Trinuclear <i>C</i>₃-Symmetric Extension of Jacobsen’s Catalyst: Synthesis, Characterization, and Catalytic Properties of a Chiral Trinuclear Mn^III Triplesalen Complex

The synthesis of a chiral version of a triplesalen ligand has been performed in two steps starting from 2,4,6-triacetyl-1,3,5-trihydroxybenzene (1). Reaction with excess trans-(1R,2R)-1,2-cyclohexanediamine and trans-(1S,2S)-1,2-cyclohexanediamine provided the chiral triplesalen half units 2,4,6-tris[1-((1R,2R)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2RR) and 2,4,6-tris[1-((1S,2S)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2SS), respectively. The two enantiomeric pure triplesalen ligands H6chandRR and H6chandSS were obtained by reaction of the triplesalen half units with 3,5-di-tert-butylsalicylaldehyde. Reaction with MnCl2·2H2O under basic aerobic conditions afforded the chiral trinuclear triplesalen complexes 3RR and 3SS. Single-crystal X-ray diffraction studies on both enantiomers showed the presence of the two ionization isomers [(chand){MnIIICl(MeOH)}3] and [(chand){MnIII(MeOH)2}3]Cl3 in the solid state, resulting in the formulation of 3 (either 3RR or 3SS) as [(chand){MnIIICl(MeOH)}3][(chand){MnIII(MeOH)2}3]Cl3. The crystal structures exhibit chiral hydrophobic channels of ∼8 Å diameter decorated with tert-butyl groups. These form left-handed helices in the 3RR enantiomer and right-handed helices in the 3SS enantiomer. Magnetic measurements are in accord with weak exchange interactions between the MnIII Si = 2 ions and strong local magnetic anisotropy as has been found in other trinuclear MnIII3 triplesalen complexes. As a proof-of principal, we have investigated the catalytic ability of the two enantiomers in the enantioselective epoxidation of unfunctionalized olefins. The chiral trinuclear MnIII3 triplesalen acts under non-optimized conditions as a catalyst with relatively good yields and moderate enantiomeric excess.