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

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