Understanding electrostatics and covalency effects in highly anisotropic organometallic sandwich dysprosium complexes [Dy(CR)] (where R = H, SiH, CH and = 4 to 9): a computational perspective.

In this article, we have thoroughly studied the electronic structure and 4f-ligand covalency of six mononuclear dysprosium organometallic sandwich complexes [Dy(CR)] (where R = H, SiH, CH; = 4 to 9; = 1, 3) using both the scalar relativistic density functional and complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) method to shed light on the ligand field effects in fine-tuning the magnetic anisotropy of these complexes. Energy decomposition analysis (EDA) and -based ligand field theory AILFT calculations predict the sizable 4f-ligand covalency in all these complexes. The analysis of CASSCF/NEVPT2 computed spin-Hamiltonian (SH) parameters indicates the stabilization of |±15/2〉 for [Dy(C(SiH))] (1), [Dy(C(CH))] (2) and [Dy(CH)] (3) complexes with the value of 1867.5, 1621.5 and 1070.8 cm, respectively. On the other hand, we observed |±9/2〉 as the ground state for [Dy(CH)] (4) and [Dy(CH)] (5) complexes with significantly smaller values of 237.1 and 38.6 cm respectively. For the nine-membered ring [Dy(CH)] (6) complex, we observed the stabilization of the |±1/2〉 ground state, with the first excited state being located ∼29 cm higher in energy. AILFT-NEVPT2 ligand field splitting analysis indicates that the presence of π-type 4f-ligand interactions in complexes 1-3 help generate the axial-ligand field, while the δ-type interactions in complexes 4-5 generate the equatorial ligand field despite the ligands approaching from the axial direction. As the ring size increases, φ-type interactions dominate, generating a pure equatorial ligand field stabilising |±1/2〉 as the ground state for 6. Calculations suggest that the nature of the ligand field mainly governs the values in the following order: 4f-L > 4f-L > 4f-L > 4f-L. Calculations were performed by replacing ligands with CHELPG charges to access the crystal field (CF) effects which suggests the stabilization of pure |±15/2〉 in all the charge-embedded models (1Q-6Q). Our findings point out that the crystal field and ligand field effects complement each other and generate a giant barrier for magnetic relaxation in the small ring complexes 1-3, while a relatively weak crystal field and adverse 4f-L/4f-L interactions diminish the SMM behaviour in the large ring complexes 4-6.