Magnetic relaxation studies in trigonal bipyramidal cobalt(II) complexes.

We report the preparation and the full characterization of a novel mononuclear trigonal bipyramidal Co-II complex [Co(NS3iPr)Br](BPh4) (1) with the tetradentate sulfur-containing ligand NS3iPr (N(CH2CH2SCH(CH3)(2))(3)). The comparison of its magnetic behaviour with those of two previously reported compounds [Co(NS3iPr)Cl](BPh4) (2) and [Co(NS3tBu)Br](ClO4) (3) (NS3tBu=N(CH2CH2SC(CH3)(3))(3)) with similar structures shows that 1 displays a single-molecule magnet behaviour with the longest magnetic relaxation time (0.051 s) at T=1.8 K, which is almost thirty times larger than that of 3 (0.0019 s) and more than three times larger than that of 2 (0.015 s), though its effective energy barrier (26 cm(-1)) is smaller. Compound 1, which contains two crystallographically independent molecules, presents smaller rhombic parameters (E=1.45 and 0.59 cm(-1)) than 2 (E=2.05 and 1.02 cm(-1)) and 3 (E=2.00 and 0.80 cm(-1)) obtained from theoretical calculations. Compounds 2 and 3 have almost the same axial (D) and rhombic (E) parameter values, but present a large difference of their effective energy barrier and magnetic relaxation which may be attributed to the larger volume of BPh4- than ClO4- leading to larger diamagnetic dilution (weaker magnetic dipolar interaction) for 2 than for 3. The combination of these factors leads to a much slower magnetic relaxation for 1 than for the two other compounds.