Maximizing Electron Exchange in a [Fe₃] Cluster

The one-electron reduction of (^tbsL)­Fe₃(thf)¹ furnishes [M]­[(^tbsL)­Fe₃] ([M]⁺ = [(18-C-6)­K­(thf)₂]⁺ (<b>1</b>, 76%) or [(crypt-222)­K]⁺ (<b>2</b>, 54%)). Upon reduction, the ligand ^tbsL^6– rearranges around the triiron core to adopt an almost ideal <i>C</i>₃-symmetry. Accompanying the (^tbsL) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe–Fe distances within the trinuclear cluster contract by ∼0.13 Å in <b>1</b>. Variable-temperature magnetic susceptibility data indicates a well-isolated <i>S</i> = ¹¹/₂ spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χ_M^″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field ⁵⁷Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (<i>U</i>_eff) of 22.6(2) cm^–1, nearly matching the theoretical barrier of 38.7 cm^–1 calculated from the axial zero-field splitting parameter (<i>D</i> = −1.29 cm^–1) extracted from the reduced magnetization data. A polycrystalline sample of <b>1</b> displays three sextets in the Mössbauer spectrum at 4.2 K (<i>H</i>_ext = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions