Though numerous mechanistic studies have shown the importance of the PdII/IV redox couple in palladium catalysts for C-H bond activation and oxidative cross-coupling reactions, more recently the PdII/III couple has been implicated as the active redox pair in related binuclear palladium catalysts. Our recent work on palladium(II) complexes with cyclometallated ligands and the trithiacrown macrocycle 9[ane]S3 (1,4,7-trithiacyclononane) has shown reversible oxidative PdII/III and PdIII/IV electrochemistry. As these stable pseudo five-coordinate complexes resemble postulated Pd(III) intermediates in catalysts, we have focused our work on elucidating fundamental relationships between ligand donor/acceptor properties and ease of oxidation at Pd(II). We have investigated the electronic influences of a range of cyclometallating ligands as well as macrocycles with homoleptic sulfur donors and heteroleptic oxygen and sulfur donors. X-ray crystallographic studies show persistent intramolecular Pd…S interactions in complexes with homoleptic sulfur donor macrocycles but no such analogous interactions with mixed oxygen/sulfur donor macrocycles. Density functional theory calculations, consistent with crystallographic studies and electronic spectroscopy, provide further evidence that the axial Pd…S interactions supply needed electron density and structural features to enable the unusual reversible PdII/III redox processes in these complexes. A series of related Au(III) congeners have also been prepared and comparison of their structural and electrochemical features with Pd(II) systems will also be presented.

Fluxional five-coordinate palladium(II) complexes with sulfur donor macrocycles: Reversible PdII/III electrochemistry, DFT calculations, and X-ray structure

251st National Meeting (2016)

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