Probing the need for a very low axial zero-field splitting in Mn ^II systems with slow relaxation of the magnetization

The effect of very small anisotropy on the slow relaxation of the magnetization was analysed for two series of structurally related Mn^II complexes with a <i>trans</i>-octahedral or trigonal bipyramid environment. Complexes with the formula [MnX₂(<i>S</i>,<i>S</i>-L1)], where L1 is a neutral tetradentate Schiff base and X = NCS^- (1_NCS), Cl^- (2_Cl), Br^- (3_Br), and I^- (4_I), and [MnX₂(L2)], where L2 is a neutral tridentate Schiff base and X = Cl^- (5_Cl) and Br^- (6_Br), and 1D [Mn(N₃)₂(L2)]_<i>n</i> (7) systems were characterized. Benefiting from the increasing spin orbit coupling (SOC) contribution of heavier halide donors in complexes 1-6, the gradation of the axial zero-field splitting parameter (<i>D</i>) was determined to be between ∼0.1 and 0.6 cm^-1 by EPR spectroscopy. Changing current measurements showed that the slow relaxation of the magnetization vanishes for larger <i>D</i> values, evidencing the crucial effect of <i>D</i> values of ∼0.2 cm^-1.