Evidence for Ligand- and Metal-Centered Reduction in Polypyridyl Dicarboxylate Complexes of Ru(II) and U(VI)

Polypyridyl dicarboxylates have been established as oxidatively robust ligands capable of effectively binding heavy metals, but the reductive electrochemical properties of complexes supported by these ligands have not been explored to date. Here, the redox properties of Ru(II) and uranyl(VI) (UO22+) complexes of 2,2′-bipyridyl-6,6′-dicarboxylate (bdc), 2,2′:6′,2″-terpyridyl-6,6″-dicarboxylate (tdc), and 4′-phenyl-2,2′:6′,2″-terpyridyl-6,6″-dicarboxylate (Phtdc) have been investigated, revealing that these ligands can enable both ligand- and metal-centered reductions. In control ruthenium complexes, electrochemical and spectroelectrochemical data supported by theoretical findings from density functional theory suggest electron density in the reduced forms primarily resides on the ligands. In bdc complexes of uranyl, electrochemical data and theoretical findings support the involvement of both ligand- and metal-centered reductive behavior. This “non-innocent” redox chemistry, along with support for the assertion that these ligands bind large metals effectively, suggests that polypyridyl dicarboxylates could be useful in new schemes for reductive activation of challenging metal-containing species. The observation of ligand-centered reduction events is also in agreement with the recognized “non-innocent” redox activity of related 2,2′-bipyridyl systems that lack appended carboxylate functionalities.