Mn-8 and Mn-18 Clusters from the Use of 2-(Hydroxymethyl)pyridine, and Comparison with the Products from Bulkier Chelates: A New High Nuclearity Single-Molecule Magnet
Résumé
The synthesis, crystal structures, and magnetochemical characterization of two new Mn clusters [Mn8O2(O2CPh)(10)-(hmp)(4)(MeOH)(2)] (1; 6Mn(II), 2Mn(III)) and (Mn16O8(OH)(2)(O2CPh)(12)(hmp)(10)(H2O)(2)](O2CPh)(2) (2; 6Mn(II), 10Mn(III)) 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 hexadeca-nuclear 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 [MR4O3] cubanes, while that of 2 consists of a linked pair of complete [Mn4O4] cubanes, on either side of which is attached a tetrahedral [Mn-4(mu(4)-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 (chi(M)'') ac susceptibility signals at temperatures below 3K suggestive of a single-molecule magnet (SMM). Magnetization versus applied dc field sweeps on single crystals of 2 center dot 10MeOH down to 0.04 K exhibited hysteresis, confirming 2 to be a new SMM. Comparison of the structure of 2 (Mn-16) with Mn-12 or Mn-6 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.