Crystal structure and heat capacity of the mixed-valence trinuclear iron monoiodoacetate complex, [Fe(III)2Fe(II)O(O2CCH2I)6(H2O)3][Fe(III)3O(O2CCH2I)6(H2O)3]I

The crystal structure of a trinuclear iron monoiodoacetate complex was determined. Although it has been incorrectly characterized as [Fe3O(O2CCH2I)(6)(H2O)(3)], the correct chemical formula turned out to be [Fe(III)(2)Fe(II)O(O2CCH2I)(6)(H2O)(3)][Fe(III)(3)O(O2CCH2I)(6)(H2O)(3)]I (1). The two kinds of Fe3O molecules (Fe(III)(2)Fe(II)O and Fe(III)(3)O) are crystallographically indistinguishable. All the Fe atoms are crystallographically equivalent because of a crystallographic threefold symmetry. Heat capacities of 1 seem to exhibit no thermal anomaly in the temperature range 5.5-309 K, although the valence detrapping phenomenon has been observed in this temperature range. This fact indicates that the valence-detrapping phenomenon in I occurs without any phase transition, leading 1 to a glassy state, probably because the crystal of 1 is just like a solid solution of distorted mixed-valence Fe(III)(2)Fe(II)O molecules and permanently undistorted Fe(III)(3)O molecules which may act as an inhibitor for a cooperative valence-trapping. (C) 2002 Elsevier Science Ltd. All rights reserved.