Substituted fullerenes as a promising capping ligand towards stabilization of exohedral Dy(iii) based single-ion magnets: a theoretical study

Abstract

Organometallic dysprosocenium-based molecular magnets are the forefront runners in offering giant magnetic anisotropy and blocking temperatures close to the boiling point of liquid nitrogen. Attaining linearity in the organometallic dysprosocenium complexes is the key to generating giant magnetic anisotropy and blocking barriers. In the present study, we have unravelled the coordination ability of the substituted fullerene (C₅₅X₅)⁻ (where X = CCH₃, B, and N) generated by fencing around the five-membered ring of fullerene towards stabilizing a new family of exohedral dysprosium organometallic complexes showcasing giant magnetic anisotropy and blockade barriers. Eight exohedral mononuclear dysprosium organometallic complexes, namely [Dy(η⁵-C₅₅X₅)(η⁴-C₄H₄)] (1), [Dy(η⁵-C₅₅X₅)(η⁵-Cp)]⁺ (2), [Dy(η⁵-C₅₅X₅)(η⁵-Cp*)]⁺ (3), [Dy(η⁵-C₅₅X₅)(η⁶-C₆H₆)]²⁺ (4), [Dy(η⁵-C₅₅X₅)(η⁸-C₈H₈)] (5), [Dy(η⁵-C₅₅X₅)₂]⁺ (6) (where X = CCH₃), [Dy(η⁵-C₅₅B₅)₂]⁺ (7) and [Dy(η⁵-C₅₅N₅)₂]⁺ (8), were studied using scalar relativistic density functional theory (SR-DFT) and the complete active space self-consistent field (CASSCF) methodology to shed light on the structure, stability, bonding and single-ion magnetic properties. SR-DFT calculations predict complexes 1–8 to be highly stable, with a strictly linear geometry around the Dy(III) ion in complexes 6–8. Energy Decomposition Analysis (EDA) predicts the following order for interaction energy (ΔE_int value): 5 > 1 > 2 ≈ 3 > 6 > 7 > 8 > 4, with sizable 4f-ligand covalency in all the complexes. CASSCF calculations on complexes 1–8 predict stabilization of m_J |±15/2〉 as the ground state for all the complexes except for 5, with the following trend in the U_cal values: 6 (1573 cm⁻¹) ≈ 3 (1569 cm⁻¹) > 1 (1538 cm⁻¹) > 8 (1347 cm⁻¹) > 2 (1305 cm⁻¹) > 7 (1284 cm⁻¹) > 4 (1125 cm⁻¹) > 5 (108 cm⁻¹). Ab initio ligand field theory (AILFT) analysis provides a rationale for U_cal ordering, where π-type 4f-ligand interactions in complexes 1–4 and 6–8 offer giant barrier height while the large (C₈H₈)²⁻ rings generate δ-type interaction in 5, which diminishes the axiality in the ligand field. Our detailed finding suggests that the exohedral organometallic dysprosocenium complexes are more linear compared to bent [DyCp*₂]⁺ cations and display a giant barrier height exceeding 1500 cm⁻¹ with negligible quantum tunnelling of magnetization (QTM) – a new approach to design highly anisotropic dysprosium organometallic complexes.