Binuclear oxidative addition of aryl halides

Article abstract of DOI:10.1021/om100811f

Reaction of LCoCH₂SiMe₃ (L = 2,6-bis[2,6- dimethylphenyliminoethyl]pyridine) with H₂ produces LCo(N ₂), presumably via intermediate LCoH. Reaction of LCo(N₂) (prepared in this way or via reaction of LCoCl₂ with Na/Hg) with aryl halides ArX (X = Cl, Br, I) produces LCoAr and LCoX in a ratio depending on the nature of Ar and X. For X = Cl, the reaction is slowest but also produces the largest amount of LCoAr. Electron-withdrawing substituents both accelerate the reaction and improve the yield of LCoAr. Computational studies support a radical mechanism for this reaction, involving displacement of N₂ to give LCo(XAr) followed by loss of the Ar radical, which then binds to a second Co(0) moiety.

Full text of DOI:10.1021/om100811f

Organometallics 2010, 29, 5759–5761 5759
DOI: 10.1021/om100811f
Binuclear Oxidative Addition of Aryl Halides
Di Zhu and Peter H. M. Budzelaar*
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada, R3T 2N2
Received August 18, 2010
Summary: ReactionofLCoCH2SiMe3 (L=2,6-bis[2,6-dimethyl-
these two products proved impossible, but their identities
were established by comparison of 1H NMR data with
independently prepared authentic samples. LCoCl was pre-
pared by reduction of LCoCl2 with Na/Hg.5 LCoC6H4-4-Me
was obtained from LCoCl2 and LiC6H4-4-Me (1:2), and its
structure was confirmed by a single-crystal X-ray diffraction
study (Figure 1); this is the first structurally characterized
diiminepyridine cobalt aryl complex reported to date.9
Conversion and yield of the reaction were determined for
the addition of ClC6H4-4-CF3, which is conveniently monitored
phenyliminoethyl]pyridine) with H2 produces LCo(N2), presum-
ably via intermediate LCoH. Reaction of LCo(N2) (prepared
in this way or via reaction of LCoCl2 with Na/Hg) with aryl
halides ArX (X=Cl, Br, I) produces LCoAr and LCoX in
a ratio depending on the nature of Ar and X. For X = Cl, the
reaction is slowest but also produces the largest amount of
LCoAr. Electron-withdrawing substituents both accelerate the
reaction and improve the yield of LCoAr. Computational studies
support a radical mechanism for this reaction, involving dis-
placement of N2 to give LCo(XAr) followed by loss of the Ar
radical, which then binds to a second Co(0) moiety.
(2) At Fe: (a) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem.
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K. H. D.; Patrick, B. O.; Smith, K. M. Organometallics 2004, 23, 1487.
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Ballem, K. H. D.; Patric, B. O.; Smith, K. M. Organometallics 2009, 28, 6798.
At Co: (e) Halpern, J.; Maher, J. P. J. Am. Chem. Soc. 1965, 87, 5361. (f) Chock,
P. B.; Halpern, J. J. Am. Chem. Soc. 1969, 91, 582. (g) Schnelder, P. W.; Phelan,
P. F.; Halpern, J. J. Am. Chem. Soc. 1969, 91, 77. (h) Halpern, J.; Phelan, P.
J. Am. Chem. Soc. 1972, 94, 1881. (i) Marzilli, L. G.; Marzilli, P. A.; Halpern, J.
J. Am. Chem. Soc. 1971, 93, 1374. At Rh: (j) Ogoshi, H.; Setsunu, J.; Yoshida,
Z. J. Am. Chem. Soc. 1977, 99, 3869.
(3) Rossi, R. A.; Pierini, A. B.; Penenory, A. B. Chem. Rev. 2003, 103, 71.
(4) Zhu, D.; Janssen, F. F. B. J.; Budzelaar, P. H. M. Organometallics
2010, 29, 1897.
(5) Bowman, A. C.; Milsmann, C.; Atienza, C. C. H.; Lobkovsky, E.;
Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 1676.
(6) LCo(N2) prepared from LCoCl2 and Na/Hg5 undergoes identical
subsequent reactions, but we find LCoCH2SiMe3 to be a more convenient
precursor.
Oxidative addition of carbon-halogen bonds is one of
the fundamental reactions of organometallic compounds; it
forms the basis for many applications in organic synthesis.
The most common mechanisms of oxidative addition of halides
RX are1 (a) SN2-like nucleophilic attack by an electron-rich
metal center (mostly for alkyl halides); (b) concerted addi-
tion via a three-center transition state (mostly for aryl
halides); and (c) radical mechanisms (usually for activated
alkyl halides). In all of these, the addition product has the
halide and organic group bound to the same metal atom,
increasing the oxidation state of the metal by 2. Examples of
binuclear oxidative addition, in which the R and X groups
end up on separate metal centers, are much rarer and mostly
involve addition of alkyl halides;2 the only example involving
an aryl halide reported to date seems to be the reaction of
2-I-C5H4N with Co(CN)53- to give ICo(CN)53- and 2-C5H4-
(7) The corresponding L0CoCH2SiMe3 (L0 = 2,6-[2,6-iPr2C6H3Nd
CMe]2C5H3N) also reacts with H2, but the product L0CoH is fairly
stable, can be observed by 1H NMR, and does not convert efficiently to
L0Co(N2), presumably for steric reasons. See: (a) Tellmann, K. F.;
Humphries, M. J.; Rzepa, H. S.; Gibson, V. C. Organometallics 2004, 23,
5503. (b) Knijnenburg, Q.; Horton, A. D.; van der Heijden, H.; Kooistra,
T. M.; Hetterscheid, D. G. H.; Smits, J. M. M.; de Bruin, B.; Budzelaar,
P. H. M.; Gal, A. W. J. Mol. Catal. A 2005, 232, 151.
NCo(CN)5 .
3- 2e,3 We here report net binuclear oxidative addi-
tion of aryl chlorides to two molecules of a Co(0) complex,
forming a mixture of Co-X and Co-R complexes via what
appears to be a free-radical process.
(8) Treatment of LCoCH2SiMe3 with H2 in the presence of PhCtCPh
produces a diamagnetic species identified as LCoCPhdCHPh on the basis
of 1H NMR; see SI for details.
€
(9) Ir analogue: (a) Nuckel, S.; Burger, P. Angew. Chem. Int. Ed. 2003, 42,
1632. For a related Fe(0) complex, see: (b) Fernandez, I.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2008, 27, 109. For Co aryls bearing
different ancillary ligands, see e.g.: (c) Yoshimitsu, S.-I.; Hikichi, S.; Akita, M.
Organometallics 2002, 21, 3762, and references therein. (d) Beck, R.; Sun, H.; Li,
X.; Klein, H.-F. Z. Anorg. Allg. Chem. 2009, 635, 99, and references therein.
(e) Will, S.; Lex, J.; Vogel, E.; Adamian, V. A.; Caemelbecke, E. V.; Kadish, K. M.
Inorg. Chem. 1996, 35, 5577, and references therein. (f) Lei, H.; Ellis, B. D.; Ni,
C.; Grandjean, F.; Long, G. J.; Power, P. P. Inorg. Chem. 2008, 47, 10205.
(g) Theopold, K. H.; Silvestre, J.; Byrne, E. K.; Richeson, D. S. Organometallics
1989, 8, 2001. (h) Kays, D. L.; Cowley, A. R. Chem. Commun. 2007, 1053.
(10) The partial “loss” of Ar means less than 2 equiv of LCo(N2) are
consumed per mole of ArCl. Most experiments were actually performed
with a 1:1 LCo(N2):ArX ratio (see Table 1) to maximize the amount of
LCoAr formed and hence left no unreacted LCo(N2) (although we
verified the final LCoAr:LCoX ratio did not depend on the initial
LCo(N2):ArX ratio). For reactions carried out using a 2:1 LCo(N2):
ArX ratio, quantification of any leftover LCo(N2) was difficult because
of (a) the broad 1H NMR resonances of this paramagnetic species; (b)
the presence of 5-10% diamagnetic side products from the LCo(N2)
synthesis; (c) limited stability of LCo(N2) in the reaction mixture.
Treatment of a solution of LCoCH2SiMe34 in benzene-d6
under nitrogen with H2 produced paramagnetic LCo(N2);5-7
formation of this complex probably involves LCoH,8 but we
have not observed this intermediate in the 1H NMR spectra.
In addition to LCo(N2), small amounts of diamagnetic side
products (typically 5-10%) are always observed.
Treatment of the dark green solution of LCo(N2) with
ClC6H4-4-Me resulted in formation of LCoC6H4-4-Me and
LCoCl in approximately 0.6:1 ratio (Table 1). Separation of
*Corresponding author. E-mail: Peter_Budzelaar@umanitoba.ca.
(1) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry, 2nd ed.;
Oxford University Press: Oxford, 2010; pp 204-226.
r
2010 American Chemical Society
Published on Web 10/07/2010
pubs.acs.org/Organometallics

5760 Organometallics, Vol. 29, No. 22, 2010
Zhu and Budzelaar
Table 1. Reactions of LCo(N2) with Organic Halidesa
Scheme 1. Proposed Mechanism for Binuclear Oxidative
Addition of ClC6H5
entry
halide
[LCoAr]:[LCoX]c
rxn timed
1
2
3
4
5
6
7
8
IC6H5
BrC6H5
ClC6H5
BrC6H2-2,4,6-Me3
BrC6H2-2,4,6-tBu3
ClC6H3-2,6-Me2
ClC6H4-4-COMe
ClC6H4-4-COOMe
ClC6H4-4-CF3
ClC6H4-4-F
ClC6H3-3,5-(OMe)2
ClC6H4-4-Cl
ClC6H4-4-Me
ClC6H4-4-OMe
IMe
Cl-n-C4H9
Br-n-C6H13
BrCH2C6H5
ClCH2C6H5
2,6-Cl2-C5H3N
CH2dCH-n-C6F13
F-n-C8H17
0.24
0.25
0.59
≈ 0
seconds
1 min
hours
seconds
seconds
days
30 min
seconds
seconds
seconds
seconds
seconds
hours
0.27
≈ 0
0.91
0.83
0.77
0.40
0.59
0.59
0.59
0.50
0.71b
0.13
0.30b
0.14b
0.45b
1.00
0.59
N.R.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2,6-dimethyl-substituted aryls form very little LCoAr
(entries 4, 6).12
hours
seconds
seconds
seconds
seconds
minutes
seconds
minutes
We believe these results are best explained by a radical pro-
cess similar to that shown in Scheme 1 (analogous mechanisms
have been proposed for alkyl halide addition to cobalt(II);2e,f,i
halide abstraction by a Co(III) complex bearing redox-active
ligands has also been reported13). The aryl halide could
displace N2 from LCo(N2) and then undergo C-X cleavage
to release the free aryl radical, which would independently
find its way to a second LCo(N2) molecule. Stabilized
radicals are less likely to undergo side reactions, explaining
the improved LCoAr yields for entries 7-9 and 20. The faster
reactions of bromides and iodides would result in higher
radical concentrations and hence formation of more biaryl side
products. The alternative sequence of initial mononuclear
oxidative addition followed by aryl transfer to unreacted
LCo(N2) seems unlikely;14 in particular, the required side-on
C-Br coordination of BrC6H2-2,4,6-tBu3 to the LCo frag-
ment is impossible for steric reasons.
a Reaction conditions: LCoCH2SiMe3 (14 mg, 27 μmol), 0.4 mL of
C6D6, 2 mL of H2 gas, then ArX (27 μmol, 1.0 equiv). b Using 14 μmol
(0.5 equiv) of RX. c From 1H NMR; estimated error margin ≈ 5%.
d Qualitative indication.
Alkyl halides RX (X=Cl, Br, I) also react, and selectivities
show similar trends: faster reactions, but less LCoR formation,
for activated halides (benzyl) and for bromides vs chlorides.
Methyl iodide, however, produced an unexpectedly high yield
of LCoMe (entry 15). Unactivated C-F bonds are not attacked
(n-C8H17F, entry 22), but the allylic fluoride n-C6F13CHd
CH2 produced a reasonable amount of an alkyl complex
tentatively identified as LCo(σ-CH2CHdCFC5F11).
DFT studies15 support the proposed reaction sequence.
Replacement of N2 from LCo(N2) by terminally Cl-bound
ClC6H5 is endergonic by about 10 kcal/mol (Figure 2). From
this complex, the transition state for C-Cl bond cleavage is
another 10 kcal/mol further uphill and shows the correct
imaginary mode; this would constitute the rate-determining
step. Further optimization results in full dissociation of an
Figure 1. Structure of LCoC6H4-4-Me (hydrogens omitted
for clarity).
by 1H and 19F NMR spectroscopy. Using a 2:1 LCo(N2):ArCl
ratio, conversion of ArCl was virtually complete (94%). Varia-
tion of the amount of ArCl (keeping [LCo(N2)] constant) did
not affect the product ratio LCoAr:LCoCl (0.77 for this aryl;
the “lost” Ar groups end up as ArH, detected by 19F NMR,
and Ar2, detected by GC/MS10). The oxidative addition was
then explored for a variety of halides with different electronic
and steric properties (Table 1). Relative amounts of LCoAr
and LCoX were determined from the characteristic11 1H
resonances for Py H4 (triplet around 10 ppm) and imine Me
(singlet around -1 ppm); for details see the SI. The reaction
rates of phenyl halides increase in the “normal” order Cl <
Br<I (entries 1-3). However, the relative amount of LCoAr
formed decreases in this order, from 0.59 to 0.24. Electron-
withdrawing groups accelerate the reaction (entries 3, 7-9,
20) and also increase the amount of LCoAr formed. Sensi-
tivity to steric factors is not very high: even BrC6H2-2,4,6-
tBu3 gave a significant amount of LCoAr (entry 5). However,
(12) This difference may be related to the fact that the •C6H2-2,4,6-tBu3
radical has a fairly long lifetime, whereas •C6H2-2,4,6-Me3 rearranges or
reacts quickly: (a) Brunton, G.; Griller, D.; Barclay, L. R. C.; Ingold, K. U.
J. Am. Chem. Soc. 1976, 98, 6803. (b) Childress, B. C.; Rice, A. C.; Shevlin,
P. B. J. Org. Chem. 1974, 39, 3056.
(13) (a) Smith, A. L.; Clapp, L. A.; Hardcastle, K. I.; Soper, J. D.
Polyhedron 2010, 29, 164. (b) Note that diiminepyridine ligands, like the
iminobenzosemiquinonate ligands used by Soper, are redox-active,5 so that in
both their work and ours one could consider the actual reductant to be the
ligand rather than the metal.
(14) We cannot exclude that the actual mechanism of oxidative addi-
tion is substrate-dependent (see e.g.: Affo, W.; Ohmiya, H.; Fujioka, T.;
Ikeda, Y.; Nakamura, T.; Yorimitsu, H.; Oshima, K.; Imamura, Y.;
Mizuta, T.; Miyoshi, K. J. Am. Chem. Soc. 2006, 128, 8068), varying from
chlorine radical abstraction for unactivated halides to three-center oxidative
addition followed by aryl radical loss for the more reactive halides. Neither our
experimental work nor preliminary DFTcalculations provide any indication of
this; so for simplicity we assume a single common mechanism in the present
work.
(11) Knijnenburg, Q.; Hetterscheid, D. G. H.; Kooistra, T. M.;
Budzelaar, P. H. M. Eur. J. Inorg. Chem. 2004, 1204.
(15) Turbomole, b3-lyp functional, optimization and vibrational
analysis using TZVP, final energies using TZVPP; for details see SI.

Communication
Organometallics, Vol. 29, No. 22, 2010 5761
acteristics of the reaction strongly suggest a radical mecha-
nism, which is supported by calculations. Further efforts will
be aimed at achieving C-C coupling of the resulting cobalt
aryls in stoichiometric and catalytic reactions.16,17
Acknowledgment. We thank Mr. M. Cooper (Univer-
sity of Manitoba) for help with the X-ray structure deter-
mination and Dr. B. de Bruin (University of Amsterdam)
for EPR measurements. Financial support from NSERC,
CFI, and MRIF (to P.H.M.B.) and a University of
Manitoba Graduate Fellowship (to D.Z.) are gratefully
acknowledged.
Supporting Information Available: Experimental and compu-
tational details, X-ray structure determination, coordinates and
energies for calculated structures. This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 2. Calculated free energy profile (b3-lyp/TZVPP//b3-
lyp/TZVP) for the binuclear oxidative addition of ClC6H5 at
two Co(0) centers according to Scheme 1. Bond lengths (for the
˚
(16) For an example of C-C bond formation involving radical
chemistry at well-defined Ni centers, see: Jones, G. D.; Martin, J. L.;
McFarland, C.; Allen, O. R.; Hall, R. E.; Haley, A. D.; Brandon, R. J.;
Konovalova, T.; Desrochers, P. J.; Pulay, P.; Vicic, D. A. J. Am. Chem.
Soc. 2006, 128, 13175, and references therein.
(17) There are numerous reports of C-C bond formation at cobalt
involving incompletely characterized organocobalt intermediates; see
for example ref 14 and (a) Cahiez, G.; Moyeux, A. Chem. Rev. 2010, 110,
1435. (b) Yorimitsu, H.; Oshima, K. Pure Appl. Chem. 2006, 78, 441; many
of these reactions most likely involved radical chemistry.
TS) are in A.
aryl radical, which then combines with another LCo frag-
ment to form the rather stable LCoAr. The calculated overall
free energy barrier for this process (23.2 kcal/mol) is compat-
ible with a process that is slow (hours) at room temperature.
In conclusion, this work demonstrates the first example of
net binuclear oxidative addition of aryl chlorides. The char-