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

The crystal structures and crystallisation behaviours of MII–TbIII–MII 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 MII 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 MII–LnIII–MII 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.