Eight-coordinate fluoride in a silicate double-four-ring

Abstract

Fluoride, nature's smallest anion, is capable of covalently coordinating to eight silicon atoms. The setting is a simple and common motif in zeolite chemistry: the box-shaped silicate double-four-ring (D4R). Fluoride seeks its center. It is the strain of box deformation that keeps fluoride in the middle of the box, and freezes what would be a transition state in its absence. Hypervalent bonding ensues. Fluoride's compactness works to its advantage in stabilizing the cage; chloride, bromide, and iodide do not bring about stabilization due to greater steric repulsion with the box frame. The combination of strain and hypervalent bonding, and the way they work in concert to yield this unusual case of multiple hypervalence, has potential for extension to a broader range of solidstate compounds.hat is the maximum number of atoms to which a maingroup element may bond? Four, one would normally think. But, hypervalence and delocalized bonding enlarge the upper range--no one bats an eyelid at silicon binding 6 ligands, and there exist cases of arguably 10-coordinate Si (1, 2). Much thinking on high coordination focuses on atomic or ionic radii, and packing considerations. Or, one can approach the problem from a quantum-chemical perspective. In this paper, we follow the latter course, and operate at the crossing point between molecular and extended systems. We explore how the upper bound for coordination number is determined by attractive and repulsive forces, adding in a factor of strain, associated with deforming a molecular cage that entraps an element in its center.The stage for this work is set by a simple and common motif in catalysis and separation: the silicate double-four-ring (D4R), one of the archetypal building blocks in large-pore zeolites. By use of relativistic density-functional theory (DFT) calculations and Kohn-Sham molecular orbital theory (3-6), we will see that within this box-like environment, fluoride coordinates, actually binds, to eight silicon atoms.

Local Environment and Dynamic Behavior of Fluoride Anions in Silicogermanate Zeolites: A Computational Study of the AST Framework

“…For both SiO 2 -AST and GeO 2 -AST, the energy minimum is located at the center of the cage, and the energy increases smoothly when moving towards a corner. There is a good correspondence between the potential energy curve obtained for SiO 2 -AST and that calculated by Goesten et al for fluoride at the center of a silsesquioxane model of a d4r unit 54. The potential well is somewhat wider for GeO 2 -AST than for SiO 2 -AST due to the larger dimensions of the d4r cage.…”

Nucleophilic Substitution (SN2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

“…Frequency analysis of the optimized C(CN) 5 − species indeed revealed an imaginary frequency, corresponding to a TS. On a separate, but related quest for hypervalence, Goesten and co‐workers found that fluoride, the smallest existing anion, was capable of coordinating to eight silicon atoms. In this case, the “box” (a key building block of large pore zeolites referred to as double‐four‐ring) had the perfect size to bind F − at its center and set the scene for hypervalent bonding and significant charge transfer.…”

A hypervalent and cubically coordinated molecular phase of IF8predicted at high pressure

“…Isoelectronic XeF 7 + and TeF 7 – have been synthesized 16. With the exception of some caged ions,17 higher coordination numbers in neutral main group compounds are, to the best of our knowledge, non-existent. XeF 8 has been shown unstable even under pressures reaching 200 GPa 18.…”