New α-diimine nickel complexes—Synthesis and catalysis of alkene oligomerization reactions

Article abstract of DOI:10.1016/j.molcata.2016.07.024

Three α-diimine complexes of Ni^II of the general formula {2,3-bis[N,N-(aryl)imino]butane}NiBr₂ with aryl groups 2,3,5,6-F₄-C₆H, 2-CF₃-C₆H₄ and 2,4-(CH₃)₂-C₆H₃ were synthesized and used as catalyst precursors in oligomerization reactions of ethylene and propylene. Oligomerization catalysts were produced by activating the complexes with AlEt₂Cl, Al₂Et₃Cl₃, and with combinations of these compounds and PPh₃. The catalysts convert ethylene into low molecular-weight materials (liquids and waxes) with oligomer chains containing 15–20?mol.% of methyl branches and 4–6?mol.% of ethyl branches, which are apparently produced in “chain-walking” reactions of active centers along oligomer chains. Oligomerization of propylene yields a variety of light products, mostly dimers, which are formed in primary and secondary insertion reactions of propylene into Ni–H and Ni–C bonds in active centers. Reaction schemes of oligomer formation are discussed.

Full text of DOI:10.1016/j.molcata.2016.07.024

Accepted Manuscript
Title: New ␣-diimine nickel complexes – Synthesis and
catalysis of alkene oligomerization reactions
Author: Laura A. Rishina Yury V. Kissin Svetlana S. Lalayan
Svetlana C. Gagieva Dmitri A. Kurmaev Vladislav A. Tuskaev
Boris M. Bulychev
PII:
S1381-1169(16)30274-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.07.024
MOLCAA 9958
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Journal of Molecular Catalysis A: Chemical
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27-5-2016
10-7-2016
11-7-2016
Please cite this article as: Laura A.Rishina, Yury V.Kissin, Svetlana
S.Lalayan, Svetlana C.Gagieva, Dmitri A.Kurmaev, Vladislav A.Tuskaev, Boris
M.Bulychev, New ␣-diimine nickel complexes – Synthesis and catalysis of
alkene oligomerization reactions, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.07.024
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New -diimine nickel complexes - synthesis and catalysis of alkene oligomerization reactions
Laura A. Rishinaa*, Yury V. Kissinb, Svetlana S. Lalayana, Svetlana C. Gagievac,
Dmitri A. Kurmaevc,d, Vladislav A. Tuskaevc,d, Boris M. Bulychevc
a Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin St., Moscow 119991,
Russia.
b Rutgers, The State University of New Jersey, Department of Chemistry and Chemical Biology, 610 Taylor Rd.,
Piscataway, NJ 08854, USA.
с Lomonosov Moscow State University, Department of Chemistry, 1 Leninskie Gory, Moscow 119992,
Russia.
d Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St.,
Moscow 119991, Russia.
----------------
*
Corresponding author. E-mail address: rishina@polymer.chph.ras.ru

Graphical abstract
Synthesis of new NiII α-diimine complexes I-III from substituted anilines, 2,3-butanedione (reaction Ra),
and NiBr2 (reaction Rb). I - R1=R2=R4=R5=F; R3=H; II - R1=CF3; R2-R5=H; III - R1=R3=CH3;
R2=R4=R5=H.
Catalysts for oligomerization of ethylene and propylene were produced by activating the complexes with
AlEt2Cl, Al2Et3Cl3, and with combinations of these compounds and PPh3.
Highlights
“New -diimine nickel complexes - synthesis and catalysis of alkene oligomerization reactions” by
Laura А. Rishina, et al.




Three new -diimine complexes of Ni are synthesized.
The complexes are used as catalyst precursors for oligomerization of ethylene and propylene.
The complexes are activated with AlEt2Cl, Al2Et3Cl3 and their combinations with PPh3.
The catalysts convert ethylene into liquids and waxes and propylene into light oligomers.
Abstract
Three -diimine complexes of NiII of the general formula {2,3-bis[N,N-(aryl)imino]butane}NiBr2 with
aryl groups 2,3,5,6-F4-C6H, 2-CF3-C6H4 and 2,4-(CH3)2-C6H3 were synthesized and used as catalyst
precursors in oligomerization reactions of ethylene and propylene. Oligomerization catalysts were
produced by activating the complexes with AlEt2Cl, Al2Et3Cl3, and with combinations of these
compounds and PPh3. The catalysts convert ethylene into low molecular-weight materials (liquids and
waxes) with oligomer chains containing 15-20 mol.% of methyl branches and 4-6 mol.% of ethyl
branches, which are apparently produced in “chain-walking” reactions of active centers along oligomer
chains. Oligomerization of propylene yields a variety of light products, mostly dimers, which are formed
in primary and secondary insertion reactions of propylene into Ni–H and Ni–C bonds in active centers.
Reaction schemes of oligomer formation are discussed.
KEYWORDS: Nickel complexes; α-diimine ligand; oligomerization, alkenes; ethylene, oligomerization;
propylene, oligomerization

1. Introduction
At the present time, the leading research area in the field of catalytic alkene polymerization is post-
metallocene catalysis, studies of homogeneous polymerization catalysts based on multidentate complexes
of early- and late-period transition metals. Some of the most interesting among them are complexes of
PdII and NiII containing bulky -diimine ligands with a particular substitution pattern in their aryl rings
which restricts access to the transition metal atom (Scheme 1, these complexes are abbreviated below as
DiiM):
When DiiM complexes of NiII containing two o,o-disubstituted aryl groups are combined with
organoaluminum compounds, such as methylalumoxane or AlR′xCl3-x (R′=Me, Et, etc., x=1 or 2), they
form extremely active catalysts for ethylene polymerization; their productivity at moderate temperatures
approaches 200 kg/g Nih. A unique feature of the catalysts is their ability to form branched polyethylene
chains [1-5].
Polymerization reactions of propylene with DiiM complexes under ambient conditions produce
polymers of radically different nature depending on the temperature and the type of the substituents R. At
low temperatures, the catalysts polymerize propylene to a syndiotactic polymer [6] but the products
formed at higher temperatures vary from mostly regioregular polymers to highly branched materials with
a chain structure bearing no resemblance to “normal” polypropylene.
This article discusses two subjects:
1. Synthesis of NiII complexes LNiBr2 (I - III in Scheme 2) containing three new types of -diimine
ligands L with the substitution pattern in the aryl rings different from that in DiiM complexes:
LI - 2,3-bis[N,N-(2,3,5,6-tetrafuorophenyl)imino]butane
LII - 2,3-bis[N,N-(2-trifluoromethylphenyl)imino]butane
LIII - 2,3-bis[N,N-(2,4-dimethylphenyl)imino]butane
2. Oligomerization reactions of ethylene and propylene with catalysts based on complexes I - III.
The aryl-ring substitution in the L ligands provides an easier access of alkene molecules to the Ni
atom in active centers derived from the complexes and a larger volume for reorganization of Ni-alkyl
species. Complexes I - III were converted into catalysts with two types of activators (cocatalysts in the
terminology of alkene polymerization catalysis); organoaluminum compounds AlEt2Cl and Al2Et3Cl3, and
combinations of these compounds and PPh3.
2. Experimental
2.1. Chemicals
Ethylene and propylene (polymerization grade, 99.9%) were purchased from Moscow Oil Refinery.
AlEt2Cl (0.8 M heptane or toluene solution), Al2Et3Cl3 (1М toluene solution) and PPh3 were purchased
from Acros. Chemicals used for the synthesis of complexes I - III (NiBr2, 2,3,5,6-tetrafluoroanilin, 2-
trifluoromethylanilin, 2,4-dimethylanilin, p-toluenesulfonic acid and 2,3-butanedione) were purchased

from Aldrich. Purification of chemicals for the synthesis of complexes I - III is described in ref. [7]. All
manipulations with air-sensitive materials were performed in Schlenk glassware on a dual manifold
Schlenk line. Toluene used in oligomerization reactions (analytic grade) was boiled over sodium wire
and distilled in an argon flow.
2.2. Oligomerization reactions
Ethylene oligomerization reactions were carried out in a 100-mL stainless-steel autoclave equipped with
a mechanical stirrer and a temperature controller. Conditions of the reactions are given in Table 1.
Toluene, a specified amount of an organoaluminum activator, and toluene solution of a Ni complex were
added to the reactor to a total volume of 50 mL under ethylene atmosphere. Once a reaction temperature
(30 °C) was reached, ethylene pressure was increased to 3 atm and was maintained at this level during
30-min reactions. The reactions were stopped by cooling the reactor in an ice bath followed by release
of excess pressure. The reaction volume was quenched with HCl/ethanol (5%) and then the mixture was
separated into a liquid and wax.
Propylene oligomerization reactions were carried out in a 200-mL stainless steel reactor in liquid
propylene. Details of the oligomerization procedure are described in ref. [8]. The reaction conditions
are given in Table 3. An organoaluminum activator and PPh3 were added to the propylene-filled reactor,
after which a glass ampoule with a Ni complex was broken inside the reactor with a mechanical device.
Oligomers and the unreacted monomer were collected in a glass trap cooled with liquid nitrogen and
then propylene was slowly released by warming the trap. The reaction products were washed with a
10% HCl/ethanol mixture and water.
To compare catalyst activity in oligomerization reactions of ethylene and propylene, catalyst
productivities per mole of Ni were normalized to monomer concentrations, CE and CPr (M).
2.3. Oligomer Analysis
GC analysis of oligomers was carried out on a gas chromatograph “Trace GC ultra” equipped with a
RTX-5ms capillary column (0.25 mm, length 30 m) and connected to a mass spectrometer Finnegan
Polaris Q (ion trap, EI, 70 eV, mass range 33 -1000 atm). The injector temperature was 270 C, the
column temperature was held at 90 °C for 4 min and then increased at a rate of 10 °C/min to 320 C.
1H and 13C NMR spectra were recorded on a Bruker AMX-400 spectrometer at 20 °C in ortho-
dichlorobenzene or in chloroform as solvents using TMS as an internal standard. The signal assignment
in NMR spectra was carried out according to refs. [5-7,9,10]. IR spectra of complexes I - III were
recorded on a Magna-IR 750 spectrophotometer and those of oligomers on a Bruker Tensor 27 FTIR
spectrometer.

3. Results
3.1. Synthesis of Ni complexes I - III
Synthesis of complexes I - III was carried out in two steps according to Scheme 3: synthesis of α-diimine
ligands LI - LIII via condensation of two equivalents of a substituted aniline with one equivalent of 2,3-
butanedione (Reaction Ra) and reactions of the ligands with NiBr2 (Reaction Rb).
3.1.1. Synthesis of ligands LI - LIII
The synthesis of ligand LI is described in ref. [7].
Ligand LII, 2,3-bis[N,N-(2-trifluoromethylphenyl)imino]butane. In a 250 mL Schlenk flask, 2.2 equiv.
of 2-trifluoromethylaniline were dissolved in 150 mL of toluene and a catalytic amount of p-
toluenesulfonic acid monohydrate (0.06 equiv.) was added. One equiv. of 2,3-butanedione was added to
the flask dropwise. After refluxing the mixture for 40 h, the solvent was removed under vacuum. The
crude product was dissolved in methylene chloride and precipitated with methanol. A yellow product
was filtered and dried under vacuum. Analysis: calc. for C18H14F6N2 (372.31): C 58.07, H 3.79, N 7.52, F
1
30.62%; found C 58.01, H 3.64, N 7.22, F 39.81%. H NMR (400 MHz, CDCl3): δ 7.74 (d, J=7.8 Hz,
2H), 7.59 (t, J=7.6 Hz, 2H), 7.26 (t, J=7.7 Hz, 2H), 6.83 (d, J=7.9 Hz, 2H), and 2.18 ppm (s, 6H). 19F
NMR (400 MHz, CDCl3): δ= -62.44 ppm (s, 6F); 13C NMR (101 MHz, CDCl3): δ 170.01, 150.07, 133.18,
126.94, 125.91, 124.84, 124.39, 123.02, 119.93, 118.51, 77.16, 16.93 ppm. M.p. 152-153 °C.
Ligand LIII, 2,3-bis[N,N-(2,4-dimethylphenyl)imino]butane. It was obtained in a similar procedure from
2,4-dimethylaniline. Analysis: calc. for C20H24N2 (292.42): C 82.15, H 8.27, N 9.58%; found C 82.00, H
1
8.01, N 9.39%. H NMR (400 MHz, CDCl3): δ 7.26, 7.21, 7.19, 6.75, 6.73, 2.52, 2.29 ppm. 13C NMR
(101 MHz, THF): δ 165.69, 145.28, 130.98, 128.96, 124.67, 115.48, 22.17, 21.73, 18.07, 15.26, 12.54
ppm. M.p. 131-132 °C (cryst. from ethanol).
3.1.2. Synthesis of complexes I - III
Complexes I - III were prepared from ligands LI - LIII and NiBr2 (Reaction Rb in Scheme 3). All the
syntheses were carried out in methanol and the complexes were crystallized from a THF/methanol
mixture.
Complex I, {2,3-bis[N,N-(2,3,5,6-tetrafluorophenyl)imino]butane}NiBr2. Anhydrous NiBr2 (0.22 g, 1.00
mmol) and LI (0.38 g, 1.00 mmol) were combined in a Schlenk flask under argon. Methanol (20 mL)
was added and the reaction mixture was refluxed for 24 h. The resulting suspension was filtered and
complex I was recrystallized from a 1:1 methanol/THF mixture; yield 0.54 g, 79%.
According to the x-ray data [7], complex I crystallizes from a THF/methanol mixture as an ionic
dimer {[L2Ni2Br3(H2O)2]+ Br}·3THF. It is shown in Scheme 4. The presence of H2O molecules in the
crystalline complex (most probably, captured from insufficiently dry methanol) indicates significant
coordination options for the NiII atom, which are not restricted by the square-plane configuration typical
for NiII complexes in general.

Analysis of I: calc. for C44H44Br4F16N4Ni2O5 (1450): C 36.45, H 3.06, N 3.86, Ni 8.10%; found C
1
36.40, H 2.94, N 3.81, Ni 8.07%. H NMR (400 MHz, THF): δ=7.31-7.27 ppm (t, 2H, ArH), 3.58 (8H,
THF), 2.26 ppm (12H, CH3), 1.73 ppm (4H, H2O). 19F NMR (400 MHz, THF): δ=-82.48, -82.54, -82.57,
-94.61, -94.67, -94.70 ppm. 13C NMR (400 MHz, THF): δ=174.13, 147.54, 145.27, 137.92, 135.65,
129.85, 101.40, 101.26, 100.79, 16.38 ppm. IR (KBr): 3150 cm-1 ν(H2O), 1660 cm-1 δ(H2O), 1627cm-1
ν(C=N).
Syntheses of complexes II and III were carried out in similar procedures.
Complex II, {2,3-bis[N,N-(2-trifluoromethylphenyl)imino]butane}NiBr2. Analysis: calc. for
C38H40Br4F12N4Ni2O2 (1250): C 36.52, H 3.23, N 4.48, Ni 9.39%; found C 36.40, H 2.94, N 3.81, Ni
1
8.07%. H NMR (400 MHz, THF): δ 7.74 ppm (d, J=7.8 Hz, 4H), 7.59 ppm (t, J=7.6 Hz, 4H), 7.26 ppm
(t, J=7.7 Hz, 4H), 6.83 ppm (d, J=7.9 Hz, 4H), 2.18 ppm (s, 12H), 1.56 ppm (s, 4H, H2O), 19F NMR (400
MHz, THF): δ=-62.44 ppm. 13C NMR (400 MHz, THF): δ 170.01, 150.07, 133.18, 126.94, 125.91,
124.84, 124.39, 123.02, 119.93, 118.51, 77.16, 16.93 ppm. IR (KBr): 1640 cm-1 ν(C=N).
Complex III, {2,3-bis[N,N-(2,4-dimethylphenyl)imino]butane}NiBr2. Analysis: calc. for
C42H60Br4N4Ni2O2 (1090): C 46.28, H 5.55, N 5.14, Ni 10.77%; found C 46.12, H 5.31, N 5.05, Ni
1
10.65%. H NMR (400 MHz, THF): δ 7.74 ppm (d, J=7.8 Hz, 2H), 7.59 ppm (t, J=7.6 Hz, 2H), 7.26 ppm
(t, J=7.7 Hz, 4H), 6.83 ppm (d, J=7.9 Hz, 4H), 2.52 ppm (s, 6H), 2.18 (s, 12H). 13C NMR (400 MHz,
THF): δ 167.49, 145.28, 130.98, 128.96, 124.67, 115.48, 22.17, 21.73, 18.07, 15.26, 12.54 ppm.
NMR, IR and analytical data showed that crystalline complexes II and III also have an ionic structure
similar to that of complex I.
3.2. Ethylene oligomerization reactions
3.2.1. General results
The principal data on ethylene oligomerization reactions are given in Table 1. Several trends in
productivity of the Ni complexes can be traced. When the oligomerization reactions are carried out in the
absence of PPh3, AlEt2Cl and Al2Et3Cl3 are nearly equally effective in activating the Ni complexes.
Addition of PPh3 leads to a significant increase of catalyst productivity. The effect is the largest, nearly 4
times, for the I - AlEt2Cl system, it is two-fold for the II - AlEt2Cl system but merely ~7% for the III -
AlEt2Cl system. The activating effect of PPh3 can be possibly explained as due to its ability to form
complexes both with the Ni atom in the active centers of oligomerization reactions and with the
organoaluminum compounds; this subject is discussed in Section 4.1.
Oligomerization reactions of ethylene yielded mixtures of liquid and waxy products. This fact
reflects a broad molecular weight (MW) distribution of the oligomers: the products of sufficiently high
MW crystallize in the course of the reactions whereas the low-MW components remain dissolved in the
reaction solvent (toluene). Complex I produces active species which yield only low-MW oligomers
irrespectively of the activator type. Complex II yields only high-MW oligomers with both
organoaluminum compounds in the absence of PPh3 but mostly low-MW oligomers in the presence of

PPh3. In the case of complex III, the yields of low-MW and high-MW oligomers are approximately
equal both with AlEt2Cl and with Al2Et3Cl3; addition PPh3 noticeably shifts the MW distribution towards
low-MW oligomers.
3.2.2. Oligomer structure
The 13C NMR spectrum of one of the oligomers produced with the III - AlEt2Cl/PPh3 system is shown in
Fig. 1 (the range of aliphatic carbon atoms).
The main signals in the spectrum belong to linear –(CH2)n– sequences formed in ethylene homo-
oligomerization reaction (signal at 29.8 ppm) and to linear chain-ends CH3CH2CH2CH2: the CH3
signal at 14.1, -CH2 22.8, -CH2 32.1, -CH2 29.5 ppm.
The spectrum also contains signals of the following groups:
1. Isolated methyl branches in linear chains, ~CH2CH2CH(CH3)CH2CH2~
(13C NMR: CH3 20.1, CH ~ 34.0, -CH2 37.5, -CH2 27.4 ppm; 1H NMR: CH3 ~0.85, CH ~1.70, -CH2
1.15 ppm).
2. Isolated ethyl branches in linear chains, ~CH2CH2CH(CH2CH3)CH2CH2~
(CH3 11.0, branch-CH2 26.0, CH 38.5, -CH2 34.3, -CH2 26.9, -CH2 26.9 ppm).
3. Isolated longer alkyl branches in linear chains (CH3 ~14, -CH2 ~30.5 ppm).
13C NMR spectra also show the presence of the following unsaturated chain-ends:
Cis- and trans-CH3CH=CHR in a 1.7:1 ratio (cis - 130.0 and 124.4 ppm, trans - 131.8 and 123.9 ppm).
Cis- and trans-R′CH=CHR″ in a ~1.3:1 ratio (cis - 131.1 ppm, trans - 130.4 ppm).
CH2=CHR (139.1 and 114.2 ppm).
IR analysis of the oligomers confirmed the NMR data. The IR spectra contain strong out-of-plane
deformation bands of the trans-CH=CH-bond at 966 cm-1, the cis-CH=CH-bond at ~700 cm-1, and weak
bands of the CH2=CH-bond at 909 and 992 cm-1 and the CH2=CR-bond at 890 cm-1.
Table 2 shows several examples of the branching level in ethylene oligomers produced with complex
III. The branching degree is given as a mole percent of two types of branches, methyl and ethyl, with
respect to all ethylene units in oligomer molecules. To decrease uncertainty related to end-groups, the
branching was measured in waxes, oligomer fractions with the highest MW. The mechanism of branch
formation is described in Section 4.2. The table also contains the data on the average oligomerization
degree of the products, which were calculated from the relative intensity of end-group signals in 13C NMR
spectra.
3.3. Propylene oligomerization reactions
3.3.1. General results
Table 3 presents basic data on propylene oligomerization reactions. In the absence of PPh3, AlEt2Cl and
Al2Et3Cl3 do not produce efficient propylene oligomerization catalysts but addition of PPh3 leads to the

formation of very active catalyst systems. The oligomer yield with the I - AlEt2Cl/PPh3 system at 40 °С
reaches ~200 kg/mol Ni and with the II - AlEt2Cl/PPh3 system ~850 kg/mol Ni.
In the case of complex I, replacement of AlEt2Cl with Al2Et3Cl3 triples the catalyst productivity
whereas the other two complexes produce 2-3 times more efficient catalysts when activated with AlEt2Cl.
The increase of the [P]:[Ni] ratio in the I - AlEt2Cl/PPh3 system from ~2 to >25 does not affect the
catalyst productivity. Kinetic data in Fig. 2 show that the I - AlEt2Cl/PPh3 system is not particularly
stable: the reaction rate at 40 °С decreases nearly 1.5 times over the first 10 minutes and then remains
approximately constant. An interplay of temperature effects on the initial activity and the deactivation
rate of this catalyst leads to an apparent maximum of its oligomerization productivity at 40 °С.
3.3.2. Oligomer structure
Oligomerization reactions of propylene yielded only light liquid products. GC analysis showed that they
mostly consist of propylene dimers and relatively small amounts of trimers (from 10 to 12 different
isomers). The 13C NMR spectrum of the oligomer mixture produced with the II - AlEt2Cl/PPh3 system at
2
40 °C is shown in Fig. 3 in the ranges of aromatic and aliphatic carbon atoms.
The spectrum is quite complex. The following propylene dimers were identified (they are listed in
the order of abundance; only non-overlapping NMR signals are given for minor components, the
identification is supported by IR):
Trans- and cis-4-methyl-2-pentenes CH3CH=CHCH(CH3)2 in a ~3.5:1 ratio [13C NMR 138.8 (tr),
138.4 (cis), 121.5 (tr), 121.3 (cis), 31.0 (tr), 26.3 (cis), 22.4-22.8, 17.8 (tr), 12.3 (cis) ppm; 1H NMR 5.42
(tr), ~5.3 (cis), 5.38 (tr), ~5.2 (cis), 2.63, 1.61, 0.98-0.94 ppm; IR 968 cm-1 (tr), 724 cm-1 (cis)].
Trans- and cis-2-hexenes CH3CH=CHCH2CH2CH3 in a ~2.5:1 ratio [13C NMR 131.4 (tr), 130.6
(cis), 124.7 (tr), 123.8 (cis), 39.8 (tr), 29.1 (cis) ppm, 1H NMR 1.47 (tr), 1.37 (cis) ppm].
2-methyl-2-pentene (CH3)2C=CHCH2CH3 [13C NMR 130.6, 126.8, 25.6, 22.0, 14.0 ppm, 1H NMR 5.1,
2.0, 1.7, ~0.93 ppm, IR 734 cm-1].
2-methyl-1-pentene CH2=C(CH3)CH2CH2CH3 [13C NMR 145.8, 109.9, 40.2, 22.3, 20.9, 13.8 ppm, 1H
NMR ~4.7 ppm, IR 889 cm-1].
Small signals of four other propylene dimers, 2,3-dimethyl-1-butene CH2=C(CH3)CH(CH3)2 (151.8
and 107.8 ppm), 1-hexene (139.2 and 114.2 ppm), 2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 (123.8 ppm),
and 4-methyl-1-pentene CH2=CHCH2CH(CH2)3 (115.4 and 43.5 ppm), were also detected in in the
spectra of some oligomer mixtures.
Relative yields of different dimers depend on the catalyst, as Table 4 shows.
The NMR results show that the oligomer mixtures do not contain regular “head-to-tail” structures
with the polypropylene motif, ~CH2CH(CH3)CH2CH(CH3)~, which is characterized by signals of
CH2 groups at ~44-46.5 ppm and CH groups at ~28 ppm [12]. Chain ends typical for such oligomer

molecules, both unsaturated, CH2=C(CH3)CH2~, and saturated (n-propyl, and isobutyl [11]), are also
absent.
4. Discussion
4.1. Mechanistic features of oligomerization reactions
In general, the mechanism of alkene polymerization reactions (including oligomerization reactions) with
soluble catalysts based on DiiM complexes of NiII and the mechanism of branch formation in ethylene
polymers produced with the catalysts was thoroughly investigated [1,13,14,21]. According to the
literature [1,13], DiiM complexes (L)NiX2 rapidly react with organoaluminum compounds (AlMe3 in
methylalumoxane or AlR′xCl3-x) and form (L)NiR′2 products (R′ = Me, Et, etc.). The cocatalysts abstract

+
the R′ anion from alkylated DiiM complexes and convert them into (L)Ni R′ cations, diamagnetic
square-plane NiII species [19,21], which are the active centers in alkene polymerization and
oligomerization reactions. The resting state in the polymerization cycle is a -complex between the
+
+
(L)M –Polymer species and an alkene molecule, (L)(Polymer)M …CH2=CHR [1,13]. The metal atom in
the resting state does not have a proximate counter-ion derived from a cocatalyst and, therefore, the
nature of a counter-ion does not significantly affect either the activity of DiiM-based catalysts or the
nature of the produced polymers [17].
Phosphines are often used to increase the productivity of various nickel-based catalysts in ethylene
oligomerization reactions [20]. Their activating effect was attributed to protection of active species
during the catalytic cycle by acting as auxiliary ligands [21]. In our case, the role of PPh3 as such a
ligand, which does not directly affect the performance of the active centers, follows from the results in
Sections 3.2 and 3.3:
1. PPh3 significantly activates the Ni - AlR′xCl3-x systems in ethylene oligomerization reactions,
especially the catalysts derived from complexes I and II (Table 1). In the case of propylene
oligomerization reactions, none of the complexes forms active catalysts without PPh3 (Table 3).
2. Introduction of PPh3 leads only to a very modest increase of the branching level in the ethylene
oligomers (Table 2) and it does not significantly affect their average oligomerization degree.
3. An increase of the [P]:[Ni] ratio above 1 does not affect the nature of the oligomer products.
One can expect that, in general, the nature of polymerization products generated by -diimine
complexes should be strongly affected by the substitution pattern in the diimine ligands. The presence of
two ortho-substituents R in DiiM complexes (Scheme 1) restricts the movement of the growing polymer
+
chain in the resting state (L)(Polymer)M …CH2=CHR and makes the polymerization reaction more
regular, both in the kinetic sense (a lower frequency of chain transfer reactions resulting in the formation
of high-MW polymers) and with respect to regioregularity (higher probability of head-to-tail chain
growth).

The substitution pattern in -diimine complexes I - III (Scheme 2) significantly increases the
movement range of the polymer chain-end attached to the Ni atom. It leads to two effects:
1. The probability of -H elimination in a (L)Ni+–Polymer species increases:
(L)Ni+CH2CHRPolymer  (L)Ni+H …CH2=CRPolymer
 (L)Ni+H + CH2=CRPolymer (R=H or an alkyl group)
(1)
This effect signifies that the MW of the reaction products decreases: oligomers with an average length of
6 to 8 ethylene units are formed (Table 2) instead of polymers [1,13,14].
2. The probability of chain isomerization via reversible dissociation of the (L)Ni+Polymer bond
increases:
(L)Ni+CH2CHRPolymer  (L)(H)Ni+…CH2=CRPolymer
 (L)(H)Ni+…CR(Polymer)=CH2  (L)Ni+CR(CH3)Polymer
Oligomerization reactions are a particular example of polymerization reactions characterized by high
probability of chain transfer reactions; in our case, -H elimination (Reaction 1).
The chain growth in reactions of 1-alkenes CH2=CHR with any multidentate Ni complex starts the
(L)Ni+H species [18,21]:
(2)
(L)Ni+H + CH2=CHR  (L)Ni+CH2CH2R
(3)
Standard chain propagation reactions in the polymerization/oligomerization reactions are consecutive
insertions of 1-alkene molecules into the (L)Ni+C bond:
(L)Ni+CH2CHR(CH2CHR)nR + CH2=CHR  (L)Ni+CH2CHR(CH2CHR)n+1R (4)
The data in Tables 1 and 3 provide information on the relative reactivity of ethylene and propylene in
Reaction 4 normalized to monomer concentration. Ethylene is always more reactive than propylene in
insertion reactions into a transition metal-carbon bond [14] and the comparison of catalyst productivity in
Tables 1 and 3 confirms this rule for the (L)Ni+C bond as well. The difference in reactivity depends also
on the type of the Ni complex. For example, the relative yield of ethylene oligomers in a 30-min reaction
with complex I activated with AlEtCl2/PPh3 at 30 °C is about ten times higher than the relative yield of
propylene oligomers in a 60-min reaction at 40 °C with the same catalyst system, whereas the difference
is much higher, over 100 times, for complex III.
4.2. Reaction scheme of ethylene oligomerization
The data in Section 3.2.2 show that ethylene oligomerization with complexes I - III produces branched
chains. Similarly to the literature on ethylene polymerization with DiiM complexes [1-5], the formation
of branches can be explained by “chain-walking” reactions [1,3,5,13,15-17], a series of -H
elimination/alkene reinsertion steps (Reaction 2). Methyl branches in the oligomer chains are formed in
Reactions 5-6:
+
+
(L)Ni CH2CH2CH2CH2R  (L)(H)Ni …CH2=CHCH2CH2R

 (L)Ni+CH(CH3)CH2CH2R
(5)
(6)
+
(L)Ni CH(CH3)CH2CH2R + CH2=CH2

+
(L)Ni CH2CH2CH(CH3)CH2CH2R
Formation of ethyl and longer branches can be rationalized in a similar way starting with the product of
Reaction 5:
+
+
(L)Ni CH(CH3)CH2CH2R  (L)(H)Ni …CH(CH3)=CHCH2R
+
 (L)Ni CH(C2H5)CH2CH2R
(7)
+
+
(L)Ni CH(C2H5)CH2CH2R + CH2=CH2  (L)Ni CH2CH2CH(C2H5)CH2R (8)
The data in Table 2 show that methyl branches dominate in ethylene oligomers produced with
complexes I - III. Their content in waxes prepared with complex III is about two times higher than in
ethylene polymers produced with Ni-based DiiM complexes under similar reaction conditions [19], most
probably due to lower space restrictions in the transition state of Reaction 5.
The growth of linear and branched oligomer chains is terminated in a -H elimination reaction
(Reaction 1), which leads to the formation of alkenes. Their double bonds are readily identified by 13C
NMR. Dissociation of the complex formed in Reaction 7 produces cis- and trans-CH3CH=CHR
alkenes, dissociation of complexes produced after a further migration of the Ni atom along the alkyl chain
produces cis- and trans-R′CH=CHR″ alkenes, and dissociation of the complex formed in Reaction 5
produces CH2=CHR alkenes. All the alkenes are formed at a [CH3CH=CHR]:
[R′CH=CHR″]:[CH2=CHR] ratio of ~1:0.7:0.08.
These results are matched by GC data for the lightest ethylene oligomers. The C4 products include 2-
butenes and 1-butene in a 1:0.06 ratio and the C6 products contain 2-hexenes, 3-hexenes and 1-hexene in
a ~1:0.3:0.2 ratio.
Alkenes constitute about 70% of the oligomer molecules; the remaining products are isoalkanes
formed in chain transfer reactions with organoaluminum compounds.
4.3. Reaction scheme of propylene oligomerization
Polymerization of propylene with Ni-based DiiM complexes at low temperatures, -78C to -45C,
produces regioregular, predominantly syndiotactic polymers. However, both the regiochemical and the
stereochemical control in these reactions deteriorate with temperature and at 50C stereo-irregular and
regio-irregular polymers are formed [6].
In the case of Ni complexes I - III, the only products formed from propylene are light oligomers,
mostly dimers. The formation of the four most abundant dimers is explained by the following reaction
+
scheme. A propylene molecule can insert into the (L)Ni H bond both in the primary and the secondary
orientation:
+
+
(L)Ni H + CH2=CHCH3  (L)Ni CH2CH2CH3
(9)

+
+
(L)Ni H + CH3CH=CH2  (L)Ni CH(CH3)2
(10)
+
As the data for ethylene oligomers show, the (L)Ni CH2CH2CH3 species can isomerize into the
+
(L)Ni CH(CH3)2 species in a chain-walking reaction similar to Reaction 5.
+
Primary insertion of a propylene molecule into the (L)Ni CH2CH2CH3 species followed by -H
elimination produces 2-methyl-1-pentene:
+
+
(L)Ni CH2CH2CH3 + CH2=CHCH3  (L)Ni CH2CH(CH3)CH2CH2CH3
+
 (L)Ni H + CH2=C(CH3)CH2CH2CH3
(11)
A chain-walking reaction in the (L)Ni+CH2CH(CH3)CH2CH2CH3 species leads to the
formation of 2-methyl-2-pentene:
+
+
(L)Ni CH2CH(CH3)CH2CH2CH3  (L)Ni C(CH3)2CH2CH2CH3
+
 (L)Ni H + (CH3)2C=CHCH2CH3
(12)
+
Secondary propylene insertion into the (L)Ni CH2CH2CH3 bond produces mostly 2-hexenes and
a small quantity of 1-hexene (GC data confirm these NMR results):
+
+
(L)Ni CH2CH2CH3 + CH3CH=CH2  (L)Ni CH(CH3)CH2CH2CH2CH3
+
 (L)Ni H + CH3CH=CHCH2CH2CH3
(13)
+
In a similar set of reactions, secondary insertion of a propylene molecule into the (L)Ni CH(CH3)2
species produces 4-methyl-2-pentenes and a small quantity of 4-methyl-1-pentene:
+
+
(L)Ni CH(CH3)2 + CH3CH=CH2  (L)Ni CH(CH3)CH2CH(CH3)2
+
 (L)Ni H + CH3CH=CHCH(CH3)2
(14)
Several kinetic features of propylene oligomerization reactions can be evaluated from relative yields
of different dimers reported in Table 4:
=
=
=
1. The ([2-Me-2-C5 ]+[2-Me-1-C5 ])/∑[2-C6 ] ratio gives an approximate measure of two probabilities
+
for propylene insertion into the (L)Ni CH2R bond, the primary (Reaction 11) vs. the secondary
(Reaction 13):
Catalyst: I - AlEt2Cl/PPh3 I - Al2Et3Cl3/PPh3 II - AlEt2Cl/PPh3 II - Al2Et3Cl3/PPh3
kprimary Ni-C
/
ksecondary Ni-C
:
3.1
1.6
1.2
1.0
Complex II produces active species which exhibit no regioselecivity whereas complex I produces active
species with some preference for primary insertion.

+
2. Regioselecivity of propylene insertion into the (L)Ni H bond can be approximately evaluated in a
=
=
similar manner from the relative yields of ∑[2-C6 ] vs. ∑[2-Me-2-C5 ]. This estimation assumes that the
+
frequency of the chain-walking reaction in the (L)Ni CH2CH2CH3 species (similar to Reaction 5) is
relatively low, as Table 2 shows.
Catalyst: I - AlEt2Cl/PPh3 I - Al2Et3Cl3/PPh3 II - AlEt2Cl/PPh3 II - Al2Et3Cl3/PPh3
kprimary Ni-H
/
ksecondary Ni-H
:
0.7
0.6
1.5
0.5
+
Thus, in most cases the probability of secondary propylene insertion into the (L)Ni H bond [which gives
+
a thermodynamically more stable (L)Ni CH(CH3)2 species] is about 1.5-2 times higher than the
probability of primary insertion.
+
3. Primary propylene insertion into the (L)Ni CH(CH3)2 species is difficult due to steric reasons and the
respective dimers, CH2=C(CH3)CH(CH3)2 and (CH3)2C=C(CH3)2, are present in the oligomer mixtures
in very small quantities.
5. Conclusions

-Diimine Ni complexes I - III (Scheme 2) containing three new types of diimine ligands were
synthesized and characterized. The ligands provide easy access of alkene molecules to the Ni atom in the
catalytically active species derived from the complexes. The complexes were converted into efficient
ethylene and propylene oligomerization catalysts with organoaluminum compounds, AlEt2Cl and
Al2Et3Cl3, and with combinations of these compounds and PPh3.
Ethylene oligomerization reactions with the catalysts at 30 °C yielded mixtures of waxy and liquid
oligomers (the oligomerization degree 6 to 9) containing 14-20 mol. % of methyl branches, 4-6 mol. % of
ethyl branches and a small number of longer branches. The branched oligomer chains are apparently
formed in chain-isomerization (“chain-walking”) steps which occur in the course of ethylene
oligomerization reactions.
Propylene oligomerization reactions with the catalysts at 30-70 °C produced mixtures of very light
oligomers (mostly dimers), which are formed in competing primary and secondary insertion reaction into
NiH and NiC bonds. The probability of the secondary propylene insertion into the NiH bond is ~1.5-
2 times higher than the probability of the primary insertion. The regioselectivity of propylene insertion
into the NiC bond depends on the type of ligand: complex II produces active species which exhibit no
regioselecivity whereas complex I produces active species with some preference for primary insertion.

Acknowledgements
This work was financially supported by Russian Science Foundation (Project No 14-13-00682).
Elemental analysis was performed in Nesmeyanov Institute of Organoelement Compounds, Moscow,
Russia. The help of Ms. A. Serdukova in preparing the figures is greatly appreciated.
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Chemical shift, ppm
Fig. 1. 13C NMR spectrum of ethylene oligomer produced with III - AlEt2Cl/PPh3 catalyst system (range
of aliphatic carbon atoms).

Fig. 2. Kinetics of propylene polymerization with I - AlEt2Cl/PPh3 catalyst system at 30 °C (1),
40 °C (2) and 50 °C (3). Ordinate - relative rate per mole of I.

A
B
Chemical shift, ppm
Chemical shift, ppm
Fig. 3. 13C NMR spectrum of propylene oligomer produced with the II - AlEt2Cl/PPh3 catalyst system at
40 °C. A - range of aromatic carbon atoms (signals at 124.4, 128.3, 129.1 and 137.8 ppm - toluene); B -
range of aliphatic carbon atoms (signal at 21.4 ppm - toluene).

R'
R'
R
R
R
N
N
M
R
X
X
Scheme 1
-Diimine complexes DiiM. M = PdII and NiII, X = Cl, Br, Me; R′+ R′ = cyclohexyl, cyclohexenyl,
dinaphthyl; R = Me, i-Pr, Ph.

F
F
F
F
F
F
F
N
N
N
N
N
CH3
CH3
N
Ni
Ni
Ni
CF3
CH3
F
CF3
CH3
Br
Br
Br
Br
Br
Br
I
III
II
Scheme 2
-Diimine complexes of NiII used in oligomerization reactions of ethylene and propylene.

R4
R4
R5
R1
R5
R4
R5
R4
R4
R5
R1
R5
R1
Rb
Ra
R3
R3
N
N
R3
N
R3
NH2
R3
N
Ni
R2
R2
R1
R2
R1
R2
R2
Br
Br
Scheme 3
Synthesis of ligands LI - LIII and complexes I - III. LI and I - R1 = R2=R4 = R5 = F; R3 = H; LII and II
- R1 = CF3; R2-R5 = H; LIII and III - R1 = R3 = CH3; R2 = R4 = R5 = H.

Scheme 4. Crystal structure of complex I.

Table 1
Ethylene oligomerization reactions with complexes I - III.a
Organoaluminum
activator
Catalyst productivity
Fraction of
liquid products,%
kg/mol Ni
Complex I
kg/(mol Ni·CE)
AlEt2Clb
449
400
~100
~100
~100
~100
161.8
b
144.1
617.3
420.5
Al2Et3Cl3
AlEt2Clc
1714
1168
d
Al2Et3Cl3
Complex II
AlEt2Clb
272.5
235.0
532.5
332.5
757
653
~0
~0
94
77
b
Al2Et3Cl3
AlEt2Clc
1479
9234
c
Al2Et3Cl3
Complex III
AlEt2Clb
412.5
422.5
462.5
480.0
1146
1174
1285
1333
58
48
98
70
b
Al2Et3Cl3
AlEt2Clc
c
Al2Et3Cl3
a All experiments at 30 °C, PE = 0.3 MPa, CE = 0.36 M, [Ni] = 4·10-2 mmol,
[Al]:[Ni] = 150, reaction time 30 min.
b Experiments without PPh3.
c Experiments at [PPh3]:[Ni] = 1.
d Experiments at [PPh3]:[Ni] = 25.

Table 2
Branchinga and oligomerization degree of ethylene oligomers
produced with complex III.
Activator
[CH3], mol. % [C2H5], mol. % Av. oligomer.
degree n
Al2Et2Cl
13.6
15.7
18.0
19.0
4.4
5.2
5.0
5.6
8.6
6.1
8.4
7.0
Al2Et3Cl3
AlEt2Cl/PPh3
Al2Et3Cl3/PPh3
a Branches per one ethylene monomer unit.

Table 3
Propylene oligomerization reactions with complexes I - III.a
[Ni]102,
mmol
Temperature,
Organo-
aluminum
activator
[Al]:[Ni], [P]:[Ni],
Catalyst productivity
°С
molar
molar
kg/mol Ni kg/(mol Ni·CPr)
Complex I
154
30
30
40
40
50
70
40
5.3
4.8
3.0
3.5
3.9
3.5
4.3
AlEt2Cl
″ - ″
0
0
0
184
1.8
1.8
26.0
2.2
1.9
2.4
124
195
198
178
120
580
10.4
17.1
17.4
15.9
12.0
50.9
″ - ″
224
″ - ″
162
″ - ″
163
″ - ″
173
Al2Et3Cl3
153
Complex II
150
40
20
40
40
5.3
9.8
4.2
6.3
AlEt2Cl
″ - ″
0
0
0
161
2.1
2.2
2.2
256
847
282
21.3
74.3
24.7
″ - ″
154
Al2Et3Cl3
156
Complex III
181
40
40
3.74
3.7
AlEt2Cl
2.6
2.7
114
59
10.0
5.2
Al2Et3Cl3
154
a Oligomerization in liquid propylene, run time 60 min; CPr (M) at 20°С - 12.4, 30°С - 11.9,
40°С - 11.4, 50°С - 11.2, 70°С - 10.0.
Table 4
Relative yields of different propylene dimers.a
=
=
=
=
Catalyst system
I - AlEt2Cl/PPh3
I - Al2Et3Cl3/PPh3
II - AlEt2Cl/PPh3
II - Al2Et3Cl3/PPh3
[4-Me-2-C5 ] [2-Me-2-C5 ] [2-C6 ] [2-Me-1-C5 ]
1
1
1
1
0.63
0.21
0.66
0.10
0.46
0.63
1.51
0.28
0.81
0.78
1.23
0.19
a All oligomers were prepared at 40 °С.