Synthesis and characterisation of sterically bulky lithium amidinate and bis-amidinate complexes

Abstract

The synthesis and characterisation of the amidines, (tript)C(NR)(NHR), R = Pr i or cyclohexyl (Cy), tript = triptycenyl, and lithium amidinate complexes, [Li(THF) 2{(tript)C(NR) 2}] bearing the bulky triptycenyl substituent on the amidine or amidinate backbone carbon is described. NMR spectroscopic studies have shown these to exist solely as their Z- syn isomeric forms in solution due to the steric effect of the triptycenyl moiety. The X-ray crystal structures of two examples confirm this is also the case in the solid state. A new bis-amidine ligand, 1,4-{(Pr i HN)(Pr i N)C} 2{2,3,5,6-C 6( p-C 6H 4Bu t ) 4}, and the corresponding lithium bis-amidinate complex, [1,4-{Li(THF) 2(Pr i N) 2C} 2{2,3,5,6-C 6( p-C 6H 4Bu t ) 4}], which incorporate a sterically bulky tetraarylphenylene spacer unit have also been prepared. In solution, the amidine undergoes facile inter-conversion between its E- syn: E- syn and Z- syn: E- syn isomers. The bis-amidinate complex has been structurally characterised and shown to chelate both of its lithium centres.