Unsymmetrical bidentate ligands of α-aminoaldimines leading to sterically controlled selectivity of geometrical isomerism in square planar coordination

Abstract

New α-aminoaldimines with the formula of Et₂NCMe₂CHNR (R = ⁱPr, ^tBu, Ph) and their dichloro or diacetato complexes of Ni, Pd, Pt are prepared and structurally characterized. A nickel complex is in a distorted tetrahedral configuration, and the Pd and Pt complexes (4–6) are of square planar form. The α-aminoaldimines can chelate to the metal in a C₂-unsymmetric bidentate motif through the hetero functionalities of amine and imine, which show comparable trans influence. Square planar organometallic palladium derivatives bearing α-aminoaldimines, including Pd–methyl, Pd–acetyl, and Pd–(η²-acetylnorboryl) (7–10), are also synthesized. The latter two species are a result of CO-insertion into Pd–methyl complexes and ensuing norbornene-insertion, respectively. The geometrical isomerism is found in the trans configuration in most studied cases. Such a stereoselectivity results from the thermodynamic stability governed predominantly by steric control. The stereoselectivity is also supported by DFT calculations.