Rationalizing the sign and magnitude of the magnetic coupling and anisotropy in dinuclear manganese(iii) complexes

Abstract

We have synthesised twelve manganese(iii) dinuclear complexes, 1-12, in order to understand the origin of magnetic exchange (J) between the metal centres and the magnetic anisotropy (D) of each metal ion using a combined experimental and theoretical approach. All twelve complexes contain the same bridging ligand environment of one μ-oxo and two μ-carboxylato, that helped us to probe how the structural parameters, such as bond distance, bond angle and especially Jahn-Teller dihedral angle affect the magnetic behaviour. Among the twelve complexes, we found ferromagnetic coupling for five and antiferromagnetic coupling for seven. DFT computed the J and ab initio methods computed the D parameter, and are in general agreement with the experimentally determined values. The dihedral angle between the two Jahn-Teller axes of the constituent MnIII ions are found to play a key role in determining the sign of the magnetic coupling. Magneto-structural correlations are developed by varying the Mn-O distance and the Mn-O-Mn angle to understand how the magnetic coupling changes upon these structural changes. Among the developed correlations, the Mn-O distance is found to be the most sensitive parameter that switches the sign of the magnetic coupling from negative to positive. The single-ion zero-field splitting of the MnIII centres is found to be negative for complexes 1-11 and positive for complex 12. However, the zero-field splitting of the S = 4 state for the ferromagnetic coupled dimers is found to be positive, revealing a significant contribution from the exchange anisotropy - a parameter which has long been ignored as being too small to be effective.

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“…Previous studies of exchange coupled clusters highlighted the impact of the exchange coupling anisotropy on the splitting of spin state multiplets at zero magnetic field. [33,34] Therefore, in our case the four ferromagnetically anisotropic exchange coupled Mn IV ions should explain the relatively large effective zero field splitting.…”

“…Therefore,t he twospin model serves as proxy of the four-spin system for the lowenergy spin states.Crucially,the weak effective ferromagnetic coupling is of the same magnitude as the local anisotropy of Mn4, therefore the usual simplified assumptions regarding the strong exchange limit do not apply and cannot justify the high effective D value.Previous studies of exchange coupled clusters highlighted the impact of the exchange coupling anisotropy on the splitting of spin state multiplets at zero magnetic field. [33,34] Therefore,inour case the four ferromagnetically anisotropic exchange coupled Mn IV ions should explain the relatively large effective zero field splitting.…”

Computational studies of the magneto-structural correlations in a manganese dimer with Jahn–Teller distortions

“…41,42 The DFT calculations were performed using the unrestricted Kohn-Sham (UKS) approach, with the B3LYP density functional, 43,44 which has been frequently used in calculations of magnetic coupling parameters for Mn III dimers. 7,[12][13][14]45 The def2-TZVP basis set was used for Mn atoms and the def2-SVP basis was used set for all other atoms. [46][47][48] Selecting a larger basis set for metal centres and more reasonable basis sets for the other atoms is a common approach for transition metal compounds.…”

“…6,[11][12][13][14] In Mn III dimers with elongated coordination environments, the coupling can range from antiferromagnetic to ferromagnetic, with the relative orientation of the JT axes significantly affecting the magnetic exchange parameter. 12,13 Ferromagnetic coupling has been reported for dimers with both Mn III centres in JT compressed coordination environments. 11,15,16 Literature examples containing both Jahn-Teller elongated and compressed octahedra in the same compound are rare, [17][18][19][20][21][22][23][24][25][26][27] with few reports of discrete complexes possessing both types of distortion.…”

“…10,11 Several reports have found the type and orientation of JT distortion influences the sign and magnitude of magnetic exchange in bridged Mn III dimers. 6,[11][12][13][14] In Mn III dimers with elongated coordination environments, the coupling can range from antiferromagnetic to ferromagnetic, with the relative orientation of the JT axes significantly affecting the magnetic exchange parameter. 12,13 Ferromagnetic coupling has been reported for dimers with both Mn III centres in JT compressed coordination environments.…”