Exchange-transferred anisotropy: an isotropic MnII ion governs single-molecule magnetism in a {Mn₉} double-decker metallacrown

A mixed-valence {MnIIMnIII₈} (1) cluster with a double-decker structure comprising a central eight-coordinate MnII atom surrounded by a coupled array of eight five-coordinate MnIII atoms was isolated. Magnetic measurements revealed an S = 5/2 spin ground state associated with a large molecular D value of −2.5 cm⁻¹. Complex 1 behaves as a single-molecule magnet (SMM) exhibiting entirely visible out-of-phase magnetic susceptibility signals (χ''_M$) and open hysteresis loops below 2 K, with an effective energy barrier for the magnetization reversal of 16.6 K. Computational analysis uncovers the origin of the unexpectedly strong magnetic anisotropy of the central MnII ion in complex 1, revealing that it emerges from a synergistic interplay between the pronounced axial anisotropy of the surrounding MnIII ions and the substantial exchange coupling that links them. To the best of our knowledge, this represents the first example where an isotropic ion (MnII) effectively controls the SMM behavior of the entire cluster. This counterintuitive phenomenon arises only when three stringent conditions are simultaneously satisfied: (i) a high density of strongly anisotropic magnetic neighbors, (ii) efficient magnetic communication through relatively strong antiferromagnetic exchange interactions, and (iii) a geometric arrangement that maximizes anisotropy transfer to the isotropic center.