Synthesis and characterization of a series of sterically-hindered amidines and their lithium and magnesium complexes

A series of sterically hindered amidinate ligands containing terphenyl substituents at the carbon atom of the amidinate backbone have been synthesized in high yields; their lithium derivatives were prepared, including the first two examples of monodentate lithium amidinates. Additionally, two independent routes to magnesium amidinates have been developed, yielding both mono- and bis-amidinate magnesium complexes. The presence of substituents at the 2', 6', 2", and 6" positions on the terphenyl moieties attached to the amidinate backbone induces a strong steric effect. In the free-base amidines, equilibria between E-syn and Z-syn isomers were dependent upon the size of these groups, while in the lithium amidinates, the steric bulk of these ligands led to the formation of the first monodentate lithium amidinates. Finally, in the magnesium amidinates, short C-N bond lengths and reduced N-C-N bond angles were observed within the amidinate backbone.