Polytypic phase transition in alkyl chain-functionalized valence tautomeric complexes

Abstract

A novel series of valence tautomeric (VT) complexes, [Co(Cnbpy)(3,5-DTBQ)2] (Cnbpy = 4,4'-dialkyl-2,2'-bipyridine (alkyl = (C(n)H(2n+1)) where n = 0 (CoC0bpy), 1 (CoC1bpy), 5 (CoC5bpy), 9 (CoC9bpy) and 13 (CoC13bpy); 3,5-DTBQ = 3,5-di-tert-butylsemiquinonate or catecholate), including two previously reported complexes (CoC0bpy and CoC1bpy), were systematically prepared and their properties examined. Introduction of alkyl groups into VT chromophores leads to unexpected variations in the VT process, depending on the lengths of the alkyl chains. An even more interesting observation is that significant polytypic phase transitions with crystallographic symmetry changes accompanying the abrupt VT conversion are induced for the first time in CoC9bpy and CoC13bpy.

Valence Tautomerism in One-Dimensional Coordination Polymers

“…Three heating-cooling cycles (up to 300 K, 320 K and 340 K, respectively) show a reversible, incomplete VT transition with the highest recorded mT value of 3.00 cm 3 mol -1 K at 340 K. Full reversibility suggests no solvent loss below 340 K, which is consistent with the thermogravimetric data ( Figure S3). The magnetic properties of 2•2EtOH differ from those reported recently by Tao has been previously achieved in molecular materials through the presence of crystallographically inequivalent interconverting molecules in the lattice, 60,61 or arising from weak electronic coupling between VT moieties in dinuclear Co-dioxolene systems. 57,62,63 In the present case, such behavior is most likely due to the existence of two non-equivalent 1D chains, with cis and trans disposition of pyridyl ligands around cobalt centers, which results in different relative stabilities of the two tautomers.…”

Coexistence of Two Thermally Induced Intramolecular Electron Transfer Processes in a Series of Metal Complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)2] (M=Co, Fe, and Ni) bearing Non‐Innocent Catechol‐Based Ligands: A Combined Experimental and Theoretical Study

“…Previous density functional studies on related catechol-based systems to assign electronic transitions have already been published. [18] Among them, the band centered at l = 390 nm is of especial interest. This band can be attributed to the radical character of the Cat-N-SQ 2À ligand giving direct information on its oxidation state and therefore on the electronic distribution of the complex at a given temperature.…”

“…[16] Even more interesting was the fact that this complex also exhibits a reversible thermally induced IET between the metal ion and the ligand, a process named with the generic term of valence tautomerism (VT). [17] Therefore, this family of complexes contains unique systems that exhibit more than one type of thermally induced, intramolecular, electron-transfer process-ligand-to-metal (IET LM ) [18] and ligand-to-ligand (IET LL ). [19] A schematic representation of the different thermally induced IET processes that take place in a metal-based complex bearing catechol-based noninnocent ligands is shown in Figure 1.…”

Modulable cooperativity in a valence tautomeric complex functionalized with branched alkyl chains

“…1,2,4 The introduction of effective intermolecular interactions such as coordination bonds, hydrogen bonds and/or p-stacking, enhances cooperativity, resulting in increased abruptness, wide hysteresis width, and large ON/OFF signal ratio in bistable transformations. 2,4,5 Moreover, further enhancement of cooperativity coexisting with structural flexibility results in macroscopic phase transformations such as solid-solid, 6,7 solid-liquid crystal 8 or liquid crystal-melt 9 transitions synchronized with the molecular bistabilities. Changes in macroscopic properties, such as viscosity, reflectance and dielectric polarization derived from assembled molecules would turn these materials into a practical reality because macroscopic signals could be readily detectable, in contrast to inherently weak physicochemical signals from individual molecules.…”

“…6,11 The observed cooperativity is derived in part from the nature 45 of the linear alkyl chains, which show relatively high crystallinity. In contrast, a key strategy toward bistable materials with modulable cooperativity would require modulable intermolecular interactions.…”