Bimodal association of a bis-1,2,3-dithiazolyl radical

Article abstract of DOI:10.1039/b517098b

The N-ethyl pyrazine-bridged bis-1,2,3-dithiazolyl radical 2 (R₁ = Et) associates at room temperature as a C-C bonded σ-dimer which, on heating, converts to a laterally S-S σ-bonded structure. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b517098b

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Bimodal association of a bis-1,2,3-dithiazolyl radical
Alicea A. Leitch,a Courtney E. McKenzie,b Richard T. Oakley,*a Robert W. Reed,a John F. Richardsonb and
Lenora D. Sawyera
Received (in Cambridge, UK) 5th December 2005, Accepted 16th January 2006
First published as an Advance Article on the web 25th January 2006
DOI: 10.1039/b517098b
The N-ethyl pyrazine-bridged bis-1,2,3-dithiazolyl radical 2
(R1 = Et) associates at room temperature as a C–C bonded
s-dimer which, on heating, converts to a laterally S–S s-bonded
structure.
The potential applications of heterocyclic thiazyl radicals in the
design of functional electronic1 and magnetic materials2 are driving
the exploration for new stable radical systems.3 In recent years we
have characterized a number of N-alkylated pyridine-bridged bis-
1,2,3-dithiazolyls 1.4,5 These resonance stabilized radicals enjoy low
on-site Coulomb potentials, and as such serve as building blocks
for single component molecular conductors. However, while the
presence of the beltline substituents R1 and R2 helps suppress
cofacial dimerization, their buffering action also militates against
the close intermolecular interactions necessary for high bandwidth,
a critical requirement for good conductivity. In search of higher
bandwidth materials we have developed a preparative route to the
related N-alkylated pyrazine-bridged bis-1,2,3-dithiazolyls 2. The
Scheme 1
ethyl derivative 2 (R1 = Et) described herein displays a rare form of
polymorphism, crystallizing as two very different dimers, the C–C
Conversion of 3 to the 3,5-bis-thiocyanate 4, and reduction of the
latter with tributylphosphine generates the phosphinimine-dithiol
5, which undergoes double cyclocondensation with thionyl
chloride to afford the protonated chloride [2][Cl] (R1 = H).
Treatment of this black, insoluble chloride salt with gallium
trichloride in MeCN produces a deep blue solution of the
corresponding gallate, which can be deprotonated with Proton
Sponge to yield the zwitterion 6. Subsequent alkylation of 6 with
triethyloxonium tetrafluoroborate provides the desired N-ethylated
salt [2][BF4] (R1 = Et), which crystallizes from acetonitrile as deep
red flakes, lmax (MeCN) 623 nm, log e = 4.5.{
s-bonded structure a-[2]2 and the laterally S–S s-bonded variant
b-[2]2 (Chart 1).
The general synthetic sequence to 2 (Scheme 1) starts from 2,6-
diaminopyrazine 3, which is conveniently prepared by reduction of
the corresponding diazide6 with iron in aqueous acetic acid.
Cyclic voltammetry on solutions of [2][BF4] (R1 = Et) in MeCN
(Pt electrodes, Bu4NPF6 supporting electrolyte) reveals a reversible
0/+1 wave with EK = 20.029 V, a value 50–100 mV more cathodic
than that found for derivatives of 1, and well within the reduction
range of decamethylferrocene (DMFc).7 Accordingly chemical
reduction of [2][BF4] (R1 = Et) with DMFc in degassed MeCN
leads to precipitation of the radical dimer a-[2]2 (R1 = Et) as a
brown crystalline solid. While the dimer is diamagnetic (by EPR)
in the solid state, it dissolves in CH2Cl2 to afford fuchsia colored
solutions that display a strong and persistent EPR signal with g =
2.0087. The spectrum (Fig. 1) has been simulated in terms of the
expected radical 2 (R1 = Et); assignment of the experimental
hyperfine splitting (hfs) constants was confirmed by comparison
with computed hfs values and spin density data obtained from a
B3LYP/6-31G** calculation.
Chart 1
aDepartment of Chemistry, University of Waterloo, Waterloo, Ontario,
N2L 3G1, Canada. E-mail: oakley@uwaterloo.ca
bDepartment of Chemistry, University of Louisville, Louisville,
Kentucky, 40292, USA
Translucent amber blocks of a-[2]2 suitable for X-ray work can
be grown by vacuum sublimation at 1024 Torr down a
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Fig. 1 Observed (CH2Cl2) and simulated EPR spectrum (SW = 3 mT) of
2 (R1 = Et). Experimentally derived hfs constants are aN = 0.313 (2N),
0.267 (1N), 0.073 mT, aH = 0.025 (2H) mT. Computed (B3LYP/6-31G**)
spin densities and hfs constants (in parentheses) are also shown.
temperature gradient from 100 uC to 50 uC. At the same time,
however, thin, jet black blades of b-[2]2 are also produced. The
relative proportions of the two phases are sensitive to temperature.
Thus, at more elevated temperatures, i.e., a gradient from 110 uC to
60 uC, the b-phase is favored. Crystals of the two phases can be
separated manually, and the b-phase can be generated almost
exclusively by resublimation of a mixture of the two. Like the
a-phase, crystals of b-[2]2 dissolve in CH2Cl2 to afford fuchsia
colored solutionswhich exhibit thecharacteristicEPR spectrumof2.
Crystals of a-[2]2 (R1 = Et) belong to the monoclinic space
group P21/n, and consist of centrosymmetric pairs of radicals
Fig. 3 Packing of a-[2]2 (R1 = Et), with interdimer contacts: d1 =
˚
3.432(1), d2 = 3.534(1), d3 = 3.317(1), d4 = 3.140(2), d5 = 2.959(2) A.
˚
linked by a C3–C39 s-bond (Fig. 2).{ At 1.607(4) A, this bond is
3
3
8
˚
significantly longer than a standard sp –sp C–C linkage (1.54 A),
and comparable to those found in other strained systems in which
a s-bond forms at the expense of a p-system.9 The concomitant
rehybridization at C3 causes a severe distortion of the otherwise
planar heterocycle into a butterfly-like conformation, and the
internal bond lengths become localized into a pattern consistent
with the valence bond formulation shown in Chart 1.
Fig. 4 The radical dimer b-[2]2 (R1 = Et), with atom numbering. Selected
…
˚
distances: S4–S49 = 2.168(4), S3 S4 = 2.817(4), C3–S3 = 1.641(7) A.
exocyclic thione group, the sulfur atom of which (S3) develops a
˚
long range (2.817(4) A) hypervalent interaction with S4. This mode
Inspection of the packing of the dimers in the unit cell (Fig. 3)
indicates that the molecules are laced together by a web of
of dimerization was recently observed for mixed bis-1,2,3-
thiaselenazolyls,10 but to date has never been seen for any 1,2,3-
dithiazolyl radical. The dimers are tightly packed into intersecting
slipped p-stacked arrays (Fig. 5), a motif that affords a series of
…
…
intermolecular S S9 (d1–d3) and S N9 (d4, d5) contacts. These
interactions play a role in stabilizing the dimers with respect to
dissociation.
…
…
close intermolecular S S9 (d1) and S N9 (d2, d3) contacts. Like
the C–C bonded a-dimer, this S–S linked b-dimer is diamagnetic in
the solid state (by EPR). It behaves as a semiconductor, with a
room temperature conductivity sRT y 1027 S/cm.
Crystals of b-[2]2 (R1 = Et) belong to the monoclinic space
group P21/c, and consist of centrosymmetric dimers possessing a
lateral S4–S49 s-bond (Fig. 4). The nearly coplanar association of
radicals is accompanied by ring opening and formation of an
To date the known examples of 1,2,3-dithiazolyl dimers
associate cofacially through their p-systems, and are linked via
11
…
˚
long (> 3.0 A) transannular S S contacts. The observation of
these new modes of dimerization is therefore unusual, but the
Fig. 2 The radical dimer a-[2]2 (R1 = Et), with atom numbering. Selected
˚
distances: C3–C39, 1.607(4); N2–C1, 1.289(3); N4–C4, 1.276(3) A.
Fig. 5 Intersecting slipped p-stacks of b-[2]2 (R1 = Et), with inter-stack
˚
contacts: d1 = 3.226(3), d2 = 2.862(6), d3 = 3.011(6) A.
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{ Crystal data at 298(2) K for a-[2]2 (R1 = Et): C12H10N8S8, M = 522.84,
˚
space group P21/n, a = 9.332(2), b = 7.4076(16), c = 14.207(3) A, b =
;
128 parameters were refined using 2203 unique reflections to give R =
100.594(3)u, V = 965.4(4) A , Z = 2, Dcalcd = 1.798 g cm23, m = 0.944 mm21
3
˚
0.0382 and Rw = 0.0899. Crystal data at 298(2) K for b-[2]2 (R1 = Et):
C12H10N8S8, M = 522.84, space group P21/c, a = 4.5411(15), b = 7.583(2),
3
˚
˚
c = 27.391(9) A, b = 94.050(5)u, V = 940.9(5) A , Z = 2, Dcalcd
=
1.845 g cm23, m = 0.969 mm21; 128 parameters were refined using 2000
unique reflections to give R = 0.0718 and Rw = 0.0913. CCDC 292072–
292073. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b517098b
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Fig. 6 B3LYP/6-31G** enthalpies of dimerization (per mole of dimer)
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nyls).12 In the present case the C–C bonded a-dimer appears to be
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…
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…
S
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{ Satisfactory chemical analyses (for C, H and N) were obtained for
compounds 4, 5, [2][BF4] (R1 = Et) and b-[2]2 (R1 = Et).
1090 | Chem. Commun., 2006, 1088–1090
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