Cobalt and Nickel Complexes Bearing 2,6-Bis(imino)phenoxy Ligands: Synthesis and Ethylene Oligomerization Study

A series of new cobalt and nickel complexes MLX2 (M = Co or Ni, X = Cl) bearing 2,6-bis(imino)phenoxy ligands have been synthesized. Treatment of the complexes with methylaluminoxane (MAO) leads to active catalysts for ethylene oligomerization. The oligomers are olefins from C4 to C6.


Introduction
7][8] Moreover, Grubbs reported new neutral Ni (II) salicylaldimiminato complexes as catalysts for the polymerization of ethylene to obtain high molecular weight polyolefin under moderate conditions. 9Modification of the substituents of imino groups results in dramatic changes to the resultant polyolefin. 10Sun also reported a series of salicylaldimiminato complexes as catalysts for the polymerization of ethylene. 11Now we focus on exploring the effect of changing the central pyridyl moiety to phenol.Herein, we report the synthesis of Co(II) and Ni(II) complexes bearing 2,6bis(imino)phenoxy ligands and their ethylene oligomerization activity.

Experimental
All manipulations were carried out under an atmosphere of nitrogen using standard Schlenk and Cannula techniques.Using HP-MOD 1106 microanalyzer performed elemental analysis.NMR spectra were recorded on a Bruker spectrometer DMX-300, with TMS as the internal standard.IR spectra were obtained as KBr pellets on a Perkin-Elmer FTIR 2000 spectrometer.Mass Spectra were measured on a Kratos AEI MS-50 instrument using electron impact (EI).Melting points were determined without further correction.Ethylene oligomerization results were recorded on a HP5890 Series II gas chromatograph (Hewlett-Packard).
Compound 2-hydroxy-5-tert-butylisophthaldehyde was prepared according to an established procedure. 14While MAO (1.4mol l -1 ) was purchased from Albemarle Corp (USA); Nickel or cobalt chloride and all of the anilines were used commercially without further purification unless stated otherwise.

Preparation of complexes
Complex (1): CoCl 2 •6H 2 O (1mmol) and L 1 (1mmol) were combined in a Schlenk flask under an N 2 atmosphere.EtOH (10 mL) was added, and the mixture was stirred at room temperature for 10 hours.The crude product was filtrated, washing with diethyl ether and the product was recrrystallized from ethanol afford complex 1 as a green powder 0.46 g in 85% yield.IR(KBr), cm

General procedure for ethylene oligomerization
A flame dried three-neck round flask was vacuated-filled three times by nitrogen.Then ethylene was charged with 30 ml of freshly distilled toluene and stirred.At the room temperature, the aluminum cocatalyst MAO was added via syringe.The solution was stirred and the precatalyst complex (1 -6, 5 µmol in 5ml toluene) was added to the reaction mixture via syringe.After stirred under 1 atm ethylene pressure for 20 min, the oligomerization was terminated by acidified ethanol.An aliquot of the reaction mixture was analyzed using gas chromatrography.Their activity and distribution of the oligomers were collected in Table 1.

Synthesis and characterization
2,6-bis(imino)phenoxy ligands L 1 -L 3 were prepared in good yields by condensation of one equivalent of the appropriate aniline with one equivalent of 2-hydroxy-5-tertbutylisophthaldehyde (Scheme 1).Compounds L 1 -L 3 were characterized by microanalysis, 1 H-NMR and mass spectrometry.The complexes 1 -6 were synthesized by dissolving nickel chloride or cobalt chloride in ethanol (Scheme 1), followed by addition of one equivalent of ligand.The complexes precipitated from the reaction solution.After recrystallized from ethanol, the complexes were obtained in good yields and high purity.All the complexes were confirmed with elemental analysis and IR spectroscopy.The elemental analysis results revealed that the components of all complexes were in accord with the formula MLX 2 .

Oligomerization of ethylene
Upon treatment with methylaluminoxane (MAO), all of the complexes are active ethylene oligomerization catalysts.Table 1 lists their activity and molecular weight distribution of the oligomers.The nature of the metal center has a major influence on catalytic activities.In general, Ni(II) catalysts are more active than their corresponding Co(II) analogues under the same condition.The most active Ni(II) catalyst is complex 1 (4.57×10 4 g(ethylene)mol -1 (Ni)h -1 ), While the Co(II) complexes are about 10 3 g(ethylene)mol -1 (Co)h -1 for oligomerization.Complexes 1 -3 (4 -6) revealed that a reduction of steric methyl at the ortho-aryl position resulted in the decrease of their activities, and electronic environmental around the central metal affected the catalytic activities.In the same condition, catalysts 1 (4) are more active than the catalysts 3 (6).Without methyl group in the aryl rings and 3 (6) displays an activity of 1.78×10 3 (1.98×10 4 ) g(ethylene)mol -1 (M)h -1 , approximately complex 1 (4) with one methyl at the ortho-aryl with an activity of 3.65×10 3 (4.57×10 4 ) g(ethylene)mol -1 (M)h -1 .] The oligomers are from C 4 to C 6 .The substituents of the complexes affect distribution of the oligomers to some extent.Comparing with the 2,6-bis(imino)pyridyl ligands [6][7][8] , when we change the central pyridyl moiety to phenol, their complexes only show oligomerization activity, no polymerization activity; at the same time, the selectivity in ethylene conversion are not very well.

Conclusions
We have synthesized six cobalt and nickel complexes.Upon treatment with methylaluminoxane, they are all active ethylene oligomerization catalysts.The oligomers are from C 4 to C 6 .

Table 1 .
Activity and Distribution of the Oligomerization a a Condition: Toluene solvent, 1atm of ethylene, T=15 º C.