Mechanism of the catalytic hydrodefluorination of pentafluoropyridine by group six triangular cluster hydrides containing phosphines: A combined experimental and theoretical study

Article abstract of DOI:10.1021/om1009878

The catalytic hydrodefluorination (HDF) of pentafluoropyridine in the presence of arylsilanes is catalyzed by the tungsten and molybdenum(IV) cluster hydrides of formula [M₃S₄H₃(dmpe) ₃]⁺, W-1⁺ for M = W and Mo-1⁺ for M = Mo (dmpe = 1,2-(bis)dimethylphosphinoethane). The reaction proceeds regioselectively at the 4-position under microwave radiation to yield the 2,3,5,6-tetrafluoropyridine. Catalytic activity is higher for the tungsten complexes with turnover numbers close to 100, while reactions catalyzed by molybdenum compounds are faster. A mechanism for the HDF reaction has been proposed that explains these differences based on DFT calculations. The mechanism involves partial decoordination of the diphosphine ligand that generates an empty position in the metal coordination sphere. This position together with its neighbor M-H site are used to activate the C-F bond of the pentafluoropyridine through a M-H/C-F σ-bond metathesis mechanism involving a four-center transition state to give 2,3,5,6-tetrafluoropyridine. Subsequent coordination of the dangling diphosphine affords the para-substituted product and the [M₃S₄F₃(dmpe) ₃]⁺, W-2⁺ for M = W and Mo-2⁺ for M = Mo, cluster fluoride. The structure of W-2⁺ has been determined by single-crystal X-ray diffraction experiments. In the presence of silanes the calculated mechanism for the cluster hydride regeneration also implies three steps: (i) partial decoordination of the diphosphine, (ii) M-F/Si-H σ-bond metathesis, and (iii) coordination of the dangling diphosphine, to afford the cluster hydride.

Full text of DOI:10.1021/om1009878

290 Organometallics 2011, 30, 290–297
DOI: 10.1021/om1009878
Mechanism of the Catalytic Hydrodefluorination of Pentafluoropyridine
by Group Six Triangular Cluster Hydrides Containing Phosphines: A
Combined Experimental and Theoretical Study
†
Tomas F. Beltran, Marta Feliz, Rosa Llusar,* Jose A. Mata, and Vicent S. Safont.
†
,†
‡
†
ꢀ
ꢀ
†
´
Departament de Quımica Fısica i Analıtica, Universitat Jaume I, Avenida Sos Baynat s/n, 12071 Castello,
Spain, and Departament de Quımica Inorganica i Organica, Universitat Jaume I, Avenida Sos Baynat s/n,
´
´
ꢀ
‡
´
ꢁ
ꢁ
12071 Castelloꢀ, Spain
Received October 15, 2010
The catalytic hydrodefluorination (HDF) of pentafluoropyridine in the presence of arylsilanes is
catalyzed by the tungsten and molybdenum(IV) cluster hydrides of formula [M3S4H3(dmpe)3]þ, W-1þ
for M = W and Mo-1þ for M = Mo (dmpe = 1,2-(bis)dimethylphosphinoethane). The reaction
proceeds regioselectively at the 4-position under microwave radiation to yield the 2,3,5,6-tetrafluor-
opyridine. Catalytic activity is higher for the tungsten complexes with turnover numbers close to 100,
while reactions catalyzed by molybdenum compounds are faster. A mechanism for the HDF reaction
has been proposed that explains these differences based on DFT calculations. The mechanism
involves partial decoordination of the diphosphine ligand that generates an empty position in the
metal coordination sphere. This position together with its neighbor M-H site are used to activate the
C-F bond of the pentafluoropyridine through a M-H/C-F σ-bond metathesis mechanism
involving a four-center transition state to give 2,3,5,6-tetrafluoropyridine. Subsequent coordination
of the dangling diphosphine affords the para-substituted product and the [M3S4F3(dmpe)3]þ, W-2þ
for M = W and Mo-2þ for M = Mo, cluster fluoride. The structure of W-2þ has been determined by
single-crystal X-ray diffraction experiments. In the presence of silanes the calculated mechanism for
the cluster hydride regeneration also implies three steps: (i) partial decoordination of the dipho-
sphine, (ii) M-F/Si-H σ-bond metathesis, and (iii) coordination of the dangling diphosphine, to
afford the cluster hydride.
Introduction
and chemo- and regioselectivity.8 In this regard it is of funda-
mental interest to gain a deeper understanding of the mecha-
nism of the catalytic C-F bond transformation in order to
design new transition metal complexes with a higher selec-
tivity and discover unusual derivatization reactions.
In the case of fluoroaromatic and fluoroheteroaromatic
compounds, group 10 metals have proved to be efficient in
activating C-F bonds, for which the dominant mechanism
involves oxidative addition of this bond to generate metal
fluorine and metal carbon bonds.9-11 Divergent behaviors
are observed regarding the preference for C-F cleavage of
pentafluoropyridine, and different reaction mechanisms have
been proposed to account for differences in regioselectivity. For
example, recent studies demonstrated that phosphino lig-
ands in nickel complexes can participate as fluorine atom
acceptors, providing alternative phosphine-assisted C-F
activation pathways through a four-centered transition state,
where the substitution at the 2-position of fluorinated pyridines
Organofluorine compounds find widespread applications
in pharmaceutical, agrochemical materials, and other indus-
tries.1-3 High electronegativity, low polarizability, and the
small covalent radius of the fluorine atom together with the
great strength of the C-F bond make difficult the chemical
modification of organic fluoro compounds, rendering cata-
lytic selective functionalization of carbon-fluorine bonds a
major challenge in synthetic chemistry.4-6 The majority of
the corresponding catalysts are based on late transition
metals, although in recent years they are competing with
early transition metal complexes and Lewis-acidic main-
group species.7 The nature of the metal has a significant
effect on the activation of C-F bonds with regard to activity
*Corresponding author. Fax: þ34 964 728066. Tel: þ34 964 728086.
E-mail: Rosa.Llusar@qfa.uji.es.
(1) Smart, B. E. J. Fluorine Chem. 2001, 109, 3–11.
(2) Muller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881–1886.
(3) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc.
Rev. 2008, 37, 320–330.
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2001–2003.
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R. H., Mingos, D. M. P., Eds.; Elsevier: 2007; Vol. 1, pp 725-758.
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r
pubs.acs.org/Organometallics
Published on Web 12/21/2010
2010 American Chemical Society

Article
Organometallics, Vol. 30, No. 2, 2011 291
is favored.12 As previously mentioned, activation of C-F
bonds is not restricted to electron-rich metals; to a lesser
extent, electron-poor metal compounds such as zirconocenes
also exhibit good performances for the hydrodefluorination
(HDF) of pentafluoropyridine. Rosenthal and co-workers
reported that zirconocene hydrides exhibit a significantly
better performance for the hydrodefluorination of penta-
fluoropyridine than rhodium hydrido complexes, for which,
in addition, high hydrogen pressures are required.13 Lentz et
al. have also shown that, in the presence of silanes, titanocene
dihalides can catalytically defluorinate fluoroalkenes, lead-
ing to less fluorinated compounds through a mechanism that
implies the formation of the titanocene hydride and further
olefin insertion followed by β-fluoride elimination.14 All these
pieces of evidence point to the potential of electronically poor
transition metal complexes in the catalytic activation of C-F
bonds.
Metal hydrides play a crucial role in the catalytic HDF of
organic fluorides promoted by early transition metals, where
activation takes place at the hydride site to yield fluoride
complexes.15 In the catalytic cycle, the fluoride ligand is
replaced by a hydride ligand upon reaction with silanes,
e.g., in Holland’s catalytic cycle, shown in Schme 1, for
the HDF of perfluoroaromatics and perfluoroalkenes
promoted by β-dikeminate iron(II) fluoride species.16
The driving force for the reaction is the formation of a
strong Si-F bond. Surprisingly, the iron hydride com-
plexes obtained by reaction of iron fluoride with silanes
do not show evidence for direct C-F activation in the
absence of silane, preventing the complete elucidation of
the reaction mechanism.
Scheme 1. Holland’s HDF Catalytic Cycle
chalcogenides. The isolation of M3S4 phosphino fluoride com-
plexes combined with a complete DFT investigation of the
potential reaction pathways has allowed us to propose a
hydrodefluorination mechanism that explains the differ-
ences in activity between molybdenum and tungsten. To
our knowledge, thisstudyconstitutesthefirstreportoncatalytic
organofluoride HDF by group 6 transition metal cluster com-
plexes. Examples of C-F bond activation by group 6
transition metal complexes are scarce. Nevertheless, the first
organometallic system for activating C-F bonds at room
temperature was based on a tungsten(0) carbonyl complex
that reacts through an intramolecular chelate-assisted
oxidative addition of a fluorinated aromatic ligand.23,24
Richmond et al. successfully extended this chemistry to
include reactions at molybdenum(0).25 C-F activation of
fluoroarenes has also been observed by molybdenum and
tungsten cyclopentadienyl complexes.26 All these exam-
ples on group 6 metals refer to electronically rich com-
plexes, in contrast with the high-valent character of the
molybdenum and tungsten cluster hydrides reported in the
present work.
In the past years we have undertaken a full mechanistic
study on the reaction of trinuclear cluster hydrides of formula
[M3Q4H3(diphosphine)3]þ (Q = S, Se; M = Mo, W) with
acids.17-21 These studies include aspects such as solvent
effects, ion pairing, phosphine basicity, and chalcogen and
metal substitution. In the course of this work we found
evidence that these trinuclear metal hydrides react with
fluorinated inorganic compounds activating B-F bonds
and potentially C-F bonds.22 We now report that HDF
of pentafluoropyridine can be carried out catalytically
using incomplete cuboidal group 6 transition metal cluster
Results and Discussion
Synthesis and Structure of Fluorinated [M3S4F3(dmpe)3]þ
(M = Mo, W) Cluster Cations. The incomplete cuboidal
M3(μ3-S)(μ-S)3 group 6 hydrides of formula [M3S4H3-
(dmpe)3]þ, which we refer to as M-1þ, react with HX (X =
Cl and Br) under mild conditions in a variety of solvents,
which results in a formal substitution of the hydride by the
coordinating chloride or bromide ligands. In this reaction
protons play an important role in the process, and mecha-
nistic studies support the formation of dihydrogen species
as intermediates or transition states.19,22 Reactions with
noncoordinating acids such as HBF4 were investigated
and allowed us to identify the solvated [M3S4(solv)3-
(dmpe)3]4þ species as intermediates. However, the identifica-
tion of these intermediates was somehow hindered, as there
was a slow formation of the fluoride substitution products
[M3S4H3-xFx(dmpe)3]þ obtained by abstraction of fluoride
from the tetrafluoroborate anion. Fluoride clusters are
scarce compared with their chloride and bromide analogues,
the only example ofathiofluoridegroup6trinuclearclusterbeing
the [Mo3S4F7(FHF)2]5- anion obtained as a potassium salt.27
(12) Nova, A.; Reinhold, M.; Perutz, R. N.; Macgregor, S. A.;
McGrady, J. E. Organometallics 2010, 29, 1824–1831.
(13) Jager-Fiedler,U.;Klahn,M.;Arndt,P.;Baumann,W.;Spannenberg,
A.; Burlakov, V. V.; Rosenthal, U. J. Mol. Catal. A: Chem. 2007, 261,
184–189.
(14) Kuhnel, M. F.; Lentz, D. Angew. Chem., Int. Ed. 2010, 49, 2933–
2936.
(15) Braun, T.; Noveski, D.; Ahijado, M.; Wehmeier, F. Dalton
Trans. 2007, 3820–3825.
(16) Vela, J.; Smith, J. M.; Yu, Y.; Ketterer, N. A.; Flaschenriem,
C. J.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2005, 127,
7857–7870.
(17) Basallote, M. G.; Estevan, F.; Feliz, M.; Fernandez-Trujillo,
M. J.; Hoyos, D. A.; Llusar, R.; Uriel, S.; Vicent, C. Dalton Trans. 2004,
530–536.
(18) Algarra, A. G.; Basallote, M. G.; Feliz, M.; Fernandez-Trujillo,
M. J.; Guillamon, E.; Llusar, R.; Vicent, C. Inorg. Chem. 2006, 45, 5576–
5584.
(19) Algarra, A. G.; Basallote, M. G.; Feliz, M.; Fernandez-Trujillo,
M. J.; Llusar, R.; Safont, V. S. Chem. Eur. J. 2006, 12, 1413–1426.
(20) Algarra, A. S. G.; Basallote, M. G.; Fernandez-Trujillo, M. J.;
Llusar, R.; Safont, V. S.; Vicent, C. Inorg. Chem. 2006, 45, 5774–5784.
(21) Algarra, A. G.; Basallote, M. G.; Fernandez-Trujillo, M. J.;
Feliz, M.; Guillamon, E.; Llusar, R.; Sorribes, I.; Vicent, C. Inorg. Chem.
2010, 49, 5935–5942.
(23) Richmond, T. G.; Osterberg, C. E.; Arif, A. M. J. Am. Chem.
Soc. 1987, 109, 8091–8092.
(24) Asplund, M. C.; Johnson, A. M.; Jakeman, J. A. J. Phys. Chem.
B 2006, 110, 20–24.
(25) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. Rev.
1994, 94, 373–431.
(26) Chernega, A. N.; Graham, A. J.; Green, M. L. H.; Haggitt, J.;
Lloyd, J.; Mehnert, C. P.; Metzler, N.; Souter, J. J. Chem. Soc., Dalton
Trans. 1997, 2293–2303.
(22) Basallote, M. G.; Feliz, M.; Fernandez-Trujillo, M. J.; Llusar,
(27) Mironov, Y. V.; Yarovoi, S. S.; Solodovnikov, S. F.; Fedorov,
R.; Safont, V. S.; Uriel, S. Chem. Eur. J. 2004, 10, 1463–1471.
V. E. J. Mol. Struct. 2003, 656, 195–199.

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292 Organometallics, Vol. 30, No. 2, 2011
Figure 1. ORTEP representation of the W-2þ cation.
Table 1. Selected Averaged Bond Distances for Compounds
[W3S4H3(dmpe)3]BPh4 ([W-1]BPh4) and [W3S4F3(dmpe)3]BPh4
([W-2]BPh4)
a
b
˚
distance (A)
[W-1]BPh4
[W-2]BPh4
W-W
2.751[4]
2.354[2]
2.329[6]
2.341[4]
2.476[9]
2.516[5]
2.750[12]
2.367[6]
2.343[5]
2.306[3]
2.517[9]
2.558[2]
W-(μ3-S)
W-μ-S)c
W-(μ-S)d
W-P(1)e
W-P(2)f
Figure 2. ESI-MS monitoring of the reaction between W-1þ
and pentafluoropyridine: (a) Initial time, (b) after 1 h under MW
radiation, (c) after 2 h.
a Standard deviations for averaged values are given in brackets.
b Data taken from ref 33. c Distance trans to the Mo-P bond. d Distance
trans to the W-X bond (X = H or F). e Distance trans to the W-(μ3-S)
bond. f Distance trans to the W-(μ-S) bond.
than the average W-(μ-S) bond lengths, and there are two
kinds of W-(μ-S) distances. The substitution of a hydride
ligand in the [W3S4H3(dmpe)3]þ starting material by fluorine
is reflected in the W-(μ-S) distance trans to that position,
˚
which decreases by approximately 0.034 A in the fluoride
The lack of solubility of this salt in all common solvents
has prevented the derivatization of this anion with other
ligands, i.e., diphosphines. However, complexes of formula
[M3S4F3(dmpe)3]þ, which we refer to as M-2þ, can be conve-
niently prepared by reacting their hydride precursors with
fluoride-containing anions in the presence of acids. Achieving
the reaction requires excess of the fluorinated anion and the
presence of protons. Quantitative transformation of [M3S4H3-
(dmpe)3]þ to the fluoride-substituted product is achieved by
reaction with aqueous solutions of HPF6 in acetonitrile/water
mixtures according to eq 1.
cluster due to the lower trans influence of the fþluoride versus
˚
the hydride ligand. The W-F distances in W-2 of 2.001[6] A
are slightly shorter than those found for the [Mo3S4F7-
(FHF)2]5- anion (2.043-2.140 A).27 The specific coordina-
˚
tion of the diphosphine ligand in W-2þ, with one phosphorus
atom trans to the capping sufur atom and the other trans to
the bridging sulfur atom, results in incomplete cubane-type
sulfido clusters with backbone chirality. These complexes
show two signals in the 31P NMR spectra that correspond to
the two sets of phosphorus atoms located above and below
the metal plane defined by the three Mo or W atoms.
Stoichiometric and Catalytic HDF of Pentafluoropyridine
with [M3S4H3(dmpe)3]þ (M = Mo, W). Reactivity of the
molybdenum and tungsten triangular cluster hydrides of
formula [M3S4H3(dmpe)3]þ with fluorinated compounds is
not restricted to acidic media, as under more severe condi-
tions these metal hydrides react with an excess of pentafluor-
opyridine upon microwave (MW) irradiation at 180 °C to
afford the fluoride-substituted cluster complex according to eq 2.
½M3S4H3ðdmpeÞ ꢀþ
3
M-1þ
þ HPF6ðexcessÞ f ½M3S4F3ðdmpeÞ ꢀþ
ð1Þ
3
M-2þ
The structure of the fluoride [W3S4F3(dmpe)3]þ (W-2þ)
complex has been determined by single-crystal X-ray diffrac-
tion methods. An ORTEP drawing of the W-2þ cation is
represented in Figure 1.
The metal and sulfur atoms in W-2þ occupy adjacent
vertices in a cube with a vacant metal position, which results
in an incomplete cubane-type structure. The three metal atoms
define an approximately equilateral triangle with W-W bond
˚
distances of 2.750 [12] A, in agreement with the presence of a
single metal-metal bond and a þ4 oxidation state for the
metal. Table 1 contains a list of important bond distances
and angles. The metal-metal and metal-sulfur distances
within the M3S4 cluster core follow the trends observed for
other trinuclear M3Q4 (M = Mo, W; Q = S, Se) species. The
The reaction was followed by ESI mass spectrometry.
Sequential substitution of hydride by fluoride ligands for
W-1þ is observed by the appearance of peaks at m/z 1151
[W3S4H2F(dmpe)3]þ, 1169 [W3S4HF2(dmpe)3]þ, and 1187
[W3S4F3(dmpe)3]þ for the tungsten cluster during the
W-(μ3-S) distance in W-2þ is approximately 0.02 A longer
˚

Article
Organometallics, Vol. 30, No. 2, 2011 293
Table 2. Catalytic Hydrodefluorination of Pentafluoropyridinea
hydride clusters present a higher activity than their fluori-
nated analogues (entries 2, 4, 6, and 7). The HDF process is
better achieved with the use of dimethyl phenyl silane than
diphenyl silane. The catalytic role of the metal cluster is
evidenced by the absence of direct reaction between the fluo-
rinated pyridine and the silane. The reaction is highly selective,
and only the 2,3,5,6-tetrafluoropyridine is detected. Degrada-
tion of the catalysts was not observed even at the end of the
process. The fact that catalytic HDF conversion is observed
for both the hydride or the fluoride cluster complexes gives
full support to the catalytic cycle postulated by Holland
(Scheme 1) for low-coordinate iron(II) complexes that in the
case of the iron system could not be fully elucidated due to
the lack of C-F bond activation of the iron hydride in the
absence of silanes.16 In our case we have been unable to
detect by ESI-MS and NMR techniques the formation of
M-1þ hydrides in appreciable amounts from the reaction
between M-2þ fluorides and silanes, and the only evidence
that reaction occurs comes from the detection of FSiMe2Ph
as reaction product by 19F NMR. We believe that the
inefficient production of hydrides by reaction of the corre-
sponding fluorides with silanes may be the reason for the lower
catalytic activities found for the fluoride clusters in front of their
hydride counterparts.
reaction
time (h)
yield
(%)b
entry
catalyst
silane
TONc
1
2
3
4
5
6
7
3
3
16
3
12
2
2
HSiMe2Ph
HSiMe2Ph
Ph2SiH2
HSiMe2Ph
Ph2SiH2
0
90
29
31
25
60
51
0
90
29
31
25
60
51
W-1þ
W-1þ
W-2þ
W-2þ
Mo-1þ
Mo-2þ
HSiMe2Ph
HSiMe2Ph
a Reaction conditions: 1% cat., 0.38 mmol of fluoroarene, 1.9 mmol
of silane, 1 mL of CH3CN. b Yield based on 19F NMR using fluoro-
benzene as internal standard. c TON (mol of HDF product/mol of cat.).
ESI-MS reaction monitoring, illustrated in Figure 2. Com-
plete transformation of the cluster hydride into the fluoride is
achieved within two hours for Mo and three hours for W. The
same reaction monitoring was observed by 1Hand19FNMR. In
the case of the 31P NMR, overlap of the resonances for the
different species makes this technique inappropriate to fol-
low the reaction. The W-1þ hydride 1H NMR signal appears
as a doublet of doublets centered at -0.80 ppm. After 1 h of
reaction, this signal disappears as two new sets of signals appear
downfied centered at -0.48 and -0.08 ppm due to the di- and
monohydride species, respectively. The 19F resonance of the
W-2þ trifluoride is also clearly observed after one hour of
reaction together with a complicated pattern of signals asso-
ciated with the mono- and bisubstituted fluoride derivatives.
At the end of the reaction (3 h for W), the only observed
signal is that of W-2þ at -199.7 ppm. A similar situation
is found for the conversion of the Mo-1þ hydride into the
Mo-2þ fluoride.
For both metal complexes, molybdenum and tungsten, the
reaction requires energetic conditions (ca. 180 °C) in spite of
its exothermicity. Hence, the process is expected to take place
through a transition structure with a high activation barrier.
DFT calculations have been conducted modeling the HDF
process on one metallic site, and the results obtained for
activation and reaction energies (ca. 40 and ca. -45 kcal/mol,
respectively) nicely agree with the experiment; see below.
The resulting organofluoride product was identified as
the 2,3,5,6-tetrafluoropyridine by GC and confirmed by
19F NMR by the appearance of two signals at δ = -140.1
and -92.2 ppm, which correspond to the two sets of fluorine
atoms characteristic of 2,3,5,6-tetrafluoropyridine.
Mechanistic Studies
A theoretical study of the mechanism of hydrodefluorina-
tion of pentafluoropyridine at the 4-position promoted by
group 6 trinuclear cluster hydrides containing phosphines
(see eq 3) and the cluster regeneration reaction by silane
HSiMe2Ph (see eq 4) has been conducted by using DFT pro-
cedures. The mechanistic study was carried out atþa singlþe
M-H site of the model cluster catalyst M3S4H3(PH3)6 , M0-1
(W0-1þ for M = W and Mo0-1þ for M = Mo), in the gas phase.
We use the prime symbol to designate the model compound used
þ
in our computational studies. In the case of M3S4H2F(PH3)6
,
þ
0
we name this model complex Msc -2 to emphasize the fact that
calculations are carried out on a single metal center. This
approach has been successfully employed in other theoretical
þ
studies involving reactions with acids of the W3S4H3(PH3)6
(W0-1þ) cluster hydride19,20 or the related hetobimetallic
W3S4PdH3(PH3)6(CO)þ complex.28
Catalytic Hydrodefluorination. The fluoride complex W-2þ
can be potentially regenerated to the corresponding hydride
in the presence of silanes, as seen for other transition metal
halides, turning the C-F activation process, represented in
eq 2, catalytic. To explore this possibility, the HDF of penta-
fluoropyridine was carried out in the presence of dimethyl
phenyl silane under microwave radiation using different hydride
and fluoride cluster catalysts. The results are summarized in
Table 2. The reaction was followed by GC and 19F NMR and
enabled us to confirm the formation of the hydrodefluori-
nated pyridine at the 4-position and the subsequent forma-
tion of the fluorinated silane coproduct. The two products
were well characterized by GC and 19F NMR by comparison
with pure commercial standards, δ=-140.1 and -92.2 ppm
for 2,3,5,6-tetrafluoropyridine and δ = -159.5 ppm for
Me2PhSiF. All clusters show some activity under the reaction
conditions used (MeCN, MW radiation, and 1% cat. loading).
Complex W-1þ proved to be the most efficient catalyst,
giving 90% yield and 90 turnovers (entry 2). In general, the
Considering the HDF reaction (eq 3), a single-step mecha-
nism involving a direct fluorine/hydrogen exchange was first
studied. However, all attempts made to find the correspond-
ing transition state (TS) led to the dissociation of the phosphine
ligand trans to μ-S. In this way, a coordinatively unsaturated
intermediate, [M3S4H3(PH3)5]þ, M-1Iþ, characteristic of a
(28) Algarra, A. G.; Basallote, M. G.; Feliz, M.; Fernandez-Trujillo,
M. J.; Llusar, R.; Safont, V. S. Chem. Eur. J. 2010, 16, 1613–1623.

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Beltran et al.
294 Organometallics, Vol. 30, No. 2, 2011
Scheme 2
mononuclear boryl phosphino rhodium complexes.32 This
transition state involves direct fluorine transfer onto the
boron center and is stabilized by short Rh N contacts.
3 3 3
The energies of M0-1TSþ lie at 41.4 and 38.6 kcal mol-1
above the reactants for W and Mo, respectively. These values
are the maxima of both reaction profiles, stressing the rate-
limiting-step nature of the HDF process. Although there is
not a significant energetic difference (1.1 kcal mol-1) between
the activation barriers (24.8 kcal mol-1 for W0-1TSþ and 23.þ7
kcal mol-1 for Mo0-1TSþ), the higher stability of Mo0-1TS
over W0-1TSþ (2.8 kcal mol-1) explains the faster conversion
observed experimentally when using the molybdenum cluster as
catalyst. The 2,3,5,6-tetrafluoropyridine product andþthe coor-
dinatively unsaturated reaction intermediates, M0-2I , are ob-
tained along the reaction coordinate as thermodynamically
favorable processes, with an energy releaseþof 76.8 kcal mol-1 for
W0-2Iþ and 72.5 kcal mol-1 for Mo0-2I . It is interesting to
point out that formation of W0-2Iþ is slightly more favorable
(1.5 kcal mol-1) than Mo0-2Iþ, pointing out that there is an
inversion of relþative stabilities between the Mo and W inter-
mediates M0-2I and their corresponding M0-1TSþ transition
structures.
In tþhe third reaction step, coordination of phosphine to
M0-2I gives [M3S4H2F(PH3)6]þ (which we name Msc0-2þ,
stressing the fact that a single metallic center is considered in
our model) as the most stable fluorinated product. This stabi-
lization is also confirmed by the isolation of various salts of
the [M3S4F3(dmpe)3]þþ(M-2þ) cations. In addition, higher
stabilization of Wsc0-2 over Mosc0-2þ by ca. 4 kcal mol-1
would explain the highest yields obtained by using the
tungsten cluster as catalyst.
dissociative mechanism, has been found. The dissociatioþn
step going from the reactants (R) to the intermediate M0-1I
is endothermic (16.7 and 14.9 kcal mol-1 for M = W and
Mo, respectively); therefore, partial decoordination of the
diphosphine in M3S4H3(dmpe)3þ would be energetically acces-
sible by microwave heating, in good agreement with the experi-
ment. Previous calculations at the QM/MM level on the
decoordination process of one of the phosphorus atoms of
the diphosphine ligand in the P-[Mo3S4Cl3(dppe)3]þ (dppe =
diphenylphosphinoethane) stereoisomer reported an activation
energy of 24.4 kcal mol .
-1 29 As in the present case, the calculated
energy demand was used to explain the cluster racemization
observed in refluxing acetonitrile through a mechanism that
implies the formation of a metal unsaturated intermediate
generated upon phosphine decoordination. Our analysis of
the HDF reaction (eq 3) is then better described as a three-
step process (red lines in Scheme 2) in which the first and third
step correspond to the decoordination and further coordination
of one phosphine ligand in the [M3S4H3(PH3)6]þ model complex.
In a secondþstep of the HDF reaction (eq 3), the inter-
mediate M0-1I reacts with one equivalent of pentafluoro-
pyridine between M-H and C-F bonds through a σ-bonþd
metathesis mechanism, where the transition state M0-1TS
can be described as a four-center structure that brings together
the fluorinated compound wiþth the meþtal-vacant site, to finally
afford the M3S4FH2(PH3)5 (M0-2I ) cluster intermediate.
This is the first example of a σ-bond metathesis mechanism
for a F/H exchange reaction promoted by group 6 metal
complexes and one of the few examples of this mechanism
reported in the literature for C-F activation.30 Recently,
Whittlesey et al. have proposed a σ-bond metathesis transi-
tion state for the C-F aromatic fluorocarbon activation by a
coordinatively unsaturated N-heterocyclic carbene Ru(II)
complex, generated by facile dissociation of PPh3.31 A four-
membered transition state has also been calculated in the
case of the boryl-assisted mechanism for the activation of
the C-F bond in pentafluoropyridine at the 2-position by
An inspection of the reactive site of M0-1Iþ shows that
hydride ligand lies in the plane defined by one phosphorus
atom of the phosphine ligand, the metal, and the tricapped
sulfur atom as shown in Figure 3 for W-1Iþ. The M3 subunit
is slightly distorted, showing shortþer intermetallic distancþes
˚
˚
than its precursor (2.772 A for W-1 and 2.755 A for Mo-1 )
˚
with deviations less than 0.05 A.
A closer analysis of the M0-1TSþ transition structure
allows us to identify a four-center plane defined by the metal,
one hydrogen, one fluorine, and one of the carbon atoms of
the fluorinated organic substrate; the plane is marked in
dashed lines in Figure 3. The fluorine and hydrogen atoms
occupy trans positions to the bridging sulfur atoms, and the
aromatic ring lies perpendicular to the four-center plane defined
above. Ananalysisof the interatomic distancesshows a M
þ
H
3 3 3
0
˚
bond elongation of ca. 0.2 A for M -1TS with regard to the
M0-1Iþ intermediate. The M H and M F distances are
3 3 3
3 3 3
˚
characteristic of a transient structure (1.96 and 2.36 A,
respectively). The C F distance of the organic moiety is
3 3 3
˚
elongated by ca. 0.2 A in relation to that of the pentafluor-
opyridine, and the C-C bonds close to the reactive carbon
˚
experience a slight elongation (ca. 0.04 A), in agreement with
the loss of the sp2 character of the reactive carbon atom.
Finally the structure of theþM0-2Iþ intermediate shows close
similarities to that of M0-1I , as shown in Figure 3. Scheme 3
summarizes the catalytic cycle at the same time that empha-
sizes the structure of the transition states referred to a single
metal center.
(29) Andres, J.; Feliz, M.; Fraxedas, J.; Hernandez, V.; Lopez-Navarrete,
J. T.; Llusar, R.; Sauthier, G.; Sensato, F. R.; Silvi, B.; Bo, C.; Campanera,
J. M. Inorg. Chem. 2007, 46, 2159–2166.
(30) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,
2578–2592.
(31) Reade, S. P.; Mahon, M. F.; Whittlesey, M. K. J. Am. Chem.
Our experimental evidence suggests that the cluster catalyst
can be regenerated by adding the silane HSiMe2Ph to the
Soc. 2009, 131, 1847–1861.
(32) Teltewskoi, M.; Panetier, J. A.; Macgregor, S. A.; Braun, T.
Angew. Chem., Int. Ed. 2010, 49, 3947-3951.
(33) Lide, D. R. Handbook of Chemistry and Physics, 80th ed.; CRC
Press LLC: Boca Raton, FL, 2003.

Article
Organometallics, Vol. 30, No. 2, 2011 295
Figure 3. Optimized geometries and some relevant characteristic distances of W0-1Iþ (left), W0-1TSþ (center), and W0-2Iþ (right).
Scheme 3
reaction mixture. The computed regeneration reaction, referred
to a single metal center, converting the fluoride into the
hydride is represented in eq 4.
respectively. In the last reaction step, coordination of the
phosþphine to the unsaturated M-1Iþ cluster affords the
M0-1 cluster hydride, closing the catalytic cycle, with an energy
release of 16.6 and 14.9 kcal mol-1 for W and Mo, respectively.
Analysis of the geometry of the M0-2TSþ transition-state
structure shows that the organosilane interacts with the
M-H site, defining a four-center moiety with the π-orbitals
of the aryl group pointing toward the hydrogen atoms of one
phosphine ligand, as depicted for W0-2TSþ inþFigure 4.
The interatomic M F distance in M0-2TS shows bond
3 3 3
þ
0
˚
elongation of ca. 0.2 A with regard to the M -2I intermedi-
ate. The M H and M F bond lengths are characteristic
3 3 3
3 3 3
˚
of a transient structure (1.81 and 2.12 A, respectively). The
þ
Si H distance in M0-2TS is elongated by ca. 0.3 A in
relation to that of the dimethyl phenyl silane.
˚
3 3 3
According to our computational analysis, regeneration
takes place in three steps, highlighteþd in blue in Schemes
2 and 3. The monofluorinated Msc0-2 complex dissociates
one phosphine ligand and gives the unsaturated M0-2Iþ
The transition states, M0-1TSþ and M0-2TSþ, schemati-
cally represented in Scheme 3, are not reached through an
oxidative addition mechanism, on the other hand, very
unlikely for a d2 metal configuration; instead, decoordina-
tion of one phosphine opens an empty site in the metal
coordination sphere. This empty site next to the hydride or
fluoride positions allows the interaction with the pentafluor-
opyridine or the silane molecules activating the C-F or the
Si-H bonds, respectively.
intermediate with an energy increase of 11-13 kcþal mol-1
.
In the second step, the organosilane reacts with M0-2I through
a σ-bond metathesis mechanism, where the TS is associated to
a F/H exchange between the silane and the fluorinated
cluster, namely, M0-2TSþ, to give the unsaturated hydride
cluster [M3S4H3(PH3)5]þ (M0-1Iþ). The activation energy for
M0-2TSþ shows that F/H exchange is a kinetically favorable
process, with an energy barrier of ca. 5 kcal mol-1 for both
the molybdenum and tungsten profiles. The M0-2TSþ transi-
tion state renders the FSiMe2Ph product and the M0-1Iþ
intermediate in a thermodynamically favorable process, with
an energy release of 8.6 and 11.9 kcal mol-1 for W and Mo,
Conclusions
In summary, addition of silane HSiMe2Ph to the reactant
mixture, NC5F5 and catalytic amounts of the [M3S4H3-
(dmpe)3]þ (M = W, Mo) complex, produces exclusively

ꢀ
Beltran et al.
296 Organometallics, Vol. 30, No. 2, 2011
The solvent was partially removed (ca. 20 mL) under reduced
pressure, and the desired product was extracted with CH2Cl2
(30 mL) and washed with water (3 ꢁ 15 mL). The resulting violet
organic phase was dried over anhydrous MgSO4, filtered, and
dried under vacuum to afford [W3S4(dmpe)3F3](PF6) as an air-
-
stable violet solid (yield: 95 mg, 90%). Anion exchange, PF6
for SO3CF3-, was achieved by elution with a KSO3CF3 solution
in acetone after absorption of a CH2Cl2 solution of [W-2](PF6)
solution in a silica gel column.
31P{1H} NMR (CD3CN, 121 MHz): δ -143.1 (septet, 1P,
1J(31P,19F) = 706.5 Hz), 9.69 (d, 3P, J(31P,19F) = 101.9 Hz,
2
1J(31P,183W) = 96.8 Hz), 12.77 (d,3P, J(31P,19F) = 61.5 Hz,
2
1J(31P,183W) = 98.3 Hz. 19F NMR (CD3CN, 282 MHz):
δ -199.7 (dd, 3F, 2J(19F,31P) = 96.6 Hz, 2J(19F,31P) = 61.3 Hz), -
71.2 (d, 6F, 1J(19F,31P) = 707.7 Hz). IR (cm-1, KBr): 1417 (s),
1286 (s) 940 (s), 901 (s), 842 (s, P-F), 558 (s, P-F), 440 (w,
W-μ3S), 418 (w, W-μ3S). UV-vis (CH3CN): λ (T) 548 (71.4), 305
(350.1), 253 nm (588.2 M-1 cm-1). ESI-MS (CH3CN, 20 V): m/z
1187 [Mþ]. Anal. Calcd for W3F9S4P7C18H48: C, 16.23; H, 3.63.
Figure 4. Optimized geometry for W0-2TSþ.
the para-substituted NC5F4H product and FSiMe2Ph, at the
same time recovering the metal cluster, turning the process
catalytic. The reaction occurs through coordinatively unsa-
turated intermediates and a σ-bond metathesis mechanism
that results in four-center transitionstates,asshowninScheme3.
The reaction is highly exothermic, and the driving force of the
reaction can be associated with the lower bond disruption
enthalpy of the Si-H bond (ca. 72 kcal/mol) in comparison
with that of the Si-F bond (ca. 132 kcal/mol).33 The reaction
profile for tungsten is more energy demanding than for
molybdenum, which explains the faster conversion observed
experimentally when using the molybdenum cluster as cata-
lyst. The higher stability of the fluorinated Wsc0-2þ cluster in
Found: C, 16.06; H, 3.80.
-
Preparation of [Mo3S4(dmpe)3F3]X, [Mo-2]X (X = PF6
,
SO3CF3-). This compound was prepared following the proce-
dure described for [W-2](PF6) except that [Mo3S4H3(dmpe)3]-
(SO3CF3) (0.100 g, 0.098 mmol) and HPF6 (15 wt %, 1.60 mL,
1.84 mmol) were used as starting materials. The resulting green
solid was redissolved in an acetone/CH2Cl2 (2:1) mixture and
loaded in a silica gel colum. Two bands separated in the column
after elution with a KPF6 (10 mg/L) solution in acetone. The first
yellow band was rejected, and the second band (green) contain-
ing the desired product was collected and taken to dryness. The
green solid was redissolved in CH2Cl2, filtered, and dried under
vacuum to afford [Mo3S4(dmpe)3F3](PF6) as an air-stable green
-
solid (0.070 mg, 67%). Anion exchange, PF6 for SO3CF3-, was
þ
0
achieved by elution with a KSO3CF3 solution in a (2:1) acetone/
CH2Cl2 mixture after absorption of a (2:1) acetone/CH2Cl2
solution of [Mo-2](PF6) in a silica gel column.
contrast with their analogous Mosc -2 complex supports the
highest yields obtained by using the tungsten cluster as catalyst.
31P{1H} NMR (CD3CN, 121 MHz): δ -143.3 (septet, 1P,
1J(31P,19F) = 706.4 Hz), 24.30 (d, 3P, 2J(31P,19F) = 83.6 Hz, 35.63
Experimental Section
2
(d, 3P, J(31P,19F) = 49,8 Hz). 19F NMR (CD3CN, 282 MHz):
General Remarks. Electrospray mass spectra were recorded
with a Quattro LC (quadrupole-hexapole-quadrupole) mass
spectrometer with an orthogonal Z-spray electrospray interface
(Micromass, Manchester, UK). The cone voltage was set at 20 V
unless otherwise stated using CH3CN as the mobile phase solvent.
Nitrogen was employed as drying and nebulizing gas. Isotope
experimental patterns were compared with theoretical patterns
obtained using the MassLynx 4.0 program.34 1H, 19F, and 31P
NMR spectra were recorded on a Varian Innova 300 and
500 MHz, using CD3CN as solvents and referenced to TMS,
CFCl3, and 85% H3PO4. Electronic absorption spectra were
obtained on a Perkin-Elmer Lambda-19 spectrophotometer in
CH3CN. Elemental analyses were carried out on a EuroEA3000
Eurovector analyzer. Gas chromatography analyses were per-
formed onaAgilent7820AGCsystemequippedwithaFIDandan
Agilent capillary column (HP-5, 30 m ꢁ 0.32 mm ꢁ 0.25 μm).
MW-promoted reactions were carried out in a Discover Lab-
mate (CEM Corp.) apparatus. Solvents were dried and degassed
δ -198.2 (dd, 3F,2J(19F,31P) = 85.1 Hz, 2J(19F,31P) = 50.3 Hz), -
71.2 (d, 6F, 1J(19F,31P) = 707.7 Hz). IR (cm-1, KBr): 1417 (s),
1285 (s) 940 (s), 899 (s), 840 (s, P-F), 557 (s, P-F), 440 (w,
Mo-μ3S), 457 (w, Mo-μ3S). UV-vis (CH3CN): λ 610 (280.4),
382 (2261.7), 342 (2355.1), 305 nm (3439.3 M-1 cm-1). ESI-MS
(CH3CN, 20 V): m/z 923 [Mþ]. Anal. Calcd for Mo3F9S4P7C18H48:
C, 20.23; H, 4.53. Found: C, 20.09; H, 4.67.
HDF of Pentafluoropyridine with [M-1](SO3CF3) (M = Mo, W).
A 10 mL capped high-pressure vessel containing a stirrer
bar was charged with pentafluoropyridine (0.46 mmol),
anisole as internal reference (0.46 mmol), 10 mg of catalyst
(0.0077 mmol for [W-1](SO3CF3) and 0.0098 mmol for [Mo-1]-
(SO3CF3)), and 1 mL of acetonitrile. The solution was heated to
180 °C under MW irradiation for the appropriate time. Reaction
monitoring was achieved by ESI-MS spectrometry and 31P{1H} and
19F NMR for the inorganic species and GC chromatography and
19F NMR for the organic compounds. The inertness of the triflate
anion as fluorinating agent under the present HDF conditions was
confirmed by the lack of reaction between the anion and the
M-1þcations.
by standard methods before use.
Synthesis. Compounds [W3S4H3(dmpe)3]X (X = PF6
-
,
SO3CF3-, [W-1]X) and [Mo3S4(dmpe)3H3](SO3CF3), [Mo-1]-
(SO3CF3) were prepared by following literature procedures for
X-ray Data Collection and Structure Refinement. Suitable
crystals for X-ray studies of the tetraphenylborate salts of
W-2þ were grown by slow diffusion of diethyl ether into a sample
PF6- or BPh4- salt analogues.21,35
-
Preparation of [W3S4(dmpe)3F3]X, [W-2]X (X = PF6
,
SO3CF3-). A 15% wt aqueous solution of HPF6 (1.40 mL,
1.61 mmol) was added dropwise to a pink solution of [W3S4-
(dmpe)3H3]PF6 (100.6 mg, 0.079 mmol) in CH3CN/H2O (2:1,
30 mL), and the mixture was stirred at room temperature for 4 h.
-
solution in CH2Cl2. Replacement of the PF6 ion was carried
out by addition of an excess of Na(BPh4) to methanol solutions
of [W-2](PF6), resulting in precipitation of the desired tetra-
phenylborate salts of the trinuclear cation W-2þ. The crystals
are air-stable and were mounted on the tip of a glass fiber with
the use of epoxy cement. X-ray diffraction experiments were
carried out on a Bruker SMART CCD diffractometer using Mo
(34) MASSLYNX, 4.0 ed.; Waters Ltd.: Milford, MA, 2005.
(35) Cotton, F. A.; Llusar, R.; Eagle, C. T. J. Am. Chem. Soc. 1989,
111, 4332–4338.
˚
KR radiation (λ = 0.71073 A) at T = 293(2) K . The data were

Article
Organometallics, Vol. 30, No. 2, 2011 297
collected with a frame width of 0.3° in Ω and a counting time of
25 s per frame at a crystal-detector distance of 4 cm. SAINT
software was used for integration of intensity reflections and
scaling, and SADABS software was used for absorption
correction.36,37 A total of 13 851 reflections were collected in
potential (RECP) from the Stuttgart group and its associated
basis set,42 augmented by an f polarization function (Mo: R =
1.043; W: R = 0.823).43 P, S, and Si atoms were represented by
the relativistic effective core potential (RECP) from the Stutt-
gart group and the associated basis set,44 augmented by a d
polarization function (P: R = 0.387; S: R = 0.503; Si: R =
0.284).45 A 6-31G(d,p) basis set was used for all the other atoms
(H, C, N, F).46The geometry optimizations were performed in
the gas phase without any symmetry constraint followed by
analytical frequency calculations to confirm that a minimum or
a transition state had been reached. The nature of the species
connected by a given transition-state structure was checked by
optimization as minima of slightly altered TS geometries along
both directions of the transition-state vector.
the θ range 1.23-27.50°, of which 10 199 were unique (Rint
=
0.0550). The structure was solved by direct methods and refined
by the full matrix method based on F2 using the SHELXTL
software package.38The non-hydrogen atoms were refined ani-
sotropically; the positions of all hydrogen atoms were generated
geometrically, assigned isotropic thermal parameters, and allowed
to ride on their respective parent carbon atoms. Least-squares
refinement on 522 parameters converged normally with R1 (I >
2σ(I)) 0.0594, wR2 (all data) = 0.1009 with a GOF = 1.040.
Crystal data: C42H68BF3P6S4W3, Mr = 1506.38, triclinic,
˚
˚
space group P1 (no. 2), a=11.137(4) A, b = 14.984(5) A, c =
˚
Acknowledgment. The financial support of the Spanish
16.850(6) A, R = 82.866(8), β = 81.808(7), γ = 88.481(7)°, V =
ꢀ
Ministerio de Ciencia e Innovacion (Grants CTQ2008-
02670), Fundacio Bancaixa-UJI (research project P1.1B2007-
3
-1
˚
2761.5(16) A , Z = 2, μ = 6.596 mm
.
ꢀ
Catalytic Studies. Catalytic experiments were carried out
under aerobic conditions. The microwave power is dynamically
adjusted to follow the defined temperature profile. All other
reagents were used as received from commercial suppliers.
Catalytic Hydrodefluorination of Pentafluoropyridine. A 10 mL
capped high-pressure vessel containing a stirrer bar was charged
with pentafluoropyridine (0.38 mmol), silane (dimethylphenyl-
silane, phenylsilane) (1.9 mmol), anisole as internal reference
(0.38 mmol), catalyst (1 mol %), and 1 mL of acetonitrile. The
solution was heated to 180 °C under MW irradiation for the
appropriate time. During the reaction monitoring, yields and con-
versions were determined by GC chromatography and 19F NMR.
The confirmation of the nature of 2,3,5,6-tetrafluoropyridine and
FSiMe2Ph was performed by comparison with a pure commercial
standard and by spectral comparison with literature data.
12), and Generalitat Valenciana (ACOMP/2010/276 and
Prometeo/2009/053) is gratefully acknowledged. The
ꢀ
authors also thank the Servei Central D’Instrumentacio
Cientifica (SCIC) of the Universitat Jaume I for providing
us with the mass spectrometry, NMR, and X-ray facilities.
Special thanks go to Dr. Cristian Vicent for collecting
diffraction data. T.B. thanks the Spanish Ministerio de
ꢀ
Ciencia e Innovacion (MICINN) for a doctoral fellow-
ship (FPI).
Supporting Information Available: Crystallographic data in
cif format (excluding structure factors), DFT-computed Cartesian
coordinates for the optimized molecules, their electronic energies,
and imaginary frequencies for transition-state structures reported
in this paper, and complete ref 39 can be obtained free of charge
via the Internet at http://pubs.acs.org.
Computational Details. All calculations were performed with
the Gaussian03 package39 at the B3PW91 level.40,41 Transition
metal atoms were represented by the relativistic effective core
(42) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123–141.
(43) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth,
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(36) SAINT, version 6.2; Bruker Analytical X-ray Systems: Madison,
WI, 2001.
€
€
(37) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,
Germany, 1996.
(38) Sheldrick, G. M. SHELXTL, version 5.1; Bruker Analytical X-ray
Systems: Madison, WI, 1997.
(39) Frisch, M. J.; et al. et al. Gaussian 03, revision D.02; Gaussian,
Inc.: Wallingford, CT, 2004.
(40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(41) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244–13249.
(44) Bergner, A.; Dolg, M.; Kuchle, W.; Stoll, H.; Preuss, H. Mol.
Phys. 1993, 80, 1431–1441.
(45) Hollwarth, A.; Bohme, M.; Dapprich, S.; Ehlers, A. W.; Gobbi,
A.; Jonas, V.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G.
Chem. Phys. Lett. 1993, 208, 237–240.
(46) Harihara, Pc; Pople, J. A. Theor. Chim. Acta 1973, 28, 213–222.