The peculiar case of conformations in coordination compounds of group V pentahalides with N-heterocyclic carbene and synthesis of their imidazolium salts

Abstract

Reaction of niobium or tantalum pentafluoride (MF5; M=Nb, Ta) with N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (LDipp) yields neutral [(LDipp)MF5] (M=Nb (1), Ta (2)) complex. Special attention in their characterization was given to the structural analysis and to the comparison of structural features to the equivalent chloride complexes. Particularly interesting is conformation of the MX5 (M=Nb, Ta; X=F, Cl) unit in relation to the bonded LDipp ligand. In presented compounds (1, 2) the MF5 group is in a staggered position, which concurs with DFT calculations of structurally optimized [(LDipp)NbF5]. In contrast, eclipsed orientation is preferred in equivalent chloride complexes, according to both experiment and DFT calculations. The preference between staggered and eclipsed conformations is attributed to an interplay between attractive C–H⋯X interactions and steric repulsion between equatorial halogen atoms and “wingtips” of the ligand. To help distinguish products detected by NMR spectroscopy, ionic [(LDipp)H][MF6] (M=Nb (3), Ta (4)) compounds were prepared in the reaction between MF5 and “naked” fluoride reagent [(LDipp)H][F]. Disorder in the anionic part of the [(LDipp)H][MF6] prevented unambiguous determination in the crystal structure. Therefore, additional crystallization systems were used and crystal structures of [(LDipp)H][MF6]·2MeCN (M=Nb (3a), Ta (4a)) and [(LDipp)H][MF6]·DCM (M=Nb (3b), Ta (4b)) solvates were obtained in which no disorder was observed.