Exchange-transferred anisotropy: an isotropic Mn$^{II}$ ion governs single-molecule magnetism in a {Mn$_9$} double-decker metallacrown

A mixed-valence {Mn$^{II}$Mn$^{III}_8$} (1) cluster with a double-decker structure comprising a central eight-coordinate Mn$^{II}$ atom surrounded by a coupled array of eight five-coordinate Mn$^{III}$ 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 ($\chi$$^{''}_M$) and open hysteresis loops below 2 K, with an effective energy barrier for the magnetization reversal of 16.6 K. Computational analysis uncovers the origin of the unexpectedly strong magnetic anisotropy of the central Mn$^{II}$ ion in complex 1, revealing that it emerges from a synergistic interplay between the pronounced axial anisotropy of the surrounding Mn$^{III}$ ions and the substantial exchange coupling that links them. To the best of our knowledge, this represents the first example where an isotropic ion (Mn$^{II}$) 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.