A Singlet-Diradical Co(III)-Dimer as a Nonvolatile Resistive Switching Device Synthesis, Redox-Induced Interconversion, and Current-Voltage Characteristics

Herein we report a ligand-centered redox-controlled strategy for the synthesis of an unusual binuclear diradical cobalt(III) complex, [Co₂^III(L^•3-)2] (1), featuring two three-electron reduced trianionic monoradical 2,9-bis(phenyldiazo)-1,10-phenanthroline ligands (L^•3-) and two intermediate-spin cobalt(III) centers having a Co-Co bond. Controlled ligand-centered oxidation of 1 afforded two mononuclear complexes, [Co^II(L^•-)(L⁰)]+([3])⁺and [Co^II(L⁰)₂]²⁺([2]²⁺), which upon further ligand-centered reduction yielded a di-azo-anion diradical complex, [Co^II(L^•-)₂] (4). In complex 1, two three-electron reduced di-azo-anion monoradical ligands (L•3-) bridge two intermediate Co(III) centers at a distance of 2.387(2) Å, while upon oxidation, one of the coordinating azo-arms of L becomes pendent, and in complexes [2]²⁺, [3]⁺, and 4, two tetradentate ligands coordinate a single Co(II) center in a tridentate meridional fashion with one uncoordinated azo-arm from each of the ligands. In the presence of reducing agents, the monomers [2]²⁺, [3]⁺, and 4 undergo ligand-centered reduction to form azo-anion radicals, and the otherwise pendent azo-arms in the presence of cobalt(II)-salts like Co(ClO₄)₂or CoCl₂bind the second Co(II)-ion; further internal electron transfer from the cobalt center to the arylazo backbone produces the binuclear complex 1. Spectroscopic analysis, DFT studies, and control experiments were performed to understand the electronic structures and the ligand-centered redox-controlled interconversion. The application of complex 1 as a molecular memory device (memristor) was also explored. Complex 1 showed encouraging results as a memristor with a current ON/OFF ratio > 10⁴ and is highly promising for resistive RAM/ROM applications.