Synthesis and X-ray single crystal study of the bi- and trimetallic alkoxides of molybdenum(VI) and tantalum, Mo2Ta4O8(OMe)16, Mo4Ta4O16(O(i)Pr)12 and LiMo4Ta3O14(O(i)Pr)9(OC2H4OMe)3

Article abstract of DOI:10.1016/S0277-5387(00)00443-5

The partial hydrolysis of a mixture of molybdenum and tantalum methoxides (Mo:Ta = 1:2) with stoichiometric amounts of water yields a new bimetallic complex Mo₂Ta₄O₈(OMe)₁₆ (I) on crystallization from toluene. The molecules of I appear to be a combination of two MoO₂(O(i)Pr)₂ units with the earlier described Ta₄O₄(O(i)Pr)₁₂ core. The thermal reduction of a mixture of MoO(O(i)Pr)₄ and Ta₂(O(i)Pr)₁₀ (in a 1:1 Mo:Ta ratio) followed by oxidation of the products by dry oxygen, provides access to Mo₄Ta₄O₁₆(O(i)Pr)₁₂ (II). The same procedure, using LiMo₂O₄(O(i)Pr)₄(OC₂H₄OMe) instead of MoO(O(i)Pr)₄, yielded trimetallic LiMo₄Ta₃O₁₄(O(i)Pr)₉(OC₂H₄OMe)₃ (III) in minor yields. The molecular structure of II can be described as a distorted cube, the corners of which are occupied in turn by [((i)PrO)₃Ta(μ-O)₃] octahedra and [MoO(μ-O)₃] tetrahedra sharing common vertexes. The molecular structure of III can be derived from that of II by replacing one [((i)PrO)₃Ta(μ-O)₃] octahedron with an [Li(μ, η²-OC₂H₄OMe)₃] trigonal prism (the coordination number of the three Mo atoms is then increased to six). (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00443-5

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1791–1798
Synthesis and X-ray single crystal study of the bi- and trimetallic
alkoxides of molybdenum(VI) and tantalum, Mo2Ta4O8(OMe)16,
Mo4Ta4O16(OiPr)12 and LiMo4Ta3O14(OiPr)9(OC2H4OMe)3
Anders Johansson, Vadim G. Kessler *
Department of Chemistry, Swedish Uni6ersity of Agricultural Sciences, PO Box 7015, S-75007 Uppsala, Sweden
Received 1 November 1999; accepted 31 May 2000
Abstract
The partial hydrolysis of a mixture of molybdenum and tantalum methoxides (Mo:Ta=1:2) with stoichiometric amounts of
water yields a new bimetallic complex Mo2Ta4O8(OMe)16 (I) on crystallization from toluene. The molecules of I appear to be a
combination of two MoO2(OiPr)2 units with the earlier described Ta4O4(OiPr)12 core. The thermal reduction of a mixture of
MoO(OiPr)4 and Ta2(OiPr)10 (in a 1:1 Mo:Ta ratio) followed by oxidation of the products by dry oxygen, provides access to
Mo4Ta4O16(OiPr)12 (II). The same procedure, using LiMo2O4(OiPr)4(OC2H4OMe) instead of MoO(OiPr)4, yielded trimetallic
LiMo4Ta3O14(OiPr)9(OC2H4OMe)3 (III) in minor yields. The molecular structure of II can be described as a distorted cube, the
corners of which are occupied in turn by [(iPrO)3Ta(m-O)3] octahedra and [MoO(m-O)3] tetrahedra sharing common vertexes. The
molecular structure of III can be derived from that of II by replacing one [(iPrO)3Ta(m-O)3] octahedron with an [Li(m,h2-
OC2H4OMe)3] trigonal prism (the coordination number of the three Mo atoms is then increased to six). © 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: Molybdenum; Tantalum; Trimetallic alkoxides; Crystal structures
1. Introduction
2. Experimental
Heterometallic alkoxides of heavy transition elements
appear to be potent precursors of fine powders of the
mixed-metal oxides and corresponding metal alloys
having a broad range of applications [1,2]. We have
reported earlier the isolation and characterization of the
mixed-metal molybdenum(VI) and Group 4, e.g.
Zr3Mo8O24(OiPr)12(iPrOH)4 [3], and Group 7, e.g.
ReMoO2(OMe)7 [4], derivatives. It appeared challeng-
ing to extend this study to the Group 5 derivatives and
we report here the isolation and structural characteriza-
tion of the bimetallic alkoxides of molybdenum(VI) and
tantalum and of the related trimetallic complex of these
elements and lithium. A preliminary communication
reporting the isolation and a short description of the
molecular structure of Mo2Ta4O8(OMe)16 has been
published [5].
All manipulations were carried out in a dry nitrogen
atmosphere using the Schlenk technique or a dry box.
The alcohols were purified by distillation over Al(OiPr)3
or Mg(OMe)2 under argon. IR spectra of Nujol mulls
were registered with a Bruker IFS-55 spectrometer. The
1H NMR spectra were obtained with a Varian 400
MHz spectrometer, using dry deaerated toluene as sol-
vent (using TMS as internal and 20 vol% solution of
CDCl3 as external standards). The metal ratio in the
samples was determined, exploiting the facilities of
Arrhenius Laboratory, Stockholm University, Sweden,
on a JEOL-820 scanning electron microscope (SEM),
supplied with a Link AN-10000 energy dispersive spec-
trometer (EDS). The total contents of the incom-
bustible residues for the samples was determined using
Perkin–Elmer TGA-7 device (in air, 10°C min−1, final
temperature 800°C).
* Corresponding author. Tel.: +46-18-671-541; fax: +46-18-673-
476.
E-mail address: vadim.kessler@keuvi.slu.se (V.G. Kessler).
The starting reagents used in this work (MoO(OMe)4
and LiMo2O4(OiPr)4(OC2H4OMe) [6] Ta(OMe)5, and
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00443-5

1792
A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
Ta(OiPr)5 [7]) were prepared by anodic oxidation of
metals in alcohols and purified according to conven-
tional techniques. An alcohol interchange reaction with
isopropanol was used to obtain MoO(OiPr)4 and was
performed by repeated (three times) dissolution of a
in a flask covered by a serum cup and it changed color
slowly to light brown. The solvent was evaporated in
vacuo and the light-brown glassy residue redissolved in
3 ml of hexane and left overnight to crystallize. The
colorless transparent crystals formed were separated by
filtration and dried in vacuo. Yield: 0.127 g (18%). IR
(cm−1): 1381 m, 1367 m, 1337 w, 1262 w, 1179 s, 1168
m, 1120 s br, 1007 s br, 966 w, 904 sh, 877 s, 840 s, 810
i
measured quantity of methoxide in an excess of PrOH
with subsequent evaporation to dryness.
1
s br, 684 sh, 583 s br, 565 sh, 455 m br. H NMR
2.1. Preparation of Mo2Ta4O8(OMe)16 (I)
(ppm): 5.28 septet (1H, CH), 1.56 doublet (6H, CH3).
MoO(OMe)4 (0.584 g, 2.47 mmol) and Ta(OMe)5
(1.734 g, 5.16 mmol) were dissolved in 20 ml MeOH
and then a solution of H2O (0.13 ml, 7.2 mmol) in 20
ml MeOH was added dropwise to the bright yellow
solution over a period of 1 h. The transparent colorless
solution formed was then evaporated to dryness in
vacuo and the glassy residue was redissolved in 5 ml of
toluene and left to crystallize at −30°C for 2 days. The
colorless crystals obtained were separated by decanta-
tion, washed with 1 ml toluene and dried in vacuo.
Yield: 1.353 g (71%). IR (cm−1): 1160 s, 1155 s, 1146 s,
1133 s, 1059 s br, 1035 sh, 1028 s, 935 s, 912 s, 837 sh,
826 s br, 732 w, 616 s br, 578 m, 547 s, 540 s, 527 s br,
2.3. Preparation of LiMo4Ta3O14(OiPr)9(OC2H4OMe)3
(III)
The procedure used was analogous to that for the
preparation of II except that LiMo2O4(OiPr)4-
(OC2H4OMe) was used instead of MoO(OiPr)4. The
crystals of III were obtained in minor yields after
prolonged storage in toluene solution in contact with
dry air (about 2 months) of the products of thermal
reduction.
2.4. Crystallography
1
502 s, 494 s, 449 m br. The H NMR spectrum of I is
shown in Fig. 1.
2.4.1. Crystal data for C16H48O24Mo2Ta4 (I)
M=1450.22, monoclinic, space group P21/n, a=
2.2. Preparation of Mo4Ta4O16(OiPr)12 (II)
9.3422(18), b=9.455(2), c=22.621(6) A, b=99.94(3)°,
,
3
V=1968.3(8) A , Dcalc=2.599 g cm−3 for Z=2, u(Mo
,
MoO(OiPr)4 (0.472 g, 1.36 mmol) and Ta(OiPr)5
(0.674 g, 1.41 mmol) were dissolved in 5 ml of toluene
and the bright yellow solution obtained was refluxed
for 30 min. The brownish–orange viscous liquid thus
formed was left at room temperature (r.t.) for 2 weeks
Ka)=0.71073 A, 4414 [Rint=0.1053] independent
,
reflections with I\2|(I) were collected at 22°C up to
2qmax=56.58° with a Bruker SMART CCD 1K diffrac-
tometer. The structure was solved by standard direct
methods; the coordinates of tantalum and molybdenum
Fig. 1. The 1H NMR spectrum of Mo2Ta4O8(OMe)16 (I) (toluene, 300 K).

A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
1793
atoms were taken from the initial solution and the
other non-hydrogen atoms were located in subsequent
Fourier syntheses. The structure was refined by full-ma-
trix least-squares in an isotropic and then anisotropic
approximation. Final discrepancy factors are R=
0.0501 and wR2=0.0711.
2.4.2. Crystal data for C36H84O28Mo4Ta4 (II)
(
M=2072.59, tetragonal, space group I4, a=b=
3
,
,
14.998(6), c=14.974(8) A, V=3368(3) A , Dcalc
=
2.044 g cm−3 for Z=2, u(Mo Ka)=0.71073 A, 785
[Rint=0.0000] independent reflections with I\2|(I)
were collected at 22°C up to 2qmax=41.58° with a
Bruker SMART CCD 1K diffractometer. The structure
was solved by direct methods, applying HKLF 5 option
for the experimental data file because of the presence of
two twinning domains (creating a pseudo 3-fold axis
and a pseudo-symmetry plane, the crystal thus having
six individual components with an ideal crystallo-
graphic symmetry). The coordinates of tantalum and
molybdenum atoms were taken from the initial solution
and refined anisotropically. The other non-hydrogen
atoms were located in subsequent Fourier syntheses
and refined isotropically, the geometrical restraints be-
ing applied to conserve the idealized geometry for the
iPr-radicals (subjected to rotational disorder at r.t.).
Final discrepancy factors are R1=0.0766 and wR2=
0.1933.
,
Fig. 2. The molecular structure of Mo2Ta4O8(OMe)16 (I).
Ta(OR)5, [9] we decided to apply the microhydrolysis as
the latter leads to oxoalkoxide species [10,11] that
might have unsaturated coordination and thus be prone
to aggregation with the formation of heterometallic
products. The Mo:Ta=1:2 ratio was chosen initially
taking into account the existence of a bimetallic
oxoalkoxotantalate of I2Zn2Ta4O4(OiPr)14 [12] compo-
sition, in which molecular structure the ZnI(m-OR)2(m3-
O) tetrahedra appeared to be as sterically demanding as
should be necessary for the MoO2(OR)(m-OR)2(m3-O)
octahedra. When the hydrolysis of a mixture of
methoxides with stoichiometric amounts of water was
carried out in MeOH and the transparent solution
obtained was left overnight to crystallize, the amor-
phous solid product collected did not contain tantalum
(according to EDS analysis) and displayed an IR spec-
trum identical with that described for MoO2(OMe)2
[13]. To enhance the formation of heterometallic spe-
cies, which could occur only via the Lewis acid–base
interaction mechanism, a non-donor solvent was used
(thus removing the concurrent Lewis base, MeOH).
Evaporation of the reaction mixture produced by the
hydrolytic treatment, to dryness and redissolution of
the residue in toluene produced a clear solution from
which the expected heterometallic complex, Mo2Ta4O8-
(OMe)16 (I), was obtained by crystallization at low
temperatures.
2.4.3. Crystal data for C36H84O29Mo4Ta3Li (III)
(
M=1914.58, triclinic, space group P1, a=14.628(4),
,
b=14.721(4), c=17.238(6) A, h=90.620(17), i=
3
,
95.908(18), k=118.354(9)°, V=3241.6(17) A , Dcalc
=
1.962 g cm−3 for Z=2, u(Mo Ka)=0.71073 A, 8399
[Rint=0.0534] independent reflections with I\2|(I)
were collected at 22°C up to 2qmax=45.00° with a
Bruker SMART CCD 1K diffractometer. The coordi-
nates of tantalum and molybdenum atoms were taken
from the initial solution and refined anisotropically.
The coordinates of all other non-hydrogen atoms were
located in subsequent Fourier synthese and refined in
anisotropic approximation for the oxygen atoms and
isotropically for the carbon atoms. Final discrepancy
factors are R1=0.0636 and wR2=0.1516.
,
The molecular structure of I, determined by an X-ray
single crystal study (see Fig. 2 and Tables 1 and 2), was
analogous to that of the chosen structural prototype,
I2Zn2Ta4O4(OiPr)14 [12]. It is composed of centrosym-
metric hexanuclear molecules, with the TaꢀO distances
(see Table 2) divided into two large groups, the first of
which includes the bonds with the bridging alkoxoli-
gands (Ta(1)ꢀO(2) 2.125(7), Ta(2)ꢀO(2) 2.128(7),
The positions of the hydrogen atoms in the structures
I–III were calculated geometrically and included in the
final cycles of the refinement in isotropic approxima-
tion. All calculations were performed with an IBM PC
using SHELXTL-NT programs [8].
,
3. Results and discussion
Ta(1)ꢀO(6) 2.125(7), Ta(2)ꢀO(4) 2.043(7) A) and the
triply-bridging oxoatom (Ta(1)ꢀO(3) 2.044(7),
,
Taking into account the absence of complex forma-
tion between the monooxoalkoxides of molybde-
num(VI), MoO(OR)4, and tantalum alkoxides,
Ta(2)ꢀO(3) 2.026(7) A), and the second includes the
bonds to the terminal alkoxide ligands (Ta(1)ꢀO(5)
1.859(8), Ta(1)ꢀO(9) 1.859(8), Ta(2)ꢀO(7) 1.852(8),

1794
A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
,
Ta(2)ꢀO(8) 1.865(8) A) and the doubly-bridging
ing to the MꢀOR vibrations (616 s br, 578 m, 547 s, 540
s, 527 s, 502 s, 494 s, 449 m br). It should be mentioned
that I supposedly does not exist in solution: it is almost
insoluble in toluene at r.t. and when dissolved under
reflux does not crystallize on cooling for several weeks.
oxoatom (Ta(1)ꢀO(1) 1.890(7) and Ta(2)ꢀO(1)c
,
,
1.931(7) A). The Ta(1)ꢀTa(2) distance of 3.155(9) A is
quite short (compared with 3.31 A in the ZnꢀTa com-
,
plex). The coordination of the doubly-bridging ox-
ogroups is also almost linear, Ta(1)ꢀO(1)ꢀTa(2)c
172.0(4)°. The coordination of the Mo atom is rather
distorted in comparison with that usually observed [14].
The MoꢀO distances themselves fall into the common
ranges: 1.686–1.700(9) for the double bonds, 1.883(8)
for the terminal alkoxide one and 2.134–2.279(8) for
the alkoxide and oxo-bridging groups. However, the
sum of the bond lengths in the trans-position differ
1
The H NMR spectra of I (see Fig. 1) are strikingly
complex: at least 20 sharp and well defined signals are
observed between 4.40 and 5.20 ppm indicating pre-
sumably the destruction of the molecules on dissolution
and the coexistence in solution of different molecular
aggregates. The dissolution in MeOH in contrast occurs
easily at r.t. and a very high solubility can in fact be
observed on dissolution, but after several hours precipi-
tation of an insoluble sediment characterized as
MoO2(OMe)2 takes place (see above). Compound I is
almost insoluble in hexane even on heating.
Analogous experiments to those used in the prepara-
tion of I were carried out with the isopropoxides of the
title elements but did not lead to isolation of any
crystalline product. At the same time it was found that
oxidation of the bimetallic alkoxides of molybde-
num(V) and tantalum, such as Mo4Ta2O8(OiPr)14 or
Mo3Ta2O8(OiPr)10, with dry air led to the formation of
a colorless crystalline product containing Mo and Ta in
,
noticeably (3.873(15), 3.979(17) and 4.017(16) A) and
the bond angles deviate strongly from 180°
(O(10)ꢀMo(1)ꢀO(6)
161.3(3),
O(11)ꢀMo(1)ꢀO(4)
164.1(4) and O(12)ꢀMo(1)ꢀO(3) 151.7(4)°).
The IR spectrum of I is in good agreement with its
molecular structure: there is a strong doublet present at
935 and 912 cm−1, corresponding to vibrations in the
cis-MoO2 fragment [14], a very strong and broad band
at 837 cm−1, that can be attributed to vibrations with
participation of oxobridging ligands, and a number of
strong and well-defined bands in the area correspond-
Table 1
Crystal data and structure refinement parameters for compounds I–III
Empirical formula
Formula weight
Temperature (K)
C8H24MoO12Ta2
770.11
295(2)
C36H84Mo4O28Ta4
2072.59
295(2)
C36H84LiMo4O29Ta3
1914.58
295(2)
,
Wavelength (A)
0.71073
monoclinic
P21/n
0.71073
tetragonal
0.71073
triclinic
Crystal system
Space group
(
(
I4
P1
Unit cell dimensions
,
a (A)
9.3422(18)
9.455(2)
22.621(6)
90
99.94(3)
90
1968.3(8)
4
14.998(6)
14.998(6)
14.974(8)
90
90
90
3368(3)
2
14.628(4)
14.721(4)
17.238(6)
90.620(17)
95.908(18)
118.354(9)
3241.6(16)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
Volume (A )
Z
Dcalc (g cm−3
)
2.599
11.766
1424
2.044
7.255
1968
1.962
5.862
1844
Absorption coefficient (mm−1
F(000)
)
Crystal size (mm)
q Range for data collection (°)
0.50×0.15×0.10
1.83–28.29
0.5×0.2×0.15
1.92–20.79
1.1×0.4×0.3
1.59–22.50
Index ranges (°)
−115h512, −125k512,
−255l529
−105h514, −105k514,
−145l514
−155h515, 155k515,
155l518
Reflections collected
11 104
785
12 866
Independent reflections
Completeness to q=28.29° (%)
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
4414 [Rint=0.1053]
90.1
785 [Rint=0.0000]
84.9
8399 [Rint=0.0534]
99.1
full-matrix least-squares on F2
4414/0/208
full-matrix least-squares on F2
785/33/88
full-matrix least-squares on F2
8399/0/473
0.867
1.039
0.838
R1=0.0501, wR2=0.0711
R1=0.1342, wR2=0.0865
1.224 and −1.299
R1=0.0766, wR2=0.1933
R1=0.0867, wR2=0.2081
0.800 and −1.201
R1=0.0636, wR2=0.1516
R1=0.1080, wR2=0.1612
1.848 and −3.048
Largest difference peak and hole
−3
,
(e A
)
Absolute structure parameter
0.14(11)

A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
1795
Table 2
Selected bond distances (A) and angles (°) of compound I
a
,
Ta(1)ꢀO(5)
Ta(1)ꢀO(9)
Ta(1)ꢀO(1)
Ta(1)ꢀO(3)
Ta(1)ꢀO(2)
Ta(1)ꢀO(6)
Ta(2)ꢀO(7)
Ta(2)ꢀO(8)
Ta(2)ꢀO(1)c1
Ta(2)ꢀO(3)
1.859(8) Ta(2)ꢀO(4)
1.859(8) Ta(2)ꢀO(2)
1.890(7) O(1)ꢀTa(2)c1
2.044(7) Ta(1)ꢀTa(2)
2.125(7) Mo(1)ꢀO(12)
2.125(7) Mo(1)ꢀO(11)
1.852(8) Mo(1)ꢀO(10)
1.865(8) Mo(1)ꢀO(6)
1.931(7) Mo(1)ꢀO(3)
2.026(7) Mo(1)ꢀO(4)
2.043(7)
2.128(7)
1.931(7)
3.3155(9)
1.686(8)
1.700(9)
1.883(8)
2.134(8)
2.187(7)
2.279(8)
O(5)ꢀTa(1)ꢀO(9)
O(5)ꢀTa(1)ꢀO(1)
O(9)ꢀTa(1)ꢀO(1)
O(5)ꢀTa(1)ꢀO(3)
O(9)ꢀTa(1)ꢀO(3)
O(1)ꢀTa(1)ꢀO(3)
O(5)ꢀTa(1)ꢀO(2)
O(9)ꢀTa(1)ꢀO(2)
O(1)ꢀTa(1)ꢀO(2)
O(3)ꢀTa(1)ꢀO(2)
O(5)ꢀTa(1)ꢀO(6)
O(9)ꢀTa(1)ꢀO(6)
O(1)ꢀTa(1)ꢀO(6)
O(3)ꢀTa(1)ꢀO(6)
O(2)ꢀTa(1)ꢀO(6)
100.2(3) O(7)ꢀTa(2)ꢀO(8)
99.5(3) O(7)ꢀTa(2)ꢀO(1)c1
94.7(3) O(8)ꢀTa(2)ꢀO(1)c1
154.7(3) O(7)ꢀTa(2)ꢀO(3)
97.1(3) O(8)ꢀTa(2)ꢀO(3)
97.3(3) O(1)c1ꢀTa(2)ꢀO(3)
88.9(3) O(7)ꢀTa(2)ꢀO(4)
170.4(3) O(8)ꢀTa(2)ꢀO(4)
101.3(3)
97.9(3)
92.0(3)
159.4(3)
93.9(3)
95.3(3)
89.2(3)
94.4(3)
Fig. 3. The molecular structure of Mo4Ta4O16(OiPr)12 (II).
86.7(3) O(1)c1ꢀTa(2)ꢀO(4) 169.3(3)
Mo(2)O(7){m-O(1)}{m-O(5)}{m-O(6)}, the latter hexaco-
ordinated, forming Ta(1){m-O(1)}{m-O(5)}{m-
73.4(3) O(3)ꢀTa(2)ꢀO(4)
87.7(3) O(7)ꢀTa(2)ꢀO(2)
90.5(3) O(8)ꢀTa(2)ꢀO(2)
170.2(3) O(1)c1ꢀTa(2)ꢀO(2)
73.8(3) O(3)ꢀTa(2)ꢀO(2)
86.9(3) O(4)ꢀTa(2)ꢀO(2)
75.8(3)
91.8(3)
166.8(3)
85.0(3)
73.6(3)
86.9(3)
92.3(3)
70.8(3)
88.0(4)
164.1(4)
85.2(3)
81.1(3)
68.0(3)
O(6)}O(2)O(3)O(4) octahedral units. The bonding
parameters (Table 3) inside the tetraoxomolybdate unit
are quite close to those usually observed for such
fragments in polyoxometallate structures [15] (MoꢀO
1.76(3)–1.77(3) for MoꢀOꢀTa bridging fragments and
Mo(2)ꢀO(7) 2.04(6) for the terminal MoꢁO bond, the
OꢀMoꢀO angles being quite close to the tetrahedral
ones, i.e. 110(4)–116(3)°). The bond lengths and angles
inside the Ta(m-O)3(OR)3 octahedron (see Table 3) are
within the ranges usually observed for tantalum
oxoalkoxides [7]. The metalꢀoxygen core in structure II
can be considered as the first representative of an
unusual oxometallate core composed by four tetrahedra
O(12)ꢀMo(1)ꢀO(11) 105.6(5) O(10)ꢀMo(1)ꢀO(3)
O(12)ꢀMo(1)ꢀO(10) 100.9(4) O(6)ꢀMo(1)ꢀO(3)
O(11)ꢀMo(1)ꢀO(10)
O(12)ꢀMo(1)ꢀO(6)
O(11)ꢀMo(1)ꢀO(6)
O(10)ꢀMo(1)ꢀO(6)
O(12)ꢀMo(1)ꢀO(3)
O(11)ꢀMo(1)ꢀO(3)
99.8(4) O(12)ꢀMo(1)ꢀO(4)
91.4(4) O(11)ꢀMo(1)ꢀO(4)
90.3(4) O(10)ꢀMo(1)ꢀO(4)
161.3(3) O(6)ꢀMo(1)ꢀO(4)
151.7(4) O(3)ꢀMo(1)ꢀO(4)
96.6(4)
a Symmetry transformations used to generate equivalent atoms:
c1 −x, −y+1, −z.
1:1 composition (according to EDS analysis) in minor
yields [9]. The solution containing MoO(OiPr)4 and
Ta(OiPr)5 in a 1:1 molar ratio in toluene was then
prepared and subjected to thermal tretament in an
analogous manner to that used for the complete reduc-
tion of Mo(VI) to Mo(V). It was then left at r.t. in a
flask covered by a serup cup that provides good protec-
tion against moisture but not against oxygen in air.
Complete oxidation, accompanied by the appropriate
color change, occurred in 2 weeks and the bimetallic
complex Mo4Ta4O16(OiPr)12 (II) was obtained on crys-
tallization from hexane in rather low yields. One of the
major difficulties in the isolation of II appears to be its
very high solubility in hydrocarbon solvents (achieving
at least 50 wt% in both toluene and hexane).
The molecular structure of II (see Figs. 3 and 4 and
Tables 1 and 3) is composed of non-centrosymmetric
octanuclear molecules, the arrangement of the metal
atoms in which can be described best as a distorted
cube. The vertexes of this cube contain the molybde-
num and tantalum atoms in turn. The former are
tetracoordinated, building tetraoxomolybdate anions,
Fig. 4. A polyhedral presentation of the metalꢀoxygen core in the
structure of II.

1796
A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
Table 3
a
,
Selected bond distances (A) and angles (°) of compound II
O(1)ꢀTa(1)
Ta(1)ꢀO(2)
Ta(1)ꢀO(3)
Ta(1)ꢀO(6)c1
Ta(1)ꢀO(5)c2
Ta(1)ꢀO(4)
2.09(3) O(5)ꢀTa(1)c3
1.83(6) O(6)ꢀTa(1)c1
1.97(11) Mo(2)ꢀO(1)
2.08(3) Mo(2)ꢀO(6)
2.09(3) Mo(2)ꢀO(5)
2.08(7) Mo(2)ꢀO(7)
2.09(3)
2.08(3)
1.77(3)
1.76(3)
1.77(3)
2.04(6)
Mo(2)ꢀO(1)ꢀTa(1)
O(2)ꢀTa(1)ꢀO(3)
158(4)
85(4)
84(3)
156(4)
172(3)
93(4)
O(6)ꢀMo(2)ꢀO(5)
O(6)ꢀMo(2)ꢀO(1)
O(5)ꢀMo(2)ꢀO(1)
O(6)ꢀMo(2)ꢀO(7)
O(5)ꢀMo(2)ꢀO(7)
O(1)ꢀMo(2)ꢀO(7)
110(4)
116(3)
110(4)
111(3)
113(4)
97(3)
O(2)ꢀTa(1)ꢀO(6)c1
O(3)ꢀTa(1)ꢀO(6)c1
O(2)ꢀTa(1)ꢀO(5)c2
O(3)ꢀTa(1)ꢀO(5)c2
O(6)c1ꢀTa(1)ꢀO(5)c2 95(3)
Mo(2)ꢀO(5)ꢀTa(1)c3 149(3)
O(2)ꢀTa(1)ꢀO(1)
O(3)ꢀTa(1)ꢀO(1)
97(3)
78(3)
Mo(2)ꢀO(6)ꢀTa(1)c1 142(2)
Fig. 5. The molecular structure of LiMo4Ta3O14(OiPr)9(OC2H4OMe)3
(III): the b-carbon atoms of the isopropoxide groups are omitted for
clarity.
a Symmetry transformations used to generate equivalent atoms:
c1 −x+2, −y+1, z; c2 −y+3/2, x−1/2, −z+3/2; c3 y+1/2,
−x+3/2, −z+3/2.
dination of the tantalum atoms is very close to that
observed in II (the distances to the oxygen atoms of the
terminal OiPr-groups are essentially the same
and four octahedra, sharing vertexes with each other
(see Fig. 4).
,
The IR spectrum of II appears to be in good agree-
ment with its molecular structure described above: it
displays three broad and intensive lines that correspond
to the vibration of the tetraoxomolybdate core at 877,
840 and 840 cm−1 and two strong lines at 583 and 455
cm−1 typical of TaꢀOR bond vibration. Structure of II
is preserved in hydrocarbon solution because the NMR
spectra obtained in toluene display only one kind of
well-defined signal for both methyne and methyl pro-
tons (all the isopropoxide groups in the solid state
structure of II are terminal and have analogous bond-
ing parameters), while the dissociation in solution
would supposedly lead to the appearance of several
different signals.
1.832(14)–1.865(11) A, while the lengths of the oxo-
,
bridges vary a little more 1.988(10)–2.114(10) A, with
no obvious difference observed between the bridges to
the tetrahedrally coordinated Mo(7) and ‘pseudoocta-
hedrally’-‘4+2’ coordinated Mo(4), Mo(5) and Mo(6)
atoms). The bonding parameters for Mo(7) indicate a
rather regular tetrahedral coordination (Mo(7)ꢀO
,
1.697(13)–1.772(13) A, the OꢀMo(7)ꢀO angles usual
for a tetrahedron 105.8(6)–113.6(6)°). The bonding
An attempt to prepare a trimetallic alkoxide complex
using LiMo2O4(OiPr)4(OC2H4OMe) instead of MoO-
(OiPr)4
yielded
trimetallic
LiMo4Ta3O14(OiPr)9-
(OC2H4OMe)3 (III) in minor yields (only a small crop
of single crystals could be isolated from the viscous
liquid media on complete removal of the solvent).
Compound III, in analogy with II, possesses very high
solubility in hydrocarbon solvents. The molecular struc-
ture of III (see Figs. 5 and 6 and Tables 1 and 4) is
closely related to that of II and can be obtained from
the latter by replacement of one of the [(iPrO)3Ta(m-
O)3] octahedra with a [Li(m,h2-OC2H4OMe)3] trigonal
prism and demonstrates a distorted trigonal symmetry.
The three [MoO(m-O)3] tetrahedra that were in contact
with the [(iPrO)3Ta(m-O)3] octahedron that is removed
get their m-O groups, which became ‘free’, fused to one
m3-O ligand. The molybdenum atoms complete their
coordination number to six by coordinating two addi-
tional m-OR 2-methoxyethoxide groups each. The coor-
Fig. 6. A polyhedral presentation of the metalꢀoxygen core in the
structure of III.

A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
1797
Table 4
Selected bond distances (A) and angles (°) of compound III
coordination analogous to that observed for one of the
symmetrically independent Li atoms in the structure of
[{LiWO2(OC2H4OCH3)3}2·2Li(HOC2H4OCH3)2]2+
cations [6]. In both cases, this coordination appears to
be forced by the parallel arrangement of the three
2-methoxyethoxide groups providing a pair of donor
atoms each.
The molecule of III also represents a new type of a
polyoxometallate core with a combination of octahe-
dral and tetrahedral elements via sharing the common
vertexes.
,
Ta(1)ꢀO(10)
Ta(1)ꢀO(23)
Ta(1)ꢀO(6)
Ta(1)ꢀO(5)
Ta(1)ꢀO(16)
Ta(1)ꢀO(20)
Ta(2)ꢀO(13)
Ta(2)ꢀO(14)
Ta(2)ꢀO(21)
Ta(2)ꢀO(9)
Ta(2)ꢀO(11)
Ta(2)ꢀO(3)
Ta(3)ꢀO(25)
Ta(3)ꢀO(8)
Ta(3)ꢀO(24)
Ta(3)ꢀO(1)
Ta(3)ꢀO(26)
Ta(3)ꢀO(7)
O(2)ꢀLi(1)
1.832(13) Mo(5)ꢀLi(1)
1.846(13) Mo(6)ꢀLi(1)
1.861(11) Mo(4)ꢀO(18)
1.988(10) Mo(4)ꢀO(7)
2.065(11) Mo(4)ꢀO(9)
2.124(11) Mo(4)ꢀO(19)
1.850(13) Mo(4)ꢀO(17)
1.857(13) Mo(4)ꢀO(15)
1.856(13) Mo(5)ꢀO(12)
2.020(11) Mo(5)ꢀO(16)
2.091(12) Mo(5)ꢀO(1)
2.114(10) Mo(5)ꢀO(19)
1.814(14) Mo(5)ꢀO(15)
1.849(12) Mo(5)ꢀO(2)
1.868(13) Mo(6)ꢀO(4)
2.026(10) Mo(6)ꢀO(3)
2.089(13) Mo(6)ꢀO(5)
2.100(11) Mo(6)ꢀO(19)
3.30(4)
3.35(3)
1.739(10)
1.771(11)
1.823(11)
2.065(10)
2.155(12)
2.184(9)
1.690(11)
1.815(10)
1.831(11)
2.075(10)
2.170(10)
2.168(11)
1.721(12)
1.781(10)
1.840(10)
2.045(11)
2.164(10)
2.168(11)
1.697(13)
1.748(11)
1.771(12)
1.772(13)
4. Supplementary material
Supplementary data are available from: The Direc-
tor, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: +44-1223-336033; e-mail: deposit@ccdc.
cam.ac.uk or www: http://www.ccdc.cam.ac.uk) on re-
quest, quoting the deposition numbers CCDC-139287
(I), CCDC-142959 (II), and CCDC-142960 (III).
2.16(3)
2.08(3)
2.17(3)
2.17(4)
2.09(3)
2.16(3)
3.33(3)
Mo(6)ꢀO(17)
Mo(6)ꢀO(2)
Mo(7)ꢀO(22)
Mo(7)ꢀO(20)
Mo(7)ꢀO(11)
Mo(7)ꢀO(26)
O(15)ꢀLi(1)
O(17)ꢀLi(1)
O(1A)ꢀLi(1)
O(2A)ꢀLi(1)
O(3A)ꢀLi(1)
Mo(4)ꢀLi(1)
O(10)ꢀTa(1)ꢀO(23)
O(10)ꢀTa(1)ꢀO(6)
O(23)ꢀTa(1)ꢀO(6)
O(10)ꢀTa(1)ꢀO(5)
O(13)ꢀTa(2)ꢀO(14)
O(13)ꢀTa(2)ꢀO(21)
O(14)ꢀTa(2)ꢀO(21)
O(13)ꢀTa(2)ꢀO(9)
O(14)ꢀTa(2)ꢀO(9)
O(21)ꢀTa(2)ꢀO(9)
O(13)ꢀTa(2)ꢀO(11) 171.1(5)
O(22)ꢀMo(7)ꢀO(20) 105.8(6)
O(22)ꢀMo(7)ꢀO(11) 106.7(6)
O(20)ꢀMo(7)ꢀO(11) 113.6(6)
O(22)ꢀMo(7)ꢀO(26) 106.5(6)
O(20)ꢀMo(7)ꢀO(26) 111.2(5)
O(11)ꢀMo(7)ꢀO(26) 112.5(6)
O(18)ꢀMo(4)ꢀO(7) 104.3(5)
O(18)ꢀMo(4)ꢀO(9) 103.8(5)
96.1(6)
95.6(6)
98.1(5)
165.2(5)
96.4(6)
94.4(6)
97.8(5)
93.6(5)
93.1(5)
165.7(5)
O(18)ꢀMo(4)ꢀO(19) 152.5(5)
O(7)ꢀMo(4)ꢀO(19)
O(9)ꢀMo(4)ꢀO(19)
O(18)ꢀMo(4)ꢀO(17)
O(7)ꢀMo(4)ꢀO(17)
O(9)ꢀMo(4)ꢀO(17)
O(19)ꢀMo(4)ꢀO(17)
O(18)ꢀMo(4)ꢀO(15)
O(7)ꢀMo(4)ꢀO(15)
O(9)ꢀMo(4)ꢀO(15)
O(19)ꢀMo(4)ꢀO(15)
O(17)ꢀMo(4)ꢀO(15)
O(15)ꢀLi(1)ꢀO(2A)
O(15)ꢀLi(1)ꢀO(2)
O(2A)ꢀLi(1)ꢀO(2)
O(15)ꢀLi(1)ꢀO(17)
O(2A)ꢀLi(1)ꢀO(17)
O(2)ꢀLi(1)ꢀO(17)
O(15)ꢀLi(1)ꢀO(3A)
O(2A)ꢀLi(1)ꢀO(3A)
91.5(4)
93.4(4)
89.7(5)
158.8(4)
86.7(5)
69.8(4)
86.8(5)
89.0(4)
159.5(4)
70.8(4)
75.8(4)
80.4(12)
79.1(13)
127.8(17)
77.8(12)
142.2(16)
77.4(12)
144.1(17)
91.7(13)
Acknowledgements
The authors express their sincerest gratitude to Dr
Regine Herbst-Irmer, who succeeded in resolving the
twinning in the structure of II and provided a lot of
help with its refinement. The authors are grateful to Dr
C. Sandstro¨m for the assistance with the NMR experi-
ments, to Dr G. Westin for providing access to EDS-
analysis and to the Swedish Council for Natural
Science Research for financial support of this work.
References
[1] M.T. Pope, A. Muller (Eds.), Polyoxometallates: From
Platonic Solids to Anti-retroviral Activity, Kluwer, Dordrecht,
1993.
O(7)ꢀMo(4)ꢀO(9)
104.8(5)
[2] G.A. Seisenbaeva, V.G. Kessler, M. Sundberg, M. Nygren,
D.V. Drobot, in: B.D. Bryskin (Ed.), Rhenium and Rhenium
Alloys, The Minerals, Metals & Materials Society, USA, 1997, p.
159.
[3] V.G. Kessler, A.N. Panov, A.Yu. Borissevitch, N.Ya. Turova, J.
Sol-Gel Sci. Tech. 12 (1998) 81.
[4] V.G. Kessler, G.A. Seisenbaeva, A.V. Shevelkov, G.V. Khvo-
rykh, J. Chem. Soc., Chem. Commun. (1995) 1779.
[5] A. Johansson, V.G. Kessler, Inorg. Chem. Commun. 3 (2000)
5.
[6] V.G. Kessler, A.N. Panov, N.Ya. Turova, Z.A. Starikova, A.I.
Yanovsky, F.M. Dolgushin, A.P. Pisarevsky, Yu.T. Struchkov,
J. Chem. Soc., Dalton Trans. (1998) 21.
parameters for Mo(4), Mo(5) and Mo(6) are very simi-
lar and indicate the same kind of distortion for the
octahedra: the shortest bonds are those to the terminal
,
oxygen atoms (1.690(11)–1.739(10) A), the oxobridges
to the tantalum atoms are also rather short (1.771(11)–
,
1.840(10) A), the triply bridging oxoatom has a longer
MoꢀO(19) distance, essentially same for all the three
,
Mo atoms (2.045(10)–2.075(10) A) and the longest
[7] N.Ya. Turova, A.V. Korolev, D.E. Tcheboukov, A.I. Belokon,
Polyhedron 15 (1996) 3869.
[8] SHELXTL-NT Reference manual, Bruker AXS, 1997.
[9] A. Johansson, M. Roman, G. Seisenbaeva, L. Kloo, Z. Szabo,
V.G. Kessler, J. Chem. Soc., Dalton Trans. (2000) 387.
distances are those to the alkoxobridges (2.155(12)–
,
2.168(11) A). It is very interesting to note that the
lithium atoms definitely do not have an octahedral
coordination but, indeed, have a trigonal prismatic

1798
A. Johansson, V.G. Kessler / Polyhedron 19 (2000) 1791–1798
[10] L.G. Hubert-Pfalzgraf, Polyhedron 13 (1994) 1181.
[11] N.Ya. Turova, E.P. Turevskaya, V.G. Kessler, M.I.
Yanovskaya, J. Sol-Gel Sci. Tech. 2 (1994) 17.
[12] S. Boulmaaz, L.G. Hubert-Pfalzgraf, S. Halut, J.C. Daran, J.
Chem. Soc., Chem. Commun. (1994) 601.
[13] N.Ya. Turova, V.G. Kessler, Russ. J. Gen. Chem. (Russ.) 60
(1990) 1, 113.
[14] M.H. Chisholm, K. Folting, J.C. Huffman, C.C. Kirkpatric,
Inorg. Chem. 23 (1984) 1021.
[15] M.I. Khan, J. Zubieta, Prog. Inorg. Chem. 43 (1995) 1.
.