Stepwise addition of difluorocarbene to a transition metal centre

Article abstract of DOI:10.1039/c3cc48468h

The Ruppert-Prakash reagent (Me₃SiCF₃) is used to introduce difluorocarbene (CF₂) and tetrafluoroethylene (TFE) ligands to cobalt(i) metal centres, whereby the TFE ligand is generated via [2+1] cycloaddition between [Co]CF₂ and CF₂.

Full text of DOI:10.1039/c3cc48468h

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Stepwise addition of difluorocarbene to a
transition metal centre†
Cite this: Chem. Commun., 2014,
50, 1128
Graham M. Lee, Daniel J. Harrison, Ilia Korobkov and R. Tom Baker*
Received 5th November 2013,
Accepted 2nd December 2013
DOI: 10.1039/c3cc48468h
www.rsc.org/chemcomm
The Ruppert–Prakash reagent (Me3SiCF3) is used to introduce difluoro-
carbene (CF2) and tetrafluoroethylene (TFE) ligands to cobalt(I) metal
centres, whereby the TFE ligand is generated via [2+1] cycloaddition
Q
between [Co] CF2 and CF2.
Among the most versatile tools for the synthesis of metal-fluoroalkyl
complexes is the Ruppert–Prakash reagent (Me3SiCF3).1 It has been
used to prepare a variety of transition metal complexes with trifluoro-
methyl (CF3) ligands, including examples of first-row (Ti, Ni, Cu),
second-row (Ru, Rh, Pd) and third-row (Pt, Au) metals.2 Recently,
conditions were reported that render Me3SiCF3 an excellent source of
Scheme 1 Previously reported synthesis (a), and reactivity (b) of cobalt
fluorocarbenes. L = phosphine or phosphite, RF = F or CF3.
difluorocarbene (CF2) (eqn (1)), as applied to the synthesis of difluoro- the CF3 group to the metal centre yields [Ru(QCF2)(F)(H)(CO)(L2)].2d
cyclopropanes and difluorocyclopropenes,3 as well as unusual This difluorocarbene complex is electrophilic at the carbenoid carbon
fluorinated carbacycle motifs.4 Iodide activates Me3SiCF3 to liberate atom, demonstrated by hydride migration in the presence of coordinat-
the trifluoromethyl anion, which decomposes into CFꢀ2 a5nd Fꢀ. The ing solvent.2d Recently, we reported the synthesis of nucleophilic cobalt
fluoride ion also reacts with Me3SiCF3 to release CF3
.
fluorocarbenes (Scheme 1a),10 using a procedure adapted from Hughes
and co-workers.9 The [Co]QCFRF complexes undergo [2+2] cyclo-
addition reactions with tetrafluoroethylene (TFE) to give perfluoro-
metallacyclobutanes.6 The metallacyclobutane compounds exhibit rich
reactivity upon activation of Cb–F bonds, including the catalytic
isomerization to alkene complexes under acid catalysis (Scheme 1b).
For the present work, we investigated CpCoL2 complexes [Cp =
Z5-C5H5; 1a, L = CO; 1b, L = PPh3; 1c, L = P(OiPr)3] as potential CF2
acceptors in reactions with the Ruppert–Prakash reagent. Compounds
of type 1 were selected based on our previous work that demonstrated
the [CpCoL] substructure can support the CF2 fragment, making it an
attractive platform for CF2-transfer screening reactions.
Me3SiCF3 ꢀNaIð!initiatorÞ Me3SiF þ : CF2
(1)
THF
Here, we present a novel application of Me3SiCF3 for directly
introducing the CF2 group to transition metal compounds, providing
new routes to metal difluorocarbene ([Co]QCF2) and metal tetrafluoro-
ethylene (TFE) complexes {[Co](Z2-C2F4)}. Such compounds are under
investigation as intermediates in potential catalytic cycles utilizing
perfluoroalkenes (e.g., metathesis and polymerization).6
Examples of metal fluorocarbenes ([M]QCFRF, RF = F or CF3) are
rare and, relative to metal alkylidenes or other types of Fischer
carbenes,7 have been the subject of few reactivity studies.8 Almost
without exception, [M]QCF2 complexes are prepared via fluoride
abstraction/elimination from metal fluoroalkyl precursors.9 Notably,
Caulton and co-workers showed that Me3SiCF3 reacts with a ruthenium
fluoride complex to give [Ru(CF3)(H)(CO)(L2)]; a fluoride migration from
Treatment of 1 with Me3SiCF3 (2 equivalents) and catalytic quan-
tities of NaI in THF at 65 1C gave a mixture of the corresponding
cobalt fluorocarbenes 2a–c, and novel cobalt tetrafluoroethylene
complexes 3a–c (Scheme 2). The products were readily identified in
solution by their distinct 19F NMR signals.
Selectivity for products 2 vs. 3 depends on the nature of
the ancillary ligands. When L = CO (i.e., 1a), the TFE complex 3a is the
major product, and only minor quantities of 2a are observed. The
19F NMR spectrum of 2a displays two characteristically downfield
resonances at d = 112.5 ppm and 83.4 ppm (2JFF = 152 Hz), consistent
with data previously reported for [CpCo(QCF2)(L)] complexes.6,10
Department of Chemistry and Centre for Catalysis Research and Innovation,
University of Ottawa, 30 Marie Curie, Ottawa, Ontario K1N 6N5, Canada.
E-mail: rbaker@uottawa.ca
† Electronic supplementary information (ESI) available. CCDC 968287–968289.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3cc48468h
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Scheme 2 Structures of cobalt fluorocarbenes 2, and TFE complexes 3
(NMR determined yields)11 from Co(I) complexes 1.
Complex 3a was isolated as a brown-yellow oil in 69% yield, whereas
the carbene complex 2a could only be observed spectroscopically. We
previously reported that attempts to prepare 2a via reduction of
[CpCo(CO)I(CF3)] were unsuccessful.10 Complexes 1b or 1c (with PPh3
or P(OiPr)3 ligands) react under the same conditions to yield cobalt
fluorocarbenes 2b (reported previously)10 or 2c, respectively, as the
minor products (although in much higher yields than 2a), along with
major products 3b and 3c. Using four equivalents of Me3SiCF3
increases the yield of alkene complexes 3b and 3c significantly, while
carbenes 2b and 2c are no longer observed in solution. The crystal
structures of 3b and 3c are presented in Fig. 1.
Scheme 3 Proposed pathways for generation of 3. Path A involves direct
addition of TFE to 1, while path B is comprised of the stepwise addition of
CF2, with 2 formed as a stable intermediate.
concomitant formation of Me3SiF (Fig. S1, ESI†). In order to probe
the feasibility of path A, complexes 1a–c were treated with TFE
(1.7 atm) in THF at 65 1C. Complexes 1a and 1c did not react under
these conditions, and the addition of NaI also had no effect. Small
amounts of 3a were observed in a complex mixture when a THF
solution of 1a was photolyzed (medium-pressure Hg lamp) in the
presence of TFE (1.7 atm), presumably through photolytically-
generated [CpCo(CO)].15 Interestingly, Stone and co-workers reported
in 1961 that 1a reacts with excess TFE in cyclohexane at high
temperatures (160 1C) to produce the perfluorocyclopentane complex
[CpCo(CF2)4(CO)] in 11% yield.16 While 3a is likely an intermediate in
this process, we did not observe the 5-membered ring product under
the conditions we explored.
The 19F NMR spectra of Z2-TFE complexes 3a–c are highly
characteristic. In THF or C6D6 at room temperature, the signals
for the TFE ligand exhibit second order coupling indicative of either
an AA0BB0 spin system for 3a, or an AA0BB0X spin system for 3b and
3c (X = 31P), and Cs symmetry for all three complexes. The observa-
tion of two resonances with well-resolved splitting patterns suggests
the C2F4 fragment does not rotate with respect to the metal on the
NMR timescale in solution, in contrast to related Z2-C2F4 complexes
of Ni and Pd described by Ogoshi and co-workers,12 or Ru and Ir
complexes described by Hughes and co-workers.13
In contrast to 1a and 1c, complex 1b reacts with TFE to produce
3b in 89% yield by 19F NMR. These results indicate that under the
conditions explored, path A does not likely contribute to the
formation of 3a and 3c, but can contribute to the formation of 3b,
if TFE is formed in appreciable quantities. The increased reactivity
toward TFE of 1b vs. 1a,c is apparently due to the increased lability of
PPh3 relative to p-accepting CO and P(OiPr)3, allowing generation of
16eꢀ complex [CpCo(PPh3)] in solution. These results suggest a
dissociative mechanism for pathway A.
Under the reaction conditions outlined in Scheme 2, we envision
two likely pathways for formation of TFE complexes 3a–c, as illustrated
in Scheme 3. In pathway A, tetrafluoroethylene, formed in situ from
two equivalents of CF2,14 reacts directly with complexes 1a–c. 19F NMR
analysis of a mixture of only Me3SiCF3 and NaI in THF confirms that
TFE is formed cleanly as the major product upon heating, with
Pathway B represents a new synthetic route to metal fluoro-
alkene complexes. In this scheme, a metal fluorocarbene inter-
mediate 2 is formed initially, which undergoes [2+1] cycloaddition
reaction with a second equivalent of CF2 to yield perfluoroalkene
complexes 3. Indeed, independently-synthesized 2b and 2c react
with Me3SiCF3/NaI, producing 3b and 3c, respectively, in high yield
(>90% by 19F NMR). Similarly, the fluoro(trifluoromethyl) carbene
complex 4 (reported previously)10 is converted to the corresponding
fluoroalkene complex 5 in high yield under the same conditions.
These reactions are summarized in Scheme 4, and the crystal
structure of 5 is presented in Fig. 2.
From these results, it can be reasoned that path B likely
contributes, along with path A, to the formation of 3b. In the
case of 3a and 3c, B appears to be the dominant pathway. The
detailed mechanism of pathway B, (difluorocarbene addition
to complexes 1 and 2) is under further investigation using
DFT calculations.
Fig. 1 Molecular structures of 3b (left) and 3c (right). The ellipsoids are set to 50%
probability, and hydrogen atoms are omitted for clarity. Selected bond distances
(Å) and angles (1): 3b: Co1–C24 1.884(3), Co1–C25 1.897(3), Co1–P1 2.1930(7),
Co1–Cp(centroid) 1.735(6), C24–F1 1.357(5), C24–F2 1.361(4), C25–F3 1.362(4),
C25–F4 1.347(5), C24–Co1–C25 44.49, Co1–C25–C24 67.27, C25–C24–
Co1 68.24. 3c: Co1–C6 1.880(2), Co1–C7 1.896(2), Co1–P1 2.1478(6),
Co1–Cp(centroid) 1.711(2), C6–F1 1.372(3), C6–F2 1.376(3), C7–F3 1.370(2),
C7–F4 1.356(3), C6–Co1–C7 43.92, Co1–C7–C6 67.45, C7–C6–Co1 68.62.
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the province of Ontario and University of Ottawa (OGS and
Vision 2020 Scholarships).
Notes and references
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Scheme 4 Synthesis of fluoroalkene complexes via [2+1] cycloaddition
between CF2 and pre-isolated cobalt fluorocarbenes. For complexes 4 and
5, L = PPh3, RF = CF3.
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Co1–Cp(centroid) 1.718(3), C24–F1 1.373(3), C24–F2 1.351(3), C25–F3 1.388(3),
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The unique [2+1] reactions described here, involving highly
electrophilic difluorocarbene,17 are consistent with the nucleo-
philic character of the CoQC bond of the Co(I) fluorocarbene
complexes.6,10 Upon addition of CF2, the Co(I) metal centre of
carbenes 2 are formally oxidized to Co(III). The Co–C (TFE) bonds
in 3b (Co1–C24 1.884 Å; Co1–C25 1.897 Å) are significantly longer
than the CoQC bond of 2b (1.7395 Å), and the same is true
for the analogous Co–C (TFE) bonds of 5 (Co1–C24 1.902 Å;
Co1–C25 1.943 Å) relative to the CoQC bond of 4 (1.751 Å).
In conclusion, we have demonstrated that cobalt difluoro-
carbenes and Z2-TFE complexes are generated via sequential
addition of CF2, generated from Me3SiCF3 and catalytic NaI, to
CpCoL2 complexes. We also note that Me3SiCF3/NaI can be
used as a safe and convenient precursor for generating tetra-
fluoroethylene. Future work will extend the methods described
here to synthesize new difluorocarbene and perfluoroalkene
transition metal complexes18 with potential relevance to cata-
lytic processes involving fluorocarbon substrates.
16 T. D. Coyle, R. B. Kings, E. Pitcher, S. L. Stafford, P. Teichel and
F. G. A. Stone, J. Inorg. Nucl. Chem., 1961, 20, 172–173.
17 Z.-Y. Yang, J. Am. Chem. Soc., 1996, 118, 8140–8141.
We thank the NSERC and the Canada Research Chairs
program for generous financial support and the University of 18 While this work was in progress, Ozerov et al. reported obtaining a
mixture of [M]QCF2 and [M](Z2-C2F4) products from the reaction of
a (PNP)Rh alkene complex with a mixture of Me3SiCF3 and CsF in
C6D6 at 70 1C: Canadian Society of Chemistry Abstract. INOR 163,
Ottawa, Foundation for Innovation and Ontario Ministry of
Economic Development and Innovation for essential infrastruc-
ture. G.M.L. and D.J.H. gratefully acknowledge support from
May 27, 2013.
1130 | Chem. Commun., 2014, 50, 1128--1130
This journal is ©The Royal Society of Chemistry 2014