A zwitterionic nickel-olefin initiator for the preparation of high molecular weight polyethylene

Article abstract of DOI:10.1016/j.jorganchem.2007.05.016

The reaction of the sodium 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato with Ni(PMe₃)₂(η¹-CH₂C₆H₅)Cl provides 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato(η¹-benzyl)(trimethylphosphine) nickel (1), which was structurally characterized. The addition of 2 equiv. of B(C₆F₅)₃ to 1 results in the formation of 2-tris(pentafluorophenyl)borate-3-(2,6-diisopropylphenylimino)-but-1-ene(η³-benzyl)nickel (2), in which the borane coordinates to the O site of the ligand and forces binding of the olefin unit to the nickel center. The solid-state structure of 2 shows a zwitterionic structure with substantial positive charge at the nickel center. Compound 2 can be used to initiate the homopolymerization of ethylene to yield high molecular weight polyethylene.

Full text of DOI:10.1016/j.jorganchem.2007.05.016

Journal of Organometallic Chemistry 692 (2007) 4745–4749
www.elsevier.com/locate/jorganchem
A zwitterionic nickel–olefin initiator for the preparation of
high molecular weight polyethylene
*
Yaofeng Chen, Brycelyn M. Boardman, Guang Wu, Guillermo C. Bazan
Center for Polymer and Organic Solids, Departments of Chemistry and Materials, University of California, Santa Barbara, CA 93106, USA
Received 11 April 2007; received in revised form 9 May 2007; accepted 10 May 2007
Available online 18 May 2007
Dedicated to Professor Gerhard Erker, for his lasting contributions to Organometallic Chemistry and for his persistent
and significant efforts in promoting the chemical sciences.
Abstract
The reaction of the sodium 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato with Ni(PMe3)2(g1-CH2C6H5)Cl provides 3-(2,6-diiso-
propylphenylimino)-but-1-en-2-olato(g1-benzyl)(trimethylphosphine) nickel (1), which was structurally characterized. The addition of
2 equiv. of B(C6F5)3 to 1 results in the formation of 2-tris(pentafluorophenyl)borate-3-(2,6-diisopropylphenylimino)-but-1-ene(g3-ben-
zyl)nickel (2), in which the borane coordinates to the O site of the ligand and forces binding of the olefin unit to the nickel center.
The solid-state structure of 2 shows a zwitterionic structure with substantial positive charge at the nickel center. Compound 2 can be
used to initiate the homopolymerization of ethylene to yield high molecular weight polyethylene.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Homogeneous catalysis; Polyethylene; Nickel; Olefin complexes
1. Introduction
ination of ligand/reactivity relationships for cationic Ni(dii-
mine) initiators [2,8] led to the design and syn-
thesis of {(H3C)C[@NAr]C[O-B(C6F5)3][@NAr]-j2N,N0}-
Ni(g3-CH2C6H5), with which high molecular weight
polyethylene (PE) can be produced for bulky aromatic
(Ar) substituents [9]. Several other ligand types have been
reported that are amenable to the concept of Lewis acid
attachment on a ligand site to redistribute electron density,
including 3-(1-arylimino-ethyl)-acetylacetonato [10], 2-
diphenylphosphinylbenzamido [11], N-(2-benzoylphenyl)-
benzamido [12], a-iminoenamido [13], 2-(alkyldeneamino)-
benzoato [14] and iminoamido pyridine [15], However, none
of these systems have been able to produce high molecular
weight PE in the absence of bulky Ar substituents.
Herein we show a zwitterionic Ni complex containing
the simple and relatively small 3-(arylimino)-but-1-en-2-
olato ligand, in which the active species is stabilized by a
chelated olefin adduct. These complexes, which have
reduced bulk, possess the ability to produce high molecular
weight PE.
Nickel-based olefin oligomerization and polymerization
initiators are of interest in industrial and academic labora-
tories [1]. Cationic versions are the most commonly
encountered [2]. Neutral species, although less active, are
under investigation because of their higher tolerance
toward functionalities [3]. Zwitterionic counterparts are
the least common and provide an intermediate range of
reactivities [4].
During our efforts at developing tandem catalytic pro-
cesses [5], we discovered that the reactivity of SHOP-type
catalysts such as [(C6H5)2PC6H4C(O)O-j2P,O]Ni(g3-CH2-
CMeCH2) [6] increases upon addition of B(C6F5)3 [7]. Car-
bonyl coordination to the borane gives zwitterionic
[(C6H5)2PC6H4C(O-B(C6F5)3)O-j2P,O]Ni(g3-CH2CMeCH2)
and removes electron density from the nickel center. Exam-
*
Corresponding author.
E-mail address: bazan@chem.ucsb.edu (G.C. Bazan).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.05.016

4746
Y. Chen et al. / Journal of Organometallic Chemistry 692 (2007) 4745–4749
2. Results and discussion
2.1. Synthesis and characterization of a zwitterionic nickel–
olefin initiator
As shown in Scheme 1, synthetic access begins with 3-(2,6-
diisopropylphenylimino)-butan-2-one [16], which is
obtained by condensation of 2,6-diisopropylaniline with
2,3-butanedione. Deprotonation with NaH in THF pro-
vides the sodium salt in 80% yield. Subsequent reaction of
the salt with Ni(PMe3)2(g1-CH2C6H5)Cl [17] provides a
1
new organometallic species, which by H NMR spectros-
copy (C6D6) contains the ligand fragment (d: 4.91 and
4.63ppm, @CH2), an g1-benzyl group (d: 7.56ppm,
CH2C6H5) and PMe3 (d: 0.90ppm). The vicinal protons on
the methylene unit adjacent to oxygen appear as singlets [18].
Single crystals of the product were obtained from a
solution of pentane and the results are shown in Fig. 1.
The product is thus (3-(2,6-diisopropylphenylimino)-but-1-
en-2-olato)(g1-benzyl)-(trimethylphosphine)-nickel (1), as
shown in Scheme 1.
Fig. 1. ORTEP drawing of 1 at the 50% probability level. Hydrogen
atoms are not shown for clarity.
Structural characterization of 1 reveals a distorted
square-planar geometry with a cis relationship between
the benzyl ligand and the imine nitrogen. The 3-(2,6-diiso-
propylphenylimino)-but-1-en-2-olato ligand coordinates to
the nickel via the nitrogen and oxygen atoms with Ni–O
benzyl ligand [15] and that the major isomer contains a
cis relationship between the vinyl group and the benzyl
group, as shown in Scheme 1.
Single crystals of compound 2 formed from a toluene
solution and the structure is shown in Fig. 2. Isomer
2(A) was obtained preferentially. The oxygen binds to
the Lewis acidic boron and by doing so, forces coordina-
tion of the olefin to the nickel center. The distances from
˚
and Ni–N bond lengths of 1.9031(16) and 1.9555(18) A,
˚
respectively. The C1–C2 (1.344(3) A) and C3–N (1.294(3)
˚
A) distances are consistent with double bond character,
˚
˚
whereas the C2–C3 (1.486(3) A), C3–C4 (1.497(3) A) and
˚
˚
˚
C2–O (1.315(3) A) distances reveal single bond character.
Ni to C1 and C2, are 2.109 (6) A and 2.455(6) A, respec-
tively. The rotation of the C2–C3 bond and the C1–C2–
C3–N torsional angle (41.6(8) ꢁ), are orientated to allow
overlap between the p-orbital of the –C@CH2 group and
These data indicate localized bonding in the chelating ring.
The addition of 2 equiv. B(C6F5)3 to 1 in toluene results
in the formation of Me3P-B(C6F5)3, which precipitates out
1
˚
of solution, and a new organometallic product. H NMR
Ni. The C2–O distance (1.319(6) A) is characteristic of a
˚
spectra of the product show the formation of two isomers
in a 9:1 ratio. The upfileld shift of the aromatic signals due
to –CH2C6H5 from 6.96–7.56 to 5.97–6.86ppm indicates
g3-coordination. The vinyl protons are observed as a pair
of doublets (J = 3.5 Hz) at 4.35 and 3.25 ppm. The 11B
NMR spectrum shows a signal at ꢀ2.6 ppm, consistent
single bond, while the C1–C2 (1.371(7) A) distance is more
indicative of a double bond. These observations are con-
sistent with the charge distribution as shown for structures
2(A) and 2(B) in Scheme 1. The C3–N distance (1.300(6)
˚
˚
A) is close to that observed in compound 1 (1.294(3) A).
˚
˚
The N–Ni bond length (1.900(4) A) is 0.055 A shorter
than that of compound 1 (1.9555(18) A), revealing a more
1
˚
with boron–oxygen coordination. H NOE spectroscopy
between the vinyl group and the benzyl group indicates
that the sets of isomers arise from pseudorotamers of the
electron deficient metal center in the zwitterionic
compound.
N
N
i)
CH Ph
2
N
O
+
2 B(C F )
N
O
6
5 3
Ni
Ni
Ni
ii)
- Me3P-B(C6F5)3
CH
CH
2
2
PMe
O
3
O
B(C F )
B(C F )
6 5 3
6
5 3
9 (A) :
1 (B)
2
1
Scheme 1. (i) NaH, (ii) Ni(PMe3)2(g1-CH2C6H5)Cl.

Y. Chen et al. / Journal of Organometallic Chemistry 692 (2007) 4745–4749
4747
3. Conclusion
In summary, we have demonstrated that a zwitterionic
nickel complex supported by the 3-(2,6-diisopropylphe-
nylimino)-but-1-en-2-olato ligand can be used to prepare
high molecular weight PE. The most noteworthy feature
of the active site is that it is supported by an olefinic
fragment, which lacks substantial steric hindrance. Such
a structure departs from previous structure/reactivity
relationships and theoretical predictions for cationic
Ni(diimine) complexes, which require sterically demand-
ing substituents to produce high molecular weight PE.
Bonding from an olefin is considerably different from
that of a nitrogen donor in that p-back bonding is a pos-
sibility. Whether this electronic feature is responsible for
increasing the ratio of the rate of chain propagation, rel-
ative to chain transfer or termination rates is unknown at
this stage and may require theoretical analysis for
elucidation.
Fig. 2. ORTEP diagrams of the molecular structure of 2 at the 50%
probability level. Hydrogen atoms are omitted for clarity.
4. Experimental
2.2. Reactivity toward ethylene
All manipulations were performed under a nitrogen
atmosphere using standard glove box and schlenk tech-
niques. Toluene and THF were distilled from benzophe-
none ketyl. Pentane and hexane were dried over Na/K
alloy. The toluene for polymerization use was purchased
from Aldrich (anhydrous grade) and purified further over
Na/K alloy. Tis(pentafluorophenyl)boron was provided
by Boulder Scientific Company, Mead, Colorado and
purified further by sublimation. Ethylene (99.99%) was
purchased from Matheson Trigas and purified by passing
through the Agilent moisture and oxygen/moisture traps.
2,3-Butanedione and 2,6-diisopropylaniline were pur-
chased from Aldrich and used without further
purification. 3-(2,6-Diisopropylphenylimino)-butan-2-one
was prepared from condensation of 2,6-diisopropylaniline
with 2,3-butanedione in benzene using a catalytic amount
of p-toluenesulfonic acid. NMR spectra were recorded on
Ethylene polymerization reactions with 1 or 2 were stud-
ied, and the results of these studies are listed in Table 1.
The addition of ethylene at 100 psi to a solution of 2 in tol-
uene at 30 ꢁC results in the quick consumption of the
monomer and polymer formation (entry 1). An additional
6 equiv. of B(C6F5)3 increases the activity of the catalyst
and decreases the PDI of the polymer (entry 2). Our cur-
rent thinking is that B(C6F5)3 acts as a scrubbing agent that
removes impurities in the reaction medium that may inter-
fere with the propagating sites. Thermal analysis indicates
that the polymer has a high melting point, consistent with
the highly linear polymer structure (7 branches/1000 car-
bons) revealed by 1H NMR spectroscopy. The activity
increases with temperature and at 75 ꢁC (entries 3 and 4)
it is comparable to those of cationic diimine nickel initia-
tors [2]. The polymers formed at high temperature have
broader PDIs, increased branching in the backbone (23
branches/1000 carbons) and lower melting points [1,19].
No reaction is observed when 1 is used alone (entry 5).
1
Varian 400, 500 or Bruker 200 spectrometers. H NMR
spectra of the polyethylene were obtained in a mixed sol-
vent (C6D6/1,2,4-trichlorobenzene 1:4 ratio in volume) at
1
115 ꢁC. H and 13C NMR spectra were calibrated using
signals from the solvent and are reported downfield from
tetramethylsilane referenced to residual solvent 1H and
13C signals. 11B NMR, 19F NMR and 31P NMR spectra
were calibrated and reported downfield from external
BF3 Æ OEt2, CFCCl3 and H3PO4, respectively. Elemental
analyses were performed by Analytic Lab, Marine Science
Institute, University of California, Santa Barbara. Gel
permeation chromatograms (GPC) were measured on
PL-GPC220 in trichlorobenzene at 135 ꢁC, and the cali-
bration curve was established against polystyrene stan-
dards. Thermal analysis was carried out on a TA
instruments DSC 2920. Tm data were measured at a scan
rate of 10 ꢁC/min.
Table 1
Polymerization of ethylenea with 1 or 2
b
Compd
(lmol)
Additive
(equiv.)
T
Time
(min)
Ac
Mwd
PDI
Tmb
2 (6)
2 (3)
2 (3)
2 (2)
1 (6)
a
0
30
30
75
75
30
10
8
8
5
10
0.45
2.2
4.0
6.9
0
135.5
134.8
149.7
132.3
–
5.8
2.5
6.9
6.8
–
135
134
117
118
–
BCF(6)
BCF(6)
BCF(6)
0
Polymerization conditions: 26 g toluene, 100 psi ethylene.
b
c
ꢁC.
Activity in kg polymer/(mmol cat h).
d
Mw (·103).

4748
Y. Chen et al. / Journal of Organometallic Chemistry 692 (2007) 4745–4749
4.1. 3-(2,6-Diisopropylphenylimino)-but-1-en-2-olato (g1-
benzyl)(trimethylphosphine)nickel (1)
(septet, J = 7.0 Hz, 1H; iPr-CH), 1.63 (s, 3H; CH3), 1.15
(d, J = 2.5 Hz, 1H; benzyl-CH2), 0.95 (d, J = 7.0 Hz, 3H;
iPr-CH3), 0.88 (d, J = 7.0 Hz, 3H; iPr-CH3), 0.81 (d,
J = 7.0 Hz, 3H; iPr-CH3), 0.58 (d, J = 2.5 Hz, 1H; benzyl-
CH2), 0.49 ppm (d, J = 7.0 Hz, 3H; iPr-CH3); minor isomer,
6.61(d, J = 7.5 Hz, 1H; benzyl-ph-H4), 6.37 (d, J = 7.5 Hz,
1H; benzyl-ph-H2), 5.02 (d, 1H; CH2), 3.27 (d, 1H; CH2),
1.50 (s, 3H; CH3), 1.18 (d, J = 7.0 Hz, 3H; iPr-CH3), 1.10
(d, J = 7.0 Hz, 3H; iPr-CH3), 1.03 (d, J = 7.0 Hz, 3H; iPr-
CH3), 0.68 ppm (d, J = 7.0 Hz, 3H; iPr-CH3), some signals
of the minor isomer were not seen due to the overlap with
those of the major isomer; 13C NMR (100 MHz, CD2Cl2,
25 ꢁC): d = 183.55 (imine), 164.40 (@C–O), 149.67, 147.27,
141.27, 139.99, 138.69, 138.20, 136.44, 135.30, 134.19,
130.04, 128.42, 124.75, 123.42, 115.74, and 113.87 (ph-C),
104.20 (CH2), 29.06, 28.92 and 28.39 (iPr-CH and CH3),
24.26, 24.22, 23.84 and 22.45 (iPr-CH3), 19.33 ppm (ben-
zyl-CH2); 19F NMR (376 MHz, C6D6, 25 ꢁC): d =
ꢀ133.70, ꢀ158.08, ꢀ164.28 ppm; 11B NMR (161 MHz,
CD2Cl2, 25 ꢁC): d = ꢀ 2.6 ppm; Elemental analysis Calc.
for C41H29NOBF15Ni: C, 54.34; H, 3.23; N, 1.55. Found:
C, 54.06; H, 2.81; N, 1.50%.
NaH (0.120 g, 5.0 mmol) was added to 3-(2,6-diisopro-
pylphenylimino)-butan-2-one (1.23 g, 5.0 mmol) in 5 g of
THF. After stirring overnight at room temperature, the sol-
vent was removed under vacuum. The residue was dissolved
in toluene and filtered through celite. Excess toluene was
removed and pentane was added, the mixture was cooled
1
to ꢀ35 ꢁC to yield a white solid (1.08 g, 80 % yield). H
NMR (400 MHz, C6D6/THF-d8 (5/1), 25 ꢁC): d = 7.14–
7.00 (m, 3H; imine-ph-H), 4.42 (s, 1H; CH2), 4.23 (s, 1H;
CH2), 2.93 (septet, J = 6.8 Hz, 2H; iPr-CH), 1.88 (s, 3H;
CH3), 1.28 (d, J = 6.8 Hz, 6H; iPr-CH3), 1.18 ppm (d, J =
6.8 Hz, 6H; iPr-CH3). The sodium salt (227 mg, 0.85 mmol)
and Ni(PMe3)2(g1-benzyl)Cl (288 mg, 0.85 mmol) were
mixed at ꢀ35 ꢁC in toluene and allowed to warm to room
temperature over 2 h. Excess toluene was then removed
under vacuum and the residue was extracted with pentane.
The solution was concentrated to ꢁ3 mL and cooled to
1
ꢀ35 ꢁC to yield an orange solid (295 mg, 74% yield). H
NMR (400 MHz, C6D6, 25 ꢁC): d = 7.56 (d, J = 7.2 Hz,
2H, benzyl-ph-H2,6), 7.03–6.96 (m, 6H; imine-ph-H and
benzyl-ph-H3,4,5), 4.91 (s, 1H; CH2), 4.63 (s, 1H; CH2),
3.60 (septet, J = 6.8 Hz, 2H; iPr-CH), 1.65 (s, 3H; CH3),
1.32 (d, J = 6.8 Hz, 6H; iPr-CH3), 1.04 (s, 2H; benzyl-
CH2), 1.01 (d, J = 6.8 Hz, 6H; iPr-CH3), 0.74 ppm (d,
J = 10.0 Hz, 9H; PCH3); 13C NMR (100 MHz, C6D6,
25 ꢁC): d = 181.88 (imine), 169.24 (@C–O), 152.23,
142.86, 140.96, 130.35, 128.09, 127.27, 124.29, 123.45 (ph-
C), 92.11 (CH2), 28.85 (iPr-CH), 24.60 (iPr-CH3), 24.37
(iPr-CH3), 18.81 (CH3), 12.87 (d, J = 28 Hz, PCH3),
9.55 ppm (d, J = 26 Hz, benzyl-CH2); 31P NMR (162
MHz, C6D6, 25 ꢁC): d = ꢀ8.69 ppm; Elemental analysis
Calc. for C26H38NOPNi: C, 66.41; H, 8.14; N, 2.98. Found:
C, 65.97; H, 8.24; N, 3.02%.
4.3. Polymerization reactions
Typical polymerization of ethylene: B(C6F5)3 (18 lmol
in 1.0 g toluene) and compound 2 (3 lmol in 1.0 g toluene)
were placed in a 100 mL steel autoclave reactor charged
with 26.0 g of toluene. The reactor was then assembled
and brought out of the glove box, ethylene was then added
at 100 psi. Reaction length and temperature were varied as
described in Table 1. The reaction was quenched by release
Table 2
Crystallographic data for compounds 1 and 2
Compound
1
2
Formula
C
470.25
Monoclinic
P2(1)/c
26H38NOPNi
C41H29BF15NONi
906.17
Monoclinic
P2(1)/c
4.2. 2-Tris(pentafluorophenyl)borate-3-(2,6-
diisopropylphenylimino)-but-1-ene(g3-benzyl)nickel (2)
Molecular weight
Crystal system
Space group
Unit cell parameters
˚
Compound 1 (94 mg, 0.20 mmol) and 2 equiv. of
B(C6F5)3 (205 mg, 0.40 mmol) were mixed in toluene
(7.0 g). The color of solution changed from orange into pur-
ple immediately along with the formation of a white precip-
itate. After 90 min, hexanes were added and the reaction was
then filtered after 15 min. The solvents were then removed
under vacuum. The residue was washed with hexanes, and
dried under vacuum to afford a purple solid (168 mg, 93%
yield). 1H NMR (500 MHz, C6D6, 25 ꢁC. Two sets of peaks
(9:1 ratio) were observed): major isomer, d = 6.86 (t, J =
7.5 Hz, 1H; imine-ph-H4), 6.77 (d, J = 7.5 Hz, 1H; imine-
ph-H3 or 5), 6.67 (d, J = 7.5 Hz, 1H; imine-ph-H3 or 5), 6.52
(t, J = 7.5 Hz, 1H; benzyl-ph-H4), 6.31 (t, J = 7.5, 1H; ben-
zyl-ph-H3 or 5), 6.30 (t, J = 7.5, 1H; benzyl-ph-H3 or 5), 6.25
(d, J = 7.5 Hz, 1H; benzyl-ph-H6), 5.97 (d, J = 7.5 Hz, 1H;
benzyl-ph-H2), 4.35 (d, J = 3.5 Hz, 1H; CH2), 3.25 (d, J =
3.5, 1H; CH2), 2.38 (septet, J = 7.0 Hz, 1H; iPr-CH), 1.96
a (A)
9.8143(6)
21.7333(13)
12.0168(7)
90
94.571(2)
90
2555.0(3)
4
1.223
0.838
0.2 · 0.1 · 0.05
Blades
orange
1.87–28.21
423
13.9249(16)
14.2751(16)
20.158(2)
90
105.457(3)
90
3862.0(8)
4
1.558
0.610
0.12 · 0.1 · 0.06
Blades
purple
1.77–26.71
561
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (Mg mꢀ3
)
k (Mo Ka) (mmꢀ1
Crystal Size (mm)
Habit
)
Color
h Range (ꢁ)
Parameters refined
F(000)
1008
1832
R1, wR2 [I>2r(I)]
0.0418, 0.0913
0.831 and ꢀ0.233
0.0641, 0.01153
0.601 and ꢀ1.160
Maximum andꢀm3 inimum
˚
density (e A
)

Y. Chen et al. / Journal of Organometallic Chemistry 692 (2007) 4745–4749
4749
[4] J.W. Strauch, G. Erker, G. Kehr, R. Fro¨hlich, Angew. Chem. Int.,
Ed. Engl. 41 (2002) 2543–2546.
[5] (a) G.C. Bazan, G. Rodriguez, A.J. Ashe III, S. Al-Ahmad, C.J.
Mu¨ller, J. Am. Chem. Soc. 118 (1996) 2291–2292;
(b) R.W. Barnhart, G.C. Bazan, T.J. Mourey, J. Am. Chem. Soc. 120
(1998) 1082–1083;
of ethylene pressure and addition of acetone. The precipi-
tated polymer was collected by filtration and dried under
high vacuum overnight (see Table 2).
Acknowledgement
(c) Z.J.A. Komon, G.M. Diamond, M.K. Leclerc, V. Murphy, M.
Okazaki, G.C. Bazan, J. Am. Chem. Soc. 122 (2002) 15280–15285;
(d) J.C. Wasilke, S.J. Obrey, R.T. Baker, G.C. Bazan, Chem. Rev.
105 (2005) 1001–1020.
This work was generously supported by the DOE
(Grant No.: DE-FG03098ER14910).
[6] M.C. Bonnet, F. Dahan, A. Ecke, W. Keim, R.P. Schultz, I.J.
Tkatchenko, Chem. Soc., Chem. Commun. (1994) 615–616.
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Appendix A. Supplementary material
CCDC 280467 and 280468 contain the supplementary
crystallographic data for 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2007.05.016.
[12] C.B. Shim, Y.H. Kim, B.Y. Lee, D.M. Shin, Y.K. Chung, J.
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