Molecular structure and magnetic properties of N,N-bis{4-methoxy-3,5-bis(N-tert-butyl-N-oxyamino)phenyl}aminoxyl. An approach to a stable and high-spin pentaradical

Article abstract of DOI:10.1039/a704160h

A stable pentaradical, N,N-bis{4-methoxy-3,5-bis(N-tert-butyl-N-oxyamino)phenyl)}aminoxyl, is prepared and its ground state determined to be a sextet (S = 5/2) by SQUID magnetization measurements of the pentaradical diluted in poly(vinyl chloride) matrix.

Full text of DOI:10.1039/a704160h

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Molecular structure and magnetic properties of
N,N-bis{4-methoxy-3,5-bis(N-tert-butyl-N-oxyamino)phenyl)}aminoxyl. An
approach to a stable and high-spin pentaradical
Junpei Fujita,a Yoshihisa Matsuoka,a Kenji Matsuo,a Masakazu Tanaka,a Takeyuki Akita,a Noboru Koga*a
and Hizu Iwamura*b
a Faculty of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-82, Japan
b Institute for Fundamental Research of Organic Chemistry, Kyushu University, Fukuoka 812-81, Japan
R
A stable pentaradical, N,N-bis{4-methoxy-3,5-bis(N-tert-
R
•
R2
butyl-N-oxyamino)phenyl)}aminoxyl, is prepared and its
ground state determined to be a sextet (S = 5/2) by SQUID
magnetization measurements of the pentaradical diluted in
poly(vinyl chloride) matrix.
2
R
•
•
R1
R1
2
R
Poly(triarylmethyl) radical 1 (S
=
7/2),1 nonacarbene 2
(S = 9),2 and trinitrene 3 (S = 3)3 are some of the highest-spin
organic molecules rivalling high-spin 3d5 MnII, FeIII and
4f7 GdIII ions. However, these organic open-shell species are
kinetically unstable and can be kept only in solid solution at
cryogenic temperature under exclusion of air. Stable counter-
parts that can be bottled in the neat state under air are limited to
perchlorinated triradical 4 (S = 3/2).4 Knowing that the
methoxy group, while weakening the ferromagnetic coupling,
increases the kinetic stability of 5,5 and that 66 is a reasonably
stable quartet, we designed pentakis(aminoxyl) radical 7 as a
stable, high-spin, and high-spin-density pentaradical.
R
•
2 R1 = R2 =
•
H
2
•
R
3
1
R =
But
3 R1 =
•
N
•
• •
N
R2 =
R
R
•
R1
R2
Pentaradical 7 was prepared in a manner similar to the
procedure6 for triradical 6 by using bis(hydroxyamino) deriva-
tives protected with tert-butyldimethylsilyl groups in place of
protected monohydroxylamines. A solution of 1-bromo-
4-methoxy-3,5-bis[N-tert-butyl-N-(tert-butyldimethylsiloxy)-
amino]benzene in THF was lithiated with n-butyllithium and
coupled with 0.5 mol equiv. of pentyl nitrite to give mono-
radical 7a† (7.7%) whose EPR spectrum (n0 = 9.509 GHz) in
benzene showed a hyperfine structure consisting of a triplet of
quintets (aN = 10 G, aH = 1.85 G). Monoradical N,N-bis[4-
methoxy-3,5-bis(N-tert-butyl{N-tert-butyldimethylsiloxy}-
amino)phenyl]aminoxyl 7a was deprotected in the presence of
R
O
R
But
R3
•
•
•
N
O
N
Cl3
R
R
O
•
•
4 R = C6Cl5
O
N
•
5 R1 =
But
R2 = –OMe R3 = But
But
O
But
•
•
N
N
6 R1 = But R2 = H
MeO
OMe
But
But
But
N
O
N
N
R3 =
O
7
O
•
•
•
NBun F followed by oxidation with silver oxide in ether and
4
N
•
But
purification by column chromatography (silica gel/n-hexane–
diethyl ether). Recrystallization from ether gave indefinitely
stable pentaradical 7 as red needles.‡ It is somewhat unstable in
dilute benzene and THF solution; a decrease of the EPR signal
intensity by 5% was observed after 24 h under ambient
conditions. Pentaradical 7 crystallizes in the monoclinic space
group P21/n (no. 14) with four molecules in the unit cell. An
ORTEP drawing and stick model for the molecular and crystal
structure of 7 are shown in Fig. 1(a) and (b), respectively.‡
The crystal was found to contain 0.5 of a molecule of diethyl
ether per molecule of 7 and it is situated near the radical unit
SA1. The molecular structure of 7 has no symmetry axis and two
sets of aminoxyl radicals SA and SB have syn and anti§
conformations, respectively, with respect to the benzene ring.
The dihedral angles of the central N,N-diarylaminoxyl radical
SC with the two aryl rings are 23.7 and 32.9° and those of SA and
two SB with the benzene rings are relatively large [60–76°;
Fig. 1(a)]. The average distances among the three aminoxyl
radical moieties within a molecule are in the range 4.96–5.74 Å
to construct a two-triangular five-spin system. As indicated in
Fig. 1(b), there are two intermolecular short distances i and ii
between the radical centers which should be responsible for the
appearance of a strong intermolecular antiferromagnetic inter-
O
action at low temperature;8 two molecules make a head-to-tail
dimer structure with a separation of 3.35 Å and the resulting
dimers align at a distance of 3.69 Å to form a chain structure
along the b axis.
The X-band EPR spectrum of pentaradical 7 in toluene
solution at room temp. showed one broad signal with
DHPP = 20 G at g = 2.0064. In the corresponding spectrum
(n0 = 9.410 GHz) of 7 in frozen solution at 6.5 K, five signals
(318.8, 327.2, 335.3, 343.4, and 351.8 mT) were observed in
addition to the signal (g = 4.019) due to a DmS = 2 transition.
The spectrum was simulated as a sextet with S = 5/2 to afford
g
=
2.0062 and zero-field splitting parameters ıD/
hcı = 0.0039 and ıE/hcı = 0.0013 cm21.¶ As the temperature
was increased from 12 to 100 K, the intensity of the signal at
g = 2 decreased in accordance with the Curie law, suggesting
that the sextet was a ground state.
The magnetic susceptibility of a crystalline sample of
pentaradical 7 was measured on a SQUID magnetometer/
susceptometer in the temperature range 2–350 K at a constant
field of 200 mT. The cmolT value at 350 K is 2.09 emu K mol21
Chem. Commun., 1997
2393

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1
SA
(a)
1
SB
SC
1.0
0.8
0.6
C(29)
O(1)
N(1)
C(14)
C(27)
O(4)
C(2)
C(30)
S
O(7)
N(3)
M
C(12)
C(9)
N(5)
C(17)
C(3)
C(1)
C(5)
C(28)
C(13)
C(11)
C(7)
M
C(15)
0.4
0.2
0.0
C(4)
C(6)
C(8)
C(16)
C(10)
O(2)
O(3)
N(2)
O(6)
N(4)
C(24)
C(26)
C(19)
O(5)
C(18)
5
15
0
10
20
25
C(22)
C(20)
–3
–1
10 (H / T) / Oe K
C(21)
C(23)
2
SA
2
SB
C(25)
Fig. 2 Field dependence of the magnetization at 2 (2), 4 (8), and 6 K («)
as expressed by M/MS vs. H/T plots for pentaradical 7. The solid curves are
theoretical ones for S = 5/2 as described in text.
(b)
i
molecules at low temperature caused by short intermolecular
contacts between radical centers in the crystalline state was very
strong.
ii
i
Footnotes and References
b
* E-mail: koga@yakukaws.phar.kyushu-u.ac.jp
Fig. 1 Molecular and crystal structure of pentaradical 7: (a) ORTEP
drawing with thermal ellipsoid plot at the 30% probability level. The diethyl
ether molecule of crystallization is omitted for clarity; torsion angles
between the benzene ring and the radical N–O bond are 75.8°, 262.9,
263.1 and 60.3° for SA1[O(1)N(1)C(3)C(2)], SA2[O(3)N(2)C(5)C(6)],
SB1[O(7)N(5)C(11)C(12)] and SB2[O(5)N(4)C(9)C(10)], respectively. The
average distances between the two nitrogens and two oxygens are 5.53, 5.56
and 4.96 Å for r(SA1SA2), r(SA1SC) and r(SA2SC) and 5.57, 5.74 and 5.04 Å
for r(SB1SB2), r(SB1SC) and r(SB2SC), respectively, and (b) a stick model of
crystal structure; But groups are omitted for clarity. Broken lines indicate
short contacts (Å) between the atoms of the radical centers; (i) 3.35 and (ii)
3.60.
† 7a: mp (decomp.) 85–87 °C, FABMS (m-nitrobenzyl alcohol matrix) m/z
1049 (M+ + 1), Anal. Calc. for C54H106N5O7Si4: C, 61.71; H, 10.18; N,
6.67. Found: C, 62.08; H, 10.17; M, 6.37%.
‡ 7: mp (decomp.) 139–141 °C, FABMS (m-nitrobenzyl alcohol matrix) m/z
592 (M+ + 3), 591 (M+ + 2), Anal. Calc. for C30H46N5O7·0.5C4H10.O: C,
61.41; H, 8.19; N, 11.18. Found: C, 61.42; H, 8.21; N, 11.19%. Crystal data
for 7: C30H46N5O70.5C4H10O, M = 625.78, monoclinic, space group P21/n
(no. 14), a = 16.101(2), b = 12.537(4), c = 19.232(2) Å, b = 113.687(8)°,
U = 3555(1) Å3, T = 296 K, Z = 4, Dc = 1.169 g cm23, m(Cu-Ka) = 6.83
cm21
, 5729 total (5516 independent) reflections, R = 0.072 and
Rws = 0.071 for 404 reflections with I > 3s(I). Direct methods and
refinement converged using the full-matrix least squares method of the
TEXAN Ver. 1.6 program (Molecular Structure Corperation). CCDC
182/633.
§ The anti conformation is the first example in this series of methoxy
substituted phenyl aminoxyl diradicals.7
¶ The simulation for the EPR fine structure was performed by using a
program based on the second-order perturbation theory and provided by
Professors K. Ito and K. Takui of Osaka City University.
∑ PVC films containing 1% of 7 were also prepared and similar M/MS vs.
H/T plots were obtained.
which is slightly greater than 1.88 emu K mol21 calculated as a
spin only value for five isolated radical centers. As the
temperature was decreased from 350 K, cmolT values increased
gradually and reached a maximum of 2.22 emu K mol21 at 120
K and then decreased sharply. The maximum value is
considerably smaller than theoretical 4.38 emu K mol21 for
S = 5/2 and suggests that intermolecular antiferromagnetic
interaction takes place predominantly below 120 K, as expected
from the X-ray crystal structure of 7.
** M = NgjmBBJ(x), where BJ(x) = [(2J + 1)/2J] coth [(2J + 1)/2J]x 2 1/2J
coth (1/2J)x and x = gJmBH/kBT.
In order to exclude the contribution of the intermolecular
antiferromagnetic interaction at low temperature, a sample of 7
doped in poly(vinyl chloride) (PVC) matrix was prepared and
employed for a SQUID measurement. The field dependence of
a PVC film∑ containing 2% 7 was investigated at 2, 4, and 6 K
and the M/MS vs. H/T plot is shown in Fig. 2 together with a
theoretical curve obtained on the basis of the Brillouin
function** for S = 5/2. The theoretical curve fitted the
experimental data reasonably well indicating that the five
aminoxyl radicals interact ferromagnetically to produce a sextet
state.
In summary, stable pentaradical 7 was successfully prepared
and its molecular structure was revealed by X-ray structure
analysis. Its magnetic properties revealed that ferromagnetic
interactions occur predominantly among the five spins within a
molecule to produce a sextet ground state. However, the
exchange coupling parameters (J) among them could not be
estimated quantitatively from a cmolT–T plot since the inter-
molecular antiferromagnetic interaction among the pentaradical
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Received in Cambridge, UK, 16th June 1997; 7/04160H
2394
Chem. Commun., 1997