The isolation, characterisation, gas phase electron diffraction and crystal structure of the thermally stable radical [CF3CSNSCCF3]

Article abstract of DOI:10.1039/b001489n

The first structural characterisation of a heterocyclic free radical in all phases has been achieved. 4,5-Bis(trifluoromethyi)-l,3,2-dithiazol-2-yl, [CF3CSNSCCF3]' 4d, a In radical, was prepared quantitatively on reduction of [4d][AsF6] with various reducing agents. It is a blue gas over a green paramagnetic liquid [19F NMR o-59.7], which freezes to a diamagnetic black-green solid. There is a remarkably large volume increase on melting implying that the liquid consists of discrete monomeric radicals, and consistently there is no tendency to dimerise in CC13F solution and the vibrational spectra attributable to the monomer are essentially identical in all phases. The thermodynamic properties obtained for the vapourisation, sublimation and melting processes are very similar to those of related diamagnetic materials. 4d is remarkably thermochemically robust giving decomposition products identical to those from photochemical decomposition, a quantitative mixture of CF3CSSCCF3 (and its oligomers) and CFjCNSNCCF3. The reaction chemistry is described. The crystal structure shows radical 4d to be a planar heterocyclic ring which associates in the solid as a diamagnetic tetramer. The structure of the gaseous monomer determined by electron diffraction is very similar to that in the solid phase. Comprehensive physical measurements were made including the UV-visible spectra of the bright blue solutions, variable temperature magnetic susceptibility and quantitative variable temperature ESR spectra both of which confirm a phase change from a diamagnetic solid to a paramagnetic liquid on melting, the vibrational spectra, and the photoelectron spectrum of the gaseous free radical. A study of the behavior of RCSNSCR' (R = H, CH3 or CF3) given by the reduction of RCSNSCR on increase of concentrations of radical is reported, including the isolation of HCSNSCCF/ at low temperature. The experimental and calculated [UMPW1PW91/6-31+G ] structures of 4d are in good agreement. These and related calculations also support the interpretation of the vibrational and photoelectron spectra, and the thermodynamic properties of 4d, as well as provide possible explanations for the relative stabilities of RCSNSCR', and imply that the weak intramolecular interactions in 4d in the solid state are largely ionic Sff interactions. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b001489n

View Article Online / Journal Homepage / Table of Contents for this issue
The isolation, characterisation, gas phase electron diffraction
and crystal structure of the thermally stable radical
ؒ
[
CF CSNSCCF ] †
3
3
a
a
b
c
Scott Brownridge, Hongbin Du, Shirley A. Fairhurst, Robert C. Haddon,
d
e
a
f
Heinz Oberhammer, Simon Parsons, Jack Passmore,* Melbourne J. Schriver,
g
h
Leslie H. Sutcliffe and Nicholas P. C. Westwood
a
Department of Chemistry, University of New Brunswick, Post Office Box 45222,
Fredericton, New Brunswick, E3B 6E2, Canada. E-mail: passmore@unb.ca
Nitrogen Fixation Laboratory, John Innes Centre, Norwich, UK NR4 7UA
Department of Chemistry and Physics, and Advanced Carbon Materials Center,
University of Kentucky, Lexington, Kentucky, 40506, USA
b
c
d
Institut fur Physikalische und Theoretische Chemie, Universitat Tuebingen,
7
2076 Tuebingen 1, Auf der Morganstralle 8, Germany
e
f
Department of Chemistry, University of Edinburgh, Edinburgh, UK EH9 3JJ
Department of Science, Atlantic Baptist University, P.O. Box 6004, Moncton, New Brunswick,
E1C 9L7, Canada
Chemistry and Physics Departments, University of Surrey, Guildford, Surrey, UK TW16 7LN
Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario,
N1G 2W1, Canada
g
h
Received 23rd February 2000, Accepted 3rd July 2000
First published as an Advance Article on the web 1st September 2000
The first structural characterisation of a heterocyclic free radical in all phases has been achieved. 4,5-
ؒ
Bis(trifluoromethyl)-1,3,2-dithiazol-2-yl, [CF CSNSCCF ] 4d, a 7π radical, was prepared quantitatively on
3
3
19
reduction of [4d][AsF ] with various reducing agents. It is a blue gas over a green paramagnetic liquid [ F
6
NMR δ Ϫ59.7], which freezes to a diamagnetic black-green solid. There is a remarkably large volume increase on
melting implying that the liquid consists of discrete monomeric radicals, and consistently there is no tendency to
dimerise in CCl F solution and the vibrational spectra attributable to the monomer are essentially identical in all
3
phases. The thermodynamic properties obtained for the vapourisation, sublimation and melting processes are very
similar to those of related diamagnetic materials. 4d is remarkably thermochemically robust giving decomposition
products identical to those from photochemical decomposition, a quantitative mixture of CF CSSCCF (and its
3
3
oligomers) and CF CNSNCCF . The reaction chemistry is described. The crystal structure shows radical 4d to
3
3
be a planar heterocyclic ring which associates in the solid as a diamagnetic tetramer. The structure of the gaseous
monomer determined by electron diffraction is very similar to that in the solid phase. Comprehensive physical
measurements were made including the UV-visible spectra of the bright blue solutions, variable temperature
magnetic susceptibility and quantitative variable temperature ESR spectra both of which confirm a phase change
from a diamagnetic solid to a paramagnetic liquid on melting, the vibrational spectra, and the photoelectron
ؒ
spectrum of the gaseous free radical. A study of the behavior of RCSNSCR (R = H, CH or CF ) given by
3
3
ϩ
the reduction of RCSNSCR on increase of concentrations of radical is reported, including the isolation of
ؒ
HCSNSCCF3 at low temperature. The experimental and calculated [UMPW1PW91/6-31ϩG*] structures of
4
d are in good agreement. These and related calculations also support the interpretation of the vibrational and
photoelectron spectra, and the thermodynamic properties of 4d, as well as provide possible explanations for the
ؒ
relative stabilities of RCSNSCR , and imply that the weak intramolecular interactions in 4d in the solid state are
δϩ
δϪ
largely ionic S ؒ ؒ ؒ N interactions.
Introduction
radicals, having novel and/or desirable physical, chemical, or
biological properties. Of particular note is the recent synthesis
2
The preparation of new ionic or neutral heterocycles that
incorporate π bonds to the heavier elements of Groups 15 and
by Oakley, Haddon and co-workers of a C/N/S neutral hetero-
3
cycle that is a π molecular conductor and by Banister, Rawson
1
1
6 has been the subject of much interest in recent years. Motiv-
ؒ
and co-workers of the beta phase of [p-NCC F CNSSN] , a
6
4
ation has included the stabilisation of new π environments
for these elements and the search for compounds, especially
4
spin canted weak ferromagnetic material below 36 K. Our
interest in this area arose from our discovery of the quanti-
tative, symmetry allowed cycloaddition of [SNS] ([SNS]-
ϩ
5,6
[
AsF ]) to a wide range of unsaturated centres as shown in
6
†
Electronic supplementary information (ESI) available: physical and
Schemes 1 and 2 in reference 7, providing general routes to
chemical properties of 4d, optimized geometries, MOs, crystallographic
experimental, etc. See http://www.rsc.org/suppdata/dt/b0/b001489n/
ϩ
[SNS] containing heterocycles, several of which were previ-
DOI: 10.1039/b001489n
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
This journal is © The Royal Society of Chemistry 2000
3365

View Article Online
ously unknown. Radicals 1, 2 and 4 are members of a larger
family of sulfur–nitrogen containing heterocyclic radicals, the
cyclic sulfurs and nitrogens, which is unprecedented in main
ϩ
group chemistry. The reduction of 3 [RCSNSCR] gives the
ϩ
archetype of which is the 7π [SNSNS] cation first structurally
ؒ
14
corresponding [RCSNSCR] 4 quantitatively in dilute solution,
8
elucidated by Banister et al. in 1974. Isovalent substitution into
readily detectable by ESR. Some radicals of this family had
the ring of this radical generates families of related ring systems
demonstrated in Scheme 1. Remarkably, most of these hetero-
been prepared previously by Sutcliffe and co-workers from
the reaction of S N2 with alkynes in dilute solution and
4
15
detected by ESR spectroscopy. The authors noted that deriv-
ؒ
ative [CF CSNSCCF ] 4d showed considerable persistence as
3
3
a free radical but no attempt to isolate and characterise the
radical as a pure material was reported. This implied stability is
consistent with a number of persistent radicals containing a
central SNS moiety attached to electronegative groups that are
reported to be stable in dilute solution, such as the dithiazolyl
ؒ
16,17
ؒ
5
radical [CF SNSCF ] .
The acyclic radical, [F SNSF ] 6
3
3
5
has also been shown to be stable enough to allow identification
and characterisation by ESR in dilute solution. The dissoci-
ation of this radical to stable diamagnetic species has made the
18
complete characterization of this compound difficult.
Very few of the simple derivatives of the radical 4 have actu-
ally been isolated and characterised. Wolmershäuser et al. were
the first to isolate and characterise a 1,3,2-dithiazolyl radical 5
1
5
(
discovered by Sutcliffe and co-workers ), fused to an aromatic
19
ring, as a pure compound. Subsequent investigation has
shown that 5 is monomeric in dilute solution with the stable
20
diamagnetic dimer actually isolated. This spawned a sub-
stantial body of research on 1,3,2-dithiazolyl radicals fused to
aromatic systems including recent research on systems with
21
multiple radical centres. In 1987 we reported, in a preliminary
22a
communication, on the isolation and initial characterisation
of a derivative of 4, the thermally stable 4d, for the first time.
23
More recently, the 4,5-dicyano-1,3,2-dithiazolyl radical 4h
has been isolated by Wolmershäuser and Kraft and shown to
have a similar stability to those of 5 and 4d.
Scheme 1
cycles are known and have been shown to have rich synthetic
Our preparative route to simple derivatives of radical 3, and
hence 4, from alkynes is very general and in this paper we
describe the preparation and relative stabilities of a range of
derivatives of 4 in solution. Possible explanations for the
relative stabilities of 4 are implied by theoretical calculations.
We were able to isolate 4c in small quantities at low temper-
ature. However, of the derivatives of 4 we attempted to prepare,
only 4d was isolable under ambient conditions. It is a green
volatile liquid under a blue gas at room temperature and herein
we give an account of the characterisation of its structure,
spectroscopic and physical properties, including a proposed
structure of the liquid inferred from its physical properties, and
the relationship between structures in all phases. The structural
and spectroscopic interpretations are assisted by theoretical
calculations. 4d is of particular interest as it is a liquid and
paramagnetic under ambient conditions, and when initially iso-
and structural chemistries. We recently reported the first deriv-
9
10
atives of ring systems 7 and 8.
22a
lated was the first example of a class of materials we called
22b
the Paramagnetic liquids‡. Other examples that are liquids
t
under ambient conditions include BuCNSNS and the isomeric
t
22b
BuCNSSN.
In addition, some related solids that are
diamagnetic solids at room temperature become paramagnetic
liquids on melting. We also report a preliminary investigation
of the chemistry of 4d.
Experimental
Reagents and general procedures
ϩ
Reduction of [RCSNSN] led to the quantitative synthesis of
the corresponding radical 1, which in some cases was isolatable
[
Apparatus, techniques and chemicals unless specified have been
t
11
R = Bu, C H or p-C H NO ]. However these derivatives,
6 5 6 4 2
‡
We define a paramagnetic phase as the lowest energy phase in which
and those radicals which were not isolatable, undergo a photo-
chemically and/or thermally symmetry allowed rearrange-
ment via a dimer intermediate to the thermodynamically
more stable isomer 2, first prepared by Banister and co-
the compound exhibits bulk paramagnetism consistent with a near
stoichiometric number of unpaired electrons. Thus a paramagnetic
gas is paramagnetic only in the gas phase, a paramagnetic liquid
is paramagnetic in the gas and liquid phases and diamagnetic in the
solid. A paramagnetic solid is paramagnetic in all phases.
12
13
workers. The net result is a switching of positions of adjacent
3
366 J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
Table 1 Preparations of radical 4d
4
d
Reactant
g, mmol)
Reducing agent or other
compound (g, mmol)
Apparent
reaction time
a
b
(
SO (g)
g
mmol
%
Non-volatile residue
2
3
d[AsF6]
c
Ϫ
5
0
.864, 13.67
.429, 1.00
Na S O (7.09, 40.80)
8.49
5.10
24 h
< 5 min
2.68
0.19
11.15
0.80
82
80
Insoluble white solid (Na S O , AsF ; IR)
2
2
4
2
2
4
6
(C H ) Sb (0.175, 0.50)
Insoluble white solid (N(CH ) AsF ; IR)
6
5
3
3 4 6
N(CH ) Cl (0.110, 1.00)
Soluble yellow solid (N(CH ) Cl, N(CH ) AsF ,
3 4 3 4 6
3
4
(
C H ) Sb and (C H ) SbCl ; IR)
6 5 3 6 5 3 2
0
.877, 2.04
KI (0.344, 2.07)
7.08
3.55
< 5 min
0.393
0.072
1.64
0.20
80
97
Insoluble white solid (KAsF , IR; 0.356 g)
6
Insoluble sublimed black solid (I , mp)
2
3
d[Cl]
d
0
.086, 0.31
(C H ) Sb (0.063, 0.18)
< 5 min
Non-volatile yellow-white solid (C H ) SbCl ,
6 5 3 2
6
5
3
(
C H ) Sb (IR, 0.07 g)
6 5 3
a
b
Assumed to be from the initial warming to r.t. until formation of an opaque black solution. The reaction yield (of the isolated triply distilled
c
product) based on 3d[AsF6]. The reactants (3d[AsF ] and Na S O ) were intimately ground to a fine powder prior to the reaction. No apparent
reaction occurred between the solids. An initial reaction was observed between the two solids prior to the addition of the solvent giving a blue
vapour over the darkened solids.
6
2
2
4
d
2
4
described.
Chemical analyses were by Beller Micro-
by the Faraday method (AT & T Bell Laboratories) using an
21
analytisches Laboratorium. Reactions were generally carried
out in two bulb Pyrex vessels incorporating two J. Young Teflon-
in-glass valves and a medium sintered glass frit separating the
two bulbs (approximately 20 mL volume each). The radical 4d
is photosensitive; therefore it was prepared and handled with a
minimum exposure to light.
apparatus that has been described previously.
Melting points were uncorrected.
25
The preparation of radical 4d
In a typical reaction a sample of 3d[AsF ] and an excess of
6
Na S O , (C H ) Sb and [(CH ) N][Cl] [Aldrich] were used as
2
2
4
6
5
3
3
4
reducing agent were loaded into one bulb of a two-bulb vessel.
Sulfur dioxide was then condensed onto the mixture and the
vessel wrapped in aluminium foil. The mixture was stirred at
room temperature for twenty-four hours, changing from an
initial clear yellow to opaque black. It was filtered and the
insoluble material washed several times with the solvent by
back condensation. The volatile materials were removed to a
U-tube trap (Ϫ196 ЊC) via a dynamic vacuum, and fractionally
distilled under a static vacuum by passage through a series of
U-tube traps held at Ϫ15, Ϫ78 and Ϫ 196 ЊC, the last two con-
received. 3d[AsF ], [CF CSNSCH][AsF ], [HCSNSCH][AsF ]
6
3
6
6
and [HCSNSCCH ][AsF ] were prepared according to refer-
3
6
ence 5(a), [CH CO CSNSCCO CH ][AsF ] and [(CH ) -
3
2
2
3
6
3 3
SiCSNSCSi(CH ) ][AsF ] according to reference 5(b). Infrared
3
3
6
spectra were recorded on a Perkin-Elmer Model 683 spec-
1
19
5
trometer, H and F NMR spectra as previously reported and
FT-Raman spectra from neat samples sealed under nitrogen
in thick walled precision 5 mm NMR tubes (Wilmad Glass,
Buena, NJ) on an FT-IR spectrometer (Bruker IFS66)
equipped with a FT-Raman accessory (Bruker FRA 106), as
taining SO2
and trace amounts of 4d (IR). The Ϫ15 ЊC U-tube
trap contained a black crystalline solid which melted to give a
black liquid at r.t. and was fractionally distilled again twice,
giving a photosensitive black liquid under a bright blue vapour in
the final Ϫ15 ЊC U-tube trap. This black liquid was shown to
be pure radical 4d, typically by mp, infrared and NMR
spectroscopy, and then stored at r.t. in a common reservoir in
the dark. Experimental details are given in Table 1. 4d: mp
24
previously described.
UV-visible spectra were recorded on a Perkin-Elmer Model
30 UV-visible spectrometer in the range 185–1000 nm. The
He(I) photoelectron spectrum of radical 4d was obtained from
the vapor above the liquid at the University of Guelph on an
instrument specifically designed to study labile species and cali-
brated with Ar and CH I. Resolution was 45 meV during data
acquisition.
Solution ESR spectra were recorded using a locally modified
version of a Varian E-4 spectrometer fitted with an external
temperature control unit (University of New Brunswick).
X-Band spectra were recorded at the Royal Holloway College
with a Varian E-4 spectrometer: details of sample manipu-
lation, spectrometer calibration and g-factor measurements
have been given previously. Curie and Q-factor corrections
were made for intensity measurements: the latter correction
was made after examining CFCl with an external sample of
polycrystalline DPPH (diphenylpicrylhydrazyl). Liquid state
X-band spectra were obtained with a Varian E9 ESR
spectrometer fitted with a dual cavity, which enabled Q-factor
corrections to be made with a polycrystalline DPPH sample in
the second cavity.
Mass spectra were recorded on a VG 70780 E mass
spectrometer with an ionising voltage of 70 eV (University of
Durham, Durham, England).
Magnetic susceptibilities were determined by the Gouy
method (University of New Brunswick) using a Newport 1–1/2
inch electromagnet in conjunction with a Stanton Instruments
Model MC5 microbalance with a sensitivity of ±0.02 mg or
3
26
3
11.9–12.5 ЊC (Found: C, 20.31; F, 46.80; N, 5.87; S, 26.84%;
ϩ
M
m/z 240. C F NS requires C, 20.00; F, 47.46; N, 5.83; S,
4 6 2
ϩ
26.70%; M m/z 240). Infrared and Raman spectra of 4d are
given in Figs. 1 and 2, respectively, with vibrational frequen-
cies and assignments given in Table 2. Mass spectrum
[direct inlet, 25 ЊC, 70 eV]: m/z (relative intensity % of highest
34
ϩ
peak, assignment) 242 (7, [CF C SNSCCF ] ), 241 (5,
3
3
13
ϩ
ϩ
15
[ CF CSNSCCF ] ), 240 (74, [CF CSNSCCF ] ), 221 (4,
3
3
3
3
ϩ
ϩ
ϩ
[CF CSNSCCF ] ), 125 (10, [CF C S] ), 113 (9, [CF CS] ), 93
3
2
3
2
3
ϩ
34
ϩ
(5, [CF C ] ), 87 (8, unassigned), 80 (10, [ SNS] ), 78 (100,
3 2
ϩ ϩ ϩ ϩ
3
[SNS] ), 76 (5, [SCS] ), 69 (22, [CF ] ), 64 (6, S ), 46 (32,
3 2
ϩ ϩ ϩ
[SN] ), 40 (11, [CN ] ) and 32 (93, S ). UV-visible spectrum:
2
Solution [quartz cell, path length = 1 mm] (hexane, 0.0453 M),
λ 560 (ε 260), 738 (110); (hexane, 0.0063 M) 227 (2260), 287
(2720) [Fig. 3]; Gas phase [quartz cell, path length = 10 cm, gas,
Ϫ1
Ϫ1
r.t.] 282 (1450), 544 (40) and 708 nm (80 M cm ). Acquisition
of more than one spectrum from a single sample led to the
appearance of bands in the region of 330 and 418 nm that were
also observed in the spectra of the photolytic decomposition
19
products. F NMR spectrum: in SO solution [0.0077 g, 0.032
2
mmol 4d, 0.022 g, 0.17 mmol CCl F, 0.93 g SO in a thick
3
2
walled 5 mm o.d. NMR tube at r.t.] δ (assignment, integration)
J. Chem. Soc., Dalton Trans., 2000, 3365–3382 3367

View Article Online
Fig. 3 The UV-visible spectrum of radical 4d, (A) in hexane after 1 h,
B) After 19 h, with 9 h of irradiation at 544 nm in the spectrometer.
(
Fig. 1 The infrared spectra of radical 4d at (A) P = 20 mmHg, (B) P =
mmHg, and of 3d[AsF ] (C).
1
6
Vapour pressure (VP). At 22 ЊC, vapour over neat liquid,
vapour pressure 25 mmHg, measured by Penning gauge. A
sample of pure radical 4d was contained in one arm of a sealed
mercury manometer immersed in a large (20 L), stirred, water
bath heated with immersion heaters in a slightly modified pro-
cedure to that given in reference 28, in a darkened room. For
the solid a shorter manometer was used with only the sample
bulb cooled in a low temperature bath. The measurements were
repeated on the same samples without significant changes in the
data, indicating that decomposition had not occurred during
the experiment. A full description is given in ref. 25. The plots
of ln VP versus 1000/T gave straight lines by least squares
analysis (correlation coefficients of 0.993 (liquid) and 0.999
(
solid) defined by eqns. (1) and (2). Experimental data are
included with the Supplementary Materials.
ln VP[4d] (l) = (Ϫ4580/T) ϩ 18.3
(1)
(2)
Fig. 2 FT-Raman spectra of radical 4d as a frozen solid at 150 K (A),
ln VP[4d] (s) = (Ϫ5844/T) ϩ 22.5
neat liquid at r.t. (B), close-up of the neat liquid at r.t. (C) and of
3
d[AsF ] at 150 K (D).
Ϫ3
6
ESR spectrum. At 10 M in CCl F, Ϫ70 ЊC: g = 2.0052,
3
N14
F19
a
= 1.125 mT, a = 0.069 mT. Pattern is a 1:1:1 triplet of
1
:6:20:25:20:6:1 septets as reported in reference 15. Neat
0
(CCl F, 0.97), Ϫ53.7 (< 0.01, a hydrolysis product, unknown),
3
liquid: radical 4d was sealed in a capillary (2 mm o.d.). The
sample was quench frozen in liquid nitrogen and kept immersed
for approximately six hours and then placed in one section of
the ESR dual cavity and warmed in stages from 121 K. The
relative concentrations of unpaired electrons in the cavity were
obtained after making Q and Curie corrections (T(K)/relative
concentration (M)): 335/11000, 315/11000, 296/11000, 278/
5400, 269/1300, 259/1600, 241/700, 211/400, 181/400 and 151/
Ϫ55.7 (< 1, unknown), Ϫ56.5 (< 1, unknown), Ϫ62.6 (< 0.01,
CF CSSCCF ) and Ϫ66.6 [< 1, a hydrolysis product, unknown]
3
3
(
the total integration for the non-CCl F resonances = 0.03); in
3
spectra obtained from other samples additional resonances
were observed at δ Ϫ42.3, Ϫ61.9 (CF CNSNCCF ) and Ϫ76.0;
3
3
Neat liquid (0.440 g, 5 mm o.d. thick walled NMR tube),
δ Ϫ59.7, ν1/2 = 200 Hz, integration = 0.98 and three smaller
resonances (δ Ϫ42, Ϫ55, Ϫ62, total integration = 0.02).
Estimated concentration of fluorine in sample by comparison
of standard solutions of CCl F in SO , 32 M (based on the
400, as shown in Fig. 4. 4.35 M in CCl F: the sample was exam-
3
ined above the freezing point of the solution and the relative
concentration of unpaired electrons did not significantly
change, T(K)/relative concentration (M) 318/4.35, 298/4.35,
3
2
density of CF CSNSCCF , 40.5 M). Conductivity: a Pyrex
vessel consisting of a 20 mm o.d. tube with a flat bottom and a
J. Young valve was fitted with platinum wire electrodes that
ended in flattened blades of approximately 0.8 cm area, 0.5 cm
apart. The blades of the electrodes were immersed in radical 4d.
In both the liquid and solid state 4d was non-conductive.
Density: liquid (22 ЊC), 1.63 ± 0.02 g cm , solid (Ϫ78 ЊC),
ρ ≥ 1.97 ± 0.02 g cm (ρ = 2.086 g cm ; calculated from
crystallographic data at Ϫ90 ЊC).
3
3
2
83/4.35, 267/4.27 and 226/4.18.
2
Magnetic susceptibility. Gouy method (calibrated with solid
Hg[Co(CNS)] ). The diamagnetic correction for radical 4d was
8
mol , µ = Ϫ0.02 (± 0.33) µ . Neat Liquid (23 ЊC); χ * = 969
(± 23) × 10
4
Ϫ6
Ϫ1
Ϫ6
4.42 × 10 cgs mol . Solid (0 ЊC); χ * = Ϫ5 (± 45) × 10 cgs
m
Ϫ1
Ϫ3
eff
B
m
Ϫ6
Ϫ1
Ϫ3
Ϫ3
cgs mol , µ = 1.50 (± 0.02) µ . Solution:
eff
B
Ϫ6
(
CCl F, 1.16 M, 23 ЊC) χ * = 1476 × 10
cgs, µeff = 1.86 µB;
3
m
3
368
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

Table 2 Comparison of vibrational data for radical 4d and 3d in 3d[AsF6]
4
d
3
d[AsF6]
Gas phase
IR
FT-Raman r.t.
liquid, neat
FT-Raman
frozen, neat
a
b
c
d
b
b
c
d
IR
FT-Raman
Calculated
Assignment
Calculated
Assignment
3
3
9
6 (vw, 0.7)
8 (vw, 1)
4 (vw, 15)
CF twist
20 (vw, 0.4)
47 (vw, 0.2)
91 (vw, 5.4)
110 (0.1, 0.1)
127 (0.6, 3)
155 (0.5, 0.1)
176 (0.2, 0.1)
253 (vw, 0.5)
253 (0.5, 4)
301 (0.3, 4)
335 (0.3, 7)
372 (vw, 0.7)
417 (0.2, 0.2)
484 (vw, 1)
CF twist
3
3
CF twist
CF twist
3
3
δCSN sym. out of plane
δCCC ϩ δSCC
δCSN out of plane
δCCC ϩ δSCC
δCCC ϩ δSNS
δSNS ϩ δCSN out of plane
ring ϩ CF rock
ring ϩ CF3
ring ϩ CF3
ring ϩ CF3
ring ϩ CF3
1
15 (100)
101 (0.4, 1)
105 (14)
109 (7)
1
42 (7)
131 (69)
157 (54)
178 (56)
1
1
2
2
3
3
3
4
59 (47)
78 (9)
20 (9)
64 (13)
11 (11)
57 (36)
71 (24)
02 (18)
135 (0.4, 9)
162 (0.2, 1)
249 (0.6, 11)
254 (vw, 0.5)
293 (0.4, 14)
336 (1, 28)
383 (1, 14)
387 (2, 6)
44 (0.3, 2)
490 (vw, 3)
33 (0.1, 3)
34 (0.1, 6)
566 (2, 69)
582 (2, 0.2)
δSCC ϩ δSNS out of plane
ring ϩ CF rock
ring ϩ CF3
ring ϩ CF3
ring ϩ CF breathe
ring ϩ CF breathe, ν [AsF ]
δSNS/ring
δSNS out of plane
δsymCSN out of plane ϩ δsymSCC
δSCC out of plane/CF3
186 (2)
265 (4)
3
3
265 (16)
291 (8)
315 (28)
356 (52)
395 (24)
441 (6)
472 (1)
522 (18)
532 (8)
3
Ϫ
315 (7)
350 (5)
390 (1)
446 (1)
475 (1)
521 (2)
3
5
6
ring
δSCC sym. out of plane
δSCC out of plane/CF3
δSNS/δCSN
4
4
74 (12)
505 (0.7, 12)
522 (0.9, 0.8)
532 (0.1, 3)
571 (0.7, 0.4)
5
5
δCF /νCS asym.
δCF /νCS asym.
3
3
Ϫ
δCF /νCS sym., ν ]AsF ]
δCF3
3
2
6
5
64 (25)
δCSN ϩ δSNS
δSCC sym. out of plane
543w
590w
557 (3)
550 (14)
607 (5)
δsym(SCC) out of plane
5
6
6
83w
08w
45m
579 (51)
623 (8)
675sh
641 (0.1, 12)
660 (1, 6)
673 (3, vw)
697 (4, 11)
830 (1, 3)
853 (1, 0.1)
906 (4, 7)
δsym(SCC) out of plane ϩ νSNS sym.
νSNS sym. ϩ δasym(SCC) out of plane
νCS sym. ϩ δCF3
664 (0.1, 7)
683 (4, 0.8)
710 (2, 42)
773 (10, 100)
857 (0.1, 9)
933 (1, 31)
980 (0.2, 5)
1053 (13, 3)
δSCC asym. out of plane
676sh
690 (5)
726 (9)
843 (2)
872 (1)
921 (9)
1000 (1)
1027 (2)
Ϫ
6
83 (79), 700sh
ν1AsF6 /δCF /νCS asym.
691 (34)
727 (42)
852 (24)
878 (6)
929 (29)
1001 (1)
1038 (14)
1149 (4)
1187 (7, br)
3
6
7
94vs
79m
742 (38)
779 (46), 800sh
8
961 (18)
993 (2)
080 (2)
νCS sym. ϩ δCF3
νSNS sym.
νCS asym./νSNS asym.
νCS sym.
νSNS asym.
νCS asym./CF3
νCF3
νCF3
νCF3
νCC asym./νCF3
νCC sym./νCF3
νCC asym./νCF3
721s
δCF /νCS asym.
3
843m
863m
917m
νCS sym. ϩ δCF3
νSNS asym./νCS asym.
νSNS asym.
80 (3)
9
9
63w
93m
1
1028m
1182vs
1006 (0.5, 2)
1142 (0.9, 2)
1155 (59, 0.5)
1167 (0.1, 2)
1174 (88, 7)
1238 (100, 9)
νCS sym.
νCS asym./CF3
νCF3
νCF3
νCF3
1
160 (32, 27)
1
195s, br
1188 (100, 0.3)
1205 (15, 11)
1233 (36, 9)
1242 (78, 17)
1259 (66, 21)
1189 (4, br)
|
|
to
|
1202 (7)
1263 (2)
1216 (3)
1246 (2)
1271 (6, sh)
1288 (27)
1263vs
1282vs
νCC sym./νCF3
1269 (2, sh)
1286 (6)
1
299s, br
1256 (52, 25)
νCC asym./νCF3
1
316w
1
1
348w
371w
1350w
1373 (1)
1368 (7)
1554 (9)
1580 (100)
1
3
1
548 (6)
C isotope effect
ν(C᎐C)
1
504m
1505 (8)
1497 (2, 88)
ν(C᎐C)
1580m
1585 (100)
1596 (12, 100)
a
b
c
d
Nujol mull. Relative intensities in parentheses. UMPW1PW91/6-31ϩG*, IR and Raman relative intensities in parentheses. vw indicates calculated relative intensities less than 0.1. Based on visualisation of
2
7
calculated vibrations using HyperChem. A plus sign indicates two vibrations of equal intensity; a solidus indicates the first listed vibration is more pronounced than the second.

View Article Online
Fig. 4 The relative intensity of the ESR of radical 4d as a function of
temperature.
Fig. 6 Experimental radial distribution function for radical 4d and
difference curve.
8
6% of 4d had decomposed to the observed products. Other
results: % decomposition [temperature (time)]; 9 [23 ЊC (3 h)];
1
6 [202 (2)]; 81 [209 (15)].
Photolytic stability. In a typical photolysis, pure opaque
black liquid radical 4d (0.18 g, 0.08 mmol) in a 5 mm o.d. NMR
tube was exposed to direct sunlight for three hours giving a
clear yellow liquid. SO (0.60 g) and CCl F (0.069 g, 0.50 mmol)
2
3
19
were added and flame sealed. F NMR spectrum of photolysis
products, to which CCl F (0.055 g, 0.40 mmol) and SO (0.58 g)
3
2
were added: δ (integration, assignment) 0 (0.55, CCl F), Ϫ55.5
3
and Ϫ56.2 (0.09, ([CF CSSCCF ] ), Ϫ61.9/Ϫ62.2 unresolved
3
3 n
(
0.36, CF CNSNCCF and CF CSSCCF ), assignments based
3 3 3 3
on reference 29 and 30, corresponding to 85% decomposition.
Fig. 5 The variable temperature magnetic susceptibility of radical 4d.
Ϫ6
Isolation and characterisation of CF CNSNCCF
3
3
(
(
CCl F, 2.00 M, 23 ЊC) χ * = 1587 × 10 cgs, µeff = 1.92 µB;
3 m
Ϫ6
CCl F, 3.26 M, 23 ЊC) χ * = 1164 × 10
cgs, µeff = 1.65 µB.
3
m
The volatile materials (0.229 g) from the photolysis of pure
radical 4d (0.344 g, 1.43 mmol) were isolated in a U-tube trap at
Ϫ196 ЊC, and melted to a pale yellow liquid at room temper-
ature. A catalytic amount of N(CH ) (0.001g, 0.01 mmol) was
A plot of mole fraction of 4d versus the effective magnetic
moment gave an extrapolated value of 1.93 µB at infinite
dilution. Faraday method: the magnetic susceptibility was
measured from 4.2 to 380 K. Low temperature data were
obtained at intervals of about 1 K by using a helium cryostat
3
3
added to polymerise the CF CSSCCF (according to ref. 30),
3
3
giving a non-volatile yellow solid and a volatile colourless liquid
0.10 g, 0.45 mmol, 31% yield based on 4d) identified as pure
CF CNSNCCF by elemental analysis and comparison of the
Ϫ1
cooled at an average rate of 1 K min . The absolute accuracy
(
of the susceptibility is ± 2% as determined by a comparison
with several standards. The relative accuracy of these measure-
ments is much higher, approximately ± 0.06%, so that small
changes can easily be detected. The applied field was 14 kOe,
and the measured susceptibility checked for field dependence at
several temperatures. A liquid sample of 4d was condensed into
a high purity quartz container on a vacuum line and the vessel
backfilled with 0.5 atm of helium. The measured magnetic sus-
ceptibility (Fig. 5) exhibits a Curie tail due to the presence of
trace amounts of paramagnetic material, likely trapped radicals
in a diamagnetic host. The complete data are included in the
Supplementary Materials.
3
3
29
infrared, NMR and mass spectra with published spectra
ϩ
(
Found: C, 21.84; N, 11.50; S, 13.18%; M m/z 222. C F N S
4 6 2
ϩ
requires C, 20.00; N, 12.61; S, 14.41%; M m/z 222).
Electron diffraction of radical 4d
The electron diffraction intensities were recorded at two camera
distances (25 and 50 cm) with a Balzers Gas Diffractograph
31
KD-G2. The accelerating voltage was about 60 kV and the
electron wavelengths were calibrated with ZnO diffraction pat-
terns. The sample, stainless steel inlet system and nozzle were at
room temperature. The camera pressure during the experiment
Ϫ6
never exceeded 5 × 10 Torr. Two plates for each camera dis-
Thermal stability. Pure, opaque black liquid radical 4d
0.035 g, 0.15 mmol) was heated (250 ЊC) for three hours in a
mm o.d. NMR tube fitted with a break seal and wrapped in
aluminium foil, giving a clear yellow liquid. F NMR spectrum
of the product, to which CCl F (0.055 g, 0.40 mmol) and SO2
0.58 g) were added: solution, δ [integration, assignment]
[0.34, CCl F], Ϫ55.5 and Ϫ56.2 [0.12, (CF CSSCCF ) ],
32
tance were analysed by the usual procedures. The averaged
(
5
molecular intensities s·M(s) in the s ranges 1.4–17 and 8–35
Ϫ1
Ϫ1
19
Å
in steps of ∆s = 0.2 Å are presented in the Supplementary
Materials.
3
Analysis of the radial distribution function (Fig. 6) resulted
in a preliminary model with a planar ring and the CF groups in
(
3
0
3
3
3 n
staggered positions with respect to the C᎐C double bond. The
᎐
Ϫ61.9/Ϫ62.2 unresolved (0.55,
CF CNSNCCF3
plus
3
torsional position of the CF groups correspond to that in
3
33
CF CSSCCF ), assignments based on references 29 and 30.
cis-CF CH᎐CHCF . In the least squares analyses a diagonal
3
3
3
3
3
370
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
a
Table 3 Structural parameters (values in Å and Њ) for radical 4d
An attempt was made to collect data at 150 K but the peak
(
gas phase)
profiles deteriorated significantly on cooling, possibly due to
thermal stress or a nearby phase change, and so this experiment
was abandoned.
CCDC reference number 186/2074.
See http://www.rsc.org/suppdata/dt/b0/b001489n/ for crystal-
lographic files in .cif format.
b
C᎐C
1.324(7)
1.749(3)
1.634(2)
1.481(3)
1.330(2)
C᎐C–S
C–S–N
S–N–S
114.8(3)
96.5(6)
117.3(5)
127.3(3)
107.4(2)
1.5(6)
b
C–S
S–N
C–C
C–F
C᎐C–C
F–C–F
c
tilt
a
Theoretical calculations
Error limits are 3σ values and include a possible scale error of 0.1% for
b
c
bond lengths. Dependent parameter. Tilt angle between C axis of
CF group and C–C bond, away from C᎐C bond.
3
Initially UB3PW91/6-31G* geometry optimisations were
3
carried out for radical 4d and 3d using the GAUSSIAN 94
39
suite of programs. Subsequently geometry optimisations were
completed using a different set of functionals and a basis set
including diffuse functions (UMPW1PW91/6-31ϩG*) avail-
weight matrix was applied to the molecular intensities and
scattering amplitudes and phases of reference 34 were used.
Possible deviations of the ring conformation from planarity
40
able in GAUSSIAN 98. The starting geometries used for 3d
and 4d were based on the electron diffraction results for the
neutral 4d. All optimised geometries were stationary points.
Dimerisation and tetramerisation energies§ of 4d were also
calculated using MPW1PW91/6-31ϩG*; these were single
point calculations using geometries taken from the crystal
structure with the major occupancies of the disordered fluorine
atoms (structure A in Fig. 8). Various possible dimers of 4a, 4d,
and 4j were also optimised at the UB3PW91/6-31G* level to
access the dimerisation free energy, with the zero point energy
and derived enthalpy corrections (to 298 K) taken from values
calculated at the UB3PW91/3-21G* level. The optimised geom-
etries and energies of 3d, 4a, 4d, 4j, 20a, 20d, 20j, 21d, 22a,
22d, 22j and 24 are included in the Supplementary Materials.
Vibrational frequencies for 4d and 3d were calculated at the
same levels of theory and scaled by 0.96. Unrestricted (U) and
restricted open-shell (RO) DFT calculations were completed for
the radical 4d to assess Koopmans’-like orbital energies,
together with UHF/6-31ϩG* and ROHF/6-31ϩG* Hartree–
Fock calculations performed at the UMPW1PW91/6-31ϩG*
geometry. In addition the first ionisation energy (IE) was calcu-
lated using an E(cation) Ϫ E(radical) approach which has been
and of the CF groups from the exactly staggered position were
3
tested in a series of refinements. The agreement between model
and experiment improves marginally for a slightly non-planar
ring structure with an out-of-plane angle α ≈ 5Њ (the angle
between the S(1)NS(2) and S(1)C(1)C(2)S(2) planes, see Fig. 6).
The agreement factors R increase for α > 10Њ. This minor devi-
ation from planarity should be interpreted in terms of a planar
equilibrium configuration with a small out-of-plane vibrational
amplitude. Torsional deviations of the CF groups from the
staggered positions up to 15Њ have no effect on the quality of
the fit between experimental and calculated intensities. In the
3
final analysis a planar ring and exactly staggered CF groups
(
3
i.e. C2v overall symmetry) were assumed. The CF groups were
3
constrained to C3v symmetry with a possible tilt angle between
the C3 axes and the C–C bond directions. Assumptions
for vibrational amplitudes are evident from Table 3 given in
the Supplementary Materials. Nine independent geometric
parameters and 14 vibrational amplitudes were refined
simultaneously. The following correlation coefficients had
41
values larger than |0.7|: C᎐C/CF, 0.83; C᎐C/FCF, 0.73 and
CF/FCF, 0.84. The final results are summarized in Table 3.
᎐
᎐
42
43
shown to provide good first IEs for atoms and molecules.
Both adiabatic (radical and cation optimised) and vertical
using optimised radical geometry for the cation) first IEs were
Crystal structure determination
(
Growth of crystals for X-ray diffraction. A sample of radical
d held in a Pyrex capillary (o.d. 0.3 mm) was mounted on a
obtained.
4
Stoe Stadi diffractometer equipped with an Oxford Cryo-
Chemistry of radical 4d
35
systems low-temperature device. It is usual in studies such as
this to cool the sample, controlling the crystal growth front by
careful adjustment of the temperature gradient. The rate of
crystal growth can normally be observed visually, but here the
intense colour of the sample made this impossible. The sample
was therefore cooled through its melting point at various rates,
and the crystallinity assessed at each stage from a rotation
photograph. The results of most of these experiments were
In a series of experiments, pure radical 4d was treated with
AsF , Cl , Br , I , O , H O, air, Hg, NO and Cu under different
5 2 2 2 2 2
conditions. The experimental details, results of reactions,
observations and product analyses are included in Table 4 and
a more complete account is included in the Supplementary
Materials.
Radical 4d and AsF gave 3d[AsF ] as clear colourless fibrous
5
6
36
polycrystalline masses, but an indexable diffraction pattern
plates that are multiple crystals: a = 6.908(3), b = 24.483(10),
c = 7.182(3) Å, α = 90, β = 108.28(3), γ = 90Њ, V = 1153.5(9) Å .
Ϫ1
3
was obtained by cooling from 280 to 270 K at 20 K h .
3
d[Cl]: orange moisture sensitive crystalline solid, mp 143–
ϩ
Crystal and data collection parameters. C F NS , M = 240.17.
146 ЊC (Found: C, 18.00; Cl, 12.90; N, 5.21; S, 22.83%; M m/z
240. C ClF NS requires C, 17.43; Cl, 12.86; N, 5.08; S, 22.26%;
M m/z 240). F NMR (SO , CCl F): δ 0 (CCl F) and 52.8
4
6
2
Triclinic, a = 7.976(3), b = 8.806(3), c = 11.830(4) Å, α =
4
6
2
19
3
1
0
02.15(2), β = 93.40(2), γ = 108.16(2)Њ, U = 764.8(4) Å (λ =
2 3 3
¯
ϩ
.71073 Å. T = 220.0(2) K, space group P1, graphite mono-
(
6F, S, [CF CSNSCCF ] ). IR spectrum [Nujol or fluorolube
3 3
mull, KBr plates]: 1511w, 1314m, 1246ms, 1193s, 1177s, 1078m,
002w, 944w, 880w, 784w, 759m, 736m, 711m, 609w, 583w,
65w and 410m cm . Mass spectrum [direct inlet, 25 ЊC,
chromated Mo-Kα radiation, Z = 4. Blue-black cylinder with
Ϫ1
radius 0.15 mm, µ = 0.752 mm . Reflections were measured
1
4
with ω–θ scans; 5384 data were measured, of which 2697 were
unique (Rint = 0.0421).
Ϫ1
70 eV]: m/z (% of highest peak, assignment) 240 (100, [CF3-
ϩ
ϩ
CSNSCCF ] ), 226 (60, [CF CSSCCF ] ), 221 (48, [CF -
3
3
3
3
Structure solution and refinement. The structure was solved
3
7
2
ϩ
ϩ
by Patterson methods (DIRDIF 96) and refined against F
CSNSCCF ] ), 194 (24, [CF CSCCF ] ), 162 (20, [CF C-
CCF
[SNSCO]
2
3
3
3
38
ϩ
ϩ
ϩ
(
SHELXTL). All trifluoromethyl groups were rotationally
3
] ), 126 (32, [CF
3
C
2
S] ), 113 (43, [CF
3
CS] ), 106 (28,
ϩ
ϩ
)
, 93 (20, [CF C ] ), 88 (28, unassigned), 78 (100,
disordered about their threefold axes. The refinement converged
to a conventional R factor of 0.0404 [based on F and 1953 data
3
2
2
with F > 4σ( F )], and wR2 = 0.1085 (based on F and all 2690
§
It was assumed that the dimerisation or tetramerisation energy was
the difference in the calculated total energy of the dimer or tetramer
minus two or four times the calculated total energy of the monomer.
data used during refinement). The final difference map extrema
Ϫ3
were 0.347 and Ϫ0.254 e Å .
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
3371

View Article Online
ϩ
ϩ
ϩ
2
[
(
SNS] ), 75 (16, unassigned), 69 (80, [CF ] ), 64 (30, S ), 46
3
ϩ
ϩ
37
ϩ
35
ϩ
88, [NS] ), 44 (16, [CS] ), 37 (8, Cl ), 35 (20, Cl ), 32 (80,
ϩ ϩ
S ) and 28 (100, N ). Peaks greater than 5% included. This
2
compound was also prepared in lower yield from the reaction
of 3d[AsF ] (0.420 g, 1.00 mmol) and KCl (0.075 g, 1.00 mmol)
6
in 3.128 g SO . The orange crystalline product (0.174 g, 0.63
2
mmol, 63% yield) was recovered by sublimation from the
non-volatile reaction residue.
3
d[Br]: air stable (IR) red-black crystalline solid, mp 135–
ϩ
1
2
36 ЊC (Found: C, 14.92; Br, 24.89; N, 4.33; S, 19.00%; M m/z
40. C F NS requires C, 15.01; Br, 24.96; N, 4.38; S, 20.03%;
4
6
2
ϩ
19
M m/z 240). F NMR (SO , CCl F): δ 0 (CCl F) and Ϫ53.4
2
3
3
ϩ
(
6F, S, [CF CSNSCCF ] ). IR spectrum [Nujol or fluorolube
3 3
mull, KBr plates]: 1510m 1304ms, 1284w, 1259s, 1185vs,
1
4
7
070m, 982w, 930m, 876w, 772m, 730m, 709ms, 608ms, 578m,
60m and 405m cm . Mass spectrum [direct inlet, 25 ЊC,
Ϫ1
0 eV]: m/z (% of highest peak, assignment) 240 (50, [CF3-
ϩ
ϩ
CSNSCCF ] ), 221 (50, [CF CSNSCCF ] ), 162 (24, [CF -
CCCF ] ), 126 (37, [CF C S] ), 113 (35, [CF CS] ), 106 (22,
3
3
ϩ
2
3
ϩ
ϩ
3
3
2
3
ϩ
ϩ
ϩ
[
[
SNSCO] ), 93 (12, [CF C ] ), 80 (28, [S O] [?]), 78 (75,
3 2 2
ϩ ϩ ϩ ϩ
SNS] ), 69 (80, [CF ] ), 46 (80, [NS] ), 32 (24, S ) and 28 (100,
3
ϩ
N ). Peaks greater than 5% included. This compound was also
2
quantitatively prepared from the reaction of 3d[AsF ] (0.86 g,
6
2
.05 mmol) and KBr (0.30 g, 2.52 mmol) in 5.50 g SO . The red-
2
black crystalline solid (0.60 g, 1.88 mmol, 94% yield) was
recovered by sublimation from the non-volatile reaction
residue.
1
2
The reduction of [R CSNSCR ][AsF ]
6
1
2
In a series of experiments, [R CSNSCR ][AsF ] (approximately
6
2
–3 mg) was mixed with either Na S O and [N(CH ) ][Cl]
2 2 4 3 4
(
(
approximately 3 mg each) or (C H ) Sb and [N(CH ) ][Cl]
approximately 3 mg each) in a quartz 3 mm o.d. ESR tube.
6 5 3 3 4
The solvent [SO and CCl F in a 1:4 w/w ratio] was condensed
2
3
Ϫ4
onto the mixture [to give a solution of approximately 10
M
concentration], which was shaken at room temperature for one
minute. The ESR spectra of these solutions were then acquired
as a function of time. The results of these reactions are given
in Table 5. Preparative scale reactions are also included for
comparison.
The generation and decomposition of [HCSNSCCF ] 4c
3
Sulfur
dioxide
(3.70
g)
was
condensed
onto
[
HCSNSCCF ][AsF ] (3c[AsF ]) (0.350 g, 1.00 mmol),
3
6
6
(
C H ) Sb (0.353 g, 1.00 mmol) and [N(CH ) ][Cl] (0.258 g, 2.30
6 5 3 3 4
mmol) giving an opaque black solution on rapidly warming to
r.t. with shaking for one minute. The volatile materials were
collected in a U-tube trap (Ϫ196 ЊC) by dynamic vacuum leav-
ing a brown-black tar. The former (a white solid [presumably
SO ] and a blue-black crystalline solid [presumably 4c]) gave a
2
black solution at r.t. changing to brown-black after one hour.
Removal of the volatile material [SO ; infrared] gave a non-
2
volatile brown tarry solid, which was recovered and heated
(
80 ЊC under vacuum) to give a non-sublimed brown solid
and a sublimed purple-black crystalline solid that was
identified as SCSNSCCF (0.025 g, 0.12 mmol, 12% yield) by
3
44
comparison with data reported for related compounds.
SCSNSCCF , 13: mp 104–106 ЊC. IR spectrum [Nujol mull,
3
KBr plates]: 1494w, 1340w, 1295m, 1282w, 1267m, 1222w,
1
7
208w, 1176m, 1151m, 1082w, 978m, 939m, 897w, 823vw,
Ϫ1
73sh, 755sh, 743sh, 728m, 692w, 438w, 400w and 360w cm .
1
9
F NMR: δ (integration, assignment) 0.0 (0.74, CCl F) and
3
Ϫ59.8 in SO solution (0.26, [SCSNSCCF ]). Mass spectrum
2
3
[
direct inlet, 25 ЊC, 70 eV]: m/z (% of highest peak, assignment)
ϩ
ϩ
2
[
(
03 (14, [CF CSNSCS] ), 157 (15, [CF CSSC] ), 113 (12,
3 3
ϩ ϩ ϩ ϩ
CF CS] ), 88 (6, [CSSC] ), 78 (5, [SNS] ), 32 (23, S ) and 28
3
ϩ
100, N ). Peaks greater than 5% included.
2
3
372
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
1
2
Table 5 The reduction of [R CSNSCR ][AsF ] in dilute solution
6
Reduction
mixture
Additional
couplings (mT)
Apparent
half-life (h)
1
2
N14
a
R
R
g
a
/mT
b
b
e
e
e
e
e
H
c
H
H
H
CF3
H
Si(CH3)3
CO CH
H
CH3
CF3
CF3
CN
Si(CH3)3
CO CH
3
2.007
2.006
2.009
2.005
2.004
2.008
2.006
1.07
1.07
1.10
1.13
1.07
1.10
1.08
a
a
a
= 0.13
= 0.14
<1
H
H
F
d
F
= a = 0.075
0.3
>200
>0.1g
0.5
a = 0.069
f
f
f
h
>200
2
3
2
a
Determined from the change in intensity of the ESR signal i.e. the time required for intensity to diminish by half assuming the signal width is
b
c
constant. Reduction with Na S O in C H O (THF). When 1.01 mmol of 3a[AsF ] was stirred with 2.81 mmol of Na S O in SO (2.38 g) at room
2
2
4
4
4
6
2
2
4
2
temperature for one month the starting materials were recovered unchanged (IR). An immediate reaction of 5.59 mmol of 3a[AsF ] with 5.59 mmol
6
of Sb(C H ) and 5.59 mmol of N(CH ) Cl in 8.38 g of SO was observed that gave an opaque red-black solution over an insoluble black solid. The
6
4
3
3
4
2
soluble brown solid was shown to contain [N(CH ) ][AsF ], Sb(C H ) Cl and Sb(C H ) by IR The insoluble black solid was washed with liquid SO
3
4
6
6
5
3
2
6
5
3
2
for a month to remove soluble impurities and the remaining black solid (0.315 g) characterised by IR [(Nujol mull, KBr plates); 1201s, br, 1090ms,
Ϫ1
1
040w, sh, 1025s, 870m, 775w, 683w, 603m, 561w, 540w cm ] and elemental analyses (Found: C, 17.50; H, 1.93; N, 7.91; S, 48.74. C H NS requires
2
2
2
d
C, 23.06; H, 1.94; N, 13.45; S, 61.55%). For complete experimental details see reference 25. When 1.00 mmol of 3b[AsF ] was stirred with 3.00 mmol
of Na S O in SO (2.94 g) at room temperature for one month the starting materials were recovered unchanged (IR). Reduction with [N(CH ) ][Cl]
6
e
2
2
4
2
3 4
f
g
and SbPh and in SO solvent. No secondary hyperfine couplings were observed. Reaction of 0.53 mmol of 3e[AsF ] with 2.50 mmol of Na S O in
3
2
6
2
2
4
3
.73 g of SO gave an opaque purple-red solution after 24 hours. The IR spectrum of the insoluble black solid gave only bands assigned to Na S O .
2 2 2 4
h
The soluble materials formed an intractable black tar. Reaction of 0.55 mmol of 3f[AsF ] with 1.97 mmol of Na S O in 2.50 g of SO gave an
6
2
2
4
2
opaque brown-black solution after 36 hours. The IR spectrum of the insoluble black solid gave only bands assigned to Na S O and 3f[AsF ]. The
2
2
4
6
soluble materials formed an intractable brown tar.
Discussion
Preparation of 4d
A typical preparative scale reduction of 3d[AsF ] in liquid
6
sulfur dioxide used sodium dithionite Na S O in large excess as
2
2
4
the reducing agent according to eqn. (3). This chemical reaction
1
9
F NMR analysis of the reduction of 3c[AsF6]
Na S O ϩ 2 3d[AsF ] →
SO (0.70 g) and CCl F (0.068 g, 0.50 mmol) were added to
2
2
4
6
2
3
3
c[AsF ] (0.036 g, 0.10 mmol), (C H ) Sb (0.036 g, 0.10 mmol)
2 4d ϩ 2 SO
2
ϩ 2 Na[AsF
]
6
(3)
6
6
5 3
and [N(CH ) ][Cl] (0.037 g, 0.30 mmol) in a 5 mm o.d. NMR
3
4
tube. The reaction mixture was warmed to room temperature
was preferred because the oxidised product is the solvent SO2
and the spectator ions precipitate as an insoluble, non-volatile
ionic salt. The relatively low purity of the reducing agent [ca.
19
and shaken for one minute. The F NMR spectra of the initial
solution contained only resonances assignable to CCl F and
3
Ϫ
45
[
AsF6] (δ Ϫ56.8, JAs-F = 921 Hz) consistent with quantitative
85% reported by the supplier ] apparently did not result
reduction of 3c to the radical. After twenty minutes there were
5 unassigned resonances with a total integrated intensity equal
in contaminating volatile or soluble impurities. Preparative
scale reductions were also observed from the reactions of the
1
to that of the original 3c, which can be found in reference 25.
hexafluoroarsenate salt 3d[AsF ] with KI, or a mixture of
6
triphenylantimony and tetramethylammonium chloride (Table
1).¶ This radical could be prepared in the absence of solvent by
the reaction of the chloride salt of 3d with triphenylantimony
by mildly heating the mixture under vacuum in the dark accord-
ing to eqn. (4). However, this route required the preparation of
The generation of radical 4c and reaction with chlorine
Sulfur dioxide (3.70 g) was condensed onto 3c[AsF ] (0.143 g,
6
0
.40 mmol), (C H ) Sb (0.173 g, 0.50 mmol) and [N(CH ) ][Cl]
6
5
3
3
4
(
0.110 g, 1.00 mmol) in one bulb of a two bulb, two valve vessel,
rapidly warmed to r.t. and shaken for one minute, giving an
opaque black solution. The volatile materials were collected in
a U-tube trap (Ϫ196 ЊC) by dynamic vacuum leaving a brown-
2
3d[Cl] ϩ Sb(C H ) → 2 4d ϩ Sb(C H ) Cl (4)
6 5 3 6 5 3 2
the intermediate chloride salt from the metathetical ion
exchange reaction between 3d[AsF6] and tetramethyl-
ammonium chloride.
black tar. The former (a white solid [presumably SO ] and a
2
blue-black crystalline solid [presumably 4c]) were treated
immediately with Cl (0.212 g, 3.00 mmol). The initially opaque
2
The radical 4d was isolated from these reaction mixtures by
a fractional distillation train of sequential U-tube traps and
isolated as a black crystalline solid in the Ϫ15 ЊC trap. It is
very reactive towards moisture and oxygen and very photo-
chemically sensitive (see below), leading to some loss of sample
during preparation and purification. Without these difficulties
the yield would be close to 100% (observed isolated yield: 82%
total mass product in Table 1). To obtain very pure samples for
analysis it was usually necessary to repeat the fractional distil-
lation three times and store the material in the dark to minimise
photolytic decomposition. The purity of the radical 4d was
routinely determined by a combination of infrared (sensitive to
black solution changed to a clear yellow in less than thirty sec-
onds. The volatile materials [SO and SO Cl ; infrared] were
2
2
2
removed to give a waxy yellow non-crystalline solid, which was
recovered and identified as 3c[Cl] (0.30 g, 0.15 mmol, 36%
yield). The product decomposed to a black tar on attempted
1
in vacuo sublimations at 70 ЊC. H NMR: δ (integration, assign-
ϩ
ment) 11.2 (0.95, [HCSNSCCF ] , 3.14 (< 0.03, impurity) and
3
19
1
.25 (< 0.03, impurity). F NMR in SO : δ (integration,
2
ϩ
assignment) 0.0 (0.30, [CCl F]), Ϫ53.6 (0.60, [HCSNSCCF ] )
3
3
ϩ
and Ϫ56.7 (0.10, impurity). Assignments of [HCSNSCCF3]
resonances were based on comparison of spectra from solu-
19
the presence of SO solvent and photolytic impurities) and
F
2
tions of [HCSNSCCF ][AsF ] in SO (this work). IR spectrum
3
6
2
[
Nujol mull, KBr plates]: 3075w, 1280ms, 1260m, 1192s, 1158vs,
1
5
039w, 990w, 955w, 857m, 776ms, 730m, 679m, 630w, 580w,
50w and 409m cm .
¶
4d was also generated in situ as one of the products of the reaction of
46
Ϫ1
3d[AsF ] with CsN in SO .
6 3 2
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
3373

View Article Online
Ϫ3
NMR spectroscopy (sensitive to the presence of photolytic
decomposition products). The purity of a routine sample was
greater than 95% by integration analysis of the F NMR
spectra against an internal concentration standard. The
absolute composition of the product was confirmed by good
elemental analyses.
The density in the liquid state (1.63 g cm at 22 ЊC) is
Ϫ3
approximately 20% less than in the solid (1.97 g cm at Ϫ78 ЊC).
19
Normal density changes on melting are about 4%, although
those for the smaller inert gases are comparable and are attrib-
uted to changes in overall structure of the material on change
49
of phase. We attribute the change in density to a change from
a diamagnetic tetramer (see below) in the solid to a para-
magnetic liquid containing isolated molecules separated by the
sums of the corresponding van der Waals radii. In the solid
state the monomers are joined by weak S ؒ ؒ ؒ S π*–π* bonds
perpendicular to the rings (average 3.14 Å). An expansion of
the distance between the rings to 3.7 Å (the anisotropic van der
Waals radii of S perpendicular to the ring estimated as 2.03
Å ) would account for the change in density. A large change in
density on melting is likely to be a distinguishing characteristic
of the paramagnetic liquids.
The vapour pressure of the black solid and liquid phases of
the radical 4d was measured over a range of temperatures
Ϫ20 < T < 100 ЊC). The extrapolated value for the boiling
The thermal and photochemical dissociation of radical 4d
The pure radical in the gas phase and in hexane solutions
Ϫ2
[
4.5 × 10 M] was shown to be unstable with respect to
irreversible decomposition by photochemical (daylight) and
thermal processes (Tdecomp = 209 ЊC, % decomp. = 81 after 15
hours). The product by either thermal or photochemical
decomposition was a diamagnetic yellow oil that contained
50
(
14 and 15; or known oligomers of 15) in exactly the same
1
9
9
stoichiometric ratios [ F NMR]. Reduction of 7d gives 16,
which decomposes to 15 and its oligomers. Therefore we
propose that 4d thermally and photochemically decomposes
30
(
(
∆Hrxn = Ϫ 429 kJ)||, according to Scheme 2 with subsequent
point of the radical was 119 ЊC. The data are consistent with the
following thermodynamic values for pure 4d: ∆H = 38.1 ±
decomposition of 16 to 15 and elemental sulfur.
vap
Ϫ1
Ϫ1
Ϫ1
0
.5 kJ mol , ∆S = 97.1 ± 1.0 J K mol ; ∆H = 49.0 ± 1.5
vap sub
Ϫ1 Ϫ1 Ϫ1
kJ mol , ∆S = 172.1 ± 2.0 J K mol ; ∆H = 10.8 ± 1.5 kJ
sub
fus
Ϫ1
Ϫ1
Ϫ1
mol , ∆S = 75.3 ± 3.0 J K mol . The values for ∆H and
Hsub were determined directly from experimental data whilst
Hfus was calculated using the assumption that the heat of
fus
vap
∆
∆
sublimation is equal to the sum of the heats of fusion and
vaporisation. This derived value for the enthalpy of fusion was
confirmed by direct measurement using Differential Scanning
Scheme 2
Calorimetry which measured the value of ∆H = 11.5 ± 0.1 kJ
fus
Ϫ1 51,52
mol .
Comparison of the physical data with those reported
for cis-2,3-dichloro-1,1,1,4,4,4-hexafluoro-2-butene (∆Hvap
=
Ϫ1
3
2.6 kJ mol ) and related compounds [included in the
Supplemental Materials] shows that 4d behaves very much
like a molecular, covalent compound in the solid, liquid
Ϫ1
Ϫ1
and gas phase (Troutons Rule, ∆Hvap/Tbp = 88 J K mol ;
Ϫ1
Ϫ1
4
d, ∆Hvap/Tbp = 97 J K mol ), with a low dipole moment
of 0.1 (UB3PW91/6-31G*) and 0.4 Debye (UMPW1PW91/6-
1ϩG*). It is as though the energy on changes of state is
Similar switching of sulfur and nitrogen positions occurs
within the ring on the rearrangement of radical 1 to 2. We
have shown that this occurs via a photochemically symmetry-
allowed rearrangement of the dimer of 1 to the dimer of 2 by
3
the same as if 4d were not a radical, i.e. the energy of inter-
molecular interaction between unpaired electrons is minimal.
Consistently 4d has no tendency to associate as revealed by
11,12
minimal atomic displacement.
A similar rearrangement of a
dimer of 4d to the observed products is not readily envisaged.
variable temperature ESR in CCl F, and the calculated
3
However, it is tempting to suggest that the dimer of 4d
48
(MPW1PW91/6-31ϩG*) energy of formation of the tetramer
rearranges to 14 and 16 (via 9d?) and hence to 14 and 15.
Ϫ1
from four monomers in the gas phase is 33 kJ mol per
Aromatic and aliphatic derivatives of the anions of sulfur
tetramer. Given the method of calculation we can conclude that
the tetramerisation energy is very small.
diimides RN᎐S᎐NR are known to be unstable with respect to
᎐
᎐
rearrangement (ref. 1e and references therein). No evidence for
dimer formation of 4d was evident from a quantitative ESR
study in solution as a function of temperature. We propose that
the decomposition occurs via an excited state of 4d either
produced photochemically or thermally, the mechanism of
which warrants further study.
The magnetic properties of radical 4d
The pure radical 4d is paramagnetic in the liquid (µeff = 1.53 µB,
2
2 ЊC) and diamagnetic in the solid state (µeff = 0 µ , 0 ЊC)
B
by magnetic susceptibility measurements (Gouy method). This
magnetic behaviour was confirmed by quantitative, dual cavity,
variable temperature ESR spectroscopy and variable temper-
ature magnetic susceptibility (Figs. 4 and 5). The ESR experi-
ment reveals that the solid material has approximately thirty
The characterisation and physical properties of radical 4d
A pure sample of the isolated radical in an evacuated vessel at
room temperature was a black liquid (mp = 11 ЊC) [black in
reflected light and greenish with transmitted light through a
thin film] under a highly visible blue vapour (measured vapour
pressure at r.t. = 25 mmHg). A similar blue colouration was
observed in dilute solutions of the radical in non-polar solvents
times less unpaired spins than the liquid at 296 K in CCl F and
3
neat liquid 4d had no tendency to form diamagnetic dimers or
polymers. The variable temperature magnetic susceptibility
measurements [Faraday method, Fig. 5] reveal that the dia-
Ϫ6
such as hexane and Freon-11 while solutions in SO were green-
magnetic solid [χ * (140 K) = Ϫ25.7 × 10 cgs] became a
2
m
Ϫ6
ish brown possibly due to formation of a weak charge transfer
complex 4dؒSO2.
paramagnetic liquid [χ * (286 K) = ϩ835 × 10 cgs] upon
m
melting. This material was the first compound to be shown to
have such profoundly different magnetic properties in the solid
and liquid states making 4d the first Paramagnetic Liquid. Sub-
sequently research has discovered other paramagnetic liquids
including the trifluoromethyl and tertiary butyl derivatives of
the 1,3,2,4-dithiadiazolyl 1 and 1,2,3,5-dithiadiazolyl 2 rad-
|
| The calculation of the enthalpy of reaction based on a simple Hess’s
47
Law analysis using bond dissociation energies E and the assumption
that the π delocalisation energies of the two heterocycles are the
same [∆Hrxn = Ebroken[4ES-N (145 kJ)] Ϫ Eformed[2ES-S(236 kJ) ϩ 2ES-N
12
(
145 kJ)].
icals. There are examples of paramagnetic solids, e.g. the
3
374
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
related 1,3,2-dithiazolyl radicals fused to aromatic systems, e.g.
Table 6 Selected bond lengths [Å] and angles [Њ] for radical 4d in the
solid phase
21
21d
1
7 and 18, and related multiradicals. A recent report of 18
suggested that these radicals could be applied in thermal
sensors, switching units, and information storage media.
S(1)–N(2)
S(1)–C(5)
N(2)–S(3)
S(3)–C(4)
C(4)–C(5)
C(4)–C(41)
C(5)–C(51)
1.646(3)
1.730(3)
1.640(3)
1.740(3)
1.351(5)
1.493(5)
1.495(5)
S(6)–N(7)
S(6)–C(10)
N(7)–S(8)
1.638(3)
1.737(3)
1.639(3)
1.734(3)
1.349(4)
1.503(4)
1.490(5)
S(8)–C(9)
C(9)–C(10)
C(9)–C(91)
C(10)–C(101)
N(2)–S(1)–C(5)
S(3)–N(2)–S(1)
N(2)–S(3)–C(4)
C(5)–C(4)–C(41)
C(5)–C(4)–S(3)
C(41)–C(4)–S(3)
C(4)–C(5)–C(51)
C(4)–C(5)–S(1)
C(51)–C(5)–S(1)
98.79(15)
114.47(16)
98.57(15)
128.2(3)
114.0(2)
117.7(2)
127.5(3)
114.0(2)
118.5(3)
N(7)–S(6)–C(10)
S(6)–N(7)–S(8)
98.47(15)
114.69(17)
98.72(15)
128.0(3)
113.8(2)
118.2(2)
127.9(3)
114.1(3)
117.9(3)
The concentration of unpaired spins in a 4.3 M solution of
N(7)–S(8)–C(9)
C(10)–C(9)–C(91)
C(10)–C(9)–S(8)
C(91)–C(9)–S(8)
C(9)–C(10)–C(101)
C(9)–C(10)–S(6)
C(101)–C(10)–S(6)
radical 4d in CCl F over the temperature range from 226 to 318
3
K does not change as measured by dual cavity, quantitative
ESR spectroscopy, revealing that there is no measurable ten-
dency to dimerise or polymerise in solution. Related radicals
have small solution dimerisation energies ([RCNSSN] 2: R =
Ϫ1
C H , ∆H
= Ϫ37 kJ mol ; R = CF , ∆H
= Ϫ37
dimerisation
Ϫ1
6
5
dimerisation
Ϫ1
3
kJ mol ; R = t-C H , ∆Hdimerisation = Ϫ31 kJ mol
Ph
4
9
Ϫ1 22,52,53
CSNSNS, ∆Hdimerisation = Ϫ19.0 (2) kJ mol ).
If the same
π*–π* spin-paired dimer 19d were to form in solution as is
observed in the solid state (one half of the tetramer shown in
Fig. 7] then the dimerisation would be hindered by electrostatic
repulsion between the relatively highly charged adjacent
atoms.** Consistently the dimer 19d is not a stationary point on
the potential hypersurface (UB3PW91/6-31G*, starting from
X-ray geometry). If a tetramer similar to that in the solid state
were formed it would be heavily disfavoured by entropy. The
most favourable σ bonded dimer is the N–N bonded dimer 20d
(
N–N distance 1.427 Å), with a gas phase dimerisation enthalpy
Ϫ
1
Ϫ
1
of Ϫ13.6 kJ mol , and ∆G (dimerisation) of 52.1 kJ mol
UB3PW91/6-31G* optimised geometry and atomic charges
(
included in the Supplemental Materials).
5
4
Fig. 7 An ORTEP drawing of the solid state tetramer of radical 4d.
Fluorine atoms have been omitted for clarity. The thermal parameters
are drawn at 30% probability.
point [Fig. 5] showing that the decrease is not due to dimer [or
tetramer] formation and strongly implies that the liquid consists
of essentially isolated monomer radicals separated by the sums
of the corresponding van der Waals radii. This thesis is also
supported by the large change of volume on melting.
The structure of radical 4d in the gas and solid phases as
determined by electron diffraction and X-ray crystallography
The high vapour pressure (25 mmHg at r.t.) and stability of the
radical 4d allow structural determination in the gaseous and
solid state. The gas phase electron diffraction structure [Table 3
and Fig. 6] and the crystal structure [Table 6, Figs. 7 and 8] were
determined and the structure of 4d and the corresponding 3d
determined by ab initio theoretical calculations. The data for the
discrete molecules are compared in Table 7. The crystal struc-
In solution the radical has an extrapolated value for the
magnetic moment of 1.9 µ at infinite dilution (Gouy method).
B
Similar changes in magnetic moment on change of concen-
tration have been observed in the related paramagnetic liquid
55
ture of 4d was originally determined by Englert et al. but the
accuracy was low and a dimer was identified and published in
ؒ
25
[
(CH ) CCNSSN ] 2 (R = C(CH ) ). The lower magnetic
a Ph.D. thesis by one of us [M. J. S.]. This dimer was reported
3
3
3 3
1a
moment of the neat liquid [µ = 1.43 µ ] is consistent with the
in a review and incorrectly attributed to our preliminary
22a
B
presence of long-range weak intramolecular antiferromagnetic
communication.
In this work the crystal structure was
interactions. Similar values of the room temperature magnetic
redetermined at much higher accuracy and shown to be a
tetramer, not a dimer as previously reported.
The structures of the two crystallographically independent
planar 4d monomers in the solid state are essentially identical.
9
moments are found for the paramagnetic solids 7d[AsF ] and
6
10
[
CF CSSSN][AsF ]. The magnetic moment of the liquid 4d
3 6
decreases with an increase of temperature above the melting
At 220 K the CF groups are not freely rotating, and the crystal-
3
lographic data are consistent with both the major and minor
sets of fluorine atoms arranged such that each set of CF3
groups within each monomer are geared, as shown in Fig 8 (A)
and (B). The ESR of the matrix-isolated 4d has been analysed
by Mattar and the fluorines shown to be magnetically non-
equivalent, consistent with the geared geometry. In the gas
*
* The atomic charges are very dependent on the basis set and the
method by which the charges are deduced. For radical 4d, Mulliken
UB3PW91/6-31G*)/Mulliken (UMPW1PW91/6-31 ϩ G*)/NBO
charges: N (Ϫ0.766/Ϫ0.429/Ϫ0.846), S(0.636/0.431/0.706), in plane C
(
56
(
(
Ϫ0.336/Ϫ0.480/Ϫ0.340), exocyclic
Ϫ0.404/Ϫ0.271/Ϫ0.346).
C (1.293/1.075/1.093), F (av.)
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
3375

View Article Online
refinement. However the fit of electron diffraction intensities
is not significantly changed with the CF groups in geared
3
positions. Energy barriers to rotations are low and there is
nearly free rotation of CF groups in the gas phase. The orien-
3
tation of the CF groups in the calculated optimised geometry
3
of 4d is almost identical to that in the monomer (Fig. 8).
The structures of the 1,3,2-dithiazolyl radicals 4d, 4h, and 5
Table 7] do not reveal any crystallographically significant
[
differences in bond length within the heterocyclic rings, with
the exception of the C–C bond. In 4d the C–C bond length
2
2
[
[
1.351(5) Å] is consistent with that expected for an sp –sp olefin
1.36 Å]. The same bond is significantly longer in the radicals 4h
and 5 due to π bond delocalisation to the substituent [4h, CN;
, fused aromatic ring] which acts to lower the relative bond
5
order from that in radical 4d.
The bond distances of radical 4d and the corresponding
cation 3d (Table 7) are consistent with the nature of the cal-
culated SOMO of 4d, i.e. S–N and C–S lengthen and C–C
shortens on addition of an electron to 3d as previously
20
discussed.
Radical 4d is planar in both the gaseous and solid phases,
and taking into account that there can be systematic differences
of ± 0.03 Å and ± 3Њ between values obtained from ED and
57
X-ray diffraction, the structures of the ring are essentially
identical in both phases (see Table 7). This is supported by the
similarity of the vibrational spectra attributed to the monomer
in the gas, liquid and solid phases (see below). This is also con-
ؒ
sistent with the similarity of the structures of [CF CNSSN]
3
58
2
d
in the gas and solid phases, the only other carbon–sulfur–
nitrogen radical for which experimental structures have been
obtained in both gas and solid phases.
Both UB3PW91/6-31G* and UMPW1PW91/6-31ϩG*
optimised geometries of radical 4d were in satisfactory agree-
ment with the structure determined by electron diffraction and
X-ray crystallography (Table 7), taking into account the known
elongation of bonds containing heavier elements at this level of
59
theory (SN bonds calculated to be too long by 0.03 to 0.04 Å).
The calculated energy of rotating the SNS fragment out of the
Ϫ1
SCCS plane is very low (e.g. 3.9 kJ mol for 15Њ rotation, 18.5
Ϫ1
Ϫ1
kJ mol for 30Њ, 51.0 kJ mol for 45Њ (UB3PW91/6-31G*). A
low energy has also been reported for the folding of 5 (5.0 kJ
Ϫ1
Ϫ1
19
mol for 10Њ, 20.7 kJ mol for 20Њ) and of a similar 1,3,2-
dithiazolidine heterocycle in which the nitrogen bears a sub-
Ϫ1
Ϫ1
stituent hydrogen atom (0.0 kJ mol for 0Њ, 4.2 kJ mol for
3
60
6Њ).
In the solid state 4d monomers are associated into face-to-
Fig. 8 ORTEP diagrams of the two independent monomers of radical
4
d, showing the orientation of the major (A) and minor (B) occupan-
face dimers, shown in Fig. 7, by interaction of the two π* SNS
based SOMOs (see Fig. 2 in Supplementary Material). How-
ever the monomers are not parallel. The dihedral angle between
the planes containing each of the two rings is 26Њ resulting from
the steric and electrostatic repulsion between the negatively
cies of the disordered fluorine atoms of the CF groups, compared to
3
the calculated structure (C). The thermal parameters are drawn at 30%
probability.
phase the electron diffraction data are consistent with the CF3
groups arranged in a staggered orientation with respect to the
charged CF groups and cyclic carbon atoms (see Footnote **).
3
C᎐C bond with overall C molecular symmetry in the final
In addition the sulfur–sulfur intermolecular distances are not
᎐
2v
3
376
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
Table 7 Comparison of bond lengths (Å) and angles (Њ) for 3d, radical 4d and related species. E.s.d.s listed in parentheses
3
d calculated
4d
c
c
4
h
5
NBO bond
order
NBO bond
order
electron
diffraction
crystal
structure
crystal
structure
crystal
structure
b
a
b
b
c
c
18
19
Parameter
Bond
a
b
calc.
calc.
C᎐C
C–S
S–N
C–C
C–F
1.371
1.717
1.609
1.532
1.330
1.371
1.714
1.602
1.531
1.328
1.55
1.26
1.34
0.93
0.91
1.349
1.753
1.674
1.508
1.341
1.349
1.749
1.665
1.507
1.341
1.65
1.10
1.09
0.97
0.89
1.324 (7)
1.749 (3)
1.634 (2)
1.481 (3)
1.330 (2)
1.350 (4)
1.735 (3)
1.641 (3)
1.495 (5)
1.315 (8)
1.370 (6)
1.727 (5)
1.641 (4)
1.422 (8)
1.394 (3)
1.744 (2)
1.646 (2)
1.393 (3)
Angle
C᎐C–S
C–S–N
S–N–S
C–C–C
C–C–S
113.3
98.8
115.9
127.8
118.9
113.2
98.7
116.3
128.2
118.6
114.4
98.9
113.4
127.4
118.1
114.4
98.6
114.0
127.8
117.8
114.8 (3)
96.5 (6)
117.3 (5)
127.3 (3)
117.9 (3)
114.0 (2)
98.6 (2)
114.6 (2)
127.9 (3)
118.0 (3)
114.0 (4)
98.4 (2)
115.3 (3)
122.9 (5)
120.5 (4)
113.0 (2)
99.3 (1)
113.9 (1)
120.3 (2)
Torsion angle
S–C–C–S
0.26
0.40
0.03
0.11
0
0
0
0
a
b
c
UB3PW91/6-31G*. UMPW1PW91/6-31ϩG*, averaged values; fully optimized geometry details have been deposited. Averaged values.
equal [S(1) ؒ ؒ ؒ S(8) 3.239 (36) Å; S(3) ؒ ؒ ؒ S(6) 3.097 (50) Å]
(
ΣvdW radii = 3.6 Å) reflecting an angle between S(1) ؒ ؒ ؒ S(3)
and S(8) ؒ ؒ ؒ S(6) of 3Њ. The dimers interact to give a tetramer,
shown in Fig. 7, via six S ؒ ؒ ؒ N contacts, two [S(3) ؒ ؒ ؒ N(2)] at
3
.190(3) Å lying in the same plane as the two centro-
symmetrically related monomers they connect, and two sets of
weaker contacts [S(6) ؒ ؒ ؒ N(2) 3.302(3), S(3) ؒ ؒ ؒ N(7) 3.199(3)
Å], one set above and the other below the plane (ΣvdW radii
47
(
S ؒ ؒ ؒ N) = 3.35 Å). The non-centrosymmetric S ؒ ؒ ؒ N con-
tacts with the shorter of the two intra dimer sulfur contacts
S(3a) ؒ ؒ ؒ S(6a), S(3b) ؒ ؒ ؒ S(6b) 3.097(2) Å], and N(2a) ؒ ؒ ؒ
N(7a) with N(7b) ؒ ؒ ؒ N(2b), define centrosymmetric
[
a
rhombohedron (Fig. 7). The sulfur–nitrogen contacts are sub-
stantially ionic (calculated natural bond order (NBO) charges
on the sulfur atoms of ϩ0.71 and nitrogen Ϫ0.85), with the
major portion of the bonding between the unpaired electrons
of the monomer occurring in the S(3)N(2)–S(6)N(7) faces of
the rhombohedron consistent with S(3) ؒ ؒ ؒ S(6) being shorter
than S(1) ؒ ؒ ؒ S(8). Thus the covalent and ionic bonding of
four monomer radicals in the observed tetramer largely occurs
within the rhombohedral cage. The heats of fusion and sublim-
ation are not significantly different from related diamagnetic
molecules, therefore the magnitude of the sum of all the long
inter-monomer interactions is very small.
The distances between the tetramer units are all significantly
longer than those within the tetramer and are greater than the
sum of the van der Waals radii of the atoms involved. A view of
the crystal lattice of radical 4d (as shown in Fig. 3 in the
Supplementary Material) reveals that the tetramers are oriented
to form layers with an internal sulfur–nitrogen rich zone
sandwiched between layers of CF groups. This would indicate
3
that even though the interactions between the SNS areas of
the tetramers are very long (and weak) they are sufficient to
determine the larger structure of the lattice itself.
Fig. 9 Intermolecular contacts in radical 4h (A) and 17 (B). Contacts
for 4h: a 3.275, b 2.126, c 3.275, d 3.222 and e 3.273 Å. Contacts for 17:
a 3.323, b 3.071, c 3.071, d 3.323, e 3.190, f 3.427, g 3.154, h 3.154,
j 3.020 and k 3.020 Å.
20
Dimers of radical 5 are connected by planar sheets of long
sulfur–sulfur contacts (four times 3.477(1), 3.781(1) Å per
23
dimer). In contrast dimers of 4h are linked into a network
polymer by ionic N–S interactions (see Fig 9A). There is no
reason to assume that the strength of the two long completely
covalent sulfur–sulfur bonds joining 4h monomers of
δϩ
δϪ
joined by other long but significant S ؒ ؒ ؒ N contacts into a
three dimensional network. The importance of inter-monomer
3
.145(2) Å [av.] are greater than the sum of largely ionic sulfur–
21
ionic S ؒ ؒ ؒ N interactions is further illustrated by 17, which is
nitrogen inter-dimer interactions. The solid state structure of
a paramagnetic solid at room temperature. The overall struc-
ture of 17 is best described as consisting of monomers joined by
largely ionic S ؒ ؒ ؒ N interactions as shown in Fig. 9(B) into a
sheet polymer. The sheets are equally separated and held apart,
ؒ
[
[
CF CNSSN] consists of monomers connected by S ؒ ؒ ؒ S
3
2.997(2) Å] and S ؒ ؒ ؒ N bonds (3.197(5), 3.242(5) Å) formed by
interaction of the two π* SOMOs. However the dimers are
58
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
3377

View Article Online
we propose, largely by electrostatic repulsions, with monomers
in each sheet arranged into slipped π stacks (Fig. 4 in reference
at 617 nm that has been assigned by Oakley and co-workers to a
61
π* → σ* transition.
2
(
2
1c). The sulfur atoms are equally separated by 3.875(38) Å
less than twice Nyberg’s isotropic sulfur vdW radius of
.03 Å perpendicular to the π network). A phase transition
The vibrational spectra of radical 4d
The spectra of radical 4d in all three states, and 3d[AsF ] salt,
are shown in Figs. 1 and 2, with assignments based on the calcu-
6
at 150 K to a ground state antiferromagnetic material takes
place in which electrons are paired by weak sulfur–sulfur π*–π*
interactions [3.48(5) Å, SNS moiety, 3.401(5) Å, NSN
fragment].
lated spectra (geometries optimised to C symmetry) given in
2
Table 2. 33 vibrations are expected, all of which are both IR
and Raman active. Of these, 27 are observed in the Raman
spectrum of 4d, 3 are calculated at lower frequency than the
58
Gleiter and co-workers showed that the two structurally
different forms of the π*–π* [CF CNSSN] dimer found in the
Ϫ1
Ϫ1
3
2
instrumental cut-off [Raman ≈ 90 cm ; IR ≈ 200 cm ].
Correlations between the observed and calculated frequen-
cies are in good agreement for gaseous and liquid 4d, and for
solid state, as well as the alternative rotomers, were not signifi-
Ϫ1
cantly different in energy (<5 kJ mol , MNDO calculations).
The dimerisation energies in solution of simple monoradicals
Ϫ1
solid 4d in the high energy region (above 400 cm ), taking
Ϫ1
15
of this class are small (<40 kJ mol ) and solvent dependent
into account that the Raman spectrum of the solid is of better
quality and that some peaks are split in the solid state. The
major stretching vibrations remain similar for all phases of 4d
implying minimal changes in intramolecular bond distances on
implying, but not proving, that the π*–π* bond energies of the
pure radical were also small. The enthalpy of sublimation of
radical 4d is equal to the sum of the π*–π*, electrostatic and
van der Waals intermolecular bond energy which is 49.0 kJ
change of phase. Of particular note is the similarity of the C᎐C
᎐
Ϫ1
mol per monomer. The diamagnetic solid melts at 11.9 ЊC to
Ϫ1
stretch for all three phases of 4d (1580 cm ). This implies simi-
Ϫ1
give a paramagnetic liquid with ∆Hfusion = 11.5 kJ mol . If we
lar intramolecular bond distances in the solid and gas phases,
although this cannot be concluded from the structural data
assume that the electrostatic and van der Waals interactions are
the same in the solid as in the liquid, then the enthalpy of fusion
is equal to the strength of the π*–π* bond in the tetramer,
because of the large deviations for the C᎐C distances, especially
᎐
in the gas phase (see Table 7). The weak peak adjacent to the
Ϫ1
Ϫ1
i.e. 11.5 kJ mol per monomer, or 23.0 kJ mol per dimer,
i.e. small and significantly less than the van der Waals
Ϫ1
strong C᎐C stretching peak (1580 cm ) in the Raman spectra
᎐
13
of 4d is reasonably assigned to C isotope effects (ca. 4–6%
observed relative peak height, 2.22% expected ( C natural
and electrostatic interactions which are approximately 49.0 Ϫ
13
Ϫ1
1
1.5 = 37.5 kJ mol per monomer. The dipole of 4d is calcu-
abundance = 1.11%) relative intensity; 1.6% observed wave-
lated to be 0.1 (UB3PW91/6-31G*) or 0.4 Debye
UMPW1PW91/6-31ϩG*), i.e. very small, and fluorine
number difference, 2–4% expected by Hooke’s law). The
(
Ϫ1
frequencies in the FT-Raman of solid 4d below 250 cm are
atoms are very non-polarisable, therefore van der Waals
forces in 4d are small. Therefore it is likely that the major
contributions to the intermolecular interactions in solid and
largely absent for the liquid and gaseous phase and, in part,
δϩ
δϪ
can be attributable to intermolecular S ؒ ؒ ؒ S and S ؒ ؒ ؒ N
vibrations between the monomers in the tetramer. Similar low
energy absorptions in the Raman spectra of sulfur–nitrogen
systems have been assigned to intermolecular long range S ؒ ؒ ؒ S
0
.64ϩ
liquid 4d are electrostatic, i.e. the interaction between S
and N
0.77Ϫ
(UB3PW91/6-31G* Mulliken charges) on adjacent
monomers in tetrameric (4d) [see Fig. 7]. This is supported
4
ؒ
62
interactions in [HCNSSN] . The calculated SN and CS dis-
tances in 4d are longer than in 3d (see below) consistent with the
antibonding nature of the SOMO of 4d and consistently 3d SN
and CS stretching frequencies are higher than those of 4d. Simi-
by dimerisation energy calculations which show the π*–π*
dimer 19d is not a stationary point. The triplet planar, cen-
trosymmetric dimer 21d given by two electrostatic
0.61Ϫ
0.53ϩ
N
ؒ ؒ ؒ S
interactions at 3.109 Å (UB3PW91/6-31G*
larly the C᎐C stretch is higher for 4d than in 3d, reflecting a
᎐
Mulliken charges, geometry and other details deposited) was
Ϫ1
shorter C᎐C distance and some bonding SOMO contribution.
stable by Ϫ7.8 kJ mol .†† The corresponding calculated
0.43ϩ
0.61Ϫ
Mulliken charges (UB3PW91/6-31G*) on 4a (S
, N
)
The structure of liquid 4d as implied from its physical and
spectroscopic properties
are also appreciable, supporting the view that electrostatic
interactions play a major role in determining the structures
and physical properties of all the derivatives of 4, and also in
The structure of liquids is not readily determined, but we are
able to deduce that of radical 4d from its physical and spectro-
scopic properties. The Raman spectrum of the liquid in the
bond stretching and bending regions is very similar indeed to
that of the solid for which the crystal structure was determined.
Thus the molecular structure is the same in both states, with the
possible exception of some changes in the relative orientations
ϩ
ؒ
many of the related radicals derived from [S N ] (Scheme
3
2
1
). We suggest that the planned synthesis of 7π C–N–S rad-
icals of this class with specific structure and properties should
take into account the importance of intramolecular electro-
static interactions.
The spectroscopic characterisation of radical 4d
of the two CF groups to one another. The distance between
3
molecules, however, is greater in the liquid as its molecular
volume is ≈ 20% greater than the solid. The liquid is para-
magnetic with weak antiferromagnetic intermolecular inter-
actions reducing the magnetic moment from 1.53 to the
observed 0 µB in the diamagnetic solid which consists of
tetramers of weakly bound monomers. These facts imply that
the liquid consists of monomers that are essentially separated
by van der Waals distances. Consistently the heat of fusion
The mass spectrum of radical 4d is consistent with that of
5
related compounds. The solution ESR spectrum has been
15
19
analysed elsewhere. The F NMR spectra of neat 4d shows
one broad peak (γ1/2 = 200 Hz) at δ Ϫ59.7, but no peak is
observed in dilute solution.
The UV-visible spectrum of radical 4d in hexane is given in
Fig. 3. The distinctive blue colour of the vapour of 4d is pre-
sumably attributed to the weak red absorption at 738 nm
Ϫ1
Ϫ1
Ϫ1
is 11.5 kJ mol , i.e. very low and similar to that of related
(
ε = 110 M cm ) in the gas phase and hexane solution,
diamagnetic molecules. This is an example of very small energy
changes leading to dramatic changes in physical properties, in
this case of volume, bulk magnetic properties and change in
and the assigned transition based on high level theoretical
56
calculations by Mattar, which will be published elsewhere. The
ؒ
UV-vis spectrum of the related [HCNSSN] has an absorption
ؒ
state. Other examples in this area include [p-NCC F CNSSN]
6
4
4
which is a spin canted antiferromagnet in one phase, and 17
†
† An overall diamagnetic tetramer may then be constructed by the
which becomes a paramagnetic solid with small structural
addition of two radicals 4d of appropriate spin to the centrosymmetric
dimer.
21
changes on change of temperature, and solid [NSNSC–
3
378
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
Table 8 Observed IEs (eV) and calculated orbital energies (eV) for radical 4d
UMPW1PW91/6-31ϩG*
UHF/6-31ϩG*
Observed
ROMPW1PW(1/6-31ϩG*
α
ROHF/6-31ϩG*
α
a,b
IE
Orbital
α
β
α
β
a: (7.46) 7.70
b: 10.85
c: 11.80
d: 12.50
e: 14.47
f: 15.55
π (b1)
2.95
9.29
9.43
5.97
9.73
2.87
11.69
11.98
13.71
13.79
16.01
16.18
16.63
8.94
12.82
13.86
13.94
14.30
16.33
16.78
18.00
b
π (a )
9.06
9.13
9.67
10.55
12.22
12.29
12.33
11.12
11.49
13.41
13.63
15.69
16.09
16.52
2
b
b
c
c
c
c
10.06
10.10
10.71
12.36
12.42
12.59
9.80
10.60
12.32
12.36
12.42
g: 16.00
h: 17.24
a
Letters correspond to labelling in Fig. 10; adiabatic first IE in parentheses. Calculated first IE [E(cation) Ϫ E(radical), UMPW1PW91/6-
b
3
1ϩG*]: adiabatic = 7.67 eV, vertical = 7.92 eV. See text. Bands b to d in Fig. 10 may comprise four π and σ orbitals (as suggested by the
c
calculations), although only three distinct peaks are observed. See text. Bands e to h in Fig. 10; maxima indicated, although total number of IEs and
orbital assignments are not feasible.
ab initio methods and a simple scaling factor can be applied
to bring the eigenvalues in accord with observed IEs. We have
not undertaken this scaling exercise, but nonetheless the DFT
results suggest, following the low lying IE, a cluster of 3 or 4
bands followed by a small gap before the remaining bands.
Thus a Koopmans’ like approach does seem to operate for such
65
radicals with DFT, although strictly it ignores the singlet–
triplet splittings of the excited states of the cations. Previous
21b,65,66
experience with similar molecules
exchange splittings are small.
suggests these
Chemistry of radical 4d
Fig. 10 Photoelectron spectrum of radical 4d.
Radical 4d does not appear to react with O at very low concen-
2
tration in CCl F solution, but at higher concentration reaction
3
2
ؒ
3
CSNSN] which becomes
a
paramagnetic diradical on
occurred under various conditions, resulting in a foul smell-
ing yellow oil containing a complex mixture [see Table 4, and
Table 4 in the Supplementary Material]. The reaction of the
radical 4d with Cl and Br quantitatively produced the halide
6
grinding.
2
2
Electronic structure and ultraviolet photoelectron spectroscopy
UPS) of radical 4d
salts according to eqn. (5), and the reaction with arsenic
(
The π and σ molecular orbitals (MOs) of radical 4d (Fig. 2 in
2 4d ϩ X → 2 3d[X] (5)
2
Supplementary Material) are consistent with lower level calcu-
20
lations for 4a and 5 and more extensive theoretical studies of
pentafluoride led to the formation of the salt 3d[AsF6]
according to eqn. (6). The halides were fully characterised by
21
aryl derivatives, albeit with some delocalisation of ring MOs
into the CF substituents. The gas phase UPS of 4d (Fig. 10),
3
typical for such a radical, has a low-lying vertical IE at 7.70 eV
2 4d ϩ 3 AsF → 2 3d[AsF ] ϩ AsF
3
(6)
5
6
(
adiabatic, 7.46 eV), corresponding to ionisation from the
singly occupied π MO of (in C ) b symmetry. Following the
elemental analysis, mass spectra, infrared and NMR spectro-
scopy. They were readily purified by sublimation and could
be reduced back to the radical with triphenylantimony. The
halides 3d[Cl] and 3d[Br] could also be prepared by the
2v
1
first IE (Table 8) is a gap of some 3 eV to a cluster of 3 or 4
bands (labeled b–d) between 10.5 and 13.5 eV, which comprise
ring π and σ MOs. Beyond 14 eV the remaining σ and π MOs
contain increased CF3 character, although the number of
IEs and their assignment remain unknown. The calculated,
E(cation) Ϫ E(radical), using UMPW1PW91/6-31ϩG* gives
very satisfactory agreement for both adiabatic (a) (calc. 7.67
eV) and vertical (v) (calc. 7.92 eV) IEs, with the a Ϫ v difference
metathetical reaction of 3d[AsF ] with CsCl and KBr , respec-
6
tively, in liquid sulfur dioxide. The higher reactivity of the
chloride salt with the solvent (giving SO Cl ) required a more
2
2
rapid isolation of the salt from the solvent and resulted in a
lower yield by this route. Indeed, the attempted preparation of
(
0.25 eV) matching that observed experimentally (0.24 eV),
the fluoride by reaction of 3d[AsF ] with various fluorides in
6
consistent with the relatively small geometry change between
radical and cation (Table 7). Table 8 also shows the calculated
orbital energies using the UMPW1PW91/6-31ϩG* and
ROMPW1PW91/6-31ϩG* methods, together with single
point ROHF and UHF (unrestricted Hartree–Fock) calcula-
tions at the UMPW1PW91/6-31ϩG* geometry. These are
provided to assess the sequencing and spacing of the higher
IEs, and to see if any Koopmans-like correspondence can be
observed. The absolute values for these calculations are very
poor, not surprisingly since Koopmans’ theorem (IE = Ϫεi)
does not strictly hold for open shell systems, and for DFT there
is the issue of the meaning of Kohn–Sham orbitals and eigen-
sulfur dioxide gave low yields of impure 4d as the only isolable
products.
Radical 4d did not react with iodine, rather the neutral 4d
was generated by in situ formation of the iodide according
to eqn. (7) in liquid sulfur dioxide solution, which then gave
2 3d[AsF ] ϩ 2 CsI → “2 3d[I]” ϩ 2 Cs[AsF ] (7)
6
6
elemental iodine and 4d according to eqn. (8). This is an
“2 3d[I]” → 2 4d (l) ϩ I (s) (8)
2
64
values. It has been suggested that the ordering of Kohn–Sham
orbitals is actually quite good when compared to conventional
alternative preparative route to the radical 4d which does not
Ϫ
occur when the [AsF6] salts of 3a and 3b are treated with
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
3379

View Article Online
iodide. In these reactions, hydrolytically stable iodide salts are
The stability of radical 4d is similar to that of the corre-
69
25
quantitatively recovered according to eqn. (7). The relatively
low first ionisation potential of 4d (7.70 eV) is consistent with
the facile oxidation of 4d to the cation 3d by elemental chlorine
sponding bis(perfluoromethyl)aminyl oxide radical (CF ) NO.
3 2
The dimethylaminyloxide is not isolable, however [(CH ) -
3
3
45
C ]NO is, whereas the related 4f, sterically protected by the
2
and bromine or AsF but not iodine. Further studies of the
thermodynamics of the system are in progress.
Radical 4d is rapidly hydrolysed by water but unreactive to
the metals Cu and Hg and there was no observable reaction
with the radical NO [Table 4, and Table 4 in the Supplementary
Material].
(CH ) Si groups, is not. Thus it does not seem that moderately
5
3 3
67
bulky substituents will lead to stable derivatives of 4 as is the
case for the aminyl oxides. It is significant that 4c‡‡ was
isolated at low temperature by rapidly removing it from the
reduction mixture. This suggests that derivatives of 4 may be
isolated if they can be rapidly removed from the reaction
mixture to either low pressure and/or low temperature condi-
tions. However, it rapidly decomposed to numerous products
when warmed to room temperature, even when removed from
the reaction mixture and dissolved in relatively dilute solution.
Similarly, Wolmershäuser and Kraft have shown that the
Evidence for isolation of small quantities of radical 4c at Ϫ20 ЊC
and its decomposition
The presence of a trifluoromethyl group as a substituent in
derivative 4c suggested that the radical may be isolable as a
23
dicyano derivative 4h is a stable species that can be isolated
19
volatile species. F NMR analysis of the in situ reduction of
c[AsF ] by a mixture of tetramethylammonium chloride and
while 4g rapidly decomposes even in dilute solution, and 4a, 4b,
4e, 4f all decomposed. This suggests all derivatives of 4 contain-
ing hydrogen are liable to have low stability with respect to
decomposition. The concentration dependence of the stabilities
3
6
triphenylantimony showed the quantitative reduction occurred
almost instantaneously. However, 4c decomposed to a myriad
of diamagnetic non-volatile compounds at these concentrations
7
of 4f and 4i and the quantitative nature of the decomposition
1
19
within twenty minutes ( H, F NMR). The radical 4c was
isolated as a blue-black crystalline solid by fractional distil-
lation of the volatiles immediately after reduction, which was
very rapid. It was stable only if kept frozen (Ϫ196 ЊC) and was
recovered as the chloride salt by reaction with chlorine. The
of 4d implies that the decomposition is bi- or multi-molecular.
19,20
Radical 5
and various other related benzenoid deriv-
atives, as well as 4d and 4h, are isolatable as pure materials
under ambient conditions. This suggests that the stability of
these radicals may be associated with a lowering of the bonding
1
19
stable chloride was shown spectroscopically ( H, F NMR) to
be a mixture that contained a major component consistent with
the cation 3c. It is clear that the combination of rapid reduction
and volatility, which allowed transfer to a trap at Ϫ196 ЊC,
rather than stability allowed the isolation of this radical.
19
π orbital energies by delocalisation into the aryl ring for 5 and
on to the substituent for 4d (e.g. MOs SOMO-9, SOMO-14 in
Fig. 2 of Supplementary Material) and implied by the structure
of 4h, as well as lowering of energy by the inductive effect of the
electronegative substituents as expected for 4d and 4h. The
dimerisation energies of 4a and 4d to monomers linked into
dimers through N–N, S–S or S–N σ bonds were expected to be
more favourable for 4a than 4d due to the stronger, more
delocalised π bonds in 4d that would be broken to form the
dimer (shown in Scheme 3 for 4d, with an N–N bonding dimer).
The decomposition of the radical 4c, both as an isolated
compound and in reaction mixtures, always produced a com-
19
plex mixture of compounds ( F NMR) in contrast to the
rather simple products given on decomposition of 4d. Sublim-
ation of the non-volatile component of the reaction mixture
allowed isolation of a diamagnetic, black crystalline solid in
approximately 12% yield. Spectroscopic analysis by multi-
nuclear NMR and mass spectroscopy indicated that the
isolated compound was the thione 13, identified by comparison
with related compounds. The parent heterocycle 1,3,2-
dithiazole-4-thione, SCSNSCH, was generated by Oakley and
co-workers from a complex mixture in low (> 5%) yield
44
and characterised by single crystal X-ray crystallography.
More recently, a systematic study of the reaction of 1,3,2-
dithiazolium chlorides with organic bases by Rees and co-
68
workers showed the thiones are produced in moderate yield.
The fact that these reactions have retained the 1,3,2-dithiazolyl
heterocyclic ring intact gives some indication of the chemical
stability of this moiety.
Scheme 3
In fact, the enthalpies (UB3PW91/6-31G*) of dimerisation
of the N–N σ bonded dimer 20 of 4a, 4j, and 4d are all
about zero, and the free energies are all unfavourable. Other
σ bonded dimers are of higher energy than that of the
N–N dimer 20. However, a triplet σ dimer 22 bonded through
ؒ
The stability of [RCSNSCR] radicals
The radicals 4a–4g had been observed in dilute solution by ESR
spectroscopy (Table 5), 4c, 4e and 4g for the first time. The
derivatives 4c and 4g were shown to have limited stability with
observed half-lives of less than twenty minutes. We also
both carbons is favoured for 4a (∆H
= Ϫ47.4 kJ
dimerisation
Ϫ1
Ϫ1
mol , ∆G
4
= 17.0 kJ mol ), thermally accessible for
dimerisation
Ϫ1 Ϫ1
j (∆Hdimerisation = 6.1 kJ mol , ∆G
= 90.1 kJ mol ),
dimerisation
Ϫ1
Ϫ1
but unfavourable for 4d (∆Hdimerisation = 44.6 kJ mol ,
attempted to prepare these radicals at higher (10 M) concen-
Ϫ1
∆
1
Gdimerisation = 124.5 kJ mol ) (C–C bond distances: for 22a,
trations by reduction of the corresponding hexafluoroarsenate
salt of 3. The reactions either generated intractable, soluble tars
.550 and 1.553; 22j, 1.572 and 1.581; 22d, 1.573 and 1.592 Å).
This implies that the decompostions of 4a and 4j may proceed
through the formation of the dimer 22. Similar reactions have
(
4c, 4e and 4f) or gave an insoluble black solid (4a), see Table 5.
The solution of 4c showed numerous peaks in the proton and/
or fluorine NMR spectra, indicative of extensive decom-
position (see below). 4d and 4f were the most stable in dilute
solutions as measured by ESR spectroscopy (Table 5). However,
when an attempt was made to isolate 4f it decomposed at
been found for the related ClCNSSCCl radical, which rapidly
4
8
dimerised via a C–C bond and led to 23. Attempts to investi-
gate the face-to-face dimer 19a optimised to a triplet state
hydrogen bonded dimer 24 through a H ؒ ؒ ؒ C bond of 2.334 Å,
≈
0.1 M concentration after thirty-six hours and on attempted
isolation after rapid reduction. This pattern of stability is very
‡
‡ We note, however, that there were no significant differences in the
similar to that observed for 4i which we reported as a stable
radical in solution with concentrations less than 0.5 M.
decomposition products when radical 4c was isolated and when it
remained in the presence of the reducing agents in the reaction mixture.
7
3
380
J. Chem. Soc., Dalton Trans., 2000, 3365–3382

View Article Online
and a dihedral angle between the rings of 88.7Њ, with the
enthalpy and free energy of dimerisation of Ϫ12.4 and 26.1 kJ
mol , respectively. This provides for additional reaction path-
12 N. Burford, J. Passmore and M. J. Schriver, J. Chem. Soc., Chem.
Commun., 1986, 140.
13 A. Vegas, A. Pérez-Salazar, A. J. Banister and R. G. Hey, J. Chem.
Soc., Dalton Trans., 1980, 1812.
Ϫ1
ways for 4a and C–H hydrogen containing derivatives of 4,
1
4 G. K. MacLean, J. Passmore, M. J. Schriver, P. S. White, D. Bethell,
R. S. Pilkington and L. H. Sutcliffe, J. Chem. Soc., Chem. Commun.,
1983, 807.
consistent with the greater reactivity of 4a. The concentration
7
dependence of the stability of 4a, 4b, 4c, 4e, 4f and 4i and the
quantitative nature of the decomposition of 4d indicates that
the decomposition is multimolecular, which is supported by our
calculations.
15 (a) S. R. Harrison, R. S. Pilkington and L. H. Sutcliffe, J. Chem.
Soc., Faraday Trans. 1, 1984, 669; (b) S. A. Fairhurst, R. S.
Pilkington and L. H. Sutcliffe, J. Chem. Soc., Faraday Trans. 1,
1
983, 925; (c) S. A. Fairhurst, R. S. Pilkington and L. H. Sutcliffe,
J. Chem. Soc., Faraday Trans. 1, 1983, 439; (d) K. F. Preston and
L. H. Sutcliffe, Magn. Reson. Chem., 1990, 28, 189; (e) Y.-L. Chung,
S. A. Fairhurst, D. G. Gilles, K. F. Preston and L. H. Sutcliffe,
Magn. Reson. Chem., 1992, 30, 666.
1
6 Y. Miura, N. Makita and M. Kinoshita, Bull. Chem. Soc. Jpn., 1977,
5
1
5
0, 482; Y. Miura, N. Makita and M. Kinoshita, Tetrahedron Lett.,
975, 127; Y. Miura and M. Kinoshita, Bull. Chem. Soc. Jpn., 1980,
3, 2395.
1
1
7 K. Schlosser and S. Steenken, J. Am. Chem. Soc., 1983, 105, 1504.
8 J. S. Thrasher and J. B. Neilsen, J. Am. Chem. Soc., 1986, 108,
1
108.
1
2
2
9 G. Wolmershäuser, M. Schnauber and T. Wilhelm, J. Chem. Soc.,
Chem. Commun., 1984, 573.
0 E. G. Awere, N. Burford, R. C. Haddon, S. Parsons, J. Passmore,
J. V. Waszczak and P. S. White, Inorg. Chem., 1990, 29, 4821.
1 (a) T. M. Barclay, A. W. Cordes, N. A. George, R. C. Haddon,
R. T. Oakley, T. T. M. Palstra, G. W. Patenaude, R. W. Reed,
J. F. Richardson and H. Zhang, Chem. Commun., 1997, 873;
(
b) T. M. Barclay, A. W. Cordes, R. H. de Laat, J. D. Goddard,
R. C. Haddon, D. Y. Jeter, R. C. Mawhinney, R. T. Oakley, T. T. M.
Palstra, G. W. Patenaude, R. W. Reed and N. P. C. Westwood, J. Am.
Chem. Soc., 1997, 119, 2633; (c) T. M. Barclay, A. W. Cordes,
N. A. George, R. C. Haddon, M. E. Itkis, M. S. Mashuta, R. T.
Oakley, G. W. Patenaude, R. W. Reed, J. F. Richardson and
H. Zhang, J. Am. Chem. Soc., 1998, 120, 352; (d) W. Futijta and
K. Auaga, Science, 1999, 286, 261.
2 (a) E. G. Awere, N. Burford, C. Mailer, J. Passmore, M. J. Schriver,
P. S. White, A. J. Banister, H. Oberhammer and L. H. Sutcliffe,
J. Chem. Soc., Chem. Commun., 1987, 66; (b) W. V. F. Brooks,
N. Burford, J. Passmore, M. J. Schriver and L. H. Sutcliffe, J. Chem.
Soc., Chem. Commun., 1987, 69.
3 G. Wolmershäuser and G. Kraft, Chem. Ber., 1990, 123, 881.
4 S. Brownridge, T. S. Cameron, J. Passmore, G. Schatte and T. C.
Way, J. Chem. Soc., Dalton Trans., 1996, 2553; M.P. Murchie,
J. Passmore and R. Kapoor, Inorg. Synth., 1997, 31, 102.
25 M. J. Schriver, Ph.D. Thesis, University of New Brunswick,
1988.
6 M. Alace and N. P. C. Westwood, J. Phys. Chem., 1994, 98, 3818.
7 HyperChem 3.0 for Windows, Autodesk, Hypercube, Inc., Gainsville,
FL, 1993.
8 W. L. Jolly, The Synthesis and Characterisation of Inorganic
Compounds, Prentice Hall, New York, 1970.
29 W. Bludssus and R. Mews, J. Chem. Soc., Chem. Commun., 1979,
35.
30 C. G. Krespan, B. C. McKusick and T. L. Cairns, J. Am. Chem. Soc.,
1960, 82, 1515; C. G. Krespan, J. Am. Chem. Soc., 1961, 83,
3434.
Acknowledgements
The authors thank Dr. Andreas Decken of the University of
New Brunswick for help in the analysis of the crystal structure
data, various figures and the original suggestion that radical
4
d was not dimeric in the solid state. Dr. Arthur Banister
2
and Dr. Zdenek Hauptman of Durham University are thanked
for the mass spectrum and the DSC measurement for radical
4
d, and much encouragement and help over many years.
Dr. Saba Mattar of the University of New Brunswick for useful
discussions and communication on the results of theoretical
calculations accounting for the colour of 4d and ESR of 4d
in a frozen matrix, prior to publication. We also thank the
Natural Sciences and Engineering Research Council (Canada)
for funding and for graduate scholarships (S. B. and M. J. S.).
2
2
2
2
References
2
1
(a) A. J. Banister and J. M. Rawson, in The Chemistry of Inorganic
Ring Systems, ed. R. Steudel, Elsevier, Amsterdam, 1992; (b) A. J.
Banister and J. M. Rawson, Adv. Heterocycl. Chem., 1995, 62, 137;
(
c) R. T. Oakley, Prog. Inorg. Chem., 1988, 36, 299; (d) A. W.
Cordes, R. C. Haddon and R. T. Oakley, in The Chemistry of
Inorganic Ring Systems, ed. R. Steudel, Elsevier, Amsterdam, 1992;
(
e) R. T. Oakley, Can. J. Chem., 1993, 71, 1775; ( f ) J. M. Rawson,
31 H. Oberhammer, in Molecular Structure by Diffraction Methods,
The Chemical Society, London, 1976.
New J. Chem., 1999, 23, 565.
2
3
Comprehensive Heterocyclic Chemistry, eds. A. R. Katritsky and
C. W. Rees, Pergamon Press, Oxford, 1984.
32 H. Oberhammer, W. Gombler and H. Willner, J. Mol. Struct., 1980,
70, 273.
33 H. Bürger, G. Pawelke and H. Oberhammer, J. Mol. Struct., 1982,
84, 49.
34 J. Haase, Z. Naturforsch., Teil A, 1970, 25, 936.
T. M. Barclay, A. W. Cordes, R. C. Haddon, M. E. Itkis, R. T.
Oakley, R. W. Reed and H. Zhang, J. Am. Chem. Soc., 1999, 121,
9
69.
4
5
A. J. Banister, N. Bricklebank, I. Lavender, J. M. Rawson,
C. I . Gregory, B. K. Tanner, W. Clegg, M. R. J. Elsegood and
F. Palacio, Angew. Chem., Int. Ed. Engl., 1996, 35, 2533.
35 J. Cosier and A. M. Glazer, J. Appl. Crystallogr., 1986, 19, 105.
36 A. J. M. Duisenberg, J. Appl. Crystallogr., 1992, 25, 92.
37 P. T. Beurskens, G. Beurskens, W. P. Bosman, R. de Gelder,
S. Garcia-Granda, R. O. Gould, R. Isreal and J. M. M. Smits,
The DIRDIF-96 program system, Crystallographic Laboratory,
University of Nijmegen, 1995.
(a) G. K. MacLean, J. Passmore, M. N. S. Rao, M. J. Schriver,
P. S. White, D. Bethell, R. S. Pilkington and L. H. Sutcliffe, J. Chem.
Soc., Dalton Trans., 1985, 1405; (b) S. Parsons, J. Passmore, M. J.
Schriver and X. Sun, Inorg. Chem., 1991, 30, 3342.
38 G. M. Sheldrick, SHELXTL version 5, Siemens Analytical
Instrumentation Inc., Madison, WI, 1995.
6
M. Sannigrahi and F. Grein, J. Mol. Struct. (Theochem), 1999, 465,
2
5.
39 GAUSSIAN 94, Revision E.3, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb,
J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery,
K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz,
J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara,
M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong,
J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox,
J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon,
C. Gonzalez and J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1995.
7
8
S. Parsons and J. Passmore, Acc. Chem. Res., 1994, 27, 101.
A. J. Banister, H. G. Clarke, I. Rayment and H. M. M. Shearer,
Inorg. Nucl. Chem. Lett., 1974, 10, 647.
9
T. S. Cameron, R. C. Haddon, S. M. Mattar, S. Parsons, J. Passmore
and A. P. Ramirez, J. Chem. Soc., Dalton Trans., 1992, 1563.
1
1
0 T. S. Cameron, R. C. Haddon, S. M. Mattar, S. Parsons, J. Passmore
and A. P. Ramirez, Inorg. Chem., 1992, 31, 2274.
1 J. Passmore, X. Sun and S. Parsons, Can. J. Chem., 1992, 70, 2972.
J. Chem. Soc., Dalton Trans., 2000, 3365–3382
3381

View Article Online
4
0 GAUSSIAN 98 (Revision A.3), M. J. Frisch, G. W. Trucks,
H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant,
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C.
Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi,
B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski,
J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong,
J. L. Andres, M. Head-Gordon, E. S. Replogle and J. A. Pople,
Gaussian, Inc., Pittsburgh, PA, 1998.
54 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge
National Laboratory, Oak Ridge, TN, 1976.
55 U. Englert, J. Passmore, M. J. Schriver and R. Strähle, unpublished
work.
56 S. Mattar and A. D. Stephens, J. Phys. Chem., 2000, 104, 3718.
57 A. Domenicano and I. Hargittai (Editors), Accurate Molecular
Structures, Oxford University Press, Oxford, 1992.
58 H.-U. Höfs, J. W. Bats, R. Gleiter, G. Hartmann, R. Mews,
M. Eckert-Maksic, H. Oberhammer and G. M. Sheldrick, Chem.
Ber., 1985, 118, 3781.
59 R. Haist, R. M. S. Alvartez, E. H. Cutin, C. O. Della Vedora and
H. Oberhammer, J. Mol. Struct., 1999, 484, 249.
60 M. J. S. Dewar, H. W. Kollmar and S. H. Suck, Theor. Chim. Acta,
1975, 36, 237; R. Boese, H. Oberhammer, P. Pulay and A. Waterfeld,
J. Phys. Chem., 1993, 97, 9625.
61 J. Campbell, D. Klapstein, P. F. Bernath, W. M. Davis, R. T. Oakley
and J. D. Goddard, Inorg. Chem., 1996, 35, 4264.
4
1 J. B. Foresman and A. Frisch, Exploring Chemistry with Electronic
Structure Methods, 2nd edn., Gaussian Inc., Pittsburgh, PA,
1
995.
4
4
2 F. de Proft and P. Geerlings, J. Chem. Phys., 1997, 106, 3270.
3 S. Joanteguy, G. Pfister-Guillouzo and H. Chermette, J. Phys.
Chem., 1999, 103, 3505.
62 C. D. Bryan, A. W. Cordes, R. C. Haddon, R. G. Hicks, D. K.
Kennepohl, C. D. MacKinnon, R. T. Oakley, T. T. M. Palstra, A. S.
Perel, S. R. Scott, L. F. Schneemeyer and J. V. Waszczak, J. Am.
Chem. Soc., 1994, 116, 1205; N. Burford, T. Chivers, P. W. Codding
and R. T. Oakley, Inorg. Chem., 1982, 21, 982.
63 W. V. F. Brooks, S. Brownridge, S. Parsons and J. Passmore,
Phosphorus Sulfur Silicon Relat. Elem., 1994, 93–94, 443; G.
Antorrena, S. Brownridge, T. S. Cameron, F. Palacio, S. Parsons,
J. Passmore and L. K. Thompson, Inorg. Chem., to be published.
64 R. Stowasser and R. Hoffmann, J. Am. Chem. Soc., 1999, 121, 3414.
65 R. T. Boeré, R. T. Oakley, R. W. Reed and N. P. C. Westwood,
J. Am. Chem. Soc., 1989, 111, 1180.
66 A. W. Cordes, C. D. Bryan, W. M. Davis, R. H. de Laat, S. H.
Glarum, J. D. Goddard, R. C. Haddon, R. G. Hicks, D. K.
Kennepohl, R. T. Oakley, S. R. Scott and N. P. C. Westwood, J. Am.
Chem. Soc., 1993, 115, 7232.
67 D. Jenkins, J. Passmore and M. J. Schriver, to be published.
68 P. J. Dunn and C. W. Rees, J. Chem. Soc., Perkin Trans. 1, 1989,
2489; M. A. Gray and C. W. Rees, J. Chem. Soc., Perkin Trans. 1,
1993, 3077; J. L. Morris and C. W. Rees, J. Chem. Soc., Perkin
Trans. 1, 1987, 217.
4
4 R. T. Oakley, University of Guelph, personal communication; R. T.
Oakley, H. Koenig and A. W. Cordes, Acta Crystallogr., Sect. C,
1
987, 43, 2468.
4
5 Aldrich Chemical Company Inc., Catalogue Handbook of Fine
Chemicals 1996–1997, Milwaukee, 1996.
4
4
6 E. Awere, N. Burford and J. Passmore, unpublished work.
7 J. E. Huheey, E. A. Kieter and R. L. Kieter, Inorganic Chemistry,
4
th edn., Harper Collins Publishers, New York, 1993.
4
8 T. M. Barclay, L. Beer, A. W. Cordes, R. C. Haddon, M. I. Itkis,
R. T. Oakley, K. E. Preuss and R. W. Reed, J. Am. Chem. Soc., 1999,
1
21, 6657.
4
5
9 E. A. Moelwyn-Hughes, Physical Chemistry, 2nd edn., MacMillan,
New York, 1961.
0 S. C. Nyburg and C. H. Faerman, Acta Crystallogr., Sect. B, 1985,
4
1, 274.
5
5
5
1 Z. V. Hauptman and A. J. Banister, personal communication.
2 M. I. Hansford, Ph.D. Thesis, Durham University, 1989.
3 S. A. Fairhurst, K. M. Johnson, L. H. Sutcliffe, K. F. Preston,
A. J. Banister, Z. V. Hauptman and J. Passmore, J. Chem. Soc.,
Dalton Trans., 1986, 1465; J. Passmore and X. Sun, Inorg. Chem.,
69 (a) A. K. Hoffmann and A. T. Henderson, J. Am. Chem. Soc., 1961,
83, 4671; (b) W. D. Blackley and R. R. Reinhard, J. Am. Chem. Soc.,
1965, 87, 802.
1
996, 35, 1313.
3
382
J. Chem. Soc., Dalton Trans., 2000, 3365–3382