Cobalt fluorocarbenes: Cycloaddition reactions with tetrafluoroethylene and reactivity of the perfluorometallacyclic products

Article abstract of DOI:10.1021/ja411503c

Cobalt fluorocarbene complexes CpCo(i - CFR^F) (PPh₂Me) (Cp = η⁵-C₅H₅, R ^F = F or CF₃) react with tetrafluoroethylene to give the metallacyclobutanes CpCo(κ²-CFR^FCF ₂CF₂-)(PPh₂Me) in the first examples of cycloaddition reactions between perfluoroalkenes and metal perfluorocarbenes. The metallacyclic products undergo a variety of reactions upon activation of the C-F bonds, including Bronsted acid-catalyzed C-F/Co-C scrambling. Implications for metal-catalyzed metathesis and polymerization of perfluoroalkenes are discussed.

Full text of DOI:10.1021/ja411503c

Communication
pubs.acs.org/JACS
Cobalt Fluorocarbenes: Cycloaddition Reactions with
Tetrafluoroethylene and Reactivity of the Perfluorometallacyclic
Products
Daniel J. Harrison, Graham M. Lee, Matthew C. Leclerc, Ilia Korobkov, and R. Tom Baker*
Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 30 Marie Curie, Ottawa, Ontario
K1N 6N5, Canada
S
* Supporting Information
Scheme 1
ABSTRACT: Cobalt fluorocarbene complexes CpCo(
CFRF)(PPh2Me) (Cp = η5-C5H5, RF = F or CF3) react
with tetrafluoroethylene to give the metallacyclobutanes
CpCo(κ2-CFRFCF2CF2-)(PPh2Me) in the first examples
of cycloaddition reactions between perfluoroalkenes and
metal perfluorocarbenes. The metallacyclic products
molecular structures are presented in Figure 1.9 Indeed, the
PPh2Me ligand renders 1a,b more reactive (see below) than the
PPh3 and P(OMe)3 analogs disclosed previously.7
undergo a variety of reactions upon activation of the C−
F bonds, including Brønsted acid-catalyzed C−F/Co−C
scrambling. Implications for metal-catalyzed metathesis
and polymerization of perfluoroalkenes are discussed.
ransition-metal alkylidenes ([M]CRR′; R and R′ = H,
T
alkyl or aryl) are important intermediates in organometallic
chemistry, most notably in catalytic alkene metathesis.1 Metal
carbenes ([M]CRR′; R and/or R′ = heteroatom donor
group), including N-heterocyclic carbene complexes, are also
implicated in a wide variety of catalytic and stoichiometric
transformations.1−3 Metal fluorocarbenes ([M]CFRF; RF = F
or perfluoroalkyl)4 have been studied in much less detail
compared to metal alkylidenes and other types of metal carbenes
and, so far, have not been utilized for catalysis. Terminal first-row
metal fluorocarbenes are very rare, with only three examples
reported prior to our work {[CpFe(CF2)(CO)L]+; L = CO or
PPh3; Cp = η5-C5H5 and [Mn(CF2)(CO)5]+}.5 These cationic
complexes are potently electrophilic at the carbene carbon
atoms. Counting the carbene ligands as neutral two-electron
donors, the metal atoms have formal d6 electron configurations,6
and the metal-carbene π bonds are expected to be polarized
toward the electron-deficient metal atoms.4 For potential
catalytic applications based on electron-poor fluoroalkenes
(e.g., metathesis or polymerization), we reasoned that more
electron-rich/nucleophilic metal fluorocarbenes would be
required.
Figure 1. ORTEP representations of the molecular structures of 1a
(left) and 1b (right) with 30% probability ellipsoids. Hydrogen atoms
are omitted for clarity, and selected bond distances are indicated.9
Cobalt fluorocarbenes 1a and 1b react with TFE (CF2CF2)
to give the perfluorometallacyclobutanes 2a and 2b, via formal [2
+ 2] cycloaddition (Scheme 2, Figure 2).9 The metallacycles
Scheme 2
were obtained in >75% isolated yields (19F NMR yields >85%)
after 4 days at room temperature under 1.5−2 atm of TFE
pressure. Compound 2b is produced as a single diastereomer
with the CF3 group on the opposite face of the metallacycle
relative to the phosphine (Figure 2). The only other reported
transition-metal perfluorocyclobutane was produced by decar-
bonylation of an iron bis(acyl) complex.10
We recently reported the first stable difluoro- and fluoro-
(perfluoroalkyl)carbenes of cobalt.7 These complexes were
synthesized by two-electron reduction of CoIII precursors
(Scheme 1), following the method developed by Hughes et al.
for making related iridium fluorocarbenes.8 The products have d8
cobalt(I) centers, and the [Co]CFRF bonds exhibited
nucleophilic character in preliminary reactivity studies.7
For this study, we synthesized new derivatives with the more
basic phosphine, PPh2Me (Scheme 1), to obtain cobalt
fluorocarbenes with enhanced nucleophilicity. Complexes 1a,b
were isolated in >70% yield (1−2 mmol scale), and their
The mechanism of alkene addition is under investigation, but
preliminary kinetic data reveal that the reaction between 1a and
Received: October 2, 2013
Published: December 2, 2013
© 2013 American Chemical Society
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are investigating a fluoro-variant of the Green−Rooney
mechanism16 as a potential route to fluoropolymers. The
proposed cycle is initiated by formal [2 + 2] fluoroalkene
addition to a metal fluorocarbene to give a perfluorometallacy-
clobutane intermediate, identical to the first step of the alkene
metathesis sequence. Ideally, activation of one of the Cα−F
bonds,4,8b,14 accompanied by 1,3-fluoride migration and opening
of the strained metallacycle, generates a new metal fluorocarbene
with a growing perfluoroalkyl chain (Scheme 3, right).
Compounds 2a,b are thermally stable, with or without TFE
present, and do not decompose or react further upon heating for
>24 h at 60 °C in C6D6. However, when 2a,b are treated with
Me3SiOTf (Tf = SO2CF3), fluoride abstraction yields perfluoro-
trans-vinyl (3)17 or perfluoro-trans-allyl (4) products (Scheme 4,
Scheme 4
Figure 2. ORTEP representations of the molecular structures of 2a
(top) and 2b (bottom) with 30% probability ellipsoids. Hydrogen atoms
are omitted for clarity. Selected bond distances [Å]: 2a: Co−
Cp(centroid) = 1.724(5), Co−P = 2.209(1). 2b: Co−Cp(centroid) =
1.723(2), Co−P = 2.226(6). For 2a, one orientation is shown for the
disordered F atoms on the central carbon atom.9
excess TFE is only marginally slower in the presence of 20 equiv
of free PPh2Me. Additionally, CpCo(CF2)[P(OMe)3] under-
goes TFE addition much slower than 1a but slightly faster than
the PPh3 complex (SI). We therefore suggest that TFE addition
proceeds without phosphine dissociation from the metal and
small, Lewis-basic ligands appear to favor relatively rapid
reactions.11 Attempts to synthesize Co fluorocarbenes with L =
PMe3 or PMe2Ph were unsuccessful using the reduction route
(Scheme 1) or ligand substitution from the PPh3 derivative under
conditions similar to those used for the TFE cycloaddition
reactions.
left; yields from 19F NMR).18 Moreover, catalytic quantities of
HNTf2 induce clean isomerization/ring-contraction reactions,
affording cobalt hexafluoropropene (5/5′, RF = F) or perfluoro-
2-butene (6/6′, RF = CF3) complexes (Scheme 4, right).
Brønsted and Lewis acids have been shown to activate Cα−F
bonds in metal perfluoroalkyls,4,8b,14 but previous examples have
used stoichiometric quantities of acid and the fluoride ion was
lost irreversibly.
Metallacycles and metal carbenes/alkylidenes are key
intermediates in numerous stoichiometric and catalytic pro-
cesses,1−3 and we are interested in harnessing perfluoroalkene
metathesis (e.g., Scheme 3, left) as a powerful technique for
The mechanistic details of the reactions of 2a,b are currently
being explored by computational methods. However, the
reaction between 2a and [HPPh2Me](NTf2) produces the β-
phosphonium metallacycle 7 (40 °C, 22 h in CH2Cl2, 81%
isolated yield), strongly suggesting that fluoride-abstraction
occurs at the β position.19 We propose that Cβ−F activation
yields HF and an electrophilic π perfluoroallyl complex (Int1 in
Scheme 5), and the latter is trapped by the phosphine.
Scheme 3
Scheme 5
organofluorine chemistry. We are also considering metal
fluorocarbenes as potential initiators for the polymerization of
tetrafluoroethylene (TFE) and other fluorinated alkenes. The
controlled polymerization of perfluoroalkenes in a metal
coordination sphere has not been achieved,12 despite being an
attractive alternative to the currently used radical polymerization
methods which require environmentally persistent fluorosurfac-
tants.13
Nucleophilic attack of PMe3 and F− at the β position of a
related η3-perfluoroallyl iron complex has been documented,
providing support for Int1.20 Thus, the ring-opening and
catalytic ring-contraction reactions likely proceed via a unique
Cβ−F activation pathway and probable allyl intermediates
(Int1), rather than the expected Cα−F activation mechanism
To avoid the problem of alkene insertion into inert [M]−RF
bonds,14a as required by the Cossee−Arlman mechanism,15 we
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Communication
observed previously for metal fluoroalkyls and fluorometallacy-
clopentanes.4,8b,14
ACKNOWLEDGMENTS
■
We thank the NSERC and the Canada Research Chairs program
for generous financial support and the University of Ottawa,
Canada Foundation for Innovation and Ontario Ministry of
Economic Development and Innovation for essential infra-
structure. Prof. Russell P. Hughes is thanked for helpful
discussions and Dr. Glenn Facey for assistance with NMR
experiments. D.J.H., G.M.L., and M.C.L. gratefully acknowledge
support from Ontario and the University of Ottawa (Vision 2020
and OGS programs).
For RF = CF3, the σ allyl species was observed directly (4,
above). In the case of RF = F, an allyl complex is likely formed
upon fluoride abstraction but undergoes isomerization to the
CF3-subsituted vinyl species (3) (Scheme 6, left), unless the
Scheme 6
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and data. This information is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
rbaker@uottawa.ca
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Notes
The authors declare no competing financial interest.
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(18) Reactions in CH2Cl2 containing water (∼0.003 wt %) were
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