Magneto-Structural Characterization of Heterometallic Ni^II₂Mn^II₂/Ni^II₂Co^II₂ Cubane Complexes with a Compartmental Ligand

We report a combined experimental and theoretical magneto-structural study of four heterometallic isostructural complexes: [Ni₂Mn₂(L)₂(OAc)₂{N(CN)₂}]·5H₂O (<b>1</b>), [Ni₂Mn₂(L)₂(OAc)₂(N₃)₂]·MeCN (<b>2</b>), [Ni₂Mn₂(L)₂(OAc)₂(NCS)₂]·H₂O (<b>3</b>), and [Ni₂Co₂(L)₂(OAc)₂(NCS)₂]·4H₂O (<b>4</b>). These complexes were synthesized using a compartmental Schiff base ligand, <i>N,N</i>'-bis(3-methoxy-5-methylsalicylidene)-2,2-dimethyl-1,3-diaminopropane (H₂L), and various pseudohalide ions. X-ray crystallography reveals cubane-like structures featuring a {Ni₂M₂(μ₃-O)₄} (M = Mn or Co) core. The Ni^II centers invariably exhibit octahedral geometry, while Mn^II and Co^II ions adopt hepta-coordinated pentagonal bipyramidal geometries. Magnetic studies, ab initio calculations, and BS-DFT analyses reveal competing ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. The heterometallic exchange via an additional μ-O,O'-acetate ligand is FM, contrasting with mostly AFM interactions through the two μ₃-O bridges. Ni^II ions in <b>1-4</b> display moderate easy-plane single-ion anisotropy, while complex <b>4</b> uniquely features hepta-coordinated Co^II ions with eas<i>y</i>-axis anisotropy of <i>D</i>_Co ≈ -23 cm^-1. Moreover, a field-induced slow magnetic relaxation was observed in <b>4</b>, governed by a direct relaxation process. Notably, this is the first report of a Ni₂Mn₂ cubane structure, with only one prior example of a Ni₂Co₂ cubane complex in the literature. Thus, this study uncovers the magneto-structural properties of rare Ni₂Mn₂ and Ni₂Co₂ systems while highlighting <b>4</b> as a unique case of hepta-coordinated Co^II ions with significant eas<i>y</i>-axis anisotropy and slow magnetization relaxation, enriching the molecular magnetism landscape.