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, [Co 2 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 • - )(L 0 )] + ([ 3 ]) + and [Co II (L 0 ) 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(ClO 4 ) 2 or CoCl 2 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 > 10 4 and is highly promising for resistive RAM/ROM applications.