Unravelling Lawesson's reagent: The structure of monomeric (4-methoxyphenyl)phosphine disulfide

Article abstract of DOI:10.1039/c8cc00034d

We describe the isolation as well as IR and UV/Vis spectroscopic characterization of (4-methoxyphenyl)phosphine disulfide in argon matrices at 10 K. The title compound proved to be highly photolabile; irradiation with UV light (λ = 334 nm) led to rearrang

Full text of DOI:10.1039/c8cc00034d

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DOI: 10.1039/C8CC00034D
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Unravelling Lawesson’s Reagent – The Structure of Monomeric (4-
Methoxyphenyl)phosphine Disulfide
Received 00th January 20xx,
Accepted 00th January 20xx
Artur Mardyukov,a* Dominik Niedek,a and Peter R. Schreinera
DOI: 10.1039/x0xx00000x
www.rsc.org/
We describe the isolation, IR and UV/Vis spectroscopic reports on the identification and characterization of 1, which we
characterization of (4-methoxyphenyl)phosphine disulfide in argon present here, together with its hitherto unexplored
matrices at 10 K. The title compound proved to be highly photochemistry.
photolabile; irradiation with UV light (λ =334 nm) led to
rearrangement to the equally unreported 3-(4-methoxyphenyl)-
1,2,3-dithiaphosphirane. Photoreversion can be achieved upon
irradiation at λ=465 nm.
Scheme 1. 4-Methoxy-phenylphosphine disulfide (
from Lawesson’s reagent via pyrolysis and trapping in an
argon matrix. Subsequent photochemistry to 3-(4-
methoxyphenyl)-1,2,3-dithiaphosphirane ). Previously
prepared lighter congeners of and and , respectively.
1) generated
Lawesson’s
dithiadiphosphetane-2,4-dithione;
Lecher and co-workers,1 is the most widely used reagent for the
synthesis of large number of S-heterocyclic compounds.1-6 The
active form of the reagent is assumed to be its monomer 4-
reagent
(2,4-bis(4-methoxyphenyl)-1,3,2,4-
), first synthesized by
2
2
(
3
5
1
3
,
4
methoxy-phenylphosphine disulfide (1, Scheme 1) but this
species has never been observed spectroscopically.3,7,8 The
difficulty in isolating compounds of type RPE2 (R = aryl, alkyl;
E=O, S, Se) is mainly due to the strong electrophilicity of the
Compound
phase (at 150 °C), where it effectively dissociates to give two
equivalents of ; these are subsequently trapped with an excess
of argon at 10 K. Dissociation of to is indeed the generally
accepted mechanism for the preparation of in solution as well
(even though it has never been identified in solution). The
excellent agreement between the experimental and unscaled
B3LYP/6-311++G(3df,3pd) computed IR spectrum is taken as
1 was prepared from 2 via evaporation into the gas
1
2
1
phosphorus center; RPE
2
species instantly undergo self-
aggregation to form cyclic or linear polymeric compounds.9-13
Numerous attempts have been undertaken to synthesize the
monomeric RPE2 species. Their detection generally is only
possible in gas-phase14, 15 or through low-temperature matrix
isolation spectroscopy.16-18 The kinetic lability of these species
can be compensated by using bulky substituents that inhibit
dimerization as successfully demonstrated with the preparation
of isolable dithioxophosphoranes.19 N-Heterocyclic carbene
1
evidence for the successful preparation of
Table S1). The most prominent IR bands attributed to
1267, and 1096 cm–1 can be assigned to the C=C stretch, C
and C P stretching vibrations. With the aid of the
computations, the additional other intense IR bands of medium
1
(Figures 1, S1, and
at 1591,
O,
1
−
−
(NHC) are strong σ-donors and can be utilized as ligands for the
stabilization ArPE2 compounds.20
intensity can also be assigned to
S1).
1
(Figure 1, Figure S1, and Table
Recently, we reported the synthesis of the previously elusive
phenyldioxophosphorane (the phosphorus analogue of
nitrobenzene, PhPO2) under matrix isolation conditions by the
reaction of triplet phenylphosphinidene with triplet molecular
oxygen (3P-O2).18,20 To the best of our knowledge, there are no
To confirm these assignments, we also prepared d3-
1
from d3-2,
which leads to characteristic isotopic shifts. For example, the IR
band at 1467 cm–1 shows a strong red shift of 357 cm–1 and can
thus be assigned to the CH3 asymmetric deformation
(computed at 1176 cm–1) of
1, in good agreement with the
computed shift of 372 cm–1. Other weak bands at 1462, 1186,
and 1035 cm–1 show large and moderate isotope shifts of 399,
228 and 41 cm–1, also in good agreement with computed shifts
of 415, 237, and 43 cm–1, respectively. The good agreement
aInstitute of Organic Chemistry, Justus-Liebig University, Heinrich-Buff-Ring 17,
35392 Giessen, Germany, Email: artur.mardyukov@org.chemie.uni-giessen.de
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
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between the computed (B3LYP/6-311++G(3df,3pd)) and level of theory). The IR frequencies, intensities,VaienwdArtiiscloetOonplinice
DOI: 10.1039/C8CC00034D
1 isotopologues shifts of two isotopologs (3 and d3-3) closely match with the
experimental frequencies of the
1
and d3-
underscores the successful preparation of
and Table S1).
1
(Figure 1, Figure S2, computed data.
Figure 2. Solid line: UV/Vis spectrum of
Dashed line: UV/Vis spectrum of at 10 K; the photochemistry
of after irradiation at = 334 nm in argon at 10 K. Inset:
computed [TD-B3LYP/6-311++G(3df,3pd)] electronic transitions
for and
1 isolated at 10 K in Ar.
3
1
λ
Figure 1. IR spectra showing the product of pyrolysis of
argon matrix with subsequent trapping in an argon matrix at 10
K. (a) IR spectrum of computed at B3LYP/6-311++G(3df,3pd)
(unscaled). (b) IR difference spectra showing the
photochemistry of after irradiation with = 334 nm in argon
at 10 K. Downward bands assigned to disappear while upward
bands assigned to appear after 5 min irradiation time. (c) IR
spectrum of computed at B3LYP/6-311++G(3df, 3pd)
(unscaled). (d) IR spectrum of d3- computed at B3LYP/6-
311++G(3df,3pd) (unscaled). (e) IR difference spectra showing
the photochemistry of d3- after irradiation with = 334 nm in
argon at 10 K. Bands pointing downwards assigned to d3-
disappear and bands pointing upwards assigned d3- appear
computed
2 in
1
3.
1
1
λ
The UV/Vis spectrum of matrix-isolated
263 nm (strong), 297 nm (weak), and 493 (weak, broad) (Figure
2, dashed line). All bands of correlate well with the values of
3 shows transitions at
1
3
3
3
the electronic excitations at 263 nm (f = 0.325), 297 nm (f =
0.063), and 442 nm (f = 0.007) computed at TD-B3LYP/6-
311++G(3df,3pd). According to the orbitals involved in the
electronic excitations, the weak absorption band in the visible
at 442 nm is an n → π* transition, while the strong band at 263
nm corresponds to a π → π* transition (Figure S3).
1
1
λ
1
3
after 5 min irradiation time. (e) IR spectrum of d3-
3
Irradiation of the sample at λ = 465 nm led to the simultaneous
at B3LYP/6-311++G(3df,3pd) (unscaled).
disappearance of both IR as well as UV/Vis bands of
concomitant appearance of the IR and UV/Vis bands of
3
1
, with
. The
and
The UV/Vis spectrum of matrix isolated
1 reveals a strong
reversible photochemistry (Scheme 1) demonstrates that
exist in a stationary equilibrium.
Next we examine the nature of the phosphorus
and how this relates to its reactivity, which often invokes
some ylide-type resonance form (1a) (Figure 3).3,4 Phosphorus
compounds featuring P S double bonds are rare and only a few
examples have been isolated and characterized by single-crystal
X-ray diffraction analysis, e.g., dithioxo(tri-tert-
butylphenyl)phosphorene ( S bond distances in
).19,20 The P
are 1.920 and 1.919 Å at B3LYP/6-311++G(3df,3pd) (1.919 and
1.918 Å at MP2/cc-pVTZ) and hence in the same range as in
(1.90 Å).18 They are only slightly shorter than the P
S bond
distances (1.978 and 1.968 Å) in the N-Heterocyclic carbene
(NHC) adduct of (phenyl)phosphine disulfide (
).20 The slight
differences in the two P=S bond lengths arises from the
nonsymmetric orientation of the para-methoxy group in . The
S bond distances in are computed to be 2.134 Å (B3LYP/6-
1
absorption band at max = 329 nm (Figure 2), which is in good
λ
3
agreement with its computed UV/Vis spectrum using time-
dependent density functional theory (TD-DFT); TD-B3LYP/6-
311++G(3df,3pd) computations exhibit two strong transitions at
−
sulfur bonds in
1
333 nm (
transitions at 290 nm (
= 0.048), and 234 nm (
f
= 0.342) and 323 nm (
= 0.017), 249 nm (
= 0.027), respectively (Figure 2 solid
f
= 0.087) as well as four weak
=
f
f
= 0.046), 237 nm (
f
f
line).
6
=
1
Irradiation (
λ
= 334 nm) of
1
in an argon matrix at 10 K resulted
and
in the almost complete bleaching of the IR bands of
1
6
formation of a new compound with strong IR absorptions at
1596, 1260, 1092, and 826 cm–1 (Figure 1b). The major
=
constituent of the photolysis products of
(4-methoxyphenyl)-1,2,3-dithiaphosphirane
of from is also strongly supported by isotopic labeling
experiments using d3- . The large shifts are found to the CH3
asymmetric deformation and CH3 rocking modes of of 350 and
225 cm–1, in good agreement with computed shifts of 379, and
1
was identified as 3-
7
3
. The formation
3
1
1
1
P
−
3
3
311++G(3df,3pd)) and 2.124 Å (MP2/cc-pVTZ), respectively
(Figure 3). The NBO charges at the phosphorus and at the sulfur
atoms in 1 are 0.99, −0.38 (anti), and −0.40 (syn). That is, there
233 cm–1 (Figure 1, Table S2). The shift of the C
−O stretching
mode is only 36 cm–1 (44 cm–1 at the B3LYP/6-311++G(3df,3pd)
2 | J. Name., 2012, 00, 1-3
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is no indication of an ylide-type structure on the basis of the reaction enthalpy of the homodesmotic reaction sVhieowwAnrticilne Oenqlin1e
–1
DOI: 10.1039/C8CC00034D
rather symmetric charge distribution. As expected, compound
(∆Hr) of −73.9 kcal mol at the B3LYP/6-311++G(3df,3pd).
1
has large singlet
–
triplet energy separation of
∆
EST = –37.3 kcal
mol− at B3LYP/6-311++G(3df,3pd), underlining its singlet
ground-state nature.
1
The nature of the P
comparison with the P
compounds. Several unstable thiophosphines (X
F) have been characterized spectroscopically either in the gas
phase21 or have been isolated in argon matrix.22,23
P=S shows
a strong P=S stretching frequency at 720 cm–1, while Br
P=S
absorbs at 712 cm–1 . The IR absorption corresponding to the
=
S bond in
S stretching modes in related
P=S; X = Cl, Br,
1 can also be judged by
=
−
F
−
−
SPS asymmetric vibration for 1
is found at 805 cm–1 (computed
at 798 cm–1) (Figure 1, Figure S2 and Table S1). Figure S4
presents the spanned vector displacements of the SPS
asymmetric vibration; the displacement vector of the P
bond from the initial position to the final position marginally
larger than that of the P Ssyn bond. That is, on spectroscopic
grounds, displays two rather similar, typical P=S double bonds.
−Santi
−
Figure 4
.
Isomerization pathways 1 → 3, and 4 → 5,
1
respectively. Stationary points and transitions structures were
computed at B3LYP/6-311++G(3df,3pd).
In summary, we present the synthesis, IR and UV/Vis
spectroscopic characterization of elusive 4-methoxy-
phenylphosphine disulfide (
1), isolated in an argon matrix at 10
K for the first time. This hitherto unreported molecule has been
postulated to as the key intermediate derived from Lawesson’s
reagent (which is its dimer) that is extremely useful in sulfur
Figure 3. Resonance structure for
1 (top). Bottom: Bond lengths
transfer reactions
and irradiation with light of
the hitherto unknown cyclic isomer 3-(4-methoxyphenyl)-1,2,3-
dithiaphosphirane ( ), which represents the first three-
membered organophosphorus–chalcogen ring incorporating a
phosphorus atom in the oxidation state
3.11,25 Structure
rearranges back to upon irradiation at = 465 nm. Since the
conversion is only +20.2 kcal
.
Structure
1
proved to be highly photolabile
(Å) of and at the B3LYP/6-311++G(3df,3pd) level. Values in
1
3
λ
= 334 nm led to rearrangement to
parentheses at MP2/cc-pVTZ. The P
are from ref 18 and 19.
−
S bond lengths in
6
and
7
3
The question then remains why
upon irradiation while
phenyldioxophosphorane ( , Scheme 1) does not rearrange
upon photoexcitation.17 To gain more information on the
photochemical reactivity of both and , we looked at the
1
reversibly rearranges to
3
+
3
the closely
related
1
λ
4
activation barrier for the
1 → 3
‡
mol–1
(
∆
H0 ), this conversion is also thermally accessible at
1
3
elevated temperatures. The structures and IR spectra of
1 and
energy differences and transition states for ring-opening
connecting these pairs of species. The relative energy ordering
3
, and their deuterated isotopologues (d3- and d3- ) were
1
3
unambiguously assigned through matching of experimental and
DFT computations at B3LYP/6-311++G(3df, 3pd) level of theory.
According to B3LYP and MP2 computations and spectral
comparisons, the sulfur atoms in the two isomers each are
essentially indistinguishable. An NBO analysis reveals negative
of the two isomers (
(including zero-point vibrational energy correction, ZPVE,
denoted as H0) above the . The transition structure for the
conversion is associated with a barrier of +20.2 kcal mol–1
1 and 3) shows that 3
lies 16.8 kcal mol–1
∆
1
1
→
3
‡
(∆
H0 ) (Figure 4).
corresponding structures of the oxygen analogues (
where the dioxophosphorane structure is much more stable
(90.7 kcal mol–1 at the B3LYP/6-311++G(3df,3pd)) than the
dioxophosphirane form 5 18
This is due to the much higher
This is in marked contrast to the
charges at the sulfur atoms of
a positive charge at the phosphorus of +0.99 for
(sulfur) and +0.64 (phosphorus) for (B3LYP/6-311++G(3df,
3pd)). The charge separation is also apparent from the
computed high dipole moment of 6.26 D for , compared to
4.37 D for . Our bonding analysis finds no indications for ylide-
−
0.38 (anti), and
−
0.40 (syn) and
4
and ),
5
1
, and 0.18
−
4
3
.
1
polarizability of sulfur and its comparable electronegativity
relative to phosphorus,24 which is supported by the large
3
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24 M. T. Molina, M. Yanez, O. Mo, R. Notario and J. VLi.ewMA.rtAicblebOonulinde,
or biradical-type species and the structure of
with two nearly equal P=S double bonds.
1 is best depicted
Wiley, 1977-1997, 1997, pp. 1355-1496.
DOI: 10.1039/C8CC00034D
25 D. Gudat, Coord. Chem. Rev., 1997, 163, 71-106.
Conflicts of interest
No competing financial interests have been declared.
Acknowledgements
The research was supported by the Justus-Liebig University.
Notes and references
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