Magneto-Structural Properties and Theoretical Studies of a Family of Simple Heterodinuclear Phenoxide/Alkoxide Bridged MnIIILnIII Complexes: On the Nature of the Magnetic Exchange and Magnetic Anisotropy

A family of Mn(III)Ln(III) strictly dinuclear complexes of general formula [Mn-III(mu-L)(mu-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)center dot 4H(2)O, Ln(NO3)(3)center dot nH(2)O, and NEt3 in MeOH. These compounds represent the first examples of fully structurally and magnetically characterized dinuclear Mn(III)Ln(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 LnO(9) coordination spheres, are linked by phenoxide/methoxide double bridging groups. Static magnetic studies show that the (MnGdIII)-Gd-III derivative exhibits a weak antiferromagnetic interaction between the metal ions, with a negative axial zero-field splitting D parameter. The (MnGdIII)-Gd-III complex shows a notable magnetocaloric effect with magnetic entropy change at 5 T and 3 K of -Delta 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.