Heterocyclic substituted methanides as promising alternatives to the ubiquitous nacnac ligand

Abstract

A series of group 13 complexes containing deprotonated bisheterocyclomethanes have been prepared and structurally as well as spectroscopically characterised. In the case of the parent neutral homo-disubstituted bisheterocyclomethanes bis-(benzoxazol-2-yl)-methane (abbreviated as (NCOC6H4)(2)CH2) (1) and bis-(benzothiazol-2-yl)-methane (abbreviated as (NCSC6H4)(2)CH2) (2), two interesting ligand systems were investigated with respect to their coordination abilities toward group 13 metals. Upon deprotonation of the acidic methylene bridge in each case a beta-diketiminate-like structure is formed, where the negative charge becomes delocalised about the whole ligand framework. There is a mesomeric stabilisation between the carbanionic and the amidic resonance structure. The ligands mentioned above were treated with AlMe3, AlMe2Cl and GaMe3 to achieve deprotonation of the backbone and coincidental metal complexation. The resulting complexes [Me2Al{(NCOC6H4)(2)CH}] (3), [Me2Al{(NCSC6H4)(2)CH}] (4), [ClMeAl{(NCOC6H4)(2)CH}] (5), [ClMeAl{(NCSC6H4)(2)CH}] (6), [Me2Ga{(NCOC6H4)(2)CH}] (7), and [Me2Ga{(NCSC6H4)(2)CH}] (8) were structurally characterized in the solid state by X-ray single crystal diffraction and in solution by different (hetero-nuclear) 1D/2D-NMR spectroscopic techniques. Each of those molecules shows a nearly planar arrangement with the group 13 metal cation coordinated by the two ring nitrogen atoms of the conjugated heterocycles. Furthermore, in this work (NCOC6H4)(3)C(C3H7) (9) could be isolated, which might turn out to be a promising analogue of the omnipresent trisoxazoline ligands in asymmetric catalysis.