Modulating electron distributions by integrating ligands with metal molecules in THF

Abstract

While the complex structures and dynamic behavior of alkali metal cations in solvents have been fully understood, insights into the structures, properties and dynamic behaviors of metal dimers in aprotic solvents (e.g. tetrahydrofuran/THF) are quite scarce, and especially the bonding characteristics, valence electron distribution and effect of co-solvents (e.g. crown ether/18-crown-6 (18C6)) are poorly understood. We here perform ab initio molecular dynamics simulation characterization on K2 molecular solute in THF with and without co-solvent molecule (18C6). Our findings indicate that K2 can be integrated by different numbers of THF molecules and 18C6 (if available), forming various chemical identities with unique structures and electron distributions in THF solvent ((THF)xK-K(THF)y, x + y ≤ 9; (THF)x(18C6)K-K(THF)y, x + y ≤ 4). These identities have considerable lifetimes in time evolution and the structure fluctuation dynamics of solution can realize interconversion among these identities through removing and/or adding THFs from/to two K subcluster moieties. In general, compared with the gaseous K2, integration of THFs considerably expands the K2 electron cloud and exhibit much more active states featuring distorted diffuse distributions which can be viewed as electron-expanded chemical entities. Further 18C6 coordination can tightly bind an end K and keep one THF coordinating a K at the axis direction and another K keeps a low coordination status. Such a coordination mode largely forces valence electrons to THF bulk acting as the electron-expanded complexes. In particular, the coordination of 18C6 greatly polarizes the bonding electron, changing the K-K covalent bond into highly ionic one with a Kδ+-Kδ− core structure. All these identities present lower ionization potentials than that of the gaseous K2, and thus have highly reductive reactivity, in agreement with the experiments. This work provides a promising strategy for how to modulate the bonding electrons around the electron-flexible metal–metal bonds.