Electron Presolvation in Tetrahydrofuran-Incorporated Supramolecular Sodium Entities

Abstract

Alkali metal atoms can repopulate their valence electrons toward solvation due to impact from solvents or microsurroundings and provide the remaining alkali metal cations for coordinating with a variety of specific solvents, forming various electron-expanded complexes or solvated ionic pairs with special interactions. Such special solute–solvent interactions not only affect their electronic structures but also enable the formation of entirely new species. Taking Na­(THF) n (n = 1–6, THF = tetrahydrofuran) and Na2@THF complexes as typical representatives, density functional theory calculations are carried out to explore the solvation of a sodium atom and its dimer in THF and characterize their complexes as solvent-incorporated supramolecular entities and particularly valence electron presolvation due to their interaction with solvent THF. Electron presolvation is caused by the Pauli repulsion between THF containing a coordinating O atom with a lone pair of electrons and the alkali metal Na or Na2 containing valence electrons, and THF coordination to them forces their valence electrons to redistribute, which can be easily realized in such solvents. Compared with strongly bound valance electrons of alkali metal atoms, THF coordination enables Na or Na2 electrons to exhibit much more active states (i.e., the presolvated states) featuring small vertical detachment energies of electrons and distorted diffuse distributions in the frames of the generally structured metal cation complexes, acting as the electron-expanded chemical entities. Furthermore, the degree of electron diffusion and the polarity of the Na–Na bond are proportional to the coordination number (n) and the coordination number difference (Δn) between two Na centers in Na2@THF. The unique properties of such entities are also discussed. This work offers a theoretical support to the supramolecular entities formed by alkali-metal atoms or their dimers with ligands containing O or N and uncovers the unique electron presolvation phenomena and also enriches our understanding of the novel metal atom complexes.

A two-electron reducing reaction of CO2 to an oxalate anion: a theoretical study of delocalized (presolvated) electrons in Al(CH3)n(NH3)m, n = 0–2 and m = 1–6, clusters

“…In contrast to the case of the hexacoordinated sodium atom by tetrahydrofuran – Na(THF) 6 – for which the average interaction energy per ligand (AIE) between Na and O ( E int / n ) remains unchanged when the number of ligands increases from one to six, 30 the AIE is almost divided by three in going from Al(NH 3 ) − E int /1 = 11.2 kcal mol −1 – to Al(NH 3 ) 6 − E int /6 = 4.5 kcal mol −1 . Our energetic results are in agreement with conclusions obtained in an electron paramagnetic resonance study of the Al(NH 3 ) compound.…”

“…22 Several works have been devoted to understanding and rationalizing the process of solvated electron formation during alkali metal solvation in ammonia. 8,10,20,30,31 In the case of Na(THF) 6 , 30,31 the valence electron 3s loses its spherical symmetry as the number of THFs encircling the neutral Na atom increases from 1 to 6. The main cause of this deformation is indeed the Pauli repulsion between the valence electron of Na and the O lone pair electrons.…”

“…[25][26][27][28] The study of solvated metals in NH 3 has recently been applied to some transition metals. [21][22][23] Experimental and theoretical works have evidenced that the solvation of metals in different solvents (for example, ammonia, water, and tetrahydrofuran (THF)) gives rise to the three categories of solvated species: tetracoordinated complexes such as Li(NH 3 ) 4 , 6,18,29 hexacoordinated species such as Na(NH 3 ) 6 , 17,30 and octacoordinated compounds such as Y(NH 3 ) 8 . 22 Several works have been devoted to understanding and rationalizing the process of solvated electron formation during alkali metal solvation in ammonia.…”

A fresh perspective on metal ammonia molecular complexes and expanded metals: opportunities in catalysis and quantum information

“…14 Expanded metals are similar to organic electrides 15 but the ligands are solvent molecules known to solvate electrons, such as ammonia, 5 water, [16][17][18][19][20] dimethyl ether, 21,22 methanol, 23,24 methylamine, [25][26][27][28][29][30][31] acetonitrile, 32 hexamethylphosphoric triamide (HMPA) 33,34 and tetra-hydrofuran. 35 Electrides and expanded metals can be unified under the general category of solvent separated electrons from metals ions. 15 Among these species, electrides have been studied quite extensively both experimentally and computationally.…”