Theoretical Study On The Magnetic Properties Of Single Molecule Magnets

The slow relaxation of magnetization and macroscopic quantum tunneling effect of single-molecule magnets （SMMs） is an important discovery in the field of molecular magnetism since the 90 s. SMMs attract great interests for their potential application in high-density information storage and quantum computing. In this paper, we selected several representative SMMs synthetized in recent years to explore the origin of the magnetic anisotropy and the magneto-structural correlations using quantum chemistry theory and methods. We hope our work could provide theoretical guidance for experimenter to synthesize new SMMs with high magnetic relaxation barriers and high blocking temperature.Firstly, we investigated the magneto-structural correlations in a mononuclear eight-coordinate complex [CoⅡ（12-crown-4）2]2+（1）.We gave the calculated D and E of complex 1 changed by two ways, and then analyzed the influence of changing a or φ on the magnetic anisotropy. Our analysis showed that the large twist angle (p far from zero may be responsible for the unexpected strong magnetic anisotropy of 1 since the combined β orbitals of the dz2 and dxy were axially elongated along the local main magnetic axis on Co（II） under the twist. What’s more, our calculations show that both of decreasing a and increasing φ may enhance its magnetic anisotropy, which was rationalized according to the variation of the combined β orbitals of the dz2 and dxy. Based on the above result, we predicted that the possible structure having the largest |D|value is the one with a= 52° and φ = 43 °.Secondly, to explore the origin of their high effective relaxation barriers in a family of two-coordinate cobalt imido complexes with NHC ligation in the form of [（NHC）CoNDmp] （NHC=IPr（1）,cyIPr （2）,sIPr （3）） using hybrid density functional theory B3LYP and difference-dedicated configuration interaction （DDCI3）, we calculated the exchange coupling interactions, the magnetic anisotropy and energy levels of them. Our calculations qualitatively indicate the super strong ferromagnetic coupling between the Co and N atoms. The energy separations of the ground and the first excited Kramers doublets (275.6,292.9,314.7, and 383.1 cm^-1, respectively) agree well with the corresponding magnetic relaxation barriers of 1-3 obtained from ac susceptibility measurements (215,254, and 363 cm^-1, respectively).What’s more, the spin-orbit energy level of [CoN]⁺(351.0 cm^-1)is in the same magnitude as those of 1-3, revealing that the inherent large magnetic anisotropy of the meta-ion [CoN]⁺ plays a dominant role on the magnetic anisotropies of the cobalt imido species. Those calculation studies revealed that the cobalt imido complexes can be viewed as the meta-ion [CoN]⁺ based SMMs, and that their high effective relaxation barriers originate from the inherent large magnetic anisotropy of the meta-ion [CoN]⁺.