Crystallization Behavior of Heterotrinuclear Zn^II-Ln^III-Zn^II Complexes Bearing Two Tripodal Nonadentate Ligands: Possibility for Absolute Spontaneous Resolution

Crystal structures and crystallization behaviors of a series of heterotrinuclear Zn^II-Ln^III-Zn^II complexes, [(L)ZnLnZn(L)]NO₃ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, and Y; H₃L = 1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane), were examined. Because the trinuclear complex cation has two asymmetric centers (Δ and Λ) at the Zn^II sites resulting from the twist of the tripodal ligand arms, it was confirmed from the single-crystal X-ray diffraction (SC-XRD) analysis that these complexes have homochiral structures (i.e., Λ,Λ- or Δ,Δ-enantiomer) in the solid state. Among the series of compounds, various types of spontaneous resolution were observed, which were classified by the SC-XRD analysis (using the Flack parameter) or solid-state circular dichroism spectroscopy. The latter technique was used to determine the chirality of every single crystal and that of an entire bulk mixture of microcrystals obtained in a particular crystallization process. It was found that the Tb^III complex, [(L)ZnTbZn(L)]NO₃·2MeOH (Zn-Tb-Zn), exclusively deposited the left-handed Λ-conglomerate in every crystallization experiment. This phenomenon may be termed “absolute spontaneous resolution.” It was also notable that only the lanthanoid(III) ions with an even number of 4f electrons (Eu, Tb, Ho, and Lu) exhibited this unusual phenomenon. It was also attempted to deposit the opposite-handed Zn-Tb-Zn Δ-conglomerate, which could not be isolated from the ambient crystallization. Seeding of a crashed single-crystal Δ-conglomerate of [(L)ZnYZn(L)]NO₃·2MeOH (Zn-Y-Zn), which was isomorphic to Zn-Tb-Zn, into a saturated methanolic solution of Zn-Tb-Zn gave, for the first time, crystals of the corresponding Δ-conglomerate.