Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)-Porphyrin Complex

We report on a novel manganese(III)-porphyrin complex with the formula [Mn-III(TPP)(3,5-Me(2)pyNO)(2)]ClO4 center dot CH3CN (2; 3,5-Me(2)pyNO = 3,5-dimethylpyridine N-oxide, H2TPP=5,10,15,20-tetraphenylporphyrin), in which the Mn-III ion is six-coordinate with two monodentate 3,5-Me(2)pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero-field splitting (D = -3.8 cm(-1)), low rhombicity (E/vertical bar D vertical bar = 0.04) of the high spin MO ion, and, ultimately, for the observation of slow magnetic-relaxation effects (E=15.5 cm(-1) at H=1000 G) in this rare example of a manganese-based single ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single-crystal diffraction studies, variable-temperature direct- and alternating-current measurements and high-frequency and -field EPR spectroscopic analysis followed by quantum chemical calculations. Slow magnetic-relaxation effects were also observed in the already known analogous compound [Mn-III(TPP)Cl] (1; E-a = 10.5 cm(-1) at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.