Influence of the Coordinated Transition Metal Ion on Magnetic Relaxation of Lanthanide Based Complexes with Imino Nitroxide Biradical Ligands.

In spite of achievement of a lot of Ln-radical SMMs, how to improve magnetic behavior of Ln-radical system remains challenging. Here, two series of Ln-radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln 2 (hfac) 6 (ImPhPyobis) 2 ] (Ln III =Gd 1, Tb 2, Dy 3) and [Ln 2 Cu 2 (hfac) 10 (ImPhPyobis) 2 ] (Ln III =Gd 4, Dy 5; hfac=hexafluoroacetylacetonate and ImPhPyobis=5-(4-oxypyridinium-1-yl)-1,3-bis(1'-oxyl-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). For these biradical-metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4-oxypyridinium units to produce a binuclear {Ln 2 O 2 } unit. Those imino nitroxide groups are free for complexes 1-3, however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5. The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(U eff ) of 74.0 K while complex 5 exhibits dual relaxation processes with U eff values for the fast- and slow-relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation.