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 (H6tren) containing three oligo-α-pyridylamine branches and 13
nitrogen donors (H6L) reacts with [Fe2(Mes)4] to yield an organic nanocage built up by two tripodal ligands
with interdigitated branches (HMes = mesitylene). The nanocage has
crystallographicD3symmetry but hosts
a remarkably unsymmetric hexairon–oxo core, with a central
Fe5(μ5-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 Fe2+–N3–bonds with nitrogen donors of oligo-α-pyridylamides.
This allowed us to formulate the compound as [Fe6O2(OH)(H3L)L], with nominally four FeIIions and two FeIIIions. Mössbauer spectra indicate
that the compound contains two unique FeIIsites, identified
as a pair of closely spaced hydroxo-bridged metal ions in the central
Fe5(μ5-O) pyramid, and a substantially
valence-delocalized FeII2FeIII2unit. Broken-symmetry DFT calculations predict strong ferromagnetic
coupling between the two iron(II) ions, leading to a localS= 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 parametersUeff/kB= 49(2) K and τ0= 4(2) × 10–10s.