Substitution behaviour of an unusual seven-coordinate iron(III) complex in different solvents Ivana Ivanovic-Burmazovic 1 , Mohamed S.A. Hamza 2 , Rudi van Eldik * Institute for Inorganic Chemistry, University of Erlangen-N€ urnberg, Egerlandstrasse 1, 91058 Erlangen, Germany Received 26 July 2002; accepted 23 August 2002 Abstract In order to clarify the solution chemistry of 3d metal hepta-coordinate complexes, a detailed mechanistic study of the two-step substitution reaction between the ‘‘tweezer-like’’ iron(III) complex, ½FeðdapsoxÞðH 2 OÞ 2  þ (H 2 dapsox ¼ 2,6-diacetylpyridine-bis- (semioxamazide)), and SCN  was performed in water and methanol as solvents. Based on the observed rate law and reported activation parameters (DH # , DS # and DV # ) for both reaction steps in aqueous solution, the operation of an I a mechanism for the diaqua,an I d mechanismfortheaquathiocyanato,andan I d orDmechanismfortheaquahydroxoformsoftheFe(III)complex,is proposed. The operation of a limiting D mechanism is proposed for both reaction steps in methanol as solvent. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Seven-coordinate complexes; Fe(III); Mechanisms; High pressure 1. Introduction Hepta-coordinate 3d metal complexes present a very interesting field of research. Such complexes are still rather rare and there are no systematic investigations concerning the behaviour of these complexes, especially in solution, reported in the literature. The substitution mechanisms of hepta-coordinate 3d metal complexes remain largely undefined, which makes the chemistry of these species a challenging topic. Thereisageneralquestionwhetherthehepta-coordi- nate3dmetalcomplexesarealwaysmorelabilethantheir octahedral species, and whether a limiting dissociative mechanism is always operative in their substitution re- actions.Inordertoresolvethesequestions,wehaveper- formed a detailed investigation of the substitution behaviouroftheiron(III)complex, ½FeðdapsoxÞðH 2 OÞ 2  ClO 4 (H 2 dapsox ¼ 2,6-diacetylpyridine-bis-(semioxamaz- ide)),withSCN  asenteringnucleophileinaqueousand methanolicsolution.Besidesthereasonsthatarebasedon their fundamental scientific significance, the mechanistic behaviour of Fe(III) hepta-coordinate complexes may also be of relevance to biological systems. For instance, Fe(III) pentagonal–bipyramidal (PBP) complexes pos- sessanexcellentsuperoxidedismutase(SOD)activity[1] andcanserveasacatalystforthesuperoxidedispropor- tionation, thus illustrating their possible pharmaceutical application. TheinvestigatedFe(III)complexhasaPBPstructure [2] with doubly deprotonated bis(acylhydrazine), dapsox 2 , (Scheme 1) as a pentadentate ligand in the equatorial plane with two water molecules in the apical positions. As far as we know, the only kinetic data re- ferring to substitution reactions of hepta-coordinate 3d metal complexes with respect to ligand displacement [3] andwaterexchangereactions[4,5],havebeenperformed on polyaminecarboxylate Fe(III) complexes. These complexes differ significantly from the complex investi- gated here, in that they contain only one aqua ligand that is positioned in the equatorial plane and can be displacedduringsubstitutionreactions.Thesestructural differences significantly influence the substitution be- Inorganic Chemistry Communications 5 (2002) 937–940 www.elsevier.com/locate/inoche * Corresponding author. Tel.: +49-9131-8527350; fax: +49-9131- 8527387. E-mail address: vaneldik@chemie.uni-erlangen.de (R. van Eldik). 1 On leave from the Department of Chemistry, University of Belgrad, Belgrad, Yugoslavia. 2 On leave from the Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt. 1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1387-7003(02)00607-X