Experimental and theoretical insights into the trans influence of organo-sulfur and -selenium ligands in 5,6-membered palladium(ii) cationic pincer complexes based on iminophosphoranes

Abstract

Novel iminophosphorane based heterotridentate SNS and SNSe ligands with the general formula MeSC₆H₄P(Ph₂) = NC₆H₄ER (ER = SMe 3a, SPh 3b, SeMe 3c, and SePh 3d) and their corresponding Pd(II) cationic complexes with tetrafluoroborate counterions 4a–d have been synthesized and fully characterized by elemental analysis, IR, multinuclear NMR (¹H, ¹³C, ³¹P, and ⁷⁷Se) and MS. The solid-state molecular structures of ligand 3d and complexes 4a–d were determined by X-ray crystallography, which confirmed the meridional tridentate SNE coordination of the iminophosphorane ligands, leading to unsymmetrical Pd(II) pincer complex structures containing 5,6-membered fused rings. The structural changes suggest that the nature of the substituent R has a greater control on the trans influence of ancillary ligands than chalcogen donor E, decreasing in the following order: SeMe (4c) > SMe (4a) > SePh (4d) > SPh (4b). Density functional theory (DFT) calculations at the PBE0/SDD/6-31+G(d) level gave optimized structures with high accuracy. Topological analysis of the electron density using quantum theory of atoms in molecules (QTAIM) gave more insight into the strength of the Pd–donor atom interactions. Theoretical data coincide with experimental observations in assigning the strongest and weakest trans influence to SeMe (4c) and SPh (4d) ligands, respectively, and show that the nature of both chalcogen atom E and organic substituent R contribute to the trans influence.