Diruthenium Tetraacetate Monocation, RuII/III2(O2CMe)4+, Building Blocks for 3-D Molecule-Based Magnets

Diruthenium tetracarboxylates monocations are utilized as building blocks for cubic 3-D network structured molecule-based magnets. Ru(II/III)(2)(O(2)CMe)(4)(3)M(III)(CN)(6) M = Cr (1a), Fe (2), Co (3) were prepared in aqueous solution. Powder X-ray diffraction indicates that they have body-centered cubic structures (space group = Imm, a = 13.34, 13.30, and 13.10 A for 1a, 2, and 3, respectively), which was confirmed for 1a by Reitveld analysis of the synchrotron powder data a = 13.3756(5) A. Ru(2)(O(2)CMe)(4)(3)M(III)(CN)(6).xMeCN M = Cr, x = 1.8 (1b); M = Mn, x = 3.3 (4) were prepared from acetonitrile. The magnetic ordering of 1a (33 K), 1b (34.5 K), 2 (2.1 K), and 4 (9.6 K) was determined from the temperature dependencies of the in-phase (chi') alternating current (AC) susceptibility. The field dependence of the magnetization, M(H), at 2 K for 1a showed an unusual constricted hysteresis loop with a coercive field, H(cr), of 470 Oe while the M(H) data for 1b, 2, and 4 showed a normal hysteresis loop with a coercive field of 1670, 10, and 990 Oe, respectively. The (57)Fe Mössbauer spectrum of 2 is consistent with the presence of low spin Fe(III) (delta = -0.05 mm/s; DeltaE = 0.33 mm/s) at room temperature, and the onset of 3-D magnetic ordering at lower temperature (<2 K). The effects of M(III) in M(III)(CN)(6)(3-), and the large zero-field splitting (D) of diruthenium tetracarboxylates are discussed. The increasing critical temperatures T(c), with increasing S could not be accounted for by mean field models without significantly different J values for 1a, 4, and 2. By fitting the T(c) data with mean field models H = -2JS(Ru).(S(M) - micro(B)(g(Ru)S(Ru) + g(M)S(M))H, J/k(B) are 4.46, 1.90, and 0.70 K for 1a, 4, and 2, respectively.