Magnetostructural study of substituted α-nitronyl aminoxyl radicals with chlorine and hydroxy groups as crystalline design elements

Article abstract of DOI:10.1039/a700589j

We present a new family of phenyl substituted α-nitronyl aminoxyl radicals which contain hydroxy- and chlorine-substituents as crystal engineering tools. The magnetic behaviour of these radicals strongly differs in dimensionality and strength, showing in all cases antiferromagnetic interactions. We have determined the X-ray crystal structure and analysed the crystal packings of these radicals. From this analysis all the observed magnetic properties can be conveniently rationalized by only considering the close contacts of NO groups of neighbouring molecules according to the generally accepted mechanisms for intermolecular magnetic interactions. Beside the strong O-H...O(-N) hydrogen bonds and the weaker C-H...O(-N) hydrogen bonds, weak Cl...H bonds also seem to play a significant role in determining the molecular arrangement in the solid state and, therefore, the magnetic properties. In only one case are close Cl...Cl contacts observed, pointing to attractive interactions between chlorine atoms.

Full text of DOI:10.1039/a700589j

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Magnetostructural study of substituted a-nitronyl aminoxyl radicals with chlorine and
hydroxy groups as crystalline design elements
Oriol J u¨ rgens, Joan Cirujeda, Montse Mas, Ignasi Mata, Araceli Cabrero, Jos e´ Vidal-Gancedo, Concepci o´ Rovira,
Elies Molins and Jaume Veciana*
Institut de Ci e` ncia de Materials de Barcelona (CSIC), Campus de la UAB, 08193-Bellaterra, Barcelona, Spain
We present a new family of phenyl substituted a-nitronyl aminoxyl radicals which contain hydroxy- and chlorine-substituents as
crystal engineering tools. The magnetic behaviour of these radicals strongly diÂers in dimensionality and strength, showing in all
cases antiferromagnetic interactions. We have determined the X-ray crystal structure and analysed the crystal packings of these
radicals. From this analysis all the observed magnetic properties can be conveniently rationalized by only considering the close
contacts of NO groups of neighbouring molecules according to the generally accepted mechanisms for intermolecular magnetic
interactions. Beside the strong OMH,O(MN) hydrogen bonds and the weaker CMH,O(MN) hydrogen bonds, weak Cl,H
bonds also seem to play a significant role in determining the molecular arrangement in the solid state and, therefore, the magnetic
properties. In only one case are close Cl,Cl contacts observed, pointing to attractive interactions between chlorine atoms.
Magnetic properties of molecular solids depend both on the
molecular electronic properties and on the intermolecular
electronic interactions present in the solid state. Since the
discovery of the b-phase of 4-nitrophenyl a-nitronyl aminoxyl,†
the first example of a purely organic free radical with a bulk
ferromagnetic transition,1 much work has been devoted to the
design of new substituted a-nitronyl aminoxyl radicals as
building blocks for new molecular magnetic materials and
bonding through OH substituents has been demonstrated to
be a powerful crystal engineering element of a-nitronyl ami-
noxyl radicals providing new supramolecular architectures
with relevant magnetic properties.2 One of these new molecular
solids was obtained with 4-hydroxyphenyl a-nitronyl aminoxyl
radical.3 This radical in the solid state forms a two-dimensional
network built up by OMH,O(MN) and CMH,O(MN)
hydrogen bonds that explains its quasi-two-dimensional ferro-
magnetic behaviour. On the other hand the 2-hydroxyphenyl
and 2,5-dihydroxyphenyl a-nitronyl aminoxyl radicals4,5
undergo bulk ferromagnetic transitions at 0.45 and 0.5 K,
respectively, being two of the rare examples of purely organic
ferromagnets. Beside their crystal packing, which is controlled
by a complex network of weak CMH,O(MN) hydrogen
bonds, the twist angles between the phenyl rings and the mean
planes defined by the ONCNO groups seem to play a signifi-
cant role in determining this unusual magnetic property.
Therefore such results clearly show that the control of the
molecular conformation and the crystal packing are key points
in molecular magnetism.
Chlorine atoms have long been known for their steering
ability in crystal engineering of molecular solids. Attractive
Cl,Cl interactions and CMH,Cl hydrogen bonds have been
described as responsible for this ability.6,7 Herein we report a
detailed magnetostructural study of a new family of phenyl a-
nitronyl aminoxyl radicals 3–5 that combine OH groups and
Cl atoms, attached to diÂerent positions of the phenyl rings,
as crystalline design elements. The combination of these two
crystalline design elements has provided an eÂective way to
modulate the crystal packing of several chlorine substituted
phenols,8,9 leading to interesting applications to magnetic
molecular solids.
O•
N
O•
N
Cl
Cl
N+
O–
N+
O–
1
2
O• Cl
N
O•
N
Cl
OH
OH
N+
N+
O–
O–
3
4
O• HO
N
N+
O–
5
Cl
Radicals with only one Cl atom at the ortho and meta
positions, radicals 1 and 2, have also been studied here as
reference compounds in order to evaluate the role played by
this bulky and electroactive atom in crystal packing.
Interestingly, the radical with one Cl atom at the ortho position
also provides an opportunity to evaluate the eÂect of a large
torsion between the two rings of the radical without introduc-
ing any strong OMH,O(MN) hydrogen bonds.
Experimental
General procedures
Radicals 1–5 were prepared using the procedure described by
Ullman and co-workers.10 The 2,3-(dihydroxylamino)-2,3-
dimethylbutane used as a precursor was obtained by following
the reported procedure.11 Melting points were determined by
diÂerential scanning calorimetry (Perkin-Elmer, DSC-7 calor-
imeter) and are given as the maxima of the observed peaks.
All radicals containing OH groups, radicals 3–5, melt with
decomposition. IR (Nicolet 710 FT-IR spectrometer) and
UV–VIS spectra (Cary 5 UV–VIS-NIR spectrometer) of the
synthesized radicals were also recorded.
†
4
a-Nitronyl aminoxyl is used throughout to indicate 4,5-dihydro-
,4,5,5-tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl.
J. Mater. Chem., 1997, 7(9), 1723–1730
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Synthesis of free radicals
-(2-Chlorophenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxido-
Superconducting SQUID susceptometer and using microcrys-
talline samples (80–115 mg) of the radicals 1–5. The diamag-
netic contributions of the sample holder and the radicals were
determined by extrapolation from the xT vs. T plots in the
high-temperature range and were used later to correct the
SQUID outputs.
2
1
H-imidazol-3-ium-1-oxyl 1. 2,3-(Dihydroxylamino)-2,3-
dimethylbutane (2.11 g; 14.2 mmol) was added to a stirred
solution of 2-chlorobenzaldehyde (2 g; 14.2 mmol) in 30 ml of
methanol. Stirring at room temp. was continued for 20 h and
the resulting white precipitate was filtered o and dried in
X-Ray measurements
vacuo. This solid was oxidized with a solution of NaIO (1.5 g;
4
7
dichloromethane. The solution was evaporated and the crude
.1 mmol) in 50 ml of water at 5 °C and extracted with
X-Ray data for single crystals of 1, 2, 4 and 5 were collected
at 293 K on an Enraf-Nonius CAD 4 FR-590 diÂractometer
˚
working at 1 kW with monochromatic Mo-Ka (l=0.71069 A)
product was purified by column chromatography (SiO ) with
ethyl acetate dichloromethane (151) as eluent (2.55 g; yield,
2
radiation. Data were collected by using an v/2h scan method.
The structures were all refined by a full-matrix least squares
method which minimized Sw(DF)2.‡ The presence of diÂerent
polymorphs in each crystalline material used for magnetic
measurements was ruled out by means of powder X-ray
diÂraction spectra by comparing the experimental spectra with
the simulated ones based on the single crystal X-ray diÂraction
structure. These spectra were simulated by using the CERIUS2
6
7% from the aldehyde). Single crystals of 1 were grown by
evaporation at room temp. from a toluene solution. Mp
68.3 °C (Found: C, 58.27; H, 6.02; N, 10.42. Calc. for
1
C H N O Cl: C, 58.32; H, 6.02; N, 10.46%); n /cm−1 (KBr)
1
3
16
2
2
max
1
7
595w, 1449m, 1404s, 1367s, 1211w, 1171m, 1133m, 1055m,
66m; UV–VIS (CH Cl ) l /nm (e): 354 (18 000), 554 (780);
2
2
max
MS (EI) m/z: 267 (M+), 179, 138, 114, 84, 69, 56.
2.0 program (Molecular Simulations Inc.). Powder diÂraction
spectra were collected on a Rigaku Dimax RC-200
2
-(3-Chlorophenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxido-
1
H-imidazol-3-ium-1-oxyl 2. Radical 2 was synthesized by the
diÂractometer with a 12 kW rotating anode generator and a
monochromator of single crystalline graphite for Cu-Ka
radiation.
same procedure as 1. Crystals were grown by slow evaporation
of a heptane–dichloromethane (1051) solution at room temp.
Mp 123.5 °C (Found: C, 58.30; H, 6.01; N, 10.40. Calc. for
C H N O Cl: C, 58.32; H, 6.02; N, 10.46%); yield, 92% from
EPR spectroscopic measurements
1
3 16 2 2
the aldehyde; n /cm−1 (KBr) 1580m, 1418m, 1395m, 1364s,
max
134m, 795m; UV–VIS (CH Cl ) l /nm (e): 271 (16 000), 367
max
18 000), 584 (480); MS (EI) m/z: 267 (M+), 179, 138, 114,
The EPR spectra of radicals 1–5 in toluene solutions under
free tumbling conditions were recorded on a Bruker ESP-300E
spectrometer operating in the X-band (9.3 GHz) with a rec-
tangular TE102 cavity and equipped with a field-frequency
1
(
2
2
8
4, 69.
(
F/F) lock accessory and a built-in NMR gaussmeter. Signal-
2
-(2-Chloro-4-hydroxyphenyl)-4,5-dihydro-4,4,5,5-
to-noise ratio was increased by accumulation of scans using
the F/F lock accessory to guarantee a high-field reproducibility.
Precautions to avoid undesirable spectral line broadening such
as that arising from microwave power saturation and magnetic
field overmodulation were taken. In order to avoid dipolar
broadening, the radical solutions were carefully degassed by
bubbling with pure argon.
tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl 3. Radical 3 was
obtained in a similar way to 1, but instead of stirring the
reactants, they were refluxed in benzene for 19 h. All attempts
to grow large single crystals of radical 3 failed, and so its X-
ray structure could not be determined. Mp 166.2 °C (decomp.)
(
5
n
Found: C, 55.21; H, 5.75; N, 9.70. Calc. for C H N O Cl: C,
13 16 2 3
5.03; H, 5.68; N, 9.87%); yield, 22% from the aldehyde;
/cm−1 (KBr): 1604s, 1456m, 1364m, 1106m, 858w; UV–VIS
max
(
CH Cl ) l /nm (e): 269 (13 000), 328 (12 000), 561 (1040);
Results and Discussion
2
2
max
MS (EI) m/z: 283 (M+), 195, 153, 114, 84, 69.
Spin density distribution of the radicals
2
-(3-Chloro-4-hydroxyphenyl)-4,5-dihydro-4,4,5,5-
The most widely accepted mechanism for rationalizing the
intermolecular magnetic interactions in organic molecular
solids is the so-called McConnell I mechanism based on the
overlap of the orbitals on atoms with large spin densities of
neighbouring molecules.12,13 According to this mechanism,
dominant contacts of atoms with spin densities having the
same sign produce an antiferromagnetic interaction between
the two neighbouring molecular units. In contrast, ferromag-
netic interactions are favoured if opposite signs in these
contacts are predominant. For this reason it is important in
magnetic molecular materials to know how the spin density of
the unpaired electron is distributed within the building block
molecules.
tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl 4. Radical 4 was
synthesized by the same procedure as for 3. Crystals were
grown by a slow diÂusion of pentane into a concentrated
toluene solution at room temp. Mp 158.4 °C (decomp.) (Found:
C, 55.31; H, 5.75; N, 9.81. Calc. for C H N O Cl: C, 55.03;
H, 5.68; N 9.87%); yield, 93% from the aldehyde; n /cm−1
max
KBr): 1605m, 1490m, 1387m, 1340s, 1300m, 1273m, 1214m,
169m, 1133m, 831m, 702w, 541w; UV–VIS (CH Cl ) l /nm
13 16 2 3
(
1
(
(
2
2
max
e): 282 (16 000), 369 (12 000), 618 (720); MS (EI) m/z: 283
M+), 195, 153, 114, 84, 69.
2
-(5-Chloro-2-hydroxyphenyl)-4,5-dihydro-4,4,5,5-
tetramethyl-3-oxido-1H-imidazol-3-ium-1-oxyl 5. Radical 5 was
obtained similarly to 3. Crystals were grown by slow evapor-
ation of a heptane–dichloromethane (1051) solution at room
temp. Mp 118.7 °C (decomp.) (Found: C, 55.78; H, 5.94; N,
Free tumbling solution EPR spectra provide the necessary
information about such spin density distributions in organic
free radicals, through the determination of the coupling con-
stants with the magnetically active nuclei of the molecules.
The EPR spectra of radicals 1–5 show basically five main
groups of lines with relative intensities of 152535251, resulting
from the coupling of the unpaired electron with two equivalent
nitrogen nuclei (I=1), as shown in Fig. 1(a) for radical 4. The
9
.05. Calc. for C H N O Cl: C, 55.03; H, 5.68; N, 9.87%);
16
13
2 3
yield, 25% from the aldehyde; n /cm−1 (KBr): 1573w, 1471s,
1
l
1
max
375m, 1341m, 1278m, 1135m, 825m, 646w; UV–VIS (CH Cl )
/nm (e): 349 (4400), 581 (440); MS (EI) m/z: 283 (M+),
2
2
max
95, 153, 114, 84, 69.
‡
Atomic coordinates, thermal parameters, and bond lengths and
Magnetic measurements
angles have been deposited at the Cambridge Crystallographic Data
Centre (CCDC). See Information for Authors, J. Mater. Chem., 1997,
Issue 1. Any request to the CCDC for this material should quote the
full literature citation and the reference number 1145/40.
DC magnetic susceptibility data from 2 to 300 K, in a magnetic
field of 1 T, were collected using a ‘Quantum Design’ MPMS
1
724
J. Mater. Chem., 1997, 7(9), 1723–1730

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agreement with those previously reported by other authors,16
who have determined by NMR measurements that the N and
O atoms carry a large and positive spin density while the a-
carbon atom has a significant negative spin density. The methyl
groups and the aromatic ring also carry a spin density, as
inferred from the observed EPR hyperfine couplings with all
these hydrogen atoms. The coupling constants for the aromatic
hydrogen atoms are smaller in radicals 1, 3 and 5 than in
radicals 2 and 4. The loss of planarity between the five- and
the six-membered rings, due to the presence of a substituent
at the ortho position in compounds 1, 3 and 5, yields a
satisfactory explanation for this fact. Assignments of the coup-
ling constants of aromatic hydrogen atoms for radicals 1–5
have been performed by comparison with a whole series of
radicals with non-magnetically active substituents located at
diÂerent positions of the phenyl rings.17
Molecular and crystal structures of radicals
General crystallographic information for radicals 1, 2, 4 and 5
is summarized in Table 2. Atomic numbering schemes used for
these radicals are shown in Fig. 2 together with their molecular
conformations.
Structure of radical 1. Radical 1 crystallizes in the orthorhom-
bic system with an asymmetric unit which contains one radical
molecule. The most relevant feature of the solid state molecular
conformation of radical 1 is the large angle (62°) formed by
the phenyl ring and the mean plane of the OMNMCMNMO
unit. This twist angle is larger than that reported for the
unsubstituted phenyl a-nitronyl aminoxyl radical (29°)18 as
well as for the p-chloro- (24°),19 the o-hydroxy- (40°)4 and the
p-hydroxyphenyl- (30°)3a substituted examples. Therefore, this
result suggests that the large angle in 1 is merely due to steric
hindrance of the bulky Cl atom at the ortho position.
˚
The intramolecular Cl,O distance in radical 1 (3.24 A) is
quite similar to intermolecular distances found between hal-
ogen and nucleophile atoms by Murray-Rust and co-workers.20
This fact suggests that, in spite of the strong steric hindrance
between the Cl and the O atom, a slightly attractive interaction
between both atoms cannot be excluded. The measured dis-
tance would be the resulting equilibrium position between the
steric repulsion and such halogen–nucleophile attracting forces.
In our case, the CMCl,O angle is obviously much smaller
Fig. 1 (a) Complete EPR spectrum of radical 4 in toluene at 293 K.
b) Experimental (upper) and simulated (lower) central groups of EPR
lines. The computer simulation was carried out using a Lorentzian
line shape with DH1/2 of 0.13 G and the hfcc valves given in Table 1.
(
values of the isotropic hyperfine coupling constants in all
cases are between 7.2 and 7.8 G; i.e. a =7.5(3) G; which is
typical for freely tumbling substituted a-nitronyl aminoxyl
N
radicals.1,14,15 A more detailed analysis of these five main
groups of signals reveals a complex pattern of lines arising
from supplementary couplings with the twelve equivalent
hydrogen atoms (I=1/2) of the four methyl groups and all the
hydrogen atoms of the phenyl ring.
Table 2 Crystallographic data for radicals 1, 2, 4 and 5
compound
1
2
4
5
˚
a/A
10.483(3)
10.921(3)
11.671(3)
—
3887/165
1336.2(6)
1.331
9.946(2)
11.522(1)
12.700(2)
109.91(1)
4130/213
1368.4(4)
1.300
13.735(1)
11.819(2)
17.153(3)
—
2441/176
2785.5(7)
1.354
Pbca
8
0.0514
9.741(6)
11.663(5)
13.324(14)
111.23(7)
2037/223
1411(2)
˚
b/A
Computer simulations of the experimental EPR spectra of
–5 yield the hyperfine coupling constants summarized in
˚
c/A
b/°
1
Table 1. All values are in agreement with those previously
reported for other a-phenyl nitronyl aminoxyl radicals,15 indi-
cating, therefore, that the chloro- and hydroxy-substituents do
not alter the electron distribution in the radicals significantly.
Consequently, the unpaired electron is mainly distributed on
both NO groups and the a-carbon atom. This result is in
data/parameters
˚
V /A3
D /g cm−3
space group
1.336
P2 /n
c
P2 2 2
P2 /c
1
0.0394
1 1
1
0.0478
1
0.0488
Z
R
4
4
4
Table 1 Summary of hyperfine coupling constants (a/G; 1 G=10−4 T) obtained for radicals 1–5 by computer simulation of the EPR spectra of
toluene solutions
compound
aN
a (methyl)
a (ortho)
a (meta)
a (para)
H
H
0.24
H
H
1
2
3
4
5
7.26 (2N)
7.40 (2N)
7.65 (2N)
7.50 (2N)
7.81, 7.40
0.20 (12H)
0.19 (12H)
0.18 (12H)
0.21 (12H)
0.20 (12H)
0.15, 0.12
0.21
0.16 (2H)
0.17
0.25
a
0.52 (2H)
0.23
0.54, 0.50
0.33
0.43
—
—
0.30
aNot observed.
J. Mater. Chem., 1997, 7(9), 1723–1730
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Fig. 2 Molecular conformations of radicals 1, 2, 4 and 5 with the atomic numbering schemes used in the text and tables
(
75°) than in the examples described by Murray-Rust and co-
perpendicular to the crystallographic [1 0 −1] direction.
Between adjacent planes no H,O contacts with
workers20 because of the intramolecular nature of that contact.
As shown in Fig. 3, the molecules of radical 1 are packed in
such a way that one of the two NO groups of each molecule
forms a hydrogen bond with a methyl group of a neighbouring
molecule [d(H11D,O14i, i=−x, 1/2+y, 5/2−z)=2.64 A;
h(C112MH11D,O14i)=154°] giving rise to twisted chains
along the b axis. Each of these chains is connected to two
˚
d(H,O)<3 A are observed. Molecules of adjacent planes are
just connected by weak C
[iii=−x,
MH11B,C11iii interactions
methyl
˚
1/2+y,
3/2−z;
d(H11B,C11iii)=3.01 A;
˚
h(C111MH11B,C11iii)=172°], and these interactions are
propagated along the b axis.
As a result of this complex crystal packing, the shortest
neighbouring chains by means of C
MH7,O14iiMN13ii
arom
hydrogen bonds [ii=1/2+x, 1/2−y, 2−z; d(H7,O14ii)=
distance between the NO groups of diÂerent molecules
˚
−5/2−z;
d(O14,O10iv)=5.39 A] occurs between neighbouring mol-
ecules in the chains along the b axis. These kinds of contacts
are responsible for the magnetic behaviour as will be dis-
cussed below.
[d(O14,N9iv)=4.94 A;
iv=−x,
−1/2+y,
˚
˚
2
.66 A; h(C7MH7,O14ii)=130°] forming molecular sheets
Structure of radical 2. This compound crystallizes in the
monoclinic P2 /c space group with an asymmetric unit con-
taining one radical molecule. Fig. 2 shows its solid state
1
molecular conformation. The torsion angle between the two
rings of radical 2 is 26°, being very similar to those of other
radicals with similar steric requirements at the ortho pos-
ition.18,19,3a The crystal structure of 2 clearly shows alternating
planes perpendicular to the crystallographic [1 0 −1] direc-
tion. Within these planes, the molecules are connected to each
other forming chains along the b axis [see Fig. 4(a)] by means
of
−
C
MH14,O2iMN2i hydrogen bonds [i=1−x,
arom
1/2+y, 1/2−z; d(H14,O2i)=2.47 A; h(C14MH14,O2i)=
˚
1
29°]. These chains are linked to each other by weak
C
−
MH44,O5iiMN5ii hydrogen bonds [ii=−x, 1/2+y,
methyl
˚
1/2−z; d(H44,O5ii)=2.55 A; h(C42MH44,O5ii)=148°].
As happens with radical 1, between adjacent planes contain-
ing the stronger CMH,O hydrogen bonds, no other H,O
˚
contacts with d(H,O)<3 A are observed. In a similar way as
Fig. 3 Crystal packing of radical 1. Crystallographic (1 0 −1) plane
formed by chains of molecules connected through
–H11D,O14i–N13i (i=−x, 1/2+y, 5/2−z) hydrogen bonds
occurs with radical 1 there are C
MH15,C113iii inter-
arom
˚
actions [iii=x, 1/2−y, 1/2−z; d(H15,C113iii)=3.12 A;
h(C15MH15,C113iii)=154°] between the planes. Also,
between chains of neighbouring (1 0 −1) planes, we have
found short C113,C113iv contacts [iv=1−x, −y, −z;
C
methyl
(dashed lines) along the b axis. The dotted line connects the nearest
NO groups of diÂerent molecules, N13–O14,N9iv (iv=−x,
1/2+y, −5/2−z).
−
1
726
J. Mater. Chem., 1997, 7(9), 1723–1730

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strong intermolecular O8MH8,O15iMN14i bond [i=1−x,
˚
−
1/2+y, 1/2−z; d(H8,O15i)=1.72 A; h(O8MH8,O15i)=
68°] which connects each molecule to two neighbours giving
1
rise to zigzag chains along the b axis. This arrangement is
˚
reinforced by a second hydrogen bond [d(H6,O15i)=2.65 A;
h(C6MH6,O15i)=128°] between the same NO group and
an aromatic H atom in the meta position which is adjacent to
the OH group which forms the strong hydrogen bond. Further,
these chains are arranged into planes perpendicular to the
crystallographic [1
0 0] direction by means of weak
C
−
MH134,O11iiMN10ii hydrogen bonds [ii=1−x, −y,
methyl
˚
z; d(H134,O11ii)=2.71 A; h(C132MH134,O11ii)=160°],
as shown in Fig. 5(a).
This pattern resembles very much the crystal packing
reported for the 4-hydroxyphenyl a-nitronyl aminoxyl radical3
but there are two basic diÂerences caused by the presence of
the Cl atom at the meta position. The first is that the rings of
radical 4 are not coplanar to the (1 0 0) plane; i.e. the molecules
are alternately canted within the chains. The second diÂerence
refers to the stacking of the planes along the crystallographic
a
direction, which in radical
4 takes place through
C
MH125,Cl1iii interactions [iii=−1/2+x, 1/2−y, −z;
methyl
˚
d(H125,Cl1iii)=3.07 A;
C
h(C122MH125,Cl1iii)=167°],
MH121,Cl1iv interactions [iv=−1/2+x, y, 1/2−z;
methyl
˚
d(H121,Cl1iv)=3.08 A;
h(C121MH121,Cl1iv)=134°]
MH131,O11vMN10v hydro-
[shown in Fig. 5(b)] and C
methyl
˚
gen bonds [v=1/2−x, 1/2+y, z; d(H131,O11v)=2.75 A;
h(C131MH131,O11v)=151°], while in the para-hydroxy sub-
stituted radical the stacking of planes occurs through van der
Waals interactions.
The lack of planarity of the molecules of radical 4, within
the twisted chains in the (1 0 0) plane, means that the shortest
intermolecular distance between NO groups occurs pairwise
Fig. 4 Crystal packing of radical 2. (a) Crystallographic (1 0 −1)
plane formed by chains of molecules connected through
C
–H14,O2i–N2i (i=1−x, −1/2+y, 1/2−z) hydrogen bonds
arom
(dashed lines) along b axis. (b) Molecules of two neighbouring (1 0 −1)
planes showing as dashed lines C113,C113iv close contacts (iv=
1−x, −y, −z). The dotted lines connect the nearest NO groups of
diÂerent molecules, N2,O5v–N5v (v=−x, 1−y, −z).
˚
d(C113,C113iv)=3.72 A], which are depicted in Fig. 4(b).
The distances of such contacts are in accordance with those
described by Desiraju7 for several other compounds which
clearly show non-covalent attractive Cl,Cl interactions.
Therefore this result suggests that the Cl,Cl interactions are
driving forces for the packing of molecules in (1 0 −1) planes.
As will be discussed below, from the magnetic point of view,
the most important aspect of the crystal packing of radical 2
is the presence of short intermolecular distances between the
NO groups of two molecules in neighbouring (1 0 −1) planes.
This kind of contact permits us to consider the molecular
system as being composed of dimeric magnetic entities which
are clearly shown in Fig. 4(b).
Structure of radical 4. Radical 4 crystallizes in the orthorhom-
bic Pbca group and also contains one molecule in the asymmet-
ric unit. In this case the phenyl ring is twisted with respect to
the mean OMNMCMNMO plane by 28.5°. This result is
again in accordance with the twist angles observed for other
radicals with similar steric requirements, that is, those without
substituents at the ortho positions.
Fig. 5 Crystal packing of 4. (a) Crystallographic (100) plane formed
by zigzag chains of molecules connected through strong
O8–H8,O15i–N14i (i=1−x, −1/2+y, 1/2−z) hydrogen bonds
along b axis depicted as dashed lines. The dotted lines connect the
nearest NO groups of diÂerent molecules, N10,O11vi–N10vi (vi=
1
−x, −y, −z). (b) Molecules of neighbouring (100) planes showing
C
C
–H125,C11iii
(iii=−1/2+x,
1/2−y,
−z)
and
methyl
methyl
–H121,C11iv (iv=−1/2+x, y, 1/2−z) interactions which are
The main feature of the crystal packing of radical 4 is the
depicted as dashed lines.
J. Mater. Chem., 1997, 7(9), 1723–1730
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between molecules of neighbouring chains as shown in Fig. 5(a)
leading to a dimeric magnetic pattern, as will be discussed
below.
Structure of radical 5. This compound does not crystallize
in the orthorhombic system, as occurs for radicals 1 and 4,
but in the monoclinic one, similarly to radical 2 which also
has one chlorine atom in the meta position. The presence of
the OH group at the ortho position results in the formation
of a strong intramolecular O51MH51,O15MN14 hydrogen
˚
bond [d(H51,O15)=1.66 A; h(O51MH51,O15)=170°].
Consequently, there is a twist angle of 35° between the two
rings of the molecule, as was also observed for the radical with
one OH group in the ortho position.4 Fig. 2 shows the molecu-
lar structure of 5, where the lack of planarity is clear. Due to
the establishment of these strong intramolecular OMH,OMN
hydrogen bonds, the molecules are packed in the crystal
through other types of weak hydrogen bonds. Thus, the
molecules are arranged into zigzag chains parallel to the [1 0
1
] direction by means of rather weak C
MH7,O11iMN10i
arom
1/2−y,
hydrogen bonds [i=−1/2+x,
d(H7,O11i)=2.22 A; h(C7MH7,O11i)=157°]. The chains
are layered into planes perpendicular to the [1 0 −1] direction
−1/2+z;
˚
by C
3
MH13B,O15iiMN14ii hydrogen bonds [ii=1/2+x,
methyl
˚
/2−y,
1/2+z;
d(H13B,O15ii)=2.82 A;
h(C131M
H13B,O15ii)=155°] as shown in Fig. 6(a).
Furthermore, the planes are connected pairwise through
other C
MH12A,O11iiiMN10iii hydrogen bonds [iii=
methyl
−x, 1−y, 1−z; d(H12A,O11iii)=2.68 A; h(C121M
˚
1
H12A,O11iii)=174°] giving rise to an alternating pattern in
which the radical molecules of two neighbouring planes form
dimers, related by an inversion centre shown in Fig. 6(b). As
will be seen below, these dimers are responsible for the magnetic
behaviour of the compound.
As can be inferred from geometric considerations, all
CMH,Cl distances present in this radical are longer than
˚
˚
˚
3
.3 A. {The shortest ones are [d(H6,Cl1i)=3.33 A;
h(C6MH6,Cl1i)=156°]
h(C122MH123,Cl1iv)=110.6°; iv=x, 1+y, z]}.
and
[d(H123,Cl1iv)=3.36 A;
The crystal packing of radical 5 is therefore very similar to
the molecular arrangement of radical 2. This result can be
rationalized by noting that both compounds have similar
crystalline design elements: in fact both radicals have one Cl
atom at the meta position and the additional OH substituent
at the ortho position of radical 5 seems to aÂect only the
intramolecular conformation and not the intermolecular inter-
actions. The fact that no Cl,Cl contacts are observed in
radical 5 may be explained as a consequence of the lower
planarity7 of this molecule due to the OH substituent at the
ortho position.
Fig. 6 Crystal packing of 5. (a) Crystallographic (1 0 −1) plane formed
by chains of molecules connected through C
−
–H7,O11i–N10i (i=
arom
1/2+x, 1/2−y, −1/2+z) hydrogen bonds parallel to the a+c
direction. (b) Molecules of two neighbouring (1 0 −1) planes forming
dimers by intermolecular
C
–H12A,O11iii–N10iii (iii=1−x,
methyl
1−y, 1−z) hydrogen bonds. The strong intramolecular
O51–H51,O15–N14 hydrogen bonds are also depicted.
Magnetic properties of radicals
Summarizing the above structural analysis, the arrangement
of molecules of the radicals 1, 2, 4 and 5 in the solid state is
mainly governed by strong OMH,OMN (only possible in
radicals 4 and 5) and weak CMH,OMN hydrogen bonds
giving rise to molecular solids with one- or two-dimensional
structural character.6,21 The CMH,Cl hydrogen bonds only
seem to play an important role in the crystal packing of the
radicals without OH substituents on the phenyl ring, as shown
by the shorter H,Cl distances in radicals 1 and 2 compared
with radicals 4 and 5. Cl,Cl interactions have only been
found for radical 2. According to Desiraju et al.,7 the lack of
planarity, especially in radicals 1 and 5, and the presence of
other stronger intermolecular interactions, as occurs for 4, may
be responsible for the absence of Cl,Cl interactions in radicals
Static magnetic susceptibility measurements of radicals 1–5
are shown in Fig. 7. Such measurements indicate that the
molecular solids studied present in all cases intermolecular
antiferromagnetic (AFM) interactions with diÂerent strengths.
As we will discuss later, the magnetic dimensionalities vary
from one compound to another in close relationship with their
crystal packings.
The magnetic susceptibility data of 1 were nicely fitted to a
1D Heisenberg model of S=1/2 molecular units having weak
AFM interactions with a magnetic exchange interaction of
J/k =−0.95 K. In the crystal structure of radical 1, all the
intermolecular distances between atoms carrying the larger
B
spin densities are quite large. As described previously, the
˚
1
, 4 and 5. As a consequence of such considerations, it
shortest one [d(O14,N9iv)=4.94 A; iv=−x, −1/2+y,
has been observed that the placement of an additional Cl
atom modulates considerably the crystal packing of phenyl
a-nitronyl aminoxyl radicals.
−5/2−z] occurs among atoms having a positive sign of spin
density and that belong to molecules forming the previously
described chains along the crystallographic b direction. In
1
728
J. Mater. Chem., 1997, 7(9), 1723–1730

View Article Online
maximum at 9 K in the x vs. T plot strongly suggests a
low magnetic dimensionality for this molecular solid. Actually,
the data can be fitted to a dimer chain model23 of S=1/2
molecular units with an intradimer exchange interaction of
J /k =−14.9 K and an interdimer exchange interaction of
1
B
J /k =−10.5 K. For radical 4, the shortest distance between
2
B
˚
NO groups of diÂerent molecules [d(N10,O11ii)=3.69 A,
˚
d(O11,O11ii)=3.81 A, ii=1−x, −y, −z] occurs within the
(
chains. The shortest distance between the spin-carrying units
1 0 0) layers among molecules belonging to neighbouring
˚
of diÂerent dimers [d(O15,O15vi)=4.61 A; vi=1−x, 1−y,
−
z] takes place along the b axis within the (1 0 0) plane.
Moreover, the remaining distances between dimers are much
˚
longer, the lowest one being 5.65 A. Therefore, all these facts
explain the goodness of the experimental data to a dimer chain
model for radical 4. This magnetostructural correlation has
short intra- and inter-dimer distances between NO units that
are responsible for the strong antiferromagnetic couplings
within and between the magnetic dimers. Both distances are
clearly shorter than those observed for radicals 1 and 2 in
agreement with the stronger antiferromagnetic couplings
observed for 4.
Fig. 7 Temperature dependence of the paramagnetic susceptibility x
of radicals (%) 1, (+) 2, (&) 3, (') 4 and ($) 5. The inset shows an
enlargement of the temperature dependence for radicals 4 and 5. The
solid lines represent the fits of experimental data to the magnetic
models explained in the text.
consequence, this molecular arrangement is in agreement with
the observed weak 1D antiferromagnetic behaviour.
The x vs. T plot of radical 5 shows a broader maximum at
ca. 50 K indicating again a low dimensionality with a very
strong antiferromagnetic interaction. The experimental data
The magnetic behaviour of 2 is properly explained by a
dimer model with antiferromagnetic interactions. However, the
fitting of experimental data is considerably improved if an
additional interdimer antiferromagnetic term is included in the
Bleaney–Bowers equation for S=1/2 molecular units with an
can be fitted to a simple dimer model with strong AFM (J/k =
B
−42.3 K) interactions.24 An additional Curie tail (C=0.0043
emu K mol−1) is necessary to fit the low temperature values,
which takes into account possible crystal surface eÂects or the
presence of dislocations in the crystals. The strength of the
intradimer interaction is remarkable, since it is one of the
largest reported to date for a purely organic compound.
As mentioned previously in the description of the molecular
packing of radical 5, there are molecules linked by
CMH,OMN hydrogen bonds that clearly form dimeric enti-
ties. The shortest distance between the spin-carrying units for
antiferromagnetic interaction of J/k =−1.82 K, by means of
a temperature correction with a Weiss constant of h=
B
−
1.39 K.22 This result means that the dominant magnetic
interactions take place within the dimers but such dimers also
interact weakly with each other in a quite isotropic antiferro-
magnetic fashion. Analysing the crystal structure of 2, we have
found that the shortest distance between NO groups of diÂerent
˚
˚
molecules
is
slightly
˚
shorter
[d(N2,O5v)=4.89 A;
radical 5 occurs inside these dimers [d(O11,O11iii)=3.37 A;
d(O2,O5v)=5.03 A; v=−x, 1−y, −z] than in radical 1,
occurring in a dimer geometry instead of in chains. This
structural pattern therefore explains the observed magnetic
characteristics of this molecular solid. The distances between
iii=1−x, 1−y, 1−z]. This short intermolecular distance is
the shortest one observed in this family of radicals and explains
the strong intermolecular magnetic interaction observed for
radical 5.
˚
NO groups of diÂerent dimers are larger than 5.24 A and can
be associated with the weaker interdimer antiferromagnetic
interactions.
From the magnetostructural study it has been possible to
establish a complete correlation between the solid state mag-
netic properties and the molecular arrangement in the crystals
for all the radicals studied. Table 3 summarizes the main
features of the structure and magnetic behaviour for radicals
1, 2, 4 and 5 showing in addition that the structural dimension-
alities, based on packing motifs linked by hydrogen bonds, are
diÂerent from the magnetic dimensionalities.
The experimental data for radical 3 can be fitted either by
a 1D AFM model with J/k =−0.62 K or by the Curie–Weiss
law with h=−0.89 K. Thus, such data clearly show the
antiferromagnetic nature of the intermolecular interactions
even though they do not have any singularity that permits us
to distinguish the magnetic dimensionality of this molecular
system.
B
It also seems clear that the magnitude of exchange inter-
actions between radical molecules in these molecular solids
strongly decreases as the mean intermolecular distances
In contrast, in the case of radical 4, the presence of a broad
Table 3 Summary of structural and magnetic dimensionalities of radicals 1, 2, 3 and 5 together with the most relevant intermolecular contacts
from the structural and magnetic points of view. See text for symmetry operations
compound
structural dimensionalitya
structurally relevant contacts
magnetic dimensionalityb
magnetically relevant contacts
˚
1
2
4
2D
2D
2D
d(H11D,O14i)=2.64 A
1D, regular chains
(J/k =−0.95 K)
0D AFM, dimers
˚
˚
d(H7,O14ii)=2.66 A
d(O14,O10iv)=5.39 Ac
B
˚
d(H14,O2i)=2.47 A
˚
˚
d(H44,O5ii)=2.55 A
(J/k =−1.82 K)
1D, dimer chains
d(O2,O5v)=5.03 Ad
B
˚
d(H8,O15i)=1.72 A
˚
˚
d(H134,)15ii)=2.71 A
(J/k =−14.9 K)
d(O15,O15vi)=4.61 A
B
˚
(
J /k =−10.5 K)
d(O11,O11ii)=3.81 Ae
2
B
˚
5
1D
d(H7,O11i)=2.22 A
0D, dimers
J/k =−42.3 K)
˚
(
d(O11,O11iii)=3.37 A
B
˚
aBased on the observed packing motifs linked by H,O hydrogen bonds with distances shorter than 2.8 A. bBased on the fits of experimental
magnetic data to magnetic models. Figures in parentheses are the strengths of dominant magnetic exchange interactions which are in all cases
˚
antiferromagnetic. cThe corresponding O,N contact is shorter at d(O14,N9iv)=4.94 A. dThe corresponding O,N contact is shorter at
˚
˚
d(N2,O5v)=4.89 A. eThe corresponding O,N contact is shorter at d(N10,O119ii)=3.69 A.
J. Mater. Chem., 1997, 7(9), 1723–1730
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between ONCNO units increases, being insignificant for O,O
distances larger than 6.0 A. Moreover, the AFM nature of
10 J. H. Osiecki and E. F. Ullman, J. Am. Chem. Soc., 1968, 90, 1078;
E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and R. J. Darcy,
J. Am. Chem. Soc., 1972, 94, 7049.
˚
such interactions can be rationalized by the McConnell I
mechanism, through the contacts of atoms with the same sign
of spin density, that belong to the NO units carrying the large
spin density. An alternative but coincident rationalization of
such magnetic interactions can be provided by the overlap of
the SOMOs of neighbouring molecules.13
1
1
1
2
M. Lamchen and T. W. Mittag, J. Chem. Soc. C, 1966, 2300.
(a) H. M. McConnell, J. Phys. Chem., 1963, 39, 1910;
(b) J. B. Goodenough, Magnetism and the Chemical Bond,
Interscience, New York, 1963, p. 163; (c) H. M. McConnell, Proc.
R. A. Welch Found, Conf. Chem. Res., 1967, 11, 144.
1
1
3
In some aspects the McConnell I mechanism is a restatement of
concepts introduced many years before by Heitler and London.
Thus, dominant contact of atoms of two neighbouring molecules
with spin densities having the same sign is equivalent to a non-
zero overlap integral between their singly occupied molecular
orbitals (SOMO) because such SOMOs are located on those
atoms.
This research was financially supported by the C.I.C. y T.
(
(
Grant, MAT 94-0797), Spain and the Generalitat de Catalunya
Grant, SGR 95/00507). O. J. and J. C. thank also the
Generalitat de Catalunya for the award of doctoral fellowships.
4
M. S. Davis, K. Morokuma and R. W. Kreilick, J. Am. Chem. Soc.,
1
974, 96, 652.
1
1
5
6
J. A. D’Anna and J. H. Wharton, J. Chem. Phys., 1970, 53, 4047.
(a) M. S. Davis, K. Morokuma and R. W. Kreilick, J. Am. Chem.
Soc., 1972, 94, 5588; (b) J. W. Neely, G. F. Hatch and
R. W. Kreilick, J. Am. Chem. Soc., 1974, 96, 652.
References
1
M. Tamaura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi,
M. Ishikawa, M. Takahashi and M. Kinoshita, Chem. Phys. L ett.,
1
991, 186, 401.
17 (a) J. Veciana, J. Cirujeda and O. J u¨ rgens, Manuscript in prep-
aration; (b) J. Cirujeda, Ph D Thesis, 1997, Universitat Ramon
Llull.
18 C. W. Wang and S. F. Watkins, J. Chem. Soc., Chem. Commun.,
1973, 888.
19 (a) J.-L. Stagner, Doctoral Thesis, 1995, Universit e´ Louis Pasteur
de Strasbourg; (b) Y. Hosokoshi, Doctoral Thesis, 1995, University
of Tokyo.
20 N. Ramasubbu, R. Parthasarathy and P. Murray-Rust, J. Am.
Chem. Soc., 1986, 108, 4308.
2
(a) J. Veciana, J. Cirujeda, C. Rovira and J. Vidal-Gancedo, Adv.
Mater., 1995, 7, 221; (b) J. Cirujeda, C. Rovira and J. Veciana,
Synth. Met., 1995, 71, 1799; (c) J. Cirujeda, E. Hern a´ ndez-Gasi o´ ,
F. Lanfranc de Panthou, J. Laugier, M. Mas, E. Molins, C. Rovira,
J. J. Novoa, P. Rey and J. Veciana, Mol. Cryst. L iq. Cryst., 1995,
2
71, 1.
3
(a) H. Hern a´ ndez-Gasi o´ , M. Mas, E. Molins, C. Rovira and
J. Veciana, Angew. Chem., Int. Ed. Engl., 1993, 32, 882;
(b) J. Cirujeda, E. Hern a´ ndez-Gasi o´ , C. Rovira, J. L. Stanger,
P. Turek and J. Veciana, J. Mater. Chem., 1995, 5, 243.
21 G. R. Desiraju, Acc. Chem. Res., 1991, 24, 290.
22 The equation used to fit the data corresponds to a modified
Bleaney–Bowers equation which includes a multiplying term to
4
5
J. Cirujeda, M. Mas, E. Molins, F. Lanfranc de Panthou,
J. Laugier, J. G. Park, C. Paulsen, P. Rey, C. Rovira and J. Veciana,
J. Chem. Soc., Chem. Commun., 1995, 709.
take
account
of
intradimer
interactions:
x=
T. Sugawara, M. M. Matsuskita, A. Izuoka, N. Wada, N. Takeda
and M. Ishikawa, J. Chem. Soc., Chem. Commun., 1994, 1723.
R. Taylor and O. Kennard, J. Am. Chem. Soc., 1982, 104, 5063.
(a) J. A. R. P. Sarma and G. R. Desiraju, Acc. Chem. Res., 1986, 19,
[Ng2m 2/3k(T −h)][1+(1/3) exp(−2J/kT )]−1. See R. L. Carlin in
B
Magnetochemistry, Springer Verlag, Berlin, 1986, p. 88.
6
7
23 T. Barnes and J. Riera, Phys. Rev. B, 1994, 50, 6817.
24 However, the fitting of the experimental data for 5 is slightly
improved if the same model as for radical 2 is employed, with an
2
22; (b) G. R. Desiraju, in Organic Solid State Chemistry, ed.,
G. R. Desiraju, Elsevier, 1987, p. 519; (c) J. A. R. P. Sarma and
G. R. Desiraju, Chem. Phys. L ett., 1985, 117, 160.
G. R. Desiraju, in Organic Solid State Chemistry, ed.,
G. R. Desiraju, Elsevier, 1987, p. 539.
intradimer interaction of J/k =−50.1 K and an additional inter-
dimer term of h=+10.4 K, corresponding to the magnetic inter-
actions among the dimers.
B
8
9
N. W. Thomas and G. R. Desiraju, Chem. Phys. L ett., 1984, 110, 99.
Paper 7/00589J; Received 27th January, 1997
1
730
J. Mater. Chem., 1997, 7(9), 1723–1730