The Effect of a Fourth Binding Site on the Stabilization of Cationic SPS Pincer Palladium Complexes : Experimental, DFT, and Mass Spectrometric Studies

A potentially tetradentate ligand was synthesized by the addition of α-Li-picoline to the 2,6-bis(diphenylphosphine sulfide)-3,5-diphenylphosphinine (SPS) ligand. The corresponding Pd complex (3) was obtained by the reaction with [(COD)PdCl2]. An air-stable cationic Pd(II) complex (4) was obtained after abstraction of Cl− with silver salt. The role of the additional binding site in the stabilization of the cationic complex was explored using NMR spectroscopic and tandem ESI-MS studies combined with DFT calculations. Additionally the picoline moiety was used to tune the electronic properties of the cationic complex 7 to modify the activity in the Lewis acid-catalyzed allylation of aldehydes.