Revealing the diastereomeric nature of pincer terdentate nitrogen ligands 2,6-bis(arylaminomethyl)pyridine through coordination to palladium

Abstract

Palladium complexes of formula [PdLCl]X (X = Cl 1, BF42) containing the pincer terdentate ligands L [L = 2,6-(ArNHCH2)2C5H3N (Ar = 4-MeC6H41a and 2a, 4-MeOC6H41b and 2b); L = 2,6-(4-MeC6H4NMeCH2)2C5H3N 1c and 2c] have been synthesised as cis–trans diastereomeric mixtures. The chiral nature of both amine arms of the pincer ligands accounts for the mixtures of isomers. All the complexes show dynamic behavior assigned to conversion between diastereomers. A ΔG‡273 of 68.9 kJ mol−1 for 2a and a ΔG‡273 of 65.8 kJ mol−1 for 2c were found through complete DNMR line-shape analysis of the –CH2– signals in the 1H spectra. The cis–trans conversion requires the inversion of the pyramidal nitrogen of one of the coordinated amine arms. Two putative pathways have been proposed for such inversion: a) dissociation of one of the amine nitrogen atoms followed by inversion of the nitrogen atom and reformation of the nitrogen–palladium bond and b) deprotonation of one of the N–H bond leading to an amido intermediate which can reprotonate on the other side. Experimental evidence is given for both mechanisms. The structural characterization of 1a is described.