In this paper we present the characterization of a complex with the formula [MnCa(hmp)(HO)(CHCN)](ClO) (1), where hmp-H = 2-(hydroxymethyl)pyridine. Compound 1 crystallizes in the monoclinic space group C2/c with the cation lying on an inversion centre. Static magnetic susceptibility, magnetization and heat capacity measurements reflect a unique Mn(iii) valence state, and single-ion ligand field parameters with remarkable large rhombic distortion (D/k = -6.4 K, E/k = -2.1 K), in good agreement with the high-field electron paramagnetic resonance experiments. At low temperature MnCa cluster behaves as a system of ferromagnetically coupled (J/k = 1.1 K) Mn dimers with a S = 4 and m = ±4 ground state doublet. Frequency dependent ac susceptibility measurements reveal the slow magnetic relaxation characteristic of a single molecule magnet (SMM) below T = 4 K. At zero magnetic field, an Orbach-type spin relaxation process (τ ∼ 10 s) with an activation energy E = 5.6 K is observed, enabled by the large E/D rhombicity of the Mn(iii) ions. Upon the application of a magnetic field, a second, very slow process (τ ∼ 0.2 s) is observed, attributed to a direct relaxation mechanism with enhanced relaxation time owing to the phonon bottleneck effect.
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