Synthesis &amp; Magnetic Properties of an Antimony Chelate (‘XantSb₂’) Supported Cobalt(II): Rare Example of a 5<i>p</i>|3<i>d</i> Paramagnetic Complex

Determining the magnetic effects of binding heavy, main group donors (E) to 3<i>d</i> transition metals has been stymied by their weak σ donor strength, hard-soft mismatch between 3<i>d</i> metal and 5<i>p</i>/6<i>p</i> element, and the instability of E-C bonds in ligand frameworks. Herein, we resolve these issues by leveraging the chelate effect in the xanthene scaffold-based bidentate triaryl-antimony ligand (Xant<b>Sb</b>₂) to enforce binding to the Co(II) ion. The resulting <i>S</i> = ³/₂ Co(II) complex [(Xant<b>Sb</b>₂)Co(I)₂] (<b>1</b>) exhibits a large magnitude zero-field splitting parameter [<i>D</i> ≈ |25-50| cm^-1] as determined by magnetometry measurements, variable-temperature electron paramagnetic resonance, and CASSCF/NEVPT2 calculations. Such <i>D</i> value is up to 10× higher than its light-atom congener [(Xant<b>P</b>₂)Co(I)₂] and is attributed to the 'heavy-atom effect' of antimony donors, as well as a minor geometric distortion. This finding represents a rare example of a 3<i>d</i>|5<i>p</i> paramagnetic system.