Coordination complexes of niobium and tantalum pentahalides with a bulky NHC ligand

The 1 : 1 molar reactions of niobium and tantalum pentahalides with the monodentate NHC ligand 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Ipr), in toluene (or benzene) at ca. 80 degrees C, afforded the complexes NbX5(Ipr) (X = F, 2; Br, 3) and TaX5(Ipr) (X = F, 4; Cl, 5; Br, 6), in generally good yields. Complexes 2-6 represent uncommon cases of stable NHC adducts of metal halides with the metal in an oxidation state higher than + 4, and also rare examples of Nb-NHC and Ta-NHC bonding systems. In particular, the X-ray molecular structure determined for 6 provides the unprecedented crystallographic characterization of a tantalum compound with a monodentate NHC ligand. DFT results indicate that the metal-carbon bond in 2-6 is a purely s one. According to NMR studies (H-1, C-13, Nb-93), the formation of 3, 5, 6, as well as the previously communicated NbCl5(Ipr), 1, proceeded with the intermediacy of [MX6](-) salts, presumably due to steric reasons. On the other hand, the intermediate formation of MF6- in the pathways to 2 and 4 was not observed, according to F-19 (and Nb-93 in the case of 2) NMR. DFT calculations were carried out in order to shed light on structural and mechanistic aspects, and allowed to trace possible reaction routes.