Theoretical study of the magnetic anisotropy and magnetic tunnelling in mononuclear Ni(II) complexes with potential molecular magnet behavior

Abstract

Magnetic molecules that present a slow decay of their magnetization (molecular magnets) are very interesting both from a fundamental and applied points of view. While many approaches focus strongly in finding systems with strong magnetic anisotropy giving rise to large spin-reversal barriers, less is known on the behavior of magnetic tunneling, which is a also fundamental component of molecular magnet behavior. In this work we propose a model to describe both the spin-reversal barrier and magnetic tunneling in Ni(II) trigonal bipyramidal complexes, that could be easily extended to other transition-metal systems. Based on this model we show the criteria that lead to optimal complexes to find molecular magnet behavior. We test our proposal with multireference configuration-interaction (MRCI) and ligand-field-density-functional-theory (LF-DFT) first-principles calculations applied over several families of mononuclear Ni(II) complexes. As a salient result we find that the complex [NiCl3(Hdabco)2]+ (dabco is 1,4-diazabicyclo[2.2.2]-octane) displays both a very large magnetic anisotropy energy, 524 cm−¹, and a small tunneling splitting, 0.2 cm−¹, when compared to other systems containing the same metal, making it a very attractive potential molecular magnet. These values are reached due to the choice of ligands that favor a complete destruction of the Jahn-Teller distortions through the spin-orbit coupling and an unquenched orbital momentum.