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Structural Control on the Nanomolecular Scale Self-Assembly of the Polyoxotungstate Wheel [{-Ti2SiW10O39}4]24.

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Angewandte
Chemie
Polyoxometalates
Structural Control on the Nanomolecular Scale:
Self-Assembly of the Polyoxotungstate Wheel
[{b-Ti2SiW10O39}4]24**
Firasat Hussain, Bassem S. Bassil, Li-Hua Bi,
Markus Reicke, and Ulrich Kortz*
Polyoxometalates are metal?oxygen clusters with an enormous structural variety combined with very interesting
properties.[1?6] This class of inorganic compounds has been
known for a long time, but the rate of discovery of novel
species is currently faster than ever before.[7, 8] Nevertheless,
the mechanism of formation of polyoxoanions is not well
understood and commonly described as self-assembly. The
search for novel polyanions is predominantly driven by the
catalytic properties of many transition-metal substituted
polyoxotungstate salts.[9] The catalytic activity (e.g. activation
of O2 and H2O2) combined with high thermal stability have
led to industrial applications of these species, mostly as
heterogeneous catalysts in the oxidation of organic substrates
(e.g. Wacker process).[3?6]
The redox-activity of titanium in the oxidation state + 4
has led to numerous catalytic studies using TiO2 as a
photocatalyst.[10] To date only a few titanium(iv)-substituted
polyoxoanions have been synthesized and most of them are of
the Keggin-type. Structural characterization in the solid state
indicates a strong tendency towards dimer formation via Ti
OTi bonds as seen in [(a-Ti3PW9O38.5)2]12, [(xTi3SiW9O38.5)2]14 (x = a, b), [(a-Ti3GeW9O38.5)2]14, and [(a1,2-Ti2PW10O39)2]10.[11] Nevertheless two monomeric species
have also been structurally characterized.[12] Some of the
above species have shown interesting catalytic (e.g. photocatalysis) as well as medicinal activity (e.g. antiviral).[13, 14]
Recently we have structurally characterized some dimeric
and tetrameric titanium-substituted polyoxotungstates based
on the Wells?Dawson fragment.[15] Soon afterwards Nomiya
et al. reported similar tetrameric species.[16]
Our group has been working extensively on the interaction of the dilacunary tungstosilicate [g-SiW10O36]8 with
low-valent, first-row transition metals.[17] Herein we investigate the system Ti4+/[g-SiW10O36]8 in some detail.
Interaction of solid TiO(SO4) with [g-SiW10O36]8 in the
ratio 2:1 in aqueous acidic medium (pH 2) resulted in the
novel, tetrameric [{b-Ti2SiW10O39}4]24 (1; Figure 1 and
Figure 2). The polyanion 1 is the first cyclic, tetrameric
[*] F. Hussain, B. S. Bassil, Dr. L.-H. Bi, M. Reicke, Prof. U. Kortz
International University Bremen
School of Engineering and Science
P.O. Box 750 561, 28725 Bremen (Germany)
Fax: (+ 49) 421-200-3229
E-mail: u.kortz@iu-bremen.de
[**] U.K. thanks the International University Bremen for research
support. U.K. also highly appreciates that the Florida State
University Chemistry Department (USA) allowed him unlimited
access to the single-crystal X-ray diffractometer. Figures 1?4 were
generated by Diamond Version 2.1e (copyright Crystal Impact GbR).
Angew. Chem. Int. Ed. 2004, 43, 3485 ?3488
DOI: 10.1002/anie.200454203
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3485
Communications
as ?wheel caps? (Figure 3). The central ion K8 (which is
disordered over two positions) is seven coordinate (KиииO =
2.91?3.25(3) D), namely by six oxo groups of the tungsten-oxo
framework of 1 and one water molecule. The two equivalent,
Figure 1. Polyhedral representation of [{b-Ti2SiW10O39}4]24 (1).
Red = WO6, green = TiO6, and blue = SiO4.
Figure 3. Side-view of 1 including the potassium ions (purple) inside
the central cavity (K8) and above and below the cavity (K9, and K9?).
Figure 2. Ball-and-stick representation of 1 (thermal ellipsoids are set
at 50 % probability). All the heavy atoms of the asymmetric unit are
labeled.
polyoxotungstate and it is also the largest titanium-substituted tungstosilicate known to date.[18] Bond valence sum
calculations (BVS) indicate that 1 is not protonated and
therefore its charge must be 24.[19] This result is fully
consistent with elemental analysis, which indicated the
presence of 24 potassium ions. Owing to disorder we could
identify only 17 potassium counterions by X-ray diffraction.
The central cavity of the wheel-shaped 1 is occupied by a
potassium ion (K8) and two additional K+ ions (K9, K9?) act
3486
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
external cations K9 and K9? are nine coordinate (KиииO =
2.66?3.22(3) D), by seven water molecules and two terminal
oxo groups of tungsten atoms. The remaining potassium ions
are distributed around 1 being coordinated to bridging and
terminal oxo-groups of 1 as well as water molecules of
hydration. The bond lengths of the central K8 ion (see above)
are somewhat longer than expected, but we discovered that in
the presence of Rb+ and K+ ions the title polyanion still
crystallizes exclusively as a potassium salt. Therefore we
believe that potassium ions exhibit a template effect during
the formation of 1. Furthermore, the selective binding of an
alkali-earth ion by the macrocyclic polyanion 1 is reminiscent
of crown ether properties.
The solid-state structure of K24[{b-Ti2SiW10O39}4]и50 H2O
(K24-1) is composed of polyanions 1 that have self-assembled
face-by-face along the crystallographic a axis, which leads to a
nanotube-like arrangement (Figure 4).
Polyanion 1 is composed of four {b-Ti2SiW10O39} Keggin
fragments that are linked via TiOTi bridges leading to a
cyclic assembly. Close inspection of 1 indicates that the four
Keggin fragments are of the beta-type, which is rare and the
first structurally characterized compound containing such a
dilacunary tungstosilicate fragment was the dimeric, nickelsubstituted polyanion [{b-Ni2SiW10O36(OH)2(H2O)}2]12.[17c]
Therefore 1 is only the second example composed of a bdecatungstosilicate Keggin fragment and it represents the first
tetrameric derivative.
The two titanium atoms of each beta-Keggin fragment are
not located in the same M3O13 triad. In fact they are separated
by four bonds (Ti2-O-W3-O-Ti1 or Ti2-O-W8-O-Ti1, see
Figure 2) and one of the two Ti atoms is located in the rotated
triad (Ti2 and Ti4, see Figure 2). Based on the IUPAC
nomenclature the two titanium atoms are located in positions
1 and 10 (the position 10 being in the rotated triad).[4, 20] This
arrangement does not allow formation of a dimeric product,
which is usually favored in titanium-polyoxotungstate chemistry.[11, 15] In 1, each of the four Keggin fragments has a mirror
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Angew. Chem. Int. Ed. 2004, 43, 3485 ?3488
Angewandte
Chemie
Figure 4. View along the crystallographic a axis showing three adjacent
wheels of 1 and the resulting tunnel-like cavity. Cations and lattice
water molecules are not shown for clarity.
plane and both titanium atoms are located in this plane of
symmetry (see Figure 1 and Figure 2). For comparison, the
nickel(ii) centers in [{b-Ni2SiW10O36(OH)2(H2O)}2]12 are in
the 4 and 10 positions.[17c]
The structure of polyanion 1 can be viewed as a larger
titanium-derivative of the trimeric, cyclic manganese(ii)substituted tungstosilicate [(b2-SiW11MnO38OH)3]15. This
product was also synthesized from [g-SiW10O36]8 and it also
contains b-Keggin fragments.[17a] However, the important
difference to 1 is that its basic building block is a monosubstituted (b-SiW11O39) Keggin-unit and as a result the Keggin?
Keggin connectivities are accomplished via MnOW bonds.
Compound 1 is not perfectly cyclic, but somewhat
ellipsoidal (see Figure 1 and Figure 2). This shape reflects
the inequivalence of the two titanium centers within each
Keggin fragment. In 1, the short axis (distance between O3Ti
and O3Ti?, see Figure 2) is 10.8 D, whereas the long axis is
13.0 D (distance between O1TT and O1TT?). The reason for
this distortion is that 1 contains two pairs of inequivalent TiO-Ti bridges: the type which involves two Ti centers in the
rotated triad (Ti2-O1TT-Ti4, Ti2?-O1TT?-Ti4?, see Figure 2)
and the type which involves two Ti centers that are not in the
rotated triad (Ti3-O3Ti-Ti3?, Ti1-O3Ti-Ti1?). The two types of
Ti-O-Ti bond angles are only marginally different in the solid
state: 152.8(12)8 (Ti3-O3Ti-Ti3?) versus 153.9(12)8 (Ti2O1TT-Ti4). All of the above indicates that 1 is best described
as a dimer of dimers.
Synthesis of 1 is accomplished by reaction of TiO(SO4)
with [g-SiW10O36]8, which means that the mechanism of
formation of 1 must occur in the following sequence: a) metal
insertion, b) rotational isomerization (g-Keggin!b-Keggin),
c) dimerization, and d) ring closure. Most likely, insertion of
two TiIV ions into [g-SiW10O36]8 leads to the monomeric
species [g-Ti2(OH)2SiW10O38]6, which isomerizes to [b(1,10)Ti2(OH)2SiW10O38]6. This species is also unstable and
dimerizes to {[b(1,10)-Ti2(OH)SiW10O38]2(O)}12, which is
equivalent to the asymmetric unit of 1. In most structurally
characterized titanium-substituted polyoxometalates the titaAngew. Chem. Int. Ed. 2004, 43, 3485 ?3488
nium atoms do not contain terminal bonds. Therefore it is not
surprising that {[b(1,10)-Ti2(OH)SiW10O38]2(O)}12 is a highly
reactive, dimeric species which reacts with other, identical
dimers leading to the formation of 1. This ?dimer-of-dimer
mechanism? excludes the possibility of a gradual growth by
individual Keggin units and it also implies that a trimeric
intermediate is not present during formation of 1. It is
virtually impossible to prove the above hypothesis and the
identity of the proposed intermediates experimentally, as
formation of 1 occurs by rapid self-assembly as soon as the
proper reaction conditions are present.
In the 183W NMR spectrum of 1 (at 16.66 MHz on a
400 MHz JEOL ECX instrument at room temperature using
D2O as a solvent) we identified 10 signals of about equal
intensity (d = 111.4, 118.0, 122.8, 125.7, 141.4,
156.2, 157.8, 170.6, 173.8, 221.0 ppm). The solidstate structure of polyanion 1 indicates Ci point-group
symmetry and as a result the asymmetric unit consists of
20 tungsten atoms (see Figure 2). However, it is expected that
in solution the apparent symmetry of 1 is C2. In this case,
polyanion 1 contains 10 magnetically inequivalent tungsten
groups (W1/W2, W3/W8, W4/W7, W5/W6, W9/W10, W11/
W12, W13/W18, W14/W17, W15/W16, W19/W20, and their
symmetry equivalents, see Figure 2). Therefore 10 signals of
equal intensity are expected for 183W NMR spectrum of 1 in
solution. Indeed, the 183W NMR spectrum of redissolved K24-1
in D2O shows a 10-line spectrum (see above), but the 183W
NMR spectrum of freshly synthesized 1 at very high concentration contains some additional signals of much lower
intensity which we cannot assign yet. Nevertheless, any solid
which precipitated or crystallized from this solution gave an
IR spectrum identical to that of K24-1.
In summary, we have synthesized a unique tetrameric and
cyclic tungstosilicate assembly using mild, one-pot reaction
conditions. Polyanion 1 was fully characterized in solution and
in the solid state by several analytical techniques. Formation
of 1 indicates that a) very large and cyclic TiIV-substituted
tungstosilicates can be formed, b) it might be possible to
construct even larger wheel-shaped polyoxotungstates, c) it
might be possible to construct inorganic nanotubes by linking
or orientating individual polyoxoanion wheels appropriately,
d) the gigantic, mixed-valence polyoxomolybdate wheels
have some smaller, fully oxidized polyoxotungstate analogues, e) the structural variety of supra- and supersupramolecular polyoxotungstates is just beginning to be explored,
and finally that f) undoubtedly no other class of inorganic
compounds allows for preparation of discrete, nanomolecular
objects of similar size, structure, and function as those made
with polyoxometalates.
Currently we are investigating the electro- and photochemical, electrocatalytic, and oxidation catalysis properties
of 1 and this work will be published elsewhere.
Experimental Section
K24[{b-Ti2SiW10O39}4]и50 H2O (K24-1): A sample of K8[g-SiW10O36]
(2.23 g, 0.75 mmol; synthesized according to ref. [21]) was added with
stirring to a solution of TiO(SO4) (0.26 g, 1.65 mmol; Merck) in H2O
(40 mL). The pH value was adjusted to 2 by addition of 4 m HCl. This
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3487
Communications
solution was heated to 80 8C for 1 h and then cooled to room
temperature and filtered. Slow evaporation of the filtrate at room
temperature resulted in a white, crystalline product after 2?3 weeks
that was collected by filtration and air-dried. Yield: 1.39 g (61 %); IR:
1000(w), 966(m), 913(s), 803(s), 657(m), 541(w), 516(w), 487(w),
467(w) cm1; elemental analysis calcd (%) for K24-1: K 7.7, W 60.4, Ti
3.1, Si 0.9; found: K 7.3, W 61.6, Ti 2.8, Si 1.2. Elemental analysis was
performed by Kanti Labs Ltd. in Mississauga, Canada.
[13]
[14]
Received: March 6, 2004 [Z54203]
[15]
.
Keywords: cluster compounds и polyoxometalates и
self-assembly и titanium и tungsten
[16]
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[6] Polyoxometalate Chemistry for Nano-Composite Design (Eds.:
T. Yamase, M. T. Pope), Kluwer, Dordrecht, 2002.
[7] J. Berzelius, Pogg. Ann. 1826, 6, 369.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[19]
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Crystal data for K24[{b-Ti2SiW10O39}4]и50 H2O (K24-1): A colorless block of K24-1 with dimensions 0.06 R 0.10 R 0.10 mm3 was
mounted on a glass fiber for indexing and intensity data
collection at 173 K on a Bruker D8 SMART APEX CCD
single-crystal diffractometer using MoKa radiation (l =
0.71073 D). Of the 23 877 unique reflections (2qmax = 56.648),
19 910 reflections (Rint = 0.093) were considered observed (I >
2s(I)). Direct methods were used to solve the structure and to
locate the tungsten and titanium atoms (SHELXS 97). Then the
remaining atoms were found from successive difference maps
(SHELXL 97). The final cycle of refinement, including the
atomic coordinates, anisotropic thermal parameters (W, Ti, K,
and Si atoms) and isotropic thermal parameters (O atoms)
converged at R = 0.104 and Rw = 0.201 (I > 2s(I)). In the final
difference map the deepest hole was 2.501 e D3 and the
highest peak 5.731 e D3. Routine Lorentz and polarization
corrections were applied and an absorption correction was
performed using the SADABS program (G. M. Sheldrick,
Siemens Analytical X-ray Instrument Division, Madison, WI,
1995). Further details of the crystal structure investigation may
be obtained from the Fachinformationszentrum Karlsruhe, D76344 Eggenstein-Leopoldshafen, Germany (Fax: (+ 49) 7247808-666; E-mail: crysdata@fiz-karlsruhe.de) on quoting the
depository number CSD-413815.
I. D. Brown, D. Altermatt, Acta Crystallogr. Sect. B 1985, 41,
244 ? 247.
Y. Jeannin, M. Fournier, Pure Appl. Chem. 1987, 59, 1529 ? 1548.
A. TQzQ, G. HervQ, Inorganic Syntheses, 1990, 27, 88 ? 89.
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Angew. Chem. Int. Ed. 2004, 43, 3485 ?3488
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