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RESEARCH PRODUCT

The Two‐Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy

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The triple helicate dinuclear iron(II) complex, [Fe-2(L)(3)](ClO4)(4)center dot 2H(2)O (1), previously reported by Tuna et al. (Chem. Eur. J. 2004, 10, 5737), was prepared and characterised by detailed SQUID and Fe-57 Mbssbauer measurements. Compound 1 exhibits a thermochromic two-step spin conversion at T-SC((1)) ca. 240 K and T-SC((2)) ca. 120 K, but does not switch its spin state further below 20 K as proven by Mossbauer spectroscopy. The sharp variation of the susceptibility below 20 K is due to zero-field splitting of the remaining iron(II) high-spin species. Applied field Fe-57 Mossbauer spectroscopy experiments at 4.2 K indicate that the gradual thermal spin conversion from [HS-HS] pairs to [LS-LS] pairs is complete for one half of the Fen active sites. The presence of a step tracked in the spin-conversion curve and through the variation of the Mossbauer parameters of an uncoupled iron(II) site call for the occurrence of an [LS-HS] intermediate spin state in the plateau region. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).