Regulating Spin Dynamics of Nitronyl Nitroxide Biradical Lanthanide Complexes through Introducing Different Transition Metals

Four biradical-Ln complexes with different transition metal ions, namely [LnM(hfac)(5)(NITPh-PyPzbis)] (M-II=Mn-II and Ln(III)=Gd 1, Dy 2; M-II=Ni-II and Ln(III)=Tb 3, Dy 4), were prepared by the reaction of Ln(hfac)(3) . 2H(2)O, Mn(hfac)(2) . 2H(2)O or Ni(hfac)(2) . 2H(2)O with NITPh-PyPzbis biradical (hfac=hexafluoroacetylacetonate, NITPh-PyPzbis=5-(3-(2-pyridinyl)-1H-pyrazol-1-yl)-1,3-bis(1'-oxyl-3'-oxido- 4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). In complexes 1-4, the NITPh-PyPzbis biradical chelates one Ln(III) ion by means of its aminoxyl moieties and the transition metal ion is introduced through the two N donors from the pyridyl pyrazolyl moiety. Magnetic investigations indicate that complex 4 displays visible maxima in frequency/temperature-dependent chi '' signals with two-step relaxation processes, but complex 2 exhibits no slow magnetization relaxation. The comparison of structure parameters of both Dy complexes indicates that the symmetries of coordination spheres of two Dy ions are D-2d for 2 and C-2v for 4, which thus probably results in different magnetic relaxation behaviors. This work provides new insight for improving properties of Ln-biradical based SMMs.