Isolation of a Chromium Hydride Single-Component Ethylene Polymerization Catalyst

Reaction of the divalent complex [(t-Bu)NPN(t-Bu)]2Cr (1) with different stoichiometric ratios of Al(i-Bu)3 afforded {μ-[(t-Bu)NP(i-Bu)N(t-Bu)]Al(i-Bu)2}2Cr (2) and [{(i-Bu)P{μ-N(t-Bu)}2Al(i-Bu)2}Cr(μ-H)]2 (3) as part of the same reaction sequence. Complex 2 arises from association of alane and the two ligands via alkylation of the P atom and retention of the AlR2 unit. Complex 3 appears to be generated from subsequent dissociation of one alkylated ligand and possible transfer of an i-Bu group to Cr followed by β-H elimination or transfer of a hydride originating from isobutyl elimination of the aluminum-containing residue. Both species are potent ethylene polymerization catalysts with no need for further activation. It is assumed that 2 is transformed in situ into 3 under the influence of ethylene. Reactions with ethylene carried out in the presence of excess of Al(i-Bu)3 switch the selectivity completely toward selective trimerization.