Ferromagnetic spin-delocalized electron donors for multifunctional materials: π-conjugated benzotriazinyl radicals

Article abstract of DOI:10.1039/c0cc04727a

We have developed new synthetic methodology for benzotriazinyl radicals that exhibit spin delocalization, low oxidation potentials, and ferromagnetic interactions in the solid state via π-π interactions, making them promising candidates for multifunctiona

Full text of DOI:10.1039/c0cc04727a

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COMMUNICATION
Ferromagnetic spin-delocalized electron donors for multifunctional
materials: p-conjugated benzotriazinyl radicalsw
a
a
b
a
Bin Yan, Jordan Cramen, Robert McDonald and Natia L. Frank*
Received 1st November 2010, Accepted 6th January 2011
DOI: 10.1039/c0cc04727a
We have developed new synthetic methodology for benzotriazinyl
radicals that exhibit spin delocalization, low oxidation potentials,
and ferromagnetic interactions in the solid state via p–p inter-
actions, making them promising candidates for multifunctional
magnetic materials.
Recent developments in multifunctional molecular materials
combining magnetic and conducting functionality are of interest
1
for spintronics and organic electronics applications. While
–7
8
,9
1,10,11
stable radicals such as nitroxides,
thiazyls,
play a central role in the development of
spin-based materials, systems exhibiting ferromagnetic
and
1
2
Scheme 1
verdazyls
1
,13–16
ꢀ7
ꢀ1
exchange are quite rare in organic materials
due to the
conductivities of 10 S cm in 1a/1b through the formation
of slipped 1-D p-stacks.
The radicals 1-phenyl-3-(3-vinylphenyl)-1,2,4-benzotriazin-
4-yl (1a) and 1-(4-bromophenyl)-3-(3-vinylphenyl)-1,2,4-
benzotriazin-4-yl (1b) were prepared as shown in Scheme 1.
The 3-vinylbenzaldehyde 2 was prepared by Stille coupling
of 3-bromo-benzaldehyde with tributylethenylstannane/
nature of orbital interactions and radical–radical contacts
1
7
dictating the sign and magnitude of magnetic exchange. As
both magnetic exchange and charge transport depend on
orbital overlap terms, large planar delocalized radicals that
are redox active are predicted to lead to effective pathways for
intermolecular magnetic exchange and charge transport.
A unique subclass of hydrazyl radicals are the 1,3 diaryl-
3 4
Pd(PPh ) in DMF to yield 2 in 76%. Condensation with
1
,2,4-benzotriazinyl radicals (DABTs, 1) which have high
phenylhydrazine or p-bromo-phenylhydrazine hydrochloride
in benzene gave the hydrazones 3a/3b followed by chlorination
via the Corey–Kim reagent at ꢀ78 1C to give the corresponding
chlorohydrazones 4a/4b in 57–67% yield. Condensation with
aniline in ethanol at 70 1C led to the amidrazones 5a/5b in
B35% yield. Oxidative cyclization catalyzed by HgO or DBU
proceeded cleanly to give the imidrazone, which upon air
oxidation yields the radicals as black polycrystalline solids in
63–71% yield. The resultant radicals were found to be stable in
solution and the solid state for months at a time.
1
thermodynamic and kinetic stability, planar topologies,
8
1
9
and delocalized spin/electron density critical for magnetic
exchange and charge transport pathways. Unusually strong
intermolecular antiferromagnetic exchange interactions have
been observed in the crystalline state (2J = ꢀ220 K) of
2
0
DABTs due to the formation of 1-D slipped p-stacks. The
electron rich benzotriazinyl skeleton leads to extremely low
reversible one-electron oxidation processes that have been
utilized for the formation of antiferromagnetic semiconducting
2
1
charge-transfer salts. In order to further exploit this unique
class of stable radicals for material applications, we have
developed synthetic methodology for DABT radicals that
allows a greater tolerance to diverse functionality. We report
here intermolecular FM exchange interactions and
Due to the unique planar open-shell electronic structure of
DABT radicals, 1a/1b exhibit strong electron-donor abilities,
low-lying states giving rise to transitions in the visible region,
and strong spin delocalization. Cyclic voltammetry of 1a/b in
acetonitrile (0.1 M TBAH) reveals one-electron reversible
oxidation (0/+1) and reduction (0/ꢀ1) processes at + 0.21 V,
a
Department of Chemistry, Box 3065, University of Victoria,
(DE
60 mV) and ꢀ0.83 V (DE
p
62 mV) vs. SCE with a
p
Victoria, British Columbia, V8W 3V6, Canada.
E-mail: nlfrank@uvic.ca; Fax: +1 250-721-7147;
Tel: +1 250-721-7154
disproportionation energy (DE) of 1.0 V (see ESIw for data
for 1b). The electronic absorption spectra of 1a/1b in CH Cl
2
2
b
Department of Chemistry, University of Alberta, 11227
exhibit absorptions in the UV region (ESIw) and visible at lmax
(e) = 370 (3900), 428 (2100), 491 (900), and 550 (470) nm with
the two lowest energy transitions assigned to HOMO–SOMO
and SOMO–LUMO (n–p*) transitions, respectively. X-band
EPR spectroscopy of 1a in CH Cl at 300 K exhibits 7
Saskatchewan Drive NW, Edmonton, Alberta, Canada T6G 2G2
w Electronic supplementary information (ESI) available: Syntheses,
spectroscopic data, computational data, crystallographic coordinates.
CCDC 799186–7. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc04727a
2
2
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Fig. 1 Crystallographic intermolecular contacts in radical 1a (left)
and 1b (right) viewed along the a-axis. Thermal ellipsoids are shown at
50% probability.
transitions due to hyperfine coupling to three nitrogen nuclei
to give hyperfine coupling constants a(N ) = 7.47, a(N) 4.76,
1
and a(N) 4.99 G with a g-value of 2.0071 (1a). The electronic
structures of the radical derivatives 1a and 1b differ only
slightly from that of the parent, suggesting little perturbation
by vinyl substitution, as expected. (See ESIw for 1b.)
Single crystals of 1a and 1b suitable for X-ray diffraction
(
Fig. 1) were grown from CH
crystallizes in the monoclinic crystal system and I2/a space
group (Z = 8), and 1b in P2 /n with Z = 12 (ESIw). The
molecular structure of 1a reveals a planar benzotriazinyl
2 2 3
Cl /CH OH. Radical 1a
1
˚
Fig. 2 (Upper) Temperature dependence of the molar magnetic
moment at 10 000 Oe and inverse susceptibility (inset) for radical 1a
with Curie–Weiss fit; (lower) magnetization vs. H(Oe) and dM/dH as a
function of field (inset) at 2 K.
(
C6–C7–N1–N2 of 1791) with bond lengths (N1–N2 1.363(2) A,
˚
˚
N2–C1 1.336(3) A and C1–N3 1.340(3) A) and bond angles
N1–N2–C1 116.02(18)1, N2–C1–N3 127.2(2)1, C1–N3–C2,
16.10(19)1 consistent with a partially delocalized radical
(
1
structure (ESIw). The phenyl ring attached to N1 is disordered
with two equally-abundant conformers showing torsional
angles of 69.2(15)1 (N2–N1–C11A–C12A) and 125.6(16)1
ꢀ
1
(w T) are 0.390 (1a) and 0.380 emu K mol (1b), slightly
M
above that for with a spin-only S =
1
2
system. As the
(
N2–N1–C11B–C12B). The twisting of the aromatic ring
attached to C1 is less pronounced (N3–C1–C21–C22 =
.6(3)1), with the two conformers of the vinyl group
in 60/40 relative abundance) exhibiting torsions of 11.2(8)1
C24–C23–C27–C28A) and ꢀ151.2(9)1 (C24–C23–C27–C28B)
temperature decreases, the moments remain almost constant
until 200 K at which point the moments increase reaching
ꢀ
1
ꢀ1
at 8.5 K (1a) and
7
maxima of 0.53 emu K Oe
mol
ꢀ1
K at 9.0 K (1b), with a further decrease in the
ꢀ
1
(
emu mol
(
magnetic moment as T approaches 2 K. In the 50–300 K
temperature range, the magnetic susceptibility for both
radicals follows Curie–Weiss behaviour to give Weiss
constants of y = (+)4.0 (1a) and y = (+)3.7 K (1b) consistent
with dominant ferromagnetic interactions.
with respect to their attached aromatic ring. While the
N1–N2–C1–N3–C2–C7 ring in 1a is essentially planar
˚
all atoms within 0.01 A of the best-fit least-squares plane),
(
for 1b a noticeable out-of-plane bending of N1 is observed
˚
0.056 A, 0.093 A, and 0.081 A for the three crystallographically-
˚
˚
(
The field dependence of the magnetization was measured at
2 K in which the M versus H plot exhibits slight sigmoidal
independent molecules). Despite this, the bond lengths and
angles within this ring are essentially identical to those
observed for 1a, suggesting little effect from the bromination
of the N1-bound phenyl group.
1
behaviour relative to a Brillouin function for an S = spin
2
system (Fig. 2). The first derivative dM/dH exhibits a
maximum at a critical field H = 16 kOe, characteristic of
c
The radicals 1a and 1b form 1-D chains via p–p interactions
in which the molecules are related by translation and slippage
˚
along the b-axis with interplanar distances of 3.30–3.50 A. For
metamagnetic behavior in which the coexistence of intrachain
ferromagnetic and interchain antiferromagnetic interactions
can give rise to chain alignment and stabilization of a ferro-
1
6,22–27
˚
a, the interplanar distance is 3.461 A with a slippage angle of
1
magnetic-like state.
The high temperature expansion
˚
4
61 and short Cꢁ ꢁ ꢁC contacts of 3.294 A, while an interplanar
of the Heisenberg linear chain expression with Pade
2
8
˚
distance of 3.341 A and slippage angle of 361 is observed for
b. The large degree of slippage in both radicals leads to direct
approximant was used to model the intrachain magnetic
interactions, with an additional mean field parameter for
inclusion of the weak interchain interactions (ESIw). The
exchange coupling estimated from a least-squares fit of the
data from 15–300 K (w vs. T, R = 0.998) gave exchange
1
face-to-face p–p interactions between the benzo and the 1,2,4-
triazinyl moieties. In addition, edge-to-face p–p interactions
are present between p stacks, leading to p–p interactions in
three dimensions.
ꢀ1
coupling constants of J = +7 cm with Weiss constant
ꢀ1
ꢀ1
DC field magnetic susceptibility measurements (w = M/H)
of polycrystalline samples of 1a/1b were carried out as a
function of field and temperature in the temperature range
y = ꢀ3.5 cm for radical 1a, and J = +7.4 cm with
ꢀ1
y = ꢀ5 cm for 1b, consistent with the intrachain ferromagnetic
interactions and weaker antiferromagnetic interchain
interactions.
2
–300 K (Fig. 2). At room temperature (300 K) the moments
3
202 Chem. Commun., 2011, 47, 3201–3203
This journal is c The Royal Society of Chemistry 2011

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to metamagnetic behaviour. These radicals are an important
class of stable open shell monomers for organic materials in
which the redox activity, spin delocalization and strong inter-
molecular interactions in the solid state make them promising
materials for multifunctional magnetic materials.
Notes and references
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1
9
parent DABT radical. Both sets of computations predict
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1
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2
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2
9
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2
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0,31
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2
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ꢀ
1
ꢀ1
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2
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We have reported new synthetic methodology for DABT
radicals that exhibit spin delocalization and low oxidation
potentials. The radicals 1a and 1b form slipped 1D chains due
to p–p interactions that give rise to intrachain ferromagnetic
and interchain antiferromagnetic exchange interactions leading
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3201–3203 3203