Computational insights into the encapsulation of a Dy(iii) bis(amide)–alkene single-ion magnet within porous frameworks

Abstract

Recent advances in achieving very high blocking temperatures in Dy(III)-based single-molecule magnets have brought the field closer to the long-standing goal of molecular alternatives to conventional magnets; however, the highest-performing systems are predominantly organometallic Dy(III) complexes, underscoring the need to integrate such systems into robust porous matrices for device-oriented applications. Here, we investigate the encapsulation of a dysprosium bis(amide)–alkene single-ion magnet [Dy{N(SiⁱPr₃)[Si(ⁱPr)₂C(CH₃)CHCH₃]}{N(SiⁱPr₃)(SiⁱPr₂Et)}]⁺ (1) within two porous hosts: an AFI-type zeolitic imidazolate framework [Zn(Im)₂] (F1) and a Cd-based MOF [Cd(L)(bpy)] (F2). As 1 is cationic, the influence of the counter-anion under confinement was explicitly examined. Encapsulation with chloride in the Cd-MOF induces compression of the N–Dy–N angle and enhances transverse crystal-field contributions, reducing the calculated barrier to U_cal = 1250.7 cm⁻¹ from 1528 cm⁻¹. As the counter-anions are also anchored to the framework by non-covalent interactions, we have tested the stability of the guest molecule using ab initio molecular dynamics simulations. This study reveals that there is a strong driving force for the Cl⁻ ions to migrate within the pore and coordinate with the Dy centre, reducing the SIM performance. A relatively bulkier anion, such as a fluorinated aluminate model anion, Al(OCF₃)₄⁻, mitigates this problem and restores axial symmetry. Density functional theory calculations reveal favourable host–guest binding (−357 kJ mol⁻¹ for 1_Cl@F1 and −246.2 kJ mol⁻¹ for 1_Al(OCF3)4@F2). Noncovalent interaction analysis confirms extensive pore–guest contacts, and comparison of binding energies computed with and without the D3 correction indicates that dispersion accounts for ∼35% of the stabilisation in 1_Cl@F1 and ∼70% in 1_Al(OCF3)4@F2. CASSCF/RASSI-SO calculations demonstrate that the magnetic anisotropy barrier is largely retained upon encapsulation (U_cal = 1528 cm⁻¹ for 1, 1507.7 cm⁻¹ for 1_Cl@F1, and 1511 cm⁻¹ for 1_Al(OCF3)4@F2). The combination of strong host–guest binding and preservation of the magnetic barrier through judicious counteranion and host selection highlights encapsulation as a promising strategy for future applications of this class of molecules.