Hierarchical Assembly of a {Mn^II₁₅Mn^III₄} Brucite Disc: Step-by-Step Formation and Ferrimagnetism

Abstract

In search of functional molecular materials and the study of their formation mechanism, we report the elucidation of a hierarchical step-by-step formation from monomer (Mn) to heptamer (Mn₇) to nonadecamer (Mn₁₉) satisfying the relation 1 + Σ_<i>n</i>6<i>n</i>, where <i>n</i> is the ring number of the Brucite structure using high-resolution electrospray ionization mass spectrometry (HRESI-MS). Three intermediate clusters, Mn₁₀, Mn₁₂, and Mn₁₄, were identified. Furthermore, the Mn₁₉ disc remains intact when dissolved in acetonitrile with a well-resolved general formula of [Mn₁₉­(<i>L</i>)_<i>x</i>­(OH)_<i>y</i>­(N₃)_{36–<i>x</i>−<i>y</i>}]²⁺ (<i>x</i> = 18, 17, 16; <i>y</i> = 8, 7, 6; H<i>L</i> = 1-(hydroxy­methyl)-3,5-dimethylpyrazole) indicating progressive exchange of N₃^– for OH^–. The high symmetry (<i>R</i>-3) Mn₁₉ crystal structure consists of a well-ordered discotic motif where the peripheral organic ligands form a double calix housing the anions and solvent molecules. From the formula and valence bond sums, the charge state is mixed-valent, [Mn^II₁₅Mn^III₄]. Its magnetic properties and electrochemistry have been studied. It behaves as a ferrimagnet below 40 K and has a coercive field of 2.7 kOe at 1.8 K, which can be possible by either weak exchange between clusters through the anions and solvents or through dipolar interaction through space as confirmed by the lack of ordering in frozen CH₃CN. The moment of nearly 50 Nμ_B suggests Mn^II–Mn^II and Mn^III–Mn^III are ferromagnetically coupled while Mn^II–Mn^III is antiferromagnetic which is likely if the Mn^III are centrally placed in the cluster. This compound displays the rare occurrence of magnetic ordering from nonconnected high-spin molecules