Synthesis, crystal structures, electrochemical and magnetic properties of polynuclear {Fe 4 } and {Fe 8 Na 4 } carboxylate/picolinate clusters L. Arizaga a , M.F. Cerdá b , R. Faccio c , A.W. Mombrú c , M.A. Novak d , R. González a , C. Kremer a , R. Chiozzone a,⇑ a Cátedra de Química Inorgánica, Departamento ‘‘Estrella Campos’’, Facultad de Química, CC 1157, Montevideo, Uruguay b Laboratorio de Biomateriales, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo, Uruguay c Laboratorio de Cristalografía, DETEMA, Facultad de Química, CC 1157, Montevideo, Uruguay d Instituto de Física, Universidade Federal do Río de Janeiro, Brazil article info Article history: Received 8 September 2010 Received in revised form 1 February 2011 Accepted 9 February 2011 Available online 16 February 2011 Keywords: Iron(III) carboxylate clusters Iron(III) picolinate complexes Electrochemical properties Magnetic properties abstract Reaction of sodium picolinate with Fe III oxo-centered carboxylate triangles in MeCN in the presence of PPh 4 Cl yields (PPh 4 )[Fe 4 O 2 (O 2 CR) 7 (pic) 2 ] (R = Ph (1), Bu t (2)). Omitting the phosphonium cation produces [Fe 8 Na 4 O 4 (O 2 CPh) 16 (pic) 4 (H 2 O) 4 ](3), which contains two Fe 4 Na 2 units bridged by two picolinate ligands. X-ray crystal structures of 1 and 3 are reported. Voltammetric profiles in MeCN show four one-electron reduction steps for complexes 1 and 2. Vari- able-temperature magnetic susceptibility measurements in polycrystalline samples of 1 and 3 reveal strong antiferromagnetic couplings leading to S = 0 ground states. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Polynuclear Fe III compounds with oxygen donor ligands have received a great attention mainly due to their relevance in bioinor- ganic chemistry and in molecular magnetism. Iron-oxo and -hydroxo complexes have been studied as synthetic models of the active sites of various non-heme metalloenzymes, such as hemerythrin and ribonucleotide reductase [1], and as model systems for iron storage proteins, as ferritin [2]. In addition, some iron clusters can possess large total spin values in their ground state, and can even behave occasionally as single-molecule magnets (SMMs) [3], which is relevant to the field of molecular magnetic materials. SMMs are molecules that can function as sin- gle-domain magnetic particles of nanoscale dimensions, exhibit a slow relaxation rate of the magnetization, and present magnetic hysteresis below a certain blocking temperature [4]. The exchange interactions between Fe III centers within multi- nuclear iron-oxo or -hydroxo clusters {Fe x } are normally antiferro- magnetic. However, in some {Fe x } topologies, large spin ground states can arise even when all the pairwise Fe 2 interactions are antiferromagnetic because of the occurrence of competing ex- change interactions that frustrate the preferred spin alignments [5]. Nuclearity and topology of the polynuclear metal complexes depend strongly on the ligands that complete the peripheral coor- dination of the cluster, acting as bridging or chelating terminal groups. In this respect, the ligand blend RCO 2  /L–L (L–L = bidentate ligand) have been intensely studied and several Fe x cluster types have been reported [6]. When L–L = picolinate (pic, the anion of the 2-pyridinecarboxylic acid), the up to now structurally charac- terized members of this family are [NBu 4 ][Fe III 4 O 2 (O 2 CMe) 7 (pic) 2 ] [7], [Fe II Fe III 3 O 2 (O 2 CBu t ) 7 (Hpic) 1.5 (Kpic) 0.5 ] [8] and [Fe III 6 Na 2 O 2 - (O 2 CPh) 10 (pic) 4 (EtOH) 4 (H 2 O) 2 ](ClO 4 ) [9]. The first two of them have the well-documented bent (‘‘butterfly’’) arrangement of four iron sites in [Fe 4 (l 3 -O) 2 ] units, while the third consists of two sym- metry-related [Fe 3 O] triangles. We have thus revisited the reaction system consisting of Fe III and RCO 2  /picolinate, and tried to investigate the effects of a change in the carboxylate ligand or in the counterion on the nuclearity and/or the topology of the iron cage. In this paper, we describe the synthesis and characterization of complexes (PPh 4 )- [Fe 4 O 2 (O 2 CR) 7 (pic) 2 ] (R = Ph (1), Bu t (2)) and [Fe 8 Na 4 O 4 (O 2 CPh) 16 - (pic) 4 (H 2 O) 4 ](3). The electrochemical properties of compounds 1 and 2 in acetonitrile solution are reported. In addition, we also present X-ray crystal structures and magnetic properties of 1 and 3. 2. Experimental 2.1. General procedure All chemicals were of reagent grade and used as received. Na(pic)3/4H 2 O was prepared from picolinic acid (Hpic) and NaOH as described [10]. [Fe 3 O(O 2 CPh) 6 (H 2 O) 3 ](O 2 CPh) was prepared 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.02.021 ⇑ Corresponding author. Tel.: +598 2924 9739; fax: +598 2924 1906. E-mail address: rchiozzo@fq.edu.uy (R. Chiozzone). Inorganica Chimica Acta 370 (2011) 427–434 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica