The effect of solvent molecules on crystallisation of heterotrinuclear MII–TbIII–MII complexes with tripodal nonadentate ligands

Abstract

The crystal structures and crystallisation behaviours of M^II–Tb^III–M^II heterotrinuclear complexes, [(L)MTbM(L)]NO₃ (M = Mn and Zn; L³⁻ stands for a conjugated base of H₃L = 1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane), obtained from various organic solvents (MeOH, EtOH, CH₂Cl₂ and CHCl₃) were investigated. The trinuclear complex cation has two asymmetric centres (Δ or Λ) at two M^II sites as a result of the twisted tripodal arms of L³⁻. Single-crystal X-ray diffraction analysis revealed that all the analysed Zn–Tb–Zn complexes had homochiral structures (Δ,Δ- or Λ,Λ-enantiomers) in each single crystal; however, the type of crystallisation behaviour showed clear differences depending on the type of solvent molecule. Specifically, crystallisation from MeOH or CH₂Cl₂ resulted in the exclusive formation of the Λ-conglomerates with the Λ,Λ-enantiomers—a phenomenon we recently termed ‘absolute spontaneous resolution’. The analogous Mn–Tb–Mn complex crystallised from MeOH also resulted in the same phenomenon as that of Zn–Tb–Zn. In contrast, the meso-type (Δ,Λ) achiral isomer of the Mn–Tb–Mn complex was deposited for the first time in a series of M^II–Ln^III–M^II trinuclear complexes from a CH₂Cl₂ or EtOH solution. Density functional theory calculations were performed to compare the thermodynamic stability of homochiral (Λ,Λ) and meso-type (Δ,Λ) complex cations of [(L)MnTbMn(L)]⁺ in MeOH and EtOH. Results were consistent with the molecular structures observed in the crystallographic analysis of the compounds deposited from these solvents.