| With the development on super-paramagnetic nanomagnets,lanthanides ions have become the important core of these magnets,due to their large intrinsic magnetic anisotropy.The nitroxide radicals show unique paramagnetism and flexible tunability,so the"Ln(?)radical approach"has been a major strategy toward the super-paramagnetic nanomagnets.Two new nitronyl nitroxide radicals,NIT-3-Br-4Py(2-(3'-bromo-4'-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)and NITPh-4-2Py(2-(4'-2'-pyridylphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)have been synthesized,and characterized with IR,elemental analyses,ESR,XRD and single-crystal X-ray diffraction.Three series of new complexes:[Ln(hfac)3(NIT-3-Br-4Py)2](Ln=Eu(1),Gd(2),Tb(3),Dy(4)),[M2(hfac)4(NIT-3-Br-4Py)2]n(M=Mn(5),Co(6),Cu(7)),[Ln(hfac)3(NITPh-4-2Py)(H2O)]2ˇ(NITPh-4-2Py)(Ln=Gd(8),Tb(9),Dy(10),Ho(11),Er(12)),have been synthesized with NIT-3-Br-4Py and NITPh-4-2Py as organic paramagnetic ligand.These complexes were characterized with IR,single-crystal X-ray diffraction,ESR and XRD,and the magnetic properties were studied.[Ln(hfac)3(NIT-3-Br-4Py)2](Ln=Eu(1),Gd(2),Tb(3),Dy(4))exhibit mononuclear tri-spin rad-Ln(?)-rad structures.[M2(hfac)4(NIT-3-Br-4Py)2]n(M=Mn(5),Co(6),Cu(7))are one-dimensional chain structures,in which the nitronyl nitroxide radicals act as bridging ligands.[Ln(hfac)3(NITPh-4-2Py)(H2O)]2ˇ(NITPh-4-2Py)(Ln=Gd(8),Tb(9),Dy(10),Ho(11),Er(12))are composed of mononuclear coordination molecules[Ln(hfac)3(NITPh-4-2Py)(H2O)]and free nitronyl nitroxide radicals(NITPh-4-2Py).There are four mononuclear coordination molecules[Ln(hfac)3(NITPh-4-2Py)(H2O)]with slightly different structural parameters and two free nitronyl nitroxide radicals in the smallest asymmetric unit of these complexes.The Ln(?)ion of the mononuclear coordination molecule is coordinated with a nitronyl nitroxide radical and a water molecule.As for the nine"Ln(?)-nitroxide radical"complexes,the ferromagnetic interactions are found between Gd(?)ion and the coordinating nitronyl nitroxide radicals,but the weak antiferromagnetic interactions between the other Ln(?)ions and the coordinating nitronyl nitroxide radicals.In the threetransition metal-nitroxide radicalcomplexes,all central M(?)ions and nitronyl nitroxide radicals exhibit weak antiferromagnetic interactions.Ac magnetic studies show that four complexes:[Ln(hfac)3(NIT-3-Br-4Py)2](Ln=Tb(3),Dy(4))and[Ln(hfac)3(NITPh-4-2Py)(H2O)]2ˇ(NITPh-4-2Py)(Ln=Tb(9),Dy(10)),exhibit frequency-dependent out-of-phase signals,suggesting single-molecule magnet behavior.Complexes 3,4,and 9 exhibit single slow magnetic relaxation process,while complex 10 displays two field-induced relaxation processes.These achievements confirm that the minor difference in the substituents of nitroxide radicals may make a big difference in the structures and magnetic properties of complexes.Both electronic effect and steric hindrance affect the structures of complexes,and the magnetic anisotropies of central ions.Finally,the magnetic properties of these complexes show essential difference. |
PDF Full Text Request