Synthesis of [N(CH3)4]2O 3SOSO2(s) and [N(CH3)4] 2[(O2SO)2SO2]·SO 2(s) Containing (SO4)(SO2)x 2- x = 1, 2, members of a new class of sulfur oxydianions

Article abstract of DOI:10.1021/ic400805c

One mole equivalent of SO₂ reversibly reacts with [N(CH ₃)₄]₂SO₄(s) to give [N(CH ₃)₄]₂S₂O₆(s) (1) containing the [O₃SOSO₂]^2-, shown by Raman and IR to be an isomer of the [O₃SSO₃]^2- dianion. The experimental and calculated (B3PW91/6-311+G(3df)) vibrational spectra are in excellent agreement, and the IR spectrum is similar to that of the isoelectronic O₃ClOClO₂. Crystals of [N(CH ₃)₄]₂(O₂SO)₂SO ₂·SO₂ (2) were isolated from solutions of [N(CH₃)₄]₂SO₄ in liquid SO ₂. The X-ray structure showed that 2 contained the [(O ₂SO)₂SO₂]^2- dianion. The characterized N(CH₃)₄⁺ salts 1 and 2 are the first two members of the (SO₄)(SO₂)_x ^2- class of sulfur oxydianions analogous to the well-known small cation salts of the SO₄(SO₃)_x^2- polysulfates.

Full text of DOI:10.1021/ic400805c

Article
pubs.acs.org/IC
Synthesis of [N(CH3)4]22O−3SOSO2(s) and [N(CH3)4]2[(O2SO)2SO2]·SO2(s)
Containing (SO4)(SO2)x x = 1, 2, Members of a New Class of Sulfur
Oxydianions
Pablo Bruna,† Andreas Decken,† Friedrich Grein,† Jack Passmore,*,† J. Mikko Rautiainen,‡,∥
Stephanie Richardson,† and Tom Whidden§,⊥
†Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
‡Department of Chemistry, University of Oulu, Oulu, P.O. Box 3000, 90014, Finland
§Atlantic Hydrogen, Fredericton, New Brunswick E3B 6E9, Canada
S
* Supporting Information
ABSTRACT: One mole equivalent of SO2 reversibly reacts
with [N(CH3)4]2SO4(s) to give [N(CH3)4]2S2O6(s) (1)
containing the [O3SOSO2]2−, shown by Raman and IR to be
an isomer of the [O3SSO3]2− dianion. The experimental and
calculated (B3PW91/6-311+G(3df)) vibrational spectra are in
excellent agreement, and the IR spectrum is similar to that of
the isoelectronic O3ClOClO2. Crystals of [N(CH3)4]2-
(O2SO)2SO2·SO2 (2) were isolated from solutions of
[N(CH3)4]2SO4 in liquid SO2. The X-ray structure showed
that 2 contained the [(O2SO)2SO2]2− dianion. The charac-
+
terized N(CH3)42−salts 1 and 2 are the first two members of
2−
the (SO4)(SO2)x class of sulfur oxydianions analogous to the well-known small cation salts of the SO4(SO3)x polysulfates.
[SO ]2−(g) + SO2(g) ↔ [O3SOSO2]2−(g)
1. INTRODUCTION
4
ΔG(298 K) = −174 kJ mol−1
The known salts of binary sulfur oxydianions with one to three
(1)
sulfur atoms were discovered prior to 1891 (Table 1)1 and are
part of the foundational facts of chemistry.2 Sulfur oxyanions
[O3SOSO2]2−(g) + SO2(g) ↔ [(O2SO)2SO2]2−(g)
are important in all areas of science and technology, and one
would assume that their chemical and physical properties have
ΔG(298 K) = −79 kJ mol−1
(2)
now been exhaustively established, especially for salts of the
The energetics for the corresponding reactions of solid
first identified hydrated sulfur oxydianion, [SO4]2−.3 Numerous
sulfate salts can be estimated using a Born−Haber cycle for
R2SO4(s) R = N(CH3)4 and Na+ (Figure 1). The lattice
+
sulfate salts having small counter cations are stable in the solid
state and several are naturally occurring. However, [SO4]2−
enthalpies and entropies of solids in the Born−Haber cycle,
even for the hitherto unknown salts, are readily estimated from
the corresponding molecular volumes using volume based
thermodynamics (V.B.T.) (See Supporting Information,
Section S1.)16
undergoes a Coulombic explosion in the gas phase, producing
an electron and the [SO4]− • radical.4
In this paper, synthesis of two new sulfur oxidianions,
[O3SOSO2]2− and [(O2SO)2SO2]2−, were obtained by addition
of one or two SO2 to the sulfate dianion. [O3SOSO2]2− is an
isomer of the long known [O3SSO3]2− (Table 1) and
isoelectronic with O3ClOClO2.5 When compared to sulfate
the repulsion between the two negative charges is reduced in
[O3SOSO2]2− that we surmise is a driving force for reaction 1.
In a paper by Chan and Grein,4 structures and energies of these
new dianions were calculated at the B3PW91/6-311+G(3df)
level of theory. Accordingly, in the gas phase reactions 1
(formation of [O3SOSO2]2−) and 2 (formation of
[(O2SO)2SO2]2−) were calculated to be favorable, with the
corresponding ΔG(298 K) [ΔH] values of −174[−211] and
−79[−117] kJ mol−1, respectively.
The ΔG values account for the facts that solid Na2SO4 does
not react with SO2(g) but the reaction of SO2(g) with solid
[N(CH3)4]2SO4 is energetically favorable as the lattice energy
change with the larger cation (76 kJ mol−1) is much smaller
than that with Na+ (296 kJ mol−1) (Figure 1 and the
Supporting Information, Section S1). We have further
estimated the free energies for reactions of SO2 with a variety
of sulfate salts having differing cation volumes (Supporting
Information, Table S1), as well as for the reaction of SO2 with
R2O3SOSO2 according to eq 5. These results, shown in Figure
Received: April 2, 2013
© XXXX American Chemical Society
A
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Table 1. Known Sulfur Oxydianions Containing One to Three Sulfur Atoms in the Solid State
1
2
3
a
discovered [cation]
struct. determination [cation]
discovered [cation]
discovered [cation]
struct. determination [cation]
anion
anion
[O3S−S]2−
struct. determination [cation]
anion
1799, Chaussie
[2Na+]
1952, Taylor
b
[2Na+]
[O2S−SO2]2−
1870, Schutzenberger
̈
[2Na+]
1956, Dunitz
c
[2Na+]
[SO3]2−
1702, Stahl
[2K+]
[O3S−SO2]2−
1797, Vauquelin
[2Na+]
1931, Zachariasen
1932, Zachariasen
d
e
[2Na+]
[2K+]
[O3S−SO3]2−
1819, Gay-Lussac
[Mn2+]
[O3S−S−SO3]2−
1841, Langlois
[2K+]
1931, Barnes
1934, Zachariasen
f
g
[2K+]
[2K+]
[SO4]2−
1625, Glauber
[Na+]
[O3S−O−SO3]2−
[O3S−O−O−SO3]2−
1861, Rosenstiehl
[2Na+/2K+]
[O3SOSO2OSO3]2−
1868, Schultz-Sellack
[2Na+]
1927, Goeder
[2K+/2Rb+/2Cs+]
1960, Truter
1954, Eriks
h
i
j
+
[2K+]
[2N2O5 ]
1891, Marshall
[2K+]
1934, Zachariasen
k
[2Cs+/2NH4 ]
+
a
b
c
d
e
f
g
h
i
j
k
Ref 1. Ref 6. Ref 7. Ref 8. Ref 9. Ref 10. Ref 11. Ref 12. Ref 13. Ref 14. Ref 15.
+
Figure 1. Born−Haber cycle [kJ mol−1] for reaction of R2SO4(s) + SO2(g) where R = N(CH3)4 (blue) and Na+ (red).
2, suggest that reactions 4 and 5 are favorable for salts of
R2(O3SOSO2)(s) + SO2(g) ↔ R2[(O2SO)2SO2](s)
(5)
cations having volumes equal to or larger than that of
+
This work is of both fundamental and practical interest. On
the fundamental side, the successful preparation of the
N(CH3)4 . Experimental support for the viability of these
reactions can be found in a 1938 report by Jander and
Mesech17 that examined the temperature/vapor pressure
characteristics of [N(CH3)4]2SO4 with SO2(g). They reported
the formation of two solvates [N(CH3)4]2SO4·xSO2 x = 3, 6,
and that the properties of [N(CH3)4]2SO4·xSO2 x = 1, 2
implied a chemical reaction of SO2 rather than a solvate
formation.17
+
N(CH3)4 salts of the two novel oxyanions, [O2SIVOSVIO3]2−
and [O2SIVOSVI(O2)OSIVO2]2−, constitutes the discovery of a
new class of salts of sulfur oxyanions, [(SO4)(SO2)x]2−,
analogous to the well-known polysulfates [(SO4)(SO3)x]2− (x
= 1−2) given on the addition of xSO3 to salts of SO42− (Table
1)18 (also known with x = 3 and 4).19 This work confirms the
earlier indications that such compounds exist17 and is
experimental confirmation of the previous computational
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Figure 2. Estimated ΔGrxn (298 K) for reactions 4 and 5 as a function of cation volume.
work by Chan and Grein.4 Furthermore, our theoretical
estimates imply that the chemistry of simple polyanions with
large cations may be very different from that of the well-known
salts of smaller cations and that stable salts containing
numerous new classes of simple binary oxyanions and related
anions can be predicted using our methodology and
subsequently prepared and studied. Thus preparation of these
novel salts constitutes a valuable confirmation of the predictive
ability of our methodology.
Within a more practical context, any materials that reversibly
absorb and release SO2 under ambient or near-ambient
conditions (i.e., Equations 4 and 5) are of potential industrial
interest. Sulfur dioxide is a byproduct of burning fossil fuels,
especially coal and emissions of SO2 produce acid rain that
acidifies water resources, degrades monuments and buildings,
and harms plant life. The current industrial methods for SO2
absorption are energetically inefficient, requiring high temper-
atures for both absorption and regeneration steps. As well,
current methods do not capture all of the SO2, with about 5%
released into the air.20
obtained at 20 °C using an Accu-Cal Plus Digital gauge (3D
Instruments) with a range of 0−1500 2.5 mmHg.
FT-IR spectra were recorded on a Thermo Nicolet NEXUS 470
FT-IR. Samples were prepared quickly, to minimize decomposition of
product, in the drybox, and the spectra obtained as Nujol mulls using
KBr plates that were wrapped on the outer edges with Teflon tape.
FT-Raman spectra were recorded on a Thermo Nicolet 6700 FT-IR
equipped with a Thermo Nicolet NXR FT-Raman accessory at 298 K
using a Nd:YVO4 laser (emission wavelength: 1064 nm; 180°
excitation). All Raman spectra were obtained in situ in vessel A.
Intensities were integrated from the area under the band. Elemental
analyses were performed using a LECO CHNS-932 analyzer.
Thermogravimetric analyses (TGA) and differential scanning
calorimetry (DSC) were performed on a TA Instruments Q50
analyzer. For TGA, approximately 5 mg of sample was loaded into an
Al crucible and crimped with an Al cover in an argon-filled drybox. A
pinhole pierced in the cover enabled the escape of SO2 that evolved
during the measurement while minimizing the exposure of the sample
to the atmosphere as it was transferred to the TGA instrument. The
sample was heated to 500 °C at a ramp rate of 5 °C min−1 under a 120
mL min−1 N2 flow. For DSC, 5 mg of material was placed in an Al pan,
which was crimped with an Al cover in an argon-filled drybox. The
sample pan was mounted on the instrument, and the sample was
heated to 500 °C at a rate of 5 °C min−1 under a N2 flow of 95 mL
min−1. The reference sample was an empty Al pan containing a cover.
2.2. Materials. Sulfur dioxide (Matheson) was stored over
molecular sieves (4 Å) in a 100 mL round-bottom flask (rbf)
equipped with a Whitey (1KS4) valve for 24 h and vacuum distilled to
a similar rbf and stored over CaH2 to guarantee that the SO2 was dry.
The SO2 was degassed using a freeze, pump, thaw method prior to use
and was freshly distilled into the reaction vessel via the vacuum line.
Tetramethylammonium sulfate (Aldrich, > 99.0%, white free-flowing
powder) was used as received. The high purity of the sulfate salt was
confirmed by elemental analysis (Found: %C 39.46, %H 9.47, %N
11.27, %S 13.29; Calculated: %C 39.32, %H 9.90, %N 11.46, %S
13.12) and by vibrational spectroscopy, the spectrum was identical to
that reported by Malchus and Jansen.22 Tetramethylammonium sulfate
(TCI) was dried under vacuum for 24 h and shown to be
spectroscopically (IR and Raman) and analytically less pure (Found:
%C 40.19, %H 9.73, %N 11.26, %S 12.50) compared to the Aldrich
sample. The Aldrich sample was the preferred sample and was the one
used unless otherwise specified. Paratone-N oil (Hampton Research,
clear pale yellow oil) was used as received.
This report describes the study of the reaction of
[N(CH3)4]2SO4(s) with various molar equivalents of gaseous
SO2. The solid products of the reaction were characterized by
vibrational spectroscopy and by X-ray crystallography.
2. EXPERIMENTAL SECTION
2.1. General Procedures. All solids were manipulated in an
MBraun Unilab drybox under a nitrogen atmosphere and/or using a
Monel vacuum line and vapor pressure gauge (V = 52 mL). The
general techniques have been fully described elsewhere.21 The reaction
vessel used for in situ Raman experiments and for vapor pressure
measurements (A) was a single 1 cm OD Pyrex tube (V = 14 mL)
equipped with a Rotaflo (HP 6K) Teflon-in-glass valve; the vessel used
to grow crystals (B) was composed of two 3 cm OD Pyrex tubes
connected by a medium glass frit to form a H-shaped vessel equipped
with two Rotaflo (HP 6K) Teflon-in-glass valves (Supporting
Information, Figure S1). Weights were obtained using either a Mettler
PM100 balance (0−110 g
0.001 g capacity) or a Mettler H311
balance (0−240 g 0.0001 g). Vapor pressure measurements were
C
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Figure 3. (A) Raman spectrum of [N(CH3)4]2[O3SOSO2](s) (1) compared with (B) the calculated [B3PW91/6-311+G(3df)] Raman spectrum of
[O3SOSO2]2−(g) in the 140−1500 cm−1 region (2048 scans; 4 cm−1 resolution; 0.205 W laser power; Ge detector). Spectrum of the 1500−3500
cm−1 region is included in Supporting Information, Figure S2. Assignments related to N(CH3)4+ and SO42− were made by comparison with those in
[N(CH3)4]2SO4(s) and SO2 of solvation with similar compounds found in Supporting Information, Table S8.32
Table 2. Experimental Vibrational Spectrum of [O3SOSO2]2− in 1 [cm−1], Compared with the Calculated (B3PW91/6-
311+G(3df)) Spectrum and the Spectrum of the Isoelectronic O3ClOClO2(g)5 (Relative Intensities in Brackets)
a
a
b
bands attributed to [O3SOSO2]2− in 1
calculated [O3SOSO2]2−
O3ClOClO2(g)
IR
c
Raman
IR
Raman
35 (1)
IR
assignments
35 (<1)
A″
A′
45 (<1)
45 (1)
132 (3)
132 (1)
A′ v(O2S−O)
215 (84)
277 (15)
389 (9)
222 (42)
279 (9)
222 (13)
279 (<1)
393 (18)
420 (<1)
479 (40)
542 (1)
A″ ρ(SO2); ρ(SO3)
A′ ρ(SO2); ρ(SO3)
A″ δas(SO3)
A′ δs(SO3); δ(SO2)
A′ δ(SO3); δ(SO2)
A″ Rocking (SO3)
A′ ρ(SO3); δ(SO2) [v(O2S−O)]
A′ v(O2S−O) [δ(SOS); δas(SO3)]
A′ vs(O3S−O)
380 (12)
429 (3)
393 (4)
420 (1)
477 (10)
550 (3)
476 (9)
479 (17)
542 (10)
575 (3)
544
549 (5)
591 (2)
590 (14)
614 (15)
634 (21)
836 (100)
1032 (47)
1103 (23)
1168 (58)
1210 (87)
1210 (87)
575 (5)
579
615 (3)
613 (16)
630 (8)
613 (2)
629
640 (3)
630 (48)
792 (84)
1039 (53)
1096 (10)
1173 (45)
1221 (76)
1222 (100)
691
842 (7)
792 (27)
1039 (41)
1096 (100)
1173 (47)
1221 (52)
1222 (12)
1024
1080
A′ vs(SO3); vs(SO2)
1033 (9)
1104 (37)
1172 (3)
1192 (19)
1192 (19)
A′ vs(SO2) [vs(SO3)]
A″ vas(SO2) [vas(SO3)]
A″ vas(SO3)
1265
A′ vas(SO3) [vas(SO2)]
1265
a
b
A full listing of IR and Raman frequencies is given in Supporting Information, Table S2. See Ref 5. No intensities were reported. For a more
c
complete comparison of [O3SOSO2]2− and O3ClOClO2 in the gas phase and solid matrices see Supporting Information, Table S3. Vibrational
bands were assigned visually using ChemCraft program; v - stretching, δ - bending, ρ - twisting/rocking/wagging. Vibration listed first is the main
contributor with secondary contribution given in square brackets. Equal contributions are separated by a semicolon.
2.3. X-ray Crystallography. A hemisphere of X-ray diffraction
data was collected for crystals of [N(CH3)4]2[(O2SO)2SO2]·SO2 using
a Bruker AXS P4/SMART 1000 diffractometer. ω and θ scans had a
width of 0.3° and 10 s exposure times. The detector distance was 5 cm.
The crystal was twinned and the orientation matrices for two
components were determined (CELL_NOW).23 The data were
reduced (SAINT)24 and corrected for absorption (TWINABS).25
The structure was solved by direct methods and refined by full-matrix
least-squares on F2 (SHELXTL)26 on all data. All non-hydrogen atoms
were refined using anisotropic displacement parameters. Hydrogen
atoms were included in calculated positions and refined using a riding
model. The reflections were of weak intensity, and it was not possible
to model any disorder in the anion. Figures depicting structures were
obtained using Diamond 3.2 program.27
optimization and frequency calculations.29 Normal modes were
visually assigned using ChemCraft program.30 There are significant
differences between the calculated intensities and the experimental
Raman intensities in the region between 1000 cm−1 and 1200 cm−1
where the calculated intensities are overestimated. Similar discrep-
ancies for the problematic S−O vibrations in this region have been
observed in other cases.31,32 Natural bond orbital analyses were
performed with NBO 5.9 program.33
2.5.1. In Situ Preparation of [N(CH3)4]2O3SOSO2(s) (1). One mole
equivalent of SO2(g) (0.256 g, 4.000 mmol) according to eq 4 (R =
+
N(CH3)4 ) was expanded into the vacuum line and onto [N-
(CH3)4]2SO4(s) (0.970 g, 3.970 mmol) in vessel A. The vapor
pressure was 1400.0 mmHg, decreasing over 4 h until it equilibrated at
95.0
2.5 mmHg over a partially clumped yellow solid. The
2.4. Quantum Chemical Calculations. All calculations were
carried out with the Gaussian 03 program package.28 The B3PW91
functional was used with the 6-311+G(3df) basis set for geometry
remaining SO2 in the line was condensed into, and isolated in, vessel
A, the contents agitated by hand and in an ultrasonic bath (20 °C)
leading to a free-flowing white homogeneous powder (1.235 g, 4.004
D
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Table 3. Experimental Raman Frequencies Attributed to [(O2SO)2SO2]2− Measured from the Solid Obtained by the Addition of
2.07 mol Equivalents of SO2 to One Mole of [N(CH3)4]2SO4(s) [cm−1] and Comparison with the Calculated [B3PW91/6-
a
311+G(3df)] Frequencies of [(O2SO)2SO2]2− (Relative Intensities in Brackets)
b
b
c
[N(CH3)4]2[(O2SO)2SO2]·SO2(s) experimental
calculated Raman [(O2SO)2SO2]2−
tentative assignments
169 sh
161 (19)
161 (6)
B vas(O2SVI(-OSO2)2)
B vas(O2SVI(-OSO2)2)
A ρt(SO2)
181 (98)
232 (4)
242 sh (40)
342 (<1)
354 <1)
243 (4)
B ρt(SO2) [ρt(SO4)]
A δ(SO4) [ρw(SO2)]
B ρw(SO2)
367 (0.8)
381 (<1)
398 (1)
459 (24)
496 (7)
A δ(SO4) [ρw(SO2)]
492 (17)
541 (<1)
564 (2)
A δ(O2SVI(-OSO2)2) [δ(SO2)]
B δ(SO2) [δ(SO4)]
544 (5)
564 (<1)
578 (<1)
593 (<1)
619 (7)
A δ(SO2) [δ(SO4)]
A δ(SO4) [δ(SO2)]
B δ(SO4) [δ(SO2)]
586 (8)
595 (2)
633 (1)
B δ(SO4) [δ(SO2)]
900 (19)
925 (7)
894 (16)
912 (2)
A vsym(SO4) [vsym(SO2)]
B vas(SO4) [vsym(SO2)]
A vsym(SO2); vsym(SO4)
B vsym(SO2)
1124 (100)
1121 (56)
1133 (2)
1158 (100)
1245 (29)
1250 (14)
1288 (26)
1145 (64)
1238 (5)
1249 (2)
1294 (7)
A vsym(SO4) [vsym(SO2)]
B vas(SO2) [vas(SO4)]
A vas(SO2) [vas(SO4)]
B vas(SO4) [vas(SO2)]
a
A full listing of IR and Raman frequencies is given in Supporting Information, Table S4. The observed symmetry is C1 while that of the calculated
b
anion is C2 (Figure 8). Ten of the expected 27 vibrations were observed experimentally while eight were calculated [B3PW91/6-311+G(3df)] too
low in frequency [12 (A), 13 (B), 29 (A), 48 (B), 110 (A), 110 (B), 161 (A), and 161 cm−1 (B)] to be observed. The remaining nine calculated
vibrations [169 (B), 342 (A), 354 (B), 459 (A), 496 (A), 564 (A), 578 (A), 593 (B), and 1294 cm−1 (B)] had very low calculated intensities and
c
therefore were not observed or were buried in cation or other anion bands. Assignment listed first is the main contributor with secondary
contribution given in square brackets. Equal contributions are separated by a semicolon. Bands were assigned visually using ChemCraft; v- stretching,
δ - bending, ρ - twisting/rocking/wagging.
mmol). The vapor pressure measurements were not taken at this point
since several experiments showed that the opening and closing of the
vessel affected the amount of time it took for the full conversion to the
[O3SOSO2]2− anion. After a 7 day period in situ Raman spectra
showed only bands attributable to 1 (see Figure 3, and Supporting
Information, Figures S2 and S3), and vapor pressure dropped to 11.0
mmHg (5−11 mmHg in related experiments). A more detailed
account of the preparation of 1 is given in Supporting Information,
Section S2.1. The IR spectrum is given in Supporting Information,
Figures S4 and S5. A complete vibrational assignment of 1 is given in
Supporting Information, Table S2. Vibrations assigned to
[O3SOSO2]2− have been compared with the experimental IR
vibrations of the isoelectronic O3ClOClO2 and the calculated
[B3PW91/6-311+G(3df)] normal modes and intensities of
[O3SOSO2]2− in Table 2. The TGA and DSC results are given in
Supporting Information, Figures S8 and S9, respectively.
2.5.3. Preparation of Crystals of [N(CH3)4]2(O2SO)2SO2·SO2(s) (2).
Crystals of 2 were prepared according to reaction 6. Approximately 13
mol equivalents of SO2(g) (1.225 g, 19.122 mmol) were condensed
directly onto [N(CH3)4]2SO4(s) (0.369 g, 1.510 mmol) in the left-
hand side of vessel B (Supporting Information, Figure S1). Upon
warming to room temperature a yellow solution was formed. The
vessel was evacuated by slowly opening the valve on the left-hand side
of the vessel under dynamic vacuum. Pumping on the yellow solution
gave clear colorless crystals in less than 2 min. The left-hand side of
the vessel was isolated as soon as crystals were observed with small
amounts of liquid remaining. The vessel was transferred to the drybox
and clear, colorless crystals suitable for X-ray diffraction were collected
using a spatula tip coated with Paratone-N oil. These crystals were
placed in a vial under Paratone-N oil for transportation. The vial lid
was wrapped with Teflon tape during transportation. Single crystals
coated in Paratone-N oil were mounted using a polyimide Micro-
Mount and cooled to −100 °C in the cold nitrogen stream of the
goniometer. Crystallographic details are given in Supporting
Information, Table S5.
2.5.2. Removal of SO2 from [N(CH3)4]2O3SOSO2(s). [N-
(CH3)4]2O3SOSO2(s) (1.317 g, 4.270 mmol), prepared from
[N(CH3)4]2SO4(s) (1.029 g, 4.213 mmol) according to eq 4 (R =
+
N(CH3)4 ), was subjected to a dynamic vacuum at room temperature.
Loss of SO2(g) was observed by the turbulence of the white powder
and weight loss: (0 min/1.317 g, 10 min/1.270 g, 30 min/1.202 g, 50
min/1.129 g, 90 min/1.042 g, 100 min/1.032 g), as illustrated in
Supporting Information, Figure S11. After 100 min the Raman
spectrum and weight (1.032 g) were, within experimental error, that of
the original starting material, [N(CH3)4]2SO4(s) (1.029 g) (Support-
ing Information, Figure S6). However, we note that samples of
[N(CH3)4]2O3SOSO2(s) (0.149 g) prepared from [N(CH3)4]2SO4(s)
(0.116 g) and SO2(g) (0.033 g) isolated in vessel A with the valve
closed for 3−4 weeks did not lose weight on pumping for one day, but
heating to 100 °C for a few minutes led to SO2 loss and complete
recovery of [N(CH3)4]2SO4(s) (0 min/0.149 g, 10 min/0.119 g), as
shown by Raman spectroscopy.
[N(CH3)4 ]2 SO (s) + excess SO2(l)
4
↔ [N(CH3)4]2 (O2SO)2 ·SO2(s)
(6)
2.5.4. Attempted Preparation of [N(CH3)4]2(O2SO)2SO2(s) from
[N(CH3)4]2SO4(s) and xSO2 (x > 2). In accordance with eq 7, 2.07 mol
equivalents of SO2 (0.233 g, 3.637 mmol) were expanded into the
vacuum line and condensed at −196 °C onto white [N-
(CH3)4]2SO4(s) (0.430 g, 1.750 mmol) in vessel A. Upon warming
to room temperature, a yellow clumped solid was observed. The
contents were agitated by hand and in an ultrasonic bath (20 °C), but
the powder never became homogeneous.
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3.3. Characterization of [O3SOSO2]2− in 1 by Vibra-
tional Spectroscopy. The experimental Raman and IR
spectra of [N(CH3)4]2O3SOSO2 (1) are given in Figure 3
and Supporting Information, Figure S4, respectively, with
frequency assignments attributable to [O3SOSO2]2− given in
Table 2. Cation bands were assigned by comparison with the
[N(CH3)4]2 SO (s) + 2SO2(g) ↔ [N(CH3)4]2 (O2SO)2SO2(s)
4
(7)
Several in situ Raman spectra were taken over a period of one
month. The bands in the spectrum were assigned to N(CH3)4 ,
+
[(O2SO)2SO2]2−, and additionally to [O3SOSO2]2− and SO2 of
solvation (Supporting Information, Figure S7). The observed Raman
bands attributed to [(O2SO)2SO2]2− are compared with the calculated
[B3PW91/6-311+G(3df)] normal modes and intensities in Table 3. A
complete listing of the Raman frequencies has been given in
Supporting Information, Table S4. Very similar Raman spectra were
obtained from reactions of [N(CH3)4]2SO4(s) with slightly higher and
lower amounts of SO2(g) than 2 mol equivalents. Bands attributed to
[(O2SO)2SO2]2− were also observed in the preparation of [N-
(CH3)4]2O3SOSO2(s) prior to completion of the reaction (Supporting
Information, Figure S3).
previously assigned N(CH3)4 in [N(CH3)4]2SO4.22 The
+
experimental data related to the anion is in good agreement
with the calculated values and assignments in Table 2. Eighteen
bands are expected to be active in the Raman spectrum of a
[O3SOSO2]2− anion having Cs symmetry, of which fifteen were
observed in the experimental spectrum. The remaining three
bands were calculated to be very weak in intensity or in the low
unobservable frequency range. Similar results were seen in the
IR, as shown in Supporting Information, Figure S4 and Table 2.
The spectra of [O3SOSO2]2− contain more bands, and the
bands are very different from those reported for the well-known
dithionate anion, [O3SSO3]2−,34,35 which is calculated to be 53
kJ mol−1 lower in energy [B3PW91/6-311+G(3df)]. The IR
bands assigned to [O3SOSO2]2− are in line with the values
The addition of 3.3 mol equivalents of SO2 to [N(CH3)4]2SO4(s)
(0.430 g, 1.750 mmol) in vessel A gave a partly yellow and clumped
solid. The contents were agitated at room temperature by hand and in
an ultrasonic bath (20 °C) but the powder never became
homogeneous. In situ Raman spectra of the solids obtained from
the reactions [N(CH3)4]2SO4 + xSO2(g) x = 2.95, 3.30, 5.50 as a
function of time were all similar, except that bands attributed to SO2 of
solvation increased in the intensity as the amount of SO2(g) reactant
increased.
5
reported for the isoelectronic O3ClOClO2 (Table 2 and
Supporting Information, Table S3), further supporting the
anion identification.
3.4. Structure and Bonding in [O3SOSO2]2−. The
excellent agreement between the experimental vibrational
spectrum and calculated normal modes of [O3SOSO2]2−
strongly supports the validity of the calculated [B3PW91/6-
311+G(3df)] structure (Figure 4). The calculated bond
3. RESULTS AND DISCUSSION
3.1. Reversible Absorption of SO2(g) by [N-
(CH3)4]2SO4(s) with Quantitative Formation of [N-
(CH3)4]2O3SOSO2(s) (1). The vapor pressure of one mole
equivalent of SO2(g) over [N(CH3)4]2SO4(s) dropped first
from 1400.0 mmHg (0 h) to 95.0 mmHg (4 h), followed by a
much reduced rate of SO2 uptake until after 7 days the vapor
pressure was decreased to about 11.0 mmHg. The resulting
white free-flowing powder was unambiguously characterized as
[N(CH3)4]2O3SOSO2 by in situ Raman and IR spectroscopy
(see Section 3.3), with the weight gain corresponding to uptake
of one mole equivalent of SO2(g) according to eq 4 (R =
+
N(CH3)4 ).
SO2 was quantitatively lost on evacuation of freshly prepared
samples, with full recovery of [N(CH3)4]2SO4(s) (Supporting
Information, Figure S6, 1.317 to 1.032 g, theoretical 1.033 g).
Periodic in situ Raman spectra of the sample were obtained
over time prior to full conversion to 1. The in situ Raman
spectra show strong bands due to the final product and weaker
Figure 4. Calculated [B3PW91/6-311+G(3df)] structure of
[O3SOSO2]2− in the gas phase; bond lengths (black) [Å], Wiberg
bond indices (red), and Natural atomic charges (blue).39 ∠O2−S1−
O1: 113.06, ∠O1−S1−O3: 113.06, ∠O3−S1−O4: 107.29, ∠O4−S1−
O2: 107.29, ∠S1−O4−S2: 121.88, ∠O4−S2−O6: 100.26, ∠O6−S2−
O5: 111.40, and ∠O5−S2−O4: 100.26°.
bands attributed to the SO2 of solvation, [(O2SO)2SO2]2− and
2−
SO4
(Supporting Information, Figure S3). Attempted
optimizations of gas phase (SO4)2−·SO2 resulted in the
covalently bound [O3SOSO2]2− structure, suggesting that the
SO2 of solvation is likely associated with 1 and/or [N-
2−
(CH3)4]2(O2SO)2SO2(s) but not SO4
.
lengths, angles, Wiberg bond indices, and natural atomic
charges in [O3SOSO2]2− are in line with those calculated for
related species, SO2, SO42−, and [O2SOH]− (Figure 5) and
show the expected trends. We note that salts of [O2SOH]− are
unknown, but evidence has been provided that it exists in
aqueous solutions.36 The bridging S−O bonds in [N-
(CH3)4]2O3SOSO2 are longer than most other known S−O
bonds as shown in Supporting Information, Figure S12, but
shorter than those found in some cyclic structures, for example,
1,6-Dioxa-6a-thiapentalene [1.866(2) Å]37 and donor−accept-
or adducts, for example, 1,4-dioxane adduct of SO3,
C4H8O2·SO3 [1.861(2) Å].38
3.2. TGA Analysis of 1. The quantitative loss of SO2 from
[N(CH3)4]2O3SOSO2(s) on evacuation of the reaction vessel
according to eq 4 (R = N(CH3)4 ) was in line with the
+
thermogravimetric analysis (TGA) and the differential scanning
calorimetry (DSC) results of this material that showed the
quantitative loss of SO2 at approximately 90 °C. At temper-
atures greater than 90 °C the remaining material exhibited
characteristics identical to those of [N(CH3)4]2SO4 as reported
by Malchus and Jansen22 (Supporting Information, Figures S8
and S9). The weight loss of a sample of 1 stored in a vial in the
drybox was consistent with the loss of SO2. A TGA of 1 held
constant at room temperature over a period of approximately
200 min suggested that SO2 was gradually released under
ambient conditions (Supporting Information, Figure S10).
The current view of the bonding in SO42− is that S−O bonds
are single σ bonds with high ionic contributions42 in contrast to
earlier views that included double bond character to S−O
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Figure 5. Comparison of calculated [B3PW91/6-311+G(3df)] bond lengths, bond angles, Wiberg bond indices, and natural atomic charges in (a)
SO2, (b) SO42−, and (c) [HOSO2]−; bond lengths (black) [Å], Wiberg bond indices (red), bond angles (green) [deg], and natural atomic charges
(blue).39 For comparison experimental bond lengths and angles reported for SO42− in [N(CH3)4]2SO4 are S−O: 1.461(2) Å, ∠OSO: 109.51(9) and
109.4(2)°22 and for SO2(s) are S−O: 1.4299(3) Å, ∠OSO 117.16(3)°.40 Structure of [HOSO2]− has been previously calculated at the B3LYP/6-
31G(2df,p) level by Steudel et al.,41 and they reported the S−O bond lengths to be 1.494, 1.482, and 1.768 Å and the O−H bond length 0.966 Å.
bonds.43 The bonding description of SO4 is in line with the
2−
more general trend of emphasizing electrostatic interactions in
the bonding explanations of the heteroatom bonds of the
heavier main groups elements.40,44
The structure of [O3SOSO2]2− can be viewed as a donor−
acceptor adduct of SO42− and SO2 where lone pair electrons of
a negatively charged oxygen atom on the Lewis base SO42− are
Figure 7. Valence bond description of [O3SOSO2]2−.
donated to the empty π* lowest unoccupied molecular orbital
(LUMO) orbital of SO2 (see Figure 6).31 The formation of a
implying the same ratio between the valence bond structures
A and B. Furthermore a similar conclusion can be drawn from
the ratio of group atomic charges −1.59 on SO4
(O1O2S1O3O4) and −0.41 on SO2 (O5S2O6) moieties in
[O3SOSO2]2−.
3.5. Preparation of Crystals of [N(CH3)4]2(O2SO)2-
SO2·SO2(s) (2). Single crystals of 2 were obtained from a
solution of [N(CH3)4]2SO4 dissolved in liquid sulfur dioxide on
removal of the solvent. The structure of 2 was unambiguously
determined by X-ray crystallography (Figure 8a). 2 is an SO2
solvate of the [(O2SO)2SO2]2− dianion and does not contain
the discrete isoelectronic [(O2SO)3(SO)]2− dianion depicted in
Figure 9. We estimate that the formation of [N-
(CH3)4]2(O2SO)2SO2(s) from [N(CH3)4]2O3SOSO2(s) and
SO2(g) is thermodynamically feasible within the accuracy of
our predictive method46 while the formation of [N-
(CH3)4]2(O2SO)3(SO)(s) from [N(CH3)4]2(O2SO)2SO2(s)
and SO2(g) is unfavorable (see Supporting Information,
Table S1 for details).
Figure 6. NBO donor−acceptor description of bonding in
[O3SOSO2]2−.39
The Raman spectrum of very small crystals obtained from
the reaction of [N(CH3)4]2SO4 and 2.07 mol equivalents of
SO2 was very similar to those obtained from other reactions
with differing molar ratios of the reactants. The main difference
in the Raman spectra was the increase in the intensity of SO2
solvent band as the ratio of SO2 reactant increased. The peaks
in the Raman spectra attributed to [(O2SO)2SO2]2− are
compared with the calculated normal modes and intensities
in Table 3. We were unable to prepare pure samples of
[N(CH3)4]2(O2SO)2SO2(s) or [N(CH3)4]2(O2SO)2SO2-
SO2(s) (2). All attempts to prepare the bulk materials gave
mixtures as demonstrated by Raman spectroscopy (For
example Supporting Information, Figure S7). However, single
crystals of 2 have been obtained.
donor−acceptor adduct may account for the observation of
[O3SOSO2]2− rather than the more stable [O3SSO3]2− isomer
(−53 kJ mol−1 at the B3PW91/6-311+G(3df) level of theory),
that would require the breaking of a strong S−O bond and
forming of an S−S bond. In [O3SOSO2]2− 0.41 electrons have
been transferred from SO42− to SO2.45 This process delocalizes
the negative charge and contributes to the driving force of
reaction 1 as predicted. The charge transfer to SO2 in
[O3SOSO2]2− is calculated to be somewhat less than that in
[HOSO2]− (0.48 electrons) making SO4 a weaker base
2−
toward SO2 than OH−.
In valence bond terms the structure of [O3SOSO2]2− can be
represented by a resonance between valence bond structures A
and B shown in Figure 7, where the structure A assumes full
electron pair donation from SO42− to SO2. The relative weights
of A and B structures can be estimated from the ratios of the
bond orders of bridging S−O bonds. The bond order ratio of
S1−O4 (0.75) and S2−O4 (0.52) is approximately 3:2
3.6. X-ray Structure of [N(CH3)4]2(O2SO)2SO2·SO2(s).
The structure of [N(CH3)4]2(O2SO)2SO2·SO2(s) consists of
+
discrete N(CH3)4 cations (Supporting Information, Figure
S13) and [(O2SO)2SO2]2− anions (Figure 8a) which weakly
interact with an SO2 molecule of solvation to give a two-
dimensional sheet (Figure 10).
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Figure 8. (a) Diamond depiction of [(O2SO)2SO2]2− in 2 (thermal ellipsoid plots are at the 50% probability level) and (b) calculated [B3PW91/6-
311+G(3df) level; C2 symmetry] structure of [(O2SO)2SO2]2− in the gas phase [bond lengths [Å] (black), bond angles [deg] (green), Wiberg bond
indices (red), and natural atomic charges (blue)].39 Selected bond angles for the calculated structure [deg]: ∠O5−S2−O6: 113.1 ∠O6−S2−O1:
101.1, ∠O1−S2−O5: 97.8, S2−O1−S1: 122.6, ∠O1−S1−O3: 109.7, ∠O3−S1−O4: 115.9, ∠O4−S1−O2: 109.7, ∠O2−S1−O1: 105.7, ∠S1−O2−
S3: 122.6, ∠O2−S3−O8: 101.1, ∠O8−S3−O7: 113.1, ∠O7−S3−O2: 97.8.
numerous O−H hydrogen bonds (Supporting Information,
Table S6). The geometry of N(CH3)4+ is very similar to that in
[N(CH3)4]2SO4 (Supporting Information, Table S7).22 The
S−O distances in the SO2 solvent molecule [S4−O10 1.394(4)
Å, S4−O9 1.344(4) Å, ∠SOS 120.0(2)°] in 2 are shorter than
in SO2(s) [1.4299(3) Å, ∠SOS 117.16(3)°]40 (Supporting
Information, Table S8). Many X-ray structures containing SO2
fragments often give S−O distances that are much shorter than
expected (e.g., see references 31 and 32 and references therein),
likely because of unresolved SO2 disorder and/or unaccounted
for librational motion.
3.7. Structure and Bonding in [(O2SO)2SO2]2−. Two
minimum conformations with almost the same energy (energy
difference 2.3 kJ mol−1) were optimized for [(O2SO)2SO2]2−.
The conformation that more resembled the experimental
structure (Figure 8a) is presented in Figure 8b while the other
Figure 9. Optimized structure of [(O2SO)3(SO)]2− (B3PW91/6-
conformation is given in the Supporting Information, Figure
31+G* level; C3 symmetry) with bond lengths [Å].4
S17. The optimized structure of [(O2SO)2SO2]2− can be
viewed as being formed by donor−acceptor interactions
2−
between SO4 and two SO2 molecules in analogous fashion
The cations reside in between the layers formed by the
anions and solvent molecules in the bc plane (Supporting
Information, Figure S14). The anion and cations are linked by
to [O3SOSO2]2− above. The negative charge on the SO4
moiety in [(O2SO)2SO2]2− (−1.42) is less than in
Figure 10. Portion of the layer of [(O2SO)2SO2]2− anions and weakly interacting SO2 molecules of solvation in the ab plane of 2, bond distances [Å]
(black) and bond orders (red)47 (thermal ellipsoid plots are drawn at the 50% probability level).
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Figure 11. Valence bond description of [(O2SO)2SO2]2−.
[O3SOSO2]2− (−1.59) and the transfer to the two terminal
SO2 moieties (−0.58) is greater than the negative charge
transfer to the single SO2 in [O3SOSO2]2− (−0.41). In addition
the negative charge is delocalized over a larger volume. The
calculated anion structure can be represented in valence bond
terms with resonance structures C, C′, and D (Figure 11). The
calculated bridging S2−O1 (2.030 Å, B.O.: 0.34) bond suggests
that the relative weights of C, C′, and D structures are close to
1:1:1.
[O3SOSO2]2− and [(O2SO)2SO2]2− oxydianions are the first
new sulfur oxydianions containing one to three sulfur atoms
that have been identified since 1891 (Table 1) and are the first
2−
members of a new class of [SO4][SO2]x (x = 1, 2) sulfur
2−
oxyanions, analogous to the well-known series of SO4[SO3]x
polysulfates (Table 1) isolated as salts of small cations.
This work illustrates the usefulness of predictive thermody-
namics and the importance of size in salt reactivity. It implies
that the chemistry of oxydianions of sulfur with large cations
will be very different from that of the more traditional salts of
small cations. This will be further illustrated in upcoming
publications. It also implies that the energetics of SO2 uptake by
sulfates can be tailored by changing the size of the cation. This
represents a unique opportunity for the engineering of
reversible SO2 uptake reagents.
The present work compliments the classical 1938 studies by
Jander and Mesech on the SO2 uptake by [N(CH3)4]2SO4, and
confirms their suggestion that [N(CH3)4]2SO4·(SO2)x x = 1
and 2 were not just SO2 solvates of [N(CH3)4]2SO4.
Compared to the optimized structure the interaction
between the central SO4 unit and terminal SO2 moieties in
the experimental X-ray structure is weaker as evidenced by the
longer bridging sulfur−oxygen bond lengths S2−O1 (2.213(3)
Å, B.O.: 0.31) and S3−O2 (2.358(2) Å, B.O.: 0.22).47 Despite
of the weaker interaction the identity of the anion is judged to
remain the same. The only difference is the higher contribution
of the nonbonding D resonance structure to bonding in the
experimental structure compared to the optimized structure.
The weaker interaction between the central SO4 unit and
terminal SO2 moieties in the experimental structure can be
attributed to numerous cation−anion H···O interactions
(Supporting Information, Table S6) and influence of the SO2
solvate molecules that are expected to delocalize some of the
negative charge on the anion. Compared to the majority of
known S−O bonds (Supporting Information, Figure S12) the
bridging S−O bonds in [(O2SO)2SO2]2− are long but bonds in
the same range (2.00−2.50 Å) have been reported.48 A related
bridging O−SO2 bond distance that is even longer than those
in [(O2SO)2SO2]2− was reported in [(η6-C6H6)2Cr]S2O6·2SO2
(2.433(4) Å, B.O. = 0.18).47,49 The authors formulated the
structure as SO2 solvate, but it can also be viewed as a sulfur
oxyanion, [S4O10]2−, that has even greater contribution of a
valence bond structure corresponding to a nonbonding
resonance form similar to D than [(O2SO)2SO2]2−. The
previously known very long S−O bonds can be regarded either
as a part of a bond/no bond resonance system or alternatively
as a part of a multicentered σ bond.50
ASSOCIATED CONTENT
* Supporting Information
■
S
Details of thermodynamic calculations, comparison spectra, and
tables with assignments for vibrational spectroscopy, exper-
imental details for in situ preparation of 1, results of TGA and
DSC measurements of 1, crystal refinement details, crystal
packing figure of 2, structural parameter comparison tables of 2,
and calculated structures of [O3SOSO2]2−, O3ClOClO2, and
second conformation of [(O2SO)2SO2]2−. CCDC 824802
contains the supplementary crystallographic data for this
paper. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: passmore@unb.ca. Fax: 1 506 453 4981. Tel.: 1 506
453 4821.
■
Notes
4. CONCLUSIONS
The authors declare no competing financial interest.
∥E-mail: Mikko.Rautiainen@oulu.fi.
⊥E-mail: tom.whidden@ah2inc.com.
We have shown that the use of Born−Haber cycles and volume
based thermodynamics to estimate lattice energies with density
functional theory (DFT) enthalpies of the corresponding gas
phase reactions correctly predicted that [N(CH3)4]2SO4 will
react with SO2 to give [N(CH3)4]2O3SOSO2 (1) and
[N(CH3)4]2(O2SO)2SO2. Subsequently [N(CH3)4]2SO4 was
experimentally shown to reversibly and quantitatively take up
gaseous SO2 at room temperature to produce [N-
(CH3)4]2O3SOSO2, an isomer of the known [O3SSO3]2−
dianion. This salt was unambiguously characterized using
v i b r a t i o n a l s p e c t r o s c o p y . C r y s t a l s o f [ N -
(CH3)4]2(O2SO)2SO2·SO2 (2) were isolated from solutions
of [N(CH3)4]2SO4 in liquid SO2. The X-ray structure of 2
showed that it contained the [(O2SO)2SO2]2− dianion. The
ACKNOWLEDGMENTS
■
We thank NSERC (Natural Sciences and Engineering Research
Council) and NBIF (New Brunswick Innovation Foundation)
for financial support, Sandra Riley at the University of New
Brunswick for assistance with TGA and DSC, Emma Harris for
vapor pressure measurements, and Birgit Muller for TGA.
̈
ACEnet, the regional high performance computing consortium
for universities in Atlantic Canada and CSC-IT Center for
Science Ltd. in Finland are acknowledged for providing
computational resources.
I
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(33) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J.
E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.9;
Theoretical Chemistry Institute, University of Wisconsin: Madison,
WI, 2012; http://www.chem.wisc.edu/∼nbo5.
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