Synthesis and crystal structure of [Cr(thd)2(OEt)]2

Article abstract of DOI:10.1002/zaac.201000241

The mixed-ligand complex [Cr(thd)₂(OEt)]₂ [(thd) ^- = anion of H(thd) = C₁₁H₂₀O₂ = 2,2,6,6-tetramethylheptane-3,5-dione] appears as by-product when EtOH/H ₂O is used as solvent during preparation of Cr(thd)₃. [Cr(thd)₂(OEt)]₂ can be difficult to separate from Cr(thd)₃ by sublimation, but separation is easily accomplished by extracting Cr(thd)₃ with acetone. A detailed account for the sublimation behavior of [Cr(thd)₂(OEt)]₂/Cr(thd) ₃ mixtures is advanced. Good yields of [Cr(thd)₂(OEt)] ₂ are obtained when CrCl₃, H(thd), and Na(EtO) react in absolute EtOH. [Cr(thd)₂(OEt)]₂ is obtained in the form of green needle-shaped crystals by recrystallization from toluene. The crystal structure is triclinic [a = 10.2919(15), b = 10.6686(16), c = 14.194(3) A, α = 106.559(2), β = 107.869(2), and γ = 98.326(2)° at 295 K; space group P1. The complex contains two crystallographic equivalent chromium atoms, which are bridged by two cis-configured ethoxy groups, the four remaining sites in the octahedral coordination around each chromium atom being occupied by oxygen atoms from two thd ligands. The bond lengths and angles concur with the findings for related molecular complexes. The temperature dependence of the magnetic susceptibility of [Cr(thd)₂(OEt)] ₂ follows Curie-Weiss law with Weiss constant θ ≈ -65 K and μ_p = 3.87 μ_B. Copyright

Full text of DOI:10.1002/zaac.201000241

ARTICLE
DOI: 10.1002/zaac.201000241
Synthesis and Crystal Structure of [Cr(thd)2(OEt)]2
Mohammed A. K. Ahmed,[a] Helmer Fjellvåg,[a] Arne Kjekshus,*[a] David S. Wragg,[a]
and Nalinava Sen Gupta[a]
Keywords: [Cr(thd)2(OEt)]2; X-ray diffraction; Magnetic properties; Sublimation; Chromium
Abstract. The mixed-ligand complex [Cr(thd)2(OEt)]2 [(thd)– = anion structure is triclinic [a = 10.2919(15), b = 10.6686(16), c
=
of H(thd) = C11H20O2 = 2,2,6,6-tetramethylheptane-3,5-dione] appears 14.194(3) Å, α = 106.559(2), β = 107.869(2), and γ = 98.326(2)° at
¯
as by-product when EtOH/H2O is used as solvent during preparation 295 K; space group P1. The complex contains two crystallographic
of Cr(thd)3. [Cr(thd)2(OEt)]2 can be difficult to separate from Cr(thd)3 equivalent chromium atoms, which are bridged by two cis-configured
by sublimation, but separation is easily accomplished by extracting ethoxy groups, the four remaining sites in the octahedral coordination
Cr(thd)3 with acetone. A detailed account for the sublimation behavior around each chromium atom being occupied by oxygen atoms from
of [Cr(thd)2(OEt)]2/Cr(thd)3 mixtures is advanced. Good yields of two thd ligands. The bond lengths and angles concur with the findings
[Cr(thd)2(OEt)]2 are obtained when CrCl3, H(thd), and Na(EtO) react for related molecular complexes. The temperature dependence of the
in absolute EtOH. [Cr(thd)2(OEt)]2 is obtained in the form of green magnetic susceptibility of [Cr(thd)2(OEt)]2 follows Curie–Weiss law
needle-shaped crystals by recrystallization from toluene. The crystal with Weiss constant θ ≈ –65 K and μp = 3.87 μB.
checkers during sublimation treatment of products. This paper
1 Introduction
focuses on this material with emphasis on formation, sublima-
The official policy of the Editorial Board of the annual series
Inorganic Syntheses is to provide the scientific community
with “detailed and foolproof procedures for preparation” of in-
organic substances [1]. Each reported procedure “should be the
best one available”, with special attention on “yield and purity
of product”. With such high ambitions the success of the
project will depend crucially on competence, practical skill,
attitude to details, skepticism to unexpected events, dedication
to commission, etc. of the originators as well as the checkers
of the reports.
In 1987 a report appeared in this periodical [2] on the synthe-
sis of Cr(thd)3 [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6-
tetramethylheptane-3,5-dione]. Strictly speaking the prepara-
tion of Cr(thd)3 does not qualify for classification among “out-
standing syntheses”, but reference [2] satisfies most of the
other criteria set for such reports. Reference [2] inter alia
makes no secret of lacking phase purity of the as-synthesized
products. The present authors [3] have confirmed that the “pur-
ple platelets” and “ruby-red needle-shaped” ingredients of the
products correspond to polymorphs of Cr(thd)3. Reference [2]
moreover mentions a “light green, slightly less volatile, con-
taminant” (alternatively referred to as “fluffy powder”), which
apparently caused some problems for both originators and
tion, and crystal and molecular structure.
Reference [2] does not specify the relative amounts of
Cr(thd)3 and [Cr(thd)2(OEt)]2 in the as-synthesized products,
but the use of the term contaminant may lead one to imagine
small amounts. The present authors, on the other hand, have
established [3] that some 5 wt.-% of the as-synthesized pro-
ducts, made according to the recipe in reference [2], consist of
[Cr(thd)2(OEt)]2. Moreover there appear to be snags (sect. 3.2)
associated with the use of sublimation to separate the main
product Cr(thd)3 from the by-product [Cr(thd)2(OEt)]2. It is
highly relevant to establish what causes the appearance of
[Cr(thd)2(OEt)]2 in synthesis of Cr(thd)3 according to the rec-
ipe of reference [2]. Other natural questions are: Where in the
procedure does it form? How does it manifest itself? Can
measures be taken to prevent its occurrence or can its abun-
dance be reduced? How can [Cr(thd)2(OEt)]2 be separated
from Cr(thd)3? Some [Cr(thd)2(OEt)]2–Cr(thd)3 relations have
been briefly touched on in reference [3].
2 Experimental Section
2.1 Reactants and Solvents
CrCl3·6H2O (Aldrich, ≥ 96 %), H(thd) (Aldrich, purum, ≥ 98 %), urea
(Aldrich, 99 %), sodium (Merck, rod), HNO3 (Prolabo, 68–70 %),
KOBu-tert (Fluka, ≥ 97.0 %), absolute (abs.) ethanol (Arcus, prima),
methanol (Merck, p.a.), 1-propanol (Aldrich, 99.5 %), 2-propanol
(Fluka, ≥ 99.5 %), tert-BuOH (Merck, p.a.), acetone (Prolabo, for
HPLC), 1,4-dioxane (Fluka, p.a., ≥ 99.5 %), acetonitrile (Merck, for
GC, ≥ 99.8 %), toluene (Merck, p.a.), tetrahydrofuran (Prolabo, p.a.),
benzene (Fluka, p.a., ≥ 99.5 %), and hexane (Fluka, p.a., ≥ 99.5 %)
* Prof. Dr. A. Kjekshus
Fax: +47-22855441
E-Mail: arne.kjekshus@kjemi.uio.no
[a] Center for Materials Science and Nanotechnology
Department of Chemistry
University of Oslo
P.O. Box 1033 Blindern
0315 Oslo, Norway
56
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 637, 56–61

Synthesis and Crystal Structure of [Cr(thd)2(OEt)]2
were used as reactants and/or solvents, mostly without further purifica-
tion. Methanol, ethanol, and toluene were dried, distilled, and stored
over molecular sieves.
2.4 X-ray Diffraction
All samples were characterized by PXD at 22 °C with a Siemens
D5000 diffractometer (capillary geometry) using monochromatic Cu-
Kα1 radiation (λ = 1.540598 Å) from an incident-beam germanium
monochromator, and position-sensitive detector (Brown); silicon (a =
5.431065 Å) served as internal standard. The diffraction patterns were
collected over the 2θ range 3–90° and indexed with use of DICVOL
[5] and TREOR [6]. Unit-cell dimensions were obtained by least-
squares refinements (METRIC [7]).
2.2 Syntheses
Syntheses were performed in round-bottomed flasks equipped with re-
flux condenser and magnetic stirrer. The reaction mixtures were heated
under reflux at boiling temperature (b.t.) or heated at fixed lower tem-
peratures for different periods of time. Solid products were filtered
through a sinter-glass funnel, washed, dried under vacuum, and finally
sublimed (Büchi-type B-580 apparatus at ca. 0.05 mbar pressure) and/
or recrystallized from various solvents. The phase purity of the final
products was checked by powder X-ray diffraction (PXD).
Crystals were mounted on thin glass fibers on brass pins. Single-crystal
X-ray diffraction (SXD) data were collected at 295 K with a Bruker
D8 Apex II diffractometer (Mo-Kα radiation). The data were integrated
with SAINT [8] and corrected for absorption using SADABS [9]. The
crystal structures were solved by direct methods with the program
SIR2004 [10] or SHELXS [11] and refined using full-matrix least-
squares against |F|2 with SHELXL as implemented in Farrugia’s
WinGX suite [12]. All non-hydrogen atoms were refined allowing for
anisotropic displacement parameters (introduced for chromium already
from the beginning and for carbon and oxygen at the penultimate stage
before the hydrogen atoms were added). Hydrogen atoms were as-
signed to idealized positions and refined with isotropic thermal param-
eters proportional to the thermal parameter for the atoms, to which
they are attached (riding model).
[Cr(thd)2(OEt)]2 was prepared along three different routes. First, it ap-
peared [3] as a by-product (ca. 5 wt.-%) during syntheses of Cr(thd)3
according to the recipe in reference [2] [modified through the use of
1:3 stoichiometric proportions of the reactants CrCl3·6H2O and
H(thd)].
Second, a parallel route started by heating Cr(thd)3 in abs. EtOH to
reflux. After some 5 days (d) the reaction mixture was cooled to room
temperature, the solvent was evaporated, afterwards the solid product
was treated with acetone to remove unreacted Cr(thd)3, and finally
recrystallized from toluene {result ca. 5 wt.-% [Cr(thd)2(OEt)]2}.
Crystallographic data (excluding structure factors) for the structure in this
paper have been deposited with the CCDC Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB21EZ. Copies of the data
can be obtained on quoting the depository number CCDC-786172.
A third approach adopted an analogous procedure to that used for the
synthesis of [Cr(thd)2(OMe)]2 [4]. CrCl3·6H2O was used instead of the
more expensive CrC13·3THF and converted to CrCl3 by heating at
200 °C for 1 d under vacuum. Anhydrous CrCl3 (0.04 mol) was added
to abs. EtOH (ca. 70 mL) in a round-bottomed flask. In another flask
sodium (0.12 mol) was added to abs. EtOH (ca. 100 mL) and stirred
until all the sodium had reacted. The clear colorless solution of sodium
ethoxide thus obtained was added together with H(thd) (0.08 mol) to
the suspension of CrCl3, and the resulting mixture was heated to reflux
with stirring for 1 d. The mixture was left to cool to room temperature,
afterwards the mixture was filtered and the green solid product was
dried under vacuum at 40 °C for 5 h and purified by recrystallization,
first from benzene or hexane and afterwards from toluene. This proce-
dure gave a yield of ca. 93 % and well developed crystals. The elemen-
tal analysis for [Cr(thd)2(OEt)]2 (C48H86O10Cr2) gave C 62.20 (calcd.
62.18), H 9.33 (9.35), O 17,40 (17.25), Cr 11.07 (11.22) %.
(Fax:
+44-1223-336-033;
www.ccdc.cam.ac.uk/data_request/cif;
E-Mail: deposit@ccdc.cam.ac.uk).
2.5 Magnetic Measurements
Magnetic susceptibility measurements were performed (temperature
range 2–300 K, magnetic field 1 kOe) with a Magnetic Property Meas-
urement System; Quantum Design. The sample mass was 15–30 mg
and the samples were contained in gelatine holders during the measure-
ments.
3 Results and Discussion
3.1 Genesis of [Cr(thd)2(OEt)]2
The preparation of [Cr(thd)2(OMe)]2 was repeated according to a com-
pletely analogous procedure to that described for [Cr(thd)2(OEt)]2. At-
tempts were also made to prepare other homologous alkoxides (viz.
[Cr(thd)2(OR)]2 with R = 1-Pr, 2-Pr, and tert-Bu).
There is much evidence for the existence of a complex with
the well defined composition [Cr(thd)2(OEt)]2. It appears in
admixture with Cr(thd)3 in syntheses of the later [2], but can
also be obtained as a pure phase by other procedures (vide
infra). It is accordingly appropriate to dwell briefly on reac-
tants and experimental terms used in these syntheses.
2.3 Elemental Analysis
The recipe of reference [2] starts with CrCl3·6H2O and
H(thd) as reactants and EtOH/H2O as reaction medium. Since
The elemental analysis was done by the standard combustion technique
at Birmingham University (UK). The analysis of chromium was deter- both reactants exhibit weak acidity, such mixtures will remain
mined as follows. Excess of 68 % HNO3 was added to an accurately
weighed amount of [Cr(thd)2(OEt)]2 and after careful stirring this mix-
ture was heated at 100–150 °C until dryness. The crust thus obtained
was transferred to an oven and heated in air at progressively increasing
temperature (to 550 °C in steps of 50 °C) until a constant weight (at
room temperature) had been reached. Calculations were based on the
postulate that the end product was stochiometeric Cr2O3 (confirmed
by PXD).
unreacted even after protracted heating at b.t. A base is clearly
required to promote the reaction. For this purpose, reference
[2] introduces urea, which gives NH3 upon hydrolysis:
CO(NH2)2(solv.) + H2O(solv.) → CO2(g) + 2NH3(solv.)
at temperatures near b.t.
(1)
The appearance of product mixtures of Cr(thd)3 and
[Cr(thd)2(OEt)]2 in these syntheses is a direct consequence of
Z. Anorg. Allg. Chem. 2011, 56–61
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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57

A. Kjekshus et al.
ARTICLE
the water-containing medium: CrCl3·6H2O and H(thd) in re- 3.2 Sublimation of Cr(thd)3 and [Cr(thd)2(OEt)]2 Mixtures
fluxing EtOH/H2O. The following scheme is suggested:
The present findings show that sublimation (under com-
Cr3+
+ 3H(thd)(solv.) + 3NH3(solv.) → Cr(thd)3(solv.) + 3NH4 (solv.)(2)
+
monly used conditions) can be a rather laborious way to sepa-
rate Cr(thd)3 and [Cr(thd)2(OEt)]2. There are indeed other
ways based on solvent extraction to perform this task. How-
ever, since reference [2] goes quite far toward recommending
use of sublimation for purification of Cr(thd)3, we will give a
brief account of our experience with the application of subli-
mation on Cr(thd)3/[Cr(thd)2(OEt)]2 mixtures.
(solv.)
Cr(thd)3(solv.) % Cr(thd)3(s)
(3)
(4)
(5)
Cr(thd)3(solv.) + 2EtOH % [Cr(thd)2(OEt)]2(solv.) + 2H(thd)(solv.)
[Cr(thd)2(OEt)]2(solv.) % [Cr(thd)2(OEt)]2(s)
Let us consider two series of experiments, one in which the
sublimation temperature (s.t.) was set at 90 °C and another
with s.t. = 180 °C. The deposition temperature (d.t.; viz. the
temperature at the cold finger) was kept at 50 °C in both series.
The seemingly trivial change of s.t. from 90 to 180 °C caused
a major change in the sublimation process. For the s.t. at 90 °C
series the process commenced with good speed, but soon after
the sublimation rate started to decline, and went asymptotic to
nil. PXD showed that the purple sublimate consisted of
Cr(thd)3. The unsublimed part had become covered at this
stage with a thin layer of green-colored [Cr(thd)2(OEt)]2. Aft-
erwards, the entire batch of unsublimed material was subjected
to gentle crushing and thorough mixing and submitted to re-
newed sublimation treatment. The evolution of the resumed
sublimation process once again followed the course just de-
scribed. In fact, the crushing/mixing-sublimation treatment had
to be repeated about 20 times over two weeks in order to fully
separate Cr(thd)3 from [Cr(thd)2(OEt)]2.
The process at s.t. = 180 °C developed differently. The subli-
mation rate remained largely unaltered during the process, and
the characteristic light green cover-layer on the remnants at the
evaporation source was reduced almost to invisibility. Most of
a 5 g batch of raw material was sublimed after 30 min treat-
ment at 180 °C. PXD of the sublimate after this treatment
showed a mixture Cr(thd)3 and [Cr(thd)2(OEt)]2. This finding
appears to conflict the following statement in reference [2]: “If
the sublimation temperature is maintained near 180 °C, the
light green solid will not sublime at the pressure specified”.
The distinction between the behavior at s.t. = 90 and 180 °C
can be explained by different volatility of Cr(thd)3 and
[Cr(thd)2(OEt)]2. In accordance with its lower molecular mass
and presumably weaker intermolecular van der Waals interac-
tions, Cr(thd)3 obtains a significant sublimation pressure at
lower temperature than [Cr(thd)2(OEt)]2. This concurs with the
fact that [Cr(thd)2(OEt)]2 is detected in the sublimates from
the 180 °C series, but absent in the 90 °C series.
As also established in reference [2], [Cr(thd)2(OEt)]2 is less
soluble in EtOH/H2O than Cr(thd)3 and this is evidently the
cause of its appearance as a minor phase in the product mix-
ture. The balance between the constituents of the product is in
any case regulated through Equations (2)–(5). The reversibility
of Equation (4) was tested by heating 1:2 stoichiometric mix-
tures of [Cr(thd)2(OEt)]2 and H(thd) dissolved in (i) EtOH/
H2O, (ii) abs. EtOH, and (iii) H(thd) heated to reflux. The
experiments with EtOH/H2O as solvent turned out somewhat
inconclusive. A faint red coloring of the liquid phase indicated
that the anticipated reaction had commenced, but the process
had then discontinued. This is in agreement with poor solubil-
ity of [Cr(thd)2(OEt)]2 in EtOH/H2O.
[Cr(thd)2(OEt)]2 is obtained from Cr(thd)3 and EtOH accord-
ing to the forward direction of Equation (4) [solv. here denot-
ing abs. EtOH; 5 d heating to reflux gave ca. 5 wt.-%
[Cr(thd)2(OEt)]2 and 95 wt.-% unreacted Cr(thd)3]. Since
[Cr(thd)2(OEt)]2 dissolves poorly in abs. EtOH, the absence of
the reverse reaction is in accordance with expectations. How-
ever, the reversibility of Equation (4) was ascertained by the
use of H(thd) as both reactant and solvent. For example, heat-
ing a mixture of [Cr(thd)2(OEt)]2 and H(thd) to reflux for one
day gave (semi-quantitatively assessed) Cr(thd)3 in amounts
corresponding to the proportions of the starting mixtures.
An essential inference from the present study is the confir-
mation of the reversibility of Equation (4), which in turn is
controlled [Equation (3) and (5)] by the relative solubilities of
Cr(thd)3 and [Cr(thd)2(OEt)]2 in the solvent concerned. The
basis for the inevitable occurrence of Cr(thd)3/[Cr(thd)2(OEt)]2
mixtures in synthesis according to the urea-based recipe of ref-
erence [2] is laid by use of water-containing ingredients. The
relative amounts in the product mixtures can to some extent
be changed, but since H2O has to be present according to
Equation (1), both complexes should appear in the product.
However, turning to water-free ingredients phase-pure
[Cr(thd)2(OEt)]2 can be prepared according to:
The participation of sublimation of [Cr(thd)2(OEt)]2 in the
s.t. = 90 °C series is manifested as the thin green cover-layer
on the source material, which gradually renders evaporation of
Cr(thd)3 difficult and eventually blocks the sublimation proc-
2CrCl3(solv.) + 6Na(OEt)(solv.) + 4H(thd)(solv.)
[Cr(thd)2(OEt)]2(s) + 4EtOH + 6NaCl(s)
→
(7)
The introduction of Na(OEt) from the very beginning makes ess. Even though both complexes are volatile at 90 °C, the
the use of urea and thus Equations (1) and (2) become super- sublimation pressure of [Cr(thd)2(OEt)]2 is low and the flight
fluous. With CrCl3 and H(thd) as other reactants and abs. EtOH range is short. At onset, the Cr(thd)3 at the evaporation source
as solvent, [Cr(thd)2(OEt)]2 is obtained phase-pure in about has large surface areas free for [Cr(thd)2(OEt)]2, viz. numerous
93 % yield. This approach mimics the synthesis of locations are accessible for sublimation. However, some
[Cr(thd)2(OMe)]2 [4] and preparation of the last mentioned [Cr(thd)2(OEt)]2 will also become vaporized, but for s.t. =
complex was repeated (some 90 % yield) according to the 90 °C these species cannot travel far from their original loca-
presently modified procedure.
tion and they will eventually deposit on available colder spots
58 www.zaac.wiley-vch.de
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Z. Anorg. Allg. Chem. 2011, 56–61

Synthesis and Crystal Structure of [Cr(thd)2(OEt)]2
left behind from the sublimation of a Cr(thd)3 species. Once a lengths and angles, in which the various categories of bonds
[Cr(thd)2(OEt)]2 species has settled on such a spot, this spot
becomes blocked for further sublimation, and when all such
surface sites are blocked the sublimation will necessarily cease.
The sublimation process can be recommenced by the creation
of unblocked Cr(thd)3 source surface (e.g., by cautious crush-
ing).
According to these findings we cannot recommend sublima-
tion as a means to separate Cr(thd)3 from [Cr(thd)2(OEt)]2. The
use of an s.t., which is low enough to ensure satisfactory sepa-
ration demands intermediate crushing of the raw material,
whereas high s.t. leads to vapor-phase transport of both com-
pounds. Instead one can take advantage of the different solubil-
ity of Cr(thd)3 and [Cr(thd)2(OEt)]2 in various solvents.
are represented by the range, over which the individual distan-
ces are distributed and the averages. The Cr–Oket and Cr–Oalk
bonds are here, as in Cr(thd)3 [3], fairly free to adopt lengths
adjusted to the size of CrIII and OII, whereas the Oket–Cket
,
Cket–Cket and Oalk–Calk bonds can retain lengths characteristic
for their bonding situation in the chelate rings and alkoxide
bridges. The crystal packing in the unit cell of [Cr(thd)2(OEt)]2
is shown in Figure 1 (c).
Valence-state accounting according to the bond-valence
scheme [16] shows an acceptable balance (Table 2). The bond-
valence sum for chromium came out consistently some 10 %
too high (3.26), whereas the corresponding sums for Oket and
Oalk deviated from two by ca. 7 % for Oket (2.14) and ca. 2 %
for Oalk (2.05). From the empirical bond length versus bond-
order relation for carbon [17, 18] it follows that the effective
valences of Cket and Calk are close to four. The CrO6 configura-
tion in the central part is almost regular octahedral and the Cr–
Oket bond length [individual values within 1.932(6)–
1.967(6) Å] is adequately accounted for by the bond-valence
model (Table 2). The very similar and nearly 90° intra- and
interligand Oket–Cr–Oket angles imply that there cannot occur
significant bonding interaction across the ca. 2.75 Å
Oket.···Oket. bite and non-bite separations. The arrangement (in-
teratomic distances and angles) in the Oket–Cket–Cket–Cket–Oket
section of the chelate ring is virtually the same as in Cr(acac)3
[19–21] and Cr(thd)3 [3]. The adjusting influence of the tert-
butyl side groups on the shape of the chelate ring, to which it
belongs, is quite marginal. Least-squares fitted planes through
the chelate ring show deviation from the mean plane by
0.16(2) Å for both non-equivalent rings of the [Cr(thd)2(OEt)]2
structure.
3.3 Crystal and Molecular Structure of [Cr(thd)2(OEt)]2
The structure of [Cr(thd)2(OEt)]2 was solved by direct meth-
ods, but in this case it was also possible to predict essential
features of the crystal and molecular structures from knowl-
edge on related complexes [3, 4, 13–15]. The refinements of
the crystal structure of [Cr(thd)2(OEt)]2 run fairly smoothly
and converged at somewhat high, but acceptable values for the
reliability factors (Table 1) without introducing explicit de-
scriptions of disorder.
Table 1. Single-crystal data and relevant parameters used in the refine-
ments of the crystal structure of [Cr(thd)2(OEt)]2. Estimated standard
deviations are given in parentheses.
Empiric formula
M
C48H86Cr2O10
927, 1894
295(3)
T /K
Crystal system
Space group
a /Å
Triclinic
¯
P1
The crystal and molecular structures of [Cr(thd)2(OEt)]2
10.2919(15)
10.6686(16)
14.194(3)
106.559(2)
107.869(2)
98.326(2)
1375.0(4)
1
b /Å
(Figure 1)
carry
all
essential
characteristics
of
c /Å
[Cr(thd)2(OMe)]2 [4], [Cr(acac)2(OMe)]2 [15], substitutional
derivatives [13, 14] thereof, and analogous complexes of iron
[22]. The crystal structure comprises dimeric [Cr(thd)2(OEt)]2
units, which are well separated from each other. The arrange-
ment of the six oxygen atoms around the central chromium
atom is octahedral [slightly more distorted than in Cr(thd)3].
α /°
β /°
γ /°
V /Å3
Z
D /g·cm–3
μ(Mo-Kα)–1 /mm
F(000)
1.120
0.443
Compared with Cr(thd)3 two ketonato oxygen atoms (Oket
)
502
Crystal size /mm
Radiation /Å
θ min., max. /°
No. reflections meas.
No. unique reflect.
Rint
0.17 × 0.22 × 0.53
0.71073
2.1, 18.3
6373
have been replaced by the oxygen (Oalk) atoms of two cis-
configured ethoxy groups, which bridge the two chromium at-
oms of the dimeric unit. The implications of the less symmetric
1963
octahedral Cr(Oket)4(Oalk 2 arrangement and the bridging
)
0.015
Cr2(Oalk)2 regions are surprisingly small. The Cr2(Oalk)2 bridge
is a molecular building brick with special bond-geometry de-
mands, which, however, appear to be fully satisfied through
Obs. data [I > 2σ(I)]
No. parameters
R1 [I > 2σ(I)]
wR2 (all data)
GOF
1734
272
0.0719
0.1907
the formation of the strictly planar Cr2(Oalk)2 arrangements
1.086
(imposed by the inversion center). The Cr–Cr distance across
the bridging region [3.053(2) Å] is short enough to permit di-
rect Cr–Cr orbital overlap. The consequent intramolecular
magnetic exchange interaction serves to couple the moments
antiferromagneticly in accordance with the low-temperature
Min. max. resid. /e·Å–3
–0.27, 0.79
Unit-cell dimensions and space groups are included in Ta-
ble 1. The molecular structure of [Cr(thd)2(OEt)]2 is shown in
Figure 1. Table 2 gives a surveying extract of selected bond magnetic susceptibility data.
Z. Anorg. Allg. Chem. 2011, 56–61
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
59

A. Kjekshus et al.
ARTICLE
Figure 1. (a) X-ray molecular structure of [Cr(thd)2(OEt)]2 (at 295 K). Hydrogen atoms are systematically omitted. Thermal displacement
ellipsoids are drawn at the 50 % probability level. See references [4, 13–17] for illustrations of related structures. (b) Atomic labeling scheme
for chromium, oxygen, and carbon atoms in the asymmetric unit. (c) Packing of the [Cr(thd)2(OEt)]2 molecules in the crystal structure.
3.4 Magnetic Susceptibility
References
The χg–1(T) relation (corrected for induced diamagnetism
[23]) for [Cr(thd)2(OEt)]2 shows a positive (upward) curvature
for T < 50 K, which evidences the presence of intramolecular
antiferromagnetic exchange interaction between the chromium
atoms (vide supra). Above this temperature the Curie–Weiss
law is followed, with θ = –65 K and μp = 3.87 μB. The mag-
netic moment of [Cr(thd)2(OEt)]2 is in good agreement with
the reported values for CrIII complexes in the literature.
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The authors are grateful to Dr. Hiroshi Okamoto for help with the
magnetic susceptibility measurements.
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 56–61

Synthesis and Crystal Structure of [Cr(thd)2(OEt)]2
Table 2. Range and average bond lengths (dmin to dmax and dav in Å) and bond angles (φmin to φmax and φav in deg.) together with individual
bond valences (vi = exp[(Di–di)/ b]; Di and b from reference [16]) and bond valence sums (V = ∑vi) for [Cr(thd)2(OEt)]2.
Cr–Oket
Oket–Cr–Oket
Range: 1.943(6)–1.967(6)
Average: 1.952
Cr–Oalk
Range: 89.0(3)–89.3(3)
Average: 89.2 [92.7]; non-bonding
Oket–Cr–Oalk
Gauge: 1.944(5)
Range: 92.5(2)–95.4(2)
Average: 94.1
Oalk–Cr–Oalk
Gauge: 76.5(2)
vCr–Oket = 0.54
vCr–Oalk = 0.55
VCr = 4·vCr–Oket + 2·vCr–Oalk = 2.16 + 1.10 = 3.26
Oket–Cket
Cr–Oket–Cket
Range: 1.181(14)–1.251(12)
Average: 1.216
Oalk–Calk
Range: 126.7(6) –130.0(7)
Average: 128.1
Cr–Oalk–Calk
Gauge: 1.408(12)
Gauge: 127.4(5)
vOket–Cket = 1.60
vOalk–Calk = 0.95
VOket = vOket–Cket + vOket–Cr = 1.60 + 0.54 = 2.14
VOalk = vOalk–Calk + 2·vOalk–Cr = 0.95 + 1.10 = 2.05
Cket–Cket
Range: 1.422(15)–1.475(16)
Average: 1.460
Oket–Cket–Cket
Range: 122.1(10) –125.2(11)
Average: 123.7
C
ket–C–Cket
Range: 121.6(10)–122.4(11)
Average: 122.0
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Z. Anorg. Allg. Chem. 2011, 56–61
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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