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Description
Magneto-structural correlations in a series of heterometallic isostructural cubane-type complexes with {Ni$_{2}$M$_{2}$($\mu_{3}$-O)$_{4}$} (M = Mn, Co) core: [Ni$_{2}$Mn$_{2}$(L)$_{2}$(OAc)$_{2}${N(CN)$_{2}$}]$\cdot 5$H$_{2}$O, [Ni$_{2}$Mn$_{2}$(L)$_{2}$(OAc)$_{2}$(N$_3$)$_{2}$]$\cdot$MeCN, [Ni$_{2}$Mn$_{2}$(L)$_{2}$(OAc)$_{2}$(NCS)$_{2}$]$\cdot$H$_{2}$O, and [Ni$_{2}$Co$_{2}$(L)$_{2}$(OAc)$_{2}$(NCS)$_{2}$]$\cdot 4$H$_{2}$O, were studied by magnetic studies, ab initio and Broken-Symmetry DFT calculations. This work marks the first-ever report of a Ni$_{2}$Mn$_{2}$ cubane structure, with only one prior structural example of a Ni$_{2}$Co$_{2}$ cubane complex. The estimated exchange interaction network in the cubane core is characterized by a competition between ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Specifically, the heterometallic exchange interaction mediated by the two $\mu_{3}$-O bridges is of AFM type, contrasting with FM exchange interaction when an additional $\mu$-O,O$^{\prime}$-acetate ligand is included in the exchange path. The Ni(II) centers exhibit octahedral coordination geometry yielding a moderate easy-plane anisotropy, while the Mn(II) and Co(II) ions adopt hepta-coordinated pentagonal bipyramidal geometries. The Ni$_{2}$Co$_{2}$ cubane features a rare example of hepta-coordinated Co(II) ions exhibiting easy-axis anisotropy with zero-field splitting parameter $D_{Co} \approx −23$ cm$^{–1}$. In addition, a field-induced slow magnetic relaxation was observed in Ni$_{2}$Co$_{2}$ cubane governed by a direct relaxation process.
Acknowledgements
Supported by CSIR, New Delhi, India (Sanction no. 01/3118/23/EMR-II), APVV-22-0172, APVV-23-0006, and National Competence Centre for High Performance Computing (project code: 311070AKF2, user project No. p696-24-2).
