Ferromagnetic interactions in a 1d alternating linear chain of π-stacked 1,3-diphenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals

Article abstract of DOI:10.1002/chem.201200248

X-ray studies show that 1,3-diphenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4- benzotriazin-4-yl (6) adopts a distorted, slipped π-stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52A). Cyclic voltammograms of 7-(thien-2-yl)benzotriazin-4-yl 6 show two fully reversible waves that correspond to the -1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie-Weiss behavior in the 5-300K region with C=0.378emuKmol^-1 and I=+4.72K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J₁=+7.12cm^-1, J ₂=+1.28cm^-1).

Full text of DOI:10.1002/chem.201200248

FULL PAPER
DOI: 10.1002/chem.201200248
Ferromagnetic Interactions in a 1D Alternating Linear Chain of p-Stacked
1,3-Diphenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl ACTHUNTGRENNGRU adicals
Christos P. Constantinides,[a, b] Panayiotis A. Koutentis,*[a] and Jeremy M. Rawson[b]
Abstract: X-ray studies show that 1,3-
diphenyl-7-(thien-2-yl)-1,4-dihydro-
cesses. EPR and DFT studies on radi-
cal 6 indicate that the spin density is
mainly delocalized over the triazinyl
fragment. Magnetic susceptibility meas-
urements show that radical 6 obeys
Curie–Weiss behavior in the 5–300 K
region with C=0.378 emuKmolꢀ1 and
q= +4.72 K, which is consistent with
1,2,4-benzotriazin-4-yl (6) adopts a dis-
torted, slipped p-stacked structure of
centrosymmetric dimers with alternate
short and long interplanar distances
(3.48 and 3.52 ꢁ). Cyclic voltammo-
grams of 7-(thien-2-yl)benzotriazin-4-yl
6 show two fully reversible waves that
correspond to the ꢀ1/0 and 0/+1 pro-
ferromagnetic interactions between S=
1
=
radicals. Fitting the magnetic sus-
2
ceptibility revealed the behavior is con-
sistent with an alternating ferromagnet-
ic chain (g=2.0071, J1 = +7.12 cmꢀ1,
J2 = +1.28 cmꢀ1).
Keywords: benzotriazinyls · hetero-
cycles · magnetic properties · radi-
cals · stacking interactions
Introduction
Persistent organic radicals have received increased attention
as promising building blocks for functional molecular mate-
rials on account of their unique physical properties.[1] The
presence of unpaired electrons in these organic molecules
gives rise to bulk solid-state properties such as conductivity
and magnetism that are traditionally associated with metals,
their oxides, and ceramics.[2] Organic-based materials offer
potential advantages over their inorganic counterparts since
efficient and versatile organic synthesis facilitates the intro-
duction of small structural changes that are needed to fine-
tune properties.[2a]
Numerous families of stable radicals (verdazyl,[2d] nitro-
xide, nitronyl nitroxide radicals,[3a] heterocyclic thiazyl,[3b] se-
lenazyl,[3c–e] and triphenylmethyl radicals[3f]) have been dis-
covered and extensively studied. Nevertheless, benzotriazin-
yl radicals, which have been known since the late 1960s
when Blatter et al. first prepared 1,3-diphenyl-1,4-dihydro-
1,2,4-benzotriazin-4-yl (1) (Blatterꢀs radical),[4] have been
less investigated and only a few examples of this radical
family have appeared in the literature.[5–9] On account of the
ease of preparation and their exceptional air and moisture
stability, benzotriazinyls have been recently revisited.[6–8]
Their potential to functionalize at the 5-8 positions of the
benzo-fused ring, as well as N1 of the triazinyl ring and C3,
offers potential synthetic and structural diversity. Five deriv-
atives have been previously reported with derivatives 1, 2,
and 3 substituted at N1, and 4 functionalized at C3. Recent-
ly, we reported the first derivative functionalized at the
benzo-fused ring 5.
The magnetic properties of these benzotriazinyls prove in-
teresting: whereas 2 exhibits short-range antiferromagnetic
interactions[9] derivatives 3–5 demonstrate low-dimensional
ferromagnetism.[7,8] Given the strict necessity for orbital or-
thogonality to propagate ferromagnetic exchange, the appar-
ent propensity for radicals of this type to exhibit ferromag-
netic interactions is unusual but seems to be intimately asso-
ciated with the formation of slipped p-stacked structures[8]
and has prompted us to prepare additional derivatives with
a view to developing a better understanding of the effects of
[a] C. P. Constantinides, Dr. P. A. Koutentis
Department of Chemistry, University of Cyprus
P.O. Box 20537, 1678 Nicosia (Cyprus)
E-mail: koutenti@ucy.ac.cy
[b] C. P. Constantinides, Prof. J. M. Rawson
Department of Chemistry
The University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200248.
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substituents on crystal packing and ultimately magnetic be-
havior.
X-ray studies: Suitable single crystals of 6 for X-ray diffrac-
tion studies (Table S1 in the Supporting Information) were
obtained by slow evaporation of a concentrated hexane so-
lution. Radical 6 crystallizes in the monoclinic space group
P21/n with one molecule in the asymmetric unit. To date,
crystal structures of benzotriazinyl radicals 1–5 have been
reported and the molecular geometry of 6 is described in re-
lation to 1–5.
The 1,2,4-amidrazonyl moiety in radical 6 is almost planar
with a deviation from planarity of approximately 3.58 as de-
fined by the angle measured between the plane of the N1,
N2, C1, N3 amidrazonyl atoms and the plane of the fused
phenyl ring (Figure 1). This is similar to radicals 1, 2, and 4,
which are planar and also radical 3 (6.88) but markedly less
than radical 5 (14.58).
As part of this effort, we now report the synthesis and
characterization of 1,3-diphenyl-7-(thien-2-yl)-1,4-dihydro-
1,2,4-benzotriazin-4-yl (6) in which the thiophene group has
the potential to generate favorable S···S contacts. Such S···S
contacts play a key role in the packing and electronic prop-
erties of sulfur-rich organic conductors such as tetrathiaful-
valene (TTF),[10a,b] and also oligo- and polythiophenes.[10c,d]
Results and Discussion
Synthesis and characterization: 1,2,4-Benzotriazinyl radicals
are typically prepared by means of a five-step product-spe-
cific synthesis that affords the radicals in moderate yields
and involves large excess amounts of HgO (toxic) or AgO
(expensive) oxidants.[4,11] Recently, we developed a mild and
high-yielding preparation of Blatterꢀs radical 1 and several
C7-substituted analogues through catalytic oxidation of the
amidrazone precursors by using palladium-on-carbon and
1,8-diazabicycloACHTUNGTRENNUNG
[5.4.0]undec-7-ene (DBU) in air.[6a] One of
the analogues, 7-iodo-1,2,4-benzotriazinyl (7), can be further
modified by means of Stille and Suzuki–Miyaura cross-cou-
pling reactions to access a range of 7-aryl and 7-heteroaryl-
1,2,4-benzotriazinyls.[6b] Both the Stille and Suzuki–Miyaura
reactions are in most cases high yielding but the latter are
cleaner; the former are faster but are accompanied by the
benzotriazin-7(H)-one 8 as side product, which arises from
the oxidation of the radical at C7 (Scheme 1). Blocking the
Figure 1. Ellipsoid diagram of radical 6 in the crystal with atom-number-
ꢀ
ing scheme (1808 disorder of the thienyl group about the C6 C8 bond
has been removed for clarity).
ꢀ
ꢀ
The C N bond lengths of radical 6 (C1 N2 1.336(2) ꢁ,
Scheme 1. Synthesis of 7-thien-2-ylbenzotriazinyl 6. a) DBU (0.1 equiv),
Pd/C (1.6 mol%), CH2Cl2 (1 mL), approximately 208C under an air at-
ꢀ
C1 N3 1.337(2) ꢁ) are intermediate between typical single
ꢀ
mosphere, 4–9 h (81%);[6a] b) Pd
ACTHNUGTRENNUG(OAc)2 (5 mol%), thien-2-ylSnBu3
and double C N bonds. The C1-N2-N1 and C1-N3-C3
angles of 115.0(1) and 116.0(1)8, respectively, are typical of
sp2-hybridized pyridine coordination complexes.[12] Radical 6
shows bond lengths and angles similar to radicals 1–5, which
is consistent with a strong delocalization within the amidra-
zoyl moiety of the heterocycle.
(2 equiv), dry DMF (2 mL), approximately 1008C, under argon, 30 min
(93%).[6b]
C7 position with a trifluoromethyl group led to higher oxi-
dative stability and, remarkably, when the 7-trifluoromethyl
benzotriazinyl radical 5 was treated with an excess amount
of MnO2 or KMnO4 in solutions of benzene heated at
reflux, the radical was recovered unchanged.[7] The introduc-
tion of heteroaryl substituents at C7 can be doubly advanta-
geous since they can inhibit formation of the benzotriazi-
none 8 and can also participate in various intermolecular in-
teractions that can affect the packing of the radicals in the
solid state. Radical 6 was considered a good candidate to
study. Using our best conditions, thien-2-ylSnBu3 (2 equiv)
The torsion angle (C21-C20-N1-N2) of the N-phenyl
group with respect to the plane of the benzotriazinyl is
60.1(2)8 as a result of steric interactions between the peri-
hydrogen H25 and the phenyl ortho-hydrogen H7. In con-
trast, the phenyl substituent at C1 is essentially coplanar
(torsion C19-C14-C1-N3 0.8(2)8) with the benzotriazinyl
ring affording two favorable but weak intramolecular hydro-
gen bonds (C15···N2 2.805(2) ꢁ, C19···N3 2.772(2) ꢁ). The
2-thienyl substituent is nearly coplanar with the benzotria-
zinyl group (ꢁ6.68) but is disordered over two sites in
ꢀ
and Pd
G
a 50:50 ratio by a 1808 rotation about the C6 C8 bond.
The packing of radical 6 comprises a centrosymmetric
means of Stille reaction conditions in 93% (Scheme 1).[6b]
dimer linked through a pair of crystallographically equiva-
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Ferromagnetic Interactions
FULL PAPER
is negligible (or at least the possible interactions it adopts in
the two orientations are near equi-energetic). It is notable,
however, that the thienyl ring is nearly coplanar with the
phenyl ring so as to optimize efficient p stacking of the mol-
ecules along the b axis. In addition, twisting of the thienyl
ring from planarity would require a disruption to the planar-
ity of the phenyl ring in a neighboring molecule in the same
ꢀ
p stack that exhibits the intramolecular C H···N interactions
already described.
Neighboring columns in radical 6 are connected through
a net of contacts (S···S 3.36 ꢁ, S···H5 2.86 ꢁ, and H25···H25’
2.39 ꢁ) that link adjacent radicals in a tail-to-tail manner to
form 2D sheets along the a axis (Figure S1 in the Supporting
Information). These sheets pack in a parallel arrangement
along the ac diagonal plane (Figure S2 in the Supporting In-
formation) and in a herringbone mode down the a axis. (Fig-
ure S3 in the Supporting Information).
Cyclic voltammetry and EPR spectroscopy: The redox be-
havior of radical 6 is similar to radicals 1–5 in that it exhibits
two fully reversible waves that correspond to the ꢀ1/0 and
0/+1 processes (Figure 3). With oxidation potentials occur-
ꢀ
Figure 2. Solid-state structure of 6. Top: Close intermolecular C H···N
contacts lead to a face-to-face interaction between benzotriazinyl radicals
in 6. Bottom: Stacking of radicals parallel to the b axis. Labels I, II and
III refer to molecules discussed in the text.
ꢀ
lent C H···N contacts between a triazinyl N (at position N4)
and an ortho-hydrogen atom of the N-phenyl ring (dC···N
3.630(2) ꢁ; Figure 2, top). These dimers pack parallel to the
crystallographic b axis (Figure 2, bottom), thus leading to
a p-stacked structure with close contacts in the range 3.5–
3.8 ꢁ between delocalized p systems. Notably, both 3 and 4
ꢀ
exhibit intra-stack ortho-C H···N interactions but to posi-
tion N2 of a neighboring triazinyl radical (see the Support-
ing Information).
The interplanar distance between the weakly hydrogen-
bonded pair of molecules (I–II, Figure 2, bottom) is 3.52 ꢁ
with a slippage angle of f2 =15.78, whereas the interplanar
distance between molecules in radical pair II–III is 3.48 ꢁ
with a slippage angle of f1 =45.98. As such, the column can
be envisaged as a slipped stack of radical pairs and can be
considered electronically to be a 1D Peierls distorted stack
of radicals with alternating interplanar distances.
Figure 3. Cyclic voltammogram of 6 (1 mm). Conditions: nBu4NBF4 0.1m,
CH2Cl2, RT, 50 mVsꢀ1
.
The crystal packing of radical 6 shares similar features
with the packing of other benzotriazinyl radicals 2–5 in that
all adopt a p-stacked motif. However, whereas radicals 3–5
form regular 1D chains through simple translation and have
shorter interplane distances, both 2 and 6 incorporate an ad-
ditional symmetry element in addition to translation. For
radicals 3–5 the interplanar distances are 3.34, 3.46, and
3.35 ꢁ and have slippage angles of 36, 46, and 288, respec-
tively, with favorable edge-to-face (3 and 5) and face-to-face
(4) p–p interactions between the N-phenyl groups. Such
contacts are absent from the packing of radicals 2 and 6 in
which the molecules are related by a center of inversion
that resides at the centre of their radical-pair structure.
The presence of disorder in the 2-thienyl group would
suggest that the structure-directing influence of the S atom
ring between +0.10 to +0.36 V versus ferrocene/ferroceni-
um (Fc/Fc+; Table S2 in the Supporting Information), ben-
zotriazinyls can act as good electron donors in charge-trans-
fer salts. Blatterꢀs radical 1 forms a charge-transfer salt with
7,7,8,8-tetracyanoquinodimethane (TCNQ), which is a pres-
sure-dependent semiconductor.[13]
Although radical 6 has an electron-donating group at-
tached at C7, a ring position with substantial contribution to
the singly occupied molecular orbital (SOMO), its oxidation
potential (+0.15 V) is slightly higher than that of Blatterꢀs
radical 1 (+0.1 V) but less than those of radicals 4 (+
0.21 V) and 3 (+0.25 V), which have substituents not direct-
ly connected to the benzotriazinyl ring. In contrast, radical 5
with an electron-withdrawing CF3 substituent at C7 has
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P. A. Koutentis et al.
a slightly higher oxidation potential (+0.36 V) on account
of the electronegative nature of this group. Although benzo-
triazinyls have an extended delocalized benzo-fused struc-
ture relative to verdazyls,[14] one less N atom in their ring
leads to higher oxidation potentials. However, benzotriazin-
yls have lower reduction potentials than verdazyls but signif-
icantly higher than their isoelectronic benzothiadiazinyls,
which reduce easily to give benzothiadiazines.[15] The cell po-
tentials for the two nitrogen-rich radical families have a simi-
lar value of around 1.0 V, which is larger than those found
for other neutral radical families (e.g., thiazyls)[2b,3b] that
have been investigated as promising building blocks for con-
ducting materials.
hydrogen and nitrogen atoms.[16] Based on these numbers,
we employed the McConnell equation[17] to estimate the
spin-density distribution around the benzotriazinyl ring for
radical 1 and compared it to that of radical 6.
It is clear from Table 1 that most of the spin density re-
sides on the triazinyl ring and radical 6 has similar spin-den-
Table 1. Spin densities (1) estimated from hyperfine coupling constants
using McConnellꢀs equation.[a]
Radical
N1
+0.300 +0.238 +0.225 ꢀ0.049 ꢀ0.043 ꢀ0.067 ꢀ0.032
+0.295 +0.232 +0.216
N2
N3
C4
C5
C6
C7
1
6
–
–
–
–
[a] AH =QH·1C and AN =QN·1N, in which AH, AN are the hfcc in Gauss;
QH =(ꢀ)27.3 G,[17] QN =21.2 G[18] (for N2 and N3), and QN =25 G[18a,b]
(for N1).
EPR studies on radical 6 revealed hyperfine coupling con-
stants (hfcc) similar to radicals 1–5 (Table S3 in the Support-
ing Information). The solution EPR spectrum of radical 6
(Figure 4) exhibits a seven-line spectrum that is consistent
sity values for the three nitrogen nuclei to radical 1. The
ENDOR measurements on 1 indicate the presence of small
quantities of negative spin density delocalized over the
benzo-fused ring. DFT calculations at the UB3LYP/6-311+
GACTHNUTRGNE(NUG d,p) level of theory on radical 6 failed to predict the mag-
nitude and the sign of experimentally estimated spin densi-
ties in accordance with previous studies, and this was attrib-
uted to the fact that DFT methods do not describe the elec-
tronic structure of the ground state correctly.[7,8]
Magnetic properties: 1,2,4-Benzotriazinyls are
a fertile
source of interesting magnetic properties. Although few rad-
icals of this family have appeared so far in the literature, the
majority exhibit short-range ferromagnetic interactions.
Radicals 3–5 exhibit 1D Heisenberg linear chain ferromag-
netism with intrachain exchange interactions of J= +7.38,[7]
+6.90, and +1.05 cmꢀ1, respectively.[8] Conversely, radical 2
exhibits strong antiferromagnetic interactions that are mod-
eled as an alternating linear chain with magnetic exchange
parameters of J1 =ꢀ76.50 cmꢀ1, J2 =ꢀ22.90 cmꢀ1, and an al-
ternation parameter of a=J2/J1 =0.3.[9] We were therefore
interested to examine the magnetic behavior of 6.
Figure 4. Experimental and simulated EPR spectrum of radical 6 at RT
in CH2Cl2. Fitting parameters: g=2.0071, aN(1)=7.38, aN(2)=4.92,
aN(3)=4.59 (lineshape: DHpp ꢁ2 G; 100% Gaussian).
The variable-temperature magnetic susceptibility of radi-
cal 6 was measured using a Quantum Design SQUID mag-
netometer in the region 5–300 K in an applied field of 0.5 T.
The data were corrected for both sample diamagnetism
(Pascalꢀs constants) and the diamagnetism of the sample
holder.
with the coupling of the unpaired electron with the three
similar but slightly inequivalent 14N nuclei. EPR spectra re-
corded in both first- (Figure 4) and second-derivative modes
(Figure S5 in the Supporting Information) were simulated to
determine the hyperfine coupling constants. By comparison
with previous studies,[6b,8,11a] the largest hfcc was assigned to
N1 (7.38 G), with the hfcc to N2 tentatively assigned to be
slightly larger than that to N3 (4.92 and 4.59 G, respective-
ly).
Atomic spin densities of the benzo-fused ring in radical 6
could not be experimentally determined since line broaden-
ing of the spectrum resulted in poor resolution of the hyper-
fine coupling to hydrogen atoms. Electron nuclear double
resonance (ENDOR) studies on the Blatter radical 1 per-
formed by Neugebauer and co-workers reported the magni-
tude and the sign of the hyperfine coupling constants of the
Radical 6 obeys Curie–Weiss behavior down to 5 K
(Figure 5, inset) with C=0.378 emuKmolꢀ1, which corre-
sponds to an S=1= radical with g=2.0071, consistent with
2
that measured in solution by EPR spectroscopy. The Weiss
constant (q= +4.72 K) is consistent with local ferromagnet-
ic interactions.
Attempts to model the magnetism of 6 using the Blea-
ney–Bowers equation[19] [Eq. (1)] for an isolated dimer of
S=1= radicals (based on the Hamiltonian h=ꢀ2J1S1S2)
ˆ ˆ
2
rapidly revealed that interdimer interactions were required
to model the low-temperature values of cT that surpass the
value of 0.5 emuKmolꢀ1 per monomer that is anticipated
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Ferromagnetic Interactions
FULL PAPER
exchange interactions in the chain is antiferromagnetic, then
there is a turning point in c versus T.[20] This corresponds to
a minimum when J1 <0<J2 and a maximum when J1 <J2 <0.
Both the position and value of the turning point in c(T) is
sensitive to the relative signs and magnitudes of J1 and J2.
Conversely, for an alternating ferromagnetic chain (J1 >J2 >
0) c versus T continues to increase monotonically with de-
creasing temperature. Curve-fitting of such functions is
likely to be rather insensitive to the values of J1 and J2, and
these two parameters are likely to be strongly correlated. To
probe the magnetism of 6 further, we investigated a regular
ferromagnetic chain of S=1= spins using Bakerꢀs high-tem-
2
perature series expansion [Eq. (3)][21] to estimate the mean
value of J:
Figure 5. Temperature dependence of cT for radical 6. The dashed line
represents the best fit to the 1D ferromagnetic linear chain model (g=
2.0071, J= +4.2 cmꢀ1), the solid line represents the corresponding fit to
Ng2b2
4kT
2=3
ð3Þ
c ¼
½N=Dꢂ
an isolated S=1= dimer with g=2.0071 and J1 = +6.6 cmꢀ1, illustrating
2
in which N=1+5.7979916x+16.902653x2 +29.376885x3 +
the requirement for a ferromagnetic interdimer exchange term. Inset:
Curie–Weiss behavior in the 5–300 K region (C=0.378 emuKmolꢀ1, q=
+4.72 K).
29.832959x4 +14.036918x5
and D=1+2.7979916x+
7.008678x2 +8.6538644x3 +4.5743114x4, and x=J/2kT.
Fitting the magnetic data to Equation (3) yielded J= +
4.2 cmꢀ1 in good agreement with the mean value estimated
from the dimer model with interdimer interactions
(+3.9 cmꢀ1). By applying the ratio of J1/J2 from the dimer
model to the one-dimensional chain, we determined estimat-
ed J1 and J2 values along the chain direction of +7.12 and
+1.28 cmꢀ1, respectively.
for a ferromagnetic interaction between S=1= spins at tem-
2
peratures below 19 K.
Ng2b2
1
ð1Þ
c ¼
kðT ꢀ qÞ ½3 þ expðꢀ2J1=kTÞꢂ
Inclusion of a mean field term (q) to model interdimer fer-
romagnetic exchange provided a much improved fit with g=
2.0071 (fixed), J1 = +6.6 cmꢀ1, and q= +1.75 K, in which J1
is the intradimer interaction. Within the mean field approxi-
mation, the mean field term (q) correlates to the interdimer
interactions according to Equation (2):
Discussion
Although DFT studies on 6 based on a single-point calcula-
tion (UB3LYP/6-311+GACTHUNGTRENNG(U d,p)) performed on the X-ray ge-
ometry coordinates do not describe accurately the electronic
structure of the ground state, they confirm the p character
of the radical (Figure 6) with most of the orbital density
being confined on the benzotriazinyl ring.
q ¼ 2zJ2SðS þ 1Þ=3k
ð2Þ
By assuming the dominant magnetic exchange interaction
occurs through p*–p* interactions along the p-stacking di-
rection (z=2), we can estimate a mean interdimer interac-
tion, J2 of +1.2 cmꢀ1. The mean of the intra- and interdimer
exchange parameters +3.9 cmꢀ1 (+5.6 K) is consistent with
the observed Weiss constant. However, the similar magni-
tude of intra- and interdimer exchange interactions (J2/J1 =
0.18) suggests that the dimer model does not adequately
represent the behavior of 6. Given the inadequacies of the
dimer model, we sought to model the behavior of 6 based
on a one-dimensional chain, which corresponds to the p-
stacking direction in which orbital overlap and hence mag-
netic exchange between radicals is likely to be most effi-
cient. Whereas there are models for alternating antiferro-
Since the SOMO of 6 is of p character, we anticipate the
strongest SOMO–SOMO interactions to correspond to the
p-stacking direction, that is, parallel to the b axis. The crys-
magnetically coupled chains (J1 <J2 <0) of S=1= ions and
2
alternating ferro-/antiferromagnetically coupled chains (J1 <
0<J2), there is a dearth of literature that describes the mag-
netism of alternating ferromagnetically coupled chains (J1 >
Figure 6. Singly occupied molecular orbital of
UB3LYP/6-311+G(d,p) level of theory.
6 calculated at the
J2 >0) for S=1= ions. In cases in which at least one of the
ACHTUNGTRENNUNG
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P. A. Koutentis et al.
tal structure reveals two types of radical···radical contacts
parallel to the stacking direction (Figure 2). We define the
staggered radical pair within the stack as dimer II–III and
the near-eclipsed pair as dimer I–II, thus leading to two dif-
ferent exchange parameters J1 and J2 along the stacking di-
rection. Normally, the magnitude of the exchange interac-
tion can be estimated by computing the energy of the triplet
and broken-symmetry singlet for pairs of radicals.[22] Howev-
er, this relies on a basis set and functional that accurately re-
produce the spin-density distribution of the isolated radical.
Whereas the current studies confirm the p* nature of the
singly occupied orbital, significant discrepancies between
the calculated and estimated (from EPR) spin-density distri-
butions caution against such an approach being reliable in
this case. Such problems with accurately reproducing the
spin distribution in these triazinyls have been noted previ-
ously and indicate the requirement for a multiconfigurational
approach.[8] Such approaches are significantly more compu-
tationally expensive, and examining a pair of radicals is cur-
rently prohibitive. Instead we have applied a more qualita-
tive approach to understand the behavior of 6.
Figure 7. SOMO orbitals of radical pairs I–II and II–III for radical 6 cal-
culated at the UB3LYP/6-311+GACTHUNTGRNEUNG(d,p) level of theory.
pair is substantially closer to eclipsed (f2 =15.78), the inter-
plane separation is simultaneously increased (3.52 versus
3.48 ꢁ in dimer II–III), which leads to reduced orbital over-
lap. As a consequence, we assign this second interaction to
the weaker ferromagnetic interaction, +1.28 cmꢀ1).
According to the McConnell-I model,[23] atoms with posi-
tive spin density are exchange-coupled to atoms with nega-
tive spin density in neighboring molecules to give ferromag-
AB
ij
netic coupling (HAB =ꢀSA·SBSJ 1iA1Bj ). In 6, the eclipsed
Conclusion
centrosymmetric radical pair I–II leads to the triazinyl moi-
eties of one radical located above the benzo-fused ring of
the other radical and vice versa. Since the spin density on
the triazinyl ring is predominantly positive but negative on
the benzo-fused ring (Table 1), this arrangement is anticipat-
ed to favor a ferromagnetic interaction.
The crystal structure and the magnetic properties of 1,3-di-
phenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl (6)
have been investigated. The radicals p-stack in one-dimen-
sional columns along the b axis and comprise slipped radical
pairs (I–II and II–III) with alternate short and long interpla-
nar distances. Magnetic susceptibility measurements reveal
the existence of weak ferromagnetic interactions within the
1D stacks. Spin-density distributions determined by EPR
spectroscopy, SOMO orbitals computed using DFT methods,
and a magneto-structural model have allowed us to correlate
the estimated exchange interactions to different dimer pairs
within the distorted p stack; J2 = +1.28 cmꢀ1 and J1 = +
7.12 cmꢀ1, which correspond to the radical pairs I–II and II–
III, respectively. The slippage of radicals along the stacking
direction causes the orthogonality of the SOMO orbitals of
the radical pairs. Elegant studies by Oakley and co-workers
on heavier p-block radicals have shown that the nature of
the magnetic exchange along the p-stacking direction is sen-
sitive to the degree of slippage and can be controlled by
fine-tuning the steric properties of the substituents.[3c] Fur-
ther modifications to the benzotriazinyl framework are in
progress to optimize both the intradimer separation and the
degree of slippage.
In the molecular orbital model,[24] the exchange coupling
interaction (JAB =2kij +4cijSij ) is proportional to the size of
2
the overlap between SOMO orbitals (Sij), which depends on
the distance and orientation of the interacting radicals.
When the two interacting SOMO orbitals are orthogonal to
each other, Sij is zero and the exchange coupling interaction
is ferromagnetic. A qualitative analysis of the SOMO orbi-
tals of radical pairs I–II and II–III of radical 6 indicates that
in radical pair II–III the orthogonality is more pronounced
due to the greater slippage of the radicals (f1 =45.98)
(Figure 7).
The benzotriazinyl moiety, which bears most of the orbital
density (and hence spin density), is located on top of the ad-
jacent thien-2-yl substituent that bears little orbital density.
The radical pair II–III of radical 6 has the same interplanar
distance and slippage angle (3.48 ꢁ, f1 =45.98) as the 1D p-
stacked molecules of radical 4 (3.46 ꢁ and f=468). The
crystal packing of radical 4 is comprised exclusively of these
slipped p-stacked molecules that interact between them, and
that interaction was ferromagnetic (J= +6.90 cmꢀ1),[8] which
is not dissimilar to that estimated from our analysis of the
magnetic susceptibility of 6 (+7.12 cmꢀ1). The radical pair
I–II is less slipped than II–III; the commencement of some
overlap of regions of positive spin density may lead to
a weakening of the ferromagnetic interaction or indeed ulti-
mately make it antiferromagnetic. However, whereas this
Experimental Section
Synthetic procedures: The synthesis and characterization of radical 6 can
be found in the Supporting Information.
Instrumental analyses: Cyclic voltammetry (CV) measurements were per-
formed using a Princeton Applied Research Potentiostat/Galvanostat
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Ferromagnetic Interactions
FULL PAPER
263A apparatus. The concentration of the benzotriazinyl radical used was
1 mm in CH2Cl2. A 0.1m solution of tetrabutylammonium tetrafluorobo-
rate (TBABF4) in CH2Cl2 was used as electrolyte. The reference elec-
trode was Ag/AgCl and the scan rate was 50 mVsꢀ1. Ferrocene was used
ertson, A. A. Leitch, K. Cvrkalj, D. J. T. Myles, R. W. Reed, P. A.
Dube, R. T. Oakley, J. Am. Chem. Soc. 2008, 130, 14791–14801;
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f) A. Rajca, Chem. Rev. 1994, 94, 871–893.
1
as an internal reference; the E = (ox) of ferrocene in this system was
2
0.352 V.[25] EPR measurements were carried out using a Bruker EMX
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meter in the temperature region of 5–300 K and in an applied magnetic
field of 5000 G. X-ray diffraction data were collected using a Nonius
Kappa-CCD diffractometer, equipped with a CCD area detector and
graphite-monochromated MoKa radiation (l=0.71073 ꢁ).
A suitable
crystal (black plate of dimensions 0.32ꢃ0.18ꢃ0.10 mm) was attached to
a glass fiber using paratone-N oil and transferred to a goniometer where
it was cooled to 180(2) K for data collection using an Oxford Instruments
cryostream. Unit-cell dimensions were determined and refined using
17991 (1.02<q<27.488), reflections. An empirical absorption correction
was applied using a multiscan method based on symmetry-related meas-
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refined on F2 using full-matrix least squares using SHELXL97.[28] Pro-
grams used: HKL Denzo and Scalepack for cell refinement and data re-
duction[29a] and MERCURY[29b] for molecular graphics. The non-hydro-
gen atoms were treated anisotropically, whereas the hydrogen atoms
were placed in calculated ideal positions and refined by using a riding
model. The thienyl group was found to be disordered over two positions.
The site occupancy factors (sofꢀs) of the two orientations were deter-
mined by isotropic refinement with common Uiso for chemically equiva-
lent atoms in each fragment to afford a ratio of 0.493:0.507. Subsequent
anisotropic refinement of the thienyl groups in the latter stages of refine-
ment used fixed sofꢀs of 0.5 for the two orientations. Unit-cell data and
structure refinement details are listed in Table S1 in the Supporting Infor-
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Acknowledgements
We thank the University of Cyprus (medium-sized grants), the Cyprus
Research Promotion Foundation (grant no. YGEIA/BIOS/0308ACTHNUGTRNEUNG(BIE)/13)
and the following organizations in Cyprus for generous donations of
chemicals and glassware: the State General Laboratory, the Agricultural
Research Institute, the Ministry of Agriculture and Biotronics Ltd. Fur-
thermore, we thank the A.G. Leventis Foundation for helping to establish
the NMR spectroscopic facility in the University of Cyprus. We thank
Dr. John Davies (University of Cambridge) for the collection of the crys-
tallographic data.
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Received: January 23, 2012
Revised: March 8, 2012
Published online: && &&, 0000
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Ferromagnetic Interactions
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Slippery characters: The 7-(thien-2-
yl)benzotriazinyl radical packs in 1D
columns with alternate short and long
interplanar distances. Slippage of radi-
cals along the stacking direction causes
the near orthogonality of their singly
occupied molecular orbitals and there-
fore leads to ferromagnetic interac-
tions between them (see figure).
Magnetic Properties
C. P. Constantinides, P. A. Koutentis,*
J. M. Rawson ................. &&&& — &&&&
Ferromagnetic Interactions in a 1D
Alternating Linear Chain of p-Stacked
1,3-Diphenyl-7-(thien-2-yl)-1,4-
dihydro-1,2,4-benzotriazin-4-yl
AHCTUNGTRENNRGUN adicals
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