Guanidinate Rare-Earth Tetraphenylborate Complexes and Their Prospects in Single-Molecule Magnetism

The pursuit of higher-nuclearity rare-earth-metal clusters necessitates adequate ancillary ligand scaffolds as well as easily dissociable counterions. To this end, guanidinate anions are advantageous owing to their highly customizable, sterically encumbering, anionic charged framework. Here, we present the two mononuclear guanidinate rare-earth complexes [{(Me3Si)2NC(NiPr)2}2RE][(μ-η6-Ph)(BPh3)], (RE = Y (1), Dy (2)), featuring inner-sphere tetraphenylborate anions. Each complex is sterically congested and comprises a metal ion that is ligated by two ancillary guanidinate ions and one tetraphenylborate ligand. The isostructural compounds, 1 and 2, were synthesized from a protonolysis reaction between guanidinate alkyl complexes and [HNEt3][BPh4]. The isolated molecules were characterized by X-ray crystallography and IR, NMR, and UV–vis spectroscopy. The crowded coordination sphere around each metal ion implemented by the ancillary guanidinate ligands causes the tetraphenylborate ion to adopt a rare η6-binding mode where the metal center interacts asymmetrically with the carbon atoms of one phenyl ring. DFT and NBO calculations carried out on 1 provide insight into a complicated bonding picture between one of the phenyl rings of the BPh4– moiety and the rare-earth ion. Furthermore, the dysprosium congener, 2, is a single-molecule magnet displaying slow magnetic relaxation under the application of a static dc field.The pursuit of higher-nuclearity rare-earth-metal clusters necessitates adequate ancillary ligand scaffolds as well as easily dissociable counterions. To this end, guanidinate anions are advantageous owing to their highly customizable, sterically encumbering, anionic charged framework. Here, we present the two mononuclear guanidinate rare-earth complexes [{(Me3Si)2NC(NiPr)2}2RE][(μ-η6-Ph)(BPh3)], (RE = Y (1), Dy (2)), featuring inner-sphere tetraphenylborate anions. Each complex is sterically congested and comprises a metal ion that is ligated by two ancillary guanidinate ions and one tetraphenylborate ligand. The isostructural compounds, 1 and 2, were synthesized from a protonolysis reaction between guanidinate alkyl complexes and [HNEt3][BPh4]. The isolated molecules were characterized by X-ray crystallography and IR, NMR, and UV–vis spectroscopy. The crowded coordination sphere around each metal ion implemented by the ancillary guanidinate ligands causes the tetraphenylborate ion to adopt a rare η6-binding mode where the metal center interacts asymmetrically with the carbon atoms of one phenyl ring. DFT and NBO calculations carried out on 1 provide insight into a complicated bonding picture between one of the phenyl rings of the BPh4– moiety and the rare-earth ion. Furthermore, the dysprosium congener, 2, is a single-molecule magnet displaying slow magnetic relaxation under the application of a static dc field.