Lanthanide Tetrazolate Complexes Combining Single-Molecule Magnet and Luminescence Properties: The Effect of the Replacement of Tetrazolate N3 by β-Diketonate Ligands on the Anisotropy Energy Barrier

• Published in: Chemistry : a European journal 2016-10, Vol.22 (41), p.14548-14559
• Main Authors: Jiménez, Juan-Ramón, Díaz-Ortega, Ismael F., Ruiz, Eliseo, Aravena, Daniel, Pope, Simon J. A., Colacio, Enrique, Herrera, Juan Manuel
• Format: Article
• Language: English
• Subjects: Anisotropia; Anisotropy; lanthanides; ligand effects; Ligands; Lligands; Luminescence; Luminescència; magnetic properties; Metalls de terres rares; Rare earth metals; single-molecule magnets
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201601457

Three new sets of mononuclear LnIII complexes of general formulas [LnL3]⋅CH3OH [LnIII=Yb (1), Er (2), Dy (3), Gd (4), and Eu (5)], [LnL2(tmh)(CH3OH)]⋅n H2O⋅m CH3OH [LnIII=Yb (1 b), Er (2 b), Dy (3 b), Gd (4 b)], and [LnL2(tta)(CH3OH)]⋅CH3OH [LnIII=Yb (1 c), Er (2 c), Dy (3 c), Gd (4 c)] were prepared by the reaction of Ln(CF3SO3)⋅n H2O salts with the tridentate ligand 2‐(tetrazol‐5‐yl)‐1,10‐phenanthroline (HL) and, for the last two sets, additionally with the β‐diketonate ligands 2,2,6,6‐tetramethylheptanoate (tmh) and 2‐thenoyltrifluoroacetonate (tta), respectively. In the [LnL3]⋅CH3OH complexes the LnIII ions are coordinated to three phenanthroline tetrazolate ligands with an LnN9 coordination sphere. Dynamic ac magnetic measurements on 1–3 reveal that these complexes only exhibit single‐molecule magnet (SMM) behavior when an external dc magnetic field is applied, with Ueff values of 11.7 K (1), 16.0 K (2), and 20.2 K (3). When the tridentate phenanthroline tetrazolate ligand is replaced by one molecule of methanol and the β‐diketonate ligand tmh (1 b–3 b) or tta (1 c–3 c), a significant increase in Ueff occurs and, in the case of the DyIII complexes 3 b and 3 c, out‐of‐phase χ′′ signals below 15 and 10 K, respectively, are observed in zero dc magnetic field. CASSCF+RASSI ab initio calculations performed on the DyIII complexes support the experimental results. Thus, for 3 the ground Kramers’ doublet is far from being axial and the first excited state is found to be very close in energy to the ground state, so the relaxation barrier in this case is almost negligible. Conversely, for 3 b and 3 c, the ground Kramers’ doublet is axial with a small quantum tunneling of the magnetization, and the energy difference between the ground and first Kramers’ doublets is much higher, which allows these compounds to behave as SMMs at zero field. Moreover, these calculations support the larger Ueff observed for 3 b compared to 3 c. Additionally, the solid‐state photophysical properties of 1, 2, 4, and 5 show that the phenanthroline tetrazolate ligand can act as an effective antenna to sensitize the characteristic YbIII, ErIII, and EuIII emissions through an energy‐transfer process. More effective: Mononuclear LnL3 complexes prepared from a tridentate phenanthroline‐tetrazolate ligand (HL) exhibit field‐induced single‐molecule magnet (SMM) behavior with very low thermal energy barriers. Replacement of one tridentate ligand L by a β‐diketonate ligand induces a significant