Crucial role of paramagnetic ligands for magnetostructural anomalies in "breathing crystals"

Article abstract of DOI:10.1021/ic301149e

Breathing crystals based on polymer-chain complexes of Cu(hfac)₂ with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu(hfac)₂ complexes with tert-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets.

Full text of DOI:10.1021/ic301149e

Article
pubs.acs.org/IC
Crucial Role of Paramagnetic Ligands for Magnetostructural
Anomalies in “Breathing Crystals”
Evgeny V. Tretyakov,† Svyatoslav E. Tolstikov,† Anastasiya O. Suvorova,† Aleksey V. Polushkin,†
Galina V. Romanenko,† Artem S. Bogomyakov,† Sergey L. Veber,† Matvey V. Fedin,† Dmitry V. Stass,‡
Edward Reijerse,§ Wolfgang Lubitz,§ Ekaterina M. Zueva,∥ and Victor I. Ovcharenko*,†
†International Tomography Center, and ‡Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of
Sciences, Institutskaya Street, 630090 Novosibirsk, Russian Federation
§Max-Planck-Institut fur chemische Energiekonversion, 45470 Mulheim/Ruhr, Germany
̈
̈
∥Department of Inorganic Chemistry, Kazan State Technological University, 68 K. Marx Street, 420015 Kazan, Russian Federation
S
* Supporting Information
ABSTRACT: Breathing crystals based on polymer-chain complexes of Cu(hfac)2 with nitroxides exhibit thermally and light-
induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis
and investigation of a new family of Cu(hfac)2 complexes with tert-butylpyrazolylnitroxides and their nonradical structural
analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper(II) coordination
units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues
with diamagnetic ligands do not undergo rearrangements in the copper(II) coordination environments. This confirms
experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper(II) ions for the
origin of magnetostructural anomalies in this family of molecular magnets.
is an essential condition for the potential polymorphic
transformation of the compound.
INTRODUCTION
■
The synthesis of heterospin complexes of transition metals and
nitroxides often involves Cu(II) as the complex-forming metal.1
Compounds of Cu(II) with nitroxides are valuable model
objects for magnetochemical studies because the analysis of
their properties does not require the account of spin−orbital
contribution.2 They are also convenient for EPR studies, which
were widely used for investigating the first coordination
compounds of transition metals with nitroxides.1a,b However,
the synthesis of heterospin compounds of Cu(II) and
multifunctional nitroxides always involves the difficulty of
predicting the coordination mode of nitroxide in a solid. This
is a multifactor effect, which depends on the acceptor pro-
perties of the metal-containing matrix, the steric accessibility
of the nitroxyl group, the electronic effects of functional groups
The majority of known Cu(II) complexes with nitroxides
are stable in polymorphic transformations.1m On cooling or
heating in a wide temperature range, XRD shows very small
changes in the geometrical parameters due to crystal com-
pression on cooling or expansion on heating. At the same
time, a large group of Cu(II) complexes with nitroxides ex-
perience structural rearrangements of heterospin coordination
units due to variation of temperature, leading to various
magnetic anomalies on the curve of the temperature depend-
ence of the effective magnetic moment (μeff).3 This group
mainly involves Cu(hfac)2 complexes, where hfac is hexa-
fluoroacetylacetonate, which is a powerful acceptor capable of
coordinating the O atom of the O−N< fragment with weak
̇
other than >N−O in the structure of nitroxide, conditions of
̇
synthesis, and packing effects in the resulting solid. The
formation of a Cu−OON coordination bond in the solid complex
Received: May 31, 2012
Published: August 14, 2012
© 2012 American Chemical Society
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donor properties comparable to the donor properties of the
acetone molecule.4
Among heterospin complexes capable of thermally induced
magnetic effects are [Cu(hfac)2LR] complexes, where LR are
pyrazolyl-substituted nitronylnitroxides. The single crystals of
these compounds possess unique mechanical plasticity, due
to which they retain their quality in the course of thermally
induced phase transitions. Their quality remains high enough
for using the same single crystal in a series of experiments on
structure determination during polymorphic transformations.
Because of this, the structure of a heterospin complex at a
definite temperature can be unambiguously correlated with its
magnetic characteristics.5 On the basis of this striking feature,
these heterospin complexes were grouped together under the
name of breathing crystals.5c The ability of [Cu(hfac)2LR]
crystals to retain their integrity during repeated cooling−
heating cycles also allows real-time observation of direct and
reverse phase transition waves along the single crystal.
Before this study, attempts to synthesize such pairs of dia- and
paramagnetic organic ligands and Cu(II) complexes with a
structure of the solid phase similar to each representative of this
pair have failed.
In this work, we have succeeded in synthesizing the topo-
logical analogues of LR, nitroxides NPzR, and their diamagnetic
analogues, pivaloylpyrazoles PPzR. [Cu(hfac)2(NPzR)] hetero-
spin complexes having a polymer chain structure in the solid
state were isolated, and their magnetic properties were studied.
These properties were compared to those of [Cu(hfac)2(PPzR)]
having a similar structure of polymer chains but containing
ketones as the structural diamagnetic analogues of NPzR. As a
result, our studies of [Cu(hfac)2(NPzR)] and [Cu(hfac)2(PPzR)]
provided first unambiguous experimental evidence that the
presence of a coordinated nitroxyl fragment in the structure of
the heterospin compound is an essential condition for the
breathing crystal effect.
Systematic studies of the relationship between the structural
dynamics and the character of variation of the magnetic pro-
perties of [Cu(hfac)2LR] serve as a basis for quantum-chemical
analysis of the electronic structure of exchange channels6 and
the development of theoretical approaches to the analysis and
description of spin transitions in heterospin exchange clusters.7
The spin state dynamics of {>N−O−Cu2+−O−N<} or {Cu2+−
̇
̇
O−N<} exchange clusters also shows itself in a specific way in
RESULTS AND DISCUSSION
̇
■
EPR spectra, providing detailed information about the character
of intracluster exchange interactions.8 Radiospectroscopic studies
of breathing crystals revealed a rare (for multispin compounds)
situation when interchain exchange interactions were higher in
energy than interactions inside the chain.9 Moreover, the spin
state of these compounds can change during photoexcitation,
stimulating interest to them in view of their ability to serve as
optical magnetic switches.10
Complexes with Nitroxides. The general procedures for
the synthesis of paramagnetic ligands NPzR are given in the
Experimental Section. The EPR spectra of NPzR were quite
similar, showing the dominant triplet from the N-oxyl nitrogen
with a substructure from the nuclei of the pyrazolyl substituent
(Figure 1 for NPzMe, and Supporting Information for NPzEt,
NPzPr, and NPzBu). The substructure involved two relatively
strong couplings with protons in the third and fifth positions
of the pyrazole ring, weaker couplings with three equivalent
methyl protons of the N-methyl group in NPzMe or with two
equivalent N-methylene protons in NPzR (R = Et, Pr, Bu), and
still weaker couplings with pyrazole nitrogens. The EPR spectra
of NPzR indicated a substantial delocalization of spin density
to the pyrazolyl substituent, which was further confirmed by
DFT calculations (Supporting Information). This distinguishes
NPzR from LR, in which spin density delocalization to the
pyrazole ring is much weaker. This circumstance is important
Coordination of the O−N< group by the copper ion was
̇
always considered to be an essential condition for thermally
induced structural rearrangements of {>N−O−Cu2+−O−N<}
̇
̇
or {Cu2+−O−N<} heterospin coordination fragments, which,
̇
in turn, give rise to anomalies on the μeff(T) curve.1 However,
this was not confirmed experimentally because it requires
synthesis and further comparative structural and magneto-
chemical studies of Cu(II) complexes with nitroxides and
diamagnetic ligands structurally related to these nitroxides.
Figure 1. Experimental (noisy trace) and simulated (up-shifted clean trace) EPR spectra for NPzMe (a) and central line (b).
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Figure 2. Polymer chain fragment in solid [Cu(hfac)2(NPzEt)]·0.5C7H16 (a) and [Cu(hfac)2(PPzEt)] (b).
for understanding the magnetic properties of Cu(hfac)2
heterospin complexes with NPzR.
Our investigation of the structure of [Cu(hfac)2(NPzEt)]·
0.5C7H16 showed that the solid phase of this compound is
formed by polymer chains with a “head-to-head” motif, resulting
from the bridging coordination of NPzEt, with the centrosym-
metric coordination of the Cu(II) atom in the Cu(hfac)2
matrixes complemented to octahedral by two NPz atoms of
the pyrazole rings or by two ONO atoms of the nitroxide groups
(Figure 2). At 240 K, the {CuO6} units are elongated octahedra
with Cu−ONO axial distances of 2.287(3) Å and Cu−Ohfac
equatorial distances of 1.925(3) and 2.015(3) Å. The
coordination polyhedron is similar for {CuO4N2} units, in
which the axial positions are occupied by the NPz atoms
(d(Cu−NPz) = 2.353(4) Å), and the equatorial positions by the
Ohfac atoms (1.956(3) and 1.982(3) Å).
The [Cu(hfac)2(NPzR)] complexes were prepared by the
reaction of Cu(hfac)2 with freshly prepared NPzR in hexane or
heptane. After the reagents were mixed, the mother solutions
were quickly cooled to −15 °C and stored at this temperature
overnight. The resulting crystals of the complexes were separated
from the mother solution and also stored at −15 °C. The heating
of the complexes, especially to 30−35 °C, stimulated their active
decomposition in both liquid and solid states.
Note that when the reaction with NPzMe or NPzEt was
performed in heptane, the solid phase of the complexes with
a polymer chain structure included the corresponding solvate
molecules in case of [Cu(hfac)2(NPzMe)]·0.5C7H16 or [Cu-
(hfac)2(NPzEt)]·0.5C7H16, whereas [Cu(hfac)2(NPzPr)] and
[(Cu(hfac)2(NPzBu)] did not contain solvate molecules. The
mass of the finely crystalline compounds [Cu(hfac)2(NPzPr)]
or [(Cu(hfac)2(NPzBu)] included large individual crystals of
two polymorphic modifications of [(Cu(hfac)2)3(NPzPr)2] or
[(Cu(hfac)2)3(NPzBu)2] trinuclear complexes (Supporting
Information). The introduction of an additional amount of
NPzPr or NPzBu in the reaction mixture did not suppress the
formation of trinuclear complexes. During crystallization of
complexes from the hexane solution, all compounds were
obtained as solvates [Cu(hfac)2(NPzR)]·0.5C6H14 (R = Me, Et,
Pr, Bu). An X-ray diffraction study of [Cu(hfac)2(NPzR)]·
0.5C7H16 (R = Me, Et) and [Cu(hfac)2(NPzR)]·0.5C6H14
(R = Me, Et, Pr, Bu) was performed for crystals separated
from the layer of the mother solution and then coated with a
layer of epoxide resin. When isolated from the mother solution,
the [Cu(hfac)2(NPzMe)]·0.5C7H16 and [Cu(hfac)2(NPzR)]·
0.5C6H14 solvates (R = Me, Pr, Bu) quickly lost the included
solvent molecules. The [Cu(hfac)2(NPzEt)]·0.5C7H16 and
[Cu(hfac)2(NPzEt)]·0.5C6H14 complexes were kinetically stable
enough; therefore, they were easy to study.
Studies of the temperature dynamics of the structure of
[Cu(hfac)2(NPzEt)]·0.5C7H16 showed that when the temper-
ature decreased from 240 to 85 K, the bond lengths and angles
in the ligands (NPzEt and hfac) changed to a small degree.
In the {CuO6} and {CuO4N2} units at 240−190 K, the axial
Cu−ONO distances decreased by 0.078 Å (from 2.287(3) to
2.209(3) Å) (Table 1). Further decrease to 150 K in the
temperature caused a drastic shortening of the Cu−ONO bonds
from 2.209(3) to 2.032(3) Å, which was accompanied by a
simultaneous increase in the Cu−Ohfac distances in {CuO6}
units from 2.078(3) to 2.288(3) Å along one of the Ohfac−Cu−
O
hfac axes. In Jahn−Teller distorted octahedral {CuO6} units at
lowered temperatures, the elongated octahedron axis actually
shifted from ONO−Cu−ONO to Ohfac−Cu−Ohfac. In contrast, in
{CuO4N2} units, the Cu−N distances increased from 2.353(4)
to 2.392(4) Å in this temperature range. When the crystal was
further cooled to 85 K, the Cu−ONO and Cu−NPz bond lengths
decreased because of the thermal compression of the crystal,
but much less significantly, by 0.024 and 0.021 Å, respectively.
The temperature dynamics of the structure of [Cu(hfac)2(NPzEt)]·
0.5C6H14 is similar to that for [Cu(hfac)2(NPzEt)]·0.5C7H16
(Table 1). Note that the structural data given in Table 1 for
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[Cu(hfac)2(NPzEt)]·0.5C7H16 and [Cu(hfac)2(NPzEt)]·
0.5C6H14 were obtained using the same single crystal. The
structural changes in [Cu(hfac)2(NPzEt)]·0.5C7H16 and [Cu-
(hfac)2(NPzEt)]·0.5C6H14 after the lowering of temperature
from 240 to 85 K explain the form of the experimental
dependence μeff(T) (Figure 3a and Supporting Information).
Figure 3. Experimental dependences μeff(T) for [Cu(hfac)2(NPzMe)]
Et
■
▼
(red ), [Cu(hfac)2(NPz )]·0.5C7H16 (blue ), and [Cu(hfac)2-
Pr
Et
●
(NPz )] ( ) at 1.0 T (a) and hysteresis for [Cu(hfac)2(NPz )]·
0.5C7H16 (b).
The rearrangement of {CuO6} coordination units (three-spin
exchange clusters {>N−O−Cu2+−O−N<}) observed at 150−
̇
̇
190 K for [Cu(hfac)2(NPzEt)]·0.5C7H16 and at 180−210 K for
[Cu(hfac)2(NPzEt)]·0.5C6H14 results in a transition of the
coordinated nitroxyl O atoms from axial to equatorial positions,
which gives rise to strong antiferromagnetic exchange inter-
actions in these compounds3 and is the reason for the observed
decrease in μeff.
At lowered temperatures (T ≈ 190 K), the EPR spectra of
the polycrystalline sample of [Cu(hfac)2(NPzEt)]·0.5C7H16
also experienced changes typical of the transition of
{>N−O−Cu2+−O−N<} exchange clusters from weakly to
̇
̇
strongly coupled state with a total spin S = 1/2.11 The EPR data
point not only to a strong antiferromagnetic exchange (|J| >
100 cm−1) in the three-spin clusters at T < 190 K, but also to a
weak (J ≈ 1 cm−1) intercluster exchange in the polymer chain
between the three-spin clusters of the {CuO6} units and the
Cu2+ ions of the {CuO4N2} units.
Because the single crystals of the [Cu(hfac)2(NPzMe)]·
0.5C7H16 and [Cu(hfac)2(NPzR)]·0.5C6H14 solvates (R = Me,
Pr, Bu) easily lose the solvent and decompose under ambient
conditions, they were extracted from under the layer of the
mother solvent immediately before the XRD experiment and
coated with epoxide resin to avoid their decomposition.
For the individual [Cu(hfac)2(NPzMe)]·0.5C7H16 and [Cu(hfac)2-
(NPzR)]·0.5C6H14 phases (R = Me, Pr, Bu), however, reliable
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magnetochemical data were not obtained because of the
insufficient kinetic stability of the solvates.
The structural characteristics of the compounds are pres-
ented in Table 2. All hexane solvates and [Cu(hfac)2(NPzMe)]·
0.5C7H16, as well as [Cu(hfac)2(NPzEt)]·0.5C7H16 (Figure 2),
are formed by polymer chains with a “head-to-head” motif.
For [Cu(hfac)2(NPzMe)]·0.5C7H16 at low temperatures (150,
105, and 85 K), the Cu−ONO distances in the {CuO6} units
are markedly shortened, although they corresponded to the
high-spin state of the {>N−O−Cu2+−O−N<} clusters over the
̇
̇
whole range of measurements. In the {CuO6} units in
[Cu(hfac)2(NPzPr)]·0.5C6H14, the Cu(1)−O(01) distances at
240 K were noticeably shorter than in [Cu(hfac)2(NPzEt)]·
0.5C7H16 (2.220(3) Å) and shortened to 2.019(2) Å when the
temperature was lowered to 85 K (Table 3). In [(Cu(hfac)2-
(NPzBu)]·0.5C6H14, the ONO atoms lie in the equatorial plane of
the Cu bipyramid already at 240 K (the distances are Cu−ONO
=
2.000(2) Å, Cu−Ohfac = 1.930(2) Å) with two Ohfac atoms
displaced to the axial positions (Cu−Ohfac = 2.358(2) Å). If we
consider the magnetostructural correlations of [Cu(hfac)2-
(NPzEt)]·0.5C7H16 (Table 1), we can conclude that the
Cu−ONO distances for [(Cu(hfac)2(NPzBu)]·0.5C6H14 (Table 3)
correspond to the geometry of the low-spin state of the {>N−O−
̇
Cu2+−O−N<} clusters already at room temperature.
̇
A comparison of the structural characteristics of the pairs
[Cu(hfac)2(NPzMe)]·0.5C7H16−[Cu(hfac)2(NPzMe)]·0.5C6H14
and [Cu(hfac)2(NPzEt)]·0.5C7H16−[Cu(hfac)2(NPzEt)]·
0.5C6H14 (Tables 1 and 2) shows that if the R substituent
is the same, the replacement of the solvent has little effect
on the structural characteristics of the polymer chain. The most
significant difference in the structural parameters in the
[Cu(hfac)2(NPzMe)]·0.5C7H16−[Cu(hfac)2(NPzMe)]·0.5C6H14
pair lies in a drastic shortening of the Cu−N distances in the
{CuO4N2} units at lowered temperatures along with pro-
nounced shortening of the Cu−ONO distances in the {CuO6}
units in [Cu(hfac)2(NPzMe)]·0.5C6H14.
The single crystals of the [Cu(hfac)2(NPzR)] complexes
containing no solvate molecules were not obtained. Studies of
the magnetic properties and temperature dynamics of the EPR
spectra of desolvated [Cu(hfac)2(NPzR)] (R = Me, Pr, Bu),
however, indicate that they contain {>N−O−Cu2+−O−N<}
̇
̇
clusters. In the case of [Cu(hfac)2(NPzMe)] at T < 85 K, the
spectra show a characteristic change, which points to a drastic
strengthening of antiferromagnetic exchange coupling and a
transition of three-spin clusters to a state with a total spin
S = 1/2.12 For [Cu(hfac)2(NPzPr)] at T < 290 K, the EPR
spectra have shape similar to the spectra of [Cu(hfac)2(NPzMe)]
and [Cu(hfac)2(NPzEt)]·0.5C7H16 at T < 85 and T < 190 K,
respectively; that is, in [Cu(hfac)2(NPzPr)], strong antiferro-
magnetic exchange interactions dominate already at 240−300 K.
The temperature dependences of the effective magnetic
moment (μeff) are presented in Figure 3a for the polycrystalline
samples of desolvated [Cu(hfac)2(NPzMe)] and [Cu-
(hfac)2(NPzPr)] and the solvate [Cu(hfac)2(NPzEt)]·0.5C7H16
and in the Supporting Information for [Cu(hfac)2(NPzEt)]·
0.5C6H14. For [Cu(hfac)2(NPzMe)], μeff, which is 2.54 μB at
310 K, corresponds to the theoretical spin only value 2.45 μB
for two noninteracting paramagnetic centers with S = 1/2 at
g = 2. When the sample was cooled to 115 K, μeff gradually
increased to 2.64 μB, indicating that ferromagnetic exchange
interactions dominated in the solid [Cu(hfac)2(NPzMe)]. Below
115 K, μeff drastically decreased to 1.88 μB, which is charac-
teristic for systems with one unpaired electron. Thereafter, it
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Table 3. Selected Bond Lengths (Å) and Angles (deg) in Solid [Cu(hfac)2(NPzPr)]·0.5C6H14 and [(Cu(hfac)2(NPzBu)]·0.5C6H14
at Different Temperatures
compound
[Cu(hfac)2(NPzPr)]·0.5C6H14
[(Cu(hfac)2NPzBu]·0.5C6H14
T, K
85
2.019(2)
150
240
2.220(3)
240
Cu(1)−O(01)
Cu(1)−Ohfac
Cu(2)−N(02)
Cu(2)−Ohfac
O(01)−N(01)
N(01)−O(01)−Cu(1)
2.024(3)
2.000(2)
1.937(2), 2.324(2)
2.389(3)
1.934(3), 2.321(3)
2.386(4)
1.914(3), 2.075(3)
2.398(4)
1.930(2), 2.358(2)
2.392(2)
1.973(2), 1.974(2)
1.302(4)
1.968(3), 1.974(2)
1.299(4)
1.948(3), 1.967(3)
1.297(4)
1.952(2), 1.967(1)
1.303(2)
125.4(2)
125.8(2)
126.5(3)
125.0(1)
did not change down to helium temperatures. For both
[Cu(hfac)2(NPzEt)]·0.5C7H16 and [Cu(hfac)2(NPzEt)]·
0.5C6H14 solvates, μeff first gradually decreased from 2.49 to
2.35 μB at lowered temperatures, then quickly decreased to
1.90 μB at 200−180 K, and then changed insignificantly down to
2 K. The drastic √2-fold decrease in μeff recorded experimentally
for [Cu(hfac)2(NPzMe)], [Cu(hfac)2(NPzEt)]·0.5C7H16, and
[Cu(hfac)2(NPzEt)]·0.5C6H14 corresponds to the vanishing of
exactly one-half of all spins in the sample.
Supporting Information demonstrates the passage of the thermal
front accompanied by a change in the color of the sample).
For [Cu(hfac)2(NPzPr)] and [(Cu(hfac)2(NPzBu)] com-
plexes with NPzR having long alkyl substituents in the pyrazole
ring, the temperature of the magnetic anomaly shifted to the
range >300 K. For this reason, for [Cu(hfac)2(NPzPr)], which
decomposes above 320 K, it is possible to record only the
starting growth of μeff at T > 270 K. For [(Cu(hfac)2(NPzBu)],
the potential spin transition effect is latent because of its
thermal instability at T > 300 K. In the series of compounds
under study formed by heterospin chains of the same type in
the solid state, the increase in the length of the alkyl substituent
R is thus formally accompanied by a pronounced increase in the
temperature of the effect.
Our structural and magnetochemical studies of Cu(hfac)2
complexes with nitroxides NPzR thus revealed a new group of
compounds whose heterospin coordination units experienced
structural rearrangements accompanied by changes in the
magnetic characteristics and color of the samples during variation
of temperature.
For [Cu(hfac)2(NPzPr)], μeff gradually decreased from 2.04
μB at 300 K and formed a plateau ∼1.93 μB below 260 K.
Finally, for [(Cu(hfac)2(NPzBu)], μeff remained ∼1.91 μB over
the whole temperature range (Figure 3a). When the heating/
cooling cycle was reversed for the same sample and in repeated
measurements for complexes obtained in different syntheses, the
dependences μeff(T) given in Figure 3 were always reproduced.
For [Cu(hfac)2(NPzEt)]·0.5C7H16, a narrow hysteresis loop was
recorded (Figure 3b).
Thus, [Cu(hfac)2(NPzMe)], [Cu(hfac)2(NPzEt)]·0.5C7H16,
and [Cu(hfac)2(NPzEt)]·0.5C6H14, whose μeff(T) curves showed
magnetic anomalies at 115, 190, and 200 K, respectively, are the
first examples of complexes with N-tert-butyl-N-alkylpyrazolyl-
nitroxides, for which spin transitions were recorded. The magnetic
anomalies inherent in [Cu(hfac)2(NPzMe)] and [Cu(hfac)2-
(NPzEt)]·0.5C7H16 were accompanied by thermochromic effects
(Figure 4). Spontaneous heating of the powder samples of
Complexes with Pivaloylpyrazoles. The driving force for
the observed rearrangements in [Cu(hfac)2(NPzR)], as well as
all compounds of this type studied previously,3,5−10 was
assumed to be the exchange interactions between the unpaired
electrons in {>N−O−Cu2+−O−N<} heterospin clusters;
̇
̇
when the nitroxyl groups pass to the equatorial positions, these
interactions sharply increase, leading to a decrease in the free
energy of the solid. The strong antiferromagnetic exchange
interactions that appear in {>N−O−Cu2+−O−N<} exchange
̇
̇
clusters actually serve as a certain increment to the Cu−O bond
energy. This exchange and hence increment are impossible for
Cu(hfac)2 complexes with diamagnetic structural analogues of
nitroxides. This was shown experimentally by comparing the
magnetostructural correlations for Cu(hfac)2 complexes with
NPzR and PPzR. Pivaloylpyrazoles PPzR synthesized in this
study are unique because, while being very close topological
analogues of nitroxides NPzR, they form the same polymers as
NPzR in their reactions with Cu(hfac)2.
The general procedure for the synthesis of PPzR is given in
the Experimental Section. The interaction of Cu(hfac)2 with
PPzR (R = Et, Me, Pr) in heptane led to the formation of
[Cu(hfac)2(PPzEt)] and solvates [Cu(hfac)2(PPzMe)]·0.5C7H16,
[Cu(hfac)2(PPzPr)]·0.5C7H16. All solids are formed by polymer
chains with a “head-to-head” motif similar to those in
[Cu(hfac)2(NPzR)]. Although the [Cu(hfac)2(PPzMe)]·
0.5C7H16 and [Cu(hfac)2(PPzPr)]·0.5C7H16 crystals were stable
in solution for a long time, they were coated with epoxide resin
after their separation from the mother solution before the XRD
experiment because they gradually lose the solvent molecules
and decompose under ambient conditions. Figure 2b shows a
Figure 4. Thermochromism of [Cu(hfac)2(NPzMe)] (left) and
[Cu(hfac)2(NPzEt)]·0.5C7H16 (right).
[Cu(hfac)2(NPzMe)] and [Cu(hfac)2(NPzEt)]·0.5C7H16 applied
to a paper and preliminarily cooled with liquid nitrogen led to a
drastic change of color from yellow-brown to brown-red and
from brown-violet to dark brown, respectively (the movie in the
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Table 4. Selected Bond Length (Å) and Angles (deg) for [Cu(hfac)2(PPzMe)]·0.5C7H16, [Cu(hfac)2(PPzEt)], and
[Cu(hfac)2(PPzPr)]·0.5C7H16 at Different Temperatures
compound
[Cu(hfac)2(PPzMe)]·0.5C7H16
85 240
2.409(3) 2.417(2)
[Cu(hfac)2(PPzEt)]
[Cu(hfac)2(PPzPr)]·0.5C7H16
85 240
2.358(1) 2.368(2)
T, K
100
2.335(1)
150
240
2.355(2)
Cu(1)−O(01)
Cu(2)−N(02)
O(01)−C(01)
C(01)−O(01)−Cu(1)
2.342(1)
2.418(1)
1.222(2)
135.2(1)
2.262(4)
1.251(5)
2.349(3)
1.236(3)
2.410(1)
1.229(1)
2.439(2)
1.219(3)
2.394(2)
1.233(2)
2.410(2)
1.221(3)
144.4(4)
145.9(2)
133.39(8)
138.2(2)
129.8(1)
137.2(2)
fragment of the [Cu(hfac)2(PPzEt)] polymer chain. The same
structure is typical for chains in [Cu(hfac)2(PPzMe)]·0.5C7H16
and [Cu(hfac)2(PPzPr)]·0.5C7H16 crystals (Supporting Infor-
mation). A comparison of the structures of chains in Cu(hfac)2
complexes with nitroxides NPzR and pivaloylpyrazoles PPzR
showed that they are very similar (Figure 2).
structure of [Cu(hfac)2(PPzMe)]·0.5C7H16, the Cu−N distances
in the {CuO4N2} units shortened on cooling (Table 4). The
magnetic moments for Cu(hfac)2 complexes with PPzR have
practically no changes in the temperature range 2−300 K and
were due only to the contribution from the Cu2+ ions.
It is realistic to expect that structural rearrangements (if any)
can be detected using EPR. If the elongated Jahn−Teller axis of
the six-coordinated copper(II) ion is flipped, the largest
component of the g tensor (gz) must also interchange with
one of the other two components (gx or gy) that can be
observed in the EPR spectrum of the single crystal. To enhance
the spectral resolution, we used high-field EPR at 244 GHz.
Figure 5a shows the single-crystal EPR spectra of [Cu(hfac)2-
(PPzEt)] measured at 244 GHz in the temperature range 10−
260 K. The spectra exhibit negligible changes with temperature,
which cannot be associated with the Jahn−Teller axis flip. The
Q-band powder EPR spectra (Figure 5b) also did not change
noticeably with temperature. For [Cu(hfac)2(PPzMe)]·0.5C7H16
and [Cu(hfac)2(PPzPr)]·0.5C7H16, we also could not observe
any manifestations of abrupt structural rearrangements
(Supporting Information).
An XRD study of [Cu(hfac)2(PPzEt)] in the range 240−85 K
did not reveal any structural changes except for a small mono-
tonic shortening of distances. The [Cu(hfac)2(PPzPr)]·0.5C7H16
solvate behaved similarly in this temperature range. In the
CONCLUSIONS
■
A new family of heterospin polymer chain complexes of
Cu(hfac)2 with 1-R-4-(N-tert-butyl-N-oxylamino)-pyrazoles
NPzR was synthesized. During the repeated cooling−heating
cycles, the solid complexes experienced reversible structural
rearrangements accompanied by abrupt changes in the effective
magnetic moment. Synthetic procedures were developed for
the diamagnetic analogues of NPzR, 2,2-dimethyl-1-(1-R-1H-
pyrazol-4-yl)propan-1-ones PPzR containing a >CO group
instead of the paramagnetic >N−O group, and for [Cu(hfac)
̇
2
(PPzR)] polymer chain complexes having the same structure as
[Cu(hfac)2(NPzR)]. A comparative analysis of the temperature
dynamics of the structure and magnetic characteristics of
[Cu(hfac)2(NPzR)] and [Cu(hfac)2(PPzR)] provided exper-
imental evidence that the presence of a coordinated nitroxyl
fragment in a solid heterospin compound is an essential condi-
tion for the breathing crystal effect. The coordination of the
O−N< fragment by the Cu(II) ion provides the potential for
̇
considerable displacements of atoms in solid Cu(hfac)2
complexes with nitroxides, favored by the appearance of strong
antiferromagnetic interactions in {>N−O−Cu2+−O−N<}
̇
̇
exchange clusters. The absence of such interactions in
[Cu(hfac)2(PPzR)] crystals leads to a negligible dependence
of {CuO6} coordination units geometry on temperature despite
the similarity of the spatial structures of NPzR and PPzR.
Figure 5. (a) 244 GHz single-crystal EPR spectra of [Cu(hfac)2-
(PPzEt)] at T = 10−260 K (νmw ≈ 243.68 GHz, arbitrary crystal
orientation). (b) Q-band powder EPR spectra of [Cu(hfac)2(PPzEt)]
at T = 10−290 K (νmw ≈ 33.21 GHz). The temperature values are
given on the right.
EXPERIMENTAL SECTION
General Procedures. 1-Methyl, 1-ethyl-, and 1-propyl-4-bromo-
1H-pyrazoles12 were synthesized by bromination of the corresponding
1-alkylpyrazoles with NBS in CCl4 and distilled in vacuum. 1-Methyl-,
■
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1
t
1-ethyl-, and 1-propyl-1H-pyrazol-4-carbaldehydes were prepared
496, 434 cm−1. H NMR (400 MHz, CDCl3): δ = 1.10 (s, 9H, Bu),
3.81 (s, 3H, N−Me), 7.17 and 7.29 (both s, 1H each, C(3)−H,
C(5)−H). 13C NMR (100 MHz, CDCl3): δ = 25.4 (C(CH3)3), 39.1
(Me), 59.9 (C(CH3)3), 124.7 and 134.7 (C(3), C(5)), 133.3 (C(4)).
Anal. Calcd for C8H15N3O: C, 56.8; H, 8.9; N, 24.8. Found: C, 57.0;
H, 8.9; N, 25.0.
from the corresponding 1-alkylpyrazoles13 by the Vilsmeier reaction.
14
Cu(hfac)2 was sublimed before use. Tetrahydrofuran (THF) was
fresh distilled over sodium hydride. Other chemicals of the highest
purity available were purchased from Acros and Aldrich and used as
received. The reactions were monitored by TLC using silica gel 60 F254
aluminum sheets, Merck. Chromatography was carried out with the
use of Merck 0.063−0.100 mm silica gel for column chromatography.
C, H, and N elemental analyses were carried out on a CHN analyzers
EA-3000 and Carlo Erba 1106 in the Chemical Analytical Center of
the Novosibirsk Institute of Organic Chemistry. The melting points
were determined on an SMP3 Stuart melting point apparatus. Infrared
spectra (4000−400 cm−1) were recorded with a Bruker VECTOR 22
N-tert-Butyl-N-(1-R-1H-pyrazol-4-yl)hydroxylamines (2b−d).
These were obtained from corresponding 4-bromo-1-R-1H-pyrazoles
(1b−d) by a procedure similar to the one used for the synthesis of 2a
(Supporting Information).
4-(N-tert-Butyl-N-oxylamino)-1-methyl-1H-pyrazole (NPzMe).
K3[Fe(CN)6] (89 mg, 0.27 mmol) was added to a stirred mixture of
hydroxylamine 2a (30 mg, 0.18 mmol), NaHCO3 (30 mg, 0.4 mmol),
CH2Cl2 (5 mL), and H2O (2 mL). The reaction mixture was stirred
at room temperature for 1 h. The bright orange organic phase was
separated, dried over anhydrous Na2SO4, and filtered. The filtrate was
diluted with heptane (10 mL). The resulting solution was evaporated
at reduced pressure and a bath temperature of ∼35 °C. The residue was
dissolved in heptane (5 mL). The solution was filtered and evaporated.
The product was red oil. HRMS, m/z: 168.1124 (M+, calcd for
C8H14N3O 168.1137). MS, m/z (%): 168 (M+, 5), 167 (21), 153 (19),
138 (61), 122 (17), 113 (14), 112 (59), 111 (9), 97 (14), 96 (20), 82
(11), 70 (30), 69 (10), 57 (100). EPR: giso = 2.0055; A>N−O(1N) = 1.269
mT, AN(2)(1N) = 0.032 mT, AN(1)(1N) = 0.018 mT, AC(5)−H(1H) =
0.254 mT, AC(3)−H(1H) = 0.155 mT, AN−Me(3H) = 0.065 mT.
4-(N-tert-Butyl-N-oxylamino)-1-R-1H-pyrazoles (NPzR). These
were obtained from corresponding N-tert-Butyl-N-(1-R-1H-pyrazol-4-yl)-
hydroxylamines (2b−d) by a procedure similar to the one used for the
synthesis of NPzMe (Supporting Information).
1
instrument in KBr pellets. H and 13C NMR spectra were recorded
at 25 °C using a Bruker Avance 400 spectrometer locked to the
deuterium resonance of the solvent; the chemical shifts are reported in
parts per million (ppm), and the solvent was used as internal standard.
HRMS were recorded on a DFS instrument using the electron impact
ionization technique (70 eV). X-Band CW EPR spectra of the
nitroxides were recorded in dilute degassed toluene solutions at room
temperature on a Bruker EMX spectrometer at MW power 2 mW,
modulation amplitude 0.01 mT at 100 kHz, single scan of 4096 points
at 1310 ms per point, time constant 1310 ms, and modeled with
Winsim v.0.96 free package as described earlier;15 the isotropic
g-values were determined using solid DPPH as standard, and
the accuracy of the hyperfine coupling constants and g-values was
0.005 mT and 0.0001, respectively. Q-Band EPR measurements were
performed using a Bruker Elexsys E580 spectrometer equipped with an
Oxford Instruments temperature control system using a ER 5106QT
resonator. A high-field EPR spectrometer16 equipped with an ICE-
Oxford cryogenic system was used for experiments at 244 GHz.
These high-field EPR experiments were carried out without a
resonator using induction mode detection. The magnetic susceptibility
of the polycrystalline complexes was measured with a Quantum
Design MPMSXL SQUID magnetometer in the temperature range
2−300 K with magnetic fields of up to 5 kOe. None of the complexes
exhibited any field dependence of molar susceptibility at low temp-
eratures. The diamagnetic corrections were applied using the Pascal
constants. The effective magnetic moment was calculated as μeff(T) =
1-(1-Ethyl-1H-pyrazol-4-yl)-2,2-dimethylpropan-1-ol (4b). A
1.7 M solution of BuLi in pentane (8 mL, 13.6 mmol) was added
t
2
[(3k/NAμB )χ′MT]1/2 ≈ (8χ′MT)1/2, where χ′M is the corrected molar
dropwise to the vigorously stirred solution of 1-ethyl-1H-pyrazole-4-
carbaldehyde (3b) (1.5 g, 12.1 mmol) in THF (25 mL) at −80 °C
under argon. The cooling was stopped. The reaction mixture was
allowed to warm to room temperature and then diluted with water
(70 mL). The product was extracted with CH2Cl2 (5 × 30 mL); the
combined extracts were dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The residue was recrystallized
from a hexane−ethyl acetate (5: 1) mixture. The pale yellow crystalline
precipitate was filtered off. Yield 0.82 g (37%); mp 79−82 °C. IR: ν =
3316, 2952, 2906, 2868, 1724, 1681, 1632, 1559, 1478, 1465, 1440,
1406, 1379, 1360, 1296, 1243, 1177, 1159, 1103, 1059, 1010, 960, 901,
susceptibility.
Syntheses of NPzR and PPzR.
864, 818, 788, 767 cm−1. H NMR (300 MHz, CDCl3): δ = 0.81
1
N-tert-Butyl-N-(1-methyl-1H-pyrazol-4-yl)hydroxylamine (2a). A
2.5 M solution of BuLi in hexane (5.47 mL, 12.7 mmol) was added
dropwise to the solution of 4-bromo-1-methyl-1H-pyrazole (1a)
(2.0 g, 12.4 mmol) in absolute THF (20 mL) vigorously stirred under
argon at −90 °C. The reaction mixture was stirred for 30 min, and a
solution of 2-methyl-2-nitrosopropane (1.08 g, 12.4 mmol) in THF
(10 mL) was added to it. After 1 h, the cooling was stopped and the
reaction mixture was allowed to warm to room temperature. Water
(15 mL) and CH2Cl2 (30 mL) were added to the resulting mixture.
The organic phase was separated, and the aqueous phase was extracted
with CH2Cl2 (5 × 20 mL). The extract and the organic layer were
combined, then dried over anhydrous Na2SO4, filtered through the
Al2O3 layer, and concentrated under reduced pressure. The residue
was crystallized by storing it under a heptane layer at −15 °C for 12 h.
The product was filtered off and recrystallized from a mixture of ethyl
acetate with heptane. Yield 0.66 g (31%); colorless needle crystals; mp
101−102 °C. IR: ν = 3211, 3139, 3086, 2982, 2933, 2868, 1681, 1556,
1460, 1441, 1418, 1387, 1360, 1348, 1320, 1210, 1168, 1115, 1073,
1041, 1017, 996, 949, 928, 886, 852, 790, 706, 685, 646, 634, 561,
(s, 9 H, tBu), 1.29 (t, J = 7.3 Hz, 3 H, CH3), 4.07 (q, J = 7.3 Hz, 2 H,
N−CH2), 4.16 and 4.86 (both d, J = 4.4 Hz, 1 H each, CH−OH,
CH−OH), 7.24 (s, 1 H, C(3)−H), 7.49 (s, 1 H, C(5)−H). HRMS,
m/z: 182.1415 (M+, calcd for C10H18N2O 182.1419). MS, m/z (%):
182 (M+, 1), 167 (4), 126 (5), 125 (100), 123 (2), 97 (3), 95 (1),
69 (5), 57 (1), 42 (1).
2,2-Dimethyl-1-(1-R-1H-pyrazol-4-yl)propan-1-ols (4a and 4c).
These were obtained by a procedure similar to the one used for the
synthesis of 4b (Supporting Information).
2,2-Dimethyl-1-(1-ethyl-1H-pyrazol-4-yl)propan-1-one (PPzEt).
PCC (0.49 g, 2.25 mmol) was added to a stirred solution of 4b
(0.25 g, 1.4 mmol) in CH2Cl2 (20 mL). The reaction mixture was
stirred at room temperature for 1.5 h, diluted with diethyl ether
(20 mL), again stirred for 10 min, and filtered through a silica gel layer
(1 × 20 cm, diethyl ether as eluent). The filtrate was concentrated in a
vacuum, and a pale yellow oil of PPzEt was obtained. Yield 0.13 g
(53%); Rf = 0.80 (SiO2, ethyl acetate). IR: ν = 3269, 3130, 2976, 2871,
1654, 1540, 1477, 1392, 1264, 1238, 1168, 1112, 1080, 1052, 988, 956,
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922, 897, 870, 810, 766 cm−1. 1H NMR (300 MHz, CDCl3): δ = 1.29
(s, 9H, Bu), 1.49 (t, J = 7.3 Hz, 3H, CH3), 4.16 (q, J = 7.3 Hz, 2H,
geometrically and included as riding groups in the refinement. All
calculations were fulfilled with the SHELXTL 6.14 program package.
Imperfection and the small size of crystals in some cases led to a small
amount of independent reflections with I > 2σI in experimental data
despite the substantial increase in scan time to 60 s per frame.
Crystallographic data and experimental details for compounds under
discussion are given in the Supporting Information.
t
N−CH2), 7.90 and 7.92 (both s, 1H each, C(3)−H, C(5)−H). HRMS,
m/z: 180.1258 (M+, calcd for C10H16N2O 180.1263). MS, m/z (%):
180 (M+, 3), 139 (2), 137 (1), 125 (1), 124 (5), 123 (100), 95 (18),
68 (1), 57 (2), 41 (2), 39 (1), 29 (1), 28 (2), 18 (2).
2,2-Dimethyl-1-(1-R-1H-pyrazol-4-yl)propan-1-ones (PPzMe and
PPzPr). These were obtained by a procedure similar to the one used for
the synthesis of PPzEt.
ASSOCIATED CONTENT
* Supporting Information
■
[Cu(hfac)2(NPzMe)]. A solution of nitroxide NPzMe in heptane
(10 mL) obtained from 2a (30 mg, 0.16 mmol) was added to the
solution of Cu(hfac)2 (74 mg, 0.16 mmol) in heptane (5 mL) and
stored for 12 h at −15 °C. The crystalline precipitate was filtered off,
washed with cold heptane, and dried in air for 10 min. The product
was stored in a refrigerator at −15 °C. Yield 90 mg (71%); yellow
brown crystals. Anal. Calcd for C18H16CuF12N3O5: C, 33.5; H, 2.5; N,
6.5; F, 35.3. Found: C, 33.2; H, 2.7; N, 6.1; F, 33.3.
S
Synthetic procedures; crystallographic data for all compounds
(in CIF form); EPR spectra for NPzEt, NPzPr, NPzBu,
[Cu(hfac)2(PPzMe)]·0.5C7H16, and [Cu(hfac)2(PPzPr)]·
0.5C7H16; dependences μeff(T) for [Cu(hfac)2(NPzEt)]·
0.5C6H14, [Cu(hfac)2(PPzMe)]·0.5C7H16, [Cu(hfac)2(PPzEt)],
and [Cu(hfac)2(PPzPr)]; the results of DFT calculations for
NPzEt and LEt; and the movie that demonstrates the passage of
the thermal front of the phase transition in [Cu(hfac)2(NPzMe)]
and [Cu(hfac)2(NPzEt)]·0.5C7H16. This material is available free
of charge via the Internet at http://pubs.acs.org.
[Cu(hfac)2(NPzEt)]·0.5C7H16 and [Cu(hfac)2(NPzEt)]·0.5C6H14.
These were obtained from heptane and hexane, respectively, by a
procedure similar to the one used for the synthesis of [Cu(hfac)2
(NPzMe)]. The yield of [Cu(hfac)2(NPzEt)]·0.5C7H16 was 83 mg
(71%); dark violet crystals with a brown tint. Anal. Calcd for C19H18-
CuF12N3O5·0.5C7H16: C, 38.0; H, 3.7; N, 5.9; F, 32.1. Found: C, 37.9;
H, 3.4; N, 6.3; F, 32.3. The yield of [Cu(hfac)2(NPzEt)]·0.5C6H14 was
77 mg (66%); dark violet crystals with a brown tint. Anal. Calcd for
C19H18CuF12N3O5·0.2C6H14: C, 35.8; H, 3.1; N, 6.2; F, 33.7. Found:
C, 35.5; H, 3.1; N, 6.2; F, 33.6.
AUTHOR INFORMATION
Corresponding Author
*E-mail: victor.ovcharenko@tomo.nsc.ru.
■
[Cu(hfac)2(NPzPr)]. This was obtained by a procedure similar to the
one used for the synthesis of [Cu(hfac)2(NPzMe)] except that hexane
solutions of Cu(hfac)2 and NPzPr (5 mL) were used. The product was
dried in air. Yield 74 mg (70%); dark violet crystals with a brown tint.
Anal. Calcd for C20H20CuF12N3O5: C, 35.6; H, 3.0; N, 6.2; F, 33.8.
Found: C, 35.2; H, 3.2; N, 5.9; F, 33.4. When the procedure was
performed in heptane, the finely crystalline [Cu(hfac)2(NPzPr)]
product always contained greenish brown crystals of two modifications
of the [(Cu(hfac)2)3(NPzPr)2] complex.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was financially supported by the Russian Foundation
for Basic Research (grant nos. 11-03-00158, 11-03-00027, 11-
03-12001, 12-03-00067, 12-03-31184, and 12-03-31289), RF
President grants (MK-4268.2010.3, MK-868.2011.3, MK-
6497.2012.3, and MK-1662.2012.3), the Russian Academy of
Sciences, and the Siberian Branch of the Russian Academy of
Sciences. We are grateful to Prof. Yuri N. Molin for initiating us
to look at this problem and Dr. Alex I. Kruppa for help in
modeling the EPR spectra.
[(Cu(hfac)2(NPzBu)]. This was obtained by a procedure similar to the
one used for the synthesis of [Cu(hfac)2(NPzPr)]. Yield 50 mg (20%);
violet-brown crystals. Anal. Calcd for C21H22CuF12N3O5: C, 36.7; H,
3.2; N, 6.1; F, 33.1. Found: C, 36.2; H, 3.0; N, 6.1; F, 33.4. When the
procedure was performed in heptane, the main product [(Cu-
(hfac)2(NPzBu)] contained perfect crystals of [(Cu(hfac)2)3(NPzBu)2].
[Cu(hfac)2(PPzEt)]. A mixture of Cu(hfac)2 (0.38 g, 0.79 mmol) and
PPzEt (0.13 g, 0.72 mmol) was dissolved in heptane (17 mL) at 50 °C.
The resulting dark green solution was slowly cooled to room
temperature. After 20 h, the resulting green needle crystals were filtered
off. Yield 0.27 g (57%); mp 120−125 °C. IR: ν = 3149, 2985, 2352,
1643, 1603, 1559, 1533, 1486, 1399, 1354, 1261, 1227, 1146, 1106,
1006, 967, 923, 880, 768, 745 cm−1. Anal. Calcd for C20H18CuF12N2O5:
C, 36.5; H, 2.8; N, 4.3. Found: C, 36.9; H, 2.7; N, 3.9.
REFERENCES
■
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[Cu(hfac)2(PPzMe)]. This was obtained by a procedure similar to the
one used for the synthesis of [Cu(hfac)2(PPzEt)]. Yield 0.42 g (44%). IR:
ν = 3142, 2977, 1643, 1559, 1536, 1487, 1399, 1354, 1261, 1225, 1145,
1106, 994, 923, 881, 801, 768, 746 cm−1. Anal. Calcd for
C19H16CuF12N2O5: C, 35.4; H, 2.5; N, 4.4. Found: C, 35.6; H, 2.5; N, 4.2.
[Cu(hfac)2(PPzPr)]. This was obtained by a procedure similar to the
one used for the synthesis of [Cu(hfac)2(PPzEt)]. Yield 0.09 g (27%).
IR: ν = 3444, 3137, 2980, 2911, 2885, 1646, 1597, 1559, 1538, 1489,
1463, 1400, 1368, 1353, 1263, 1219, 1145, 1107, 1040, 1004, 923, 883,
815, 798, 766, 745 cm−1. Anal. Calcd for C21H20CuF12N2O5: C, 37.5;
H, 3.0; N, 4.2. Found: C, 37.1; H, 3.0; N, 4.2.
X-ray Crystallography. Crystals for an XRD analysis were selected
directly from solution and immediately coated with a layer of epoxide
resin. The intensity data for the single crystals were collected by the
standard procedure on SMART APEX II CCD and SMART APEX II
DUO (Bruker AXS) automated diffractometers (Mo Kα radiation,
graphite monochromator). The structures were solved by direct methods
and refined by the full-matrix least-squares procedure anisotropically for
non-hydrogen atoms. The H atoms were partially located in difference
electron density syntheses, and the other atoms were calculated
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(b) Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, 1986.
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