Magnetic Interactions in Dimeric Zn(II):Cu(II)(thiodiacetate)(phenanthroline)

NICOLAS NEUMAN; EMERSON BURNA; ALBERTO C. RIZZI; CARLOS D. BRONDINO

Chascomús

Encuentro; IV LABIC Fourth Latin American Meeting on Biological Inorganic Chemistry V WOQUIBIO Fifth Workshop on Bioinorganic Chemistry; 2014

UBA, UNR, CONICET, AGENCIA, Workshop Argentino de Química Bioinorgánica

O1.-Magnetic Interactions in Dimeric Zn(II):Cu(II)(thiodiacetate)(phenanthroline) Nicolás I. Neuman,1,2 Emerson Burna,1 Alberto C. Rizzi1 and Carlos D. Brondino1 1Departamento de Física, FBCB, Universidad Nacional del Litoral, Santa Fe, Argentina 2INQUIMAE/DQIAyFQ, FCEN, Universidad de Buenos Aires, Buenos Aires, Argentina niconeuman@gmail.com INTRODUCTION The study of spin pairs coupled by exchange interaction is a research field that constitutes a crossing point between biology and physical chemistry.1-3 Weak exchange interactions can be correlated with chemical paths joining paramagnetic metals,4 and particularly the magnetic dipolar interaction (which has a 1/r 3 dependence) is useful to determine distances between paramagnetic centers, which can be native metallic sites or organic radicals, or covalently attached spin labels. Also, when two anisotropic metal centers are coupled by dipolar interaction (which is inherently anisotropic), the coupling´s analysis can provide with a certain accuracy relative orientations between the gtensors of each site.5 Although the theoretical principles governing magnetic interactions are well known,6 it is necessary to perform detailed studies of how magnetic properties are affected by subtle structural characteristics, in both lowmolecular weight coordination complexes and metalloproteins. In this work we study Cu(II)- doped dimeric compound [Zn(II)2(tda)2(phen)2] ?H2tda (tda = thiodiacetate, phen = 1,10- phenanthroline), isostructural to the pure Cu(II) compound7 (see Fig. 1) and analyze the magnetic properties in the diluted compound and their relation to the properties previously studied in pure Cu(II)(tda)phen. EXPERIMENTAL METHODS Synthesis and Crystallization. M(II) acetate (M = Cu, Zn), thiodiacetic acid and 1,10- phenanthroline were dissolved in an H2O/EtOH (85/15) mixture. The solutions were filtered and mixed in 1:5 Cu:Zn ratios. After one or two days prismatic light-blue crystals appeared, which were separated from the solution. Single Crystal EPR measurements. A single crystal was mounted on a Rexolite sampleholder and introduced into a Quartz EPR tube which was inserted, attached to a goniometer, in the resonant cavity of a Bruker EMX Plus EPR spectrometer. This allowed us to acquire EPR spectra for different magnetic field orientations in three mutually orthogonal crystal planes. Density Functional Calculations. Calculations on the Cu2(tda)2(phen)2 dimer were performed using the ORCA program8 and the B3LYP, PBE and B2PLYP functionals, together with the Ahrlichs group triple-zeta basis with polarization functions (TZVP).9 Fig.1. Structure of the Cu2(tda)2phen2 dimer. RESULTS AND DISCUSSION Single crystal spectra in the ca* plane of Cu(II):Zn(II)(tda)phen are shown in Fig. 2. These show a central group of one to four resonances corresponding to a single Cu(II) ion with hyperfine structure (I (63,65Cu) = 3/2). This signal arises from heterodinuclear CuZn(tda)2(phen)2 units. The spectra also show two groups of one to seven resonances (partially or totally resolved), which arise from homodinuclear Cu2(tda)2(phen)2 units. The groups are separated by zero-field splitting (ZFS) of the S = 1 triplet state of the dimer, and this ZFS is composed mainly by dipole-dipole magnetic interaction between the spins localized at each Cu(II) ion. 250 300 350 400 122º 57º Magnetic Field (mT) EPR signal (a.u.) 47º Fig. 2. Selected single-crystal EPR spectra of Cu(II):Zn(II)(tda)phen in the ca* plane. From the angular dependence of the gravity center of the resonances we determined the gtensor of the Cu(II) ion. The hyperfine A-tensor was determined from the hyperfine splitting of the central quartet. The ZFS tensor of the homodimer was determined by simulation of the spectra using EasySpin.10 The g-, A- and Dtensors are summarized in Table 1. g-factors A-factors (MHz) D-tensor 1 g = 2.055(4) A1 = 29 D = -0.0641 cm-1 2 g = 2.073(4) A2 = 29 |E/D| = 0.01 3 g = 2.307(5) A3 = 452 Table 1. g, A and D tensor components. The g and A tensors are nearly axial, with the largest value (g3 and A3) lying along the perpendicular to the equatorial ligand plane (see Fig. 2). The principal value of the D-tensor lies nearly along the Cu-Cu direction, but the experimental D-tensor cannot be described using the point-dipole approximation, which assumes each spin to be localized at the copper nucleus. This constitutes an experimental proof of covalency effects, making necessary the use of quantum-chemical methods to calculate the D-tensor. Table 2 shows a selection of magnetic parameters calculated with DFT using the structure of the dimer depicted in Fig. 1. Our results indicate that the double-hybrid B2PLYP functional11 provides a good approximation to calculate the g- and dipolar part of the D-tensor. gx gy gz D(cm-1) E/D PBE 2.034 2.038 2.105 -0.044 0.043 B3LYP 2.050 2.051 2.156 -0.050 0.038 B2PLYP 2.084 2.089 2.288 -0.061 0.016 Exptl. 2.055 2.073 2.307 -0.0641 0.01 Table 2. Calculated and experimental g and D tensor components. CONCLUSIONS The intradimeric magnetic interaction in Cu2(tda)2phen2 units is mainly due to a dipolar term affected by covalency. Detailed analysis of weak magnetic interactions in dimeric systems can be correlated with their structural characteristics and provide experimental evidence of delocalization of the magnetic orbitals. REFERENCES 1. Kahn O., Molecular Magnetism (1993). 2. Solomon E.I. et al., CR (2014), 114(7):3659- 3853. 3. Cox N., JACS (2010), 132:11197-11213. 4. Santos-Silva T. JACS (2009), 131:7990-7998 5. Gómez C. et al., JBIC, manuscript in preparation. 6. Bencini A. et al. EPR of Exchange Coupled Systems, 1991. 7. Neuman N.I. et al., JPCA (2010), 114(59):13069-13075. 8. Neese, F. WIR: CMS (2012), 2: 73-78. 9. Schaefer A. et al., JCP (1992), 97(4): 2571- 2577. 10. Stoll, S. et al., JMR (2006), 178(1):42-55. 11. Grimme S., JCP (2006), 124:034108. ACKNOWLEDGMENTS We thank FONCYT, CONICET, and CAI+DUNL for financial support. NIN thanks CONICET for a fellowship grant. CDB is a member of CONICET-Argentina