Molecular Twist‐Induced Single‐Crystal Isomerization and Valence Tautomeric Transitions in a Cobalt‐Dioxolene Complex

Abstract A mononuclear valence tautomeric (VT) complex, [Co(pycz) 2 (Sq)(Cat)] ( 1‐ trans ), where pycz = 9‐(pyridin‐4‐yl)‐9 H ‐carbazole, Sq⋅ − = 3,5‐di‐ tert ‐butyl‐semiquinonato, and Cat 2− = 3,5‐di‐ tert ‐butyl‐catecholato, is synthesized in the trans configuration, which undergoes one‐step valence tautomeric transition above room temperature. Remarkably, 1‐ trans can transform into its isomeric structure, [Co(pycz) 2 (Sq)(Sq)] ( 1‐ cis ), at temperature above 350 K in a single‐crystal‐to‐single‐crystal way by in situ molecular twist, and the resulting 1‐ cis exhibits a pronounced two‐step VT transition during magnetic measurements that is rare for mononuclear VT complexes. Such drastic solid‐state structural transformation is reported in VT compounds for the first time, which is actuated by a crystal surface's melting‐recrystallization induced phase transition process. DFT calculations offer an underlying mechanism suggesting a concerted bond rotation during the structural transformation. The results demonstrate an unconventional approach that realizes structural transformation of VT complexes and the control of VT performance.