Singlet-triplet bistability in a 1,3-phenylene-based bis(aminoxyl) diradical

Article abstract of DOI:10.1039/a901906e

Rapid quenching of the title bis(aminoxyl) 1 in 2-methyltetrahydrofuran from ambient temperature to cryogenic temperatures (5 K or below) produces 1 in its singlet state, which slowly converts at low temperatures, with an S-shaped time-dependence, to the

Full text of DOI:10.1039/a901906e

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Singlet–triplet bistability in a 1,3-phenylene-based bis(aminoxyl) diradical
Andrzej Rajca,* Kan Lu, Suchada Rajca and Charles R. Ross, II†
Department of Chemistry, University of Nebraska, Lincoln, NE 68588-0304, USA. E-mail: arajca@unlinfo.unl.edu
Received (in Columbia, MO, USA) 8th March 1999, Accepted 26th April 1999
Rapid quenching of the title bis(aminoxyl) 1 in 2-methylte-
trahydrofuran from ambient temperature to cryogenic
temperatures (5 K or below) produces 1 in its singlet state,
which slowly converts at low temperatures, with an S-
shaped time-dependence, to the triplet ground state.
signal is also observed (Fig. 2). In the 1H NMR spectrum of a
0.05 mol dm23 solution of 1 in CDCl3, two prominent peaks at
216 (broad) and 1.1 ppm (narrow) have a relative intensity of
36+1, they are assigned to the tert-butyl groups associated with
the aminoxyl moiety and the diamagnetic impurity (presumably
a hydroxyamine moiety), respectively.12 Molecular ions in the
EI MS for 1 and the dihydroxyamine 2 (Scheme 1) show a m/z
difference corresponding to two hydrogen atoms.
1,3-Phenylene-based diradicals are ubiquitous building blocks
for both high-spin polyradicals and molecular magnets.1,2
Although 1,3-phenylene is the most reliable mediator of
ferromagnetic coupling in organics, it is well established that
severe bond torsions in 1,3-phenylene-based diradicals may
result in singlet ground states.3–5 Therefore, different bond
torsions associated with selected conformers may allow for the
same diradical to be obtained in either a singlet or triplet ground
state. Such a phenomenon was recently reported for a different
class of organic diradicals, which have been obtained in either
a singlet or triplet state, depending on the photon energy in the
photochemical generation or the solvent.6–8 However, the rate
of such conformationally mediated conversion between the
different spin states in diradicals was not observed.8 Now we
report the singlet and triplet ground state in a 1,3-phenylene-
based diradical, 4,6-bis(trifluoromethyl)-N,NA-di-tert-butyl-
1,3-phenylenebis(aminoxyl) 1, with an observable rate of
conversion between the low- and high-spin states.
Synthesis of 1 is outlined in Scheme 1. X-Ray crystallog-
raphy, EPR, 1H NMR, MS, IR and elemental analysis data
confirm the structure and suggest a purity of > 95%.
Fig. 1 Molecular structure of 1 in the solid state at 50 K; thermal ellipsoids
at the 50% probability. The hydrogens are not shown.
X-Ray crystallography indicates that the aminoxyl moieties
are twisted out of the plane of the 1,3-phenylene with torsional
angles of 270 (C5–C6–N2–O2) and 49° (C3–C2–N1–O1);
both oxygens point toward the trifluoromethyl groups.9 The
geometry and conformation of the trifluoromethyl groups
suggests negligible radical–fluorine hyperconjugation (Fig.
1).10
The Dms = 1 region of the X-band EPR spectrum of 1 in
frozen 2-MeTHF has six broad peaks consistent with a triplet
state. The two inner most resonances show resolved hyperfine
coupling with the two nitrogens; the center resonance, which
would correspond to a monoradical impurity, is negligible. The
spectral width of 0.0321 Tesla is constant in the 10–65 K range
and it is of similar magnitude to that found for other
1,3-phenylene-connected bis(aminoxyl)s.3,4,11 The Dms = 2
Fig. 2 The EPR spectrum of 0.001 mol dm23 1 in 2-MeTHF. Zero field
splitting parameters for an S = 1 state are: ID/hcI ≈ 0.015 cm21 and IE/hcI
≈ 0.001 cm21. Lower insert: plot of the product (IT) of the intensity (I) for
the Dms = 2 resonance and the temperature (T) vs. T at three settings of
microwave power (10, 20 and 30 dB); the 20 and 30 dB data are amplified
by the consecutive factors of 3.2 ≈ (10)1/2. The values of I are obtained by
Scheme 1 Reagents and conditions: i, Br2, Fe, CH2Cl2; ii, NaNO2, H2SO4,
EtOH (45% for 2 steps); iii, ButLi, Et2O; iv, (ButNO)2; v, Ag2O, CHCl3
(20% for 3 steps).
numerical double integration of the Dms
= 2 region. The solid line
corresponds to numerical fit with the singlet–triplet energy gap
a
† Current address: Department of Structural Biology, St. Jude Children’s
Hospital, 332 N. Lauderdale St., Memphis, TN 38105, USA.
corresponding to 2J/k = 80 K (ref. 14) (J is a coupling constant from the
Heisenberg Hamiltonian, H = 22JS1·S2 with IS1I = IS2I = 1/2).
Chem. Commun., 1999, 1249–1250
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time span for the completion of the conformational change are
associated with various glassy matrix effects, e.g. random
distribution of non-equivalent sites in the matrix.15 A somewhat
vexing problem is the preference for the singlet ground state in
the high temperature conformer. This would require over-
coming the entropic advantage of the triplet state (ms degen-
eracy) by other sources of entropy or enthalpy, e.g. relatively
low energy vibrational modes in the singlets or different
solvation in the liquid vs. glassy 2-MeTHF. Unfortunately, there
are no structural data for the singlet state (or states) of 1.
Although conformational isomers were invoked in studies of
some organic di- and poly-radicals, the time-dependence of the
magnetic susceptibility of 1 in 2-MeTHF is unprecedented and
calls for prudent interpretation of magnetic data for organic
molecules, especially in glassy matrices.6 Ultimately, a rational
design of spin-bistable diradicals (or polyradicals) may provide
organic analogues of metal ion based spin crossover com-
pounds.16
This research was supported by the National Science
Foundation (CHE-9510096 and CHE-9806954).
Fig. 3 Plot of molar magnetic susceptibility (cmol) vs. time, following rapid
cooling of 1 (0.62 ± 0.07 mg) in 2-MeTHF (ca. 0.05 ml) from ambient
temperature to 5 K. The applied magnetic field is 0.5 Tesla.
Notes and references
1 A. Rajca, J. Wongsriratanakul, S. Rajca and R. Cerny, Angew. Chem.,
Int. Ed., 1998, 37, 1229; K. K. Anderson and D. A. Dougherty, Adv.
Mater., 1998, 10, 688; K. Sato, M. Yano, M. Furuichi, D. Shiomi, T.
Takui, K. Abe, K. Itoh, A. Higuchi, K. Katsuma and Y. Shirota, J. Am.
Chem. Soc., 1997, 119, 6607; A. Rajca, S. Rajca and S. R. Desai,
J. Chem. Soc., Chem. Commun., 1995, 1957; A. Rajca, Chem. Rev.,
1994, 94, 871.
2 H. Iwamura, K. Inoue and N. Kaga, New J. Chem., 1998, 201.
3 M. Dvolaitzky, R. Chiarelli and A. Rassat, Angew. Chem., Int. Ed. Engl.,
1992, 31, 180.
4 F. Kanno, K. Innoue, N. Koga and H. Iwamura, J. Am. Chem. Soc.,
1993, 115, 847; J. Fujita, M. Tanaka, H. Suemune, N. Koga, K. Matsuda
and H. Iwamura, J. Am. Chem. Soc., 1996, 118, 9347.
5 S. Fang and M.-S. Lee, J. Am. Chem. Soc., 1995, 117, 6727; K. Okada,
T. Imakura, M. Oda, H. Murai and M. Baumgarten, J. Am. Chem. Soc.,
1996, 118, 3047; A. Rajca and S. Rajca, J. Chem. Soc., Perkin Trans. 2,
1998, 1077.
A sample of 1 in 2-MeTHF (1022 mol dm23) was inserted, at
a moderately rapid rate (5–10 min), into a SQUID magnetome-
ter/susceptometer sample chamber kept at approximately 5 K.
After establishing a stable temperature in the sample chamber
(ca. 5 min), and an additional brief equilibration (2–5 min), the
molar magnetic susceptibility (cmol) was measured at a constant
applied magnetic field (H = 0.05–0.5 Tesla) as a function of
time.13 S-shaped plots were obtained, in which cmol increases
about five times (Fig. 3).13 The onset of a sharp rise in cmol is
random and varies from several minutes to several hours.
Gradual cooling of 1 in 2-MeTHF, e.g. temperature sweep from
290 to 5 K at 3, 5 or 10 K min21, gives the equilibrium value of
cmol at 5 K. Analogous S-shaped plots are obtained at 1.8 and 2
K. The observed phenomena are solvent dependent; the time-
dependence of cmol was not found for 1 in tetrahydrofuran
(THF) or toluene (constant cmol vs. time).
Diradical 1 in 2-MeTHF was kept at 1.8 or 5 K until cmol (vs.
time) reached a constant value, and then magnetization (M) was
measured at T = 1.8 and 5 K as a function of H = 0–5 Tesla.
The fit of the M vs. H/T data to the Brillouin functions gives S
= 1, indicating a triplet ground state. Furthermore, the plateau
value of cmol = 0.19 at 5 K (Fig. 3) is in good agreement with
cmol = 0.20, as expected for a predominantly triplet ground
state at 5 K. The plot of IT vs. T, where I is the intensity of the
EPR signal in the DmS = 2 region, slightly decreases at the
higher end of the 10–75 K range (Fig. 2). Therefore, the triplet
ground state is slightly depopulated at higher temperatures,
suggesting a small energy gap between the triplet and the
excited singlet state within the same conformer (weak ferro-
magnetic coupling) or conversion to another conformer with
different ordering of states.
6 L. C. Bush, R. B. Heath and J. A. Berson, J. Am. Chem. Soc., 1993, 115,
9830; L. C. Bush, L. Maksimovic, X. W. Feng, H. S. M. Lu and J. A.
Berson, J. Am. Chem. Soc., 1997, 119, 1416.
7 D. A. Schultz, A. K. Boal and G. T. Farmer, J. Am. Chem. Soc., 1997,
119, 3846.
8 J. A. Berson, Acc. Chem. Res., 1997, 30, 238.
9 Crystal data for 1: C16H20F6N2O2, M = 386.34, monoclinic, a =
5.849(2), b = 31.360(200), c = 9.436(6) Å, b = 93.8600(5)°, V =
1726.9(111) Å3, T = 50(2) K, space group P21/n (14), Z = 4, m(AgKa)
= 0.08 mm21. Structure solved by direct methods and refined by full-
matrix least-squares on F2. 5018 reflections observed [Fo > 4s(Fo)].
Final refinement statistics: wR(F2) = 0.1168, R(F) = 0.0502, GooF =
1.079. CCDC 182/1278. See http://www.rsc.org/suppdata/cc/
1999/1249/ for crystallographic files in .cif format.
10 On the side of the relatively less twisted aminoxyl moiety in 1, C8 (CF3
group) and N1 (aminoxyl) are 0.16 and 0.15 Å, respectively, from the
plane defined by the six carbons of the benzene ring (RMS 0.014 Å). All
C–F bond lengths are 1.34 Å.
One of the most straightforward rationalizations for these
results is by invoking two conformers (conformational isomers)
for 1 with different torsions about the C(1,3-phenylene)–N
bonds.3 The low temperature conformer has a triplet ground
state, probably with a rather small (Heisenberg 2J/k < 100 K)
singlet–triplet energy gap.14 The high temperature conformer
has a singlet ground state (at least, at low temperatures), with a
non-negligible singlet–triplet energy gap. Upon rapid quench-
ing from ambient temperature to 5 or 1.8 K, the high
temperature conformer (or a large fraction of) is frozen in
2-MeTHF glass. Because the singlet–triplet thermal equilibra-
tion within the same conformer will remain very fast, the singlet
ground state is predominantly populated in the 1.8–5 K range
for the high temperature conformer. The observed S-shaped
increase of cmol vs. time in the 1.8–5 K range would involve a
conformational change from the high temperature to the low
temperature conformer. The resultant dramatic change between
the population of the singlet vs. triplet states is responsible for
a typically five-fold increase in cmol. The S-shape and random
11 A. Calder, A. R. Forrester, P. G. James and G. R. Luckhurst, J. Am.
Chem. Soc., 1969, 91, 3724.
12 The chemical shift of 217 ppm is found for protons of the But group in
2-methyl-N-tert-butylphenylaminoxyl: A. Calder, A. R. Forrester, J. W.
Emsley, G. R. Luckurst and R. A. Storey, Mol. Phys., 1970, 18, 481.
Treatment of 1 in CDCl3 in an NMR tube with phenylhydrazine gives
quantitatively dihydroxyamine 2, as expected.
13 A Quantum Design MPMS5S SQUID instrument was used with either
standard or continuous temperature control. An approximate correction
for diamagnetism (cdia) for each sample was obtained from extrapola-
tion of c vs. 1/T (c = total magnetic susceptibility) plots using the data
in the T = 160–280 K range in the cooling mode of the MPMS5S.
Typical values cdia ≈ 4 3 1028 emu are an order of magnitude less than
the total c in the 5–1.8 K range.
14 The estimate of the Heisenberg coupling constant, J/k = 40 K (Fig. 2),
can only be very approximate, as conformational effects may interfere
with a simple two-state model.
15 W. Siebrand and T. A. Wildman, Acc. Chem. Res., 1986, 19, 238.
16 O. Kahn and C. J. Martinez, Science, 1998, 279, 44.
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Chem. Commun., 1999, 1249–1250