Magneto-Structural Correlations in a Series of Pseudotetrahedral [Co^II(XR)₄]^2–Single Molecule Magnets: An ab Initio Ligand Field Study

Abstract

Over the past several decades, tremendous efforts have been invested in finding molecules that display slow relaxation of magnetization and hence act as single-molecule magnets (SMMs). While initial research was strongly focused on polynuclear transition metal complexes, it has become increasingly evident that SMM behavior can also be displayed in relatively simple mononuclear transition metal complexes. One of the first examples of a mononuclear SMM that shows a slow relaxation of the magnetization in the absence of an external magnetic field is the cobalt(II) tetra-thiolate [Co(SPh)4](2-). Fascinatingly, substitution of the donor ligand atom by oxygen or selenium dramatically changes zero-field splitting (ZFS) and relaxation time. Clearly, these large variations call for an in-depth electronic structure investigation in order to develop a qualitative understanding of the observed phenomena. In this work, we present a systematic theoretical study of a whole series of complexes (PPh4)2[Co(XPh)4] (X = O, S, Se) using multireference ab initio methods. To this end, we employ the recently proposed ab initio ligand field theory, which allows us to translate the ab initio results into the framework of ligand field theory. Magneto-structural correlations are then developed that take into account the nature of metal-ligand covalent bonding, ligand spin-orbit coupling, and geometric distortions away from pure tetrahedral symmetry. The absolute value of zero-field splitting increases when the ligand field strength decreases across the series from O to Te. The zero-field splitting of the ground state of the hypothetical [Co(TePh)4](2-) complex is computed to be about twice as large as for the well-known (PPh4)2[Co(SPh)4] compound. It is shown that due to the π-anisotropy of the ligand donor atoms (S, Se) magneto-structural correlations in [Co(OPh)4](2-) complex differ from [Co(S/SePh)4](2-). In the case of almost isotropic OPh ligand, only variations in the first coordination sphere affect magnetic properties, but in the case of S/SePh ligand, variations in the first and second coordination sphere become equally important for magnetic properties.

Modelling spin Hamiltonian parameters of molecular nanomagnets

“…40 DFT calculations have been employed to extract J values in a linear {Mn II -Mn III -Mn II } cluster [Mn 3 (Hcht) 2 (bpy) 4 ] 3+ (11) (HCht=cis,cis-1,3,5-cyclohexanetriol; see Figure 4b). 45 46 It is worth to note here that this Mn 6 cluster broke the record U eff barrier of the classical Mn 12 Figure 6b) are given in Table 1 and they yield excellent fit to the susceptibility data (see Figure 6c). Calculations in this case clearly suggest that the nextnearest neighbour interactions cannot be neglected as it can be some time strong enough to compete with the nearest-neighbour interactions.…”

“…The D parameter is particularly associated with Spin-Orbit coupling (SOC), Spin-Spin coupling (SSC) and structural distortion 12 observed in the coordination sphere (such as Jahn-Teller distortion; J-T). These parameters stabilize energy levels with large z-components of the magnetic moment.…”

“…In another attempt, calculations were carried out on [Gd III 6 Cu II 12 (OH)12 (L 3 ) 6 (NO 3 ) 7 -(OAc)3 (H 2 O) 12 ](OH)8 (76) 75 {here L 3 = Schiff base ligand, see reference 75 for details) wheel like structure. In another attempt, calculations were carried out on [Gd III 6 Cu II 12 (OH)12 (L 3 ) 6 (NO 3 ) 7 -(OAc)3 (H 2 O) 12 ](OH)8 (76) 75 {here L 3 = Schiff base ligand, see reference 75 for details) wheel like structure.…”

Magnetic Anisotropy in CoIIX4 (X=O, S, Se) Single‐Ion Magnets: Role of Structural Distortions versus Heavy Atom Effect

“…It is to be noted here that we have not considered another important torsional angle parameter of X‐Co‐X‐Y ( ω ) as the ligands here are bidentate in nature and therefore ω does not vary much across the structures. It is observed that the axial and rhombic anisotropies ( D and E / D ) strongly depend on angle 2 θ and dihedral angle θ d …”

A toolchain for the automatic generation of computer codes for correlated wavefunction calculations

“…The resulting excitation energies are 952, 4657, and 4703 cm 21 which are very close to previously obtained SORCI values of 954, 4771, and 4771 cm 21 . [48,49] Hence the FIC MRCI method, implemented via the ORCA-AGE or comparable tools, is probably the most rigorous variational post-CASSCF treatment routinely applicable to chemically relevant molecules.…”