Modular “Click” Preparation of Bifunctional Polymeric Heterometallic Catalysts

Article abstract of DOI:10.1002/anie.201600999

Heterobimetallic molecular complexes or strictly alternating metallated polymers are obtained by a click reaction between mononuclear metal complexes (secondary building units, SBUs) bearing NHCs functionalized with either p-azidophenyl or p-ethynylphenyl

Full text of DOI:10.1002/anie.201600999

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201600999
German Edition: DOI: 10.1002/ange.201600999
Bifunctional Catalysts
Modular “Click” Preparation of Bifunctional Polymeric
Heterometallic Catalysts
Wenlong Wang, Liyuan Zhao, Hui Lv, Guodong Zhang, Chungu Xia, F. Ekkehardt Hahn,* and
Fuwei Li*
Abstract: Heterobimetallic molecular complexes or strictly
alternating metallated polymers are obtained by a click
reaction between mononuclear metal complexes (secondary
building units, SBUs) bearing NHCs functionalized with either
p-azidophenyl or p-ethynylphenyl wingtips. With a copper-
NHC complex as SBU the formation of molecular or
polymeric compounds did not require any additives as the
copper complex catalyzes the click reaction. Transmetallation
from heterobimetallic Cu/Ag derivatives to Cu/Pd derivatives
was achieved. The linker between the SBUs (flexible or rigid)
influences the catalytic activity of the heterobimetallic com-
pounds. The polymer with alternating copper-NHC and silver-
NHC units and a flexible methylene-triazole bridge between
them shows the highest activity in the catalytic alkynylation of
trifluoromethyl ketones to give fluorinated propargylic alco-
hols.
N-heterocyclic carbene (NHC) complexes have attracted
considerable attention over the last two decades owning to
their high stability and outstanding applications in homoge-
neous catalysis.[7] We developed an interest in the immobili-
zation and modification of NHC-metal catalysts with the dual
purpose of creating a reactive environment for effective
catalysis and of facilitating the recovery of the catalyst.[8]
As an extension of this chemistry we envisioned the
preparation of polymers featuring two different NHC-M
tethers arranged in an alternating and constantly spaced
fashion at the polymer backbone. Such derivatives are
potentially very useful as they might function as bifunctional
heterometallic catalysts while at the same time offering the
possibility of facile catalyst recovery. The strictly alternating
metallation of a polymer with two different metal complexes
is rather difficult and has not been demonstrated yet. For the
preparation of such polymers we have developed a new
synthetic methodology based on a “bottom-up” strategy.[9]
This strategy involves the synthesis of azide and ethynyl
functionalized NHC-metal complex monomers A and B as
secondary building units (SBUs) which are subsequently
joined together to form the polymer by classical click
chemistry.[10] This approach guarantees the desired strictly
equidistant and alternating sequence of the metal complexes
tethered to the polymer (Scheme 1). While the click poly-
merization of various organic alkynes and azides has been
demonstrated,[10] this synthetic approach has not yet been
used for the preparation of metal-complex-tethered polymers.
Four monometallic NHC-metal building blocks 1–4
(SBUs, Scheme 2) were used for the preparation of the
I
n nature, rather complicated organic compounds are often
constructed elegantly via catalysis in environments containing
multiple catalytically active sites.[1] Cooperative catalysis,
wherein two (or more) catalytic sites work in concert to
activate selected substrates and to create new chemical bonds
efficiently, has recently emerged as a powerful mechanistic
approach for catalytic reaction engineering.[2] The design and
preparation of multifunctional catalysts featuring synergisti-
cally[3] or cooperatively[4] acting catalytically active sites has
become a challenge in the field of catalysis and synthesis.
To mimic the cooperative catalytic efficiency found for
selected natural transformations, recent interest has turned
towards homo and heterometallic coordination polymers and
metal catalysts immobilized on polymers.[5] However, the
incorporation of multiple metal sites into a single polymer
support is quite rare.[5d,6] Even less common are polymers
functionalized with metals complexes at preselected positions
in an exactly predetermined sequence. This is largely due to
the generally employed “post-synthesis” strategy where the
metal centers are introduced after polymer formation.
[*] Dr. W. Wang, L. Zhao, H. Lv, G. Zhang, Prof. C. Xia, Prof. F. Li
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Chinese Academy of Sciences
Lanzhou 730000 (P.R. China)
E-mail: fuweili@licp.cas.cn
Prof. F. E. Hahn
Institut für Anorganische und Analytische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstrasse 30, 48149 Münster (Germany)
E-mail: fehahn@uni-muenster.de
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600999.
Scheme 1. Click preparation of polymeric catalysts alternatingly func-
tionalized with two different NHC-metal complexes.
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Figure 1. Molecular structure of complexes 1 (left) and 2 (right, ther-
mal ellipsoids set at 30% probability, hydrogen atoms have been
omitted for clarity).
reduction of the PdII by the terminal alkyne, the Cu/Pd
tethered polymers poly-1/3(Pd) and poly-2/3(Pd) were
obtained in good yield of 87% by transmetallation of the
Cu/Ag polymers (poly-1/3 or poly-2/3, no alkyne present
anymore) with allylpalladium(II) chloride dimer[13]
(Scheme 3, see also the Supporting Information).
Owing to their linear structure, the heterometallic poly-
mers are highly soluble in DMF. They have been character-
ized by NMR spectroscopy showing the typical resonance
Scheme 2. Complexes 1 and 2 bearing phenylazide wingtips and NHC-
silver(I) (3) and NHC-gold(I) (4) complexes with phenylethynyl wing-
tips.
polymeric catalysts. Two of these are NHC-copper(I) com-
plexes functionalized with p-azidophenyl groups as NHC
wingtips (1 and 2) while the other two are NHC-silver(I) (3)
and NHC-gold(I) (4) complexes featuring p-ethynylphenyl
NHC wingtips. Complexes 1 and 2 were obtained by the
reaction of CuCl with the suitable imidazolidinium salts in the
presence of 1.0 equivalent of NaOCH3 in methanol (see the
Supporting Information, Scheme S1).[11a,d,e] In these com-
plexes, the azide groups are attached either directly (2) or via
a flexible methylene linker (1) to the aromatic ring. NHC-
gold(I) complex 4 was obtained by transmetalation of the
NHC ligand from silver(I) complex 3[11b] to [AuCl(THT)]
(THT= tetrahydrothiophene, see the Supporting Informa-
tion, Scheme S2).[11c] Note that NHC-PdII complexes as SBUs
cannot be obtained by transmetallation from 3 to the
allylpalladium(II) chloride dimer as this method leads to the
formation of palladium black as a result of
À
signals for the polymeric backbone (d ꢀ 9.1–9.7 ppm, C H
triazole) and, in selected cases, the CNHC resonances (d-
(CNHC) = 201.8 for poly-1/3, only one resonance detected;
d(CNHC) = 197.9 and 194.7 for poly-2/4). The problems in
detecting selected CNHC resonances are possibly due to the
polymeric nature of the compounds and in accord with
previous reports on coinage metal NHC complexes.[11] In
addition, elemental analyses and AAS spectrometry (for
metal content) confirmed the formation of the polymers (see
the Supporting Information, Table S2). Gel permeation
chromatography (GPC) was employed to determine the
molecular weight of the polymers (see the Supporting
PdII reduction by the terminal alkyne groups.
Formation of complexes 1–4 was con-
firmed by NMR spectroscopy showing the
resonances for the CNHC carbon atoms in the
À
expected ranges of d = 203.5 ppm (Cu CNHC
for 2),[11a] d = 206.7 ppm (Ag CNHC
)
[11b], and
À
[11c]
À
d = 196.3 ppm (Au CNHC
)
and by X-ray
diffraction studies with 1 and 2 (Figure 1, see
also the Supporting Information).[12] Compa-
rable metric parameters found in 1 and 2 fall
in the range previously observed for related
NHC-copper(I) complexes.[11a]
With the azide-tagged NHC-CuI (1, 2)
and ethynyl-tagged NHC-AgI (3) and NHC-
AuI (4) SBUs in hand, the preparation of
polymers tethered with alternating metal
sites was attempted. Reaction of equimolar
amounts of the SBUs 1 with 3, 2 with 3, or 2
with 4 yielded the polymers poly-1/3, poly-2/
3, and poly-2/4 by click chemistry in yields of
90–93% (Scheme 3). The polymerizations
proceeded at ambient temperature and no
additional polymerization catalyst was
needed owing to the catalytic activity of the
copper(I) containing SBUs 1 and 2.[10] While
a PdII containing SBU similar to 3 could not
Scheme 3. Preparation of metallated polymers with alternating metal centers by click
be prepared by transmetallation owing to polymerization from SBUs (a–c) and transmetallation (d,e).
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Information, Figure S1 and Table S1). The click polymeri-
zation resulted in heterometallic polymers with high molec-
ular weights (MW) in the range of 20–30 kDa and a relatively
narrow MW distribution. The high MW and relatively low
polydispersity index (PDI) are particularly gratifying since
the polymers are presumably formed in a step-growth
polymerization process. Note that polymers poly-1/3 and 1/
3(Pd) feature a flexible methylene linker in between the NHC
ligands while the other polymers are linked by a more rigid
linker.
Figure 2. Molecular structure of complexes 5 (left) and 6 (right, ther-
mal ellipsoids set at 30% probability, hydrogen atoms have been
omitted for clarity).
While metal-NHC containing polymers have been de-
scribed, they are either formed by coordination of rigid
benzobiscarbenes to selected metals or by the metalation of
polymeric polyazolium salts.[14] These methods offer no
control regarding the type of metals or their sequence in the
polymer and are in these regards inferior to our click
polymerization of metal complex SBUs.
In an attempt to illustrate the efficiency and versatility of
the SBU coupling procedure, NHC complexes 5 and 6 bearing
only one wingtip substituent each (azide or ethynyl) for click
coupling were prepared from the monofunctionalized imida-
zolidinum salts (Scheme 4). They were fully characterized by
NMR spectroscopy and X-ray diffraction studies (Figure 2,
see also the Supporting Information).[12] The monofunction-
alized building blocks 5 and 6 were used for the preparation of
the dinuclear heterobimetallic complex 7 via a click reaction
(Scheme 4a). Transmetallation of 7 with [PdCl(allyl)]2 gave
the heterobimetallic Cu/Pd complex 8.
the trinuclear Ag/Cu/Ag-NHC complex 9 which reacts with
[PdCl(allyl)]2 in a transmetallation reaction to give the
trinuclear Pd/Cu/Pd complex 10 (Scheme 4b). The molecular
heterobimetallic complexes 7–10 have been characterized by
NMR spectroscopy und ESI HRMS spectrometry (see the
Supporting Information). The molecular heterobimetallic
complexes are fragments of the polymers poly-1/3 and poly-
1/3(Pd) featuring a flexible linker (Scheme 3). Their synthesis
demonstrates once more the versatility of the SBU/click
procedure. In addition, the molecular complexes constitute
a useful addition to the toolbox for subsequent catalytic
investigations.
The catalytic alkynylation of trifluoromethyl ketones to
give fluorinated propargylic alcohols is an important reaction
in organic synthesis, pharmaceutical science, and organo-
fluorine chemistry.[15] Copper complexes have been shown to
catalyze this reaction via the formation of Cu–alkyne
intermediates.[15a,16] We assumed that Lewis acids, such as
silver-NHC complexes, could support the catalytic alkynyla-
tion via an activation of the carbonyl group of the
trifluoromethyl ketone. For a cooperative Cu/Ag
catalysis the copper and silver metal centers would
have to be located in close proximity to allow the
concurrent activation and reaction/activation of the
alkyne and the trifluoromethyl ketone. Such a sit-
uation exists in the dinuclear, flexibly linked
molecular complex 7 and the polymers poly-1/3
and poly-2/3 described above. We therefore studied
the catalytic activity of these derivatives in the
catalytic alkynylation of 2,2,2-trifluoro-1-phenyl-
ethanone with phenylacetylene.
Reaction of the doubly azide-functionalized copper com-
plex 1 with the mono-ethylnyl functionalized complex 5 gave
The dinuclear complex 7 represents a fragment
of the polymer poly-1/3 (Scheme 3) featuring
a flexible methylene-triazole linker between the
NHC donors. It is freely soluble in THF and DMF
and was first used as a catalyst in the homogeneous
catalytic alkynylation of 2,2,2-trifluoro-1-phenyl-
ethanone (11) with phenylacetylene (12a). As
shown in Table 1, the use of 1.0 mol% or
0.5 mol% of 7 in THF resulted in almost quanti-
tative yields of the fluorinated propargylic alcohol
13a within 16 h (TON of 97 and 192, respectively).
Reduction of the amount of catalyst to 0.3 mol%
led to 62% yield in THF and a slightly lower yield
in DMF (Table 1, entries 1–4).
Since compounds poly-1/3 and poly-2/3 are
highly soluble in DMF they were used as catalysts
in this solvent for the alkynylation of 2,2,2-
Scheme 4. Synthetic routes to heterodinulear and heterotrinclear complexes.
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Table 1: Catalytic alkynylation of 2,2,2-trifluoro-1-phenylethanone 11 with
phenylacetylene 12a using different catalysts.[a]
In addition, we studied the catalytic activity of the
mononuclear complexes SIPr-CuI, SIPr-AgI and a 1:1 mixture
of these complexes (Table 1, entries 8–13). In all cases and
both in THF or DMF a much lower catalytic activity of these
complexes or mixtures of them was observed compared to 7
or poly-1/3.
A plot of the GC yield versus reaction time for the six
catalysts tested in shown in Figure 3. This plot nicely
Entry Catalyst
Loading (mol%) Yield [%][c] TON
1
2
3
4
5
6
7
8
7
7
1
0.5
0.3
0.3
0.5
0.3
0.5
0.5
0.5
0.5
0.5
0.5+0.5
97
96
62
55
95
61
67
18
10
21
17
65
39
97
192
207
183
190
203
134
36
20
42
34
65
7
7[b]
poly-1/3[b]
poly-1/3[b]
poly-2/3[b]
SIPr-CuI
SIPr-CuI[b]
SIPr-AgI
SIPr-AgI[b]
SIPr-CuI + SIPr-AgI
Figure 3. Reaction profile for the catalytic alkynylation of 2,2,2-trifluoro-
1-phenyletanone with phenylacetylene using different catalyst.
9
10
11
12
13
illustrates the significant differences in the catalytic activities.
The lowest yields were observed for the individual mono-
nuclear catalysts SIPr-CuI and SIPr-AgI. If complexes SIPr-
CuI and SIPr-AgI were used as a 1:1 mixture, the resulting
yield corresponds well to the one obtained with the rigidly
bridged metalpolymer poly-2/3 as a catalyst. The highest
catalytic activity was observed for the flexibly bridged
complex 7 and the metalpolymer poly-1/3. These data indicate
to us that indeed cooperative effects exist for the flexibly
linked catalysts which enable the NHC-Cu and NHC-Ag
metal centers to approach each other. To our knowledge, the
Cu/Ag bimetallic molecular complex 7 and the polymeric
compound poly-1/3 are the most active catalysts reported to
date for the alkynylation of trifluoromethyl ketones.
Apart from the model reaction (2,2,2-trifluoro-1-phenyl-
ethanone plus phenylacetylene) we have studied the substrate
scope of the catalytic reaction. Terminal alkynes with
electron-donating or electron-withdrawing groups as well as
alkyl alkynes afforded good yields of fluorinated propargyl
alcohols in the presence of the dinuclear catalyst 7 or the
metal polymer poly-1/3. For all substrates a mixture of SIPr-
CuI and SiPr-AgI complexes was also examined but the yields
were always significantly lower than those obtained for the
heterobimetallic catalysts 7 or poly-1/3. The results are
summarized in the Supporting Information, Figure S3.
These results indicate once more that some catalytic cooper-
ativity between the NHC-Cu and NHC-Ag units in 7 and
poly-1/3 exists.
SIPr-CuI + SIPr-AgI[b] 0.5+0.5
39
[a] Reaction conditions: 2,2,2-trifluoro-1-phenylethanone (1.0 mmol),
phenylacetylene (2.0 mmol), NaOtBu (2 mol%), 1 mL THF, T=508C,
t=16 h. [b] 1 mL DMF instead of 1 mL THF. [c] Determined by GC
analysis with tridecane as an internal standard.
trifluoro-1-phenylethanone. Metal polymer poly-1/3 proved
to be a very active catalyst. When used in a concentration of
0.5 mol% (this number was calculated from the weight% of
copper in the polymer, which is 5.41%) compound 13a was
obtained in 95% yield (TON = 190). Reduction of the
catalyst loading to 0.3 mol% yielded 13a in 61% yield with
a TON of 203 (Table 1, entries 5–6). The catalytic reaction in
THF produced only trace amounts of 13a owing to the poor
solubility of poly-1/3 in this solvent thus verifying the
homogenous nature of the catalytic reaction.
Both of the most active catalysts, 7 and poly-1/3, feature
a flexible linker between metal-NHC groups potentially
enabling a close approach of two neighboring Cu and Ag
centers and thus some cooperative interaction. Contrary to
this situation, metal-polymer poly-2/3 contains a rigid triazole
linker. The catalytic performance of this polymer was lower in
the test reaction compared to poly-1/3 under otherwise
identical conditions (entry 7). Apparently, the flexible linker
is beneficial for an optimal catalytic performance. This would
indicate the presence of some synergism between the metal
centers in the catalytic action of poly-1/3 which due to
geometric constraints is absent in poly-2/3.
Based on our observations and in accord with literature
reports[15,16] we propose the reaction mechanism depicted in
Scheme 5 for the cooperative catalytic alkynylation of
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them show the highest catalytic activity. Control experiments
using a very similar polymer with a rigid linker between the
NHC-copper and NHC-silver units (poly-2/3) or with 1:1
mixtures of mononuclear NHC-CuI and NHC-AgI complexes
produced lower yields of propargylic alcohols. Linkage of the
two NHC-metal complex fragments with a flexible linker in
such a way, that the metal centers can approach each other
appears greatly beneficial for the outcome of the catalytic
reaction. Similar observations were made with flexibly
methylene-triazole bridged molecular NHC-CuI/NHC-AgI
complex 7 and mixtures of mononuclear NHC-CuI NHC-
AgI complexes where the flexible linkage of the two NHC-
metal centers again proved beneficial for the catalytic out-
come. A reaction mechanism for the alkynylation of trifluoro-
methyl ketones based on a cooperative substrate activation is
proposed based on these observations. Efforts towards the
synthesis of new heterobimetallic tagged molecular com-
pounds and polymers by click linkage of suitable SBUs are
currently in progress.
Acknowledgements
This work was supported by the Chinese Academy of
Sciences, the National Natural Science Foundation of China
(21373246, 21403258, and 21522309), and the Deutsche
Forschungsgemeinschaft (SFB 858 and IRTG 2027).
Scheme 5. Proposed mechanism for the cooperative catalytic alkynyla-
tion of trifluoromethyl ketones using the bimetallic Cu/Ag-complex 7.
Keywords: bifunctional catalysts · catalyst design ·
trifluoromethyl ketones. Previously, it has been shown that an
NHC-CuI center in the presence of a base activates terminal
alkynes with formation of a copper–acetylide intermediate C.
The neighboring NHC-AgI center can then act as a Lewis acid
and interacts with the carbonyl group of the trifluoromethyl
ketone to give D. This double activation of two substrate
molecules in close proximity will facilitate the subsequent
nucleophilic addition of the acetylide to the carbonyl group.
click chemistry · cooperative catalysis · N-heterocyclic carbene
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Angew. Chem. 2016, 128, 7795–7800
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Received: January 28, 2016
Published online: March 11, 2016
7670 www.angewandte.org
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