New oxovanadium(IV) complexes with mixed ligands - Synthesis, thermal stability, and crystal structure of (VO)2(acac)2(μ-OEt) 2 and (VO)2(thd)2(μ-OEt)2

Article abstract of DOI:10.1002/zaac.200400369

Synthesis, crystal structure, thermal stability, and magnetic properties of the new oxovanadium(IV) complexes [(VO)₂L₂(μ-OEt) ₂] with mixed β-diketonato [L = acac = C₅H ₇O₂ or L = thd = C₁₁H₁₉O ₂^-] and ethanolato (OEt) ligands are reported. The structure determinations of (VO)₂(acac)₂(μ-OEt) ₂ [P2₁/c, with a = 14.775(2), b = 8.788(1), c = 15.742(2) A, β = 112.43(1)°] and (VO)₂(thd)₂(μ- OEt)₂ [P1, with a = 8.871(3), b = 9.988(4), r = 10.299(4) A, α = 73.51(1), β = 68.70(1), γ = 65.85(1)°] are based on single-crystal X-ray diffraction data collected at 105 K. Structure data for VO(acac)₂ and VO(thd)₂ are redetermined at 105 and 295 K. (VO)₂(acac)₂(μ-OEt)₂ and (VO) ₂(thd)₂(μ-OEt)₂ comprise binuclear five-coordinated molecular units. The configuration of five oxygen atoms around a central vanadium atom is approximately square pyramidal in all the four studied complexes. (VO)₂(thd)₂(μ-OEt)₂ exhibits conspicuous changes in unit-cell dimensions (expansion in a and c, shrinkage in b, and appreciable variation in α, β, and γ) on going from 105 to 295 K, which appear to reflect adjustments in the configuration of its (anti-arranged) ligands. The interatomic distances in the crystal structures are analyzed in terms of the bond-valence concept. The four-valent state of vanadium is verified by magnetic susceptibility measurements. All complexes are subject to sublimation.

Full text of DOI:10.1002/zaac.200400369

New Oxovanadium(IV) Complexes with Mixed Ligands ؊
Synthesis, Thermal Stability, and Crystal Structure of (VO)2(acac)2(µ-OEt)2
and (VO)2(thd)2(µ-OEt)2
˚
M. A. K. Ahmed, H. Fjellvag, A. Kjekshus*, and B. Klewe
Oslo/Norway, Department of Chemistry, University of Oslo
Received June 17th, 2004.
Dedicated to Professor Martin Jansen on the Occasion of his 60th Birthday
Abstract. Synthesis, crystal structure, thermal stability, and mag-
vanadium atom is approximately square pyramidal in all the four
netic properties of the new oxovanadium(IV) complexes
studied complexes. (VO)2(thd)2(µ-OEt)2 exhibits conspicuous
changes in unit-cell dimensions (expansion in a and c, shrinkage in
b, and appreciable variation in α, β, and γ) on going from 105 to
295 K, which appear to reflect adjustments in the configuration of
its (anti-arranged) ligands. The interatomic distances in the crystal
structures are analyzed in terms of the bond-valence concept. The
four-valent state of vanadium is verified by magnetic susceptibility
measurements. All complexes are subject to sublimation.
Ϫ
[(VO)2L2(µ-OEt)2] with mixed β-diketonato [L ϭ acac ϭ C5H7O2
or L ϭ thd ϭ C11H19O2Ϫ] and ethanolato (OEt) ligands are
reported. The structure determinations of (VO)2(acac)2(µ-OEt)2
˚
[P21/c, with a ϭ 14.775(2), b ϭ 8.788(1), c ϭ 15.742(2) A, β ϭ
¯
112.43(1)°] and (VO)2(thd)2(µ-OEt)2 [P1, with a ϭ 8.871(3), b ϭ
˚
9.988(4), c ϭ 10.299(4) A, α ϭ 73.51(1), β ϭ 68.70(1), γ ϭ
65.85(1)°] are based on single-crystal X-ray diffraction data col-
lected at 105 K. Structure data for VO(acac)2 and VO(thd)2 are
redetermined at 105 and 295 K. (VO)2(acac)2(µ-OEt)2 and
(VO)2(thd)2(µ-OEt)2 comprise binuclear five-coordinated molecu-
lar units. The configuration of five oxygen atoms around a central
Keywords: Oxovanadium(IV) complexes; Mixed ligands; Crystal
structure; Thermal stability; Magnetic properties
Neue Oxovanadium(IV) Komplexe mit gemischten Liganden ؊ Synthese, thermische
Stabilität und Kristallstrukturen von (VO)2(acac)2(µ-OEt)2 und (VO)2(thd)2(µ-OEt)2
Inhaltsübersicht. Es wird über die Synthese, Kristallstrukturen,
thermische Stabilität und magnetische Eigenschaften der neuen
Oxovanadium(IV)-Komplexe [(VO)2L2(µ-OEt)2] mit β-Diketonat-
[L ϭ acac ϭ C5H7O2Ϫ oder L ϭ thd ϭ C11H19O2Ϫ] und Ethanolat-
(OEt) Liganden berichtet. Die Strukturbestimmungen von
(VO)2(acac)2(µ-OEt)2 [P21/c, mit a ϭ 14.775(2), b ϭ 8.788(1), c ϭ
Moleküleinheiten. Die Konfiguration der fünf Sauerstoffatome am
zentralen Vanadiumatom ist annähernd quadratisch-pyramidal in
allen vier untersuchten Komplexen. (VO)2(thd)2(µ-OEt)2 zeigt
einen deutlichen Wechsel in den Parametern der Einheitszelle
(Expansion in a und c, Abnahme in b, und deutliche Variation
bei α, β und γ) bei 105 bis 295 K, was die Anpassungen in der
Konfiguration seiner (anti-positionierten) Liganden zu reflektieren
scheint. Die interatomaren Abstände in den Kristallstrukturen wer-
den hinsichtlich der Valenzbindungstheorie analysiert. Die Vier-
wertigkeit des Vanadiumatoms wird durch Messungen der magneti-
schen Suszeptibilität nachgewiesen. Alle Komplerxe sind sublimier-
bar.
˚
¯
15.742(2) A, β ϭ 112.43(1)°] und (VO)2(thd)2(µ-OEt)2 [P1, mit a ϭ
˚
8.871(3), b ϭ 9.988(4), c ϭ 10.299(4) A, α ϭ 73.51(1), β ϭ 68.70(1),
γ ϭ 65.85(1)°] basieren auf bei 105 K gemessenen Einkristall-Rönt-
genstrukturdaten. Die Strukturdaten für VO(acac)2 und VO(thd)2
wurden neubestimmt bei 105 und 295 K. (VO)2(acac)2(µ-OEt)2 und
(VO)2(thd)2(µ-OEt)2 enthalten zweikernige fünffach-koordinierte
Introduction
tive of H(thd) ϭ C11H20O2 ϭ 2,2,6,6-tetramethylheptane-
3,5-dione; alternatively dpm for dipivaloylmethane] and
(acac)Ϫ [ϭ C5H7O2Ϫ; derivative of H(acac) ϭ C5H8O2 ϭ
acetylacetone ϭ pentane-2,4-dione] are conveniently used
in ALCVD (atomic layer chemical vapor deposition) reac-
tions owing to their suitable sublimation properties. The
present contribution concerns synthesis, crystal structure,
thermal stability, and magnetic properties of vanadyl (VIV)
complexes with (acac)Ϫ or (thd)Ϫ as ligands (L), alone or
in admixture with ethanolato (OEt)Ϫ.
Metal-organic molecular compounds are frequently used as
precursors in the growth of thin films. Precursors based on
β-diketonato ligands such as (thd)Ϫ [ϭ C11H19O2Ϫ; deriva-
* Prof. A. Kjekshus
Department of Chemistry, University of Oslo
P.O. Box 1033
Blindern
N-0315 Oslo / Norway
Tel.: ϩ47-22855560; Fax: ϩ47-22855441.
E-mail address: arne.kjekshus@kjemi.uio.no
The chemically simplest vanadyl β-diketonato complex,
VO(acac)2, was prepared already in 1876 by Guyard [1],
whereas the (thd)Ϫ variant has attracted much less atten-
Z. Anorg. Allg. Chem. 2004, 630, 2311Ϫ2318
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˚
M. A. K. Ahmed, H. Fjellvag, A. Kjekshus, B. Klewe
tion. VO(acac)2 is easily obtained by reaction between
H(acac) and an oxovanadium(IV) salt, such as VOCl2 [2]
or VOSO4 [3], or by a direct reaction between V2O5 and
H(acac) [4]. It can also be prepared by oxidation of
V(acac)3 in air [2]. The preparation of VO(thd)2 can be
achieved by analogous methods, e.g., by reaction between
VOCl2 and H(thd) [5].
independent of the presence or absence of O2. For V(acac)3
and V(thd)3 the main decomposition route goes through
H(acac) and H(thd), respectively [15, 16]. In conclusion it
seems well documented that VL3 complexes are more stable
than the corresponding VOL2 complexes.
Experimental Section
To explain the chemical and physical properties of
VO(acac)2 and related chelates Jones [6] postulated that the
central VIV atoms in these complexes are tetragonal pyrami-
dally five coordinated to oxygen atoms of the ligands. Jones’
inferences have subsequently been confirmed [7Ϫ9] by
structure determinations of VO(acac)2. The VϪO bond of
the vanadyl group [9] is directed toward the apex of the
Reactants and solvents: Vanadium(V) oxide (Riedel-de Hae¨n, pure),
H(acac) (Fluka, p.a., Ն 99.5 %), H(thd) (Fluka, 98 %), absolute
ethanol (Arcus, prima), acetone (Prolabo, p.a.), diethylether (Den
norske Eterfabrikk, p.a.), H2SO4 (Merck, p.a., 95Ϫ97 %), sodium
acetate trihydrate (Merck, p.a.) and anhydrous sodium carbonate
(Merck, p.a.) were used as reactants and/or solvents without
further purification.
˚
pyramid and is significantly shorter [1.584(2) A at 294 K]
˚
than the other four VϪO bonds [average 1.968(1) A at
294 K]. While the present work was in progress an account
of the crystal structure of VO(thd)2 appeared in the elec-
tronic literature [10]. As expected the molecular structures
of VO(acac)2 and VO(thd)2 show pronounced similarities
(see below).
Syntheses
All syntheses were performed in round-bottomed flasks equipped
with reflux condenser and magnetic stirrer. The reaction mixtures
were refluxed at the b. p. for different periods of time. Unless other-
wise mentioned all solid products were filtered through a sinter-
glass funnel and washed. These products were then dried for 1 day
(d) in a Heraeus vacuum drier and finally cleaned in a Büchi type
B-580 sublimation apparatus under ca. 0.05 mbar pressure. The
phase purity of all final products was ascertained by powder X-ray
diffraction (PXD). PDF-type data for characterization are listed in
the electronic deposit.
Thermogravimetric data [11] for V(tfa)3 and VO(tfa)2
[(tfa)Ϫ
ϭ
1,1,1-trifluoropentane-2,4-dionato ion
ϭ
C5H4O2F3Ϫ] in a flow of helium or helium-containing
H(tfa) vapor show that both compounds vaporize without
decomposing. The molecular shape of a complex appears
to be a key to its volatility, complexes with sphere-like
shapes (e.g., tetrahedral and octahedral arrangements) be-
ing generally more volatile than those with less enclosing
shapes (e.g., square-planar, square-pyramidal etc. geo-
metries). Basically it is of course the intermolecular interac-
tions in the solid state that determine the volatility of a
given complex, e.g., it is the polar VϪO bond in the vanadyl
group that makes VO(tfa)2 less volatile than V(tfa)3.
Dilli and Patsalides [12] found that some acetylacetonato
and fluorinated β-diketonato VIIIL3 complexes volatilize
without significant decomposition in air, and are stable in
the absence of air up to fusing at around 300 °C. In con-
trast, the corresponding VIVOL2 complexes start to decom-
pose into inorganic vanadium(III) compounds at some
150 °C with complete decomposition at 180Ϫ240 °C. Mass
spectrometric (MS) and differential scanning calorimetric
data [13] confirm that, e.g., V(acac)3 is thermally more
stable than VO(acac)2. In general, VIIIL3 complexes are
more volatile than VIVOL2 complexes despite their larger
molecular weights.
Several batches of the complexes VO(acac)2 [bis(acetylacetonato)-
oxovanadium(IV) (1)], (VO)2(acac)2(µ-OEt)2 [bis(acetylacetonato)-
bis(µ-ethoxo)bis{oxovanadium(IV)} (2)], VO(thd)2 [bis(2,2,6,6-
tetramethylheptane-3,5-dionato)oxovanadium(IV) (3)], and (VO)2-
(thd)2(µ-OEt)2 [bis(2,2,6,6-tetramethylheptane-3,5-dionato)bis(µ-
ethoxo)bis{oxovanadium(IV)} (4)] were prepared. In the following
text the complexes are usually referred to by the bold-faced num-
bers. Preparation details are kept to a minimum for the earlier re-
ported complexes 1 and 3. However, certain details are of signifi-
cance for successful syntheses of these complexes and our recipes
for 1 and 3 as well as 2 and 4 can be obtained upon request.
Complex 1 was prepared phase pure (overall yield of ca. 70 %)
according to the recipe of Bryant and Fernelius [4]. Complex 1 was
alternatively synthesized by reacting V2O5 (0.011 mol) and H(acac)
(0.051 mol; a certain excess) in 50 mL 50Ϫ80 % aqueous ethanol
for 5 d. After cooling to r. t. the liquid phase was filtered off (some-
times difficult owing to small particle size), the dark-colored pre-
cipitate was washed with 10 mL 50Ϫ80 % aqueous ethanol, vac-
uum dried at 50Ϫ60 °C, and cleaned by sublimation at 95 °C. Yield
ca. 25 %; dark turquoise-colored crystals. Elemental analysis for
C10H14O5V: C 45.52 (calc. 45.30), H 5.29 (5.32), V 19.26 (19.21) %.
The decomposition [12] of the VIVOL2 complexes in-
volves an intramolecular red/ox reaction with electron
transfer between the VO and L parts of the molecules. On
the basis of MS data [14] it was first suggested that
VO(thd)2 decomposes to H(thd) as principal volatile prod-
uct and CO and 2-methylpropene as byproducts, leaving be-
hind a black non-volatile residue of VO or VO2. More re-
cently it was found [15, 16] that VO(thd)2 remains unde-
composed below 200 °C. On increasing temperature the
progressive decomposition of VO(thd)2 leads to formation
of increasingly larger amounts of the more stable V(thd)3,
Complex 2 was prepared by mixing 0.011 mol V2O5 and 0.029 mol
H(acac) in ca. 65 mL absolute ethanol and refluxed for 5 d. The
color of the solution gradually darkened during the first day, but
up to 4 d small amounts of unreacted V2O5 could still be seen.
After 5 d treatment the dark-blue reaction mixture was cooled
overnight to r. t., the liquid phase was filtered off, and a dark-
colored solid was obtained. The solid was washed with 30 mL abso-
lute ethanol and dried under vacuum at 40Ϫ50 °C and cleaned by
sublimation under vacuum at 130 °C. Yield ca. 65 %; shining blue
crystals. Elemental analysis for C14H24O8V2: C 40.07 (calc. 39.83),
H 5.75 (5.73), V 23.83 (24.13) %.
2312
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Z. Anorg. Allg. Chem. 2004, 630, 2311Ϫ2318

New Oxovanadium(IV) Complexes with Mixed Ligands
Complex 3 was preferentially obtained by refluxing a mixture of
V2O5, H(thd), and aqueous ethanol (see complex 1) The solid prod-
uct was cleaned by sublimation at 85 °C. Yield ca. 65 %; needle-
shaped green crystals. Preparation of 3 was furthermore carried out
according to Refs. [3] (using a somewhat modified procedure; our
yield ca. 45 %) and [5] (our yield ca. 20 %). The three synthesis
methods gave identical PXD diagrams. Elemental analysis for
C22H38O5V: C 61.22 (calc. 60.96), H 8.70 (8.84), V 11.40 (11.75) %.
solution was then diluted (ca. 5:1) with distilled H2O. The clear
blue solution was gently heated on a hot plate at 50Ϫ70 °C for 1 d
and the temperature was thereafter gradually increased to 90 °C
where the rest of the liquid phase was evaporated. The container
with the resulting crust was then transferred to a furnace whose
temperature was gradually increased to 190 °C, kept at this tem-
perature for 1 d, cooled to r. t., and weighed. To verify that this
procedure leads to a product with a constant well-defined compo-
sition, a sample was heated in the thermogravimetric (TG) instru-
ment (O2 atmosphere; 2 °C/min) up to 700 °C. The TG showed
that no weight change had occurred and PXD confirmed that well-
crystalline V2O5 had been obtained. This knowledge was used to
determine the vanadium contents in 1Ϫ4.
Complex 4 was prepared according to the procedure used for com-
plex 2. After 1 d a dark-blue, almost black, solution was obtained,
but small amounts of unreacted V2O5 were seen after 2 d. After
another 3 d no unreacted V2O5 could be detected and the reaction
was stopped. During the overnight cooling to r. t. a dark-blue solid
precipitated. After filtering the solid product was washed with
absolute ethanol and dried at 50 °C under vacuum. Some batches
contained small amounts of green crystals of 3 which could be
removed by repeated sublimation at 85Ϫ105 °C. All fractions of
the blue crystalline substance obtained after a final sublimation at
160Ϫ170 °C proved to be phase pure 4. Yield ca. 85 %; blue shining
crystals. Elemental analysis for C26H48O8V2: C 53.39 (calc. 52.88),
H 8.19 (8.19), V 16.74 (17.25) %.
Powder X-ray diffraction (PXD)
All samples were characterized by PXD at 22 °C with a Siemens
D5000 diffractometer in reflection geometry using monochromatic
˚
CuKα1 radiation (λ ϭ 1.540598 A) from an incident-beam ger-
manium monochromator. The detector was a Brown PSD. The dif-
fraction patterns were collected over the 2θ range 5Ϫ90°, autoind-
exed with help of the DICVOL [17] and TREOR [18] programs,
and compared with simulated patterns on the basis of data from
the single-crystal structure determinations. PXD data collected in
capillary geometry were used for the Rietveld analyses by means
of the GSAS [19] software. PDF-type data are listed in the elec-
tronic deposit.
Suitable single crystals of 4 were obtained by recrystallization of
sublimed materials from absolute ethanolic solutions (saturated at
b. p.) at Ϫ20 to 20 °C. [We record that the examination of crystals
of 4 from different fractions of as-sublimed material revealed severe
twinning or a kind of polycrystalline fragmentation (4Ϫ5 or more
crystallites per specimen).]
Single-crystal X-ray diffraction (SXD) analysis
Elemental analysis
Carbon and hydrogen were determined by standard combustion
analysis (at Ilse Beetz). For the vanadium analyses the given com-
plex was first destroyed with excess conc. H2SO4. The resulting
Diffraction data covering more than a hemisphere of the reciprocal
space were collected with a Siemens SMART [20] CCD dif-
fractometer at 105 and 295 K by ω scans (graphite-monochromat-
Table 1 Single-crystal data and relevant parameters for the structure refinements of complexes 1Ϫ4 at 105(2) K; including unit-cell
dimensions from SXD at 295(2) K. Calculated standard deviations are given in parentheses. For unit-cell dimensions at 295(2) K from
PXD see the electronic deposit.
Complex no.
1
2
3
4
empiric formula
M
C10H14O5V
265.15
C14H24O8V2
422.21
monoclinic
P21/c
C22H38O5V
433.46
monoclinic
P21
C26H48O8V2
590.52
crystal system
space group
Z
triclinic
triclinic
¯
¯
P1
P1
2
4
2
1
˚
a/A
7.346(1)
8.149(1)
11.207(2)
72.86(1)
72.20(1)
66.94(1)
575.89(3)
7.5178(9)a)
8.2116(7)a)
11.229(1)a)
73.187(7)a)
71.493(8)a)
66.664(7)a)
592.9(1)a)
5394
2682
2547
0.0227
0.0628
0.0239
0.0639
14.775(2)
8.788(1)
15.742(2)
Ϫ
112.43(1)
Ϫ
1889.4(4)
15.083(6)
8.901(3)
15.953(6)
Ϫ
113.16(2)
Ϫ
1969.1(13)
30432
4683
3861
9.415(1)
10.833(1)
12.743(2)
Ϫ
108.81(1)
Ϫ
1230.3(3)
9.649(2)
11.032(3)
12.819(3)
Ϫ
108.84(1)
Ϫ
1291.4(5)
21724
6065
5932
8.871(3)
9.988(4)
10.299(4)
73.51(1)
68.70(1)
65.85(1)
765.8(5)
9.121(2)
9.527(2)
10.708(2)
71.90(1)
72.00(1)
68.34(1)
801.9(3)
6290
3101
2515
0.0396
0.1028
0.0545
0.1109
˚
b/A
˚
c/A
α/°
β/°
γ/°
3
˚
V/A
˚
a/A [295(2) K]
˚
b/A [295(2) K]
˚
c/A [295(2) K]
α/° [295(2) K]
β/° [295(2) K]
γ/° [295(2) K]
3
˚
V/A [295(2) K]
reflections collected
independent reflections
independent reflections [I2π(I)]
R1 [I2σ(I)]
wR2 [I2σ(I)]
R1 (all data)
0.0326
0.0833
0.0444
0.0890
0.0228
0.0615
0.0235
0.0619
wR2 (all data)
a) Quoted from Ref. [9] after unit-cell transformation.
Z. Anorg. Allg. Chem. 2004, 630, 2311Ϫ2318
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M. A. K. Ahmed, H. Fjellvag, A. Kjekshus, B. Klewe
ized MoKα radiation). Data for several crystals were collected.
Data reduction (SAINT [21]), absorption correction (multiscan ap-
proach according to SADABS [22]), space group establishment,
structure determination (by direct methods), and refinements
(SHELXTL [23]) were performed. All non-hydrogen atoms were
refined with anisotropic displacement factors except for the minor
components of disordered atoms. The hydrogen atoms were con-
strained to ride on their parent atoms. Crystal data, data collection
information, unit-cell dimensions, and R factors for the reported
structures at 105 K are listed in Table 1, which also includes the
SXD unit-cell dimensions obtained at 295 K. (Further crystallo-
graphic data in CIF format are found in the electronic deposit.)
The existence of complexes 2 and 4 were detected during
attempts to vary the aqueous content of the ethanolic solu-
tions. The complexes 2 or 4 have different colors from 1
and 3 (pronounced blue versus turquoise/green) and have
higher sublimation temperatures. The distinctions were
further confirmed by elemental analyses, PXD, and SXD
(see the Experimental Section and below). Complexes 2 and
4 are highly soluble in hot (slightly in cold) absolute etha-
nol. It is difficult to conclude on whether 1 and 3 occur as
distinguishable intermediates in the formation of 2 and 4,
respectively. However, the fact that reaction rates for the
formation of 2 and 4 are generally low and that 2 and 4
can, respectively, be obtained by treatment of 1 and 3 in
absolute ethanol lends some support to 1 and 3 acting as
intermediates. If the synthesis period of 2 and 4 is too short
and the water content of the ethanol is too high mixtures
of 1 and 2 or 3 and 4 are obtained.
Thermal analysis
TG analysis was performed with a Perkin Elmer TGA7 system in
N2 atmosphere or under 0.7 mbar pressure. Al containers were gen-
erally used as sample holders; quartz containers were used for the
vanadium determinations. The heating rate was 2Ϫ10 °C/min and
the sample mass 15Ϫ40 mg.
Sublimation properties
Sublimation is a convenient remedy to purify the reaction
products or to separate mixtures of 1 and 2 or 3 and 4.
Suitable sublimation temperatures for this purpose are
given in the Experimental Section. The results from the TG
studies in N2 atmosphere or under vacuum (Fig. 1) show
that all four complexes sublime over appreciable ranges of
temperature. The dynamic conditions during the TG treat-
ment lead to somewhat higher onset temperatures for the
sublimation than those established by the sublimation ap-
paratus.
Magnetic susceptibility
Magnetic susceptibility measurements were systematically performed
at 5Ϫ300 K in a magnetic field of 0.1 T using a Quantum Design
MPMS instrument. The sample mass was 12Ϫ40 mg, sample holders
of gelatin were used, and the maximum field was 5.5 T.
Results and Discussion
Syntheses
The complexes 1 and 3 can be synthesized from various
vanadium reactants and in different solvents. The chemi-
cally simplest route is to react a compound that already
contains the VO2ϩ species with H(acac) or H(thd) [2, 3].
The formation of 1 or 3 then proceeds as a one-step acid/
base reaction, the dissolution of the oxovanadium(IV) salt
in H(acac) or H(thd) acting as the rate-determining factor.
With V2O5 as reactant the process must necessarily be a
two-step reaction, involving a red/ox stage in addition to
the mentioned acid/base reaction. V2O5 is clearly reduced
to VO2ϩ by H(acac) or H(thd) whereas the oxidation prod-
ucts have not been identified. [Triketones of the type
(CH3CO)2CO (!) have been introduced in Ref. [4] to balance
the red/ox equation for formation of 1.]
The attempt to introduce ethanol as solvent for the reac-
tants was economically motivated. As ethanol is far less ex-
pensive than H(acac) or H(thd) the successful preparation
of 1 and 3 in 50Ϫ80 % (the limits may be extended some-
what in both directions) aqueous ethanolic solutions rep-
resent a major saving of expenditure. The yields of 1 and 3
in ethanolic solutions were comparable to those with pure
H(acac) or H(thd) as solvents (the highest ethanolic con-
centrations give even better yields). The advantage of pure
H(acac) or H(thd) as solvent concerns the reaction rate;
completed reaction after 1 d versus 5 d in aqueous ethanol.
It should be recorded that 1 and 3 are highly soluble in hot
H(acac) and H(thd), respectively, and moderately soluble in
the cold solvents.
Fig. 1 Thermogravimetric (TG) data for the complexes (a) 1 and
2 and (b) 3 and 4 in N2 atmosphere and under vacuum. Heating
rate 2 °C/min.
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New Oxovanadium(IV) Complexes with Mixed Ligands
According to TG, onset of sublimation occurs at tem-
peratures between 100 and 200 °C depending on the heating
rate, the composition of the complex, and to the lesser ex-
tent on the atmosphere (N2 or vacuum; note that the subli-
mation rate differs appreciably). For TG under vacuum
conditions the sublimation process is completed at ca. 160,
190, 205, and 220 °C for 3, 1, 2, and 4, respectively. The
residues after completed sublimation under vacuum are ne-
gligibly small for 1, 3, and 4 (1 Ϫ ∆m/m0 ഠ 0 in Figure 1;
up to 5 % residue in some experiments; ca. 10 % residue for
most runs on 2). The main weight losses in N2 atmosphere
are shifted 50Ϫ100 °C to higher temperatures than in vac-
uum. The increased amounts of residue for complexes 1, 2,
and 4 in N2 atmosphere (Fig. 1; up to 20Ϫ50 % residue
including results for repeated experiments) reflects the in-
creasing thermal decomposition of the complexes at higher
temperatures. (The black-to-gray residues consisted mainly
of mixtures of vanadium oxides, which, however, were not
properly characterized in this study.) The TG behavior of 3
differs from this trend by just showing a slight increase in
residue on turning to N2 atmosphere. Hence the thermal
decomposition of these complexes is evidently composite
processes that involve many factors.
a)
b)
Magnetic properties
c)
The reciprocal magnetic susceptibility versus temperature
relationships for the complexes 1 and 3 closely follow the
Curie law, whereas 2 and 4 satisfy the CurieϪWeiss law above
ca. 95 and 120 K, with θ ഠ Ϫ80 and Ϫ110 K, respectively.
The former set of values may be taken as the antiferromag-
netic ordening temperature for the vanadium moments
within the binuclear complexes 2 and 4. Below the respect-
ive ordering temperature the χϪ1(T) curves for 2 and 4 show
slight negative and positive deviations, respectively, from
CurieϪWeiss-law relationships. No attempts have been
made to extract magnetic exchange-interaction parameters
from the moment-ordered regions of the χϪ1(T) curves.
After correction for induced diamagnetism [24] the para-
magnetic moment values (from repeated experiments for all
complexes: µP ϭ 1.62Ϫ1.75 µB) came out reasonably close
d)
1
to the expected spin-only value for S ϭ /2 (µP ϭ 1.73 µB).
This confirms that the complexes 1Ϫ4 are proper VIV com-
pounds.
Crystal structures
The crystal structures (Fig. 2 and Table 1) were determined
(2 and 4) or redetermined (1 and 3) by SXD at 105 and
295 K, but the following presentation will be based on the
105 K data with reference to the 295 K data when relevant.
The present structural data for complexes 1 and 3 are in
good agreement with those reported in the literature
[7Ϫ10], in particular the findings of the most recent studies
[9, 10]. The derived atomic coordinates from SXD were
fixed during Rietveld refinements of PXD data (295 K),
and the perfect fits thus obtained confirmed the correctness
Fig. 2 Molecular structure and atomic labeling scheme for com-
plexes (a) 1, (b) 2, (c) 3, and (d) 4 (note that 4 has inversion symme-
try). Vanadium, oxygen, and carbon atoms are represented by ther-
mal ellipsoids drawn at the 50 % probability level, hydrogen atoms
(see text) being omitted for clarity. Disorder is indicated for C(11),
C(12) in part b and C(8)ϪC(11) in part c. Note the variation in
the numbering of the ligand atoms.
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˚
M. A. K. Ahmed, H. Fjellvag, A. Kjekshus, B. Klewe
˚
ted planes), but the two acac skeletons of the same molecule
are not coplanar and form an angle of 15.7° with each
other; viewed from the acac skeleton planes vanadium is
Table 2 Selected bond distances (in A) at 105 K for VO(acac)2
(1), (VO)2(acac)2(µ-OEt)2 (2), VO(thd)2 (3), and (VO)2(thd)2(µ-OEt)2
(4). Calculated standard deviations in parentheses. Complex 4 has
inversion symmetry, the label A referring to atoms at x¯, y¯, z¯. Sup-
plementary data in the electronic deposit and Refs. [7Ϫ10].
˚
located 0.432 and 0.290 A above these planes. The planarity
within each acac ring also approximately comprises the car-
bon atoms of the methyl groups linked to the ring skeletons.
The structural characteristics of complex 1 are prominent
also in the structure of complex 3 (see Fig. 2a,c), i.e.,
square-pyramidal coordination around vanadium and
planar ring systems (with virtually identical building bricks)
that meet at an angle of 13.0°. The change from methyl to
tertiary butyl in the attachments to the ketonato rings im-
plies that the entire carbon skeleton of the thd ligand no
longer can be planar (see Fig. 2c). However, the central car-
bon atoms of the tertiary butyl groups in 3 are also approxi-
mately coplanar with the ketonato rings (deviation from
1
VϪO(1)
VϪO(2)
VϪO(3)
VϪO(4)
VϪO(5)
O(2)ϪC(2)
O(3)ϪC(4)
1.600(1)
1.974(1)
1.979(1)
1.969(1)
1.976(1)
1.290(2)
1.284(2)
O(4)ϪC(7)
O(5)ϪC(9)
C(2)ϪC(3)
C(3)ϪC(4)
C(7)ϪC(8)
C(8)ϪC(9)
1.285(2)
1.281(2)
1.393(2)
1.404(2)
1.396(2)
1.404(2)
2
V(1)ϪO(1)
V(1)ϪO(3)
V(1)ϪO(4)
V(1)ϪO(7)
V(1)ϪO(8)
V(1)ϪV(2)
O(3)ϪC(2)
O(4)ϪC(4)
O(7)ϪC(11)
O(8)ϪC(13)
C(2)ϪC(3)
C(3)ϪC(4)
1.590(2)
1.980(1)
1.967(1)
1.958(1)
1.970(1)
3.064(1)
1.280(2)
1.286(2)
1.410(1)
1.434(2)
1.389(3)
1.392(3)
V(2)ϪO(2)
V(2)ϪO(5)
V(2)ϪO(6)
V(2)ϪO(7)
V(2)ϪO(8)
O(5)ϪC(7)
O(6)ϪC(9)
C(7)ϪC(8)
C(8)ϪC(9)
C(11)ϪC(12)
C(13)ϪC(14)
1.588(2)
1.970(1)
1.977(2)
1.966(1)
1.962(1)
1.281(2)
1.280(2)
1.391(3)
1.393(3)
1.495(2)
1.509(3)
˚
planarity ranging between Ϫ0.211 and 0.224 A). The struc-
ture determination of 3 unveiled disorder (22 % at 105 K)
within the methyl groups of the tertiary butyl units [illus-
trated at the C(8) side branch in Fig. 2c].
The essential difference between the structures of com-
plexes 2 and 4 (Fig. 2b,d), on the one hand, and 1 and 3
(Fig. 2a,c), on the other, concerns the binuclear nature of
the former complexes. However, the main characteristics of
the mononuclear complexes 1 and 3 are retained in 2 and
4. The square-pyramidal coordination around vanadium
and the virtually planar ketonato ring systems of acac and
thd are found also in the structures of 2 and 4, but the
vanadium coordination polyhedra have become somewhat
more distorted owing to the different bonding situation of
the ethanolato and ketonato oxygens. However, the base of
the individual pyramids of 2 and 4 is still largely planar.
One of the ethanolato ligands of 2 is subject to some 20 %
disorder at 105 K (see Fig. 2b).
There is also a striking mutual difference between the
structures of 2 and 4 (Fig. 2b,d) in that the vanadyl oxygen
atoms [O(1) and O(2)] and the ethanolato carbon atoms
[C(11), C(12) and C(13), C(14)] of 2 are in syn configuration
whereas those [O(1) and C(12), C(13), respectively] of 4 are
in anti configuration. In fact, also the characteristic rings
of the β-diketonato ligands can be regarded to adopt syn
configuration in 1؊3 and anti configuration in 4. The mo-
lecular structure of 4 has inversion symmetry (Ci), whereas
the individual molecules of 1 and 3 exhibit approximately
C2v symmetry. The distortion of the pyramidal base plane
around vanadium and the overall deviation from planarity
of complex 4 are smaller than in 2 (see Fig. 2b,d and the
electronic deposit).
3
VϪO(1)
VϪO(2)
VϪO(3)
VϪO(4)
VϪO(5)
O(2)ϪC(5)
O(3)ϪC(7)
1.591(1)
1.972(1)
1.963(1)
1.967(1)
1.980(1)
1.284(1)
1.281(1)
O(4)ϪC(16)
O(5)ϪC(18)
C(5)ϪC(6)
C(6)ϪC(7)
C(16)ϪC(17)
C(17)ϪC(18)
1.281(1)
1.278(1)
1.394(2)
1.397(2)
1.392(2)
1.407(2)
4
VϪO(1)
VϪO(2)
VϪO(3)
VϪO(4)
VϪO(4A)
VϪV(A)
1.580(2)
1.973(2)
1.956(2)
1.962(2)
1.962(2)
3.089(1)
O(2)ϪC(5)
O(3)ϪC(7)
O(4)ϪC(12)
C(5)ϪC(6)
C(6)ϪC(7)
C(12)ϪC(13)
1.275(3)
1.283(3)
1.431(3)
1.394(3)
1.389(3)
1.497(3)
of the structure solutions obtained by SXD as well as de-
monstrating absence of preferred orientation effects in the
PXD data provided for product characterization (see the
electronic deposit).
Although a description of the crystal and molecular
structure of 1 is already available [7] we briefly recapitulate
on a few aspects, which reappear in the structures of com-
plexes 2Ϫ4. The vanadium atom is virtually square-pyrami-
dally coordinated to five oxygen atoms (Fig. 2a) with va-
nadium near the center of gravity of the pyramid (actually
The molecular-type structures of 1Ϫ4 have only van der
Waals interaction between the individual molecular units,
and the main effect of going from 105 to 295 K is to
lengthen the intermolecular van der Waals distances (see
Table 1 and deposited data). The axes of the unit cells
(Table 1) of 1Ϫ3 expand 0.3Ϫ2.5 % between 105 and 295 K
whereas only minor changes are seen in the cell angles.
More extensive changes occur for complex 4, in particular
a 4.6 % shrinkage of b and significant alterations of α, β,
˚
0.537 A above the basal plane of the pyramid; distances
quoted in the text without standard deviations refer to aver-
ages). The vanadyl oxygen atom (Fig. 2) is located at the
apex of the pyramid whereas the ketonato oxygen atoms of
the acac ligands constitute the base of the pyramid. Each
acac skeleton is virtually planar (specifications for planes in
the following structure descriptions refer to least-square fit-
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Z. Anorg. Allg. Chem. 2004, 630, 2311Ϫ2318

New Oxovanadium(IV) Complexes with Mixed Ligands
and γ on going from 105 to 295 K. These impacts on the
unit-cell metrics are reflected in the appreciable adjustments
of features in the molecular architecture that are uncon-
strained by the bond lengths and angles (see above and be-
low). The large number of unsuitable SXD specimens,
which were detected for as-sublimed crystals of 4 can poss-
ibly also be explained by interior stress enforced during the
adjustments of the geometrical shape of the complex dur-
ing cooling.
Table 3 Bond-valence data for VO(acac)2 (1), (VO)2(acac)2-
(µ-OEt)2 (2), VO(thd)2 (3), and (VO)2(thd)2(µ-OEt)2 (4) at 105 K.
Data for the crystallographically different vanadium atoms of 2 are
separated by semicolons.
Complex no.
1
2
3
4
VϪOapic.
VϪOket.
VϪOeth.
1.65
1.69; 1.70
1.69
1.74
0.59Ϫ0.61 0.59Ϫ0.61; 0.59Ϫ0.61 0.59Ϫ0.62 0.60Ϫ0.63
0.61Ϫ0.63; 0.61Ϫ0.62
1.31Ϫ1.34 1.32Ϫ1.35
0.62Ϫ0.62
1.33Ϫ1.35 1.34Ϫ1.37
0.90Ϫ0.90
The influence of temperature on the molecular structure
of the different complexes is reflected in the variation of the
angle between the planes of the β-diketonato ligands. In 1
and 3 this angle changes from, respectively, 15.7 and 13.0°
at 105 K to 20.4 and 20.9° at 295 K. The temperature effects
are less pronounced in 2 and 4. The angle between the acac
rings in 2 goes from 62.1° at 105 K to 64.6° at 295 K. The
thd rings in 4 are related by a centre of symmetry and these
planes are accordingly parallel with an interplanar distance
O
O
V
ket.ϪCket.
eth.ϪCeth.
0.89Ϫ0.89
4.12; 4.13
1.69; 1.70
4.04
1.65
4.10
1.69
4.21
1.74
Oapic.
Oket.
Oeth.
1.91Ϫ1.94 1.93Ϫ1.94; 1.94Ϫ1.95 1.94Ϫ1.95 1.97Ϫ1.97
2.11Ϫ2.13 2.13Ϫ2.13
˚
of 0.725 and 0.707 A at 105 and 295 K, respectively.
come out somewhat below and above 2, respectively
(Table 3). Each ketonato oxygen atom in complexes 1Ϫ4 is
bonded to one vanadium atom and one carbon atom, the
corresponding bonds appearing to be appreciably weaker
(average VOϪV ϭ 0.60) and stronger (average VOϪC ϭ 1.34)
than single bonds. The situation for the ethanolato oxygen
atoms in complexes 2 and 4 is more uncommon by having
oxygen atoms bonded to three other atoms. The bond val-
ence for the (ethanolato) oxygen linkages between the two
vanadium atoms (average VOϪV ϭ 0.61) remains virtually
identical with VOϪV for the ketonato oxygen atoms (see
above). The bond valence directed toward the neighboring
ethanolato carbon atoms (average VOϪC ϭ 0.89) is, how-
ever, reduced to about 2/3 of VOϪC for the ketonato oxygen
atoms. The message is accordingly that it is the ethanolato
oxygen atoms which are burdened with all consequences of
the bonding adjustments following the extension from two
(for Oket.) to three (for Oeth.) directed bonds per oxygen
atom (see Fig. 2aϪd). In this respect the bonding situation
in 2 and 4 is different from that in oxovanadium(V) alkox-
ide complexes [26Ϫ28] with a similar square-pyramidal
configuration of five oxygen atoms around the vanadium
central atoms. However, here the alkoxide is connected to
just one vanadium atom and the total bond valence for
vanadium is approximately five. The essential structural
distinction between the two bonding situations is just the
location of the oxygenϪcarbon bond in the inner part or
outskirts of the complex framework.
Briefly summarized it is found that the geometrical con-
figuration of the five square-pyramidally-arranged O atoms
around the central V atom is essentially identical in 1؊4.
The mutual variation within the different kinds of VϪO
bonds is remarkably invariant (see Table 2). The mean value
˚
for the apical VϪO bond length is 1.590 A and for the
˚
equatorial VϪO bond lengths 1.971 A, V being located 0.55
˚
and 0.60 A above the base of the pyramid in 1, 3 and 2, 4,
˚
respectively. The O····O bite distance is 2.734 and 2.704 A
in 1 and 3, respectively, whereas the corresponding lengths
of the other O····O edges of the pyramid bases are 2.640
˚
and 2.637 A. The O····O bite distances are shortened
˚
˚
ϳ 0.02 A (to 2.716 and 2.681 A, respectively) in 2 and 4,
the O····O distance between the ethanolato oxygen atoms is
˚
2.441 and 2.421 A, and the other O····O edges are increased
by ϳ 0.09 A on going from 1 to 2 and 3 to 4. The carbon
˚
skeletons of the acac and thd ligands are planar except for
the terminal methyl and tertiary butyl groups whose central
˚
carbon atom is up to 0.11, 0.08, 0.22, and 0.24 A out of the
plane at 105 K in 1, 2, 3, and 4, respectively. Normal CϪO
and CϪC bond lengths (see Table 2) and OϪCϪC and
CϪCϪC bond angles (see the electronic deposit) were
found for the organic ligands.
It is instructive to consider the crystal structures of 1Ϫ4
in relation to the bond-valence data for the involved
vanadium and oxygen atoms. The bond valence, Vij, for a
given bond distance, dij, is given by [25]:
The bond valences show unequivocally that apical oxygen
Vij ϭ exp[(DijϪdij) / b]
atoms (average VO
ϭ 1.69) are in a multiple-bonding
apec.ϪV
situation. However, whereas the bond-valence sums for the
vanadium atoms come out reasonably close to four (see Table
3; average for 1Ϫ4: VV ϭ 4.12), the apical oxygen atoms ap-
pear to be heavily underbonded. A similar situation is found
in other crystal structures which comprise vanadyl groups
[26Ϫ28]. Significant bond-valence contributions from sur-
rounding inter- or intramolecular arranged atoms in the
complexes are excluded since the interatomic distances in
question are too long to remedy the actual deficiency.
IV
˚
˚
using b ϭ 0.37 A and Dij ϭ 1.784 and 1.39 A for V ϪO
and CIVϪO, respectively. The bond-valence data at 105 K
(Table 3) show excellent mutual constancy, and confirm
that the vanadium and oxygen bond valences are largely in
conformity with the assumed formal oxidation states four
and two, respectively. Probably not too much significance
should be attached to the finding that the bond valences of
the ketonato and ethanolato oxygen atoms systematically
Z. Anorg. Allg. Chem. 2004, 630, 2311Ϫ2318
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2317

˚
M. A. K. Ahmed, H. Fjellvag, A. Kjekshus, B. Klewe
Acknowledgement. This project has received financial support from
the Research Council of Norway.
An obvious source of error in bond-valence consider-
ations originates from the bond-valence “constants” Dij.
These are neither precisely established nor proper constants
for a given pair of atoms. With some 4 % upward adjust-
ment of Dij for the VIVϪO bond it is, isolated seen, indeed
possible to obtain a bond valence close to two for the
VϪOapec. bond, but an inevitable consequence of such an
adjustment would be to enforce surplus bond-valence sums
of around five for the vanadium atoms. One is accordingly
left with the inescapable conclusion that essential features
of the multiple-bond characteristics (i.e., σ versus π partici-
pation) are not caught by the bond-valence concept. (Note
that this discrepancy cannot be resolved by turning to
Browns [29, 30] alternative approach to bond valence, ac-
cording to which nearly the same bond valences as given in
Table 3 are obtained.)
References
[1] A. Guyard, Bull. Soc. Chim. Fr. 1876, 25, 350.
[2] G. T. Morgan, H. W. Moss, J. Chem. Soc. 1913, 103, 78.
[3] R. A. Rowe, M. M. Jones, Inorg. Synth. 1957, 5, 114.
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of the studied complexes of four-valent vanadium with β-
diketonato and mixed β-diketonato and ethanolato ligands
show distinct points of resemblance. The occurring distinc-
tions concern mainly the atomic arrangement within and
between the ligands whereas the configuration of the five
square-pyramidally-arranged oxygen atoms around va-
nadium remains virtually identical. The bonding VϪO dis-
tances occur in two categories, one reflecting the multiple
bonding to the terminal oxygen atom and another the frac-
tional bonding to ketonato and/or ethanolato oxygen
atoms. The individual VϪO distances within each category
remain constant, in most cases the scatter is well within
[10] U. K. Urs, K. C. Anitha, K. L. Raghunathan, S. A. Shivashan-
kar, W. T. Robinson, T. N. G. Row, Acta Crystallogr. 2001,
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[12] S. Dilli, E. Patsalides, Aust. J. Chem. 1976, 29, 2369.
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[17] A. Boultif, D. Louer, J. Appl. Crystallogr. 1991, 24, 987.
[18] P.-E. Werner, Program TREOR, Version 5, University of
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[20] SMART Area-detector Control, Version 5.054, Bruker AXS
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[21] SAINT Integration Software, Version 6.22, Bruker AXS, Inc.,
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[22] G. M. Sheldrick, SADABS, Empirical Absorption Correction
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Madison, WI, 2000.
˚
0.02 A. Except for the ethanolato OϪC bond distances, the
bond distances within the organic ligands show even
smaller mutual variation. The virtual planarity of the β-
diketonato rings and the bond angles therein signalize that
the carbon atoms involved may be regarded as sp2 hy-
bridized. The features which have most noticeable individu-
ality are the angle between the ketonato planes in the
mononuclear β-diketonato and binuclear β-diketonato/
ethanolato complexes and the mutual syn/anti distinction
between complexes 1؊3 on one hand and 4 on the other.
By and large, the overall molecular geometry of these com-
plexes is a compromise between the square-pyramidal con-
figuration demanded by the vanadium central atom and co-
planarity between the β-diketonato rings which would have
promoted conjugation between the two rings. All in all the
valence demands of vanadium are best taken care of in the
actual molecular arrangements of 1Ϫ4, viz. vanadium is the
conqueror of the competition for bonding advantages.
[24] P. E. Selwood, Magnetochemistry, Interscience, New York Ϫ
London, 1956.
[25] N. E. Brese, M. O. O’Keeffe, Acta Crystallogr. 1991, B47, 192.
[26] R. J. H. Clark, The Chemistry of Titanium and Vanadium, El-
sevier, Amsterdam Ϫ London Ϫ New York, 1968.
[27] H. Lütjens, G. Wahl, F. Möller, P. Knochel, J. Sudermeyer,
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Tahir, D. C. Crans, Inorg. Chem. 1998, 37, 5439.
[29] I. D. Brown, K. K. Wu, Acta Crystallogr. 1976, B32, 1957.
[30] I. D. Brown, in: Structure and Bonding in Crystals, Vol. 2, M.
O’Keeffe, A. Navrotsky (eds), Academic Press, New York Ϫ
London Ϫ Toronto Ϫ Sydney Ϫ San Francisco, 1982, p. 1.
Available supporting information: Crystallographic data for the
structure(s) have been deposited with the Cambridge Crystallo-
graphic Data Centre, deposition numbers: CCDC 230339 Ϫ
230345. Copies of the data can be obtained free of charge on
application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: int. code ϩ(1223)336-033; e-mail:
deposit@ccdc.cam.ac.uk).
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