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Abstract

Coordination chemistry of N‑heterocyclic carbenes (NHC) with early transition metals is often overlooked and in the shadow of the reactions done with the flagship NHC complexes with coinage and other mid to late transition metals. The attention toward NHC complexes with fluoride or oxide fluoride reagents of oxophilic metals further diminishes, due to their reactivity as well as special expertise and equipment required for their manipulation. Our intent was to expand the library of NHC‑based complexes with group 5 fluorides and to shed some light onto their characteristics. Newly prepared neutral compounds were [(NHC)VOF3], [(NHC)NbF5] and [(NHC)TaF5]. Recently, our laboratory synthesized from the NHC substrate a new “naked” fluoride reagent. The goal was to expand the study of fluoride donating capabilities for our imidazolium fluoride [(NHC)H][F] from the main group fluorides to transition metal fluorides and oxide fluorides. We correctly predicted that previously established sterically demanding properties of the imidazolium cation could be utilized to prepare and isolate long sought [VOF4]− anion in crystalline form. Furthermore, we also treated the pentafluoride reagents with the [(NHC)H][F] salt, which yielded otherwise abundantly represented [MF6]− anions, yet additional tests demonstrated the limits of fluoride donation potential. Isolated aforementioned neutral NHC complexes as well as [(NHC)H][VOF4], [(NHC)H][NbF6] and [(NHC)H][TaF6] salts were structurally characterized and analyzed with NMR and vibrational spectroscopy. Additionally, their stability in solid form and in solution was investigated. In the case of vanadium species, their hydrolysis afforded air stable [(NHC)H][VO2F2] salt.
Through careful observation of crystal structures and their comparison to the related compounds, several peculiarities were noted. To help us elaborate on these unusual features, a theoretical approach was employed. Part of calculations was the elucidation of structural difference in conformation of fluoride and chloride moiety in [(NHC)MX5] (M = Nb, Ta; X = F, Cl) and shift of fluorine atoms toward the NHC ligand. An especially interesting feature of oxophilic metals in their highest oxidation state is a formally empty valence orbitals and a hypothetically impossible situation for any kind of back‑donation to occur urged us to scrutinize the bonding situation in the square pyramidal [(L)VOF3] complexes. A minute π‑back‑donation of fluoride ligands to carbenic carbon atom was theoretically determined. Calculations also showed that elongation of the V–Ftrans bond is due to the internal crystal packing and should not be oversimplified and invariably ascribed to the trans effect.
Newly prepared salt with the [VOF4]− anion was used for the preparation of relatively rare anionic complexes [VOF4(THF)]−, [VOF4(py)]− and [(VOF4)2(pyz)]2−. We were able to associate the electron donating ligand onto the [VOF4]− anion, however their instability outside their mother liquor averted analysis beyond structural characterization. As such, the structural data was used to compare and speculate about possible contributing factors to the observed anion–ligand interaction.
Finally, during the preparation of soluble precursors (nitrile adducts of group 5 fluorides) we stumbled upon a system that produced easily and under atmospheric conditions a product of cyclotrimerization of nitriles [(PhTZN)H][MF6]. Oxonium salt [H3O][SbF6] was used for acidic initiation of the trimerization process. In specific reaction conditions with benzonitrile, we managed to obtain crystals of otherwise only theoretically proposed twice‑protonated triazine framework in [(PhTZN)H2][SbF6]2. Interestingly, in the same system tert‑butylnitrile yielded unusual carbonylamidine in [(tBuCA)H][SbF6], which is a hydrolyzed by‑product in the process of the ongoing cyclotrimerization.