Intermolecular ferromagnetic interaction in the crystal of a diphenyl nitroxide derivative. The role of spin-polarized hydrogen atoms located near a neighboring N-O site

Article abstract of DOI:10.1016/S0009-2614(99)00116-5

The crystal of 9,9-dipropyl-9,10-dihydroacridin-10-yloxyl exhibited a positive Weiss constant (+0.77 K) ascribable to a three-dimensional network of ferromagnetic interactions. The orbital orthogonality between the antibonding N-O π^* and H 1s orbitals was found in a nearly isosceles triangle geometry of neighboring H atoms and the N-O site. The following spin polarization scheme is proposed: N(↑)-C(↓)=C_o(↑)-C_m(↓)-H _m(↑)O(↑)-N, where o- and m-positions are defined with respect to the N-O group.

Full text of DOI:10.1016/S0009-2614(99)00116-5

12 March 1999
Ž
.
Chemical Physics Letters 302 1999 125–131
Intermolecular ferromagnetic interaction in the crystal of a
diphenyl nitroxide derivative. The role of spin-polarized hydrogen
atoms located near a neighboring N–O site
)
Mitsutoshi Nakagawa, Takayuki Ishida , Motohiro Suzuki, Daisuke Hashizume,
Masanori Yasui, Fujiko Iwasaki, Takashi Nogami
Department of Applied Physics and Chemistry, The UniÕersity of Electro-Communications, Chofu, Tokyo 182-8585, Japan
Received 14 November 1998; in final form 6 January 1999
Abstract
Ž
.
The crystal of 9,9-dipropyl-9,10-dihydroacridin-10-yloxyl exhibited a positive Weiss constant q0.77 K ascribable to a
three-dimensional network of ferromagnetic interactions. The orbital orthogonality between the antibonding N–O p) and H
1s orbitals was found in a nearly isosceles triangle geometry of neighboring H atoms and the N–O site. The following spin
Ø
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž .
polarization scheme is proposed: aN ≠ –C x 5Co ≠ –Cm x –Hm ≠ PPP O ≠ –Nb, where o- and m-positions are
defined with respect to the N–O group. q 1999 Elsevier Science B.V. All rights reserved.
w x
1. Introduction
5 . For developing high-TC organic magnets, we
turn our attention to p-conjugated radicals, espe-
The research on purely organic magnetic materi-
als has greatly progressed in recent years 1 . We
have reported organic ferro- and metamagnets hav-
ing a 2,2,6,6-tetramethylpiperidin-1-yloxyl TEMPO
cially diphenyl nitroxides.
w x
w x
As reported recently 6 , the magnetism of 9,9-
Ž
.
Ž
.
bis p-tolyl -9,10-dihydroacridin-10-yloxyl BTAO
Ž
.
obeys a ground triplet-state dimer model with 2 JrkB
sq18 K, which is consistent with the dimeric
molecular arrangement. This interaction is much
stronger than those observed for TEMPO-based
magnets. We have studied the magnetism of its
derivatives having less bulky substituents than p-tolyl
in order to realize stronger ferromagnetic interaction.
However, antiferromagnetic interactions are domi-
w
x
radical group 2–4 . Although TEMPO radicals are
very stable and the TEMPO-based materials are eas-
Ž
.
ily accessible, they have low TC or TN -0.3 K ;
sterically bulky substituents such as four methyl
groups in a TEMPO moiety pull the N–O radical
centers apart, and the spin densities on aliphatic
substituents are generally small. The intermolecular
exchange interaction is considered to be proportional
to the polarized spin densities at contacting atoms
Ž
nant for DMAO, DEAO and DBAO these formulas
are shown below as indicated by the negative Weiss
.
Ž
.
temperatures y13, y4.6 and y1.6 K, respectively
)
w x
Ž
7 . Among them a propyl derivative, DPAO 9,9-di-
propyl-9,10-dihydroacridin-10-yloxyl , showed inter-
Corresponding author. E-mail: ishi@pc.uec.ac.jp; fax: q81
.
424 43 5501
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 99 00116-5

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)
126
M. Nakagawa et al.rChemical Physics Letters 302 1999 125–131
Ž
.
molecular ferromagnetic interaction. We will report
here the structure and magnetism of the DPAO
crystal and discuss its mechanism of the interaction
based on the crystal structure.
DC magnetometers Quantum Design equipped with
7 and 9 T superconducting magnets, respectively.
3. Results
3.1. Crystal structure
We initially expected pancake-type stacking of
benzene rings, which is favorable for strong mag-
netic interaction according to McConnell’s model
w x
5 . In fact, such a stacking of acridine skeletons was
w x
found in a crystal of BTAO 6 . However, neighbor-
ing acridine skeletons were not arranged parallel in
the DPAO crystal. Fig. 1a depicts the molecular
Ž
structure of DPAO. The alkyl chain CH3CH2-
2. Experimental
.
CH2 2C has all-trans conformation. Fig. 1b shows
the molecular arrangement of DPAO, in which the
Ž
.
The precursor 9,9-dipropyl-9,10-dihydroacridine
was prepared according to the method reported by
CH3CH2CH2 2C moieties are omitted for the sake
of clarity.
The nearest intermolecular O PPP O distances are
w x
Noyori et al. 8 . The photochemical reaction of
˚
Ž .
acridine and butanoic acid gave 9-propyl-9,10-dihy-
6.161 2 A. There seems to be no direct magnetic
interaction among the N–O sites, because model
calculations indicate that the through-space exchange
and dipole interactions between two nitroxide radi-
cals are negligible when the O PPP O distance is )5
droacridine with loss of CO2. After oxidation of
Ž
.
9-propyl-9,10-dihydroacridine with K3Fe CN , the
6
resultant 9-propylacridine was subjected again to the
photoreaction with butanoic acid, giving 9,9-dipro-
pyl-9,10-dihydroacridine. Radical DPAO was ob-
tained by oxidation of the amine with m-chloroper-
benzoic acid in dichloromethane and isolated by
˚
w
x
A 2,11 . Rather short atomic distances are found
between the N–O sites and the aromatic hydrogen
atoms of a neighboring molecule. Three shortest
Ž
Ž .
Ž .
Ž .
passing through an HPLC Jaigel 1Hq1H, Japan
O PPP H distances are 2.67 4 , 2.93 4 and 2.96 4
˚
.
Analytical Industry eluted with chloroform. The
A, which we call contacts A, B and C, respectively,
specimen was purified by repeated recrystallizations
as shown with broken lines in Fig. 1b.
Ž
.
from dichloromethane-hexane m.p. 146–1488C .
X-ray diffraction data of a single crystal of DPAO
were collected on a RIGAKU AFC-5R diffractome-
Contacts A, B and C run along the b, c and a
axes, respectively, in a zigzag manner with 21 sym-
metry operation. All of these relations are repeated
uniformly and infinitely. The DPAO crystal seems to
have a three-dimensional network of magnetic inter-
actions. In view of the intermolecular atomic dis-
tances and the direction of the N–O p) orbital lobe
w x
ter and analyzed by SHELXS-86 9 and SHELXL-97
w
x
10 program packages. The crystallographic data of
Ž .
DPAO were: orthorhombic, P212121, as12.060 3 ,
3
˚
˚
Ž .
Ž .
Ž .
bs12.466 3 , cs10.892 3 A, Vs1637.6 7 A ,
Zs4, Dcalc s1.137 g cmy3, Rs0.049 for 2092
observed reflections.
Ž
.
see below , contacts A and B may play a main role
of magnetic coupling. A closer inspection of the
molecular arrangement shows that only contacts A
and B can form a three-dimensional network.
An X-band ESR spectrum was measured on an
Ž
.
ESP-300E spectrometer Bruker . A dilute benzene
solution of DPAO was purged with nitrogen. Hyper-
fine splitting constants were determined by computer
simulation.
Magnetic properties of a polycrystalline sample of
DPAO were measured on MPMS SQUID and PPMS
Fig. 1c shows the detailed geometries around
these contacts, where intermolecular atomic dis-
tances of N PPP H and O PPP H are indicated. The
˚
Ž .
N–O bond length is 1.291 4 A. The Cartesian coor-
dinates of selected atoms are listed in Table 1. For

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M. Nakagawa et al.rChemical Physics Letters 302 1999 125–131
127
Table 1
Cartesian coordinates of selected atoms neighboring an N–O sitea
Atom
x
y
z
˚
A
˚
A
˚
Ž .
A
Ž
.
Ž
.
Ø
(
)
C1 – N – O –C2 group:
N
O
0
1.29
0
0
0
0
C1
C2
y0.67
y0.68
y1.26
1.21
0
0.03
H –C moiety in contact Ab:
H
C
1.22
1.27
0.87
1.30
y2.52
y3.52
H –C moiety in contact Bc:
H
C
0.44
0.42
y0.22
y0.52
2.79
3.77
H –C moiety in contact Cd:
H
C
3.54
4.28
y1.88
y2.42
0.45
y0.09
a For contacts A–C, see Fig. 1.
bSymmetry operation is y xq1, yq1r2, y zq1r2.
cSymmetry operation is y xq3r2, y yq1, zy1r2.
dSymmetry operation is xy1r2, y yq3r2, y z.
both contacts A and B, the hydrogen atoms are
located just above the nitroxide group, and three
atoms H, O and N approximately form an isosceles
triangle. The hydrogen atoms correspond to m-H
atoms in both cases, where the o-, m- and p-posi-
tions are defined with respect to the nitroxide group.
On the other hand, for contact C the hydrogen atom
is located almost on the nitroxide aN–OØ plane.
This hydrogen atom also corresponds to another
m-H atom. Therefore, we have to pay attention to the
role of the contacting m-H atoms for clarifying the
mechanism of ferromagnetic interaction.
Ž .
Fig. 1. a Molecular structure of DPAO. Thermal elipsoids of C,
Ž .
N and O atoms are shown at the 50% probability level.
b
Ž
.
Molecular arrangement in the crystal of DPAO. CH3CH2CH2 2C
moieties are omitted for clarity. Dotted lines indicate relatively
Ž .
c
short distances between nitroxide O and aromatic H atoms.
Detailed geometries of contacts A, B and C. NPPPH and OPPPH
distances are shown with dotted lines. Standard deviations of the
˚
atomic distances are 0.04 A.

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128
M. Nakagawa et al.rChemical Physics Letters 302 1999 125–131
3.2. ESR spectrum
In order to determine the spin densities on the
m-H atoms, we measured an ESR spectrum of DPAO.
Ž
The solution ESR spectrum benzene, room tempera-
.
ture indicated gs2.0050, aN s8.82, ao-H sap-H
X
s2.37 and am-H sam -H s0.77 G, which are com-
w
parable to those of typical diphenyl nitroxides 6,12–
x
14 . The sign of the hyperfine splitting constants or
of the spin densities could not be determined by the
ESR data alone. McConnell’s equation with QC s
w
x
Ž
.
y22.5 G 15,16 gave the spin densities r’s of the
<
<
<
<
<
<
carbon atoms: ro-C s rp-C s0.105 and rm-C
s
0.034.
The MacLachlan calculation of a diphenyl nitrox-
w
x
ide portion by use of ls1.2 17,18 indicated ro-C
sq0.104, rp-C sq0.097 and rm-C sy0.041.
They are converted by McConnell’s equation to be
ao-H sy2.34, ap-H sy2.18 and am-H sq0.92 G.
These absolute values agree with the observed ones.
The calculation suggests that the m-C and m-H
atoms have negative and positive spin densities,
respectively. Furthermore, the spin polarization is
well known to take place from a radical center
throughout the conjugated benzene rings, giving an
w
x
alternating spin density 19 . We can safely assume
that the contacting H atoms, i.e. m-H atoms in
neighboring molecules, have a positive spin density.
3.3. Magnetic properties
Ž
.
Static magnetic susceptibility xmol was mea-
sured on a SQUID magnetometer down to 1.8 K at
0.5 T. Fig. 2a shows that the product xmolT of
DPAO monotonically increases with decreasing tem-
perature and reaches 0.59 cm3 K moly1 at 1.8 K. A
plot of reciprocal magnetic susceptibility against
y1
Ž .
Fig. 2. a Temperature dependence of xmolT and xmol of DPAO.
Ž .
b
M – H curves of DPAO measured at 2.0 and 4.5 K. Solid lines
are calculated Brillouin functions.
Ž .
temperature gives a positive Weiss temperature u
These findings clearly indicate the presence of
intermolecular ferromagnetic interaction in the DPAO
crystal. Since intermolecular contacts A, B and C in
Fig. 1b form an infinite three-dimensional network,
the magnetic dimensionality can be defined by JA,
JB and JC. However, we cannot determine these
values separately. Although all of JA, JB and JC
should not be ferromagnetic for u)0, the molecular
arrangement suggests that intermolecular antiferro-
magnetic interaction, if any, will be small, as dis-
cussed below.
of q0.77 K. The xmolT value larger than 0.5 cm3 K
moly1 at 1.8 K means that more than two molecules
are ferromagnetically correlated.
The magnetization curves were measured at 2.0
and 4.5 K with an applied field up to 9 T. Fig. 2b
shows the data points as well as calculated Brillouin
functions. The data measured at 4.5 K fall between
the Ss1r2 and 1 Brillouin functions. The data
exceed the Ss1 function at 2.0 K, in accordance
with the result of the product xmolT.

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M. Nakagawa et al.rChemical Physics Letters 302 1999 125–131
129
4. Discussion
orbital overlaps among the contacting atoms shown
in Fig. 1c, because a few nearest atomic orbital
overlaps mainly contribute to the total molecular
orbital overlap. The SOMO of DPAO has a coeffi-
cient at m-H atoms, indicated by the positive spin
density at the m-H atoms, and also supported by the
semiempirical UHFrPM3 MO calculation. In all
cases of contacts A, B and C, therefore, the hydro-
gen 1s orbitals located nearest an N–O site and the
N–O p) orbital are selected on behalf of the SO-
MOs of neighboring molecules.
When a hydrogen atom approaches a nitroxide on
the nodal plane which bisects the N–O bond at the
center, the hydrogen 1s and nitroxide p) orbitals
cannot have any appreciable net overlap due to
orthogonality; the H PPP N and H PPP O atomic or-
The exchange interaction J is generally expressed
as the sum of a positive contribution JF , stabilizing
the triplet state, and a negative contribution JAF
stabilizing the singlet state Eq. 1 20 . The JF
term originates in the exchange integral K. The JAF
term approximately increases in proportion to the
product of the overlap integral S and resonance
integral b.
,
Ž
Ž .. w x
JsJF qJAF fKq2bS .
1
Ž .
Ž
Fig. 3 shows the SOMO singly occupied molecu-
.
lar orbital surface of a DPAO molecule calculated
w
x
by a semiempirical UHFrPM3 MO method 21
based on the determined molecular structure. The
Ž
SOMO has antibonding p) character at the NO site;
bital overlaps are almost cancelled contacts A and
.
B . In contact A the H PPP O distance is somewhat
Ž .
Ž .
the coefficients of 2p O and 2p N being q0.41 and
y0.59, respectively. This figure also confirms the
presence of SOMO coefficients at every other car-
bons on the phenyl rings. These types of molecules
are known to have negative spin densities on the
carbons possessing no SOMO coefficients. Nega-
tively polarized spin densities are suggested on the
m-C atoms, and consequently the m-H atoms have a
positive spin density.
shorter than the H PPP N one, but the overlap cancel-
lation is rationalized in view of the larger coefficient
of the N atom than that of the O atom. When a
hydrogen atom approaches a nitroxide on another
nodal plane defined by the aN–OØ plane, the hy-
drogen 1s and nitroxide p) orbitals have no orbital
Ž
.
overlap also by symmetry contact C . In the DPAO
Ž .
crystal, therefore, the second term JAF in Eq. 1 is
negligible and the ferromagnetic term JF survives in
JA, JB and JC. The polarized spin density at the
m-H atom is positive, and the spin density at the
N–O site in a neighboring molecule should also be
positive by through-space ferromagnetic coupling.
For contacts A and B the spin polarization scheme is
shown in Fig. 4. A similar scheme can be drawn for
In the following discussion of SOMO–SOMO
overlaps of DPAO we have to focus on atomic
w x
contact C. In our previous paper on BTAO 6 , the
m-H contact was reported to cause antiferromagnetic
Ž . Ž .
Ž .
Ž .
interaction, such as aN ≠ –C x 5Co ≠ –Cm x –
Ø
Ž .
Ž .
Hm ≠ PPP O x –Nb. A crucial difference is that
the intermolecular SOMO–SOMO overlap is present
at the contact of m-H PPP ØO in the previous case.
Yoshizawa and Hoffmann discussed the magnetic
coupling of stacking dimer models of nitroxide radi-
w
x
cals 22 . An anti-stacking mode favors ferromag-
netic coupling because the N PPP N interactions are
of opposite sign to the N PPP O interactions. Their
discussion holds for the present system; the H atoms
in place of the N atom approach an N–O site. To our
knowledge, the DPAO crystal is the first example
possessing ferromagnetic interaction due to the lack
Fig. 3. SOMO surface of a DPAO molecule calculated by a
UHFrPM3 method using the X-ray crystal data.

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)
130
M. Nakagawa et al.rChemical Physics Letters 302 1999 125–131
etry of H PPP N–O that there is a considerable orbital
overlap between the hydrogen 1s and N–O p or-
bitals in the case of contacts A and B. For contact C
the oxygen lone-pair orbitals are located on the
aN–OØ plane and accordingly an orbital overlap
between the hydrogen 1s and oxygen lone-pair is
also possible. The situation where SOMO–SOMO
interaction is absent and, at the same time, apprecia-
ble SOMO–NHOMO interaction is present may fa-
vor ferromagnetic coupling in view of CT configura-
tions among the radicals. Namely, a b-spin transfer
from NHOMO to SOMO of a neighboring molecule
stabilizes a triplet state within a dimer.
A spin-unrestricted picture may give an insight
into CT interactions bringing about intermolecular
ferromagnetic interaction. Awaga et al. suggested
Fig. 4. Schematic drawing of a possible mechanism of intermolec-
ular ferromagnetic interaction between the neighboring DPAO
molecules.
Ž .
that large orbital overlaps between NHOMO b and
Ž .
Ž .
Ž .
SOMO b and between SOMO a and NLUMO a
were responsible for the ferromagnetic coupling of
the galvinoxyl radical on which they made a spin-un-
of SOMO–SOMO overlap in an H PPP N–O triangu-
lar geometry.
Gatteschi and co-workers reported a successful
application of orbital orthogonality to the transition
metal complexes in which the nitroxide O atom was
w
x
restricted MO calculation 25 . The intermolecular
orbital overlaps between SOMO and NHOMO and
between SOMO and NLUMO seem to be necessary
for ferromagnetic interaction. In the present case, the
latter contribution has not yet been clarified. A more
detailed theoretical calculation of orbital overlaps is
needed for further discussion.
w
x
coordinated to the metal center 23 . For example,
Ž
.
the TEMPOL-Cu hfac complex has 2 JrkB s19
2
"7 K between Ss1r2 spins of a dx2yy2 orbital of
the copper II ion and the axially coordinated nitrox-
ide 24 . In the present study, the interaction J is
estimated to be as much as us0.77 K, which is
smaller than that of the TEMPOL-Cu hfac com-
Ž .
w
x
5. Summary
Ž
.
2
plex, probably because the exchange occurs in a
non-covalent manner and because the polarized spin
density at the H atom is small. As described in
Section 1, the intermolecular exchange interaction is
considered to be proportional to the product of polar-
The DPAO crystal exhibited intermolecular ferro-
magnetic interaction. The positively spin-polarized
hydrogen atoms located near a neighboring N–O site
play an important role in the ferromagnetic exchange
mechanism. Orbital orthogonality was found in the
H PPP N–O geometry.
Singly occupied molecular orbitals extended on
p-conjugated radicals are likely to overlap in crystal
packing to give rise to intermolecular antiferromag-
Ž
Ž .. w x
ized spin densities at contacting atoms Eq. 2 5 .
In the DPAO crystal the contacting H atoms has a
spin density of 1.52=10y3, as estimated by Mc-
Connell’s equation with QH s508 G. The weak
ferromagnetic interaction arises from a small spin
density on the m-H atoms.
w
x
netic interactions 26,27 . The present work provides
an example of ferromagnetic interaction in p-con-
jugated nitroxide radicals, in addition to those on
HAB sySA PSB JABriArjB .
2
Ž .
Ý
i j
w x
three other ferromagnetic compounds 6 . We sup-
Ž
.
The NHOMO or a lower doubly-occupied MO
of DPAO has bonding p character at the N–O site.
We can point out from the isosceles triangular geom-
pose that ferromagnetic crystals of p-conjugated
radicals are not exceptional; a search for more
strongly ferromagnetic materials is now underway.

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M. Nakagawa et al.rChemical Physics Letters 302 1999 125–131
131
w
w
w
w
w
x
10 G.M. Sheldrick, A program for the refinement of the crystal
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