A Mn4 single-molecule magnet with the defective-dicubane structure from the use of pyrenecarboxylic acid

Abstract

The reaction between Mn(pc)2·4H2O (pcH is 1-pyrenecarboxylic acid), N-methyldiethanolamine (mdaH2), and NEt3 in a 2:2:3 molar ratio in CH2Cl2 or CHCl3 gives the tetranuclear complex [Mn4(pc)4(mda)2(mdaH)2], isolated as the ·2CH2Cl2 (3) and ·2CHCl3 (4) solvates, respectively. Only crystals of 4 were of sufficient quality for single-crystal X-ray crystallography. Complex 4 possesses a [MnII2MnIII2] core with the Mn ions arranged in a planar rhombus that can be described as two Mn3 triangles fused at one edge. Additional monoatomic bridging by mda2− or mdaH−μ- and μ3-O atoms on each edge and at the center of the Mn3 triangular units gives a defective-dicubane core structure. Ligation is completed by two bridging and two monodentate pc− groups, each of which is involved in π–π stacking interactions with those on neighboring Mn4 molecules to give a 2D network. Variable-temperature solid-state magnetic susceptibility studies of 3 and 4 in the temperature range 5.0–300K. Various fitting and simulation models were employed in analyzing the data for 3, which shows no evidence of significant intermolecular interactions, and it was concluded that it contains three symmetry-inequivalent exchange interactions, J=+2.34cm−1, J′=+7.70cm−1, J″=−1.33cm−1 and g=1.99, where J′ is the MnIII⋯MnIII interaction, and J and J″ are the MnII⋯MnIII interactions. This indicates an S=8 ground state with a very low-lying an S=9 excited state. This conclusion was supported by fits of reduced magnetization data, which gave an S=8 ground state with axial zero-field splitting parameter D=−0.19(1)cm−1, and g=1.94(1). Ac susceptibility studies on 3 show that it is a single-molecule magnet (SMM). Complex 4 shows different magnetic behavior from 3 due to the intermolecular interactions.