Theoretical exploration of single-molecule magnetic and single-molecule toroic behaviors in peroxide-bridged double-triangular {M II3Ln III3} (M = Ni, Cu and Zn; Ln = Gd, Tb and Dy) complexes

Abstract

Detailed state-of-the-art ab initio and density functional theory (DFT) calculations have been undertaken to understand both Single-Molecule Magnetic (SMM) and Single-Molecule Toroic (SMT) behaviors of fascinating 3d–4f {M₃Ln₃} triangular complexes having the molecular formula [M^II₃Ln^III₃(O₂)L₃(PyCO₂)₃](OH)₂(ClO₄)₂·8H₂O (with M = Zn; Ln = Dy (1), Tb (2) & Gd (3) and M = Cu; Ln = Dy (4), Tb (5) & Gd (6)) and [Ni₃Ln₃(H₂O)₃(mpko)₉(O₂)(NO₃)₃](ClO₄)·3CH₃OH·3CH₃CN (Ln = Dy (7), Tb (8), and Gd (9)) [mpkoH = 1-(pyrazin-2-yl)ethanone oxime]. All these complexes possess a peroxide ligand that bridges the {Ln^III₃} triangle in a μ³-η³:η³ fashion and the oxygen atoms/oxime of co-ligands that connect each M^II ion to the {Ln^III₃} triangle. Through our computational studies, we tried to find the key role of the peroxide bridge and how it affects the SMM and SMT behavior of these complexes. Primarily, ab initio Complete Active Space Self-Consistent Field (CASSCF) SINGLE_ANISO + RASSI-SO + POLY_ANISO calculations were performed on 1, 2, 4, 5, 7, and 8 to study the anisotropic behavior of each Ln(III) ion, to derive the magnetic relaxation mechanism and to calculate the Ln^III–Ln^III and Cu^II/Ni^II–Ln^III magnetic coupling constants. DFT calculations were also performed to validate these exchange interactions (J) by computing the Gd^III–Gd^III and Cu^II/Ni^II–Gd^III interactions in 3, 6, and 9. Our calculations explained the experimental magnetic relaxation processes and the magnetic exchange interactions for all the complexes, which also strongly imply that the peroxide bridge plays a role in the SMM behavior observed in these systems. On the other hand, this peroxide bridge does not support the SMT behavior. To investigate the effect of bridging ions in {M₃Ln₃} systems, we modeled a {Zn^II₃Dy^III₃} complex (1a) with a hydroxide ion replacing the bridged peroxide ion in complex 1 and considered a hydroxide-bridged {Co^III₃Dy^III₃} complex (10) having the formula [Co₃Dy₃(OH)₄(OOCCMe₃)₆(teaH)₃(H₂O)₃](NO₃)₂·H₂O. We discovered that as compared to the LoProp charges of the peroxide ion, the greater negative charges on the bridging hydroxide ion reduce quantum tunneling of magnetization (QTM) effects, enabling more desirable SMM characteristics and also leading to good SMT behavior.