Trinuclear, oxo-bridged, mixed-valence iron mercaptocarboxylates Ashutosh K. Pandey, Tarkeshwar Gupta and Balram P. Baranwal* Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur - 273 009, India Received 23 September 2003; accepted 28 October 2003 Abstract Mixed carboxylates and mercaptocarboxylates of mixed-valence Fe II Fe III with general formula [Fe II - Fe III 2 O(OOCR) 3 (OOCR¢SH) 3 L 3 ] (where R ¼ C 13 H 27 ,C 15 H 31 or C 17 H 35 ;R¢ ¼ CH 2 or C 6 H 4 and L ¼ pyridine) were successfully prepared by substitution of [Fe II Fe III 2 O(OAc) 6 (H 2 O) 3 ] with straight chain carboxylic acids and mercaptocarboxylic acids. The title products have been characterized by elemental analyses, molar conductances and magnetic susceptibility measurements, i.r., electronic and Mo¨ ssbauer spectral data. The i.r. spectra showed resolved bands arising from the m asym (OCO) and m sym (OCO) vibrations of the bridging carboxylate and mercaptocarboxylate ligands along with the m asym (Fe 3 O) vibrations. Mo¨ ssbauer spectroscopic studies revealed two resolved quadrupole doublets due to Fe II and Fe III at 80, 200 and 295 K, confirming the valence-localized type of compound. This fact has also been supported by the observed electronic spectral bands in the complexes. Magnetic susceptibility measurements showed net anti-ferromagnetic coupling related to mixed-valence pairs. A structure for the complexes has been established on the basis of physico-chemical studies. Introduction There is considerable current interest in the coordina- tion chemistry of iron complexes with carboxylato ligands owing to the fact that there is an increased recognition of the role of this metal in biological systems [1–3]. Trinuclear, oxo-centered carboxylate complexes of iron, chromium or manganese of general formula [M III 3 O(OOCR) 6 L 3 ] Æ X, where L ¼ H 2 O, py or MeOH and X ¼ Cl ) , NO  3 or RCOO ) etc. have been studied extensively [4]. The mixed-valence, Fe II Fe III complexes have been of special interest for examining the factors that influence the role of electron transfer in mixed- valence compounds [5]. Examples of [Fe II Fe III 2 O- (OOCR) 6 L 3 ] Æ sol. compounds exist valence-localized, even above 300 K, whereas others undergo a transition from valence localization at low temperature to valence delocalization at room temperature [5]. On the other hand, iron complexes with sulfur ligands show signifi- cant biological activity and possess medicinal properties [6, 7]. Among sulfur ligands mercaptocarboxylic acids (HSR¢COOH) display very interesting structural and bonding properties as they contain both ‘soft’ sulfur and ‘hard’ oxygen donor sites which may lead to aggregation of soft and hard, or to an intermediate class of metal centers. It has been observed that the mixed-valence iron complexes with sulfur donor ligands have received almost no attention. As a part of the research in our laboratory directed towards cobalt, nickel and copper mercaptocarboxylates and hydroxycarboxylates [8–10], we report here the synthesis of mixed-ligand, mixed-valence iron complexes containing carboxylates and mercaptocarboxylates ligands. This was achieved by substitutions in [Fe 3 O- (OAc) 6 (H 2 O) 3 ] with higher carboxylic acids and mer- captocarboxylic acids, reported for the first time. In the process, a non-polar solvent (PhMe) was used and was found to be a suitable choice for the synthesis of these complexes in view of the fact that coordinating H 2 O molecules and liberated AcOH were removed by frac- tionation with PhMe as an azeotrope which drives the reaction forward. This method of synthesis is convenient and particularly suitable for [Fe II Fe III 2 O(OOCR) 3 - (OOCR¢SH) 3 L 3 ] complexes, which cannot be easily prepared by any other route. The isolated complexes were characterized in order to study the coordination behavior of the ligands towards the metal site. Their structure has been established. Experimental Materials Glass apparatus with interchangeable joints was used throughout. The organic solvents (Qualigens) were purified by standard procedures before use. l 3 -Oxo- hexakis(acetato)tri(aqua)iron(II)diiron(III)dihydrate, [Fe II Fe III 2 O(OAc) 6 -(H 2 O) 3 ] Æ 2H 2 O was prepared by the literature procedure [11]. The non-coordinated H 2 O molecules were removed by drying under vacuum (0.1 mm) for 2 h at 80 °C. The straight chain fatty * Author for correspondence Transition Metal Chemistry 29: 370–375, 2004. 370 Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.