Prediction of a model crystal structure for Ni₂F₅ by first-principles calculations

Abstract

Electrochemical fluorination in anhydrous HF, also known as the Simons process, is a widely used industrial method for fluorination of organic compounds. Its mechanism, being not so well understood, has long been debated and is believed to involve higher valent nickel fluorides formed on the nickel-plated anode during the process. One of these is speculated to be Ni2F5, which was previously reported in the literature and assigned via infrared spectroscopy, but its crystal structure is not yet known. We have identified known crystal structures of compounds with similar stoichiometries as Ni2F5 and utilized them as a starting point for our periodic DFT investigations, applying the PBE+U method. Ni2F5 as the most stable polymorph was found to be of the same crystal structure as another mixed valent fluoride, Cr2F5. The calculated lattice parameters are a = 7.24 Å, b = 7.40 Å, c = 7.08 Å and β = 118.9° with an antiferromagnetic ordering of the nickel magnetic moments.

Unravelling highly oxidized nickel centers in the anodic black film formed during the Simons process by in situ X-ray absorption near edge structure spectroscopy

“…The comparison of the averaged Ni-F distances of 1.88 and 1.83 Å for Ni III and Ni IV , respectively, with 2.02 Å in NiF 2 , supports our assignment of the features in the XANES spectra. In agreement with these theoretical results and the literature, 38,[44][45][46] we determined Ni-F distances of 1.75 (Ni IV ) and 1.88 Å (Ni II ) for NiF 3 and 1.80 Å for K 2 NiF 6 by their EXAFS (Fig. S40 and Table S2 †).…”