Single Crystal EPR Study of the Dinuclear Cu(II) Complex [Cu(tda)(phen)] 2 · H 2 tda (tda ) Thiodiacetate, phen ) Phenanthroline): Influence of Weak Interdimeric Magnetic Interactions Nicola ´s I. Neuman, † Mireille Perec, ‡ Pablo J. Gonza ´lez, § Mario C. G. Passeggi, †,| Alberto C. Rizzi, † and Carlos D. Brondino* ,† Departamento de Fı ´sica, Facultad de Bioquı ´mica y Ciencias Biolo ´gicas, UniVersidad Nacional del Litoral, Ciudad UniVersitaria, Paraje El Pozo, S3000ZAA Santa Fe, Argentina, Departamento de Quı ´mica Inorga ´nica Analı ´tica y Quı ´mica Fı ´sica, Facultad de Ciencias Exactas y Naturales, INQUIMAE, UniVersidad de Buenos Aires, Ciudad UniVersitaria, Pabello ´n II, 1428, Buenos Aires, Argentina, Departamento da Quimica, Requimte/ CQFB, Faculdade de Cie ˆncias e Tecnologia, UniVersidade NoVa de Lisboa, 2829-516 Caparica, Portugal, and Intec, Conicet-UNL, Gu ¨emes 3450, 3000 Santa Fe, Argentina ReceiVed: September 13, 2010; ReVised Manuscript ReceiVed: NoVember 9, 2010 We report powder and single crystal EPR measurements of [Cu(tda)(phen)] 2 · H 2 tda (tda ) thiodiacetate, phen ) phenanthroline) at 9.7 GHz. This compound consists of centrosymmetric copper(II) ion dimers, weakly ferromagnetically exchange-coupled (J )+3.2 cm -1 ), in which the dimeric units are linked by hydrophobic chemical paths involving the phen molecules. EPR revealed that the triplet spectra are collapsed by interdimeric exchange interactions mediated by that chemical path. Analysis and simulation of the single crystal EPR spectra were performed using Anderson’s exchange narrowing model, together with statistical arguments. This approach allowed us to interpret the spectra modulated by the interdimeric interactions in situations of weak, intermediate, and strong exchange. We evaluated an interdimeric exchange constant J′ ) 0.0070(3) cm -1 , indicating that hydrophobic paths can transmit weak exchange interactions between centers at relatively long distances of the order of ∼10 Å. Introduction The study of exchange-coupled copper(II) dinuclear systems is an important subject of research in both inorganic and bioinorganic chemistry, due to their relevance in molecular magnetism and in the study of copper proteins. 1-4 The physi- cochemical theories that explain the magnetic coupling phe- nomenon in these type of systems have been established in the middle of the past century 5-8 and have been widely used in the characterization of dinuclear copper compounds. 9-12 For a dinuclear copper(II) system (S ) 1/2 ion pair), the spin-spin coupling is determined by the isotropic exchange constant J and by anisotropic interactions such as dipolar and anisotropic and antisymmetric exchange. The isotropic exchange interaction (H ex )-JS 1 · S 2 ) splits the singlet state from the triplet state of the dimer by an energy amount J, whereas the remaining interactions cause the threefold degeneration of the triplet state to be removed even in zero magnetic field (ZFS). Additional splitting of the energy levels can be produced by the hyperfine interaction with the copper nucleus (I ) 3/2). Furthermore, in weakly exchange-coupled dimers the anisotropic interactions mix appreciably the singlet and triplet states, which therefore cannot be considered pure in character. However, singlet-triplet nomenclature will be used throughout the text for simplicity. The magnetic characterization of dimeric systems is usually performed on solid state samples by magnetic susceptibility measurements and also, though less frequently, by EPR spec- troscopy. While the former is used principally for evaluating the intradimeric isotropic exchange interaction J, the latter is more appropriate to evaluate the above-mentioned anisotropic interactions. Particularly, when EPR spectroscopy is performed on oriented single crystal samples, it can be used to calculate the eigenvalues and eigenvectors of the tensorial magnitudes associated with such interactions. 6,7,13-17 In undiluted magnetic systems, the dimeric units of the crystal lattice may interact with their neighbors through different kinds of chemical paths that may transmit weak exchange interactions, hereafter designated J′. These interactions should be considered in the analysis of the data, since they can introduce modifications in the magnetic behavior of the dimer. 14,16,17 For thermodynamic magnetic mea- surements, the problem is well-known and can be easily accounted for by including in the Hamiltonian an energetic term describing the intermolecular interactions J′ in the molecular-field approxima- tion. 7 In contrast, the analysis by EPR can be more complicated because interdimeric exchange interactions may produce merging effects of the EPR triplet spectra, a phenomenon that has been previously well documented. 14,16-23 For weak exchange (J′< ZFS), the two EPR resonance lines of the triplet state are shifted to the center of the spectra and broadened; the larger J′, the larger the shift and broadening. In contrast, for strong exchange (J′> ZFS) the two lines merge into one. Similar spectral changes have also been observed in EPR measurements of interacting dimers as a function of temperature, since the effective J′-value depends on the thermal equilibrium density F of triplet states, a phenomenon which has been documented in moderately strong antiferromag- netically coupled dimers. 16,17,20 The EPR spectra of these systems at temperatures with low F values (kT < J) show well-resolved triplet spectra, which are averaged out at higher temperatures * To whom correspondence should be addressed. E-mail: brondino@ fbcb.unl.edu.ar. Fax: +54-342-4575221. Tel.: +54-342-4575213. † Universidad Nacional del Litoral. ‡ Universidad de Buenos Aires. § Universidade Nova de Lisboa. | Conicet-UNL. J. Phys. Chem. A 2010, 114, 13069–13075 13069 10.1021/jp108736p  2010 American Chemical Society Published on Web 11/30/2010