A Remarkably Unsymmetric Hexairon Core Embraced by Two High-Symmetry Tripodal Oligo-α-pyridylamido Ligands.

Oligo-α-pyridylamides offer an appealing route to polyiron complexes with short Fe-Fe separations and large room-temperature magnetic moments. A derivative of tris(2-aminoethyl)amine (H 6 tren) containing three oligo-α-pyridylamine branches and 13 nitrogen donors (H 6 L) reacts with [Fe 2 (Mes) 4 ] to yield an organic nanocage built up by two tripodal ligands with interdigitated branches (HMes = mesitylene). The nanocage has crystallographic D 3 symmetry but hosts a remarkably unsymmetric hexairon-oxo core, with a central Fe 5 (μ 5 -O) square pyramid, two oxygen donors bridging basal sites, and an additional Fe center residing in one of the two tren-like pockets. Bond valence sum (BVS) analysis, density functional theory (DFT) calculations, and electrochemical data were then used to establish the protonation state of oxygen atoms and the formal oxidation states of the metals. For this purpose, a specialized set of BVS parameters was devised for Fe 2+ -N 3- bonds with nitrogen donors of oligo-α-pyridylamides. This allowed us to formulate the compound as [Fe 6 O 2 (OH)(H 3 L)L], with nominally four Fe II ions and two Fe III ions. Mössbauer spectra indicate that the compound contains two unique Fe II sites, identified as a pair of closely spaced hydroxo-bridged metal ions in the central Fe 5 (μ 5 -O) pyramid, and a substantially valence-delocalized Fe II 2 Fe III 2 unit. Broken-symmetry DFT calculations predict strong ferromagnetic coupling between the two iron(II) ions, leading to a local S = 4 state that persists to room temperature and explaining the large magnetic moment measured at 300 K. The compound behaves as a single-molecule magnet, with magnetization dynamics detectable in zero static field and dominated by an Orbach-like mechanism with activation parameters U eff / k B = 49(2) K and τ 0 = 4(2) × 10 -10 s.