Experimental and Theoretical Studies of Hemilabile (Imino)Phosphine Palladium(ii) Complexes as Ethylene Oligomerization Catalysts

Reactions of P∧N-donor (imino)­phosphine ligands bearing potential hemilabile donor pendant arms, Y = OH (L1), N­(et)2 (L2), OCH3 (L3), CH2CH3O (L4), and CH3 (L5) with [Pd­(cod)­ClX] (X = Cl or Me), afforded the respective palladium complexes [Pd­(L1)­Cl2] (Pd1); [Pd­(L2)­Cl2] (Pd2); [Pd­(L3)­Cl2] (Pd3); [Pd­(L4)­Cl2] (Pd4); [Pd­(L3)­ClMe] (Pd5); and [Pd­(L5)­Cl2] (Pd6) in moderate yields. Solid-state structures of complexes Pd1, Pd3, Pd4, Pd5, and Pd6 established the P∧N bidentate coordination mode of the ligands L1, L3-L5 to give the respective neutral mononuclear complexes.On the other, a P∧N∧N tridentate coordination mode of ligand L2 was established to give the cationic complex Pd2. Activation of complexes Pd1–Pd6 with either EtAlCl2 or MAO as co-catalysts afforded active catalysts in the oligomerization of ethylene to give predominantly C4 and C6 oligomers. The catalytic activities of the complexes were largely controlled by the nature of the pendant donor arm in the ligand motif. In general, complexes bearing weakly coordinating donor atoms (Y = OCH3) formed more active catalysts, consistent with ethylene coordination to the active sites being the rate-determining step. Theoretical studies using DFT calculations were in good agreement with the experimental results and firmly established the role of the pendant donor atoms in regulating the resultant catalytic activities and stability of the active species.