3- and 4-(α-diazobenzyl)pyridine-: N -oxides as photoresponsive magnetic couplers for 2p-4f heterospin systems: Formation of carbene-Tb^III and carbene-Dy^III single-molecule magnets

3- and 4-(α-diazophenyl)pyridine-N-oxides, 3- and 4-D1pyO, were prepared as new photoresponsive magnetic couplers in heterospin systems. Lanthanide dinuclear complexes, [Ln^III(tta)₃(4-D1pyO)]₂; Ln^III = Gd (1d), Tb (2d), and Dy (3d) and tta = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate, bridged with 4-D1pyO in μ₂ coordination mode were obtained. The obtained 1d, 2d, and 3d were isostructures. The isomeric complex, [Tb^III(tta)₃(3-D1pyO)]₂ was also prepared. In the carbene-Ln^III complexes [Ln^III(tta)₃(4- and 3-C1pyO)]₂ generated by photolysis, C1pyO regioselectively interacted with the Ln^III ions through pyridine-N-oxide, in which the magnetic coupling of 4-C1pyO was weakly ferromagnetic, while that of 3-C1pyO was insignificant. Before and after irradiation of 1d, no SMM behavior was observed. In contrast, the Tb^III and Dy^III complexes being anisotropic functioned as heterospin SMMs. Before irradiation, 2d showed no SMM behavior. After irradiation, two species showing slow magnetic relaxations were produced and one of them exhibited SMM behavior with the thermal activation barrier, U_eff/k_B = 30 K, and τ₀ = 5.8 × 10^-8 s. In 3d, SMM behaviors were observed before and after irradiation, and the U_eff/k_B value of 102 K (τ₀ = 3.6 × 10^-7 s) before irradiation was reduced to 39 K (τ₀ = 1.5 × 10^-8 s) after irradiation.