Tuning the coordination behavior of an unexplored asymmetric multidentate ligand for developing diverse heterometallic architectures with luminescent and magnetic properties

Abstract

Recently, by tuning the coordination behavior of a new asymmetric multidentate ligand, H₃L, containing two nitrogen and six oxygen donors by both metal centres and anions, H₃L = 1-(2-hydroxy-3-methoxy-benzamido)-2-(2-hydroxy-3-methoxy-benzylideneamino)-2-hydroxy propane, three types of heterometallic compounds with the chemical composition of [Zn₈Ln₂L₂(OH)₄(OAc)₁₂]·xCH₃OH·yCH₃CN·zH₂O (Zn₈Ln₂, where Ln = Nd, Tb, Dy and Er), [Ni₃Tb₂(HL)₂(OH)₄(OAc)₆]·3CH₃CN·H₂O (Ni₃Tb₂), and [Ni₂Tb₂(HL)₂(OAc)₄(NO₃)₂(H₂O)₂]·4CH₃CN·3H₂O (Ni₂Tb₂) were developed. Structural analysis revealed that four ellipsoidal decanuclear Zn^II–Ln^III clusters resulted from the assembly of H₃L with Zn(OAc)₂·2H₂O and Ln(OAc)₃·6H₂O. Replacement of Zn(OAc)₂·2H₂O with Ni(OAc)₂·4H₂O rendered a butterfly pentanuclear Ni^II–Ln^III cluster, and upon further using Tb(NO₃)₃·6H₂O to replace Tb(OAc)₃·6H₂O, a tetranuclear Ni^II–Ln^III metallocycle was obtained. The luminescence studies on these compounds indicated that the Zn^II-containing chromophore is a good antenna for sensitizing the characteristic emission of Ln³⁺ effectively from the visible to near-infrared region as a result of its energy match, perfect shielding and less concentration quenching compared to its Ni^II analogues, where the d → d transitions characteristic of Ni²⁺ were observed in the excitation spectra when irradiated. Notably, weak emission bands characteristic of Tb³⁺ were observed in the spectrum of Ni₂Tb₂ but not in that of Ni₃Tb₂ when excited by the d→d transition, which may be attributed to the difference in crystal field. Furthermore, the magnetic susceptibility studies indicate that the magnetic exchange for the Tb^III ions in Zn₈Tb₂, Ni₃Tb₂ and Ni₂Tb₂ is ferromagnetic, whereas that for the Dy^III ions in Zn₈Dy₂ is antiferromagnetic. Our results are of great significance for the rational design and synthesis of novel heterometallic supramolecular structures with interesting luminescence and magnetic properties.