Aromatic C-F activation by complexes containing the {Pt2S 2} core via nucleophilic substitution: A combined experimental and theoretical study

Article abstract of DOI:10.1039/b901697j

The C-F bond activation of perfluorobenzene and perfluoropyridine have been achieved by means of the complex [Pt₂(μ-S)₂(dppp) ₂], where dppp denotes 1,3-bis(diphenylphosphino)propane. The reaction with the first substrate

Full text of DOI:10.1039/b901697j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Aromatic C–F activation by complexes containing the {Pt2S2} core via
nucleophilic substitution: a combined experimental and theoretical study†
Ainara Nova,a Rube´n Mas-Balleste´,a,b Gregori Ujaque,a Pilar Gonza´lez-Duarte*a and Agust´ı Lledo´s*a
Received 26th January 2009, Accepted 13th May 2009
First published as an Advance Article on the web 27th May 2009
DOI: 10.1039/b901697j
The C–F bond activation of perfluorobenzene and perfluoropyridine have been achieved by means of
the complex [Pt2(m-S)2(dppp)2], where dppp denotes 1,3-bis(diphenylphosphino)propane. The reaction
with the first substrate requires a long time (five days) and high temperature (reflux in toluene) to yield
[Pt(o-S2C6F4)(dppp)] and [Pt3(m3-S)2(dppp)3]F2, and involves replacement of two fluorides in the ortho
position. In contrast, the reaction with perfluoropyridine is much faster (15 min at 0 ◦C) yielding
[Pt2(m-S){m-(p-SC5F4N)}(dppp)2]F, which implies the C–F activation in the para position with respect
the pyridine nitrogen. The mechanism of both reactions has been studied computationally and the
geometries of the transition states are consistent with an SNAr mechanism where a sulfido bridging
ligand replaces the fluoride anion. The energy barriers corresponding to the first and the second
fluoride substitution are 131.7 and 137.1 kJ mol-1 for perfluorobenzene and 85.9 and 142.7 kJ mol-1 for
perfluoropyridine, respectively. The different energy barrier of the first substitution explains the
different experimental conditions required and the various products obtained for these reactions.
not obvious.9,10 In addition, it has been reported intramolecular
Introduction
C–F bond activation on the coordination sphere of metallic
Organofluorine chemistry is an active field of research due to
centres by the nucleophilic attack of coordinated ligands such
as H2O,17 OH,18 SH2 or NEt2.20 A novel mechanism involving
nucleophilic attack of an electron-rich organometallic compound
and trapping of the displaced fluoride by a phosphine ligand
to form a metallophosphorane intermediate has been recently
identified with the help of DFT calculations.21–23 The strategy
proposed herein is to confine the nucleophilic group between two
metal centres, which will direct selective activation of only one or
two C–F bonds of aromatic perfluorinated compounds.
19
the interesting properties that fluorine furnishes to the carbon
skeleton.1 As a consequence, these compounds show a great
variety of uses in the pharmaceutical industry and in material
science.2 One strategy to synthesize organofluoride derivatives
is the functionalization of polyfluorinated organic compounds.
Two main different routes to achieve this goal have been widely
explored: (1) C–F bond activation at transition metal centers,3–8
and (2) nucleophilic substitution of the fluoride anion by organic
nucleophiles.9,10 The advantages of the former strategy are the
selectivity of the reactions and the catalytic role of metallic
centers.11–14 However, the reluctance of C–F bonds to undergo
oxidative additions with a metal centre and possible C–H acti-
vation as a competitive reaction are important drawbacks.4,15,16
Concerning the second strategy, the C–F bond is often the
easiest C-halogen bond to be activated by nucleophilic aromatic
substitution (addition + elimination) as a consequence of the
major polarity of this bond that facilitates the addition step,
which usually is the rate-determining step.10 This mechanism
takes place in reactions with organic nucleophiles such as sulfur,
oxygen and nitrogen-based systems.10 However, controlling the
selectivity using organic nucleophiles is a main challenge because
the control of the number of fluorine atoms to be activated is
Within this context, the reactivity of the {Pt2S2} core in [L2Pt(m-
S)2PtL2] compounds with organic electrophiles becomes specially
attractive.24,25 It has been reported over the last decade that
sulfur atoms in the {Pt2S2} core are highly nucleophilic and, as
a consequence, C–X (X= Cl, Br, I) bonds can be activated.26–33
Thus, it is possible to make use of the high nucleophilicity of
the bridging sulfido ligand to achieve C–X activation by taking
advantage of the metallic framework that solubilises the sulfide
anion in organic solvents, modulates its reactivity and induces
selectivity on the corresponding reactions.
Recently, the reaction of [Pt2(m-S)2(dppp)2] (I) with 1,3-difluoro-
2-propanol (Scheme 1) allowed us to provide the first example of
C–F activation by the {Pt2S2} core.34 The computational study
aDepartament de Qu´ımica, Universitat Auto`noma de Barcelona, 08193
Bellaterra, Barcelona, Catalonia, Spain. E-mail: agusti@klingon.uab.es,
pilar.gonzalez.duarte@uab.es; Fax: +34-935812920; Tel: +34-935811363
bDepartamento de Qu´ımica Inorga´nica, Universidad Auto´noma de Madrid,
28049, Madrid, Spain
1
† Electronic supplementary information (ESI) available: 31P{ H}, 19F
NMR and MS spectra. Energies and optimized geometries (Cartesian
coordinates). CCDC reference numbers 717503 (II) and 717504 (IV). For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b901697j
Scheme 1 Reaction between [Pt2(m-S)2(dppp)2] (I) and 1,3-difluoro-
2-propanol.
5980 | Dalton Trans., 2009, 5980–5988
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The Royal Society of Chemistry 2009
©

◦
˚
of the reaction mechanisms showed that, in this reaction, the
assistance of the OH group on the fluorine departure was a
main factor to facilitate the Csp3–F bond activation. The reaction
proceeds through a SN2 mechanism where the O–H ◊ ◊ ◊ F hydrogen
bond established from the alcohol group of the organic substrate
is essential stabilizing the departure of the fluoride anion.34
Extending our previous work, we have explored the reactivity of
I with unsaturated substrates such as perfluorobenzene (pfb) and
perfluoropyridine (pfp). Remarkably, the conditions required to
achieve these reactions and the products obtained are substrate-
dependent. Synergy between experiments and theory is illustrated
in this work by the mechanistic insights obtained from DFT
calculations, which complete our understanding of the reactions
carried out in the laboratory.
Table 1 Selected bond lengths (A) and angles ( ) for complex II
II
˚
Bond lengths/A
Pt(1)–P(1)
Pt(1)–P(2)
Pt(1)–S(1)
2.2638(14)
2.2691(14)
2.3031(14)
Pt(1)–S(2)
S(1)–C(1)
S(2)–C(6)
2.3044(13)
1.747(5)
1.727(6)
Bond angles/◦
P(1)–Pt(1)–P(2)
P(1)–Pt(1)–S(1)
P(2)–Pt(1)–S(1)
P(1)–Pt(1)–S(2)
89.88(5)
176.43(5)
89.80(5)
90.75(5)
P(2)–Pt(1)–S(2)
S(1)–Pt(1)–S(2)
C(1)–S(1)–Pt(1)
C(6)–S(2)–Pt(1)
177.45(5)
89.73(5)
103.15(19)
103.26(18)
X-Ray quality crystals were obtained by slow evaporation of a
DMSO–acetone solution. The structure of complex II, depicted
in Fig. 1, consists of mononuclear [Pt(o-S2C6F4)(dppp)] discrete
molecules devoid of crystallographic symmetry elements. The
listing of the main geometric parameters for II appears in Table 1.
The structure can be considered as formed by two rings that share a
platinum atom. One is an essentially planar five-membered PtS2C2
ring (dihedral PtSCC angle of 3.6◦), which has a common edge
with the aromatic cycle of the chelating [o-S2C6F4]2- ligand. The
other is the C3P2Pt ring that includes six atoms and shows a chair
conformation. The platinum atom has square planar coordination,
with a S–Pt–S angle of 89.73(5)◦, very close to the ideal value of
90◦. The bite angle of the diphosphine ligand, 89.88(5)◦, dictates
the values of the remaining P–Pt–S angles about the platinum
atom (89.88(5)◦ and 90.75(5)◦).
Results and discussion
Double C–F bond activation in perfluorobenzene
Experimental study. Treatment of [Pt2(m-S)2(dppp)2] (I) with
an excess of perfluorobenzene in toluene at reflux for 5 days
yielded two products: [Pt(o-S2C6F4)(dppp)] (II) and [Pt3(m3-
S)2(dppp)3]+2(F-)2 (III-F2) (Scheme 2). The species present in
the reaction mixture were monitored by 31P{ H} NMR. The
1
series of spectra recorded as a function of time indicated that I
completely evolved into II without detection of any intermediate.
While complex II is soluble in the reaction medium, complex
III-F2 is insoluble and was isolated by filtration. The formu-
lation of III-F2 as [Pt3(m3-S)2(dppp)3]+2(F-)2 was confirmed by
31P-NMR measurements in d6-DMSO, which showed identical
spectroscopic features than those obtained for complex [Pt3(m3-
S)2(dppp)3](PF6)2, whose synthesis, X-ray structure and NMR
parameters have been previously reported.35
Scheme 2
Complex II, which comes from the double C–F bond activation
in perfluorobenzene, was isolated by evaporating to dryness the
solution obtained after filtering off the solid product III-F2. II was
1
characterized by NMR (31P{ H} and 19F), FAB mass spectrometry
1
and X-ray diffraction. The P{ H} NMR spectrum in d6-DMSO
shows a set of signals centred at -5.37 ppm (J1
= 2684 Hz)
Pt–P
consistent with a complex with chemically equivalent phosphorus
nuclei. The fine structure of the central signal discloses a triplet,
which is consistent with the coupling of phosphorus with two
equivalent F nuclei located at a distance of 5 bonds (J5
Fig. 1 Displacement ellipsoid plot (20% probability) of [Pt(o-S2C6F4)-
(dppp)] (II). H atoms omitted.
=
P–F
6 Hz). This long distance coupling is also observed in the related
[Os(SC6F5)2(S2C6F4)(PMe2Ph)] complex.36 In the 19F NMR of II
only two signals appeared (d = 140.2 ppm; 167.4 ppm), which is
consistent with the formation of a complex by activation of two
C–F bonds in ortho position. Finally, the FAB MS spectrum, which
showed a major peak at m/z = 820.7 Da, is also in agreement
with the theoretical expected value for [Pt(o-S2C6F4)(dppp)]H+
(820.1 Da).
Theoretical study. The theoretical study of the reaction of
[Pt(m-S)2(dppp)2] (I) with perfluorobenzene (pfb) has been per-
formed using DFT methods. In this study the dppp ligands have
been modelled by H2P(CH2)3PH2 (dhpp). As shown in Fig. 2 and
3, this reaction initiates with the attack of the bridging sulfido
ligand in [Pt(m-S)2(dhpp)2] (1) to one of the carbon atoms of
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 5980–5988 | 5981
©

strong interaction between both centres. The importance of this
interaction in several C–F bond activation reactions has been
already reported by Macgregor et al.21–23 The energy barrier of
this first fluoride substitution is 131.7 kJ mol-1 and the energy
difference between reactant and product is 2.1 kJ mol-1 (Fig. 2).
From 1f, the direct second C–F bond activation is not possible
and a previous endo–exo isomerisation is required. It has been
shown that this process is usually easy for complexes containing
the {Pt2S(SR)} core (vide infra).37–39
Intermediate 2f, with C6F5 group in exo position and perpen-
dicular to the plane defined by SSC, is 6.4 kJ mol-1 more stable
than the endo isomer. From this intermediate it takes place the
substitution of the second fluoride anion in ortho position through
˚
˚
TS2f (S ◊ ◊ ◊ C = 2.049 A and C ◊ ◊ ◊ F = 1.588 A) with an energy
barrier of 137.1 kJ mol-1 (Fig. 2 and 3). The dihedral angle of the
{Pt2S2} core in this transition state is 115.7◦ and the SCF angle
91.2◦. In this case, the fluoride departure occurs in the direction
opposite to the first one. By this way, the fluoride ion can be better
stabilized by the other dhpp phosphine. The product of the second
substitution is 3f, an ion pair where the group S2C6F4 is bridging
the two Pt atoms. This species is 35.2 kJ mol-1 more stable than
the reactants, which makes the reaction to be exothermic.
The next step of the reaction to obtain product II is the
fragmentation of 3f to give [Pt(o-S2C6F4)(dhpp)] (4f) and the
trinuclear complex [Pt3(m3-S)2(dhpp)3]+2 (T), which models II and
III, respectively. This step was calculated by considering the
reaction 3f + 1 → 4f + T, and the distances and angles obtained
by 4f were compared with those coming from the crystal structure
of II. As shown in Fig. 3, the optimized geometry fits well the
crystallographic structure. Considering the energy of this step, the
fragmentation is strongly exothermic as it requires 99.6 kJ mol-1.
Overall, the mechanism of this reaction can be described
as a double aromatic nucleophilic substitution (SNAr). This
mechanism is usually believed to follow a two-step process:
addition of the nucleophile to the aromatic ring, forming the
so called Meisenheimer intermediate, and elimination of the
leaving group.40 However in this case the Meisenheimer complex
is not a real intermediate; instead, it is the transition state.
Such behaviour has already been found in theoretical studies
of nucleophilic aromatic substitutions.41–43 This reaction requires
time (5 days) and temperature (110.6 ◦C) to evolve 1 + pfb
into [Pt(o-S2C6F4)(dppp)] (II). The high energy barriers obtained
(131.7 kJ mol-1 and 137.1 kJ mol-1 for the first and second C–F
nucleophilic substitutions, respectively) agree with these hard
conditions. The fragmentation of the product obtained through
these steps, modelled by 3f, results in formation of II and [Pt3(m3-
S)2(dppp)3]+2(F-)2 (III-F2). Considering that the overall observed
reaction is very slow (5 days at 110 ◦C) it is reasonable to consider
that the fragment [Pt(dppp)]2+ formed by the fragmentation of 3f
reacts with unreacted I to afford III-F2 as the final product.
Fig. 2 Energy profile in toluene of the C–F bond activation mechanism
of perfluorobenzene (pfb) by [Pt2S2(dhpp)] (1). All energies in kJ mol-1.
Fig. 3 Optimized geometries of transition states (TS1f and TS2f) and
˚
complex 4f with the most representative distances (A) calculated for this
model structure and found in the crystal structure of II (numbers in italics).
perfluorobenzene and the concomitant departure of the fluoride
˚
ion in transition state TS1f (S ◊ ◊ ◊ C = 2.015 A and C ◊ ◊ ◊ F =
Single C–F bond activation in perfluoropyridine
˚
1.572 A). In this transition state, the {Pt2S2} core is almost planar
with a dihedral angle of 171.7◦ and an SCF angle of 93.3◦.
In this way the leaving fluoride is stabilized by interaction with
the aliphatic chain of the phosphine (see Fig. 3). This geometry
induces the product of this step, the ion pair 1f, to have the
C6F5 group in endo position with respect to the {Pt2S2} core
Experimental study. Addition of an excess of perfluoropyri-
dine to a solution of [Pt2(m-S)2(dppp)2] in toluene at 0 ◦C yielded
after 15 min a yellow solid, which was isolated by filtration. This
solid (IV-F) was firstly characterised by 19F NMR in d6-DMSO,
where two signals were observed, at d -94.9 and -127.4 ppm,
consistently with the substitution in pfp of the fluorine atom in
˚
with a P ◊ ◊ ◊ F distance remarkably short (1.996 A) indicating a
5982 | Dalton Trans., 2009, 5980–5988
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The Royal Society of Chemistry 2009
©

the para position to the nitrogen. The spectrum also exhibited a
broad signal at d -148.8, which is within the expected chemical
shift range for fluoride ions forming hydrogen bonds in species
such as [F2H]- of [(FH)nF]-.34,44–46 The disappearance of this broad
signal when the anion in IV-F was replaced by ClO4- confirmed the
proposed C–F bond activation in the perfluoropyridine molecule
(Scheme 3).
Scheme 3 Reaction between [Pt2(m-S)2(dppp)2] (I) and perfluoropyridine
(pfp)
1
The 31P{ H} NMR spectra of IV-F showed two signals with the
same multiplicity and intensity, and both with two satellites (d =
2.4 ppm, J1Pt–P = 3412 Hz; d = 1.6 ppm, J1Pt–P = 2567 Hz). This is
indicative of the presence of two unequivalent pairs of phosphorus
in [Pt2(m-S){m-(p-SC5F4N)}(dppp)2]+, one pair trans to the thiolate
bridge and the other pair trans to the sulfide bridge. The ESI-MS
spectrum, which showed a major peak at m/z = 1428.4 Da is
also in good agreement with the expected theoretical value for IV
cation (1428.2 Da).
X-Ray quality crystals were obtained by slow evaporation from
methanol solution. The structure of the dinuclear [Pt2(m-S){m-
(p-SC5F4N)}(dppp)2]+ (IV) cation was unequivocally determined.
However, disordered distribution of anions and solvent molecules
hampered the modelling of their position. Selected angles and
distances in IV are given in Table 2. The structure of the cations
consists of a hinged central Pt2(m-S)(m-SR) ring fused to two
Pt2P2C3 rings (Fig. 4). The dihedral angle between the two PtSS¢
planes is 132.7◦ With the thiolate group in endo position with
respect to the {Pt2SS¢} ring. Unlike IV, the R group is in exo
position in a closely related complex containing the {Pt2(m-S)(m-
SR)} core, with R= CH3.28 Thus, while in the exo structure the SSC
Fig. 4 Displacement ellipsoid plot (20% probability) of [Pt2(m-S)-
{m-(p-SC5F4N)}(dppp)2]+ (IV). H atoms omitted.
angle is 85.1◦, in IV this angle is 210.6◦. The origin of the preference
for the endo isomer in IV could be the stabilizing interactions
between the {SC5F4N} fragment and the phenyl groups of the
dppp ligand. The endo geometry allows p–p stacking interactions
between the aromatic C5F4N ring and two phenyl groups, each
one from a different phosphine ligand. In addition, the fluorine
atoms of the pfp could establish C–F ◊ ◊ ◊ H–C interactions with the
phenyl hydrogens.
Theoretical study. The computational study of the reaction
of [Pt(m-S)2(dhpp)2] (1) with perfluoropyridine (pfp) has been
performed. In contrast to pfb, this substrate has three different
positions susceptible of fluoride substitution: ortho, meta and para.
Giving the preference of ortho and para positions for nucleophilic
substitution,9,47 only the reactions in these two positions have been
considered. The energy profiles obtained for both reactions are
depicted in Fig. 5. Similarly to pfb, the reaction of 1 with pfp
initiates with the nucleophilic substitution of the fluoride in para
position through TS1n, with an energy barrier of 85.9 kJ mol-1.
The analogous reaction but replacing the fluoride in ortho position
has a barrier 32.8 kJ mol-1 higher in energy (TS3n). This energy
difference accounts for the C–F bond activation taking place
only in para position. In addition, this position is not only the
most preferred kinetically, but also thermodynamically. Thus, the
product 1n (3.7 kJ mol-1) is more stable than 4n (31.9 kJ mol-1)
(Fig. 5). The geometries of TS1n and 1n are similar to those of the
related species in the reaction with pfb, TS1f and 1f, respectively.
In TS1n the {Pt2S2} core has a dihedral angle of 166.4◦ and an
SCF angle of 95.3◦ (Fig. 6). The S ◊ ◊ ◊ C and C ◊ ◊ ◊ F distances are
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for cation IV
IV
˚
Bond lengths/A
Pt(1)–S(1)
Pt(1)–S(2)
Pt(1)–P(3)
Pt(1)–P(4)
Pt(2)–S(1)
2.320(2)
2.373(2)
2.285(2)
2.254(2)
2.333(2)
Pt(2)–S(2)
Pt(2)–P(1)
Pt(2)–P(2)
S(2)–C(1)
2.390(2)
2.298(2)
2.249(2)
1.773(9)
Bond angles/◦
S(1)–Pt(1)–S(2)
S(1)–Pt(1)–P(3)
S(1)–Pt(1)–P(4)
S(2)–Pt(1)–P(3)
S(2)–Pt(1)–P(4)
P(3)–Pt(1)–P(4)
S(1)–Pt(2)–S(2)
S(1)–Pt(2)–P(1)
77.03(8)
174.39(8)
88.89(8)
102.62(8)
163.63(8)
92.20(8)
76.46(1)
171.62(8)
S(1)–Pt(2)–P(2)
S(2)–Pt(2)–P(1)
S(2)–Pt(2)–P(2)
P(1)–Pt(2)–P(2)
Pt(1)–S(1)–Pt(2)
Pt(1)–S(2)–Pt(2)
Pt(1)–S(2)–C(1)
Pt(2)–S(2)–C(1)
89.05(8)
101.96(8)
165.11(8)
92.87(9)
93.21(8)
90.46(8)
115.6(3)
110.4(3)
˚
˚
1.894 A and 1.683 A, shorter and longer, respectively, than in TS1f,
showing a more advanced transition state.
In order to analyse why the second fluoride substitution does
not take place with perfluoropyridine, the computational study
of this step has been also pursued. Only the reaction starting
from intermediate 1n, which is the most favoured kinetically
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 5980–5988 | 5983
©

Fig. 6 Optimized geometries of transition states (TS1n and TS2n) and
˚
complex endo-IV with the most representative distances (A) for this
structure and for the crystal structure of IV (numbers in italics).
to that obtained in pfb (137.1 kJ mol-1). This energy barrier is
not feasible at 0 ◦C and consequently the reaction stops before
reaching this stage. At higher temperatures the reaction gives a
mixture of platinum complexes that have not been characterized.
Probably, the absence of I in solution, which reacts quickly with
pfp, impairs the formation of [Pt3(m3-S)2(dppp)3]+2, thus directing
the system towards several other by-products resulting from the
cleavage of the {Pt2S2} core.
Fig. 5 Energy profile in toluene of the C–F bond activation mechanism
of perfluoropyridine (pfp) by [Pt2S2(dhpp)] (1). All energies in kJ mol-1.
and thermodynamically, and also the only one experimentally
observed, has been considered. This intermediate requires a
previous endo–exo isomerization process to enable the second
fluoride substitution. From the exo intermediate (2n, 11.1 kJ mol-1
below 1n) the sulfide bridge can attack only a carbon in ortho
position related to sulfide ligand (meta related to nitrogen). The
energy barrier of this second substitution is 142.7 kJ mol-1 (TS2n),
much higher than the first substitution (85.9 kJ mol-1). In this
Endo–exo exchange in cation IV. As it has been described in
the experimental section, IV crystallizes with the R group of the
{Pt2(m-S)(m-SR)} core in endo position. The stabilization of this
conformer comes from the interaction between the {SC5F4N}
fragment and the phenyl groups of the dppp ligand. Thus, the
model system used to study the reaction mechanism, in which the
Ph groups in dppp have been replaced by H, can not reproduce this
interaction. Consequently, the exo isomer 2n is more stable than
the endo 1n by 11.1 kJ mol-1 (Fig. 5). To estimate the importance
of this interaction, the dynamic process endo–exo arising from the
flexibility of the {Pt(m-S)(m-SR)Pt} has been studied for cation
IV considering the full system, including the phenyl rings in
the theoretical calculation (Fig. 6 and 7). This study has been
performed using a hybrid QM/MM method (see Computational
details). The analogous system used in the mechanistic study was
described at the quantum mechanics (QM) level, whereas the
phenyl groups were described at the molecular mechanics (MM)
level.
˚
transition state, the S ◊ ◊ ◊ C and C ◊ ◊ ◊ F distances are 2.060 A and
˚
1.597 A, respectively (Fig. 6). Remarkably, in all transition states
(with pfp and pfb), the energy barrier increases with longer C ◊ ◊ ◊ S
distances. The final product of the double fluoride substitution is
complex 3n, 34.0 kJ mol-1 more stable than reactants. In this case,
the reaction of 3n + 1 → 5n + T (where 5n = [(dhpp)Pt(S2C6F4)])
is also exothermic by 107.5 kJ mol-1.
These theoretical results are in agreement with the experimental
observations for the reaction of I with pfp. Thus, this reaction
is fast and requires low temperature (15 min at 0 ◦C) to afford
[Pt2(m-S){m-(p-SC5F4N)}(dppp)2]F (IV-F). The energy barrier for
the first fluoride substitution (85.9 kJ mol-1) is significantly lower
than that for the reaction with pfb (131.7 kJ mol-1); this can
explain the different experimental conditions required in these
two reactions. This difference between the energy barrier of pfb
and pfp in the first fluoride substitution is not observed for the
second substitution. Thus, the energy barrier to activate the second
C–F bond in perfluoropyridine is 142.7 kJ mol-1 (TS2n), similar
Considering the full system for complexes 1n and 2n, there is a
change in the relative stability of these complexes, the endo isomer
5984 | Dalton Trans., 2009, 5980–5988
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©

Fig. 7 Energy profile in the gas phase of the endo–exo dynamic process
arising from the flexibility of the {Pt(m-S)(m-SR)Pt} core in IV. All energies
in kJ mol-1.
Fig. 8 VT-19F NMR spectrum of [Pt2(m-S){m-(p-SC5F4N)}(dppp)2]F
(IV-F).
becoming more stable than the exo one, as it was expected from
the X-ray conformation of IV. In addition, a new isomer with the
fluoropiridyl substituent in the exo position has been found as an
energy minimum.
Experimental
Materials and methods
All the manipulations were carried out at room temperature
under an atmosphere of pure dinitrogen, and conventionally
dried and degassed solvents were used throughout. These were
Purex Analytical Grade from SDS. The synthesis of [Pt2(m-
S)2(dppp)2] (I) has already been reported.26 Perfluorobenzene and
perfluoropyridine reagents were purchased from Aldrich in their
purest presentation (>99.5%). The purity of these reagents was
examined by 19F-NMR, observing that no signals other than those
corresponding to the reagent itself could be detected.
The structural difference between the two exo isomers, exo(A)
and exo(B) in Fig. 7, arises from the relative position of the
fluoropyridine ring relative to the SSC plane, almost parallel
in exo(A) (PtSCC dihedral angle (q2) of 71.3◦), and almost
perpendicular in exo(B) (q2 = -42.2◦). Exo(B) isomer is only
slightly less stable than endo (2.5 kJ mol-1), exo(A) being placed
6.3 kJ mol-1 above exo(B). Exo(A) may be obtained from the
endo isomer by the hinging motion of the {Pt2S2} core, while
exo(B) implies the rotation of the ring around the S–C s-bond.
The energy barriers for these transformations have been estimated
considering in each case the corresponding reaction coordinate:
q1 from endo to exo(A) and q2 from exo(A) to exo(B), obtaining
values around 20 kJ mol-1, which point out a fast interconversion
between the different isomers.
Elemental analyses were performed on a Carlo-Erba CHNS
EA-1108 analyzer. The ESI-MS and FAB-MS measurements were
performed on a VG Quattro Micromass Instrument by using 1 : 1
acetonitrile–water as carrier (ESI-MS) or 3-nitrobenzil alcohol
(NBA) as matrix in the positive ion mode (FAB-MS). 31P{ H}, 19
F
1
NMR spectra were recorded from samples in d6-DMSO solution
at room temperature using a Bruker AC250 spectrometer. 1H and
13C chemical shift are relative to SiMe4. 19F chemical shift is relative
to CFCl3 and 31P{H} NMR spectra were referenced to external
85% H3PO4.
With these results, a dynamic endo–exo process is expected at
room temperature. This is in good agreement with the 19F NMR
data. While at room temperature only two signals at d -94.9 and
◦
-127.4 ppm are observed, it is necessary to go down to -90 C,
as shown in Fig. 8, to separate the signals corresponding to two
different isomers of IV. These two isomers appear in almost the
same proportion, indicating a very ◦close thermodynamic stability.
Thus, the signals observed at -90 C (d = -94.2, -94.8, -125.7,
-128.2 ppm) probably correspond to the endo and the most stable
exo (i.e. exo(B)) isomers.
Synthesis of [Pt(dppp)(o-S2C6F4)] (II)
Perfluorobenzene (1 mL, 8.7 mmol) was added to a solution of
[Pt2(m-S)2(dppp)2] (340 mg, 0.3 mmol) (I) in dry toluene (30 mL).
The reaction mixture was stirred at reflux (111 ◦C) and after
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 5980–5988 | 5985
©

5 days it was filtered. The toluene solution was evaporated to
dryness under vacuum. The remaining yellow solid was washed
with ether. From this solid, X-ray quality crystals were obtained
by slow evaporation from an acetone DMSO mixture solution.
Yield: 42 mg (39%). Anal. calcd for C33F4H26P2PtS2 (819): C 48.35,
H 3.20, S 7.82. Found: C 48.31, H 3.52, S 7.42.
the crystal structure in lower symmetry space groups do not solve
the disorder problem. There were very high correlations among
symmetry related geometrical parameters in the highest symmetry
space group (P21/n). The electron density maxima found inside
the voids were equally unexplainable by means of reasonable
molecular models. Therefore, the SQUEEZE52,53 function was
used to eliminate the contribution of the electron density in
Synthesis of [Pt2(dppp)2(l-S){l-(p-SC5F4N)}]F (IV)
the disordered solvent region from the measured intensity data
3
˚
(two 690 A voids per unit cell containing 319 electrons each)
Perfluoropyridine (0.84 mL, 7.65 mmol) was added to a solution
of [Pt2(m-S)2(dppp)2] (98 mg, 0.08 mmol) in dry toluene (15 mL).
After 15 min, a pale yellow solid appeared. The solid product
was collected by filtration, washed with toluene, and dried with
diethyl ether. Yield: 48 mg (43%). X-Ray quality crystals were
obtained by slow evaporation of a CH2Cl2 solution. Anal. calcd
for C59F5NH52P4Pt2S2·H2O·CH2Cl2 (1551.17): C 46.46, H 3.64, N
0.90. Found: C 46.37, H 3.54, N 0.88.
and therefore the solvent-free data was employed in the final
refinement. The method to determine the number of electrons
per void is the previously reported as the ‘BYPASS’ procedure,52
implemented in the PLATON software as SQUEEZE calculation.
The lack of high resolution data (lower data/parameter ratio) and
the use of the SQUEEZE procedure to account for the high level
of disorder inside the voids caused problems in the least-square
refinement (mainly in the Uij behaviour). To minimize this effect
the structure was submitted to further refinement cycles including
a complete set of geometrical restraints (a mixture of SIMU and
ISOR instructions). The high number of restraints finally used
(696) significantly improved the anisotropic atomic displacement
parameters obtained from the initial refinement (12 restraints).
Molecular graphics are represented by Ortep-3 for Windows.
X-Ray crystallographic characterization†
A summary of crystal data, data collection, and refinement
parameters for the structural analysis of complexes II and
IV-F is given in Table 3. Measurements of diffraction intensity
data were collected respectively on a Bruker SMART CCD-1000
and APPEX-II CCD area-detector diffractometer with graphite-
˚
monochromated Mo Ka radiation (l = 0.71073 A). Absorption
Computational details
correction was carried out by semiempirical methods based on
redundant and symmetry-equivalent reflections with the aid of the
SADABS program.48 The structure was resolved by direct methods
(DIRDIF49 and SIR9750) and refined by full-matrix least-squares
based on F2, with the aid of SHELX-9751 software. All hydrogen
atoms were included at geometrically calculated positions with
thermal parameters derived from the parent atoms. In compound
This study was made using two different models: one small, where
all phenyl substituents of the phosphine ligands were replaced
by H; and one complete, where the full system was considered.
The calculations for the small system were performed using the
Gaussian03 package.54 In this case, the geometries of the minima
and transition states were fully optimized, without any symmetry
restriction. Frequency calculations were performed to characterize
the stationary points as minima or transition states. Single point
calculations including solvent effects were performed at the opti-
mized gas-phase geometries, using the CPCM approach,55,56 which
is an implementation of the conductor-like screening solvation
model (COSMO) in Gaussian03. Toluene, the solvent used in all
experiments, was chosen as solvent (dielectric constant e = 2.379).
QM/MM IMOMM calculations57 for the complete system
were performed with a program build from modified version of
two standard programs: Gaussian98 for the quantum mechanics
part58 and mm3(92) for the molecular mechanics part.59 The
QM region was [Pt(m-S)2(dhpp)2] plus perfluorobenzene and
perfluoropyridine. The phenyl groups of the phosphine were
computed at the MM level. Optimizations were full except for
the connection between the QM and the MM region where P–H
˚
IV-F the best crystal found only diffracts up to 0.95 A resolution
at 22.02◦ in theta, Mo Ka, and thus, data were only collected
out to 22◦. The solvent and fluoride anion molecules appear
to be highly disordered thus making it difficult to model its
position and distribution reliably. Attempts to solve and refine
Table 3 Crystallographic data for complexes II and IV
II
IV
Formula
Mr
C33H26S2P2F4Pt
819.69
C59H52F4NP4Pt2S2
1429.2
Monoclinic
P21/n
12.7777(5)
19.9091(6)
24.4992(9)
90
95.906(2)
90
6199.3(4)
100.0(1)
4
1.531
4.73
Crystal system
Space group
Monoclinic
P21/c
13.393(4)
21.653(6)
12.975(3)
90
113.829(4)
90
3442.0(16)
120.0(1)
4
1.582
4.33
28 890
7064(0.0614)
379/0
˚
a/A
˚
b/A
˚
c/A
˚
a/◦
b/◦
g /◦
distance was fixed to 1.420 A.
Quantum calculations in the small and complete systems were
performed at the B3LYP level.60,61 For the Pt, P and S atoms,
the lanl2dz effective core potential was used to describe the
innermost electrons,62 whereas their associated double-z basis
set was employed for the rest of the electrons. An extra series
of d-polarization functions were also added for P (exp. 0.387)
and S (exp. 0.503) atoms.63 The carbon and nitrogen atoms of
perfluorobenzene and perfluoropyridine were described by the 6–
31G(d) basis set.64 The rest of the C and H atoms were described
by the 6–31G basis set. The fluorine atoms were described by the
6–31G+(d) basis set.65
3
˚
V/A
T/K
Z
r
calcd/g cm-3
m/mm-1
Reflections collected
Unique reflections (Rint
Parameters/restraints
Goodness-of-fit on F2
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
69 141
)
7575(0.126)
649/696
0.928
0.0369, 0.0771
0.0596, 0.0821
1.398
0.0341, 0.0784
0.0548, 0.0847
5986 | Dalton Trans., 2009, 5980–5988
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The MM calculations in the full system were carried out with
the MM3 force field.66 The van der Waals parameters for platinum
were taken from UFF force field.67 Torsional contributions
involving dihedral angles with the metal atom in terminal position
were set to zero.
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Conclusions
In the present work the C–F bond activation of aromatic
compounds (perfluorobenzene and perfluoropyridine) has been
achieved by the [Pt2(m-S)2(dppp)2] complex. Recently, the same
complex has been used to activate the aliphatic substrate 1,3-
difluoro-2-propanol (Scheme 1).34 These two examples show the
efficiency of [Pt2(m-S)2(dppp)2] to activate C–F bonds of organic
substrates by nucleophilic substitution. In the case of 1,3-difluoro-
2-propanol, the computational study of the reaction mechanism
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10 Organo-Fluorine Compounds, ed. B. Baasner, H. Hagemann and
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2
departure in a SN process. In the case of perfluoropyridine and per-
fluorobenzene, this requirement is not necessary and consequently
the C–F bond activation in aromatic compounds seems to be
more general. The theoretical study has revealed that the reaction
proceeds through a SNAr mechanism, accounting for the different
experimental condition required in each reaction and for the
various products obtained. According to these results the different
reactivity of perfluorobenzene and perfluoropyridine towards I
resides in the kinetics of these reactions, their thermodynamics
being quite similar. The products of the first substitution (2f and
2n) are few kJ mol-1 below reactants (-4.3 and -7.4 kJ mol-1,
respectively) and the products of the second substitutions are
around 35 kJ mol-1 below in the two cases. On the contrary,
the energy barriers for the first C–F substitution are remarkably
different: 131.7 kJ mol-1 with perfluorobenzene and 85.9 kJ mol-1
with perfluoropyridine.
C–F bond activation in both organic substrates is also selective.
With perfluorobenzene the double C–F bond activation takes
place in ortho position. This is the only possibility considering the
disposition of the two sulfur atoms in complex I. This orientation
is not usual in reactions with thiolates where the substitution
of two fluorides is always in positions 1,4.10 In the case of
perfluoropyridine, the activation in para position with respect to
nitrogen is more common.9 As it has been shown in this study,
the reason in this case is not only kinetic but also thermodynamic
being 1n 28.2 kJ mol-1 more stable than 4n.
24 P. Gonzalez-Duarte, A. Lledos and R. Mas-Balleste, Eur. J. Inorg.
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Acknowledgements
We are grateful to Dr A. L. Llamas and Dr B. Dacun˜a (Unidade
de Raios X RIAIDT, Universidade de Santiago de Compostela,
Spain) for crystal structure determinations. The authors thank the
financial support from the “Ministerio de Ciencia e Innovacio´n” of
Spain (projects CTQ2004-01463, CTQ2008-06866-C02-01/BQU
and Consolider Ingenio 2010 CSD2007-00006). R. M. B. thanks
the Spanish MICINN for funding through the “Ramo´n y Cajal”
program. The use of the computational facilities of the “Centre de
Supercomputacio´ de Catalunya” is greatly appreciated.
35 R. Mas-Balleste, G. Aullon, P. A. Champkin, W. Clegg, C. Megret, P.
Gonzalez-Duarte and A. Lledos, Chem.–Eur. J., 2003, 9, 5023.
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5988 | Dalton Trans., 2009, 5980–5988
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©