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]+ (1, 76%) or [(crypt-222)­K]+ (2, 54%)). Upon reduction, the ligand tbsL6– rearranges around the triiron core to adopt an almost ideal C3-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 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 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 (χM″) 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 (Ueff) of 22.6(2) cm–1, nearly matching the theoretical barrier of 38.7 cm–1 calculated from the axial zero-field splitting parameter (D = −1.29 cm–1) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (Hext = 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.