NHC Complexes of High Valent Niobium and Tantalum Halides

Despite the outstanding properties that N-heterocyclic carbene ligands (NHC) may confer to the relevant metal complexes, the chemistry of NHC ligands has primarily focused on low and medium valent metals, while the isolation of derivatives of high valent metals is a harder task. This is particularly true for the halides of high oxidation state transition elements, due to the extreme moisture sensitivity usually exhibited by the related reaction systems [1]. We have synthesized a series of coordination adducts of MX5 (M = Nb, Ta; X = F, Cl, Br), NbOF3 and NbOX2 (X = Cl, Br) with bulky, stable NHC ligands. Rare examples of crystallographic characterization of Nb(Ta)NHC frames have been provided [2]. Moreover, DFT studies have been extensively carried out in order to shed light on mechanistic and structural features, the latter contributing to the knowledge of the yet poorly investigated high-valent metal-NHC bonding systems. A view of the X-ray structure of NbCl5(Ipr) is shown in the Figure (displacement ellipsoids are at the 30% probability level).