Molecular structures of dinuclear zinc(II) complexes of chiral tridentate imine and amine ligands: Effect of ligand geometry on diastereoselectivity

Abstract

Racemic mixtures of Schiff bases (L1H and L2H) were reduced to respective amines (L1′H and L2′H). Compounds of composition [Zn2(L1)2(NCO)2] (1), [Zn(L1)N3]n (2), [Zn2(L2)2(NCO)2] (3), [Zn2(L1′)2(NCO)2] (1′), [Zn2(L1)2(N3)2]·2DMF (2′·2DMF) and [Zn2(L2′)2(NCO)2]·DMF (3′·DMF) were synthesized using these ligands, Zn(NO3)2·6H2O and NCO− or N3− ions. Molecular structures of all six compounds have been established. Both imine and amine ligands have N2O donor set, but coordinate, respectively, in meridional and facial fashions. First four complexes are monochelates and last two are bis-chelates. The Zn2O2 unit in 1–3 has RS combination of imine where as in 1′ has RR(SS) combinations of amine. In 2′ and 3′, zinc(II) bis-chelates have cis,cis,trans-(OP)2(NA)2(NY)2 stereochemistry and cis-(OP)2 disposition further coordinate to tetrahedral zinc. Compound 1′ exhibits 1D hydrogen-bonded polymer by enantioselective H-bonding interaction. In 2′, DMF molecules assemble RR and SS bis-chelates through N–H⋯O interactions.