Direct synthesis of cis-dihalido-bis(NHC) complex of nickel(ii) and catalytic application in olefin addition polymerization: Effect of halogen co-ligands and density functional theory study

Article abstract of DOI:10.1039/c3dt50957e

Two novel amine-containing N-heterocyclic carbene ligand precursors [H(1a-b)]Br have been prepared in good yield and fully characterized. Direct syntheses of cis- and trans-dihalido-bis(NHC) nickel complexes [Ni(NHC) ₂X₂] (X = Cl, Br

Full text of DOI:10.1039/c3dt50957e

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Direct synthesis of cis-dihalido-bis(NHC) complex of
nickel(II) and catalytic application in olefin addition
polymerization: Effect of halogen co-ligands and
density functional theory study†
Cite this: Dalton Trans., 2013, 42, 12020
Dao Zhang,* Sen Zhou, Zhiming Li, Quanrui Wang and Linhong Weng
Two novel amine-containing N-heterocyclic carbene ligand precursors [H(1a–b)]Br have been prepared in
good yield and fully characterized. Direct syntheses of cis- and trans-dihalido-bis(NHC) nickel complexes
[Ni(NHC)2X2] (X = Cl, Br) are reported. The solid structures of trans-[Ni(1a–b)2Br2] (2a–b) and cis-
[Ni(1a)2Cl2] (3) were determined by single-crystal X-ray analysis and 3 was found to be the first example
of cis-configuration coordination of monodentate NHC ligands to a metal center for dihalido-bis(NHC)
nickel complexes. DFT calculations were conducted to determine the energy difference between cis- and
trans-isomers of complexes 2a and 3 bearing bromide and chloride co-ligands. The cis-[Ni(1a)2Cl2] (cis-3)
is 1.77–1.55 kcal mol−1 lower in energy than its trans-isomer in polar solvents including CH2Cl2 and THF,
while the trans-[Ni(1a)2Br2] (trans-2a) is more stable than the cis-isomer similarly in the gas phase. The cis
nickel complex 3 with two coordinated monodentate NHCs was tested for olefin addition polymerization
at standard conditions. It was found that cis-3 was inactive in ethylene polymerization but showed
moderate catalytic activities (0.5–3.0 × 106 g of PNB (mol of Ni)−1 h−1) in the addition polymerization of
norbornene in the presence of methylaluminoxane (MAO) as cocatalyst.
Received 11th April 2013,
Accepted 15th May 2013
DOI: 10.1039/c3dt50957e
www.rsc.org/dalton
Introduction
N-Heterocyclic carbene (NHC) ligands have been studied inten-
sely recently and are now used widely as strongly basic phos-
phine analogues, to support transition metal complexes, and
many applications, e.g., in homogeneous catalysis, have been
reported as described in recent reviews.1 The overwhelming
majority of NHCs are functionalized (with respect to the N and
N′ substituents) by incorporating neutral donor or anionic
groups.2 Recently, several kinds of amine group substituted
NHCs ligands were reported to stabilize transition metal
complexes.3–9 The reactivity and catalytic behavior of these
well-defined NHCs metal complexes were also explored.
Although NHCs are now ubiquitous in the field of olefin
metathesis, including ring-opening metathesis polymeriz-
ation,10 their success as ancillary ligands in catalysts for olefin
addition polymerization is still very limited.11 Until now the
effective olefin polymerization catalysts so far developed are
Chart 1
cis-geometry because the trans-disposition of two labile X
ligands is unfavorable for efficient olefin insertion. The cis-
complexes of nickel of the type [(bisNHC)NiX2] (X = halide)
(Chart 1), in which bisNHC is a chelating bidentate bis(imid-
azol-2-ylidene) ligand, haven been proved to be highly active as
catalyst precursors in coupling reactions because the cis-
arrangement of NHCs could translate into a faster catalyst
initiation due to their trans-effect.12 However, these complexes
show the tendency to undergo ligand disproportionation or
autoionization to form the inactive bis(chelates) nickel moi-
eties [Ni(bisNHC)2].12b We thus became interested in studying
cis-metal complexes containing two monodentate NHCs and
their catalytic applications in olefin addition polymerization.
To the best of our knowledge, dihalido-bis(monodentate-NHC)
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433,
People’s Republic of China. E-mail: daozhang@fudan.edu.cn; Fax: +86 21 6564 1740;
Tel: +86 21 6564 3021
†Electronic supplementary information (ESI) available. CCDC 877556–877559,
933724–933725. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt50957e
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Scheme 3 Synthesis of cis-dichloride-bis(NHC) nickel complexes.
Scheme 1 General synthesis of the NHC precursors [H(1a–b)]Br.
Fig. 1 Molecular structure of preligand H(1a)Br with thermal ellipsoids drawn
at the 50% level. Selected distances (Å) and angles (°): N(1)–C(10) 1.342(7),
N(2)–C(10) 1.293(7), N(3)–C(19) 1.358(8), N(3)–C(20) 1.463(8), N(1)–C(10)–N(2)
110.6(5), C(19)–N(3)–C(20) 122.9(6).
Scheme 2 Synthesis of trans-dibromide-bis(NHC) nickel complexes.
Results and discussion
Synthesis of amine-substituted NHC precursors
sively form trans-complexes,13 whereas cis-isomers remain In general, the amine-NHC ligand precursors were synthesized,
Ni(II) complexes of the type [Ni(monoNHC)2X2] (Chart 1) exclu-
unknown to date.14
as outlined in Scheme 1. The key benzyl halide 4 containing a
Previously we have developed aryloxyl modified hybrid BOC-protected-amine group was prepared straightforwardly
NHCs ligands and studied their coordination chemistry in the from commercially available N-methyl anthranillic acid methyl
area of s-block (Li, Na, K, Mg) and transition metal (Ti, Zr, Ni) ester (A) by a three-step process of continuous reduction of
complexes.15 Some of these NHC-metal complexes were found ester, BOC-protection of amine and bromination of benzol.16
to be active for ethylene and norbornene homogeneous poly- Reactions of 1-mesityl- and 1-tert-butylimidazoles (5a–b)17 with
merization.15c Recently we designed and synthesized a class 4 produce the BOC-protected-amine substituted imidazolium
of new amine-containing N-heterocyclic carbene ligands 1a–b salts [H(6a–b)]Br in high yields. Deprotection of the BOC-
(Scheme 1). We envisioned that the introduced amine amine group of [H(6a–b)]Br with HCl–CH2Cl2 gave the desired
groups would make the coordination structure more restricted amine-containing NHC precursors [H(1a–b)]Br as white solids.
1
or bring hydrogen bonds, which may be beneficial to the
The NHC precursors [H(1a–b)]Br were characterized by H,
formation of cis-dihalido-bis(NHC) complexes. We herein 13C NMR spectra and elemental analysis. Their 1H NMR
describe the direct preparation, structural characterization and spectra exhibit the signal for the NCHN proton in the range
theoretical study of cis- and trans-dihalido-bis(NHC) complexes of δ 10.74–11.02 ppm; the 13C NMR spectra exhibit the NCN
of nickel(II) (trans-2a–b and cis-3) (Schemes 2 and 3), along resonances between δ 148.5 and 148.9 ppm. The structure of
with the catalytic performance of the nickel complex cis-3 in [H(1a)]Br was further confirmed by X-ray single crystal analysis
olefin addition polymerization.
(Fig. 1). Its molecular structure belongs to a monoclinic unit
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cell. There exist hydrogen bonding interactions between the
halide and protons of both NCHN and MeNH-Ar in [H(1a)]Br.
The bond distances and angles of the five-membered imid-
azolium ring are in the expected range.
Synthesis of trans-dibromide-bis(NHC) nickel complexes
At the beginning of metallization, the synthesis of cis-dihalido-
bis(NHC) nickel complexes was attempted by deprotonation of
t
imidazolium salts [H(1a)]Br using BuOK or NaOAc as bases
in the presence of nickel precursors of Ni(OAc)2 or Ni(PPh)3X2
(X = Cl, Br), but only complex mixtures were obtained. The
cyclopentadiene elimination reactions of imidazolium salts18
for the preparation of NHC nickel complexes were also unsuc-
cessful. The treatment of (Cp)2Ni with 1 equiv. of [H(1a)]Br in
1,2-dimethoxyethane (DME) solution at 85 °C overnight gave a
red solution in which large brown insoluble solids were sus-
pended. After workup, the bisimidazole dibromide nickel
complex (7) was isolated from the filter as red crystals in
ca. 37% yield (Scheme 2). Soon afterwards the strong bases
were tested for the synthesis of nickel carbene complexes.
With 1 equiv. of n-BuLi or NaN(SiMe3)2 in THF, the deproton-
ation of [H(1a)]Br leads to hydrocarbon-soluble NHCs with
neutral amine moieties; the in situ resultant NHC continuously
reacted with Ni(PPh)3Br2 to afford trans-[Ni(1a)2Br2] (2a) in
65% yield (Scheme 2). Complex trans-[Ni(1b)2Br2] (2b) was
obtained from [H(1b)]Br in 56% yield using a similar way
to 2a.
Fig. 2 Molecular structure of trans-complex 2a with thermal ellipsoids drawn
at the 50% level. All hydrogen atoms are omitted for clarity. Selected distances
(Å) and angles (°): Ni(1)–C(1) 1.909(2), Ni(1)–Br(1) 2.3274(7), C(1)–Ni(1)–Br(1)
88.67(7).
The 1H NMR spectra of trans-complexes 2a–b of nickel
lacked the characteristic imidazolium NCHN resonance, indi-
cating successful carbene generation. The other peaks present
in the NMR spectra of [H(1a–b)]Br could successfully be identi-
fied, albeit shifted slightly from their original position. Similar
to previously reported trans-nickel complexes bearing amine-
linked NHC ligands,13i a syn–anti conformational equilibrium
was confirmed to be exhibited in the solution of 2a. For
example, two doublet methyl (connected to amine) group
peaks are present at δ 2.68 (minor) and 2.73 ppm (major) with
an approximate ratio of 2 to 3, respectively, integrating to a
total of 6 protons each. The carbene carbon atom of complexes
is observed in their 13C NMR spectra at 167.3–164.7 ppm.
Single crystals of 2a–b suitable for X-ray diffraction studies
were grown from a concentrated toluene solution upon vapor
diffusion of hexane. Crystallographic analysis revealed homolo-
gous structures for 2a (Fig. 2) and 2b (Fig. 3) and confirmed
their assignment as trans-configuration. The carbene ring
planes are oriented almost perpendicularly to the square
coordination geometry. Although the Ni–C distances [1.909(2)–
1.940(4) Å] measured in 2a–b occurred within the range of
values [1.909–1.933 Å] observed with other trans-[NiX2(NHC)2]
complexes, their relatively short distances presumably reflect
the greater π-acidity19 of [H(1a–b)]Br compared to typical
imidazolylidene-type NHCs.13 Similarly, the Ni–Br distances
[2.3274(7) and 2.3247(10) Å] and N–C–N angles [104.43(19) and
104.1(3)°] measured in 2a–b are consistent with those
(2.258–2.320 Å and 103.23–104.35°) measured in analogous
NHC-supported complexes.13
Fig. 3 Molecular structure of trans-complex 2b with thermal ellipsoids drawn
at the 50% level. All hydrogen atoms are omitted for clarity. Selected distances
(Å) and angles (°): Ni(1)–C(1) 1.940(4), Ni(1)–Br(1) 2.3247(10), C(1)–Ni(1)–Br(1)
89.42(11).
Synthesis of cis-dichloride-bis(NHC) nickel complexes
In general there are two reliable synthetic pathways for the syn-
thesis of NHC-Ni complexes. Given that we were unable to
isolate cis-dihalido-bis(NHC) metal compounds with the
above-mentioned in situ deprotonation of imidazolium salts,
we undertook to use the silver-NHC complex as transmetalat-
ing agent. A mixture of imidazolium [H(1a–b)]Br with silver
oxide in CH2Cl2 provided the corresponding NHC-silver
complex [Ag(1a–b)Br] (8a–b) (Scheme 3). Compound 8b could
be generated from [H(1b)Br] in almost quantitative yield, while
[H(1a)Br] reacted with Ag2O very slowly and less than 50%
yield of pure 8a was achieved after a long reaction time of
three days and recrystallization twice from hexane–dichloro-
methane. In the 13C NMR spectra of 8a–b, the NHC
carbon atoms normally resonate at 182.5 and 178.3 ppm,
respectively.
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Fig. 4 The photographs of the manually separated cis- complex of nickel 3
(left) and silver complexes 9a–b (right) (recorded by Canon eos digital slr
camera).
The silver NHC compound 8a was firstly treated with
Ni(PPh)3Cl2 in dichloromethane to afford the desired nickel
complex cis-[Ni(1a)2Cl2] (3) as well as a tetranuclear silver
complex [Ag(PPh3)(μ3-Br)]4 (9) (Scheme 3). Interestingly,
similar NHC transfer metalation reaction of 8b did not occur
at the same reaction and the silver NHC could be retrieved in
60–70% yield. Similar solubility properties of 3 and 9 prohib-
ited us to separate them completely. However, recrystallization
of the crude products from toluene–hexane at low temperature
resulted in the mixture of the orange crystals of 3 and the
colorless crystals of 9; the pure nickel and silver complexes
could be easily obtained under the microscope through
manual separation (Fig. 4).
Fig. 6 Molecular structure of cis-complex 3 with thermal ellipsoids drawn at
the 50% level. All hydrogen atoms are omitted for clarity. Selected distances (Å)
and angles (°): Ni(1)–C(1) 1.881(3), Ni(1)–C(21) 1.876(4), Ni(1)–Cl(1) 2.2123(12),
Ni(1)–Cl(2) 2.2198(13), C(1)–Ni(1)–C(21) 92.54(14), C(1)–Ni(1)–Cl(1) 88.52(10),
C(21)–Ni(1)–Cl(1) 178.86(11), C(1)–Ni(1)–Cl(2) 177.57(10), C(21)–Ni(1)–Cl(2)
89.64(11), Cl(1)–Ni(1)–Cl(2) 89.31(5).
It is well known that reactions of metal halides with imida-
zolium halides often result in the exchange of the halide at the
metal; one may expect the reaction of bromide silver NHC
compounds 8a–b with Ni(PPh)3Cl2 to give a mixture of
Ni(NHC)2Br2, Ni(NHC)2Cl2 and Ni(NHC)2BrCl. We tried but
failed to completely convert [Ag(1a–b)Br] to [Ag(1a–b)Cl], and
a half-exchanged silver compound [Ag(4b)Br0.67Cl0.33] (8c)
was analysed by X-ray crystal analysis (Fig. 5). The 1H NMR
spectrum of complex 3 is too complicated to be accurately
assigned, while in its 13C NMR spectrum, the NHC carbon
atoms normally resonate at 166.9 and 166.0 ppm respectively.
The molecular structures of 3 and 9 were determined by
X-ray diffraction as shown in Fig. 6 and 7. Complex 3 shows a
Fig. 7 Molecular structure of silver complex 9 with thermal ellipsoids drawn at
the 50% level. All hydrogen atoms are omitted for clarity. Selected distances (Å)
and angles (°): Ag(10)–P(1) 2.400(5), Ag(10)–Br(1) 2.726(8), Ag(10)–Br(2)
2.703(7), Ag(10)–Br(3) 2.770(7), P(1)–Ag(10)–Br(2) 119.88(14), P(1)–Ag(10)–
Br(1) 130.70(10), Br(2)–Ag(10)–Br(1) 91.76(6), P(1)–Ag(10)–Br(3) 120.65(18),
Br(2)–Ag(10)–Br(3) 93.03(6), Br(1)–Ag(10)–Br(3) 91.28(18).
cis-geometry around
a
slightly distorted square-planar
nickel center, with the two chloride ions twisted out of the
C(1)–Ni(1)–C(21) plane by 3.7(2)–4.1(2)°. The carbene
ring planes are twisted away from the coordination plane with
dihedral angles of ca. 65°. The coordination Ni–Ccarbene bond
Fig. 5 Molecular structure of NHC-silver complex 8c with thermal ellipsoids
drawn at the 50% level. All hydrogen atoms are omitted for clarity. Selected dis-
tances (Å) and angles (°): Ag(1)–C(1) 2.112(4), Ag(1)–Br(1) 2.4284(12), Ag(1)–
Br(1)# 2.9984(15), C(1)–Ag(1)–Br(1) 157.69(12), C(1)–Ag(1)–Br(1)# 113.19 (12).
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lengths [1.881(3) Å and 1.876(4) Å] in 3 are rather similar to
those in cis-[Ni(NCS)2(monoNHC)2] [1.879(2) Å and 1.881(3)
Å].14 These distances are slightly longer than those [1.855(8) Å–
1.872(2) Å] in cis-[Ni(bisNHC)Br2] with chelating bidentate
NHC ligands,12a,b but amazingly obviously shorter than those
in trans-complexes 2a–b [1.909(2)–1.940(4) Å] and other related
trans-[Ni(monoNHC)2X2] [1.888(12) Å–1.943(9) Å].12 The Ni–Cl
bond distances [2.2123(12) Å and 2.2198(13) Å] are notably
elongated as compared to their trans-derivatives trans-
[Ni(monoNHC)2Cl2] [2.167(3) Å–2.1875(6) Å],13a,d,f,i,j owing to
the stronger trans-influence of the carbene ligands. The cis-
complex 3 has a more open Ccarbene–Ni–Ccarbene bite angle of
92.54(14)° than cis-[Ni(NCS)2(monoNHC)2] [90.61(14)° and
90.80(18)°]13 and cis-[Ni(bisNHC)Br2]12a,b whose bite angles
change slightly upon lengthening of the bridging moiety, from
85.8(2)° for the propyl bridge to 91.0(2)° for the butyl bridge.
The Ccarbene–Ni–Cl angles [178.86(11)° and 177.57(10)°] deviate
slightly from linearity, this is a relatively small difference
with the Ccarbene–Ni–N angles [173.16(10)–173.30(13)°] in cis-
[Ni(NCS)2(monoNHC)2].13 Moreover, in the solid state there
exist hydrogen bonds (N(3)–H(3)⋯Cl(1), 2.24(5) Å; N(6)–H(6)
⋯Cl(2), 2.41(5) Å) between the chloride coligand and hydrogen
atom of amine, which maybe another reason for the formation
of cis complex 3 besides the effect of halide coligands.
Table 1 Calculated energies (sum of electronic and thermal free energies,
hartree)
Solvent
[Ni(1a)2X2]a Gas phase
THF
CH2Cl2
cis-2a
trans-2a
ΔEb
−534.2089088
−8534.2146803
−8534.289225
−8534.289123
−8534.288325
−8534.288525
+3.62 kcal mol−1 −0.06 kcal mol−1 +0.13 kcal mol−1
cis-3
−4306.7850002
−4306.7889726
−4306.86508
−4306.862604
−4306.866044
−4306.863228
−1.77
trans-3
ΔEb
+2.49 kcal mol−1 −1.55
a X = Br, 2a; Cl, 3. b ΔE = (Ecis) − (Etrans).
favored in polar solvents. Thus, the solvent effect would favor
the formation of the cis-isomers in polar solvents as observed
in our experiments. For 2a with bromide ligands, trans-2a is
also more stable than the cis-isomer similarly in gas phase.
Though solvent effect (THF) can stabilize the cis-isomer more,
trans-2a is still more stable than cis-complex, which is in line
with our experiment as well.
In X-ray structure of cis-3 (Fig. 6), the four atoms connected
with Ni directly is almost in the same plane, the dihedral
angle of Cl(1)–Cl(2)–C(1)–C(21) is 1°. This indicates that cis
isomer adopts a singlet ground state, so in our calculation
work, a net charge of zero and a multiplicity of one was
assumed. It can be concluded from the geometry parameters
that the optimized structure of cis isomer is completely sym-
metric, while the X-ray structure is a little distorted. The
largest difference in bond length is 0.085 Å for C(carbene)–N
distance, and the maximum difference in bond angle is about
1.2° for Cl(1)–Ni(1)–C(21). This means that the theoretical
result is in good agreement with the experimental one.
X-ray analysis (Fig. 7) revealed that the silver bromide crys-
tallized as a tetrameric PPh3 adduct. In the solid-state struc-
ture of complex 9, the asymmetric unit consists of four
formula units, where Ag and Br form an almost ideal hetero-
cubane cage with an average Ag–Br bond distance of 2.733 Å
(2.703–2.770 Å) and a slightly large Br–Ag–Br angle of 92.02°
(91.28–93.03°). The central cube is shielded by four bulky PPh3
groups.
The NBO analysis indicates that, in the cis-3, Ni(1)–C(1) and
Ni(1)–C(21) are polar σ bonds. These two bonds have a weight
of 70.5% at the carbon sides, which means that the bonds are
strongly polarized toward carbon atoms and the carbene
carbon is a relatively weak σ-donor. In the Ni–C bonds, cis-3
has 40.0% s, 11.2% p, and 48.8% d contribution at the Ni
atom, respectively, the corresponding electronic configuration
is sp0.28d1.22. The carbon atoms with sp1.36 hybridization form,
has 47.3% s and 57.3% p characters. Based on the above
results, one can conclude that the Ni(II) ion coordination with
C atoms is mainly on 3d and 4s orbits, C atoms form coordi-
nation bonds with Ni(II) ion by using 2s and 2p. The LUMO
orbit of cis-3 is composed of d nickel orbital and the orbitals
of two Cl atoms and the NHC fragment, whereas the HOMO
orbit is mainly from the d orbit of nickel (Fig. 8).
DFT study
The sole composition difference of the nickel complexes cis-3
and tran-2a is the halide coligands (Cl− and Br−). To get more
insight into the influence of the halogen co-ligands on the for-
mation of cis- and trans-complexes, we conducted DFT calcu-
lations to determine the energy difference between cis- and
trans-isomers of complexes (2a and 3) bearing bromide and
chloride co-ligands,20,21 and their relative energies are sum-
marized in Table 1.
As one may expect, the trans-isomers of 2a and 3 were
found to be more stable than their cis-isomers in the gas
phase by 2.49, 3.62 kcal mol−1. Since the cis isomer with larger
dipole moment is generally more favored in polar solvents, the
solvent effect was taken into consideration. For 3 with chloride
ligands, when considering solvent effect of CH2Cl2, the stabi-
lities of the isomers are inversed. The relative energies (sum of
electronic and thermal free energies) of cis and trans-3 in
Catalytic study
CH2Cl2 solvent are −4306.866044 and −4306.863228 hartree Since the targeted cis-complex of nickel 3 has chloride atoms
respectively. Obviously, cis-3 is 1.77 kcal mol−1 lower in energy in the desired cis arrangement, we envisioned it would have
than its trans-isomer in CH2Cl2 solvent. Moreover, changing potential use in olefin polymerization catalysis. The catalytic
the solvent from CH2Cl2 to THF does not affect the energy activity of cis-3 toward ethylene polymerization were firstly
difference much (1.55 kcal mol−1). This may be attributed to studied in toluene at atmospheric pressure and various reac-
the larger dipole moment of the cis-isomer, which is more tion temperatures of 0–50 °C in the presence of 1000
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at 15 °C. The activity increased with increasing the Al/Ni ratio
from 1000 to 3500 (entries 4–6), but decreased when the Al/Ni
ratio further increased from 3500 to 5500 (entries 4 vs. 1).
A possible reason for this is that, the presence of a large excess
of MAO would lead to the transmetalating process of NHCs
from Ni to Al. When the reaction time was prolonged from
15 min to 30 min, the activity decreased from 3.0 to 1.8 × 106 g
of PNB (mol of Ni)−1 h−1, which suggested that the active
species could exist stably for a certain time.
Fig. 8 The HOMO (left) and LUMO (right) orbits of cis-3.
The above obtained polymers are sparingly soluble at room
temperature in chlorobenzene, o-dichlorobenzene, and
dichloroethylene, which prohibited us from measuring their
actual molecular weights (Mw) and molecular weight distri-
butions (Mw/Mn). The resonances appearing at 2.6–0.9 (m,
maxima at 1.2, 1.4, 1.6, 2.3 ppm) in the 1H NMR spectra of the
partially soluble PNB product (Table 2, entry 3) and the
absence of double bands (CvC) at 1680–1620 cm−1 in the IR
spectra indicated that the polymerization initiated by the
nickel complex cis-3/MAO system adopts a vinyl-type addition
mechanism. Considering the active species, one may argued
that the NHC ligands are cis in the dihalide precursor, but
rearrange to trans in the MAO-activated complex. Related to
this, a polymerization experiment was run using the trans-
complex 2a. Its catalytic activity of 4.7 × 105 g of PNB (mol of
Table 2 Addition polymerization of norbornene with complex cis-3 activated
by MAOa
Entry
T (°C)
MAO (Al/Ni)
PNB (g)
Activityb
1
2
3
4
15
30
40
15
15
15
15
15
5500
5500
5500
3500
2500
1000
3500
3500
0.487
0.358
0.165
0.605
0.296
0.108
0.703
0
2.4
1.8
0.8
3.0
1.5
0.5
1.8
0
5
6
7c
8d
a Polymerization conditions: in 15 mL of chloromethane and toluene;
0.8 μ mol of [Ni] (4 × 10−4 mol L−1 in dichloromethane); 1.0 g of
norbornene; MAO (1.5 mol L−1 in toluene); t = 15 min. b In units
of (106 g of PNB) (mol of Ni)−1 h−1 c t = 30 min. d Without nickel
.
complex 3.
6
Ni)−1 h−1 is lower than that (3.0 × 10 g of PNB (mol of Ni)−1
h−1) of cis-complex 3 at the same conditions (see Table 2, entry
equivalents of methylaluminoxane (MAO) as cocatalyst. Unfor- 4). Until now, no other useful information has been attained
tunately, no polyethylene could be obtained. The generation of
to propose that the two cis NHC ligands remain in the MAO-
solid black particulates observed in some cases and attributed activated complex.
to the formation of nickel(0) suggests catalyst decomposition,
possibly through reductive elimination of a 2-alkylimidazolium
Conclusions
salt, as reported by others.11e,22 However, the GC-MS analysis
revealed that a small amount of ethylene oligomer of C4 could New amine-containing NHC precursors [H(1a–b)]Br and the
be found in the reaction mixture. Inspired by this, we under-
corresponding trans- and cis-NHC nickel complexes 2a–b and 3
took the addition polymerization of norbornene using the cis- were successfully synthesized, and X-ray crystal structure analy-
complex 3 as catalyst by treatment with MAO. The results are sis revealed that 3 is the first example of a dihalido-bis(mono-
listed in Table 2. Moreover, we even tried running the polymer-
dentate-carbene) nickel complex. DFT calculations revealed
ization of norbornene in the presence of ethylene. However, that the bromide and chloride co-ligands have an influence on
we are not sure whether the polymer is a copolymer or not the energy difference between cis- and trans-isomers of the
from the NMR spectra due to the sparingly soluble nature of
the obtained polymer.
NHC nickel complexes. In polar solvents the bromide complex
retains its trans-geometry as in the gas phase while the chlor-
In general, the cis-complex of nickel 3 showed a moderate ide complex tends to adopt a cis-isomer due to its low energy.
catalytic activity of 0.5–3.0 × 106 g PNB (mol of Ni)−1 h−1 in the
Preliminary studies using the cis nickel complexes 3 with co-
addition polymerization of norbornene with methylaluminox- ordinated NHCs showed no activity in ethylene polymerization
ane (MAO) as cocatalyst. The low reaction temperature is favor- but moderate catalytic activities (106 g of PNB (mol of Ni)−1
h−1) in the addition polymerization of norbornene with methyl-
able for norbornene polymerization with cis-3/MAO. With the
Al/Ni molar ratio of 5500 in dichloromethane, the activity aluminoxane (MAO) as cocatalyst. Future work will aim at
decreased with increasing temperature from 15 to 40 °C understanding the poor ethylene polymerization performance
(entries 1–3). This indicated that the active species is unstable
of the cis-complex 3 through the synthesis, characterization,
at high temperature and decomposes. This catalytic behavior and reactivity study of expected reaction intermediates.
is similar to the bis(aryloxide-NHC) nickel complexes,11f but
quite different from that of NHC nickel complex [Ni(C^N)2]Cl2
Experimental section
Materials and general considerations
(C^N is picolyl-functionalized NHC bidentate ligand)
that showed the highest activity of 2.6 × 107 g of PNB (mol of
Ni)−1 h−1 at 80 °C reported by Jin et al.23 According to Table 2,
Unless noted otherwise, all manipulations of metal complexes
the ideal Al/Ni ratio is ca. 3500 for norbornene polymerization were carried out under an inert atmosphere using standard
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Schlenk techniques. THF, toluene, hexane and dichloro- 7.23 (m, 2H, CH), 6.98 (dd, J = 2.0 Hz, 3H, CH), 6.69 (m, 1H,
methane were distilled from sodium–benzophenone or CaH2 N–CHv), 6.62 (d, J = 8.2 Hz, 1H, N–CHv), 6.50 (br s, 1H, NH),
under argon. Chemicals, 4,16 5a–b,17 (Scheme 1) and [Ni- 5.94 (s, 2H, NCH2), 2.80 (s, 3H, NCH3), 2.35 (s, 3H, CH3), 2.06
(PPh3)2X2] (X = Cl, Br)24 were prepared according to published (s, 6H, CH3). 13C NMR (100.5 MHz, CDCl3): δ 148.9, 141.3,
procedures. Other commercially available reagents were pur- 138.1, 134.4, 131.5, 131.4, 130.7, 129.9, 129.2, 122.7, 122.3,
chased and used without purification. The 1H and 13C NMR 117.3, 116.2, 111.1, 52.1, 29.9, 21.2, 17.5, 17.4. Anal. Calcd For
spectra were recorded on a Varian Unity INOVA 500 spectro- C20H24BrN3 (M = 386.33 g mol−1): C 62.18, H 6.26, N 10.88.
meter. Chemical shifts were referenced to the residual 1H, and Found: C 62.13, H 6.30, N 10.93.
13C solvent resonances, respectively. Elemental analysis was
performed on a Perkin-Elmer 240C analyzer.
1-tert-Butyl-3-[2-MeNH-benzyl]-imidazolium bromide ([H(1b)]Br).
White solid, Yield: 81.2%. H NMR (400 MHz, CDCl3): δ 11.02
1
(s, 1H, NCHN), 7.28 (d, J = 7.3 Hz, 1H, CH), 7.19 (d, J = 7.3 Hz,
1H, CH), 7.14 (s, 1H, CH), 7.08 (s, 1H, CH), 6.65 (t, J = 7.2 Hz,
1H, N–CHv), 6.59 (d, J = 8.2 Hz, 1H, N–CHv), 6.36 (br s, 1H,
General procedure for synthesis of 1-R-3-[2-MeN(BOC)-benzyl]-
imidazolium bromide ([H(6a–b)]Br)
Under
a N2 atmosphere 1-substituted imidazole (5a–b) NH), 5.66 (s, 2H, NCH2), 2.83 (s, 3H, NCH3), 1.70 (s, 9H,
(1.10 mmol) was charged into a flame-dried three neck flask CH3 tBu). 13C NMR (100.5 MHz, CDCl3): δ 148.5, 135.4, 131.7,
(100 mL) and THF (10 mL) was added. After the resultant clear 131.3, 121.8, 119.1, 116.8, 116.1, 110.9, 60.1, 50.8, 30.5, 30.1.
solution was heated to reflux, 2-MeN(BOC)-benzyl bromide Anal. Calcd For C15H22BrN3 (M = 324.26 g mol−1): C 55.56,
(1.13 mmol) dissolved in THF (10 mL) was added dropwise. H 6.84, N 12.96. Found: C 55.49, H 6.80, N 13.01.
The reaction mixture was stirred overnight and the solvent was
Synthesis of [Ni(C3H3N2)-C6H2-Me3-2,4,6]2Br2] (7)
removed in vacuum. The crude product was washed with
toluene to afford [H(3a–b)]Br as white solids.
Under N2 atmosphere [H(1a)]Br (193 mg, 0.5 mmol) and NiCp2
1-[2,4,6-Me3(C6H2)]-3-[2-MeN(BOC)-benzyl]-imidazolium bromide (95 mg, 0.5 mmol) were charged into a flame-dried Schlenk
([H(6a)]Br). White solid, Yield: 68.4%. 1H NMR (400 MHz, tube and DME (10 mL) was added. The resultant mixture was
CDCl3): δ 10.45 (s, 1H, NCHN), 7.52 (s, 1H, CH), 7.44–7.41 (m, heated to reflux at 85 °C overnight to give a red solution in
2H, CH), 7.35 (t, J = 7.2 Hz, 1H, CH), 7.16 (m, 2H, CH), 6.62 (br which a large brown insoluble solid was suspended. After cen-
s, 1H, NCHH′), 5.30 (br s, 1H, NCHH′), 3.23 (s, 3H, NCH3), trifugation, the clear solution was layered with hexane to
2.35 (s, 3H, CH3), 2.08 (s, 6H, CH3), 1.51 (s, 9H, CH3 tBu). 13C afford red crystals of 7 which were determined by X-ray single
NMR (100.5 MHz, CDCl3): δ 142.8, 141.4, 134.3, 130.8, 129.9, crystal analysis. Yield: 120 mg (41%). 1H NMR (400 MHz,
129.6, 128.8, 128.0, 110.9, 81.4, 48.9, 38.8, 31.6, 28.4, 26.8, CDCl3): δ 8.12 (bs, 6H), 7.10 (bs, 4H), 2.30 (s, 6H), 1.64
21.1, 17.7, 17.4. Anal. Calcd For C25H32BrN3O2 (M = 486.44 g (s, 12H). Anal. Calcd For C24H28Br2N4Ni (M = 591.01 g mol−1):
mol−1): C 61.73, H 6.63, N 8.64. Found: C 61.63, H 6.70, C 48.77, H 4.78, N 9.48. Found: C 48.88, H 4.88, N 9.57.
N 8.82.
General procedure for synthesis of trans-[Ni(1a–b)2Br2] (trans-
2a–b)
1-tert-Butyl-3-[2-MeN(BOC)-benzyl]-imidazolium
bromide
([H(6b)]Br). White solid, Yield: 75.3%. 1H NMR (400 MHz,
CDCl3): δ 10.85 (s, 1H, NCHN), 7.73 (s, 1H, CH), 7.47–7.40 (m, n-BuLi or Na[N(SiMe3)2] (1.32 mmol) was added dropwise to a
2H, CH), 7.33 (t, J = 7.4 Hz, 1H, CH), 7.19 (s, 2H, CH), 6.05 (br suspension of [H(1a–b)]Br (1.32mmol) in THF (35 ml) at
s, 1H, NCHH′), 5.20 (br s, 1H, NCHH′), 3.20 (s, 3H, NCH3), −78 °C under N2 atmosphere. The mixture was allowed to
1.73 (s, 9H, CH3 tBu), 1.49 (s, 9H, CH3 tBu). 13C NMR warm slowly to room temperature and stirred for another
(100.5 MHz, CDCl3): δ 142.7, 136.0, 130.7, 128.6, 128.0, 119.3, 3 hours. To the obtained red solution Ni(PPh3)2Br2 (981 mg,
81.3, 60.5, 48.6, 30.2, 28.4. Anal. Calcd For C20H30BrN3O2 (M = 1.32 mmol, 1.0 equiv.) was added. The solution turned dark-
424.38 g mol−1): C 56.60, H 7.13, N 9.90. Found: C 56.50, purple immediately and was stirred constantly overnight. The
H 7.21, N 9.95.
volatiles was removed in vacuum to give a dark-purple crude
that was washed with hexane (15 mL) three times and extracted
with toluene (10 mL) twice to afford trans-2a–b.
General procedure for the synthesis of 1-R-3-[2-MeNH-benzyl]-
imidazolium bromide (H[(1a–b)]Br
trans-[Ni(1a)2Br2] (trans-2a). Complex trans-2a was obtained
1
To a CH2Cl2 (5 mL) solution of H(6a–b)]Br (0.5 mmol) was as an orange solid. Yield: 356 mg (65%). H NMR (400 MHz,
added a premixed solution of CH2Cl2 (14 mL)–12 M HCl CDCl3): syn-form, δ 7.29 (4H, Ar), 6.69 (4H, Ar), 6.90 (4H, Ar),
(8 mL) under an ice-water bath. The resultant mixture was vig- 6.56 (4H, Ar), 6.47 (s, 4H, CH), 5.90 (s, 4H, CH2), 4.47 (d, J =
orously stirred at room temperature for 2 h. After the reaction 5.9, 2H, NH), 2.68 (d, J = 4.9 Hz, 6H, CH3N), 2.18 (s, 12H,
mixture was neutralized with solid NaHCO3, a small amount CH3), 2.08 (s, 6H, CH3); anti-form, δ 7.30 (4H, Ar), 6.91 (4H,
of water was added. The organic phase was separated, dried Ar), 6.60 (4H, Ar), 6.54 (m, 4H, CH), 6.41 (s, 4H, CH2), 4.90 (d,
over anhydrous Na2SO4 and filtered. The solvent was evapor- J = 4.9, 2H, NH), 2.73 (d, J = 4.9 Hz, 6H, CH3N), 2.54 (s, 6H,
ated under vacuum to give 1a–d as white powder.
CH3), 1.96 (s, 12H, CH3). 13C NMR (100.5 MHz, CDCl3): syn-
1-[2,4,6-Me3(C6H2)]-3-[2-MeNH-benzyl]-imidazolium bromide form, δ 167.3 (NCN), 148.4, 137.8, 136.8, 135.5, 132.0, 130.5,
([H(1a)]Br). White solid, Yield: 85.4%. 1H NMR (400 MHz, 129.2, 123.9, 119.9, 118.2, 115.5, 110.0, 53.9, 53.2, 29.6, 21.5,
CDCl3): δ 10.74 (s, 1H, NCHN), 7.30 (t, JHH = 8.2 Hz, 1H, CH), 19.8. anti-form, δ 167.3 (NCN), 148.3, 137.8, 136.3, 135.5,
12026 | Dalton Trans., 2013, 42, 12020–12030
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132.0, 130.5, 129.0, 123.8, 119.8, 118.0, 115.2, 109.7, 53.9, 53.2, was removed in vacuum. The obtained crude was washed with
29.6, 20.8, 19.3.Anal. Calcd For C40H46Br2N6Ni (M = 829.33 g hexane (15 mL) three times and extracted with toluene (10 mL)
mol−1): C 57.93, H 5.59, N 10.13. Found: C 57.88, H 5.62, twice. Hexane (15 mL) covered the toluene extract to diffusion
N 10.15.
trans-[Ni(1b)2Br2]
crystallize colorless crystals, determined as the silver cluster 9
was by X-ray crystallography. Residual solution was placed at low
(trans-2b). Complex
trans-2b
obtained as an orange solid. Yield: 287 mg (53%). 1H NMR temperature for several days, accompanied by colorless crystals
(400 MHz, CDCl3): syn-form, δ 7.69 (2H, Ar), 6.89 (2H, Ar), 6.60 of 9, the orange crystals was formed, conformed as the cis-
(1H, Ar), 6.39–6.37 (m, 1H, CH), 5.29 (s, 2H, NCH2), 2.63 complex of nickel (3) by X-ray crystallography. The reaction was
(s, 3H, NCH3), 2.05 (s, 1H, NH), 1.26 (s, 9H, CH3). anti-form, almost in quantitative yield according to the total of cis-3 and
δ 7.67 (2H, Ar), 6.89 (2H, Ar), 6.38 (1H, Ar), 6.25–6.21 (m, 2H, 9 and the given yields were only based on the manually separa-
CH), 5.28 (s, 2H, NCH2), 2.63 (s, 3H, NCH3), 2.05 (s, 1H, NH), ted compounds.
1.26 (s, 9H, CH3). 13C NMR (100.5 MHz, CDCl3): syn-form,
Complex cis-3. Yield: 167 mg (58%). 1H NMR (400 MHz,
δ 164.7 (NCN), 148.2, 147.0, 130.0, 129.5, 119.2, 118.8, 116.1, CDCl3): δ 7.17, 6.93, 6.85, 6.82, 6.68, 6.55, 5.97, 5.46, 5.07,
115.5, 110.0, 59.8, 57.6, 53.7, 50.5, 31.9, 30.4. anti-form, 4.61, 3.48, 3.44, 2.72–2.44, 1.92, 2.14, 2.09, 1.59, 1.46. 13C NMR
δ 164.7 (NCN), 148.1, 147.0, 130.0, 129.5, 119.0, 118.7, 116.0, (100.5 MHz, CDCl3): δ 166.9, 166.0, 159.7, 149.1, 148.4, 148.3,
115.4, 110.0, 59.7, 57.6, 53.5, 50.5, 31.8, 30.3. Anal. Calcd For 140.0, 139.0, 137.7, 136.8, 136.3, 135.7, 135.5, 134.5, 131.6,
C30H42Br2N6Ni (M = 705.2 g mol−1): C 51.10, H 6.00, N 11.92. 131.1, 130.5, 130.4, 129.0, 128.9, 123.3, 121.2, 114.8, 109.9,
Found: C 50.98, H 6.00, N 11.88.
53.1, 51.5, 29.6, 21.2, 19.8, 18.5, 18.2. Anal. Calcd For
C40H46Cl2N6Ni (M = 740.43 g mol−1): C 64.88, H 6.26, N 11.35.
Found: C 64.70, H 6.13, N 11.23.
General procedure for synthesis of [Ag(1a–b)Br] (8a–b)
Ag2O (232 mg, 10 mmol) and several 4 Å molecular sieve were
Complex 9. Yield: 188 mg (54%). 1H NMR (400 MHz,
added to a solution of [H(1a–b)]Br (0.40 mmol) in chloro- CDCl3): δ 7.47 (m, 24H), 7.33 (m, 12H), 7.28–7.24 (m, 24H). 13
C
methane (5 mL). After it was stirred for a certain time (48 h NMR (100.5 MHz, CDCl3): δ 134.2 (d, J = 17 Hz), 137.3 (d, J =
and 24 h, respectively), the mixture was filtered through Celite 13 Hz), 130.0, 128.7 (d,
J = 10 Hz). Anal. Calcd For
to give an off-white crude, which was purified, if necessary, C72H60Ag4Br4P4 (M = 1800.24 g mol−1): C 48.04, H 3.36. Found:
and dried under reduced pressure to afford a colorless C 48.33, H 3.51.
material.
[Ag(1a)Br] (8a). Pure 8a was obtained by recrystallization
twice from hexane–dichloromethane in ca. 50% yield. 1H NMR
DFT study
(400 MHz, CDCl3): δ 7.22–7.08 (m, 5H, CH), 6.86 (s, 3H, CH), The DFT Calculations were carried out with Gaussian 09
6.76 (m, 1H, NH), 6.58 (m, 2H, CH) 5.37 (s, 2H, NCH2) , 2.75 quantum chemistry software.21 The molecular geometries of
(s, 3H, NCH3), 2.32 (s, 3H, CH3), 1.82 (s, 6H, CH3). 13C NMR the two isomers were fully optimized using PBEPBE functional
(100.5 MHz, CDCl3): δ 182.5 (NCN), 147.7, 139.0, 135.7, 134.9, of Perdew–Burke–Ernzerhof22 in combination with 6-311+G(d,p)
130.0, 129.2, 122.2, 119.9, 116.2, 110.4, 52.8, 30.3, 21.2, 17.7. basis set. The effect of solvation on the reaction energetics
Anal. Calcd For C20H23AgBrN3 (M = 493.19 g mol−1): C 48.71, H was determined by means of single-point self-consistent reac-
4.70, N 8.52. Found: C 48.87, H 4.89, N 8.61.
tion field (SCRF) calculations using the polarized continuum
[Ag(1b)Br] (8b). Pure 8b was obtained in almost quantitative model (PCM). Gas-phase optimized structures were used in
yield. 1H NMR (400 MHz, CDCl3): δ 7.24–7.22 (m, 2H, CH), the single point calculations with the method PBEPBE/
7.08 (s, 1H, CH), 6.82 (s, 1H, CH), 6.79 (m, 1H, NH), 6.60–6.58 6-311+G(df,p).
(m, 2H, CH), 5.26 (s, 2H, NCH2), 2.76 (s, 3H, NCH3), 1.68 (s,
6H, CH3). 13C NMR (100.5 MHz, CDCl3): δ 178.3 (NCN), 148.6,
147.4, 130.1, 129.7, 119.6, 119.1, 118.9, 116.6, 115.8, 110.7,
General procedure for ethylene polymerization
110.6, 60.0, 57.8, 54.0, 50.9, 31.9, 30.5. Anal. Calcd For Ethylene polymerization was performed at atmospheric
C15H21AgBrN3 (M = 431.12 g mol−1): C 41.79, H 4.91, N 9.75. pressure and reaction temperature of 0–50 °C in a 100 mL
Found: C 41.97, H 5.00, N 9.88.
flamed Schlenk flask containing a magnetic stir bar. After
backfilling with ethylene three times under dynamic vacuum,
the flask was charged with 30 mL of dry toluene or dichloro-
Synthesis of cis-[Ni(1a)2Cl2] (cis-3) and [Ag(PPh3)(η3-Br)]4 (9)
An oven-dried 50 mL Schlenk flask containing a stir bar was methane and 1000 equivalents of MAO (10 wt% in toluene).
charged with [H(1a)]Br (300mg, 0.776 mmol), 4 Å molecular Then the nickel catalyst (10 μmol in 2.0 mL of toluene) was
sieves (0.3 g), silver(I) oxide (90 mg, 0.388 mmol) and CH2Cl2 introduced into the flask via syringe. The reaction mixture (the
(5 mL). After the mixture was stirred at room temperature in total volume is 50 mL) was vigorously stirred for 30 min at the
the absence of light for 24 h, Ni(PPh3)2Cl2 (254 mg, desired temperature and subsequently quenched with concen-
0.388 mmol) was added. The solution turned dark-purple in trated HCl–methanol (v/v = 1 : 9). The resulting mixture was fil-
colour immediately and was kept at room temperature over- tered, and any solid collected was washed with alcohol and
night. After centrifugation, the resulting upper clear solution dried overnight at 40 °C under vacuum. The filtrate was ana-
was transferred into another centrifuge bottle and the volatile lyzed by GC-MS.
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Table 3 Crystal data and structure refinements of H(1a)Br, trans-2a, trans-2b, cis-3, 8c and 9
H(1a)Br
20H24BrN3
386.33
0.10 × 0.08 ×
0.05
trans-2a
trans-2b
cis-3
8c
9
Formula
Fw
C
C40H46Br2N6Ni
829.36
0.15 × 0.13 ×
0.12
C30H42Br2N6Ni
705.22
0.25 × 0.15 ×
0.10
C40H46Cl2N6Ni
740.44
0.24 × 0.15 ×
0.13
C30H42Ag2BrClN6 C72H63Ag4Br4P4
817.79
0.45 × 0.40 ×
0.18
1803.22
0.20 × 0.18 ×
0.12
Cryst size/mm3
Crystal syst
Space group
Monoclinic
P2(1)/c
Monoclinic
P21/n
Monoclinic
P2(1)/c
Monoclinic
P2(1)/c
Monoclinic
P2(1)/c
Monoclinic
C2/c
a/Å
b/Å
c/Å
10.403(4)
9.077(3)
20.526(7)
96.017(5)
1927.5(12)
4
1.331
800
2.138
8.573(3)
15.870(6)
14.351(5)
91.024(5)
1952.2(12)
2
1.411
852
2.579
10.292(6)
13.510(8)
12.261(7)
111.141(8)
1590.1(15)
2
1.473
724
3.152
21.256(9)
8.957(4)
19.814(8)
95.989(6)
3752(3)
12.868(5)
12.316(5)
11.078(5)
108.620(5)
1663.7(12)
2
1.632
820
2.487
24.39(8)
12.08(3)
48.24(13)
100.90(7)
13 956.2(70)
8
1.716
7064
3.527
β/°
V/Å3
Z
4
ρcalcd/Mg m−3
F(000)
1.311
1560
0.696
μ/mm−1
θ range/°
Reflns collected
R(int)
1.97 to 25.01
7748
0.0521
1.91 to 26.30
8928
0.0414
2.12 to 26.01
7123
0.0443
1.93 to 26.01
16 480
0.0635
1.67 to 26.00
7364
0.0379
1.72 to 25.01
23 746
0.0524
Data/restraints/parameters
Goodness of fit
R indices (I > 2σ(I))a
3391/0/217
0.915
3961/0/231
0.857
3114/8/207
0.842
7348/0/442
0.908
3258/0/186
1.059
12 185/8/783
0.994
R1 = 0.0761
wR2 = 0.2109
R1 = 0.1283
wR2 = 0.2331
0.542/−0.540
R1 = 0.0341
wR2 = 0.0684
R1 = 0.0559
wR2 = 0.0715
0.525/−0.233
R1 = 0.0419
wR2 = 0.0914
R1 = 0.0808
wR2 = 0.0998
0.415/−0.267
R1 = 0.0533
wR2 = 0.1029
R1 = 0.1009
wR2 = 0.1174
0.401/−0.339
R1 = 0.0457
wR2 = 0.1361
R1 = 0.0629
wR2 = 0.1455
1.201/−0.800
R1 = 0.0753
wR2 = 0.2255
R1 = 0.1182
wR2 = 0.2444
0.960/−2.406
R indices (all data)a
Lgst diff. peak and hole e
Å−3
a R1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = [Σ(|Fo|2 − |Fc|2)2/Σ(Fo2)]1/2
.
General procedure for norbornene polymerization
Acknowledgements
Norbornene polymerization was performed in a 50 mL flamed
Schlenk flask containing a magnetic stir bar. After backfilling
with argon three times under a dynamic vacuum, the flamed
flask was charged with dichloromethane, norbornene (1.0 g)
and MAO (10 wt% in toluene). Then the nickel catalyst
We thank the financial support from the Natural Sciences
Foundation of China (No. 20972031, 21272040).
(0.4 μmol mL−1 in CH2Cl2) was introduced into the flask via Notes and references
syringe. The reaction mixture (the total volume is 15 mL) was
1 Selected reviews: (a) F. E. Hahn and M. C. Jahnke, Angew.
vigorously stirred for 15 min at the desired temperature after
the addition of the catalyst and subsequently quenched with
concentrated HCl–methanol (v/v = 1 : 9). The resulting mixture
was filtered, and the solid polymer collected was washed with
alcohol and dried overnight at 40 °C under vacuum.
Chem., Int. Ed., 2008, 47, 3122; (b) S. Diez-Gonzalez,
N. Marion and S. P. Nolan, Chem. Rev., 2009, 109, 3612;
(c) M. Poyatos, J. A. Mata and E. Peris, Chem. Rev., 2009,
109, 3677; (d) P. L. Arnold and I. J. Casely, Chem. Rev.,
2009, 109, 3599; (e) L. Benhamou, E. Chardon, G. Lavigne,
S. Bellemin-Laponnaz and V. César, Chem. Rev., 2011, 111,
2705.
X-ray data collection, structure solution, and refinement.
2 (a) N-Heterocyclic Carbenes: From Laboratory Curiosities to
Efficient Synthetic Tools, ed. S. Diez-Gonzalez, RSC Catalysis
Series, RSC, Cambridge, 2010; (b) N-Heterocyclic Carbenes in
Transition Metal Catalysis and Organocatalysis. Catalysis by
Metal Complexes, ed. C. S. J. Cazin, Springer, Heidelberg,
2010, vol. 32; (c) O. Kühl, Functionalised N-Heterocyclic
Carbene Complexes, John Wiley and Sons, New York, 2010;
(d) F. Glorius, N-Heterocyclic Carbenes in Transition Metal
Catalysis, Springer-Verlag, Heidelberg, Germany, 2007;
(e) N-Heterocyclic Carbenes in Synthesis, ed. S. P. Nolan,
Wiley-VCH, Weinheim, 2006.
The single crystals suitable for X-ray analysis was sealed into a
glass capillary and the intensity data of the single crystal were
collected on the CCD Bruker Smart APEX system. Data
obtained with the ω–2θ scan mode were collected on a Bruker
SMART 1000 CCD diffractometer with graphite-monochro-
matic Mo Kα radiation (λ = 0.710 73 Å) at 293(2) K. The struc-
tures were solved using direct methods, while further
refinement with full-matrix least squares on F2 was obtained
with the SHELXTL program package.25 All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
introduced in calculated positions with the displacement
factors of the host carbon atoms. The crystallographic data of
these compounds are summarized in Table 3.
3 (a) L. P. Spencer, S. Winston and M. D. Fryzuk, Organo-
metallics, 2004, 23, 3372; (b) L. P. Spencer and
M. D. Fryzuk, J. Organomet. Chem., 2005, 690, 5788;
12028 | Dalton Trans., 2013, 42, 12020–12030
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
(c) H. Jong and M. D. Fryzuk, Can. J. Chem., 2008, 86, 803;
(d) H. Jong, B. O. Patrick and M. D. Fryzuk, Organometallics,
2011, 30, 2333.
J. H. H. Ho, T. C. Neo and L. L. Koh, J. Organomet. Chem.,
2005, 690, 3854.
13 For trans-[Ni(monoNHC)2X2] (X
=
halide), see:
4 (a) P. L. Arnold, S. A. Mungur, A. J. Blake and C. Wilson,
Angew. Chem., Int. Ed., 2003, 42, 5981; (b) S. T. Liddle,
I. S. Edworthya and P. L. Arnold, Chem. Soc. Rev., 2007, 36,
1732.
5 (a) R. E. Douthwaite, J. Houghton and B. M. Kariuki, Chem.
Commun., 2004, 698; (b) L. G. Bonnet, R. E. Douthwaite,
R. Hodgson, J. Houghton, B. M. Kariuki and S. Simonovic,
Dalton Trans., 2004, 3528; (c) J. Houghton, G. Dyson,
R. E. Douthwaite, A. C. Whitwood and B. M. Kariuki,
Dalton Trans., 2007, 3065.
6 (a) L. Busetto, M. Cristina Cassani, C. Femoni,
A. Macchioni, R. Mazzoni and D. Zuccaccia, J. Organomet.
Chem., 2008, 693, 2579; (b) L. Busetto, M. Cristina Cassani,
C. Femoni, M. Mancinelli, A. Mazzanti, R. Mazzoni and
G. Solinas, Organometallics, 2011, 30, 5258.
(a) W. A. Herrmann, G. Gerstberger and M. Spiegler, Organo-
metallics, 1997, 16, 2209; (b) D. S. McGuinness, W. Mueller,
P. Wasserscheid, K. J. Cavell, B. W. Skelton, A. H. White
and U. Englert, Organometallics, 2002, 21, 175;
(c) A. L. MacKinnon and M. C. Baird, J. Organomet. Chem.,
2003, 683, 114; (d) K. Matsubara, K. Ueno and Y. Shibata,
Organometallics, 2006, 25, 3422; (e) B. R. Dible and
M. S. Sigman, Inorg. Chem., 2006, 45, 8430;
(f) H. V. Huynh, C. Holtgrewe, T. Pape, L. L. Koh and
E. Hahn, Organometallics, 2006, 25, 245; (g) E. F. Hahn,
B. Heidrich, A. Hepp and T. Pape, J. Organomet. Chem.,
2007, 692, 4630; (h) H. V. Huynh, L. R. Wong and P. S. Ng,
Organometallics, 2008, 27, 2231; (i) Y.-P. Huang, C.-C. Tsai,
W.-C. Shih, Y.-C. Chang, S.-T. Lin, G. P. A. Yap, I. Chao and
T.-G.
Ong,
Organometallics,
2009,
28,
4316;
7 W. Wei, Y. Qin, M. Luo, P. Xia and M. S. Wong, Organo-
metallics, 2008, 27, 2268.
8 (a) W. B. Cross, C. G. Daly, R. L. Ackerman, I. R. George
and K. Singh, Dalton Trans., 2011, 40, 495; (b) W. B. Cross,
C. G. Daly, Y. Boutadla and K. Singh, Dalton Trans., 2011,
40, 9722.
9 (a) W. N. O. Wylie, A. J. Lough and R. H. Morris, Organo-
metallics, 2009, 45, 6755–6761; (b) W. N. O. Wylie,
A. J. Lough and R. H. Morris, Chem. Commun., 2010, 8240–
8242.
( j) A. G. Tennyson, V. M. Lynch and C. W. Bielawski, J. Am.
Chem. Soc., 2010, 132, 9420; (k) Z.-H. Liu, Y.-C. Xu,
L.-Z. Xie, H.-M. Sun, Q. Shen and Y. Zhang, Dalton Trans.,
2011, 40, 4697; (l) J. Berding, J. A. van Paridon, V. H. S. van
Rixel and E. Bouwman, Eur. J. Inorg. Chem., 2011, 2450.
14 Note that an example of cis-diisothiocyanato-bis(NHC)
complexes of nickel(II) has been reported by a trans–cis iso-
merization occurring upon halido-isothiocyanato exchange
with trans-dihalido-bis(carbene) Ni(II) complexes bearing
less bulky carbene ligands, see: R. Jothibasu, K.-W. Huang
and H. V. Huynh, Organometallics, 2010, 29, 3746.
10 (a) T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34,
18; (b) G. C. Vougioukalakis and R. H. Grubbs, Chem. Rev., 15 (a) D. Zhang and H. Kawaguchi, Organometallics, 2006, 25,
2010, 110, 1746; (c) J. C. Conrad and D. E. Fogg, Curr. Org.
Chem., 2006, 10, 185; (d) S. Beligny and S. Blechert, N-Het-
erocycl. Carbenes Synth., 2006, p. 1; (e) Y. Schrodi and
5506; (b) D. Zhang, H. Aihara, T. Watanabe, T. Matsuo and
H. Kawaguchi, J. Organomet. Chem., 2007, 692, 234;
(c) D. Zhang and N. Liu, Organometallics, 2009, 28, 499.
R. L. Pederson, Aldrichimica Acta, 2007, 40, 45; 16 G. Babu, A. Orita and J. Otera, Org. Lett., 2005, 7, 4641.
(f) E. Despagnet-Ayoub and T. Ritter, Top. Organomet. 17 (a) J. P. Liu, Z. Y. Ren, Y. H. Zhao and H. B. Zhang,
Chem., 2007, 21, 193; (g) E. Colacino, J. Martinez and
F. Lamaty, Coord. Chem. Rev., 2007, 251, 726;
(h) C. Samojlowicz, M. Bieniek and K. Grela, Chem. Rev.,
Chin. J. Org. Chem., 2004, 24, 1091; (b) J. P. Liu, J. B. Chen,
J. F. Zhao, Y. H. Zhao, L. Li and H. B. Zhang, Synthesis,
2003, 2661.
2009, 109, 3708; (i) C. Fischmeister and C. Bruneau, Beil- 18 (a) C. D. Abernethy, J. A. C. Clyburne, A. H. Cowley and
stein J. Org. Chem., 2011, 7, 156.
11 (a) For review, see: D. McGuinness, Dalton Trans., 2009,
6915 and references therein; (b) Y. Nagai, T. Kochi and
R. A. Jones, J. Am. Chem. Soc., 1999, 121, 2329;
(b) C. D. Abernethy, A. H. Cowley and R. A. Jones, J. Organo-
met. Chem., 2000, 596, 3.
K. Nozaki, Organometallics, 2009, 28, 6131; (c) Y. Kong, 19 M. D. Sanderson, J. W. Kamplain and C. W. Bielawski,
H. Ren, S. Xu, H. Song, B. Liu and B. Wang, Organometal- J. Am. Chem. Soc., 2006, 128, 16514.
lics, 2009, 28, 5934; (d) S. Shanmuganathan, O. Kuehl, 20 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
P. G. Jones and J. Heinicke, Cent. Eur. J. Chem., 2010, 8,
992; (e) Y. Kong, L. Wen, H. Song, S. Xu, M. Yang, B. Liu
and B. Wang, Organometallics, 2011, 30, 153; (f) Y. Kong,
M. Cheng, H. Ren, S. Xu, H. Song, M. Yang, B. Liu and
B. Wang, Organometallics, 2011, 30, 1677; (g) X. Zhou and
R. F. Jordan, Organometallics, 2011, 30, 4632.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
12 For cis-nickel complexes of the type [Ni(bisNHC)X2]
employing alkyl-bridged bidentate NHC ligands, see:
(a) J. Berding, M. Lutz, A. L. Spek and E. Bouwman, Organo-
metallics, 2009, 28, 1845; (b) H. V. Huynh and R. Jothibasu,
Eur. J. Inorg. Chem., 2009, 1926; (c) H. V. Huynh,
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 12020–12030 | 12029

View Article Online
Paper
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
Dalton Transactions
V. V. Grushin, Organometallics, 2003, 22, 1591;
(d) D. S. McGuinness and K. J. Cavell, Organometallics,
2000, 19, 4918; (e) D. S. McGuinness, W. Mueller,
P. Wasserscheid, K. J. Cavell, B. W. Skelton, A. H. White
and U. Englert, Organometallics, 2002, 21, 175;
(f) D. S. McGuinness, N. Saendig, B. F. Yates and
K. J. Cavell, J. Am. Chem. Soc., 2001, 123, 4029.
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
GAUSSIAN 09 (Revision A.1), Gaussian, Inc., Wallingford CT,
2009.
21 (a) J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 23 X. Wang and G. Jin, Organometallics, 2004, 23, 6319.
1996, 77, 3865; (b) J. P. Perdew, K. Burke and M. Ernzerhof, 24 (a) P. A. Sehun, Inorg. Synth., 13, 124; (b) M. Hidai,
Phys. Rev. Lett., 1997, 78, 1396.
22 (a) R. E. Douthwaite, M. L. H. Green, P. J. Silcock and
T. Kashiwagi, T. Ikeuchi and Y. Uchida, J. Organomet.
Chem., 1971, 30, 279.
P. T. Gomes, J. Chem. Soc., Dalton Trans., 2002, 1386; 25 G. M. Sheldrick, SHELXL-97, Program for the Refinement
(b) B. E. Ketz, X. G. Ottenwaelder and R. M. Waymouth,
Chem. Commun., 2005, 5693; (c) W. J. Marshall and
of Crystal Structures, Universität Göttingen, Germany,
1997.
12030 | Dalton Trans., 2013, 42, 12020–12030
This journal is © The Royal Society of Chemistry 2013