A Mn^II₆Mn^III₆ single-strand molecular wheel with a reuleaux triangular topology: Synthesis, structure, magnetism, and DFT studies

The use of the anion of 3-methyl-1,3,5-pentanetriol (mpt^3-) in manganese carboxylate chemistry has afforded the new Mn^II/III ₁₂ cluster [Mn^II₆Mn^III ₆(mpt)₆(CH₃CO₂)₁₂(py) ₆]·3CH₃CN (1·3CH₃CN). Complex 1 was isolated in moderate yield by the reaction of Mn(CH₃CO ₂)₂·4H₂O and H₃mpt in a 2.6:1 molar ratio in a solvent mixture of acetonitrile and pyridine. The structure of 1 consists of alternating [Mn^II₂(CH₃CO ₂)₃(py)]⁺ and [Mn^III ₂(μ-OR)₂(CH₃CO₂)(py)] ³⁺ dimeric units (three of each dimer), linked at each end by two alkoxo and one acetate bridges; the mpt^3- ligands adopt the η²:η²:η²:μ₄ coordination mode. The overall metal topology of this new Mn₁₂ wheel resembles a guitar plectrum, or a Reuleaux triangle. Complex 1 displays an unprecedented structural topology, being the first example of a Mn ^II₆Mn^III₆ wheel constructed from alternating homovalent dimers and the only known Mn₁₂ loop with the trigonal symmetry of a Reuleaux triangle (all other reported loops were ellipsoids). Variable-temperature, solid-state direct- and alternating-current magnetization studies were carried out on complex 1, revealing the presence of antiferromagnetic exchange interactions between the metal ions in the molecule, which lead to a spin ground-state value S = 0; the exchange coupling parameters J were calculated using density functional theory employing a hybrid B3LYP functional.