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

Abstract

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 °C, afforded the complexes NbX5(Ipr) (X = F, ; Br, ) and TaX5(Ipr) (X = F, ; Cl, ; Br, ), in generally good yields. Complexes 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 provides the unprecedented crystallographic characterization of a tantalum compound with a monodentate NHC ligand. DFT results indicate that the metal-carbon bond in is a purely σ one. According to NMR studies ((1)H, (13)C, (93)Nb), the formation of , , , as well as the previously communicated NbCl5(Ipr), , 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 and was not observed, according to (19)F (and (93)Nb in the case of ) NMR. DFT calculations were carried out in order to shed light on structural and mechanistic aspects, and allowed to trace possible reaction routes.

Early/Late Heterobimetallic Tantalum/Rhodium Species Assembled Through a Novel Bifunctional NHC‐OH Ligand

“…Crystallographic structures of Ta-NHC complexes are scarce in the literature. [62][63][64][65][66] The TaÀC NHC bond distances reporteds o far span the range 2.22-2.40 .T he TaÀC NHC bond distance in 3 (2.447(5) )i st hus the longest reported to date. This suggests that the Ta -NHC association might be loose and labile, and could eventually be displaced by another metal centre with a higher affinity for the NHC moiety (see below).…”

“…Crystallographic structures of Ta–NHC complexes are scarce in the literature . The Ta−C NHC bond distances reported so far span the range 2.22–2.40 Å.…”

“…Thed ifference in the metal-carbene interactions is well reflected by the metrical parameters. The TaÀC NHC bond distance in 4 (2.355(5) )i sr ather long and falls in the upper range of TaÀC NHC bond distances reported in the literature so far, [62][63][64][65][66] although slightly shorter than the correspondingd istance in 3 (2.447 (5) ). In contrast, the Ta=C21 bond distance is drastically shorter (1.921 (5) ), which reveals considerable multiple bond character and falls in the range of previously reported d 0 tantalum-alkylidene complexes.…”

Back-Donation in High-Valent d0 Metal Complexes: Does It Exist? The Case of NbV

“…[VOCl3(IMes)] (IMes = 1,3-dimesitylimidazol-2-ylidene), 13 [NbCl5(IPr)] (IPr = 1,3-bis(2,6diisopropyl-phenyl)imidazol-2-ylidene), 14 [TaBr5(IPr)], 15 [TaCl5(NHC)], 16 [MCl4(NHC)2][MCl6] (M = Nb, Ta, NHC = N-Heterocycle Carbene = IPr or IMes), 16 [NbOF3(IPr)]2 17 and [WO2Cl2(IPr)]. 18 All of the chloride complexes exhibit a peculiar structural arrangement: the chlorides cis to the carbene, forming a plane almost perpendicular to the NHC ring (Clax), are slightly bent toward the carbene itself, the angle Ccarbene-M-Clax ranging from 81.84° to 85.09° for [NbCl4(IPr)2] + and [TaCl4(IMes)2] + .…”

Stable coordination complexes of α-diimines with Nb(v) and Ta(v) halides

“…The 1 H and 13 C NMR spectra clearly exhibited one set of resonances due to the two equivalent α-diimine ligands. The 19 F NMR pattern consists of the typical decet related to the [NbF 6 ] − anion around 102 ppm, 26,27,28 and one additional resonance ascribable to the cation, occurring in the range 144.4-156.1 ppm. The 93 Nb spectra confirmed the presence of [NbF 6 ] − (heptet centred at 1555 ppm).…”