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

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), t Bu (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 MnII 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 [MnIII2MnII(μ3-O)]6+ and [MnIII3(μ3-O)]7+ units, respectively, linked by two pdpd2- groups. Complexes 5 and 6 contain four [MnIII3(μ3-O)]7+ units linked by six pdpd2- groups into a rectangular tetramer [MnIII3]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/kB = -0.011 K (Ĥ = -2Jŝi ·ŝj convention). The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.