Solvent enhancement of the magnetic ordering temperature (Tc) of the room temperature V[TCNE]x · S (S = solvent, TCNE = tetracyanoethylene; x ～ 2) magnet

Article abstract of DOI:10.1016/j.poly.2005.12.007

The reaction of V(CO)₆ and TCNE in CH₂Cl₂, hexane, toluene (PhMe), and trifluoromethylbenzene (PhCF₃) form an immediate black precipitate of V(TCNE)_x · yS (S = solvent; x ～ 2; y ～ 0.5) composition, which have a magnetic ordering onset temperatures, T_on, in the range of 385-390 K (S = CH₂Cl₂, hexane, PhMe). For S = PhCF₃ in addition to T_on being enhanced it exhibits greater thermal stability.

Full text of DOI:10.1016/j.poly.2005.12.007

Polyhedron 25 (2006) 1927–1930
www.elsevier.com/locate/poly
Solvent enhancement of the magnetic ordering temperature (Tc)
of the room temperature V[TCNE] Æ S (S = solvent,
x
TCNE = tetracyanoethylene; x ꢀ 2) magnet
Matthew S. Thorum, Konstantin I. Pokhodnya, Joel S. Miller
*
Department of Chemistry, University of Utah, 315 S. 1400 East, RM 2020, Salt Lake City, UT 84112-0850, USA
Received 6 October 2005; accepted 7 December 2005
Available online 19 January 2006
Abstract
The reaction of V(CO)
6 2 2 3
and TCNE in CH Cl , hexane, toluene (PhMe), and trifluoromethylbenzene (PhCF ) form an immediate
black precipitate of V(TCNE) Æ yS (S = solvent; x ꢀ 2; y ꢀ 0.5) composition, which have a magnetic ordering onset temperatures,
x
T
on, in the range of 385–390 K (S = CH
thermal stability.
2005 Elsevier Ltd. All rights reserved.
2 2 3
Cl , hexane, PhMe). For S = PhCF in addition to Ton being enhanced it exhibits greater
ꢀ
Keywords: Magnetic ordering; TCNE
1
. Introduction
Since the discovery of the first organic-based magnet
the thermal stability, as well as exploit organic magnets,
we investigated the solvent (S) dependence of the magnetic
behavior of V[TCNE] Æ yS (x ꢀ 2) prepared from V(CO) .
x
6
ꢀ+
ꢀÀ
[
Fe(C Me ) ]
[TCNE]
(TCNE = tetracyanoethylene)
Coordinating solvents like tetrahydrofuran (THF) or
MeCN substantially reduce the magnetic ordering temper-
5
5 2
[
1,2], new materials have been synthesized with enhanced
and controllable magnetic properties [3–10]. Subsequently,
the reaction of V(C H ) [11a,11b] and V(CO)6 [11d]
ature, T , to ꢀ215 [16a] and ꢀ138 K [16b], respectively.
c
These lower T s with respect to CH Cl (T ꢀ 400 K [11])
6
6 2
c
2
2
c
with TCNE formed V[TCNE] Æ yCH Cl (x ꢀ 2; y ꢀ 0.5),
are due to the ability of these solvents to coordinate to V
x
2
2
ꢀÀ
an inhomogeneous, non-stoichiometric, amorphous,
disordered room-temperature organic-based magnet.
Recently, solvent-free thin films of the room temperature
sites and block them from bonding to the [TCNE] ’s Ns.
II
ꢀÀ
Thus, this limits the number of the V Á Á Á[TCNE] spin
coupling pathways, and consequently reducing T . In con-
c
V[TCNE] (x ꢀ 2) magnet have been prepared via chemical
trast, studies of V[TCNE] Æ yCH Cl and solvent-free
x
x
2
2
vapor deposition [12], and structurally characterized by
EXAFS and Xanes studies [13]. Magnetotransport studies
indicate that electrons in valence and conducting bands of
CVD-synthesized V[TCNE] [12b] reveal that the presence
x
of non-coordinating solvent like CH Cl does not substan-
2
2
tially influence T , although its presence creates additional
c
the V[TCNE] films are spin polarized [14]. Hence, this mag-
structural disorder resulting in a spin-glass behavior at low
temperatures. While the non-coordinating CH Cl solvent
x
net is an attractive material for future generations of mag-
netic applications including spintronics [15]. As part of
our overall thrust to discover, understand, and enhance
2
2
enabled formation of V[TCNE] Æ yCH Cl magnets with
x
2
2
the highest T , V(CO) slowly decomposes in it (vide infra),
c
6
and occlusion of even small amounts of impurities with the
isolated V[TCNE] Æ yCH Cl may affect the bulk magnetic
*
Corresponding author. Tel.: +1 801 585 5455; fax +1 801 581 8433.
x
2
2
E-mail address: jsmiller@chem.utah.edu (J.S. Miller).
behavior.
0
277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.12.007

1928
M.S. Thorum et al. / Polyhedron 25 (2006) 1927–1930
Table 1
as a function of solvent, S, for V[TCNE]
pentoxide under a nitrogen atmosphere. Hexacarbonyl-
vanadium(0), V(CO)6, was prepared by oxidizing
tetraethylammonium hexacarbonylvanadium(À1) with
phosphoric acid and purified by vacuum sublimation
(25 ꢁC, 0.1 Torr) [19]. Tetracyanoethylene (TCNE) was
purchased from Acros and purified by vacuum sublimation
from activated carbon (80 ꢁC, 0.1 Torr).
T
c
x
Æ yS prepared from V(C
6
H
6 2
)
and V(CO)
6
V-source
Solvent (S)
CH Cl
THF
MeCN
None (CVD)
Ton (K)
Ref.
b
V(C
V(C
V(C
6
6
6
H
H
H
6
6
6
6
6
6
6
6
)
2
)
2
)
2
2
2
ꢀ350
215
138
370
387
390
396
388
[16a]
[16b]
[12a]
this work
this work
this work
this work
a
V(CO)
V(CO)
V(CO)
V(CO)
V(CO)
a
CH
PhMe
PhCF
hexane, C
2
Cl
2
2.1. V[TCNE] Æ yS (x ꢀ 2, y ꢀ 0.5)
x
3
6
H
14
All materials were prepared with the desired solvent
using a nominally identical procedure exemplified by the
5
K.
b
Extrapolated from 150 Oe data presented in Ref. [11b].
following reaction performed in PhCF . A yellow solution
3
of V(CO) (44.7 mg, 0.204 mmol) dissolved in ꢀ5 mL of
6
PhCF was added dropwise to a stirred suspension of
3
The interpretation of T is problematic, as it is deter-
2 equiv. TCNE (52.3 mg, 0.408 mmol) in ꢀ10 mL PhCF .
c
3
mined from low-field magnetization, M(T), measurements
taken on warming a sample toward the expected magnetic
transition temperature. An abrupt drop of M(T) was
observed above ꢀ350 K [12]; however, its value did not
recover upon cooling, suggesting an irreversible structural
transformation (which rapidly degrades the magnetic prop-
erties) occurs at almost the same temperature as the mag-
netic transition (vide infra). Therefore, to evaluate the Tc
of the materials, the temperature dependence of the satu-
The mixture immediately became black and CO evolved.
The mixture was allowed to stir for 12 h. (As the reaction
is rapid, the amount of time reaction is stirred does not
have a significant impact on the yield or magnetic proper-
ties of the product, and was variable between samples.)
The black precipitate was collected by suction filtration
and dried under vacuum. Yield: 68.5 mg (88%) of
À1
V(TCNE) Æ 0.5PhCF . IR (KBr, cm ): 2190, 2151, 2089
2
3
(1324, only observed in samples prepared from PhCF3).
Elemental Anal. Calc. C, 48.97; H, 0.66; N, 29.47. Found:
C, 48.59; H, 0.73; N, 29.22%. Thermogravimetric analysis
shows broad nearly featureless decomposition above 50 ꢁC.
3
/2
rated magnetization, M (T), was fit to the Bloch T law
s
[
12], and T s were ꢀ400 K.
c
Due to the lack of data on the influence other non-
coordinating solvents have on T of the V[TCNE] mag-
c
x
nets, alternative solvents were studied with the objective
of extending the ferrimagnetic behavior to higher temper-
atures, which is important for a practical applications. It
is possible that different solvent molecules trapped in the
V[TCNE]x structure may strengthen (or weaken) the
2.2. Properties characterization
The thermal properties were studied on a TA Instru-
ments Model 2050 thermogravimetric analyzer (TGA)
equipped with a TA-MS Fison triple filter quadrupole mass
spectrometer, to identify gaseous products with masses less
than 300 amu. This instrument was located in a Vacuum
Atmospheres DriLab under nitrogen to protect air and
moisture sensitive samples. Samples were placed in an alu-
minum pan and heated at 20 ꢁC/min under a continuous
10-mL/min nitrogen flow. A TA Instruments Model 2910
differential scanning calorimeter was used to carry out
DSC experiments. Samples were loaded into an aluminum
pan with a lid and hermetically sealed with a special press
inside the box. The samples were loaded into the instru-
ment and DSC experiments were performed under nitrogen
gas flow.
V[TCNE] network (for instance, via short contacts) sta-
x
bilizing it against thermal degradation. Alternative sol-
vents might also reduce the occlusion of impurities
arising from the decomposition of V(CO)6 in CH Cl .
2
2
Hence, the effect of several non-coordinating solvents
including toluene (PhMe), trifluoromethylbenzene
PhCF ), and hexane, were studied. PhCF was particu-
(
3
3
larly intriguing as it has a dielectric constant (9.2) compa-
rable to CH Cl (8.9) [17]. Herein, we report the solvent
2
2
dependence of the magnetic properties of V[TCNE] Æ yS
x
in the vicinity of magneto-structural transition tempera-
ture (see Table 1).
The 5–300 K magnetic susceptibility was determined on
a Quantum Design MPMS-5XL 5 T SQUID (sensitiv-
2
. Experimental
À8
À12
ity = 10 emu or 10
emu/Oe at 1 T) as previously
All manipulations were performed under an inert nitro-
described [20]. In addition to correcting for the diamag-
netic contribution from the sample holder, core diamag-
netic corrections based on summing Pascal’s constants
gen atmosphere (<1 ppm O and <1 ppm H O) using stan-
2
2
dard Schlenk techniques and Vacuum Atmospheres
DriLab gloveboxes. Methylene chloride (CH Cl ), PhMe,
À1
À1
2
2
were used. Infrared spectra (400–4000 cm ; with 1 cm
and hexanes were sparged with nitrogen and passed over
two columns of activated alumina prior to use to remove
traces of oxygen and water [18]. Trifluoromethylbenzene
resolution) were obtained on a Bruker Tensor 37 FT IR-
40 spectrophotometer as KBr pellets. Elemental analyses
were performed by Complete Analysis Laboratories Inc.
of Parsippany, NJ.
(
PhCF ) was purified by distillation from phosphorous
3

M.S. Thorum et al. / Polyhedron 25 (2006) 1927–1930
1929
3
. Results and discussion
Solutions of V(CO) in CH Cl were observed to gradu-
1400
6
2
2
1200
1000
ally decompose over time. The initially bright yellow
solutions became green or brown in color after about
1
5–30 min, and after 24 h a fine black precipitate formed
leaving a nearly colorless supernatant. The IR spectrum
8
6
4
2
00
00
00
00
0
M
of the precipitate was featureless between 400 and
À1
4
000 cm indicating the loss of all CO, and that the mate-
rial does not contain vanadium oxides or hydroxides (from
perhaps reaction with trace amounts of O or H O). The
2
2
material may be a chloride, possibly formed via a chlorine
radical intermediate. This decomposition is not unprece-
dented, as V(CO) PPh was reported to be unstable in
5
3
the presence of CH Cl or toluene [21]. The identity of
2
2
the decomposition product or the mechanisms of its forma-
tion have not been fully investigated, to our knowledge. No
solutions showing visible signs of decomposition were used
to prepare the magnetic materials in this study.
300
320
340
360
380
400
Temperature, T, K
Fig. 1. M(T) for V[TCNE]
and PhMe (+)] in an applied field of 5 Oe.
x
Æ yS [S = CH Cl ) PhCF
2
2
(
3
(d), hexane (·),
ꢀÀ
ꢁ
Due to the small sample-to-sample variations observed
in the physical properties of the materials, a survey of the
effects that altering various synthetic protocols have on
these properties was undertaken in order to determine if
the variations were caused by inadvertent slight variation
in the synthetic methods. It was generally found that the
volume of solvent used for the reaction should be as small
as is practical. It is surprising to note that incomplete dis-
solution of the TCNE is not detrimental to the reaction.
As a result of this observation, all of the materials were pre-
pared from TCNE slurries, as TCNE is only moderately
soluble in CH Cl and PhMe, slightly soluble in PhCF ,
II
tion from S = 3/2 V and two S = 1/2 [TCNE] }. Using
this criterion, Ton was determined to be 387, 388, 390,
and 396 K for V[TCNE] Æ yS synthesized in CH Cl , hex-
anes, PhMe, and PhCF , respectively. The T for the
materials prepared from CH Cl , hexanes, and PhMe are
not significantly different given that the observed Ton
values for V[TCNE] Æ yS vary slightly between samples
(due to extreme air sensitivity), and the similarity of
M(T) curve shapes when approaching the transition. This
x
2
2
3
on
2
2
x
suggests that solvent has a minimal effect on T . In addi-
on
tion, there is no correlation between the stability of V(CO)6
in a solvent and the ordering temperature of the material
prepared from that solvent.
2
2
3
and essentially insoluble in hexane. The use of large vol-
umes of CH Cl to completely dissolve the TCNE typically
2
2
resulted in slightly reduced magnetic ordering temperature.
The order of combination of the reagents is, however, crit-
We believe that the sample-to-sample variations are pri-
marily a result of the extreme air sensitivity of V[TCNE]x
and that slight amounts of oxidation occasionally contam-
inate the materials as they are being loaded into instru-
ments for analysis, in spite of rigorous precautions taken
to prevent as much oxidation as possible.
ical, and the V(CO) solutions must be added dropwise to
6
the TCNE solutions or slurries (although the rate of addi-
tion is not important). The amount of time that the reac-
tion mixture is allowed to stand after combination of the
reagents (1 min to 48 h) had no affect on the magnetic
properties of the materials although several hours were
generally allowed to ensure that all of the TCNE had dis-
solved and reacted in order to prevent the inclusion of
unreacted TCNE into the products collected, which was
monitored by IR spectroscopy.
The reaction of V(CO) and TCNE in CH Cl , hexanes,
6
2
2
PhMe, and PhCF form an immediate black precipitate,
3
ꢀ
À
II
with IR spectra characteristic of [TCNE] bonded to V
À1
(Fig. 2), i.e., mCN absorptions at 2150 and 2188 cm , as
well as two poorly resolved shoulders at ꢀ2000 and
À1
ꢀ2210 cm . The position and peak intensity (with respect
À1
The zero-field cooled (ZFC) magnetization of
V[TCNE] Æ yS synthesized from PhMe, PhCF , hexanes,
to the 2150 cm
band) of
a
broad feature at
À1
ꢀ2090 ± 10 cm is solvent dependent. However, no dis-
x
3
and CH Cl is nearly temperature independent below
tinct correlation between its position and T was identi-
2
2
on
3
00 K, showing a ꢀ10% decrease from 300 to 5 K. In this
fied. We also observed similar variations in the mCN
region, the M(T) is reproducible and the variation between
samples is not solvent dependent. Above 300 K the M(T)
gradually decreases with different rates for different sol-
vents (Fig. 1) suggesting a magnetic transition. We define
the onset temperature, T , as a temperature at which the
magnetization is an order of magnitude greater than its
paramagnetic limit of 0.05 emuOe/mol {at 400 K and
absorptions of V[TCNE] Æ yS from different batches syn-
x
thesized using the CH Cl solvent with similar T ’s. Ear-
2
2
on
lier we tentatively assigned the 2150 and broad
À1
ꢀÀ
2100 cm
bands to mCN of trans-l-[TCNE] and l4-
ꢀÀ
[TCNE] bonded to V, respectively [12a]. The relative
insensitivity of the high frequency bands positions to sol-
vent substitution most probably implies that the non-coor-
dinating solvents do not change the effective electron
on
5
Oe external field {assuming only paramagnetic contribu-

1930
M.S. Thorum et al. / Polyhedron 25 (2006) 1927–1930
material is more than twice the 56 kJ/mol for the material
made in CH Cl , This suggests that it is more robust against
Hexane
2
2
thermal degradation. The reason(s) of this effect are not
entirely clear, but is under investigation. Hence, a new sol-
vent has been identified that enables the preparation of a
more thermally stable organic-based magnet.
CH Cl2
2
Toluene
Acknowledgments
PhCF3
The continued partial support by the Department of En-
ergy Division of Materials Science (Grant No. DE-FG03-
93ER45504) and Army Research Office (Grant No.
DAAD19-01-1-0562) is appreciated.
2300
2200
2100
2000
1900
1800
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