Diruthenium Tetraacetate Monocation, [Ru^II/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. [RuII/III2(O2CMe)4]3[MIII(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 = Im3̄m, a = 13.34, 13.30, and 13.10 Å for 1a, 2, and 3, respectively), which was confirmed for 1a by Reitveld analysis of the synchrotron powder data [a = 13.3756(5) Å]. [Ru2(O2CMe)4]3[MIII(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 (χ‘) 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, Hcr, 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 57Fe Mössbauer spectrum of 2 is consistent with the presence of low spin FeIII (δ = −0.05 mm/s; ΔE = 0.33 mm/s) at room temperature, and the onset of 3-D magnetic ordering at lower temperature (<2 K). The effects of MIII in [MIII(CN)6]3-, and the large zero-field splitting (D) of diruthenium tetracarboxylates are discussed. The increasing critical temperatures Tc, 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 Tc data with mean field models [H = −2JSRu.SM − μB(gRuSRu + gMSM)H], J/kB are 4.46, 1.90, and 0.70 K for 1a, 4, and 2, respectively.