Chromium complexes ligated by amino-salicylaldimine ligands: synthesis, structures and ethylene oligomerisation behaviour

Article abstract of DOI:10.3184/030823410X12628736648105

A series of amino-salicylaldimine ligands, 5-tert-butyl-3-(R-1-ylmethyl)- salicylaldimine (L1-4, R = morpholine, piperidine, pyrrolidine, 4-methylpiperazine) have been synthesised and structurally analysed. The molecular structure of compound (L1, R = piperidine) has been determined by single-crystal X-ray diffraction. These ligands reacted with [CrCl ₃(THF)₃] in THF to form chromium complexes, which were isolated in high yields as stable green solids and characterised by means of IR and elemental analysis. Activated with methylaluminoxane (MAO), these complexes showed moderate catalytic activities for ethylene oligomerisaton.

Full text of DOI:10.3184/030823410X12628736648105

JOURNAL OF CHEMICAL RESEARCH 2010
RESEARCH PAPER 41
JANUARY, 41–43
Chromium complexes ligated by amino-salicylaldimine ligands:
synthesis, structures and ethylene oligomerisation behaviour
Jin Cui and Mingjie Zhang*
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, P.R.China
A series of amino-salicylaldimine ligands, 5-tert-butyl-3-(R-1-ylmethyl)-salicylaldimine (L1-4, R = morpholine, piperi-
dine, pyrrolidine, 4-methylpiperazine) have been synthesised and structurally analysed. The molecular structure of
compound (L1, R = piperidine) has been determined by single-crystal X-ray diffraction. These ligands reacted with
[CrCl3(THF)3] in THF to form chromium complexes, which were isolated in high yields as stable green solids and
characterised by means of IR and elemental analysis. Activated with methylaluminoxane (MAO), these complexes
showed moderate catalytic activities for ethylene oligomerisaton.
Keywords: amino-salicylaldimine, chromium(III) complexes, ethylene oligomerisation
Olefin polymerisation using transition-metal complex
catalysts has received great attention over the past decade.1
Especially, the development of chromium-based catalysts
for olefin oligomerisation/polymerisation has received con-
siderable attention.2 In general, two classes of chromium-based
catalysts are commercially used: Phillips catalyst3 and Union
Carbide catalyst.4 Recently, various chromium complexes
coordinated by anionic ligands such as NO (salicylaldiminato,
amino-phenol),5,6 NN (imino-pyrrolide, β-diketiminate, bis(ph
osphoranimine)methanide)7,8,9 and NNN (salicylaldiminato,
bis(imino)pyridines, (2-pyridylmethyl)amines, triazacyclo-
hexane)10,11,12,13 have been reported. Chromium complexes
coordinated to various symmetrical ligands such as PNP
(bridged-diphosphine),14 PPP (triphosphacyclododecane),15
SNS [amine bis(thioether)],16 CNC [bis(carbine)pyridine]17
and NSN (iminodiphenylsulfide)18 ligands have also been
investigated. When combined with appropriate cocatalyst,
some of those chromium complexes catalyse ethylene poly-
merisation to form oligomeric products with high catalytic
activity.
of one equivalent of diisopropylaniline with one equivalent
of 5-tert-butyl-3-(R-1-ylmethyl)-salicylaldehyde19 in ethanol,
yielding yellow crystals or oily products after purified through
column chromatography on Al2O3. All the ligands were well
characterised by IR spectrometry, 1H NMR, elemental analysis
and an X-ray structure of the representative compound (L2,
R = piperidine). A perspective view of the molecular structure
of L2 with atom numbering scheme is shown in Fig. 1. The
yellow crystal of L2, suitable for X-ray diffraction analysis
was obtained by slow evaporation of a saturated ethanol
We report here a series of new amino-salicylaldimine ligands
that have been designed and synthesised. The chromium com-
plexes were obtained by reaction of amino-salicylaldimine
with [CrCl3(THF)3]. In the presence of methylaluminoxane
(MAO), all the chromium complexes showed moderate
ethylene oligomerisation activities. Here the synthesis, charac-
terisation of the chromium complexes are reported with their
catalytic properties for ethylene activation investigated under
various reaction conditions.
The synthetic procedures for ligands 5-tert-butyl-3-(R-1-
ylmethyl)-salicylaldimine (L1-4, R = morpholine, piperidine,
pyrrolidine, 4-methylpiperazine) and their chromium com-
plexes are shown in Scheme 1. Ligands were conveniently pre-
pared in good yields (91–95%) by the condensation reaction
Fig. 1 Molecular structure of ligand 2. Hydrogen atoms are
omitted for clarity.
Scheme 1 Synthesis of amino-salicylaldimine ligands L1-4 and their Cr (III) complexes C1-4.
* Correspondent. E-mail: Mjzhangtju@163.com

42 JOURNAL OF CHEMICAL RESEARCH 2010
solution at room temperature. In the molecular structure of
L2 (Fig. 1), the imino group in the solid state is in the E
conformation with the typical imino C=N double-bond length
of 1.281(2) Å. The dihedral angle between phenoxy and
phenylimino rings is 85.43° (or 89.37°). The piperidine ring
possesses a stable chair conformation.
The chromium complexes C1–C4 were prepared by treating
a THF solution of [CrCl (THF) ]20 with the corresponding
ligands L1–L4 under refl3ux. Th3e obtained complexes were
precipitated from the reaction solution by adding ethyl ether
and separated as green powders in reasonable yields. These
complexes showed high stability in both solution and solid and
have been identified by FT-IR and elemental analysis. The IR
spectra of the ligands showed that the C=N stretching frequen-
cies appeared in the range of 1619–1624 cm−1, while the C=N
stretching vibrations shifted toward lower frequency, giving
bands between 1600 and 1609 cm−1 with weak intensities
for complexes C1–C4. The results indicate an effective coordi-
nation interaction between the imino nitrogen atom and the
chromium centre.
To probe the effects of reaction parameters on the ethylene
reactivity, oligomerisation behaviour was typically investi-
gated via changing the amount of Al/Cr, reaction temperature
and reaction time. The oligomers range from C4 to C18
with good selectivity for α-olefins, and the results are given in
Table 1. Guided by the experience of the catalytic system with
MAO, the various Al/Cr molar ratios were studied with com-
plex C1 at 30 atm of ethylene. When the Al/Cr molar ratio was
varied from 300 to 1500 (entries 1–4), the catalytic activities
initially increased and then decreased, with the optimum
activity being at an Al/Cr molar ratio of 1000 (entry 3). This
may be a consequence of MAO scavenging adventitious water
and impurities in the solvent, and attained the highest value
at an Al/Cr ratio of 1000. Increasing the Al/Cr molar ratio to
1500 led to lower activity.
As the ethylene oligomerisation is a highly exothermic reac-
tion, the reaction temperature significantly affects the catalytic
activity. The effect of temperature on oligomerisation activity
was investigated (entry 5). Elevation of the reaction tempera-
ture from 40 to 60 °C resulted in a decrease of catalytic activ-
ity, which might be caused by instability of the active species
or a lower concentration of ethylene in the reaction solution.
The proportion of C4 increased at higher temperature because
β-hydrogen elimination was faster than ethylene propagation.
The catalytic activity decreased along with the prolonged
reaction time and meanwhile higher order olefins were obtained
(entry 6).
(entries 7–9). Changing the cyclic amine substituents on the
phenol ring resulted in little differences in productivity and
α-olefin selectivity. C4 catalyst gave the highest activity
of 2.97 × 105 g mol−1(Cr)h−1. Their corresponding amino-
salicylaldimine chromium complexes displayed productivities
in the range of 2.43 × 105–2.71 × 105 g mol−1(Cr) h−1.
In summary, a series of new amino-salicylaldimine ligands
have been successively synthesised and characterised,
and consequently used to form their chromium complexes.
The molecular structure of the 5-tert-butyl-3-(piperidin-1-
ylmethyl)-salicylaldimine ligand has been analysed. Upon
activation with MAO, under mild conditions, the chromium
complexes have moderate catalytic activities in the range of
2.43 × 105 to 2.97 × 105 g mol−1(Cr)h−1, in producing α-olefin
(C4-C18) with good selectivity.
Experimental
Solvents were refluxed over an appropriate drying agent then distilled
and degassed prior to use. All other chemicals were obtained com-
mercially without purification unless stated otherwise. 5-tert-butyl-
3-(R-1-ylmethyl)-salicylaldehyde precursors and [CrCl3(THF) ] were
prepared according to the established procedures.20 IR spectr3a were
obtained as KBr pellets on a Perkin-Elmer FTIR 2000 Spectrometer.
Elemental analyses were performed on a Flash EA 1112 micro-
1
analyser. H NMR spectral data were recorded on a Varian Inova
500 MHz spectrometer using TMS as internal standard and CDCl as
solvent. The distribution of oligomers obtained was measured o3n a
Varian VISTA 6000GC spectrometer and the yield of oligomers was
calculated by referencing to the mass of the solvent on the basis of
the prerequisite that the mass of each fraction was approximately
proportioned to its integrated area in the GC trace.
Synthesis of ligands
2,6-diisopropylaniline (0.01mol) was added to a solution of 5-tert-
butyl-3-(R-1-ylmethyl)-salicylaldehyde (0.01mol) in ethanol (30ml)
and heated to reflux for 4 hours. TLC analysis showed completed con-
version of starting materials. The solvent was removed under reduced
pressure and the resulting yellow oil was eluted with petroleum ether/
ethyl acetate (5:1) on an alumina column.
(E)-4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-6-(morph
olinomethyl)}phenol (L1): Yellow oil, yield 95%. IR (KBr, cm−1):
1619 (C=N). 1H NMR (500 MHz, CDC13): δ 8.278 (s, 1H,Ar-CH=N),
7.534–7.539 (d, J = 2.5Hz, 1H, H-Ar-CH=N), 7.320–7.325 (d,
J = 2.5Hz, 1H, H-Ar-CH=N), 7.204 (s, 3H, C=NArH), 3.792–3.810(t,
J = 4.5Hz, 4H, NCH2CH O), 3.694 (s, 2H, Ar-CH2N), 2.933–2.995
(m, 2H, Ar(CH(CH3)2)2),22.619 (s, 4H, NCH2CH2O), 1.384 (s, 9H,
ArC(CH3)3), 1.190–1.203 (d, J = 6.5Hz, 12H, Ar(CH(CH3)2))2. Calcd
for C28H40N2O2: C, 77.02; H, 9.23; N, 6.42. Found: C, 77.15; H, 9.30;
N, 6.53%. (E)-4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-
6-(piperidin-1-ylmethyl})phenol l(L2): Yellow crystal, yield 94%. IR
The catalytic behaviour of these tridentate chromium
complexes with MAO is rooted in their ligands with different
substituents. In the presence of MAO as cocatalyst, complexes
C1–4 showed moderate activities for ethylene oligomerisation
1
(KBr, cm−1): 1621 (C=N). H NMR (500 MHz, CDC13): δ 8.410 (s,
1H, ArCH=N), 7.560 (s, 1H, H-ArCH=N), 7.451 (s,1H, H-ArCH=N),
7.183–7.217 (m, 3H, C=NArH), 3.742 (s, 2H, ArCH2N), 3.028–3.078
(m, 2H, Ar(CH(CH3)2)2), 2.587 (m, 4H, NCH2CH2CH2), 1.690–1.711
Table 1 Ethylene oligomerisation with 1-4/MAO systemsa
Entry
Cat.
Al/Ni
T/°Cb
T/minc
Oligomer distributiond (wt%)
Activitye
C4/∑C
C6/∑C
C8/∑C
≥C10/∑C
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
2
3
4
300
500
40
40
40
40
60
40
40
40
40
30
30
30
30
30
60
30
30
30
45.1
39.2
31.3
25.3
37.7
30.1
38.4
42.5
34.1
19.8
24.2
33.6
35.3
30.4
24.1
28.7
28.4
30.8
8.4
12.3
13.1
11.5
12.2
16.3
13.1
14.2
14.6
26.7
24.3
22.0
27.9
19.7
29.5
19.8
14.9
20.5
1.23
1.77
2.55
2.11
0.93
1.97
2.71
2.43
2.97
1000
1500
1000
1000
1000
1000
1000
aConditions: 5µmol of catalyst; toluene (100ml); 30atm ethylene.
bReaction temperature.
cReaction time. dDetermined by GC. eOligomer activity: 105 g mol−1 (Cr) h−1.

JOURNAL OF CHEMICAL RESEARCH 2010 43
Table 2 Crystallographic parameters
Ligand 2
cm−1): 1600 (C=N). Calcd for C28H39Cl CrN2O: C, 58.19; H, 6.80; N,
4.85. Found: C, 58.26; H, 6.88; N, 4.913%
[4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-6-((4-
methylpiperazin-1-yl)methyl)}phenol]CrCl3 (C4): Green solid, Yield
81%. IR (KBr, cm−1): 1604 (C=N). Calcd for C H42Cl3CrN3O: C,
57.38; H, 6.97; N, 6.92. Found: C, 57.45; H, 7.02; 2N9 , 6.98%
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C29 H42 N2 O
434.65
113(2)
0.71073
Procedure for ethylene oligomerisation
Triclinic
Ethylene oligomerisation at 30 atm ethylene pressure was carried out
in a 250-mL autoclave stainless steel reactor equipped with a mechan-
ical stirrer and a temperature controller. Toluene, the desired amount
of cocatalyst and a toluene solution of the catalyst precursor (the total
volume was 100 mL) was added to the reactor in this order under an
ethylene atmosphere. Upon reaching 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 30 min, the reaction was stopped. The catalytic reaction mixture
was quenched with HCl-acidified ethanol (5%) in an ice-water bath
in accordance with the oligomers produced. Then the solution was
analysed by gas chromatography for determining the distribution of
oligomers obtained.
P-1
a (Å)
10.700(2)
14.447(3)
17.502(4)
77.83(3)
b (Å)
c (Å)
α (°)
β (°)
86.63(3)
88.34(3)
γ (°)
Vol.(Å3)
2639.8(9)
Z
4
Dcalc(Mg m−3)
1.094
µ(mm−1)
0.065
Reflections collected/unique [Rint]
F(000)
19552/9261
952
Crystal size (mm)
θ Range for data collection (°)
Data/restraints/parameters
Final R indices [I > 2σ(I)]
R indices (all data)
Goodness-of-fit on F2
Largest differences in peak and
hole(eÅ−3)
0.26 × 0.24 × 0.16
1.44–25.02
9261/0/594
R1 = 0.0567, WR2 = 0.1527
R1 = 0.0739, WR2 = 0.1671
1.037
X-ray crystallographic studies
Ligand L2 intensity data sets were collected at 113(2) K on a Rigaku
RAXIS Rapid IP diffractometer with graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). Cell parameters were obtained by
the global refinement of the positions of all collected reflections.
Intensities were corrected by Lorentz and polarization effects and
empirical absorptions were applied. The structure was solved by
direct method, and refined by full-matrix least-squares on F2 using
SHELXS-97 package [G. M. Sheldrick, SHELXTL Version 5.1,
Bruker Analytical X-rayInstruments Inc, Madison, WI, USA, 1998].
0.342 and –0.234
(t, 4H, NCH CH2CH2), 1.514 (m, 2H, NCH2CH CH2), 1.379 (s, 9H,
ArC(CH3)3), 21.209–1.223 (d, J = 6.5Hz, 12H, Ar2(CH(CH3)2) ). Calcd
for C29H42N2O: C, 80.13; H, 9.74; N, 6.44. Found: C, 79.98;2H, 9.68;
N, 6.49%
Received 13 October 2009; accepted 30 November 2009
Paper 090826 doi: 10.3184/030823410X12628736648105
Published online: 22 January 2010
(E)-4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-6-(pyrro-
lidin-1-ylmethyl}- phenol (L3): Yellow crystals, yield 91%. IR (KBr,
1
cm−1): 1620 (C=N). H NMR (500 MHz, CDC13): δ 8.399 (s, 1H,
ArCH=N), 7.578 (s, 1H, H-ArCH=N), 7.453 (s, 1H, H-ArCH=N),
7.199–7.211 (m, 3H, C=NArH), 3.752 (s, 2H, ArCH N), 2.981–3.023
(m, 2H, Ar(CH(CH ) )2), 2.584 (m, 4H, NCH2CH2), 1.702(m, 4H,
NCH CH2), 1.388 (3s,2 9H, ArC(CH3)3), 1.198-1.2121 (d, J = 6.5Hz,
12H, 2Ar(CH(CH3) )2). Calcd for C28H N O: C, 79.95; H, 9.59; N,
6.66. Found: C, 792.83; H, 9.51; N, 6.634%0 . 2(E)-4-tert-butyl-2-{(2,6-dii
sopropylphenylimino)methyl)-6-((4-methylpiperazin-1-yl)methyl)}
phenol (L4): Yellow oil, yield 95%. IR (KBr, cm–1): 1624 (C=N). 1H
NMR (500 MHz, CDC13): δ 8.312 (s, 1H, ArCH=N), 7.533 (s, 1H,
H-ArCH=N), 7.302 (s, 1H, H-ArCH=N), 7.182 (s, 3H, C=NArH),
3.786 (s, 2H, ArCH2N), 2.977–3.005 (m, 2H, Ar(CH(CH3)2)2), 2.582–
2.887 (s, 8H, NCH2CH2N), 2.398–2.433 (m, 3H, NCH2CH2NCH3),
1.345 (s, 9H, ArC(CH ) ), 1.172–1.186 (d, J = 7.0Hz, 12H,
Ar(CH(CH3)2)2). Calcd fo3r 3C29H43N O: C, 77.46; H, 9.64; N, 9.34.
Found: C, 77.66; H, 9.56; N, 9.28%.3
References
1
2
V.C. Gibson and S.K. Spitzmesser, Chem. Rev., 2003, 103, 283.
E. Groppo, C. Lamberti, S. Bordiga, G. Spoto and A. Zecchina, Chem.
Rev., 2005, 105, 115.
3
4
A. Clark, Catal. Rev., 1969, 3, 145.
F.J. Karol, G.L. Karapinka, C. Wu, A.W. Dow, R.N. Johnson and W.L.
Carrick, J. Polymer. Sci., Part A: Polymer. Chem., 1972, 10, 2621.
V.C. Gibson, S. Mastroianni, C. Newton, C. Redshaw, G.A. Solan, A.J.P.
White and D.J. Williams, J. Chem. Soc., Dalton Trans., 2000, 1969.
V.C. Gibson, C. Newton, C. Redshaw, G.A. Solan, A.J.P. White and D.J.
Williams, J. Chem. Soc., Dalton Trans., 1999, 827.
V.C. Gibson, P.J. Maddox, C. Newton, C. Redshaw, G.A. Solan, A.J.P.
White and D.J. Williams, Chem. Commun., 1998, 1651.
L.A. McAdams,W.-K. Kim, L.M. Liable-Sands, I.A. Guzei,A.L. Rheingold
and K.H. Theopold, Organometallics, 2002, 21, 952.
5
6
7
8
9
P. Wei and D.W. Stephan, Organometallics, 2002, 21, 1308.
10 D.J. Jones, V.C. Gibson, S.M. Green and P.J. Maddox, Chem. Commun.,
2002, 1038.
11 H. Sugiyama, G. Aharonian, S. Gambarotta, G.P.A. Yap and P.H.M.
Budzelaar, J. Am. Chem. Soc., 2002, 124, 12268.
12 M.J. Carney, N.J. Robertson, J.A. Halfen, L.N. Zakharov and A.L.
Rheingold, Organometallics, 2004, 23, 6184.
13 R.D. Köln, M. Haufe, G. Kociak-Köhn, S. Grimm, P. Wasserscheid and
W. Keim, Angew. Chem. Int. Ed., 2000, 39, 4337.
14 D.S. McGuinness, P. Wasserscheid, D.H. Morgan and J.T. Dixon,
Organometallics, 2005, 24, 552.
15 R.J. Baker and P.G. Edwards, J. Chem. Soc., Dalton Trans., 2002, 2960.
16 D.S. McGuinness, P. Wasserscheid, W. Keim, D. Morgan, J.T. Dixon,
A. Bollmann, H. Maumela, F. Hess and U. Englert, J. Am. Chem. Soc.,
2003, 125, 5272.
17 D.S. McGuinness, V.C. Gibson, D.F. Wass and J.W. Steed, J. Am. Chem.
Soc., 2003, 125, 12716.
18 J. Liu, Y. Li, J. Liu and Z. Li, J. Mol. Catal. A: C, hem., 2006, 244, 99.
19 M. Fennie, E. DiMauro, E. O’Brien, V. Annamalai and M. Kozlowski,
Tetrahedron, 2005, 61, 6249.
Synthesis of complexes
0.20 mmol of [CrCl3(THF)3] and 0.21 mmol of ligand were dissolved
in THF (20 mL). The solution was refluxed for 2 h and then cooled
to room temperature. The solution was filtered, then part of solvent
was removed in vacuum. Ether (100 mL) was added to the reaction
mixture, and this mixture was allowed to stand for an appropriate
period of time, after which it was filtered and the solid was obtained.
[4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-6-(morpholi
nomethyl}phenol]CrCl3 (C1): Green solid, Yield 71%. IR (KBr,
cm−1): 1604 (C=N). Calcd for C28H Cl3CrN O2: C, 56.62; H, 6.62;
N, 4.72. Found: C, 56.87; H, 6.78; N3,94.79%. 2
[4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-6-(piperidin-
1-ylmethyl}phenol]CrCl3 (C2): Green solid, Yield 75%. IR (KBr,
cm−1): 1609 (C=N). Calcd for C29H41Cl CrN2O: C, 58.84; H, 6.98; N,
4.73. Found: C, 58.95; H, 6.87; N, 4.863%.
[4-tert-butyl-2-{(2,6-diisopropylphenylimino)methyl)-6-(pyrro-
lidin-1-ylmethyl}phenol]CrCl3 (C3): Green solid,Yield 69%. IR (KBr,
20 W. Herwig and H.H. Zeiss, J. Org. Chem., 1958, 23, 1404.

Copyright of Journal of Chemical Research is the property of Science Reviews 2000 Ltd. and its content may
not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written
permission. However, users may print, download, or email articles for individual use.