Solution and Solid-State Characterization of Donor-Free Alkali Metal Salts of Iminoenamide Ligands

Monoanionic [N,N] donor ligands, typified by the delocalized β-1,3-diketiminates, are important ligands for stabilizing metal complexes from across the periodic table. In contrast, non-delocalized iminoenamides maintain asymmetric donor atoms. We report the synthesis and structures of alkali metal salts of three iminoenamines: 2-(arylamido)-5-methylcyclopent-2-en-1-arylimine (aryl = 2,6-diisopropylphenyl, Dipp) and 2-(arylamido)-cyclohex-2-en-1-arylimine (aryl = mesityl, Mes, or Dipp). The successful deprotonation of all proligands was achieved when non-nucleophilic benzyl or tetramethylpiperidide bases were used. The mesityl-substituted proligand reacted with nBuLi or PhLi via deprotonation to give the desired lithium iminoenamide salt, whereas the Dipp-substituted proligands underwent nucleophilic addition. X-ray diffraction revealed dimeric structures through [N,N] coordination and through η6-arene interactions to a neighboring aryl group. However, a coordination polymer was formed for the potassium salt of the iminoenamide featuring N-Dipp substituents and a cyclohexene backbone because, unlike the methylcyclopentene backbone ligand, it does not feature a methyl group blocking this side of the enamide N-Dipp ring. The potassium salt of the N-mesityl ligand revealed coordination through only the N-atoms. DOSY NMR revealed a mixture of aggregation states in benzene solution, including monomers, benzene-coordinated monomers, and the presence of monomer/dimer equilibria. Sterically bulkier Dipp groups were more likely to lead to monomeric structures in solution.