First principles studies of the electronic and magnetic structures of ½FeðpzÞ 2  x complex L. Kabalan, S.F. Matar * Université de Bordeaux, CNRS, ICMCB, 87 Avenue du Dr. Albert Schweitzer, F-33608 Pessac Cedex, France article info Article history: Received 23 December 2008 Accepted 18 February 2009 Available online 4 March 2009 This paper is dedicated to the memory of Professor Olivier Kahn. Keywords: DFT Exchange parameters Antiferromagnets Gaussian03 IR Raman ASW abstract Polybis(pyrazolato)iron(II), ½FeðpzÞ 2  x is characterized by doubly bridged iron with pyrazolate ligands. An original DFT study at both the molecular and extended solid levels is carried out for this complex system with the purpose of identifying the magnetic interactions and chemical bonding characteristics. From molecular calculations, the exchange parameter J is obtained, pointing to the expected weakly antiferro- magnetic ground state. The IR and Raman spectra have been calculated with relevant assignments, namely for the stretching modes. Computations for the extended solid in different magnetic configura- tions point to the total moment of 4l B  fu 1 , identifying Fe as divalent in high spin configuration, in accordance with a weak tetrahedral crystal field. From relative energies the ground state is antiferromag- netic. Analyses of the chemical bonding (COOP) and of electron localization function (ELF) illustrate the interactions between Fe and the cycle as well as within the pz cycle. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction In recent years, there has been a growing interest in preparing and characterizing ‘‘molecular materials” such as coordination systems for their possible application in a number of different fields [1]; indeed some metal-containing coordination species have been shown to be photo-chemically active or to possess interesting electrical, electrochemical, magnetic properties and even antimicrobial activity [2]. Such systems typically contain a backbone of transition metal ions in close proximity, joined by polydentate ligands [3]. In this respect, the pyrazolato anion (and its ring-substituted derivatives) is an exobidendate ligand, showing the ability of bridging a variety of metal centers [4–6]. Single crystal X-ray diffraction studies on a number of binary 1,2-diazolates of copper(II) have revealed linear quasi-1D struc- tures in which the metal ions are double bridged by pyrazolate li- gands [7]. The magnetic properties of these pyrazolate systems are generally characterized by the presence of weak antiferromag- netic coupling between metal centers [8]. In contrast to the situ- ation for 1,2-diazolates, structural studies on polymeric iron(II) 1,3-diazolate systems reveal only single imidazolate bridge [9]. One important consequence of the single azolate bridging in imi- dazolates has been the generation of extended structures with 3D covalent connectivities. Another important characteristic prop- erty, not seen in corresponding pyrazolates, is that they exhibit antiferromagnetic coupling. T C and long-range ferromagnetic ordering below that temperature. Such a behavior characterizes them as low-temperature molecular magnets [10]. ½FeðpzÞ 2  x syn- thesis and magnetic susceptibility measurements were done by Patrick et al. [11] at an applied field of 10 kGauss over the tem- perature range 2–300 K. The plots of vðT Þ and l eff ðT Þ led to the observation of no maxima in the susceptibility plot on one hand and a decrease of l eff at low temperature on the other hand, which lets suggest a very weak antiferromagnetic coupling. A va- lue of J equal to 0:59 cm 1 was obtained. A section of the ex- tended structure of the complex is shown in Fig. 1. The two iron ions are linked by double pyrazolate bridges in the structure. The N pz —Fe—N other pz angles are very close to the value for a reg- ular tetrahedron. The empirical formula as given by Patrick et al. [11] is C 6 H 6 FeN 4 and the system crystallizes in orthorhom- bic symmetry with space group (SG) Ibam;]72. In this work, we carry out an original and complementary study at the molecular and extended solid levels within DFT for ½FeðpzÞ 2  x . Exchange parameters J are obtained from molecular cal- culations in order to identify the ground state. The vibrational spectra (IR, Raman) are calculated providing a signature for the system. Non-magnetic calculations in the solid state allow addressing the instability of the system in such a configuration and assessing the chemical bonding. To stress the latter properties 0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2009.02.012 * Corresponding author. Fax: +33 540002761. E-mail address: s.matar@u-bordeaux1.fr (S.F. Matar). Chemical Physics 359 (2009) 14–20 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys