Tunable Synchronicity of Molecular Valence Tautomerism with Macroscopic Solid-Liquid Transition by Molecular Lattice Engineering.

The combination of a cobalt-dioxolene core that exhibits valence tautomerism (VT) with pyridine-3,5-dicarboxylic acid functionalized with the chains bearing two, four, or six oxyethylene units led to new complexes Co n EGEspy ( n = 2, 4, and 6). These complexes commonly form violet crystals of the low-spin ( ls )-[Co III ( n EGEspy) 2 (3,6-DTBSQ)(3,6-DTBCat)] ( ls -[Co III ], 3,6-DTBSQ = 3,6-di- tert -butyl semiquinonato, 3,6-DTBCat = 3,6-di- tert -butyl catecholato). Interestingly, violet crystals of Co2EGEspy in the ls -[Co III ] transitioned into a green liquid, accompanied by an almost complete VT shift (94%) to the high-spin ( hs )-[Co II ( n EGEspy) 2 (3,6-DTBSQ) 2 ] ( hs -[Co II ]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls -[Co III ] exhibited partial VT (33%) and only a 9.3% VT shift after melting, respectively. These data demonstrate the tunability of the synchronicity of the molecular VT and macroscopic solid-liquid transitions by optimizing the tethered chains, thus establishing a new strategy for coupling bistable molecules with the macroscopic world.