Cobalt fluorocarbene complexes

Article abstract of DOI:10.1021/om3010959

Unusual nucleophilic cobalt fluorocarbene complexes ([Co^I]=CFR; R = F, CF₃) are described. Only a handful of fluorocarbenes of first-row transition metals have been reported, and these have exhibited electrophilic reactivity at the metal-carbene bonds. The title compounds are the first persistent/isolable cobalt fluorocarbenes and also rare examples of metal fluorocarbenes with relatively nucleophilic M=C bonds (vs other metal fluorocarbene complexes), demonstrated by their reactions with H⁺ and Me⁺ and their unusual stability toward water. X-ray crystal structures reveal the shortest terminal cobalt-carbene bonds reported to date, and DFT studies provide details into the nature of these bonds.

Full text of DOI:10.1021/om3010959

Communication
pubs.acs.org/Organometallics
Cobalt Fluorocarbene Complexes
Daniel J. Harrison, Serge I. Gorelsky, Graham M. Lee, 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
ABSTRACT: Unusual nucleophilic cobalt fluorocarbene
complexes ([CoI]CFR; R = F, CF3) are described. Only a
handful of fluorocarbenes of first-row transition metals have
been reported, and these have exhibited electrophilic reactivity
at the metal−carbene bonds. The title compounds are the first
persistent/isolable cobalt fluorocarbenes and also rare
examples of metal fluorocarbenes with relatively nucleophilic
MC bonds (vs other metal fluorocarbene complexes),
demonstrated by their reactions with H+ and Me+ and their
unusual stability toward water. X-ray crystal structures reveal
the shortest terminal cobalt−carbene bonds reported to date, and DFT studies provide details into the nature of these bonds.
Scheme 1
ransition-metal carbene complexes feature divalent carbon
Tcenters that play important roles in organometallic
chemistry and organic synthesis,1 such as annulations,2
catalyzed alkene metathesis,3 and cyclopropanation.4 While
much effort has focused on the reactivity of metal carbene
complexes with electronic structures ranging from early-metal
alkylidenes5 to Fischer carbenes6 and even to metal N-
heterocyclic carbene complexes,7 less attention has been paid
to metal fluorocarbenes. The first transition-metal difluor-
ocarbenes and fluoro(perfluoroalkyl)carbene ([M]CFR; R =
F, perfluoroalkyl)8 were synthesized, but not isolated, in 1978,9
and surprisingly few (<20) isolable [M]CFR complexes have
surfaced since then. Most metal fluorocarbenes are based on
precious metals (Ru, Os, or Ir).10 To date, only two terminal11
first-row metal difluorocarbenes (containing Fe) have been
isolated,12 and first-row metal complexes with terminal
fluoro(perfluoroalkyl)carbene ligands have not been reported.
The known first-row metal difluorocarbenes are monocations
with formal d6 electron configurations (CFR fragments are
counted as neutral two-electron donors throughout13), at least
one carbonyl coligand, and decidedly electrophilic reactivity at
the MC bond.10,12,14 In fact, nucleophilic behavior is unusual
for metal fluorocarbenes and has been encountered only for
precious metal complexes with formal d8 electron configura-
tions.13
hydride migrates to the carbene carbon atom in the presence of
coordinating anions (Scheme 1b).16−18
As part of our efforts to develop new transition-metal
catalysts to make or modify fluorocarbon derivatives, we are
interested in synthesizing nucleophilic fluorocarbenes based on
first-row metals. Here, we describe the first persistent19
terminal cobalt fluorocarbenes. These new complexes provide
the first examples of (a) fluoro(perfluoroalkyl)carbenes of a
first-row metal, (b) first-row [M]CFR complexes (R = F,
fluoroalkyl) with formal d8 electron configurations, and (c)
nucleophilic non-precious-metal difluoro-/fluoro-
(perfluoroalkyl)carbenes. X-ray crystal structures were obtained
in two cases, and they reveal remarkably short Co−C(carbene)
bond distances. DFT (density functional theory) calculations
are presented to provide insight into the cobalt−carbene
bonding arrangement. Finally, preliminary reactivity studies
establish the CoC bonds in these complexes as nucleophilic
rather than electrophilicsites.
Scheme 2 summarizes our synthetic approach. Starting from
commercially available [CpCo(CO)2] (Cp = cyclopentadienyl),
several cobalt difluoro- and fluoro(perfluoroalkyl)carbenes were
made in three steps, the first two having been described
previously.20 The final step, two-electron reduction, was
developed by Hughes and co-workers for making [Ir]CFR
complexes16,18 and uses analogous starting materials. Sodium−
In 1983, Roper and co-workers reported the first nucleophilic
metal difluorocarbene ([F2CRu(CO)2(PPh3)2], Ru(0),
d8),13 which was synthesized from [Ru(CO)2(PPh3)3] and
[Cd(CF3)2·(glyme)].15 More recently, Hughes’ group de-
scribed a novel reductive method for making [Ir]CFR
complexes from I−[Ir]−CF2R precursors by treatment with
alkali metals (Scheme 1a).16 The products have formal d8
electron counts, and their nucleophilic character was
demonstrated by H+X− addition to the MC bonds. The
proton initially adds to the iridium center, but the resulting
Received: November 14, 2012
Published: December 24, 2012
© 2012 American Chemical Society
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Communication
CpCo(CO)2 and CpCo(CO)(NHC) (NHC = various N-
heterocyclic carbene ligands) range from 1.64 to 1.74 Å.25 The
sum of the angles around the carbene carbon atoms in 1a and
2a is very close to 360° (≤359.9°), pointing to the near-
planarity of the CoCFR substructures.
Scheme 2
The unusually short cobalt−carbene bond lengths in 1a and
2a reflect strong π interactions between the cobalt and the
carbene ligands (cf. DFT results below). Most of the previously
reported terminal cobalt carbene complexes have been of the
Fischer type,24,26 with at least one π-electron-donating group
(e.g., −NR2, −OR) attached to the carbene carbon atoms. In
qualitative terms, the nonbonding electrons of such groups
compete with metal-based electrons to populate the carbene C
2p orbitals, leading to less effective metal−carbene π overlap
and lower metal−carbon bond orders. Fluorine atoms are also
π-electron donors, but the effect is mitigated, relative to amino
and alkoxy groups, by the extreme electronegativity of fluorine
and the associated positive inductive effects. Hammett para
substituent constants (σp) are useful approximate indicators of
the electronic influence of substituents on adjacent carbon-
based π systems. The σp values for −F, −OMe, and −NMe2 are
0.06, −0.27, and −0.83, respectively.27 The small positive σp for
fluorine indicates that electron-withdrawing inductive effects
are slightly more significant than π donation (mesomeric
effects), corresponding to net electron-withdrawing properties
relative to para hydrogen substituents. In contrast, more
negative mesomeric effects for the amino and alkoxy cases lead
to negative σp values (i.e., net electron-donating properties).
Trifluoromethyl groups are negligible π-electron donors and
very effective inductive electron-withdrawing groups (σp(CF3)
= 0.43).27 Thus, the CFR (R = F, CF3) ligands, particularly
with R = CF3, should be better π acceptors than amino- or
alkoxy-substituted carbenes, which likely enables stronger
metal→carbene π overlap and explains the remarkably short
Co−C(carbene) bonds in complexes 1a and 2a.
mercury amalgam proved to be the most effective reductant, in
toluene or THF, giving isolated yields of 63−74% for
compounds 1a,b and 2a,b (1.3−1.4 mmol scale)21 when
triphenylphosphine and trimethyl phosphite were employed as
ancillary ligands. Attempts to generate the carbonyl (L = CO)
derivatives were unsuccessful.22
The 19F NMR spectra of the cobalt difluoro-/fluoro-
(perfluoroalkyl)carbenes show characteristic positive chemical
shifts for fluorine atoms attached directly to terminal metal
carbenes (e.g., 69.5 and 94.1 ppm for 1a; 22.7 ppm for 2a);
coupling was observed between the nonequivalent fluorine
environments and the phosphorus atoms of the ancillary
ligands. The 31P{1H} NMR spectra exhibited broad peaks for
the P-donor ligands, due to their one-bond proximity to the
quadrupolar 59Co nuclei. See the Supporting Information for
complete spectroscopic details (NMR, IR, UV−vis).
Compounds 2a,b, with the CF(CF3) ligand, were formed
exclusively as E double-bond isomers, as determined from X-ray
data (for 2a, see below) and 19F{1H} HOESY NMR
experiments16,18a (Supporting Information). Similarly, the
[Ir]CFR complexes made by Hughes’ group can be
generated stereoselectively, provided the reactions are con-
ducted in the dark.16,17,18a Complexes 2a,b are photolytically
stable by comparison and do not isomerize upon standing in
ambient light for several days (room temperature, in C6D6
solution). The E isomer, in which the CF3 group is trans to the
phosphine or phosphite ligand, is favored partially for steric
reasons, and DFT calculations show that the Gibbs free energy
of the Z isomer, for 2a, is 4.4 kcal mol−1 higher than that of the
E isomer.
DFT computations were performed for 1a and 2a, at the
PBE28/TZVP29 level of theory, which provided good agree-
ment with the experimentally determined structures (Support-
ing Information). The DFT results indicate donor−acceptor
bonding between the metal and carbene fragments in 1a and
2a, as expected on the basis of the large singlet→triplet energy
gaps for the CFR ligands (R = F, 51 kcal mol−1; R = CF3, 10
kcal mol−1).10,30 Three components in the cobalt−carbene
bonding are identified from the fragment molecular orbital
analysis31 of 1a and 2a: σ donation from the highest occupied
fragment orbital (HOFO) of the carbene to the lowest
unoccupied fragment orbital (LUFO) of CpCo(PPh3) (Figure
2A), π back-donation from the HOFO of CpCo(PPh3) to the
LUFO of the carbene ligand (Figure 2B), and a second, weaker
π interaction involving donation from HOFO-3 of CpCo-
(PPh3) to the CFR LUFO+1 (Figure 2C). The primary π
component of the metal−carbene bonds, interaction B in
Figure 2, is stronger in 2a, in comparison with 1a, as evidenced
by the MPA-derived32 population of the carbene ligand LUFO
(44% in 2a and 33% in 1a). Replacing a carbene fluorine
substituent with a CF3 group results in stronger CpCo(PPh3)
HOFO→carbene LUFO interaction, manifested by the natural
population analysis (NPA)33-derived net charges of the carbene
ligands (+0.109 au for 1a vs −0.104 au for 2a), Mayer bond
orders34 for metal−carbon bonds (1.50 for 1a vs 1.55 for 2a),
and rotational free energy barriers for the carbene ligands
The molecular structures of 1a and 2a (Figure 1),
determined by single-crystal X-ray diffraction,23 feature Co−
Figure 1. ORTEP drawings of the molecular structures of 1a (left) and
2a (right). The ellipsoids are set at the 30% probability level, and
hydrogen atoms are omitted. Selected bond distances [Å]: 1a: Co1−
C24(carbene) = 1.7395(14), Co1−P1 = 2.1620(5), Co1−Cp-
(centroid) = 1.706(2). 2a: Co1−C1(carbene) = 1.751(3), Co1−P1
= 2.1638(8), Co1−Cp(centroid) = 1.745(7).23
C(carbene) bond distances of 1.740(1) and 1.751(3) Å,
respectively, approximately 0.1 Å shorter than the most closely
related [Ir]CFR complexes.16,18a These appear to be the
shortest structurally characterized cobalt−carbene bonds to
date, by a substantial margin of at least 0.06 Å and greater than
0.1 Å for most examples.24 For comparison, terminal cobalt−
carbonyl bond distances in somewhat related complexes
(ΔG⧧
= 14.1 kcal mol−1 for 1a vs 17.6 kcal mol−1 for 2a),
298 K
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Communication
consistent with nucleophilic vs electrophilic character of the
CoC bonds. Electrophilic metal difluorocarbenes (with dn, n
≤ 6),13 and metal dihalocarbenes in general ([M]CX2),
readily undergo hydrolysis at the C−F/−X bonds to furnish a
CO ligand and 2 equiv of HX.10
To summarize, we have synthesized the first persistent/
isolable difluoro- and fluoro(perfluoroalkyl)carbenes of cobalt.
The new complexes are rare examples of terminal first-row
metal fluorocarbenes, especially unique by virtue of their formal
d8 electron configurations.13 High-valent metal fluorocarbenes
(formally dn, where n ≤ 6), and Fischer metal carbenes in
general, usually exhibit electrophilic reactivity at the carbene
carbon atom.10 The complexes reported herein are part of an
emerging class of metal carbenes, with some characteristics of
Fischer carbenes (e.g., low-valent metal ions and heteroatom
carbene substituent(s)) but unusual nucleophilic reactivity at
metal−carbon bonds. The X-ray crystal structures for 1a and 2a
contain the shortest structurally characterized cobalt−carbene
bonds yet reported and DFT data show donor/acceptor M-
C(carbene) bonds, each involving six electrons. We continue to
explore the reactivity of these and related complexes with the
goal of finding catalytic methods to generate useful polymeric
and small-molecule fluorocarbon targets.
Figure 2. Interactions between fragment molecular orbitals (FOs) of
CpCo(PPh3) (left) and CF2 (middle) or CF(CF3) (right) in their
singlet spin states, responsible for cobalt−carbene bonding (from
PBE/TZVP calculations). The isosurface plots for the CF(CF3) FOs
(not shown) are generally similar to those displayed for CF2. The
significant σ (A) and π (B and C) donor/acceptor bonding
interactions are labeled in the figure. The percentage values in
parentheses represent changes in FO occupancies (decrease in the
occupancy for occupied FOs and increase in the occupancy for
unoccupied FOs) when the complex is formed from the CpCo(PPh3)
and carbene fragments. Values in red and green correspond to
complexes 1a and 2a, respectively.
ASSOCIATED CONTENT
* Supporting Information
■
S
consistent with the increased π-accepting abilities of the
CF(CF3) ligand relative to CF2.
Text, figures, tables, and CIF files giving synthetic and
spectroscopic details and additional crystallographic and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
Compounds 1a,b and 2a,b can be protonated with LutH+Br−
(Lut = 2,6-lutidine), and the protons reside on the carbon
centers, rather than the metal, in the observed products
(Scheme 3). It is possible that the reactions proceed through
AUTHOR INFORMATION
Corresponding Author
*E-mail: rbaker@uottawa.ca.
■
Scheme 3
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval.
Notes
hydride intermediates, [H−[Co]CFR]+, by analogy to the
protonation of [Ir]CFR (Scheme 1b). The highest occupied
MOs for 1a and 2a have >50% cobalt character and smaller
coefficients on the carbene atoms (Supporting Information),
suggesting that proton attack at cobalt may be kinetically
preferred. However, our efforts to observe an intermediate by
1H and 19F NMR spectroscopy in the reaction between 1a and
triflic acid (−80 °C, in CD2Cl2) were unsuccessful (Supporting
Information). Thus, it is unclear whether the cobalt or the
carbon atom is protonated first, but the CoCFR bonds are
clearly nucleophilic sites, especially in relation to all other
known first-row metal fluorocarbenes. The protonation of
complexes 2a,b gave mixtures of diastereomers containing the
CF(CF3)H ligand in 8:1 and 2:1 ratios, respectively. Attempts
to methylate the cobalt fluorocarbene complexes using MeOTf
gave complex reaction mixtures; however, for the reaction of
1b, the [Co]−CF2Me substructure was clearly visible in the
The authors declare no competing financial interest.
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 infrastructure. D.J.H. and G.M.L. gratefully acknowl-
edge support from the province of Ontario and University of
Ottawa (Vision 2020 and OGS scholarships).
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(22) Attempts to make complexes with L = 2,6-dimethylphenyl
isocyanide, 1,3-bis(isopropyl)imidazol-2-ylidene or PMe2Ph were
unsuccessful.
(23) For 1a, one of two crystallographically independent molecules
in the unit cell is shown. Single orientations are depicted for the
rotationally disordered P−Ph and Cp groups in the structure of 2a. A
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dx.doi.org/10.1021/om3010959 | Organometallics 2013, 32, 12−15