Mono-anionic acetophenone imine ligands: synthesis, ortho-lithiation and first examples of group (v) metal complexes

Abstract

A series of niobium and tantalum imido complexes with mono-anionic ortho-metallated acetophenone imine ligands have been prepared and characterized using NMR spectroscopy, mass spectrometry and elemental analysis. These low symmetry complexes are produced with only one or two structural isomers in all cases and display interesting correlations between the steric bulk of the ligands employed and the isomers formed. Crystal structures of several new niobium and tantalum complexes are presented as confirmation of the connectivity in these structural isomers.

Structural Characterization of Novel ortho‐Lithiated Imines

“…The only exception was H-L 2 , which, because of the low boiling point of tert-butylamine, was synthesized from an imido/oxo exchange reaction between Ti(NtBu)Cl 2 py 3 and 3Ј,4Ј-methylenedioxyacetophenone. [23] The subsequent lithiation of H-L 1-8 was performed in pentane at reduced temperature (Scheme 1), although for the imines with sterically very bulky groups bound to the nitrogen atom, such as 2,6-iPr 2 C 6 H 3 , lithiation was effective at room temperature with no reduction in yield or purity. Of this series of eight ortho-lithiated imines, Li-L 3 was found to be comparatively unstable, decomposing into unidentifiable products even at low temperature.…”

“…[18,23,24] Ligand Lithiation: The imine (1 equiv.) was dissolved in pentane, cooled to -78°C, and nBuLi (0.99 equiv.)…”

“…Compounds Li-L 1-2 , Li-L 4 , and Li-L 6-8 were prepared according to previously published methods. [23,24] H-L 3 : Viscous red oil (14 g, 80 %). Tables 5 and 6.…”

“…ortho-Lithiation of these imines proceeds readily to yield the deprotonated species, where regiospecific lithiation occurs at the position mutually ortho to both the methylenedioxy and imine groups. [23,24] To date, we have investigated the use of these anionic units as ancillary ligands in compounds of titanium, zirconium, niobium, and tantalum. [23][24][25] Due to the interesting NMR spectroscopic features observed for these ortho-lithiated imines and the general lack of structurally characterized examples of this class of molecules, we embarked on a study of the molecular structure of these ortho-lithiated imines, crystallized from a wide variety of nonpolar solvents.…”

“…[23,24] To date, we have investigated the use of these anionic units as ancillary ligands in compounds of titanium, zirconium, niobium, and tantalum. [23][24][25] Due to the interesting NMR spectroscopic features observed for these ortho-lithiated imines and the general lack of structurally characterized examples of this class of molecules, we embarked on a study of the molecular structure of these ortho-lithiated imines, crystallized from a wide variety of nonpolar solvents. In this contribution, we show the results of the crystallization of these species from both coordinating and noncoordinating solvents, the impact of steric bulk of the imine substituent, and the overall clustering observed in a series of eight ortholithiated imines.…”