Crystal structures and crystallization behaviors of a
series of
heterotrinuclear ZnII–LnIII–ZnII 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 ZnII 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 TbIII 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.