Chiral Recognition and Conglomerate Crystallization Induced by Self-Organization of Cobalt(III) Complexes of a Tripodal Ligand Containing Three Imidazole Groups

The effect of a counteranion on chiral recognition inducing conglomerate crystallization of a cobalt(III) complex is
reported. An achiral tripodal ligand involving three imidazole groups, tris{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H3L), was prepared by condensation of tris(2-aminoethyl)amine and 4-formylimidazole in a 1:3 mole
ratio. The reaction of H3L and trans-[CoIIICl2(py)4]+ afforded the chiral (Δ or Λ) [CoIII(H3L)]3+ complex. The formally
hemideprotonated complexes [CoIII(H1.5L)]X1.5·nH2O (where X = Cl, Br, I, BF4, ClO4, or PF6) were synthesized by
controlled deprotonation of the uncoordinated imidazole NH groups of [Co(H3L)]3+. In crystals of the hemideprotonated
complex, two components, [Co(H3L)]3+ and [Co(L)], with the same absolute configuration are linked by imidazole−imidazolate hydrogen bonds to form an extended homochiral 2D sheet structure, which is composed of a hexanuclear
unit with a trigonal void. There are two ways of stacking the sheets:  One is via homochiral stacking, and the other
is via heterochiral stacking. When the size of the counterion is small (i.e., X = Cl, Br, I, or BF4), adjacent homochiral
sheets with the same chirality are stacked to form a homochiral crystal (conglomerate). With large anions (i.e.,
ClO4- and PF6-), a homochiral sheet consisting of Δ enantiomers and a sheet consisting of Λ enantiomers are
stacked alternately to give a heterochiral crystal (a racemic crystal). Optically active Λ-[Co(H1.5L)](ClO4)1.5·H2O
was synthesized from Λ-[Co(H3L)]3+, and the crystal structure was compared to that of the racemic complex.
There are two conflicting factors governing the crystal structure:  the skeletal density; the size of the channels. The
2D sheets are more closely packed in the homochiral crystal than in the heterochiral crystal. However, the channels,
where the counterions are accommodated, are smaller in the homochiral crystal, and the steric congestion between
the anions increases with increasing anion size. The heterochiral crystal has a flexible, zigzag channel structure,
and the size of the channels can increase to accommodate larger anions. Thus, complexes with large anions (i.e.,
ClO4- and PF6-) preferentially form heterochiral crystals rather than homochiral crystals.