Magnetic Relaxation Studies on Trigonal Bipyramidal Cobalt(II) Complexes

We report the preparation and the full characterization of a novel mononuclear trigonal bipyramidal Co^II complex [Co(NS₃ ^iPr )Br](BPh₄ ) (1) with the tetradentate sulfur-containing ligand NS₃ ^iPr (N(CH₂ CH₂ SCH(CH₃ )₂ )₃ ). The comparison of its magnetic behaviour with those of two previously reported compounds [Co(NS₃ ^iPr )Cl](BPh₄ ) (2) and [Co(NS₃ ^tBu )Br](ClO₄ ) (3) (NS₃ ^tBu =N(CH₂ CH₂ SC(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.