Highly stable and differentially arranged hexanuclear lanthanide clusters: structure, assembly mechanism, and magnetic resonance imaging

Abstract

The directed and controllable synthesis of lanthanide clusters with precise structures has received considerable research attention, however, progress in such remains sluggish. The steps performed in a reaction system under “black-box” conditions are unpredictable and have very low controllability. The precise customization of lanthanide clusters with the same number of cores but different arrangements is particularly difficult. Using bis-acylhydrazone-derived multidentate chelating ligands with different substituents, differentially arranged hexanuclear lanthanide clusters (Dy₆ and HNP-Dy₆) with identical core connections but different template-motif arrangements were constructed herein for the first time using a multidentate chelating coordination method. Specifically, Dy₆ with face-to-face and dislocation-arrangement template motifs was obtained using –N(Et)₂-substituted bis-acylhydrazone ligands with a strong steric hindrance effect. Changing –N(Et)₂ to a benzene ring with a strong π–π interaction yielded HNP-Dy₆ with inverted and coplanar arrangements of template motifs. The controllable construction of these two hexanuclear dysprosium clusters represented great progress in the precise synthesis of lanthanide clusters. High-resolution electrospray ionization–mass spectrometry (HRESI–MS) with different ion-source energies demonstrated the high stabilities of Dy₆ and HNP-Dy₆ in solutions. Time-dependent HRESI–MS tracked the formation processes of Dy₆ and HNP-Dy₆ and led to the following possible self-assembly mechanisms: L¹ + 2Dy → Dy₂L¹ → Dy₃L¹ → Dy₆(L¹)₂ and L² + 2Dy → Dy₂L² → Dy₃L² → Dy₅(L²)₂/Dy₆(L²)₂ → Dy₆(L²)₂. At 1-T magnetic field, the longitudinal and transverse relaxation rates of Gd₆ were 12.06 and 24.10 mM⁻¹ s⁻¹, respectively. Gd₆ with highly aggregated Gd(III) exhibited high relaxation rates, indicating its great potential as a T₁-weighted magnetic resonance imaging contrast agent. This work provides an example of the design and synthesis of lanthanide clusters with high stabilities and relaxation rates, taking a big step toward the precise and controllable synthesis of lanthanide clusters.