A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)­FeCl2] (1)
(PNP = N[2-P­(CHMe2)2-4-methylphenyl]2–), prepared from one-electron oxidation
of [(PNP)­FeCl] with ClCPh3, displays an unexpected S = 3/2 to S = 5/2 transition above 80
K as inferred by the dc SQUID magnetic susceptibility measurement.
The ac SQUID magnetization data, at zero field and between frequencies
10 and 1042 Hz, clearly reveal complex 1 to have frequency
dependence on the out-of-phase signal and thus being a single molecular
magnet with a thermally activated barrier of Ueff = 32–36 cm–1 (47–52 K).
Variable-temperature Mössbauer data also corroborate a significant
temperature dependence in δ and ΔEQ values for 1, which is in agreement with the
system undergoing a change in spin state. Likewise, variable-temperature
X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S =
5/2 being close in energy. Multiedge XAS absorption spectra suggest
the electronic structure of 1 to be highly covalent with
an effective iron oxidation state that is more reduced than the typical
ferric complexes due to the significant interaction of the phosphine
groups in PNP and Cl ligands with iron. A variable-temperature single
crystal X-ray diffraction study of 1 collected between
30 and 300 K also reveals elongation of the Fe–P bond lengths
and increment in the Cl–Fe–Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar
orbital pictures except for the d­(z2)
orbital, which has the most sensitivity to change in the geometry
and bonding, where the quartet (4B) and the sextet (6A) states are close in energy.