1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes

Abstract

Another slice of pi: the addition of a second π-donor ligand engenders 1,2-addition and [2 + 2] cycloaddition reactivity across Nb-imido and Nb-oxo bonds.

Tantalum, easy as Pi: understanding differences in metal–imido bonding towards improving Ta/Nb separations

“…Reported Ta-imido complexes have been shown to react with CO 2 to form isocyanates and Ta-oxo/hydroxo products; 63,66 the reaction to form 2-Ta is unprecedented in tantalum chemistry. Additionally, despite Nb-carbamate complexes being reported, 59,67 2-Ta represents the first isolated Ta-carbamate complex. The TriNOx 3− tert -butyl groups in 2-Ta may be essential for stabilizing the Ta-carbamate moiety; steric demands between the carbamate tert -butyl group and the TriNOx-ligand tert -butyl groups are proposed to kinetically prevent an intramolecular rearrangement to extrude tert -butyl isocyanate product.…”

A Diverse Array of C–C Bonds Formed at a Tantalum Metal Center

“…Indeed, while there have been a handful of reports of 1,2-addition across highly polarized, group 4 metal–oxo bonds, − examples of such reactivity in group 5 metal complexes are uncommon. We recently reported an oxo imido Nb­(V) complex that was capable of adding secondary and tertiary silanes across the NbO bond, where we employed a π-loading strategy specifically to enhance the reactivity of the oxo ligand. In the present work, it is likely that capping the TaO moieties of 2-X with a silyl group serves to remove electron density from the already electron-rich Ta center, which is supported by several strongly electron-donating groups (i.e., the bis­(NHC)­borate, oxo, and enolate ligands).…”

Engendering reactivity at group 5-heteroatom multiple bondsviaπ-loading

“…Finally, in 2020, Bergman, Arnold, and co-workers reported the synthesis of a reactive, p-loaded oxo-imido complex. 30 As described earlier, imido-oxaazaniobacycle 42 can be accessed from bis(imido) 40 (Scheme 13); addition of Lewis bases to 42 resulted in expulsion of isocyanate to yield ve-coordinate oxoimido complexes 59 and 61 (Scheme 18). The pyridine derivative 59 was found to be unstable in solution, liberating half an equivalent of pyridine and dimerizing to give bis(m-oxo)-imido dimer 60, which was observed to be quite unreactive.…”

“…to a single metal center; these ligands compete to interact with the symmetrically available d p orbitals on the metal center, destabilizing and polarizing metalimido groups and thus engendering imido-based reactivity. [29][30][31][32] The p-loading strategy has found great utility in the realm of early transition metal imido chemistry, in particular through combinations of an imido group and the orbitally related cyclopentadienyl (Cp) ligand. 9,14,24,25,33,34 The p-loading strategy has proven particularly successful in expanding imido-based reactivity in group 5 bis(imido) transition metal complexes, relative to their unreactive mono(imido) congeners.…”

“…9,14,24,25,33,34 The p-loading strategy has proven particularly successful in expanding imido-based reactivity in group 5 bis(imido) transition metal complexes, relative to their unreactive mono(imido) congeners. 29,30 In this Perspective, we communicate how to implement p-loading to increase reactivity at group 5 transition metal-imido bonds, as well as highlight how these design principles can be extended to other p-donor ligands, such as oxos and suldos. This Perspective is organized as follows: we rst detail the electronic structure of bis(imido) complexes, and outline synthetic routes to bis(imido) complexes.…”