Supramolecular aggregates of single-molecule magnets: exchange-biased quantum tunneling of magnetization in a rectangular [Mn(3)](4) tetramer

The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn(3) units are reported: dimeric [Mn(6)O(2)(O(2)CMe)(8)(CH(3)OH)(2)(pdpd)(2)] (3) and [Mn(6)O(2)(O(2)CMe)(8)(py)(2)(pdpd)(2)](ClO(4))(2) (4), and tetrameric [Mn(12)O(4)(O(2)CR)(12)(pdpd)(6)](ClO(4))(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 (pdpdH(2)), either in direct synthesis reactions involving oxidation of Mn(II) salts or in metathesis reactions with the preformed complex [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) (1); complex 6 was then obtained by carboxylate substitution on complex 5. Complexes 3 and 4 contain two [MnIII2Mn(II)(μ(3)-O)](6+) and [MnIII3(μ(3)-O)](7+) units, respectively, linked by two pdpd(2–) groups. Complexes 5 and 6 contain four [MnIII3(μ(3)-O)](7+) units linked by six pdpd(2–) groups into a rectangular tetramer [MnIII3](4). Solid-state dc magnetic susceptibility studies showed that the Mn(3) subunits in 3 and 4 have a ground-state spin of S = 3/2 and S = 2, respectively, while the Mn(3) 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 Mn(3) single-molecule magnets (SMMs). High-frequency EPR studies of a microcrystalline powder sample of 5·2CH(2)Cl(2) 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·xCH(2)Cl(2) 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(3) unit is exchange-coupled to the two neighbors linked to it by the pdpd(2–) linkers, with an antiferromagnetic inter-Mn(3) 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.