Triplet ground state derivative of aza- m -xylylene diradical with large singlet-triplet energy gap

Article abstract of DOI:10.1021/ja200708b

Organic molecules with a strong preference for triplet ground states, in which the triplet state is below the lowest singlet state by ≥10 kcal/mol, are typically short-lived and mostly detected as reactive intermediates. We now report a triplet ground sta

Full text of DOI:10.1021/ja200708b

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Triplet Ground State Derivative of Aza-m-xylylene Diradical with Large
SingletꢀTriplet Energy Gap
Andrzej Rajca,* Arnon Olankitwanit, and Suchada Rajca
Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, United States
S Supporting Information
b
We examined model diradicals, including the octahydropyr-
ABSTRACT: Organic molecules with a strong preference
for triplet ground states, in which the triplet state is below
the lowest singlet state by g10 kcal/mol, are typically short-
lived and mostly detected as reactive intermediates. We now
report a triplet ground state derivative of aza-m-xylylene
diradical with a large singletꢀtriplet energy gap (ΔEST) of
∼10 kcal/mol, which is comparable to ΔEST for the well-
known reactive intermediate m-xylylene diradical. The
aminyl diradical persists in solution at room temperature
on the time scale of minutes.
idoquinoline (OHPQ) diradical computed by Schreiner and co-
workers.12 In particular, we considered planar derivatives of aza-
m-xylylene diradical in which the π delocalization is largely
limited to m-phenylene in order to maximize the spinꢀspin
interactions. The estimated values of ΔEST between the triplet
ground states and the lowest open-shell singlet states (computed
at the DFT level) for OHPQ diradical, diradical 1, aza-m-xyly-
lene, and m-xylylene are 13.5,12 11.0, 15.8, and 12.0 kcal/mol,
1
respectively.13,14 In addition, the EPR H hyperfine splittings
(aH) for N-tert-butylanilino radical indicate that the para and
ortho aH values are slightly larger than those of benzyl radical (by
a factor of 1.1ꢀ1.2).15,16 On the basis of the empirical correlation
ΔEST ∼ |aH|2,17 the values of ΔEST in planar derivatives of aza-
m-xylylene and m-xylylene should be comparable.
ylylene is a well-known high-spin diradical with a
-
Xvery strong spinꢀspin interaction, and its triplet
m
ground state lies below the lowest singlet state by 9.6 ( 0.2
kcal/mol,1ꢀ3 one of the largest known singletꢀtriplet energy
gaps (ΔEST) for an organic molecule. Such molecules typically
are short-lived, as in the case of m-xylylene, which is observable in
solution at room temperature for hundreds of nanoseconds.4
Derivatives of m-xylylene with improved stability have been
prepared to provide numerous high-spin diradicals. Over the past
decades, m-xylylene-based diradicals have been employed as
building blocks in the designs of very high spin polyradicals
and magnetically ordered organic polymers.5,6 However, the
strength of spinꢀspin interactions in these radicals is reduced
by 1ꢀ3 orders of magnitude relative to the ΔEST value of 9.6
kcal/mol in m-xylylene5 because of dissipation of spin density
over the extended π-system and/or resonance stabilization that
renders improved stability. Therefore, triplet ground state dir-
adicals that possess both large ΔEST (g10 kcal/mol) and
stability at room temperature remain elusive. Such diradicals
may be viewed as building blocks for high-spin polyradicals with
strong spinꢀspin interactions at room temperature that could
enable the development of robust magnetic polymers and
organic spintronics.5,7,8
Herein we describe the synthesis and magnetic characteriza-
tion of a sterically shielded planar derivative of aza-m-xylylene,
aminyl diradical 2, which possesses a triplet ground state having a
ΔEST comparable to that in m-xylylene but is persistent at room
temperature in solution on the time scale of minutes.
The synthetic route to 2 started with Fisher indole synthesis to
provide indolenine derivative 3 (Scheme 1).18 Addition of an
excess of allyl Grignard reagent yielded diamine 4 as a mixture of
diastereomers. Analogous additions of methyl Grignard reagents
did not produce the corresponding diamines,19 and thus, the
synthesis of diradical 1 was not feasible. In the next step, the allyl
groups in 4 were catalytically reduced with H2 to the correspond-
ing n-propyl (n-Pr) groups in 5. Diastereomers 5 were bromi-
nated at low temperature to yield 6, which was subjected to
Suzuki cross-coupling with 4-tert-butylphenyl boronic acid to
give 7. Treatment of diastereomers 7 with n-BuLi produced an
orange solution of the dianion. Oxidation of the dianion with
iodine at about ꢀ115 °C, which was monitored by EPR
spectroscopy, provided a dark-red solution of aminyl diradical
2 in 2-methyltetrahydrofuran (2-MeTHF). The last step was
Nitrogen-centered (aminyl) radicals are typically short-lived,9
and only two aminyl monoradicals are known to be stable under
ambient conditions.10 Recently, we reported the isolation of a
triplet ground state aminyl diradical under an inert atmosphere.11
The diaazapentacene-derived aminyl diradical was estimated
by density functional theory (DFT) computations to possess
ΔEST ≈ 7 kcal/mol,11 which is significantly greater than 0.6 kcal/
mol, the thermal energy at room temperature. This result
encouraged us to explore stable aminyl diradicals with ΔEST g
10 kcal/mol.
Received: January 24, 2011
Published: March 14, 2011
r
2011 American Chemical Society
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Figure 1. EPR spectrum (X-band, ν = 9.4780 GHz) of 2 in 2-MeTHF at
133 K. Selected parameters for the S = 1 state are |D/hc| = 1.749 ꢁ
Figure 2. SQUID magnetometry data for ∼20 mM 2 in 2-MeTHF. The
main plot and inset show M/Msat vs H/(T ꢀ θ) and χT vs T,
respectively, with θ = ꢀ0.2 K, Msat = 0.89 μB, S = 1.0, and χT = 0.89
10ꢀ2 cmꢀ1, |E/hc| = 3.60 ꢁ 10ꢀ3 cmꢀ1, |Azz/2hc| = 1.134 ꢁ 10ꢀ3 cmꢀ1
,
gx = 2.0046, gy = 2.0030, and gz = 2.0019. Further details concerning the
simulation of the EPR spectrum, including the center lines for an S = 1/2
byproduct, may be found in Figure S2 in the SI.
emu K molꢀ1
.
B3LYP/EPR-II computations of the D and A tensors in 1
confirmed the approximately parallel orientation of their largest
principal values to the direction of the 2pπ orbitals on the
nitrogens.22 Excellent agreement between the calculated Azz/
2hc value of 1.131 ꢁ 10ꢀ3 cmꢀ1 in 1 and the experimental
pentuplet 14N splitting, |Azz/2hc| = 1.134 ꢁ 10ꢀ3 cmꢀ1 in 2, was
obtained. Although the calculated D/hc value of 3.4 ꢁ
10ꢀ2 cmꢀ1 for 1 overestimated the experimental |D/hc| value
Scheme 1. Synthesis of Aminyl Diradical 2
of 1.749 ꢁ 10ꢀ2 cm
,
ꢀ1 22,23 the positive sign (D > 0) indicates an
“oblate-like” shape for the overall spin density, as expected for
spin density that is largely delocalized into the π system of
m-phenylene.24
For comparison, the overall spin density of the analogous
nitroxide diradical 8 with a planar nitroxideꢀm-phenyleneꢀnitr-
oxide moiety attains “prolate-like” shape because about half of
the spin density is localized on the oxygens of the NO moieties.25
The elongated shape of the spin density was supported by the
calculated result D/hc < 0 in 8.22 The computed D and A
tensors22 and the EPR spectra25,26 indicate that the second-
largest principal value of the D tensor and the largest principal
value of the A tensor are approximately parallel to the direction of
the 2pπ orbitals on the nitrogens (y axis).
typically carried out on a 0.9ꢀ2 μmol scale in custom-made
quartz tubes, enabling sequential examination of each sample
of 2 in 2-MeTHF by EPR spectroscopy and SQUID
magnetometry.
The EPR spectra of 2 in 2-MeTHF at 133 K showed six
symmetrically disposed side peaks in the |Δms| = 1 region and an
intense half-field transition (|Δms| = 2), as expected for a triplet
state with significant D and E zero-field splitting parameters
(Figure 1).
The two outermost peaks (z turning points) appear as
pentuplets, which may be simulated by the 14N hyperfine
coupling (Azz) of two nitrogen nuclei. Azz is the largest principal
value of the 14N hyperfine tensor (the A tensor), so it coincides
with the direction of the nitrogen 2pπ orbital.20 This implies that
the 2pπ orbital is approximately parallel to the z axis, which is the
direction of the largest principal value of the D tensor, as
observed in diaazapentacene-derived aminyl diradicals.11,21
Therefore, the aminyl moieties and the m-phenylene in diradical
2 adopt an approximately coplanar conformation.
These qualitatively different spin density distributions in
planar aminyl and nitroxide diradicals may correlate with
the much larger ΔEST values for triplet ground state
aminyls.11,21,25,27
The triplet ground state for 2 in 2-MeTHF was unequivocally
confirmed by SQUID magnetometry, as evidenced by the S = 1
paramagnetic behavior for both the magnetization (M) as a
function of magnetic field (H) and the magnetic susceptibility
(χ) as a function of temperature (T) (Figure 2). A small
antiferromagnetic coupling, as measured by the mean-field
parameter θ ≈ ꢀ0.2 K, is assigned to very weak intermolecular
interactions in 20 mM solutions of 2.
Specifically, the numerical fits to the curvature of the M/Msat
vs H/(T ꢀ θ) plots at 1.8, 3, and 5 K (where Msat is the
magnetization at saturation) gave the value S = 1.0, which was
independent of the spin concentration of the sample. After the
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’ AUTHOR INFORMATION
Corresponding Author
arajca1@unl.edu
’ ACKNOWLEDGMENT
We thank the National Science Foundation for support of this
work through Grants CHE-0718117 and CHE-1012578.
’ REFERENCES
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Figure 3. Persistence of aminyl diradical 2 in 2-MeTHF monitored by
EPR spectroscopy at 134ꢀ136 K: (A) before annealing; (B) after 10 min
at ꢀ27 °C; (C, D) after 10 and 20 min at room temperature.
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expected for an S = 1 diradical. Furthermore, the χT vs T plot was
flat up to 150 K, the highest temperature of the measurement,
providing a lower limit of 200 K (∼0.4 kcal/mol) for ΔEST.
We investigated the reactivity/stability of aza-m-xylylene
diradical 2 in 2-MeTHF by EPR spectroscopy. In the presence
of iodine at ꢀ115 °C, a decay of the EPR signals corresponding
to the triplet diradical was observed. Brief warming to ꢀ78 °C led
to the disappearance of the EPR signals for 2. The diradical
reactivity toward iodine at low temperature made generation of 2
challenging [Scheme 1; also see Table S4 in the Supporting
Information (SI)]. In another experiment, brief exposure of 2 in
2-MeTHF to O2 at ꢀ105 °C resulted in a sharp decrease of the
EPR signal intensity and, in particular, complete disappearance of
the triplet resonances for 2.
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spectra of 2 in 2-MeTHF after brief annealing at ꢀ27 °C were
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detectable after 20 min (Figure 3 and Figure S9 in the SI).
We also carried out similar experiments using SQUID mag-
netometry. Magnetic measurements after a sample of 2 in THF
was annealed at ꢀ27 °C for 30 min showed data corresponding
to the triplet diradical. After consecutive annealing sequences of
30 min at 0 °C, 30 min at 22 °C, and another 30 min at 22 °C, the
data indicated a sharp decrease in the diradical signal, though a
trace of the diradical was still detectable after total of 1 h at room
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UB3LYP/6-311þG(d,p) þ ZPVE level and the OHPQ diradical at the
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After extended annealing of 2 in 2-MeTHF or THF at room
temperature, we isolated the starting diamine 7 as the main product
(see the SI), thus suggesting a hydrogen abstraction mechanism
similar to that observed for other triplet aminyl diradicals.11,28
In summary, a planar derivative of aza-m-xylylene diradical was
prepared in solution, thus providing first example of an organic
diradical that is persistent at room temperature in solution on the
time scale of minutes and possesses a triplet ground state with a
singletꢀtriplet energy gap of the order of 10 kcal/mol.
’ ASSOCIATED CONTENT
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Supporting Information. Complete ref 13 and experi-
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mental details. This material is available free of charge via the
Internet at http://pubs.acs.org.
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(21) Rajca, A.; Shiraishi, K.; Pink, M.; Rajca, S. J. Am. Chem. Soc.
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(22) (a) Neese, F. ORCA: An Ab Initio, Density Functional, and
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EPR-II level on the basis of electron spinꢀspin dipolar coupling.
(23) (a) At a similar level of theory, the DFT-computed D value of
∼0.03 cmꢀ1 overestimated the experimental value of 0.011 cmꢀ1 in
m-xylylene. See: Sinnecker, S.; Neese, F. J. Phys. Chem. A 2006,
110, 12267–12275. (b) A D value of 0.013 cmꢀ1 for m-xylylene was
computed at the CASSCF(6,6)/cc-pVDZ level. See: Havlas, Z.; Michl, J.
J. Chem. Soc., Perkin Trans. 2 1999, 2299–2303.
(24) The values of D, E, and Azz for 2 are significantly greater than
those for the diaazapentacene-derived aminyl diradical: |D/hc| = 9.18 ꢁ
10ꢀ3 cmꢀ1, |E/hc| = 2.10 ꢁ 10ꢀ3 cmꢀ1, and |Azz/2hc| = 1.00 ꢁ
10ꢀ3 cmꢀ1
.
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(28) In the presence of oxygen, the nitroxide diradical corresponding
to 2 would be expected because N-tert-butylanilino monoradicals react
with oxygen to provide nitroxides (see ref 15). However, we isolated
only diamagnetic products (see the SI).
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