Post-functionalization of an Anderson-type polyoxomolybdate using a metal-free Diels-Alder click reaction

Article abstract of DOI:10.1002/ejic.201201399

Functionalization and post-functionalization of polyoxometalates (POMs) by organic groups or biological moieties can provide versatile platforms for creating novel functional hybrids with synergetic properties. Such materials can easily be envisioned as having many advanced applications in biology and materials science. Nowadays, the organic groups or biological moieties used for conjugation with POMs, can readily become increasingly complicated in terms of structure, size and properties. Accordingly, a major challenge lies in the development of efficient conjugation methods. In this work, we report our application of metal-free Diels-Alder click reactions under mild conditions to covalently link an Anderson-type polyoxomolybdate with some organic groups or biological moieties possessing complex structures or different properties. High coupling efficiencies and facile purification methods guarantee > 90 % yields. Clearly, this is an ideal methodology for the synthesis of the post-functionalized POM hybrids. With this technology now in hand, finding valuable applications for such post-functionalized POM hybrids will be a highly significant focus of future investigations. Copyright

Full text of DOI:10.1002/ejic.201201399

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DOI:10.1002/ejic.201201399
Post-Functionalization of an Anderson-Type
Polyoxomolybdate Using a Metal-Free Diels–Alder Click
Reaction
[
a]
[a]
[a]
[a]
Hai-Kuan Yang, Ming-Ming Su, Li-Jun Ren, Jing Tang,
[
a]
[a]
[a]
[a]
Yu-Kun Yan, Wen-Ke Miao, Ping Zheng, and Wei Wang*
Keywords: Metal-free / Click chemistry / Polyoxometalates / Functionalization / Diels–Alder reaction / Conjugation
Functionalization and post-functionalization of polyoxomet-
alates (POMs) by organic groups or biological moieties can
provide versatile platforms for creating novel functional hy-
brids with synergetic properties. Such materials can easily be
envisioned as having many advanced applications in biology
and materials science. Nowadays, the organic groups or bio-
logical moieties used for conjugation with POMs, can readily
become increasingly complicated in terms of structure, size
tions under mild conditions to covalently link an Anderson-
type polyoxomolybdate with some organic groups or bio-
logical moieties possessing complex structures or different
properties. High coupling efficiencies and facile purification
methods guarantee Ͼ 90% yields. Clearly, this is an ideal
methodology for the synthesis of the post-functionalized
POM hybrids. With this technology now in hand, finding
valuable applications for such post-functionalized POM hy-
and properties. Accordingly, a major challenge lies in the de- brids will be a highly significant focus of future investi-
velopment of efficient conjugation methods. In this work, we
report our application of metal-free Diels–Alder click reac-
gations.
Introduction
(OD) POM hybrids to display a greater number of possible
activities by manipulating and then optimizing their struc-
Polyoxometalate clusters (POMs) are a class of structur- tures and functions.[5–7] Recently, post-functionalization ef-
ally well-defined metal–oxo anions exhibiting versatile forts of OD-POM hybrids containing reactive organic
physical, chemical, and biological properties. Consequently groups, have been highly focused on imparting hybrids with
POMs offer a wide range of capabilities and thus have many more value-added functionalities for advanced applications.
[
7]
potential applications in medicine, catalysis, materials sci- In a recent review, Proust and co-workers summarized
ence, and nanoassembly.[
1–3]
Functionalization of POM synthetic methodologies that have been successfully devel-
[8]
clusters using organic groups has attracted a great deal of oped for post-functionalization as follows: amidation, im-
[
9]
[10]
attention during the past decades, in large part, because idization, Sonogashira cross coupling, Heck cross cou-
[4–7]
[11]
[12]
novel hybrids could now be created.
More importantly, pling,
esterification,
Huisgen 1,3-dipolar cycload-
and the Diels–Alder reaction.
[
13]
[14]
these hybrids may constitute a unique class of hybrid mate- dition reactions,
De-
rials with attractive activities or functionalities. Moreover, veloping different methods aims to overcome system-spe-
these hybrids may be much more than a simple combination cific obstacles, thus enabling overall realization of these
of inorganic and organic groups and their corresponding covalent post-functionalizations. Major obstacles are likely
features; there is a distinct possibility for POMs to benefit to result from specific characteristics of POM clusters.
from unexpected synergetic effects attributable to their These might include: (i) generally poor solubility in most
unique and varied combinations of various functional common solvents limiting the use of most organic reactions,
groups. Among several functionalization methods, the cova- (ii) strong electrophilicity greatly lowering reactivity of or-
lent functionalization of POM clusters by organic groups is ganic groups attached, and (iii) interactions with some met-
a smart approach enabling unique organically derivatized allic additives or catalysts that slow down or even terminate
reactions. These obstacles could become serious when or-
ganic groups or biological moieties become large in size,
[
a] Center for Synthetic Soft Materials, Key Laboratory of
Functional Polymer Materials of Ministry of Education and complicated in structure, or inconsistent in property. Fi-
Institute of Polymer Chemistry, Nankai University,
nally, some technical problems might be met during the fi-
Tianjin 300071, China
Fax: +86-22-23498126
E-mail: weiwang@nankai.edu.cn
Homepage: http://wangwei.polymer.cn/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201399.
nal stages of purification, tough issues such as the removal
of by-products and/or activators or catalysts would ideally
be circumvented altogether by one’s choice of POM modifi-
cation chemistry prior to purification. With these consider-
Eur. J. Inorg. Chem. 2013, 1381–1389
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ations in mind, it is clear that developing a new and effec- and their derivatives). In order to assess the DA reaction
tive method for post-functionalization of POM clusters is as an efficient post-functionalization method for the POM
indeed welcomed.
cluster between maleimide’s derivatives and derivatives of
The Diels–Alder (DA) cycloaddition reaction[15] between furan or anthracene, we initially synthesized three kinds of
reactive diene and dienophile is a metal-free click reac- OD-POM hybrids as precursors. These OD-POMs contain
[
16]
tion. Due to this important property, the DA click reac- the reactive dienophile maleimide, diene furan, and diene
tion should be especially suitable for post-functionlization anthracene groups in their organic parts, respectively.
of polyoxometalates with various targets. We note that this Scheme 1 shows the synthetic route and chemical structures
method has been used for post-functionalization of Keggin- of the OD-POM hybrids with dienophile or diene groups:
and Wells–Dawson-type OD-POMs by several protein-reac- maleimide–POM hybrid (2a), furan–POM hybrid (2b), and
tive groups for preparation of small, individually distin- anthracene–POM hybrid (2c).
guishable labels for conventional transmission electron mi-
[
14]
croscopy. In this work we report post-functionlization of
an Anderson-type polyoxomolybdate using the DA click re-
action. The choice of Anderson-type POM is dictated by
the fact that molybdenum POM clusters demonstrate sig-
nificant medical potential. For instance, molybdenum
POMs have applications as inorganic drugs for cancer
[
17]
therapeutics
and are compatible with organic modifica-
[
18]
tions. Herein we have selected biotin (vitamin H), tris(hy-
droxymethyl)aminomethane (Tris), the polyhedral oligo-
meric silsesquioxane (POSS) cluster, and a second-genera-
tion poly(urethane amide) (G2-PUA) dendron as the bio- Scheme 1. Synthetic route and chemical structures of OD-POM hy-
brids: dienophile maleimide–POM hybrid (2a) and diene furan–
and anthracene–POM hybrids (2b and 2c).
logical moieties or organic groups. The reasons for these
selections are as follows: (i) biotin is a coenzyme necessary
for cell growth, production of fatty acids, and the metabo-
In our experiments, dienophile– and diene–POM hybrids
lism of fats and amino acids, and (ii) Tris is extensively used
were synthesized by two procedures: In the first step, the
in biochemistry and molecular biology as a buffering salt.
corresponding organic derivatives of maleimide (1a), furan
The conjugation of biotin and Tris with POM clusters is
(
1b) and anthracene (1c) were prepared. Compound 1a was
envisioned to extend the application of POM clusters to
numerous biological fields. Additionally, (iii) cage-like
POSS is one of the most important nanoscale building
blocks for constructing functional hybrids,[ and the G2-
PUA dendron has a more complex structure than the li-
gands used in covalent linkages with POM clusters and is
an excellent building block for diverse supramolecular as-
[21]
prepared according to a reported protocol, whereas acyl-
ation of 2-(aminomethyl)furan and 9-anthracenylmethanol
[22]
with succinic anhydride afforded 1b and 1c.
Tris-modi-
19]
+
fied Anderson-type POM cluster {[(nBu) N ] (MnMo O )
4
3
6
18
3–
[
(OCH ) CNH ] } (Tris-POM-Tris) was also prepared ac-
2 3 2 2
[18b]
cording to a reported method.
Notably, in Tris-POM-
Tris two amino groups were attached on the two sides of
the POM cluster and three cationic tetrabutylammonium
TBA) surfactants were used to cover the anionic POM
[
20]
semblies. The selected moieties or groups may be difficult
to covalently conjugate onto the POM cluster using pre-
viously described methods. Low activity, reactive hydroxy
groups, large size or complex structure are all factors lead-
ing to this postulate. Using the metal-free DA system we
now have the chance to study the effects of such factors on
POM coupling chemistry. Moreover, POM post-function-
alization in high yields under mild and catalyst-free condi-
tions means that the DA reaction is an ideal method for
the creation of diverse POM hybrids. Investigations of the
biological and/or chemical functions and supramolecular
assemblies of these new POM hybrids will be important in
determining a potentially wide range of advanced applica-
tions for these modified POMs.
(
cluster improving its solubility in some organic solvents.
[8a]
Then, applying a reported method,
1a, 1b and 1c were
linked onto the two sides of the Tris-POM-Tris to create
dienophile– or diene–POM hybrids through amidation be-
tween the carboxyl groups of the organic derivatives and
the amino groups on the two sides of the Tris-POM-Tris.
The reaction was performed in a refluxing acetonitrile
(
CH CN) solution for 24 h in the presence of 2-ethoxy-1-
3
ethoxycarbonyl-1,2-dihydroquinoline (EEDQ). After care-
fully purifying by precipitation and recrystallization, the fi-
nally obtained dienophile–POM hybrid is 2a (maleimide–
POM hybrid), and diene–POM hybrids are 2b (furan–POM
hybrid) and 2c (anthracene–POM hybrid).
Results and Discussion
Preparation of Dienophile– and Diene–POM Hybrids
Effect of POM Electrophilicity on the DA Reaction
The DA reaction generally involves a cycloaddition of an
POM clusters are normally electrophilic, and the normal
electron-poor dienophile (usually maleimide and its deriva- DA reaction is favoured by electron-withdrawing groups on
tives) and an electron-rich diene (usually furan, anthracene, the electrophilic dienophile and by electron-donating
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groups on the nucleophilic diene. So, it is necessary to know groups. After they are covalently conjugated onto the two
the effect of POM electrophilicity on the DA reaction with sides of the POM cluster, the strong electrophilicity of the
electron-withdrawing dienophiles and electron-donating POM cluster should have an influence on the generation of
dienes. In this work, dienophile 2a was selected to react six-membered rings resulting from the DA reaction.
with dienes 1c and 1b; also, dienes 2b and 2c were selected
for couplings with dienophile 1a. Our aim was to assess
whether an efficient post-functionalization of the OD-POM Effect of Anthracene Derivatives on the DA Reaction
hybrids could be performed by application of the DA reac-
tion.
As shown in Scheme 2, we found that 2a reacted com-
As noted, the DA reaction between maleimide-modified
POM and anthracene groups can lead to efficient POM
post-functionalization. In consideration of this finding,
there are many commercially available anthracene deriva-
tives. Consequently, we further studied the effect of anthra-
cene substitutions upon the DA reaction with the malei-
mide groups in 2a; four anthracene derivatives were evalu-
ated, and from these reactions evolved an understanding of
anthracene reactivity with maleimide-modified POM 2a.
As shown in Table 1, the four anthracene derivatives we
selected are: 9-anthracenylmethanol, anthracene, 9-anthra-
cenecarboxylic acid, and 2-anthracenecarboxylic acid. The
DA products of post-functionalized 2a are denoted as 3a–
d. Note that these anthracene derivatives have a strong UV/
Vis absorption. Since the DA cycloaddition between an-
thracene and maleimide destroys the π-conjugated structure
of anthracene, a decrease in UV/Vis absorption can be used
to track the reaction. In this study, the reaction completion
time was determined by monitoring the UV/Vis absorption
change (see Figure S1, Supporting Information). As listed
in Table 1, the reaction completion time of the post-func-
tionalized 2a with the four anthracene derivatives are dif-
ferent, and also their yields are different: 9-anthracenyl-
methanol shows the highest yield (96%) at the shortest re-
action time, whereas 2-anthracenecarboxylic acid reacted
with 2a to afford the lowest yield at the longest time. This
is not surprising, because the hydroxymethyl group in 9-
anthracenylmethanol is an electron-donating group that
boots the formation of six-membered rings, whereas the
carboxylic acid group is an electron-withdrawing group that
pletely with 1c in refluxing CH CN overnight, as monitored
3
by ESI-MS. When 2a reacted with 1b and 2b reacted with
1a under the same conditions, to our surprise, we only ac-
quired a mixture of single- and double-clicked hybrids, re-
gardless of how much we attempted to optimize reaction
conditions by changing reactant ratios, temperatures, and
reaction times. More interestingly, we found that the an-
thracene groups in 2c completely reacted with the maleim-
ide group in 1a under the same conditions. This result indi-
cates that, in the system studied, the DA reaction is an ef-
ficient post-functionalization method for the POM when
coupling maleimide and anthracene groups. However, the
DA approach is not suited as an efficient post-functionali-
zation method when using maleimide and furan groups as
the coupling partners. This is because the maleimide group
in 1a is an electron-poor organic group, whereas the furan
and anthracene groups in 1b and 1c are electron-rich
Table 1. DA reaction of the maleimide–POM hybrid 2a with an-
thracene derivatives.[
a]
Scheme 2. The DA reactions between the maleimide–POM hybrid
[a] At the temperature indicated, 2a (1.0 equiv.) was treated with
anthracene derivative (2.4 equiv.) in DMF solution. [b] The reac-
tion completion time was determined by monitoring the UV/Vis
absorption change. [c] Isolated yield, based on starting 2a, isolated
(2a) and the furan or anthracene derivatives (1b or 1c) and between
the furan–POM hybrid (2b) or anthracene–POM hybrid (2c) and
the maleimide derivative (1a). Note that the reaction between male-
imide and furan groups produces a mixture of the single- and
double-clicked hybrids.
1
product was always Ͼ 97% pure (400 MHz, H NMR spec-
troscopy).
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can depress the forming ability of six-membered rings. signed (see Figures S37–S48, Supporting Information). It is
Thus, 9-anthracenylmethanol was selected as a parent com- encouraging to see that the excellent results also confirm
pound for the further study.
that the DA reaction between the maleimide–POM hybrid
and the anthracene derivatives fulfills the requirements for
post-functionalization of POMs.
The POM Post-Functionalization
Creation of Novel Post-Functionalized POM Hybrids
Thus far, we have: (i) confirmed that the DA reaction is
an efficient and general method to implement post-func-
tionalization of OD-POMs, and (ii) identified the optimal
combination of modified POM 2a and the 9-anthracenyl-
methanol derivatives for coupling. Through a couple of
elaborate syntheses, we prepared starting materials of four
Up to now, our results have demonstrated that the DA
reaction is an efficient synthetic methodology for post-func-
tionalization of Anderson-type POM clusters. Unfortu-
nately, the functional groups we selected to evaluate DA
compatibility with POMs (see Table 2) are relatively simple
and have been or can be attached to the two sides of Tris-
POM-Tris through normal amidation. In order to highlight
the DA reaction as a powerful and versatile method for
production of post-functionalized POM, we selected biotin,
Tris, aminopropylisobutyl POSS, and G2-PUA dendron as
biological moieties or organic groups of interest to be
POM-coupled. We initially prepared 9-anthracenylme-
thanol derivatives 6a–d of these moieties using either esteri-
fication or amidation, as shown in Table 3. These com-
pounds were then conjugated onto maleimide–POM 2a by
the metal-free DA reaction. Encouragingly, the four deriva-
9-anthracenylmethanol derivatives (1c, 4a–c), as shown in
Table 2, in which carboxyl group, alkyl chain, benzene ring,
and adamantane are attached as the functional groups onto
9-anthracenylmethanol.
Table 2. Post-functionalization of the maleimide–POM hybrid 2a
with 9-anthracenylmethanol derivatives.
Table 3. Creation of new post-functionalized POM hybrids using
the DA reaction.[
a]
[
[
a] The reaction completion time was monitored by ESI-MS.
b] Isolated yield, based on starting 2a, isolated product was always
1
Ͼ 97% pure (400 MHz, H NMR spectroscopy).
The typical DA reaction procedure for production of
post-functionalized POM hybrids (5a–d) using these deriva-
tives is as follows: anthracene derivative (2.4 equiv.) was
treated with 2a (1.0 equiv.) in a refluxing CH CN solution.
3
After the reaction was completed, as determined on the ba-
sis of ESI-MS, the concentrated solution was precipitated
from ethyl acetate to give the desired hybrids with relatively
high yields (normally Ͼ 90%). The post-functionalized
POM hybrids were confirmed in detail with ESI-MS, FT-
1
IR spectroscopy, H NMR spectroscopy, and elemental
analysis. The consistency between the measured and calcu-
lated m/z values helps to validate the successful synthesis of ative (2.4 equiv.) was treated with 2a (1.0 equiv.) in refluxing
the desired post-functionalized POM hybrids, FT-IR spec- CH
[
a] In a typical experiment (Table 3, Entries 1–3), anthracene deriv-
3
CN. [b] Anthracene derivative (2.4 equiv.) was treated with 2a
(
1.0 equiv.) in DMF solution at room temp., as the PUA dendron
tra indicate that the POM cluster is still intact after the DA
is not stable at high temperature. [c] The reaction completion time
was determined by ESI-MS. [d] Isolated yield, based on starting
1
reaction, these data combined with H NMR spectroscopy
and elemental analysis provide evidence that the post-func-
tionalized hybrids we have synthesized, are exactly as de- spectroscopy).
1
2
a, isolated product was always Ͼ 97% pure (400 MHz, H NMR
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tives were conjugated with the maleimide groups in 2a af- troscopy, and elemental analysis. In the FT-IR spectra, the
fording post-functionalized POM hybrids 7a–d in high important characteristic bands centered at 941, 923, 901
yields (see Table 3).
and 672 cm–1 for the Anderson-type POM cluster before
Although 6a–d are easily conjugated with the maleimide and after DA reaction demonstrated that the cluster re-
groups in 2a in high yields (see Table 3) it should be noted mains intact and does not decompose upon reaction (see
that we have tried to use other methods to link their car- Figures S49–S52, Supporting Information). Mass spec-
boxyl-containing derivatives directly to Tris-POM-Tris by trometry characterization provides the most direct evidence
amidation. Disappointingly, yields for such couplings are that we acquired the targeted hybrids. Figure 1 shows the
relatively low. The reasons may be that amidation can be ESI mass spectra of post-functionalized POM hybrids 7a
affected by the following factors: (i) the solubility difference (A), 7b (B), 7c (C), and 7d (D) having mass-centered signals
from Tris-POM-Tris, (ii) the effect of reactive group effects at m/z = 1163.8, 1140.7, 1260.9, 1262.1, 1893.7, and 1617.3,
(
such as free hydroxy groups in Tris), and (iii) repulsive ste- respectively. The signal for 7a can be assigned to cluster
ric effects attributable to their large size and complex struc- anion [M – 3 TBA + H]2– with calcd. m/z = 1163.6, the
2–
tures (especially with POSS and G2-PUA dendron). It is signals for 7b are assigned as [M – 3 TBA + H] and [M –
worth noting that purification of the targeted products is 2 TBA]2– with calcd. m/z = 1140.5 and 1261.1; the signals
relatively easy.
3–
for 7c are assigned as [M – 3 TBA] and [M – 3 TBA +
2–
The post-functionalized POM hybrids were fully charac- H] with calcd. m/z = 1262.2 and 1893.9; and the signal
1
3–
terized using FT-IR spectroscopy, ESI-MS, H NMR spec- for 7d is assigned as [M – 3 TBA] , calcd. m/z = 1617.3.
Figure 1. Corresponding ESI mass spectra of the novel post-functionalized POM hybrids 7a–d.
1385
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1
These results, combined with H NMR spectroscopy and have a larger size, a more complicated structure, and/or
elemental analysis data, clearly demonstrate the successful properties inconsistent with POM clusters, relative to those
creation of these novel post-functionalized POM hybrids.
used in the previous study. Therefore, direct coupling of
these species to Tris-POM-Tris by amidation is not easy, if
even possible. We believe that the features mentioned above
are major features dictating the choice of POM modifica-
tion and that these are best dealt with through application
Significance of the DA Reaction for the POM Post-
Functionalization
As mentioned in the Introduction, Proust and co- of metal-free DA chemistry. For instance, the complicated
workers have summarized the synthetic methodologies de- structure of the G2-PUA dendron or the nanosized struc-
veloped for direct- and post-functionalization of POM clus- ture of the POSS cluster may create great steric hindrance
[
7]
ters. Meanwhile, a great number of organically function- for the conjugation with POM clusters. The biological moi-
alized POM hybrids have been created. What then is the eties or organic groups used for the further POM post-func-
significance of the method we have developed for post-func- tionalization will become larger and larger and more and
tionalization of POM clusters?
more complicated. Our data suggest the DA conjugation
the DA approach to be best suited for dealing with such issues of
[14]
As demonstrated in this and previous work,
reaction is more suitable to POM post-functionalization increasing complexity and size when carrying out POM
under mild conditions, because it has all the advantages of post-functionalization.
the “conventional” click reaction in addition to being free
The products we can attain in this study are also very
of metals, catalysts and other additives; one or more of significant. To the best of our knowledge, such POM hy-
these are trademarks of other click reactions. The high brids are seldom reported. Scientists now desire POM clus-
yields and absence of by-products are surely the most im- ters and functionalized POM hybrids for applications to
portant factors for all such work, including this report. The medicine, biology, supramolecular assembly, nanoscience,
ability to circumvent chromatographic purification is ex- materials science, and other interdisciplinary endeavors. We
tremely important in implementing the POM post-function- believe that the novel hybrids presently synthesized and
alization based on the physicochemical properties of many soon to be synthesized, will play a crucial role in these
+
POMs. For instance, three cationic Bu N surfactants nec- fields. For instance, biotinylated or Tris-buffered POM hy-
4
essarily used to cover the anionic POM cluster, may be in- brids are both projected to play central roles in future bio-
advertently removed during chromatographic purifications; logical investigations. Additionally, metal-catalyst-free sam-
this would dramatically alter the solubility of the hybrids, ples are pivotal to elimination of cytotoxicity caused by
thus lowering product yields. This is a main concern with metal impurities. Taking into account the special 3D archi-
POM modifications, as stated by Proust and co-workers in tectures of POM, POSS cluster, and G2-PUA dendron, the
[
7]
their review. We have our own first-hand experience. In ability to match or mismatch the shapes and dimensions of
one of our previous works,[ we prepared the hybrid of the these building blocks is a most interesting point in future
G2-PUA and the Tris-POM-Tris through amidation using studies of their diverse supramolecular assemblies. These
dicyclohexylcarbodiimide (DCC) as an activator. When we works are currently underway.
purified the product using conventional silica gel
8g]
chromatography, the yield was much lower than if
[
13e]
Conclusions
chromatography could be avoided. In another work,
we
used the CuAAC-click reaction to link a modified Wells–
We have successfully carried out the post-functionaliza-
Dawson-type POM with two polymer chains. IR data tion of an organic-modified polyoxomolybdate cluster with
showed a high yield for coupling of the alkynyl group. How- very different kinds of biological moieties or organic groups
ever, after purification by conventional alumina chromatog- by using a metal-free Diels–Alder click reaction. The high
raphy to remove the Cu catalyst, the final yield was ca. efficiency of the reaction and ease of purification guarantee
75%. Another tedious problem was the effect of the reduc- yields higher than 90%. This is an ideal methodology for
ing agent and Cu cations on the POM cluster. The absence the synthesis of novel post-functionalized POM hybrids
of metals and other additives in the DA reaction minimizes conjugated with organic groups or biological moieties large
negative impacts of such reaction components to a great in size, complicated in structure, and/or inconsistent in
degree. The disadvantage of the DA approach is that at le- properties. Using this versatile platform, increasingly novel
ast three steps are needed from the preparation of the pre- post-functionalized POM hybrids with synergetic properties
cursors of derivatives and OD-POM hybrids to the actual can be envisioned. Finding valuable applications of hybrids
execution of DA cycloaddition. However, relatively speak- synthesized in this work and many others whose construc-
ing, reactions prior to the DA reaction are simple, and tion is now enabled by this method, will be a significant
product purification is easy compared to the purification of endeavor in our future investigations.
the finalized POM product. In other words, the DA click
reaction is the key step for determining the yield of the final
products.
Experimental Section
As shown in Table 3, the biological moieties or organic Materials: Aminopropylisobutyl POSS was purchased from Hybrid
groups used for POM post-functionalization in this study Plastics as crystalline powders and used as received. Compounds
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1
a, 1b, 1c and the Tris-POM-Tris were prepared according to pre-
Hybrid 3c: Following the DA reaction, the product was isolated as
an orange powder (yield 121 mg, 92%). FT-IR (KBr pellet): ν˜ =
3343, 2959, 2930, 2872, 1703, 1464, 1403, 1368, 1229, 1160, 1116,
[18b,21,22]
viously published procedures.
tained from Alfa Aesar and used as received.
All other chemicals were ob-
–
1 1
1
070, 1024, 940, 923, 902, 670, 561 cm . H NMR (400 MHz, [D
DMSO): δ = 0.93 (t, J = 6.7 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA),
.56 (m, 24 H, TBA), 1.73 (m, 4 H, CH), 3.16 (m, 28 H, TBA,
CH ), 3.24 (m, 4 H, CH ), 4.78 (s, 2 H, CH), 7.09–7.38 (m, 12
H, Ben-H), 7.53 (m, 2 H, Ben-H), 7.94 (m, 2 H, Ben-H) ppm.
154MnMo 34 (2628.92): calcd. C 45.69, H 5.90, N 3.73;
found C 45.55, H 6.12, N 3.87. ESI-MS (negative mode, DMF):
6
]-
Methods: Thin-layer chromatography (TLC) was performed using
precoated silica gel 60 F254, and compounds were visualized with
UV light (254 nm); Merck 60 (230–400 mesh) silica gel was used
for column chromatography. FT-IR spectra were recorded with a
Bio-Rad FTS-135 FT-IR spectrometer using the KBr pellet tech-
nique. ESI mass spectra were recorded with a Finnigan LCQ ad-
vantage mass spectrometer in negative ion mode. UV/Vis absorp-
tion measurements were recorded using a U-3900H spectropho-
tometer. H and C NMR spectra were recorded at room temp.
with a 400 MHz (100 MHz) Bruker 400 spectrometer. Chemical
shifts are given in ppm (δ). Coupling constants (J) are given in
Hz. Multiplicities are indicated by the following abbreviations: s =
singlet, d = doublet, t = triplet, dd = doublet of doublets, m =
multiplet, br. = broad. Elemental analyses were performed using
an Elementary Vario Micro Cure instrument.
1
2
2
C
100
H
6 7
N O
2
–
2–
m/z = 1071.1 [M – 2 TBA] , 951.1 [M – 3 TBA + H] , 968.9 [M –
3 TBA + K] .
2
–
1
13
Hybrid 3d: Following the DA reaction, the product was isolated as
an orange powder (yield 121 mg, 87%). FT-IR (KBr pellet): ν˜ =
3
1
339, 2959, 2931, 2872, 1704, 1475, 1399, 1369, 1236, 1161, 1116,
074, 1024, 941, 923, 901, 673, 561 cm . H NMR (400 MHz, [D ]-
6
–1
1
DMSO): δ = 0.93 (t, J = 6.5 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA),
.56 (m, 24 H, TBA), 1.75 (m, 4 H, CH), 3.16 (m, 28 H, TBA,
CH ), 3.25 (s, 4 H, CH ), 4.88 (s, 4 H, CH), 7.17–7.35 (m, 8 H,
Ben-H), 7.48 (s, 2 H, Ben-H), 7.58 (s, 2 H, Ben-H), 7.77 (s, 2 H,
Ben-H) ppm. C100 154MnMo 34 (2628.92): calcd. C 45.69, H
5.90, N 3.73; found C 45.35, H 6.05, N 3.92. ESI-MS (negative
1
Maleimide–POM Hybrid 2a: Compound 1a (2.6 equiv.) and EEDQ
2
2
3
(3.9 equiv.) were dissolved in refluxing CH CN and, after 40 min,
Tris-POM-Tris (1.0 equiv.) was added to the solution, and the mix-
ture was refluxed for 24 h. Then, the concentrated solution was
added to tetrahydrofuran and precipitation induced to give the de-
sired hybrids (1.88 g, 0.86 mmol, 81%). FT-IR (KBr pellet): ν˜ =
H
6 7
N O
2
–
mode, DMF): m/z = 1072.1 [M – 2 TBA] , 951.5 [M – 3 TBA +
2
–
2–
H] , 969.7 [M – 3 TBA + K] .
3
1
366, 2957, 2932, 2872, 1706, 1536, 1478, 1370, 1321, 1229, 1116,
073, 1025, 940, 920, 902, 669, 564 cm . H NMR (400 MHz, [D ]-
6
–1
1
Hybrid 5a: Following the DA reaction, the product was isolated as
an orange powder (yield 163 mg, 95%). FT-IR (KBr pellet): ν˜ =
DMSO): δ = 0.93 (t, J = 7.0 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA),
3
1
338, 2959, 2931, 2873, 1733, 1701, 1465, 1402, 1360, 1232, 1160,
1
3
.57 (m, 24 H, TBA), 2.71 (m, 4 H, NCH
.56 (m, 4 H, CH CO), 7.01 (s, 4 H, CH), 63.4 (br., 12 H, CH
134MnMo 30 (2184.44): calcd. C 38.49, H 6.18, N
.49; found C 38.66, H 5.82, N 4.38. ESI-MS (negative mode,
2
), 3.15 (m, 24 H, TBA),
–
1
1
117, 1071, 1024, 941, 922, 901, 671, 564 cm . H NMR
]DMSO): δ = 0.94 (t, J = 6.8 Hz, 36 H, TBA), 1.31
m, 24 H, TBA), 1.57 (m, 24 H, TBA), 1.76 (m, 4 H, CH), 2.60
), 3.17 (m, 28 H, TBA, CH ), 3.31 (m, 16 H, H O,
), 4.79 (s, 2 H, CH), 5.29–5.51 (m, 4 H, CH ), 7.20–7.26 (m,
2 H, Ben-H), 7.36 (m, 2 H, Ben-H), 7.51 (m, 2 H, Ben-H), 63.5
br., 12 H, CH O) ppm. C108 166MnMo 38 (2801.10): calcd.
C 46.31, H 5.97, N 3.50; found C 46.53, H 5.66, N 3.30. ESI-MS
2
2
O)
(
400 MHz, [D
6
ppm. C70
4
H
6 7
N O
(
–
(m, 4 H, CH
CH
1
2
2
2
DMF): m/z = 1701.4 [M – 2 TBA + H] .
2
2
General Procedure for the DA Reaction: The anthracene derivative
3
(2.4 equiv.) was treated with 2a (1.0 equiv.) in a refluxing CH CN
(
2
H
6 7
N O
solution. Once the reaction was complete as determined by ESI-
MS, the concentrated solution was added to ethyl acetate. From the
resulting solution were precipitated the post-functionalized POM
hybrids.
2–
(
negative mode, DMF): m/z = 1037.5 [M – 3 TBA + H] , 1052.1
2–
[M – 3 TBA + K] .
Hybrid 5b: Following the DA reaction, the product was isolated as
an orange powder (yield 112 mg, 91%). FT-IR (KBr pellet): ν˜ =
Hybrid 3a: Following the DA reaction, the product was isolated as
an orange powder (yield 84 mg, 96%). FT-IR (KBr pellet): ν˜ =
3
348, 2958, 2926, 2866, 1702, 1465, 1402, 1368, 1233, 1160, 1114,
3
1
355, 2958, 2933, 2872, 1698, 1467, 1368, 1232, 1159, 1114, 1068,
025, 942, 921, 901, 672, 562 cm . H NMR (400 MHz, [D ]-
6
–
1 1
–1
1
1069, 1025, 942, 923, 902, 670, 563 cm . H NMR (400 MHz, [D
DMSO): δ = 0.85 (t, J = 5.8 Hz, 6 H, CH ), 0.94 (t, J = 5.9 Hz,
6 H, TBA), 1.23, 1.31 (m, 72 H, TBA, CH
6
]-
3
DMSO): δ = 0.93 (t, J = 6.9 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA),
.56 (m, 24 H, TBA), 1.75 (m, 4 H, CH), 3.16 (m, 28 H, TBA,
3
CH
CH
2
), 1.57 (m, 28 H, TBA,
2
), 1.75 (m, 4 H, CH), 2.38 (m, 4 H, CH ), 3.16 (m, 32 H, TBA,
1
2
CH
CH
2
), 3.23 (s, 4 H, CH
), 5.35 (s, 2 H, OH), 7.08–7.27 (m, 12 H, Ben-H), 7.44 (d, J
2
), 4.70 (s, 2 H, CH), 4.77–4.91 (m, 4 H,
2
2
), 4.79 (s, 2 H, CH), 5.30–5.48 (m, 4 H, CH ), 7.07–7.31 (m,
2
1
(
2 H, Ben-H), 7.32 (m, 2 H, Ben-H), 7.50 (m, 2 H, Ben-H), 63.5
br., 12 H, CH O) ppm. C132 218MnMo 34 (3077.78): calcd.
C 51.51, H 7.14, N 3.19; found C 51.44, H 7.56, N 3.54. ESI-MS
=
C
6.5 Hz, 2 H, Ben-H), 7.67 (d, J = 6.8 Hz, 2 H, Ben-H) ppm.
158MnMo 32 (2600.95): calcd. C 46.18, H 6.12, N 3.77;
2
H
6 7
N O
100
H
6 7
N O
found C 46.39, H 6.08, N 3.67. ESI-MS (negative mode, DMF):
2
–
2
–
2–
(negative mode, DMF): m/z = 1175.3 [M – 3 TBA + H] .
m/z = 1056.6 [M – 2 TBA] , 937.6 [M – 3 TBA + H] , 953.3 [M –
3
2–
3–
TBA + K] , 624.4 [M – 3 TBA] .
Hybrid 5c: Following the DA reaction, the product was isolated as
an orange powder (yield 144 mg, 92%). FT-IR (KBr pellet): ν˜ =
3348, 2959, 2932, 2872, 1704, 1464, 1401, 1369, 1271, 1159, 1114,
Hybrid 3b: Following the DA reaction, the product was isolated as
an orange powder (yield 99 mg, 95%). FT-IR (KBr pellet): ν˜ =
–
1 1
3
9
=
2
(
(
346, 2958, 2932, 2872, 1700, 1474, 1342, 1158, 1117, 1071, 1024,
6
1067, 1025, 941, 923, 901, 671, 562 cm . H NMR (400 MHz, [D ]-
–1 1
44, 924, 903, 672, 561 cm . H NMR (400 MHz, [D
0.94 (t, J = 7.2 Hz, 36 H, TBA), 1.32 (m, 24 H, TBA), 1.57 (m,
4 H, TBA), 1.77 (m, 4 H, CH), 3.16 (m, 28 H, TBA, CH
s, 4 H, CH ), 4.77 (s, 4 H, CH), 7.15–7.19 (m, 8 H, Ben-H), 7.21
m, 4 H, Ben-H), 7.46 (t, J = 3.7 Hz, 4 H, Ben-H) ppm.
154MnMo 30 (2540.90): calcd. C 46.32, H 6.11, N 3.86;
found C 46.55, H 6.08, N 3.92. ESI-MS (negative mode, DMF):
6
]DMSO): δ
DMSO): δ = 0.93 (t, J = 7.2 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA),
1.56 (m, 24 H, TBA), 1.76 (m, 4 H, CH), 3.16 (m, 28 H, TBA,
2
), 3.21 CH
4 H, CH
(br., 12 H, CH
C 48.74, H 5.96, N 3.49; found C 48.87, H 5.78, N 3.76. ESI-MS
2
), 3.35 (m, 10 H, H
), 7.22–7.66 (m, 22 H, Ben-H), 7.95 (m, 4 H, Ben-H), 63.5
O) ppm. C114 166MnMo 34 (2809.17): calcd.
2 2
O, CH ), 4.84 (s, 2 H, CH), 5.52–5.79 (m,
2
2
2
H
6 7
N O
C
98
H
6 7
N O
2
–
(negative mode, DMF): m/z = 1040.9 [M – 3 TBA + H] , 1057.7
2–
2–
2–
m/z = 907.8 [M – 3 TBA + H] , 926.4 [M – 3 TBA + K] .
Eur. J. Inorg. Chem. 2013, 1381–1389
[M – 3 TBA + K] .
1387
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
Hybrid 5d: Following the DA reaction, the product was isolated as
an orange powder (yield 98 mg, 91%). FT-IR (KBr pellet): ν˜ =
(m, 72 H, CONHCH
2
CH
2
CH
2
NCO, TBA), 3.90 (m, 16 H,
CH CH OCO), 4.78 (s, 2 H, CH), 5.00–5.43 (m, 8 H, CH ,
2
2
2
3
9
346, 2957, 2907, 2871, 1703, 1464, 1400, 1325, 1248, 1149, 1025,
42, 923, 902, 672, 562 cm . H NMR (400 MHz, [D ]DMSO): δ
6
NHCO), 6.94 (br., 2 H, NHCO), 7.12 (br., 2 H, NHCO), 7.21–7.32
(m, 12 H, Ben-H), 7.51 (m, 2 H, Ben-H), 7.62 (m, 2 H, Ben-H),
–1 1
=
(
0.93 (t, J = 6.8 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA), 1.57–1.78 7.72 (br., 2 H, NHCO), 7.85 (br., 2 H, NHCO), 63.5 (br., 12 H,
m, 52 H, TBA, CH, CH ), 1.92 (s, 6 H, CH), 2.16 (s, 4 H, CH ), CH O) ppm. C264 464MnMo 60 (5579.28): calcd. C 56.83, H
), 3.25 (m, 4 H, CH ), 4.79 (s, 2 H, CH), 8.38, N 6.28; found C 56.68, H 7.99, N 6.28. ESI-MS (negative
), 7.13–7.29 (m, 12 H, Ben-H), 7.34 (m, 2 mode, DMF): m/z = 1617.2 [M – 3 TBA] .
2
2
2
H
6 25
N O
3
5
.16 (m, 28 H, TBA, CH
.28–5.46 (m, 4 H, CH
2
2
3
–
2
H, Ben-H), 7.51 (m, 2 H, Ben-H), 63.5 (br., 12 H, CH
190MnMo 34 (2953.47): calcd. C 50.43, H 6.48, N 3.32;
found C 49.96, H 6.50, N 3.17. ESI-MS (negative mode, DMF):
2
O) ppm.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and full characterization
data of the starting materials.
C
124
H
6 7
N O
2–
2–
m/z = 1114.7 [M – 3 TBA + H] , 1131.7 [M – 3 TBA + K] .
Hybrid 7a: Following DA reaction, the product was isolated as an
orange powder (yield 107 mg, 89%). FT-IR (KBr pellet): ν˜ = 3290,
Acknowledgments
2
9
958, 2906, 2872, 1700, 1553, 1461, 1245, 1159, 1024, 941, 923,
We appreciate the financial support of the National Natural Sci-
ence Foundation of China for a grant (Grants NSFC20734001,
–1 1
01, 672, 565 cm . H NMR (400 MHz, [D
6
]DMSO): δ = 0.94 (t,
), 1.57 (m, 32
), 1.75 (m, 4 H, CH), 2.41 (m, 4 H, CH ), 2.56
m, 2 H, Vita-H), 2.80 (m, 2 H, Vita-H), 3.07 (m, 2 H, Vita-H),
.16 (m, 32 H, TBA, CH , CH ), 4.10 (m, 2 H, CH), 4.28 (m, 2 H,
CH), 4.79 (s, 2 H, CH), 5.32–5.46 (m, 4 H, CH ), 6.34, 6.68 (br., 4
H, NHCO), 7.14–7.27 (m, 12 H, Ben-H), 7.34 (m, 2 H, Ben-H),
.51 (m, H, Ben-H), 63.5 (br., 12 H, CH O) ppm.
186MnMo (3053.54): calcd. C 47.20, H 6.14, N
.05; found C 46.97, H 6.19, N 4.83. ESI-MS (negative mode,
J = 7.0 Hz, 36 H, TBA), 1.31 (m, 28 H, TBA, CH
H, TBA, CH , CH
(
2
20974057 and 21274069), the Research Fund for the Doctoral Pro-
2
2
2
gram of Higher Education of China (20100031110015) and the
Open Research Fund of State Key Laboratory of Polymer Physics
and Chemistry, Changchun Institute of Applied Chemistry.
3
2
2
2
[
1] Special thematic issue on polyoxometalates: Chem. Rev. 1998,
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C
5
2
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98, 1–398.
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[
[
2] B. Hasenknopf, Front. Biosci. 2005, 10, 275–287.
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2–
DMF): m/z = 1163.8 [M – 3 TBA + H] .
36, 105–121.
Hybrid 7b: Following DA reaction, the product was isolated as an
orange powder (yield 131 mg, 91%). FT-IR (KBr pellet): ν˜ = 3320,
[4] T. R. Zhang, J. Brown, R. J. Oakley, C. F. J. Faul, Curr. Opin.
Colloid Interface Sci. 2009, 14, 62–70.
[
5] W. Qi, L. X. Wu, Polym. Int. 2009, 58, 1217–1225.
2
9
959, 2908, 2873, 1700, 1542, 1465, 1367, 1236, 1160, 1024, 941,
23, 901, 672, 565 cm . H NMR (400 MHz, [D ]DMSO): δ = 0.94
6
–1 1
[6] a) A. Proust, R. Thouvenot, P. Gouzerh, Chem. Commun.
2
008, 1837–1852; b) A. Dolbecq, E. Dumas, C. R. Mayer, P.
(
t, J = 7.2 Hz, 36 H, TBA), 1.31 (m, 24 H, TBA), 1.57 (m, 24 H,
TBA), 1.76 (m, 4 H, CH), 2.57 (m, 4 H, CH ), 3.16 (m, 28 H, TBA,
CH ), 3.23 (m, 4 H, CH ), 3.52 (d, J = 5.3 Hz, 12 H, CCH ), 4.69
m, 6 H, OH), 4.79 (s, 2 H, CH), 5.27–5.47 (m, 4 H, CH
Mialane, Chem. Rev. 2010, 110, 6009–6048.
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[
7] A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh,
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2
2
G. Izzet, Chem. Soc. Rev. 2012, 41, 7605–7622.
(
2
), 7.18– [8] a) S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacôte,
7
.26 (m, 14 H, NHCO, Ben-H), 7.39 (m, 2 H, Ben-H), 7.50 (m, 2
O) ppm. C116 184MnMo
3007.34): calcd. C 46.33, H 6.17, N 4.19; found C 46.26, H 6.44,
N 3.97. ESI-MS (negative mode, DMF): m/z (%) = 1260.9 [M – 2
S. Thorimbert, M. Malacria, J. Am. Chem. Soc. 2005, 127,
6788–6794; b) Y. F. Song, N. McMillan, D. L. Long, J. Thiel,
Y. L. Ding, H. S. Chen, N. Gadegaard, L. Cronin, Chem. Eur.
J. 2008, 14, 2349–2354; c) J. Zhang, Y. F. Song, L. Cronin, T. B.
Liu, J. Am. Chem. Soc. 2008, 130, 14408–14409; d) Y. F. Song,
N. McMillan, D. L. Long, S. Kane, J. Malm, M. O. Riehle,
C. P. Pradeep, N. Gadegaard, L. Cronin, J. Am. Chem. Soc.
H, Ben-H), 63.4 (br., 12 H, CH
(
2
H
6 9 42
N O
2–
2–
TBA] , 1140.7 [M – 3 TBA + H] .
Hybrid 7c: Following DA reaction, the product was isolated as a
light orange powder (yield 158 mg, 92%). FT-IR (KBr pellet): ν˜ =
2009, 131, 1340–1341; e) Y. L. Wang, X. L. Wang, X. J. Zhang,
N. Xia, B. Liu, J. Yang, W. Yu, M. B. Hu, M. A. Yang, W.
Wang, Chem. Eur. J. 2010, 16, 12545–12548; f) Y. Yan, H. B.
Wang, B. Li, G. F. Hou, Z. D. Yin, L. X. Wu, V. W. W. Yam,
Angew. Chem. 2010, 122, 9419–9422; g) B. Liu, J. Yang, M.
Yang, Y. L. Wang, N. Xia, Z. J. Zhang, P. Zheng, W. Wang, I.
Lieberwirth, C. Kübel, Soft Matter 2011, 7, 2317–2320; h) H.-
K. Yang, Y.-X. Cheng, M.-M. Su, Y. Xiao, M.-B. Hu, W.
Wang, Q. Wang, Bioorg. Med. Chem. Lett. 2013, DOI: 10.1016/
j.bmcl.2012.12.081.
3
9
329, 2956, 2905, 2873, 1702, 1537, 1464, 1229, 1116, 1030, 941,
–1 1
23, 901, 673, 564 cm . H NMR (400 MHz, [D
, CH ), 0.92 (t, J = 4.3 Hz, 36 H, TBA),
), 1.31 (m, 24 H, TBA), 1.43 (m, 4 H, CH ), 1.56
6
]DMSO): δ = 0.57
(d, J = 6.6 Hz, 32 H, SiCH
2
2
0
.95 (s, 84 H, CH
3
2
(
m, 24 H, TBA), 1.69–1.88 (m, 18 H, CH, CH), 2.39 (m, 4 H,
COCH ), 2.57 (m, 4 H, CH CO), 2.99 (m, 4 H, CH ), 3.16 (m, 28
H, TBA, CH ), 3.26 (s, 4 H, CH ), 4.79 (s, 2 H, CH), 5.29–5.44
m, 4 H, CH ), 7.15–7.31 (m, 12 H, Ben-H), 7.36 (m, 2 H, Ben-H),
.51 (m, 2 H, Ben-H), 7.87 (br., 2 H, NHCO), 63.5 (br., 12 H,
2
2
2
2
2
(
7
2
[
[
9] a) I. Bar-Nahum, H. Cohen, R. Neumann, Inorg. Chem. 2003,
42, 3677–3684; b) M. H. Rosnes, C. Musumeci, C. P. Pradeep,
2 6 9
CH O) ppm. C170H304MnMo N O60Si16 (4514.25): calcd. C 45.23,
H 6.79, N 2.79; found C 45.23, H 7.02, N 2.93. ESI-MS (negative
mode, DMF): m/z = 1894.7 [M – 3 TBA + H] , 1261.4 [M – 3
TBA] .
J. S. Mathieson, D. L. Long, Y. F. Song, B. Pignataro, R.
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S. Renaudineau, L. M. Chamoreau, C. Afonso, G. Izzet, A.
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J. H. Kang, F. C. Chen, Y. Yang, Z. H. Peng, Chem. Mater.
2–
3–
Hybrid 7d: Following DA reaction, the product was isolated as a
light orange powder (yield 87 mg, 88%). FT-IR (KBr pellet): ν˜ =
3
9
=
1
CH
317, 2925, 2909, 2857, 1705, 1636, 1532, 1461, 1250, 1154, 1024,
–1 1
40, 923, 902, 669, 565 cm . H NMR (400 MHz, [D
0.85 (t, J = 5.7 Hz, 24 H, CH ), 0.93 (t, J = 6.7 Hz, 36 H, TBA),
.11–1.39 [m, (, 168 HCH , TBA], 1.44–1.87 (m, 68 H,
CH NCO, CH, TBA), 2.29–2.42
CO), 2.90 (m, 8 H, CH , CH ), 2.98–3.24
6
]DMSO): δ
3
2005, 17, 402–408; e) B. Xu, M. Lu, J. H. Kang, D. Wang, J.
2 9
)
Brown, Z. H. Peng, Chem. Mater. 2005, 17, 2841–2851; f) S.
Chakraborty, A. Keightley, V. Dusevich, Y. Wang, Z. H. Peng,
Chem. Mater. 2010, 22, 3995–4006; g) B. B. Xu, Y. G. Wei,
2
CH
2
OCO, OCONHCH
CH
2
CH
2
2
(m, 12 H, NCOCH
2
2
2
2
Eur. J. Inorg. Chem. 2013, 1381–1389
1388
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
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Xu, C. L. Barnes, Z. H. Peng, J. Am. Chem. Soc. 2001, 123,
[17] a) T. Yamase, H. Fujita, K. Fukushima, Inorg. Chim. Acta
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