Synthesis, structures and magnetic properties of a series of 3d-4f tetranuclear Co2IILn2III cubanes

The reaction of 2,6-pyridinedimethanol (pdmH(2)), CoCl2 center dot 6H(2)O, Ln(NO3)(3)center dot 6H(2)O and pivalic acid (pivH) in the presence of triethylamine in MeCN and CH2Cl2 under ambient conditions results in the tetranuclear heterobimetallic compounds [Co(2)(II)Ln(2)(pdmH)(4)(Piv)(6)]center dot 4CH(3)CN (Ln = Y (1), Gd (2), Tb (3), Ho (4)) and [(Co2Dy2)-Dy-II(pdmH)(4)(Piv)(6)]center dot 2CH(3)CN (5). The molecular structures of 1-5 were determined by single-crystal X-ray diffraction and reveal that they are isostructural. However, compounds 1-4 crystallize in the monoclinic space group P2(1)/n, while compound 5 crystallizes in the orthorhombic space group Pnna. In all of these compounds, the four metal ions are held together by four alkoxide mu(3)-RCH2O- bridges to form novel Co(2)Ln(2)O(4)-heterocubanes. Magnetic properties of 1-5 have been investigated using dc and ac susceptibility measurements. Of these compounds, only compound 5 displays an out-of-phase signal in the ac susceptibility and exhibits slow relaxation of the magnetization, which is attributed to the presence of the anisotropic Dy-III ions.