Improved Synthesis of Unsymmetrical N‐Aryl‐N′‐alkylpyridyl ß‐Diketimines Using Molecular Sieves and their Lithium Complexes

The synthesis of unsymmetrical β‐diketimines with N‐aryl‐N′‐alkyl substitution require at least two condensation steps and tedious purification procedures. An improvement in the preparation of four unsymmetrical N‐aryl‐N′‐alkylpyridyl β‐diketimine ligands (HL1–HL4) using 5‐Å molecular sieves is reported. The synthesis of four Li complexes (LiL1, LiL2, LiL3, and LiL4) containing N‐aryl‐N′‐alkylpyridyl β‐diketimines ligands is also presented. All four Li complexes were fully characterized by 1H NMR spectroscopic, 13C NMR spectroscopic, and elemental analyses. X‐ray structure analysis and spectroscopic results indicate that the Li β‐diketimine structures and ligand binding modes are governed by the bulkiness of the N‐aryl substituent and the length of the pyridyl arm of the N‐aryl‐N′‐alkyl β‐diketiminate ligand. Four unsymmetrical N‐aryl‐N′‐alkylpyridyl β‐diketiminate ligands HL1–HL4 were synthesized. Deprotonation of N‐aryl‐N′‐alkylpyridyl β‐diketimines using nBuLi yields the corresponding Li complexes. The observed binding mode differences were rationalized by considering the steric interference of the N‐aryl substituent and the length of the pyridyl arm of the N‐aryl‐N′‐alkyl β‐diketiminate ligand./