Mn(III)-tris(benzoylacetonate) complex exhibiting a static rotational disorder: synthesis, crystal structure, magnetic properties, and DFT computations

The mixture of manganese acetates and the asymmetrical benzoylacetone ligand (bzacH) crystallized into a monomeric Mn(III)-tris(benzoylacetonato) complex, [Mn(bzac)3]. The crystal structure of [Mn(bzac)3] revealed a static rotational disorder, where half [Mn(bzac)3] molecule adopts the Λ configuration and the other is in the Δ configuration. Both molecules are mer-isomers, exhibiting Jahn-Teller elongation along two opposing Mn–O–CMe bonds (mer-Me-Me). [Mn(bzac)3] was further characterized by IR spectroscopy, thermogravimetric analysis (TGA/DTA), magnetic susceptibility measurements, UV-Vis spectroscopy, Hirshfeld surface analysis and Density functional theory (DFT) calculations. The Hirshfeld surface analysis, complemented by 2D fingerprint plots, confirms the presence of significant intermolecular interactions, including H···H, H···O, H···C hydrogen bonding, and C—H···π stacking interactions. Magnetic measurements reveal a high-spin Mn(III) center with an S = 2 state with relevant Zero-Field Splitting (D = -4.50 cm⁻¹, g = 2.00) and slow magnetization relaxation at low temperatures. DFT calculations support the experimentally observed high-spin state of [Mn(bzac)₃] and provide insights into its electronic structure. HOMO analysis suggests that the Jahn-Teller elongation along the axial Mn–O bonds arises from electrostatic interactions between the Mn 3dz² orbital and the pz orbitals of the axial oxygen atoms, leading to the energetic stabilization of the HOMO relative to the dx²−y² LUMO. DFT also identifies four possible Jahn-Teller elongation isomers: one facial (fac) and three meridional (mer) forms. Among these, the experimentally observed mer-Me-Me isomer becomes increasingly favorable at elevated temperatures.