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

A family of new structurally manipulable pentagonal-bipyramidal (PBP) DyIII 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), which is compared with the reported imine-based DyIII analogues (1’-3’). Upon fine-modulating the type of nitrogen donor on 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 1CH3exhibit slower relaxation and enhanced anisotropy than the imine-based analogue, accompanied with the reorientation of magnetic easy axes. Conversely, the poor magnetic property is observed in 3 after reduction of imine. The changes of SMMs behavior and uniaxial anisotropies are rationalized by both experimental and theoretical studies. The H-substituted amine has formally larger magnitude of negative charge than that of imine due to polarized N-H σ bond. However the repulsion generated by sp3 N of amine to DyIII ion is also affected by the orientation of lone pair electron. The present work provides a feasible way to rational optimize the SMMs performance of DyIII complexes, highlighting the importance of the electronic property of equatorial donor in controlling the quantum tunneling and the magnetic relaxation for PBP DyIII-SMMs.