Mixed-Valence Tetra- and Hexanuclear Manganese Complexes from the Flexibility of Pyridine-Containing β-Diketone Ligands

The reactions of [Mn₃O(O₂CCCl₃)₆(H₂O)₃] with 1-phenyl-3-(2-pyridyl)propane-1,3-dione (HL¹) and 1-(2-pyridly)-3-(p-tolyl)propane-1,3-dione (HL²) in CH₂Cl₂ afford the mixed-valence Mn^II₂Mn^III₂ tetranuclear complexes [Mn₄O(O₂CCCl₃)₆(L¹)₂] (1) and [Mn₄O(O₂CCCl₃)₆L₂²] (2), respectively. Similar reactions employing [Mn₃O(O₂CPh)₆(H₂O)(py)₂] with HL¹ and HL² give the Mn^II₃Mn^III₃ hexanuclear complexes [Mn₆O₂(O₂CPh)₈(L¹)₃] (3) and [Mn₆O₂(O₂CPh)₈L₃²] (4), respectively. Complexes 1·2CH₂Cl₂, 2·2CH₂Cl₂·H₂O, 3·1.5CH₂Cl₂·Et₂O·H₂O, and 4·2CH₂Cl₂ crystallize in the triclinic space group P1̄, monoclinic space group P2₁/c, monoclinic space group P2₁/n, and monoclinic space group P2₁/n, respectively. Complexes 1 and 2 consist of a trapped-valence tetranuclear core of [Mn^II₂Mn^III₂(μ₄-O)]⁸⁺, and complexes 3 and 4 represent a new structural type, possessing a [Mn^II₃Mn^III₃(μ₄-O)₂]¹¹⁺ core. The magnetic data indicate that complexes 3 and 4 have a ground-state spin value of S = ⁷/₂ with significant magnetoanisotropy as gauged by the D values of −0.51 cm⁻¹ and −0.46 cm⁻¹, respectively, and frequency-dependent out-of-phase signals in alternating current magnetic susceptibility studies indicate their superparamagnetic behavior. In contrast, complexes 1 and 2 are low-spin molecules with an S = 1 ground state. Single-molecule magnetism behavior confirmed for 3 the presence of sweep-rate and temperature-dependent hysteresis loops in single-crystal M versus H studies at temperatures down to 40 mK.