Magnetic Properties of 1:2 Mixed Cobalt(II) Salicylaldehyde Schiff-Base Complexes with Pyridine Ligands Carrying High-Spin Carbenes (<i>S</i>_car = 2/2, 4/2, 6/2, and 8/2) in Dilute Frozen Solutions: Role of Organic Spin in Heterospin Single-Molecule Magnets

The 1:2 mixtures of Co­(<i>p</i>-tolsal)₂, <i>p</i>-tolsal = <i>N</i>-<i>p</i>-tolylsalicylideniminato, and diazo-pyridine ligands, <b>DXpy</b>; <b>X</b> = <b>1</b>,<b> 2</b>,<b> 3l</b>,<b> 3b</b>, and <b>4</b>, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt–carbene complexes Co­(<i>p</i>-tolsal)₂(<b>CXpy</b>)₂, with <i>S</i>_total = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co­(<i>p</i>-tolsal)₂(<b>CXpy</b>)₂, <b>X</b> = <b>1</b>,<b> 2</b>,<b> 3l</b>,<b> 3b</b>, and <b>4</b>, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, <i>U</i>_eff/<i>k</i>_B, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, <i>H</i>_c, of ∼0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, τ_Q, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co­(<i>p</i>-tolsal)₂(<b>C1py</b>)₂, ∼2.0 × 10³ s for Co­(<i>p</i>-tolsal)₂(<b>C2py</b>)₂, and >10⁵ s for Co­(<i>p</i>-tolsal)₂(<b>CXpy</b>)₂; <b>X</b> = <b>3b</b>,<b> 3l</b>, and <b>4</b>. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the τ_Q value increased with increasing the <i>S</i>_total values