Slow Magnetic Relaxation of a 12-Metallacrown-4 Complex with a Manganese(III)-Copper(II) Heterometallic Ring Motif

The heterobimetallic metallacrown (MC), (TMA)$_2${Mn(OAc)$_2$[12-MC$_{Mn(III)Cu(II)N(shi)}$-4](CH$_3$OH)}$\cdot$2.90CH$_3$OH, 1, where TMA$^+$ is tetramethylammonium, $^-$OAc is acetate, and shi$^{3-}$ is salicylhydroximate, consists of a Mn$^{II}$ ion captured in the central cavity and alternating unambiguous and ordered manganese(III) and copper(II) sites about the MC ring, a first for the archetypal MC structure design. DC-magnetometry characterization and subsequent simulation with the Spin Hamiltonian $$H = -J_1(\mathbf{s}_1+\mathbf{s}_3)\cdot\mathbf{s}_5-J_2(\mathbf{s}_2+\mathbf{s}_4)\cdot\mathbf{s}_5-J_3\sum_{i=1}^{4}\mathbf{s}_i\cdot\mathbf{s}_{i+1} + d(s_{z,1}^2 + s_{z,3}^2) + \mu_B\sum_{j=1}^5 g_j\mathbf{s}_j\cdot\mathbf{B}$$ indicates an S = 5/2 ground state and a sizable axial zero-field splitting on Mn$^{III}$. AC-susceptibility measurements reveal that 1 displays slow magnetization relaxation akin to single-molecule magnet (SMM) behavior.