Synthesis, structure and redox reactions of a new crowded benzodithiolium salt: First isolation and characterization of a stable dithiolyl radical with a 7Π electron framework

Article abstract of DOI:10.1039/a905276c

The molecular structure of the sterically crowded 3-(2,4,6-triisopropilphenyl)-1,2,benzodithiolium cation has been determined by crystallographic studies on the BF₄^- salt and the corresponding novel 1,2-dithiolyl radical with a 7π el

Full text of DOI:10.1039/a905276c

Synthesis, structure and redox reactions of a new crowded benzodithiolium
salt: first isolation and characterization of a stable dithiolyl radical with a 7p
electron framework
Satoshi Ogawa,*a Mieko Kikuchi,a Yasushi Kawai,b Shigeya Niizumac and Ryu Sato*a
a Department of Applied Chemistry and Molecular Science, Faculty of Engineering, Iwate University, Morioka
020-8551, Japan. E-mail: ogawa@iwate-u.ac.jp
b Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
c College of Humanities and Social Science, Iwate University, Morioka 020-8550, Japan
Received (in Cambridge, UK) 30th June 1999, Accepted 9th August 1999
The molecular structure of the sterically crowded
3-(2,4,6-triisopropylphenyl)-1,2-benzodithiolium cation has
been determined by crystallographic studies on the BF42 salt
and the corresponding novel 1,2-dithiolyl radical with a 7p
electron framework has been isolated by one-electron
reduction.
the resulting dithiol 4 was carried out in the presence of I2/Et3N
after displacement of the solvent from toluene to CH2Cl2. After
usual work-up, the crude product was purified by column
chromatography (silica gel, n-hexane) to give 3-(2,4,6-triiso-
propylphenyl)-3H-1,2-benzodithiole 5 quantitatively. The cor-
responding dithiolium salt 1 was prepared by a two-electron
oxidation of dithiole 4 with 2 equiv. of NOBF4 in quantitative
yield.†
The structure of new dithiolium salt 1 has been fully
characterized by physical and spectroscopic means, and its
solid-state structure (recrystallized from benzene) was deter-
mined by single-crystal X-ray diffraction (Fig. 1).‡ In the solid
state, the benzodithiolium unit is almost coplanar (the torsion
angles S2S1C8C7 and S2C3C9C4 are 178.9 and 2178.9°,
respectively), with the 2,4,6-triisopropylbenzene ring very
nearly orthogonal to the planar unit (the torsion angle
S2C3C10C11 is 89°), which may arise from the steric repulsion
between the bulky 2,6-isopropyl groups on benzene and the
ortho proton on the benzene ring fused to the dithiolium ring.
This conformation plays an important role in blocking radical
dimerization, which is adopted via a cofacial alignment,
resulting in the isolation of the corresponding radical (vide
infra).
The unusual 7p electron structure of certain five-membered
rings has allowed the construction of a unique reversible one-
electron redox system. Recently, we reported reversible one-
electron redox couples by the use of 4,7-disubstituted benzo-
trichalcogenoles and found by EPR spectroscopy that their
radical cation salts obtained on treatment with a one-electron
oxidant had unusual 7p frameworks.1 However, to date,
dithiolyl radicals with 7p electrons have so far only been
obtained in solution as unstable species by electrochemical
reduction or laser flash photolysis.2 Herein we present the
synthesis and structural determination by X-ray crystallo-
graphic analysis of the new sterically crowded dithiolium salt,
3-(2,4,6-triisopropylphenyl)-1,2-benzodithiolium tetrafluoro-
borate 1, and the first isolation of a stable 7p radical,
3-(2,4,6-triisopropylphenyl)-1,2-benzodithiolyl 2, by one-elec-
tron reduction. In addition, we have succeeded in the construc-
tion of a new type of one-electron redox system between 1 and
2 by means of both chemical and electrochemical methods.
Cyclic voltammetry of 1 in MeCN at 20 °C under an Ar
atmosphere exhibited well-defined reversible one-electron
redox waves at E1/2 = 20.51 V vs. Ag / 0.01 mol dm23
AgNO3.§ This result implies that dithiolium 1 provides a stable
3-(2,4,6-Triisopropylphenyl)-1,2-benzodithiolium
tetra-
fluoroborate 1 was synthesized as follows (Scheme 1). We
employed commercially available thiophenol as a starting
material to prepare the desired cyclic disulfide compound.
2-Mercapto-2A,4A,6A-triisopropylbenzhydrol 3 was obtained in
59% yield by ortho lithiation3 of thiophenol followed by the
addition of 2,4,6-triisopropylbenzaldehyde, which was readily
prepared by general methods.4 Thiolation was performed upon
treatment of the benzhydrol with P2S5 and in situ cyclization of
Fig. 1 ORTEP view of 1. Benzene molecule is omitted for clarity. Selected
bond lengths (Å) and angles (°): S(1)–S(2) 2.020(4), S(1)–C(8) 1.70(1),
S(2)–C(3) 1.69(1), C(3)–C(9) 1.38(1), C3–C10 1.47(1), C(8)–C(9) 1.44(2);
S(1)–S(2)–C(3) 97.8(4), S(2)–S(1)–C(8) 95.3(4), S(2)–C(3)–C(9) 115.0(9),
S(1)–C(8)–C(9) 115.0(8), C(3)–C(9)–C(8) 116(1); S(2)–S(1)–C(8)–C(7)
178.9(10), S(2)–C(3)–C(9)–C(4) 2178.9(8), S(2)–C(3)–C(10)–C(11)
89(1).
Scheme 1 Reagents and conditions: i, TMEDA (2.2 equiv.), BuLi (2.2
equiv.), cyclohexane, ii, 2,4,6-Pri3C6H2CHO; iii, P2S5, toluene; iv, I2, Et3N,
CH2Cl2; v, NOBF4 (2.0 equiv.), THF–MeCN.
Chem. Commun., 1999, 1891–1892
1891

signals gradually decreased at low temperatures, but their
hyperfine structure was unchanged, which suggests that the
bulky substituent serves as an efficient protective group for
spin-dimerization even at low temperature. Although it has not
yet been possible to grow single crystals suitable for X-ray
diffraction, unpaired p-electron density was delocalized over
both the coplanar benzene and the five-membered heterocycle.
Interestingly, the salt 2 undergoes one-electron oxidation to
give 1 quantitatively by treatment with 1 equiv. of NOBF4
(Scheme 2). Thus, the facile interconversion in the redox
reactions of 1 and 2 has been ascribed to the unusual
stabilization of the radical by the 7p electron framework.
Scheme 2 Reagents and conditions: i, Na (1.0 equiv), THF; ii, NOBF4 (1.0
equiv), THF–MeCN.
Notes and references
† Selected data for 1: yellow plates (CH2Cl2–n-hexane); mp 219–225 °C
(decomp.) (Found C, 59.41; H, 5.90. C22H27S2BF4 requires C, 59.73; H,
6.15%); nmax(KBr)/cm21 3449, 2959, 2871, 1593, 1458, 1431, 1380, 1084,
1035, 877, 761, 722; dH(400 MHz, CD3CN) 1.02 (d, 6H, J 6.7, o-CH3), 1.15
(d, J 6.7, 6H, o-CH3), 1.32 (d, J 6.9, 6H, p-CH3), 2.18 (sept, J 6.7, 1H, o-
CH), 3.06 (sept, J 6.9, 1H, p-CH), 7.39 (s, 2H, m-ArH), 7.85 (dd, J 8.5, 0.7,
1H, 4-ArH), 7.91 (ddd, J 8.6, 6.8, 0.7, 1H, 6-ArH), 8.30 (ddd, J 8.5, 6.8, 1.3,
1H, 5-ArH), 8.67 (d, J 8.6, 1.3, 1H, 7-ArH); dC(101 MHz, CD3CN) 24.0 (o-
CH3), 24.1 (o-CH3), 24.9 (p-CH3), 32.5 (o-CH), 35.3 (p-CH), 121.5, 123.6,
126.5, 129.8, 131.3, 138.9, 143.8, 150.0, 155.4, 163.7, 197.4 (3-C).
‡ Crystal data for 1: C22H27S2BF4•C6H6, M = 520.49, monoclinic, space
group C2/c (no. 15), a = 26.387(8), b = 13.177(7), c = 19.851(7) Å, b =
122.91(2)°, U = 5794(4) Å3, T = 293 K, Z = 8, Dc = 1.193 g cm23, m(Cu-
Fig. 2 VT-EPR spectra of 2 at (a) 18, (b) 250, (c) 281 °C, and (d)
simulated spectrum.
Ka)
= = 2192. A yellow prismatic crystal of
20.03 cm21, F(000)
dimensions 0.40 3 0.30 3 0.20 mm was used. 4659 reflections were
measured of which 4544 were unique using a Rigaku AFC7R diffractometer
with Cu-Ka radiation using w–2q scans. The structure was solved by direct
methods (SIR92) and expanded using Fourier techniques (DIRDIF94). The
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
included but not refined. All calculations were performed using the teXsan
crystallographic software package. The final cycle of full-matrix least-
squares refinement was based on 2083 observed reflections [I > 1.50s(I)]
and 316 variable parameters with R = 0.105, Rw = 0.151. CCDC 182/1370.
See http://www.rsc.org/suppdata/cc/1999/1891/ for crystallographic data in
.cif format.
neutral radical even at room temperature. The novel dithiolyl
radical 2¶ was isolated in the one-electron reduction of 1 with
sodium metal in THF (Scheme 2). The structure of the first
isolable dithiolyl radical 2, a green solid, was determined by
high-resolution MS and EPR spectroscopy. The EPR spectra of
2 in THF solution showed the multiple signals (g = 2.0049 G)
attributable to a radical, and the aH values were evaluated by the
fitting of simulated spectra to the experiment spectrum (Fig. 2).
In addition, the experimental values of the unpaired p spin
density from the hyperfine splitting are in good agreement with
those calculated by the simple Hückel and McLachlan’s
methods (Table 1). Using variable-temperature EPR spectra
normalized by the Mn2+ on MgO standard, the intensities of the
§ Cyclic voltammograms of 1 (2.0 mmol dm23) were measured in MeCN
at 20 °C containing 0.1 mol dm23 NBu4ClO4 as a supporting electrolyte
using a glassy-carbon working electrode and Ag/0.01 mol dm23 AgNO3
couple in MeCN as a reference electrode; scan rate in the range from 50 to
500 mV s21
.
¶ Selected data for 2: green crystals (n-hexane); mp 76 °C (decomp.)
(Found: M+ 355.1565. C22H27S2 requires 355.1554); X-band EPR (THF)
Table 1 Unpaired spin populations (rp) of 2
g = 2.0049, aH1 = 0.461 mT, aH2 = 0.394 mT, aH3 = 0.123 mT, aH4
0.121 mT, aH5 = 0.065 mT.
=
1 S. Ogawa, T. Kikuchi, S. Niizuma and R. Sato, J. Chem. Soc., Chem.
Commun., 1994, 1593; S. Ogawa, T. Kikuchi, A. Sasaki, S. Chida and R.
Sato, Tetrahedron Lett., 1994, 35, 5469; S. Ogawa, S. Saito, T. Kikuchi,
Y. Kawai, S. Niizuma and R. Sato, Chem. Lett., 1995, 321; S. Ogawa, S.
Nobuta, R. Nakayama, Y. Kawai, S. Niizuma and R. Sato, Chem. Lett.,
1996, 757; S. Ogawa, T. Ohmiya, T. Kikuchi, Y. Kawai, S. Niizuma and
R. Sato, Heterocycles, 1995, 43, 321; D. Schröder, H. Schwarz, B.
Löbrecht, W. Koch and S. Ogawa, Eur. J. Inorg. Chem., 1998, 983.
2 C. T. Pedersen and C. Lohse, Tetrahedron Lett., 1972, 5213; K.
Bechgaard, V. D. Parker and C. T. Pedersen, J. Am. Chem. Soc., 1973, 95,
4373; C. T. Pedersen and C. Lohse, Acta Chem. Scand., Ser. B, 1975, 29,
831.
rp
Position Atom Exp.
Hückel
McLachlan
3 G. D. Figuly, C. K. Loop and J. C. Martin, J. Am. Chem. Soc., 1992, 111,
645; E. Block, V. Eswarakrishnan, M. Gernon, G. Ofori-Okai, C. Saha,
K. Tang and J. Zubieta, J. Am. Chem. Soc., 1992, 111, 658; K. Smith,
C. M. Lindsay and G. J. Pritchard, J. Am. Chem. Soc., 1992, 111, 665.
4 B. B. Wayland, A. E. Sherry, G. Paszmik and A. G. Bunn, J. Am. Chem.
Soc., 1992, 114, 1673.
1
2
3
4
5
6
S
S
—
—
0.146
0.244
0.149
0.107
0.012
0.176
0.004
0.210
0.335
0.180
0.089
20.019
0.200
C
C
C
C
C
0.175
0.055
0.054
0.205
0.029
12,14
20.007
Communication 9/05276C
1892
Chem. Commun., 1999, 1891–1892