A mixed-valence Mn6 cluster capped by nitronyl nitroxide units

A mixed-valence Mn₆ cluster, Mn₆O₂(t-BuCO₂)₁₀(H₂O)(NNPy)₂] (2), has been synthesized by the assembly of Mn₆O₂(t-BuCO₂)₁₀(THF)₄] · THF (1) with a nitronyl nitroxide radical molecule, 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NNPy). Complex 2 consists of Mn₆O₂]¹⁰⁺ core and two NNPy radical attachments, where the Mn₆O₂]¹⁰⁺ core is composed of two Mn^III ions (i.e., the inner Mn ions) bridged by two μ₄-O^2? and four Mn^II ions (i.e., the outer Mn ions) surrounding the inner Mn^III ions via the two μ₄-O^2?, forming an edge-shared double-tetrahedral polygon (vertices: Mn ions, centers: μ₄-O^2?). Bridges by pivalate groups in μ₂- and μ₃-modes assist the coordination spheres around these Mn ions. Two of the outer Mn^II ions in trans position are, to the sixth coordination site, occupied by NNPy molecule (pyridine-N coordination). One of the outer Mn^II ions is occupied by a water molecule to complete octahedral geometry, but the rest of them is vacancy taking a square-pyramidal geometry. The variable-temperature direct current (DC) magnetic susceptibility was collected in the temperature range of 1.81 to 300 K at a 1 kOe appled field. The molar magnetic susceptibility (χ_M) steadily increases from 0.053 emu mol^?1 at 300 K to 0.29 emu mol^?1 at 20 K, showing a kink (peak), and then increases rapidly to 0.70 emu mol^?1 at 1.81 K. The profile of this magnetic behavior indicates that the intracluster spin cancellation takes place bellow 20 K to derive an S_Mn6 = 0 by strong antiferromagnetic interaction between Mn ions. This is consequently leading to paramagnetic isolation of only the two nitronyl nitroxide units as if remaining as exchange-uncorrelated spin carriers. The variable-temperature ESR spectra show a sharp signal below 30 K with g = 2.00, proving the presence of free nitronyl nitroxide radical, in which double integral of such signal obeys the Curie–Weiss law that follows the susceptibility result in the low-temperature region.