Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand-VOF3 Bonding with Evidence of a Minute π Back-Donation of Fluoride

Reaction of vanadium(V) oxide trifluoride (VOF3) and the new naked fluoride reagent [(L-Dipp)H][F] (L-Dipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) leads to the isolation of [(L-Dipp)H][VOF4] (1) where the long sought discrete [VOF4](-) anion was finally obtained. The neutral [(L-Dipp)VOF3] (2) complex was synthesized by a similar reaction between VOF3 and bulky N-heterocyclic carbene (NHC) ligand LDipp. In this context, we analyzed, by means of DFT calculations, intermolecular interactions between [(L-Dipp)VOF3] (2) complexes in the crystal structure and realized that these interactions have a significant effect on the V-F-trans bond length. We further scrutinized ligand bonding within [(L-Dipp)VOF3] (2) and related complexes, because, in this kind of complexes, a rather short distance between CNHC and cis-halogen atoms has spurred some discussion about the type of interactions between them. We provide evidence of a minute p back-bonding into NHC ligands, which is larger for chloride [(NHC)VOCl3] than fluoride [(NHC)VOF3] complexes, although the fluoride ions are, counterintuitively and to a larger degree, involved in back-bonding than chloride ions. The influence of p back-bonding on V-F-trans and V-F-cis bond lengths was also rationalized. Finally, the hydrolysis of [(L-Dipp)VOF3] (2) product was studied and [(L-Dipp)H][VO2F2] (3) salt was obtained and characterized as the most stable product in this system.