr) by using the "click" post-functionalization. The functionalized MOF could be used as a platform for metalation to yield highly active and stable single-site heterogeneous metal (e.g., Ru(iii)) catalysts.

Full text of DOI:10.1039/c5cc02741a

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DOI: 10.1039/C5CC02741A
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“Click” Post-Functionalization of Metal-Organic Framework for
Engineering Active Single-Site Heterogeneous Ru(III) Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Suntao Wu, Liyu Chen, Biaolin Yin and Yingwei Li*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We present a general strategy for incorporating π-conjugated potential coordination of the functional groups to the metal
NNN-chelators (e.g., terpyridyl moiety) into a porous metal- ions during MOFs formation.15-17 In this regard, postsynthetic
organic framework (MOF) MIL-101(Cr) by using the “click” post- modification (PSM) represents a powerful tool to incorporate
functionalization. The functionalized MOF could be used
a
functional groups in a MOF backbone for the preparation of
platform for metalation to yield highly active and stable single-site supported organometallics.18-20 Among the developed PSM
heterogeneous metal (e.g., Ru(III)) catalysts.
methods, the “click” chemistry has been discovered as a useful
methodology for post-functionalization of MOFs, which would
largely enrich the chemical diversity of MOFs.21-23 More
recently, homogeneous catalysts based on simple bipyridyl and
phenanthryl ligands have been successfully incorporated on
MOFs to elicit significantly enhanced catalytic activity over
their homogeneous counterparts. Considering the extensive
use of terpyridyl ligands in coordinating with many transition
metal ions (e.g., Ru, Pd, Ni, and Cu) to construct stable
complexes with interesting catalytic, photovoltaic, and
electrochemical properties, it is highly desirable if such
terpyridyl ligands could be immobilized on MOFs and
reused.24-27 However, up to date, the incorporation of
terpyridyl ligands on MOFs has never been explored.
Chelating ligands containing pyridyl moieties such as
bipyridines, phenanthrolines, and terpyridines, have been
extensively employed in coordination chemistry and
homogeneous catalysis.1-3 Owing to their robust redox stability,
strong coordinating ability with metal ions, and easy
functionalization, these ligands have offered a useful platform
to alter the catalytic activity of the anchored metal ions.4,5
However, these homogeneous catalysts based on nitrogen
donor ligands are prone to deactivation via intermolecular
pathways.6 Thus, the immobilization of these metal-pyridine
homogeneous catalysts on a solid support (e.g., zeolites,
mesoporous aluminosilicates, and other porous inorganic or
organic materials) has been developed as an efficient way to
isolate each catalytic site to enhance the stability and catalytic
activity.7-9
Metal-organic frameworks (MOFs) have emerged as a new
type of functional materials owing to their high surface areas,
porosity, and chemical tunability.10-12 The modification of the
pore surfaces with desired functional groups could provide
various anchoring sites for the immobilization of metal ions on
MOFs to develop novel single-site heterogeneous catalysts for
a variety of organic transformations.13,14
Currently, the modification of MOF pore surface is mostly
based on the use of pre-designed ligands with specific
functional groups. But this pre-synthesis method still remains
relatively limited by the complicated multi-step preparation of
bridging ligands, and the unpredictable framework caused by
Scheme 1 Schematic representation of the post-synthesis modification of MIL-101(Cr)
and the synthesis of a single-site heterogeneous Ru(III) catalyst.
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China. E-mail: liyw@scut.edu.cn
† Electronic Supplementary Information (ESI) available: Experimental details,
catalyst characterization, and additional reaction results. See
DOI: 10.1039/x0xx00000x
In this work, we report the successful incorporation of
2,2’:6’,2’’-terpyridine (tpy), a tridentate oligopyridine σ-donor
chelator, onto a new azides-tagged MIL-101(Cr)-N3 through a
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click PSM method. Transition metal species, e.g., RuCl3, could
be then easily immobilized on the tridentate chelator to afford
single-site heterogeneous catalysts. The as-synthesized Ru
catalyst is demonstrated to be highly active and selective in
the oxidation of benzyl alcohols, which is much superior to the
homogeneous counterpart (i.e., RuCl3). Furthermore, metal
leaching is inhibited because of the strong interaction between
Ru(III) ions and the chelators in the MOF network. To the best
of our knowledge, this work represents the first example of
successful incorporation of terpyridyl ligands on MOFs.
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DOI: 10.1039/C5CC02741A
Fig. 2 LC-HRMS spectrogram (A) and TIC chromatogram (B) of the linker molecule of the
MIL-101(Cr)-NH2 was prepared from the nitration of MIL- digested MIL-101(Cr)-tpy (positive mode).
101(Cr) to MIL-101(Cr)–NO2 and the subsequent reduction by
using SnCl2·2H2O and ethanol (Scheme 1).28,29 Powder X-ray
The presence of azido groups on the MIL-101(Cr)-N3 could
diffraction (PXRD) measurements confirmed that MIL-101(Cr)-
NH2 was isostructural to MIL-101(Cr) (Fig. S1).30 1H-NMR
spectroscopy of the digested MIL-101(Cr)-NH2 material in
deuterated dimethyl sulphoxide (DMSO-d6) showed the same
signals as that of 2-aminoterephthalic acid (H2BDC-NH2). This
result demonstrated that all the terephthalate ligands in the
MIL-101(Cr) were converted to 2-aminoterephthalate in the
as-prepared MIL-101(Cr)-NH2.
Using the similar procedures for the functionalization of
IRMOF-3,31 MIL-101(Cr)-NH2 was treated with tBuONO and
TMSN3 in THF overnight at room temperature to give a new
azido functionalized MOF material MIL-101(Cr)-N3 (Scheme 1).
From XRD, no significant loss of the framework integrity was
observed after the transformation of -NH2 to azido (-N3) group.
FT-IR spectroscopy verified the presence of azido group on the
MIL-101(Cr)-N3, showing a peak at ca. 2123 cm-1 that can be
ascribed to the -N3 asymmetric stretching vibration (Fig. 1). 1H-
NMR spectroscopy of the digested MIL-101(Cr)-N3 showed
totally new aromatic signals at 7.74-7.83 ppm, demonstrating
the complete conversion of the amino groups into the
corresponding azido groups (Fig. S2). HRMS provided
additional proof to this modification. The negative mode
spectrogram of digested MIL-101(Cr)-N3 gave values of m/z
around 206 and 413 (Fig. S3), corresponding to [(N3-BDC)-H]–
and [2(N3-BDC)-H]–, respectively (N3-BDC: 2-azido-1,4-
benzenedicarboxlate).
allow for covalent attachment of tridentate ligands through
post-synthetic modifications (Scheme 1). We next modified the
MIL-101(Cr)-N3 employing the efficient azide-alkyne
cycloaddition protocol (CuAAC) with 4’-ethynyl-2,2’:6’,2’’-
terpyridine. The obtained material was denoted as MIL-
101(Cr)-tpy. FT-IR data indicated that the intensity of the
characteristic peak of the azido group at 2123 cm-1 decreased
after this modification (Fig. 1). Using the positive mode, LC-
HRMS spectrogram of the MIL-101(Cr)-tpy showed that a base
peak at m/z ca. 465 ([M+H]+) was appeared as the major
species, which could be attributed to the triazole product of
the cycloaddition (Fig. 2). These results indicated that the azide
MOF crystals had been successfully modified by the alkyne
derivatives in the click reaction. PXRD confirmed the modified
MOF had identical crystallinity to the MIL-101(Cr)-NH2,
demonstrating the maintenance of the framework structure
(Fig. S1).
It should be note that due to the large size of the chelating
terpyridyl unit, only one part of the azido groups in the MIL-
101(Cr)-N3 could be transformed into the triazole groups
(about 14.8% based on the elemental analysis). The occupancy
of the pore volume by the large terpyridyl moiety was evident
from N2 adsorption isotherms and pore-size distribution (Fig.
S4), which showed a remarkable decrease in N2 adsorption
capacity and pore diameter for MIL-101(Cr)-tpy as compared
to MIL-101(Cr)-N3.
The open terpyridyl groups in the MIL-101(Cr)-tpy would
offer an excellent platform for the chelation of transition-
metal complexes. Taking Ru as an example, we used the as-
synthesized MIL-101(Cr)-tpy to bind the ruthenium trichloride
in methanol to obtain RuCl3@MIL-101(Cr)-tpy (Scheme 1).
PXRD and TEM results showed that the crystalline structure of
the parent MIL-101(Cr) was mostly retained after the doping of
RuCl3 (Fig. S1 and Fig. S5).32 N2 adsorption isotherms also
confirmed the porous structure of the framework (Fig. S4). The
Ru content in the RuCl3@MIL-101(Cr)-tpy was ca. 1.0 wt%, as
determined by atomic absorption spectroscopy (AAS). To
confirm that the Ru(III) ions were coordinated to the 2,2’:6’,2’’-
terpyridine chelating groups, we performed X-ray
photoelectron spectroscopy (XPS) analysis on RuCl3, MIL-
101(Cr)-tpy, and RuCl3@MIL-101(Cr)-tpy. Accordingly, both
RuCl3@MIL-101(Cr)-tpy and RuCl3 exhibited Ru 3p3/2 and 3p1/2
bands at around 464 eV and 486 eV (Fig. 3A). However, the Ru
3p lines of RuCl3@MIL-101(Cr)-tpy were shifted by ca. 0.5 eV
Fig. 1 Infrared spectra of MIL-101(Cr)-NH2 (a), MIL-101(Cr)-N3 (b), MIL-101(Cr)-tpy (c),
and RuCl3@MIL-101(Cr)-tpy (d).
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toward lower binding energies, as compared to the pristine preliminary study using benzyl alcohol as the model substrate
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DOI: 10.1039/C5CC02741A
RuCl3. Such shifts suggested that a strong coordination was performed for optimizing the reaction conditions, and the
interaction between the terpyridine chelating groups and the results are summarized in Table 1. First, the effect of solvents
Ru atoms in RuCl3@MIL-101(Cr)-tpy.
on the oxidation reaction was investigated. It was observed
that water was the best among the examined solvents with
respect to both conversion and selectivity (Table 1, entries 1-6),
achieving almost quantitative yield of benzyl aldehyde within 6
h. For comparison, we also carried out the oxidation reaction
using the parent MIL-101(Cr)-tpy or homogeneous RuCl3·3H2O
as catalyst under the same reaction conditions. Much lower
activities were obtained for both the MIL-101(Cr)-tpy and
RuCl3 as compared to RuCl3@MIL-101(Cr)-tpy (Table 1, entries
7-8). The observation indicated that the excellent catalytic
performance of RuCl3@MIL-101(Cr)-tpy compared to its
homogeneous counterpart could be related to the difference
in the electron configuration and σ-donor ligands of the tpy
moiety in the MOF.
Table 2 Oxidation of various alcohols catalyzed by RuCl3@MIL-101(Cr)-tpy a
Entry
1
Substrate
Time (h)
6
Yield (%)
99
Fig. 3 (A) XPS spectra of the Ru 3p region for RuCl3 (a), and RuCl3@MIL-101(Cr)-tpy (b).
(B) XPS spectra of the N 1s region for MIL-101(Cr)-tpy (a), and RuCl3@MIL-101(Cr)-tpy
(b). (C) XPS survey spectra of RuCl3@MIL-101(Cr)-tpy.
2
3
6
6
99
97
The N 1s spectra indicated three types of chemical
environments for nitrogen in the materials (Fig. 3B). The peak
at 401.7 eV was attributed to the three nitrogen atoms of the
triazole ring.33 Another peak at around 398.1 eV could be
ascribed to the three nitrogen atoms of the terpyridine in the
parent MIL-101(Cr)-tpy. Because of the strong interaction
between the terpyridine and RuCl3 in the RuCl3@MIL-101(Cr)-
tpy, this peak was observed to shift toward higher binding
energy by ca. 0.6 eV. The peak at 399.9 eV may be assigned to
the residual azide groups, which showed a lower binding
energy compared to the triazole ring.34
4
5
6
99
99
6.5
6
7
8
6
98
99
99
99
99
8
5.5
6
Table 1 Results of the oxidation of benzyl alcohola
9
Entry
Catalyst
Solvent
DCE
Toluene
DCM
THF
CH3CN
H2O
H2O
Conv. /sel. (%)
41/92
10
7
1
2
3
4
5
RuCl3@MIL-101(Cr)-tpy
RuCl3@MIL-101(Cr)-tpy
RuCl3@MIL-101(Cr)-tpy
RuCl3@MIL-101(Cr)-tpy
RuCl3@MIL-101(Cr)-tpy
RuCl3@MIL-101(Cr)-tpy
RuCl3∙3H2O
64/>99
25/>99
31/>99
82/91
>99/>99
49/98
11b
12
5.5
6
99
99
6
7
13
9
95c
8
MIL-101(Cr)-tpy
RuCl3@MIL-101(Cr)-tpy
H2O
H2O
11/>99
99/>99
9b
a Reaction conditions: benzyl alcohol (0.5 mmol), catalyst (Ru 1 mol%), solvent (5
mL), 100 °C, H2O2 (0.75 mmol), 6 h. Fifth run to test the reusability of
RuCl3@MIL-101(Cr)-tpy under the conditions of entry 6.
a
b
Reaction conditions: alcohol (0.5 mmol), RuCl3@MIL-101(Cr)-tpy (Ru 1 mol%),
H2O (5 mL), 100 °C, H2O2 (0.75 mmol). b H2O2 (1.25 mmol). C Isolated yield.
Various aromatic alcohols were further employed in the
reaction to investigate the range of alcohols that can be
tolerated in the reaction over this novel RuCl3@MIL-101(Cr)-
tpy catalyst. Substituted benzyl alcohols containing both
Selective oxidation of alcohols is a significant process in
organic synthesis and industrial manufacture.35-37 The
synthesized RuCl3@MIL-101(Cr)-tpy material was investigated
as catalyst for the oxidation of various alcohols with H2O2. A
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6
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F. Shi, A. M. Coffey, K. W. WadDdOeIl:l,10E..10Y3.9/CCh5eCkCm02e7n4e1Av
electron-donating
and
electron-withdrawing
groups
View Article Online
Chem. Soc., 2015, 137, 2665.
underwent the oxidation smoothly, affording the desired
oxidation products in almost quantitative yields (Table 2,
entries 1-10). With the presence of more amount of H2O2, 1,4-
benzenedimethanol was selectively oxidized to 1,4-
phthalaldehyde in 99% yield (Table 2, entry 11). 1-
Naphthalenemethanol and 9-anthracenemethanol also
reacted well to provide the desired aldehydes in excellent
yields (Table 2, entries 12-13). It was noteworthy that the
reaction of 9-anthracenemethanol took more time to
complete, probably due to the space steric hindrance.
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The heterogeneous nature of RuCl3@MIL-101(Cr)-tpy was
investigated by using a hot filtration experiment. At ca. 30%
conversion, the reaction solution was quickly filtrated at the
reaction temperature in order to avoid re-deposition of
dissolved Ru upon cooling. The content of Ru in the filtrate was
below the detection limit of the AAS analysis. The isolated
solution was allowed to further react for 8 h. It was observed
the reaction was essentially stopped after the removal of the
catalyst (Fig. S6). This result strongly suggested that the
reaction would proceed mostly on the heterogeneous surface.
Moreover, the Ru content of the recycled catalyst was almost
identical to the fresh one. These studies indicated that the
leaching of Ru active sites was negligible, which could account
for the preservation of the catalytic activity of this catalytic
system after being reused for at least 5 times (Table 1, entry 9).
In summary, we have successfully incorporated terpyridyl
moiety into a new azido-functionalized MOF, MIL-101(Cr)-N3,
by using the CuAAC click reaction. The as-prepared MOF with
open terpyridyl unit may be used as a useful platform to
synthesize various single-site heterogeneous metal catalysts.
Preliminary metalation investigation demonstrated the
synthetic utilization of MIL-101(Cr)-tpy for the construction of
highly active single-site Ru(III) catalyst. Moreover, the catalyst
was easily recoverable and reusable for at least five times
without any apparent loss of catalytic efficiency. The proposed
PSM methodology starting from the aromatic rings that are
contained in the majority of MOFs, is believed to be applicable
to a wide range of MOFs that might bring new opportunities in
the development of highly active heterogeneous single-site
metal catalysts.
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This work was supported by the NSF of China (21322606
and 21436005), the Doctoral Fund of Ministry of Education of
China (20120172110012), the FRFCU (2015PY0002), and the
Guangdong NSF (S2011020002397, 2013B090500027, and
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