Mix and match: templating chiral Schiff base ligands to suit the needs of the metal ion

Abstract

One-pot reactions of 2,2′-bipyridine-6-carbaldehyde, (1S,2S)-(−)-1,2-diphenyl-1,2-diaminoethane and FeCl₂·4H₂O or Zn(OAc)₂·2H₂O (2 : 1 : 1) at room temperature in MeOH lead to [Fe{(S,S)-5}₂][PF₆]Cl or [Zn{(S,S)-5}₂][PF₆]₂ in which (S,S)-5 contains an imidazolidine ring, produced by intramolecular cyclization. This has been confirmed with the single-crystal structure of 2{P-[Fe{(S,S)-5}₂][PF₆]Cl}·H₂O. The diastereoselectivity observed in the solid state has been confirmed by NMR spectroscopy for solutions of [Fe{(S,S)-5}₂][PF₆]Cl and [Zn{(S,S)-5}₂][PF₆]₂. At room temperature, a minor product competes with the formation of [Fe{(S,S)-5}₂][PF₆]Cl, and the preference for these complexes is switched by carrying out the reaction in MeOH at reflux. In this case the major product is M-[Fe₂{(S,S)-4}₂][PF₆]₄ in which (S,S)-4 is the hexadentate Schiff base ligand formed by condensation of two equivalents of 2,2′-bipyridine-6-carbaldehyde with (1S,2S)-(−)-1,2-diphenyl-1,2-diaminoethane; the single-crystal structure of 4{M-[Fe₂{(S,S)-4}₂][PF₆]₄}·8Me₂CO·5MeCN·3H₂O confirms the assembly of a double helicate. When pyridine-6-carbaldehyde replaces 2,2′-bipyridine-6-carbaldehyde in the iron(II)-templated reaction with (1S,2S)-(−)-1,2-diphenyl-1,2-diaminoethane, the product is [Fe{(S,S)-7}₂][PF₆]₂ (3 : 2 mixture of diastereoisomers in solution) in which (S,S)-7 is an asymmetrical Schiff base, formed by reaction of only one of the amine groups in (1S,2S)-(−)-1,2-diphenyl-1,2-diaminoethane. The solid state structure of P-[Fe{(S,S)-7}₂][PF₆]₂·MeCN is presented.