The First Organosilver(III) Fluoride, [PPh4][(CF3)3AgF]

Article abstract of DOI:10.1002/chem.201905771

Organosilver(III) fluoride complexes have been assigned a key role in different fluorination processes. To the best of our knowledge, however, none of them seem to have been isolated or even detected thus far. Here we report on the successful synthesis of the trifluoromethyl derivative [PPh₄][(CF₃)₃AgF], which has been isolated in high yield. The thermodynamic stability of the Ag?F bond is shown by calculation and demonstrated by multistage mass spectrometry (MSⁿ) under collision-induced dissociation (CID) conditions. Nevertheless, the substantial elongation found in the Ag?F bond (X-ray) is correlated with a marked nucleophilic character of the terminal F ligand. This Ag?F bond is, in fact, quite reactive: it suffers hydrolysis and is also solvolyzed by thiols.

Full text of DOI:10.1002/chem.201905771

A Journal of
Accepted Article
Title: The First Organosilver(III) Fluoride, [PPh4][(CF3)3AgF]
Authors: Babil Menjón, Daniel Joven-Sancho, Miguel Baya, Antonio
Martín, and Jesús Orduna
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The First Organosilver(III) Fluoride, [PPh4][(CF3)3AgF]
Daniel Joven-Sancho,[a] Miguel Baya,[a] Antonio Martín,[a] Jesús Orduna,[b] and Babil Menjón*[a]
Dedicated to Dr. Milagros Tomás on the occasion of her 65th birthday
Abstract: Organosilver(III) fluoride complexes have been assigned a
key role in different fluorination processes. To the best of our
knowledge, however, none of them seem to have been isolated or
even detected thus far. Here we report on the successful synthesis
of the trifluoromethyl derivative [PPh4][(CF3)3AgF], which has been
isolated in high yield. The thermodynamic stability of the Ag–F bond
is shown by calculation and demonstrated by multistage mass
spectrometry (MSn) under collision-induced dissociation (CID)
conditions. Nevertheless, the substantial elongation found in the Ag–
F bond (X-ray) is correlated with a marked nucleophilic character of
the terminal F ligand. This Ag–F bond is, in fact, quite reactive: it
suffers hydrolysis and is also solvolysed by thiols.
competitive system.[13] Interestingly, the importance of Ag(III)
intermediates in silver-catalyzed C–X, C–C, C–N, and C–O
cross-couplings was recently demonstrated.[14] The handful of
organosilver(III) compounds currently known are only stabilized
by either macrocyclic ligands[15] or by fluorinated methyl
groups.[16] In fact, the non-fluorinated complexes [(CH3)3AgI]−
and [(CH3)4Ag]− were found to be unstable.[17] On the other hand,
aside from the paramagnetic phase Cs2KAgF6,[18] the known
silver(III) fluorides are restricted to the binary compound AgF3
and a number of salts of its diamagnetic [AgF4]− complex
anion.[19] We report now on the synthesis and structural
characterization of an organosilver(III) fluoride, which is, as far
as we know, the first compound to incorporate both the Ag(III)–C
and Ag(III)–F functionalities.
Silver-mediated fluorination of organic molecules is attracting
much current interest in chemical synthesis owing to an
advantageous combination of activity and selectivity.[1] The
binary fluoride AgF2 is known to behave as a strong fluorinating
agent towards organic substrates.[2,3] Moreover, along with the
monovalent binary fluoride AgF,[1,4] there is a number of Ag(I)-
mediated processes operating in the presence of electrophilic
fluorinating reagents, where organometallic Ag(II) and mainly
Ag(III) fluorides bearing both the Ag–C and Ag–F functionalities
have been recently invoked as key intermediates (Scheme 1).[5,6]
The fact that fluoride forms labile bonds with coinage metals
makes this kind of compound very attractive from the reactivity
point of view, but at the same time, difficult to isolate. As a
matter of fact, no organosilver(III) fluoride seems to have been
isolated or even detected to date.[7] This is in sharp contrast with
the field of organogold fluorides, which emerged in the early
years of the present century and is rapidly developing.[8] It is also
worth noting that Ag(III) is the highest oxidation state currently
available for silver.[9,10]
Scheme 1. Selected silver-mediated fluorination processes in which fluoride
intermediates of high-valent silver have been invoked: a) Ref. [5a], b) Ref. [5b],
c) Ref. [5c], d) Ref. [6].
The only well-established organosilver fluorides of molecular
nature reported to date,[11] namely (SIDipp)AgF and
[{(SIDipp)Ag}2(µ-F)][BF4], contain silver(I) centres stabilized by
NHC-ligands.[12] The lack of precedents in silver(III) chemistry
can be ascribed to the unavailability of suitable synthetic
methods, which is probably related to the higher lability of silver
compared to gold. The substantially faster reaction kinetics
usually achieved with silver together with its much lower price
makes this 4d coinage metal a very attractive, promising and
Based on our previous experience, the CF3 group was selected
here to stabilize Ag(III).[16a] Access to the (CF3)3Ag fragment,
however, was not so easy. The only reported procedure leading
to an isolable compound of this organosilver(III) moiety, namely
[N(PPh3)2][(CF3)3AgCl], relied on the intriguing precursor
(CF3)3Ag·NCMe,[20] which was itself obtained in about 8%
estimated yield.[16b] A better synthetic method was obviously
needed. While working with the homologous gold system, we
had previously found that the photoinduced oxidative addition of
CF3I to the linear organogold(I) complex [CF3AuCF3]− cleanly
afforded the square-planar organogold(III) derivative [(CF3)3AuI]−
in high yield and under mild conditions.[21] In contrast, no
reaction was observed by treating the organosilver(I) complex
[CF3AgCF3]− with CF3I under various conditions (Scheme 2).
This is also at variance with the recently reported oxidative
addition of CH3I to the non-fluorinated complex [CH3AgCH3]−,
which resulted in the highly unstable [(CH3)3AgI]−. The latter
compound was detected (NMR and MS) but not isolated.[17]
[a]
[b]
M. Sc. D. Joven-Sancho, Dr. M. Baya, Dr. A. Martín, Dr. B. Menjón
Instituto de Síntesis Química y Catálisis Homogénea (iSQCH)
CSIC–Universidad de Zaragoza
C/ Pedro Cerbuna 12, ES-50009 Zaragoza (Spain)
E-mail: menjon@unizar.es
Dr. J. Orduna
Instituto de Ciencia de Materiales de Aragón (ICMA)
CSIC–Universidad de Zaragoza
C/ Pedro Cerbuna 12, ES-50009 Zaragoza (Spain)
Supporting information for this article is given via a link at the end of
the document.
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nucleophilic character at the terminal F atom. Another factor
contributing to the Ag–F bond elongation is of intrinsic character,
namely the high trans influence of the CF3 group.[26] The
involved Ag–C distance (203.6(2) pm) is, in turn, substantially
shorter than that corresponding to the mutually trans CF3 groups
(Ag–C 209.8(2) pm av.). The latter value exactly coincides with
the mean value observed in the homoleptic derivative
[PPh4][(CF3)4Ag].[16a] There is also excellent agreement between
the structure of the [(CF3)3AgF]− anion experimentally found in
crystals of 2 and that calculated in the gas phase by theoretical
methods (DFT/M06). Our current results (Figure S17) are also in
good agreement with previous calculations by Preiss and
Krossing.[27]
Scheme 2. Synthetic procedures to prepare complexes 1 and 2 in pure form.
Full details are given in the Experimental Section (see Supporting Information).
The cation is [PPh4]+ in every case.
After much trial,
a
convenient method to prepare
[PPh4][(CF3)3AgCl] (1) was found, which involved chlorination of
the linear organosilver(I) complex [CF3AgCF3]− at low
temperature (Scheme 2). A mixture of the desired species 1 and
the stoichiometrically required [PPh4][AgCl2] was thereby
obtained, which could be cleanly separated making use of the
low solubility of the latter in DME (see Experimental). Once
obtained and isolated, compound
1
was successfully
transformed into the fluoro-complex [PPh4][(CF3)3AgF] (2) by
treatment with AgF (Scheme 2). It is worth noting that all
previous attempts to obtain or even detect this fluoro-complex
using reasonable procedures and careful experimental
conditions had failed.[16b,16d]
Figure 1. Displacement-ellipsoid diagram (50% probability) of the [(CF3)3AgF]−
anion as found in single crystals of 2. Only the most populated set of the
rotationally disordered F atoms found in every CF3 group is shown. Selected
bond lengths [pm] and angles [°] with estimated standard deviations: Ag–F(10)
198.41(10), Ag–C(1) 210.02(18), Ag–C(2) 203.61(19), Ag–C(3) 209.63(18),
C(1)–Ag–C(2) 90.50(8), C(1)–Ag–C(3) 177.42(7), C(1)–Ag–F(10) 89.22(6),
C(2)–Ag–C(3) 89.35(8), C(2)–Ag–F(10) 179.46(7), C(3)–Ag–F(10) 90.91(6). A
view of the C(sp2)–H···F–Ag hydrogen bonds is shown in Figure S16.
In solution, compound 2 has a very rich and characteristic
19F NMR spectrum (Figure S3), where all the ligands provide
information relating to their respective environments. The
spectral pattern is best resolved at low temperature. The less
shielded signal (δF = −24.43 ppm) corresponds to the CF3 group
located trans to the F ligand and the signal appearing at δF =
−37.60 ppm is assigned to the mutually trans CF3 groups. These
signals are more than 13 ppm apart, which denotes very
different chemical and magnetic environments for each
inequivalent CF3 group. Finally, the terminal F ligand is highly
shielded and gives rise to a complex upfield signal at δF =
−236.22 ppm.[22,23] All of these signals are mutually coupled and
belong to the same spin system. Furthermore, they all show
additional coupling to the 107/109Ag nuclei of the central atom.
Each pair of nJ(107Ag,F) and nJ(109Ag,F) coupling constants
shows the appropriate relationship required by the ratio of the
corresponding gyromagnetic constants: γ(109Ag)/γ(107Ag) ≈ 1.15.
The experimental spectrum has been satisfactorily simulated
(Figure S4) with the parameters given in Table S2. The
1J(107/109Ag,F) value (60.8 / 69.9 Hz) is much lower than that
observed in the inorganic complex anion [AgF4]− (370.4 / 425.8
Hz)[23] suggesting that the Ag–F bond in compound 2 might be
elongated (see below).
We wanted also to evaluate the thermodynamic strength of the
Ag–F bond. With this purpose in mind, the most relevant
decomposition paths that might be reasonably expected to
operate on the [(CF3)3AgF]− anion at the unimolecular level were
mapped by calculation (DFT/M06). The results are shown in
Scheme S1 and summarized in Scheme 3. Non-reductive ionic
dissociation of F− is highly endergonic: ΔG0 = 74.5 kcal/mol. This
value is the reverse of the FIA corresponding to the (CF3)3Ag
fragment, which thus appears as a molecular Lewis acid of
considerable strength.[28] The overall thermodynamic balance for
the concerted reductive elimination of CF4 or C2F6 is strongly
favoured (ΔG0 < −52 kcal/mol). These concerted processes,
however, involve high activation barriers (ΔG⧧ ~40 kcal/mol) that
might well hinder the occurrence of these single-step two-
electron reductions in favour of less energetically demanding
•
The crystal structure of 2, as established by X-ray diffraction
methods, consists of tetrahedral [PPh4]+ and square-planar
[(CF3)3AgF]− (Figure 1) ions connected by a network of C(sp2)–
H···F–Ag hydrogen bonds (Figure S16). A minimum H···F
distance of 221 pm and an associated C–H···F angle of 155.7°
denote fairly strong interactions.[24] This can contribute to the
elongated Ag–F bond observed in the anion (198.41(10) pm)
when compared to that found in the purely inorganic salt K[AgF4]
(188.9(3) pm).[25] Moreover, the presence of these noteworthy
hydrogen bonds can be taken as evidence of substantial
paths. Radical dissociation of CF3 was found to be less
endergonic in general and to exhibit significant stereochemical
dependence. Thus, the energy required for radical dissociation
of the CF3 group trans to F is still rather high (ΔG0 = 35.3
kcal/mol), in keeping with the short experimental Ag–C bond
length: 203.61(19) pm. In contrast, dissociation of any of the
mutually trans CF3 groups is substantially less endergonic
(ΔG0 = 24.8 kcal/mol), which nicely correlates with the
appreciably longer Ag–C bonds found in this case (209.8(2) pm
av.). Dissociation of a second CF3• from the resulting open-shell,
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T-shaped [(CF3)2AgF]− transient species should be energetically
inexpensive (ΔG0 = 1.1 kcal/mol). Thus, following our
calculations, we conclude that this two-step radical process
should be the preferred decomposition path.
by gentle warming (40 °C). Trifluoromethylation of thiols (or
thiolate anions) can be accomplished by various existing
procedures, which include the use of CF3I[32] or convenient
surrogates thereof, CF3I·D,[33] as well as different electrophilic
trifluoromethylating reagents.[34] It is worth noting, however, that
with our system even the 'deactivated' thiol C6F5SH is efficiently
trifluoromethylated. Moreover, the experimental detection of the
intermediate Ag(III) thiolate compounds 3–6 is of certain interest.
To the best of our knowledge, the only compound bearing
Ag(III)–S bonds reported to date is the neutral complex
(CF3)2Ag(S2CNEt2), the stability of which is likely enhanced by
the κ2S-didentate ligand.[16b]
Scheme 3. Most relevant unimolecular fragmentation processes in
[(CF3)3AgF]− with calculated ΔG0 (ΔG⧧) values (kcal/mol). Dissociation of F• at
the first stage is less favored. There is no experimental evidence for CF2
extrusion. An unabridged mapping is given in Scheme S1.
The unimolecular fragmentation of the anionic complex
[(CF3)3AgF]− has been experimentally determined by multistage
mass spectrometry (MSn) under collision-induced dissociation
(CID) conditions. A very simple pattern is observed (Figure S8)
consisting exclusively of the mixed organosilver(I) fluoride
complex [CF3AgF]−. This pattern confirms our proposed
dissociation based on the energy profiles calculated for the
different decomposition paths (Scheme 3). It further
demonstrates that the Ag–F bond in compound 2 has marked
thermodynamic stability, at least in comparison with the
accompanying Ag–C bonds.[29,30]
Scheme 4. Solvolysis of 2 with thiols, RSH, and evolution of the resulting
thiolato complexes (3–6) into the corresponding trifluoromethyl thioethers
RSCF3 under the conditions indicated (full details given in the Experimental
Section). The cation is [PPh4]+ in every case.
In summary, an unprecedented organosilver(III) fluoride complex,
[PPh4][(CF3)3AgF] (2), has been prepared and isolated. This
compound lends experimental support to several previous
proposals on the existence of this interesting kind of chemical
species operating in silver-mediated fluorination processes.[1,5]
The Ag–F bond in 2 is strong, yet reactive. Although significantly
elongated (Figure 1), it withstands harsh CID conditions in the
gas phase, which gives proof of its strength. On the other hand,
it suffers solvolysis with thiols, RSH, giving rise to the
corresponding thiolato complexes [(CF3)3AgSR]−, which readily
eliminate the corresponding RSCF3 thioethers. Further
experiments aiming to exploit the nucleophilic character of this
terminal fluoride ligand are in progress.
In the solid state, compound 2 is appreciably less stable (dec.
145 °C; Figure S14) than the homoleptic compound
[PPh4][(CF3)4Ag] (dec. 188 °C)[16a] and much less so than its gold
homologue [PPh4][(CF3)3AuF] (mp 180 °C, dec. 267 °C).[21] Upon
thermolysis of 2 in the solid state, no sign of CF4 or C2F6 was
observed, which is against a concerted mechanism. Instead,
formation of CF3H together with trifluoromethylation of the
•
[PPh4]+ cation (Figure S15) are in keeping with CF3 generation,
thus favouring the radical path.[31] Furthermore, predominant
formation of the homoleptic [(CF3)4Ag]− derivative (Figure S15)
gives unequivocal proof for intermolecular processes operating
in the condensed phase, which are necessarily absent in the gas
phase.
Acknowledgements
This work was supported by the Spanish MICIU/FEDER (Project
PGC2018-094749-B-I00) and the Gobierno de Aragón (Group
The structural evidence for substantial nucleophilicity at the
terminal fluoride ligand in 2 together with the ease with which it
suffers hydrolysis (Figures S5 and S6) prompted us to explore
its reactivity towards thiols, RSH. Representative R substituents
were selected (Scheme 4) and, in all cases, reaction takes place
under formation of the corresponding thiolato complexes
[(CF3)3AgSR]− (3–6). These compounds, which were detected,
but not isolated (see Supporting Information), consistently
undergo elimination of the corresponding trifluoromethyl
thioethers RS–CF3 in good-to-excellent yields and under very
mild conditions. Elimination proceeds readily at room
temperature with the alkyl groups n-decyl and benzyl; in the
case of the aryl groups C6H5 and C6F5, it was greatly accelerated
E17_R17). The Instituto de Biocomputación
Sistemas Complejos (BIFI) and the
y
Física de
Centro de
Supercomputación de Galicia (CESGA) are acknowledged for
allocation of computational resources. D. J.-S. also thanks the
Spanish MICIU for a grant (BES-2016-078732).
Keywords: fluorides • high oxidation state • organosilver(III) •
thiols • trifluoromethylation
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[3]
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Ref. [30]). Thus, every Ag–C bond in compound 2 is broken with
preference over the Ag–F bond.
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10.1002/chem.201905771
Chemistry - A European Journal
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D. Joven-Sancho, M. Baya, A. Martín, J.
Orduna, B. Menjón*
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The First Organosilver(III) Fluoride,
[PPh4][(CF3)3AgF]
Strong yet reactive: The Ag–F bond in the unprecedented organosilver(III) fluoride
complex [(CF3)3AgF]− readily undergoes solvolysis with thiols giving rise to thiolato
complexes, which themselves furnish RSCF3 molecules under very mild conditions.
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