RESUMEN
To study the electronic structures and properties of [Crn(L)4Cl2] (n = 3, L = dpa: di(2-pyridyl)amido; n = 5, L = tpda: tripyridyldiamido; n = 7, L = teptra: tetrapyridyltriamine) metal string complexes, the BP86 method was used by considering the influence of the electric field (EF) applied parallel to the metal axis. As the EF increases, the migration of more positively charged Crodd is more significant than that of Creven, which results in alternating long-short Cr-Cr bonds. This happens because of the natural charges on the Crodd of 1-3, which are more electropositive than those on Creven. The electrons are pulled to the Cr and Cl(r) atoms at the high-potential side from Cl(l) at the low-potential side by the EF, which leads to asymmetrical FMOs. After the critical electric field (Ec), the configuration turns into a remarkably asymmetric one with alternating Cr-Cr quadruple bonds and weak interactions. The electrons are transferred from equatorial ligands (L) to metal chains. In the meantime, the asymmetry of the FMOs increases and the delocalization is further reduced, which affects the conductivity. Especially for [Cr7(teptra)4Cl2], the delocalized electrons of HOMO are completely transformed into a localized model after the critical electric field. It is observed that this supports the electric switching phenomenon ascribed to the conformations of delocalized and localized electrons. In addition, the longer the length of the metal chain, the smaller the Ec and the easier is for the complexes to be polarized by the EF.
RESUMEN
In recent years, metal-organic frameworks (MOFs) have been widely used in the field of electrochemical energy storage and conversion because of their excellent properties, such as high specific surface area, adjustable pore size, high porosity, structural diversity, and functional controllability. This paper reviews the applications of metal-organic framework-derived composites such as nitrogen-doped carbon, transition metal sulfides, transition metal selenides, transition metal phosphides and metal selenium compound modifications in potassium ion batteries (PIBs) as anode electrode materials. A variety of MOF-derived composites with different structures and morphologies based on several types of ligands, including 2-methylimidazole, aromatic carboxylic acids, and ferricyanide, have been discussed. Moreover, the current challenges faced by MOF-derived materials and possible countermeasures are proposed.