Stabilization of a Triplet Diradical on a Binuclear palladium(II) Dipyrrindione

Porphyrins and other macrocyclic oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. The stabilization of unpaired electrons within oligopyrrolic π systems adds to the available reactivity pathways and spin states of metal complexes for applications in catalysis and functional materials. In this context, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged dinuclear complex. Electrochemical, spectroelectrochemical, and computational analyses indicate the accessibility of two reduction processes on the dipyrrindione frameworks of the dinuclear complex. The product of a two-electron reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltoceni- um salt features a folded dianionic complex maintaining the hydroxo bridges between the divalent palladium centers. X- band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a triplet diradical supported by the dipyrrindione ligands. These dipyrroles, which are also known as propentdyopents and were initially isolated as urinary pigments and heme metabolites, extend the rich chemistry of bidentate dipyrrin ligands to include the stabilization of ligand-centered radicals.