Supramolecular aggregates of single-molecule magnets: exchange-biased quantum tunneling of magnetization in a rectangular [Mn3]4 tetramer

Abstract

The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn₃ units are reported: dimeric [Mn₆O₂(O₂CMe)₈(CH₃OH)₂(pdpd)₂] (3) and [Mn₆O₂(O₂CMe)₈(py)₂(pdpd)₂](ClO₄)₂ (4), and tetrameric [Mn₁₂O₄(O₂CR)₁₂(pdpd)₆](ClO₄)₄ (R = Me (5), ^tBu (6)). They were all obtained employing 3-phenyl-1,5-di(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH₂), either in direct synthesis reactions involving oxidation of Mn^II salts or in metathesis reactions with the preformed complex [Mn₃O(O₂CMe)₆(py)₃](ClO₄) (1); complex 6 was then obtained by carboxylate substitution on complex 5. Complexes 3 and 4 contain two [Mn^III₂Mn^II(μ₃-O)]⁶⁺ and [Mn^III₃(μ₃-O)]⁷⁺ units, respectively, linked by two pdpd²⁻ groups. Complexes 5 and 6 contain four [Mn^III₃(μ₃-O)]⁷⁺ units linked by six pdpd²⁻ groups into a rectangular tetramer [Mn^III₃]₄. Solid-state dc magnetic susceptibility studies showed that the Mn₃ subunits in 3 and 4 have a ground-state spin of S = 3/2 and S = 2, respectively, while the Mn₃ subunits in 5 and 6 possess an S = 6 ground state. Complexes 5 and 6 exhibit frequency-dependent out-of-phase (χ′′_M) ac susceptibility signals indicating 5 and 6 to be tetramers of Mn₃ single-molecule magnets (SMMs). High-frequency EPR studies of a microcrystalline powder sample of 5·2CH₂Cl₂ provided precise spin Hamiltonian parameters of D = −0.33 cm⁻¹, |E| = 0.03 cm⁻¹, B⁰₄ = −8.0 × 10⁻⁵ cm⁻¹, and g = 2.0. Magnetization vs. dc field sweeps on a single crystal of 5·xCH₂Cl₂ gave hysteresis loops below 1 K that exhibit exchange-biased quantum tunneling of magnetization (QTM) steps with a bias field of 0.19 T. Simulation of the loops determined that each Mn₃ unit is exchange-coupled to the two neighbors linked to it by the pdpd²⁻ linkers, with an antiferromagnetic inter-Mn₃ exchange interaction of J/k_B = −0.011 K (Ĥ = −2JŜ_i·Ŝ_j convention). The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.