Development and Validation of Effective Computational Strategies for the Study of Metal Nitroxide Complexes Andrea di Matteo and Vincenzo Barone* Dipartimento di Chimica, UniVersita ` di Napoli “Federico II”, Via Mezzocannone 4, I-80134 Napoli, Italy ReceiVed: April 6, 1999; In Final Form: July 9, 1999 The structures and spectromagnetic properties of some model copper nitroxide complexes were studied by both density functional and multireference post-Hartree-Fock models in order to assess the reliability of different computational procedures. Next, the most promising density functional was employed for a systematic study of the effect of structural parameters and choice of the model system on the magnetic properties of copper nitroxides. These results provided the reference data for the extension of our recent nitroxide force field (J. Am. Chem. Soc. 1998, 120, 7069) to copper complexes. The resulting computational tool (consisting of magnetic properties evaluated by density functional methods possibly using geometries optimized by cheaper moleculear mechanics approaches) seems particularly effective and well suited for systematic use also by nonspecialists. Extension to other metals is quite straightforward along the same lines. 1. Introduction The experimental investigation of exchange interactions between magnetic centers both in a vacuum and in condensed phases represents one of the main research topics in modern chemistry. 1 For instance, magnetostructural correlations are widely used to interpret the magnetism of solids 2 and to develop synthetic strategies affording compounds with suitable magnetic proper- ties. Furthermore, in bioinorganic chemistry the understanding of the magnetic interactions between metal centers provides information about the coordination environment and gives assessments about the geometry of active sites in enzymes. 3 In the past few years, a number of magnetic systems containing organic free radicals (like nitroxide derivatives) and paramagnetic transition metal ions have been synthesized as possible precursors for ferromagnetic materials. 4 Here, we will concentrate on copper(II) complexes. Although both σ and π bonding may occur between the nitroxide and the metal, only one example of the latter situations has been characterized, namely, the CuBr 2 (TEMPO) 5 complex (TEMPO is 2,2,6,6- tetramethyl-piperidinyl-1-oxy) in which the ligand is bound in a η 2 fashion. In all the other reported nitroxide complexes the free radical is bound to the metal through the oxygen atom only. 6 The complexes can be grouped into two classes, each possessing characteristic structural and magnetic features. Complexes with short-bonded equatorial ligands exhibit strong antiferromagnetic coupling, 7 while those containing a long-bonded axial nitroxide 8 have ferromagnetic behavior. Schematic drawings of both classes of complexes are shown in Figure 1. Although X-ray structures are available for complexes of each class, their magnetic characteristics are often difficult to analyze from an experimental point of view. For instance in the case of equatorial complexes Gatteschi et al. 6 emphasize the difficulty in obtaining reliable J values, since the magnetic susceptibility is essentially constant in the entire temperature range. At the same time the computation of reliable magnetic couplings represents a severe challenge for quantum mechanical methods since they are obtained as differences between much larger quantities where both dynamic and non dynamic correlation effects play a significant role. Since the chemically interesting systems are quite large and the magnetic properties are quite demanding from a computa- tional point of view, two limiting strategies have been employed. * Corresponding author. Voice: +39-081-5476503. Fax: +39-0815527771. E-mail: enzo@chemna.dichi.unina.it. Figure 1. Schematic drawing of complexes studied in the present work together with labeling of atoms and main geometrical parameters. In the simplified models R groups are H atoms or methyl groups in place of methylene groups. 7676 J. Phys. Chem. A 1999, 103, 7676-7685 10.1021/jp991151v CCC: $18.00 © 1999 American Chemical Society Published on Web 09/08/1999