High-Spin [Fe^I₃] Cluster Capable of Pnictogen Atom Capture

Using a new hexanucleating anildophosphine ligand LH₃ (1,3,5-C₆H₉(NHC₆H₃-5-F-2-P(tBu)₂)₃), the all-monovalent [Fe^I₃] compound (L)Fe₃ (1) was isolated and characterized by X-ray diffraction analysis, SQUID magnetometry, ⁵⁷Fe Mössbauer spectroscopy, and cyclic voltammetry. The molecular structure of 1 reveals very close Fe-Fe distances of 2.3825(7), 2.4146(8), and 2.3913(8) Å which results in significant Fe-Fe interactions and a maximum high-spin S = ⁹/₂ spin state as determined by SQUID magnetometry and further supported by quantum chemical calculations. Compound 1 mediates the multielectron, oxidative atom transfer from inorganic azide ([Bu₄N][N₃]), cyanate (Na[NCO]), and phosphonate (Na(dioxane)_2.5[PCO]) to afford the [Fe₃]-nitrido (N^3-) and [Fe₃]-phosphido (P^3-) pnictides, (L)Fe₃(μ³-N) (2) and [(L)Fe₃(μ³-P)(CO)]^- (3), respectively. Compounds 1-3 exhibit rich electrochemical behavior with three (for 1), four (for 2) and five (for 3) distinct redox events being observed in the cyclic voltammograms of these compounds. Finally, the all-monovalent 1 and the formally Fe^II/Fe^II/Fe^I compound 3, were investigated by alternating current (ac) SQUID magnetometry, revealing slow magnetic relaxation in both compounds, with 3 being found to be a unique example of a [Fe₃]-phosphido single-molecule magnet having an energy barrier relaxation reversal of U = 30.7(6) cm^-1 in the absence of an external magnetic field. This study demonstrates the utility of an all low-valent polynuclear cluster to perform multielectron redox chemistry and exemplifies the redox flexibility and unique physical properties that are present in the corresponding midvalent oxidation products.