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Supramolecular Aggregates of Single-Molecule Magnets: Exchange-Biased Quantum Tunneling of Magnetization in a Rectangular [Mn3]4 Tetramer

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The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn3 units are reported dimeric Mn6O2O2CMe8CH3OH2pdpd2 3 and Mn6O2O2CMe8py2pdpd2ClO42 4, and tetrameric Mn12O4O2CR12pdpd6ClO44 R Me 5, tBu 6. They were all obtained employing 3-phenyl-1,5-dipyridin-2-ylpentane-1,5-dione dioxime pdpdH2, either in direct synthesis reactions involving oxidation of MnII salts or in metathesis reactions with the preformed complex Mn3OO2CMe6py3ClO4 1 complex 6 was then obtained by carboxylate substitution on complex 5. Complexes 3 and 4 contain two MnIII2MnII3-O6 and MnIII33-O7 units, respectively, linked by two pdpd2 groups. Complexes 5 and 6 contain four MnIII33-O7 units linked by six pdpd2 groups into a rectangular tetramer MnIII34. Solid-state dc magnetic susceptibility studies showed that the Mn3 subunits in 3 and 4 have a ground-state spin of S 32 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 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 52CH2Cl2 provided precise spin Hamiltonian parameters of D 0.33 cm1, E 0.03 cm1, B04 8.0 105 cm1, and g 2.0. Magnetization vs. dc field sweeps on a single crystal of 5xCH2Cl2 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 JkB 0.011 K 2Jij convention. The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum com

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