Unique Hexanuclear Mixed Bimetallic Mn^III<inf>2</inf>Ni^II<inf>4</inf> Complex Possessing Both Jahn-Teller Tetragonally Elongated and Compressed Mn^III Ions: Spin Frustration

Herein, we report the synthesis, magnetic and theoretical studies of a mixed bimetallic and mixed-valence [MnIII2NiII4(N3)4(hep)4(OMe)2(OAc)4]·MeOH (1·MeOH; hep = 2-ethylhydroxypyridine) complex sharing a common MnIII-MnIII vertex between two defect dicubane NiII2MnIII2O4N2 cores. The single-crystal X-ray data were collected at two different temperatures, confirming that two Jahn-Teller axes of two adjacent MnIII ions in 1 are nearly parallel. One MnIII ion possesses an axis of Jahn-Teller elongation, while the other MnIII ion undergoes Jahn-Teller compression. DC and AC susceptibility measurements were carried out to predict the nature of magnetic exchange interactions between the metal ions and to understand the anisotropic behavior to rationalize its SMM behavior. DC measurements suggest that this ferromagnetically coupled Ni-Mn complex possesses a spin ground state (S) = 6-7 at 2 K, showing a maximum in χT vs T plot at 9.5 K. Fitting the susceptibility data using the PHI program yields three ferromagnetic J values and an antiferromagnetic J value between the metal ions and the reasonable magnetic anisotropy (D) for all the Ni(II) and Mn(III) metal ions. The dominant three ferromagnetic interactions, i.e., (i) the exchange interaction between Ni1-Ni4 and Ni2-Ni3 metal ions is +20.2 cm-1 (J1); (ii) the J2 exchange interaction pertains to the interaction between the Mn2 metal ion with all four Ni metal ions is +3.7 cm-1, (iii) the J4 exchange interaction between the Mn1 and Mn2 metal ions is +6.8 cm-1, which align all the metal ions spin in spin-up orientation, and the presence of a small antiferromagnetic exchange (−6.8 cm-1) contribution (J3; Mn1-Ni3/Ni4) aligns the Ni3 and Ni4 ion spin in between the spin-up/down orientation; representing the possible spin frustration and suggesting a possible spin ground state (S) of 6-7. The existence of spin frustration in the MnIII-NiII triangles cannot be completely ruled out since 1 is made of four hetero bimetallic MnIII-NiII triangular units. To fully comprehend the magnetic properties, extensive density functional theory (DFT) calculations were performed by using the B3LYP/TZV functional setup, yielding very good numerical estimates of Js, which are in agreement with the experimental values. Furthermore, high-level ab initio calculations were performed to calculate the zero-field splitting (D) for all Ni and Mn ions. These calculations predict different sign D values for both Mn(III) ions, which further supports the presence of two different JT-distorted Mn ions in this complex.