Neutral and cationic chiral NCN pincer nickel(II) complexes with 1,3-bis(2′-imidazolinyl)benzenes: Synthesis and characterization

Article abstract of DOI:10.1039/c1dt10329f

Chiral 1,3-bis(2′-imidazolinyl)benzenes 1a-e easily undergo direct nickelation at the C2 position of the central benzene ring via the C-H bond activation in the reaction with anhydrous NiCl₂ giving neutral NCN pincer nickel(ii) complexes 2a-e in 40-87% yields. Treatment of the nickel pincers 2a or 2c with AgBF₄ in CH₃CN-CH₂Cl ₂ afforded the cationic nickel pincers 3a or 3c in good yields. All the complexes were characterized by elemental analysis, ¹H, ¹³C NMR, and IR spectra. Molecular structures of the neutral complexes 2a, 2b and 2c as well as the cationic complex 3c have been determined by X-ray single-crystal diffraction. The cationic nickel pincers 3 are found to be effective catalysts for the Michael addition of ethyl 2-cyanopropionate to methyl vinyl ketone in the presence of i-Pr₂NEt base with a catalyst loading of 5 mol% even at -78 °C, producing the adduct in >99% yield after 24 h albeit with no ee. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10329f

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Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
Published in issue 35, 2011 of Dalton Transactions
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Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
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Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
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Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
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Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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PAPER
Neutral and cationic chiral NCN pincer nickel(II) complexes with
1,3-bis(2¢-imidazolinyl)benzenes: synthesis and characterization†
Dan-Dan Shao, Jun-Long Niu, Xin-Qi Hao, Jun-Fang Gong* and Mao-Ping Song*
Received 25th February 2011, Accepted 11th May 2011
DOI: 10.1039/c1dt10329f
Chiral 1,3-bis(2¢-imidazolinyl)benzenes 1a–e easily undergo direct nickelation at the C2 position of the
central benzene ring via the C–H bond activation in the reaction with anhydrous NiCl2 giving neutral
NCN pincer nickel(II) complexes 2a–e in 40–87% yields. Treatment of the nickel pincers 2a or 2c with
AgBF4 in CH3CN–CH2Cl2 afforded the cationic nickel pincers 3a or 3c in good yields. All the
complexes were characterized by elemental analysis, 1H, 13C NMR, and IR spectra. Molecular
structures of the neutral complexes 2a, 2b and 2c as well as the cationic complex 3c have been
determined by X-ray single-crystal diffraction. The cationic nickel pincers 3 are found to be effective
catalysts for the Michael addition of ethyl 2-cyanopropionate to methyl vinyl ketone in the presence of
i-Pr2NEt base with a catalyst loading of 5 mol% even at -78 ◦C, producing the adduct in >99% yield
after 24 h albeit with no ee.
reactions. For example, the Rh(Phebox) complexes are efficient
catalysts for conjugate reduction of a,b-unsaturated esters with
hydrosilanes (up to 98% ee),4a,c and for asymmetric reductive aldol
reaction of acrylates and aldehydes with hydrosilanes (up to 96% ee
for anti-products).4b,c The Pt(Phebox)s have been used in alkylation
of aldimines (up to 82% ee)6a and Ru(Phebox)s in hydrogenation
(up to 98% ee) and transfer hydrogenation of ketones (up to 97%
ee).7a,b On the other hand, replacing an oxazoline oxygen atom by
a NR group gives an imidazoline, which is a structural analogue
of oxazoline and has demonstrated unique advantages of further
tuning electronic and conformational properties of the ligand by
the proper choice of the group on the additional nitrogen atom.
This N-substitution can also serve as a linker for attaching the
ligand to a solid support. Moreover, similar to the oxazolines, the
chirality in the imidazolines can be easily introduced and tuned
through using different chiral aminoalcohols.10 Consequently, we
have studied the synthesis and characterization of a series of Pt(II)
and Pd(II) pincer complexes with anionic tridentate 1,3-bis(2¢-
imidazolinyl)phenyl (abbreviated as Phebim, Chart 2) ligands.11
In the preparation of these complexes, it was found that the
Phebim-H ligands showed different reactivities from Phebox-H
Introduction
Aryl-based pincer metal complexes with anionic terdentate ligands
have been widely applied in organic synthesis, organometallic
catalysis, materials science and the related areas since the pioneer-
ing work of Shaw,1 van Koten and Noltes2 on so-called PCP and
NCN type complexes in the 1970s. Among them, the complexes
with anionic tridentate 1,3-bis(2¢-oxazolinyl)phenyl (abbreviated
as Phebox, Chart 1) ligands have emerged as one versatile chiral
pincer complex family due to the ample opportunities to introduce
chirality in the oxazoline rings by using readily available, opti-
cally active aminoalcohols.3 Numerous metal-Phebox complexes
including Rh,4 Ir,4c Pd,5 Pt,6 Ru,7 Ni8 and even Fe9 have been
reported and some show high performance in asymmetric catalytic
Chart
1 Phebox-H, Phebox and the corresponding metal-Phebox
complexes.
Department of Chemistry, Henan Key Laboratory of Chemical Biology
and Organic Chemistry, Zhengzhou University, Zhengzhou, 450052, China.
E-mail: gongjf@zzu.edu.cn, mpsong@zzu.edu.cn
† Electronic supplementary information (ESI) available. CCDC reference
numbers 810478, 810120, 809675 and 811901. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1dt10329f
Chart 2 Phebim-H, Phebim and the corresponding Pt- and Pd-Phebim
complexes.
9012 | Dalton Trans., 2011, 40, 9012–9019
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The Royal Society of Chemistry 2011
©

ligands towards direct C–H activation of the ligand’s central
benzene ring in the reaction with Pd(II) or Pt(II) precursors,
where the activation of the Phebim-H ligands was more facile.
For instance, the Pd(Phebim)s could be prepared by this aryl
C–H activation method (mostly in 28–54% yields),11b while such
process did not seem to be applicable to the Phebox-H ligands5,8c
and a Pd(Phebox) complex was isolated in only 3% yield by this
method.6c Additionally, the yields of Pt(Phebim)s were obviously
higher (51–84%)11 than those of the related Pt(Phebox)s (9–
49%)6c,d when this direct metalation methodology was used. In the
meanwhile, we noticed that up to now the reported Ni(Phebox)
complexes could only be prepared by oxidative addition reac-
tion of 2-halo-1,3-bis(oxazolinyl)benzenes with Ni(COD)2 or by
lithiation–transmetalation also starting from the appropriate 2-
halo derivatives.8 From a synthetic point of view, the direct metal-
induced Caryl–H bond activation should be the most simple and
convenient method for the construction of aryl-based pincer metal
complexes since it is unnecessary to prepare appropriate 1,2,3-
trisubstituted benzenes (in the oxidative addition and transmetala-
tion methods) and the synthetic procedures are thus simplified. On
the basis of the above findings and also considering that examples
on the chiral pincer nickel(II) complexes are rather limited,12 we
have investigated the synthesis and preliminary application of
pincer Ni(Phebim) complexes. Herein, we report the convenient
preparation of neutral Ni(Phebim) complexes 2a–e by direct C–H
activation of the related Phebim-H ligands 1a–e and their cationic
pincers 3a and 3c as well as the catalytic activity of the cationic
complexes 3 in Michael addition.
Scheme 1 Synthesis of the neutral and cationic chiral pincer Ni(Phebim)
complexes.
yields, respectively (Scheme 1). It should be noted that the result
was in contrast to that of the neutral Pt(Phebim)Cl complexes,
from which pure samples of the cationic Pt complexes could not
be isolated by the reaction with commonly used Ag(I) salts such
as AgBF4 or AgOTf or AgSbF6 under various conditions.11b
All of the neutral complexes 2a–e are air- and moisture-stable
both in the solid state and in solution. While it seems that the
cationic complexes 3, particularly 3a have a tendency to partially
exchange their CH3CN ligand for H2O existing in trace amounts
in the wet air or the undried solvent including deuterium solvent
and solvent used during work-up. The exchange was somewhat
1
confirmed by the H NMR spectra of complex 3a. It was found
that sometimes the signal at d 2.57 ppm corresponding to the
CH3CN protons became very small and a new broad singlet at
d 3.59 ppm possibly due to the metal-coordinated water protons
was observed. For this reason, satisfactory elemental analysis data
could not be obtained for complex 3a. In addition, the possibility
of generating water-coordinated cationic species from 3 was briefly
investigated. Thus, the cationic complex 3c was treated with more
than quantitative water in CH2Cl2/H2O (10 : 1, v/v) at room
temperature for 0.5 h. After evaporation of solvent under reduced
pressure, the 1H and 13C NMR spectra of the obtained solids were
determined. Although the signal of coordinated water protons
could not be identified undoubtedly from the 1H NMR spectrum,
it could be clearly seen that the signal at d 2.56 ppm (1H NMR) and
d 2.9 ppm (13C NMR) corresponding to the protons and carbon
atom of the CH3- group, respectively in coordinated CH3CN for
complex 3c disappeared almost completely. The related NMR
spectra are collected in the ESI.† All the other complexes including
the cationic 3c were well characterized by elemental analysis,
1H NMR, 13C NMR, and IR spectra. The formation of the
pincer Ni(II) complexes 2a–e was confirmed by the disappearance
of the signal at d 7.50–8.09 ppm corresponding to the central
Results and discusssion
Synthesis and spectroscopic characterization of pincer Ni(Phebim)
complexes
The needed 1,3-bis(2¢-imidazolinyl)benzene namely Phebim-H
ligands 1 can be readily prepared from isophthalyl chloride,
chiral amino alcohol and amine in two steps using the published
procedure.11 Three known Phebim-H ligands 1a–c and two new
ones 1d–e were thus synthesized by treatment of isophthalyl
chloride with L-valinol, L-phenylalaninol or L-phenylglycinol,
respectively, to give the corresponding bis(amido alcohol)s, which
reacted with excess thionyl chloride, followed by the addition of p-
toluidine or cyclohexylamine and basic workup with 10% NaOH
solution. The direct nickelation of the Phebim-H ligands 1 for
preparation of the NCN pincer nickel(II) complexes via the C–H
activation was then attempted. Accordingly, 1 reacted with easily
available, cheap NiCl2 in dry toluene at reflux for 40–48 h. We
were delighted to find that the new chiral Ni(Phebim)Cl pincers
2a–e could be isolated in 40–87% yields after chromatography on
silica gel (Scheme 1). It should be mentioned that NiCl2 is an
often-used metallating reagent in the preparation of achiral PCP
pincer Ni(II) complexes via the C–H activation.1,13 Focusing our
initial attention on the Lewis acid-catalyzed reactions,11b we wish
to replace the chloride ligand in 2 by an exchangeable neutral
ligand, and generate the corresponding cationic pincer metal
complexes. Thus, treatment of 2a or 2c with 2.0 eq. of AgBF4
in CH3CN/CH2Cl2 (v/v 1 : 1) at room temperature for 12 h easily
removed the chloride and resulted in the formation of the cationic
[Ni(Phebim)·NCCH3][BF4] complexes 3a or 3c in 45% and 88%
1
aryl proton located ortho to both imidazoline rings in the H
NMR spectra. Similar to the M(Phebim)Cl (M = Pd and Pt)
complexes, the signals of central aryl protons for the Ni(Phebim)Cl
complexes were strongly shifted upfield relative to those in the
corresponding free ligands 1a–e, and signals of the N-aryl protons
as well as those of the Cy-proton directly attached to imidazoline-
N shifted downfield due to the N–M coordinations and C–M
bond formation in the complexes. The protons of the imidazoline
ring in 2a–e appeared in three different positions, of which the
multiplet or doublet of doublets at d 4.00–5.14 ppm was for the
CH proton and the signals for the CH2 protons were observed
at d 3.56–4.42 and 3.45–3.92 ppm, respectively as one apparent
triplet and one doublet of doublets in a 2 : 2 ratio. Among them,
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Dalton Trans., 2011, 40, 9012–9019 | 9013
©

◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for the neutral complexes 2a, 2b·CH2Cl2·0.5H2O, 2c and the cationic complex 3c
2a
2b·CH2Cl2·0.5H2O
2c
3c
Ni(1)–C(1)
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–Cl(1)/Ni–N(5)
C(1)–Ni(1)–N(1)
C(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
C(1)–Ni(1)–Cl(1)/C(1)–Ni(1)–N(5)
N(1)–Ni(1)–Cl(1)/N(1)–Ni(1)–N(5)
N(3)–Ni(1)–Cl(1)/N(3)–Ni(1)–N(5)
1.846(4)
1.912(3)
1.894(3)
2.2408(14)
81.19(19)
81.56(17)
162.67(17)
177.91(13)
100.39(14)
96.82(12)
1.848(4)
1.878(4)
1.891(4)
2.2462(12)
81.55(19)
81.93(18)
163.48(17)
177.76(19)
96.98(13)
99.51(12)
1.836(4)
1.914(3)
1.920(3)
2.2249(11)
81.65(14)
81.10(14)
162.75(12)
179.45(12)
97.83(10)
99.42(10)
1.831(6)
1.902(5)
1.894(4)
1.904(6)
81.8(2)
81.7(2)
163.3(2)
176.2(3)
96.4(2)
100.2(2)
the doublet of doublets at d 3.45–3.92 ppm corresponding to one
of the CH2 protons (trans to R1 substituent) were also significantly
shifted downfield compared to the parent ligands. Comparison of
1
the H NMR spectra of the cationic [Ni(Phebim)·NCCH3][BF4]
complexes 3a and 3c with those of their corresponding Phebim-
H ligands revealed the similar phenomena. Some evidence for
the coordination of CH3CN to the Ni center in 3a and 3c were
obtained from NMR spectra in which the singlet arising from
the CH3CN protons was observed at d 2.55 and 2.56 ppm,
respectively, together with the resonance due to the carbon atom
of CH3 appearing at d 2.9 ppm in the 13C NMR spectra. Also,
the downfield shift of the CH3CN protons in the complexes
relative to free CH3CN (2.10 ppm) can be seen. It was thought
that relative Lewis acidity of pincer complexes could be indirectly
measured by the downfield shift that occurred to methyl protons
of the CH3CN compared to free CH3CN.8c Therefore, the bigger
downfield shift of the CH3 protons (0.45 and 0.46, respectively) in
[Ni(Phebim)·NCCH3][BF4] complexes 3a and 3c may mean that
they should be more acidic than the related [Ni(Phebox)][ClO4]
complexes (values of downfield shift: 0.25–0.31).8c
Fig. 2 Molecular structure of complex 2b·CH2Cl2·0.5H2O. Hydrogen
atoms and noncoordinated molecules (CH2Cl2 and H2O) are omitted for
clarity.
Molecular structures of 2a–c and 3c
The molecular structures of the neutral Ni(Phebim)Cl complexes
2a–c and the cationic one 3c were determined by X-ray single
crystal analysis. The molecules are shown in Fig. 1–4, respectively.
Selected bond lengths and angles are collected in Table 1.
Fig. 3 Molecular structure of complex 2c. Hydrogen atoms are omitted
for clarity.
The structural features of 2a–c and 3c are directly related to
the Pt(Phebim) and Pd(Phebim) complexes.11 Phebim ligand in
each complex is coordinated to the Ni(II) center in a tridentate
manner, and the formed two five-membered-ring metallacycles
as well as two imidazoline rings are approximately coplanar
with the central aryl ring. The metal center adopts a typical
distorted square-planar geometry with bond angles of N–Ni–N
being around 163◦ and C–Ni–Cl(C–Ni–N) being around 178◦.
All of the bond lengths and angles around the Ni(II) center in
complexes 2a–c are similar, which are also comparable to those
in the related Ni(Phebox) complexes.8a–c A comparison of 2c and
Fig. 1 Molecular structure of complex 2a. Hydrogen atoms are omitted
for clarity.
9014 | Dalton Trans., 2011, 40, 9012–9019
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©

Table 2 Asymmetric Michael addition of ethyl 2-cyanopropionate to
methyl vinyl ketone catalyzed by the cationic Ni(Phebim) complexes 3a
Entry
Cat.
Temp. (◦C)
Solvent
Time (h)
Yield (%)b
1
2
3a
3a
3a
3c
3c
rt
rt
-30
rt
THF
Toluene
THF
THF
CH2Cl2
5
5
24
5
24
92
> 99
81
89
> 99
3
4c
5
-78
a Reactions were performed with 5 mol% of the cationic pincer Ni(II)
complex, 0.2 mmol of 5, 0.3 mmol of 6, and 0.02 mmol of i-Pr2NEt in 2
mL solvent. b Isolated yield. c 2 mol% of 3c.
Fig. 4 Molecular structure of the cationic complex 3c. Hydrogen atoms
and the anion are omitted for clarity.
inherent electronegativity of Ni(II) vs. Pd(II).8c In fact, catalysis
of the Michael addition of ethyl 2-cyanopropionate to methyl
vinyl ketone in THF at room temperature with 5 mol% of an
achiral cationic [Ni(Phebox)·NCCH3][BF4] complex, in which the
nickel center seemed to be electron deficient enough to work as an
efficient Lewis acid from the cyclic voltammetry study, resulted in
the formation of racemic product in 99% yield after 4.5 h.8d As
a preliminary investigation, the activity and the stereocontrolling
potential of the cationic Ni(Phebim) complexes 3a and 3c in the
Michael addition were evaluated, and the addition of ethyl 2-
cyanopropionate (5) to methyl vinyl ketone (6) was chosen as
the model reaction. As shown in Table 2, the results indicated
that the cationic nickel pincers 3 gave highly effective but not
stereoselective catalysts for the Michael addition, and in all cases
the racemic product 4 was obtained. In the presence of 5 mol% of
3a, the reaction proceeded in THF at room temperature to afford
product 4 in a 92% isolated yield after 5 h (entry 1), and the yield
increased to > 99% yield by employing toluene as solvent (entry
2). When the reaction temperature was lowered to -30 ◦C, the yield
dropped to 81% after 24 h, partially due to the poor solubility of
3a in THF at lower temperature (entry 3). The cationic complex 3c
was also found to be active for this reaction with a catalyst loading
of 2 mol%, affording the Michael adduct 4 in a 89% yield in THF
at room temperature after 5 h (entry 4). Furthermore, the Michael
addition smoothly proceeded with 5 mol% of 3c in CH2Cl2 at
temperatures as low as -78 ◦C, affording the product in a nearly
quantitative yield (> 99% yield, entry 5). As a control reaction, the
Michael reaction was also conducted in THF or toluene at room
temperature for 5 h in the absence of the cationic pincer Ni(II)
complex. It was found that only trace amount of the expected
adduct was detected by GC analysis.
its Pt(Phebim)Cl complex reveals that the metal–tridentate ligand
bond lengths in the two complexes follow the expected pattern of
Ni < Pt. For example, the Ni complex 2c has a Ni–C bond length
˚
˚
of 1.836(4) A, Ni–N bond lengths of 1.914(3) [1.920(3)] A, and a
˚
Ni–Cl bond length of 2.2249(11) A. The corresponding values in
the Pt(Phebim)Cl complex are 1.907(10), 2.031(8) [2.039(9)], and
11b
˚
2.454(2) A, respectively. The two phenyl groups of the Phebim
ligand in 2c reside either side of the metallacyclic plane and are thus
trans orientated due to the free rotation of carbon-carbon single
bond. A striking structural difference in Phebim ligand between
3c and 2c is the arrangement of the two phenyl groups, which are
mutually cis in 3c. The metal–tridentate ligand bond lengths in
the cationic complex 3c are slightly shorter than that seen in the
corresponding neutral complex 2c, which can be explained by the
reduced trans influence of the NCCH3 ligand.
Catalytic studies
Chiral NCN pincer metal complexes have been applied as Lewis
acid catalysts in the asymmetric Michael addition of activated
nitriles to Michael acceptors, which has received increasing
attention since the products bear a quaternary carbon center with
various functionalities.14 For example, the in situ generated cationic
chiral bis-aldimine NCN pincer Pd(II) and Pt(II) complexes only
gave racemic products in the Michael reaction between methyl
vinyl ketones and methyl 2-cyanopropionate.15 While the cationic
Pd(Phebox) complexes showed catalytic activity for the reaction
of 2-cyanocarboxylates and various Michael acceptors to form the
adducts with up to 34% ee.5d Remarkably, the reaction could be
efficiently catalyzed by the chiral pyrroloimidazolone-based NCN
pincer Pd(II) complexes with high levels of enantioselectivities (up
to 83% ee).16 Additionally, the addition of 2-cyanocarboxylates
to acrolein proceeded in the presence of the in situ generated
Rh(III)(Phebox)-(SnMe3)Cl complex under mild and neutral con-
ditions to afford high yields of the products with up to 86%
ee.17 It was reported that there was a correlation between Lewis
acidity and reactivity of the cationic pincer complexes in the
Michael reaction,6c and the palladium pincer complexes showed
less Lewis acidity than the Ni(II) complexes due to the greater
Conclusions
In summary, reaction of chiral 1,3-bis(2¢-imidazolinyl)benzenes
with anhydrous NiCl2 provided direct access to the neutral NCN
pincer nickel(II) complexes via C–H activation. These complexes
could be readily transformed into analytically pure cationic
complexes containing CH3CN in the exchangeable coordination
site. The identity of neutral and cationic complexes was confirmed
by X-ray crystal structure analysis. The cationic complexes proved
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9012–9019 | 9015
©

to be effective Lewis acid catalysts for the Michael addition of ethyl
2-cyanopropionate to methyl vinyl ketone even at temperature as
low as -78 C. Although they show the lack of stereocontrol in
the model reaction, their facile synthesis offers the benefits of
simple procedure and low cost. Additionally, it is reasonable to
assume that a combination of isophthalyl chloride or substituted
isophthalyl chloride, chiral aminoalcohols and amines provides a
high diversification of chiral NCN pincer Ni(Phebim) complexes
and some assistance in extending the scope of pincer-Ni(II)
catalyzed reactions.
1H, central Ar(5)), 6.97 (d, J = 8.4 Hz, 4H, NAr), 6.69 (d, J = 8.4
Hz, 4H, NAr), 5.30 (dd, J = 8.0, 10.8 Hz, 2H, NCH), 4.47 (dd, J =
9.4, 10.8 Hz, 2H, NCHH), 3.78 (dd, J = 8.0, 9.4 Hz, 2H, NCHH),
2.25 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3): d 162.3, 143.8,
140.3, 133.7, 131.4, 130.4, 129.7, 129.4, 128.7, 127.8, 127.3, 126.8,
123.3, 67.6, 62.1, 20.9. HRMS (m/z, ESI+), found for M + H =
547.2858, C38H35N4 requires 547.2862.
◦
General procedure for the synthesis of Ni(Phebim)Cl complexes
2a–e
To a well-stirred solution of bis(imidazoline)benzene 1 (0.3 mmol)
in anhydrous toluene (30 mL) was added Et3N (65 mL, 0.45 mmol)
and anhydrous NiCl2 (58 mg, 0.45 mmol). The reaction mixture
was then refluxed for 40–48 h. After cooling and concentrating in
vacuo, the residue was purified by silica gel flash chromatography
using ethyl acetate (for 2a–d) or CH2Cl2/MeOH 10 : 1 (for 2e) as
the eluent.
Experimental
General
Reactions involving air-sensitive reagents were performed under a
nitrogen atmosphere. Solvents were dried with standard methods
and freshly distilled prior to use if needed. The 1,3-bis(2¢-
imidazolinyl)benzene ligands 1a–e were prepared according to
the literature method reported by us.11b All other chemicals were
used as purchased. Melting points were measured on an XT4A
melting point apparatus and are uncorrected. Infrared spectra
were obtained with a Bruker VECTOR 22 spectrophotometer
in KBr pellets. NMR spectra were recorded on a Bruker DPX
400 instrument using TMS as an internal standard. HRMS
were determined on a Waters Q-Tof Micro MS/MS System ESI
spectrometer. Elemental analyses were measured on a Thermo
Flash EA 1112 elemental analyzer. Optical rotations were recorded
on a Perkin Elmer 341 polarimeter.
2,6-Bis((S)-4-isopropyl-1-p-tolyl-4,5-dihydro-1H-imidazol-2-yl)
phenylchloronickel(II) (2a). Orange solid in 79% isolated yield.
mp: > 260 ◦C. [a]D20 +289 (c 0.144 in CH2Cl2). IR (KBr): nmax/cm-1
2956, 2868, 1574, 1536, 1512, 1429, 1298, 1158, 1040, 821, 723. 1H
NMR (400 MHz, CDCl3): d 7.20 (d, J = 8.1 Hz, 4H, NAr), 7.09
(d, J = 8.1 Hz, 4H, NAr), 6.51 (t, J = 7.7 Hz, 1H, central Ar (4)),
6.34 (d, J = 7.7 Hz, 2H, central Ar (3,5)), 4.10–4.00 (m, 4H, NCH
and NCHH), 3.79 (dd, J = 2.8, 8.9 Hz, 2H, NCHH), 2.81–2.74
(m, 2H, CH(CH3)2), 2.39 (s, 6H, CH3), 0.88 (d, J = 7.0 Hz, 12H,
CH(CH3)2). 13C NMR (100 MHz, CDCl3): d 177.4, 167.0, 137.2,
137.0, 132.7, 130.0, 125.5, 124.2, 121.6, 64.7, 55.3, 30.3, 21.1, 18.7,
14.3. Anal. calcd for C32H37ClN4Ni: C, 67.22; H, 6.52; N, 9.80.
Found: C, 67.00; H, 6.78; N, 9.51%.
Characterization of new ligands 1d and 1e
1,3-Bis((S)-4-benzyl-1-cyclohexyl-4,5-dihydro-1H-imidazol-2-
yl)benzene (1d). Yellow oil in 45% isolated yield. [a]2D0 -13 (c 0.2 in
CH2Cl2). IR (KBr): nmax/cm-1 3420, 3028, 2930, 2855, 2365, 1707,
2,6-Bis((S)-1-cyclohexyl-4-isopropyl-4,5-dihydro-1H-imidazol-
2-yl)phenylchloronickel(II) (2b). Orange solid in 40% isolated
yield. mp: 257–258 ◦C. [a]D20 +283 (c 0.146 in CH2Cl2). IR (KBr):
1
n
max/cm-1 2925, 2854, 1569, 1532, 1509, 1459, 1379, 1269, 1070,
1569, 1492, 1447, 1400, 1276, 1085, 996, 888, 815, 749, 703. H
1011, 720. 1H NMR (400 MHz, CDCl3): d 7.19 (d, J = 7.7 Hz, 2H,
central Ar (3,5)), 6.97 (t, J = 7.7 Hz, 1H, central Ar (4)), 4.05–4.00
(m, 2H, NCH), 3.88–3.84 (m, 2H, N-Cy), 3.57 (app t, J = 10.5 Hz,
2H, NCHH), 3.45 (dd, J = 4.1, 9.8 Hz, 2H, NCHH), 2.70–2.58
(m, 2H, CH(CH3)2), 1.92–1.78 (m, 10H, Cy), 1.61–1.50 (m, 4H,
Cy), 1.41–1.34 (m, 4H, Cy), 1.19–1.08 (m, 2H, Cy), 0.82 (d, J =
7.0 Hz, 6H, CH3CHCH3), 0.72 (d, J = 6.8 Hz, 6H, CH3CHCH3).
13C NMR (100 MHz, CDCl3): d 178.6, 167.9, 133.6, 122.7, 122.4,
63.3, 54.7, 45.6, 31.9, 30.74, 30.65, 25.6, 25.4, 25.3, 18.6, 14.2.
Anal. calcd for C30H45ClN4Ni·0.2CH2Cl2: C, 63.32; H, 7.99; N,
9.78. Found: C, 63.39; H, 8.40; N, 9.73%.
NMR (400 MHz, CDCl3): d 7.50 (s, 1H, central Ar(2)), 7.43 (d, J =
6.7 Hz, 2H, central Ar(4,6)), 7.36 (t, J = 6.7 Hz, 1H, central Ar(5)),
7.25–7.14 (m, 10H, Ph-H), 4.34–4.27 (m, 2H, NCH), 3.35 (app t, J
= 10.0 Hz, 2H, NCHH), 3.14–3.06 (m, 6H, NCHH, CHHPh and
N-Cy), 2.69 (dd, J = 8.5, 13.6 Hz, 2H, CHHPh), 1.61–1.55 (m,
6H, Cy), 1.46–1.43 (m, 2H, Cy), 1.29–1.13 (m, 6H, Cy), 1.06–0.83
(m, 6H, Cy). 13C NMR (100 MHz, CDCl3): d 165.0, 138.5, 132.2,
129.4, 129.0, 128.1, 128.0, 127.6, 125.9, 64.8, 54.5, 48.3, 42.1, 31.0,
30.0, 25.3, 25.2, 25.1. MS (m/z, ESI+): 559.5 (M+H), HRMS (m/z,
ESI+), found for M + H = 559.3801, C38H47N4 requires 559.3801.
1,3-Bis((S)-4-phenyl-1-p-tolyl-4,5-dihydro-1H-imidazol-2-yl)
◦
benzene (1e). Yellow solid in 76% isolated yield. mp: 81–83 C.
2,6-Bis((S)-4-benzyl-1-p-tolyl-4,5-dihydro-1H-imidazol-2-yl)-
phenylchloronickel(II) (2c). Orange solid in 87% isolated yield.
[a]D20 -10 (c 0.12 in CH2Cl2). IR (KBr): nmax/cm-1 3027, 2922, 2864,
1608, 1568, 1512, 1380, 1300, 815, 759, 701. 1H NMR (400 MHz,
CDCl3): d 8.09 (s, 1H, central Ar(2)), 7.43 (dd, J = 1.6, 7.6 Hz,
2H, central Ar(4,6)), 7.34–7.23 (m, 10H, Ph), 7.14 (t, J = 7.8 Hz,
◦
mp: 239–241 C. [a]2D0 +297 (c 0.181 in CH2Cl2). IR (KBr):
n
max/cm-1 3025, 2918, 1573, 1533, 1511, 1430, 1302, 1262, 1158,
1
1090, 1018, 817, 724, 699, H NMR (400 MHz, CDCl3): d 7.48
9016 | Dalton Trans., 2011, 40, 9012–9019
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The Royal Society of Chemistry 2011
©

(d, J = 7.0 Hz, 4H, Ph), 7.26–7.17 (m, 6H, Ph), 7.13 (d, J = 8.1
Hz, 4H, NAr), 6.81 (d, J = 7.8 Hz, 4H, NAr), 6.46 (t, J = 7.7
Hz, 1H, central Ar (4)), 6.24 (d, J = 7.7 Hz, 2H, central Ar (3,5)),
4.44–4.39 (m, 2H, NCH), 4.10 (app t, J = 10.0 Hz, 2H, NCHH),
3.78 (dd, J = 3.1, 9.9 Hz, 2H, NCHH), 3.40 (dd, J = 3.0, 13.3 Hz,
2H, CHHPh), 3.07 (dd, J = 7.9, 13.3 Hz, 2H, CHHPh), 2.36 (s,
6H, CH3). 13C NMR (100 MHz, CDCl3): d 177.9, 167.5, 137.9,
137.3, 136.5, 132.8, 130.1, 129.9, 128.2, 126.2, 125.5, 124.3, 121.7,
60.9, 58.9, 41.1, 21.1. Anal. calcd for C40H37ClN4Ni: C, 71.93; H,
5.58; N, 8.39. Found: C, 71.79; H, 5.80; N, 8.27%.
7.09 (d, J = 8.1 Hz, 4H, NAr), 6.52 (t, J = 7.7 Hz, 1H, central Ar
(4)), 6.25 (d, J = 7.7 Hz, 2H, central Ar (3,5)), 4.10 (app t, J = 10.2
Hz, 2H, NCHH), 4.03–4.00 (m, 2H, NCH), 3.81 (dd, J = 3.4,
9.7 Hz, 2H, NCHH), 2.55 (br s, 3H, CH3CN), 2.40 (s, 6H, CH3),
2.10 (br s, 2H, CH(CH3)2), 0.95 (d, J = 5.2 Hz, 12H, CH(CH3)2).
13C NMR (100 MHz, CDCl3): d 173.7, 167.1, 137.9, 136.3,
132.8, 130.2, 125.7, 124.8, 122.6, 63.6, 55.4, 31.1, 21.2, 18.2, 14.6,
2.9.
Cationic Ni(Phebim) complex 3c
88% isolated yield. mp: 191–193 ◦C. [a]D20 +234 (c 0.09 in CH2Cl2).
IR(KBr): nmax/cm-1 3026, 2938, 2679, 2490, 1577, 1539, 1511,
1430, 1302, 1158, 1088, 1055, 823, 706, 517. 1H NMR (400 MHz,
CDCl3): d 7.47 (d, J = 6.9 Hz, 4H, Ph), 7.29–7.24 (m, 6H, Ph),
7.10 (d, J = 8.2 Hz, 4H, NAr), 6.66 (br s, 4H, NAr), 6.47 (t, J = 7.6
Hz, 1H, central Ar (4)), 6.11 (d, J = 8.0 Hz, 2H, central Ar (3,5)),
4.35–4.33 (m, 2H, NCH), 4.27 (app t, 2H, J = 10.0 Hz, NCHH),
3.79 (dd, J = 2.6, 9.7 Hz, 2H, NCHH), 2.95 (d, 4H, J = 4.6 Hz,
CH2Ph), 2.56 (s, 3H, CH3CN), 2.35 (s, 6H, CH3). 13C NMR (100
MHz, CDCl3): d 175.2, 168.5, 138.2, 136.2, 135.2, 132.8, 130.1,
130.0, 128.4, 126.8, 125.6, 125.1, 123.3, 60.0, 58.9, 41.0, 21.1, 2.9.
Anal. calcd for C42H40BF4N5Ni: C, 66.35; H, 5.30; N, 9.21. Found:
C, 66.53; H, 5.51; N, 9.21%.
2,6-Bis((S)-4-benzyl-1-cyclohexyl-4,5-dihydro-1H-imidazol-2-
yl)phenylchloronickel(II) (2d). Orange solid in 48% isolated yield.
◦
mp: 210–211 C. [a]2D0 +191 (c 0.112 in CH2Cl2). IR (KBr):
n
max/cm-1 3057, 3023, 2930, 2853, 1600, 1568, 1527, 1434, 1326,
1
1259, 1188, 1165, 1088, 1054, 996, 891, 789, 742, 720, 700. H
NMR (400 MHz, CDCl3): d 7.38 (d, J = 7.2 Hz, 4H, Ph), 7.27–
7.16 (m, 8H, Ph and central Ar (3,5)), 6.99 (t, J = 7.7 Hz, 1H,
central Ar (4)), 4.21–4.15 (m, 2H, NCH), 3.98–3.91 (m, 2H, N-
Cy), 3.56 (app t, J = 10.0 Hz, 2H, NCHH), 3.50 (dd, J = 3.2, 13.1
Hz, 2H, CHHPh), 3.45 (dd, J = 3.2, 9.9 Hz, 2H, NCHH), 2.65
(dd, J = 9.4, 13.1 Hz, 2H, CHHPh), 1.85–1.67 (m, 10H, Cy), 1.44–
1.25 (m, 8H, Cy), 1.13–1.08 (m, 2H, Cy). 13C NMR (100 MHz,
CDCl3): d 179.1, 168.4, 138.5, 133.8, 129.9, 128.2, 126.1, 123.0,
122.6, 60.2, 54.6, 49.6, 41.0, 31.8, 30.6, 25.6, 25.4, 25.2. Anal. calcd
for C38H45ClN4Ni·2.5CH2Cl2: C, 56.28; H, 5.83; N, 6.48. Found:
C, 56.03; H, 5.75; N, 6.46%.
General procedure for the asymmetric Michael addition of ethyl
2-cyanopropionate to methyl vinyl ketone
2,6-Bis((S)-4-phenyl-1-p-tolyl-4,5-dihydro-1H-imidazol-2-yl)-
phenylchloronickel(II) (2e). Orange solid in 76% isolated yield.
Under a nitrogen atmosphere, the catalyst (0.01 mmol, 5.0 mol%)
was dissolved in 2 mL of solvent. N-ethyldiisopropylamine
(0.02 mmol, 3.5 mL) was added, followed by methyl vinyl ketone
(0.3 mmol, 24 mL), and finally ethyl 2-cyanopropionate (0.2 mmol,
25.5 mL). The resulting solution mixture was stirred for the allotted
times and temperatures (Table 2). The solvent was removed, and
the residue was purified by flash column chromatography to give
the product 4 (ethyl ether/petroleum ether, 1 : 1). The enantiomeric
excess was determined by chiral-phase HPLC (Daicel Chiralpak
AD-H column).
◦
mp: 190–191 C. [a]2D0 +200 (c 0.096 in CH2Cl2). IR (KBr):
n
max/cm-1 3028, 2921, 1570, 1511, 1426, 1299, 1157, 821, 761,
697. 1H NMR (400 MHz, CDCl3): d 7.42 (d, J = 7.4 Hz, 4H, Ph),
7.33 (t, J = 7.6 Hz, 4H, Ph), 7.22 (t, J = 8.0 Hz, 2H, Ph, partially
overlapped with NAr), 7.20 (d, J = 8.2 Hz, 4H, NAr, partially
overlapped with Ph), 7.14 (d, J = 8.2 Hz, 4H, NAr), 6.60 (t, J =
7.6 Hz, 1H, central Ar (4)), 6.46 (d, J = 7.6 Hz, 2H, central Ar
(3,5)), 5.14 (dd, J = 3.5, 10.6 Hz, 2H, NCH), 4.42 (app t, J = 10.3
Hz, 2H, NCHH), 3.92 (dd, J = 3.5, 9.7 Hz, 2H, NCHH), 2.39 (s,
6H, CH3). 13C NMR (100 MHz, CDCl3): d 179.3, 168.1, 143.8,
137.6, 136.8, 132.7, 130.1, 128.5, 127.2, 126.7, 125.7, 124.7, 121.8,
64.2, 62.8, 21.1. Anal. calcd for C38H33ClN4Ni: C, 71.33; H, 5.20;
N, 8.76. Found: C, 70.99; H, 5.51; N, 8.45%.
Ethyl 2-cyano-2-methyl-5-oxohexanoate 45d
1
Colorless oil. H NMR (400 MHz, CDCl3): d 4.27 (q, J = 7.1
Hz, 2H, OCH2), 2.75–2.61 (m, 2H, CH2), 2.19 (s, 3H, COCH3),
2.02–2.24 (m, 2H, CH2), 1.62 (s, 3H, C(CN)CH3), 1.29 (t, J = 7.1
Hz, 3H, CH3).
General procedure for the synthesis of the cationic Ni(Phebim)
complexes 3a and 3c
Crystal structure determination and data collection
In a flask protected from light, Ni(Phebim)Cl (0.35 mmol) and
AgBF4 (136 mg, 0.7 mmol) were stirred in CH3CN/CH2Cl2 (20
mL, v/v 1 : 1) at room temperature for 12 h. After the solvent was
removed under reduced pressure, the residue redissolved in CH2Cl2
and filtered through Celite. The resulting solution was evaporated
to dryness and the residue washed with Et2O, yielding pure 3 as a
yellow solid.
Crystals of 2a and 2c were obtained by recrystallization from ethyl
acetate and those of 2b and 3c from CH2Cl2/n-hexane at ambient
temperature. The data of 2a–c were collected with Rigaku-
IV imaging plate area detector with graphite-monochromated
˚
Mo-Ka radiation (l = 0.71073 A) and those of 3c were col-
lected on a Oxford diffraction Gemini E diffractometer with
˚
graphite-monochromated Cu-Ka radiation (l = 1.54184 A) at
Cationic Ni(Phebim) complex 3a
ambient temperature. The diffraction data were corrected for
Lorentz and polarization factors. The structures were solved
by direct methods and expanded using Fourier techniques and
refined by full-matrix least-squares methods. The non-hydrogen
atoms were refined anisotropically, and the hydrogen atoms were
◦
45% isolated yield. mp: 188 C (dec.). [a]2D0 +306 (c 0.066 in
CH2Cl2). IR(KBr): nmax/cm-1 3031, 2925, 2866, 1575, 1537, 1512,
1430, 1297, 1158, 1107, 1039, 820, 764, 722, 670, 647, 580, 519.
1H NMR (400 MHz, CDCl3): d 7.22 (d, J = 8.1 Hz, 4H, NAr),
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9012–9019 | 9017
©

Table 3 Summary of crystal structure determination for complexes 2a, 2b·CH2Cl2·0.5H2O, 2c and 3c
2a
2b·CH2Cl2·0.5H2O
2c
3c
Empirical formula
C32H37ClN4Ni
571.82
291(2)
0.71073
Monoclinic
0.20 ¥ 0.18 ¥ 0.16
28.671(6)
9.971(2)
10.646(2)
90
100.00(3)
90
2997.3(10)
4
C2
1.267
C31H48Cl3N4NiO0.5
649.79
291(2)
C40H37ClN4Ni
667.90
291(2)
0.71073
Monoclinic
0.20 ¥ 0.18 ¥ 0.17
6.5204(13)
22.603(5)
11.813(2)
90
95.40(3)
90
1733.3(6)
2
P21
1.280
C42H40BF4N5Ni
760.31
293(2)
Mr
T/K
˚
l/A
0.71073
1.54184
Crystal system
Orthorhombic
0.20 ¥ 0.20 ¥ 0.18
13.976(3)
22.955(5)
10.551(2)
90
Monoclinic
0.20 ¥ 0.13 ¥ 0.10
11.0806(2)
12.41579(19)
13.7889(3)
90
101.4050(18)
90
1859.54(6)
2
P21
1.358
1.233
3.27–61.08
792
5855
4030
0.0264
R1 = 0.0509
wR2 = 0.1355
R1 = 0.0584
wR2 = 0.1479
0.335 and -0.436
0.02(5)
Crystal size/mm
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
3
˚
V/A
3384.7(12)
4
P21212
1.275
0.837
1.71–25.00
1380
10501
5709
Z
Space group
Dc/g cm-3
m/mm-1
0.763
1.44–27.52
1208
6073
6044
0.670
1.73–27.53
700
7082
6777
q range (◦)
F(000)
No. of data collected
No. of unique data
Rint
0.0279
0.0533
0.0185
Final R indices (I > 2s(I))
R1 = 0.0486
wR2 = 0.1143
R1 = 0.0632
wR2 = 0.1271
0.473 and -0.253
0.508(19)
R1 = 0.0614
wR2 = 0.1476
R1 = 0.0668
wR2 = 0.1511
0.446 and -0.512
0.0(6)
R1 = 0.0426
wR2 = 0.1066
R1 = 0.0540
wR2 = 0.1183
0.279 and -0.478
0.481(16)
R indices (all data)
-3
˚
Largest diff. peak and hole/e A
Flack parameter
included but not refined. Their raw data were corrected and the
structures were solved using the SHELXS-97 program.18 Details
of crystal structure determination are summarized in Table 3.
CCDCs 810478, 810120, 809675 and 811901 contain the crys-
tallographic data for complexes 2a, 2b·CH2Cl2·0.5H2O, 2c and 3c,
respectively.†
Motoyama and H. Nishiyama, Tetrahedron Lett., 2007, 48, 3397; (f) T.
Kimura and Y. Uozumi, Organometallics, 2008, 27, 5159.
6 (a) Y. Motoyama, Y. Mikami, H. Kawakami, K. Aoki and H.
Nishiyama, Organometallics, 1999, 18, 3584; (b) Y. Motoyama,
H. Kawakami, K. Shimozono, K. Aoki and H. Nishiyama,
Organometallics, 2002, 21, 3408; (c) J. S. Fossey and C. J. Richards,
Organometallics, 2004, 23, 367; (d) J. S. Fossey, G. Jones, M. Motevalli,
H. V. Nguyen, C. J. Richards, M. A. Stark and H. V. Taylor, Tetrahedron:
Asymmetry, 2004, 15, 2067.
7 (a) J. Ito, S. Ujiie and H. Nishiyama, Chem. Commun., 2008, 1923;
(b) J. Ito, S. Ujiie and H. Nishiyama, Organometallics, 2009, 28,
630; (c) J. Ito, S. Ujiie and H. Nishiyama, Chem.–Eur. J., 2010, 16,
4986; (d) J. Ito, R. Asai and H. Nishiyama, Org. Lett., 2010, 12,
3860.
8 (a) J. S. Fossey and C. J. Richards, J. Organomet. Chem., 2004, 689, 3056;
(b) M. Stol, D. J. M. Snelders, M. D. Godbole, R. W. A. Havenith, D.
Haddleton, G. Clarkson, M. Lutz, A. L. Spek, G. P. M. van Klink
and G. van Koten, Organometallics, 2007, 26, 3985; (c) A. Bugarin
and B. T. Connell, Organometallics, 2008, 27, 4357; (d) K. Mitsudo,
T. Imura, T. Yamaguchi and H. Tanaka, Tetrahedron Lett., 2008, 49,
7287.
Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (No. 20972141) and Key Technologies R & D
Program of Henan Province (102101210200) for financial support
of this work.
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©