Controlling the Nature of Exchange Interaction in Lanthanide-Based Single-Molecule Magnets

Lanthanide-based single-molecule magnets (Ln-SMMs) display slow relaxation of magnetization which may be manifested as magnetic hysteresis below a blocking temperature (<i>T</i>_b) which is theoretically the highest operational temperature for these Ln-SMM-based devices.¹ Because of this magnetic memory effect, Ln-SMMs have been proposed as building blocks for next-generation high-density information storage devices. Currently, the best strategy for maximizing the blocking temperature of Ln-SMMs is to increase the magnetic anisotropy of the lanthanide ion by optimizing its interactions with the ligand field.² A promising alternative strategy however is to enhance the magnetic properties of lanthanide ions via exchange coupling.^3,4 This can be achieved through the use of one or more bridging radical-ion ligands.<br/><br/>In the previously reported [Dy₂(<i>µ</i>-(bpytz)₂)(TMHD)₄] complex⁵ (bpytz = 3,6-bis(3,5-dimethyl-pyrazolyl)-1,2,4,5-tetrazine; TMHD = 2,2,6,6-tetramethyl-3,5-heptanedionate), the radical-radical coupling between the two bridging radical-anions (bpytz^●^-) is strongly antiferromagnetic (AFM) and this suppresses the intended radical-Dy coupling, resulting in little improvement in its magnetic properties through exchange interaction. We propose that by choosing two different bridging radical ligands with different orbital symmetries it may be possible to modify the nature of exchange interaction between them. Our current work is focused on the investigation of exchange interaction in a model system, namely [Ln₂((<i>µ</i>-(bpytz)(bpym))(PD)₄] complexes (Ln = Gd (<b>1</b>), Y (<b>2</b>); bpym = 2,2’-bipyrimidine; PD = propanedionate) containing two different bridging radicals and determining if it is possible to induce ferromagnetic (FM) coupling between both of the radicals and also the metal centers.<br/><br/>Broken symmetry DFT calculations were employed to determine the exchange coupling constants in the model gadolinium dimer <b>1</b> and its yttrium analogue <b>2</b>. Our calculations show that the exchange couplings between all the magnetic centers in <b>1</b> and <b>2 </b>are FM. The calculated exchange coupling constants agree with previous observed trends where out-of-plane coordination affords larger FM coupling between the lanthanide and radical.⁶ The radical-radical FM coupling can be rationalized by the symmetry-imposed orthogonality.^7–9 The combination of these two effects could open new paths for the design of radical-based Ln-SMMs with FM interactions between all magnetic centers and improved performance of the subsequent SMMs.<br/><br/>1 J. D. Rinehart and J. R. Long, <i>Chem. Sci.</i>, 2011, <b>2</b>, 2078.<br/>2 F.-S. Guo, B. M. Day, Y.-C. Chen, M.-L. Tong, A. Mansikkamäki and R. A. Layfield, <i>Science (80-. ).</i>, 2018, <b>362</b>, 1400–1403.<br/>3 T. Rajeshkumar and G. Rajaraman, <i>Chem. Commun.</i>, 2012, <b>48</b>, 7856–7858.<br/>4 S. Demir, M. I. Gonzalez, L. E. Darago, W. J. Evans and J. R. Long, <i>Nat. Commun. 2017 81</i>, 2017, <b>8</b>, 1–9.<br/>5 M. A. Lemes, N. Mavragani, P. Richardson, Y. Zhang, B. Gabidullin, J. L. Brusso, J. O. Moilanen and M. Murugesu, <i>Inorg. Chem. Front.</i>, 2020, <b>7</b>, 2592–2601.<br/>6 T. Kanetomo, T. Yoshitake and T. Ishida, <i>Inorg. Chem.</i>, 2016, <b>55</b>, 8140–8146.<br/>7 O. Kahn, P. Tola, J. Galy and H. Coudanne, <i>J. Am. Chem. Soc.</i>, 1978, 100, 3931–3933.<br/>8 O. Kahn, Y. Journaux, I. Morgenstern-Badarau, J. Galy and J. Jaud, <i>J. Am. Chem. Soc.</i>, 1982, <b>104</b>, 2165–2176.<br/>9 A. Mansikkamäki and H. M. Tuononen, <i>J. Phys. Chem. Lett.</i>, 2018, <b>9</b>, 3624–3630.