P-Functionalized tetrathiafulvalenes from 1,3-dithiole-2-thiones?

Article abstract of DOI:10.1039/d0nj02984j

A protocol for the synthesis of mono- and bis-phosphanylated 1,3-dithiole-2-thiones 2 and 3 is presented. The reaction of 2 or 3 with hydrogen peroxide-urea or elemental sulfur led to the corresponding P(v) chalcogenide 1,3-dithiole-2-thiones 4-7. All compounds were characterized by 31P, 1H and 13C NMR and IR spectroscopy and elemental analyses. Additionally, compounds 3 and 4 were analysed by single-crystal X-ray diffraction analysis. The reaction of 3 with P(OEt)3 led selectively to the tetrakis-phosphanylated tetrathiafulvalene derivative IIvia a reductive C2,C2 coupling reaction. A thorough computational analysis for the mechanism of phosphite-mediated reductive dimerization of 1,3-dithiole-2-thiones was performed. This journal is

Full text of DOI:10.1039/d0nj02984j

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P-Functionalized tetrathiafulvalenes from
1,3-dithiole-2-thiones?†
Cite this:DOI: 10.1039/d0nj02984j
b
A. Gese,a M. Akter,a G. Schnakenburg,a A. Garcıa Alcaraz,b A. Espinosa Ferao
*
´
a
and R. Streubel
*
A protocol for the synthesis of mono- and bis-phosphanylated 1,3-dithiole-2-thiones 2 and 3 is
presented. The reaction of 2 or 3 with hydrogen peroxide–urea or elemental sulfur led to the corres-
1
ponding P(V) chalcogenide 1,3-dithiole-2-thiones 4–7. All compounds were characterized by 31P, H and
13C NMR and IR spectroscopy and elemental analyses. Additionally, compounds 3 and 4 were analysed
by single-crystal X-ray diffraction analysis. The reaction of 3 with P(OEt)3 led selectively to the tetrakis-
phosphanylated tetrathiafulvalene derivative II via a reductive C2,C2 coupling reaction. A thorough
computational analysis for the mechanism of phosphite-mediated reductive dimerization of 1,3-dithiole-
2-thiones was performed.
Received 13th June 2020,
Accepted 3rd September 2020
DOI: 10.1039/d0nj02984j
rsc.li/njc
this context also backbone-phosphorus functionalized TTFs such
as II and III (Fig. 1) were synthesized and their redox properties
Introduction
The chemistry of tetrathiafulvalene (TTF) I (Fig. 1), first pub- revealed.5,6
lished in 1970,1 attracted a great deal of interest in 1973 with
Previously, P-functionalized TTFs were synthesized via
the discovery of the first ‘‘organic metal’’, a purely organic back-bone lithiation and phosphanylation, thus allowing the
compound showing electrical conductivity at room temperature introduction of one or four phosphorus groups into the TTF
comparable with those of classical metals. In this organic metal backbone.5,7 For the preparation of derivatives containing two
tetrathiafulvalene forms a charge transfer complex together PPh2 moieties (IV–VI, Fig. 2), the use of ‘‘ortho’’ backbone
with the organic acceptor tetracyano-p-quinodimethane (TCNQ). methyl-substituted TTF was necessary. Similarly, attempts to
The electrical conductivity is anisotropic and for one reason achieve E- or Z-type bisphosphanyl derivatives via delithiation
caused by a unique stacking pattern where oxidized TTF units of (unsubstituted) TTF I led to a mixture of polylithiated TTFs
and reduced TNCQ units create segregated co-linear stacks.2,3
A few years later, superconducting organic charge transfer
and, hence, to an unselective polyphosphanylation.
The synthesis of phosphanylated TTFs is of interest
complexes based on TTF derivatives such as TMTSF (tetramethyl- with regard to obtaining materials with special electrical and
tetraselenafulvalene), BEDT-TTF (bisethylenedithiotetrathiafulva- magnetic properties as coordination polymers.5 Several com-
lene) or DMET (dimethyldiselenethylenedithiodithiafulvalene) plexes of bis- and tetrakisphosphanyl TTFs with coinage metals,
were discovered, so that the interest in the chemistry of TTF
derivatives increased even further. Due to the unique properties of
the TTF system to act as an electron donor, a huge effort was
made to create new TTF-based materials. Therefore, a large variety
of synthetic pathways was developed to access backbone-
functionalized TTFs. Among them are the deprotonation of dithio-
lium salts and reductive desulfurization of dithiole-2-thiones.4 In
a
¨
¨
Institut fur Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universitat Bonn,
Gerhards-Domagk-Str. 1, 53121 Bonn, Germany. E-mail: r.streubel@uni-bonn.de;
Fax: +49-228-73-9616
b
´
´
´
Departamento de Quımica Organica, Facultad de Quımica, Universidad de Murcia,
Campus de Espinardo, 30100 Murcia, Spain. E-mail: artuesp@um.es
† Electronic supplementary information (ESI) available: Experimental details,
X-ray diffraction and computational studies. CCDC 2001006 and 2001007. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0nj02984j
Fig. 1 Molecular structures of tetrathiafulvalene (TTF) I, backbone-
tetraphosphanylated TTF II and P-bridged TTF-derivative III.
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Fig. 3 Backbone-phosphanylated 1,3-imidazole-2-thione VII and 1,3-
thiazole-2-thione VIII.10,11
Fig. 2 Examples of bisphosphanylated TTF derivatives obtained via back-
bone lithiation and phosphanylation.
solvent evaporation. Compound 3 crystallized in the triclinic
%
system (space group P1) with two molecules per unit cell (Fig. 4).
palladium or platinum,7,8 or carbonyl complexes of group VI
Use of the phosphanylated dithiole-2-thiones 2 and 3 and
metals, iron and rhenium,9 have been investigated and their selective protocols for P-oxidation furnished P-oxides 4 and 5
electrochemical, magnetic and optical properties studied, with (using the H2O2–urea adduct) and P-sulfides 6 and 7 (using
elemental sulfur). Whereas the oxidation of the mono substi-
tuted derivatives took 5 hours, those of the bisphosphanylated
dithiole-2-thione 3 needed about 2 days for completion to
TTF being the redox-active organic compound and the incorpo-
rated transition metal acting as a redox-active inorganic
component.
As part of a larger program to investigate novel opportunities obtain 4 and 6 (Scheme 2).
resulting from backbone P-functionalization of heterocyclic
Single crystals of compound 4, suitable for X-ray diffraction
thiones,10–12 1,3-dithiole-2-thiones were now explored, espe- study, were obtained from a saturated solution in CH2Cl2 after
cially as phosphorus derivatives were unknown, so far. Herein, slow solvent evaporation; compound 4 crystallized in the tri-
%
synthesis of P(III)- and P(V)-functionalized dithiole-2-thiones is clinic system (space group P1) (Fig. 5).
As described beforehand, the direct C2,C2 coupling of 1,3-
described, including first results on a new route to form
dithiole-2-thiones to furnish TTF derivatives using phosphites
is a widely used method, but was not applied before to C3 and/
or C3,4-phosphanylated 1,3-dithiole-2-thiones. The overall
model reaction of trimethyl (instead of triethyl) phosphite with
either model phosphanylated dithiole-2-thiones 20 and 40, or
real species 2 (Scheme 3) was explored by means of quantum
chemical calculations (see the Computational details).
phosphanylated TTF derivatives.
Results and discussion
Derivatization of 1,3-dithiole-2-thione 1 was achieved via a
single (i) or double (ii) deprotonation protocol using one or
two molar equiv. of LDA at ꢀ78 1C, followed by reaction with
one or two equiv. of chloro(diphenyl)phosphane, thus leading
to the mono- (2) or bisphosphanylated (3) 1,3-dithiole-2-thiones
(Scheme 1).
Both compounds were isolated in good yields after re-
crystallization from hot toluene and washing with n-pentane
(53% for 2, 40% for 3). The 31P{1H} NMR spectroscopic shift of 2
revealed a dD value of 19.4 compared to the related imidazole-2-
thione VII10 and 10.9 compared to the thiazole-2-thione VIII11
(Fig. 3), thus showing a clear trend.
In all cases the process is expected to be highly exergonic
and slightly favouring the E over the Z-bisphosphanyl TTF
derivatives for these particular diastereogenic examples. Com-
pared to the overall energetics in the case of parent dithiole-2-
thione 1 to give unsubstituted TTF I, DG = ꢀ44.0 kcal molꢀ1
,
this phosphite-promoted reductive dimerization seems to be
thermodynamically favoured by the mesomeric +M effect of
Single crystals of compound 3, suitable for an X-ray diffrac-
tion study, were obtained from a solution of CH2Cl2 via slow
Fig. 4 Molecular structure of 3; selected bond lengths [Å] and angles [1]:
C1–S3 1.648(2), C2–P1 1.834(2), S2–C1–S1 112.33(12), S1–C2–P1
124.16(13), C3–C2–P1 120.61(17), P1–C2–C3–P2 1.748(5).
Scheme 1 Synthesis of P-functional dithiole-2-thiones 2 and 3.
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Scheme 2 Synthesis of P(V)-substituted dithiole-2-thiones 4–7.
Scheme 4 Reactions used for the estimation of ISE for methyl-1,3-
dithiolium isomers (a–c) and hyperhomodesmotic reaction (d) for the
evaluation of ASE in the parent compound.
the inversely polarized TS compared to the other (‘‘normal’’) TS
is very likely due to the stabilization of the evolving positive
charge at the aromatic (vide infra) partially formed 1,3-
dithiolium ring14 attached to S.15 Opposite to the behaviour
displayed by ketones towards phosphanes,16 reaction of tri-
methyl phosphite with the thiocarbonyl p-system does not
produce the expected cheletropic cycloaddition but a nucleo-
philic addition to carbon, thus affording zwitterion 11 through
a lower barrier process. Ring closure by P–S bond formation
leads to spirothiaphosphirane 12, which bears the S atom in the
equatorial position around the pentacoordinated P atom (ring
closure is coupled to a pseudo-rotation around the trigonal
bipyramidal P centre). Despite the low ring strain energy
expected for the s5l5-thiaphosphirane moiety, according to
the reported values for the parent or trimethyl-substituted
s3l3-analogues (13.75 and 10.05 kcal molꢀ1, respectively),17
ring opening to betaine 13 is facilitated due to the aromaticity
of the generated dithiolium ring.
Fig. 5 Molecular structure of 4; selected bond lengths [Å] and angles [1]:
S3–C3 1.6476(16), P–C1 1.8050(15), P–O 1.4823(10), S2–C3–S1 112.54(9),
S1–C1–P 122.34(8), C2–C1–P 121.85(12).
Scheme 3 Model coupling reactions for 2, 20 and 30. Computed (COS-
MOTHF/CCSD(T)/def2-TZVPP) overall variation of Gibbs free energies
(kcal molꢀ1) in square brackets for Z and E isomers.
phosphanyl substituent lone pair (l.p.) but disfavoured by
electron withdrawing phosphanoyl groups.
There are few reports on the aromaticity of bis(1,3-
dithiadiazoliumyl),18 the two-electron oxidation product of
In order to get rid of isomerism problems and for the sake of TTF, and only one for the parent 1,3-dithiazolium cation,18b
computational efficiency, the mechanism of the reductive the latter displaying a typical very negative NICS(1)zz value of
dimerization of 1,3-dithiole-2-thiones was studied using the ꢀ28.35 ppm and a relatively high ISE (isomerization stabili-
parent (unsubstituted) compound 1 and trimethyl phosphite as zation energy)19 of ꢀ16.3 kcal molꢀ1 according to reaction ‘‘a’’
models. The reaction can proceed by direct interaction of the (Scheme 4). At the working level of theory (DLPNO-CCSD(T)/def2-
phosphorus atom of the reagent and the thiocarbonyl S atom of 1, TZVPP//B3LYP-D3/def2-TZVP) an average ISE of ꢀ27.0 kcal molꢀ1
affecting its desulfurization to 1,3-dithiole-2-ylidene 10, was found for the two methyl-1,3-dithiolium isomers A and B.
similar to the recently described desulfurization of thiiranes The remarkable differences found for the three individual iso-
by P(III) reagents.13 However, this slightly exergonic transforma- merization reactions a–c20 suggest certain unsuitability of the ISE
tion occurs via a high energy transition state (TS) (Scheme 4), method for estimating the ASE (aromatic stabilization energy).
which was shown13 to be a signature of desulfurizations The latter was more accurately computed for the parent com-
occurring via an inversely polarized TS resulting upon attack pound C using an appropriate hyperhomodesmotic reaction d
from S to P and geometrically characterized by a see-saw (Scheme 5), similar to that used for other five-membered aromatic
environment for the negatively charged P atom. Lowering of heterocycles21 that gave a value of ꢀ11.7 kcal molꢀ1
.
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Scheme 5 Proposed mechanism (preferred pathway in black; alternative pathways in grey) for the trimethyl phosphite-mediated reductive dimerization
of 1,2-dithiole-2-thione 1 to TTF (I). Relative DG values (kcal molꢀ1) indicated in square brackets, in blue, for both minima and TS (the latter indicated with
‡
superscript).
Moreover, at the standard GIAO/B3LYP/6-311+G** level rate-determining 1 - 11 step (DG‡ = 34.4 kcal molꢀ1). Thus,
(optimizations at the working level of theory), NICS(1)zz and reaction of 10 with trimethyl phosphite furnishes phosphor-
NICS(1) values of ꢀ29.33 and ꢀ11.77 ppm were obtained for C, ane 14, which as previously reported for model derivatives,13-
which fall in the range for highly aromatic rings.22 The aro- can undergo a Wittig-type reaction with the thiocarbonyl
matic electronic structure and, hence, the reason for the high group of 1 through 1,2s5l5-thiaphosphetane 15. Phosphorane
stability of 1,3-dithiole-2-ylidene 10 are brought to light by the 14 displays pronounced pyramidal geometry at the 1,3-
P
very negative values of these two descriptors: NICS(1)zz = ꢀ30.85 ppm dithiolen-2-yl C2 atom ( C = 343.11) reflecting its significant
and NICS(1) = ꢀ12.17 ppm. Also the starting material 1,3- carbanionic character due to the extra stabilization by the
dithiole-2-thione 1 exhibits some aromatic character according two adjacent S (besides the P) atoms. This pyramidalization
to its NICS(1)zz and NICS(1) values of ꢀ9.02 and ꢀ4.70 ppm, together with folding of the ring along the Sꢁ ꢁ ꢁS axis (by 36.21)
in good agreement with NICS(1) = ꢀ6.76 ppm reported21b for accounts for its required distortion in order to avoid an 8p-
1-methyl-1,3,4-thiadiazole-2-thione (at the HF/6-311+G** level). electron antiaromatic structure. Thiaphosphetane 15 features
Elimination of trimethyl thiophosphate from betaine 13 the expected trigonal bipyramid arrangement at the penta-
affords the same 1,3-dithiole-2-ylidene 10 through a lower energy coordinate P atom, in agreement with the high value of the
path. Intermediacy of this carbene species might account for the geometric index24 t5 = 0.51, and a rather elongated axial P–S
reported isolation of products of a-addition of methanol.23 bond (d = 2.433 Å; WBI = 0.517; r = 7.92 ꢂ 10ꢀ2 e a0ꢀ3) with
Therefore, the lowest energy path for the phosphite-promoted equatorial P–C bond (d = 1.904 Å; WBI = 0.724; r = 16.24 ꢂ
reductive coupling of 1,3-dithiole-2-thiones was found to occur 10ꢀ2 e a0ꢀ3). No pseudo-rotamer with equatorial P–S and axial
via symmetric 1,3-dithiole-2-ylidene 10 carbene coupling, this P–C bonds could be located in the potential energy surface
carbene being preferentially formed through three-step phos- (PES), though an isomer 15sp with square pyramidal geometry
phite insertion into the exocyclic CQS bond.
around P (t5 = 0.29) was found to have slightly higher energy
The latter is initiated by the rate-determining nucleophilic (DDG = 5.4 kcal molꢀ1). The latter shows the required stronger
attack to the C2 ring atom and P–C bond cleavage of the P–S (d = 2.336 Å; WBI = 0.598; r = 9.45 ꢂ 10ꢀ2 e a0
)
ꢀ3
intermediately formed spiro-s5l5-thiaphosphirane 12. How- and weaker P–C bond (d = 1.914 Å; WBI = 0.706; r = 15.90 ꢂ
ever, several other higher energy pathways can be reached 10ꢀ2 e a0ꢀ3) to favour its breakdown into TTF I and trimethyl
in the temperature regime required for circumventing the thiophosphate.
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C–C bond leading to 1,2-thiaphosphetane 15 or, more favour-
ably, split into phosphorane 14 and 1,3-dithiole-2-thione 1.
A second higher energy alternative for the reaction between
1,3-dithiole-2-thione 1 and 1,3-dithiole-2-ylidene 10 leads to an
unsymmetrical electron distribution in zwitterion 20 bearing a
1,3-dithiolium ring and a carbanionic centre stabilized by three
S-substituents (Scheme 4). This intermediate can undergo an
electron redistribution process affording the most stable sym-
metric isomer 16 through a relatively low-energy barrier.
In order to explore the possibility of formation of 1,3,2-
dithiophospholane intermediates as reported in previous
mechanistic proposals,23 the reaction of dithioliumthiolate 18
with the trimethyl thiophosphate side-product was studied. All
reactions seem to involve higher-energy intermediates, the
most stable of which is dispiro-1,3,2-dithiaphospholane 21,
formed by relatively high-barrier concerted addition of 18 to
the PQS unit (Scheme 4). However, the C–S bond cleavage
required to afford the final TTF product I or intermediate 22
only produces the backward reaction to 18. The abovemen-
tioned intermediate 22 is a very unstable minimum that can
indeed be formed directly from 18 through a rather high barrier
and decomposes leading to TTF and trimethyl dithioperoxo-
phosphate, (MeO)2P(S)–SOMe 23, through a very high-energy TS
(Scheme 4). The other lower barrier way in which 18 reacts with
trimethyl thiophosphate furnishes the Sꢁ ꢁ ꢁS bridged betaine 24
(dSꢁ ꢁ ꢁS = 2.904 Å; WBI = 0.105; r = 2.86 ꢂ 10ꢀ2 e a0ꢀ3) that splits
in a barrierless process into trimethylphosphite and dispiro-
1,2-dithietane 25, which is the dimerization product of the
starting 1,3-dithiole-2-thione 1 via formal head-to-head [2+2]
cycloaddition of the exocyclic CQS bonds.
The rate-determining initial steps of reaction of 1,2-dithiole-
2-thione 1 with trimethylphosphite have been also studied
with substituted derivatives bearing one model phosphanyl
–PH2 or phosphanoyl –P(O)H2 substituent (20 and 40, respec-
tively; Scheme 3). Both the direct desulfurization to the corres-
ponding carbenes (DG‡ = 38.9 and 38.8 kcal molꢀ1 for 20 and 4,
respectively) and the C-addition furnishing betaines (DG‡ = 33.5
and 34.1 kcal molꢀ1 for 20 and 4, respectively) were found to be
slightly kinetically (also thermodynamically, see the ESI†)
favoured with respect to the parent system 1 (DG‡ = 39.5 and
34.4 kcal molꢀ1, respectively).
The full lowest-energy path 1 - 11 - 12 - 13 - 10 - I
as (Scheme 4) was re-examined, together with the competing
(close in energy) direct desulfurization 1 - 10 for three
different cases, using a more inexpensive computational
method (see Computational details): the parent dithiole-2-
thione 1 and two phosphanyl-mono-substituted dithiole-2-
thiones with PMe2 1PMe2 and P(O)Me2 1POMe2 groups (as models
for the real PPh2 and P(O)Ph2-substituted derivatives, respec-
tively). As previously reported for other reactions with different
organo-phosphorus compounds,25 the similar trends and
energy values to those reported (Scheme 4) at the reference
level for the parent derivative 1 (R = H) validate the new
method, where again desulfurization of 1 via spirothiapho-
sphirane 12 is the lowest energy pathway (Scheme 6). In
the case of 1PMe2 and 1POMe2, the direct P(OMe)3-promoted
Fig. 6 Computed
(0.05 a.u.) for HOMOꢀ1 (a) and HOMO (b), as well as BCPs (small green
spheres) and bond paths (c) for 16.
(B3LYP/def2-TZVPP)
Kohn–Sham
isosurfaces
Almost the same energy barrier from 10 was found for the
reaction (in this case endergonic) with the starting material 1
furnishing a symmetric thiocarbonyl ylide 16 that readily
undergoes sequential cyclization to dispirothiirane 17 and
one-step desulfurization by the action of a second trimethyl
phosphite unit, thus yielding the final TTF (I). As expected,
the central S atom in ylide 16 holds a formal positive charge
(qN = +0.325 a.u.), locating its only p-type lone pair (LP)
perpendicular to the C–S–C plane (Fig. 6a), whereas both
adjacent C (spiro)atoms are the main contributors to the
HOMO (Fig. 6b) and formally bear half negative charge
(qN = ꢀ0.755 a.u.) (compared with natural charges in 17:
+0.086 and ꢀ0.456 a.u. at S and C, respectively). The exocyclic
C–S bond order (d = 1.759 Å; WBI = 1.030; r = 19.29 ꢂ
10ꢀ2 e a0ꢀ3) in between the typical single bond border ꢀi3n
thiirane 17 (d = 1.865 Å; WBI = 0.856; r = 15.40 ꢂ 10ꢀ2 e a0
)
and the double bond in 1,2-dithiole-2-thione 1.15 Both dithio-
lenyl rings in 16 are additionally brought together by attractive
interring Sꢁ ꢁ ꢁS contacts establishing formal S–S bonds, as
evidenced by the location of the corresponding bond critical
points (BCP) (Fig. 6c) (d = 3.333 Å; WBI = 0.056; r = 1.39 ꢂ
10ꢀ2 e a0ꢀ3) that push not only S but also even the negatively
charged C atoms (d = 2.664 Å) below the sum of van der Waals
radii (3.60 and 3.40 Å for S–S and C–C, respectively). This
results in a slightly acute C–S–C angle (98.41; compared with
the computed equilibrium bond angle of 99.71 for Me2S at the
same level) and slight pyramidalization at the ring C2 atoms
P
(
C = 350.61). An additional BCP is found between the two
distal CQC bonds (Fig. 6c).
Further evolution from 17 occurs through a very low-barrier
ring-opening reaction to the slightly less stable zwitterionic
isomer 18, whose stability arises, as in 13, from the formation
of a 1,3-ditholium aromatic ring (Scheme 5). Opposite to the
behaviour of dithiole-2-thione 1, this intermediate undergoes
desulfurization to TTF I via the normal nucleophilic P-to-S
attack featuring a planar (almost linear in this case) S and
tetrahedral P geometries in the TS.13 According to the relative
orientation of both dithiolene rings, dispirothiirane 17 also
exists as a slightly less stable (DDG‡ = 0.3 kcal molꢀ1) out–out
isomer (only the most stable in–out isomer is presented in
Scheme 5). The alternative attack of the P(III) reagent to the
dithiolium C atom of 18 proceeds through a higher barrier,
giving rise to thiabetaine 19. The latter can rotate around the
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Scheme 8 C,C-Coupling reaction of phosphanylated dithiole-2-thione 2
leading to TTF derivative 10.
Scheme 6 Relative DG values (kcal molꢀ1) for the formation of I (grey),
IPMe2 (blue) and IPOMe2 (red) from 1, 1PMe2 and 1POMe2, respectively, at the
CCSD(T)/def2-TZVPP//PBEh-3c level of theory.
Scheme 9 C,C-Coupling reaction of bis-phosphanoylated dithiole-2-
thione 5 leading to TTF derivative 26.
desulfurization leading to 1,3-dithiole-2-ylidenes 10PMe2 and
10POMe2 was also kinetically hampered over the nucleophilic
attack to thiocarbonyl carbon atoms of 1PMe2and 1POMe2 that the conversion into II was proven by 13C{1H} NMR spectroscopy
are faster than in the case of the parent compound affording as compound II showed a signal at 110.5 ppm for the C2
spirothiaphosphiranes 12PMe2 and 12POMe2 through zwitterions carbons and 142.9 ppm for the backbone carbon nuclei,
11PMe2 and 11POMe2. Betaines 13PMe2 and 13POMe2 were similarly compared to 216.7 and 152.1 ppm in the case of 3. The NMR
formed in an endergonic low-barrier process. The elimination data in different solvents are in good agreement with those
of SQP(OMe)3 proceeded through an almost barrierless step reported in the literature.
giving rise to P-substituted 1,3-dithiole-2-ylidenes 10PMe2 and
Under the same conditions (P(OEt)3, THF, 60 1C) the C,C-
10POMe2. Given that P-substitution is far away from the reactive coupling reaction of the mono-phosphanylated 1,3-dithiole-2-
centre (the 1,3-dithiolen-2-yl C2 atom), the final C2,C2 homo- thione 2 was less selective, leading to a mixture of products, but
coupling in 10PMe2 and 10POMe2 takes place through very similar the cis(Z-8) and trans(E-8) isomers of the bis-phosphanylated
energy barriers for both Z and E configurations, where only for TTF derivatives (Scheme 8) could be assigned in the 31P{1H}
the Me2P(O)-substituted TTF IPOMe2 a small kinetic preference NMR spectrum at ꢀ12.8 and ꢀ12.9 ppm (ratio 1.3 : 1.0); for the
of the Z over the E isomer could be expected, in agreement with Ph2P mono-substituted TTF derivative a signal at ꢀ22.7 ppm
the experimental results for mono-phosphanyl dithiole-2- was reported.8
thione (see below).
Under the same conditions, again, an unselective reaction
So far, only the reaction of TTF with 4 equiv. of LDA and was observed when compound 5 was used, but also here
subsequent treatment with chloro(diphenyl)phosphane leading product 26 could be observed in the 31P{1H}-NMR spectrum
to tetrakis-diphenylphosphanylated TTF II had been reported of the reaction mixture (Scheme 9) as a signal at 18.7 ppm in
previously. To test our conceptual approach, outlined before- minor intensity (12%).
hand, the reaction of mono- or bis-phosphanylated dithiole-2-
thiones 2 and 3, respectively, with triethylphosphite in toluene
at elevated temperatures was studied. Initial treatment of 3 in
Conclusions
toluene and 110 1C led to a very unselective reaction as revealed
by the 31P{1H} NMR spectrum of the reaction solution. By using
A facile access to the first examples of phosphorus-
THF as solvent and at 60 1C (Scheme 7), the reaction turned out functionalized dithiole-2-thiones 2 and 3 was described. These
to be more selective in the case of 3, and the tetrakis- P(III) compounds could selectively be oxidized to the corres-
phosphanylated TTF derivative II could be isolated in good ponding P–O and P–S P(V) derivatives 4–7. Furthermore, it was
yields (53%) after filtration and washing with n-pentane. Inter- shown that bis-phosphanylated compound 3 could successfully
estingly, the 31P{1H} resonance of the starting material 3 (THF: be converted to the corresponding tetrathiafulvalene derivative
ꢀ18.4 ppm) and product is almost identical (ꢀ18.1 ppm), but using P(OEt)3. In contrast, the C2,C2 coupling reactions of
mono-phosphanylated compound 2 or bis-phosphanoylated
derivative 5 were rather unselective. DFT calculations revealed
a high exergonicity of the reductive dimerization of dithiole-2-
thiones by phosphites and a preferred direct carbene coupling
path to the final TTF I. This carbene 10 is preferentially formed
through a three-step phosphite insertion into the exocyclic CQS
bond, initiated by the rate-determining nucleophilic attack to the
C2 ring atom and P–C bond cleavage of the intermediately formed
Scheme 7 C,C-Coupling reaction of bisphosphanylated 1,3-dithiole-2-
thione 3 leading to TTF derivative II.
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s5l5-thiaphosphirane. Phosphorus-substituted derivatives are
expected to undergo faster rate-determining steps, with final
homocoupling reactions promoting little (or none) discrimina-
tion between the formation of Z- or E-configured disubstituted
model TTF derivatives.
11 I. Begum, G. Schnakenburg and R. Streubel, Dalton Trans.,
2016, 45, 2955–2962.
12 A. Koner, G. Pfeifer, Z. Kelemen, G. Schnakenburg,
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L. Nyulaszi, T. Sasamori and R. Streubel, Angew. Chem.,
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13 A. Espinosa Ferao and R. Streubel, Inorg. Chem., 2020, 59,
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Conflicts of interest
There are no conflicts to declare.
15 The 1,3-dithiole-2-thione 1 displays a significantly elon-
gated exocyclic C–S bond (1.653 Å; WBI 1.554; r = 22.37 ꢂ
10ꢀ2 e a0ꢀ3) compared to the thiocarbonyl group in trithꢀio3-
Acknowledgements
carbonic acid (1.640 Å; WBI 1.662; r = 23.06 ꢂ 10ꢀ2 e a0
)
Financial support from the University of Bonn is gratefully
acknowledged. A. G. A. and A. E. F. wish to thank the computa-
at the same level, owing to a significant contribution of a
dithiolium-thiolate resonance structure which is also
responsible for a relatively high negative natural charge
(ꢀ0.12 a.u.) at the exocyclic S atom (compare to thiocarbo-
nyl S atom in trithiocarbonic acid: ꢀ0.05 a.u.).
´
´
tion centre at Servicio de Calculo Cientıfico (SCC – University of
Murcia) for technical support and the computational resources
used. We also appreciate the support for X-ray determination by
Prof. A. C. Filippou and Prof. K. Menche.
16 A. Espinosa Ferao, Inorg. Chem., 2018, 57, 8058–8064.
17 A. Espinosa Ferao and R. Streubel, Inorg. Chem., 2016, 55,
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New J. Chem.
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