Access to novel fluorovinylidene ligands via exploitation of outer-sphere electrophilic fluorination: New insights into C-F bond formation and activation

Article abstract of DOI:10.1039/c5dt04596g

Metal vinylidene complexes are widely encountered, or postulated, as intermediates in a range of important metal-mediated transformations of alkynes. However, fluorovinylidene complexes have rarely been described and their reactivity is largely unexplored. By making use of the novel outer-sphere electrophilic fluorination (OSEF) strategy we have developed a rapid, robust and convenient method for the preparation of fluorovinylidene and trifluoromethylvinylidene ruthenium complexes from non-fluorinated alkynes. Spectroscopic investigations (NMR and UV/Vis), coupled with TD-DFT studies, show that fluorine incorporation results in significant changes to the electronic structure of the vinylidene ligand. The reactivity of fluorovinylidene complexes shows many similarities to non-fluorinated analogues, but also some interesting differences, including a propensity to undergo unexpected C-F bond cleavage reactions. Heating fluorovinylidene complex [Ru(η⁵-C₅H₅)(PPh₃)₂(CC{F}R)][BF₄] led to C-H activation of a PPh₃ ligand to form an orthometallated fluorovinylphosphonium ligand. Reaction with pyridine led to nucleophilic attack at the metal-bound carbon atom of the vinylidene to form a vinyl pyridinium species, which undergoes both C-H and C-F activation to give a novel pyridylidene complex. Addition of water, in the presence of chloride, leads to anti-Markovnikov hydration of a fluorovinylidene complex to form an α-fluoroaldehyde, which slowly rearranges to its acyl fluoride isomer. Therefore, fluorovinylidenes ligands may be viewed as synthetic equivalents of 1-fluoroalkynes providing access to reactivity not possible by other routes.

Full text of DOI:10.1039/c5dt04596g

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Access to novel fluorovinylidene ligands via
exploitation of outer-sphere electrophilic
fluorination: new insights into C–F bond
formation and activation†
Cite this: DOI: 10.1039/c5dt04596g
Lucy M. Milner, Lewis M. Hall, Natalie E. Pridmore, Matthew K. Skeats,
Adrian C. Whitwood, Jason M. Lynam* and John M. Slattery*
Metal vinylidene complexes are widely encountered, or postulated, as intermediates in a range of impor-
tant metal-mediated transformations of alkynes. However, fluorovinylidene complexes have rarely been
described and their reactivity is largely unexplored. By making use of the novel outer-sphere electrophilic
fluorination (OSEF) strategy we have developed a rapid, robust and convenient method for the preparation
of fluorovinylidene and trifluoromethylvinylidene ruthenium complexes from non-fluorinated alkynes.
Spectroscopic investigations (NMR and UV/Vis), coupled with TD-DFT studies, show that fluorine in-
corporation results in significant changes to the electronic structure of the vinylidene ligand. The reactivity of
fluorovinylidene complexes shows many similarities to non-fluorinated analogues, but also some interest-
ing differences, including a propensity to undergo unexpected C–F bond cleavage reactions. Heating
fluorovinylidene complex [Ru(η5-C5H5)(PPh3)2(CvC{F}R)][BF4] led to C–H activation of a PPh3 ligand to
form an orthometallated fluorovinylphosphonium ligand. Reaction with pyridine led to nucleophilic attack
at the metal-bound carbon atom of the vinylidene to form a vinyl pyridinium species, which undergoes
both C–H and C–F activation to give a novel pyridylidene complex. Addition of water, in the presence of
chloride, leads to anti-Markovnikov hydration of a fluorovinylidene complex to form an α-fluoroaldehyde,
which slowly rearranges to its acyl fluoride isomer. Therefore, fluorovinylidenes ligands may be viewed as
synthetic equivalents of 1-fluoroalkynes providing access to reactivity not possible by other routes.
Received 23rd November 2015,
Accepted 14th December 2015
DOI: 10.1039/c5dt04596g
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complementary to other methodologies and/or the ability to
synthesise C–F bonds in less common environments.
Introduction
The unique properties of the carbon–fluorine bond can
The development of novel catalytic procedures for C–F
enhance many fundamental properties of biologically relevant bond formation is highly desirable and can offer significant
molecules including bioavailability, metabolic stability and advantages over traditional stoichiometric processes. Recent
lipophilicity, with approximately 30% of agrochemicals and years have seen many advances using enzymatic-,12 organo- or
20% of all pharmaceuticals now containing fluorine.1–9 The photo-catalytic13 methods for the synthesis of fluoro-organic
use of 18F-labelled imaging agents in positron emission compounds. Alongside these, transition-metal-promoted reac-
tomography (PET) is also a highly active area of medical tions have played an increasingly prominent role,14 particularly
research.10,11 As such, new facile and selective synthetic in the formation of aryl fluorides.15 Complementary studies on
methods for the introduction of fluorine into organic frame- C–F bond cleavage4,16–20 and hydrodefluorination by metal
works are highly desirable. Of particular interest are examples complexes provide insight into the functionalization of readily
which offer high selectivity, controlled regiochemistry that is available fluorine-rich substrates.13,21–23
Metal-mediated C–F bond forming methodologies using
both nucleophilic and electrophilic sources of fluorine have
been developed.5,6,15,24 Our interest has focused on reactions
Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
E-mail: jason.lynam@york.ac.uk, john.slattery@york.ac.uk; Fax: +44 (0)1904322516;
involving electrophilic fluorinating reagents, a number of
which are commercially available.25,26 They have been utilized
Tel: +44 (0)1904322534, +44 (0)1904322610
†Electronic supplementary information (ESI) available: CIF data, experimental
in metal-catalyzed C–F bond formation reactions, which are
details, DFT methods and coordinates. CCDC 1040309–1040312 and
believed to proceed either by oxidative fluorination of
1419777–1419780. For ESI and crystallographic data in CIF or other electronic
palladium27–29 or silver30 to form a high-oxidation-state metal-
format see DOI: 10.1039/c5dt04596g
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with a range of substitution patterns is restricted, this
approach does not represent a general method for the syn-
thesis and exploitation of fluorovinylidenes complexes, and
hence their reactivity is largely unexplored. However, the
potential importance of these ligands as intermediates in cata-
lytic processes is highlighted by the large number of catalytic
reactions that proceed via non-fluorinated vinylidene
intermediates.49–52
We now demonstrate the successful preparation of fluoro-
vinylidene complexes by OSEF from readily available, non-
fluorinated alkynes. Studies of their reactivity clearly demon-
strate that these complexes may indeed provide important
routes to fluorinated compounds. The fluorovinylidenes
ligands may act as fluoroalkyne equivalents therefore offering
access to reactivity patterns of compounds which are otherwise
difficult to prepare and handle.
Scheme 1 Outer sphere electrophilic fluorination of an organo-ruthe-
nium complex. “F+” = [FTMP]BF4, Selectfluor or N-fluorobenzenesulfon-
imide (NFSI).
fluoride complex,31 followed by C–F reductive elimination, or
through single-electron processes with either copper32 or palla-
dium catalysts.2,33–35 In addition, a wealth of Lewis-acidic-
metal-mediated electrophilic fluorination reactions have been
developed, which do not involve metal-based redox
chemistry.36–39
We have recently described a new mechanism for the for-
mation of C–F bonds within the coordination sphere of a
redox-active metal complex using electrophilic fluorinating
reagents (Scheme 1a).40,41 Reaction of pyridylidene complex [I]
with a latent source of “F+” results in selective C–F bond for-
mation to give [II]+. Reaction of [II]+ with a further equivalent
of 1-fluoro-2,4,6-trimethylpyridinium (FTMP) salt (in the pres-
ence of a catalytic amount of a base), or [I] with two equi-
valents of FTMP, gives the difluorinated analogue [III]+.
Experimental and computational studies indicate that this
process involves direct fluorination of the ligand, without
Ru–F bond formation, and is hence referred to as an outer-
sphere electrophilic fluorination (OSEF) reaction.
We believe that the OSEF reaction in Scheme 1a is a general
process and recent reports that an outer-sphere fluorination
step may be involved in palladium-catalysed C–F bond for-
mation42 and the fluorination of alkyne ligands in the coordi-
nation sphere of platinum43 support this idea. It is therefore
important to gain a deeper understanding of the scope of
OSEF reactions and the reactivities of the novel fluorinated
ligand systems that are formed.
Results and discussion
Synthesis and characterization of fluorine- and
trifluoromethyl-containing vinylidene complexes
By analogy with related electrophilic additions (H+,53 Cl2, Br2,
I2,54 Au(PPh3)+ (ref. 55)), the electrophilic fluorination of the
alkynyl ligand in [Ru(η5-C5H5)(–CuCR)(PPh3)2] to give fluori-
nated vinylidenes was targeted. Complexes based on the
[Ru(η5-C5H5)(–CuCPh)(PPh3)2] were selected as metal-based
precursors as they are readily available from the reaction
between [Ru(η5-C5H5)Cl(PPh3)2] and a wide range of terminal
alkynes.56 Treatment of a CH2Cl2 solution of [Ru(η5-C5H5)
(–CuCPh)(PPh3)2], [1a], with [FTMP]BF4, [2a]BF4, resulted in
an instantaneous colour change from a yellow to a dark green
solution from which the fluorinated vinylidene complex
[Ru(η5-C5H5)(vCvC{F}Ph)(PPh3)2]BF4, [3a]BF4, could be iso-
lated in excellent yield (Scheme 2a). No intermediates were
observed by NMR spectroscopy between 195 K and 295 K, sup-
porting the hypothesis that [3a]BF4 is being formed by direct
fluorination of the alkynyl ligand. Complex [3a]BF4 could also
be prepared in a one-pot procedure by treatment of the vinyl-
idene complex [Ru(η5-C5H5)(vCvC{H}Ph)(PPh3)2]+, [4a]+, with
base to generate [1a] in situ followed by reaction with [2a]BF4.
This reaction also proceeds in the absence of added base, but
the reaction is slightly slower, requiring ca. 30 min to reach
completion.
Analogous reactions of ruthenium alkynyl complexes con-
taining both electron-releasing and electron-withdrawing
substituents ([1b]–[1d], Scheme 2a) and with a range of
electrophilic fluorinating reagents (NFSI, [2b]BF4 and Select-
fluor, [2c](BF4)2) proceeded with no notable difference in reac-
tion outcome, demonstrating the generality of the approach.
However, reaction of [1a] or [1b] with 1-fluoropyridinium tetra-
fluoroborate, [2d]BF4, resulted in the formation of the disubsti-
tuted vinylidene complexes [5a]BF4 and [5b]BF4 respectively
(Scheme 2b), the structure of the latter was confirmed by X-ray
crystallography (ESI†). Complex [5a]BF4 appears to have been
The conversion of [I] to [II]+ may be viewed as the trans-
formation of an alkenyl ligand to a fluorinated carbene
(Scheme 1b) and by analogy we postulated that the corres-
ponding reaction of an alkynyl ligand via OSEF might provide
facile and selective formation of very unusual fluorovinylidene
ligands (Scheme 1c). Fluorovinylidene complexes have rarely
been described – limited examples of dimeric iridium44–47 and
iron48 complexes have been reported via activation of fluori-
nated organic precursors such as H2CvCHF (iridium) and
CF2(COCl)2 (iron). Since the availability of these precursors
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Fig. 1 LUMO (top) and HOMO (bottom) of [3a]+.
Scheme 2 (i) CH2Cl2 solution, -2,4,6-trimethylpyridine. *NFSI was used
as the fluorinating agent. (ii) CH2Cl2 solution, –“HF” (iii) CH2Cl2 solution,
4 h, r.t., -dibenzothiophene.
ca. 30 ppm and 70 ppm downfield, respectively, when com-
pared to the hydrogen-substituted analogue [4a]PF6. Indeed
the metal-bound carbon is observed at one of the lowest
reported chemical shifts for a vinylidene α-carbon.59,60 In
CD2Cl2 solution the 1H, 31P and 19F NMR spectra of [3a]BF4
showed no change on cooling to 195 K, indicating that the
vinylidene was undergoing rapid rotation on the NMR
timescale.
The deep green colour of the fluorovinylidene complexes is
due to an absorption band at ca. 690 nm, which showed no
significant solvatochromism (ESI†). With the aid of TD-DFT
studies this transition was shown to be dominated by HOMO
→ LUMO character. In the HOMO (Fig. 1) a fluorine lone pair
has an anti-bonding interaction with the π-system of the vinyl-
idene ligand (which is itself anti-bonding with respect to a
metal-centred orbital). The LUMO has a contribution from a
p-orbital on the fluorine and an orbital based on the metal-
bound carbon.
formed by formal nucleophilic aromatic substitution of [2d]
BF4 by the alkynyl ligand in [1a] with concomitant loss of HF.
In a similar manner to electrophilic fluorination, the electro-
philic incorporation of –CF3 groups into the framework of
organic compounds is also desirable.10,39,57,58 There are only
limited examples of CF3-substituted vinylidene complexes, all
of which are dimeric species formed from the activation of
H2CvCHCF3.46 The reaction between [1a] and Umemoto’s
+
reagent, [6]BF4, (a latent source of “CF3 ”) resulted in the selec-
tive formation of [Ru(η5-C5H5)(vCvC{CF3}Ph)(PPh3)2]BF4,
[7a]BF4 (Scheme 2c). The structure of [7a]BF4 was confirmed
by a single crystal X-ray diffraction study (ESI†). To the best of
our knowledge complexes [3]BF4 are the first examples of fluoro-
vinylidene ligands in mononuclear metal species, and access
to these complexes has allowed an evaluation of the effects of
fluorine incorporation into vinylidene ligands. For example,
the 19F NMR spectrum of [3a]+ displayed a singlet resonance at
δ −208.7 for the fluorine atom of the vinylidene ligand and the
13C{1H} NMR spectrum exhibits a doublet of triplets at δ 389.0
Additional spectroscopic data of the other halogen-substi-
tuted vinylidene complexes [Ru(η5-C5H5)(vCvC{E}C6H4-4-R)
(PPh3)2]X54 (E = Cl,‡ Br and I) and the CF3-containing complex,
[7a]BF4, support an interplay between σ- and π-effects on the
2
(2JCF = 39.3 Hz, JCP = 16.2 Hz) for the metal-bound carbon
atom and a doublet at δ 196.7 (1JCF = 222.1 Hz) for the
‡[8aCl]BF4 was prepared by a new method from [1a] and N-chlorosuccinamide,
fluorine-bearing carbon atom. These resonances are shifted [8aBr]BF4 was synthesised from one equivalent of Br2 and NaBF4, see ESI.
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Table 1 Summary of 13C NMR data and lowest energy bands in the
absorption spectra of vinylidene complexes. Experimental data were
recorded at a concentration of ca. 1 mmol dm−3 in CH2Cl2 solution
Complex
13C δ M = C/ppm
λmax/nm
λmax/nm (calc.)
[3a]BF4
[3b]BF4
[3c]BF4
[3d]N(SO2Ph)2
[4a]PF6
389.0
383.5
389.9
392.4
355.5
340.0
353.5
340.3
339.3
323.6
691
684
698
644
524
488
615
627
626
619
730
720
764
655
537
452
702
647
—
[7a]BF4
[8aCl]BF4
[8aBr]BF4
[8Br]Br3
[8aI]I3
668
energy of the HOMO-LUMO transition (Table 1). Good
π-donors (e.g. F, Cl) raise the energy of the HOMO, thereby low-
ering the energy of the HOMO-LUMO transition. In contrast,
the strongly (−I) electron-withdrawing groups, such as CF3 in
[7a]BF4 display a higher energy transition, presumably due to an
opposite effect. This effect is supported by the bathochromic
shift observed in methoxy-substituted [3c]BF4 and the hypso-
chromic shift in [3b]BF4. The interplay between σ- and π-effects
appears to also be manifested in the chemical shift of the metal
bound carbon atom, as the CF3-subsituted vinylidene ligand
has a chemical shift similar to [4a]PF6, but the halogen-substi-
tuted variants have a chemical shift range of 66 ppm.
Complex [3a]PF6 crystallises as two different polymorphs,
which have significantly different photophysical properties in
the solid state (Fig. 2). One is orange (λmax = 742 nm) and one
green (λmax = 692 nm) and they differ primarily in the orien-
tation of the fluorine atom and phenyl group of the vinylidene
ligand and the geometry around the β-carbon (ESI†). Dissol-
ution of both crystals in CH2Cl2 gave essentially identical UV-
vis and NMR spectra showing that the two crystalline forms
convert to the same species in solution. We propose that the
observed species is a time-averaged signal for the different
rotameric forms. Different agostic isomers of the same organo-
metallic complex have been shown to produce crystals with
markedly different electronic spectra,61 however, in the case of
[3a]PF6 subtle changes to the structure in the solid state has a
pronounced effect on the photophysical properties of the
system by altering the energy of the HOMO-LUMO transition.
Fig. 2 Solid-state structures of the [3a]+ cation found in (a) green crys-
tals and (b) orange crystals. Hydrogen atoms removed for clarity and
thermal ellipsoids (where shown) are at the 50% probability level.
of these species to evaluate the effects of fluorine incorpor-
ation into vinylidene ligands. In the case of half-sandwich
ruthenium complexes with either mono-62,63 or di-substituted
vinylidene ligands64 reaction with NCMe has been shown to
reverse the alkyne-vinylidene tautomerisation and generate
[Ru(η5-L)(NCMe)(PPh3)2]+, (L = C5H5, [9]+) liberating the corres-
ponding alkynes HCuCR and PhCuCR respectively. These
studies have permitted detailed kinetic insight into the nature
of the alkyne-vinylidene interconversion.65 However, heating a
sample of [3a]+ in NCMe-d3 did not result in an analogous
reaction to form [9]+ and FCuCPh. Instead, the major product
from the reaction was [10a]BF4, in which a coordinated alkene
ligand has been formed from a combination of vinylidene and
phosphine ligands, the latter having undergone an ortho-
metallation process (Scheme 3). The 31P NMR spectrum of
3
[10a]BF4 exhibits two resonances at δ 46.6 (dd, JPF = 48.6 Hz;
3
3
3JPP = 4.4 Hz) and δ 42.3 (dd, JPF = 16.2 Hz; JPP = 4.4 Hz)
Reactions of fluorine-containing vinylidene complexes
Attempted displacement of FCuCPh. The facile syntheses
of complexes [3]BF4 has allowed an exploration of the reactivity Scheme 3 (i) NCMe, 50 °C, 14 d. X− = BF4− or N(SO2Ph)2
−
.
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confirming the presence of two different phosphorus environ-
3
3
ments. Resonances at δ −143.4 (app. dt, JPF = 48.6 Hz; JHF
=
16.9 Hz; JPF = 16.2 Hz) in the 19F NMR spectrum, δ 4.69 (ddd,
3
3JHF = 16.9 Hz; JPH = 5.0 Hz; JPH = 1.7 Hz) in the 1H NMR
spectrum and 113.7 (dt, 1JCF = 243.0 Hz; 2JCP = 5.6 Hz) and 25.9
(ddd, 1JCP = 75.1 Hz; 2JCF = 7.8 Hz; 2JCP = 2.7 Hz) in the 13C{1H}
spectrum demonstrated that the fluorinated alkene ligand was
present.
2
3
Other products form on heating at higher temperature
(80 °C), but after ca. two weeks at 50 °C, the reaction was
reasonably selective (70% conversion based on 31P NMR
spectroscopy). A complex related to [10a]BF4 has been reported
previously from the nucleophilic attack of PPh3 at a co-
ordinated (protio)vinylidene.66 It is proposed that [10a]BF4 is
formed by initial loss of PPh3, followed by attack of the free
phosphine at the metal-bound carbon of the fluorovinylidene.
C–H activation and hydride migration subsequently leads to
[10a]BF4. This is supported by the fact that the dppe-contain-
ing fluorovinylidene complex [Ru(η5-C5H5)(vCvC{F}Ph)
(dppe)]N(SO2Ph)2, [3e]N(SO2Ph)2 (prepared from [Ru(η5-C5H5)
(–CuCPh)(dppe)] and 2b) is unchanged on heating at 100 °C
for 14 days in NCMe-d3. These data indicate that the conver-
sion of the fluorovinylidene to its alkyne form does not
proceed under any of these conditions.
Reaction of fluorovinylidene complex with pyridine: C–H
and C–F bond activation. Many of the catalytic applications of
ruthenium vinylidene complexes exploit the electrophilicity of
the metal-bound carbon atom.49,50,52,67 For example, we have
recently demonstrated that nucleophilic attack by pyridine at a
coordinated vinylidene was a key step in ruthenium-mediated
C–H functionalization reactions,68 a process whereby the for-
mation of pyridylidene-containing intermediates plays an
important role. Dissolution of [3a]BF4 in pyridine results
in the immediate formation of the vinyl complex [Ru(η5-C5H5)
Fig. 3 Solid-state structure of the cation [11]+, hydrogen atoms omitted
for clarity, thermal ellipsoids (where shown) are at the 50% probability
level.
trum of [11]BF4 exhibited a doublet resonance at δ −71.1
(4JPF = 20.1 Hz) and the 31P{1H} a corresponding resonance at
4
δ 44.0 (d, JPF = 20.3 Hz): the ESI-MS demonstrated that a
single pyridine had added to the ruthenium complex. The
structure of [11]BF4 was confirmed by X-ray crystallography
(Fig. 3), which showed that the alkenyl ligand had adopted a
Z-configuration. We have previously proposed that this type of
alkenyl complex was a key intermediate in the direct C–H
functionalization of the pyridine,68 and the previous inability
to observe such a species was principally owing to its low
intrinsic concentration due to competitive deprotonation of
the vinylidene ligand. Evidently, the presence of the fluorine
atom allows for the observation and characterization of this
species.
On standing in pyridine solution, [11]BF4 underwent a
further reaction to give [12]BF4 and free PPh3. Monitoring this
reaction by NMR spectroscopy indicated that this process pro-
ceeds through a number of phosphorus- and fluorine-contain-
ing intermediates. The molecular structure of [12]BF4 (Fig. 4 as
its pyridine solvate) showed that this species was a novel
pyridylidene complex had been formed by activation of a
C–H bond of the pyridine group in [11]BF4, incorporation of a
second pyridine molecule and formal loss of HF, the fate of
which could not be determined.
(–C{py}vC{F}Ph)(PPh3)2]BF4, (py
=
pyridine), [11]BF4
(Scheme 4), demonstrating that the incorporation of the fluor-
ine substituent does not inhibit nucleophilic attack at the
metal-bound carbon. In CD2Cl2 solution the 19F NMR spec-
A potential mechanism for the conversion of [3a]BF4 to
[12]BF4 was investigated by a DFT study. Given the changes in
molecularity during the proposed mechanism there are signifi-
cant differences in the relative zero-point-energy-corrected
electronic energies (ESCF+ZPE) and Gibbs energies of different
states. The following discussion refers solely to the Gibbs ener-
gies at 298 K, although both are presented in Fig. 5. In the first
step, nucleophilic attack of pyridine occurs at the metal-bound
carbon atom of the vinylidene in [3a]+ (taken as the reference
point for the calculations) to give [11-Z]+ through transition
Scheme 4 (i) Pyridine, 20 °C, <5 min. (ii) Pyridine, –HF, 20 °C, 10 days.
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give (after coordination of the triple bond in [G]+ to the metal)
alkyne complex [H]+. Nucleophilic attack by pyridine through
TS[H]+[12]+ then gives the observed product [12]+, which is
also the lowest point on the potential energy surface
(−45 kJ mol−1).
The calculations indicate that the nucleophilic attack by
pyridine of [3a]+ to give the E-isomer ([11-E]+) of the complex is
both kinetically (TS[3a]+[11-E]+ +155 kJ mol−1) and thermo-
dynamically unfavorable (+65 kJ mol−1) when compared to the
formation of the Z-isomer. This appears to be due to an
unfavourable interaction between the PPh3 ligands and the Ph-
group of the alkenyl ligand in the E-isomer. In contrast,
however, defluorination from [F] is far more accessible from
the E-isomer of the alkene complex ([F-E]) than from the
Z-isomer (TS[F-E][G]+ +140 kJ mol−1; TS[F-Z][G]+ +173 kJ mol−1, see
ESI† for full E- and Z-pathways). There are a number of poss-
ible points on the PES where interconversion of the E- and
Z-isomers may occur. For example, previous calculations indi-
cate that the vinylidene complexes, [I-Z]+ and [I-E]+ (Fig. 6a)
would be readily accessible from complexes such as [A]+.39
Rotation of vinylidene ligands, which will interchange the E-
and Z-isomers, is a low energy process.70,71 We tentatively
propose that at some point following state [11-Z]+ the system
isomerises and follows the E-isomer pathway.
Fig. 4 Solid-state structure of one of the two crystallographically inde-
pendent cations in the structure of [12]BF4·pyridine, hydrogen atoms
omitted for clarity, thermal ellipsoids (where shown) are at the 50%
probability level.
state TS[3a]+[11-Z]+. The fact that [11]+ is predicted to be lower in
energy than [3a]+ and TS[3a]+[11-Z]+ is reasonably accessible
(ΔG‡
+ 100 kJ mol−1) is consistent with the experimental
298
observation of fast formation of [11]BF4.
The next stages in the reaction pathway are directly related
to previous studies on pyridine C–H activation.68,69 Loss of
PPh3 from [11]+ leads to intermediate [A]+. Formation of
agostic complex [B]+ followed by C–H activation via TS[B]+[C]+
leads to hydride complex [C]+ which may then undergo a sub-
sequent migration to afford [D]+. A subsequent isomerisation
to the lower energy isomer [E]+ then occurs. Formal elimin-
ation of HF must then occur to give [12]+ although attempts to
model elimination of HF from [E]+ via a pyridine-induced E2-
process were unsuccessful. However, deprotonation of [E]+ (or
hydride complex [C]+) may occur in pyridine solution to afford
[F] and [HNC5H5]+. Defluorination may now occur through
TS[F][G]+ in which the [HNC5H5]+ aids the loss of the fluorine to
The formation of [11]+ is rapid and a second, slower, reac-
tion subsequently gives [12]+. In addition to supporting our
previous observation that the [Ru(η5-C5H5)(PPh3)]+ fragment
may selectively activate pyridine in the 2-position,68,72 the DFT
study also provides an explanation as to why no alkenylated
pyridine is observed in the fluorine-containing system. The key
transition state in this process involves C–N bond cleavage in
[F-E] via TS[F-E][J-E] which at +150 kJ mol−1 lies higher in energy
than TS[F-E][G]+ (Fig. 6b) implying that defluorination is
preferred to C–N bond cleavage.
It appears that C–F bond cleavage in this system occurs by a
pyridinium-assisted fluoride abstraction. This process is related
Fig. 5 Potential energy surface for the proposed mechanism of formation of [12]+ from [3a]+. ESCF+ZPE (top) and Gibbs energies at 298.15 K
(bottom), relative to [3a]+ and two molecules of pyridine, are shown in kJ mol−1. Calculations are at the (RI-)PBE0-D3/def2-TZVPP//(RI-)BP86/SV(P)
level with COSMO solvation in pyridine. [Ru] = [Ru(η5-C5H5)(PPh3)].
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Fig. 6 Calculations at the (RI-)PBE0-D3/def2-TZVPP//(RI-)BP86/SV(P)
level with COSMO solvation in pyridine. [Ru] = [Ru(η5-C5H5)(PPh3)].
ESCF+ZPE (top) and Gibbs energies at 298.15 K (bottom), relative to [3a]+,
pyridine and PPh3.
to Hughes’ studies on half-sandwich iridium complexes in
which α-fluoride abstraction leads to a cationic carbene
complex.73,74
In this case, a cationic alkyne complex is the product of the
fluoride loss from the β-position of complex [F], which then
undergoes nucleophilic attack by pyridine.
Hydrolysis of fluorovinylidene complexes: formation of an
α-fluoroaldehyde and acyl-fluoride. The observation that fluoro-
vinylidenes ligands are susceptible to nucleophilic attack, in
a similar way to their non-fluorinated analogues, prompted us
to explore this type of reaction with a view to generating fluori-
nated organic molecules. Vinylidene complexes are well known
to undergo hydrolysis and hydration via nucleophilic attack of
water at the α-carbon of the vinylidene.75,76 The products of
hydration depend on the ligands present at the metal. Careful
design has allowed the development of Ru-catalysts for the
anti-Markovnikov hydration of terminal alkynes via vinylidene
intermediates.77–81,82 We were therefore interested in under-
standing the reactivity of [3]BF4 with water. However, [3a]BF4
proved to be relatively stable in the presence of water, in the
absence of other additives. Addition of either stoichiometric or
excess water to a CH2Cl2 solution of [3a]BF4 at room temperate
did not result in an immediate reaction. Addition of one equi-
valent of a base, 1,4-diazabicyclo[2.2.2]octane (DABCO), and
water (either stoichiometric water or ten equivalents) resulted
in the formation of a number of new compounds. The major
new species were identified, by 31P{1H} NMR spectroscopy, as
[Ru(η5-C5H5)Cl(PPh3)2], [13], and, remarkably, [Ru(η5-C5H5)
(CuCPh)(PPh3)2], [1a] the latter corresponding to the formal
loss of “F+” from the fluorovinylidene.83 Monitoring the reac-
tions over an extended period (ca. two weeks) at room tempera-
ture showed almost total conversion to [13]. Heating the
reactions at 50 °C increased the rate of conversion but not the
final product distribution. This should be contrasted with the
reaction of [4a]+ with water which results in the formation of
[Ru(η5-C5H5)(CO)(PPh3)2]+ and toluene.76
Scheme 5 (i) CD2Cl2, +NnBu4Cl, –NnBu4BF4, R. T., 14 d. (ii) CD2Cl2,
+NnBu4Cl, –NnBu4(SO2Ph)2, R. T. (iii) 6 d, R. T.
In contrast, hydrolysis of [3a]BF4 in the presence of chloride
as an additive (introduced as NnBu4Cl·H2O§) results in the
selective, if slow, formation of [Ru(η5-C5H5)Cl(PPh3)2], [13] and
the acyl-fluoride 14 (Scheme 5a), which has resulted from
formal hydration of the vinylidene ligand and isomerisation of
the resulting α-fluoroaldehyde (vide infra). Compound 14 was
identified on the basis of characteristic resonances in the
3
1
3
19F (δ 43.7, t, JHF = 1.5 Hz), H (δ 3.84, d, JHF = 1.5 Hz), and
13C{1H} NMR spectra (δ 39.2, d, JCF = 54 Hz, CH2; 162.1, d,
2
2JCF = 362.3 Hz, COF) matching those previously reported.84
Monitoring the reaction by NMR spectroscopy demonstrated
that [13] and 14 were formed at approximately the same rate.
Compound 14 was isolated by sublimation and proved to be
highly moisture sensitive.
The formation of 14 is somewhat surprising as it appears
that either fluoro- or phenyl-migration must have occurred.
Given that the reaction of [3a]BF4 with NCMe did not give rise
to the alkyne FCuCPh and [9]BF4 (which would also require
§The ratio of NnBu4Cl to H2O was shown to be ca. 1 : 1 by elemental analysis.
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F or Ph migration) it might be presumed that these are high- formation and may be applicable to a range of organometallic
energy pathways in this system, although phenyl-migration in systems.
di-substituted vinylidene complexes is well established.64,85–88
As the new fluorinated ligands were retained within the
In order to gain further mechanistic insight into this process, coordination sphere of ruthenium it was possible to evaluate
the 13C-enriched vinylidene complex [3a-13C]BF4 was prepared the effects of incorporation of the fluorine and explore sub-
and subjected to an identical reaction with NnBu4Cl·H2O sequent reactivity. Reactions of fluorovinylidene complexes
(Scheme 5b). Analysis of the resulting 19F NMR spectra demon- with nucleophiles such as pyridine and water show many
strated that the resonance for 14 now appeared as a doublet similarities to the reactivity of non-fluorinated vinylidenes. As
(1JCF = 361.4 Hz) with no evidence of a species with the these25 are important intermediates in a range of metal-cata-
labelled atom in the alternative position. Repeating the reac- lysed reactions, this suggests that access to fluorinated ana-
tion with [3a]+ as the benzenesulfonimide salt (prepared from logues offers new opportunities for the formation of novel
[1a] and NFSI) also gave 14, indicating that the source of the fluorinated organic compounds without re-writing the rule
fluorine was the vinylidene ligand not the [BF4]− counteranion. book in terms of their reactivity. Indeed, our observation of
Hydrolysis of the dppe-containing complex [3e]N(SO2Ph)2 anti-Markovnikov hydration of fluorovinylidenes, a key reaction
in the presence of chloride (as NnBu4Cl·H2O) is much faster of non-fluorinated analogues, to form both new functionality
than the PPh3-containing analogue. At room temperature and a new stereocentre in the α-fluoroaldehyde products sup-
rapid formation of [Ru(η5-C5H5)Cl(dppe)], [15] and the α-fluoro- ports this argument. In addition to applications in synthesis,
1
aldehyde 16 (identified by resonances in the H and 19F NMR the introduction of fluorine into ligands of organometallic
3
5
spectra at δ 9.72 (dd, JHF = 7.6 Hz, JHH = 0.7 Hz and δ −191.7 complexes may significantly alter metal–ligand bonding and,
2
3
(dd, JHF = 47.0 Hz, JHF = 7.6 Hz) respectively) was observed, for example, may be used to blue-shift the emission from
Scheme 5c.89 Over the course of 6 days the resonances for 16 luminescent iridium complexes.91–94 In summary, it has been
decreased in intensity to be replaced by those for 14. It is pro- demonstrated how OSEF allows for the facile formation of C–F
posed that anti-Markovnikov hydration of the vinylidene77–79,82 (and C–CF3 bonds) in metal alkynyl complexes to be achieved
occurs to give 16 which then undergoes a slower isomerisation and that C–F bond cleavage, assisted by a pyridinium salt, may
to 14. DFT calculations indicate that 16 is 97 kJ mol−1 higher occur from the β-position of an organic ligand.
in energy (ΔG298) than 14 so this conversion is thermo-
dynamically favoured and aldehydes such as 16 have been
reported to be unstable.90 The overall process is summarised
in Scheme 5d.
Acknowledgements
The reaction of metal alkynyl complexes (derived from
We thank the EPSRC (DTA studentship to LMM and compu-
terminal alkynes) with “F+” followed by hydrolysis and liber-
tational equipment EP/H011455/1 and EP/K031589/1) and the
ation of α-fluoroaldehydes represents a stepwise functionali-
University of York (PhD Studentship to LMH and summer
zation of alkynes with the incorporation of both new
studentship to MKS) for funding. The insightful comments of
functionality and the generation of a new stereocentre. This
Professors Duncan Bruce and Robin Perutz on this work are
type of reactivity is highly desirable as the fluorinated vinyl-
gratefully acknowledged.
idene ligands are effectively acting as synthetic equivalents of
1-fluoroalkynes. Work is underway in our groups to exploit
these stoichiometric observations for the development of cata-
lytic protocols for combined alkyne functionalization and
fluorination.
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