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-con-trolled strategy for the synthesis of an unusual binuclear diradical cobalt(III) complex, [Co2III(L center dot 3-)2] (1), featuring two three-electron reduced trianionic monoradical 2,9-bis(phenyldiazo)-1,10-phenanthroline ligands (L center dot 3-) and two intermediate-spin cobalt(III) centers having a Co-Co bond. Controlled ligand-centered oxidation of 1 afforded two mononuclear complexes, [CoII(L center dot -)(L0)]+ ([3])+ and [CoII(L0)2]2+ ([2]2+), which upon further ligand-centered reduction yielded a di-azo-anion diradical complex, [CoII(L center dot-)2] (4). In complex 1, two three-electron reduced di-azo-anion monoradical ligands (L center dot 3-) bridge two intermediate Co(III) centers at a distance of 2.387(2) angstrom, 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)2 or CoCl2 bind 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.

Automated summary

In this study, a ligand-centered redox-controlled strategy was used to synthesize an unusual binuclear diradical cobalt(III) complex. The complex showed promising potential for application as a molecular memory device.