Density Functional Description of the Ferromagnetic Exchange Interactions between Semiquinonato Radicals Mediated by Diamagnetic Metal Ions Alessandro Bencini,* Ilaria Ciofini, and Elisa Giannasi Dipartimento di Chimica, Universita ` di Firenze, Firenze, Italy Claude A. Daul and Karel Doclo Institut de Chimie Inorganique et Analytique, Universite ´ de Fribourg, Fribourg, Switzerland ReceiVed July 24, 1997 The electronic structures of Ti(CatNSQ) 2 and Sn(CatNSQ) 2 , where CatNSQ 2- is the tridentate radical ligand (3,5-di-tert-butyl-1,2-semiquinonato 1(2-hydroxy-3,5-di-tert-butyl-phenyl)immine), were investigated with density functional (DF) calculations, using the local approximation for the exchange-correlation functional. The crystal structure of Sn(CatNSQ) 2 was solved. The complex crystallizes in the orthorhombic space group, C222 1 , with Z ) 8 in a unit cell of the following dimensions: a ) 19.580(5) Å, b ) 24.310(5) Å, c ) 23.690(5) Å. The crystals are not isomorphous with similar M(CatNSQ) 2 (M ) Ti, V) complexes previously reported. DF calculations showed that the triplet (S ) 1) spin state is stabilized with respect to the first excited singlet (S ) 0) state and the computed exchange coupling constant J is in semiquantitative agreement with the values obtained from magnetic susceptibility measurements. Using a symmetry-based multiplet structure decomposition in terms of states defined by a single determinant (single determinant method, SD) the energies of the excited singlet states were also computed in agreement with the experimental data. The calculations have shown that the main exchange mechanism between the organic radicals, responsible for the ferromagnetism of these complexes, is a superexchange pathway mediated by the 3d orbitals of Ti and the 4p empty orbitals of Sn. Magnetostructural correlations between the exchange coupling constant and the M-O and M-N bond distances have been established. Introduction Exchange interactions between organic radicals are currently under investigation by material chemists who continuously synthesize new compounds with the aim of stabilizing ferro- magnetic interactions and extending the interactions to long- range order. 1-4 In general, organic molecules are closed-shell systems and possess no net magnetic moment. In the case of pure organic radicals, often antiferromagnetic interactions are observed due to the overlap between the magnetic orbitals localized onto adjacent molecules in the solid state. In a few cases ferromagnetic interactions between stable organic radicals have been measured, for example, in galvinoxyl, 5 nitronyl nitroxides 6 or polycarbenes 7-10 and polyarylmethyl. 11 Binding metal ions to organic radicals has been a synthetic route followed to investigate the possibility of stabilizing ferromagnetic states. In particular nitronyl nitroxide-transition metal or -rare earth complexes have been prepared and studied. 12,13 Another kind of organic radicals which can bind to metal ions are the polyoxolene radicals, and a number of complexes have been synthesized and studied. 14 Most often the high-spin state, resulting from parallel alignment of the unpaired electrons localized on the metal center and those on the radical ligands, is the ground state of the system. The high-spin state can be stabilized by hundreds of wavenumbers, giving rise to ferro- magnetic interactions larger than those observed in nitronyl nitroxide systems. For these reasons metal-polyoxolene sys- tems have been regarded as potential candidates for obtaining bulk ferromagnets. 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Chem. 1998, 37, 3719-3725 S0020-1669(97)00906-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/03/1998