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

Five <i>N</i>-aryl-<i>N</i>-pyridylaminoxyls, which have no substituent (<b>PhNOpy</b>), one substituent (<b>MeOPhNOpy</b> and <b><i>tert-</i></b><b>BuPhNOpy)</b> at the 4-position, and three substituents (<b>TPPNOpy</b> and <b>TBPNOpy)</b> 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)<sub>2</sub>(<b>PhNOpy</b>)<sub>4</sub>] (<b>1</b>), [Co(NCS)<sub>2</sub>(<b>MeOPhNOpy</b>)<sub>4</sub>] (<b>2</b>), [Co(NCS)<sub>2</sub>(<b><i>tert</i></b><b>BuPhNOpy</b>)<sub>4</sub>] (<b>3</b>), [Co(NCS)<sub>2</sub>(<b>TPPNOpy</b>)<sub>4</sub>] (<b>4</b>), [Co(NCS)<sub>2</sub>(<b>TBPNOpy</b>)<sub>4</sub>] (<b>5a</b>), and [Co(NCO)<sub>2</sub>(<b>TBPNOpy</b>)<sub>4</sub>] (<b>5b</b>), were obtained as single crystals. The molecular geometry revealed by X-ray crystallography for all complexes except <b>4</b> is a compressed octahedron. In the crystal structure of <b>1</b>, <b>2</b>, and <b>3</b>, 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 <b>5a</b> and <b>5b</b> have no significant short intermolecular contacts, indicating that they are magnetically isolated. <b>1</b> and <b>2</b> behaved as a 3D antiferromagnet with a Neel temperature, <i>T</i><sub>N</sub>, of 22 K and as a weak 3D antiferromagnet with a <i>T</i><sub>N</sub> of 2.9 K and a spin-flop field at 1.9 K, <i>H</i><sub>sp</sub>(1.9), of 0.7 kOe, respectively. <b>3</b> was a canted 2D antiferromagnet (a weak ferromagnet) with <i>T</i><sub>N</sub> = 4.8 K and showed a hysteresis loop with a coercive force, <i>H</i><sub>c</sub>, of 1.3 kOe at 1.9 K. On the other hand, the trisubstituted complexes <b>4</b>, <b>5a</b>, and <b>5b</b> functioned as single-molecule magnets (SMMs). <b>5b</b> had an effective activation barrier, <i>U</i><sub>eff</sub>, value of 28 K in a microcrystalline state and 48 K in a frozen solution.