Design and synthesis of "dumb-bell" and "triangular" inorganic-organic hybrid nanopolyoxometalate clusters and their characterisation through ESI-MS analyses

Article abstract of DOI:10.1002/chem.201100257

A series of tris(hydroxymethyl)aminomethane (TRIS)-based linear (bis(TRIS)) and triangular (tris(TRIS)) ligands has been synthesised and were covalently attached to the Wells-Dawson type cluster [P₂V₃W ₁₅O₆₂]^9- to generate a series of nanometer-sized inorganic-organic hybrid polyoxometalate clusters. These huge hybrids, with a molecular mass similar to that of small proteins in the range of ≈10-16 kDa, were unambiguously characterised by using high-resolution ESI-MS. The ESI-MS spectra of these compounds revealed, in negative ion mode, a characteristic pattern showing distinct groups of peaks corresponding to different anionic charge states ranging from 3^- to 8^- for the hybrids. Each peak in these individual groups could be unambiguously assigned to the corresponding hybrid cluster anion with varying combinations of tetrabutylammonium (TBA) and other cations. This study therefore highlights the prowess of the high-resolution ESI-MS for the unambiguous characterisation of large, nanoscale, inorganic-organic hybrid clusters that have huge mass, of the order of 10-16 kDa. Also, the designed synthesis of these compounds points to the fact that we were able to achieve a great deal of structural pre-design in the synthesis of these inorganic-organic hybrid polyoxometalates (POMs) by means of a ligand design route, which is often not possible in traditional "one-pot" POM synthesis. Copyright

Full text of DOI:10.1002/chem.201100257

DOI: 10.1002/chem.201100257
Design and Synthesis of “Dumb-bell” and “Triangular” Inorganic–Organic
Hybrid Nanopolyoxometalate Clusters and Their Characterisation through
ESI-MS Analyses
Chullikkattil P. Pradeep,[a, b] Feng-Yan Li,[a, c] Claire Lydon,[a] Haralampos N. Miras,[a]
De-Liang Long,[a] Lin Xu,[c] and Leroy Cronin*[a]
Abstract: A series of tris(hydroxyme-
thyl)aminomethane (TRIS)-based
linear (bis(TRIS)) and triangular (tris-
(TRIS)) ligands has been synthesised
and were covalently attached to
the Wells–Dawson type cluster
[P2V3W15O62]9À to generate a series of
nanometer-sized inorganic–organic
hybrid polyoxometalate clusters. These
huge hybrids, with a molecular mass
similar to that of small proteins in the
range of ꢀ10–16 kDa, were unambigu-
ously characterised by using high-reso-
lution ESI-MS. The ESI-MS spectra of
these compounds revealed, in negative
ion mode,
a
characteristic pattern
and other cations. This study therefore
highlights the prowess of the high-reso-
lution ESI-MS for the unambiguous
characterisation of large, nanoscale, in-
organic–organic hybrid clusters that
have huge mass, of the order of
10–16 kDa. Also, the designed synthe-
sis of these compounds points to the
fact that we were able to achieve a
great deal of structural pre-design in
the synthesis of these inorganic–organ-
ic hybrid polyoxometalates (POMs) by
means of a ligand design route, which
is often not possible in traditional
“one-pot” POM synthesis.
showing distinct groups of peaks corre-
sponding to different anionic charge
states ranging from 3À to 8À for the hy-
brids. Each peak in these individual
groups could be unambiguously as-
signed to the corresponding hybrid
cluster anion with varying combina-
tions of tetrabutylammonium (TBA)
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Keywords: mass spectrometry
·
nanostructures · organic–inorganic
hybrid composites · polyoxometa-
lates · tungsten
Introduction
ganic units and inorganic clusters.[4] This means that cova-
lent functionalisation could also lead to the development of
POM hybrids with pre-designed topologies.[5] This structural
pre-design is often not possible in traditional “one-pot”
POM synthesis.[1]
Polyoxometalates (POMs) are an important class of inor-
ganic compounds containing anionic metal oxide clusters of
Mo, W, V and Nb, with interesting structural, physical and
chemical properties.[1,2] One way of expanding the horizons
of POM chemistry is through the development of inorganic–
organic hybrids by combining POM clusters with various or-
ganic functionalities.[3] For instance the covalent binding of
organic groups onto POM clusters is an important synthetic
strategy towards functional hybrids, as the covalent func-
tionalisation often allows synergistic interaction between or-
Thus far, the majority of the inorganic–organic hybrid
POMs reported in recent years are based on smaller cluster
types, such as Lindqvist, Keggin and Anderson type clusters
and/or their derivatives.[6] However, some of the recent at-
tempts in this field are focussing on larger clusters, such as
Wells–Dawson type clusters, which are more redox and cata-
lytically active compared to smaller cluster types.[7–12] For ex-
ample, the Wells–Dawson type clusters have the capability
of accepting a number of electrons reversibly without cluster
disintegration.[13] Nevertheless, compared to hybrids based
on smaller cluster types, reports on Wells–Dawson cluster
based hybrids are limited in the literature; one possible
reason for this could be the difficulties associated with the
characterisation of such huge hybrids. One major problem
often faced in the characterisation of large POM hybrids is
the difficulty in getting good quality crystals suitable for
single-crystal X-ray analysis. For example, it is observed that
POMs with a large number of organic cations, such as tetra-
butylammonium (TBA), often do not form good quality
crystals and even if they do, the crystals tend to crack imme-
diately due to solvent loss or are poorly diffracting due to
disorder associated with the many charge balancing cat-
[a] Dr. C. P. Pradeep, Dr. F.-Y. Li, C. Lydon, Dr. H. N. Miras,
Dr. D.-L. Long, Prof. Dr. L. Cronin
WestCHEM, Department of Chemistry
The University of Glasgow, G12 8QQ (UK)
Fax : (+44)141-330-4888
E-mail: L.Cronin@chem.gla.ac.uk
[b] Dr. C. P. Pradeep
Present Address: School of Basic Sciences
Indian Institute of Technology—Mandi
Mandi 175 001, Himachal Pradesh (India)
[c] Dr. F.-Y. Li, Prof. Dr. L. Xu
Key Laboratory of Polyoxometalate Science of Ministry of Education
Department of Chemistry, Northeast Normal University
Changchun, Jilin (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100257.
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FULL PAPER
ions.[14] On the other hand, it is generally agreed that the
traditional solution characterisation techniques such as
NMR spectroscopy, polarography, optical spectrophotome-
try and so forth are of limited application in the case of
polyoxometalates with large size and high symmetry. In this
context, there is a need for developing alternative character-
isation techniques that can be reliably employed for a wide
range of POM clusters, especially hybrid POMs that have
huge masses, for example, reaching that of proteins.
Electrospray ionisation mass spectrometry (ESI-MS) is a
successful technique for the characterisation of very large
molecules of biological and pharmaceutical importance, and
also for the characterisation of large supramolecular systems
as well as coordination compounds.[15,16] For instance it has
been found that ESI-MS could be a valuable tool in the
analysis of polyoxometalates dissolved in organic or aque-
ous–organic solvents, as it involves a soft ionisation tech-
nique with respect to FAB MS, and serves to transfer/desorb
ions from solution to the gas phase with minimal fragmenta-
tion along the ion path.[16] Following the pioneering works
of early researchers in this area,[17] we have extensively uti-
lised cryospray and electrospray ionisation MS techniques
for the characterisation and elucidation of cluster transfor-
mation mechanisms in POM chemistry.[18] However, in order
to establish high-resolution ESI-MS as a reliable characteri-
sation technique in POM chemistry, more studies are
needed covering a wide variety of compounds, especially
hybrid POMs that have different size and mass ranges.
Results and Discussion
Synthesis and spectroscopic characterisation: Tris(hydroxy-
methyl)aminomethane (TRIS) is an important linker mole-
cule in POM chemistry, capable of binding an organic func-
tionality at one end and a particular POM cluster at the
other.[9,20,21] This bifunctional property of TRIS has led to
the development of a number of TRIS-functionalised hybrid
POMs, the majority of which are based on the Anderson
type cluster. For example, Hasenknopf et al. have reported a
series of TRIS-functionalised Mn-Anderson cluster hybrids
in connection with light-harvesting and gel-formation prop-
erties.[21] Studies from our group have shown that TRIS-
functionalised Mn-Anderson clusters can form interesting
framework materials in the solid state and can exhibit sur-
factant type behaviour in solution, depending on the organic
groups attached.[22] Also, asymmetric functionalisation of
Mn-Anderson clusters, that is, attaching two different TRIS
functionalities onto either side of the disc-shaped Anderson
cluster, has also been achieved by our group, which ulti-
mately led to the development of surface-grafted functional-
ised POMs, capable of exhibiting cell-adhesion properties.[23]
TRIS-grafted [P2V3W15O62]9À clusters have been reported by
Hill et al., followed by a report on “super-POMs” contain-
ing four [P2V3W15O62]9À units by the same group.[7] Some of
these [P2V3W15O62]9À-based hybrids are shown to exhibit cat-
alytic properties towards peroxide oxidations of tetrahydro-
thiophene. Recently, TRIS-grafted [P2V3W15O62]9À cluster
hybrids have been utilised in the development of POM–
polymer hybrids[11] as well as hybrid gels.[12] Studies from our
group have shown that the self-assembly of [P2V3W15O62]9À
cluster in solutions and gas phase can be controlled by graft-
ing TRIS derivatives with different hydrogen-bonding func-
tionalities -NO2, -NH2 and -CH3. ESI-MS studies on such
hybrids could detect hydrogen-bonded higher aggregates
formed in solution and in gas phase.[8] In a recent study, we
have also shown that the [P2V3W15O62]9À cluster could be
used for the development of hybrids with amphiphilic prop-
erties. For example, “dumb-bell”-shaped hybrids, developed
by connecting two [P2V3W15O62]9À clusters together by using
In this paper, we report the synthesis of a series of nano-
meter-sized hybrid POMs based on Wells–Dawson type clus-
ter [P2V3W15O62]9À that have masses of the order of 10–
16 kDa, and their unambiguous characterisation using the
ESI-MS technique. These “dumb-bell”- and “triangular”-
shaped inorganic–organic hybrids are developed from tris(h-
ydroxymethyl)aminomethane (TRIS)-based linear (bis-
ACHTUNGTRENNUNG(TRIS)) and “triangular” (trisACHTUNGTERN(NUGN TRIS)) ligands and contain
tetrabutylammonium (TBA) as the major counterion. Inor-
ganic–organic hybrids containing one as well as four
[P2V3W15O62]9À units have been reported by groups of Hill
and Hasenknopf.[7,10] The hybrids reported in the present
paper containing two and/or three [P2V3W15O62]9À cluster
units are therefore, complementary to the above studies and
are logical extension of our own previous report of “dumb-
bell”-shaped hybrids containing two such units.[9] The main
interest in [P2V3W15O62]9À-based “dumb-bell”-type hybrids
lies in their amphiphilic properties such as surfactant-like
self-assembly behaviour in solutions leading to vesicle for-
mation.[9] Although, this paper concentrates mainly on the
synthesis and ESI-MS characterisation of some new
[P2V3W15O62]9À-based hybrids, detailed studies have shown
that some of these compounds exhibits interesting self-as-
sembly behaviour in solution, as expected.[19]
bisACHTNUTRGNEUNG(TRIS) linkers, are shown to exhibit interesting self-as-
sembly behaviour in solution leading to the formation of
vesicles having dimensions of the order of 100 nm.[9] The so-
lution characterisation of such “dumb-bell” clusters by ESI-
MS analyses revealed that such large nanoclusters exhibit
clear ESI-MS spectra consisting of groups of peaks corre-
sponding to differently charged cluster anions, see Figure 1.
This led us to explore this class of compounds further by
varying the bisACHTUNTRGNEUNG(TRIS) linker ligands and also by extending
this class of compounds further by developing tris
linker ligands.
ACHTUNGTRENNUNG(TRIS)
In the present study, we have synthesised a series of bis-
(TRIS) and tris
(TRIS) polyol ligands L1–L6 (shown here).
The syntheses of bis
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(TRIS) ligands L1–L3 were achieved by
using reported procedures,[24,25] involving the condensation
of tris(hydroxymethyl)aminomethane with appropriate di-
or tri-carboxylic acid esters in methanol at reflux tempera-
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7473

L. Cronin et al.
ported synthetic approaches for such molecules so far in-
volved the use of peptide coupling agents such as EEDQ, or
the use of activated aromatic carboxylic acid cores, which
necessitates the protection of TRIS moiety prior to amide
bond formation. However, the synthetic procedure reported
in this paper represents a relatively simple and straight for-
ward approach for such compounds.
The
Wells–Dawson
based
(OCH2)3CNHCO}
59A(OCH2)3CNHCO}2CH2]
59A(OCH2)3CNHCO}2A(CH2)2]
59A(OCH2)3CNHCO}2A(C5H3N)2]
59A(OCH2)3CNHCO}3A
59A(OCH2)3CNHCO}3A(C3N3)] (6) were
hybrid
2ACHTUNGTNER(NUNG CHOH)2]
clusters
TBA10H2[{P2V3W15O
TBA10H2[{P2V3W15O
TBA10H2[{P2V3W15O
TBA10H2[{P2V3W15O
TBA15H3[{P2V3W15O
TBA15H3[{P2V3W15O
59A
(1),
(2)
(3),
(4),
R
N
N
E
E
R
CHTUNGTRENN(UGN C6H3)] (5) and
H
CHTUNGTRENNUNG
Figure 1. X-ray crystal structure as well as the ESI-MS spectrum of the
synthesised in good yields by refluxing TBA5H4[P2V3W15O62]
with L1–L6, respectively, in dry acetonitrile. The compounds
thus obtained were characterised by elemental analyses, and
IR and NMR spectroscopic techniques. The analytical data
revealed that these hybrids contain V3-capped Wells–
[9]
hybrid polyoxometalate anion [{P2V3W15O
G
.
W:
cyan; V: yellow; P: purple; C: black; N: blue; O: red. This colour scheme
is continued in the diagrammatic non-X-ray pictures in Figure 2–6.
Dawson type clusters linked together by bis
ACHTUNGTREN(NUNG TRIS)/tris-
AHCTUNGTREG(NNUN TRIS) ligands as expected. Initial investigations into the
thermal stability of compounds 1–6 were carried out using
thermogravimetric analyses (TGA). The TGA profiles of
these hybrids were similar and showed that they are ther-
mally stable up to ꢀ2008C. At temperatures above 2008C,
the hybrids start to decompose, most probably due to the
loss of TRIS ligands and TBA counterions. The decomposi-
tion continues up until ꢀ5008C at which point a weight loss
of approximately 27% is observed, and this corresponds to
the loss of the organic ligand and TBA cations (see Support-
ing Information). The TGA profiles of compounds 1–6 are
comparable to those of similar hybrid compounds reported
before.[9] The electrochemical studies of compounds 1–6 re-
vealed that their redox behaviours are also quite compara-
ble to that of their parent hybrid cluster [H2NCACHTUNGTRENNUNG(CH2O)3
P2V3W15O59]6À, see Supporting Information for further de-
tails.
ESI-MS analyses of compounds 1–6, general observations:
Compounds 1–6 belong to the class of [P2V3W15O62]9À-based
“dumb-bell”
ACHTUNGTRENNUNG
hybrid
cluster
anions—[{P2V3W15O59-
tures. The synthesis of bipyridine based bis
N
(OCH2)3CNHCO}2]12À—the TBA salt of which (7) has been
and the tris
G
characterised by both single-crystal XRD and ESI-MS tech-
niques.[9] The hybrid cluster in 7, which is ꢀ3.4 nm long, ex-
hibited a clear ESI-MS spectrum as shown in Figure 1. All
of the major peaks in this spectrum could be assigned to the
ing the same synthetic procedure as that of L1–L3, starting
from corresponding di- or tri- carboxylic acid esters.
Organic molecules containing the TRIS triol moiety are
useful in diverse areas. In general, non-ionic polyol deriva-
tives such as tris(hydroxymethyl)amidomethane derivatives
are useful as sugar macronutrient substitutes, employed in
the formulation of low-calorie food products.[24] Molecules
containing TRIS tripodal alcohol moieties have been used
as ligands in the synthesis of single-molecule magnetic clus-
ters.[26] The compounds L5 and L6, containing three TRIS
moieties arranged around an aromatic platform, could be
synthetically important to develop highly branched ligands,
since such moieties may be used to anchor multiples of den-
drons in the production of arborol dendrimers.[27] The re-
(OCH2)3CNHCO}2]12À
parent cluster anion [{P2V3W15O59ACTHUNGTERNUNGN ,
with varying numbers of TBA, Na+ and H+ counterions.
Therefore, one can expect similar structural features for the
compounds 1–6, to those of 7, if their ESI-MS spectra are
comparable. Dilute solutions of compounds 1–6 in acetoni-
trile with concentrations in the range of 10À3–10À5 m were
used for the ESI-MS analyses in the negative ion mode. In-
terestingly, most of these compounds gave similar spectral
pattern as that of 7, showing distinct groups of peaks corre-
sponding to different charge states of the parent cluster
anions. The detection of such a collection of multiply
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Organic–Inorganic Hybrid Composites
FULL PAPER
charged ions of different charge states due to the attach-
ment of different number/type of counterions is quite
normal in the ESI-MS spectra of POMs.[17] As expected, the
majority of the peaks in a particular group follow a isotopic
distribution with a Gaussian shape, because of the presence
of many tungsten atoms in compounds 1–6. One other im-
portant observation of the ESI-MS spectra of compounds
1–6 is their simplicity, probably due to the non-fragmenta-
tion of the samples under the experimental conditions in
spite of their large size. Also, no major impurity peaks were
observed, and this gives some indication of the purity of the
samples (with respect to the presence of other intrinsically
charged POM species). In all six compounds reported here,
the observed experimental values are consistent, within the
experimental accuracy of Æ2 in the m/z range of 1000–3500.
Finally it has been observed that a number of peaks corre-
sponding to one- or two-electron reduced species were ob-
served in the ESI-MS spectra of most of the hybrids 1–6.
spectively (see Supporting Information, Figure S3a–d and
Table S3 for assignment of all the peaks in the spectrum).
Figures 3 and 4 show the ESI-MS spectra of compounds 2
and 3, respectively, in acetonitrile. Since the hybrid cluster
ESI-MS analysis of “dumb-bell”-shaped hybrids 1–4: Bis-
ACHTUNGTRENNUNG(TRIS)-ligand-based “dumb-bell” hybrids 1–3 show similar
spectral features in the mass range m/z ꢀ1400–3400. Com-
pound 1 shows three groups of peaks in the range 1600–
3400 with charges of 5À, 4À and 3À, while compounds 2 and
3 show four such groups of peaks with charges of 6À, 5À, 4À
and 3À in the range m/z 1400–3400.
Figure 3. The molecular diagram and the negative-ion ESI mass spectrum
(OCH2)3CNHCO}2CH2]12À
of cluster anion in compound 2, [{P2V3W15O59ACTHUNGTERNUNGN .
The spectrum was recorded in MeCN at a concentration ꢀ10À5 m. The
inset is an expansion of the peak centred at 1561.9 to show the 6À charge
state.
Figure 2 shows the ESI-MS spectrum of compound 1 in
acetonitrile. All the major peaks in the spectrum could be
satisfactorily assigned to the formula [{P2V3W15O59-
ACHTUNGTRENNUNG(OCH2)3CNHCO}2ACHTUNGTRENNUNG
(CHOH)2]12À with varying combinations
of TBA, H+ and Na+ counterions. For example, peaks ob-
anions differ by only one -CH2- unit in their organic linker
chains, the ESI-MS spectra of compounds 2 and 3 are very
similar in their appearance and peak positions. Unlike com-
pound 1, these compounds exhibit four distinct groups of
peaks corresponding to negatively charged species (6À, 5À,
4À and 3À) in the m/z region 1400–3500. In Figure 3, each
peak represents the cluster anion [{P2V3W15O59-
served at m/z values 3308.5, 2420.8 and 1931.8 correspond to
TBA7Na2[{P2V3W15O
TBA6Na2[{P2V3W15O
TBA6Na[{P2V3W15O
59A
59A(OCH2)3CNHCO}
59A(OCH2)3CNHCO}2A
2A
2A
(CHOH)2]4À and
(CHOH)2]5À,
CHTUNGTNERNUNG re-
G
CHTUNGTRENNUNG
U
AHCTUNGTRENNUNG
(OCH2)3CNHCO}2CH2]12À, with varying number of TBA,
H+ and Na+ counterions. For example, prominent peaks ob-
served at m/z values 3294.6, 2464.5, 1923.3 and 1561.9 corre-
spond to TBA7Na2[{P2V3W15O CHUTNGTRENNUNG
(OCH2)3CNHCO}2CH2]3À,
59A
(OCH2)3CNHCO}2CH2]4À,
59A
(OCH2)3CNHCO}2CH2]5À
59A
(OCH2)3CNHCO}2CH2]6À, respectively
TBA7Na[{P2V3W15O
TBA6Na[{P2V3W15O
TBA5Na[{P2V3W15O CTHUGNTRENNNUG
59ACHTUGNTRENNNUG
C
and
(see Supporting Information, Figure S4a–e and Table S4 for
assignment of all the peaks in the spectrum).
Similarly, all the peaks in the spectrum of compound 3
(Figure 4) could be satisfactorily assigned to the cluster
anion [{P2V3W15O
ple, peaks observed at m/z values 3296.9, 2467.9, 1925.7 and
(CH2)2]12À. For exam-
59ACHTUTGNRENNUG(OCH2)3CNHCO}2AHCTUNGTRENNGUN
1564.5
(OCH2)3CNHCO}
(OCH2)3CNHCO}
(OCH2)3CNHCO}
(OCH2)3CNHCO} HCTUNGTRENNUNG
correspond
2A
(CH2)2]3À,
2A
(CH2)2]4À,
2A
2A
(CH2)2]6À, respectively (see Supporting
to
TBA7Na2[{P2V3W15O59-
TBA7Na[{P2V3W15O59-
TBA6Na[{P2V3W15O59-
G
CHTUNGTRENNUNG
E
CHTUNGTRENNUNG
CHTUNGTRENNUNG
(CH2)2]5À and TBA5Na[{P2V3W15O59-
Figure 2. The molecular diagram and the negative-ion ESI mass spectrum
of cluster anion in compound 1, [{P2V3W15O59A(OCH2)3CNHCO}2-
N
CTHUNGTRENNUNG
N
ACHTUNGTRENNUNG
(CHOH)2]12À. The spectrum was recorded in MeCN at a concentration
ꢀ10À5 m. The inset is an expansion of the peak centred at 1931.8 to show
Information, Figure S5a–e and Table S5 for assignment of
all the peaks in the spectrum).
the 5À charge state.
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L. Cronin et al.
Figure 4. The molecular diagram and the negative-ion ESI mass spectrum
of cluster anion in compound 3, [{P2V3W15O59A(OCH2)3CNHCO}2-
CTHUNGTRENNUNG
ACHTUNGTRENNUNG
(CH2)2]12À. The spectrum was recorded in MeCN at a concentration
ꢀ10À5 m. The inset is an expansion of the peak centred at 1564.5 to show
the 6À charge state.
Figure 5. The molecular diagram and the negative-ion ESI mass spectrum
of cluster anion in compound 4, [{P2V3W15O59ACTHUNGTRENUNG(OCH2)3CNHCO}2-
“Dumb-bell” hybrid 4, containing a bipyridyl moiety in
the linker unit, exhibits a different spectral pattern com-
pared to compounds 1–3. This compound exhibits a clear
spectrum in the m/z range ꢀ1700–2700 showing two differ-
ent groups of peaks corresponding to 5À and 4À charge
states of the parent cluster anion. All of the peaks in these
groups could be satisfactorily assigned to the formula
AHCTUNGTRENNUNG
(C5H3N)2]12À. The spectrum was recorded in MeCN at a concentration
ꢀ10À5 m. The inset is an expansion of the peak centred at 1994.3 to show
the 5À charge state.
charged species such as 8À and 7À as major peaks in their
ESI-MS spectra. These two compounds showed four groups
of peaks in the m/z range ꢀ1700–3200 corresponding to 8À,
7À, 6À and 5À charge states of the respective cluster anions.
The ESI-MS spectrum of compound 5 in acetonitrile is
shown in Figure 6. Almost all the peaks in this spectrum
could be assigned to the formula [{P2V3W15O59-
(C5H3N)2]12À with various
[{P2V3W15O59ACHTUNGTRENNUNG(OCH2)3CNHCO}2ACHTUNGTRENNUGN
combinations of TBA, H+ and Na+ counter ions, see
Figure 5. Peaks observed at m/z 2554.4 and 1994.3 corre-
spond to TBA8[{P2V3W15O
U
(C5H3N)2]4À
2ACHTUNGTRENNUNG
and TBA7[{P2V3W15O59A(OCH2)3CNHCO} HCTUNGTRENNNUG
G
(C5H3N)2]5À, re-
spectively (see Supporting Information, Figure S6a–c and
Table S6 for assignment of all the peaks in the spectrum).
On comparing the ESI-MS spectra of “dumb-bell” hybrids
1–4, one could observe slight differences in their spectral
features such as peak shape, position, charge states and so
forth. The major reason for these observed differences is the
presence/absence of different functionalities such as alcohol,
(C6H3)]18À, with a varying number of
ACHTUNGTREN(NUG OCH2)3CNHCO}3ACHUTNGTRENNUGN
TBA, H+ and Na+ counterions. Examples being the peaks
observed at m/z values 3042.3, 2495.2, 2104.6 and 1773.5 cor-
responding to TBA12Na2[{P2V3W15O CHUTTGNERN(NUNG OCH2)3CNHCO}3-
59A
59A(OCH2)3CNHCO}3-
59A(OCH2)3CNHCO}3-
59A(OCH2)3CNHCO}3-
ACHTUNGTRENNUNG
(C6H3)]8À, respectively (see Supporting Information, Fig-
A
TBA11Na2[{P2V3W15O
CHTUNGTRENNUNG
A
TBA10Na2[{P2V3W15O CHTUGNTRENNUNG
(C6H3)]7À and TBA8H2[{P2V3W15O
ACHUTNGRENNUG HCTUNGTRENNUNG
bipyridyl and so forth on their bis
N
of course each set of compounds can be precisely modelled
using an isotopic molecular-weight calculator. Our detailed
studies have also shown that the differences in the nature of
the organic linker units can also affect the self-assembly be-
haviour of [P2V3W15O62]9À-based “dumb-bell” hybrids in so-
lution.[19] However, despite the small differences in spectral
features, the ESI-MS technique was highly successful in
characterising these huge “dumb-bell” hybrids 1–4 unambig-
uously.
ure S7a–e and Table S7 for assignment of all the peaks in
the spectrum).
The ESI-MS spectrum of compound 6 in acetonitrile is
shown in Figure 7. The peaks could be satisfactorily assigned
(C3N3)]18À cluster ion
to the [{P2V3W15O59ACTHNUTRGENN(UG OCH2)3CNHCO}3CAHTUNGTRENNGUN
with varying combinations of TBA, H+ and Na+ counter-
ions. Prominent peaks observed at m/z values 3043.3,
2452.8,
TBA12Na2[{P2V3W15O
TBA10NaH[{P2V3W15O
TBA9H2[{P2V3W15O
TBA8H2[{P2V3W15O
2062.2
and
59A(OCH2)3CNHCO}
59A(OCH2)3CNHCO}
(OCH2)3CNHCO}3A
(C3N3)]7À
59A(OCH2)3CNHCO}3A
(C3N3)]8À,
1774.9
correspond
(C3N3)]5À,
3A
(C3N3)]6À,
to
G
3ACHTUNGTRENNUNG
E
CHTUNGTRENNUNG
ESI-MS analysis of “triangular” hybrids 5 and 6: Tris-
(TRIS)-ligand-based “triangular” hybrids 5 and 6 not only
R
N
and
E
respec-
exhibited similar spectral features to those of some of the
“dumb-bell” hybrids 1-4, but also exhibited some highly
tively (see Supporting Information, Figure S8a–e and
Table S8 for assignment of all the peaks in the spectrum).
7476
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Chem. Eur. J. 2011, 17, 7472 – 7479

Organic–Inorganic Hybrid Composites
FULL PAPER
Figure 7. The molecular diagram and the negative-ion ESI mass spectrum
Figure 6. The molecular diagram and the negative-ion ESI mass spectrum
of cluster anion in compound 6, [{P2V3W15O59ACTHUNGTRENUNG(OCH2)3CNHCO}3-
AHCTUNGTRENNUNG
(C3N3)]18À. The spectrum was recorded in MeCN at a concentration
of cluster anion in compound 5, [{P2V3W15O59ACTHUNGTRENUNG(OCH2)3CNHCO}3-
ACHTUNGTRENNUNG
(C6H3)]18À. The spectrum was recorded in MeCN at a concentration
ꢀ10À5 m. The inset is an expansion of the peak centred at 1774.9 to show
ꢀ10À5 m. The inset is an expansion of the peak centred at 1773.5 to show
the 8À charge state.
the 8À charge state.
As mentioned earlier, the overall appearance of the spec-
tra of “triangular” hybrids 5 and 6 is quite comparable to
that of some of the “dumb-bell” hybrids 1–4. However, one
major difference between the spectra of both these classes
of hybrids is the presence of some higher charged species
such as 8À and 7À in the spectra of hybrids 5 and 6. One pos-
sible reason for this could be the inherently high charge
state of the cluster anions (18À) in 5 and 6 due to the pres-
ence of three [P2V3W15O62]9À clusters in them.
The ESI-MS spectra of the majority of these compounds in
the negative ion mode were clean without excessive frag-
mentation peaks, and the samples tend to exhibit groups of
peaks corresponding to different anionic charge states of the
hybrid cluster anions, ranging from 3À to 8À. This study
therefore shows that ESI-MS is a reliable technique for the
characterisation of huge nanometer-sized hybrid clusters
with relatively large masses. In further work we will present
new types of functional hybrids that exploit the design and
characterisation approach described here, in addition to
studies on the self-assembly and amphiphilic surfactant
properties in solution.
Conclusion
A series of tris(hydroxymethyl)aminomethane (TRIS)-based
linear (bisACHTUNGTRENNUNG(TRIS)) and triangular (trisACHUTNGTREN(NGUN TRIS)) polyol ligands
Experimental Section
has been synthesised and were successfully grafted on to
Wells–Dawson type cluster, [P2V3W15O62]9À to generate a
series of nanometer-sized inorganic–organic hybrid polyoxo-
metalate clusters. These hybrids containing two or three
[P2V3W15O62]9À cluster units are complementary to the previ-
ous reports of similar hybrids containing one, two and four
such cluster units. The synthesis of these hybrids shows that
a large amount of structural design is possible in the synthe-
sis of covalently functionalised hybrid POM clusters. High-
resolution ESI-MS spectrometry was successfully employed
for the unambiguous characterisation of these nanometer-
sized hybrids, that have masses of the order of 10–16 KDa.
General: All reagents and chemicals including deuterated solvents were
purchased from Aldrich-Sigma Company and were used without further
purification. NMR spectra were recorded on a Bruker DPX 400 spec-
trometer at room temperature (University of Glasgow). Elemental Anal-
yses were completed by using an Elemental Analyser MOD 1106 at Uni-
versity of Glasgow. IR spectra were measured using a JASCO FTIR 410
spectrometer. ESI-MS measurements were carried out at 1808C. The sol-
utions of the samples were diluted so that the maximum concentration of
the cluster ions was of the order of 10À5 m, and these were infused at a
flow rate of 180 mLhÀ1. The mass spectrometer used for the measure-
ments was Bruker micro TOF-Q, and the data were collected in negative
mode. The spectrometer was previously calibrated with the standard tune
mix to give a precision of about 1.5 ppm in the region of 500–5000 m/z.
Chem. Eur. J. 2011, 17, 7472 – 7479
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7477

L. Cronin et al.
The standard parameters for a medium mass data acquisition were used.
Typical working conditions: the end plate voltage was set to À500 V and
the capillary to +4500 V; the collision cell was set to collision energy
À8.0 eVzÀ1 with a maximum gas flow rate of 4.0 LhÀ1. Acetonitrile
(Merck) containing the electrolyte Bu4NBF4 (Fluka of Puriss Grade) was
used for all voltammetric studies. Solutions used for electrochemical stud-
ies were degassed with argon for at least 10 min to remove dioxygen.
C172H380N12O128P4V6W30: C 19.47, H 3.61, N 1.58; found: C 19.66, H 3.77,
N 1.60.
Synthesis of 2: The synthetic procedure was the same as that used for 1
except that L2 was used instead of L1. Yield 0.94 g (0.09 mmol, 92%,
based on TBA5H4[P2V3W15O62]); 1H NMR (400 MHz, CD3CN): d=3.129
(s, 2H; CH2C), 5.72 (s, 12H; -CH2O), 6.87 ppm (s, 2H; NH), in addition
to the TBA resonances; FT-IR (KBr): n˜ =3452.7 (w), 2962.7 (m), 2935.7
(m), 2874.0 (m), 1631.8 (w), 1562.3 (w), 1465.9(m), 1381.0 (w), 1084.0 (s),
949.0 (s), 902.7 (s), 790.8 (s), 717.5 cmÀ1 (s); elemental analysis calcd (%)
for C171H378N12O126P4V6W30: C 19.44, H 3.61, N 1.59; found: C 19.24, H
3.65, N 1.55.
Electrochemical experiments were undertaken at (19Æ1)8C with
a
Model Versastat 4 electro analysis system by Princeton Applied Research
using a standard three-electrode configuration. The working electrode
used for cyclic voltammetry was glassy carbon (1.5 mm diameter, BAS).
The auxiliary electrode was a Pt mesh. The reference electrode consisted
of a silver wire dipped into CH3CN (electrolyte) but separated from the
test solution by a porous frit. The glassy carbon working electrode was
polished with alumina (3 mm) on polishing pads, rinsed with distilled
water and sonicated in acetone solution before each experiment.
Synthesis of 3: The synthetic procedure was the same as that used for 1
except that L3 was used instead of L1. Yield 0.92 g (0.09 mmol, 90%,
based on TBA5H4[P2V3W15O62]); 1H NMR (400 MHz, CD3CN): d=6.31
(s, 2H; NH), 5.72 (s, 12H; -CH2O), 2.43 ppm (s, 4H,-CH2-CO-), in addi-
tion to the TBA resonances. FT-IR: n˜ =3452.7 (w), 2958.9 (m), 2931.9
(m), 2874.0 (m), 1678.1 (w), 1608.6 (m) 1462.0 (m), 1377.2 (w)1157.3 (w),
1084.0 (s), 949.0 (s), 902.7 (s), 790.8 (s), 709.8 cmÀ1 (s); elemental analysis
calcd (%) for C172H380N12O126P4V6W30: C 19.53, H 3.62, N 1.59; found:
C19.51, H 3.64, N 1.56.
Synthesis of ligands: Ligands L1, L2 and L3 were synthesised according to
published procedures.[24,25] L5 was synthesised using synthetic procedure
similar to that of L1–L3 by refluxing triethyl 1,3,5-benzenetricarboxylate
(294 mg, 1 mmol) with tris(hydroxymethyl)aminomethane (363 mg,
3 mmol) in methanol (40 mL) for 6 days. The desired product, which
came out of the mother liquor as a white precipitate, was collected by fil-
tering the mother liquor while hot and recrystallised from a mixture of
EtOH/H2O. Yield: 0.2 g, 38%. The formation of the product was con-
firmed by comparing the analytical data with the literature report.[28]
Synthesis of L4: Diethyl 2,2’-bipyridine-4,4’-dicarboxylate was prepared
according to the published procedures.[29] A mixture of diethyl 2, 2’-bipyr-
idine-4, 4’-dicarboxylate (0.15 g, 0.5 mmol) and (HOCH2)3CNH2 (0.18 g,
1.5 mmol) in methanol (50 mL) was refluxed for 48 h. A white precipitate
was isolated by filtration from hot solution and dried under vacuum.
Yield 0.10 g (0.22 mmol, 44%); m. p. 144–1458C; 1H NMR (400 MHz,
[D6]DMSO): d=3.74 (d, J=5.6 Hz, 12H; -CH2OH), 4.73 (t, J=5.6 Hz,
6H; -OH), 7.72 (s, 2H; -NH-), 7.80 (d, J=8.4 Hz, 2H; Ar-H), 8.73 (s,
2H; Ar-H), 8.86 ppm (d, J=8.4 Hz, 2H; Ar-H); IR (KBr, 400–
4000 cmÀ1): n˜ =3289 (s), 2940 (m), 1645 (m), 1590 (w), 1550 (m), 1461
(m), 1397 (w), 1364 (w), 1308 (w), 1288 (w), 1243 (w), 1041 (s), 1022 (s),
874 (w), 788 (w), 754 (w), 701 (w), 629 cmÀ1 (m); elemental analysis calcd
(%) for C20H26N4O8: C 53.33, H 5.82, N 12.44; found: C 53.40, H 5.84, N
12.53.
Synthesis of L6: Ligand L6 was synthesised by using a synthetic procedure
similar to that used for L5 by refluxing triethyl 1,3,5-triazine-2,4,6-tricar-
boxylate (297 mg, 1 mmol) with tris(hydroxymethyl)aminomethane
(363 mg, 3 mmol) in methanol (40 mL) for 3 days. The desired product,
which came out of the mother liquor as a white precipitate, was collected
by filtering the mother liquor while hot and recrystallised from a mixture
of EtOH/H2O. Yield: 0.31 g (59%). 1H NMR (400 MHz, [D6]DMSO):
d=3.74 (d, J=5.6 Hz, 12H; -CH2OH), 4.86 (t, J=5.6 Hz, 6H; -OH),
8.39 ppm (s, 3H; -NH-); IR (KBr): n˜ =3344.6 (m), 2947.3 (w), 2889.4 (w)
1681.9 (s), 1523.8 (s), 1465.9 (m), 1419.6 (m) 1357.9 (m), 1253.7 (m),
1114.8 (m) 1037.74(s) 1014.5 (s), 891.1 (w), 798.5 (w), 744.5 (m), 628.8 (s)
cmÀ1; elemental analysis calcd (%) for C18H30N6O12: C 41.38, H 5.79, N
16.09; found: C 40.91, H 5.80, N 15.95.
Synthesis of 4: The synthetic procedure was the same as that used for 1
except that L4 was used instead of L1 and the solution was heated to
reflux for 9 days. The yellow precipitate was isolated by filtration and
dried overnight under vacuum. Yield 0.95 g (0.09 mmol, 91%, based on
TBA5H4[P2V3W15O62]; 1H NMR (400 MHz, CD3CN): d=9.16 (d, J=
8.4 Hz, 2H; Ar), 8.52 (m, 2H; Ar), 8.42 (d, J=8.4 Hz, 2H; Ar), 7.10 (s,
2H; NH), 5.89 ppm (s, 12H; -CH2O), in addition to the TBA resonances;
FT-IR (KBr): n˜ =3423 (w), 2961 (m), 2933 (m), 2877 (m), 1655 (w), 1557
(w), 1544 (m), 1482 (m), 1379 (w), 1086 (s), 953 (s), 913 (s), 813 (s), 735
(s), 528 (w), 471 cmÀ1 (w); elemental analysis calc (%) for
C180H382N14O126P4V6W30: C 20.20, H 3.60, N 1.83; found: C 20.00, H 3.55,
N 2.13.
Synthesis of 5: The synthetic procedure was the same as that used for 1
except that L5 was used instead of L1 in a 1:3 mole ratio. Yield 0.95 g
(0.06 mmol, 92%, based on TBA5H4[P2V3W15O62]); 1H NMR (400 MHz,
CD3CN): d=8.41 (s, 3H; Ar), 6.98 (s, 3H; NH), 5.91 ppm (s, 18H;
-CH2O), in addition to the TBA resonances; FT-IR (KBr): n˜ =3348.5
(w), 2962.7 (m), 2935.7 (m), 2874.0 (m), 1662.6 (w), 1481.3(m), 1381.0(w),
1265.3 (w), 1084.0 (s), 949.0 (s), 902.7 (s), 786.9 (s), 725.2 cmÀ1 (s); ele-
mental analysis calcd (%) for C261H567N18O189P6V9W45: C 19.72, H 3.59, N
1.59; found: C19.27, H 3.67, N 1.61.
Synthesis of 6: The synthetic procedure was the same as that used for 1
except that L6 was used instead of L1 in a 1:3 mole ratio. Yield 0.96 g
(0.06 mmol, 93%, based on TBA5H4[P2V3W15O62]); 1H NMR (400 MHz,
CD3CN): d=8.11 (s, 3H; NH), 5.92 ppm (s, 18H; -CH2O), in addition to
the TBA resonances; FT-IR: n˜ =3464.2 (w), 2962.7 (m), 2935.7 (m),
2874.0 (m), 1631.8 (w), 1519.9 (w), 1481.3 (m), 1381.0 (w), 1084.0 (s),
949.0 (s), 902.7 (s), 790.8 (s), 725.2 cmÀ1 (s); elemental analysis calcd (%)
for C258H564N21O189P6V9W45: C 19.49, H 3.57, N 1.85; found: C19.71, H
3.68, N 1.92.
Synthesis of hybrid POMs
Synthesis of 1: TBA5H4[P2V3W15O62][30] (1.0 g, 0.2 mmol) was dissolved in
MeCN (30 mL), then L1 (0.030 g, 0.1 mmol) was added to the solution.
The reaction mixture was heated to reflux for 6 days in dark. The result-
ing yellow solution was added drop-wise to excess of diethyl ether with
vigorous stirring. The resulting yellow solid was collected and re-dis-
solved in minimum volume of MeCN, then re-precipitated by adding
dropwise to excess of diethyl ether. The yellow precipitate thus obtained
was isolated by filtration and recrystallised from acetonitrile by ether dif-
fusion. Yield 0.92 g (0.09 mmol, 89%, based on TBA5H4[P2V3W15O62]);
1H NMR (400 MHz, CD3CN): d=4.07 (d, J=6 Hz, 2H), 4.38 (d, J=
6 Hz, 2H), 5.78 (s, 12H; -CH2-), 6.82 ppm (s, 2H; -NH-) in addition to
the TBA resonances; FT-IR (KBr): n˜ =3356.2 (w), 2962.7 (w), 2935.7
(w), 2874.0 (w), 1662.6 (m), 1462.09 (m), 1381.0 (w), 1084.0 (s), 949.0 (s),
902.7 (s), 790.84 (s), 709.83 cmÀ1 (s); elemental analysis calcd (%) for
Acknowledgements
We thank the EPSRC, WestCHEM and the China Scholarship Council
for supporting this work. We also thank Bruker Daltonics for collabora-
tion using the microTOFQ. L.C. thanks the Royal Society and Wolfson
foundation for a merit award. H.M. thanks the Royal Society of Edin-
burgh and Marie Curie actions for the financial support.
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Received: January 24, 2011
Published online: May 18, 2011
Chem. Eur. J. 2011, 17, 7472 – 7479
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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