Resonance-stabilized 1,2,3-dithiazolo-1,2,3-dithiazolyis as neutral π-radical conductors

Article abstract of DOI:10.1021/ja026118s

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]pyridine (CIBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:-5′,4′-e]pyridin- 2-ium triflates [ClBPR]-[OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G**) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by x-ray crystallography (at 293 K). All three structures consist of slipped π-stacks of undimerized radicals, with many close intermolecular S···S contacts. CIBPMe undergoes a phase transition at 93 K to a slightly modified slipped π-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hueckel band calculations. The room-temperature conductivities of ClBPR systems (σ_RT ≈ 10^-5 to 10^-6 S cm^-1), as well as the weak 1D ferromagnetism exhibited by CIBPMe, are interpreted in terms of weak intermolecular overlap along the π-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.

Full text of DOI:10.1021/ja026118s

Published on Web 07/17/2002
Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls as
Neutral π-Radical Conductors
Leanne Beer,1a Jaclyn L. Brusso, A. Wallace Cordes, Robert C. Haddon,1c
1a
1b
Mikhail E. Itkis, Kristin Kirschbaum, Douglas S. MacGregor,1a
Richard T. Oakley,*, A. Alan Pinkerton,1d and Robert W. Reed
1c
1d
1a
1a
Contribution from the Department of Chemistry, UniVersity of Waterloo, Waterloo, Ontario N2L
3
G1, Canada, Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille,
Arkansas 72701, Department of Chemistry and Center for Nanoscale Science and Engineering,
UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry,
UniVersity of Toledo, Toledo, Ohio 43606-3390
Received March 6, 2002
Abstract: Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]pyridine (ClBP)
with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]pyridin-2-ium triflates [ClBPR]-
[OTf] (R ) Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR
radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic
voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-
31G**) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to
charge transfer in the solid state. The crystal structures of ClBPR (R ) Me, Et, Pr) have been determined
by X-ray crystallography (at 293 K). All three structures consist of slipped π-stacks of undimerized radicals,
with many close intermolecular S‚‚‚S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly
modified slipped π-stack arrangement, the structure of which has also been established crystallographically
(at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the
results interpreted in light of extended H u¨ ckel band calculations. The room-temperature conductivities of
-5
-6
-1
ClBPR systems (σRT ≈ 10 to 10 S cm ), as well as the weak 1D ferromagnetism exhibited by ClBPMe,
are interpreted in terms of weak intermolecular overlap along the π-stacks. The latter is caused by slippage
of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine
ring.
Introduction
conductor (NRC) would function like atoms in an elemental
metal and, on the basis of a simple band model of electronic
5
For over 30 years the development of organic conductors,
both molecular and polymeric, has relied almost exclusively
on the use of charge transfer (CT) to generate charge carriers.
Accordingly, conductive systems have required two components,
i.e., a donor and an acceptor, although both can be incorporated
into a single molecule. Carriers can also be injected directly
structure, the bulk material should possess a half-filled energy
band, as in elemental sodium. There are, however, several
shortcomings to this model. First, any 1D half-filled energy band
is prone to a Peierls instability; i.e., the radicals may associate
2
6
3
into closed shell dimers. Second, and perhaps more importantly,
4
if dimerization can be suppressed, e.g., by steric bulk, the
resulting low bandwidth W, coupled with the high on-site
Coulomb repulsion U associated with a half-filled band, leads
into an otherwise closed shell material using field effect doping.
An alternative to the CT paradigm is to use neutral π-radicals
as the building blocks for a single-component molecular
material. A stacked array of radicals in a neutral radical
7
to a Mott insulating state. Essentially the liberated spins are
trapped on the radicals and, while interesting magnetic interac-
(
1) (a) University of Waterloo. (b) University of Arkansas. (c) University of
California. (d) University of Toledo.
8
tions can be observed, conductivity remains low. Many
9
(
2) (a) Marsitzky, D.; Mullen, K. In AdVances in Synthetic Metals; Bernier,
P., Lefrant, S., Bidan, G., Eds.; Elsevier: New York, 1999; p 1. (b) Grossel,
M. C.; Weston, S. C. Contemp. Org. Synth. 1994, 1, 317. (c) Williams, J.
M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D.; Geiser, U.; Wang, H. U.;
Kini, A. M.; Whangbo, M.-H. Organic Superconductors (Including
Fullerenes); Prentice Hall: Upper Saddle River, NJ, 1992. (d) Ferraro, J.
R.; Williams, J. M. Introduction to Synthetic Electrical Conductors;
Academic Press: New York, 1987; p 25.
examples of such materials, based on phenalenyl frameworks
(5) (a) Haddon, R. C. Nature 1975, 256, 394. (b) Haddon, R. C. Aust. J. Chem.
1975, 28, 2333. (c) Haddon, R. C. Aust. J. Chem. 1975, 28, 2334.
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1953; p 108.
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(8) (a) Banister, A. J.; Bricklebank, N.; Lavendar, I.; Rawson, J. M.; Gregory,
C. I.; Tanner, B. K.; Clegg, W.; Elsegood, M. R.; Palacio, F. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 2533. (b) Fujita, W.; Awaga, K. Science 1999,
286, 261. (c) McManus, G. D.; Rawson, F. M.; Feeder, N.; van Duijn, J.;
McInnes, E. J. L.; Novoa, J. J.; Burriel, R.; Palacio, F.; Oliete, P. J. Mater.
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994, 265, 1684. (c) Dagoto, E. Science 2001, 293, 2410. (d) Sch o¨ n, J. H.;
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9498
9
J. AM. CHEM. SOC. 2002, 124, 9498-9509
10.1021/ja026118s CCC: $22.00 © 2002 American Chemical Society

Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls
A R T I C L E S
the nature of the 4-substituent.16 By contrast, the redox properties
of 1,3,2- and 1,2,3-dithiazolyl (DTA) radicals 2 and 3 can be
significantly modified by the electron-widthdrawing/releasing
power of the 4,5-substituents. We have shown that while simple
monofunctional derivatives have relatively large disproportion-
ation energies, attachment of electron-withdrawing groups at
the 4,5-positions, as in the tricyclic compounds 4 and 5, leads
to a significant reduction in ∆Hdisp; experimental Ecell values
follow suit. Structural analysis of 4 revealed an undimerized
structure (at room temperature), but the spins remained isolated;
Chart 1
1
7
i.e., the material was a Mott insulator. By contrast, while
molecules of 5 dimerize in the solid state, magnetic and
conductivity measurements indicated that the free spins present
1
8
as defects serve as charge carriers.
While providing encouraging results, i.e., good ion energetics
and a lower U, electron delocalization in compounds such as 4
and 5 arose from the attachment of a highly electron withdraw-
ing substituent to an otherwise localized DTA radical (2 or 3).
There are limits to this approach; e.g., is it useful, or even
possible, to build an even more effective electron-withdrawing
substituent? As an alternative design strategy we are now
exploring radicals in which the extent of spin delocalization is
enhanced by a resonance interaction between two DTA rings,
one a 1,2,3-dithiazolyl radical and the other a closed shell 1,2,3-
dithiazole. As an example of this hitherto unknown arrangement
we have developed the N-alkylpyridine-bridged 1,2,3-dithiazolo-
and heterocyclic thiazyl/selenazyl skeletons,10 have been char-
acterized.
Improved conductivity thus requires the design of materials
with a high W/U ratio, i.e., systems with a large bandwidth W
and a small on-site repulsion U. While a priori estimation of
the magnitude of both W and U is not practical, trends in the
11
gas-phase disproportionation enthalpy ∆Hdisp for a series of
related radicals provide a working mirror to the trends in U.
For heterocyclic thiazyl radicals a limited database of experi-
12
mentally obtained ionization potentials has been compiled, and
reliable estimates of both IP and EA values can now be easily
obtained by computation.1 Alternatively, solution-based cell
potentials Ecell, which are generally accessible by experiment,
3,14
1
,2,3-dithiazolyl framework ClBPR 6. On the basis of the two
equivalent resonance structures shown in Chart 1, such radicals
should have highly delocalized spin distributions, and display
1
5
can be used. Using either criterion, the working prescription
for NRCs requires radicals with good ion energetics, i.e., low
∆
Hdisp and Ecell values superior to those observed for conven-
∆Hdisp and Ecell values. In addition, the radicals must not
tional DTA-based π-radicals. Here we describe the preparation
and the electrochemical, spectroscopic, and structural charac-
terization of three ClBPR radicals (R ) Me, Et, and Pr).
Variable-temperature magnetic susceptibility and single-crystal
conductivity measurements have also been performed. The
results are discussed in light of the design criteria for NRC
materials.
dimerize in the solid state, and yet should exhibit a strong
network of intermolecular interactions, so that sufficient elec-
tronic bandwidth is generated to offset the on-site Coulomb
barrier U.
Much of our early work with NRCs focused on 1,2,3,5-
dithiadiazolyl (DTDA) radicals 1 (Chart 1), but these materials
suffered from rather large ∆Hdisp and Ecell values regardless of
Results
(
9) For recent examples, see: (a) Koutentis, P. A.; Chen, Y.; Cao, Y.; Best,
C. P.; Itkis, M. E.; Beer, L.; Oakley, R. T.; Cordes, A. W.; Brock, C. P.;
Haddon, R. C. J. Am. Chem. Soc. 2001, 123, 3864. (b) Koutentis, P. A.;
Haddon, R. C.; Oakley, R. T.; Cordes, A. W.; Brock, C. P. Acta Crystallogr.
Synthesis. 1,2,3-DTA radicals 3 have been known for many
19,20
years,
but few examples have been isolated and structurally
2
001, B57, 680. (c) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato,
characterized; their thermal stability is extremely dependent on
K.; Shiomi, D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; Ouyang,
J. J. Am. Chem. Soc. 1999, 121, 1619.
21
the nature of the 4,5-substituents. Typically, the radicals are
(
10) For recent examples, see: (a) Beer, L.; Cordes, A. W.; Myles, D. J. T.;
Oakley, R. T.; Taylor, N. J. CrystEngComm 2000, 20. (b) Britten, J. F.;
Clements, O. P.; Cordes, A.W.; Haddon, R. C.; Oakley, R. T.; Richardson,
J. F. Inorg. Chem. 2001, 40, 6820. (c) Cordes, A. W.; Haddon, R. C.;
Oakley, R. T. AdV. Mater. 1994, 6, 798. (d) Cordes, A. W.; Haddon, R.
C.; Oakley, R. T. In The Chemistry of Inorganic Ring Systems; Steudel,
R., Ed.; Elsevier: Amsterdam, 1992; p 295.
generated by reduction of salts of the corresponding closed shell
1
,2,3-DTA cations, for which a variety of synthetic routes are
2
2
known. One of the most well-known approaches is via the
(
(
(
16) (a) Boer e´ , R. T.; Moock, K. H. J. Am. Chem. Soc. 1995, 117, 4755. (b)
Chandrasekhar, V.; Chivers, T.; Parvez, M.; Vargas-Baca, I.; Ziegler, T.
Inorg. Chem. 1997, 36, 4772.
(11) ∆Hdisp is the enthalpy change for the conversion of two gas-phase radicals
+
-
R into a cation/anion pair, i.e., 2R a R + R , and accordingly is equal
17) Barclay, T. M.; Cordes, A. W.; George, N. A.; Haddon, R. C.; Itkis, M.
E.; Mashuta, M. S.; Oakley, R. T.; Patenaude, G. W.; Reed, R. W.;
Richardson, J. F.; Zhang, H. J. Am. Chem. Soc. 1998, 120, 352.
18) Barclay, T. M.; Cordes, A. W.; Haddon, R. C.; Itkis, M. E.; Oakley, R. T.;
Reed, R. W.; Zhang, H. J. Am. Chem. Soc. 1999, 121, 969.
to the difference between the ionization potential (IP) and electron affinity
(
EA). The solution-based cell potential Ecell ) E1/2(ox) - E1/2(red) is the
difference between the half-wave potentials for the oxidation and reduction
processes.
(
12) (a) Boer e´ , R. T.; Oakley, R. T.; Reed, R. W.; Westwood, N. P. C. J. Am.
Chem. Soc. 1989, 111, 1180. (b) Cordes, A. W.; Goddard, J. D.; Oakley,
R. T.; Westwood, N. P. C. J. Am. Chem. Soc. 1989, 111, 6147. (c) Cordes,
A. W.; Bryan, C. D.; Davis, W. M.; de Laat, R. H.; Glarum, S. H.; Goddard,
J. D.; Haddon, R. C.; Hicks, R. G.; Kennepohl, D. K.; Oakley, R. T.; Scott,
S. R.; Westwood, N. P. C. J. Am. Chem. Soc. 1993, 115, 7232. (d)
Brownridge, S.; Du, H.; Fairhurst, S. A.; Haddon, R. C.; Oberhammer,
Parsons, S.; Passmore, J.; Schriver, M. J.; Sutcliffe, L. H.; Westwood, N.
P. C. J. Chem. Soc., Dalton Trans. 2000, 3365.
(
(
19) Preston, K. F.; Sutcliffe, L. H. Magn. Reson. Chem. 1990, 28, 189.
20) (a) Mayer, R.; Domschke, G.; Bleisch, S. Tetrahedron Lett. 1978, 4003.
(
b) Mayer, R.; Domschke, G.; Bleisch, S.; Bartl, A.; St aˇ sko, A.; Z. Chem.
1
981, 21, 146, 264. (c) Mayer, R.; Domschke, G.; Bleisch, S.; Fabian, J.;
Bartl, A.; St aˇ sko, A. Collect. Czech. Chem. Commun. 1984, 49, 684. (d)
Mayer, R.; Bleisch, S.; Domschke, G.; Tr a´ cˇ , A.; St aˇ sko, A. Org. Magn.
Reson. 1979, 12, 532. (e) Harrison, S. R.; Pilkington, R. S.; Sutcliffe, L.
H. J. Chem. Soc., Faraday Trans. 1 1984, 80, 669.
(
21) Barclay, T. M.; Beer, L.; Cordes, A. W.; Oakley, R. T.; Preuss, K. E.;
Taylor, N. J.; Reed, R. W. Chem. Commun. 1999, 531.
(
(
13) Kaszynski, P. J. Phys. Chem. A 2001, 105, 7626.
14) Cordes, A. W.; Mingie, J. R.; Oakley, R. T.; Reed, R. W.; Zhang, H. Can.
J. Chem. 2001, 79, 1352.
(
22) (a) Rawson, J. M.; McManus, G. D. Coord. Chem. ReV. 1999, 189, 135.
(
b) Torroba, T. J. Prakt. Chem. 1999, 341, 99. (c) Kim, K. Sulfur Rep.
(
15) Boer e´ , R. T.; Roemmele, T. L. Coord. Chem. ReV. 2000, 210, 369.
1
998, 21, 147.
J. AM. CHEM. SOC.
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VOL. 124, NO. 32, 2002 9499

A R T I C L E S
Beer et al.
Scheme 1
Scheme 2
material so produced was high, its purity was not as good as
that generated by the route outlined in Scheme 1. It did not, for
example, undergo a clean metathesis with AgSbF6. The crude
chloride salt could nonetheless be purified by reacting it with
GaCl3 in CH3CN. This afforded the gallate salt [ClBPH][GaCl4]
(13), from which impurities could be removed by washing with
glacial acetic acid (in which the gallate salt is insoluble).
Reprecipitation of the chloride, by addition of pyridine to an
acetonitrile solution of the gallate, gave material which under-
went a very smooth metathesis with AgSbF6 in CH3CN to afford
pure [ClBPH][SbF6]. Subsequent treatment with Proton Sponge
2
7
afforded ClBP in good overall yield. This method is superior
for large-scale preparations of ClBP.
Herz reaction, i.e., the cyclocondensation of an aromatic amine
with S2Cl2.23 While we were eventually able to access the
ClBPR framework using a variation of Herz technology, our
initial attempts (Scheme 1) to produce a resonance-stabilized
bis(1,2,3-DTA) radical started from the pyridine bis(aminothiol)
Regardless of how the free base ClBP was produced, the final
step in the assembly of the ClBPR framework involved the
conversion of ClBP into a series of N-alkylated derivatives
+
[ClBPR] (R ) Me, Et, Pr). This was easily performed in
9
, itself prepared by a procedure developed for the analogous
diethyl ether using a slight excess of the appropriate alkyl triflate
ROTf, although for R ) Et and Pr prolonged reaction times
were necessary. The triflates [ClBPR][OTf] (R ) Me, Et, Pr)
precipitated from solution as metallic red microcrystals which
are stable in air and readily dissolve in acetonitrile to afford
deep blue solutions. They were recrystallized for analytical
purposes from glacial acetic acid.
benzene compound, i.e., oxidative thiocyanation of 2,6-diami-
24
nopyridine 7 to the 3,5-bis(thiocyanate) 8, followed by
2
5
hydrolysis. Condensation of 9 with excess S2Cl2 in refluxing
chlorobenzene then afforded the protonated salt [ClBPH][Cl]
(10) as a black insoluble solid. This crude chloride salt could
be converted by metathesis with AgSbF6 into the corresponding
hexafluoroantimonate [ClBPH][SbF6] (11). The latter salt, which
is stable in air, is very soluble in CH3CN, and partially soluble
in glacial acetic acid and SO2(l), giving rise to intensely blue
solutions. Recrystallization of 11 from SO2(l) afforded lustrous
golden blocks suitable for structural analysis.26 Both 10 and 11
derive from the antiaromatic 16π-electron base ClBP (12),
which, as expected from its zwitterionic formulation, is more
basic than a typical pyridine heterocycle. Deprotonation of
The N-alkyl triflates [ClBPR][OTf] (R ) Me, Et, Pr), along
with the protonated salt [ClBPH][SbF6], are based on 16π-
electron frameworks which should, upon reduction, afford 17π-
electron ClBPR radicals 6. Preliminary attempts at chemical
reduction were not, however, encouraging. For example, triph-
enylantimony, a reagent which has proven effective for reducing
a wide range of 1,2,3-DTA cations, failed to react at all with
+
any of the [ClBPR] salts. Resolution of the problem required
[ClBPH][SbF6] could not therefore be effected with triethy-
examination of the necessary electrochemical potentials. The
results of such a study, described in the following section,
indicated the need for a more powerful reducing agent; the
properties of decamethylferrocene (Cp2*Fe) (vide infra) sug-
lamine, but Proton Sponge readily liberated the parent zwitterion.
The above sequence of reactions represents the method
initially developed to construct the ClBP framework. Later,
however, we discovered that the chloride salt 10 could be
prepared in a single step with a “double Herz” reaction, i.e., by
heating 2,6-diaminopyridine with excess S2Cl2 at reflux in
dichloroethane (Scheme 2). However, while the yield of the
28
gested a perfect match. To our satisfaction treatment of
[ClBPR][OTf] (R ) Me, Et, Pr) with 1 equiv of Cp2*Fe in
degassed CH3CN afforded a metallic green/black microcrys-
talline precipitate of the corresponding ClBPR radical 6 in high
yield (Scheme 1). The crude radicals were purified, for structural
characterization and transport property measurements, by re-
crystallization from degassed C2H4Cl2. In contrast to the
(
23) (a) Herz, R. Chem. Zentralbl. 1922, 4, 948. (b) Warburton, W. K. Chem.
ReV. 1957, 57, 1011.
(
24) (a) Baker, A. J.; Hill, S. A. J. Chem. Soc. 1962, 3464. (b) Lochon, P.;
M e´ heux, P.; N e´ el, J. Bull. Soc. Chim. Fr. 1967, 11, 4387. (c) Okada M.;
Marvel, C. S. J. Polym. Sci., Part A-1 1968, 6, 1259.
(
(
25) Huestis, L. D.; Walsh, M. L.; Hahn, N. J. Org. Chem. 1965, 2763.
26) A preliminary account of the preparation and structural characterization of
(27) The gallate salt 13 could not be converted directly to 12. Treatment of 13
-
with Proton Sponge led to partial decomposition of the GaCl
4
anion, and
[
ClBPH][SbF
6
] and ClBP has been published. See: Beer, L.; Cordes, A.
precipitiation of a mixture of 12 and 10.
W.; Oakley, R. T.; Mingie, J. R.; Preuss, K. E.; Taylor, N. J. J. Am. Chem.
Soc. 2000, 122, 7602.
(28) Novriandi, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T. Lay, P. A.;
Masters, A. F.; Phillips, L. J. Phys. Chem. 1999, 103, 2713.
9500 J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002

Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls
A R T I C L E S
Figure 2. B3LYP/6-31G** SOMO (top) and spin density distribution
(
bottom) for ClBPH. Atomic spin densities are S(C) 0.120, S(N) 0.264,
N(S) 0.170, C(S) 0.288, C(N) -0.086, and C(Cl) -0.167 (other atoms
0.02).
<
Figure 1. Calculated (B3LYP/6-31G**) gas-phase ion energetics for ClBP
+
and [ClBPH] .
behavior of the alkylated salts, reduction of the protonated salt
[
[
ClBPH][SbF6] under the same conditions as those used for
ClBPR][OTf] (R ) Me, Et, Pr) produced a black uncharac-
terized solid. This apparent anomaly was later rationalized in
light of the cyclic voltammetric behavior (vide infra) of the
+
[
ClBPH] cation.
Electronic Structure Calculations. We have explored the
electronic structures and energies of several derivatives based
on the ClBP and ClBPH frameworks by means of a series of
density functional theory (DFT) calculations at the B3LYP/6-
31G** level; the results are summarized in Figure 1. ClBP itself
represents a rare (to our knowledge, the only) example of an
antiaromatic sulfur nitrogen heterocycle with a zwitterionic
ground state. While both ClBP and its protonated variant
Figure 3. CV scans of [ClBPH][SbF6] in CH3CN, [n-Bu4N][PF6] sup-
porting electrolyte: (A) +1.0 to -0.25 V sweep and (B) +1.0 to -0.80 V
sweep.
29
+
[
ClBPH] are predicted to be closed shell species, the singlet
1
-1
A1 ground state of ClBP lies only 2.4 kcal mol below the
B2 diradical triplet (Figure 1); for [ClBPH] the singlet/triplet
splitting is much larger (16.0 kcal mol ). As expected from
its dipolar formulation, ClBP is a relatively strong base; its
3
+
compared to monofunctional DTAs can be attributed, in valence
bond parlance, to resonance between the two 1,2,3-DTA rings.
The extent of spin delocalization can be more easily visualized
in molecular orbital terms from the distribution of the a2 SOMO
-
1
-1
calculated proton affinity (PA ) 247.7 kcal mol ) is estimated
to be very close to that of Proton Sponge (PA ) 246 kcal
(Figure 2), which is an out-of-phase combination of two 1,2,3-
-
1 30
mol ), and it was on this basis that the latter base was selected
DTA SOMOs; derived total spin densities are also shown in
Figure 2.
+
for the deprotonation of [ClBPH] .
Sequential reduction of [ClBPH] leads to the 17π-electron
radical ClBPH and the 18π-electron anion [ClBPH] ; computed
+
Cyclic Voltammetry. While the DFT calculations described
above provide an encouraging picture of the ion energetics of
-
(
∆SCF) IP and EA values for the radical are, respectively, 6.30
and 1.6 eV. Within the context of NRC design the resulting
Hdisp of 4.70 eV represents a dramatic improvement on any
previously studied 1,2,3-DTA radical; indeed it is comparable
the [ClBPH]+
cation, examination of the electrochemical
behavior of [ClBPH][SbF6] yielded unexpected results. A cyclic
voltammetric (CV) sweep on an acetonitrile solution of this salt
over the range +1.0 to -0.25 V vs SCE, with Pt wire electrodes
and [Bu4N][PF6] as supporting electrolyte, revealed a single
reversible reduction wave with E1/2 ) 0.035 V (Figure 3A).
∆
31
to that found for metallic elements, e.g., sodium. As noted
earlier the superior ion energetics of the ClBPH radical
+
This we ascribe to the reduction of the [ClBPH] cation to the
(
29) Hutchison, K.; Srdanov, G.; Hicks, R.; Yu, H.; Wudl, F.; Strassner, T.;
ClBPH radical, i.e., the +1/0 couple. When the CV sweep range
was extended to -0.8 V, a second, irreversible reduction process
with a cathodic peak potential Epc ) -0.500 V was observed;
this we assign to the 0/-1 couple. Reversal of the voltage sweep
Nendel, M.; Houk, K. N. J. Am. Chem. Soc. 1998, 120, 2989.
(
30) Maier, J. P. HelV. Chim. Acta 1974, 57, 994.
(
31) For sodium ∆Hdisp ) IP - EA ) 5.14 - 0.55 ) 4.59 eV. See: (a) Moore,
C. E. Natl. Stand. Ref. Data Ser. (U.S. Natl. Bur. Stand.) 1970, 34, 1. (b)
Hotop, H.; Lineberger, W. C. J. Phys. Chem. Ref. Data 1985, 14, 731.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002 9501

A R T I C L E S
Beer et al.
Scheme 3
Figure 4. CV scan of [ClBPMe][OTf] in CH3CN, [n-Bu4N][PF6] supporting
electrolyte.
Table 1. Half-Wave Redox and Cell Potentialsb
a
ClBPH
ClBPMe
ClBPEt
ClBPPr
E1/2(ox) (V)
E1/2(red) (V)
Ecell (V)
0.035
c
0.005
-0.835
0.830
-0.018
-0.845
0.827
-0.0116
-0.834
0.822
a
In CH3CN, reference to SCE. b Ecell ) E1/2(ox) - E1/2(red). c Irreversible
afforded an oxidation wave for the 0/-1 couple with peak
potential Epa ) -0.385 V. At the same time the reversibility of
the +1/0 couple process was lost, to be replaced by a single
irreversible oxidation wave with Epa ) 0.755 V (Figure 3B).
This dramatic change in the CV, occasioned simply by a change
in sweep width, could be entirely reversed by returning to the
original voltage sweep. Thus, a repeat CV scan over the range
behavior; see the text.
As expected from the above interpretation of the CV behavior
+
of the [ClBPH] system, replacement of the N-H proton by a
+
nonmigratory alkyl group, as in the [ClBPR] cations (R ) Me,
Et, Pr), eliminates the tautomerism issue, and produces far more
straightforward electrochemistry. Using voltage ranges of +1.2
to -1.2 V vs SCE, cyclic voltammetry of all three [ClBPR]-
[OTf] salts showed two reversible waves corresponding to the
1/0 and 0/-1 couples; the CV scan of [ClBPMe][OTf] in
Figure 4 is representative. Analyses of the half-wave and Ecell
+1.0 to -0.25 V vs SCE afforded exactly the same results as
seen initially, i.e., a single reversible process corresponding to
E1/2(ox) of ClBPH. These two scans were reproducible even
after repeated cycling between the two voltage ranges.
+
values provided in Table 1 allow two important conclusions.
i) The +1/0 couple occurs at a potential far more anodic than
This seemingly unusual behavior is the fingerprint of a system
exhibiting electrochemical irreversibility with chemical revers-
(
15
that observed for monofunctional 1,2,3-DTA derivatives; i.e.,
ClBPR radicals are more powerful reducing agents. Generation
of the radicals from the cations on a synthetic scale (vide supra)
required a chemical reducing agent with sufficient potential to
ibility (EI-CR). The condition for EI-CR arises when a
chemical change is occurring much faster than the cycling rate
of the CV scan, as, for example, in the reduction of dithioles,
which leads to S-S bond cleavage and formation of a
-
sulfidothioketone.1
5,32
produce the radical state ClBPR but not the anion [ClBPR] ,
In the present case the redox chemistry
i.e., a redox couple within the range of 0 to -0.8 V vs SCE; to
of ClBPH is perturbed by the migratory capability of the N-H
proton. A possible, but by no means exclusive, interpretation
of the structural changes that occur during the CV scan of
(
0/+1)
this end we selected decamethylferrocene, for which E1/2
2
8
)
-0.13 V. (ii) The cell potentials Ecell for ClBPR radicals
are near 830 mV, a value far smaller than the previous best
for two reversible couples), namely, the 1,2,3-DTA system 5,
[ClBPH][SbF6] is shown in Scheme 3. Electrochemical revers-
(
ibility of the +1/0 couple is possible as long as the cathodic
sweep does not reach potentials that can generate the anion. If
such a potential is reached, a rapid structural change occurs
for which Ecell ) 0.97 V. These results, coupled with the gas-
phase-computed ∆Hdisp value of the parent ClBPH radical,
provide compelling evidence for believing that in the solid state
ClBPR radicals should enjoy a much lower on-site Coulomb
repulsion U than any previously reported heterocyclic NRC.
EPR Spectra. We have been unable to observe any EPR
spectrum for ClBPH, e.g., by in situ reduction of [ClBPH][SbF6].
Calculated (B3LYP/6-31G**) a values for this species are
provided in Table 2, and provide a satisfying cross match35 with
those observed experimentally for the N-alkylated radicals. The
X-band EPR spectra of ClBPR (R ) Me, Et, Pr) have been
recorded at 293 K on CH2Cl2 solutions. The spectrum of
ClBPMe shown in Figure 5 is representative; g values and
hyperfine coupling constants are listed in Table 2. The spectra
are characterized by a five-line pattern arising from spin
-
following the formation of the [ClBPH] anion. The structural
change is precipitated by a proton shift from the pyridine
nitrogen to the DTA ring. There are other tautomers possible
in addition to the one shown, but regardless of which, or how
many, are involved, the net effect of anion formation is to
produce a more localized electronic structure.33 This is probably
34
accompanied by cleavage of one of the S-S (or S-N) bonds.
In the anodic sweep oxidation of the rearranged anion to a
radical, and of this radical to a cation, will take place at
potentials quite different from those observed in the cathodic
sweep. Presumably upon re-formation of the cation, a proton
shifts back to the pyridine nitrogen to regenerate the original
+
[
ClBPH] framework.
14
coupling to the two equivalent N nuclei on the two DTA rings.
As expected from the fact that the spin density is partitioned
equally between the two rings, aN is much smaller than in
monofunctional 1,2,3-DTAs. Smaller coupling to the pyridine
(
32) Bechgaard, K.; Parker, V. D.; Pedersen, C. T. J. Am. Chem. Soc. 1973, 95,
4
373.
(
33) In most monofunctional 1,2,3-DTAs the 0/-1 couple is irreversible.
34) Alternatively N-S bond cleavage could occur. See, for example: Barclay,
T. M.; Cordes, A. W.; Goddard, J. D.; Mawhinney, R. C.; Oakley, R. T.;
Preuss, K. E.; Reed, R. W. J. Am. Chem. Soc. 1997, 119, 12136.
(
(35) Kaszynski, P. J. Phys. Chem. A 2001, 105, 7615.
9502 J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002

Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls
A R T I C L E S
Table 2. EPR Hyperfine Coupling Constants (mT) and g Values
Crystals of ClBPEt and ClBPPr consist of slipped π-stacks
of undimerized radicals running parallel to z. As such they
represent the first examples of undimerized 1,2,3-DTA radi-
ClBPHa
ClBPMe
ClBPEt
ClBPPr
N(S)
N(R)
Cl
H(CH2)
H(X ) H) aH
g value
aN
0.346 0.310
0.310
0.060
0.030/0.024
<0.020
0.310
1
8,21
aN -0.066 0.060
0.060
cals.
Figure 6 shows the packing of ClBPEt stacks viewed
b
b
0.030/0.024b
aCl -0.052 0.030/0.024
down the z axis, and illustrates the clustering of the dithiazole
heads about the 4h center. The ClBPMe structure (both phases)
also consists of slipped π-stacks of undimerized radicals,
although now the stacking direction is along x (Figure 7).
In the low-temperature phase ClBPMe-LT the b and c cell
dimensions are shorter than in the room-temperature phase
ClBPMe-RT, while the a axis is lengthened. At the molecular
level this lateral compression and longitudinal elongation lead
to the mean plane of the radicals in ClBPMe-LT being more
sharply inclined (a decrease in the tilt angle τ) to the stacking
axis than in ClBPMe-RT; the mean plane separation δ also
contracts. This decrease in δ is, we believe, the driving force
for the phase transition. In the absence of change in the tilt angle
τ a general, and relatively uniform, contraction of all structural
parameters (concomitant with a decrease in the cell constants
a, b, and c) is normally observed on cooling. Such a process
would, however, inevitably lead to closer Cl‚‚‚Cl and Me‚‚‚
Me interactions, which are equal to the unit cell translation a.
Avoiding such interactions, while still bringing the molecular
planes closer together, requires an increase rather than a decrease
in a, and this can only be brought about (if the space group is
to be preserved) by a decrease in τ (a sharper tilt). In essence
the phase change from ClBPMe-RT to ClBPMe-LT is remi-
niscent of the closing of venetian blinds.
aH
0.030
0.30
2.008274
<0.020
c
2.008208
2.008209
a
B3LYP/6-31G** values from a C2V geometry optimization; the single
3
5
b
aCl value refers to the Cl isotope. The two aCl values correspond to the
3
5/37
c
Cl isotopes, respectively. Where R ) CH2X. Thus, X ) H for R )
Me and X ) Me and Et for R ) Et and Pr, respectively.
In all three ClBPR structures there are numerous close
intermolecular S‚‚‚S contacts d1-d4 between neighboring
dithiazole rings, about the 4h center in ClBPEt/Pr (Figure 8) and
the 21 axes in ClBPMe (Figure 9). Most of these contacts are
inside the van der Waals contact for sulfur (3.6 Å),37 and some
of those in the ClBPMe, notably d2 and d3, are among the
shortest nonbonded S‚‚‚S contacts that we have ever observed
in an undimerized heterocyclic sulfur-nitrogen radical.
At the molecular level the bond lengths in all three radicals
show classical trends relative to those seen in, for example, the
Figure 5. EPR spectrum of ClBPMe in CH2Cl2 (top) and simulation
(bottom). SW ) 3.0 mT, L/G ratio 0.50, and LW ) 0.023 mT.
nitrogen, and to the basal chlorine, can be extracted by spectral
simulation. ClBPMe also shows a small, equivalent coupling
to the three methyl protons. No coupling is discernible to the
methylene protons in either ClBPEt or ClBPPr.36
Crystal Structures. In the solid state the ClBPR radicals (R
)
Me, Et, Pr) are remarkably stable in air, but in solution they
26
oxidized ring of [ClBPH][SbF6]. Occupation of the antibonding
SOMO (Figure 2) leads to a general elongation of the S-S,
S-N, and S-C bonds, but the changes are smaller than those
seen between monofunctional DTA radicals and their cations,
where the SOMO is localized over a single DTA ring,21 and
comparable to those seen between closed shell benzobis-
all decompose rapidly by reaction with dissolved oxygen.
Crystals (green-black needles) of all three compounds suitable
for X-ray analysis could nonetheless be obtained by recrystal-
lization from carefully degassed (five freeze-pump-thaw
cycles) toluene or dichloroethane. Crystals of ClBPMe could
-
3
also be obtained by slow vacuum sublimation at 120 °C/10
3
8
(dithiazoles) and their radical cations.
Torr. Crystals of ClBPEt and ClBPPr are isomorphous, and
belong to the tetragonal space group P 4h 21m, while those of
ClBPMe belong to the orthorhombic space group P212121.
Magnetic measurements (vide infra) revealed a phase change
for ClBPMe near 93 K, as a result of which its crystal structure
has been determined both above (293 K ) ClBPMe-RT) and
below (25 K ) ClBPMe-LT) this temperature. Crystal data for
all structural determinations are listed in Table 3, and summaries
of pertinent intramolecular bond lengths and intermolecular
contacts compiled in Table 4. Packing diagrams are provided
in Figures 6-9.
Magnetic and Conductivity Measurements. Variable-
temperature magnetic susceptibility (ø) measurements have been
carried out on microcrystalline samples of ClBPR (R ) Me,
Et, Pr). The temperature dependence of ø for ClBPEt and
3
9
1
ClBPPr shows normal Curie-Weiss behavior for an S ) /2
system between 5 and 300 K; values of ø0, C, and Θ are
provided in Table 5. For ClBPMe, the temperature dependence
of ø exhibits Curie-Weiss behavior from ambient temperatures
down to 93 K (Figure 10), whereupon a hysteretic phase change
occurs, leading to an increase in ø. Plots of the function øT vs
T for the three compounds are shown, for comparative purposes,
(
36) A B3LYP/6-31G** calculation on ClBPMe in C
values of -0.095 mT for the single axial methyl proton, and -0.032 mT
for the two equivalent gauche methyl protons. The observed a represents
s H
symmetry afforded a
(37) Bondi, A. J. Phys. Chem. 1964, 68, 441.
H
(38) (a) Barclay, T. M.; Cordes, A. W.; Oakley, R. T.; Preuss, K. E.; Reed, R.
W. Chem. Mater. 1999, 11, 164. (b) Barclay, T. M.; Cordes, A. W.; Mingie,
J. R.; Oakley, R. T.; Preuss, K. E. Cryst. Eng. Chem. 2000, 15.
(39) Carlin, R. L. Magnetochemistry; Springer-Verlag: New York, 1986.
a time-averaged value for a freely rotating methyl group. In the case of
ClBPEt and ClBPPr the methylene groups are locked with the two
hydrogens in gauche positions, so that no coupling is observed.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002 9503

A R T I C L E S
Beer et al.
Table 3. Crystallographic Data
ClBPMe-RT
ClBPMe-LT
ClBPEt
ClBPPr
empirical formula
fw
C6H3N3ClS4
280.80
C6H3N3ClS4
280.80
C7H5N3ClS4
294.83
C8H7N3ClS4
308.86
a, Å
b, Å
c, Å
4.2464(12)
15.194(5)
15.069(4)
972.2(5)
1.918
4.5386(4)
14.2658(10)
14.3779(11)
930.92(13)
2.004
15.886(4)
15.886(4)
4.1088(11)
1036.9(5)
1.889
15.9601(19)
15.9601(19)
4.4494(6)
1133.4(2)
1.810
3
V, Å
-
3
F(calcd), g cm
space group
Z
P212121
4
P212121
4
P 4h 21m
4
P 4h 21m
4
temp, K
µ, mm
λ, Å
no. of data/restraints/params
solution method
R, Rw (on F )
293(2)
1.208
0.71073
1915/0/128
direct methods
0.0314, 0.0544
25(5)
1.261
0.71073
1657/0/127
direct methods
0.0493, 0.1063
293(2)
1.137
0.71073
1064/0/87
direct methods
0.0279, 0.0571
293(2)
1.045
0.71073
700/0/82
direct methods
0.0308, 0.0453
-
1
2
a
a
R ) [∑||Fo| - Fc||]/[∑|Fo|] for I > 2σ(I); Rw ) {[∑w||Fo| - |Fc| | ]/[∑(w|Fo| )]}1/2.
2
2 2
4
Table 4. Summary of Intra- and Intermolecular Structural
Parameters
ClBPMe-RT
ClBPMe-LT
ClBPEt
293(2)
ClBPPr
293(2)
temp (K)
293(2)
25(5)
Intramolecular Distances (Å)a
S-S
2.101(6)
2.110(7)
1.671(5)
1.738(6)
1.309(9)
1.435(28)
1.380(19)
1.382(7)
2.1049(12)
2.1066(18)
1.661(4)
1.724(4)
1.300(4)
1.428(5)
1.387(4)
1.398(5)
S-N
1.661(7)
1.731(8)
1.303(5)
1.438(5)
1.377(5)
1.385(14)
1.663(3)
1.728(3)
1.300(4)
1.439(4)
1.382(3)
1.382(3)
S-C
N(S)-C
C(S)-C(N)
C(S)-C(Cl)
C-N(R)
b
Intermolecular Contacts (Å) and Tilt Angles (deg)
d1
d2
d3
d4
3.541(2)
3.324(2)
3.369(2)
3.546(2)
54.81(13)
3.470(5)
3.570(2)
3.234(2)
3.283(2)
3.449(2)
48.73(12)
3.411(8)
3.742(1)
3.573(1)
3.560(1)
3.401(1)
57.9(3)
3.819(2)
3.505(2)
3.634(2)
3.563(2)
51.0(3)
c
τ
d
δ
3.483(12)
3.458(9)
a
Bond lengths in ClBPMe are an average of two values; numbers in
b
parentheses are the larger of the ESD or the range. See Figures 6-9 for
c
definitions of d1-d4. τ is the tilt angle between the mean molecular plane
d
and the stacking axis. δ is the mean interplanar separation between radicals
along the π-stack.
in Figure 11. At ambient temperatures all three compounds
exhibit values of øT near or slightly below 0.375 (the expected
value for an S ) /2 system). The low-temperature “tails” in
Figure 6. A projection of the packing of ClBPEt (top) and ClBPPr (bottom)
in the xy plane. Intermolecular S‚‚‚S contacts (dashed lines) are defined in
Figure 8 and Table 4.
1
ClBPPr and ClBPEt herald, respectively, the onset of weak
antiferromagnetic and ferromagnetic interactions. In ClBPMe,
the phase change at 93 K can now be observed more clearly,
as can the low-temperature antiferromagnetic tail below 14 K.
The enhanced øT values between 14 and 93 K are characteristic
Band Structure Calculations. To place the above structural,
magnetic, and conductivity results in context, we have performed
a series of extended H u¨ ckel theory (EHT) band calculations on
the four crystal structures under consideration, ClBPMe (LT
and RT), ClBPEt, and ClBPPr. The results must be viewed with
caution, as the EHT method cannot be expected to succeed in
systems where the tight-binding approximation fails, i.e., in Mott
insulators. The results are summarized in Figure 13, which
shows the dispersion curves along the stacking direction42 for
the four crystal orbitals (COs) arising from the SOMOs of the
four radicals in the unit cell, i.e., the putative half-filled
conduction band of the molecular metal. Clearly, none of the
materials is metallic, but the dispersion curves nonetheless
provide insight into the extent of the intermolecular interactions
along and perpendicular to the slipped π-stacks.
4
0,41
of weak 1D ferromagnetic interactions along the π-stacks.
Electrical conductivity (σ) measurements along the needle
axes of ClBPR (R ) Me, Et, Pr) were performed over the
temperature range 150-330 K; Figure 12 shows log plots of σ
against 1/T for the three compounds. As can be seen the
conductivity is activated, with values of σ at 300 K increasing
-
6
-1
-5
progressively from near 10 S cm for R ) Pr to 10
S
-
1
cm for R ) Me (Table 5). The derived activation energies Eg
half of the band gap for an intrinsic semiconductor) steadily
(
diminish from 0.48 (R ) Pr) to 0.43 (R ) Et) to 0.40 (R )
Me) eV.
(
40) (a) Lang, A.; Pei, Y.; Ouahab, L.; Kahn, O. AdV. Mater. 1996, 8, 60. (b)
Kahn, O. Molecular Magnetism; VCH: New York, 1993.
41) The magnetic properties of the ClBPR series will be discussed in more
detail elsewhere.
(42) In all four structures R ) â ) γ ) 90°. There is therefore an exact
equivalence of the directions of the unit cell vectors of the real and
reciprocal space.
(
9504 J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002

Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls
A R T I C L E S
Table 5. Magnetic Susceptibility and Conductivity Data
a
ClBPMe
ClBPEt
ClBPPr
-
1
-6
-6
-6
-6
-6
-7
ø0, emu mol
-137.5 × 10
-160.0 × 10
0.326
-162.4 × 10
0.349
-
1
0.357b
C, emu mol
Θ, K
-13b
7
-11.5
-
1
σ(300 K), S cm
5.4 × 10
3.2 × 10
4.0 × 10
c
Eg, eV
0.395
0.43
0.48
a
C and Θ obtained from the Curie-Weiss fits, i.e., ø ) C/(T - Θ).
b
c
From a Curie-Weiss fit to data above 93 K. Eg corresponds to the
activation energy of the conductivity; for an intrinsic semiconductor and
temperature-independent mobility, the band gap is twice this value.
Figure 7. A projection of the packing of ClBPMe-RT in the yz plane.
Intermolecular S‚‚‚S contacts (dashed lines) are defined in Figure 9 and
Table 4.
Figure 10. Magnetic susceptibility ø of ClBPMe as a function of
temperature.
Figure 8. Side view of slipped π-stacks in ClBPEt, showing intermolecular
S‚‚‚S contacts d1-d4 (see Table 4). The ClBPPr structure is isomorphous.
Figure 11. Temperature dependence of øT for ClBPMe (top) and ClBPEt
and ClBPPr (bottom).
Figure 9. Side view of slipped π-stacks in ClBPMe-RT, showing
intermolecular S‚‚‚S contacts d1-d4 (see Table 4).
There are several salient features in Figure 13. (i) The slope
of the curves, i.e., the sign of dE/dk, for ClBPEt and ClBPPr is
the reverse of that seen in ClBPMe. (ii) Dispersion along the
stack in ClBPPr and ClBPEt is approximately the same as, if
not somewhat greater than, that observed in ClBPMe, in the
region of 0.3-0.4 eV depending on the CO. The progressively
increasing bandwidth along the series R ) Pr, Et, and Me arises
from the onset of lateral dispersion (the spreading of the four
COs) in the latter. (iii) ClBPMe is a remarkably isotropic
structure; lateral interactions appear to be as strong as those
along the stack. The phase change from ClBPMe-RT to
Figure 12. Log plots of σ vs 1/T for ClBPR (R ) Me, Et, Pr).
ClBPMe-LT is accompanied by an expected increase in disper-
sion both along and across the stack, but the ratio remains about
the same. The interpretation of these findings is developed
below.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002 9505

A R T I C L E S
Beer et al.
Figure 13. EHT dispersion curves for ClBPR radicals.
Discussion
On the basis of computed ion energetics and electrochemical
data, the ClBPR system represents a major advance in the design
of NRC materials. It provides a chemically robust framework
which should, in the solid state, afford a much lower value of
U than in other heterocyclic radicals. The observation of good
conductivity requires, however, a large W/U ratio, i.e., strong
intermolecular interactions and a large electronic bandwidth W.
In the absence of the latter, a Mott insulating state will still
prevail, regardless of the presence of favorable ion energetics.
Figure 14. Projections of planes of nearest-neighbor rings in ClBPR
structures (only the C1 carbon of the R group is shown). Shown at the bottom
are sketches of the SOMO overlaps in ClBPMe-LT/RT (left) and ClBPEt/
Pr (right).
atom43 of its neighbor. These two S‚‚‚C overlaps are, however,
of opposite sign, so that the net effect is minimal. The
longitudinal dispersion seen in ClBPMe (both phases) should
therefore be viewed as arising from minor differences in the
two S‚‚‚C overlaps, and some small C‚‚‚C overlaps. The
-5
The relatively low room-temperature conductivities (σRT ≈ 10
-
6
-1
to 10 S cm ) within the ClBPR series point to such a
situation. The observation of 1D ferromagnetism in ClBPMe is
also, we believe, a reflection of the narrow bandwidth of the
material.
4
4
combined interaction remains weak.
The origins of this problem, the failure to generate strong
overlap between adjacent radicals along the π-stack and the
consequently small value of W, can be traced back to the
molecular shape of ClBPR radicals. Thus, while the cyclic
framework is flat, the presence of the N-alkyl group and the
chlorine atom trans to it create a sterically bulky “belt” which
prevents direct superposition (and dimerization) of the radicals.
Slippage of the radical stacks can occur along one of two
directions (Figure 14); examples of each are found in the ClBPR
structures. In ClBPMe the radicals slide sideways, perpendicular
to the Cl‚‚‚NR direction, while in ClBPEt and ClBPPr, they
slide parallel to the Cl‚‚‚NR direction. Within the context of
the band structure calculations, these two motions lead to
dramatically different types of overlaps between the SOMOs
of adjacent rings. In the ClBPEt/Pr pair, the slippage is sufficient
to remove all possible interactions save those between the two
sets of S1‚‚‚S2′ atoms (separated by d1). The in-phase combina-
tion of SOMOs in this arrangement (i.e., k ) 0) is bonding, the
reverse of that normally found for pπ-pπ overlap, and this leads
to the seemingly anomalous π-type dispersion (dE/dk > 0) seen
in Figure 13. The total dispersion is small, less than 0.5 eV,
and is a direct result of the long d1 values (Table 4); a Mott
insulating state follows.
The trends in σRT values and activation energy gaps are in
accord with the trends in EHT bandwidths, but the improved
bandwidth in ClBPMe is largely a result of improved lateral
interactions. The effective orthogonality of the S‚‚‚C interac-
tions, and the resulting poor overlap along the π-stacks in
ClBPMe, also provide a simple orbital rationale for the weak
1
D ferromagnetism in this compound. The observation of this
phenomenon in organic (nonmetal-containing) structures is not
common, but in metal-based systems the onset of ferromag-
45
netism is often associated with an orthogonal orbital effect.
Summary
Our pursuit of stable heterocyclic π-radicals suitable for NRC
materials evolved from attempts to generate perfectly superim-
10
posed π-stacked assemblies of DTDA 1 radicals. While such
materials were thermally very stable, their electronic structures
were extremely 1D, and in the absence of steric constraints spin-
pairing dimerization was inevitable. Even when intradimer bonds
were ruptured, i.e., at elevated temperatures, the liberated spins
were not charge carriers; the free electrons were trapped on the
radical centers, resulting in a Mott insulating state. In an attempt
(43) There are two carbons in each 1,2,3-DTA ring; the ClBPR radical SOMO
(
Figures 2 and 14) is almost nodal at one of these.
In the case of ClBPMe, sideways slippage (Figure 14) seems,
at first glance, to retain a reasonable opportunity for interaction
between adjacent molecules; the close plane-to-plane separation
δ (3.470 Å in the RT phase and 3.411 Å in the LT phase)
certainly suggests so. In the absence of any short S‚‚‚S or S‚‚
(
44) To assess what a strong interaction might be, we carried out a model
dispersion calculation on a 1D stack of perfectly superimposed ClBPH
radicals (with Cl replaced by H to avoid steric problems) set 3.45 Å apart.
The resulting dispersion was 2.5 eV, much larger than that seen in any of
the present ClBPR structures.
(
45) (a) Landrum G. A.; Dronkowski, R. Angew. Chem., Int. Ed. 2000, 39, 1560.
(b) Verdaguer, M. Polyhedron 2001, 20, 1115. (c) Hicks, R. G.; Lemaire,
M. T.; Richardson, J. R.; Thompson, L. K.; Xu, Z. J. Am. Chem. Soc.
‚
N contacts, the most plausible combinations for good overlap
2
001, 123, 7154. (c) Manson, J. L.; Arrif, A. M.; Miller, J. S. Chem.
are between the two sulfurs of one DTA ring and a carbon
Commun. 1999, 1479.
9506 J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002

Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls
A R T I C L E S
(w), 573 (w), 490 (m), 458 (w) cm- . H NMR (δ, DMSO): 7.87 (s,
1
1
to overcome the Mott insulator problem, more delocalized
radicals such as the fused ring 1,3,2-DTA and 1,2,3-DTA
derivatives 4 and 5 were developed.17,18 These exhibited
improved ion energetics and conductivities. However, while the
crystal structures of compound 417 and related compounds8b,c
+
+
aromatic), 6.95 (s, amino). MS (m/e): 223 (M , 48), 197 ([M - CN] ,
+
10), 165 ([M - SCN] , 86). Anal. Calcd for C
H
7 5
N
3
5
S : C, 37.65; H,
2
.26; N, 31.37. Found: C, 37.65; H, 2.18; N, 31.42.
Synthesis of 2,6-Diaminopyridine-3,5-dithiol, 9. A slurry of 2,6-
diaminopyridine-3,5-bis(thiocyanate) (1.99 g, 8.93 mmol) and sodium
sulfide nonahydrate (8.74 g, 36.4 mmol) was warmed in 90 mL of
degassed, deionized water for 1 h. The brown-yellow solution was
cooled on ice, and to this was added glacial acetic acid, dropwise, until
a pH of 5.5-6.0 was attained. The mixture was stirred for 30 min and
the yellow precipitate collected by filtration. The crude product was
redissolved in 30 mL of glacial acetic acid and the solution cooled in
an ice/water bath. Concentrated sodium hydroxide solution was added
dropwise until a pH of 5.5-6.0 was attained. The mixture was left to
stand under an inert atmosphere for 16 h. The reprecipitated product 9
was collected by filtration and dried in vacuo; yield 1.25 g (7.21 mmol,
consist of undimerized slipped π-stacks at ambient temperatures,
they too exhibit a propensity to slide into spin-paired dimers at
low temperatures. The possibility of utilizing these phase
8
b,c
changes in magnetic switching devices has been suggested.
Highly delocalized dithiazolodithiazolyl radicals, of which
the ClBPR series (R ) Me, Et, Pr) described here are the first
examples, constitute a new generation of molecular building
blocks with potential applications in single-component magnetic
and conductive materials. Their calculated gas-phase dispro-
portionation enthalpies and measured electrochemical cell
potentials are superior to those of all previously known
heterocyclic radicals, and should afford materials with a low
on-site repulsion U. However, while the Cl and R groups in
the sterically bulky “belt” prevent dimerization, they also militate
against well-developed electronic band dispersion along the
π-stack. The paramagnetic to weakly 1D ferromagnetic phase
transition at 93 K in ClBPMe is a manifestation of this weak
bandwidth W. Interestingly, and in contrast to the phase changes
observed for DTDA and DTA radicals, the structural change
associated with this phase transition in ClBPMe involves an
adjustment to a more (rather than less) acutely slipped π-stack
array. We are currently pursuing variants of the dithiazolodithia-
zolyls described here, derivatives in which the exocyclic
substituents are either modified or removed entirely. Larger W/U
ratios and improved conductivities will hopefully follow.
8
1%). IR: 3429 (m), 3325 (m, br), 2473 (w), 1694 (s), 1654 (s), 1613
(
(
s, br), 1402 (s), 1329 (w), 1294 (m), 1255 (m), 655 (w), 637 (w), 611
w), 453 (m) cm . MS (m/e): 173 (M , 100). This material was used
-
1
+
without further purification in subsequent steps.
Preparation of 8-Chloro-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]py-
ridin-2-ium Chloride, [ClBPH][Cl], 10. This compound was prepared
by two different methods. Route 1 (Scheme 1): 2,6-Diaminopyridine-
3
2 2
,5-dithiol (1.20 g, 6.93 mmol) and excess S Cl (11 mL, 13.9 mmol)
were stirred at reflux in 60 mL of chlorobenzene under an inert
atmosphere for 16 h. The reaction mixture was cooled to room
temperature and the (relatively pure) [ClBPH][Cl] collected by filtration,
washed with 20 mL of methylene chloride and then 20 mL of
acetonitrile, and dried in vacuo. Recrystallization of the purple-black
powder was not possible due to its insolubility, but metathesis with
AgSbF
6
(see below) afforded a pure hexafluorantimonate, yield 1.57 g
5.19 mmol, 75%), mp > 186 °C dec. IR: 3084 (vw), 2741 (vw), 1655
w), 1624 (w), 1507 (m), 1328 (s), 1105 (m), 842 (vw), 831 (s), 761
(
(
(
-
1
s), 721 (w), 708 (w), 665 (s), 630 (s), 519 (m), 501 (w), 480 (s) cm
MS (m/e): 266 (M , 35), 265 ([M - H] , 92)]. Route 2 (Scheme 2):
2,6-Diaminopyridine (10.0 g, 91.6 mmol) and excess S Cl (25 mL,
.
Experimental Section
+
+
General Procedures and Starting Materials. The reagents 2,6-
diaminopyridine, ammonium thiocyanate, bromine, sodium sulfide
hydrate, sulfur monochloride, gallium trichloride, silver hexafluoroan-
timonate, Proton Sponge, methyl and ethyl triflates, and decamethyl-
ferrocene were obtained commercially (all from Aldrich) and used as
received. Sulfur dioxide gas (reagent grade, Matheson) was also used
2
2
312 mmol) were stirred at reflux in 500 mL of dichloroethane under
an inert atmosphere for 16 h. The reaction mixture was cooled to room
temperature and the (highly impure) black powder collected by filtration,
washed with 200 mL of hot chlorobenzene and then 200 mL of
methylene chloride, and dried in vacuo; crude yield 29.02 g, whose IR
spectrum was characteristic of [ClBPH][Cl]. Gallium trichloride (10.0
g, 56.8 mmol) was then added to a slurry of crude [ClBPH][Cl] (14.05
g, 46.5 mmol) in 150 mL of acetonitrile. The mixture was stirred for
30 min under an inert atmosphere to afford a deep turquoise solution.
The solvent was removed by flash distillation and the deep red [ClBPH]-
4
6
as received. Propyl triflate was prepared according to literature
methods. All solvents were of at least reagent grade, and dried by
2 5
distillation from P O (for acetonitrile, chlorobenzene, and dichloro-
ethane). All reactions were performed under an atmosphere of dry
nitrogen. Melting points are uncorrected. Infrared spectra (Nujol mulls,
KBr optics) were recorded on a Nicolet Avatar FTIR spectrometer at
[GaCl
4
] (13) dried in vacuo. The solid was washed with 3 × 150 mL
-
1
2
cm resolution. Low-resolution mass spectra (70 eV, EI/DEI and
of glacial acetic acid and the red, microcrystalline solid collected by
filtration and dried in vacuo; yield 14.1 g (29.5 mmol, 64%). The
product 13 was redissolved in 200 mL of acetonitrile, and to this was
added 20 mL of pyridine to afford a dark purple precipitate of 10. The
powder was collected by filtration, washed with 2 × 50 mL of ethanol,
CI/DCI) were run on a Finnigan 4500 quadrupole mass spectrometer
at the McMaster Regional Centre for Mass Spectrometry. Elemental
analyses were performed by MHW Laboratories, Phoenix, AZ.
Preparation of 2,6-Diaminopyridine-3,5-bis(thiocyanate), 8. A
4
and dried in air; yield 8.7 g (28.8 mmol, 97% from [ClBPH][GaCl ]).
2
solution of Br (19.0 mL, 0.358 mol) in 100 mL of methanol was added
dropwise over 30 m to a stirred solution of 2,6-diaminopyridine (10.0
g, 0.092 mol) and ammonium thiocyanate (56.0 g, 0.736 mol) in 400
mL of methanol. The reaction mixture was kept on ice until the addition
was complete and then allowed to stir at room temperature for 1 h to
afford a white precipitate. The mixture was poured onto 1 L of ice/
water and left for 1 h. The resulting gray precipitate was collected by
filtration, washed with 3 × 100 mL of water, and dried in air. The
product 8 was recrystallized from hot glacial acetic acid as yellow
crystals; yield 14.1 g (0.063 mol, 68%), mp > 200 °C dec. IR: 3494
Preparation of 8-Chloro-4H-bis[1,2,3]dithiazolo[4,5-b:5′,4′-e]py-
ridin-2-ium Hexafluoroantimonate, [ClBPH][SbF6], 11. This com-
pound was prepared by two different methods. Route 1: 10 (1.20 g,
3.97 mmol) and AgSbF
SO for 48 h to afford a turquoise solution with bronze microcrystals
and a white precipitate of AgCl. The product was extracted 10 times
with SO and the solvent removed by evaporation. The product was
6
(1.47 g, 4.28 mmol) were stirred in 15 mL of
2
2
washed with 40 mL of glacial acetic acid and dried in air to yield a
bronze, microcrystalline product, yield 1.40 g (2.78 mmol, 70%).
Crystals of 11 for analytical purpose were obtained by recrystallization
(m), 3429 (m), 3381 (m), 3325 (w), 3140 (m, br), 2152 (s), 1659 (s),
1
629 (s), 1601 (s), 1561 (s), 1540 (s), 1249 (w), 953 (w), 756 (m), 663
from hot glacial acetic acid; mp > 250 °C. UV-vis (CH
61 nm, log ꢀ ) 4.5. IR: 3259 (w), 1660 (w), 1548 (w), 1329 (s),
1103 (m), 1030 (w), 949 (w), 841 (s), 725 (w), 716 (w), 668 (s), 653
3
CN): λmax )
6
(46) Beard, C. D.; Baum, K.; Grakauskas, V. J. Org. Chem. 1973, 38, 3673.
J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002 9507

A R T I C L E S
Beer et al.
+
(
2
8
s), 637 (m), 558 (w), 522 (m), 472 (s) cm-1. MS (m/e): 266 (M , 5),
The product was recrystallized from hot glacial acetic acid (1.0 g in
100 mL and then concentrated to 50 mL); yield 1.368 g (3.08 mmol);
mp > 250 °C dec. IR: 1520 (m), 1363 (m), 1269 (m), 1241 (m), 1189
(w), 1169 (w), 1082 (w), 1026 (s), 891 (w), 821 (w), 793 (w), 765
(vs), 667 (s), 637 (vs), 609 (w), 573 (w), 553 (w), 515 (m), 477 (vs)
+
65 ([M - H] , 34). Anal. Calcd for C
.36. Found: C, 12.16; N, 8.50. Route 2: 10 (7.90 g, 26.1 mmol) and
(9.88 g, 28.7 mmol) were stirred at gentle reflux in 150 mL of
5 6 3 4
HClF N S Sb: C, 11.95; N,
AgSbF
6
acetonitrile for 24 h to afford a deep turquoise solution and a white
AgCl precipitate. The solution was suction filtered and the filtrate
combined with 100 mL of chlorobenzene. The solution was concen-
trated to 50 mL on a rotary evaporator and the bronze, microcrystalline
product collected by filtration, washed with 50 mL of chlorobenzene,
and dried in air; yield 11.13 g (22.1 mmol, 85%). Materials prepared
by either of routes 1 and 2 were of sufficient purity to be used in the
synthesis of ClBP (12).
-
1
cm . Anal. Calcd for C
8 5 3 3 3 5
H ClF N O S : C, 21.64; H, 1.14; N, 9.47.
Found: C, 22.00; H, 1.01; N, 9.63.
Preparation of 8-Chloro-4-ethyl-4H-bis[1,2,3]dithiazolo[4,5-b:
′,4′-e]pyridin-3-yl, ClBPEt, 6b (R ) Et). Solid decamethylferrocene
0.387 g, 1.18 mmol) was added to a solution of [ClBPEt][OTf] (0.507
5
(
g, 1.14 mmol) in 25 mL of degassed CH CN, and the mixture stirred
3
under an inert atmosphere for 1 h. Green/brown microcrystals of 6b
(R ) Et) were collected by filtration, washed with acetonitrile (3 × 15
mL), and dried in vacuo; yield 0.333 g (1.13 mmol, 99%). Recrystal-
lization from hot, degassed, dichloroethane (0.152 g, 0.52 mmol, in 50
mL) afforded green/bronze needles, yield 0.083 g (0.28 mmol, 54%
from crude material), mp > 250 °C dec. IR: 1495 (w), 1421 (w), 1313
Synthesis of 8-Chlorobis[1,2,3]dithiazolo[4,5-b:5′,4′-e]pyridine,
ClBP, 12. A solution of Proton Sponge (0.99, 4.66 mmol) in 2 mL of
methylene chloride was added, via syringe, to a stirred solution of 11
1.00 g, 1.98 mmol) in 15 mL of acetonitrile. A blue/gray precipitate
formed immediately. After 5 min the product 12 was collected by
filtration, washed with 20 mL of acetonitrile, and dried in vacuo; yield
.52 g (1.95 mmol, 98%); mp > 250 °C dec. IR: 1407 (w), 1267 (m),
097 (w), 862 (vw), 830 (s), 754 (s), 732 (m), 641 (m), 525 (m), 488
w), 469 (m) cm . MS (m/e) 265 (M , 82). Recrystallization was not
(
(w), 1217 (m), 1180 (w), 1083 (w), 1073 (w), 1003 (m), 789 (vs), 743
0
1
(
(
s), 686 (vs), 658 (m), 596 (s), 524 (w), 509 (m), 495 (w), 466 (s)
-1
7 5 3 4
cm . Anal. Calcd for C H N S Cl: C, 28.51; H, 1.71; N, 14.25.
-
1
+
Found: C, 28.70; H, 1.79; N, 14.32.
possible due to the insolubility of the product. Crystals suitable for
X-ray structural determination were grown by layered diffusion. A
solution of 11 (0.100 g, 0.20 mmol) in 5 mL of acetonitrile was added
slowly to an unstirred solution of Proton Sponge (0.125 g, 0.58 mmol)
in 10 mL of methylene chloride. Green/black needles of the solvate
Preparation of 8-Chloro-4-propyl-4H-bis[1,2,3]dithiazolo[4,5-b:
′,4′-e]pyridin-2-ium Trifluoromethanesulfonate, [ClBPPr][OTf].
5
Propyl trifluoromethanesulfonate (5.0 g, 0.026 mol) was added, via
syringe, to a stirred slurry of 12 (1.75 g, 6.58 mmol) in anhydrous
diethyl ether (20 mL) under an inert atmosphere. The reaction mixture
was stirred at room temperature for 4 days, and the red, microcrystals
of [ClBPPr][OTf] were collected by filtration, washed with anhydrous
diethyl ether (2 × 10 mL), and dried in air; crude yield 2.0 g (4.36
mmol, 66%). The crude product was dissolved in acetonitrile, filtered,
and evaporated. The evaporate was recrystallized from hot glacial acetic
acid (1.0 g in 50 mL); yield 1.37 g (2.99 mmol, 68% from crude yield);
mp > 250 °C dec. IR: 3158 (w), 1554 (w), 1518 (w), 1342 (s), 1244
2 2 2
[ClBP] ‚CH Cl grew at the interface of the two solutions; mp > 250
°
C dec. IR: 1405 (vw), 1265 (m), 1103 (w), 864 (vw), 832 (s), 751
-
1
(s), 729 (vs), 639 (s), 536 (m), 523 (s), 490 (m), 461 (s) cm . This
solvate was characterized crystallographically.
Preparation of 8-Chloro-4-methyl-4H-bis[1,2,3]dithiazolo[4,5-b:
5
′,4′-e]pyridin-2-ium Trifluoromethanesulfonate, [ClBPMe][OTf].
Methyl trifluoromethanesulfonate (1.4 mL, 12.4 mmol) was added, via
syringe, to a stirred slurry of 12 (2.58 g, 9.7 mmol) in 80 mL of
anhydrous diethyl ether. The reaction mixture was stirred for 12 h,
and the red microcrystals of [ClBPMe][OTf] were collected by filtration,
washed with 2 × 30 mL of anhydrous ether, and dried in vacuo; crude
yield 3.70 g (8.6 mmol, 89%). The material was recrystallized from
hot acetonitrile (1.0 g per 60 mL). The recrystallized yield varied from
(
7
(
s), 1180 (m), 1109 (w), 1028 (s) 969 (w), 925 (w), 849 (w), 839 (m),
67 (vs), 748 (w), 671 (m), 635 (vs), 573 (w), 522 (w), 513 (m), 476
-1
9 7 3 3 3 5
s) cm . Anal. Calcd for C H ClF N O S : C, 23.60; H, 1.54; N, 9.18.
Found: C, 23.82; H, 1.34; N, 8.94.
Preparation of 8-Chloro-4-propyl-4H-bis[1,2,3]dithiazolo[4,5-b:
′,4′-e]pyridin-3-yl, ClBPPr, 6c (R ) Pr). Solid decamethylferrocene
5
(
6
1
1
7
0% to 70% (from crude material); mp > 250 °C dec. IR: 1516 (w),
491 (s), 1424 (s), 1347 (s), 1277 (s), 1237 (vs), 1171 (s), 1120 (m),
057 (m), 1025 (vs), 985 (w), 924 (w), 874 (w), 832 (m), 768 (vs),
17 (w), 674 (s), 637 (s), 612 (w), 575 (w), 540 (w), 515 (m), 477 (s)
0.23 g, 0.71 mmol) was added to a solution of [ClBPPr][OTf] (0.30
g, 0.65 mmol) in 15 mL of CH CN and the mixture stirred, under an
inert atmosphere, for 1 h. The resulting green/brown microcrystals of
3
6
1
c (R ) Pr) were collected by filtration, washed with acetonitrile (3 ×
-
1
cm . Anal. Calcd for C
7 3 3 3 3 5
H ClF N O S : C, 19.56; H, 0.70; N, 9.77.
5 mL), and dried in vacuo; yield 0.17 g (0.55 mmol, 84%).
Found: C, 19.80; H, 0.56; N, 9.86.
Recrystallization from hot, degassed, dichloroethane (0.17 g, 0.55 mmol,
in 20 mL) afforded green/bronze needles, yield 0.095 g (0.31 mmol,
Preparation of 8-Chloro-4-methyl-4H-bis[1,2,3]dithiazolo[4,5-b:
′,4′-e]pyridin-3-yl, ClBPMe, 6a (R ) Me). Solid decamethylferrocene
0.670 mg, 2.05 mmol) was added to a solution of [ClBPMe][OTf]
0.860 g, 2.00 mmol) in 15 mL of degassed acetonitrile, under an inert
5
(
(
5
1
7
4
6% from crude material), mp > 250 °C dec. IR: 1493 (w), 1422 (w),
313 (w), 1273 (w), 1216 (s), 1081 (m), 970 (m), 891 (w), 795 (vs),
42 (vs), 687 (s), 655 (m), 627 (m), 594 (w), 529 (m), 509 (s), 468 (s),
atmosphere, for 45 min. Green/brown microcrystals of 6a (R ) Me)
were collected by filtration, washed with 3 × 15 mL of acetonitrile,
and dried in vacuo; yield 0.535 g (1.90 mmol, 95%). Recrystallization
from hot, degassed, dichloroethane (0.150 g, 0.53 mmol, in 30 mL)
afforded green/bronze needles; yield 0.115 g (0.41 mmol, 77% from
crude material); mp > 250 °C dec. IR: 1455 (s), 1400 (vw), 1240 (s),
-1
8 7 3 4
58 (m) cm . Anal. Calcd for C H N S Cl: C, 31.11; H, 2.16; N, 13.60.
Found: C, 31.13; H, 2.16; N, 13.70.
EPR Spectra. X-Band EPR spectra were recorded at ambient
temperature using a Bruker EMX-200 spectrometer; samples of the
2 2
radicals were dissolved in degassed CH Cl . Hyperfine coupling
4
7
constants were obtained by spectral simulation using Simfonia and
WinSim. The sweep width (SW), Lorentzian/Gaussian (L/G) ratio, and
line width (LW) used in the simulation of the ClBPMe spectrum are
provided in the caption to Figure 5.
1
194 (m), 1045 (m), 972 (m), 794 (s), 745 (s), 679 (s), 655 (s), 592
-
1
6 3 3 4
(s), 511 (s), 476 (w), 458 (m) cm . Anal. Calcd for C H N S Cl: C,
2
5.66; H, 1.08; N, 14.96. Found: C, 25.72; H, 0.86; N, 15.11.
Preparation of 8-Chloro-4-ethyl-4H-bis[1,2,3]dithiazolo[4,5-b:
′,4′-e]pyridin-2-ium Trifluoromethanesulfonate, [ClBPEt][OTf].
Density Functional Calculations. All DFT calculations on ClBP
5
and ClBPH were run on PC workstations using the B3LYP DFT
Ethyl trifluoromethanesulfonate (2.0 mL, 0.018 mol) was added, via
syringe, to a stirring slurry of 12 (2.48 g, 9.34 mmol) in anhydrous
diethyl ether (170 mL) under a nitrogen atmosphere. The reaction
mixture was stirred for 4 days, and the red, microcrystals of [ClBPEt]-
4
8
method, as contained in the Gaussian 98W suite of programs.
Geometries were optimized using the 6-31G** basis set, within the
constraints of C2V symmetry.
[OTf] were collected by filtration, washed with anhydrous diethyl ether
(2 × 30 mL), and dried in air; crude yield 3.70 g (8.3 mmol, 89%).
(47) WinEPR Simfonia, Bruker Instruments, Inc., Billerica, MA.
9508 J. AM. CHEM. SOC.
9
VOL. 124, NO. 32, 2002

Resonance-Stabilized 1,2,3-Dithiazolo-1,2,3-dithiazolyls
A R T I C L E S
Cyclic Voltammetry. Cyclic voltammetry was performed using a
PINE Bipotentiostat, model AFCClBP1, with scan rates of 50-100
Conductivity Measurements. Single-crystal (along the needle axis)
conductivities were measured in a four-probe configuration, with in-
line contacts made using silver paint. Conductivity was measured in a
custom-made helium variable-temperature probe using a Lake Shore
340 temperature controller. A Keithley 236 unit was used as a voltage
source and current meter, and two 6517A Keithley electrometers were
used to measure the voltage drop between the potential leads in the
four-probe configuration.
-
1
mV s on solutions of [ClBPH][SbF
6
] and [ClBPR][OTf] (R ) Me,
) containing 0.1 M
Et, Pr) in CH CN (dried by distillation from P O
3
2 5
tetra-n-butylammonium hexafluorophosphate. Potentials were scanned
with respect to the quasi-reference electrode in a single-compartment
+
cell fitted with Pt electrodes and referenced to the Fc/Fc couple of
4
9
ferrocene at 0.38 V vs SCE. The Epa-
E
pc separations of the reversible
+
couples were within 10% of that of the Fc/Fc couple.
Band Calculations. Band electronic structure calculations were
51
X-ray Measurements. X-ray data for all three structures were
collected with monochromated Mo KR radiation on diffractometers
using CCD detection. The three room-temperature data sets (Table 3)
were collected at the University of Arkansas on a Rigaku Mercury
diffractometer, while the 25 K data set was collected at the University
of Toledo on a Siemens SMART unit using a prototype helium
performed with the EHMACC suite of programs using the Coulomb
52
parameters of Baasch, Viste, and Gray and a quasi-split valence basis
53
set adapted from Clementi and Roetti; numerical values are tabulated
elsewhere.54 The off-diagonal elements of the Hamiltonian matrix were
calculated with the standard weighting formula.5 Atomic positions were
taken from the crystallographic data.
5
5
0
cryostat. Crystals were mounted onto glass fibers with silicone or
epoxy. The structures were solved using direct methods and refined
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERCC), the U.S.
Office of Basic Energy Sciences, Department of Energy (Grant
No. DE-FG02-97ER45668), the Office of Naval Research
2
2
by full-matrix least-squares which minimized ∑w(∆F ) . The ClBPMe
crystal cracked into two domains upon cooling; the larger component
(
92%) was well defined, while the data for the second one (8%) were
relatively poor. A refinement on both components converged to 8.3%
52° cutoff in 2θ). A better model was obtained by refining on only
(Contract No. N00014-99-1-0392), and the Arkansas Science
(
and Technology Authority for financial support. We also thank
the NSERCC for a postgraduate scholarship to L.B.
those reflections that originated from the dominant component.
Magnetic Susceptibility Measurements. Magnetic susceptibilities
were measured over the temperature range 5-380 K on a George
Associates Faraday balance operating at 0.5 T.
Supporting Information Available: Details of X-ray crystal-
lographic data collection and structure refinement and tables
of atomic coordinates, bond distances and angles, anisotropic
thermal parameters, and hydrogen atom positions for ClBPR
(R ) Me, Et and Pr) (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(
48) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
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