What Controls the Sign and Magnitude of Magnetic Anisotropy in Tetrahedral Cobalt(II) Single-Ion Magnets?

A family of mononuclear tetrahedral
cobalt­(II) thiourea complexes,
[Co­(L1)4]­(NO3)2 (1) and [Co­(Lx)4]­(ClO4)2 where x = 2 (2), 3 (3), 4 (4) (where L1 =
thiourea, L2 = 1,3-dibutylthiourea, L3 = 1,3-phenylethylthiourea,
and L4 = 1,1,3,3-tetramethylthiourea), has been synthesized
using a rationally designed synthetic approach, with the aim of stabilizing
an Ising-type magnetic anisotropy (−D). On
the basis of direct-current, alternating-current, and hysteresis magnetic
measurements and theoretical calculations, we have identified the
factors that govern the sign and magnitude of D and
ultimately the ability to design a single-ion magnet for a tetrahedral
cobalt­(II) ion. To better understand the magnetization relaxation
dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues,
which are characterized by single-crystal X-ray diffraction with the
general molecular formulas of [Zn­(L1)4]­(NO3)2 (5) and [Zn­(L2)4]­(ClO4)2 (6). Interestingly,
intermolecular interactions are shown to play a role in quenching
the quantum tunneling of magnetization in zero field, as evidenced
in the hysteresis loop of 1. Complex 2 exhibits
the largest Ueff value of 62 cm–1 and reveals open hysteresis loops below 4 K. Furthermore, the influence
of the hyperfine interaction on the magnetization relaxation dynamics
is witnessed in the hysteresis loops, allowing us to determine the
electron/nuclear spin S(Co) = 3/2/I(Co) = 7/2 hyperfine coupling
constant of 550 MHz, a method ideally suited to determine the hyperfine
coupling constant of highly anisotropic metal ions stabilized with
large D value, which are otherwise hard to determine
by conventional methods such as electron paramagnetic resonance.