Potential Hemilabile (Imino)pyridine Palladium(II) Complexes as Selective Ethylene Dimerization Catalysts: An Experimental and Theoretical Approach

The (imino)­pyridine ligands bearing pendant arms 2-methoxy-N-(1-(pyridin-2-yl)­ethylidene)­ethanamine (L1), 2-methoxy-N-((pyridin-2-yl)­methylene)­ethanamine (L2), 3-methoxy-N-((pyridin-2-yl)­methylene)­propan-1-amine (L3), N,N-diethyl-N-((pyridin-2-yl)­methylene)­ethane-1,2-diamine (L4), and N-((6-bromopyridin-2-yl)­methylene)-2-methoxyethanamine (L5) were synthesized by condensation of the appropriate aldehyde and the corresponding amines. Reactions of L1–L5 with [PdCl­(Me)­(cod)] afforded the corresponding palladium complexes [PdCl­(Me)­(L1)] (1), [PdCl­(Me)­(L2)] (2), [PdCl­(Me)­(L3)] (3), [PdCl­(Me)­(L4)] (4), and [PdCl­(Me)­(L5)] (5) in good yields. Treatment of the neutral complexes 1–5 with one molar equivalent of Na­[BAr4] (Ar = 3,5-(CF3)2C6H3) led to in situ formation of the corresponding cationic compounds [Pd­(Me)­(L1)]+ (1a), [Pd­(Me)­(L2)]+ (2a), [Pd­(Me)­(L3)]+ (3a), [Pd­(Me)­(L4)]+ (4a), and [Pd­(Me)­(L5)]+ (5a). In the presence of ethylene, the catalyst systems 1–5/MAO and 1a–5a exhibited high catalytic activities in selective ethylene dimerization producing mainly butenes. 1H NMR spectroscopy and DFT studies provided valuable insight into the role of ligand architecture on ethylene-binding energies, catalytic activities, and stabilities of the resultant catalysts.