Glycoligands tuning the magnetic anisotropy of Ni(II) complexes

Two organic ligands based on a sugar-scaffold derived from galactose and possessing three O-CH(2)-pyridine pendant arms at the 3-, 4-, and 5-positions of the galactopyranose that act as chelates afford mononuclear complexes when reacted with a Ni(II) salt. The magnetization behavior in the form of M=f(H/T) plots suggests the presence of appreciable magnetic anisotropy within the two complexes. The analysis of the EPR spectra performed at two different temperatures (7 and 17 K) and at three frequencies (190, 285, and 380 GHz) leads to the conclusion that the anisotropy has a high degree of axiality (E/D=0.17 for the two complexes), but with a different sign of the D parameter. The spin hamiltonian parameters D and E were reproduced for the two complexes by using calculations based on the angular overlap model (AOM). The structural difference between the two complexes responsible of the sign of the D parameters was also determined using AOM calculations. A thorough analysis of the structures showed that the structural differences in the coordination sphere of the two complexes responsible of the different D parameter sign result from the nature of the sugar scaffolds. In complex 1, the sugar scaffold imposes an intramolecular hydrogen bond with one of the atoms linked to Ni(II); this arrangement leads to a distorted coordination sphere and positive D value, while the absence of such a hydrogen bond in complex 2 leads to a less distorted environment around the Ni center and to a negative D value.