Novel mixed-valence manganese cluster with two distinct Mn3(II/III/II) and Mn3(III/II/III)) trinuclear units in a pseudocubane-like arrangement

The reaction between Mn(ClO4)(2) and di-(2-pyridyl)-ketone in the presence of the sodium salt of propanediol as a base in MeOH leads to the formation of a hexanuclear manganese cluster. This cluster has been characterized by the formula [Mn(II)(3)Mn(III)(3)O(OH)(CH(3)pdol)(3)(Hpdol)(3)(pdol)](ClO4)(4) (1). Molecular conductance measurements of a 10(-3) M solution of compound 1 in CH3CN, DMSO, or DMF give Lambda m = 529, 135, or 245 mu S/cm, respectively,which suggests a 1:4 cation/anion electrolyte. The crystal structure of hexanuclear manganese cluster 1 consists of two distinct trinuclear units with a pseudocubane-like arrangement. The trinuclear units show two different valence distributions, Mn(II)/Mn(III)/Mn(II) and Mn(III)/Mn(II)/Mn(III). Additional features of interest for the compound include the fact that (a) two of the Mn(III) ions show a Jahn-Teller elongation, whereas the third ion shows a Jahn-Teller compression; (b) one bridge between Mn(III) atoms is an OXO (O2-) ion, whereas the bridge between Mn(II) and Mn(III) is a hydroxyl (OH-) group; and (c) the di-(2-pyridyl)-ketone ligand that is methanolyzed to methyl-Hpdol and R(2)pdol (R = CH3, H) acts in three different modes: methyl-pdol(-1), Hpdol(-1), and pdol(-2). For magnetic behavior, the general Hamiltonian formalism considers that (a) all of the interactions inside the two "cubanes" between Mn(II) and Mn(III) ions are equal to the J(1) constant, those between Mn(II) ions are equal to the J(2) constant, and those between the Mn(III) ions are equal to the J(3) constant and (b) the interaction between the two cubanes is equal to the J(4) constant. The fitting results are J(1) = J(2) = 0.7 cm(-1), J(3) approximate to 0.0, J(4) = -6.2 cm(-1), and g = 2.0 (fixed). According to these results, the ground state is S = 1/2, and the next excited states are S = 3/2 and 5/2 at 0.7 and 1.8 cm(-1), respectively. The EPR spectra prove that the spin ground state at a low temperature is not purely S = 1/2 but is populated with the S = 3/2 state, which is in accordance with the susceptibility and magnetization measurements.