On the selectivity of silver mediated thiolate transfer and the crystal structure of an unprecedented thiolate chloride Argentate(I) anion

Article abstract of DOI:10.1016/j.inoche.2018.07.025

The reactivity of a series of fluorinated silver(I) thiolates (AgSR_F) towards thiolation of dichloromethane by nucleophilic substitution in presence of bis(diphenylphosphane)methane (dppm) has been studied. Our results indicate that in the presence of an excess of CH₂Cl₂ the thiolate phosphine silver derivatives are capable of exchange thiolate and chloride to form, in different proportions, the correspondent chloromethylthioether (CH₂Cl(SR_F)) a or methyldithioether (CH₂(SR_F)₂) b with the fluorinated SR_F groups: SC₆F₄H 1, SC₆H₄(CF₃)-2 2, SC₆H₃F₂-3,4 3, SC₆H₃F₂-2,4 4, SC₆H₃F₂-3,5 5 and SC₆H₄F-2 6. Despite the kinetically preferred monosubstitution reaction, some of the thiolate derivatives tend to form selectively the disubstitution products, indicating a non-innocent role of the silver center in the formation of the product. Furthermore, we have isolated and structurally characterized, by X-ray diffraction, an ionic compound [Ag₃(μ³-Cl)₂(μ-Ph₂PCH₂PPh₂)₃][AgCl(SC₆F₄H)] that could be an intermediary product in the chloride-thiolate exchange reactions.

Full text of DOI:10.1016/j.inoche.2018.07.025

Inorganic Chemistry Communications 95 (2018) 149–153
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Inorganic Chemistry Communications
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Short communication
On the selectivity of silver mediated thiolate transfer and the crystal
structure of an unprecedented thiolate chloride Argentate(I) anion
⁎
Guillermo Moreno-Alcántar , Guillermo Romo-Islas, Marcos Flores-Álamo, Hugo Torrens
Faculty of Chemistry, Universidad Nacional Autónoma de México, Cuidad Universitaria, Mexico City 04510, Mexico
G R A P H I C A L A B S T R A C T
A B S T R A C T
The reactivity of a series of fluorinated silver(I) thiolates (AgSR
phosphane)methane (dppm) has been studied. Our results indicate that in the presence of an excess of CH
exchange thiolate and chloride to form, in different proportions, the correspondent chloromethylthioether (CH
the fluorinated SR groups: SC H 1, SC (CF )-2 2, SC -3,4 3, SC -2,4 4, SC -3,5 5 and SC
substitution reaction, some of the thiolate derivatives tend to form selectively the disubstitution products, indicating a non-innocent role of the silver center in the
F
) towards thiolation of dichloromethane by nucleophilic substitution in presence of bis(diphenyl-
Cl the thiolate phosphine silver derivatives are capable of
Cl(SR )) a or methyldithioether (CH (SR ) b with
F-2 6. Despite the kinetically preferred mono-
2
2
2
F
2
F 2
)
F
6
F
4
6
H
4
3
6
H
3
F
2
6
3
H F
2
6 3
H F
2
6 4
H
3
formation of the product. Furthermore, we have isolated and structurally characterized, by X-ray diffraction, an ionic compound [Ag
AgCl(SC H)] that could be an intermediary product in the chloride-thiolate exchange reactions.
3
(μ -Cl)
2 2 2 2 3
(μ-Ph PCH PPh ) ]
[
6 4
F
Contemporary chemical synthesis is devoting a growing attention to
search of silver(I) coordination compounds bearing aminodipho-
sphanes, the solvent (CH Cl or CHCl ) C(sp )eCl bond cleavage
2 2 3
3
silver reagents [1–7]. For example, some of the frequently studied re-
actions are the activation of multiple bonds [8–10]. Recently, the study
of group transfer and bond cleavage reactions is attracting considerable
attention [11–13]. The nucleophilic substitution of aliphatic halides by
thiolate groups is a typical reaction to obtain thioether derivatives
mediated by Ag(I) is possible. Some of the silver phosphine products of
the reaction have been identified and successfully characterized. This
peculiar reactivity was observed to be exclusive for compounds bearing
thioether or ether groups in the ligands, but the organic by-products of
the reactions have not been identified, maintaining the reaction me-
chanism enigmatic [20]. This, of course, is evidence of the rising po-
tential of silver compounds as CeX bond cleaver agents.
[
3
14–16]. Until now, AgSCF is one of the main sources of nucleophilic
trifluoromethylthiolate which is an important group in organic che-
mical synthesis and lately the silver mediating effects are being a topic
of interest [17–19]. In a recent study, it has been observed that, in the
In this work, we explore the reactivity and selectivity of silver(I)
⁎
Corresponding author.
E-mail address: lgma@comunidad.unam.mx (G. Moreno-Alcántar).
https://doi.org/10.1016/j.inoche.2018.07.025
Received 25 April 2018; Received in revised form 3 July 2018; Accepted 17 July 2018
Available online 18 July 2018
1387-7003/ © 2018 Elsevier B.V. All rights reserved.

G. Moreno-Alcántar et al.
Inorganic Chemistry Communications 95 (2018) 149–153
Scheme 1. General reaction of chloride substitution by the silver(I) fluorothiolates.
fluorothiolate derivatives towards dichloromethane in the presence of a
diphosphane ligand (dppm), for the reaction of substitution of chloride
by the thiolate group. The general view of the reaction is shown in
Scheme 1.
methylene on the thioethers are a good indicator of the ratio of for-
mation of the distinct products a and b. The different relations are
probably an indicative of the different selectivity of the thiolate species
towards the formation of either monosubstitution or disubstitution
products. The formation of disubstitution products, in some cases as
main products, is unexpected considering that dichloromethane is in a
large excess and could be indicative of a non-innocent role of the silver
center in the reaction mechanism. i.e. for the synthesis of the dis-
ubstituted products in basic media, it is reported that stoichiometrical
quantities of the thiolate are required together with long times of re-
action (7 days). Furthermore, under the last mentioned conditions an
1. General reaction procedure
A solution of 124.2 mg (0.336 mmol) of bis(diphenylphosphane)
methane in 5 mL of dried dichloromethane, was fast added to
.336 mmol of the solid silver thiolate AgSR , the mixture was stirred
0
F
by sonication for 5 min. The initial insoluble silver thiolate dissolves
and a clear solution was obtained. The residual dichloromethane was
2 2
excess of CH Cl rather yields the monosubstitution adduct, chlor-
omethyltioether [14, 21]. Thus, the silver mediated reaction is not only
selective but faster than the analogous procedure using alkaline earth
thiolates. The chemical shifts of all 11 thioether derivatives and the
1
evaporated by reduced pressure and the crude mixture analyzed by H,
1
9
31
F and P NMR.
The analysis of the 1H NMR spectrum, shown in Fig. 1, reveals the
1
ratios of formation of the products, from H NMR, are shown in Table 1.
presence of the substitution products a and b in almost all the reaction
mixtures. The proton chemical shifts are in agreement with previous
data for compounds 1a, 1b [21]. The signals corresponding to the
The identity of the products was confirmed by mass spectrometry and
elemental analysis as reported in the ESI.
Fig. 1. Detail of the 1H NMR spectra of the products of the reaction, showing the signals of products a (low field) and b (high field) of the six different fluorinated
thiolates.
150

G. Moreno-Alcántar et al.
Inorganic Chemistry Communications 95 (2018) 149–153
Table 1
Proportions of products a and b from the six substitution reactions mediated by
1
silver together with its δ H NMR.
SR
F
δ
1H NMR
% a (CH
2
Cl(SR
F
))
δ
1H NMR
2 F 2
% b (CH (SR ) )
SC
SC
SC
SC
SC
SC
6
6
6
6
6
6
F
4
H
4.86
4.96
4.93
4.84
–
92.6
36.4
39.8
9.0
–
15.4
4.38
4.38
4.33
4.20
4.35
4.31
7.4
H
H
H
H
H
4
(CF
3
)-2
63.6
60.2
91.0
100.0
84.6
3
3
3
4
F
F
F
2
2
2
-3,4
-2,4
-3,5
F-2
4.92
On the other hand, the 31P NMR spectra (Fig. 2) contain valuable
insights as well, all six reaction mixtures show a signal with the ap-
pearance of a broad pseudo-double centered in δ = −5.0 ppm. This
signal is due to the phosphorus atoms coordinated to silver, the ob-
1
served coupling constant
JAg-P has a value around 440 Hz which is in
the expected range for this kind of compounds [22, 23]. This signal
indicates the formation of the same species in all six reactions, the lack
3
1
of variation in the δ P NMR indicates that this specie does not con-
tains the diverse fluorinated fragments. There are other much less
abundant signals in the region of 22–24 ppm, these signals do not show
a coupling pattern indicative of the AgeP coordination. These addi-
tional signals change through the different systems indicating possible
side-reactions whose products have together the phosphorus atoms
Fig. 3. ORTEP view of the ionic structure observed for the compound.
Probability of displacement ellipsoids at 50%, H atoms are omitted for clarity.
Atoms in the phenyl groups of the phosphines are displayed as wireframes for
clarity. Color code: C, gray; Cl, green; P, orange; F, lime; Ag, silver. (For in-
terpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
3
1
(
probably as phosphine oxides judging by the δ P NMR range) and the
31
fluorinated moieties that induce the different shielding in the P NMR
24]. The presence of other smaller signals in the spectra show the
[
existence of minor products whose identity is mysterious. Un-
fortunately, the small amounts of formation of all these by-side pro-
ducts did not allows us to separate them and characterize them, further
study in this respect is undergoing.
From reaction 1 it was possible to isolate a single crystal suitable for
X-ray structural analysis, a few small crystals were formed during the
In contrast, the analysis of the 19F NMR spectrum (Fig. 4) shows that
in the systems there are at the most two main products displaying the
fluorination patterns, this is congruent with the presence of thioether
products. Indeed, the integration relations are coincident for reactions 2
1
solvent evaporation in the flask wall. The X-ray analysis of that crystals
and 3 with the proportions observed in H NMR, although in the other
3
reveals the formation of the ionic product [Ag
Ph PCH PPh ][AgCl(SC
in Fig. 3. The cationic motive observed in the structure was previously
3
(μ -Cl)
2
(μ-
systems the overlapping of the signals made the comparison inaccurate.
19
2
2
2
)
3
F
6 4
H)]. The corresponding structure is shown
All the δ F NMR signals are in the expected range for this kind of
adducts [28]. The differences between the signals corresponding to the
corresponding products a and b are regular; in general, fluorine nuclei
in the ortho and meta positions of the thiolate are shielded in the
3
1
reported [25–27]. The obtained structural and NMR ( P NMR) char-
acterization is consistent with those previously reported.
Fig. 2. 31P NMR spectra of the reaction products.
151

G. Moreno-Alcántar et al.
Inorganic Chemistry Communications 95 (2018) 149–153
Fig. 4. 19F NMR spectra of the reaction mixtures of silver thiolates.
methyldithioether about 0.5 ppm in relation to the chlor-
omethylthioether, while the signals of atoms located in para are un-
shielded between 0.5 and 1.0 ppm in the same comparison. Even if
of the phenyl rings at the phosphine (distance C9 -F3: 3.39(2)).
Altogether, our results provide evidence of the non-innocent role of
the silver(I) centers in the substitution mechanism of chlorine groups
for thiolate moieties in dichloromethane, which is manifested affecting
the speed, mildness and selectivity of the reactions. The selectivity of
the substitution reaction to monosubstitution or disubstitution products
seems to be affected also by the fluorination degree of the thiolate
moiety. Finally, the observations reported herein provide evidence of
the potential of silver as C-X bond cleaver and serve as encouragement
to explore this kind of reactivity towards other substrates of interest.
there are some minor signals that could correspond to the mixed ar-
−
F
gentate anions [AgCl(SR )] , their low abundance indicates that they
are not quantitative products. In consequence, it is possible that the
argentate anion we observed is still capable to transfer its thiolate group
to dichloromethane. Thus, this kind of adducts are not bulky products
but probable intermediary adducts of the partial substitution reactions.
The X-ray characterization exhibits the cation formed by a tri‑silver
3
trigonal planar core caped by two μ chloride ligands. One of the
AgeAg distances is slightly shorter than the other two (3.3019(4),
3
.4385(6) and 3.4832(6) Å), all six AgeCl distances are quite similar in
the range of 2.67–2.73 Å and both chlorides have virtually the same
distance to the Ag generated centroid (Cl1: 1.849 Å and Cl2 1.853 Å).
None of the phosphorus lies on plane of the Ag core and phosphine
3
3
disilver arrangements have different torsion angles for all three phos-
phines: Ag1-Ag2-P5-P6:1.36(3)°, Ag3-Ag2-P5-P6: 7.40(3)° and Ag1-
Ag3-P3-P4: 13.48(3)° these distortions seem to arise mainly by supra-
molecular interactions stablished by the neighbor phenyl rings.
−
6 4
The general features of the anion ([AgCl(SC F H)] ) structure are
shown in Fig. 5. The AgeS distance is 2.326(6) Å whereas that of AgeCl
is 2.311(1) Å. The observed S-Ag-Cl angle of 170.93(6) Å shows a
considerable distortion from the expected linearity for this moiety. This
unexpected small angle could be explained taking to account that the
distance Ag4-F1 (2.883(2) Å) is considerably shorter than the sum of
van der Waals radii of Ag and F (3.19 Å) [29, 30], indicating a relatively
strong interaction of pseudo-bonding characteristics. On this system,
the silver seems to approach to tree coordination with a weak AgeF
contact. The sulfur atom has a nearly tetrahedral geometry with an Ag-
3
S-C angle of 110.6(2) ° as expected for a sp hybridization.
The position of the anion with respect to the cation in the crystalline
form (Fig. 5) seems to be determined by the formation of three hy-
drogen bonds: one between one of the chlorine atoms on the cation and
the hydrogen of the fluorinated ring in the anion (C84-Cl1 distance:
−
6 4
Fig. 5. View of the X-ray structure of the heteroleptic [Ag(SC F H)Cl] anion
and hydrogen bonds distances between anion and cation. Probability of dis-
placement ellipsoids at 50%, H atoms are omitted for clarity. Atoms in the
phenyl groups of the phosphines are omitted for clarity. Color code: C, gray; Cl,
green; P, orange; F, lime; Ag, silver. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this
article.)
3.389(5) Å), another between the fluorine atom in the position 3 of the
fluorinated ring and the hydrogen of the bridge methylene of the
phosphine (C1-F3 distance: 3.535(5)) and the last one between the
mentioned fluorine with one of the hydrogen atoms on position 2 of one
152

G. Moreno-Alcántar et al.
Inorganic Chemistry Communications 95 (2018) 149–153
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