An Antimony(V) Dication as a Z-Type Ligand: Turning on Styrene Activation at Gold

With the intent to demonstrate that the charge of Z-type ligands can be used to modulate the electrophilic character and catalytic properties of coordinated transition metals, we are now targeting complexes bearing polycationic antimony-based Z-type ligands. Toward this end, the dangling phosphine arm of ((o-(Ph₂ P)C₆ H₄ )₃ )SbCl₂ AuCl (1) was oxidized with hydrogen peroxide to afford [((o-(Ph₂ P)C₆ H₄ )₂ (o-Ph₂ PO)C₆ H₄ )SbAuCl₂ ]⁺ ([2 a]⁺ ) which was readily converted into the dicationic complex [((o-(Ph₂ P)C₆ H₄ )₂ (o-Ph₂ PO)C₆ H₄ )SbAuCl]²⁺ ([3]²⁺ ) by treatment with 2 equiv AgNTf₂ . Both experimental and computational results show that [3]²⁺ possess a strong Au→Sb interaction reinforced by the dicationic character of the antimony center. The gold-bound chloride anion of [3]²⁺ is rather inert and necessitates the addition of excess AgNTf₂ to undergo activation. The activated complex, referred to as [4]²⁺ [((o-(Ph₂ P)C₆ H₄ )₂ (o-Ph₂ PO)C₆ H₄ )SbAuNTf₂ ]²⁺ readily catalyzes both the polymerization and the hydroamination of styrene. This atypical reactivity underscores the strong σ-accepting properties of the dicationic antimony ligand and its activating impact on the gold center.