A mixed-valence Mn6 cluster capped by nitronyl nitroxide units

Abstract

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 MnIII ions (i.e., the inner Mn ions) bridged by two μ4-O2− and four MnII ions (i.e., the outer Mn ions) surrounding the inner MnIII ions via the two μ4-O2−, forming an edge-shared double-tetrahedral polygon (vertices: Mn ions, centers: μ4-O2−). Bridges by pivalate groups in μ2- and μ3-modes assist the coordination spheres around these Mn ions. Two of the outer MnII ions in trans position are, to the sixth coordination site, occupied by NNPy molecule (pyridine-N coordination). One of the outer MnII 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 (χM) 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 SMn6 = 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.