Exchange coupling and magnetic blocking in dilanthanide complexes bridged by the multi-electron redox-active ligand 2,3,5,6-tetra(2-pyridyl)pyrazine
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© the Partner Organisations 2014. The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp∗2Ln)2(μ-tppz)]+ (Ln = Gd, 1; Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp∗2Ln)2(μ-tppz)]- (Ln = Gd, 4; Tb, 5, Dy, 6) are reported. Cyclic voltammograms for compounds 1-3 reveal that the tppz ligand can reversibly undergo multiple redox changes. Hence, in the two sets of compounds isolated, 1-3 and 4-6, the redox-active ligand tppz exists in the monoanionic (tppz-) and trianionic (tppz3-) forms, respectively. Substantial LnIII-tppz- exchange coupling is found for the cationic tppz- radical-bridged species of 1-3, as suggested by a rise in χMT at low temperatures. For the Gd compound 1, fits to the data yielded a coupling constant of J = -6.91(4) cm-1, revealing antiferromagnetic coupling to give an S = 13/2 ground state. Both of the TbIII and DyIII-containing compounds 2 and 3 exhibit single-molecule magnet behavior under zero applied dc field. Importantly, the Dy congener shows a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 2.8 K and magnetic hysteresis below 3.25 K. Interestingly, the coupling constant of J = -6.29(3) cm-1 determined for the trianionic tppz3- radical-bridged Gd compound 4 is of similar magnitude to that of the tppz--bridged analogue 1. However, the anionic tppz3--bridged species containing TbIII and DyIII centers, compounds 5 and 6, do not exhibit slow magnetization dynamics under zero and applied dc fields. Computational results indicate a doublet ground state for the bridging tppz3- unit, with a different distribution for the spin density orientation towards the LnIII centers. These results have important implications for the future design of molecule-based magnets incorporating exchange-coupled lanthanide-radical species. This journal is
author list (cited authors)
Demir, S., Nippe, M., Gonzalez, M. I., & Long, J. R.