Why are (NN2)Ni pincer complexes active for alkyl-alkyl coupling: β-H elimination is kinetically accessible but thermodynamically uphill

Article abstract of DOI:10.1021/om1007506

Isomerization and olefin exchange experiments show that β-H elimination is kinetically viable but thermodynamically unfavorable in [( ^MeNN₂)Ni-alkyl] complexes. The intermediacy of Ni-hydride species was corroborated by a trapping experiment. The alkyl complex [( ^MeNN₂)Ni-propyl] catalyzes olefin isomerization.

Full text of DOI:10.1021/om1007506

3686 Organometallics 2010, 29, 3686–3689
DOI: 10.1021/om1007506
Why Are (NN2)Ni Pincer Complexes Active for Alkyl-Alkyl Coupling:
β-H Elimination Is Kinetically Accessible but Thermodynamically Uphill
ꢀ
Jan Breitenfeld, Oleg Vechorkin, Clemence Corminboeuf, Rosario Scopelliti, and Xile Hu*
ꢀ ꢀ
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne (EPFL),
SB-ISIC, Lausanne, CH 1015, Switzerland
Received July 30, 2010
Summary: Isomerization and olefin exchange experiments
show that β-H elimination is kinetically viable but thermo-
dynamically unfavorable in [(MeNN2)Ni-alkyl] complexes. The
intermediacy of Ni-hydride species was corroborated by a
trapping experiment. The alkyl complex [(MeNN2)Ni-propyl ]
catalyzes olefin isomerization.
We recently developed a well-defined Ni pincer14-22 com-
plex, [(MeNN2)NiCl] (1),23 as an efficient catalyst for
C-C cross-coupling of alkyl halides.24-30 For alkyl-alkyl
Kumada-Corriu-Tamao type coupling, our mechanistic
study suggested a catalytic cycle shown in Figure 1.26,27
The intermediacy of [(MeNN2)Ni-alkyl] species was con-
firmed in both stoichiometric and catalytic reactions.26
Because coupling reactions proceeded in high yields, β-H
elimination must be slow or nonoccurring. This is consistent
with the stability of the isolated complex [(MeNN2)Ni-Et]
(2), with which no β-H elimination was observed at 80 °C.26
Whereas the stability against β-H elimination is a crucial
factor for the efficiency of the catalyst in alkyl-alkyl
coupling, the origin of this stability was not clear. Classic
organometallic chemistry suggests two possibilities:31,32
(1) The square-planar NiII center in the pincer complex is
coordinatively saturated and has no open site for β-agostic
interaction prior to H-elimination. (2) β-H elimination is
thermodynamically unfavorable. The second scenario is
common for early transition metals, but less encountered
for late transition metals.19,20,33,34 Liang et al. recently
showed several such examples in the (PNP)Ni pincer
system.19,20 Here we report a study of the isomeriza-
tion and olefin exchange reactions of the Ni alkyl com-
plexes. The reactions occur through a transient Ni-H
Metal-catalyzed C-C cross-coupling has become one of
the most powerful tools in organic synthesis.1 However, the
coupling of nonactivated and β-hydrogen-containing alkyl
halides has been challenging because the corresponding
metal alkyl intermediates often undergo unproductive β-H
elimination.2-4 In the past few years, advances in metal
catalysis,2-4 especially Ni catalysis,5-12 have started to im-
prove the scope and utility of cross-coupling reactions of alkyl
halides. The mechanism of many Ni-catalyzed reactions,
however, remains speculative. Furthermore, very few studies
on isolated catalytic intermediates have been reported.11,13
*To whom correspondence should be addressed. E-mail: xile.hu@
epfl.ch.
(1) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere,
A.; Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004.
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(22) Chakraborty, S.; Krause, J. A.; Guan, H. Organometallics 2009,
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(23) Complex 1 is now commercially available from Sigma-Aldrich
bearing the synonym “Nickamine”. The name reflects the combination
of nickel and amine ligands.
(24) Csok, Z.; Vechorkin, O.; Harkins, S. B.; Scopelliti, R.; Hu, X. L.
J. Am. Chem. Soc. 2008, 130, 8156–8157.
(25) Vechorkin, O.; Barmaz, D.; Proust, V.; Hu, X. L. J. Am. Chem.
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Commun. 2005, 4211–4213.
(14) See refs 15-22 for selected examples of Ni pincer complexes.
(15) The Chemistry of Pincer Compounds; Morales-Morales, D.; Jensen,
C. M., Eds.; Elsevier: Amsterdam, 2007.
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r
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Published on Web 08/17/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 17, 2010 3687
intermediate. The work shows that β-H elimination also
occurs in the [(MeNN2)NiII] system, but it is energetically
uphill and, thus, does not affect the efficiency of alkyl-
alkyl coupling.
determined, but its quality suffered from disorder. Never-
theless, the connectivity of 5 was confirmed.
The most probable reaction pathway for the isomerization
of 4 to 3 is first β-H elimination from 4 to form a Ni-H bond
and propene, which reinserts to form the more stable iso-
mer 3 (eq 4). Indeed, DFT computations (at the M06-2X/
cc-pVDZ level)35 show that 3 is more stable than 4 by 5.8
kcal/mol, which agrees with the fact that 3 is the only isolated
product in eq 2. The isomerization reaction implies that β-H
elimination is kinetically accessible for these Ni alkyl com-
plexes. The stability of the alkyl complexes should arise from
a thermodynamic origin.
While exploring the scope of alkyl-alkyl coupling, we
found that coupling of octyl-Br with isopropylMgCl in the
presence of 3 mol % of 1 led to n-undecane in 91% yield
(eq 1).35 The n-propyl group in the product originated from
the isopropyl group of the Grignard reagent. Presumably,
the isomerization occurred at the Ni center and before
reaction of octyl bromide with the Ni alkyl intermediate
(Figure 1). To confirm this hypothesis, we sought to study
the isomerization in a stoichiometric manner. Reaction of 1
with isopropylMgCl gave the isomerized Ni alkyl complex
[(MeNN2)Ni-Prn] (3) in 75% yield (eq 2, Figure 2). The direct
transmetalation product, [(MeNN2)Ni-Pri] (4), could not be
observed. Likewise, reaction of 1 with tert-butylMgCl gave
the isomerized complex [(MeNN2)Ni-Bui] (5) in 73% yield
(eq 3, Figure 2). Complexes 3 and 5 could be prepared by
direct transmetalation of 1 with n-propylMgCl and isobu-
tylMgCl (Figure 2).
To further verify the possibility of β-H elimination, olefin
exchange experiments were carried out (eq 5).35 Reaction of
3 with ethylene (5 to 20 bar) at 80 °C in benzene produced the
Ni ethyl complex 2 and 1-propene. The reaction is reversible
and reaches equilibrium in ca. 5 h (Figure 4). A concentra-
tion equilibrium constant Keq(conc) of 0.68(3) was found by
The structure of 3 (Figure 3) was confirmed by X-ray
analysis.35 The Ni center is square planar. The Ni-C(1)
bond length is 1.965(2) A, similar to that in 2 and other pincer
˚
NiII alkyl complexes.20,26 No short-distance interaction was
˚
observed for Ni-H(β) (3.123 A). The structure of 5 was also
Figure 3. X-ray structure of complex 3. The thermal ellipsoids
˚
are displayed at 50% probability. Selected bond lengths (A):
Figure 1. Proposed catalytic cycle for alkyl-alkyl coupling
catalyzed by Ni pincer complex 1.
Ni1-N1, 1.9914(17); Ni1-N2, 1.8906(17); Ni1-N3, 2.0075(17);
Ni1-C1, 1.965(2); C1-C2, 1.534(3).
Figure 2. Synthesis and isomerization of Ni alkyl complexes.

3688 Organometallics, Vol. 29, No. 17, 2010
Breitenfeld et al.
Table 1. Thermodynamic and Kinetics Data for Olefin Exchange
(eq 5)
pressure of
ethylene (bar)
k  104 (s-1)
a
b
entry
Keq(conc)
Keq (p)
1
2
3
4
5
10
15
20
0.69
0.71
0.65
0.67
0.50
0.47
0.44
0.26
1.4
1.5
2.4
3.6
a Calculated using the concentrations of gases. b Calculated using the
partial pressures of gases.
and β-H elimination is a fast pre-equilibrium, then the rate
constant should be first-order on the concentration of ethyl-
ene. If the rate of β-H elimination is comparable to that of
ethylene insertion, a nonlinear dependence of the rate con-
stant on ethylene is expected. The latter was indeed observed
for eq 5 (Table 1). Thus, the ethylene exchange reaction with
3 seems not to have a clear rate-determining step.
The mechanism of β-H elimination is subject to further
study. A possibility is that one of amine donors from the
tridentate NN2 ligand dissociates from the Ni center, thereby
creating a necessary open site. We have yet to observe this
type of ligand dissociation in the [(MeNN2)Ni] system. How-
ever, such a pathway could not be discounted, as it could
occur via high-energy intermediate species or transition
states. We are currently employing DFT methods to probe
this possibility.
The olefin exchange experiments give more support to the
viability of β-H elimination from the [(MeNN2)Ni-alkyl]
species. As [(MeNN2)Ni-H] (6) would be the product of
elimination, we sought to prepare it independently. Reaction
of 1 with (nBu)4NBH4 or LiBH(Et)3 in THF-d8 at -50 °C
produced a species with a 1H signal at -23.16 ppm, attribu-
table to a Ni-H moiety. This species is tentatively assigned
to complex 6. It is thermally unstable and escapes isolation
efforts. Nonetheless, its existence could be confirmed by a
trapping experiment. Reaction of in situ generated 6 with
ethylene gave the ethyl complex [(MeNN2)Ni-Et] (2), iden-
tical to an independently synthesized sample.
Figure 4. Concentration profile for reaction 5 at 80 °C. The
initial ethylene pressure is 5 bar at 25 °C. The concentrations
were monitored by NMR.
measuring the concentrations of all species in solution using
NMR (Table 1).35 A similar equilibrium constant Keq(p) of
0.42(11) was obtained if the concentrations of ethylene and
1-propene were expressed as their partial pressures.35 DFT
computations suggest a ΔHr°(298 K) of -2.69 kcal/mol in
the gas phase (-2.64 kcal/mol with benzene treated as a
continuum).35 The small numeric discrepancy between the
computed and experimental values may result from the pre-
sence of π-π noncovalent interactions between solvent
molecules and olefins, which are not taken into account by
implicit solvent models. The reaction followed a pseudo-
first-order kinetics on the concentration of 3 when ethylene
was in excess. The pseudo-first-order rate constant, however,
depends on the pressure of ethylene. The rate constant
increases with higher pressure, but not linearly (Table 1).
The mechanism of olefin exchange is proposed in eq 6. As
both 2 and 3 are stable species and do not undergo obser-
vable β-H elimination in pure solutions, the elimination step
should be reversible and thermodynamically uphill. This
hypothesis is supported by DFT computations,35 which
indicate that 2 is more stable than 6 plus ethylene by ca.
22 kcal/mol. This value is comparable to that of three-
coordinate β-diketiminate Fe ethyl complex (ΔH = 21.8
kcal/mol).34 The entropic change (TΔS) for β-H elimination
is favorable, but on the order of 10 kcal/mol.34 Thus, the
overall thermodynamics disfavor β-H elimination for
[(MeNN2)Ni-alkyl]. Comparison with related NiII alkyl com-
plexes indicates that the thermodynamics are strongly influ-
enced by ligands: β-H elimination is also disfavored in pin-
cer [(PNP)Ni-alkyl] complexes,20 but is favorable in four-
coordinate (anilinotropone)(PPh3)Ni-alkyl complexes.36
If β-H elimination is the rate-determining step in this reac-
tion, the rate constant should be independent of the concen-
tration of ethylene, as in the isomerization of β-diketiminate
Fe alkyl complexes.33,34 If olefin insertion is rate determining
The isomerization and olefin exchange experiments sug-
gest that the [(MeNN2)Ni-alkyl] species may be catalysts for
olefin isomerization reactions. Indeed, 3 catalyzed the iso-
merization of 1-allylbenzene to 2-methylstyrene at 70 °C
(10 mol % catalyst, 60% conversion in 30 h, eq 7). The
thermodynamically more stable trans-isomer was the major
product (ca. 90%). Figure 5 shows a possible catalytic cycle.
The hydride complex 6 is first generated by β-H elimination
from 3. Allylbenzene then inserts into the Ni-H bond to
form both terminal and internal Ni alkyl species 7 and 8,
which are in equilibrium via isomerization. Judging from the
relative stability of analogous complexes 3 and 4, the ratio of
8 to 7 must be very small. β-H elimination from 8 then gives
2-methylstyrene and regenerates 6. Complex 3 also catalyzed
the isomerization of cis-stilbene to trans-stilbene (10 mol %
catalyst, 70% conversion in 20 h at 70 °C).
In summary, the isomerization and olefin exchange reac-
tions of [(MeNN2)Ni-alkyl] complexes have been studied
to show that β-H elimination of these alkyl complexes is
kinetically accessible but thermodynamically unfavorable.
(35) See Supporting Information.
(36) Jenkins, J. C.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 5827–
5842.

Communication
Organometallics, Vol. 29, No. 17, 2010 3689
Figure 5. Proposed mechanism for the catalytic isomerization of allylbenzene.
The intermediacy of Ni-hydride species was corroborated by a
trapping experiment. The alkyl complex 3 can catalyze olefin
isomerization. This work reveals one of the key factors for the
efficiency of (MeNN2)Ni pincer complexes in alkyl-alkyl
cross-coupling reactions. Work to elucidate further details
of the mechanism as shown in Figure 1 is underway.
200021_126498 to X.L.H.). We thank Prof. Lothar Helm
and Prof. Gabor Laurenczy (EPFL) for help with high-
pressure NMR measurements.
Supporting Information Available: Experimental and compu-
tational details, crystallographic data in cif format for 3 and 5, a
structural drawing of 5, and Cartesian coordinates and absolute
energies of the computed structures. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work is supported by the EPFL
and the Swiss National Science Foundation (project no.