Magneto-Structural Properties and Theoretical Studies of a Family of Simple Heterodinuclear Phenoxide/Alkoxide Bridged Mn^IIILn^IIIComplexes: On the Nature of the Magnetic Exchange and Magnetic Anisotropy

Abstract

A family of Mn^IIILn^III strictly dinuclear complexes of general formula [Mn^III(μ-L)(μ-OMe)(NO₃)Ln^III(NO₃)₂(MeOH)] (Ln^III = Gd, Dy, Er, Ho) has been assembled in a one pot synthesis from a polydentate, multipocket aminobis(phenol)ligand [6,6'-{(2-(1-morpholyl)ethylazanediyl)bis(methylene)}bis(2-methoxy-4-methylphenol)], Mn(NO₃)₂·4H₂O, Ln(NO₃)₃· nH₂O, and NEt₃ in MeOH. These compounds represent the first examples of fully structurally and magnetically characterized dinuclear Mn^IIILn^III complexes. Single X-ray diffraction studies reveal that all complexes are isostructural, consisting of neutral dinuclear molecules where the Mn^III and Ln^III metal ions, which exhibit distorted octahedral MnN₂O₄ and distorted LnO₉ coordination spheres, are linked by phenoxide/methoxide double bridging groups. Static magnetic studies show that the Mn^IIIGd^III derivative exhibits a weak antiferromagnetic interaction between the metal ions, with a negative axial zero-field splitting D parameter. The Mn^IIIGd^III complex shows a notable magnetocaloric effect with magnetic entropy change at 5 T and 3 K of -Δ S_m = 16.8 J kg^-1 K^-1. Theoretical studies were performed to support the sign and magnitude of the magnetic anisotropy of the Mn^III ion ( ab initio), to predict the value and nature of J_MnGd, to disclose the mechanism of magnetic coupling, and to establish magneto-structural correlations (DFT calculations). The results of these calculations are corroborated by quantum theory of atoms in molecule analysis (QTAIM). Finally, Mn^III-Dy^III and Mn^III-Er^III complexes show field-induced slow relaxation of the magnetization but without reaching a maximum above 2 K in the out-of-phase ac susceptibility. Ab initio calculations were also performed on Mn^III-Dy^III/Ho^III systems to unravel the origin behind the weak SMM characteristics of the molecules possessing two strongly anisotropic ions. The mechanism of magnetic relaxation was developed, revealing a large QTM/tunnel splitting at the single-ion level. Furthermore, the anisotropy axes of the Mn^III and Ln^III ions were calculated to be noncollinear, leading to reduction of the overall anisotropy in the molecules. Hence, the herein reported complexes demonstrate that a combination of two anisotropic metal ions does not guarantee SMM behavior.