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.