Mn₈ and Mn₁₆ Clusters from the Use of 2-(Hydroxymethyl)pyridine, and Comparison with the Products from Bulkier Chelates: A New High Nuclearity Single-Molecule Magnet

The synthesis, crystal structures, and magnetochemical characterization of two new Mn clusters [Mn8O2(O2CPh)10(hmp)4(MeOH)2] (1; 6MnII, 2MnIII) and [Mn16O8(OH)2(O2CPh)12(hmp)10(H2O)2](O2CPh)2 (2; 6MnII, 10MnIII) are reported. They were obtained from the use of 2-(hydroxymethyl)pyridine (hmpH) under the same reaction conditions but differing in the presence or absence of added base. Thus, the reaction of hmpH with Mn(O2CPh)2 in CH2Cl2/MeOH led to isolation of octanuclear complex 1, whereas the analogous reaction in the presence of NEt3 gave hexadecanuclear complex 2. Complexes 1 and 2 possess either very rare or unprecedented core structures that are related to each other: that of 1 can be described as a linked pair of incomplete [Mn4O3] cubanes, while that of 2 consists of a linked pair of complete [Mn4O4] cubanes, on either side of which is attached a tetrahedral [Mn4(μ4-O)] unit. Solid-state direct current (dc) and alternating current (ac) magnetic susceptibility measurements on 1 and 2 establish that they possess S = 5 and 8 ground states, respectively. Complex 2 exhibits frequency-dependent out-of-phase (χM′′) ac susceptibility signals at temperatures below 3 K suggestive of a single-molecule magnet (SMM). Magnetization versus applied dc field sweeps on single crystals of 2·10MeOH down to 0.04 K exhibited hysteresis, confirming 2 to be a new SMM. Comparison of the structure of 2 (Mn16) with Mn12 or Mn6 clusters previously obtained under the same reaction conditions but with two Me or two Ph groups, respectively, added next to the alkoxide O atom of hmp− indicate their influence on the nuclearity and structure of the products as being due to the overall bulk of the chelate plus the decreased ability of the O atom to bridge.