Understanding electrostatics and covalency effects in highly anisotropic organometallic sandwich dysprosium complexes [Dy(CmRm)2] (where R = H, SiH3, CH3 and m = 4 to 9): a computational perspective

Abstract

In this article, we have thoroughly studied the electronic structure and 4f-ligand covalency of six mononuclear dysprosium organometallic sandwich complexes [Dy(C_mR_m)₂]^n+/− (where R = H, SiH₃, CH₃; m = 4 to 9; n = 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 ab initio-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 m_J |±15/2〉 for [Dy(C₄(SiH₃)₄)₂]⁻ (1), [Dy(C₅(CH₃)₅)₂]⁺ (2) and [Dy(C₆H₆)₂]³⁺ (3) complexes with the U_cal value of 1867.5, 1621.5 and 1070.8 cm⁻¹, respectively. On the other hand, we observed m_J |±9/2〉 as the ground state for [Dy(C₇H₇)₂]³⁻ (4) and [Dy(C₈H₈)₂]⁻ (5) complexes with significantly smaller U_cal values of 237.1 and 38.6 cm⁻¹ respectively. For the nine-membered ring [Dy(C₉H₉)₂]⁺ (6) complex, we observed the stabilization of the m_J |±1/2〉 ground state, with the first excited state being located ∼29 cm⁻¹ 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 m_J |±1/2〉 as the ground state for 6. Calculations suggest that the nature of the ligand field mainly governs the U_cal values in the following order: 4f-L_σ > 4f-L_π > 4f-L_δ > 4f-L_φ. Calculations were performed by replacing ligands with CHELPG charges to access the crystal field (CF) effects which suggests the stabilization of pure m_J |±15/2〉 in all the charge-embedded models (1_Q–6_Q). 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_δ/4f-L_φ interactions diminish the SMM behaviour in the large ring complexes 4–6.