Transition-metal(ii) complexes with a tripodal hexadentate ligand, 1,1,1-tris[2-aza-3-(imidazol-4-yl)prop-2-enyl]ethane, exhibiting incomplete total or absolute spontaneous resolution

Abstract

Crystal structures and crystallisation behaviours of a series of first-row transition-metal(II) complexes bearing 1,1,1-tris[2-aza-3-(imidazol-4-yl)prop-2-enyl]ethane (H₃L), [M^II(H₃L)]Cl(ClO₄) (M = Mn, Fe, Co, Ni and Zn) were examined. These compounds crystallise in an orthorhombic crystal system with a non-enantiogenic (Sohncke) space group P2₁2₁2₁, resulting in spontaneous resolution of the chiral complex cations. Hydrogen bonds between the imidazole N–H atoms in the tripodal ligand and chloride anions give enantiomorphic crystals with a homochiral three-dimensional network structure. In order to verify the spontaneous resolution of these compounds, solid-state circular dichroism spectra of the resulting single crystals were measured (KBr disk method). Unexpectedly, the observed spectra indicated that imbalanced formation of the enantiomorphic crystals (i.e., left-handed Λ-form vs. right-handed Δ-form complex cations) in all cases. Moreover, in the cases of Ni^II and Zn^II compounds, predominant enantiomorphic crystals formed by spontaneous resolution were always the same (in at least ten of our recrystallisation experiments). These observations suggest that there is a certain (but as yet unknown) factor that affects the predominant deposition of either enantiomorphic crystal when spontaneous resolution takes place from a solution of a racemic mixture in which rapid racemisation occurs.