Cobalt and Nickel Complexes Bearing 2-(2-pyridyl)- Benzimidazole: Synthesis and Ethylene Oligomerization

. A series of cobalt and nickel complexes MLX2 (M = Co or Ni, X = Cl) bearing 2-(2-pyridyl)benzimidazole ligands were synthesized. Treatment of the complexes with methylaluminoxane (MAO) leads to active catalysts for ethylene oligomerization. The oligomers were olefins from C4 to C6.


Introduction
6] Grubbs group reported new neutral Ni(II) salicylaldiminato complexes as catalyst for the polymerization of ethylene under moderate conditions. 7][10] We were interested in exploring the effect of changing the central moiety to other nitrogen-containing heterocycles. 11Here we describe the synthesis of Co(II) and Ni(II) complexes bearing 2-(2-pyridyl) benzimidazole ligands, as well as their oligomerization of ethylene study with MAO as co-catalyst.

Synthesis and characterization
The 2-(2-pyridyl)benzimidazole ligands 1a~1c were prepared as white or pale yellow solids in good yields by the condensation of one equivalent of the appropriate o-phenylenediamine with one equivalent of picolinic acid (Scheme 1).Compounds 1a~1c were characterized by microanalysis and 1 H NMR. Complexes of these ligands were synthesized by dissolving nickel or cobalt chloride in ethanol (Scheme 1), followed by addition of one equivalent of the ligand in ethanol.The complexes precipitated from the reaction solution.After washing with diethyl ether, the complexes 2a~c and 3a~c were obtained in good yield and high purity.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 same condition.The most active Ni(II) catalyst is complex 3a (1.14×10 5 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.Steric and electronic environmental around the central metal affected the catalytic activities.In the same condition, catalysts (2c, 2b, 2a; 3c, 3b, 3a) reveal that electron-donating group replacing the proton results in decreases in activity.The complex 2a (3a) contains two methyl groups in the aryl rings and displays an activity of 5.78×10 3 (0.52×10 5 ) g(ethylene)mol -1 (M)h -1 , approximately complex 2b (3b) with one methyl in the aryl ring with an activity of 6.25×10 3 (1.04×10 5 ) g(ethylene)mol -1 (M)h -1 .2] The oligomers are from C 4 to C 6 .The substituents of the complexes affect distribution of the oligomers to some extent.

Experimental
MAO (1.4 mol l -1 ) was purchased from Albemarle Corp (USA); Nickel or cobalt chloride and all of the î -phenylenediamines were used commercially without further purification.C, H, N elemental analysis was performed using CHN-RAPID microanalyzer. 1 H NMR spectra were recorded on a Bruker spectrometer DMX-300, with TMS as the internal standard.Melting points were determined without further correction.Ethylene oligomerization results were recorded on a HP5890 Series II gas chromatograph (Hewlett-Packard).

General procedure of synthesis ligands
A mixture of î -phenylenediamine (5 mmol), picolinic acid (5 mmol) and PPA (4.5 g) was stirred at 160 •C f or 5 h under N 2 .After cooled to 100•C, i t was poured i nt o 30 mL wa te r.T he s ol u t i on was cooled and added Na 2 CO 3 to adjust pH to 7. The precipitated solid was filtered and recrystallized from AcOEt. (

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 (2a~c or 3a~c, 10 ì mol in 5 mL t o lue ne ) was adde d to the reaction mixture via syringe.Afte r stirre d under 1 atm ethylene pressure for 20 min, the oligomerization was terminated by acidified ethanol.An aliquot of the reaction mixture was analyzed by gas chromatrography.Their activity and distribution of the oligomers were collected in Table 1