Trigonal antiprismatic Co(II) single molecule magnets with large uniaxial anisotropies: importance of Raman and tunneling mechanisms

The air-stable mononuclear Co(II) compounds [Co-II(Tpm)(2)][ClO4](2) (1, Tpm = tris(pyrazol-1-yl)methane), [Co-II(Tpm)(2)][BPh4](2).2MeCN (2) with trigonal antiprismatic geometry (trigonally elongated octahedral geometry) are reported. Magnetic and theoretical studies reveal that the complexes exhibit single-molecule magnet behavior with uniaxial anisotropy and a huge energy difference between ground and first excited Karmers' doublets (similar to 200 cm(-1)). Under applied DC fields, compounds 1 and 2 exhibit frequency and temperature dependence of the imaginary susceptibility. The fit of the data to an Orbach relaxation process yields effective energy barriers of 30.6(1) and 44.7(6) cm(-1) for 1 and 2, respectively, but there is no real state at that energy. The inclusion of tunneling, direct and Raman relaxation processes leads to the conclusion that the inclusion of an Orbach process is not required to provide a good fit to the data. More interestingly, a detailed study of the dependence of the relaxation time with field shows that for these Kramers' ions, tunneling is the predominant process at low temperature and that differences in the counteranion allow for a tuning of the Raman process at higher temperatures. These findings underscore the fact that large uniaxial anisotropy can be achieved in hexacoordinate Co(II) trigonal antiprismatic complexes which is an unexplored geometry in mononuclear single molecule magnets.