One-Step versus Two-Step Valence Tautomeric Transitions in Tetraoxolene-Bridged Dinuclear Cobalt Compounds.

The syntheses of valence tautomeric compounds with multistep transitions using new redox-active ligands are the long-term goal of the field of bistable materials. The redox-active tetraoxolene ligand, 2,7-di- tert -butylpyrene-4,5,9,10-tetraone (pyrene Q-Q ), is now developed to synthesize a pair of dinuclear compounds {[CoL 2 ] 2 (pyrene Sq-Sq )}[Co(CO) 4 ] 2 · x CH 2 Cl 2 ·2C 6 H 5 CH 3 ( 1 , x = 2, L = 1,10-phenanthroline, phen; 2 , x = 1.5, L = 2,2'-bipyridine, bpy). Variable-temperature magnetic susceptibilities and single-crystal X-ray diffraction measurements indicate a partial one-step valence tautomeric transition for 1 and a rare two-step valence tautomeric transition for 2 , respectively. DFT calculation results are consistent with the experimental data, revealing the correlation between thermodynamic parameters and the one-step/two-step valence tautomeric behaviors.