1352
Quinoxaline-1,2,3-dithiazolyls — Synthesis, EPR
characterization, and redox chemistry
A. Wallace Cordes, James R. Mingie, Richard T. Oakley, Robert W. Reed, and
Hongzhou Zhang
Abstract: Oxidation of quinoxalineaminothiol with SCl2 or S2Cl2/Cl2 affords a series of compounds based on the quinoxaline-1,2,3-dithiazole framework QDTA. Under highly oxidizing conditions, the 1,2,3-dithiazolyl ring is opened to afford the acyclic dichlorosulfimino-sulfenyl chlorides Clx-QDTA-Cl3 (x = 0, 1, 2). Reduction of these “trichloro”
compounds leads to ring closure. For x = 2, reduction using S2Cl2 affords the dithiazolylium chloride [Cl2-QDTA][Cl].
For all values of x, reduction with iodide ion (3 mol equiv) affords the corresponding dithiazolyl radical [Clx-QDTA].
The radicals can be isolated in good yield in crude form, but attempts to purify them by vacuum sublimation lead to
thermal degradation. The radicals have nonetheless been fully characterized by EPR spectroscopy, and the assignments
of the observed hyperfine coupling constants cross-matched with those obtained by computation at the B3LYP/6-31G**
level. The structures of the trichloro compounds Clx-QDTA-Cl3 (x = 1, 2) have been confirmed by X-ray crystallography. Crystal data: Cl-QDTA-Cl3, monoclinic, space group C2/c, a = 30.561(5) Å, b = 4.9764(9) Å, c = 22.247(4) Å,
b = 131.822(14)°, V = 2521.4(8) Å3, Z = 8, R(F) = 0.043, and Rw(F) [I ³ s (I)] = 0.049; Cl2-QDTA-Cl3, orthorhombic,
space group Pnma, a = 18.627(12) Å, b = 6.848(4) Å, c = 10.926(7) Å, V = 1393.7(15) Å3, Z = 4, R(F) = 0.047, and
Rw(F) [I ³ 3s(I)] = 0.060.
Key words: thiazyl radicals, molecular conductors, EPR spectroscopy, quinoxaline, DFT calculations.
Résumé : L’oxydation du quinoxalineaminothiol avec du SCl2 ou du S2Cl2/Cl2 conduit à la formation d’une série de
produits dérivés du squelette quinoxaline-1,2,3-dithiazole (QDTA). Dans des conditions d’oxydation extrêmes, le cycle
1,2,3-dithiazolyle s’ouvre pour conduire à la formation de chlorures de dichlorosulfimine-sulfényles, Clx-QDTA-Cl3 (x =
0, 1, 2). La réduction de ces composés “trichloro” conduit à une fermeture de cycle. Lorsque x = 2, la réduction à
l’aide de S2Cl2 conduit à la formation du chlorure de dithiazolylium [Cl2-QDTA][Cl]. Pour toutes les valeurs de x, la
réduction à l’aide de l’ion iodure (trois équivalents) conduit à la formation du radical dithiazolyle correspondant
[Clx-QDTA]. Les radicaux ont été isolés avec de bons rendements à l’état brut, mais les essais en vue de leur purification par sublimation sous vide à conduit à leur dégradation thermique. On a toutefois pu faire une caractérisation complète des radicaux par spectroscopie RPE et les attributions des constantes de couplage hyperfins observées
correspondent bien à celles qui peuvent être faites sur la base de calculs au niveau B3LYP/6-31G**. Les structures des
composés Clx-QDTA-Cl3 (x = 1, 2) ont été confirmées par diffraction des rayons X. Les données cristallographiques:
Cl-QDTA-Cl3, monoclinique, groupe d’espace C2/c, a = 30,561(5) Å, b = 4,9764(9) Å et c = 22,247(4) Å, b =
131,822(14)°, V = 2521,4(8) Å3, Z = 8, R(F) = 0,043 et Rw(F) [I ³ s(I)]] = 0,049; Cl2-QDTA-Cl3, orthorhombique,
groupe d’espace Pnma, a = 18,627(12) Å, b = 6,848(4) Å et c = 10,926(7) Å, V = 1393,7(15) Å3, Z = 4, R(F) = 0,047
et Rw(F) [I ³ 3s(I)] = 0,060.
Mots clés : radicaux thiazyles, conducteurs, spectroscopie RPE, quinoxaline, calculs DFT.
[Traduit par la Rédaction]
Cordes et al.
1359
Introduction
During the last decade, research into the design of neutral
radical conductors has developed along two slightly different
paths, one searching for materials comprised of purely carReceived April 23, 2001. Published on the NRC Research Press
Web site at http://canjchem.nrc.ca on September 17, 2001.
A.W. Cordes. Department of Chemistry and Biochemistry,
University of Arkansas, Fayetteville, AR 72701, U.S.A.
J.R. Mingie, R.T. Oakley,1 R. Reed, and H. Zhang.
Department of Chemistry, University of Waterloo, Waterloo,
ON N2L 3G1, Canada.
1
Corresponding author (telephone: (519) 888-4582; fax: (519)
746-0435; e-mail: [email protected].).
Can. J. Chem. 79: 1352–1359 (2001)
bon-based p-frameworks, the other exploring radicals with
heterocyclic (non-carbon based) systems. The two approaches share common challenges, including the requirement for materials in which: (i) solid state dimerization is
suppressed; and (ii) intermolecular charge transfer is energetically facile. Several variations of the phenalenyl radical,
the archetypal model behind the organic radical conductor
concept (1), have recently been developed to address these
issues, with encouraging results (2–4). In the heterocyclic
arena several structural databases have been established,
first on 1,2,3,5-dithiadiazolyls (DTDAs) 1 (5), and more recently on 1,3,2- and 1,2,3-dithiazolyls (DTAs) 2 (6–9) and 3
(10–12). Cyclic thioaminyls have also been pursued (13).
While the initial focus of this work was on how to inhibit
dimerization, the importance of the energetics of charge
DOI: 10.1139/cjc-79-9-1352
© 2001 NRC Canada
Cordes et al.
1353
Table 1. Computed (B3LYP/6-31G**) ion energetics (in eV) for
radicals 1–7.
Compound
IP
EA
DHdisp
c
1
2
3
4
5
6
7
7.82
6.77
7.01
7.60
7.81
6.95
7.24
1.02
–0.41
0.34
2.14
2.57
1.33
1.93
6.81
7.18
6.67
5.46
5.23
5.63
5.32
4.42
3.15
3.68
4.87
5.19
4.14
4.60
transfer is now well-recognized (14), and efforts to understand and control molecular ionization potentials (IP), electron affinities (EA), and the corresponding solution redox
potentials (15) are being vigorously pursued.
With this latter end in mind we prepared and characterized the 1,3,2- and 1,2,3-dithiazolyls 4 (8) and 5 (11). The
thiadiazolopyrazine (TDP) substituent on these radicals is
extremely electron withdrawing, and causes an increase in
both the molecular IP and EA relative to those of simple
(monocyclic) DTAs. More importantly the substitution
causes a dramatic decrease in the disproportionation
enthalpy DHdisp (DHdisp = IP – EA) of the radical and, insofar
as the gas-phase DHdisp can be related to the coulombic barrier to charge transfer in the solid state, the observed trend is
encouraging. Structural analysis on 4 revealed that the radical does not dimerize (at room temperature), but the molecular spins remain isolated, and the material can be considered
as a Mott–Hubbard (16) insulator. By contrast, molecules of
5 dimerize in the solid state, but magnetic and conductivity
measurements indicate that the free spins present as defects
serve as carriers of charge (11). As a continuation of this
work, i.e., the development of structure and (or) property relationships in heterocyclic radicals, we wished to compare
the two quinoxaline based systems 6 and 7. The 1,3,2compound 6 has been known for some time (17), although
its solid-state structure and magnetic properties were reported only recently (7). By contrast the 1,2,3-compound 7
was hitherto unknown. Herein, we report the synthesis and
characterization of a series of derivatives based on this
“1,2,3-QDTA” skeleton.
Scheme 1.
Results and discussion
Ion energetics
As noted above, one of the crucial factors in the design of
neutral radical conductors is the requirement for materials
with low disproportionation energies (DHdisp) (8, 11, 14, 15).
Within the parlance of Pearson (18), we seek soft radicals,
i.e., molecules which exhibit a low absolute hardness parameter h (where h = (IP – EA)/2). At the outset of this work,
we set about exploring by computation, within an ab initio
context, the ion energetics of series of 1,3,2- and 1,2,3dithiazolyls. In Table 1 we provide the computed (B3LYP/631G**) IP and EA values for a range of such compounds.
The results refine earlier (largely MNDO) estimates, and
confirm that a simple 1,2,3,5-DTDA radical 1 has a lower
DHdisp value, it is softer than a simple 1,3,2- or 1,2,3-DTA 2
or 3. While on this basis both 2 and 3 systems appear to be
less effective than 1, the properties of the latter cannot be
modified appreciably by substitution (the radical SOMO is
nodal at the substituted carbon). By contrast the fusion of
electron-withdrawing groups onto either of the DTA frames
2 or 3 leads to marked changes in ion energetics; both the
IPs and EAs are increased. Accordingly the radicals become
far more electronegative, i.e., c = (IP + EA)/2 increases, but
because the change in EA outpaces the change in IP, the radical disproportionation energy DHdisp (DHdisp= IP – EA) is
decreased; in essence the radicals become softer. The most
powerful electron sink that we have found is the TDP group,
but the radicals so produced (4 and 5) are extremely
electronegative. The quinoxaline residue affords slightly
harder, but less electronegative systems, i.e., 6 and 7. Having
fully characterized the electrochemical properties, EPR, and
solid-state properties of 4, 5, and 6 (1,3,2-QDTA), radical 7
(1,2,3-QDTA), therefore, became a desirable target for synthesis.
Synthesis
Quinoxalineaminothiol (QAT), the preferred starting material for the synthesis of QDTA 7, is potentially accessible
by a number of routes (Scheme 1) (19). The first involves
the conversion of dichloroquinoxaline QCl2 to the corresponding aminochloro derivative QACl, followed by thiation
of the latter with sodium sulfide. However, while the first
step is straightforward, the second provides only a low yield
(10%) of QAT. The alternative pathway, involving the amination
© 2001 NRC Canada
1354
Scheme 2.
of the intermediate dithiol QT2, which is easily accessible
from QCl2, affords QAT in much higher overall yields.
Cyclization of QAT to a 1,2,3-dithiazole by treatment
with sulfur chlorides proved to be a far more complex process than we had anticipated. Elaboration of the overall sequence of events is facilitated by reference to the manifold
of oxidation states and compounds illustrated in Scheme 2.2
Broadly speaking the nature of the product is a function of
the oxidizing power of the sulfur chloride employed. Based
on our experience with 5, we were aware that sulfur
monochloride would only produce an imide, e.g., QDTAH.
It was therefore not surprising to find that reaction of QAT
with S2Cl2, in stoichiometric amounts or in excess, afforded
an insoluble orange powder exhibiting u(NH) stretching
bands in its IR spectrum. Sulfur dichloride proved to be a
stronger but less selective oxidant. Thus, when heated at reflux in acetonitrile with excess SCl2, QAT was oxidized directly to the open chain dichlorosulfimino-chlorosulfenyl or
“trichloro” derivative Cl-QDTA-Cl3. Attempts to prevent oxidation of the benzene ring were, for a long time, unsuccessful. Eventually, and somewhat to our surprise, we discovered
that treatment of QAT with chlorine gas, followed by the addition of excess sulfur monochloride (at room temperature),
2
Can. J. Chem. Vol. 79, 2001
gave rise to the precipitation of yellow crystals of QDTACl3. Attempts to recrystallize this material for analytical purposes were always thwarted by its partial conversion, upon
dissolution in CH3CN, into Cl-QDTA-Cl3. That the quinoxaline ring was unsubstituted was confirmed by hydrolysis of
the compound and recovery of QAT.
Cl-QDTA-Cl3 is relatively resistant to further oxidation;
indeed, it is conveniently recrystallized from acetonitrile saturated with chlorine gas. However, when Cl-QDTA-Cl3 is
heated at reflux in acetonitrile with excess sulfur
monochloride, a dark orange crystalline precipitate of the
salt [Cl2-QDTA][Cl] (as a CH3CN solvate) is produced. In
essence the open chain trichloro unit is closed to a
dithiazolylium cation, while the quinoxaline ring is further
chlorinated. [Cl2-QDTA][Cl] was then easily oxidized with
chlorine to the corresponding trichloro derivative Cl2QDTA-Cl3.
With the set of three trichloro compounds in hand, we
explored their reactions with reducing agents. While Cl2QDTA-Cl3 was easily reduced with a stoichiometric
amount of iodide ion or excess S2Cl2 to Cl2-QDTA-Cl, reduction of Cl-QDTA-Cl3 required a more careful choice of
reducing agent. Eventually, we found that hexamethyldisilane reacts with Cl-QDTA-Cl3 to afford what we believe
to be to [Cl-QDTA][Cl]. However, attempts to recrystallize
this material led to its conversion to an insoluble solid exhibiting u(NH) stretching bands and which, upon hydrolysis, afforded the dichlorinated aminothiol Cl2-QAT. An analogous
sequence of events was encountered upon reduction of
QDTA-Cl3. We interpret these observations in terms of the
susceptibility of the dithiazolylium salts [QDTA][Cl] and
[Cl-QDTA][Cl] to rearrange, with chlorination of the quinoxaline ring, to an imide (Scheme 3).
In our attempts to generate the radical oxidation states
QDTA, Cl-QDTA, and Cl2-QDTA we explored a variety of
reducing agents (KI, Bu4NI, Ph3Sb, Zn), solvents
(CH3CN, SO2), and starting oxidation states (Clx-QDTACl3 or [Clx-QDTA][Cl]). The most reliable method involved
the use of potassium iodide in CH3CN or SO2 (see Experimental section). All three radicals could be isolated in crude
form, as black crystalline solids, but purification for analytical purposes was extremely inefficient. Vacuum sublimation
of the products from the crude matrix, the preferred purification method for heterocyclic radicals, inevitably led to decomposition and low recovery of sublimed product (<10%).
Curiously, once isolated, the sublimed material could be
resublimed with little or no loss.
Crystal structures
The initial intent of this work was to characterize the
structure and transport properties of the 1,2,3-QDTA radical
in the solid state. However, the lability of this material, and
its chlorinated derivatives Clx-QDTA (x = 1, 2), militated
against the isolation of the material in sufficient quantity and
quality to allow single crystal X-ray analysis. Nonetheless
we were able to establish the crystal structures of the two
trichloro derviatives (Clx-QDTA-Cl3). These determinations
The trichloro compounds depicted in Scheme 2 are identified by means of general formulas, e.g., Clx-Q-Cl3; the degree of chlorination of
the quinoxaline residue is indicated by the value of x. The dithiazole derivatives are labelled [Clx-QDTA][Cl] (cationic state), [Clx-QDTA]
(radical state), [Clx-QDTAH] (imide state). The corresponding aminothiols are labelled Clx-QAT.
© 2001 NRC Canada
Cordes et al.
Scheme 3.
1355
Figs. 2–4. The spectra are extremely complex; in the case of
1,2,3-QDTA 7 itself, coupling is observed to all three nitrogens as well as all four protons. In the chlorinated derivatives, some simplification is achieved by virtue of a
reduction in the proton count, and 35/37Cl coupling is negligible. Nonetheless, extraction of hyperfine coupling required
full spectral simulation; these are also shown in Figs. 2–4.
The observed hyperfine coupling constants drawn from
these simulations are summarized in Table 4 along with
computed values at the B3LYP/6-31G** level; the latter
served as an aid in making assignments. The correspondence
between the observed and computed aN values is relatively
good (the latter being consistently about 15% higher), so
that absolute assignments are relatively unequivocal. Such is
not the case for the various aH values. While we have been
able, in all cases, to observe the expected number of 1H couplings, the computed aH values are several times larger than
those derived experimentally. Thus, while we are relatively
confident with the matches shown in Table 4 for H1 and H3,
those for H2 and H4 are less certain.
Experimental
confirmed the presence of the dichlorosulfimino-sulfenyl
chloride functional group in both compounds, and also established unequivocally the position(s) of chlorination on the
benzene ring in the QDTA framework. Crystal data for these
two compounds is provided in Table 23, and a summary of
pertinent intramolecular structural features is given in Table 3. ORTEP drawings of the two molecules, showing atom
numbering, are given in Fig. 1. In Cl-Q-Cl3, the quinoxaline
framework is planar to within 0.018 Å, with the N-1 and S-2
atoms lying, respectively, –0.058 and +0.016 Å from this
plane. In Cl2-Q-Cl3, all the atoms save Cl-1 lie on a crystallographic mirror plane. The structural features of the
dichlorosulfimino-sulfenyl chloride unit are very similar,
with a short (localized) N=S double bond, and mirror the
trends observed in compound 8 (11).
EPR spectra
The X-band EPR spectra of the three quinoxaline-based
1,2,3-DTA radicals, recorded at ambient temperature on
samples dissolved in oxygen-free CH2Cl2, are shown in
3
Quinoxalineaminothiol (QAT) was prepared by the conversion of commercially available (Aldrich) dichloroquinoxaline into quinoxalinedithiol (QT2) (19), followed by
the treatment of the latter with aqueous ammonia; the crude
product was purified by recrystallization from benzonitrile.
Sulfur monochloride, potassium iodide, and dichloroquinoxaline were obtained commercially (Aldrich), and used
as received. Chlorine gas and sulfur dioxide gas (Matheson)
were also used directly. The solvents dichloroethane (Fisher)
and acetonitrile (Fisher HPLC grade) were dried by distillation from P2O5. All reactions were carried out under an atmosphere of nitrogen. Melting points are uncorrected.
Elemental analyses were performed by MHW Laboratories,
Phoenix, Arizona. Infrared spectra were recorded (at 2 cm–1
resolution on Nujol mulls) on a Nicolet Avatar FT-IR spectrometer. Low resolution mass spectra (70 eV, EI, DEI and
CI, DCI, FAB) were run on a Finnigan 4500 quadrupole
mass spectrometer (at McMaster University) and a VG 7070
mass spectrometer (at the University of Waterloo).
Preparation of QDTA-Cl3
A gentle stream of chlorine gas was passed over a slurry
of QAT (1.0 g, 5.64 mmol) in 10 mL of acetonitrile held at
0°C (ice bath) for 10 min. Within 60 s of initiating the
chlorine flow, a solution of excess S2Cl2 (ca. 1.0 g) in 5 mL
of CH3CN was added dropwise to the mixture. After 5 min,
the chlorine flow was halted, and the flocculent yellow
solid filtered off under nitrogen, washed with 2 × 5 mL
acetonitrile saturated with chlorine, and dried in vacuo. The
crude (moisture-sensitive) product, QDTA-Cl3 (0.845 g,
2.70 mmol, 48%) was used directly in all subsequent reactions. Attempts to purify this compound by recrystallization
Copies of material on deposit may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A OS2, Canada. Tables of crystal data, data collection, refinement parameters, interatomic distances and angles, and atomic positional parameters reported in this paper have been deposited with the Cambridge Crystallographic Data
Centre. Copies of this data can be obtained, free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. (fax: 44-1223-336033 or e-mail: [email protected]).
© 2001 NRC Canada
1356
Can. J. Chem. Vol. 79, 2001
Table 2. Crystal, data collection, and refinement parameters.
Formula
FW
a (Å)
b (Å)
c (Å)
b (deg)
V (Å3)
D(calcd) (g cm–3)
Space group
Z
l (Å)
Temperature (K)
Linear abs. coeff. (mm–1)
R(F), Rw(F)a [I > ns(I)]
Cl-QDTA-Cl3
Cl2-QDTA-Cl3
C8H3Cl4N3S2
347.06
30.561(5)
4.9764(9)
22.247(4)
131.822(14)
2521.4(8)
1.83
C2/c
8
0.7107
293
1.24
0.043, 0.049 (n = 1)
C8H2Cl5N3S2
381.51
18.627(12)
6.848(4)
10.926(7)
—
1393.7(15)
1.82
Pnma
4
0.7107
293
1.32
0.047, 0.060 (n = 3)
R = [S**Fo* – *Fc**]/[S*Fo*]; Rw = {[Sw**Fo* – *Fc**2]/[S(w*Fo*2)]}1/2.
a
Table 3. Summary of bond lengths (Å) in Cl-QDTA-Cl3, Cl2QDTA-Cl3, and 8.
(N)S—Cl
N—S
(S)N—C
(C)S—Cl
C—S
Cl-QDTA-Cl3
Cl2-QDTA-Cl3
8
2.09(3)a
1.498(4)
1.400(5)
1.993(2)
1.766(4)
2.091(3)
1.531(9)
1.394(13)
1.995(4)
1.719(10)
2.0784(11)
1.516(3)
1.403(5)
1.990(2)
1.754(4)
Fig. 1. ORTEP drawings (30% probability ellipsoids) of Cl-Q-Cl3
(above) and Cl2-Q-Cl3 (below). In Cl2-Q-Cl3, atoms Cl1 and Cl1*
are related by a mirror plane.
Note: esds are in parentheses.
a
Range over two values.
led to its partial conversion to Cl-QDTA-Cl3, but the integrity of the quinoxaline skeleton was confirmed by hydrolysis
of QDTA-Cl3 to QAT. IR (1600–400 cm–1): 1329 (s), 1292
(w), 1228 (m), 1161 (m), 1113 (m), 1017 (w), 903 (w), 805
(w), 754 (vs), 714 (w), 698 (w), 625 (vw), 593 (vw), 553
(vw), 539 (s), 458 (s).
Preparation of Cl-QDTA-Cl3
Excess (4 mL) of SCl2 was added to a slurry of QAT
(1.00 g, 5.64 mmol) in 30 mL of CH3CN at room temperature. The mixture was stirred for 1 h during which time it
changed color from red to yellow. The canary yellow precipitate was filtered off, washed with 2 × 10 mL portions of
CH3CN, and dried in vacuo; (1.3 g, 3.75 mmol, 66%). The
product Cl-QDTA-Cl3 (0.500 g, 1.44 mmol) was
recrystallized from acetonitrile containing 2 mL of SCl2 (to
enhance the solubility of the compound). On cooling, lustrous yellow needles were formed: dec > 85°C. IR (1600–
400 cm–1): 1599 (m), 1556 (w), 1525 (w), 1481 (m), 1426
(w), 1320 (s), 1280 (w), 1211 (m), 1145 (m), 1119 (s), 1067
(m), 1015 (m), 964 (w), 926 (m), 878 (m), 830 (s), 710 (m),
651 (w), 622 (m), 588 (m), 557 (m), 521 (w), 494 (w), 457
(m), 425 (m). Anal. calcd. for C8H3Cl4N3S2: C 27.68,
H 0.87, N 12.11; found: C 27.92, H trace, N 12.07.
Preparation of Cl2-QDTA-Cl3
Excess S2Cl2 (4 mL) was added to a slurry of QAT
(2.00 g, 11.3 mmol) in 60 mL of acetonitrile. The mixture
was warmed gently to produce a red precipitate. Chlorine
gas was then passed over the solid until a bright yellow pre-
cipitate was produced. Following the addition of more S2Cl2
(4 mL) the mixture was heated at reflux for 0.5 h. As the
mixture reached the boiling point the solid dissolved to give
© 2001 NRC Canada
Cordes et al.
Fig. 2. X-band EPR spectrum (above) of 1,2,3-QDTA 7, g =
2.0079, in CH2Cl2, and simulation (below) with L:G = 0.01, LW =
0.058 mT. Derived coupling constants are provided in Table 4.
a dark red-brown solution. The mixture was then cooled to
room temperature and exposed to a flow of chlorine gas to
form the final product. The yellow precipitate was filtered
off under nitrogen, washed with chlorinated acetonitrile, and
dried in vacuo. The product Cl2-QDTA-Cl3 (2.83 g,
7.42 mmol, 66%) was recrystallized from chlorinated
acetonitrile a yellow needles, mp 120°C. IR (1600–400 cm–1):
1590 (m), 1529 (w), 1334 (w), 1304 (s), 1281 (w), 1216 (m),
1191 (m), 1137 (s), 1082 (m), 1029 (m), 864 (s), 807 (s),
785 (m), 676 (m), 634 (m), 586 (m), 577 (m), 534 (m), 468
(s), 433 (m). Anal. calcd. for C8H2Cl5N3S2: C 25.19, H 0.53,
N 11.01; found: C 25.30, H trace, N 10.93.
Preparation of [Cl2-QDTA][Cl]·CH3CN
Excess S2Cl2 (4 mL) was added to a slurry of Cl-Q-Cl3
(1.50 g, 4.32 mmol) in 50 mL of CH3CN. As the mixture
was heated to reflux the solid dissolved to give a dark redbrown solution. After half an hour at reflux the solution was
allowed to cool to room temperature. The orange brown
crystalline solid which precipitated on cooling was filtered
off, washed with 2 × 10 mL portions of CH3CN, and dried
in vacuo. The product [Cl2-QDTA][Cl]·CH3CN (1.25 g,
3.55 mmol, 82%) was recrystallized from acetonitrile containing a few drops of SCl2. Infrared and elemental analysis confirmed that the compound (dec > 85°C) was solvated with 1
mol of acetonitrile. MS m/z: 274 (M+, 100%). IR (1600–
400 cm–1): 2247 (w), 1661 (w), 1589 (m), 1537 (w), 1299
(w), 1276 (m), 1191 (m), 1091 (m), 1004 (m), 878 (m), 858
(m), 808 (w), 791 (w), 715 (m), 612 (m), 584 (w), 544 (w),
451(m). Anal. calcd. for C10H5Cl3N4S2: C 34.15, H 1.43, N
15.93; found: C 33.89, H 1.20, N 15.98.
Characterization data for Clx-QAT (x = 0, 1, 2)
Hydrolysis of any of the dithiazoles described above with
10% aq NaOH afforded an orange solution which, upon
treatment with glacial acetic acid, yielded a yellow precipitate of the corresponding quinoxaline aminothiol. These
compounds can be recrystalllized from hot benzonitrile as
yellow flakes. The following characterization data apply.
QAT (starting material): dec > 250°C. IR (cm–1): 3392 (s),
3274 (m), 3110 (w), 1636 (s), 1612 (w), 1592 (w), 1486 (m),
1136 (m), 1087 (s), 940 (w), 853 (m), 838 (w), 750 (w), 724
1357
Fig. 3. X-band EPR spectrum (above) of Cl-QDTA, g = 2.0082,
in CH2Cl2, and simulation (below) with L:G = 0.03, LW = 0.041
mT. Derived coupling constants are provided in Table 4.
(w), 604 (s), 444 (w). Anal. calcd. C8H7N3S: C 54.22,
H 3.98, N 23.71; found: C 54.16, H 4.13, N 23.62. Cl-QAT:
dec > 250°C. IR (cm–1): 3413 (m), 3273 (w), 3117 (m),
1625 (s), 1587 (m), 1416 (m), 1353 (m), 1334 (m),
1232 (w), 1134 (m), 1101 (s), 1077 (m), 929 (m), 857 (m),
81 (s), 666 (w), 628 (m), 479 (m), 456 (s). Anal. calcd. for
C8H6ClN3S: C 45.39, H 2.86, N 19.85; found: C 46.66,
H 3.10, N 19.96. Cl2-QAT: dec > 250°C. IR (cm–1): 3425
(m), 3286 (w), 3214 (w), 3117 (w), 1639 (w), 1599 (w),
1358 (m), 1241 (w), 1208 (w), 1110 (s), 975 (w), 846 (s),
681 (s), 657 (w), 587 (m), 541 (w), 439 (m). Anal. calcd. for
C8H5Cl2N3S: C 39.04, H 2.05, N 17.07; found: C 38.88,
H 1.94, N 16.96.
Generation of radicals Clx-QDTA (x = 0, 1, 2)
The same basic procedure was used for all three radicals.
Three mol equiv of finely powdered and oven dried (100°C)
potassium iodide were added to a slurry of Clx-Q-Cl3 (ca.
1.0 g) in 10 mL of acetonitrile at room temperature. The
mixture was then stirred for 20 min at room temperature,
and the black solid (a mixture of Clx-QDTA and KCl) was
filtered off, washed with cold acetonitrile, and dried in
vacuo. The radicals were separated from the KCl matrix by
gradient sublimation at 110–60°C per 1 × 10–2 torr (1 torr =
133.322 Pa). Overall recovery of the radicals was, however,
extremely low; yields were generally <10% based on the
amount of Clx-Q-Cl3 used.
QDTA: dec > 150°C. IR (1600–400 cm–1): 1582 (w), 1537
(vw), 1340 (w), 1323 (w), 1223 (m), 1140 (s), 1066 (m), 949
(m), 873 (w), 845 (w), 786 (w), 753 (s, br), 639 (w), 590
(w), 577 (w), 491 (w), 427 (w). Insufficient quantity of this
material was obtained in a pure (sublimed) form for elemental analysis; but the integrity of the quinoxaline skeleton was
confirmed by hydrolysis to QAT (vide supra). The radical itself was characterized by its EPR spectrum.
© 2001 NRC Canada
1358
Fig. 4. X-band EPR spectrum (above) of Cl2-QDTA, g = 2.0094,
in CH2Cl2, and simulation (below) with L/G = 0.01, LW = 0.028
mT. Derived coupling constants are provided in Table 4.
Can. J. Chem. Vol. 79, 2001
Table 4. Observed and calculateda hyperfine coupling constants
(in mT) in QDTA, Cl-QDTA, and Cl2-QDTA radicals.b
N1
N2
N3
H1
H2
H3
H4
Cl1c
Cl3c
QDTA
Cl-QDTA
Cl2-QDTA
0.653
0.301
0.006
0.137
0.070
0.218
0.004
0.639
0.309
0.040
0.145
0.072
0.618 (0.730)
0.308 (0.361)
0.039 (–0.061)
(0.760)
(0.355)
(–0.071)
(0.351)
(–0.286)
(0.510)
(–0.388)
(0.746)
(0.359)
(–0.067)
(0.356)
(–0.282)
0.21 (–0.398)
— (0.048)
0.72 (–0.287)
0.19 (–0.396)
— (0.023)
— (0.048)
a
The calculated (B3LYP/6-31G**) coupling constants are shown in
parentheses.
b
The atom numbering scheme is provided below.
c
Coupling constants refer to the 35Cl isotope.
Cl-QDTA: dec > 163°C. IR (1600–400 cm–1): 1589 (w),
1533 (w), 1343 (w), 1304 (w), 1278 (w), 1199 (w),
1137 (w), 1077 (m), 943 (w), 906 (m), 814 (m), 792 (m),
664 (m), 649 (m), 596 (m), 577 (m), 513 (m), 458 (w),
484 (m), 435 (m). Anal. calcd. for C8H3ClN3S2: C 39.92,
H 1.26, N 17.47; found: C 40.12, H 1.38, N 17.27.
Cl2-QDTA: dec > 158°C. EI-MS (m/z): 274 (M+, 100%),
228 ([M – NS]+, 34%), 210 (10%), 158 (31%). IR (1600–
400 cm–1): 1588 (w), 1527 (w), 1303 (m), 1277 (m),
1195 (m), 1087 (m), 966 (m), 922 (m), 860 (m), 782 (w),
656 (w), 607 (w), 575 (w), 486 (w). Anal. calcd. for
C8H2Cl2N3S2: C 34.92, H 0.73, N 15.27; found: C 35.12,
H 0.90, N 15.09.
X-ray measurements
X-ray data were collected at 293 K on ENRAF-Nonius
CAD-4 and Rigaku Mercury CCD diffractometers with
monochromated Mo Ka radiation. Crystals were mounted on
glass fibers with silicone and data were collected using q/2q
(Cl-QDTA-Cl3) and w (Cl2-QDTA-Cl3) scans. The structures
were solved using direct methods and refined by full-matrix
least-squares which minimized Sw(DF)2.
EPR spectra
X-band EPR spectra were recorded (on radicals dissolved
in degassed CH2Cl2) on a Bruker EMX-200 spectrometer;
hyperfine coupling constants were obtained by spectral simulation using Simfonia (20) and WinSim (21).
Lorentzian:Gaussian (L:G) ratios and line widths (LW) used
in the simulations are provided in the figure captions.
Density functional calculations
Adiabatic ionization potentials and electron affinities were
calculated by means of DSCF calculations run on Pentium II
workstations using the B3LYP-DFT method, as contained in
the Gaussian 98W suite of programs (22). Geometries were
optimized using the 6-31G** basis set, within the constraints
of Cs (planar) symmetry; frequency calculations were not
performed. Elsewhere (23), we have noted that the release of
planarity in 1,2,3-dithiazole anions can lead to S—N bond
cleavage (a prediction echoed by electrochemical experiments). The present electron affinity results should therefore
be viewed as estimates only.
Summary and conclusions
The development of cyclic and heterocyclic p-radicals
suitable for applications as conductive molecular materials
requires the pursuit of highly delocalized systems with favorable ion energetics, i.e., a low DHdisp. In this regard, the
1,2,3-QDTA systems investigated here represent appealing
targets. However, from a synthetic standpoint, the construction of a 1,2,3-dithiazole ring on a quinoxaline framework is
by no means trivial. The oxidizing conditions that successfully allow the synthesis of the ternary radical 5 by
cyclization of an aminothiol are, in the present case, sufficiently harsh to cause chlorination of the benzene residue of
the quinoxaline framework. This leads to a rich array of partially chlorinated derivatives, both cyclic and acyclic. We
have nonetheless been able to isolate and characterize, by
EPR spectroscopy, three 1,2,3-QDTA radicals. The EPR
spectra of these radicals reveal a high degree of spin
delocalization, similar to that observed in 5, and much more
so than observed in corresponding 1,3,2-system 6.
While all the QDTA radicals reported here are stable indefinitely at room temperature, attempts to purify them by
vacuum sublimation, and hence investigate their transport
properties, have been thwarted by their tendency to thermally decompose. We attribute this instability to the
delocalization of spin density onto the benzene residue, with
a consequent tendency for the radicals to associate via C—C
linkages, and thence rearrange. In the light of the present results, the thermal stability of 5 is remarkable, and empha© 2001 NRC Canada
Cordes et al.
sizes the importance of heteroatoms in suppressing radical
coupling.
Acknowledgements
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the State of Arkansas, for
financial support in the form of operating and equipment
grants. We also acknowledge the Arkansas Science and
Technology Authority for support of the X-ray facilities.
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© 2001 NRC Canada