Polymorph-Dependent Molecular Valence Tautomerism Synchronized with Crystal-Melt Phase Transitions

This paper describes a molecular valence tautomeric (VT) bistability synchronized with polymorphic crystal-melt phase transitions of a newly designed VT complex, [Co(C9Espy)(2)(3,6-DTBQ)(2)] (CoC9Espy, C9Espy = dinonyl-pyridine-3,5-dicarboxylate and 3,6-DTBQ = 3,6-di-tert-butyl-semiquinonate (3,6-DTBSQ) or catecholate (3,6-DTBCat)). Two polymorphic crystals which are the thermodynamically stable phase (K1) and the kinetically stable phase (K2), commonly containing a [low spin-Co-III(C9Espy)(2)(3,6-DTBSQ)(3,6-DTBCat)] (ls-[Co-III]) tautomer with a low-spin Co-III atom and mixed-valence ligands. These polymorphic crystals show a molecular VT interconversion that is synchronized with crystal-to-melt phase transitions at different temperatures, 368.2 and 362.6 K for the ls-[Co-III]subset of K1 and the ls-[Co-III]subset of K2 phases, respectively. Interestingly, the ls-[Co-III]subset of K2 obtained from a melt of a [high spin-Co-II(C9Espy)(2)(3,6-DTBSQ)(2)] (hs-[Co-II]) tautomer with a high-spin Co-II atom with two 3,6-DTBSQ exhibits a double-melting phenomenon that includes, in part, the thermodynamically unfavorable hs-[Co-II]-to-ls-[Co-III] VT interconversion. Eventually, three types of VT interconversions synchronized with macroscopic crystal-melt phase transitions appear either in equilibrium or nonequilibrium conditions: (1) the thermodynamically stable ls-[Co-III]subset of K1 to the hs-[Co-II]subset of melt, (2) the metastable ls-[Co-III]subset of K2 to the hs-[Co-II]subset of melt, and (3) the relaxing process of the metastable hs-[Co-II]subset of melt to the thermodynamically stable ls-[Co-III]subset of K1. A new strategy for simultaneous control of molecular states and macroscopic phases using thermodynamically and kinetically formed polymorphic crystalline phases is presented.