Synthesis, Structures, and Ethylene Oligomerization
Activity of Bis(phosphanylamine)pyridine Chromium/Aluminate Complexes
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Abstract
A trivalent chromium
complex of a PN(pyridine) ligand system, {[(2,6-Ph<sub>2</sub>P-NH)<sub>2</sub>C<sub>5</sub>H<sub>3</sub>N]CrCl<sub>3</sub>}(THF)<sub>2</sub> (<b>1</b>), was prepared and tested as a
catalyst for ethylene oligomerization and polymerization, with the
purpose of probing the ability of a pyridine ring substituent as a
stabilizing factor on catalytically active intermediates. Its nonselective
catalytic behavior indicated that ready reduction of the metal center
to the divalent state occurred during the activation process. To substantiate
this point, we have reacted <b>1</b> with a few common aluminate
activators and isolated both the divalent complexes {[2,6-Ph<sub>2</sub>PNHC<sub>5</sub>H<sub>3</sub>NAlClEt<sub>2</sub>NPPh<sub>2</sub>]Cr(μ-Cl)<sub>2</sub>AlEt<sub>2</sub>}(toluene) (<b>3</b>) and {[2,6-Ph<sub>2</sub>PNHC<sub>5</sub>H<sub>3</sub>NAlCl-<i>i</i>-Bu<sub>2</sub>NPPh<sub>2</sub>]Cr(μ-Cl)<sub>2</sub>Al-<i>i</i>-Bu<sub>2</sub>}<sub>2</sub> (toluene) (<b>4</b>) and the trivalent
complexes {[2,6-Ph<sub>2</sub>PNHC<sub>5</sub>H<sub>3</sub>NAlClMe<sub>2</sub>NPPh<sub>2</sub>]CrMe(μ-Cl)<sub>2</sub>AlMe<sub>2</sub>}(toluene)<sub>1.5</sub> (<b>2</b>) and {[2,6-Ph<sub>2</sub>PNHC<sub>5</sub>H<sub>3</sub> NHNPPh<sub>2</sub>]CrEt(μ-Cl)<sub>2</sub>AlEt<sub>2</sub>}AlEtCl<sub>3</sub>(hexane)<sub>0.5</sub> (<b>5</b>). The reaction of the ligand with the divalent chromium
precursor CrCl<sub>2</sub>(THF)<sub>2</sub> in the presence of alkylaluminum
afforded {[2,6-Ph<sub>2</sub>PNHC<sub>5</sub>H<sub>3</sub>NCl<sub>2</sub>EtAlNPPh<sub>2</sub>]Cr(μ-Cl)<sub>2</sub>AlEt<sub>2</sub>}<sub>2</sub>(toluene) (<b>6</b>) containing aluminate residues,
where the metal has preserved the initial divalent state. All of these
species showed moderate to high activities toward ethylene oligomerization
