Rare nuclearities in Mn/oxo cluster chemistry: Synthesis and characterization of a mixed-valence {MnII/III11} complex bearing acetate and salicylhydroximate(-3) bridging/chelating ligands

Abstract

The one-pot reaction between Mn(O2CMe)2·4H2O, salicylhydroxamic acid (shaH2) and Me4NOH·5H2O in a molar ratio of 2:1:2, and a solvent mixture comprising MeCN and MeOH, affords the new mixed-valence cluster compound (Me4N)4[MnII2MnIII9O4(O2CMe)9(shi)6] (1) in moderate yields. The anion of the tetradentate (N,O,O,O) chelating/bridging ligand shi3− resulted from the in-situ metal ion-assisted amide-iminol tautomerism of shaH2. The [MnII2MnIII9(μ3-O)4(μ-OR)3(μ3-NO)6]14+ core of 1 comprises four vertex-sharing {MnIII3(μ3-O2−)}7+ triangular subunits of different types, which are linked to each other and the MnII atoms through the oximato arms and the carboxylate O-atoms of six η1:η1:η1:η2:μ3 shi3− and three η1:η2:μ3 MeCO2− groups, respectively. Peripheral ligation about the core is provided by six η1:η1:μ MeCO2− groups and the chelating parts of the shi3− ligands. Solid-state direct-current (dc) magnetic susceptibility studies revealed the presence of predominant antiferromagnetic exchange interactions between the Mn centers, mainly promoted by the coplanarity of the μ3-O2− within the {Mn3} triangular planes and the insignificant distortion of the MnNOMn bridging units. Low-lying excited states precluded obtaining a good fit from the magnetization data, and the ground state was instead determined from the alternating-current (ac) data, which indicated an S = 3 ground state for 1. The combined results emphasize the continuing supply from the use of the salicylhydroxime chelating/bridging ligand of new 3d-metal clusters of very rare nuclearities and beautiful molecular structures.