The changing structural chemistry of lithium anilide on solvation by pyridine, 4-methylpyridine or 4-tert-butylpyridine

Abstract

A family of crystalline lithium anilide solvates, [{PhN(H)Li·(pyr)2}2] 1, [{PhN(H)Li·(4-Me-pyr)2}2] 2 and [{PhN(H)Li}4·(4-But-pyr)6] 3 has been synthesised by reacting the aromatic primary amide with two molar equivalents of the appropriate pyridine-based solvent (pyridine, 4-methylpyridine and 4-tert-butylpyridine, respectively) in hexane–toluene solution. X-Ray crystallographic studies have revealed three contrasting structures: 1 adopts a dinuclear, dimeric [(anilido)N–Li]2 ring arrangement with a transoid (anti) conformation of amido substituents; 2 adopts a similar arrangement but with a cisoid (syn) conformation of amido substituents; and 3 adopts a novel tetranuclear arrangement with a central [(anilido)N–Li]2 transoid ring, separating two mixed ligand [(anilido)N–Li-(pyr)N–Li] rings, made possible by the unusual μ-bonding of a 4-tert-butylpyridine ligand. A combination of 1H, 7Li and 13C NMR spectroscopic studies at 300 K suggests similar environments exist for corresponding atoms in [2H8]-toluene solutions of 1, 2 or 3. Further examination of the solution of 3 over the temperature window (300–193 K) has detected a fluxional structure involving the intramolecular exchange of two distinct types of anilido ligand, consistent with those present in the molecular structure of crystalline 3.