Theoretically Predicted and Experimentally Observed Relaxation Pathways of two Heterodinuclear 3d-4f Complexes

The dinucleating ligand ( 1,4,7,10- tetraazacyclododecane1,4,7,10- tetrayl)- tetrakis-( methylene)- tetrakis( 2- methoxy- 4- methylphenol), H4L, is well preorganized with an N4O2 octahedral site for a divalent transition metal ion and an O4 site ( phenolates and methoxo donors, completed by added pivalates and solvent molecules) for lanthanide ions. Two 3d- 4f complexes ([ DyIII{NiII( mu- H2L)} piv2( OH2)] ClO4 and [ DyIII{CoII( mu- H2L)} piv2( OH2)] ClO4) were prepared and fully charac- terized by X- ray molecular structures and electronic properties determined by ac and dc SQUID magnetometry. While the NiII based complex shows single- molecule magnetism ( SMM), the CoII complex does not. With an extensive quantum- chemical analysis, this is shown to be due to a misalignment of the single ion anisotropies in the latter complex, and reasons for this are discussed.