Fine-tuning the type of equatorial donor atom in pentagonal bipyramidal Dy(iii) complexes to enhance single-molecule magnet properties

Abstract

A family of new structurally manipulable pentagonal-bipyramidal (PBP) Dy^III SMMs, with formulas [Dy(Hbpen)(Cl)₃] (1), [Dy(Hbpen)Cl(OPhBr₂NO₂)₂] (2), [Dy(Hbpen)(OPhCl₂NO₂)₃] (3) and [Dy(Mbpen)(Cl)₃] (1CH₃), were controllably prepared based on a H-substituted amine-based ligand N,N′-bis(2-pyridylmethyl)-ethylenediamine (Hbpen) or a CH₃-substituted amine-based ligand N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-ethylenediamine (Mbpen) and are compared with the reported imine-based Dy^III analogues (1′–3′). Upon fine-modulating the type of nitrogen donor on the pentagon plane, the electronic effect is efficiently implemented to significantly modify the magnetic anisotropy and SMM behavior of PBP complexes. Notably, the amine-based 2 shows a three-fold improved energy barrier and an observable hysteresis opening up to 3 K. 1 and 1CH₃ exhibit slower relaxation and enhanced anisotropy compared to the imine-based analogue, accompanied by the reorientation of magnetic easy axes. Conversely, poor magnetic properties are observed in 3 after the reduction of imine. The changes in SMM behavior and uniaxial anisotropies are rationalized by both experimental and theoretical studies. The H-substituted amine has formally a larger magnitude of negative charge than imine due to the polarized N–H σ bond. However the repulsion generated by the sp³ N of amine towards the Dy^III ion is also affected by the orientation of lone pair electrons. The present work provides a feasible way to rationally optimize the SMM performance of Dy^III complexes, highlighting the importance of the electronic properties of an equatorial donor in controlling the quantum tunneling and the magnetic relaxation of PBP Dy^III-SMMs.