Synthesis, characterization, and reactivity studies of (cyclohexenyl) nickel(ii) complexes

Article abstract of DOI:10.1021/om100391v

The synthesis and characterization of the complexes [(cyclohexenyl)Ni(NCMe) ₂][B(Ar_F)₄] (1) and [(cyclohexenyl) Ni(mesitylene)][B(Ar_F)₄] (2) are reported. Abstraction of the nitrile ligands in 1 with B(C₆F₅)₃ yields a "ligand-free" [(cyclohexenyl)Ni^II]⁺ species which coordinates 1,3-dienes, including butadiene (BD), isoprene (IP), 1,3-cyclohexadiene (1,3-CHD), 1,4-cyclohexadiene (1,4-CHD), and 2,3-dimethyl-1,3-butadiene (DMBD) at low temperatures. Diene insertion is observed at temperatures above ca. -20 °C. An s-trans diene complex is formed upon coordination of DMBD. Hydrogen transfer is observed when complex 2 is exposed to 1-hexene or cyclopentadiene to generate new allyl nickel complexes and 1 equiv of cyclohexene. Polymerization of 1,3-CHD and DMBD with 2 is described, and a coupling product formed between DMBD and 2 is observed and characterized.

Full text of DOI:10.1021/om100391v

5382 Organometallics 2010, 29, 5382–5389
DOI: 10.1021/om100391v
Synthesis, Characterization, and Reactivity Studies of
(Cyclohexenyl)nickel(II) Complexes†
Abby R. O’Connor, Peter S. White, and Maurice Brookhart*
Department of Chemistry, The University of North Carolina, Chapel Hill, Chapel Hill,
North Carolina 27599-3290
Received April 30, 2010
The synthesis and characterization of the complexes [(cyclohexenyl)Ni(NCMe)2][B(ArF)4] (1) and
[(cyclohexenyl)Ni(mesitylene)][B(ArF)4] (2) are reported. Abstraction of the nitrile ligands in 1 with
B(C6F5)3 yields a “ligand-free” [(cyclohexenyl)NiII]þ species which coordinates 1,3-dienes, including
butadiene (BD), isoprene (IP), 1,3-cyclohexadiene (1,3-CHD), 1,4-cyclohexadiene (1,4-CHD), and
2,3-dimethyl-1,3-butadiene (DMBD) at low temperatures. Diene insertion is observed at tempera-
tures above ca. -20 °C. An s-trans diene complex is formed upon coordination of DMBD. Hydrogen
transfer is observed when complex 2 is exposed to 1-hexene or cyclopentadiene to generate new allyl
nickel complexes and 1 equiv of cyclohexene. Polymerization of 1,3-CHD and DMBD with 2 is
described, and a coupling product formed between DMBD and 2 is observed and characterized.
Introduction
containing labile ligands have been synthesized and charac-
terized.2-21 These Ni complexes serve as precursors to highly
reactive Ni(II) species shown to be active for polymerization
and oligomerization reactions.5-13,19 In early work Tkatchenko
showed that [(2-R-allyl)Ni(L)n]þ complexes, where L=1,5-
cyclooctadiene (COD), acetonitrile (CH3CN), tetrahydrofuran
(THF), or benzonitrile, are active catalysts for ethylene oligo-
merization.14 Extensive pioneering work from the groups of
(Allyl)nickel(II) complexes are important catalysts and in-
termediates in organometallic chemistry.1 A variety of neutral
(allyl)nickel(II) complexes as well as [(allyl)NiL2]þ complexes
† Part of the Dietmar Seyferth Festschrift. Dedicated to Dietmar Seyferth,
whose vision, commitment, and firm hand have maintained Organometallics
as the preeminent journal in the field.
ꢀ19
Taube,5-11 Porri,12,13 and Teyssie explored the behavior and
*To whom correspondence should be addressed. E-mail:
mechanistic details of butadiene polymerization catalyzed by
numerous (allyl)nickel species. Additionally, Goodall16-18 and
ꢀ
mbrookhart@unc.edu.
(1) Crabtree, R. H. In The Organometallic Chemistry of the Transition
Metals; Wiley: New York, 2001.
(2) Wilke, G.; Bogdanovic, B.; Hardt, P.; Heimbach, P.; Keim, W.;
Campora20,21 have shown that cationic (allyl)nickel complexes
bearing labile ligands are efficient initiators for norbornene
and 1,3-diene polymerizations. We recently investigated the use
of [(2-R-allyl)M(L)n]þ (M = Ni, Pd; L = mesitylene (mes),
CH3CN; R = H, Me) for diene and norbornene insertion
polymerization.3,4,15,22
Much less is known about the cyclohexenyl analogues
of the acyclic (allyl)nickel(II) cations. Wilke was the first to
report the synthesis of (cyclohexenyl)2Ni and [(cyclohexenyl)-
NiBr]2 dimers.2 More recently, Hartwig identified (cyclo-
hexenyl)nickel(II) and -palladium(II) phosphine complexes
as intermediates in the hydroamination of 1,3-dienes by
alkylamines.23,24 Additionally, we previously have observed
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pubs.acs.org/Organometallics
Published on Web 07/15/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5383
hydrogen atom transfer from cyclohexene to the (allyl)Ni(mes)þ
complex to generate (cyclohexenyl)Ni(mes)þ in situ.25
Scheme 1. Synthesis of Cationic (Cyclohexenyl)nickel(II)
Complexes
(Cyclohexenyl)palladium(II) species are more common in
the literature, as they have been identified as intermediates in
asymmetric allylic alkylation reactions.26 Additionally, cyc-
lic dienes or olefins have been shown to react with group 10
metals to generate cyclic enyl metal complexes. Maitilis27-29
and co-workers observed the hydropalladation of 1,3-cyclo-
hexadiene to generate the (cyclohexenyl)Pd(dppe)þ complex.
This hydropalladation reaction was applied to 1,3-cyclo-
pentadiene and 1,3- or 1,4-cyclooctadiene to generate (cyclo-
pentenyl)- and (cyclooctenyl)palladium complexes, respec-
tively.27,28 We recently reported hydrogen atom transfer from
cyclohexene to the (allyl)Ni(mes)þ complex to generate (cyclo-
hexenyl)Ni(mes)þ and 1 equiv of propene.25
Previously, we showed that the nitrile complexes [(2-R-
allyl)Ni(NCMe)2][B(ArF)4] (R = H, Me; ArF = (C6H3)-
(CF3)2) serve as precursors to “naked” nickel allyl cations
by treatment with B(C6F5)3. In the presence of 1 equiv of
butadiene (BD) and 2 equiv of B(C6F5)3, η4 coordination of
BD occurs at low temperatures and the (2-R-allyl)Ni-
(η4-butadiene)þ complexes can be observed. However, upon
exposure to excess butadiene rapid insertion of 3 equiv of
BD occurs to yield wrap-around complexes.15 Additionally,
we have shown that [(2-R-allyl)Ni(mes)][B(ArF)4] (R = H,
CH3) complexes exhibit interesting reactivity with olefins25
and are highly active catalysts for polymerization of 1,3-
dienes and styrene.22
We report here the synthesis of [(cyclohexenyl)Ni-
(NCMe)2][B(ArF)4] (1) and [(cyclohexenyl)Ni(mes)][B(ArF)4]
(2) and an investigation of the reactivity of these complexes
with 1,3-dienes and R-olefins. Similar to previous experiments
using acyclic allyl complexes, a cationic ligand-free (cyclo-
hexenyl)nickel(II) complex, in which a single ArF ring is
coordinated to nickel, can be observed at low temperature.
Interestingly, in contrast to the acyclic allyl complexes, the
(cyclohexenyl)Ni(η4-diene)þ complexes react with dienes only
at temperatures above ca. -20 °C in the presence of excess
diene. The reactions of 2 with olefins and 1,3-dienes and
intramolecular hydrogen atom transfer are discussed. Addi-
tionally, we describe here the first observation of Ni(II)
binding a 1,3-diene in an s-trans conformation.
Figure 1. Molecular structure of complex 2. Thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms and
[B(ArF)4]- are omitted for clarity. Selected bond lengths (A):
˚
Ni(1)-C(2) = 1.934(5), Ni(1)-C(3) = 2.015(5), Ni(1)-C(1) =
2.063(6), Ni(1)-C(8) = 2.149(5), Ni(1)-C(12) = 2.154(4),
Ni(1)-C(11) = 2.157(5), Ni(1)-C(7)=2.159(4), Ni(1)-C(10) =
2.193(5), Ni(1)-C(9) = 2.224(5).
The solid-state structure of 2 was established using X-ray
crystallography (Figure 1).30 The mesitylene ligand of 2
exhibits no puckering or bending upon coordination. The
C5 methylene group of the cyclohexenyl ligand is bent out of
the plane and points away from the mesitylene ring. The
Ni-C1, Ni-C2, and Ni-C3 bond lengths of the allyl moiety
are nearly identical, consistent with similar bond distances
observed for the [(allyl)Ni(mes)][B(ArF)4] complex.25 Inter-
estingly, this structure shows variation in the Ni-C bond
lengths of the arene. Ni-C11, Ni-C12, Ni-C7, and Ni-C8
Results and Discussion
Catalyst Synthesis and Structure. The synthesis of
[(cyclohexenyl)Ni(NCMe)2][B(ArF)4] (1) and [(cyclohexenyl)-
Ni(mes)][B(ArF)4] (2) was achieved using methods similar to
those previously described for the preparation of the analogous
acyclic allyl compounds.15,25 Salt metathesis of [(cyclohexenyl)-
NiBr]2 (readily synthesized via literature methods)2 with 2 equiv
of NaB(ArF)4 in diethyl ether in the presence of either mesitylene
or acetonitrile generates compounds 1 and 2 in good yields
(Scheme 1). Characterization of the compounds was achieved
using 1H and 13C NMR spectroscopy and elemental analysis.
˚
exhibit bond lengths in the range 2.15-2.16 A, while the
Ni-C10 and Ni-C9 bond lengths are elongated to 2.19
˚
and 2.22 A, respectively. The significant differences in the
bond lengths suggest a ring slippage away from an ideal η6-
binding mode and a stronger interaction of Ni with the
C8-C7-C11-C12 diene unit relative to the C10-C9 olefinic
unit.
Ligand Exchange Reactions. The lability of the mesitylene
group coordinated to the (cyclohexenyl)nickel(II) fragment
was probed through the addition of various ligands. Addi-
tion of 5 equiv of mesitylene-d12 to complex 2 at room
temperature generates a resonance for free mesitylene at δ
6.79 and indicates exchange of free and bound mesitylene.
(25) O’Connor, A. R.; Urbin, S. A.; Moorhouse, R. A.; White, P. S.;
Brookhart, M. Organometallics 2009, 28, 2372–2384.
(26) Trost, B. M.; Lee, C. In Catalytic Asymmetric Synthesis; Ojima,
I., Ed.; Wiley-VCH: New York, 2000; pp 593-649.
(27) Mabbott, D. J.; Maitlis, P. M. J. Organomet. Chem. 1975, 102,
C34–C36.
(28) Mabbott, D. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1976,
2156–2160.
(29) Mabbott, D. J.; Mann, B. E.; Maitlis, P. M. J. Chem. Soc.,
Dalton Trans. 1977, 294–299.
(30) See the Supporting Information for X-ray structure data.

5384 Organometallics, Vol. 29, No. 21, 2010
O’Connor et al.
Scheme 2. Generation of [(cyclohexenyl)Ni][B(ArF)4] (3) and [(cyclohexenyl)Ni(η4-butadiene)][B(ArF)4] (4) at Low Temperatures
Scheme 3. Coordination of 1,3-Dienes to Ligand-Free 3 at Low
Temperatures
Equilibrium is reached within 4 h. The ratio of coordinated:
free mesitylene is 3:7 approximately 10 min after mixing
complex 2 with mesitylene-d12. After 4 h, the ratio of
coordinated to free mesitylene changes to 1:6 and remains
1
constant. No line broadening is observed in the H NMR
spectrum when protio mesitylene is added to complex 2,
indicating arene exchange is slow on the NMR time scale,
consistent with exchange on the time scale of minutes at
23 °C. Complete displacement of the mesitylene by 5.5 equiv
of hexamethylbenzene (hmb) occurs to form [(cyclohexenyl)-
Ni(hmb)]þ within 4 h at room temperature. The ratio of
2:[(cyclohexenyl)Ni(hmb)]þ was 1:9 within 10 min of adding
hmb to compound 2. Complete conversion to the hexa-
methylbenzene complex is achieved within 1 h and is much
slower than the analogous exchange reaction reported for
the [(allyl)Ni(mes)]þ analogue.25 Exposure of 2 to 9 equiv of
diethyl ether results in very minor displacement of mesitylene
after 1 day, indicating that diethyl ether does not effectively
compete with mesitylene for binding to Ni.
Reactions with 1,3-Dienes. The reaction of 1 in the presence
of 5 equiv of B(C6F5)3 generates [(cyclohexenyl)Ni][B(ArF)4]
(3) at low temperatures (Scheme 2). This species was iden-
tified by 1H and 19F NMR spectroscopy and shows the
presence of four new resonances in the 1H spectrum for the
coordinated [B(ArF)4]- anion. The ratio of the new proton
signals in the 1H NMR spectrum is 1:6:3:2. In the 19F NMR
spectrum there are two signals for coordinated [B(ArF)4]- in
a 1:3 ratio, corresponding to the coordinated and uncoordi-
nated ArF rings, respectively. This pattern indicates one ArF
ring is bound to nickel, while the remaining three aryl rings
are unbound, as previously observed for [(allyl)Ni][B(ArF)4]
and [(2-methallyl)Ni][B(ArF)4].15
1
complex 4, is observed in the H NMR spectrum and this
cis η4 species is the most stable structure predicted by
DFT calculations.31 At temperatures below -20 °C, inser-
tion reactions do not take place in the presence of excess
butadiene. This is in contrast with our previous result
using [(allyl)Ni][B(ArF)4], which rapidly inserts 3 equiv of
butadiene to form stable wrap-around complexes below
-110 °C.15 The cyclohexenyl insertion product formed
1
at -20 °C has a characteristic H NMR spectrum for a
wrap-around complex, but the spectrum is too complex to
assign all resonances.
Addition of 2 equiv of isoprene (IP), 1,3-cyclohexadiene
(1,3-CHD), and 1,4-cyclohexadiene (1,4-CHD) to ligand-
free species 3 at -80 °C yields [(cyclohexenyl)Ni(η4-IP)]-
[B(ArF)4] (5), [(cyclohexenyl)Ni(η4-1,3-CHD)][B(ArF)4] (6),
and [(cyclohexenyl)Ni(η4-1,4-CHD)][B(ArF)4] (7), respec-
tively (Scheme 3). Complexes 5 and 6 undergo insertion
of diene at -20 °C, while 7 is stable at room temperature.
Each of the CH2 groups of the cyclohexenyl ring is diastereo-
topic when diene or arene is coordinated, facilitating
their assignment in the 1H NMR spectrum (see the Experi-
mental Section and the Supporting Information for spectral
characterizations).
To compare the stability of the cyclohexenyl derivatives
to that of the (allyl)nickel(II) complexes, we generated the
(2-methallyl)Ni(η4-1,3-cyclohexadiene)þ complex in situ. Ex-
posure of 1 equiv of 1,3-CHD to the ligand-free species
[(2-methallyl)Ni]þ at -80 °C produces [(2-methallyl)Ni-
(η4-1,3-cyclohexadiene)][B(ArF)4] (Me-6) (Scheme 4). The 1H
NMR spectrum at -70 °C exhibits two broad singlets at
6.31 (2H) and 6.11 (2H) ppm corresponding to the vinylic
hydrogens of the coordinated cyclohexadiene. There are three
Intramolecular exchange is observed for the ArF rings
at -40 °C through broadening of the coordinated ArF
resonances.15 1H variable-temperature NMR experiments
between -70 and -35 °C showed the barrier to intramole-
cular Ni migration among the rings to be ΔGq = 11.9 kcal/
-
mol. Intermolecular exchange of coordinated [B(ArF)]4
with free [B(ArF)4]- occurs on the NMR time scale at
temperatures above 0 °C. Details of this dynamic process
are described in the Experimental Section. Species 3 is in
equilibrium with starting complex 1, and the equilibrium is
temperature-dependent, shifting toward species 3 at higher
temperatures. At -20 °C, the ratio of 3:1 is 4:1.
Species 3 rapidly coordinates butadiene at -80 °C to
form the η4-diene adduct [(cyclohexenyl)Ni(η4-butadiene)]-
[B(ArF)4] (4) (Scheme 2). Only one isomer, the s-cis η4-diene
€
(31) Tobisch, S.; Bogel, H.; Taube, R. Organometallics 1996, 15,
3563–3571.

Article
Organometallics, Vol. 29, No. 21, 2010 5385
Scheme 4. Reaction of Cyclohexadiene with [(2-methallyl)Ni][B(ArF)4]
Scheme 5. Reaction of 3 with 2,3-Dimethyl-1,3-butadiene
singlets at 5.14 (2H), 2.59 (2H), and 2.15 (3H) ppm that are
assigned to the syn, anti, and CH3 protons of the allyl ligand,
respectively. The final resonance is a roofed multiplet at
1.77 ppm assigned to the diastereotopic methylene hydrogens.
At temperatures above -30 °C, insertion of CHD into the
allyl group is observed. Thus, the allyl η4-diene complexes
insert diene at lower temperatures than do the cyclohexenyl
derivatives.
where high fractions of 1,4-trans-enchained polybutadiene
are formed (coordinating anions or ligands present) trans
enchainment results from insertion of s-cis-butadiene to
yield an anti-allyl species followed by anti to syn isomeriza-
tion prior to the next diene insertion.11,13,32-36 Under ligand-
free conditions using noncoordinating counteranions very
low fractions of trans enchainment (ca. 2%) are always
observed.5,8,9,15,22 While the observed equilibrium between
an η4-s-trans-1,3-diene complex, 9, and the s-cis form, 8,
certainly does not imply an s-trans-butadiene Ni complex is
involved in chain growth, it is worth careful consideration as
to whether a high energy insertion pathway via the s-trans
isomer may be responsible for the small fraction of trans-
enchainment under ligand-free conditions.
Qualitative experiments were performed to assess the bind-
ing strengths of 1,3-dienes to the (cyclohexenyl)nickel(II) frag-
ment. Exposing a solution of η4-butadiene species 4 at -60 °C
to 2 equiv of IP leads to a 3:1 ratio of complexes 5 and 4,
indicating a slight preference for binding the more electron-rich
IP to the nickel center. We have observed a similar trend for
preferential coordination of more electron-rich arenes to ca-
tionic (allyl)nickel(II) and (allyl)palladium(II) fragments.25 No
insertion of BD or IP is observed at this temperature. When the
temperature is raised, the resonances for the η4-BD coordinated
species 4broaden, while those for the η4-IP species remain fairly
sharp. At -10 °C, the resonances for 4 are broadened into the
baseline, and at 0 °C both species exhibit broad resonances,
indicating exchange of dienes on the NMR time scale.
When complex 3 is exposed to DMBD, an unusual result is
obtained. Two isomers of an η4-diene species, one major
and one minor, in addition to the ligand-free complex 3 are
observed at -80 °C, and upon warming to -20 °C, there is
no spectral evidence for 3 in solution. Through spectro-
scopic data provided from 1H-1H-COSY NMR experi-
ments, the identities of the two isomers were established as
[(cyclohexenyl)Ni(η4-s-cis-DMB)]þ (8), the major isomer,
and [(cyclohexenyl)Ni(η4-s-trans-DMB)]þ (9), the minor
isomer (Scheme 5). This is the first observation of an η4-s-
trans-coordinated nickel(II) diene complex. In the 1H NMR
spectrum at -20 °C, s-trans complex 9 exhibits four signals
(each integrating for 1H) that appear in the region for
coordinated olefin (δ 5.19, 5.04, 4.46, and 4.30) as well as
two methyl resonances (δ 1.94, 1.50), each correlating to
1
a different set of olefinic signals. These H NMR character-
istics point to an η4-diene complex in which the DMBD moiety
is in the s-trans geometry. Additionally, for the minor isomer,
separate terminal allyl proton resonances (each integrating for
0
1H) appear at 6.32 (H1) and 6.30 (H1 ) while the central allyl
proton (H2) appears at 5.32 ppm. The 1H NMR spectrum for
the s-cis coordinated diene fragment of species 8 exhibits two
olefinic signals at δ 4.97 (2H) and 2.93 (2H) and a single
6H methyl resonance (δ 2.21). The ratio of the two isomers
(cis:trans) is 4:1 at -20 °C. This ratio does not change over
time, suggesting this is the thermodynamic ratio.
An interesting reaction is observed upon the addition of
cyclopentadiene to ligand-free 3 or mesitylene complex 2.
Exposure of species 3 to 2 equiv of cyclopentadiene at -80 °C
results in a rapid color change from yellow-brown to green-
yellow. The 1H NMR spectrum indicates the formation
of [(cyclopentadienyl)Ni(η4-cyclopentadiene)][B(ArF)4] (10),
which was independently synthesized through protonation
of Cp2Ni with [H(OEt2)][B(ArF)4]. [(cyclopentadienyl)Ni-
(η4-cyclopentadiene)]þ with BF4- 37 or OTf - 38 as counteranions
The identification of an η4-trans diene coordinated to Ni
may have implications concerning the mechanism for trans-
1,4-enchainment of polybutadiene. DFT calculations by
Tobisch and Taube indicate that the barrier to coupling of
a π-allyl unit with an η4-s-trans-butadiene is substantially
higher than coupling with an η4-s-cis-butadiene and that
trans-enchained 1,4-polybutadiene is unlikely to result from
insertion of η4-s-trans-butadiene.32 Indeed, there is good
theoretical and experimental evidence that under conditions
(33) Tobisch, S.; Taube, R. Organometallics 1999, 18, 3045–3060.
(34) Jolly, P. W.; Wilke, G. The Organic Chemistry of Nickel. In
Organic Synthesis; Academic Press: New York, 1975; Vol. 2, p 225.
(35) Furukawa, J. Pure Appl. Chem. 1975, 42, 495–508.
(36) Dolgoplosk, B. A.; Beilin, S. I.; Korshak, Y. V.; Makovetsky,
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Chem. 1975, 102, C9–C11.

5386 Organometallics, Vol. 29, No. 21, 2010
O’Connor et al.
Scheme 6. Addition of Cyclopentadiene to Ligand-Free Species 3
at -80 °C
comparison to the case for the [(allyl)Ni(mes)]þ system.
Similar to previous experiments, the addition of ethylene
produced ethylene oligomers.25
The mechanism to form complex 11 involves intramole-
cular hydrogen atom transfer from hexene to the cyclo-
1
hexenyl moiety. H NMR studies show that rapid isomeri-
zation of 1-hexene to the 2- and 3-hexene isomers occurs
prior to hydrogen atom transfer; however, the details of the
isomerization mechanism are still unclear. We know from
previous experiments that the internal cis-2-hexene and
trans-2-hexene isomers also undergo hydrogen atom transfer
reactions with (allyl)Ni(mes)þ.25 It is uncertain as to which
isomer or isomers of hexene are needed to generate complex
11. Following isomerization, hydrogen atom transfer from
hexene generates 1 equiv of cyclohexene and complex 11
after trapping with mesitylene.
Reaction of 2 with DMBD. When complex 2 is treated with
DMBD, a new (allyl)nickel(II) product which results from
carbon-carbon coupling is formed. The reaction of 40 equiv
of DMBD with 2 was monitored at -20 °C using 1H NMR
spectroscopy. A new allyl species displaying two allyl reso-
nances, a syn resonance at 3.38 ppm (s, 1H) and an anti
resonance at 2.65 ppm (s, 1H), is formed. This species also
exhibits two broad olefinic triplets characteristic of an un-
symmetrically substituted cyclohexene at 5.43 and 5.66 ppm.
Structure 12 is consistent with these NMR data. There is no
evidence of cyclohexene (δ 5.66, 1.61, 1.26) in the 1H NMR
spectrum, as would be expected if hydrogen atom transfer to
the cyclohexenyl ligand occurred. The expected upfield
resonances for species 12 are masked by excess DMBD and
oligomers present in solution; therefore, the structure assign-
ment is not fully confirmed.
Scheme 8 shows a plausible sequence to form presumed 12.
η4 coordination of DMBD occurs first, and this species then
undergoes C-C coupling followed by trapping with mesity-
lene. Although the formation of η4-diene complexes has
never been observed at low temperature by 1H NMR when
2 is exposed to 1,3-dienes, we believe DMBD likely coordi-
nates in an η4 fashion to facilitate C-C coupling. This new
allyl species, 12, is similar to the previously reported assign-
ment of the coupling product of an allyl moiety and DMBD
formed in the reaction of [(allyl)Ni(mes)][B(ArF)4] with 40
equiv of DMBD.22 The strong similarity of these reactions
supports the structural assignment of 12.
The above experiments support the hypothesis that chain
transfer in 1,3-diene polymerization probably occurs
through intramolecular hydrogen atom transfer in which a
discrete Ni-H intermediate is unlikely.25,39 Observation of
intramolecular hydrogen atom transfer from added olefin to
the cyclohexenyl fragment supports a mechanism in which
chain transfer in Ni(II) butadiene polymerizations occurs via
a hydrogen atom transfer, adding experimental credence to
Tobisch’s DFT calculations.40
has previously been reported. This species is quite robust and
isnot active forbutadienepolymerization. Similarly, exposure
of complex 2 to 3 equiv of cyclopentadiene at 0 °C leads to the
direct formation of species 10 and 1 equiv of cyclohexene. This
product forms via hydrogen atom transfer from cyclopenta-
diene to the cyclohexenyl group. In the 1H NMR spectrum, all
mestiylene is now uncoordinated, as indicated by a downfield
shift of the arylsingletresonance to 6.79 ppm. We propose this
reaction occurs through a mechanism similar to that observed
for the hydrogen transfer reactions when 1-hexene is ex-
posed to [(2-R-allyl)Ni(mes)]þ25 (Scheme 6). Coordination
of η4-cyclopentadiene likely occurs prior to hydrogen atom
transfer. This speculation is supported by observation of a
minor amount of a transient complex tentatively assigned
1
as [(cyclohexenyl)Ni(η4-cyclopentadiene)]þ in the H NMR
spectrum at -80 °C.
Polymerization Screening. [(cyclohexenyl)Ni(mes)][B(ArF)4]
(2) is active for the polymerization of 1,3-dienes. The polymer-
ization of 850 equiv of 1,3-CHD is achieved within 1 h at room
temperature in methylene chloride. Upon monomer addition,
the solution changes color from red-orange to yellow. The
reaction is highly exothermic, and within 2 min the solution
temperature has risen substantially. The polymer produced is
insoluble in CH2Cl2 and precipitates as a white powder (85%
isolated). Additionally, catalyst 2 is active for DMBD poly-
merization at room temperature in CH2Cl2. Within 6 h, 32%
conversion is achieved using 2 as the initiator. The polymer
produced is also insoluble in CH2Cl2 and precipitates from
solution. Within 6 h catalyst decomposition has occurred, as
evidenced by a colorless solution. The mesitylene catalyst is not
active for polymerization of 1,3-CHD or DMBD at tempera-
tures much below 0 °C. Efforts to further characterize the
highly insoluble polymers were unsuccessful. Details of these
polymerizations can be found in the Supporting Information.
Reaction with Olefins. Reaction of mesitylene complex 2
with olefins leads to hydrogen atom transfer products,
analogous to the findings reported for the [(2-R-allyl)-
Ni(mes)][B(ArF)4] complexes (R = H, Me).25 Exposure of
4-5 equiv of 1-hexene to species 2 at room temperature
generates 1 equiv of cyclohexene and a new allyl complex,
[(2-hexenyl)Ni(mes)][B(ArF)4] (11), after 24 h (Scheme 7).
Rapid isomerization of 1-hexene to the internal 2- and
3-isomers is observed within 10 min of olefin addition, and
only minor formation of the new allyl species occurs at 10 min.
Summary
The reactions of cationic (cyclohexenyl)nickel(II) com-
plexes with olefins and 1,3-dienes were investigated. Exposure
1
After 2.5 h, the H spectrum shows a signal for the olefinic
hydrogens of cyclohexene at 5.66 ppm and a new allyl reso-
nance integrating for 2H at 3.11 ppm, with complex 2 still
present in solution. We previously reported the characterization
of species 11.25 This intramolecular hydrogen atom trans-
fer reaction in the cyclohexenyl system is much slower in
(39) Taube, R.; Wache, S.; Langlotz, J. Mol Mass Regulation in the
Allyl Nickel Complex Catalyzed 1,4-cis Polymerization of Butadiene. In
Catalyst Design for Tailor-Made Polyolefins. Studies in Surface Science
and Catalysis; Soga, K., Terano, M., Eds.; Elsevier: Tokyo, 1994; pp
315-325.
(40) Tobisch, S. Macromolecules 2003, 36, 6235–6244.

Article
Organometallics, Vol. 29, No. 21, 2010 5387
Scheme 7. Reaction of 1-Hexene with Complex 2 at Room Temperature
Scheme 8. C-C Coupling of Complex 2 with DMBD
of [(cyclohexenyl)Ni(NCMe)2][B(ArF)4] to 2 equiv of
B(C6F5)3 generated the ligand-free species [(cyclohexenyl)-
Ni][B(ArF)4] (3) at low temperatures. Stable (η4-diene)-
Ni(cyclohexenyl)þ complexes (diene=BD, IP, DMBD, 1,3-
CHD, and 1,4-CHD) have been observed for the first time
by reaction of 3 with dienes. These (cyclohexenyl)nickel(II)
1,3-diene complexes are more thermally stable than their
acyclic analogues. The mesitylene complex 2 serves as an
initiator for polymerization of 1,3-CHD and DMBD.
Significantly, the first example of an η4-s-trans 1,3-diene
coordinated to a Ni(II) center has been observed.41 Addi-
tionally, we have observed intramolecular hydrogen atom
transfer reactions from olefin to the cyclohexenyl moiety
when [(cyclohexenyl)Ni(mes)][B(ArF)4] was treated with
1-hexene and η4-cyclopentadiene. Such hydrogen transfer
reactions are likely closely related to chain transfer reac-
tions in Ni(II)-catalyzed diene polymerizations.
Materials. All solvents were deoxygenated and dried by
passage over columns of activated alumina.42 CD2Cl2, pur-
chased from Cambridge Laboratories, Inc., was dried over
CaH2, vacuum-transferred to a Teflon-sealable Schlenk flask
˚
containing 4 A molecular sieves, and degassed via three freeze-
pump-thaw cycles. Butadiene and isoprene were purchased
from Aldrich and purified by vacuum transfer through a column
˚
of 4 A molecular sieves and stored under argon at -35 °C. 1,3-
Cyclohexadiene was purified by drying over NaBH4 overnight,
vacuum-transferred to a Teflon-sealable flask, and degassed
via three freeze-pump-thaw cycles. 2,3-Dimethyl-1,3-buta-
diene and 1,4-cyclohexadiene were purified by degassing using
three freeze-pump-thaw cycles. Ni[COD]2 and B(C6F5)3 were
purchased from Strem. NaB(ArF)4 (ArF = 3,5-(CF3)2C6H3),43
nickelocene (Cp2Ni),44 and [H(OEt2)][B(ArF)4]45 were syn-
thesized according to literature methods. Cyclopentadiene
was distilled fresh prior to use through cracking of dicyclo-
pentadiene purchased from Aldrich and stored cold. 1-Hexene
was purchased from Aldrich, dried over CaH2, and vacuum-
transferred to a sealable flask. Acetonitrile, mesitylene (mes),
hexamethylbenzene (hmb), mesitylene-d12, and 3-bromocyclo-
hexene were purchased from Aldrich or Acros and used with-
out further purification. [(cyclohexenyl)NiBr]2 was synthesized
according to literature methods.2
Experimental Section
General Considerations. All reactions, unless otherwise sta-
ted, were conducted under an atmosphere of dry, oxygen-free
argon using standard high-vacuum, Schlenk, or drybox tech-
niques. Argon was purified by passage through BASF R3-11
1
Spectral Data for [B(ArF)4]-. The H and 13C spectral data
˚
for the B(ArF)4- counteranion were unchanged for all Ni(II)
cationic complexes and are not included in the characteriza-
tion unless otherwise stated. 1H NMR (400 MHz, CD2Cl2,
20 °C): δ 7.72 (s, 8H, ArF Ho), 7.57 (s, 4H, ArF1Hp). 13C{1H}
NMR (101 MHz, CD2Cl2, 20 °C): δ 162.1 (q, JC-B = 49.8
Hz, ArF Cipso), 135.2 (s, ArF Co), 129.2 (qq, 3JC-B = 3.0 Hz,
2JC-F = 34.4 Hz, ArF CF3), 122.3 (q, 1JC-F = 273.2 Hz, ArF
CF3), 117.8 (bt, ArF Cp).
catalyst (Chemalog) and 4 A molecular sieves. All nickel
catalysts were stored under argon in an M. Braun glovebox
at -35 °C. 1H, 13C, and 19F NMR spectra were recorded on a
Bruker DRX 500 MHz, a Bruker DRX 400 MHz, or a Bruker
300 MHz spectrometer. Chemical shifts are referenced relative
1
1
to residual CHCl3 (δ 7.24 for H), CH(D)Cl2 (δ 5.32 for H),
13CD2Cl2 (δ 53.8 for 13C), 13CDCl3 (δ 77.0 for 13C). Elemental
analyses were performed by Robertson Microlit Laboratories
of Madison, NJ.
(42) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(43) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579–
3581.
(44) Wilkinson, G.; Pauson, P. L.; Cotton, F. A. J. Am. Chem. Soc.
1954, 76, 1970–1974.
(41) For examples of mononuclear s-trans-butadiene complexes of
€
G.; Jacobsen, H.; Berke, H. Organometallics 1999, 18, 3082–3812. (b)
early metals see: (a) Strauch, H. C.; Wibbeling, B.; Frohlich, R.; Erker,
ꢀ
€
Strauch, J. W.; Faure, J.-L.; Bredeau, S.; Wang, C.; Kehr, G.; Frohlich, R.;
Luftmann, H.; Erker, G. J. Am. Chem. Soc. 2004, 126, 2089–2104. (c)
Wang, L.; Fettinger, J. C.; Poli, R.; Meunier-Prest, R. Organometallics 1998,
17, 2692–2701.
(45) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3290–3922.

5388 Organometallics, Vol. 29, No. 21, 2010
O’Connor et al.
Synthesis of (Cyclohexenyl)nickel(II) Complexes. [(Cyclo-
hexenyl)Ni(NCMe)2][B(ArF)4] (1).
In Situ Generation of [(cyclohexenyl)Ni(η4-diene)][B(ArF)4]
Complexes. [(Cyclohexenyl)Ni(B(ArF)4] (3).
A flame-dried Schlenk tube was charged with [(cyclo-
hexenyl)NiBr]2 dimer (0.070 g, 0.159 mmol) and NaB(ArF)4
(0.282 g, 0.318 mmol) and cooled to -78 °C. The solids were
dissolved in diethyl ether (10.0 mL) to yield an orange solution
with a fine precipitate. Acetonitrile (33.2 μL, 0.636 mmol) was
added, and the solution turned yellow. The reaction mixture was
then warmed to room temperature and stirred for 1 h. The
solution was filtered via cannula filter, and the volatiles were
removed in vacuo to yield a yellow oily product. The oil was
washed with pentane (6.0 mL) and stirred vigorously for 30 min.
At this point a yellow powder was allowed to settle and the liquid
layer was decanted. The powder was dried in vacuo. Excess
NaB(ArF)4 was removed through an additional filtration step in
which the yellow powder was dissolved in CH2Cl2 (5.0 mL) and
the solution filtered through a Celite pad. The volatiles were
removed in vacuo to yield a yellow oil. Pentane (5.0 mL) was
added, and the product was again stirred until a yellow powder
formed. The pentane layer was decanted, and the yellow powder
was dried in vacuo overnight to yield 56% of 1 (0.190 g, 0.175
A screw-top NMR tube was charged with 1 (0.010 g,
0.009 mmol) and B(C6F5)3 (0.024 g, 0.047 mmol). The NMR
tube was cooled to -80 °C in a dry ice-acetone bath. The tube
was placed under an argon stream, and the two solids were
dissolved in CD2Cl2. The NMR tube was quickly inverted to
allow for mixing and placed again in the cold bath. The solution
turned from bright yellow to yellowish brown, which is indica-
tive of ligand-free complex formation. The tube was placed
into a precooled NMR probe, and the reaction was monitored
by 1H NMR spectroscopy. An equilibrium exists between
starting complex 1 and species 3. The ratio of 1:3 is 1:4
1
at -20 °C. H NMR (500 MHz, CD2Cl2, -60 °C): δ 8.04 (s,
1H, ArFp coordinated), 7.74 (s, 6H, ArFm), 7.66 (s, 3H, ArFp),
7.22 (s, 2H, ArFm coordinated), 4.60 (br s, 2H, H1), 4.00 (br t,
1H, H2), 2.70 (s, borane adduct of CH3CN), 2.32 (s, 2H,
mmol). 1H NMR (500 MHz, CD2Cl2, 25 °C): δ 5.70 (t, 3JH-H
=
H3), 1.10 (br. s, 4H, H3 , H4, H4 ). 19F NMR (471 MHz, CD2Cl2,
-40 °C): δ -62.7 (s, 6F), -62.9 (s, 18F).
0
0
6.5 Hz, 1H, H2), 5.86 (t, 3JH-H = 6.5 Hz, 2H, H1), 2.23 (s, 6H,
CNCH3), 1.71 (br. m, 2H, H3), 1.61 (m, 1H, H4), 1.41 (m, 2H,
1H NMR Line-Broadening Experiments To Determine Barrier
to Ni Migration among the ArF Rings of [B(ArF)4]- in Species 3.
A solution of 3 was prepared in CD2Cl2 as described above.
Broadening of the coordinated ArF 1H resonance, correspond-
ing to ArF Hp (8.04 ppm), was monitored between -70 and
-35 °C. The initial half-height line width was 10 Hz at -70 °C,
12 Hz at -56 °C, 15 Hz at -46 °C, and 23 Hz at -35 °C. The rate
constants, calculated from the slow exchange approximation
(kex = πΔω) at the temperatures listed above, are kex = 6.3,
H3 ), 0.78 (m, 1H, H4 ). 13C{1H} NMR (100 MHz, CD2Cl2,
25 °C): δ 107.6 (s, C2), 77.6 (s, C1), 29.1 (s, C3), 17.6 (s, C4), 3.5
(br. s, NCCH3). The nitrile carbon could not be located. Anal.
Calcd for C42H27N2BNi: C, 46.49; N, 2.58; H, 2.47. Found: C,
46.40; N, 2.41; H, 2.34.
0
0
[(Cyclohexenyl)Ni(mes)][B(ArF)4] (2).
15.7, 40.8 s-1, where Δω = 2, 5, 13 Hz, respectively (ΔGq
11.9 kcal/mol, average of three values).
=
avg
[(Cyclohexenyl)Ni(η4-butadiene)][B(ArF)4] (4).
A flame-dried Schlenk flask was charged with [(cyclo-
hexenyl)NiBr]2 dimer (0.150 g, 0.314 mmol) and NaB(ArF)4
(0.604 g, 0.682 mmol) and cooled to -78 °C in a dry ice-
isopropyl alcohol bath. Dry diethyl ether (10.0 mL) was
added, yielding an orange solution with a fine brown pre-
cipitate. After approximately 5 min of stirring, mesitylene
(190 μL, 1.36 mmol) was added dropwise via syringe. The
solution was warmed to 0 °C and stirred for 1 h. The solution
was filtered via cannula filter, and the solvent was removed in
vacuo to yield a red powder. The solid was purified by
dissolving in a minimal amount of methylene chloride and
filtering through Celite. The volatiles were then removed in
vacuo to give 2 in 65% yield (0.491 g, 0.437 mmol). Brick red
X-ray-quality crystals of the complex were obtained by dis-
solving complex 2 in CH2Cl2 (2.0 mL), layering with pentane
(6.0 mL), and storing in a freezer at -35 °C for 4 days.
1H NMR (300 MHz, CD2Cl2, 25 °C): δ 6.52 (s, 3H, mesCH),
A dry screw-top NMR tube was charged with 1 (0.010 g,
0.009 mmol) and B(C6F5)3 (0.009 g, 0.018 mmol) and cooled to
-78 °C. The tube was placed under argon, and the solids were
dissolved in CD2Cl2 and quickly mixed by inversion of the tube.
Two equivalents of butadiene (0.37 M in CD2Cl2; 50 μL, 0.019
mmol) was added via syringe, and the NMR tube was quickly
inverted to allow for mixing as previously described.15 The
NMR tube was placed in a precooled probe and monitored by
1H NMR spectroscopy. 1H NMR (500 MHz, CD2Cl2, -53 °C):
3
δ 6.77 (t, JH-H = 6.5 Hz, 2H, H1), 6.25 (m, 2H, H7), 5.91
3
3
(t, 3JH-H = 6.5 Hz, 1H, H2), 5.20 (d, 3JH-H = 7.0 Hz, 2H, H5),
3.26 (dd, JH-H = 14.0 Hz, JH-H = 1.5 Hz, 2H, H6), 2.09
5.86 (t, JH-H = 6.3 Hz, 1H, H2), 4.81 (td, JH-H = 6.3 Hz,
3JH-H = 1.8 Hz, 2H, H1), 2.34 (s, 9H, mesCH ), 1.57-1.17 (m,
3
2
3
6H, H3, H3 , H4, H4 ). 13C NMR (100 MHz, CD2Cl2,
0
0
0
(br m, 2H, H3), 1.21 (m, 1H, H4), 0.73 (m, 1H, H4 ), 0.59 (m, 2H,
0
H3 ). 13C{1H} NMR (126 MHz, CD2Cl2, -53 °C): δ 111.4
25 °C): 123.9 (s, mesCAr), 109.7 (s, mesCArCH ), 100.4 (s, C2),
3
78.4 (s, C1), 28.5 (s, C3), 20.2 (s, C4), 20.2 (s, mesCH3, over-
lapping with C4). Anal. Calcd for C47H33F24BNi: C, 50.26; H,
2.97. Found: C, 49.04; H, 2.97.
(s, C2), 109.2 (s, C6), 90.4 (s, C1), 83.0 (s, C5), 28.8 (s, C3), 14.3
(s, C4). The solution remained yellow until insertion occurred,
and then the solution turned orange (∼ -20 °C).

Article
Organometallics, Vol. 29, No. 21, 2010 5389
[(Cyclohexenyl)Ni(η4-s-cis-2,3-dimethyl-1,3-butadiene)][B(ArF)4]
25 °C) δ 6.44 (s, 3H, mesArH), 5.66 (br s, free cyclohexene), 3.11
(m, 2H, Hanti), 2.32 (s, 9H, mesCH3).25
In Situ Generation of Coupling Product 12.
(8) and [(cyclohexenyl)Ni(η4-s-trans-2,3-dimethyl-1,3-butadiene)]-
[B(ArF)4] (9).
This experiment was conducted using the procedure de-
scribed above for the formation of complex 5. Complex 1
(0.010 g, 0.009 mmol) and B(C6F5)3 (0.009 g, 0.018 mmol) were
dissolved in CD2Cl2 (600 μL), and 2,3-dimethyl-1,3-butadiene
(1.1 μL, 0.010 mmol) was added. Species 9 is observed in
solution along with 3 and 8 at -80 °C. At -20 °C only 8 and
9 are observed in solution. The ratio 8:9 in solution at -20 °C is
4:1. Complex 8: 1H NMR (500 MHz, CD2Cl2, -80 °C) δ 6.54 (t,
3JH-H = 6.8 Hz, 2H, H1), 5.77 (t, 3JH-H = 6.5 Hz, 1H, H2), 4.97
(s, 2H, H5), 2.93 (s, 2H, H6), 2.21 (s, 6H, CH3), 2.18 (br m, 2H,
Complex 2 (0.010 g, 0.009 mmol) was added to a screw-cap
NMR tube, and the compound was dissolved in CD2Cl2
(600 μL). The NMR tube was cooled to -80 °C and then charged
with 2,3-dimethyl-1,3-butadiene (60.4 μL, 0.534 mmol) and
quickly mixed by inversion of the tube. The solution remained
deep red. The reaction was monitored at low temperatures using
1H NMR spectroscopy. Polymerization of DMBD occurred
upon warming to 10 °C, with polyDMBD precipitating out of
solution. Formation of 12, the C-C coupling product of DMBD
and complex2, isobservedat 10 °C. Complete identification ofall
resonances for 12 could not be established, due to the presence of
excess DMBD, which masks many of the signals. Here we report
0
0
H3), 1.27 (br s, 1H, H4), 0.85-0.48 (br m, 3H, H4 , H3 ). Complex
9: 1H NMR (500 MHz, CD2Cl2, -20 °C) δ 6.32 (br m, 1H, H1),
6.30 (br m, 1H, H1 ), 5.33 (t, 1H, H2, under CDHCl2), 5.19 (s,
0
0
1H, H5), 5.04 (s, 1H, H5 , under free DMBD), 4.46 (s, 1H, H6),
the observable characteristic peaks assigned to complex 12.25 1
H
0
0
4.30 (s, 1H, H6 ), 1.94 (s, 3H, CH3 ), 1.50 (s, 3H, CH3), CH2
groups masked by cis isomer and oligomers of DMBD. Inser-
tion of DMBD occurs upon warming to -10 °C.
NMR (400 MHz, CD2Cl2, 10 °C): δ 6.42 (s, 3H, mesArH), 5.66
(br t, 1H, Holefin), 5.43 (t, 1H, JH-H = 8.5 Hz, Holefin), 3.38
3
(s, 1H, Hsyn), 2.65 (s, 1H, Hanti), 2.31 (s, 9H, mesCH3), 0.89 (s, 3H,
CH3). The resonances for the methyl and methylene groups are
masked by excess DMBD and oligomers in solution.
Reaction of 3 with Cyclopentadiene To Form [(cyclopenta-
dienyl)Ni(η4-cyclopentadiene)][B(ArF)4] (10).
Assessment of the Lability of Coordinated Mesitylene in Com-
plex 2. Reaction of Complex 2 with Hexamethylbenzene (hmb).
A screw-top NMR tube was charged with 2 (0.005 g,
0.004 mmol) and hmb (0.004 g, 0.022 mmol), and these com-
pounds were dissolved at room temperature in CD2Cl2 (500 μL).
The NMR tube was shaken to allow for mixing. Within 10 min
of addition of CD2Cl2 to the tube, the 1H NMR spectrum was
recorded for this reaction. The reaction was then monitored by
1H NMR spectroscopy every 1 h until complete conversion.
After 10 min, the ratio 2:[(cyclohexenyl)Ni(hmb)]þ was 1:9, and
after 1 h complete conversion to the hmb complex was achieved.
1H NMR (300 MHz, CD2Cl2, 25 °C): δ 5.68 (t, 3JH-H = 6.6 Hz,
1H, H2), 4.26 (br t, 2H, H1), 2.28 (s, 18H, CH3), 1.47-1.37
Complex 2 or 3 (0.010 g) was added to a screw-cap NMR tube
and cooled to -80 °C in a cold bath. The complex was dissolved
in 600 μL of CD2Cl2 and mixed. Cyclopentadiene (2-3 equiv)
was added, and the reaction mixture was quickly mixed again.
The solution remained red until the temperature rose to 0 °C. At
temperatures above 0 °C, the solution turned yellow-green and
complex 10 was formed. 1H NMR (300 MHz, CD2Cl2, 25 °C):
δ 7.09 (br s, 2H, H2), 6.79 (s, free mesitylene), 5.78 (s, 5H, CpH),
0
0
(m, 3H, H3, H4), 1.11-0.99 (m, 3H, H3 , H4 ).
3
5.66 (s, free cyclohexene), 5.41 (t, 2H, JH-H = 2.4 Hz, H1),
Reaction of 2 with Mesitylene-d12. A screw-cap NMR tube
was charged with 2 (0.005 g, 0.004 mmol), and the compound
was dissolved in CD2Cl2 (500 μL) in the glovebox. Mesitylene-
3.30 (roofed dt, 1H, 2JH-H = 22.0 Hz, 3JH-H = 2.4 Hz, Hendo),
3.03 (roofed d, 1H, JH-H = 22.0 Hz, Hexo), 1.61 (bs, free
2
cyclohexene), 1.26 (bs, free cyclohexene). These data match
literature values37,38 and those of an independently generated
sample (see the Supporting Information).
Crystallographic Information. The molecular structure for
complex 2 was determined using a Bruker APEX II diffracto-
meter at -173 °C equipped with Mo radiation. All necessary
information regarding the crystal structure data can be found in
the Supporting Information and Figure 1.
d
12 (3.1 μL, 0.022 mmol) was then introduced via syringe. The
reaction was monitored at room temperature by 1H NMR
spectroscopy. The first spectrum was recorded 10 min after
the addition of mesitylene-d12, and at this point the ratio 2:2-d12
was 3:7, as determined from integrating free (δ 6.79) vs bound
mesitylene (δ 6.55). After 4 h, the ratio changed to 1:6, and this
value did not change overnight.
In Situ Observation of [(2-hexenyl)Ni(mes)][B(ArF)4] (11). A J.
Young tube was charged with complex 2 (0.010 g, 0.009 mmol),
and the compound was dissolved in 500 μL of CD2Cl2. 1-Hexene
(5.0 μL, 0.040 mmol) was then added, and the tube was inverted
to allow for mixing. The reaction was monitored by 1H NMR
spectroscopy. Only the observable resonances for complex 11,
which are not masked by excess hexene, are reported. Forma-
tion of cyclohexene is also observed in the 1H NMR spec-
trum. Characteristic resonances: 1H NMR (300 MHz, CD2Cl2,
Acknowledgment. We thank the National Science
Foundation (No. CHE-0615794) for financial support.
Supporting Information Available: Text, tables, figures, and a
CIF file giving experimental details and X-ray crystallographic
data. This material is available free of charge via the Internet at
http://pubs.acs.org.