Maximizing Electron Exchange in a [Fe3] Cluster

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• 作者：Raúl Hernández Sánchez ; Amymarie K. Bartholomew ; Tamara M. Powers ; Gabriel Ménard ; Theodore A. Betley
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：February 24, 2016
• 年：2016
• 卷：138
• 期：7
• 页码：2235-2243
• 全文大小：640K
• ISSN：1520-5126

文摘

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 ^tbsL^6– 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 (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 (U_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 (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 (H_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.