Structural and ethylene oligomerization studies of chelated N?O (imino/amino)phenol nickel(II) complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.05.050

Condensation reactions of 2-aminoethanol with the appropriate aldehyde gave ligands 2-[1-[(2-hydroxyethyl)imino]ethyl]phenol (L1) and 2-[(2-hydroxyethyl)imino] methyl]phenol (L2) respectively. Subsequent reductions of L1 and L2 with NaBH₄ affor

Full text of DOI:10.1016/j.jorganchem.2017.05.050

Accepted Manuscript
Structural and ethylene oligomerization studies of chelated N^O (imino/amino)phenol
nickel(II) complexes
Makhosonke Ngcobo, Stephen O. Ojwach
PII:
S0022-328X(17)30356-X
10.1016/j.jorganchem.2017.05.050
JOM 19976
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 4 April 2017
Revised Date: 20 May 2017
Accepted Date: 25 May 2017
Please cite this article as: M. Ngcobo, S.O. Ojwach, Structural and ethylene oligomerization studies of
chelated N^O (imino/amino)phenol nickel(II) complexes, Journal of Organometallic Chemistry (2017),
doi: 10.1016/j.jorganchem.2017.05.050.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Structural and ethylene oligomerization studies of chelated N^O
(imino/amino)phenol nickel(II) complexes
*
Makhosonke Ngcobo and Stephen O. Ojwach
School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg Campus,
Private Bag X01 Scottsville, 3209, South Africa
Abstract
Condensation reactions of 2-aminoethanol with the appropriate aldehyde gave ligands
2
-[1-[(2-hydroxyethyl)imino]ethyl]phenol
(L1)
and
2-[(2-hydroxyethyl)imino]
methyl]phenol (L2) respectively. Subsequent reductions of L1 and L2 with NaBH afforded
4
the corresponding amine ligands 2-[1-[(2-hydroxyethyl)amino]ethyl]phenol (L3) and 2-{[(2-
hydroxyethyl)amino]methyl}phenol (L4). Reactions of L1-L4 with either NiCl2 or
NiBr (DME) produced the nickel(II) complexes Ni(L1)Br ] (1), [Ni(L1)Cl ] (2), [Ni(L2)Cl ]
2
2
2
2
(
3) and [Ni(L3)Br ] (4) respectively. Structural elucidation of the compounds were
2
performed using NMR, IR, mass spectrometry, elemental analyses and single crystal X-ray
crystallography for complex 3. All the nickel(II) complexes formed active catalysts in
ethylene oligomerization reactions upon activation with EtAlCl co-catalyst to afford butenes
2
(20% - 100%) and hexenes (31% - 80%) as the major products. Higher catalytic activities of
-1
-1
up to 11 830 kg mol h and formation of exclusively butenes were realized depending on
the complex structure and reactions conditions.
Keywords: imine ligands; nickel; structures; ethylene; oligomerization
*
Corresponding author: Tel.: +27 (33) 260 5239; Fax: +27 (33) 260 5009
E-mail: ojwach@ukzn.ac.za (S. O. Ojwach)
1

ACCEPTED MANUSCRIPT
Introduction
Catalyst design constitute a great interest in the development of new late transition
1
-2
metal based complexes for ethylene transformation reactions. Most research revolve
around the variation of the ligand architecture in order to control the electronic and steric
3
-7
properties of the resulting complex. Over the past decades, nickel complexes of range of
8
9-11
12-14
15-17
18-20
ligands including P^O (the SHOP process), N^N,
N^P,
N^N^N
and O^N
donor ligands have been applied as catalyst in olefin oligomerization reactions with varied
outcomes. To date, numerous reports show nickel(II) complexes as promising olefin
oligomerization catalysts due to their high catalytic activity and selectivity especially in the
2
1
oligomerization of ethylene. For example, Yu et al, reported N-(-2-substituted-5,6,7-
trihydroquinolin-8-ylidene)arylamino nickel complexes as highly active catalysts upon
activation with ethylaluminium sesquichloride in ethylene dimerization and trimerization
reactions.
However, despite the progress made with nickel complexes, challenges such as
2
2
balancing the catalytic activity, stability and selectivity still remain unsolved. The use of
hybrid ligands (N^P, N^O, N^P or N^O^P) appear to be an attractive avenue to achieve this
catalytic balance due to tunable electronic effect of the catalysts in addition to conferring
4
, 12, 23-24
6
possible hemilibility.
For instance, Gao et al, recently reported salicylaldimito N^O
ligated nickel(II) complexes to as highly active ethylene polymerization catalystscatalysts.
On the other hand, Zhou and his co-workers reported another series of nickel(II) complexes
ligated
by N,O donor ligands based on 4,6-didenzihydryl-2-[(arylimino)methyl]phenol
7
derivatives, which exclusively dimerize ethylene to butenes.
We recently reported the use of N^N-donor (imino/amino)pyridine palladium(II) and
25-26
nickel(II) complexes as ethylene dimerization and oligomerization catalysts.
In order to
2

ACCEPTED MANUSCRIPT
further investigate the role of the ligand architecture and effect of mixed donor ligands on the
catalytic behaviour of these complexes, we now report the use of N^O donor nickel(II)
complexes based on (imino/amino)phenol ligands as ethylene dimerization and trimerizations
catalysts. The structural elucidation of these nickel(II) complexes and their catalytic
behaviour towards ethylene oligomerization under various reaction conditions have been
investigated and would be discussed.
Experimental section
Materials and methods
All synthetic manipulations were performed using standard Schlenk-line techniques
under a nitrogen atmosphere. The solvents were obtained from Merck and dried or distilled
using appropriate methods. The chemicals; 2’-hydroxy acetophenone (98%), 2-methoxy
amine (99%), ethanolamine (99 %), N,N-(diethyl)ethylenediamine (99%), sodium
borohydride (98%), nickel(II) bromide (98%), nickel(II) bromide-1,2-dimethoxyethane
(97%) and nickel(II) chloride (98%) were obtained from Sigma Aldrich and used as received.
1
13
1
H NMR and C { H} NMR (100 MHz) spectra were recorded on a 400 MHz Bruker Ultra
shield NMR spectrometer in CDCl and DMSO-d solvents. The infrared spectra were
3
6
recorded on a Perkin Elmer, Spectrometer 100. LC Premier micro-mass Spectrometer model
LCMS-2020, was used for mass spectral analyses. Elemental analyses were performed on a
Thermal Scientific Flash 2000. The magnetic moments were determined using Evans balance
(
Sherwood MK-1). Varian CP-3800 gas chromatograph equipped with a CP-Sil 5 CB (30 m x
.2 mm x 0.25 µm) capillary column was used for GC analyses while GC-MS analyses were
performed on a Shimadzu GCMS-QP2010SE.
0
3

ACCEPTED MANUSCRIPT
Syntheses of ligands and their respective nickel(II) complexes
2
-[1-[(2-hydroxyethyl)imino]ethyl]phenol, (L1)
To a solution of 2’-hydroxy acetophenone (1.50 g, 11.00 mmol) in ethanol (30 ml)
was added ethanolamine (0.67 g, 11.00 mmol) and a catalytic amount of para tolyl sulfonic
acid (5.00 mg) to give a light green solution which was refluxed for 24 h at 60 °C. After the
reaction period, the solvent was removed under vacuum to give L1 as light brown oil. Yield:
1
3
2
8
1
.58 g (80%). H NMR (400 MHz, CDCl ): δ /ppm: 2.37 (s, 3H, C-CH ); 3.75 (t, 2H, J
=
HH
3
H
3
3
3
.0 Hz, N-CH ); 3.99 (t, 2H, J = 8.0 Hz, O-CH ); 6.75 (d, 1H, J = 8.0 Hz, 6-ph); 6.92 (d,
2
HH
2
HH
3
3
3
H, J = 8.0 Hz 4-ph); 7.28 (dd, 1H, J = 8.0 Hz, 5-ph), 7.48 (d, 1H, J = 8.0 Hz, 3-ph).
HH
HH
HH
1
3
1
C{ H} NMR (CDCl ): δ/ppm: 42.6, 58.3, 63.7, 116.6, 128.3, 130.7, 133.1, 136.5, 162.4,
3
-1
1
73.4. FT-IR (cm ): ν(O-H): 3164; ν(O-H): 2865; ν(C=N):1604. TOF MS ESI: m/z (%), 180.1028
+
(
[M + H] , 100%). HRMS calc for C H NO , 180.1025. Found, 180.1028
1
0
13
2
2
-[[(2-hydroxyethyl)imino]methyl]phenol (L2)
Compound L2 was synthesized following the same procedure adopted for L1 using
salicyladehyde (1.50 g, 0.01 mol) and ethanolamine (0.73 g, 0.01 mol) to give L2 as a brown
1
3
oil. Yield: 1.87 g (84%). H NMR (400 MHz, CDCl ): δ /ppm: 3.79 (t, 2H, J = 4.0 Hz, N-
3
H
HH
3
3
3
CH ); 3.96 (t, 2H, J = 4.0 Hz, O-CH ); 6.89 (t, 1H, J = 8.0 Hz, 6-ph); 6.98 (d, 1H, J
HH
2
HH
2
HH
3
3
=
8.0 Hz, 4-ph); 7.28 (d, 1H, J = 8.0 Hz, 5-ph); 7.36 (d, 1H, J = 8.0 Hz, 3-ph), 8.42 (s,
HH HH
1
3
1
1
1
H, N=CH). C { H} NMR (CDCl ): δ/ppm: 61.7, 62.1, 117.1, 118.7, 131.5, 132.5, 161.2,
3
-
1
66.94. FT-IR (cm ): ν(O-H): 3353; ν(O-H): 3057; ν(C=N):1631. TOF MS ESI: m/z (%), 166.09
+
(
[M + H] , 100%). HRMS for C H NO , 166.0868. Found, 166.0866
9
11
2
4

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2
-[1-[(2-hydroxyethyl)amino]ethyl]phenol (L3)
To a solution of compound L1 (0.50 g, 2.79 mmol) in methanol (30 ml) was added
NaBH (0.54 g, 13.90 mmol) to give a clear solution which was stirred under reflux for 4 h at
4
5
0 °C. The solvent was reduced under vacuum and the residue re-dissolved in chloroform (30
ml) and washed with distilled water (3 x 20 ml) to remove excess NaBH . The organic layer
4
was then separated and dried over MgSO , filtered and solvent removed under reduced
4
1
pressure to afford L3 as a light yellow oil. Yield: 0.16 g (32%). H NMR (400 MHz, CDCl ):
3
3
3
δ /ppm: 1.51 (d, 3H, J = 4.0 Hz, C-CH ); 2.74 (t, 2H, J = 4.0 Hz, N-CH ); 3.79 (t, 2H,
H
HH
3
HH
2
3
3
3
JHH = 4.0 Hz, N-CH ); 4.00 (q, 1H, J = 4.0 Hz, N-CH); 6.80 (d, 1H, J = 8.0 Hz, 6-ph);
2
HH
HH
3
3
3
6
.98 (dd, 1H, J = 8.0 Hz, 4-ph), 7.15 (d, 1H, J = 8.0 Hz, 5-ph); 7.20 (d, 1H, J = 8.0
HH HH HH
1
3
1
Hz, 3-ph). C { H} NMR (CDCl ): δ/ppm: 22.4, 49.1, 58.91, 61.3, 116.8, 119.2, 126.4,
3
-1
1
28.1, 128.4, 157.1. FT-IR (cm ): ν(N-H): 3309; ν(O-H): 3047; ν(O-H): 2963. TOF MS ESI: m/z
+
+
(
%), 180.1023 ([M - H] , 100%), 181.1078 ([M] , 14%). HRMS calc. for C H NO ,
10 15 2
1
80.1025. Found, 180.1023.
2
-{[(2-hydroxyethyl) amino]methyl}phenol (L4)
Compound L4 was prepared according to the procedure described for L3 using L2 (0.54 g,
1
3
.25 mmol) and NaBH (0.62 g, 16.25 mmol). Yield: 0.058 g (11%). H NMR (400 MHz,
4
3
3
CDCl ): δ /ppm: 2.89 (t, 2H, J = 4.0 Hz, N-CH ); 3.83 (t, 2H, J = 4.0 Hz, O-CH ); 4.06
3
H
HH
2
HH
2
3
3
(
s, 2H, N-CH ); 6.85 (p, 2H, J = 8.0 Hz, 3,5-ph); 7.02 (d, 1H, J = 8.0 Hz, 6-ph); 7.21
2
HH
HH
3
13
1
(
dd, 1H, J = 8.0 Hz, 4-ph). C { H} NMR (CDCl ): δ/ppm: 50.45, 52.3, 61.2, 116.4,
HH
3
-1
1
19.1, 12.4, 128.43, 128.8, 158.1. FT-IR (cm ): ν(N-H): 3263; ν(O-H): 3228; ν(O-H): 2837. TOF
+
MS ESI: m/z (%), 166.09 ([M - H] , 100%). HRMS calc. for C H NO , 166.0868. Found,
11
17
2
1
66.0864.
5

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[
Ni(L1)Br ] (1)
2
Complex 1 was synthesized by adding a solution of [NiBr (DME)] (0.10 g, 0.33
2
mmol) in dichloromethane (5 ml) to a solution of L1 (0.12 g, 0.66 mmol) in dichloromethane
5 ml). Then the solution was allowed to stir for 24 h at room temperature to give a yellow
precipitate which was filtered and washed with dichloromethane (10 ml). Yield: 0.04 g
(
-1
(
28%). TOF MS ESI: m/z (%), 397.1000 ([M-CH ], 18%). FT-IR (cm ): ν(O-H): 3292; ν(O-H)
:
3
.
2
4
858; ν(C=N): 1626. µobs = 3.10 BM. Anal. Calcd for C H NO NiBr 3H O: C, 26.59, H,
11
17
2
2
2
.24, N, 3.10. Found (%): C 26.58, H 4.37, N 3.26.
Complexes 2 - 5 were synthesized following procedure described for complex 1 using
appropriate ligand and nickel(II) salts.
Ni(L1)Cl ] (2)
[
2
NiCl (0.10 g, 0.80 mmol) and L1 (0.15 g, 0.80 mmol). Green solid. Yield: 0.07 g (30 %).
2
+
-1
TOF MS ESI: m/z (%), 236.02 ([M - 2Cl] , 100%). FT-IR (cm ): ν(O-H): 3307; ν(O-H): 3009;
.
2
ν(C=N): 1630. µobs = 3.14 BM. Anal. Calcd. for C H NO NiCl CH Cl : C, 33.55; H, 3.84; N,
10
13
2
2
2
3
.56. Found (%): C 33.23, H 3.93, N 3.81.
[
Ni(L2)Cl ] (3)
2
L2 (0.22 g, 1.32 mmol) and NiCl2 (0.17 g, 1.32 mmol). Recrystallization from
methanol/diethyl-ether solution mixture afforded blue crystals suitable for single-crystal X-
+
ray analysis.. Yield: 0.22g (59 %). TOF MS-ESI: m/z (%), 222.00 ([M - 2Cl] , 100%). FT-IR
-1
(
cm ): ν(O-H): 3426; ν(O-H): 2922; ν(C=N): 1644. µobs = 3.55 BM. Anal. Calcd. for
.
2
C H NO NiCl H O: C 34.56, H 4.19, N 4.48. Found (%): C 34.92, H 4.39, N 4.02.
9
11
2
2
6

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[
Ni(L3)Br ] (4)
2
Ligand L3 (0.10 g, 0.56 mmol and [NiBr (DME)] (0.15 g, 0.55 mmol). Blue solid.
2
+
-1
Yield: 0.20 g (90%). TOF MS ESI: m/z (%), 237.99 ([M-2Br] , 50%). FT-IR (cm ): ν(N-H):
3
237; ν(O-H): 3058; ν(O-H): 2837. µobs = 3.75 BM. Anal. Calcd. for C H NO NiBr : C, 30.05;
10 15 2 2
H, 3.78; N, 3.50. Found (%): C, 30.24; H, 3.95; N, 3.28.
[
Ni(L4)Cl ] (5)
2
.
NiCl 6H O (0.12 g, 0.48 mmol) and L4 (0.08 g, 0.48 mmol). Green solid. Yield: 0.03
2
2
+
+
g (21 %). TOF MS-ESI: m/z (%), 224.01 ([M - 2Cl] , 100%), 225.02 ([M - 2Cl] , 40 %). FT-
-1
IR (cm ): ν(N-H): 3250; ν(O-H): 3179; ν(O-H): 2932. µobs = 3.58 BM. Anal. Calc. for
C H NO NiCl (%): C 36.42, H 4.41, N 4.72. Found (%): C 36.39, H 4.38, N 4.65.
9
13
2
2
X-ray crystallography data collection
X-ray crystallographic data collection for compound 3a was recorded on a Bruker
Apex Duo diffractometer equipped with an Oxford Instrument Cryojet operating at 100(2) K
and an Incoatec microsource operating at 30 W power. Crystallographic and structure
refinements data of 3a is provided in Table 1. The data was collected with Mo Kα (λ =
0
.71073 Å) radiation at a crystal-to-detector distance of 50 mm. The data collections were
performed using omega and phi scans with exposures taken at 30 W X-ray power and 0.50˚
2
7
27
frame width using APEX2. The data was reduced with the program SAINT using outlier
rejection, scan speed scaling, as well as standard Lorentz and polarisation correction factors.
A SADABS semi-empirical multi-scan absorption correction was also applied to the data.
7

ACCEPTED MANUSCRIPT
2
8
29
Direct methods, SHELXS-2014 and WinGX were used to solve the structure. All non-
hydrogen were located in the difference map and refined anisotropically with SHELXZ-
2
8
2
014. All the hydrogen atoms were included as idealized contributors in the least squares
process. Their positions were calculated using a standard riding model with C-Haromatic
distances of 0.93 Å and U = 1.2 U and C-H distances of 0.99 Å and U = 1.2 U
iso
eq
methylene
iso
eq
and C-Hmethyl distances of 0.98 Å and Uiso = 1.5 U . The amine N-H and hydroxyl O-H
eq
hydrogen atoms were located in the difference density map and refined isotropically.
General procedure for ethylene oligomerization reactions
Ethylene oligomerization reactions were performed in a 400 ml stainless steel Parr
reactor equipped with a mechanical stirrer, temperature controller and an internal cooling
system. The reactor was pre-heated to 100 °C in vacuo and then cooled to room temperature.
The appropriate amount of the synthesized catalyst precursor (10.0 µmol) was weighed out
and transferred into a dry Schlenk tube under nitrogen and toluene (20 ml) was added
utilizing a syringe. The required amount of a co-catalyst was then injected into the Schlenk
tube containing the pre-catalyst. The mixture was then transferred via a cannula into the
reactor followed by addition of 60 ml of toluene via a cannula to the reactor to give a total of
8
0 ml. The reactor was then flashed three times with ethylene and the appropriate
temperature and pressure was set and the reaction started by switching the stirrer. After the
reaction period, the reactor was cooled to approximately -10 °C using ice and liquid nitrogen
and the excess ethylene vented off. The reaction was then quenched by the addition of 10 %
hydrochloric acid (5 ml) and a portion of the reaction mixture was sampled in a GC-vial for
GC and GC-MS analyses to determine the product distribution. The mass of the product
formed was determined from the calibration curve of the R-factors for the standards vs the
3
0
number of carbons and using n-heptane as an internal standard.
8

ACCEPTED MANUSCRIPT
Results and discussion
Syntheses of N^O ligands and their respective nickel (II) complexes
The (imine)phenol ligands L1 and L2 were synthesized by reacting equimolar
amounts of 2-aminoethanol with 2’-hydroxy acetophenone and salicylaldehyde using para
tolyl sulphonic acid as a catalyst (Scheme 1). Reductions of compounds L1 and L2 with
NaBH afforded the corresponding (amine)phenol ligands L3 and L4 respectively (Scheme
4
1
). The imine ligands L1 and L2 were obtained in high yields (84% and 80%),. On the other
hand, the corresponding amine compounds L3 and L4 were isolated in very low yields of
2% and 11% respectively, pointing to possible low efficiency of the NABH reducing agent
3
4
used. Reactions of ligands L1-L4 with appropriate nickel(II) salts gave the corresponding
nickel(II) complexes 1-5 (Scheme 2). All the complexes were isolated as hygroscopic solids
in low to high yields (21%-90%).
OH
H2N
NaBH4
R
R
R
p -TsOH
3
50 °C, CH OH
EtOH, 50-60 °C
HN
HO
OH
O
HO
OH
N
OH
R = CH , L1
R = CH , L3
3
3
R = H, L2
R = H, L4
Scheme 1: Synthesis of N^O donor ligands and their respective nickel(II) pre-catalysts 1–5.
1
13
The compounds were characterized by a combination of H and C NMR
spectroscopy (L1-L4), mass spectrometry, IR spectroscopy, micro-analyses and single crystal
1
X-ray crystallography for complex 3. As an illustration, H NMR spectrum of L2 (Fig. S1)
showed a singlet peak around 8.42 ppm, which was diagnostic of the imine proton. Reduction
9

ACCEPTED MANUSCRIPT
of L2 to give the amine ligand L4, resulted in the appearance of a new signal at 4.06 ppm in
1
13
1
the H NMR spectrum, consistent with the presence of an amine proton. The C { H} NMR
1
spectra (Fig. S2) of the ligands were consistent with the H NMR spectral data.
R
R
NiCl
2 2
/ NiBr (DME)
HO
X
N
OH
N
CH Cl
2 2
Ni
X
O
H
HO
R=CH , X=Br, ( 1) R = CH , X = Cl, ( 2) R = H, X = Cl, ( 3)
3
3
R
R
H
NiBr
2
2 2
(DME) / NiCl .6H O
OH HN
CH
2
Cl
2
HO
X
N
Ni
X
O
H
HO
R = H, X = Br, ( 4) R = CH , X = Cl, ( 5)
3
Scheme 2: Synthesis of the N^O ligated nickel(II) complexes 1-5
IR spectroscopy was also used in the structural elucidation of ligands L1–L4 and their
nickel(II) complexes 1-5. For example, the formation of imine ligands was established from
-1
-1
the presence of the ν(C=N) signal at 1 615cm and 1 604 cm for L1 and L2 respectively (Fig.
S3). The absence of these signals in the IR spectra of L3 and L4 confirmed the successful
reduction of the corresponding ligands L1 and L2. Formation of the nickel(II) complexes 1-5
was also deduced by comparing their IR spectra to those of the respective ligands. For
-1
-1
instance, a shift of the v(C=N) signals from 1 615 cm in L1 to 1 630 cm in complex 1 was
-1
-1
observed. Similarly, the v(O-H) signals at around 3 059 cm and 3 292 cm in L1 and its
corresponding complex 1 respectively established the coordination of the phenolic O atom to
nickel(II) atom; and more importantly, absence of deprotonation and coordination of the
1
0

ACCEPTED MANUSCRIPT
ligands in their neutral forms. Fig. S4 shows the IR spectra of ligand L4 and its corresponding
complex 5.
Mass spectrometry was also used to establish the identity of the ligands their
corresponding complexes. For example, the mass spectrum of complex 3 showed an m/z
-
value at 443.0172 amu formed after the loss of the two Cl ligands and stabilization of the
fragment by another ligand unit. The base peak at 222.0113 amu results from the loss of
secondary ligand unit (Fig. S5). The magnetic moments of complexes 1-5 were obtained in
the range of 3.10 – 3.76 BM. These values were slightly higher than spin only magnetic
3
1
moments of 2.83 BM but fall within the expected range for high spin nickel(II) complexes
3
1
of 2.9-4.2 BM. The elemental analyses data of complexes 1-5 were found to be consistent
with one ligand motif per nickel(II) atom in good agreement with the proposed structures in
Scheme 2.
Solid state structure of nickel(II) complex 3a
Single crystals suitable for X-ray analysis of complex 3a (derivative of 3) were grown
by slow diffusion of diethyl ether into methanol solution of complex 3 at room temperature
and used for its solid state structure determination. Table 1 gives a summary of the
crystallographic data and structure refinement parameters, while Figure 1 represents the
molecular structure and selected bond parameters for complex 3a. The solid state structure of
3
a revealed the formation of a dinuclear species in which the coordination sphere around
each nickel(II) atom contains one tridentate bound ligand unit, one terminal chloride ligand
and one methanol solvent to give an octahedral arrangement. The phenolate O atom bridges
the two nickel atoms, confirming deprotonation of the O-H group during crystallization. The
empirical formula of the solid state structure of 3a thus differs from that of complex 3, which
contains one ligand unit and two chloride ligands per nickel(II) center. Such transformations
1
1

ACCEPTED MANUSCRIPT
have been reported in literature and relate to relative stabilities of the dinuclear and
mononuclear species and may also be solvent driven. For example, the structure of 3a
3
2
compares favorably to the one reported by Dey et al.
for bis(µ -2-((2-
2
Hydroxyethyl)iminomethyl)phenolato-N,O,O,O’)-bis(acetate-O)-diaqua-di-nickel(II)
complex.
Table 1: Crystal data and structure refinement for complex 3a.
Parameter
Value
Empirical formula
C H Cl N Ni O
6
2
0
28
2
2
2
Formula weight
580.72 g/mol
100(2) K
Temperature
Wavelength
0.71073 Å
Monoclinic
Crystal system
Space group
P 2 /n
1
a
8.3440(7) Å
8.9628(7) Å
b
c
16.4951(13) Å
90°
α
β
101.127(2)°
90°
γ
Volume
3
1210.41(17) Å
Z
2
3
Density (calculated)
Absorption coefficient
F(000)
1.593 Mg/m
-
1
1.814 mm
600
3
Crystal size
0.520 x 0.240 x 0.150 mm
Theta range for data collection
Reflections collected
2.517 to 28.299°.
10761
Completeness to theta = 25.242°
Max. and min. transmission
100.0 %
0.789 and 0.446
1
2

ACCEPTED MANUSCRIPT
Data / restraints / parameters
2994 / 2 / 154
1.044
2
Goodness-of-fit on F
Final R indices [I>2sigma(I)]
R1 = 0.0197, wR2 = 0.0472
R indices (all data)
R1 = 0.0206, wR2 = 0.0476
-3
Largest diff. peak and hole
0.450 and -0.281 e.Å
Figure 1. Molecular structure diagram of complex 3a with 50 % probability ellipsoid.
Selected bond lengths (Å) and angles (˚): Ni(1)-N(1), 2.0012(10); Ni(1)-O(1), 2.0619(8);
Ni(1)-O(2), 2.1400(9); Ni(1)-O(3), 2.1303(9); Ni(1)-Cl(1), 2.4083(4); N(1)-Ni(1)-O(1),
9
1.45(4); O(1)-Ni(1)-O(1), 80.40(4); N(1)-Ni(1)-O(2), 80.65(4); O(1)-Ni(1)-O(2), 107.15(3);
O(3)-Ni(1)-Cl(1), 173.20(3); O(2)-Ni(1)-Cl(1), 90.85(3).
1
3

ACCEPTED MANUSCRIPT
The average Ni-O bond length of complex 3a of 2.0619(8) Å was found to be longer
than the average Ni-O bond length of 2.039 Å reported in 78 similar structures, a trend that
3
3
could result due to the bridging O atom. In contrast, the Ni-N
bond length of 2.0012(10)
imine
Å obtained was found to be shorter than the average Ni-Nimine bond length of 2.003 Å
3
3
reported literature for 78 similar structures. The selected bond angles of 3a for O(1)-Ni(1)-
O(1), N(1)-Ni(1)-O(2), O(3)-Ni(1)-Cl(1) of 80.40(4)˚ and 80.65(4)˚ respectively significantly
o
33
deviate from the expected 90 and thus demonstrate a distorted octahedral geometry.
Catalytic behavior of nickel(II) complexes 1–5 in ethylene oligomerization reactions
The catalytic abilities of the nickel(II) complexes 1-5 in ethylene oligomerization
reactions were investigated using EtAlCl as the activator in chlorobenzene solvent. Table 2
2
gives a summary of the results obtained for the nickel(II) pre-catalysts 1-5. The ethylene
oligomerization reactions predominantly produced C and C oligomers as the main products.
4
6
The oligomeric products were characterized using a combination of GCand GC-MS (Fig. S6).
a
Table 2: Ethylene oligomerization data obtained for nickel(II) complexes 1-5 using EtAlCl2.
e
Entry Catalyst T /T
Yield
Activity
Product distribution (%)
min max
b
c
-1
-1 d
(°C)
(g)
(kg.mol .h )
C4
27
29
31
20
22
C6
73
71
69
80
78
α-C4
85
α-C6
1
2
3
4
1
2
3
4
5
25/30
25/27
25/29
25/26
25/33
25
26
19
20
15
2 500
2 600
1 900
2 000
1 500
91
74
75
87
86
99
87
98
5
93
a
Reaction conditions: [Ni] =10 µmol; solvent, chlorobenzene, 80 ml (90 g); temperature, 25
C; time, 1 h; pressure,10 bar; Al/Ni=250.
°
1
4

ACCEPTED MANUSCRIPT
b
Initial temperature was 25 °C, Tmin and Tmax = lowest and highest temperatures obtained
during the reaction period.
c
Determined using n-heptane as an internal standard.
d Activity, kg oligomer produced per mol catalyst per hour
e
Determined by GC.
From Table 2, it was evident that the ligand architecture and the identity of the halides
influenced the catalytic activities of the nickel(II) complexes. For instance, catalytic activities
-1
-1
-1 -1
of 2 600 kg.mol .h and 1 900 kg.mol .h were observed for the methyl substituted and
unsubstituted complexes 2 and 3 respectively (Table 2, entries 2 and 3). This can be
attributed to improved solubility of complex 2 in comparison to the non-substituted complex
3
4
3
. Significantly, the imine catalysts were more active compared to their amine analogues.
For example, the imine and amine complexes 1 and 4 exhibited catalytic activities of 2 500
-1
-1
-1 -1
kg.mol .h and 2 000 kg.mol .h respectively (Table 2, entries 1 vs 4) This was expected
.
due to greater electrophilicity of the imine complexes. It was also observed that the chloride
complex 2 was more active than its bromide analogue complex 1, in good agreement with
35
the reports of Zhang et al. using 2, 6-pyridicarboxamide nickel(II) complexes. In general,
the selectivity towards the formation of C (20%-31%) and C (69% -80%) oligomers was not
4
6
significantly affected by the ligand motif. This is reasonable from the comparable steric
parameters around the nickel atom in complexes 1-5.
Investigation of the effect of reaction conditions on the catalytic behavior complex 3
The effect of varying the reaction parameters was investigated using 3/EtAlCl2
catalyst system and the results are summarized in Table 3. Both the catalytic activities and the
product distribution were significantly affected by the variation of pressure, Al/Ni ratio, time
1
5

ACCEPTED MANUSCRIPT
and solvent medium. The Al/Ni ratio was varied from 150 to 300 (Table 3, entries 1 - 4) to
-1
-1
give an optimum catalytic activity of 3 900 kg.mol .h at Al/Ni ratio of 200. A decrease in
the catalytic activity was observed at higher Al/Ni ratios of 250 and 300, which has been
attributed to increased alkylaluminium impurities which might result in catalyst
3
5
deactivation. As the Al/Ni ratio was increased from 150 to 300, the selectivity of C4
oligomer was observed to increase from 32% to 73% due to possible increased chain transfer
to the co-catalyst. Another plausible explanation for increased C oligomers at higher Al/Ni
4
2
2
ratios could be increased chain termination arising from enhanced catalytic activity.
a
Table 3. Ethylene oligomerization reactions of the 3/EtAlCl system
2
b
d
Entry Time Pressure Al/Ni
Yield (g)
Activity
Product Distribution (%)
-1
-1 c
(
h)
(Bar)
(kg.mol .h )
C4
C6
68
55
69
27
38
77
34
22
-
α-C4
73
α-C6
40
3
2
1
2
1
1
10
10
150
200
29
39
2 900
3 900
45
31
73
62
33
66
78
100
85
87
57
3
4
5
6
7
8
9
1
10
10
10
10
20
30
10
250
300
200
200
200
200
200
19
17
15
57
62
118
21
1 900
1 700
3 000
2 800
6 200
11 800
2 100
75
1
97
87
0.5
2
84
49
91
72
1
88
69
1
90
75
e
1
>99
<1
a
Reaction conditions: [3]= 10 µmol; solvent, chlorobenzene, 80 ml; Temperature, 25°C.
Determined using n-heptane as an internal standard.
Activity, kg oligomer produced per mol catalyst per hour.
Determined by Gas Chromatography.
b
c
d
e
In toluene solvent;
To probe the stability of the resultant catalysts, we varied the reaction times from 0.5 h to 2
h using 3/EtAlCl catalyst system (Table 3, entries 2, 5 and 6). Increased catalytic activities
2
-1
-1
-1 -1
from 3 000 kg mol h to 3 900 kg mol h with reaction time from 0.5 h to 1 h was
1
6

ACCEPTED MANUSCRIPT
3
observed, consistent with an induction period within 1 h. However, prolonged reaction times
-1
-1
was marked by decreased catalytic activities (2 800 kg.mol .h within 2 h), indicative of
3
6
catalyst deactivation. With respect to product distribution, there was a general decrease of
C oligomer with reaction time followed by a concomitant increase in the C fraction. For
4
6
instance, percentage compositions of C of 62%, and 45% were reported at 0.5 h and 1 h
4
respectively. This can be apportioned to chain reinsertion/C incorporation with ethylene
4
3
7
monomer over prolonged reaction times to give the C oligomers. As expected, variation of
6
ethylene pressure from 10 bar to 30 bar resultant in a significant increase in catalytic activity
-1
-1
-1 -1
from 3 900 kg.mol .h to 11 800 kg.mol .h (Table 3, entries 2 and 8 respectively). The
oligomer distribution was also affected by pressure changes as seen from selectivities of C4
oligomer of 33% and 78% at pressures of 10 bar and 30 bar respectively. The observed trends
in selectivity of C over C is consistent with increased catalytic activities which
4
6
3
8
subsequently results in rapid chain termination.
The effect of solvent on ethylene oligomerization reactions was also investigated
using 3/EtAlCl system in chlorobenzene and toluene solvents. The solvent used significantly
2
affected both the catalytic activity and selectivity of complex 3. For example, catalytic
-1
-1
-1 -1
activities of 2 100 kg.mol .h and 3 900 kg.mol .h were obtained in toluene and
chlorobenzene solvents respectively (Table 3, entries 2 vs 9). The higher catalytic activities
observed in chlorobenzene could be attributed to improved solubility of the complex 3 in
chlorobenzene. On the other hand, the use of toluene resulted in higher chemoselectivity for
C oligomer (100%) compared to compositions of 45% (C ) and 55% (C ) reported in
4
4
6
chlorobenzene respectively (Table 3, entries 2 vs 9). While the difference in selectivities
observed can be attributed to higher catalytic activities in chlorobenzene compared to toluene
solvent,
the
drastic
change
is
rather
unusual.
1
03

ACCEPTED MANUSCRIPT
4. Conclusion
The N^O donor imine and amine ligands and their respective nickel(II) complexes
were successfully synthesized and characterized. The solid state structure of complex 3a
confirmed the formation of a dinuclear species in which the ligand is tridentate and bridges
the two metal atoms via the phenolate O atom. The nickel(II) complexes formed active
catalysts for ethylene oligomerization reactions upon activation with EtAlCl co-catalyst to
2
afford mainly butenes and hexenes. The catalytic activities of the nickel(II) complexes were
greatly influenced by the structure of the catalyst and the reaction parameters. Higher
catalytic activities resulted in lower product selectivity, while lower catalytic activities
showed better selectivity of the catalysts.
5
. Acknowledgements
The National Research Foundation Scheme (NRF) for funding the project and the University
of KwaZulu-Natal.
6
. References
Bianchini, C., Giambastiani, G., Rios, I. G., Mantovani, G., Meli, A., Segarra, M. A.,
Coord. Chem. Rev. 2006, 250, 1391-1418.
1
.
2
3
4
2
5
.
.
.
Piel, C., Kaminsky, W., Kulickle, Ing. W. -M., Dissertation: Hamburg, Chem. 2005.
Brookhart, M., Jenkins, J. C., Organometallics 2003, 22, 250-286.
Carlini, C., Isola, M., Liuzzo, V., Galletti, A. M. R., Sbrana, G., Appl. Catal. A: Gen
002, 231, 307-320.
Elowe, P. R., Mc Cann, C., Pringle, P. G., Spitzmesser, S. K., Bercaw, J. E.,
.
Organometallics. 2006, 25, 5255-5260.
1
04

ACCEPTED MANUSCRIPT
6
7
1
8
9
6
1
.
Gao, R., Sun, W-H., Redshaw, C., Catal. Sci. Technol. 2013, 3, 1172-1179.
.
Zhou, Z., Hao, X., Redshaw, C., Chen, L., Sun, W-H., Catal. Sci. Technol. 2012, 2,
340-1345.
.
.
Kermagoret, A., Braunstein, P., Dalton Trans. 2008, 4, 1564-1573.
Azoulay, J. D., Schneider, Y., Galland, G. B., Bazan, G. C., Chem. Commun. 2009,
177-6179.
0. Laine, T. V., Lappalainen, K.,Piironen, U., Aitola, E.,Klinga, M., Leskela, M., J.
Organomet. Chem. 2000, 606, 112-114.
1
5
1
1.
136-5143.
2. Ainooson, M. K., Ojwach, S. O., Guzei, I. A., Spencer, L. C., Darkwa, J., J.
Zhang, D., Nadres, E. T., Brookhart, M., Daugulis, O., Organometallics. 2013, 32,
Organomet. Chem. 2011, 696, 1528-1535.
1
3.
4.
Speicer, F., Braunstein, P., Saussine, L., Organometallics. 2004, 23, 2633-2641.
Sun, W.-H., Li, Z., Hu, H., Wu, B., Yang, H., Zhu, N., Leng, X., Wang, H., New. J.
1
Chem. 2002, 26, 1474-1478.
5. Adewuyi, S., Li, G., Zhang, S., Wang, W., Hao, P., Sun, W.-H., Tang, N., Yi, J. , J.
Organomet. Chem. 2007, 692, 3532-3541.
6. Kunrath, F. A., De Souza, R. F., Casagrande Jr., O. L., Brooks, N. R., Young Jr, V.
G., Organometallics. 2003, 22, 4739-4743.
7. Zhang, M., Zhang, S., Hao, P., Jie, S., Sun, W. -H., Li, P., Lu, X., Eur. J. Inorg.
Chem. 2007, 3816-3826.
1
1
1
1
2
1
8.
Hu, T., Tang, L. -M., Li, X.-F., Li, Y.-S., Hu, N.-H, Organometallics. 2005, 24, 2628-
631.
9.
Sun, W.-H., Zhang, W., Gao, T., Tang, X., Chen, L., Li, Y., Jin, X., J. Organomet.
Chem. 2004, 698, 917-920.
1
05

ACCEPTED MANUSCRIPT
2
2
2
3
2
2
2
3
2
2
2
2
2
2
3
3
0.
Wang, L., Sun, W. -H., Han, L., Li, Z., Hu, Y., He, C., Yan, C., J. Organomet. Chem.
002, 650, 59-63.
1.
Yu, J., Hu, X., Zeng, Y., Zhang, L., Ni, C., Hao, X., Sun, W-H., New. J. Chem. 2011,
5, 178-183.
2.
Chen, E. Y.-X., Marks, T. J., Chem. Rev. 2000, 100, 1391-1434.
3.
Redshwaw, C., Tang, Y., Chem Soc. Rev. 2012, 41, 4484-4510.
4.
Ziegler, K., Gellert, H. G., Holzkamp, E., Wilke, G., Brennst. Chem. 1954, 35, 321-
29.
5.
Nyamato G. S., O., S. O., Akerman, M. P., Organometallics. 2015, 34, 5647-5657.
Nyamato, G. S., Ojwach, S. O., Akerman, M. P., Dalton Trans. 2016, 45, 3407-3417.
Bruker, A., SAINT and SADABS, Bruker AXS Incl., Madison, Wisconsin, USA. ,
6.
7.
012.
8.
Sheldrick, G. M., Acta Crystallogr., Sect. A: Found. Crystallogr. 2007, 64, 112-122.
Farrugia, L. J., Appl. Crystallogr. 2012, 45, 849-854.
9.
0.
Rome, K., Mclntyre, A., Chromatography Today. 2012, 52, 52-55.
Cotton, F. A., Wilkinson, G., Murillo, C. A., Bochmann, M., Advanced Inorganic
1.
Chemistry 6th Ed. John Wiley and Sons: New York, 1999; p 835.
3
9
3
3
2.
Dey, M., Rao, C. P., Saarenketo, P. K., Rissanen, K., Inorg. Chem. Commun. 2002, 5,
24-929.
3.
4.
Allan, F. H., Acta Cryst. 2002, B58, 380-388.
Nyamato George, S., Alam, M. G., Ojwach, S. O., Akerman, M. P., J. Organomet.
Chem. 2015, 783, 64-72.
3
5.
6.
Zhang, J., Liu, S., Li, A., Ye, H., Li, Z., New. J. Chem. 2016, 40, 7027-7030.
3
Doherty, M. D., Trudeau, S., White, P. S., Morken, J. P., Brookhart, M. ,
Organometallics. 2007, 26, 1261-1269.
1
06

ACCEPTED MANUSCRIPT
3
2
3
7.
74-282.
8. Killian, C. M., Johnson, L. K., Brookhart, M., Organometallics. 1997, 16, 2005-2007.
Nyamato, G. S., Ojwach, S. O., Akerman, M. P., J. Mol. Catal. A: Chem. 2014, 394,
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Highlights
•
•
•
•
•
N^O donor nickel(II) complexes successfully synthesized and characterized
Tridentate coordination and formation of dinuclear complexes established
The nickel(II) complexes formed active catalysts for ethylene oligomerization
The imine complexes more active than amine analogues
Oligomeric products mains butenes and hexenes depending on reaction conditions