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Synthesis and Structural Characterization of Redox-Active Dodecamethoxoheptaoxohexavanadium Clusters.

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Angewandte
Chemie
Mixed-Valent Cluster Compounds
Synthesis and Structural Characterization of
Redox-Active Dodecamethoxoheptaoxohexavanadium Clusters
Johann Spandl, Charles Daniel, Irene Brdgam, and
Hans Hartl*
The chemical behavior of alkoxo-oxovanadium clusters lies
between that of metal oxides and metal alkoxides.[1, 2] These
compounds are important as precursors for the synthesis of
new polyoxometalates and as reagents for sol–gel processes
and chemical vapor deposition (CVD) methods for the
generation of metal oxides.[3] The interest in this substance
class for basic research as well as for technical applications is
attributed to vanadium's facile and often reversible change in
its oxidation state.[4] By varying the alkoxo ligands and the
VIV/VV ratio in such clusters, it is possible to control the
properties and the composition of the metal oxides obtained
by their thermal decomposition, which is of particular
importance, for example, for the development of heterogeneous catalysts based on vanadium oxides.
It is remarkable that up to now, [NnBu4][V6O12(OCH3)7][5]
and [V6O7(OC2H5)12][6] were the only two examples of
structurally characterized alkoxo-oxovandium clusters that
contain no further substituents other than monodentate
alkoxo ligands. In all other cases, stabilization is achieved
either by multidentate alkoxo ligands[2b] or by complementary
ligands such as squarate,[7] oxalate, acetylacetonate, or
phenylphosphonate[2b] as well as metal[8] and organometallic[9]
complexes.
Recently we synthesized a series of dodecamethoxohepV
(4 n)
taoxohexavanadium compounds [VIV
n V6 nO7(OCH3)12]
IV
V
which, while differing in their V /V ratios (n = 3, 4, 5, 6),
remain identical in their structural composition. Noteworthy
is the fact that these compounds were not only observed as
intermediates by electrochemical and spectroscopical methods, but have also been isolated and characterized by X-ray
structural analysis.
Besides vanadium oxides and vanadates,[1] polyoxovanadates containing large organic cations,[10] and thus soluble in
organic solvents, as well as orthovanadic acid esters VO(OR)3
of primary alcohols have also proven to be suitable precursors
for the synthesis of alkoxo-oxovanadium compounds.[7, 11] The
V
uncharged coumpound of mixed valency [VIV
4 V2 O7(OCH3)12]
(1; Figure 1) for example, is formed under solvothermal
conditions by reaction of tert-butyl orthovanadate
[VO(OtBu)3] with methanol. The structure of 1 can be
[*] Prof. Dr. H. Hartl, Dr. J. Spandl, Dipl.-Chem. C. Daniel, I. Br9dgam
Institut f9r Chemie/Anorganische und Analytische Chemie
Freie Universit=t Berlin
Fabeckstrasse 34–36, 14195 Berlin (Germany)
Fax: (+ 49) 30-8385-4003
E-mail: hartl@chemie.fu-berlin.de
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, No. 10
Figure 1. Ball-and-stick model of the uncharged cluster 1. Bond lengths
[]: V-Ot 1.585(2)–1.592(2), V-Ob 1.896(2)–2.009(2), V-Oc 2.266(1)–
2.296(1); Ot : terminal oxo groups, Ob : bridging O-CH3 groups,
Oc : central oxygen atom).
derived from the hexametalate(v) anion [M6O19]8 (M = Nb,
Ta),[12] yet unknown for vanadium, by replacing the 12 m2bridging oxygen atoms with methoxo groups. In this cluster,
four vanadium atoms are present in the oxidation state + IV
and the remaining two in the oxidation state + v. As could be
expected from the strong molar mass peak in the mass
spectrum (see Experimental Section), 1 can be sublimed
under reduced pressure without noticeable decomposition.
This makes 1 an interesting candidate for CVD experiments.
During the thermal decomposition of the compound in the
absence of oxygen, continuous mass loss occurs beginning at
200 8C and ending with the formation of V2O3 at approximately 380 8C (mass loss (%): found: 43.0; calcd: 43.1).
The cyclic voltammogram[13] of 1 (Figure 2) turns out to be
a challenge for synthetic chemists because it reveals a series of
electronic transitions assigned to hexametalate clusters
Figure 2. Cyclic voltammogram of 1 (10 3 m, acetonitrile; supporting
electrolyte [NnBu4]PF6 (0.1 m), sweep rate = 200 mVs 1).
V
(4 n)
[VIV
of identical composition and strucn V6 nO7(OCH3)12]
ture, yet having different VIV/VV ratios.
Starting from 1 (n = 4) and using suitable reducing and
oxidizing agents, it was indeed possible to synthesize the
singly (n = 5) and doubly (n = 6) reduced clusters in compounds 3 b, 3 a, 2 a, and 2 b respectively, as well as the singly
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V
oxidized form (n = 3) in 4 a, 4 b, [VIV
3 V3 O7(OCH3)12]Br3 (4 c),
[14]
and 4 d (Scheme 1).
For the preparation of the singly reduced cluster in 3 b
tetrabutylammonium tetrabutylborate was used, which acts as
a mild reducing agent[15] and furthermore provides the desired
Scheme 1. Synthesis of the reduced (2 a–3 b) and oxidized (4 a–5)
cluster compounds starting from 1.
tetrabutylammonium ion. Surprisingly, the reaction of a
solution of 1 in acetonitrile with an excess of tetrabutylammonium hydroxide leads upon heating to the tetrabutylammonium salt of the dinegative species 2 a; the hydroxide
ion presumably acts as the reducing agent. When 1 is treated
with a potassium hydroxide solution of varying concentration,
both forms can be isolated as their potassium salts (2 b and 3 a,
respectively) depending on the amount of KOH used. In these
reactions the astonishingly high stability of the
[V6O7(OCH3)12] framework towards hydroxide ions is apparent.
The synthesis of the singly oxidized species (n = 3) by the
oxidation of 1 succeeded with bromine as well as with iodine.
V
+
In both cases, the resulting cation [VIV
is
3 V3 O7(OCH3)12]
embedded in a polyhalide matrix (4 a or 4 b). Under reduced
pressure, the bromine incorporated in compound 4 b partially
escapes resulting in the formation of the corresponding
tribromide salt 4 c.
If the reduction of 1 by tetrabutylammonium hydroxide in
acetonitrile and by KOH in aqueous solution was unexpected,
the conversion of 1 to the singly oxidized cluster 4 d by
bubbling hydrogen chloride through a solution of 1 in
dichloromethane was even more surprising. The unusual
redox behavior of 1 in water as well as in aprotic solvents
reveals a reaction potential that could lead to the use of these
substances as renewable reduction and oxidation reagents or
even as homogeneous redox catalysts. The differing solubility
of the uncharged cluster 1 in aqueous solution and organic
solvents considerably extends the field of potential applications by also allowing the investigation of its catalytic activity
1164
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
in two-phase systems. The solubility behavior of the ionic
V
(4 n)
clusters [VIV
is determined by their
n V6 nO7(OCH3)12]
respective counterions (e. g. K+ or [NnBu4]+ in the anionic
and polyhalides or VVOCl4 in the cationic varieties).
Accordingly, diverse solvents ranging from water to organic
solvents such as acetonitrile, acetone, alcohols, diethyl ether,
or dichloromethane can be used.
Compounds 4 a–4 d are among the first examples of
structurally characterized, positively charged polynuclear
oxometalate clusters belonging to the classic “polyoxo
metals” vanadium, niobium, tantalum, molybdenum, and
tungsten.[16] Attempts to synthesize the dipositive hexavanaV
2+
dium cluster [VIV
(n = 2) by means of
2 V4 O7(OCH3)12]
oxidation of 1 with chlorine have yet been unsuccessful. To
increase the oxidation potential of chlorine, it was used in the
presence of the Lewis acid SbCl5. The X-ray structural
analysis of the reaction product revealed that 1 had indeed
been oxidized twice, yet an oxo ligand had taken the place of a
methoxo group to reduce the cluster's net positive charge,
resulting in the formation of 5 (Scheme 1). This substitution
can take place by nucleophilic attack of a chloride ion, leading
to the elimination of CH3Cl.
Compound 5 indicates that it is possible to replace the
V
(4 n)
methoxo ligands in the [VIV
clusters by
n V6 nO7(OCH3)12]
other groups. This opens up a new way for synthesizing novel
compounds in which the ligand system or supporting materials can vary, while the {V6O19} framework is maintained. This
modification capacity “offers a way to fine-tune potential
catalytic properties of the polyoxovanadate unit,” as was
recently discussed using the crystal structure of [{VO(bmimpm)(acac)}2{V6O13(OCH3)6}] (bmimpm = bis(1-methylimidazol-2-yl)-4-methoxyphen-1-ylmethanol, acac = acetylacetonate) as an example.[8] A major advantage of the present
system is not only the fact that vanadium's oxidation states in
the polyoxovanadate framework can be modified, but also
that vibrational spectroscopy provides a powerful analytical
tool for the investigation of the hexavanadium clusters and
their redox reactions. The differently charged clusters in
compounds 1–4 can be distinguished simultaneously in both
solid state and solution infrared spectra by the frequencies of
their respective V–Ot stretching and V-Ob-V bending vibrations. Thus it is possible to monitor and analyze in situ the
course of a redox reaction during its process. A comparison of
the characteristic frequencies shows that in the
V
(4 n)
[VIV
series (n = 3–6), the V–Ot stretching
n V6 nO7(OCH3)12]
as well as the V-Ob-V bending vibrations are shifted to lower
wavenumbers with increasing n, that is, with rising VIV
content. This is accompanied by an increase in the respective
V Ot and V Ob average bond lengths (Table 1).
The fact that clusters 1–4 are isostructural and only differ
in the oxidation states of their vanadium centers makes these
compounds interesting for the investigation of the principles
that direct the organization of metal centers of different
valency. For yet unknown reasons, short regions of antiferromagnetic spin–spin coupling only seem to occur in oxygen
compounds of vanadium(iv).[17]
As bond-valence calculations for the vanadium atoms
show, the VIV and VV centers are not statistically distributed in
the presented compounds. Bond-valence sums for several of
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Angewandte
Chemie
Experimental Section
Table 1: Characteristic IR frequencies and associated average bond
lengths in 1 and its derivatives.
Compound
VIV
VV
n(V-Ob-V) [cm 1]
(V Ob distance [])
n(V-Ot) [cm 1]
(V Ot distance [])
4c
1
3b
3a
2a
2b
3
4
5
3
2
1
6
0
600 (1.945)
587 (1.963)
584 (1.990)
580 (1.983)
578 (1.999)
574 (2.000)
988 (1.570)
975 (1.589)
957 (1.601)
958 (1.591)
941 (1.606)
946 (1.612)
these compounds are listed in Table 2. The calculations were
performed twice for each individual vanadium atom using
bond-valence parameters presented by Brese and O'Keeffe[18]
1: [VO(OtBu)3] (1.43 g, 5 mmol) in methanol (25 mL) was heated for
24 h at 125 8C in a 50-mL Teflon-lined autoclave (Berghof). The
resulting solution of the solvothermal reaction was evaporated to
dryness under reduced pressure. The black residue was extracted with
hexane (ca. 50 mL) in a Soxhlet apparatus until the extracting solvent
remained nearly colorless. The dark green solution was concentrated
to half of its volume and subsequently cooled to 30 8C. The resultant
crystals were filtered and washed with a small amount of cold hexane.
Yield: 0.41 g (0.51 mmol), 62 %. Elemental analysis calcd (%) for
H36C12O19V6 (M = 790.04 g mol 1): C 18.24, H 4.59; found: C 18.07, H
4.51; IR (KBr, 400–1100 cm 1): ñ = 411 (s), 436 (s), 587 (vs), 714 (m),
975 (vs), 1031 cm 1 (vs); MS (EI, 80 eV): m/z (%): 790 (60) [M]+, 630
(32) [M VO(OCH3)3]+, 470 (100) [M V2O2(OCH3)6]+.
The syntheses and experimental values of compounds 2 a–5 are
given in the Supporting Information.
Received: June 20, 2002
Revised: November 26, 2002 [Z19599]
Table 2: Bond-valence sums for vanadium atoms of selected methoxooxohexavanadium clusters.
Compound
(counterion)
VIV
VV
5
([SbCl6] )
2
4
4d
([VVOCl4] )
3
4c
([Br3] )
V1
Bond-valence sums
V2
V3
V4
V5
VIV
VV
4.80
5.06
4.75
5.00
4.74
4.99
4.73
4.98
4.25
4.47
4.24
4.47
3
VIV
VV
4.39
4.62
4.29
4.51
4.74
4.99
4.70
4.95
4.27
4.50
4.61
4.86
3
3
VIV
VV
4.60
4.84
4.45
4.68
4.50
4.73
4.62
4.86
–
–
–
–
1
4
2
VIV
VV
4.19
4.41
4.71
4.96
4.41
4.64
–
–
–
–
–
–
3a
(K+)
5
1
VIV
VV
4.13
4.35
4.27
4.46
4.87
5.12
4.15
4.37
4.12
4.33
4.14
4.36
2b
(K+)
6
0
VIV
VV
4.04
4.26
4.07
4.28
4.09
4.30
4.02
4.23
4.06
4.27
4.06
4.28
Keywords: cluster compounds · cyclic voltammetry · redox
chemistry · solvothermal synthesis · vanadium
V6
for VIV and VV. The calculated bond-valence sums for
identical vanadium atoms vary only by approximately 0.25.
In compounds with harder counterions (K+, VVOCl4 ,
SbCl6 ), the assignment of the oxidation states of the
vanadium atoms is more reliable than in those containing
softer counterions ([NnBu4]+, polyhalide). The reason for this
may be the increasing tendency to rotational disorder of the
hexavanadium clusters in the solid phase of the latter
compounds. In their octahedral coordination around the
central oxygen atom, an even number of VIV and VV centers
causes the vanadium atoms of equal valency to adopt a trans
arrangement. Compounds with a VIV/VV ratio of 3:3 display a
mer-configuration of these centers. Both EPR-spectroscopic
studies of these compounds and the investigation of their
magnetic properties should provide a better understanding of
the exchange interactions between VIV and VV centers in
oxovanadium clusters of mixed valency.
Angew. Chem. Int. Ed. 2003, 42, No. 10
.
[1] D. C. Bradley, R. C. Mehrotra, I. P. Rothwell, A. Singh Alkoxo
and Aryloxo Derivatives of Metals, Academic Press, San Diego,
2001.
[2] a) P. Gouzerh, A. Proust, Chem. Rev. 1998, 98, 77 – 111; b) M. I.
Khan, J. Zubieta, Prog. Inorg. Chem. 1995, 43, 1 – 149.
[3] a) D. E. Katsoulis, Chem. Rev. 1998, 98, 359 – 388; b) W. A.
Hermann, N. W. Huber, O. Runte, Angew. Chem. 1995, 107,
2371 – 2390; Angew. Chem. Int. Ed. Engl. 1995, 34, 2187.
[4] Polyoxometalate Chemistry (Eds.: M. T. Pope, A. MOller),
Kluwer, Dordrecht, 2001.
[5] D. Hou, Gyu-Shik Kim, K. S. Hagen, C. L. Hill, Inorg. Chim.
Acta 1993, 211, 127 – 130.
[6] V. G. Kessler, G. A. Seisenbaeva, Inorg. Chem. Commun. 2000,
3, 203 – 204. Note: According to the author's declarations,
crystals of this compound were found in an inadequately
sealed flask of [VO(OEt)3] (Aldrich); there is no information
on the synthesis of this compound.
[7] J. Spandl, I. BrOdgam, H. Hartl, Angew. Chem. 2001, 113, 4141 –
4143; Angew. Chem. Int. Ed. 2001, 40, 4018 – 4019.
[8] M. Piepenbrink, M. U. Triller, N. H. J. Gormann, B. Krebs,
Angew. Chem. 2002, 114, 2633 – 2635; Angew. Chem. Int. Ed.
2002, 41, 2523 – 2525.
[9] H. K. Chae, W. G. Klemperer, V. W. Day, Inorg. Chem. 1989, 28,
1423 – 1424.
[10] V. W. Day, W. G. Klemperer, D. J. Maltbie, J. Am. Chem. Soc.
1987, 109, 2991 – 3002.
[11] J. Spandl, I. BrOdgam, H. Hartl, Z. Anorg. Allg. Chem. 2000, 626,
2125 – 2132.
[12] a) F. Pickhard, H. Hartl, Z. Anorg. Allg. Chem. 1997, 623, 1311 –
1316; b) H. Hartl, F. Pickhard, F. Emmerling, C. RPhr, Z. Anorg.
Allg. Chem. 2001, 627, 2630 – 2638.
[13] Cyclic voltammetry was conducted by using a potentiostat/
galvanostat PGSTAT 12 from Metrohm/Autolab. The results
were analyzed with the supplied software. Measurements were
conducted in anhydrous acetonitrile with tetrabutylammonium
hexafluorophosphate as supporting electrolyte (0.1m) and a
10 3 m concentration of 1 (reference electrode: Ag/AgCl doublechamber electrode, inner chamber: 3 m KCl solution, outer
chamber: 0.1m [NnBu4]PF6 solution; working electrode: platinum).
[14] Crystal structure determination of compounds 1–5: Bruker XPS
diffractometer (CCD area detector, MoKa radiation, l =
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[15]
[16]
[17]
[18]
0.71073 Q, graphite monochromator), empirical absorption
correction using symmetry-equivalent reflections (SADABS),
direct methods, difference Fourier syntheses. The initial structures were refined against F2 (Bruker-SHELXTL, version 5.1,
1998). The hydrogen atoms were calculated in geometrically
idealized positions. The crystals were picked directly out of the
mother liquor under a cold nitrogen stream. 1: Crystal size:
0.33 R 0.32 R 0.31 mm, T = 193 K, monoclinic P21/n, a =
10.140(2), b = 13.081(2), c = 10.438(2) Q, b = 97.050(2) Q, V =
1373.9(2) Q3, Z = 2, 1calcd = 1.910 g cm 3, 2qmax = 60.028, 16 192
collected reflections, 4009 unique reflections, 176 parameters,
m = 2.034 mm 1, absorption correction, effective transmission
max./min. = 0.64/0.53, R1 [I > 2s(I)] = 0.0422, wR2 [I > 2s(I)] =
0.1243, R1 (all data) = 0.0541, wR2 (all data) = 0.1331, largest
difference peaks 0.837/ 0.691 e Q 3. Abridged versions of the
crystallographic data of compounds 2 a–5 are given in the
Supporting Information. CCDC-186491 (1), CCDC-186494 (2 a),
CCDC-186495 (2 b), CCDC-186493 (3 a), CCDC-186492 (3 b),
CCDC-186496 (4 a), CCDC-186497 (4 b), CCDC-186498 (4 c),
CCDC-186499 (4 d), and CCDC-186500 (5) contain the supplementary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+ 44) 1223-336033; or deposit@ccdc.cam.ac.uk).
R. Damico, J. Org. Chem. 1964, 29, 1971 – 1976.
C. H. Ng, C. W. Lim, S. G. Teoh, H.-K. Fun, A. Usman, S. W. Ng,
Inorg. Chem. 2002, 41, 2 – 3.
A. MOller, F. Peters, M. T. Pope, D. Gatteschi, Chem. Rev. 1998,
98, 239 – 271.
N. E. Brese, M. O'Keeffe, Acta Crystallogr. Sect. B 1991, 47,
P
192 – 197. The oxidation state of atom i is given by
nij = V
j
with nij = exp[(Rij dij)/b]. Here b is taken to be a ’universal’
constant equal to 0.37 Q, vij is the valence of a bond between two
atoms i and j, Rij is the empirical parameter, and dij is the
observed bond length.
1166
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Angew. Chem. Int. Ed. 2003, 42, No. 10
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