Formation of Radical-like NH Ligand from NH3 at Ambient Conditions Mediated by Dialkyl Rare-Earth Complexes

Abstract

Although intensive work on ammonia activation has been carried out in recent decades, generating nitrogen-centered radicals from NH₃ under ambient conditions remains quite challenging. In the presented research, the conversion of NH₃ to radical-like NH ligand has been achieved by the reactions of a series of dialkyl rare-earth (RE) complexes (1-RE, RE = Tb, Dy, Y, Ho, Er, Yb, and Lu) supported by β-diketiminate ligands with NH₃ in n-hexane at room temperature, resulting in the formations of the radical-like μ₃-NH ligands containing trinuclear RE complexes (2-RE). The radical-like feature of the μ₃-NH ligand was revealed by electron paramagnetic resonance and magnetic measurements, radical trapping experiments, and computational spin density analysis. In addition, H₂ was detected to form during the reaction of 1-RE with NH₃, indicating that the radical-like μ₃-NH ligand was likely to be generated via N–H bond homolysis. Moreover, the solvents and coordination pattern of β-diketiminate ligands are crucial for the formation of the radical-like μ₃-NH ligand from NH₃. When toluene instead of n-hexane was used in the reaction of 1-RE with NH₃, an array of octaamido tetranuclear RE complexes (3-RE) was obtained. The reaction of the dialkyl yttrium complex (4-Y) bearing a modified β-diketiminate ligand, in which the two mesityl substituents are replaced by a 2,6-diisopropylphenyl group and a 2-(dimethylamino)ethyl group, with NH₃ in both n-hexane and toluene only yielded a tetranuclear yttrium complex carrying the dianionic closed-shell μ₃-NH ligands (5-Y).