Synthesis of neutral dimethylaluminum complexes bearing imino-phenoxide ligands

Article abstract of DOI:10.14233/ajchem.2013.15642

Synthesis of neutral dimethylaluminum complexes ligated by bidentate N,O salicylaldimine ligands is reported. Treatment of the iminophenoxide ligands 3-R-2-(OH)C₆H₃CH=N(2,6-iPr₂C₆H ₃) [R = 9-anthracen

Full text of DOI:10.14233/ajchem.2013.15642

Asian Journal of Chemistry; Vol. 25, No. 15 (2013), 8785-8787
http://dx.doi.org/10.14233/ajchem.2013.15642
Synthesis of Neutral Dimethylaluminum Complexes Bearing Imino-Phenoxide Ligands
AMER EL-BATTA
Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia
Corresponding author: Fax: +966 3 8604277; Tel: +966 3 8607346; E-mail: aelbatta@kfupm.edu.sa
(Received: 16 April 2013;
Accepted: 29 August 2013)
AJC-14049
Synthesis of neutral dimethylaluminum complexes ligated by bidentate N,O salicylaldimine ligands is reported. Treatment of the imino-
phenoxide ligands 3-R-2-(OH)C6H3CH=N(2,6-iPr2C6H3) [R = 9-anthracenyl (1a) and 1-naphthyl (1b)] with Al(CH3)3 in toluene affords
the corresponding yellow dimethylaluminum compounds {3-R-2-(OH)C6H3CH=N(2,6-iPr2C6H3)}Al(CH3)2 [R = 9-anthracenyl (2a) and
1-naphthyl (2b)] in high yields. Reactions cleanly proceed by protodealumination of the Al-CH3 bond with concomitant elimination of
methane. Toluene solutions of 2a-b, when activated with 1 equivalent of B(C6F5)3, polymerized ethylene to solid polyethylene with
low activity.
Key Words: Organometallic, Salicylaldimine, Phenoxy-imine, Aluminum, Complex, Ligand.
Additionally, upon complexation within the sal framework,
the substituents (particularly R and Ar, Fig. 1) are properly
situated around the metal center for strategically controlling
the polymerization reactions. Furthermore, these ligands are
structurally flexible due to the labile imine donors and, there-
fore, can be electronically flexible, too, allowing phenoxy-
imine ligands to show both electron-receptive and electron-
donating properties, depending on the state of the metal
center4. These inherent features pertaining to phenoxy-imine
ligands have resulted in their great ability to be olefin poly-
merization catalysts independent of the early to late transition
metal centers.
INTRODUCTION
Transition metal mediated olefin polymerization catalysts
have grown dramatically over the past half a century and they
have been the main cause behind the development of the
polyolefin industry. Well-defined distinct molecular catalysts
equipped with appropriately designed organic ligands have
allowed researchers to understand and even control the reac-
tivity and the selectivity of olefin polymerizations in a rational
manner, which was unattainable with the traditional ill-
defined Ziegler-Natta catalysts1. Because of this the discovery
of metallocene catalysts is such a big breakthrough within the
development of olefin polymerization catalysis.
In recent years there has been great interest in the chem-
istry of transition metal complexes of Schiff bases5, since these
ligands offer opportunities for inducing substrate chirality,
tuning the metal-centered electronic factor and enhancing the
solubility and stability of either homogeneous or heteroge-
neous catalysts6. Moreover, the Schiff base complexes have
become increasingly important as biochemical, analytical and
antimicrobial reagents7. Salicylaldimine ligands have also been
employed to afford olefin polymerization catalysts for aluminum8,
group IV9, group VI10 and group X11 metal systems.
Post-metallocene catalysts2 have sterically and electroni-
cally offered a much higher degree of freedom for catalyst
design than metallocene catalysts. Hence these catalysts have
offered further diversity and opportunities for the development
of olefin polymerization catalysis.Among the post-metallocene
catalysts developed, salicylaldimine (sal) ligands3 (Fig. 1)
possess a particularly large diversity of structures because of
their straightforward synthesis and modular properties. Such
variability has led to remarkable progress in phenoxy-imine
based catalyst development.
Salicylaldimine ligand is usually reacted with B(C6F5)3 in
hydrocarbon or THF solvent to generate stable cationic alumi-
num alkyls8. These cationic species are highly electrophilic
and therefore more active as olefin polymerization catalysts.
As part of our research investigation into the coordi-
nation chemistry of these novel ligand platforms, we describe
herein the synthesis of neutral dimethylaluminum complexes
M
Ar
H
O
N
R
Fig. 1. Sal (phenoxy-imine) framework

8786 El-Batta
Asian J. Chem.
carrying bidentate sal ligands and their activity in ethylene
polymerization.
methanol and stirred overnight. The polymer was recovered
by filtration, washed with fresh methanol and dried under
vacuum.
Synthesis of {3-(9-anthracenyl)-2-(O)C6H3CH=N(2,6-
iPr2C6H3)}Al(CH3)2 (2a): Trimethylaluminium solution (169
µL, 0.337 mmol, 2.0 M in toluene) was slowly added to a
solution of 3-(9-anthracenyl)-2-(OH)C6H3CH=N(2,6-
iPr2C6H3) (147 mg, 0.321 mmol) in toluene (5 mL) at 0 °C.
The reaction was allowed to warm to room temperature and
stirred for 12 h. The volatiles were then removed under
reduced pressure, to afford 2a as a yellow solid in 88 % yield
The particular interest in bulky sal ligands stems from
neutral nickel ethylene polymerization catalysts, (sal)Ni(Ph)PPh3,
developed by Grubbs11a,b. Salicylaldimine ligands 3-R-2-
(OH)C6H3CH=N(2,6-iPr2C6H3) [R = 9-anthracenyl (1a) and
1-naphthyl (1b)]11a,12 were prepared by condensation of the
corresponding salicylaldehyde and 2,6-diisopropylaniline.
Treatment of ligands 1a-b with 1.0 equivalent of Al(CH3)3
in toluene at room temperature readily affords the correspon-
ding aluminum complexes {3-R-2-(O)C6H3CH=N(2,6-
iPr2C6H3)}Al(CH3)2 [R = 9-anthracenyl (2a) and 1-naphthyl
(2b)], respectively (yields 2a = 88 %, 2b = 92 %) (Scheme-I).
The Al complexes (2a-b) proved to be yellow solids, which is
typical of the bidentate N,O salicylaldimine complexes ofAl8.
1
(145 mg). The H NMR (500 MHz, C6D6): δ 8.33 (s, Anth-
C10-H, 1H), 8.15 (s, CH=N, 1H), 8.13 (d, J = 7.6 Hz, Anth-
C4,5-H, 2H), 7.95 (d, J = 7.1 Hz, Anth-C1,8-H, 2H), 7.48 (d,
J = 7.3 Hz, OAr-H, 1H), 7.43-7.28 (m, Anth-C2,3,6,7-H and
OAr-H, 5H), 7.19-7.06 (m, NAr-H, 3H), 6.84 (t, J = 7.5 Hz,
OAr-H, 1H), 3.31 [(sept, J = 6.7 Hz, CH(CH3)2, 2H)], 1.29
[(d, J = 6.8 Hz, CH(CH3)2, 6H)], 1.06 [(d, J = 6.8 Hz, CH(CH3)2,
6H)], -0.49 [(s, Al(CH3)2, 6H)]; 13C NMR (125 MHz, C6D6): δ
174.1 (CH=N), 164.8 (C-O), 142.7 (C-N), 135.9 (NArC-iPr),
134.2, 132.8 (both OArC-4,6), 131.6, 129.5, 129.0, 128.9,
128.44, 128.3, 127.4, 126.7, 126.5 (all Anth-C and OArC-3),
126.0, 125.1 (both NArC-3,4), 120.2 (OArC-5), 118.6 (OArC-
1), 29.1 [CH(CH3)2], 26.6 [CH(CH3)2], 23.2 [CH(CH3)2], -8.6
[Al(CH3)2]; Anal. Calcd. for C35H36NOAl: C, 81.84; H, 7.06;
N, 2.73. Found: C, 81.55; H, 6.86; N, 2.74.
H3C CH3
Al
Dipp
H
Dipp
OH
N
O
N
Al(CH3)3
toluene
R
R
H
1a-b
2a-b
Synthesis of {3-(1-naphthyl)-2-(O)C6H3CH=N(2,6-
iPr2C6H3)}Al(CH3)2 (2b): Trimethylaluminium solution (169
µL, 0.337 mmol, 2.0 M in toluene) was slowly added to a
solution of 3-(1-naphthyl)-2-(OH)C6H3CH=N(2,6-iPr2C6H3)
(131 mg, 0.321 mmol) in toluene (5 mL) at 0 ºC. The reaction
was allowed to warm to room temperature and stirred for 12
h. The volatiles were then removed under reduced pressure,
to afford 2b as a yellow solid in 92 % yield (137 mg). The 1H
NMR (500 MHz, C6D6): δ 7.98 (d, J = 8.3 Hz, Naph-C4-H,
1H), 7.90 (s, CH=N, 1H), 7.61 (2 × d, J = 8.1 Hz, 7.8 Hz,
Naph-C5,8-H, 2H), 7.49 (d, J = 7.1 Hz, OAr-H, 1H), 7.47-
7.20 (m, OAr-H, Naph-C3,6,7-H, 4H), 7.12-6.94 (m, NAr-H,
3H), 6.84 (d, J = 7.9 Hz, Naph-C2-H, 1H), 6.63 (t, J = 7.5 Hz,
OAr-H, 1H), 3.22 [(sept, J = 6.7 Hz, CH(CH3)2, 1H)], 3.03
[(sept, J = 6.7 Hz, CH(CH3)2, 1H)], 1.19 [(d, J = 6.8 Hz,
CH(CH3)2, 3H)], 1.09 [(d, J = 6.8 Hz, CH(CH3)2, 3H)], 0.95
[(d, J = 6.8 Hz, CH(CH3)2, 3H)], 0.79 [(d, J = 6.7 Hz, CH(CH3)2,
3H)], -0.45 [(s, Al(CH3)2, 3H)], -0.64 [(s, Al(CH3)2, 3H)]; 13C
NMR (125 MHz, C6D6): δ 174.2 (CH=N), 164.2 (C-O), 143.3
(C-N), 143.0 (NArC-iPr), 141.0 (NaphC-1), 135.7 (OArC-6),
133.9 (OArC-4), 132.5, 132.3, 129.9, 129.4, 129.1, 128.7,
128.5, 128.3, 127.8, 126.6 (all Naph-C and OArC-3), 126.2,
125.1 (both NArC-3,4), 119.9 (OArC-5), 118.5 (OArC-1), 29.1
[CH(CH3)2], 26.5 [CH(CH3)2], 23.0 [CH(CH3)2], -8.6
[Al(CH3)2]; Anal. Calcd. (%) for C31H34NOAl: C, 80.31; H,
7.39; N, 3.02. Found (%): C, 79.98; H, 7.26; N, 2.98.
Dipp = 2,6-diisopropylphenyl
R = 9-anthracenyl, a
R = 1-naphthyl, b
Scheme-I: Synthesis of Al salicylaldimine complexes
The disappearance of the O-H signal of the ligands at δ =
13.3 in the 1H NMR spectra of 2a-b (room temperature, C6D6)
and the appearance of a high-field signals (δ = 0.49, 6H, 2a)
and (δ = 0.64, 0.45, 6H, 2b) compatible withAl(CH3)2 protons,
reveal that the reaction proceeds by protodealumination of the
Al-CH3 bond with concomitant elimination of methane. The
methyl groups bound to the aluminum atom show characteristic
resonances at δ = -8.60 (2a) and -8.63 (2b) in the 13C NMR
spectra. Diagnostic signals for the imine function appear at δ
= 8.15 (2a) and 7.90 (2b) in the 1H NMR spectra and at δ ≈ H
174 in the 13C NMR spectra. The elemental analysis is consistent
with the expected products 2a-b.
Preliminary polymerization tests were conducted for the
synthesized Al complexes. Complexes 2a-b were combined
with 1.0 equivalent of B(C6F5)3 in toluene and polymerized
ethylene to solid polyethylene with low activity [2a: 78 g PE
(mmol cat)-1 (h)-1; 2b: 105 g PE (mmol cat)-1 (h)-1]. It is possible
that the bulk at the site ortho to the phenoxide moieties retards
ethylene binding and catalytic activity, by rendering the metal
less electrophilic. It is also possible that the extreme bulk of
the two Dipp substituents flanking the complex active site
provides too much steric hinderance, thus averting ethylene
association at that position.
A typical polymerization experiment was performed in a
30 mL glass reactor charged under nitrogen with a stir bar and
5 mL of toluene. The inert gas was replaced by ethylene at 20
psi, then 11.25 mg (22 µmol) of 2a and 11.26 mg (22 µmol) of
B(C6F5)3 were injected. The reactor was fed with constant
monomer pressure (100 psi) and maintained for 1 h at 40 ºC.
The reaction was then stopped by the addition of methanol.
The solution was poured into 100 mL of 5 vol % HCl in
It has been demonstrated the synthesis of neutral aluminum
alkyls 2a-b bearing bidentate N,O sal ligands cleanly and in
high yields. These complexes, when treated with 1 equivalent
of B(C6F5)3, did not undergo methyl abstraction to afford
alkylaluminum cations. Thus ethylene does not polymerize

Vol. 25, No. 15 (2013)
Synthesis of Neutral Dimethylaluminum Complexes Bearing Imino-Phenoxide Ligands 8787
(i) S. Adsule, V. Barve, D. Chen, F. Ahmed, Q.P. Dou, S. Padhye and
appreciably under these conditions. We are currently trying to
develop more reliable ethylene polymerization systems using
less bulky and less electron rich salicylaldimine ligands.
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ACKNOWLEDGEMENTS
The author acknowledged the support provided by King
Fahd University of Petroleum and Minerals (KFUPM). The
support of the Department of Chemistry at KFUPM is also
highly appreciated.
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