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

The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn units are reported: dimeric [MnO(OCMe)(CHOH)(pdpd)] () and MnO(OCMe)(py)(pdpd) (), and tetrameric MnO(OCR)(pdpd) (R = Me (), Bu ()). They were all obtained employing 3-phenyl-1,5-di(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH), either in direct synthesis reactions involving oxidation of Mn salts or in metathesis reactions with the preformed complex MnO(OCMe)(py) (); complex was then obtained by carboxylate substitution on complex . Complexes and contain two [MnIII2Mn(μ-O)] and [MnIII3(μ-O)] units, respectively, linked by two pdpd groups. Complexes and contain four [MnIII3(μ-O)] units linked by six pdpd groups into a rectangular tetramer [MnIII3]. Solid-state dc magnetic susceptibility studies showed that the Mn subunits in and have a ground-state spin of = 3/2 and = 2, respectively, while the Mn subunits in and possess an = 6 ground state. Complexes and exhibit frequency-dependent out-of-phase ('') ac susceptibility signals indicating and to be tetramers of Mn single-molecule magnets (SMMs). High-frequency EPR studies of a microcrystalline powder sample of ·2CHCl provided precise spin Hamiltonian parameters of = -0.33 cm, || = 0.03 cm, 04 = -8.0 × 10 cm, and = 2.0. Magnetization dc field sweeps on a single crystal of ·CHCl 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 Mn unit is exchange-coupled to the two neighbors linked to it by the pdpd linkers, with an antiferromagnetic inter-Mn exchange interaction of / = -0.011 K ( = -2 · convention). The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.