Crystal Design of Monometallic Single-Molecule Magnets Consisting of Cobalt-Aminoxyl Heterospins

Five N-aryl-N-pyridylaminoxyls, which have no substituent (PhNOpy), one substituent (MeOPhNOpy and tert-BuPhNOpy) at the 4-position, and three substituents (TPPNOpy and TBPNOpy) at the 2,
4, and 6-positions of the phenyl ring, were prepared as new ligands for cobalt-aminoxyl heterospin systems.
The 1:4 complexes, [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2(MeOPhNOpy)4] (2), [Co(NCS)2(tertBuPhNOpy)4]
(3), [Co(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4] (5a), and [Co(NCO)2(TBPNOpy)4] (5b), were
obtained as single crystals. The molecular geometry revealed by X-ray crystallography for all complexes
except 4 is a compressed octahedron. In the crystal structure of 1, 2, and 3, the organic spin centers have
various short contacts within 4 Å with the neighboring molecules to form 3D and 2D spin networks. On the
other hand, complexes 5a and 5b have no significant short intermolecular contacts, indicating that they
are magnetically isolated. 1 and 2 behaved as a 3D antiferromagnet with a Neel temperature, TN, of 22 K
and as a weak 3D antiferromagnet with a TN of 2.9 K and a spin-flop field at 1.9 K, Hsp(1.9), of 0.7 kOe,
respectively. 3 was a canted 2D antiferromagnet (a weak ferromagnet) with TN = 4.8 K and showed a
hysteresis loop with a coercive force, Hc, of 1.3 kOe at 1.9 K. On the other hand, the trisubstituted complexes
4, 5a, and 5b functioned as single-molecule magnets (SMMs). 5b had an effective activation barrier, Ueff,
value of 28 K in a microcrystalline state and 48 K in a frozen solution.