Methanesulfonyl Iodide

Article abstract of DOI:10.1021/acs.inorgchem.9b02441

Methanesulfonyl iodide is produced in aqueous solutions from the reaction of triiodide with methanesulfinate. Dichroic crystals of (CH₃SO₂I)₄·KI₃·2I₂ are formed from KI/I₂ solutions with high concentrations of CH₃SO₂^-, while dichroic crystals of (CH₃SO₂I)₂·RbI₃ are formed from RbI/I₂ solutions. X-ray crystallography of these two compounds shows that the CH₃SO₂I molecules coordinate through their oxygen atoms to the metal cations and that the S-I bond length is 2.44 ?. At low concentrations of CH₃SO₂^-, the solutions remain homogeneous and the sulfonyl iodide is formed in a rapid equilibrium: CH₃SO₂^- + I₃^- ? CH₃SO₂I + 2I^-, K_MSI = 1.07 ± 0.01 M at 25 °C (μ = 0.1 M, NaClO₄). The sulfonyl iodide solutions display an absorbance maximum at 309 nm with a molar absorptivity of 667 M^-1 cm^-1. Stopped-flow studies reveal that the equilibrium is established within the dead time of the instrument (～2 ms). Solutions of CH₃SO₂I decompose slowly to form the sulfonate: CH₃SO₂I + H₂O → CH₃SO₃^- + I^- + 2H⁺, k_hyd. In dilute phosphate buffer, this decomposition occurs with k_hyd = 2.0 × 10^-4 s^-1 the decomposition rate shows an inverse-squared dependence on [I^-] because of the K_MSI equilibrium.

Full text of DOI:10.1021/acs.inorgchem.9b02441

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Methanesulfonyl Iodide
Pradeepa Rajakaruna, John D. Gorden, and David M. Stanbury*
Dept. of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States
S
* Supporting Information
ABSTRACT: Methanesulfonyl iodide is produced in aqueous solutions
from the reaction of triiodide with methanesulfinate. Dichroic crystals of
(CH3SO2I)4·KI3·2I2 are formed from KI/I2 solutions with high
−
concentrations of CH3SO2 , while dichroic crystals of (CH3SO2I)2·
RbI3 are formed from RbI/I2 solutions. X-ray crystallography of these
two compounds shows that the CH3SO2I molecules coordinate through
their oxygen atoms to the metal cations and that the S−I bond length is
−
2.44 Å. At low concentrations of CH3SO2 , the solutions remain homogeneous and the sulfonyl iodide is formed in a rapid
equilibrium: CH3SO2 + I3 ⇌ CH3SO2I + 2I−, KMSI = 1.07 0.01 M at 25 °C (μ = 0.1 M, NaClO4). The sulfonyl iodide
solutions display an absorbance maximum at 309 nm with a molar absorptivity of 667 M−1 cm−1. Stopped-flow studies reveal
that the equilibrium is established within the dead time of the instrument (∼2 ms). Solutions of CH3SO2I decompose slowly to
form the sulfonate: CH3SO2I + H2O → CH3SO3− + I− + 2H+, khyd. In dilute phosphate buffer, this decomposition occurs with
khyd = 2.0 × 10−4 s−1; the decomposition rate shows an inverse-squared dependence on [I−] because of the KMSI equilibrium.
−
−
H2O (Baker Analyzed); Na2HPO4 anhydrous (MCB); NaClO4, NaI,
INTRODUCTION
■
KI, HClO4, KBr, KBrO3, and HCl (all from Fisher); 3-
(trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS; Sigma-
Aldrich); and D2O 99.8% isotopic (Alfa Aesar) were used as supplied.
I2 was purified by sublimation.
Compounds having covalent sulfur-halogen bonds have a long
history,1−4 and some, such as SF6, are impressively stable.
Their stability tends to decrease down the halogen group, and
the stable compounds with sulfur−iodine bonds are mostly
limited to organo-sulfenyl and -sulfonyl iodides. The organo-
sulfonyl iodides are almost entirely limited to arylsulfonyl
iodides (ArSO2I). A few alkane sulfonyl iodides (RSO2I) have
been reported and have received some use, but they have not
been well characterized.1 Methanesulfonyl iodide is probably
the best known of these,5 having received significant use as a
reagent in organic synthesis.6 However, the solid form is
reported to decompose on isolation,7 and its exact composition
has not been determined. 33S and 17O NMR spectra have been
reported for a species believed to be CH3SO2I in benzene
solution, but no evidence was provided to support this belief.8
To our knowledge, there are no examples of alkane sulfonyl
iodides that have been characterized structurally or energeti-
cally. Herein, we report the crystal structures of two
compounds containing methanesulfonyl iodide, the equili-
brium constant for its formation from triiodide and
methanesulfinate in aqueous solution, and the kinetics of its
formation and decomposition to methanesulfonate.
All the aqueous solutions were prepared using purified deionized
water from a Barnstead Nanopure Infinity system. Iodine and iodide
stock solutions were protected from light. All other solutions were
freshly prepared for each experiment. Iodide concentrations of the
solutions were adjusted using a stock solution of sodium iodide. To
prepare the stock solution of iodine, potassium iodide was used. Stock
solutions of methanesulfinate (MSA) were prepared and stored in the
refrigerator. Fresh stock solutions of MSA were prepared weekly,
standardized by titration with potassium bromate in the presence of
potassium bromide in strongly acidic media with methyl orange as an
indicator, and diluted to prepare the MSA solutions of known
concentrations required for equilibrium and kinetic experiments.
Instrumentation. All UV−vis spectra were obtained with an HP-
8453 diode array spectrophotometer using a quartz cuvette with 1 cm
1
path length. H NMR spectra of the reactants and products were
collected using D2O as a solvent and DSS as the reference on a Bruker
AV 400 MHz spectrometer. The progress of kinetic experiments was
monitored by observing the change in absorbance at 352 nm using a
Hi-Tech SF-51 stopped-flow spectrophotometer in the 1 cm path
length configuration and with a 312 nm optical filter that blocks
visible light; data were acquired and analyzed with OLIS 4300
software. A Thermo Scientific Nicolet iS50 FT-IR instrument was
used to obtain infrared spectra. A Corning 450 pH/ion meter with a
Mettler Toledo Inlab 421 pH electrode was used to obtain pH
measurements.
An unanticipated outcome of our efforts to crystallize
methanesulfonyl iodide was its isolation as inclusion
compounds in alkali metal polyiodide hosts. The crystal
structures of these compounds reveal some unusual polyiodide
interactions and provide a rationale for the high iodine content
and stability of these substances.
Synthesis of (CH3SO2I)4·KI3·2I2. This compound was prepared as
described by Field et al. for the synthesis of “methanesulfonyl
iodide.”5 A solution of sodium methanesulfinate (MSA) was prepared
in an ignition tube by dissolving 0.5 g of MSA in 2 mL of deionized
EXPERIMENTAL SECTION
■
Reagents and Solutions. Sodium methanesulfinate (MSA, 95%
Alfa Aesar); sodium methanesulfonate (Alfa Aesar); RbI (TCI);
Na2C2O4 (Allied Chemical); Na2HPO4·2H2O (MCB); NaH2PO4·
Received: August 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02441
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Article
water. Next, a triiodide solution prepared by mixing 1.3 g of KI and
0.685 g of solid I2 in 2 mL of deionized water was then slowly added
down the wall of the ignition tube and into the sodium
methanesulfinate solution. Lustrous brown/green dichroic needles
suitable for single crystal X-ray diffraction started to form
immediately. The tube was covered with aluminum foil to protect
the crystals from light and stored in a refrigerator for a few days.
Crystals were preserved for X-ray analysis by coating them in
Paratone oil with care to prevent excessive exposure to light. About 4
h after isolation from the mother liquor and exposure to air, the
needles became less defined. Except for the IR spectrum, results from
previous studies using this synthetic procedure have not been
amenable to analysis due to the tendency for the solid to decompose
at room temperature when removed from the mother liquor.
Table 1. Selected Crystallographic Data
formula
(CH3SO2I)2·RbI3
(CH3SO2I)4·KI3·2I2
crystal system
space group
a (Å)
b (Å)
c (Å)
β (deg)
V (Å3)
Z
monoclinic
P21/m
monoclinic
P21/c
9.4977(5)
14.7549(8)
12.3947(7)
94.818(2)
1730.83
4
14.6211(6)
26.1593(9)
9.5131(3)
108.9673(11)
3440.99
4
R factor %
2.45
3.33
Synthesis of (CH3SO2I)2·RbI3. This compound was prepared
similarly to (CH3SO2I)4·KI3·2I2. A solution of MSA was prepared in
an ignition tube by dissolving 0.25 g of MSA in 1 mL of deionized
water. Next, a triiodide solution was prepared by mixing 0.828 g of
RbI and 0.3425 g of solid I2 in 1 mL of deionized water, and the
triiodide solution was slowly added down the wall of the ignition tube
into the sodium methanesulfinate solution. Lustrous long brown/
green dichroic needles suitable for single crystal X-ray diffraction
started to form immediately. The tube was covered with aluminum
foil to protect the crystals from light and stored in a refrigerator for a
few days. X-ray quality crystals were preserved for weeks when coated
with Paratone and stored in the dark at 2 °C. Decomposition of the
crystals occurred a few days after isolation and exposure to air.
Crystallographic Studies. To collect X-ray crystallographic data,
a Bruker D8 VENTURE κ-geometry diffractometer system with Mo
for irradiation at 100 K was employed. The Bruker SAINT software
package was used to combine frames, and the Bruker SHELXTL
Software Package was used to refine the structure.
(CH3SO2I)2·RbI3. The rubidium compound consists of two
crystallographically independent molecules of MSI, two
−
independent nearly linear I3 ions, and two independent Rb+
ions. These components are arranged with MSI and I3− ligands
bridging adjacent Rb+ cations to form infinite chains that
extend along the a axis. Within the two MSI molecules, the S−
I bond lengths are almost identical (2.444 Å), as are the S−O
(1.44−1.45 Å) and S−C bonds (1.76 Å).
−
The I3 ions are asymmetric, having short and long bonds
(2.84 and 3.05 Å in one of the I3− ions and 2.78 and 3.15 Å in
−
−
the other). They have long I3 −I3 contacts (3.78, 3.82 Å)
forming approximately linear chains, and these chains are
aligned parallel to the a axis of the unit cell. The observed
−
dichroism of the crystals can be attributed to this I3
alignment. Somewhat surprisingly, there are rather short I−I
contacts between the iodine atoms of the MSI molecules and
Solution-Phase Studies. Solution-phase studies of the reaction
−
the I3 ions (3.38 and 3.43 Å). Similar weak interactions
−
of MSA with I3 were performed with NaI instead of KI or RbI in
between covalently bound I and polyiodide anions have been
order to prevent precipitation of the product. The equilibrium
reported elsewhere.11
constant for formation of CH3SO2I was determined by fitting the
The two Rb+ ions reside on a mirror plane. They have
approximate hexagonal bipyramidal coordination with six
equilibrium I3 concentrations to eq 7 with Datafit 9.1.9 Kinetic
−
studies on the hydrolysis of CH3SO2I were performed by pH
measurement; the pH values were converted to [H+] in the pH-
dependent experiments, while the production of H+ in the iodide-
dependent experiments was calculated from the conversion of
−
equatorial MSI oxygen atoms, one axial I3 iodine atom, and
an axial I3 η2 bonded. The Rb(2)−O distances range from
−
2.83 to 3.09 Å and thus are comparable to the Rb−O distances
2−
−
in Rb2S2O6.12
HPO4 to H2PO4 as determined by the pH.
Computational Studies. Calculations were performed with the
Gaussian 16 software package.10 Molecular structures were optimized
in the gas phase at the MP2-aug-cc-pVQZ level with the Frozen Core
approximation. Effective Core Potentials were used for calculations on
iodine-containing molecules. Solvated species were calculated by
using the PCM method.
As is shown in Table 2, the solid-state IR spectrum of
(CH3SO2I)2·RbI3 has features quite similar to those reported
for the other methane sulfonyl halides.13
(CH3SO2I)4·KI3·2I2. In this compound, all the K+ ions are
equivalent. There are four symmetry independent MSI
molecules, three symmetry independent I2 molecules, and
one unique I3− ion. The MSI molecules are coordinated to the
K+ ions through their O atoms in a bridging mode, resulting in
RESULTS AND DISCUSSION
■
+
Crystals containing methanesulfonyl iodide (MSI, CH3SO2I)
were obtained upon mixing concentrated aqueous solutions of
MSA and RbI/I2 or KI/I2. X-ray crystallography revealed that
these crystals were inclusion compounds of MSI in matrices of
alkali metal polyiodide salts. Selected crystallographic data for
the two compounds are presented in Table 1, and structural
diagrams are shown in Figures 1 and 2. In an effort to obtain
a n a n a l o g o u s C a 2 + c o m p o u n d , c r y s t a l s o f
Ca2(CH3SO2)2(CH3SO3)(I3)(H2O)3 were obtained. This
compound is an interesting mixed sulfinate/sulfonate with
polyiodide sheets, but it contains no sulfonyl iodide;
crystallographic data on this compound are presented in the SI.
Both of the sulfonyl iodide compounds contain alkali metal
cations coordinated to the oxygen atoms of the MSI molecules.
However, despite their crystallization under very similar
conditions, they are fundamentally different in that the ratio
of M+/MSI differs as does the iodine content.
(K(CH3SO2I)4 )∞ chains running parallel to the c axis. These
−
chains are sandwiched between pleated sheets of (I7 )∞ as
shown in Figure 3.
The MSI S−I bond lengths are quite similar at 2.440, 2.442,
2.447, and 2.440 Å, and they are essentially identical to those
in the rubidium compound. Likewise, the S−O bond lengths
are very similar (between 1.43 and 1.44 Å), as are the S−C
bond lengths (1.76−1.77 Å), and they are also essentially
identical to those in the rubidium compound.
−
−
The (I7 )∞ sheets can be regarded as consisting of I2 and I3
subunits. The I2 subunits have I−I bond lengths ranging from
2.73 to 2.74 Å, which is somewhat longer than the I−I bond
length in the free molecule (2.67 Å).14 The I3− ions are nearly
linear (176.1°) and almost symmetric with I−I bond lengths of
2.917 and 2.934 Å; these are typical I3− bond lengths.11 Longer
−
I−I contacts (3.32−3.6 Å) form the sheets. Layered I7
networks have been reported previously.11 The oriented
B
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Figure 1. Crystal structure of (CH3SO2I)2·RbI3 showing the infinite chain that extends along the a-axis and the two different coordination modes of
−
I3 .
3.79 Å, which could provide some additional stabilization to
the compound.
The K+ ions are octacoordinate, being bonded to one O
atom from each of eight MSI molecules. The K−O bond
lengths range from 2.73 to 2.80 Å, which are similar to those in
K2S2O6 (2.75−2.97 Å).15
We infer that the compound first reported by Field et al. as
CH3SO2I is actually (CH3SO2I)4·KI3·2I2.5 This inference is
based on the fact that our crystals of (CH3SO2I)4·KI3·2I2 were
obtained under the same conditions as were used to obtain
“CH3SO2I.” Additional supporting evidence is that the IR
spectrum reported by Field et al. had strong bands at 1300,
1150, 960, and 730 cm−1, which are similar to those reported
here for (CH3SO2I)2·RbI3. In addition, similarly to the prior
workers, we found that the compound has a dark green/brown
color and is unstable when removed from its mother liquor.
Quantum Structure, Polarity, and Energetics of
CH3SO2I. Structural optimization in the gas phase leads to a
CH3SO2I molecule with Cs symmetry in the staggered
configuration. Tables 3 and 4 compare the structural
parameters of this molecule as calculated for the gas phase
and as found in the two crystal structures.
Figure 2. Crystal structure of the (CH3SO2I)4·KI3·2I2 excluding the
−
I3 and I2 components.
These comparisons show that the gas-phase and solid-state
crystal structures of CH3SO2I are very similar. The gas-phase
molecule has a calculated dipole moment of 4.48 D with the
oxygen atoms having a significant accumulation of negative
charge and the iodine atom having some positive charge.
Figure 4 displays the calculated charge distribution. This
charge distribution leads to a charge-dipole interaction
between the metal cations and the oxygen atoms that provides
a stabilizing force for the crystals.
Table 2. Comparison of Vibrational Frequencies Obtained
for (CH3SO2I)2·RbI3 Crystals with Published Vibrational
Assignments for Methanesulfonyl Halides
cm−1
vibrational
assignment
(CH3SO2I)2·
a
a
a
b
CH3SO2F CH3SO2Cl CH3SO2Br
RbI3
CSO2, b
CSO2, b
C−S, st
CH3, r
493
534
733
985
493
540
755
970
480
525
749
974
456
518
720
943
Calculations were also performed on CH3SO2H, I2, and HI.
The optimized structure for CH3SO2H(g) has a H−O−S−O
dihedral angle of 169°, which corresponds to the species
MSIA1 calculated by Carvalho et al.16 Gibbs energies
calculated for the various species are summarized in Table 5.
From the data in Table 5, the Gibbs energy change for
reaction 1 is calculated to be ΔG° = 6.26 kJ mol−1. Combining
this result with NBS data17 for the ΔfG° of I2(g) and HI(g)
leads to ΔfG°(CH3SO2I (aq)) − ΔfG°(CH3SO2H (aq)) =
23.89 kJ mol−1. From this Gibbs energy difference and NBS
SO2, st
CH3, d
SO2, st
CH3, d
CH3, st
CH3, st
1177
1324
1367
1425
2955
3045
1175
1324
1375
1411
2938
3045
1170
1326
1369
1411
2932
3043
1104
1269
1300
1392
2905
3023
a
b
Reference 13. This work.
ΔfG° data for I3 (aq), H+(aq), and I−, a value of ΔG° = −27.9
−
kJ mol−1 is calculated for eq 2. With use of the reported pKa of
CH3SO2H(aq) = 2.28,18 a value of ΔG° = −40.9 kJ mol−1 is
then calculated for eq 3. In view of the challenges in
calculations of iodine-containing molecules and of hydration
polyiodide structure provides a rationale for the compound’s
dichroism. Additional I−I contacts can be found between the
sulfonyl iodide and polyiodide atoms in the range of 3.70 to
C
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−
+
Figure 3. Packing diagram of (CH3SO2I)4·KI3·2I2 viewed down the c axis illustrating the (I7 )∞ pleated sheets and (K(CH3SO2I)4 )∞ chains.
Table 3. Comparison of the Bond Lengths Obtained from
a
the Crystal Structures with the Values Calculated
bond
length
(CH3SO2I)2·
RbI3, Å
(CH3SO2I)4·KI3·
CH3SO2I
(Gaussian), Å
2I2, Å
S−I
2.44
1.44−1.45
(0.98)
2.44−2.45
1.43−1.44
(0.98)
2.4404
1.4383
1.0844, 1.0884
1.7709
S−O
C−H
C−S
1.76−1.77
1.76−1.77
a
Parenthetical values imposed in the crystallography.
Table 4. Comparison of the Bond Angles Obtained from the
a
Crystal Structures with the Values Calculated
bond
angle
(CH3SO2I)2·RbI3,
(CH3SO2I)4·KI3·
CH3SO2I
(Gaussian), deg
deg
2I2, deg
H−C−H
H−C−S
C−S−O
O−S−I
O−S−O
I−S−C
(109.4)
(109.5)
108.9−109.4
108.1−109.8
115.9−116.4
103.3−104.6
(109.4)
(109.5)
108.9−109.6
105.5−107.8
118.7−119.0
104.2−105.5
111.4805, 112.2873
105.290, 107.965
109.0016
107.4933
121.5184
Figure 4. Calculated charge distribution in CH3SO2I.
Table 5. Gibbs Energies Calculated with Gaussian 16
species
G, au
100.2206
CH3SO2H(g)
CH3SO2H(aq)
CH3SO2I(g)
CH3SO2I(aq)
I2(g)
−588.416161
−588.426880
−882.803081
−882.811792
−589.953578
−295.566281
a
Parenthetical values imposed in the crystallography.
energies, we suggest an uncertainty of 40 kJ mol−1 for this
last result (ΔG°, eq 3).
HI(g)
CH3SO2H (aq) + I2 (g) → CH3SO2I (aq) + HI (g)
(1)
D
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Figure 5. UV−vis spectra of triiodide in H2O, MSA in H2O, and 0.1 mM iodide.
Figure 6. UV−vis spectra of the product mixtures at various [MSA]. [I2]0 = 0.22 mM and [I−] = 1 mM. All solutions were unbuffered. (a) [MSA]
= 8 mM. (b) [MSA] = 6.6 mM. (c) [MSA] = 5 mM. (d) [MSA] = 3 mM.
CH3SO2H (aq) + I3− (aq)
CH3SO2I Formation in Solution. When dilute solutions of
MSA are mixed with triiodide solutions, no precipitation
occurs, but a reaction does occur as indicated by an immediate
→ CH3SO2I (aq) + H+ (aq) + 2I− (aq)
(2)
(3)
loss of color intensity. Stopped-flow experiments show that the
CH3SO2− (aq) + I3− (aq) → CH3SO2I (aq) + 2I− (aq)
−
absorbance due to I3 decreases immediately within the dead
time (∼2 ms) of the instrument. For example, upon mixing a
E
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solution of 4 mM MSA, 20 μM I2, and 0.01 M I−, the initial
absorbance is 0.017 at 352 nm. In such solutions, the I2 reacts
19
−
extensively with I− to form I3 (KI = 721 M−1):
−
3
I2 + I− F I3− KI
−
(4)
3
I3− has been reported to have ε352 = 2.7 × 104 M−1 cm−1,19 and
we find ε352 = 2.5 × 104 M−1 cm−1. Under the conditions
mentioned above and if no reaction with MSA had occurred,
−
the equilibrium concentration of I3 should have been 18 μM
and the absorbance should have been 0.48. It is clear that most
−
of the I3 was consumed by the MSA.
The UV/vis spectra of I3 , MSA, and I− are shown in Figure
−
−
Figure 8. [I−] dependence of [I3 ]eq. [I2]0 = 0.02 mM, [MSA]0 = 4.05
−
5. From these spectra, it can be seen that I3 has two
mM. Slope = (178 5) M, y intercept = (4.4 2.0) × 104 M−1.
characteristic absorption peaks at 288 and 352 nm, while
neither MSA nor I− absorbs significantly at wavelengths longer
than 250 nm.
−
−
−
[I3 ] = [I2] + [I3 ], so [I3 ]0 = [I2]0. Also, the concentrations
of I− and MSA are in large excess over the [I2]0. With these
approximations, the corresponding equilibrium relationship is
Spectra of product mixtures at various MSA concentrations
are shown in Figure 6. These spectra show that at a low I−
−
concentration, the characteristic two-peak spectrum of I3 is
entirely depleted and replaced by a weaker feature with a single
absorption peak having λmax at 309 nm. This new absorption
feature is ascribed to MSI. Similar results are obtained at 5.3
mM MSA with 66 μM I2 and 0.33 mM I−, from which we
calculate ε309 = 667 M−1 cm−1 and ε352 = ∼197 M−1 cm−1. This
spectrum is remarkably similar to that reported for SO3I− (ε300
= 500 M−1 cm−1).20
([I−3]0 − [I−3])[I−]2
KMSI
=
[I−3][CH3SO−2 ]
(6)
[I−3]0[I−]2
or [I−3] =
KMSI[CH3SO−2 ] + [I−]2
(7)
−
A fit of the data with eq 7 yields KMSI = 1.07
0.01 M.
As noted above, under certain conditions, the I3 is only
Experiments in 2 mM phosphate buffers at pH 5.8, 6.5, and 6.9
showed that the position of equilibrium is independent of pH
in this range. The magnitude of KMSI determined here is
comparable to the ab initio value obtained above, given the
large uncertainties in the latter.
partially consumed. A series of experiments was performed to
explore the degree of consumption as a function of the initial
concentrations of I− and MSA. These experiments were
performed at 25.0 °C and μ = 0.1 M (NaClO4). The results are
reported in the SI as the absorbance at 352 nm immediately
after mixing in the stopped-flow instrument and the
−
Equilibria involving I3 analogous to eq 5 have been
21
22
2−
reported for other nucleophiles, including SCN−, S2O3
,
−
corresponding I3 concentration after correction for the
absorbance of MSI. Figure 7 shows a linear relationship
H2O,23 HCN,24 and Br−.19 In all cases, the reactions proceed
to equilibrium extremely rapidly, as we have also found for
MSA. A reaction to form an I2 addition species (I2Nu−) instead
of NuI is also reported as in eq 8 for various nucleophiles,25,26
but the linear dependence on 1/[I−]2 in Figure 7 shows that
formal net I+ transfer as in eq 5 is the observed process with
MSA.
Nu− + I3− F I2Nu− + I−
(8)
We note that eqs 5 and 8 represent the equilibria in terms of
−
I3 as the major species, but the solutions also contain minor
amounts of I2. It is possible that the actual mechanism of
−
formation of MSI entails I2 rather than I3 as the species that
reacts with MSA. To identify which of these two species is the
actual reactant would require kinetic data on the reaction. Such
data are lacking because the equilibrium is established within
the dead time of the stopped-flow instrument.
−
Figure 7. MSA dependence of [I3 ]eq, [I2]0 = 0.02 mM, [I−]0 = 48
mM. Slope = (22.5 0.8) × 106 M−2; y intercept = (54 2) × 103
CH3SO2I Decomposition in Solution. As Figures 6−8
M−1.
−
demonstrate, mixtures of I3 and MSA immediately generate
MSI. However, on a longer time scale, the ultimate result is the
−
1
−
formation of methanesulfonate: CH3SO3 . H NMR spectra
conclusively identify this species as the major product by
between 1/[I3 ]eq and [MSA] with a nonzero intercept, while
Figure 8 shows a linear dependence of 1/[I3 ]eq on 1/[I−]2
−
−
comparison with an authentic sample of CH3SO3 and its
with a marginally significant intercept.
These two linear dependences are consistent with formation
of MSI in an equilibrium as in eq 5:
−
appearance as a singlet at δ 2.81 ppm. The overall reaction is
thus
CH3SO2 + I3− F CH3SO2I + 2I− KMSI
−
CH3SO2 + I3− + H2O → CH3SO3− + 3I− + 2H+
(5)
(9)
Under the conditions of Figures 7 and 8, the triiodide
equilibrium (eq 4) lies well to the right, so we approximate that
Further evidence for this stoichiometry is obtained by pH
measurements, which indicate a nearly quantitative yield of H+
F
DOI: 10.1021/acs.inorgchem.9b02441
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
for initial conditions of 2 mM MSA; 0.2 mM I2; 10 mM I−; and
pH 2.35, 3.53, and 4.7 (HClO4).
CONCLUSIONS
■
Here, we report the first characterization of an alkanesulfonyl
iodide, CH3−SO2I. It can be precipitated from aqueous mixtures
of CH3SO2 , KI, and I2 as a KI3/I2 inclusion compound or
from RbI as a RbI3 inclusion compound. In both compounds,
the cations are coordinated to sulfonyl oxygens, but additional
stabilization is achieved through weak interactions with the
polyiodide anions. The material previously claimed to be
CH3SO2I is actually the KI3/I2 inclusion compound. In NaI/I2
solutions, CH3SO2I does not precipitate and is formed very
rapidly (ms) in an equilibrium. To our knowledge, measure-
ment of this equilibrium constant provides the first report on
the thermodynamic properties of the S−I bond in aqueous
The time dependence of the pH in the above three
experiments led to pseudo-first-order kinetics with half-lives of
several hours. Values of kobs are given in the SI. These results
indicate that the rates are accelerated at low pH but are
independent of pH above a pH of 3.5. Additional pH
measurements in phosphate buffer were performed to
determine the dependence on [I−]. These results are shown
in Figure 9 (data in SI) as a linear plot of 1/kobs vs [I−]2.
−
solution. CH3SO2I solutions hydrolyze to CH3SO3 over
several hours, similarly to CH3SO2Cl solutions, although the
hydrolysis of CH3SO2I is made more complex by the
−
concurrent equilibrium with I3 .
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02441.
■
S
Equilibrium data for formation of CH3SO2I (aq); kinetic
data for hydrolysis of CH3SO2I; derivation of eq 12;
crystallographic methods; crystallographic data for
(CH3 SO2 I)2 ·RbI3 , (CH3 SO2 I)4 ·KI3 ·2I2 , and
Ca2(CH3SO2)2(CH3SO3)(I3)(H2O)3 (PDF)
Figure 9. Kinetics of H+ production during hydrolysis of CH3SO2I.
[MSA] = 2 mM, [I2]tot = 0.2 mM, 2 mM phosphate buffer, μ = 0.1 M
(NaClO4). 1/kobs vs [I−]2. Linear fit with slope = (4.00 0.05) × 103
hr M−2 and intercept = 2.71 0.05 h.
Accession Codes
CCDC 1866943, 1917585, and 1890340 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge CB2 IEZ, UK; fax: + 44
1223 336033.
This rate law is consistent with a simple two-step
mechanism:
−
CH3SO2 + I3− F CH3SO2I + 2I− KMSI
(10)
CH3SO2I + H2O → CH3SO3− + I− + 2H+ khyd
(11)
As mentioned above, the first step (eq 10) may actually
−
proceed through reaction of I2−instead of I3 , but we show it
AUTHOR INFORMATION
Corresponding Author
*E-mail: stanbury@auburn.edu.
−
■
here in terms of I3 because I3 is the major species, and our
data do not distinguish between the two pathways.
The corresponding rate law is
ORCID
kobs = khydKMSI[MSA]/([I−]2 + KMSI[MSA])
(12)
David M. Stanbury: 0000-0002-3892-9048
Notes
or
The authors declare no competing financial interest.
1/kobs = [I−]2 /(khydKMSI[MSA]) + 1/khyd
(13)
ACKNOWLEDGMENTS
From the linear fit in Figure 9, the data yield khyd = 1.0 × 10−4
■
s−1 and KMSI = 0.34 M. The minor deviation of this value for
We thank Isuru Ariyarathna (Auburn University) for his
assistance with the quantum calculations. We thank Charles
Campagna (formerly at Bruker AXS) for help with the
crystallography. This research was supported by a grant from
ACS-PRF: PRF 55078-ND3.
K
MSI from that derived from the equilibrium stopped-flow data
is attributed to the phosphate buffer. The value of kobs at 10
mM I− is about 9 times greater in phosphate buffer than the
corresponding value at pH 4.7 without phosphate buffer; this
difference may be due to phosphate buffer catalysis, which has
also been reported for the hydrolysis of IBr19 and ISCN.21
The rate constant for hydrolysis of MSI reported above is
quite small and similar to that reported for hydrolysis of
CH3SO2Cl.27 Nevertheless, solutions of MSI are much more
labile and complex because they require significant concen-
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