Nonanuclear Oxide-Bridged Manganese Complex. Preparation, Structure, and Magnetic Properties of (salH2 = Salicylic Acid; pyr = Pyridine)

Article abstract of DOI:10.1021/ja00211a022

The preparation of a mixed-valence nonanuclear manganese complex with mixed salicylate/benzoate/pyridine ligation is described.Treatment of with 3 equiv of salicylic acid (salH2) in CH3CN leads to the formation of (5), where pyr is pyridine.Complex 5 crystallizes in the monoclinic space group C2/c with, at -155 deg C, a=17.625(13) Angstroem, b=38.314(35) Angstroem, c=19.886(16) Angstroem, β=99.17(4) deg, and Z=4.The stucture was solved and refined with a total of 3168 unique reflections with F>2.33?(F).Final values of conventional discrepancy indices R and R_w are 9.70 and 9.92percent, respectively.The structure consists of two butterfly-like units, containing octahedral Mn^III ions, linked together by a bridging Mn(sal)4 central unit.The latter contains an eight-coordinate Mn^II ion and is the site of a crystallographic C2 axis relating the halves of the molecule.The sal^2- groups linking the Mn^II and units have a rare μ3-η³-bridging mode; peripheral ligation to the complete molecule is provided by eight μ2-PhCOO^-, four terminal pyr, and two μ2-salH^- groups, employing only their carboxylate functions with the phenoxide oxygen atoms protonated and not bound to a manganese ion.The complex possesses an average metal oxidation state of +2.89 and is a rare example of a discrete high-nuclearity manganese species.The results of solid-state magnetic susceptibility studies in the temperature range 300.9-5.0 K are described.The effective magnetic moment per molecule (μ_eff) gradually decreases from a value of 11.82 μ_B at 300.9 K to a value of 5.83 μ_B at 5.0 K.The derivation of a theoretical model to account for the intramolecular magnetic exchange interactions in 5 by the Kambe vector coupling approach is described.Least-squares fitting of the susceptibility data to the model gives exchange parameters for the butterfly-like units of J=-26.2 cm^-1 for the (μ-O)2Mn^III interaction, J=-11.2 cm^-1 for the hinge - wing-tip (μ-O)2Mn^III2 interaction, and J=-0.97 cm^-1 for the eight Mn^II...Mn^III interactions.Of the 209 725 states of 5, the ground state is found to be a S=3/2 state, with more than 3000 states found within 150 cm^-1 of the ground state.