Chromium(III) complexes bearing 2-imino-1,10-phenanthrolines: Synthesis, molecular structures and ethylene oligomerization and polymerization

Article abstract of DOI:10.1016/j.molcata.2007.07.012

A series of chromium(III) complexes ligated by tridentate ligands of 2-imino-1,10-phenanthrolines, LCrCl₃ (L = 2-(2,6-(R¹)₂C₆H₃N = CR)-1,10-phen) was synthesized and characterized by elemental and spectroscopic analyses as well as single-crystal X-ray crystallography. X-ray crystallographic analysis of 2 (R = Ph, R¹ = Et), 4 (R = Me, R¹ = Me) and 12 (R = H, R¹ = i-Pr) reveals that the geometry around the chromium atom is octahedral. Upon activation with MAO, these complexes exhibited high activities for ethylene oligomerization (up to 1.15 × 10⁷ g mol^-1 (Cr) h^-1) and moderate activities for ethylene polymerization. The oligomers and polymers obtained were α-olefins, and the distribution of oligomers resembled the Schultz-Flory rule. Various reaction parameters were investigated in detail, and the results revealed that both the steric and electric effects of ligands affected the catalytic activities of their chromium complexes as well as the distribution of the products.

Full text of DOI:10.1016/j.molcata.2007.07.012

Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
Chromium(III) complexes bearing 2-imino-1,10-phenanthrolines:
Synthesis, molecular structures and ethylene
oligomerization and polymerization
Shu Zhang, Suyun Jie, Qisong Shi, Wen-Hua Sun∗
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received 17 February 2007; received in revised form 6 July 2007; accepted 8 July 2007
Available online 12 July 2007
Abstract
A series of chromium(III) complexes ligated by tridentate ligands of 2-imino-1,10-phenanthrolines, LCrCl3 (L = 2-(2,6-(R1)2C6H3N = CR)-
1,10-phen) was synthesized and characterized by elemental and spectroscopic analyses as well as single-crystal X-ray crystallography. X-ray
crystallographic analysis of 2 (R = Ph, R1 = Et), 4 (R = Me, R1 = Me) and 12 (R = H, R1 = i-Pr) reveals that the geometry around the chromium atom
is octahedral. Upon activation with MAO, these complexes exhibited high activities for ethylene oligomerization (up to 1.15 × 107 g mol−1 (Cr) h−1
)
and moderate activities for ethylene polymerization. The oligomers and polymers obtained were ␣-olefins, and the distribution of oligomers
resembled the Schultz–Flory rule. Various reaction parameters were investigated in detail, and the results revealed that both the steric and electric
effects of ligands affected the catalytic activities of their chromium complexes as well as the distribution of the products.
© 2007 Elsevier B.V. All rights reserved.
Keywords: N∧N∧N tridentate ligands; 2-Imino-1,10-phenanthrolines; Chromium complex; Ethylene oligomerization; Ethylene polymerization
1. Introduction
ous chromium complexes coordinated by anionic ligands such
as N∧O (salicylaldiminato (A, Scheme 1) [6], amino-phenol 3,5-
(tBu)2-2-(OH)C6H2CH2NH(2,6-Me2C6H3) [7]), N∧N (imino-
pyrrolide [8], ␤-diketiminate [8b,9], bis(phosphoranimine)-
methanide [10]) and N∧N∧N (salicylaldiminato [6b]) have been
reported. Chromium complexes coordinated to various N∧N
(bis(imidazole) [11]), P∧P or P∧N∧P (bridged-diphosphine
(B and C, Scheme 1) [12]), P∧P∧P (triphosphacyclodode-
cane [13]), S∧N∧S (amine bis(thioether) [12c,12d,14]), C∧N∧C
(bis(carbine)pyridine (D, Scheme 1) [15]), N∧N∧O (2-acetyl-
6-iminopyridine (E, Scheme 1) [16], 2-(ethylcarboxylato)-
6-iminopyridine (F, Scheme 1) [17]), N∧N∧N∧N (tris(2-
pyridylmethyl)amine [18]) and N∧S∧N (iminodiphenylsulfide
[19]) ligands have also been investigated. When combined with
appropriate cocatalyst, some of those chromium complexes cat-
alyze ethylene to form polymeric and oligomeric products with
high catalytic activities. Chromium complexes coordinated to
N∧N∧N ligands, such as bis(imino)pyridines (G, Scheme 1)
[16,20], (2-pyridylmethyl)amines (H, Scheme 1) [21], tri-
azacyclohexane (I, Scheme 1) [22], bis(oxazolinyl)pyridine
(J, Scheme 1) [23] and 2,6-bis(benzimidazolyl)pyridines (K,
Scheme 1) [24], have attracted great attentions due to their
Olefin polymerization using transition-metal complex cat-
alysts has received great attention over the past decade
[1]. Especially, the development of chromium-based cata-
lysts for olefin oligomerization/polymerization has received
considerable attention [1a,1c,2]. In general, two classes of
chromium-based catalysts are commercially used: Phillips cat-
alyst [3] and Union Carbide catalyst [4]. The Phillips catalysts
are inorganic chromium catalysts supported on silica; there-
fore their active species have not been fully established due
to their heterogeneity [2b]. Recently, soluble chromium com-
plexes including Cp-based ligands [1a,1c,5] and other ligands
[1a,1c,2a] have been reported as catalysts for ethylene polymer-
ization. Cp-free complexes have begun to figure prominently
among chromium-based catalysts due to the fact that carefully
tuning of the ligand environment can tremendously affect the
catalytic activities and properties of products [1a,1c,2a]. Vari-
∗
Corresponding author. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.07.012

S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
175
Scheme 1. Representative chromium catalysts.
potential catalytic activities for ethylene oligomerization and
polymerization.
KBr pellets on a Bruker DMX-300 spectrometer. GC analyses
were performed with a Carlo Erba Strumentazione gas chro-
matograph equipped with a flame ionization detector and a 30 m
(0.2 mm i.d., 0.25 ␮m film thickness) DM-1 silica capillary col-
umn. The yield of oligomers was calculated by referencing with
the mass of the solvent. GC–MS analyses were performed with
HP 5890 SERIES II and HP 5971 SERIES mass detectors. 1H
NMR and 13C NMR of the polymer samples were recorded on a
Bruker DMS-300 instrument at 110 ◦C in 1,2-dichlorobenzene-
d4 using TMS as an internal standard.
Solvents were dried by the appropriate drying reagents and
distilled under nitrogen prior to use. Methylaluminoxane (MAO)
was purchased from Albemarle as a 1.4 M solution in toluene.
Modified methylaluminoxane (MMAO, 1.93 M in heptane) was
purchased from Akzo Corp. Triethylaluminum (2 M in toluene)
and diethylaluminum chloride (Et2AlCl, 1 M in hexane) were
purchased from Arcros Chemicals. All other chemicals were
obtained commercially and used without further purification
unless otherwise stated.
Recently, two different kinds of chromium complexes
containing 2,6-bis(benzimidazolyl) pyridines K [24b] and 2-
(ethylcarboxylato)-6-iminopyridines F [17] were investigated
for ethylene reactivity with moderate to high catalytic activities
in our group. The new achievement of designing and synthe-
sizing 2-imino-1,10-phenanthroline derivatives and their metal
complexes of iron(II), cobalt(II) and nickel(II) provides alter-
native model catalysts with high activities [25]. Subsequently,
it is necessary to investigate the scope of their chromium ana-
logues and their catalytic behavior. These chromium complexes
were synthesized by reactions of 2-imino-1,10-phenanthrolines
with chromium(III) trichloride. In the presence of methylalu-
minoxane (MAO), all the chromium complexes showed high
ethylene oligomerization activities and moderate polymeriza-
tion activities. Herein, the synthesis, characterization of these
chromium complexes are reported with their catalytic proper-
ties for ethylene activation investigated under various reaction
conditions.
2.2. Synthesis of Complexes 1–12
2. Experimental
2.2.1. General procedure
2.1. General
All the 2-imino-1,10-phenanthrolines used as ligands were
prepared according to our reported procedure [25]. A solution
of CrCl3·3THF in dichloromethane was dropwise added to a
solution of the ligand in dichloromethane. The color of the solu-
tion changed immediately and some precipitate was formed. The
reaction mixture was stirred at room temperature for 9 h and then
Allmanipulationsofairand/ormoisturesensitivecompounds
were carried out under a nitrogen atmosphere using standard
Schlenk techniques. Elemental analyses were performed on a
Flash EA 1112 microanalyzer. The IR spectra were obtained as

176
S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
C26H27Cl3CrN3 (539.87): C, 57.84; H, 5.04; N, 7.78. Found:
C, 57.55; H, 5.37; N, 7.79.
2.2.8. 2-Acetyl-1,10-phenanthroline(2,6-difluoroanil)CrCl3
(7)
Green powder in 70% yield. FT-IR (KBr disc, cm−1): 3053,
2969, 1605, 1531, 1460, 1413, 863, 709. Anal. Calc. for
C20H13Cl3CrF2N3 (491.69): C, 48.85; H, 2.66; N, 8.55. Found:
C, 48.66; H, 2.38; N, 8.35.
Scheme 2. The synthesis of complexes 1–12.
2.2.9. 2-Acetyl-1,10-phenanthroline(2,6-dichloroanil)
CrCl3 (8)
diethyletherwasadded. Thegreenprecipitatewascollectedwith
filtration, washed with diethyl ether and dried in vacuum. All the
complexes were prepared in high yield (Scheme 2).
Green powder in 86% yield. FT-IR (KBr disc, cm−1): 3059,
2960, 1607, 1529, 1462, 1402, 877, 780. Anal. Calc. for
C20H13Cl5CrN3 (524.60): C, 45.79; H, 2.50; N, 8.01. Found:
C, 45.69; H, 2.32; N, 7.74.
2.2.2. 2-Benzoyl-1,10-phenanthroline(2,6-dimethylanil)
CrCl3 (1)
Green powder in 83% yield. FT-IR (KBr disc, cm−1): 3047,
1591, 1542, 1487, 1450, 1405, 1294, 866, 702. Anal. Calc. for
C27H21Cl3CrN3 (545.83): C, 59.41; H, 3.88; N, 7.70. Found: C,
59.62; H, 3.65; N, 7.54.
2.2.10. 2-Acetyl-1,10-phenanthroline(2,6-dibromoanil)
CrCl3 (9)
Green powder in 97% yield. FT-IR (KBr disc, cm−1): 3058,
2965, 1609, 1522, 1457, 1405, 867, 779. Anal. Calc. for
C20H13Br2Cl3CrN3 (613.50): C, 39.15; H, 2.14; N, 6.85. Found:
C, 39.43; H, 2.22; N, 6.55.
2.2.3.
2-Benzoyl-1,10-phenanthroline(2,6-diethylanil)CrCl3
(2)
2.2.11. 2-Formyl-1,10-phenanthroline(2,6-dimethylanil)
CrCl3 (10)
Green powder in 82% yield. FT-IR (KBr disc, cm−1): 3057,
2962, 1602, 1544, 1489, 1441, 1403, 1294, 866, 785, 703. Anal.
Calc. for C29H25Cl3CrN3 (572.05): C, 60.69; H, 4.39; N, 7.32.
Found: C, 60.91; H, 4.30; N, 7.33.
Green powder in 84% yield. FT-IR (KBr disc, cm−1): 3060,
2922, 1605, 1506, 1464, 1442, 1405, 863, 776. Anal. Calc.
for C21H17Cl3CrN3·THF (541.84): C, 53.70; H, 3.65; N, 8.95.
Found: C, 53.54; H, 3.41; N, 9.26.
2.2.4. 2-Benzoyl-1,10-phenanthroline(2,6-diisopropylanil)
CrCl3 (3)
2.2.12. 2-Formyl-1,10-phenanthroline(2,6-diethylanil)
CrCl3 (11)
Green powder in 87% yield. FT-IR (KBr disc, cm−1): 3058,
2967, 1602, 1531, 1489, 1460, 1439, 1402, 1292, 871, 785, 701.
Anal. Calc. for C31H29Cl3CrN3 (601.94): C, 61.86; H, 4.86; N,
6.98. Found: C, 61.56; H, 4.70; N, 6.96.
Green powder in 69% yield. FT-IR (KBr disc, cm−1):
3051, 2972, 1604, 1507, 1458, 1406, 859. Anal. Calc. for
C23H21Cl3CrN3 (497.79): C, 55.49; H, 4.25; N, 8.44. Found:
C, 55.38; H, 3.96; N, 8.13.
2.2.5.
2-Acetyl-1,10-phenanthroline(2,6-dimethylanil)CrCl3
(4)
2.2.13. 2-Formyl-1,10-phenanthroline
(2,6-diisopropylanil)]CrCl3 (12)
Green powder in 85% yield. FT-IR (KBr disc, cm−1): 3056,
2967, 1602, 1511, 1455, 1403, 1293, 865, 775. Anal. Calc. for
C22H19Cl3CrN3 (483.76): C, 54.62; H, 3.96; N, 8.69. Found: C,
54.35; H, 4.28; N, 8.35.
Green powder in 78% yield. FT-IR (KBr disc, cm−1): 3061,
2957, 2867, 1607, 1511, 1461, 1407, 864. Anal. Calc. for
C25H25Cl3CrN3 (525.84): C, 57.10; H, 4.79; N, 7.99. Found:
C, 57.15; H, 4.80; N, 7.66.
2.2.6. 2-Acetyl-1,10-phenanthroline(2,6-diethylanil)CrCl3
(5)
2.3. General procedure for ethylene oligomerization and
polymerization
Green powder in 74% yield. FT-IR (KBr disc, cm−1): 3056,
2966, 1602, 1510, 1456, 1405, 1290, 862, 790. Anal. Calc. for
C24H23Cl3CrN3 (511.81): C, 56.32; H, 4.53; N, 8.21. Found: C,
56.26; H, 4.89; N, 7.97.
Ethylene oligomerization and polymerization at 1 atm of
ethylene pressure was carried out as follows: The catalyst pre-
cursorwasdissolvedintolueneinaSchlenktubeandthereaction
solutionwasstirredat1 atmofethylenewiththereactiontemper-
ature being controlled by water bath. The reaction was initiated
by adding the desired amount of cocatalyst. After the desired
period of time, the catalytic reaction mixture was quenched with
5% hydrochloric acid in an ice-water bath in accordance with
2.2.7. 2-Acetyl-1,10-phenanthroline(2,6-diisopropylanil)
CrCl3 (6)
Green powder in 83% yield. FT-IR (KBr disc, cm−1): 3055,
2968, 1604, 1519, 1460, 1410, 861, 770. Anal. Calc. for

S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
177
the oligomers of C4 and C6 produced. An analysis by gas chro-
matography (GC) was carried out to determine the distribution
of oligomers obtained. The remaining solution was quenched
with HCl-acidified ethanol (5%), and the precipitated polyethy-
lene was filtered, washed with ethanol, dried in a vacuum at
60 ◦C until constant weight.
˚
graphite-monochromated Mo K␣ radiation (λ = 0.71073 A).
Single-crystal X-ray diffraction studies for complex 12 were
carried out on a Rigaku RAXIS Rapid IP diffractometer with
˚
graphite-monochromated Mo K␣ radiation (λ = 0.71073 A).
Unit cell dimensions were obtained with least-squares refine-
ments. Intensities were corrected for Lorentz and polarization
effects and empirical absorption. The structures were solved
by direct methods, and refined by full-matrix least-square on
F2. Each hydrogen atom was placed in a calculated position,
and refined using a riding model. All non-hydrogen atoms
were refined anisotropically. Structure solution and refine-
ment were performed using SHELXL-97 package [26]. Crystal
data and processing parameters were summarized in Table 1.
CCDC-631242 (2), -631243 (4) and -631244 (12) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data request/cif.
Ethyleneoligomerizationathigherethylenepressurewascar-
ried out in a 250-mL autoclave stainless steel reactor equipped
with a mechanical stirrer and a temperature controller. Briefly,
toluene, the desired amount of cocatalyst and toluene solution of
the catalyst precursor (the total volume was 100 mL) was added
to the reactor in this order under an ethylene atmosphere. Reach-
ing the desired reaction temperature, ethylene with the desired
pressure was introduced to start the reaction, and the ethylene
pressure was kept by constant feeding of ethylene. After 1 h,
the reaction was stopped. The catalytic reaction mixture was
quenched with 5% hydrochloric acid in an ice-water bath in
accordance with the oligomers of C4 and C6 produced. Then the
solution was analyzed by gas chromatography (GC) for deter-
mining the distribution of oligomers obtained. The remaining
solution was quenched with HCl-acidified ethanol (5%), and
the precipitated polyethylene was filtered, washed with ethanol,
dried in a vacuum at 60 ◦C until constant weight.
3. Results and discussion
3.1. Synthesis of the chromium complexes (1–12)
The ligands were efficiently synthesized according to
our reported procedure [25a]. Their chromium complexes
were easily synthesized by the reaction of CrCl3·3THF
and corresponding ligands of 2-imino-1,10-phenanthrolines in
dichloromethane (Scheme 2). The resultant products were
2.4. X-ray crystal structural determination of 2, 4 and 12
Intensity data for complexes 2 and 4 were collected
on a Brucker SMART 1000 CCD diffractometer with
Table 1
Crystal data and structure refinement for 2, 4 and 12
2
4·CH2Cl2
12·2H2O
Empirical formula
Formula weight
Crystal color
C29H25Cl3CrN3
573.87
Green
C22H19Cl3CrN3·CH2Cl2
C25H25Cl3CrN3·2H2O
568.68
557.83
Green
Green
Temperature (K)
293(2)
293(2)
293(2)
˚
Wavelength (A)
0.71073
0.71073
0.71073
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Monoclinic
P21/c
˚
a (A)
10.3895(6)
17.8363(1)
14.6809(1)
92.275(4)
2718.4(3)
4
1.402
0.739
1180
8.0117(2)
24.9597(6)
12.4017(3)
94.8900(1)
2470.94(1)
4
1.529
1.021
1156
10.362(2)
11.376(2)
23.634(6)
108.30(3)
2645.0(1)
4
1.401
0.762
1148
˚
b (A)
˚
c (A)
β (◦)
3
˚
Volume (A )
Z
Dcalc(Mg m−3
)
μ (mm−1
F(0 0 0)
)
Crystal size (mm)
θ range (◦)
0.30 × 0.10 × 0.10
0.33 × 0.10 × 0.06
0.39 × 0.13 × 0.04
1.80–28.36
1.63–28.30
2.07–27.40
Limiting indices
−12 ≤ h ≤ 13,
−23 ≤ k ≤ 23,
−19 ≤ l ≤ 19
−10 ≤ h ≤ 10,
−33 ≤ k ≤ 31,
−15 ≤ l ≤ 16
0 ≤ h ≤ 13,
−14 ≤ k ≤ 0,
−30 ≤ l ≤ 28
Completeness to θ (%)
Absorption correction
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
98.8 (θ = 28.36)
Empirical
6723/0/325
99.9 (θ = 28.30)
Empirical
6139/0/289
97.5 (θ = 27.40)
Empirical
5871/0/297
0.796
1.109
1.012
R1 = 0.0574, wR2 = 0.1403
R1 = 0.1845, wR2 = 0.1842
0.623 and −0.346
R1 = 0.0363, wR2 = 0.1030
R1 = 0. 0477, wR2 = 0.1074
0.484 and −0.603
R1 = 0.0618, wR2 = 0.1668
R1 = 0.0942, wR2 = 0.1848
1.652 and −0.669
−3
˚
Largest diff peak and hole (e A
)

178
S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
isolated as green air-stable powders in good yields and character-
ized by IR spectra and elemental analysis. In the IR spectra, the
stretching vibration bands of C N of these chromium complexes
(1591–1609 cm−1) apparently shifted to lower wave numbers
and the peak intensity was greatly reduced, as compared to
the corresponding ligands (1618–1646 cm−1) [25a], which indi-
cates the coordination interaction between the imino nitrogen
atom and the metal cation. The molecular structures of com-
plexes 2, 4 and 12 were confirmed by the single-crystal X-ray
diffraction studies.
3.2. Molecular structures
Crystals of complexes 2, 4 and 12 suitable for X-ray struc-
ture determination were obtained through the slow diffusion of
diethyl ether into their dichloromethane solution. Analysis of
X-ray crystallography reveals that all complexes display the dis-
torted octahedronal geometry, in which the chromium center is
coordinated to the tridentate ligand and three chlorides. Their
selected bond lengths and angles are collected in Table 2, and
meanwhile their molecular structures are shown in Figs. 1 and 2.
In the molecular structure of complex 2 (Fig. 1), three
coordinated nitrogen atoms and three chlorides are situ-
ated around the chromium center in a meridianal manner.
The N1, N3, Cl1 and Cl3 atoms could be located in
an equatorial plane and the two axial bonds nearly form
a straight angle (N2 Cr Cl2, 178.48(1)◦). Most of bond
Fig. 1. Molecular structure of complex 2 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
angles in the equatorial plane are very close to a right
angle (N(1) Cr Cl(1), 86.02(1)◦; N(1) Cr Cl(3), 86.81(1)◦;
N(3) Cr Cl(1), 92.58(1)◦; N(3) Cr Cl(3), 91.80(1)◦). The
˚
axial Cr N(2) (phenanthrolinyl) bond length (1.993(3) A) is
˚
shorter about 0.11 A than the Cr N(1) (phenanthrolinyl) bond
˚
˚
length (2.101(3) A) and about 0.16 A than the Cr N(3) (imino)
˚
bond length (2.151(3) A). The Cr Cl bond lengths span a rel-
atively narrow range, with the Cr Cl(2) bond (2.2701(1) A)
˚
located at trans position to N(2) being shorter than the Cr Cl(1)
Table 2
◦
˚
Selected Bond Lengths (A) and Angles ( ) for 2, 4 and 12
2
4
12
Bond lengths
Cr N(1)
Cr N(2)
Cr N(3)
Cr Cl(1)
2.101(3)
1.993(3)
2.151(3)
2.3270(1)
2.2701(1)
2.3088(1)
1.320(5)
1.366(5)
1.337(5)
1.337(5)
1.295(5)
1.475(5)
2.1079(2)
1.9873(2)
2.1474(2)
2.3048(6)
2.2834(6)
2.3311(6)
1.325(3)
1.360(3)
1.327(2)
1.340(3)
1.297(2)
1.455(2)
2.101(3)
1.989(3)
2.158(3)
2.3319(1)
2.2731(1)
2.3160(1)
1.334(5)
1.369(5)
1.329(5)
1.332(5)
1.282(5)
1.454(4)
Cr Cl(2)
Cr Cl(3)
N(1) C(1)
N(1) C(12)
N(2) C(10)
N(2) C(11)
N(3) C(13)
N(3) C(14)
Bond angles
N(1) Cr N(2)
N(2) Cr N(3)
N(1) Cr N(3)
N(1) Cr Cl(1)
N(1) Cr Cl(2)
N(1) Cr Cl(3)
N(2) Cr Cl(1)
N(2) Cr Cl(2)
N(2) Cr Cl(3)
N(3) Cr Cl(1)
N(3) Cr Cl(2)
N(3) Cr Cl(3)
Cl(1) Cr Cl(2)
Cl(1) Cr Cl(3)
Cl(2) Cr Cl(3)
78.63(1)
75.90(1)
79.24(6)
76.00(6)
78.61(1)
76.26(1)
154.80(1)
86.91(1)
98.43(1)
89.56(1)
84.87(1)
176.85(1)
88.65(1)
92.62(9)
106.73(9)
88.06(1)
93.92(5)
173.12(4)
92.43(5)
154.52(1)
86.02(1)
101.39(1)
86.81(1)
85.32(1)
178.48(1)
88.49(1)
92.58(1)
104.09(1)
91.80(1)
93.16(5)
171.32(5)
93.03(5)
155.16(6)
86.67(5)
98.18(5)
86.76(5)
87.78(5)
177.32(5)
85.34(5)
90.47(4)
106.61(4)
93.08(4)
92.79(2)
171.26(2)
93.85(2)
Fig. 2. Molecular structures of complexes 4 and 12 with thermal ellipsoids at
the 30% probability level. Hydrogen atoms and solvent have been omitted for
clarity.

S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
179
Fig. 4. The different dihedral angles between the coordination plane and the
phenyl plane. (a) 82.2◦ in 2; (b) 106.5◦ in 4; (c) 65.9◦ in 12.
154.80(1)◦, respectively. The structure is similar to that
of 2 and 4. The observed similarities include the short-
˚
est Cr N(2)(phenanthrolinyl) bond (1.989(3) A) compared
˚
with Cr N(1)(phenanthrolinyl) bond (2.101(3) A) and
Fig. 3. A view of the crystal packing for complex 2, projected along the a-axis.
˚
Cr N(3)(imino) bond (2.158(3) A). The Cr Cl(2) bond (oppo-
site N(2)) is the shortest of the three Cr Cl bonds. The most
noticeable differences are the dihedral angle (65.9◦) between
the coordination plane and the phenyl plane (Fig. 4) and
the shorter imino N(3) C(13) bond length (1.282(5) A). The
differences are probably due to the less bulky hydrogen on the
imino-C atom.
˚
˚
(2.3270(1) A) and Cr Cl(3) (2.3088(1) A) bond at cis posi-
tion. The similar phenomenon was observed with the reported
bis(imino)pyridine chromium complexes [16,20]. The imino
˚
˚
N(2) C(13) bond length is 1.295(5) A with the typical char-
acter of a C N double bond. The atoms of 1,10-phenanthroline
ring and C(13), N(3) of the imino group as well as the chromium
atom form a perfect plane (the mean deviation is 0.0509 A), with
the largest deviation from the plane being 0.1073 A at N(3). The
phenyl ring linking to the imino group is oriented nearly per-
pendicularly to the coordination plane with the dihedral angle
of 82.2◦. Interestingly, the C6 central rings of phenanthroline
between two adjacent chromium moieties interact in a “face-
to-face” fashion with the relevant plane-to-plane distance of
3.390 A and center-to-center distance of 3.508 A. It is notewor-
thy that the molecular packing of 2, viewed from the a-axis, is
in a perfect stacking (Fig. 3). The well-stacked structure in the
solid state is formed by self-assembly of molecules, presumably
driven by intermolecular ␲–␲ stacking and hydrogen bonds.
Similarobservationwasobtainedintheirnickelanalogues [25b].
In complex 4, the chromium atom is quasi-octahedral,
the three coordinated chlorides dispose mer in the coordina-
tion sphere. It displays nearly equivalent N(2) Cr Cl angles
(N(2) Cr Cl(1) = 87.78(5)◦ and N(2) Cr Cl(3) = 85.34(5)◦).
The presence of a methyl substituent on the imino-C atom
instead of the phenyl group and that of methyl groups on the
imino-N phenyl ring instead of ethyl groups confers no obvious
influence on the molecular geometry, which can be observed
from the similar bond lengths and angles of complexes 2 and
4. The Cr N and Cr Cl bond distances in complex 4 have a
similar trend to those in complex 2: the Cr N(2) bond distance
is the shortest among the three Cr N bonds and the Cr Cl(2)
bond length is also the shortest among the three Cr Cl bonds.
˚
3.3. Ethylene oligomerization and polymerization at
ambient pressure
˚
The chromium complexes were initially studied for their
catalytic activities of ethylene oligomerization and polymer-
ization at ambient pressure (Table 3). Complex 2 was used to
select suitable reaction parameters. When combined with vari-
ous organo-aluminum compounds such as methylaluminoxane
(MAO), modified methylaluminoxane (MMAO), triethylalu-
minum (Et3Al) and diethylaluminum chloride (Et2AlCl), it
showed different catalytic performance. The catalytic sys-
tem with Et3Al or Et2AlCl only produced butenes in
lower oligomerization activities. However, complex 2 with
MAO or MMAO oligomerized ethylene to obtain oligomers
following the Schulz–Flory rule with remarkably higher activ-
ities. Further investigations were carried out with MAO as
cocatalyst.
˚
˚
3.3.1. Effects of reaction parameters
The influences of the Al/Cr molar ratio and reaction tempera-
ture on ethylene reactivity were studied with the 2/MAO system.
Increasing the Al/Cr molar ratio from 300 to 1500 led to great
enhancement of ethylene oligomerization activities (entries 5–8,
Table 3). A further increase in the Al/Cr molar ratio to 2000
resulted in decreased oligomerization activity (entry 9, Table 3).
The decrease in oligomerization activity might be caused by
the impurities in commercial MAO such as alkyl aluminum,
which led to the deactivation of active catalytic sites. However,
the polymerization activities were not strongly affected by the
Al/Cr molar ratio. The distribution of oligomers and the ␣-olefin
selectivity were also significantly affected by the Al/Cr molar
ratio. Larger amounts of longer chain oligomers were produced
and decreased selectivity for ␣-olefins was observed at higher
Al/Cr molar ratios.
˚
˚
Both the mean (0.017 A) and the largest (0.0721 A) deviations
of the plane formed by 1,10-phenanthroline, C(13), N(3) and Cr
atoms are smaller than those in complex 2, which is probably
due to the less bulky methyl group (C(22)). The different dihe-
dral angles between the phenyl plane on the imino-N and the
coordination plane in 2 (82.2◦) and 4 (106.5◦) is probably due
to the less bulky methyl group on the imino-C atom.
In complex 12, the Cl Cr Cl and N Cr N angles are
93.92(5)◦, 92.43(5)◦, 173.12(4)◦ and 78.61(1)◦, 76.26(1)◦,

180
S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
Table 3
Ethylene reactivity at ambient pressure with 1–12a
e
f
Entry
Complex
Cocat
Al/Cr
Tb (◦C)
tc (min)
Oligomer distributiond (%)
Ao
Ap
ꢀ
ꢀ
ꢀ
C4/
C
C6/
C
≥C8/
C
␣-Olefin (%)d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
3
4
5
6
7
8
9
10
11
12
Et3Al
Et3Al
Et2AlCl
MMAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
500
1000
500
1000
300
20
20
20
20
20
20
20
20
20
0
40
60
20
20
20
20
20
20
20
20
20
20
20
20
20
20
60
60
60
60
60
60
60
60
60
60
60
60
30
90
120
60
60
60
60
60
60
60
60
60
60
60
100
100
100
–
–
–
–
–
–
16.0
4.3
–
–
–
0.03
0.27
0.30
2.11
2.16
2.56
7.50
7.91
5.32
2.11
7.42
7.35
9.75
7.29
7.01
4.36
0.71
1.34
1.72
0.45
4.73
5.31
6.16
1.12
1.89
7.35
No
No
No
Trace
0.8
2.1
1.9
2.2
2.0
Trace
2.7
1.4
–
64.2
19.8
25.1
26.5
28.4
43.8
55.1
22.2
40.4
37.4
24.2
28.4
33.2
23.6
10.5
25.8
24.1
8.8
>90
>97
>96
>90
>80
>79
>90
>78
>79
>91
>94
>93
>95
>85
>90
>96
>93
>90
>91
>93
>91
>92
>95
70.6
69.2
50.4
40.4
24.9
74.1
39.2
32.0
60.6
51.4
43.2
72.4
85.1
49.2
55.4
83.7
85.8
33.8
31.9
43.0
45.1
42.1
500
4.3
1000
1500
2000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
21.2
15.8
20.0
3.7
20.4
30.6
15.2
20.2
23.6
4.0
–
–
0.8
0.6
2.1
2.1
1.4
2.1
1.6
1.7
1.0
Trace
Trace
4.3
24.9
20.5
7.5
11.3
40.4
26.5
34.6
26.5
31.1
2.9
25.8
41.6
22.4
28.4
26.8
a
Conditions: 5 ␮mol of catalysts; solvent: toluene (30 mL).
Reaction time.
Reaction temperature.
b
c
d
e
f
Determined by GC.
Oligomer activity: 105 g mol−1 (Cr) h−1
.
Polymer activity: 104 g mol−1 (Cr) h−1
.
The butenes have low boiling points and potentially affect the
responding methyl-ketimine (R = Me) complexes. This was a
completely reversed observation compared with iron(II) ana-
logues bearing the same ligands [25a]. Furthermore, the R
substituent had different influences on the catalytic perfor-
mance of methyl- or phenyl-ketimine and aldimine analogues.
The phenyl-ketimine complex 2 with 2,6-diethyl groups and
the aldimine complex 12 with 2,6-diisopropyl groups showed
much higher oligomerization activity than their correspond-
ing analogues. However, for the methyl-ketimine complexes,
no obvious difference was observed between 4 (with Me sub-
stituents) and 5 (with Et substituents). The ␣-olefin selectivity
and polymerization activities were not greatly affected by the R
group.
fractional pressure of ethylene over catalytic time course; how-
ever, the high solubility of butenes in toluene will keep produced
butenes in solution. In fact, oligomerization and polymerization
proceed with only solved ethylene, then coordination with active
sites in the catalytic system. The resultant observation was the
catalytic activity decreasing along with the prolong reaction time
(entries 7, 13–15, Table 3), and meanwhile higher order olefins
were obtained.
Elevating the reaction temperature from 0 to 20 ◦C speeded
the combined catalytic reactions of oligomerization and poly-
merization. No significant variation was observed by elevating
the reaction temperature to 60 ◦C, indicating the stability of
the catalysts at higher temperature. However, lower ␣-olefin
selectivity was obtained at higher reaction temperature. A sim-
ilar influence of temperature on the oligomers produced with
chromium-based systems had been reported [24].
The R1 substituents on the imino-N aryl ring had a great
influence on the catalytic performance of both ketimine and
aldiminecomplexes. Forphenyl-ketiminecomplexes1–3, some-
what reduced catalytic activity was observed for the sterically
bulkier catalysts. This was clearly revealed by comparing the
2,6-diisopropyl substituted 3 with 2,6-dimethyl substituted 1
or 2,6-diethyl substituted 2 (entry 17 versus entry 16 or 7,
Table 3). The same trend was also observed for the methyl-
ketimine complexes 4–6 (entries 18–20, Table 3). The more
bulky isopropyl groups at the ortho-positions of the imino-N aryl
3.3.2. Effect of the ligand environments
The variation of the R substituent on the imino-C of lig-
ands, 2-(ArN = CR)-1,10-phen, resulted in changes of catalytic
performance. Aldimine (R = H) and phenyl-ketimine (R = Ph)
complexes showed relatively higher activities than the cor-

S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
181
ring and R (R = Ph, Me) on the imino-C may prevent the inser-
tion of ethylene to the catalytic sites, therefore leading to lower
catalytic activity. Complexes 7–9, containing 2,6-dihalogen-
substituted ligands, exhibited relatively higher oligomerization
activities than complexes 4–6 bearing alkyl groups (entries
21–23, Table 3). Aldimine complexes 10–12 also displayed high
activities for ethylene oligomerization along with considerable
activities for ethylene polymerization (entries 24–26, Table 3).
Based on these data, their oligomerization activities varied in
the orders of 10 (with Me) < 11 (with Et) < 12 (with i-Pr), which
was different from ketimine complexes. This could be possibly
caused by the less bulkiness of hydrogen on the imino-C. In
addition, there was no regular influence of ligand environment
on the polymerization activities and ␣-olefin selectivity.
alytic activities could be reached with increasing pressure of
ethylene. At 10 atm of ethylene, the catalytic activities for both
oligomerization and polymerization increased by nearly one
order of magnitude. The higher ethylene concentration increased
the chain propagation leading to longer chain oligomers up to
the range of C4 to C28 along with improved activities.
According to observed data (entries 1–4, Table 4 with 10 atm
of ethylene), elevation of the temperature from 20 to 80 ◦C
resulted in an increased catalytic activities and K values. How-
ever, lower selectivity of ␣-olefins was obtained at higher
reaction temperature because of the faster chain transformation.
In general, the suitable active sites were formed at elevated tem-
perature up to 80 ◦C. Due to the high activity and acceptable
␣-olefin selectivity, further investigations were carried out at
80 ◦C.
3.4. Ethylene oligomerization and polymerization at 10 atm
of ethylene
3.4.2. Effect of ligands environment
Changes in the ligand environments [2-(ArN = CR)-1,10-
phen] of these systems led to different catalytic behavior.
Changing the substituent on the imine functional group from
ketimine (R = Ph, 1–3; R = Me, 4–9) to aldimine (R = H,
10–12) resulted in the differences in productivity, K value and
␣-olefin selectivity. Methyl-ketimine catalysts were less pro-
ductive than their phenyl-ketimine and aldimine analogues.
The phenyl-ketimine precatalyst 2 gave the highest activity
of 1.15 × 107 g mol−1 (Cr) h−1 (entry 4, Table 4). The corre-
sponding aldimines 10–12 displayed productivities in the range
(2.73–5.00) × 106 g mol−1 (Cr) h−1 (entries 13–15, Table 4).
However, the highest activity obtained from methyl-ketimine
was 2.14 × 106 g mol−1 (Cr) h−1 of 4 (entry 7, Table 4). Fur-
thermore, the phenyl-ketimine complexes displayed larger K
value and methyl-ketimine complexes showed higher ␣-olefin
selectivity.
Upon activation with methylaluminoxane (MAO), all the
chromium complexes exhibited high catalytic activities for
oligomerization and moderate activities for polymerization at
10 atm of ethylene. The detailed results are listed in Table 4. The
distribution of olefin oligomers resembled the Schulz–Flory rule
and was characterized by the constant K, where K represented
the probability of chain propagation (K = rate of propaga-
tion/((rate of propagation) + (rate of chain transfer)) = (moles of
C
n+2)/(moles of Cn)) [27]. The K values were determined by the
molar ratio of C14 and C12 fractions.
3.4.1. Effects of reaction parameters
Apparently, the ethylene concentration significantly affected
the catalytic behavior of all the complexes. Comparing with the
catalytic behavior at ambient pressure (Table 3), the higher cat-
Table 4
Ethylene reactivity at 10 atma
Tb (◦C)
Oligomers
Ap
e
Entry
Complex
ꢀ
ꢀ
ꢀ
c
Ao
K
C4/
C
C6/
C
≥C8/
C
␣-Olefin (%)d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
2
2
2
2
1
3
4
5
6
7
8
9
10
11
12
20
40
60
80
80
80
80
80
80
80
80
80
80
80
80
1.76
2.42
5.15
11.5
6.66
0.46
2.14
2.02
1.69
1.57
1.72
1.98
2.73
4.29
5.00
0.38
0.38
0.42
0.48
0.58
0.74
0.34
0.40
0.30
0.50
0.43
0.55
0.36
0.38
0.27
32.7
32.4
17.6
20.9
15.3
31.3
26.7
34.7
28.2
39.2
40.5
29.1
29.8
18.3
30.6
42.8
47.4
46.3
38.6
31.2
16.4
44.0
36.7
41.2
30.4
35.2
31.2
34.0
45.3
40.2
24.5
20.2
36.1
40.5
53.5
52.3
29.3
28.6
30.6
30.4
24.3
39.7
36.2
36.4
29.2
>96
>96
>92
>87
>86
>86
>89
>93
>91
>88
>78
>80
>89
>84
>79
2.2
1.0
2.7
4.1
3.6
4.5
3.4
3.6
3.1
2.6
2.8
2.5
1.9
1.7
Trace
a
Conditions: 5 ␮mol of catalysts; cocatalyst: MAO; Al/Cr = 1000; reaction time: 1 h; solvent: toluene (100 mL).
b
c
d
e
Reaction temperature.
Activity for oligomers: 106 g mol−1 (Cr) h−1
.
% ␣-olefin content determined by GC and GC–MS.
Activity for polymers: 105 g mol−1 (Cr) h−1
.

182
S. Zhang et al. / Journal of Molecular Catalysis A: Chemical 276 (2007) 174–183
At 10 atm of ethylene, the influence of the substituent on the
lower activity; however, reversed results were observed for the
aldimine complexes. In addition, the substituents on the N-aryl
ring imino-N aryl showed no significant effect for the methyl-
ketimine complexes.
imino-N aryl ring on the oligomerization activity was similar to
the results obtained at 1 atm of ethylene. The phenyl-ketimine
complex 2 with ethyl groups on the imino-N aryl ring displayed
the highest oligomerization activity among its corresponding
analogues (entry 4, Table 4). For aldimine complexes, it was
observed that an increase in the steric hindrance of the R1 group
led to an enhanced activity (entries 13–15, Table 4); however,
no significant effect was noticed for methyl-ketimine complexes
(entries 7–9, Table 4). The introduction of electron-withdrawing
halogen groups to the methyl-ketimine complexes resulted in
a little influence on the oligomerization activities, which were
different from the results at 1 atm of ethylene. The complexes
containing halogen groups gave the higher K values and lower
␣-olefin selectivity. In addition, the polymerization activities
were also affected by the substituents on the imino-N aryl ring.
For the phenyl-ketimine complexes 1–3, somewhat increased
activity was observed for the sterically bulkier catalyst system
(entries 4–6, Table 4), which was different from the aldimine
complexes10–12(entries13–15, Table4). Complexes4–6, bear-
ing alkyl groups, displayed higher polymerization activities than
their analogues bearing halogen groups.
Acknowledgements
This work was supported by NSFC No. 20674089 and MOST
2006AA03Z553. We are grateful to Prof. Dr. Takashi Tatsumi
for his kindly editorial and English corrections.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.molcata.2007.07.012.
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