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

Abstract

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-CoIII(C9Espy)2(3,6-DTBSQ)(3,6-DTBCat)] (ls-[CoIII]) tautomer with a low-spin CoIII 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-[CoIII]⊂K1 and the ls-[CoIII]⊂K2 phases, respectively. Interestingly, the ls-[CoIII]⊂K2 obtained from a melt of a [high spin-CoII(C9Espy)2(3,6-DTBSQ)2] (hs-[CoII]) tautomer with a high-spin CoII atom with two 3,6-DTBSQ exhibits a double-melting phenomenon that includes, in part, the thermodynamically unfavorable hs-[CoII]-to-ls-[CoIII] 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-[CoIII]⊂K1 to the hs-[CoII]⊂melt, (2) the metastable ls-[CoIII]⊂K2 to the hs-[CoII]⊂melt, and (3) the relaxing process of the metastable hs-[CoII]⊂melt to the thermodynamically stable ls-[CoIII]⊂K1. A new strategy for simultaneous control of molecular states and macroscopic phases using thermodynamically and kinetically formed polymorphic crystalline phases is presented.