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(CH₂CH₂SCH(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(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.