Original palladium pincer complexes deriving from 1,3-bis(thiophosphinoyl)indene proligands: Csp3–H versus Csp2–H bond activation

Abstract

A series of original 2-indenylidene palladium pincer complexes {PdL[Ind(Ph₂PS)₂]} (L = HNCy₂, PPh₃, Cl⁻) have been prepared by double C–H activation of a 1,3-bis(thiophosphinoyl)indene proligand. Crystallographic analyses and DFT calculations indicate that the bonding situation of the {Pd[Ind(Ph₂PS)₂]} fragment is essentially governed by the conjugated and rigid nature of the dianionic pincer ligand, the nature of the coligand having little influence. The formation of the 2-indenylidene complexes involves either a 2-indenyl pincer or a four-membered cyclometalated complex as an intermediate, suggesting that C_sp²–H or C_sp³–H bond activation takes place. However, deuterium labelling experiences show that in all cases, C_sp³–H bond activation occurs followed eventually by a Pd/H exchange. Nevertheless, evidence for direct C_sp²–H bond activation under mild conditions is obtained when a methyl group is introduced at the indene proligand to prevent C_sp³–H bond activation. The ensuing dissymmetrical 2-indenyl palladium pincer complex has been fully characterized.