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

Determining the magnetic effects of binding heavy, main group donors (E) to 3d transition metals has been stymied by their weak σ donor strength, hard–soft mismatch between 3d metal and 5p/6p 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 (XantSb2) to enforce binding to the Co(II) ion. The resulting S = 3/2 Co(II) complex [(XantSb2)Co(I)2] (1) exhibits a large magnitude zero-field splitting parameter [D ≈ |25–50| cm–1] as determined by magnetometry measurements, variable-temperature electron paramagnetic resonance, and CASSCF/NEVPT2 calculations. Such D value is up to 10× higher than its light-atom congener [(XantP2)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 3d|5p paramagnetic system.