Metal-organic conjugated microporous polymers

Article abstract of DOI:10.1002/anie.201005864

Two versatile strategies for preparing metal-organic conjugated microporous polymers (MO-CMPs) containing metals such as rhenium, rhodium, and iridium are described (see example). These materials combine the uninterrupted extended electronic conjugation i

Full text of DOI:10.1002/anie.201005864

Communications
DOI: 10.1002/anie.201005864
Porous Materials
Metal–Organic Conjugated Microporous Polymers**
Jia-Xing Jiang, Chao Wang, Andrea Laybourn, Tom Hasell, Rob Clowes, Yaroslav Z. Khimyak,
Jianliang Xiao, Simon J. Higgins, Dave J. Adams, and Andrew I. Cooper*
Microporous materials have potential applications in areas
including molecular separation, gas sorption and catalysis.[1]
Materials such as metal–organic frameworks (MOFs),[2]
covalent organic frameworks (COFs),[3] microporous organic
polymers,[4] and porous organic molecular solids[5] all contain
organic functionalities which, in principle, can allow signifi-
cant synthetic diversification. A particular advantage of
microporous organic polymers[4] is the potential to introduce
a range of useful chemical functionalities into the pores.[6]
This stems from the chemical and thermal stability of these
networks which facilitates a variety of chemical transforma-
tions.
the introduction of vacant metal sites and a range of
chemically active functionalities. The introduction of pendant
metal sites rather than metal nodes also allows the prepara-
tion of materials with unbroken extended conjugation.
The MO-CMP networks were prepared either by post-
treating a bipyridine-functionalized CMP precursor with a
metal complex or by the direct Sonogashira–Hagihara cross-
coupling of 1,3,5-triethylbenzene or 1,4-dibromobenzene and
a halogenated metal–organic co-monomer. These two strat-
egies can be defined as post-synthetic metalation and direct
metal incorporation by copolymerization. The representative
structures of the target MO-CMP networks are shown in
Scheme 1. These polymers combine conjugation along the
main chain with functional units such as bipyridine or
phenylpyridine in the backbone in order to provide sites for
incorporating various metal complexes. We note that neither
bipyridine nor phenylpyridine units have been incorporated
previously within CMP networks.
Conjugated microporous polymers (CMPs)[4a] can exhibit
extended p-conjugation and have been the subject of much
recent interest. A variety of CMPs (and closely related
structures) have been developed.[4a,7] The incorporation of
metal sites into CMPs could open up second-generation
porous materials with useful combined chemical and physical
properties such as catalytic activity, electrical conductivity, or
light-absorption/emission.[8] For example, metalated CMP
materials might be of interest in heterogeneous catalysis or
photocatalysis, where high surface areas would be beneficial.
There are, however, few demonstrations of the functionaliza-
tion of CMP networks with metals at the molecular level. The
incorporation of metal nanoparticles into microporous net-
works has been demonstrated.[4e,7e,9a] Also, a lithiated CMP
showing very high H2 sorption was described recently but the
precise nature of the metal incorporation at the molecular
level was not clear.[9b] Another recent report details a
porphyrin-derived microporous organic polymer which
shows high catalytic activity for the oxidation of thiols.[10]
Beyond this, there are no reports on the purposeful synthesis
of metal-functionalized CMPs. Here, we report two versatile
strategies for preparing metal–organic CMPs (MO-CMPs).
Unlike MOFs,[2] the resulting metal-containing conjugated
polymers are amorphous. Another significant difference is
that the metal sites need not be nodes in the network but can
also be attached pendant to the polymer chains, thus allowing
Scheme 1. Representative structures of the target metal–organic con-
jugated microporous polymers (MO-CMPs); Cp* is pentamethylcyclo-
pentadiene.
To prepare MO-CMP A, we first synthesized the bipyr-
idine-functionalized precursor network (CMP-Bpy) by Sono-
gashira–Hagihara cross-coupling of 1,3,5-triethynylbenzene
(1) with 5,5-dibromo-2,2’-bipyridine (2) and 1,4-dibromoben-
zene (3) to produce a series of porous networks with varying
incorporations of the bipyridine monomer, 2. Monomer 3 can
therefore be considered as a co-monomer which contributes
to porosity but does not incorporate metal binding sites. After
purification, the resulting polymeric precursors were treated
with [Re(CO)5Cl] in toluene under reflux to produce net-
works containing the fac-[ReCl(CO)3(bipy)] (CMP-BpyRe)
moiety. As a control reaction, the bipyridine-free network,
CMP-2, was prepared as described previously[4a,7a] and was
[*] Dr. J. X. Jiang, C. Wang, A. Laybourn, Dr. T. Hasell, R. Clowes,
Dr. Y. Z. Khimyak, Prof. J. L. Xiao, Dr. S. J. Higgins, Dr. D. J. Adams,
Prof. A. I. Cooper
Department of Chemistry and Centre for Materials Discovery
University of Liverpool, Crown Street, Liverpool (UK)
Fax: (+44)151-794-2304
E-mail: aicooper@liv.ac.uk
[**] We gratefully acknowledge the EPSRC for funding (EP/F057865/1
and EP/H000925).
Supporting information for this article (MO-CMP synthesis, gas
sorption and SEM data) is available on the WWW under http://dx.
doi.org/10.1002/anie.201005864.
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Table 1: Physical properties for Re-containing MO-CMPs compared to
those of the parent nonmetalated networks.
Polymer
SABET
SAmicro
[m2 gÀ1 [b]
Vtotal
Recalcd
ReICP
[m2 gÀ1 [a]
]
]
[cm3 gÀ1 [c]
]
[wt.%][d] [wt.%][e]
CMP-Bpy10
CMP-BpyRe10
CMP-Bpy20
CMP-BpyRe20
CMP-Bpy50
CMP-BpyRe50
822
744
859
664
652
328
339
335
374
295
469
239
0.52
0.47
0.69
0.63
0.33
0.29
0
0
13.03
0
20.54
0
12.19
0
15.06
0
33.36
15.34
[a] BET surface area (SA) calculated over the pressure range (P/P0)
0.025–0.1. [b] Micropore surface area calculated from the N2 adsorption
isotherm using the t-plot method based on the Harkins Jura Equation.
[c] Total pore volume at P/P0 =0.99. [d] Data was calculated based on
100% reaction of bipyridine unit. [e] Data was obtained by inductively
coupled plasma-optical emission spectroscopy (ICP-OES).
increasing bipyridine content in the polymers (Table 1) and
the metal incorporation appeared to saturate in CMP-
BpyRe20 at around 15 wt.%. No rhenium was detected in
the control network, CMP-Re-2. The IR data show that the
rhenium complex is incorporated into the main chain of the
polymers as fac-[ReCl(CO)3(bipy)].
The porous properties of the networks were investigated
by nitrogen adsorption analyses at 77.3 K. All of the rhenium-
containing MO-CMPs gave rise to type I nitrogen gas
sorption isotherm with H3 hysteresis loops which were similar
to the isotherms exhibited by the parent CMP-Bpy polymers
(see Figure S2 in the Supporting Information), indicating that
these metalated polymers consist of both micropores and
some mesopores. No significant change in pore size distribu-
tion was observed between the bipyridine-containing CMP
precursors and the Re-containing MO-CMPs (see Figure S3
in the Supporting Information). The apparent Brunauer–
Scheme 2. Synthetic routes for the bipyridine-containing polymer pre-
cursors and the Re-containing polymer networks.
also post-treated with [Re(CO)5Cl] to afford CMP-Re-2
(Scheme 2).
Compared with the FT-IR spectra of the CMP-Bpy
polymer precursors, the rhenium-containing polymers show
three additional strong vibration peaks located at 2020, 1920
and 1885 cmÀ1 (Figure 1), characteristic features of carbonyl
Emmett–Teller (BET) surface areas, SABET, of CMP-
BpyRe10 and CMP-BpyRe20 were somewhat lower than
that of the non-metalated parent networks (Table 1) but the
materials were still significantly microporous. The decrease in
specific surface area with increased metal loading can be
ascribed both to partial pore filling and to simple increase in
mass.
We employed an alternative direct synthesis strategy to
prepare MO-CMPs B and C (Scheme 1) from preformed
metal–organic monomers. Iridium- and rhodium-containing
polymers were prepared by Sonogashira–Hagihara coupling
of 1,3,5-triethynylbenzene or 1,4-diethynylbenzene with the
metalated monomers shown in Scheme 3. We synthesized two
different types of metalated monomer. The first monomer, a
cyclometalated Ir or Rh complex, was tetrafunctional with
respect to the Sonagashira–Hagihara network formation
reaction (Scheme 3a). A second, bifunctional monomer
(Scheme 3b) comprised an Ir–Cp* complex. The resulting
MO-CMPs prepared from the tetrafunctional cyclometalated
complexes exhibited specific surface areas, SABET, of 507, 721,
423 and 556 m2 gÀ1 for networks IrCMP-1, IrCMP-2, RhCMP-
1 and RhCMP-2, respectively. For the MO-CMPs formed
from the bifunctional monomer (Scheme 3b), specific surface
areas of 864, 698 and 469 m2 gÀ1 were obtained for CMP-CpIr-
1, CMP-CpIr-2 and CMP-CpIr-3, respectively.
Figure 1. The FT-IR spectra of CMP-Bpy networks and the correspond-
ing rhenium-containing MO-CMPs. The spectra are shown offset for
clarity.
groups in the fac-[ReCl(CO)3(bipy)] moiety.[11] As expected,
these carbonyl vibrations did not appear in the FT-IR
spectrum of CMP-Re-2, which does not contain the bipyridine
unit (see Figure S1 in the Supporting Information). Rhenium
was detected in all of the MO-CMP networks by inductively
coupled plasma-optical emission spectroscopy (ICP-OES)
analysis and the content of rhenium increased with the
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CMP-CpIr-3, which did not include 1,4-dibromobenzene (3),
showed the highest activity. This network performed well with
respect to homogeneous catalysis using the soluble compound
6 (see Table S1 in the Supporting Information). A range of
substrates were subsequently surveyed using CMP-CpIr-3 as
catalyst for reductive amination (Table 2). It was found that
Table 2: Reductive amination catalyzed by CMP-CpIr-3.[a]
Entry
1
Ketone
Amine
Yield [%][b]
95
2
3
91
90
Scheme 3. a) Direct synthetic routes for Ir- and Rh-containing MO-
CMPs produced from cyclometalated Ir and Rh complexes, respec-
tively; see also Scheme 1. b) MO-CMPs produced from Cp*-containing
Ir monomer. i) [Pd(PPh3)4], CuI, DMF and Et3N, 908C, 72 h.
4
5
93
86
The MO-CMPs prepared from the tetrafunctional cyclo-
metalated monomers have structures which differ from
previous CMPs.[4a,6,7,10,12] In particular, the metal atoms (i.e.,
Ir, Rh) act as linkers or spiro-centers to produce the cross-
linked microporous networks (Scheme 3a). This may be
compared with bonding patterns in MOFs[13] where the metal
atoms are integral nodes in the network structure. By
contrast, the metal node in these MO-CMPs does not break
the extended conjugation pathway in the network as it does in
MOFs: to give just one example, the recently reported
arylethynylene-connected material MOF-210[14] exhibits such
“broken” conjugation. Another key distinction is that most
MOFs are crystalline[15] whereas these MO-CMP networks
are amorphous.
Polymers based on cyclometalated Ir complexes have
been studied widely as components in light-emitting diodes,[16]
but no porous examples have been reported until now. The
full N2 isotherms for these polymers give rise to type I
nitrogen gas sorption isotherms, indicating that these MO-
CMPs are also microporous (see Figures S4–S7 in the
Supporting Information).
These two strategies—post-synthetic metalation and
direct metal incorporation by copolymerization—offer the
possibility to design conjugated microporous materials with a
range of functions. As a proof of concept, we have evaluated
one of the networks as a heterogeneous catalyst. Recently, a
few examples of CMPs showing catalytic activity have been
reported.[10,17] Polymer C contains a cyclometalated IrIII
complex which is closely related to a class of successful
reductive amination catalysts.[18] We therefore examined the
heterogeneous catalytic activity of the CMP-CpIr networks.
All of the networks were found to be catalytically active but
[a] Reaction conditions: ketone (0.5 mmol), amine (0.6 mmol),
HCOOH/Et3N (2.5:1 molar)=0.5 mL, MeOH (3 mL), 1 mol% Ir
based on ketone (the Ir content was determined from ICP analysis),
808C, 12 h. [b] Yield of isolated product.
CMP-CpIr-3 indeed displayed high catalytic activity across a
range of substrates with yields of isolated product higher than
90%, which are comparable with the results obtained for a
related homogeneous Ir catalyst.[18]
In summary, we have demonstrated two simple and
versatile strategies for preparing metal–organic conjugated
microporous polymers (MO-CMPs). Microporous networks
containing rhenium, rhodium, and iridium have been pre-
pared but the methodology should be readily extended to
other metals. The metal loading is tunable and substantial
metal incorporation can be achieved in microporous solids
which retain high surface areas. We also give the first example
of a microporous polymer network comprising a cyclometa-
lated complex as a framework node. The resulting metal-
containing networks are amorphous and thermally robust. A
proof-of-concept study shows that this class of polymers is
promising for heterogeneous catalysis, although it is likely in
this case that simpler porous supports (e.g., silica, macro-
porous resins) would be much more cost effective. Uniquely,
however, these MO-CMPs combine highly conjugated frame-
work backbones with the incorporation of metal–organic
moieties. We therefore foresee a modular synthetic platform
for the preparation of porous materials with unique phys-
icochemical properties. For example, other researchers have
already demonstrated the potential for light harvesting and
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Angew. Chem. Int. Ed. 2011, 50, 1072 –1075

energy transfer within nonmetalated CMP networks.[8] We
believe that MO-CMPs materials could be a platform for
applications such as light harvesting, photocatalysis, and
photocatalytic hydrogen evolution.[19]
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Experimental Section
Synthesis; typical procedure for IrCMP-1: 1,4-diethynylbenzene
(190 mg, 1.5 mmol), monomer 5a (460 mg, 0.5 mmol), tetrakis-
(triphenylphosphine)palladium (20 mg), and copper iodide (10 mg)
were dissolved in the mixture of DMF (4 mL) and Et3N (4 mL). The
reaction mixture was heated to 908C, stirred for 72 h under a nitrogen
atmosphere, and then cooled to room temperature. The precipitated
network polymer was filtered and washed four times with chloroform,
water, methanol, and acetone to remove any unreacted monomer or
catalyst residues. Further purification of the polymer was carried out
by Soxhlet extraction with dichloromethane for 24 h. The product was
dried in vacuum for 24 h at 708C as red powder (yield: 422 mg, 65%).
[8] Ref. [7g].
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Received: September 18, 2010
Published online: December 22, 2010
Keywords: conjugated microporous polymers ·
.
coordination polymers · polymerization · porous materials
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