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 chains bearing two, four, or six oxyethylene units led to new complexes ConEGEspy (n = 2, 4, and 6). These complexes commonly form violet crystals of the low‐spin (ls)‐[CoIII(nEGEspy)2(3,6‐DTBSQ)(3,6‐DTBCat)] (ls‐[CoIII], 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‐[CoIII] transitioned into a green liquid, accompanied by an almost complete VT shift (94 %) to the high‐spin (hs)‐[CoII(nEGEspy)2(3,6‐DTBSQ)2] (hs‐[CoII]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls‐[CoIII] 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. The synchronicity of molecular valence tautomerism (VT) and macroscopic solid‐to‐liquid phase transition has been rationally tuned by crystal lattice engineering in a new series of bistable valence tautomeric cobalt complexes [Co(3,6‐DTBQ)2(L)2] (3,6‐DTBQ = 3,6‐di‐tert‐butylsemiquinonato/catecholato) with two ancillary pyridine ligands (L) substituted with di‐, tetra‐, and hexa(ethylene glycol) groups.