Isolable Monocyclic N-Heterocyclic Radicals Supported by Rare-Earth Organometallics

Monocyclic N-heterocyclic radicals are the elementary reactive intermediates in synthetic chemistry and biochemical processes, but their isolation remains a central challenge due to extremely high reactivity. Here we report the first structurally characterized example of monocyclic pyridine radical supported by metal ions, [K(crypt-222)][Cp*₂RE(PyS^2-•)] (2-RE, PyS^2-• = radical anion of pyridine-2-thiolate), and the first stable monocyclic triazine radical for any species, [K(crypt-222)][(Cp*₂RE)₃(TrizS^4-•)] (4-RE, TrizS^4-• = radical anion of 1,3,5-triazine-2,4,6-tris(thiolate)), based on rare earth thiolate systems. Detailed structural, computational, UV-vis, and EPR data support the presence of heterocyclic radicals, which show a complicated, uneven spin density distribution at both pyridine and triazine rings. Remarkably, the unusual bonding characters between the lanthanide centers and the SOMO π*-orbital of the radical promote strong ferromagnetic coupling from radical to lanthanide ions and achieve the largest gadolinium-radical ferromagnetic coupling observed to date, J_Gd-rad = +28.55(57) cm^-1 (Ĥ = -2J_Gd-Rad Ŝ_Gd·Ŝ_Rad) in 2-Gd, while compound 4-Dy exhibits slow magnetic relaxation. Furthermore, the initial exploration on reactivity revealed the potential ability of the pyridine radical to activate inert bonds. Those results contribute to a better understanding of highly reactive species involving monocyclic heterocyclic radicals.