High-field EPR study and crystal and molecular structure of trans-RSSR-[CrCl2(cyclam)]nX (X=ZnCl4 2-, Cl- and Cl-·4H 2O·0.5HCl)

Article abstract of DOI:10.1039/b405789a

For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III) coordination compounds. The large zero-field splitting parameters of the dark-green photochromic trans-RSSR-[CrCl₂(cyclam)]₂ZnCl₄,1, the red-purple trans-RSSR-[CrCl₂(cyclamy)Cl, 2 and the red-purple trans-RSSR-[CrCl₂-(cyclan)]Cl·4H₂O·0.5HCl, 3, where cyclam = 1,4,8,11-tetraazacyclotetradecane, have been obtained. A full analysis of EPR spectra at 94.5 GHz of diluted complexes 1,2 and 3 at 300 K revealed that they are extremely sensitive to D and E values. The rhombic distortion was precisely determined for each compound. For l, g = 2.01, D = -0.305 cm^-1, E = 0.041 cm^-1 and λ = |E/D| = 0.1396; for 2, g = 2.01; D = -0.348 cm^-1, E = 0.042 cm^-1 and λ = |E/D| = 0.1206 and for 3, g = 1.99, D = -0.320 cm^-1, E = 0.041 cm^-t and λ = |E/D| = 0.1281. The EPR study at 94.5 GHz at 10 K allowed us to confirm the sign of the D value for all compounds. These data indicate that at room temperature the crystal field is mainly rhombic and as the temperature decreases, the rhombicity of the D tensor increases slightly. These found differences between 1, 2 and 3 allowed us to establish the importance of the intermolecular interactions in the solid state due to different hydrogen bonding networks in their crystalline arrangement.

Full text of DOI:10.1039/b405789a

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F U L L P A P E R
High-field EPR study and crystal and molecular structure
2−
of trans-RSSR-[CrCl2(cyclam)]nX (X=ZnCl4 , Cl− and
Cl−·4H2O·0.5HCl)†
Alejandro Solano-Peralta,a Martha E. Sosa-Torres,*a Marcos Flores-Alamo,a
Hassane El-Mkami,b Graham M. Smith,b Rubén A. Toscanoc and Takato Nakamurad
a
División de Estudios de Posgrado, Facultad de Química, Universidad Nacional Autónoma de
México, Ciudad Universitaria, Coyoacán, D. F. 04510, México. E-mail: mest@servidor.unam.mx
School of Physics and Astronomy, University of St-Andrews, Fife, Scotland, UK
Instituto de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria,
b
c
Coyoacán, D. F. 04510, México
Dept. of Materials Sciences and Technology, Faculty of Engineering, Shizuoka University,
d
3-5-1 Johoku, Hamamatsu, 432-8561, Japan
Received 19th April 2004, Accepted 17th June 2004
First published as an Advance Article on the web 15th July 2004
For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III)
coordination compounds. The large zero-field splitting parameters of the dark-green photochromic trans-RSSR-
[CrCl2(cyclam)]2ZnCl4, 1, the red–purple trans-RSSR-[CrCl2(cyclam)]Cl, 2 and the red–purple trans-RSSR-[CrCl2-
(cyclam)]Cl·4H2O·0.5HCl, 3, where cyclam = 1,4,8,11-tetraazacyclotetradecane, have been obtained. A full analysis
of EPR spectra at 94.5 GHz of diluted complexes 1, 2 and 3 at 300 K revealed that they are extremely sensitive to D
and E values. The rhombic distortion was precisely determined for each compound. For 1, g = 2.01, D = −0.305 cm−1,
E = 0.041 cm−1 and  = |E/D| = 0.1396; for 2, g = 2.01; D = −0.348 cm−1, E = 0.042 cm−1 and  = |E/D| = 0.1206 and
for 3, g = 1.99, D = −0.320 cm−1, E = 0.041 cm−1 and  = |E/D| = 0.1281. The EPR study at 94.5 GHz at 10 K allowed
us to confirm the sign of the D value for all compounds. These data indicate that at room temperature the crystal field
is mainly rhombic and as the temperature decreases, the rhombicity of the D tensor increases slightly. These found
differences between 1, 2 and 3 allowed us to establish the importance of the intermolecular interactions in the solid
state due to different hydrogen bonding networks in their crystalline arrangement.
Experimental
Introduction
All reagents and solvents had the highest available commercial
quality and were used without further purification. Elemental
analyses (C, H, N, Cl) were performed in the Chemistry Department
at University College London, UK.
Most of the EPR measurements on Cr(III) complexes have been
carried out at 9.45 GHz, X-band and to a lesser extent at 34.5 GHz,
Q-band. However, for Cr(III) when the second-order perturbation
is similar to the Zeeman term: gB ≈ |D|, it is at times difficult to
perform even a qualitative assignment of the transitions because
many “forbidden” transitions may be present. At a higher magnetic
field the Zeeman interaction becomes larger relative to the zero-field
splitting (zfs), and since h is larger, additional transitions become
observable,1 which permits the determination of larger values of
zfs.2 At high microwave frequency and low temperature h > kBT
can be achieved, thus increasing spin polarization, i.e., almost all of
the spins are in the Zeeman ground state. The changing population
of the levels as a function of temperature generates changes in the
relative intensity of transitions, thus allowing the determination of
the sign of the zfs values.3
In our group three chromium(III) complexes presenting
interesting optical and magnetic properties have been isolated.4a,5
The photochromism exhibited by the dark-green compound:
trans-RSSR-[CrCl2(cyclam)]2ZnCl4, 1, compared to the red–purple
trans-RSSR-[CrCl2(cyclam)]Cl, 2 and the red–purple trans-RSSR-
[CrCl2(cyclam)]Cl·4H2O·0.5HCl, 3 has been attributed to inter-
molecular interactions in the crystal packing in which the chromium
atoms interact through the zinc atom of the tetrachlorozincate
counterion via hydrogen bonding.5
Preparation of the compounds
trans-RSSR-[CrCl2(cyclam)]2ZnCl4 (tetragonal) 1, was prepared
as reported in the literature4b Elemental analysis for 1, found: C,
28.30; H, 5.54; N, 13.17; Cl, 33.81%. Calc. for C20H48Cl8Cr2N8Zn:
C, 28.14; H, 5.67; N, 13.30; Cl, 33.22%.
trans-RSSR-[CrCl2(cyclam)]Cl (tetragonal) 2. This compound
was prepared by a slight modification of the reported technique
for the preparation of trans derivative.4b Anhydrous chromium(III)
chloride (0.50 g, 5.7 mmol) was extracted with zinc–mercury
amalgam into a Soxhlet apparatus and reacted with a boiling
ethanolic solution of cyclam (0.74 g, 3.7 mmol) under nitrogen.
The reaction time was increased from 1 to 72 h.
Subsequently, a small amount of lithium chloride was added
and the reaction mixture was allowed to react for a further 24 h
under reflux. Then, the volume was reduced to 1/4 of its original
value. The solid formed was recovered and recrystallised from 1 M
HCl. Slow evaporation of the solvent produced bright red–purple
crystals, which were washed with ice-cold methanol (yield 0.44 g,
1.2 mmol, 60%). Alternatively, crystals of 2 could be grown by
slow evaporation of the solution that was used for the purification
of crude compound.4b These crystals are stable to air, in contrast to
those of 3. Elemental analysis for 2, found: C, 33.24; H, 6.75; N,
15.26; Cl, 29.56%. Calc. for C10H24Cl3CrN4: C, 33.49; H, 6.74; N,
15.62; Cl, 29.61%.
As a part of a research project on the magnetic characterization
of several coordination compounds,6 a HF-EPR (at 94.5 GHz)
study of magnetically diluted solid samples of 1, 2 and 3 at
different temperatures has been carried out. The crystal and
molecular structures of 2 and 3 were determined and discussed.
trans-RSSR-[CrCl2(cyclam)]Cl·4H2O·0.5HCl (monoclinic) 3.
Crystals of 3 were grown from the recrystallisation of the trans-
compound reported4a in 1 M HCl (4 °C). Although stable in
† This paper is dedicated to the memory of Professor Virgilio Beltrán-
López.
2 4 4 4
D a l t o n T r a n s . , 2 0 0 4 , 2 4 4 4 – 2 4 4 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 Summary of crystallographic data for 2 and 3
Formula
C10H24Cl3CrN4, 2
C10H24Cl3CrN4·4H2O·0.5 HCl, 3
Formula weight
Color/habit
Crystal size/mm
Crystal system
Space group
Z
358.68
447.15
Red–purple/hexagonal prism
Red–purple/rectangular prism
0.48 × 0.42 × 0.30
Monoclinic
P21/n
0.60 × 0.32 × 0.24
Tetragonal
P42/m
2
2
T/K
293(2)
293(2)
a/Å
b/Å
c/Å
7.6245(7)
7.6245(7)
13.623(1)
6.470(2)
7.479(2)
21.079(4)
90.67(2)
1019.9(5)
470
/°
V/Å3
792.0(1)
374
1.504
F(000)
Dc/g cm−3
1.456
Radiation
Mo-K
0.71073
1.218
Mo-K
0.71073
1.032
/Å
/mm−1
Abs. factor min./max.
Std. variation (%)
(sin/)max/Å−1
Final R (on F)
Final wR (on F2)
Data/parameters
Residuals min./max./e Å−3
0.637/0.747
2.8
0.7035
0.031
0.070
1199/72
−0.25/0.32
0.613/0.670
5.6
0.7035
0.054
0.107
2969/184
−0.49/0.50
solution, these crystals lose crystallinity almost instantaneously
when they are exposed to air.
trans-RSSR-[CoCl2(cyclam)]Cl·4H2O·0.47HCl (monoclinic) 4
was prepared and crystallised as published.7
CCDC reference numbers 195378 and 195379.
See http://www.rsc.org/suppdata/dt/b4/b405789a/ for crystallo-
graphic data in CIF or other electronic format.
trans-RSSR-[CoCl2(cyclam)]Cl (tetragonal), 5 was prepared and
crystallised8 to get the analogous form of 2.
trans-RSSR-[CoCl2(cyclam)]2ZnCl4, 6 was prepared by adding a
stoichiometric amount of zinc chloride to a solution of 4 and allow-
ing the solution to crystallise out.
Solid solutions
The diluted samples for magnetic studies were prepared by slow
evaporation of a liquid concentrated solution of 6 containing ~1%
(w/w) of 1, and 5 containing ~1% (w/w) of 2, and 4 containing ~1%
(w/w) of 3. The obtained solids were finely ground in order to have
a homogeneous sample.
X-Band EPR was used to confirm the purity of the chromium(III)
compounds as well as the diamagnetism of the cobalt complexes.
EPR measurements
Single-crystal X-ray diffraction
These studies were carried out on polycrystalline samples of 1, 2
and 3, and on solid solutions of their magnetically diluted samples
at X-band (9.45 GHz) on a JEOL JES -SX spectrometer operating
at 100 kHz. The magnetic field was calibrated with a NMR
ES-CE 50 device. The g values were calculated from the accurate
measurements of the magnetic field and frequency parameters. The
high-frequency (94.5 GHz) spectra were measured at 300, 20 and
10 K, using a novel quasi-optical induction mode spectrometer that
was designed and built at the University of St.Andrews, UK. The fit
of the powder pattern EPR spectra was performed with a program
written by Weihe and coworkers11 which diagonalises the full
Hamiltonian matrix with the bases of four S = 3/2 spin functions,
eqn. (1). This program can be used to calculate the energies and
the transition probabilities, the eigenvalues and eigenvectors,
respectively. The resonant fields were determined by the differences
of the energy between electronic levels. The SIM program also
takes into account the Boltzmann distribution factor in calculating
transition probability and signal intensity.
A single crystal of compound 2 was attached to a thin glass fibre
using epoxy resin. The data collection and structure refinement
follow a routine procedure.
A single crystal of 3 was sealed in a Lindemann capillary filled
with HCl–H2O mother-liquor. The X-ray data collection and the
structure refinement followed the procedure described elsewhere.7
An empirical absorption correction based on  scans9b was applied.
The chromium atom of the complex lies on an inversion centre, so
that the chloride counterion must have an occupancy of 0.5 in order
to preserve charge balance. Since the chloride anion is located on
a general position, its occupation and thermal displacement factors
were refined by keeping one of them constant and varying the other
one alternately.
At an advanced anisotropic refining step, four and three well
defined peaks with approximately equal electron densities were
located surrounding the water O1 and O2 atoms, respectively.
Those peaks near to the water molecule O1 were assigned to the
two disordered H atoms, and refined with partial occupancy of 0.5.
For those peaks near the water molecule O2, applying the same
restrained refinement used for the chloride ion [O–H distance
0.85(1) Å], the occupation factors for these H atoms added up
to 2.24 and gave reasonable values for the thermal parameters
(0.04–0.09 Å2). In the final model a fixed site-occupation factor of
0.746 and a refined isotropic temperature were included for each
H atom in the partially protonated water molecule. The decision in
favour of a partially protonated water instead of an orientationally
disordered water molecule was taken based on the significance in
the content of Cl−.
Results and discussion
The complex cation of 2 has crystallographically imposed 2/m
symmetry, while the chloride ion lies at a site with 4 symmetry.
The configuration for the trans-[CrCl2(cyclam)]Cl has been
suggested to be RSSR, since it is the most stable for these types of
complexes.7,12 Moreover, the molecular structure of the cation has
been found to have this configuration in the nitrate and bromide
salts.4a,c The bright red–purple compound, 2, shows that the cyclam
moiety provides a tetradentate nitrogen ligand, forming with the
two axial chlorine ligands a tetragonal bipyramid coordinated
chromium complex, which also possesses RSSR configuration on
the secondary nitrogen atoms (Fig. 1).
In both cases the structures were solved using SIR979a and refined
using SHELXL97.10 A comparison of the details on the crystal data
and experimental conditions for 2 and 3 can be found in Table 1.
D a l t o n T r a n s . , 2 0 0 4 , 2 4 4 4 – 2 4 4 9
2 4 4 5

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Table 2 Hydrogen bonding interactions found for 1, 2 and 3
Compound
N–H/Å
HCl/Å
NCl/Å
N–HCl/°
N–HCl
1
0.72(5)
0.85(4)
0.83(3)
0.77(5)
0.77(5)
0.81(5)
2.73(4)
2.57(4)
2.70(3)
2.65(5)
2.71(4)
2.49(5)
3.074(3)
3.292(3)
3.340(2)
3.055(3)
3.317(3)
3.241(3)
112(4)
148(3)
135(2)
115(4)
136(4)
155(5)
N1–H1Cl2a
N4–H4Cl1b
N1–H1Cl2c
N1–H1Cl1d
N1–H1Cl1e
N4–H4Cl2
2
3
Symmetry transformations used to generate equivalent atoms: a −x + 1, −y + 1, −z. b −x + 1/2, −y + 1/2, z. c −x + 1, −y, −z. d −x + 1, −y, −z + 1. e x + 1, y, z.
Table 3 Selected bond lengths (Å) and bond angles (°) for 2 and 3
2
3
Cr–N(1)
2.061(2)
2.3295(6)
1.487(3)
1.485(3)
Cr–N(1)
Cr–N(4)
2.061(3)
2.065(3)
2.3292(9)
1.488(5)
1.492(4)
1.482(5)
1.481(5)
Cr–Cl(1)
N(1)–C(1)
N(1)–C(2)
Cr–Cl(1)
N(1)–C(2)
C(3)–N(4)
N(4)–C(5)
N(1)–C(7′)d
N(1B)b–Cr–N(1)
N(1C)c–Cr–N(1)
N(1)–Cr–Cl(1)
N(1A)a–Cr–Cl(1)
N(1)–Cr–N(1)c
Cl(1)–Cr–Cl(1)b
85.1(1)
94.9(1)
91.7(1)
88.3(2)
180.0
N(1)–Cr–N(4)
N(1)–Cr–Cl(1)
N(4)–Cr–Cl(1)
N(1)–Cr–N(4′)d
N(1)–Cr–Cl(1′)d
N(4)–Cr–Cl(1′)d
N(1′)d–Cr–N(1)
N(4′)d–Cr–N(4)
Cl(1′)d–Cr–Cl(1)
85.3(2)
92.0(8)
88.7(1)
94.7(1)
88.0(1)
91.3(2)
180.0
180.0
Fig. 1 ORTEP-like diagram of 2 showing a [133133]13 conformation
and a distorted octahedral coordination geometry. Thermal ellipsoids are
at the 30% probability level; atoms labelled with suffix A are generated
by −x+1, −y, −z, atoms labelled with suffix B are generated by −x+1, −y, z
and atoms labelled with suffix C are generated by x, y, −z.
180.0
180.0
Symmetry transformations used to generate equivalent atoms: a −x + 1,
−y, −z. b −x + 1, −y, z. c x, y, −z. d −x + 1, −y, −z + 1.
The Cr–N ligand and Cr–Cl distances are 2.061(2) and
2.329(6) Å, respectively, agreeing with the literature values.4a The
trans angles are exactly linear on account of symmetry requirements.
The cyclam moiety has the expected geometry and assumes a chair
conformation with exact 2/m symmetry. The ligand and the central
chromium ion are coplanar, this plane being perfectly perpendicular
to the Cl1–Cr–Cl1′ axis.
Inthe crystal lattice four symmetry related trans-[CrCl2(cyclam)]+
cations are hydrogen bonded in a tetrahedral fashion to the chloride
counterion (Fig. 2 and Table 2).
Fig. 3 ORTEP-like diagram of 3 showing a [133133]13 conformation and
a distorted octahedral coordination geometry. Thermal ellipsoids are at the
30% probability level and primed atoms are generated by crystallographic
1 symmetry (1−x, −y, 1−z).
is 2.329(9) Å, modifying the ideal octahedron to a tetragonally
distorted coordination array.
The cyclam moiety exists in a chair conformation with the five-
membered chelate rings adopting a twist conformation, whereas the
six-membered chelate rings are in a chair conformation.
In the crystal lattice the trans-[CrCl2(cyclam)]+ cations stack
along the b direction (Fig. 4) and are held together by N–HCl
hydrogen bonds. The channels left by this arrangement are filled
by chloride ions (arising from the counterion of the complex and
the hydrochloric acid used for crystallisation) and the protonated
solvate water molecules, forming a rather complicated hydrogen
bonding network, in which all possible hydrogen bonds are formed
(Table 2).
Fig. 2 View looking down the crystallographic c direction of complex 2.
The structure found for trans-[CrCl2(cyclam)]Cl·4H2O·0.5HCl,
3 is depicted in Fig. 3 and confirms the RSSR configuration of the
trans-[CrCl2(cyclam)]+ cation. This Cr(III) cation is isomorphous
to the recently reported trans-RSSR-[CoCl2(cyclam)]Cl·4H2O·
0.47HCl.7 The chromium ion is located at a crystallographic centre
of symmetry and is surrounded by four N atoms at nearly identical
Cr–N distances of 2.061(3) and 2.065(3) Å in the equatorial plane
(see Table 3 for selected interatomic distances). The Cr–Cl distance
The structural characterisation of 1 has already been published4a
and showed the same absolute configuration on the secondary
nitrogen atoms; the same [133133] arrangement13 as observed for
2 and 3.
2 4 4 6
D a l t o n T r a n s . , 2 0 0 4 , 2 4 4 4 – 2 4 4 9

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Table 4 Spin-Hamiltonian parameters of 1, 2 and 3 obtained by simulation
of their X-band EPR spectra at 293 K
Complex
g
|D|/cm−1
E/cm−1
1
2
3
1.99
1.99
1.99
0.305
0.310
0.305
0.04
0.05
0.04
The solid-state interactions are very important in these
compounds; in fact, when 1 was irradiated 14 with an X-ray source
(using regular X-ray diffraction powder doses), its colour changed
from dark green to a dark grey-greenish, and the original EPR
spectrum changed showing a “burning hole-like” effect on the
original broad signal. New peaks appeared, a singlet at g = 2.003
and a multiplet centered at g = 2.217. The former is assigned to a
radical species while the latter, to a Cr(III) cation. It is important
to mention that when 2 and 3 were irradiated with the same X-ray
source, no change was observed in their EPR spectrum.
Diluted samples of 1, 2 and 3 were studied at 300 K at 9.45 GHz.
Their spectra are similar, showing well defined lines from zero to
10000 G, with no hyperfine structure. As a first approximation, all
the spectra may be described as isotropic with g = 1.99. Thus, the
spin-Hamiltonian that describes these spectra is typical for systems
with S = 3/2:
Fig. 4 View of the unit cell of 3 looking down the crystallographic b
direction.
It should be noted that in the case of 1, there is a crystallographic
array which includes the chromium and zinc from the tetrachloro-
zincate hydrogen bonded counterion, Fig. 5, Table 2.




1
2
z
2
x
2
y

˘

H = gβBS + D S − S S +1 + E S + S
(1)

(
)


(
)


3
where the first term corresponds to the electronic Zeeman and the
second and third terms correspond to zero-field splitting: axial (D)
and rhombic (E) components. It has been assumed that the principal
axes of the g tensor and the D tensor are parallel to the molecule
axes.15,16
These spectra were simulated, and the results for compound 1 are
shown in Fig. 7.
Fig. 5 View looking down the crystallographic c direction for 1.4a
From the above results it can be shown that 1, 2 and 3 have simi-
lar molecular geometry but different hydrogen bonding networks.
X-Band EPR spectra at room temperature
Another important difference among 1, 2 and 3 in the solid state
is shown by their X-band EPR spectra of their polycrystalline
samples which have different effective g values. A very broad
isotropic peak is centred at g = 1.991 for 1 and 3; while 2 shows
a rhombic-like spectrum: g = 4.309, 3.107 and 1.223. Since the
spectra, Fig. 6, are very broad with high exchange magnetic
interactions (linewidth >500 G) little can be said about their local
magnetic environments.
Fig. 7 Experimental and simulated X-band EPR spectra (top) and the
angular variation of the resonance position plot (bottom) for compound 1
diluted in a cobalt(III) matrix.
The Hamiltonian parameters found for 1, 2 and 3 are listed in
Table 4.
X-Band EPR at low temperature
Measurements carried out at 123 K for 1, 2 and 3 showed no change
relative to the room-temperature spectra.
Since in these systems the zfs energy term is approximately of
the same magnitude as the Zeeman energy, h ≈ zfs, it is possible to
observe a large number of signals in a wide range of magnetic field,
including close to zero magnetic fields and off-axis resonances
(“looping transitions”), which are of particular importance when
we consider the powder spectra17 as they correspond to non-
continuous resonance lines in the considered ranges of  and φ, i.e.
they are closed transitions that generate signals off the axis. Then,
Fig. 6 X-Band EPR spectra of the polycrystalline samples 1, 2 and 3 at
300 K.
D a l t o n T r a n s . , 2 0 0 4 , 2 4 4 4 – 2 4 4 9
2 4 4 7

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Table 5 Spin-Hamiltonian parameters of 1, 2 and 3 obtained by simulation
of their HF-EPR (94.5 GHz) spectra at different temperatures
the X-band EPR spectra are more complex under these conditions.
The E values were found to be non-zero, which is associated with a
distortion in the xy plane.18 However, in this case it was not possible
to assign small differences under these conditions. Therefore, these
samples were studied at HF-EPR in order to determine more precise
D and E parameters.
W-Band (94.5 GHz)
Compound
T/K
g
|D|/cm−1
E/cm−1
|D/E|
1
2
3
293
10
293
10
293
10
2.01
1.99
1.99
1.99
2.01
1.98
−0.305
−0.292
−0.320
−0.292
−0.348
−0.290
0.041
0.0515
0.041
0.0499
0.042
0.0519
0.1396
0.1759
0.1281
0.1709
0.1206
0.1789
HF-EPR (94.5 GHz) at room temperature
In EPR spectra at 94.5 GHz for systems of S = 3/2 the condition
gB  |D| is satisfied and at this limit the resonance lines appear
more resolved and in general the HF-EPR spectrum is much simpler
than that obtained at X-band.19 This is a great advantage since the
spectra can be more easily calculated and readily interpreted.
Working at high frequencies the situation zfs/h ≈ 0.1 is reached,
and under these conditions it is possible to separate “forbidden”
(Ms = ±2, ±3) from “allowed” (Ms = ±1) transitions.20 Thus,
perturbation theory at second-order approximation21 leads to the
fact that zero-field parameters can be easily obtained directly from
the spectrum. Additionally, the spectrum is extremely sensitive to
the values of D and E, thus allowing an accurate determination of
these parameters, Fig. 8.
the population of the energy levels is large enough to show differ-
ences in the relative intensity of the transitions of fine structure.
At low temperature, if D is positive, the energy levels will
become more populated with a concomitant increase in the
intensity of the central transition while the side transitions become
less intense in the spectrum, if there are no other changes in the
relaxation processes. In the case where D has a negative value, the
energy diagrams are inverted; therefore, the side transitions are now
more intense.19,21,22
EPR spectra at 94.5 GHz of complexes 1, 2 and 3 were also
obtained at 10 K. The comparison of the spectra obtained at differ-
ent temperatures shows that there is a magnetic dependence as the
temperature decreases, Fig. 10.
Fig. 8 Position of the signals (Ms =±1) in the HF-EPR (=94.5 GHz)
spectrum in a rhombic field, g=2.00, |D|=0.3 and E=0.04 cm−1 obtained
from our theoretical calculations.
From all these theoretical calculations, it was possible to simulate
the experimental HF-EPR spectra of diluted 1, 2 and 3 samples
obtained at 300 K. The results for compound 1 are shown in Fig. 9.
The D and E parameters for compounds 1, 2 and 3 are listed in
Table 5.
Fig. 10 HF-EPR (94.5 GHz) spectra of 1 at variable temperature showing
a slight magnetic dependence as temperature decreases.
The experimental and simulated spectra at 10 K for 1 are shown
in Fig. 11.
From the spectral analysis and theoretical calculations, the best fit
at 293 K zero-field splitting parameters is, for 1, D = −0.305 cm−1,
Fig. 9 Powder HF-EPR spectra at 300 K (top) and its angular variation
diagram for 1 diluted into a cobalt(III) matrix (bottom).
HF-EPR (94.5 GHz) at low temperature
In order to verify the sign of the D and E parameters, measurements
at low temperature were carried out. This can be explained by the
Boltzmann distribution. At low temperature the difference between
Fig. 11 Experimental and simulated HF-EPR (94.5 GHz) spectra of 1 at
10 K (see Table 5).
2 4 4 8
D a l t o n T r a n s . , 2 0 0 4 , 2 4 4 4 – 2 4 4 9

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E = 0.041 cm−1,  = |E/D| = 0.1396. At 10 K both zero-field
parameters D = −0.292 cm−1 and E = 0.0515 cm−1 change; thus,
 = |E/D| = 0.1759 increased, indicating a larger rhombic distortion
as the temperature decreased. This behaviour appeared in all
complexes.
2002, 58, 1113; (c) A. L. Barra, D. Gatteschi, R. Sessoli, G. L. Abatí,
A. Cornia, A. C. Fabretti and M. N. Uytterhoeven, Angew. Chem., Int.
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An article23 dealing with the determination of the crystal field
parameters of trans-[CrCl2(cyclam)]Cl at X-band EPR in frozen
water–ethylene glycol solution appeared recently. It is interesting
to note that the values reported by these authors: g = 1.99, |D| =
0.470 cm−1 and E = 0.068 cm−1 are higher than ours. This could be
explained by changes in the environment of the cation, since in that
case the EPR measurements were performed on frozen solutions.
It is clear that the series of compounds: trans-RSSR-
[CrCl2(cyclam)]nX, where X = ZnCl42−, Cl− and Cl−·4H2O·0.5HCl
are distinct in the solid state. They have the same trans-RSSR-
configuration of the coordinated ligand and the molecular structure
of the cations are practically the same. However, we have shown
by crystal analysis, that these compounds have different hydrogen
bonding networks due to different supramolecular interactions.
Thus, the differences in g values in the spectra of the polycrystalline
samples at X-band EPR and in the spectroscopic parameters, D and
E, found for 1, 2 and 3 can be attributed to these supramolecular
interactions. It has been recognized that hydrogen bonding is
responsible for important magnetic interactions in the solid state.
2 (a) A. L. Barra, L. C. Brunel, D. Gatteschi, L. Pardi and R. Sessoli,
Acc. Chem. Res., 1998, 31, 460; (b) W. R. Hagen, Coord. Chem. Rev.,
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Conclusions
The series of compounds: trans-RSSR-[CrCl2(cyclam)]nX, where
X = ZnCl42−, Cl− or Cl−·4H2O·0.5HCl were characterised by EPR
spectroscopy. X-Band EPR spectra were highly broadened. Our
calculations revealed that the HF-EPR (94.5 GHz) spectra are
extremely sensitive to the D and E values and the full analyses of
the spectra of 1, 2 and 3 at room temperature showed clear differ-
ences in their zero-field splitting parameters, revealing that they
have different rhombic distortion.As the temperature was decreased
(10 K), D and E changed slightly, producing a larger rhombicity
in all cases. The HF-EPR study at low temperature allowed us to
confirm unambiguously the sign of D, which in all cases was found
to be negative.
The differences in the effective g values observed in the EPR
spectra of the polycrystalline samples permitted the characterisation
and identification of compounds 1, 2 and 3. Additionally, they
allowed us to establish the importance of intermolecular interactions
in the solid state as they have different hydrogen bonding networks
in their crystalline arrangements.
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ligantes tetraazamacrocíclicos, Facultad de Química, UNAM, 2004.
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Acknowledgements
We are very grateful to Prof. Rowlands for his invaluable help in
the development of this work. We also express our thanks to the
DGAPA-UNAM research project IN117200 for economic support.
M. C. Virginia Gómez-Vidales (Instituto de Química-UNAM) is
also acknowledged for the X-band EPR facilities. We also thank
Dr H. Weihe (H. C. Ørsted Institute, University of Copenhagen,
Denmark) for the facilities of the EPR simulation software.
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