Magnetic Relaxation in a Dysprosium-Copper Heterometallic Cluster Involving Nitronyl Nitroxide Biradicals

Utilizing nitronyl nitroxide biradical NITPhPyObis [5-(3-pyridinyloxy)- 1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1 H-imidazol-2-yl)-benzene], a family of isomorphic 2p-3d-4f complexes [Ln(2)Cu(2)(hfac)(10)-(NITPhPyObis)(2)] (hfac = hexafluoroacetylacetonate; Ln(III) = Gd (1), Dy (2), and Ho (3)) were successfully produced. The biradical NITPhPyObis of 1-3 is ligated to the Ln(III) ion through its two NIT moieties in a chelating mode while the N donor of pyridine and one NO group of this biradical bind one Cu-II(I) ion, respectively, yielding a tetranuclear heterometal cyclic cluster. Magnetic studies indicate that ferromagnetic exchanges dominate in these 2p-3d-4f clusters which origin from the Ln-NO and axial NO-Cu couplings. Spin dynamics studies show that the chi' and chi '' signals of Cu-Dy derivative exhibit both temperature dependence and frequency dependence in 0 Oe dc field, demonstrating slow magnetic relaxation behavior. The obtained energy barrier for demagnetization is U-eff = 15.8 K. The value almost is three times larger than that of the related Dy cluster [Dy-4(hfac)(12) (NITPhPyObis)(2)] (U-eff = 5.2 K) which indicates the magnetization relaxation rate of the birad-Ln cluster is reduced through introducing Cu(II) ion.