A Design Criteria to Achieve Giant Ising‐Type Anisotropy in Co^II‐Encapsulated Metallofullerenes

Abstract

Discovery of permanent magnetisation in molecules just like in hard magnets decades ago led to the proposal of utilising these molecules for information storage devices and also as Q‐bits in quantum computing. A significant breakthrough with a blocking temperature as high as 80 K has been recently reported for lanthanocene complexes. While enhancing the blocking temperature further remains one of the primary challenges, obtaining molecules that are suitable for the fabrication of the devices sets the bar very high in this area. Encouraged by the fact that our earlier predictions of potential single‐molecule magnets (SMMs) in lanthanide‐containing endohedral fullerenes have been verified, here we set out to undertake a comprehensive study on CoII‐ion‐encapsulated fullerene as potential SMMs. To study this class of molecules, we have utilised an array of theoretical methods ranging from density functional to ab initio CASSCF/NEVPT2 methods for obtaining reliable estimate of zero‐field splitting parameters D and E. Additionally, we have also employed, for the first time a combination of molecular dynamics based on DFT methods coupled with CASSCF/NEVPT2 methods to seek the role of conformational isomers in the relaxation of magnetisation. Particularly, we have studied, Co@C28, Co@C38 and Co@C48 cages and their isomers as potential target molecules that could yield substantial magnetic anisotropy. Our calculations categorically reveal a very large Ising anisotropy in this class of molecules, with Co@C48 cages predicted to yield D values as high as −127 cm−1. Our calculations on the smaller cages reveal the free movement of CoII ion inside the cage, leading to the likely scenario of faster relaxation of magnetisation. However, larger fullerene cages were found to solve this issue. Further models with incorporating units such as {CoOZn}, {CoScZnN} inside larger fullerenes yield axial zero‐field splitting values as high as −200 cm−1 with negligible E/D values. As these units represent a strong axiality coupled with a viable way to obtain air‐stable low‐coordinate CoII complexes, this opens up a new paradigm in the search of SMMs in this class of molecules.

Trigonal Prismatic Cobalt(II) Single-Ion Magnets: Manipulating the Magnetic Relaxation Through Symmetry Control

“…The negative sign of the D parameter is attributed to this transition, which occurs between orbitals with the same magnetic quantum number (m l ) value. 27 The larger magnitude of the D parameter for 2 can be explained using the ground to the first excited state energy separation. The smaller the energy separation between the ground to the first excited states, the larger will be the magnitude of the D parameter and vice versa (693.5 and 387.2 cm −1 respectively for 1 and 2, Figure S25 and Tables S11 and S12).…”

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

“…Recently, similar Co analogues have been found to possess very large negative D parameters. A comprehensive theoretical study including DFT bonding analysis, molecular dynamics (MD) combined with modern CASSCF/NEVPT2 methods was performed on various sized Co endohedral metallofullerenes (Co‐EMFs) starting from C 28 to C 82 cages to unravel the zero‐field splitting on these next‐generation potential SIMs [50] . Initial calculations on Co@C 28 ( 48 ), Co@C 38 ( 49 ) and Co@C 48 ( 50 ) cages and their isomers revealed a negative D parameter of −129 cm −1 with a small rhombic E / D value for the Co@C 38 isomer (see Figure 9 a and 9 b).…”

A Dicobalt(II) Single-Molecule Magnet via a Well-Designed Dual-Capping Tetrazine Radical Ligand

“…The largest contribution to the negative and large D parameter comes from the transition from the ground to the first excited state, mainly involving the magnetic orbitals of d x 2 – y 2 → d xy with the same magnetic quantum number ( m l ) value and the smallest energy separation (Figure a and Table S5). ,, It is important to mention that the estimated transverse component of anisotropy (| E | = 3.8 cm –1 ) for the Co­(II) ions is found to be significantly high, suggesting a higher probability of ground-state QTM at the single-ion level. ,, …”