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)(2)C6H3) 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. H-1 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.