Triplet (S = 1) ground state aminyl diradical

Article abstract of DOI:10.1021/ja071881d

Aminyl diradical, which is stable in solution at low temperatures, is prepared. EPR spectra and SQUID magnetometry indicate that the diradical is planar and it possesses triplet ground state, with strong ferromagnetic coupling. Copyright

Full text of DOI:10.1021/ja071881d

Published on Web 05/17/2007
Triplet (S ) 1) Ground State Aminyl Diradical
Andrzej Rajca,*,† Kouichi Shiraishi, Maren Pink, and Suchada Rajca
†
‡
†
Department of Chemistry, UniVersity of Nebraska, Lincoln, Nebraska 68588-0304, and IUMSC, Department of
Chemistry, Indiana UniVersity, Bloomington, Indiana 47405-7102
Received March 16, 2007; E-mail: arajca1@unl.edu
Aminyl radicals (R
N•) are important reactive intermediates.1,2
2
Such radicals are generally short-lived; only recently metal- or
heteroatom (O, S) stabilized aminyl radicals were isolated.3,4 To
date, there is only one report of an aminyl diradical (Platz diradical)
in a triplet state, though the ground state for this matrix isolated
5
diradical is not established. This is in contrast to nitroxide radicals,
a popular choice for applications in materials science and biology.6
We examine aminyl radicals as building blocks in the design of
7
high-spin molecules and polymers for organic magnetic materials.
Our analysis of the EPR 1H-hyperfine splittings (a
) for 9,9-
dimethyl-9,10-dihydroacridine aminyl radical (DDAN) indicate that
the para a and ortho a are greater than those of the corresponding
nitroxide radical (DDANO) by a factor of 2. From this factor
and the empirical relationship between the singlet-triplet energy
H
Figure 1. Molecular structure and conformation of diamine 5. Carbon and
nitrogen atoms are depicted with thermal ellipsoids set at the 50% probability
level. Disorder for tert-butyl groups and solvent of crystallization (benzene)
are omitted for clarity.
H
H
8,9
2
10
gaps (2J/k) and a
H
, 2J/k ≈ |a | , we postulate that m-phenylene
H
aminyl diradical 1 should possess triplet (S ) 1) ground state with
large singlet-triplet energy gap (2J/k), greater by a factor of 4 than
that for the corresponding nitroxide diradical.11 Thus, m-phenylene
aminyl diradicals, especially if constrained to planarity, should
possess triplet ground states with extraordinary strong ferromagnetic
coupling.
The next challenging problem is the design of the diradical with
adequate stability, while maintaining ultrastrong ferromagnetic
coupling. Because of the high degree of spin delocalization in
m-phenylene aminyl diradicals, we predict that all ortho and para
positions in the m-phenylene should be sterically shielded, but
without perturbing the planarity of the cross-conjugated π-system.
Figure 2. EPR (X-band, ν ) 9.6376 GHz) spectrum of 2 in 2-MeTHF at
1
1
32 K. The simulation parameters for the S ) 1 state are |D/hc| ) 9.18 ×
-3 -1 -3 -1 -3 -1
0
)
cm , |E/hc| ) 2.10 × 10 cm , |Azz/2hc| ) 1.00 × 10 cm , gx
2.0030, gy ) 2.0042, gz ) 2.0019, Gaussian line (Lx ) 0.6, Ly ) 0.65,
1
Lz ) 0.55 mT). The center lines correspond to an S ) /2 byproduct,
-
3
-1
simulated with the identical g-values, and |Azz/hc| ) 2.00 × 10 cm
Scheme 1. Synthesis of Aminyl Diradical 2a
.
Herein we describe the synthesis and magnetic characterization
of aminyl diradical 2 that is long-lived and possesses an S ) 1
ground state with strong ferromagnetic coupling.
Bromination of diamine 312 at its most activated position for
electrophilic substitution gives 4, which is subjected to Suzuki cross-
coupling with 4-tert-butylphenyl boronic acid, to give 5 (Scheme
a
Conditions: (i) NBS in chloroform, NaHCO3 (excess), Na2SO4 (excess),
-40 °C for 1 h; (ii) 4-tert-butylphenylboronic acid (1.3 equiv), Pd(PPh3)4
(5 mol %), Na2CO3, benzene/EtOH/water, 2:1:2, 80 °C for 18 h; (iii) n-BuLi
(2.4 equiv), THF/hexane, -20 °C for 1 h, (iv) I2, THF, -105 °C (2-MeTHF,
1). The structure of diamine 5 is confirmed by X-ray crystallography
(
Figure 1). The five collinearly annelated six-membered rings form
13a
an approximately planar cross-conjugated π-system; the 4-tert-
butylphenyl group is nearly perpendicular to this π-system,1
providing steric shielding. Treatment of diamine 5 with n-BuLi
produces a yellow suspension of dianion. The dianion is oxidized
by iodine at about -105 °C to provide a red solution of aminyl
diradical 2 (Scheme 1); the oxidation process is being monitored
by EPR spectroscopy.
-
105 or -112 °C).
3b
EPR spectra of 2 in 2-MeTHF at 133 K show six symmetrically
disposed side peaks, corresponding to the zero-field splitting
-3
-1
-3
parameters |D/hc| ) 9.15 × 10 cm and |E/hc| ) 2.10 × 10
-1
cm in the |∆m
s
| ) 1 region and an intense transition in the |∆m |
s
)
2 region (Figure 2). The two outermost peaks (z-turning points)
14
†
appear as pentuplets, which may be simulated by the N hyperfine
coupling of two nitrogen nuclei with spacings of |Azz/2hc| ) 1.00
University of Nebraska.
Indiana University.
‡
7232
9
J. AM. CHEM. SOC. 2007, 129, 7232-7233
10.1021/ja071881d CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
mol- is corrected by the spin concentration (Msat ) 0.84 µ
1
B
, per
-
1
aminyl radical site), øT ≈ 1.0 emu K mol is obtained, which is
exactly the value expected for an S ) 1 diradical. Furthermore,
the øT versus T plot is flat up to 150 K, just below the softening
temperature of solid THF. These results unequivocally establish a
triplet ground state for diradical 2 and provide a lower limit for
the singlet-triplet gap, 2J/k > 200 K (∼0.4 kcal mol ).
The stability (persistence) of 2 in THF is monitored by SQUID
magnetometry; after 30 min at 203 K, Msat and øT decrease to about
-
1
half of their original values, though the average S is still ∼0.9.
1
After 2 h at room temperature, only an S ) /
2
radical is detected.
Furthermore, EPR studies show that a solution of 2 in 2-MeTHF
is inert toward O gas at 165 K (Supporting Information).
In summary, a triplet ground-state aminyl diradical with strong
ferromagnetic coupling is efficiently prepared in solution, thus
providing a promising pathway to high-spin aminyl polyradicals.
Figure 3. SQUID magnetometry of ∼15 mM 2 in THF. The main and
inset plots show M/Msat versus H/(T - θ) and øT versus T, with θ ) -0.1
2
-
1
K, Msat ) 0.84 µB, S ) 0.99, and øT ) 0.84 emu K mol
.
3
×
10- cm-1 (|Azz/2gµ
B
| ≈ 1.1 mT). As the largest principal value
1
4
Acknowledgment. This research was supported by the National
Science Foundation (Grant CHE-0414936) and the Air Force Office
of Scientific Research (Grant FA9550-04-1-0056).
N hyperfine tensor in nitrogen-based radicals coincides with the
-orbital,14 the N-hyperfine splitting
14
direction of the nitrogen 2p
of the outermost lines implies that the 2p
parallel to the z-axis, which is the direction of the largest principal
value of the magnetic dipole tensor. Therefore, the aminyl moieties
and the m-phenylene in diradical 2 adopt approximately coplanar
conformation.
π
π
-orbital is approximately
Supporting Information Available: Complete ref 11b, experi-
mental section, X-ray crystallographic file in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
15
References
Notably, in planar nitroxide diradicals, the largest principal value
of the magnetic dipole tensor (z-axis) is in the direction connecting
(
1) Piech, K.; Bally, T.; Sikora, A.; Marcinek, A. J. Am. Chem. Soc. 2007,
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nitroxide moieties, that is, the direction perpendicular to the 2p
orbital.16 The fact that the z-axis is parallel to the 2p
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π
-
(2) Chatgilialoglu, C.; Guerra, M.; Mulazzani, Q. G. J. Am. Chem. Soc. 2003,
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(
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aminyl diradical 2, precludes the possibility that 2 could be a
nitroxide diradical. This conclusion is supported by the g-values.
Anisotropic g-values for 2 (Figure 2) correspond to the isotropic g
(
(
1989, 30, 1225-1228.
(
6) (a) Itoh, K.; Kinoshita, M.; Eds. Molecular Magnetism; Gordon and
Breach: Kodansha, Japan, 2000; 1-337. (b) Eaton, G. R., Eaton, S. S.,
Berliner, L. J., Eds. Distance Measurements in Biological Systems by EPR;
Biological Magnetic Resonance, Vol. 19; Kluwer: New York, 2000; pp
)
(g
x
+ g
y
+ g )/3 ) 2.0030, which is comparable to g ) 2.0031
z
for DDAN and g ) 2.0027 for DDANH, but it is significantly
different from g ) 2.0051 for the nitroxide radical DDANO.8
Additional evidence for the aminyl diradical is based on the analysis
,9,17
1-614.
(
7) Rajca, A.; Wongsriratanakul, J.; Rajca, S. Science 2001, 294, 1503-1505.
1
1
of H-hyperfine splittings. Doubletlike isotropic H-splitting |A/
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9) Aurich, H. G.; Hahn, K.; Stork, K.; Weiss, W. Tetrahedron 1977, 33,
(
gµ
B
| ) 1.006 mT for the R-hydrogen of the NH moiety in
17
969-975.
DDANH suggests that the corresponding S ) 1 aminium diradical
(10) (a) Rajca, A. Chem. ReV. 1994, 94, 871-893. (b) Bushby, R. J.; Taylor,
1
N.; Williams, R. A. J. Mater. Chem. 2007, 17, 955-964.
should have tripletlike anisotropic H-splittings from two NH
(
11) (a) These predictions are supported by the UB3LYP/6-31G* calculations,
1
-
moieties with the spacings of |Azz/2gµ
.75 mT, and |Axx/2gµ | ≈ 0.25 mT. In particular, the spacing of
yy/2gµ
| ≈ 0.75 mT is expected to be discernible, as it exceeds
the observed line-width (L ) 0.65 mT) in the spectrum of 2.
B
| ≈ 0.5 mT, |Ayy/2gµ
B
| ≈
providing extraordinary large value 2J/k ) 4000 K (8.0 kcal mol ) in
aminyl diradical 1, which is four times greater than the 2J/k calculated
for the corresponding nitroxide diradical. (b) Frisch, M. J.; et. al. Gaussian
03, Gaussian, Inc.: Wallingford, CT, 2004.
14
0
B
|A
B
(
12) Shiraishi, K.; Rajca, A.; Pink, M.; Rajca, S. J. Am. Chem. Soc. 2005,
y
127, 9312-9313.
Another set of samples of 10-30 mM 2 in tetrahydrofuran (THF)
is prepared in homemade tubes that allow for strictly low-
temperature handling, as well as for sequential examination of the
same sample by both EPR spectroscopy and SQUID magnetometry
(13) (a) In diamine 5, the mean deviation from a calculated least-square plane
including all five co-linearly annelated six-membered rings (C1-C20, N1,
N2) is 0.0763 Å. (b) The angle between the least-square plane, defined
by C1-C20, N1, N2, and the least square plane of the benzene ring of
the 4-tert-butylphenyl group (C33-C38) is 75.82(4)°.
(
14) Morton, J. R. Chem. ReV. 1964, 64, 453-471.
15) Rajca, A.; Mukherjee, S.; Pink, M.; Rajca, S. J. Am. Chem. Soc. 2006,
128, 13497-13507.
(
Figures S3-S8, Supporting Information).18 The SQUID magne-
(
tometry unequivocally reveals the S ) 1 paramagnetic behavior
for both magnetization (M) versus magnetic field (H) and magnetic
susceptibility (ø) versus temperature (T); only small intermolecular
antiferromagnetic coupling, as measured by a mean-field parameter,
(
16) Rajca, A.; Takahashi, M.; Pink, M.; Spagnol, G.; Rajca, S. J. Am. Chem.
Soc., submitted for publication, 2007.
(17) Bamberger, S.; Hellwinkel, D.; Neugebauer, F. A. Chem. Ber. 1975, 108,
2416-2421.
(
18) Rajca, A.; Shiraishi, K.; Vale, M.; Han, H.; Rajca, S. J. Am. Chem. Soc.
2005, 127, 9014-9020.
1
9
θ ≈ -0.1 K, is observed (Figure 3).
(
19) Annealing the THF matrix at low temperatures (e.g., 170 K) does not
significantly change values of S, Msat, and øT but it increases values of θ,
for example, θ ≈ -0.2 K; in EPR spectra at 140 K, the intensity of the
center peak significantly increases several times relative to the side bands.
The numerical fits to the curvature of the M/Msat versus H/(T -
θ) plots at 1.8, 3, and 5 K (Msat, magnetization at saturation) give
the value of S ) 0.99, which is independent of the spin concentra-
tion of the sample. When the measured value of øT ) 0.84 emu K
JA071881D
J. AM. CHEM. SOC.
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VOL. 129, NO. 23, 2007 7233