Unexpected Ccarbene-X (X: I, Br, Cl) reductive elimination from N-heterocyclic carbene copper halide complexes under oxidative conditions

Article abstract of DOI:10.1021/om1005726

The non-spectator roles of NHC ligands have attracted wide attention due to their important implications for reaction mechanisms and subsequent impact on catalyst design. Herein, we report facile C_carbene-halogen reductive eliminations from NHC copper halide complexes at RT under oxidative conditions. Density functional calculations on a simplified model system suggest that the reactions occur through oxidation of Cu(I) species to Cu(III) species followed by C_carbene-halogen reductive eliminations from NHC Cu(III) halide complexes. Remarkably short C_carbene-chloride contacts and rare interactions between the chloride lone pair electrons and the C _carbene p_μ orbital were found for the calculated NHC Cu(III) chlorides. The facile C_carbene-X reductive elimination reported here warrants consideration as a potential decomposition pathway in reactions involving NHC-supported high-valent metal complexes, especially with late transition metals.

Full text of DOI:10.1021/om1005726

Organometallics 2010, 29, 3683–3685 3683
DOI: 10.1021/om1005726
Unexpected Ccarbene-X (X: I, Br, Cl) Reductive Elimination
from N-Heterocyclic Carbene Copper Halide Complexes Under
Oxidative Conditions
Bo-Lin Lin, Peng Kang, and T. Daniel P. Stack*
Department of Chemistry, Stanford University, Stanford, California 94305
Received June 9, 2010
Summary: The non-spectator roles of NHC ligands have
elimination and ligand dissociation/displacement, are docu-
mented with the metal-Ccarbene bonds.2-4
attracted wide attention due to their important implications
for reaction mechanisms and subsequent impact on catalyst
design. Herein, we report facile Ccarbene-halogen reductive
eliminations from NHC copper halide complexes at RT under
oxidative conditions. Density functional calculations on a sim-
plified model system suggest that the reactions occur through
oxidation of Cu(I) species to Cu(III) species followed by
Ccarbene-halogen reductive eliminations from NHC Cu(III)
halidecomplexes. Remarkably short Ccarbene-chloride contacts
and rare interactions between the chloride lone pair electrons
and the Ccarbene pπ orbital were found for the calculated NHC
Cu(III) chlorides. The facile Ccarbene-X reductive elimination
reported here warrants consideration as a potential decomposi-
tion pathway in reactions involving NHC-supported high-valent
metal complexes, especially with late transition metals.
Carbon-halogen (C-X) reductive elimination of metal-
Calkyl or metal-Caryl bonds is an important elementary org-
anometallic reaction,5 but no well-defined example is reported
for Ccarbene-X reductive elimination from NHC metal halide
complexes,6 although such complexes are ubiquitous in NHC
chemistry.1 Herein, we report facile formation of 2-halo-
imidazoliums from NHC Cu(I) halide complexes at room
temperature (RT) under oxidative conditions as well as
computational evidence to support a mechanism involving
Ccarbene-X reductive elimination from NHC Cu(III) halide
complexes.
As a part of our continuing efforts in the characterization
of reactivity of Cu(III) complexes,7 we postulated that NHC
N-Heterocyclic carbene (NHC) metal complexes are used
widely as catalysts in organic reactions.1 Compared to other
neutral type ligands, NHCs usually form stronger bonds with
metals due to excellent σ-donating ability. The conjugation
between the carbene carbon (Ccarbene) pπ orbital and nitrogen
lone pair electrons in the heterocycle further stabilizes the
metal-Ccarbene bonds. As a result, NHCs are often better
at suppressing catalyst decompositions. The metal-Ccarbene
bonds are remarkably inert compared to other metal-carbon
bonds at catalytic metal centers, which are prone to undergo
various reactions such as migratory insertion and olefin meta-
thesis.1 In contrast, only a small number of elementary org-
anomellic reactions, mostly limited to Ccarbene-C reductive
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(a) Ku, R.-Z.; Huang, J.-C.; Cho, J.-Y.; Kiang, F.-M.; Reddy, R.; Chen,
Y.-C.; Lee, K.-J.; Lee, J.-H.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T.
Organometallics 1998, 28, 863–870. (b) Titcomb, L. R.; Caddick, S.; Cloke,
F. G. N.; Wilsona, D. J.; McKerrecher, D. Chem. Commun. 2001, 1388–
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21, 2958–2963. (d) de, K.; Lewis, A. K.; Caddick, S.; Cloke, F. G. N.;
Billingham, N. C.; Hitchcock, P. B.; Leonard, J. J. Am. Chem. Soc. 2003,
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J. Am. Chem. Soc. 2003, 125, 10490–10491. (f) Torres, O.; Martín, M.; Sola,
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Chem. Soc. 2010, 132, 4050–4051.
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5108. (b) Sheppard, T. D. Org. Biomol. Chem. 2009, 7, 1043–1052.
(6) Halogenolysis of the metal-Ccarbene bonds to form Ccarbene-X
bonds does occur, presumably through either direct electrophilic clea-
vage of metal-Ccarbene bonds by X2 or oxidative addition of X2 followed
by Ccarbene-X reductive elimination. DFT calculations support a direct
electrophilic cleavage of the Pd-Ccarbene bond by I2.9f (a) Lappert,
M. F.; Pye, P. L. J. Chem. Soc., Dalton Trans. 1977, 1283–1291. (b) Liu,
S.-T.; Ku, R.-Z.; Liu, C.-Y.; Kiang, F.-M. J. Organomet. Chem. 1997, 543,
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Trans. 2001, 2451–2452. (d) Fooladi, E.; Dalhus, B.; Tilset, M. Dalton
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*To whom correspondence should be addressed. E-mail: stack@
stanford.edu.
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41, 1290–1309. (b) Kantchev, E. A. B.; O'Brien, C. J.; Organ, M. G. Angew.
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Nolan, S. P. Chem. Rev. 2009, 109, 3612–3676. (d) Poyatos, M.; Mata, J. A.;
Peris, E. Chem. Rev. 2009, 109, 3677–3707. (e) Somojlowicz, C.; Bieniek, M.;
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r
2010 American Chemical Society
Published on Web 08/16/2010
pubs.acs.org/Organometallics

3684 Organometallics, Vol. 29, No. 17, 2010
Lin et al.
Scheme 1
Scheme 2
Cu(III) complexes might be isolable because of the relatively
inert Cu-Ccarbene bond. In that regard, oxidations of IPr-
CuICl (1Cl, IPr: 1,3-bis(2, 6-diisopropylphenyl)imidazol-2-
ylidene) by various oxidants were investigated. No signifi-
cant reaction was observed between 1Cl and [Ph2I]þPF6- or
[Cp2Fe]þPF6- (0.64 V vs NHE)8 in acetonitrile (MeCN). By
contrast, mixing 1Cl with Selectfluor (g1.5 equiv) or CuII-
(CF3SO3)2 (g2.0 equiv, 1.30 V vs NHE)9 in MeCN at RT
rapidly and quantitatively forms 2-chloroimidazolium 2Cl
(Scheme 1).10 Quantitative formation of 2-halo-imidazolium
2Br or 2I was realized for IPrCuIBr (1Br) or IPrCuII (1I) under
similar conditions.11
the ca. 80% formation of 2Cl by reacting 1Cl with [(1,10-
phenanthroline)3FeIII 3þ (g2 equiv, 1.22 V vs NHE)12 in
]
MeCN at RT.13 However, all these reactions are fast even at
low temperatures (t1/2 ≈ seconds at ca. -40 °C), and no
intermediate species could be detected by UV-vis spectros-
copy on a time scale of seconds.
DFT calculations provide mechanistic insights into the
oxidation by Selectfluor. The calculated free-energy profiles
corresponding to Ccarbene-X reductive elimination from
either a Cu(II) or Cu(III) species using a simplified model
system (3) are shown in Scheme 2.14 The oxidation of 3 by
Selectfluor to form 5, a three-coordinate Cu(III) species, is
thermodynamically favorable.15 The Cu(III) species is further
stabilized by coordination of an MeCN ligand to form 7.16
These reactions appear to occur through either an inner- or
outer-sphere oxidation followed by Ccarbene-X reduc-
tive elimination. An outer-sphere oxidation is supported by
(13) Protonolysis of the Cu-Ccarbene bond was also observed, leading
to ca. 20% yield of the corresponding imidazolium. In addition, no well-
defined oxidation occurs with 1Cl by cyclic voltammetry at low tem-
peratures (ca. -40 °C).
(8) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.
(9) Inamo, M.; Kumagai, H.; Harada, U.; Itoh, S.; Iwatsuki, S.;
Ishihara, K.; Takagi, H. D. Dalton Trans. 2004, 1703–1707.
(10) 2Cl was characterized by NMR (1H, 13C, and 19F), mass spec-
troscopy, and X-ray crystallography (2Cl SbF6 CH2Cl2).19 Reactions
(14) All calculations were performed at the level of B3LYP/
6-311þG** by the Gaussian 03 program. The solvation effect was
estimated by the PCM/UA0 model. Relative free energies are used in
the computational discussion. See Supporting Information for more
detailed descriptions of the computational methods. (a) Frisch, M. J.;
et al. Gaussian03, Revision E.01; Gaussian, Inc.: Wallingford, CT, 2004.
(b) The gas-phase entropies were converted to corresponding entropies19
(1 M in CH3CN) according to the method in: Wertz, D. H. J. Am. Chem. Soc.
1980, 102, 5316–5322.
3
3
with lesser amounts of oxidants led to unreacted 1Cl and lower yields of
2Cl. The amounts of oxidant and the color of the reaction solution
suggest that the side product is Cu(II) (blue) and Cu(I) (colorless) species
for the reaction with Selectfluor and CuII(CF3SO3)2, respectively.
(11) 2Br and 2I were characterized by NMR (1H, 13C, and 19F), mass
spectroscopy, and X-ray crystallography (2Br CF3SO3 CH2Cl2 and
3
3
2I I3).19 The reaction of IPrCuIF with CuII(CF3SO3)2 led to the forma-
3
tion of IPrCuI(CF3SO3) (a crystal structure was obtained),19 while the
reaction with Selectfluor afforded a species with broad 1H NMR signals
similar to that of a reported NHC-ligated Cu(II) dimer,20b suggesting
that C-F formation is slower than the comproportionation between
Cu(I) and Cu(III).
(15) A 12.8 kcal mol-1 activation free energy was calculated for this
step (fluorine transfer) at a lower-level method (B3LYP/6-311þG**//
B3LYP/ LANL2DZ).19
(16) 6 is proposed as an intermediate in the reaction. It might ligate to
copper in the final uncharacterized Cu(II) product. Coordination of
MeCN to 3 might facilitate the oxidation. However, the corresponding
three-coordinate species could not be located computationally.
(12) Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5593–
5603.

Communication
Organometallics, Vol. 29, No. 17, 2010 3685
The activation barrier for Ccarbene-Cl reductive elimination
from 7 via TS1 to form 2-chloroimidazolium 10 is remarkably
low at 3.5 kcal mol .
-1 17 The overall reaction is favorable by
-54.2 kcal mol-1. Other possible Cu(III) species18 are less
stable than 7 and lead to higher activation barriers. Alterna-
tively, 7 could react with another equivalent of 3 to form
Cu(II) intermediates 8 and 9, the two most stable Cu(II)
species.19 Although no transition state from 9 to 10 could be
Figure 1. Optimized structures of 5, 7, and 8.
located (the energy monotonically increases as the Ccarbene
-
Clseparation shortens), the lower limit of itsactivation barrier
can be estimated by the corresponding reaction free energy,
44.1 kcal mol-1, the least endothermic among various Cu(II)
species.19 Therefore, the calculations on the simplified model
system favor a mechanism of Ccarbene-Cl reductive elimina-
tion from NHC Cu(III) chloride complexes for the oxidation
of 1Cl by Selectfluor.
The thermodynamically unfavorable Ccarbene-Cl reduc-
tive elimination from 9 is consistent with isolable NHC
Cu(II) chloride complexes.20 On the basis of these results,
we prefer a mechanism with two sequential inner- or outer-
sphere 1e- oxidations followed by Ccarbene-Cl reductive
elimination from NHC Cu(III) chloride complexes for the
reaction with Cu(CF3SO3)2,21 although the detailed mecha-
nism is still unclear. Furthermore, the quantitative forma-
tion of 2Cl instead of IPrCu(II) chloride complexes from the
reaction of 1Cl and Selectfluor suggests that Ccarbene-Cl
reductive elimination from IPrCu(III) chloride complexes
is much faster than reactions of 1Cl to form IPrCu(II) halide
complexes. This reactivity is consistent with the calculated
3.5 kcal mol-1 activation barrier from 7 to 10. Such a low
barrier is probably a consequence of the electrophilic nature
of a Cu(III) center that renders the NHC Ccarbene susceptible
to nucleophilic attack, as suggested by the remarkably close
Figure 2. HOMO-3 of 5 and HOMO-1 of 7.
has been invoked to rationalize the short Ccarbene-Cl contact
V
(ca. 2.85 A) in NHC V (O)Cl3.22 In contrast, no evidence of
˚
such an interaction exists for 8 (Figure 1).
In summary, we have demonstrated that Ccarbene-halogen
reductive eliminations readily occur from NHC copper
halides at RT under oxidative conditions. These reactions
provide new examples for the well-known oxidation-induced
reductive eliminations. DFT calculations on a simplified
model system suggest that the involvement of NHC Cu(III)
halides is essential for these reactions and the reductive
eliminations might be facilitated by the interaction between
Ccarbene and the halogen lone pair. Given the ubiquity of
NHC metal halide complexes, the facile Ccarbene-X reduc-
tive elimination reported here warrants consideration as a
potential decomposition pathway in reactions involving
NHC-supported high-valent metal complexes, especially
with late transition metals under oxidative conditions.
˚
Ccarbene-Cl contacts (ca. 2.7 A) in 5 and 7 (Figure 1) as well
as the interactions between the chloride lone pair electrons
and the Ccarbene pπ orbital (Figure 2). A similar interaction
(17) η2-Arene-coordination intermediates have been reported in a
related computational study for oxidative addition of Ph-Br to various
CuI species. Zhang, S.-L.; Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics
2007, 26, 4546–4554. By contrast, no analogous intermediate was located
computationally for our system.
Acknowledgment. Acknowledgment is made to the
NIH (GM50730) and the donors of the American Chemi-
cal Society Petroleum Fund for support of this research.
We thank Dr. Allen G. Oliver for the crystal structures and
Professor Xingwei Li and Dr. George S. Chen for their
suggestions in the preparation of the manuscript.
(18) These include two isomers of 7, one five-coordinate Cu(III)
species, and a complex with 6 coordinated to 5.19
(19) See Supporting Information.
(20) (a) Arnold, P. L.; Rodden, M.; Davis, K. M.; Scarisbrick, A. C.;
Blake, A. J.; Wilson, C. Chem. Commun. 2004, 1612–1613. (b) Larsen,
A. O.; Leu, W.; Oberhuber, C. N.; Campbell, J. E.; Hoveyda, A. H. J. Am.
Chem. Soc. 2004, 126, 11130–11131.
Supporting Information Available: Detailed experimental and
calculational data. This material is available free of charge via
the Internet at http://pubs.acs.org.
(21) Previous work indicated that AgSbF6 (1.35 V vs NHE)8 can
oxidize CuII-Salen to [CuIII-Salen](SbF6) in CH2Cl2.7g Therefore, the
oxidation of 1Cl to Cu(III) by CuII(CF3SO3)2 (1.30 V vs NHE)9 seems
possible.
(22) Abernethy, C. D.; Codd, G. M.; Spicer, M. D.; Taylor, M. K.
J. Am. Chem. Soc. 2003, 125, 1128–1129.