Selective sensing of phosphates by a new bis-heteroleptic Ru^II complex through halogen bonding: a superior sensor over its hydrogen-bonding analogue

Abstract

The selective phosphate-sensing property of a bis-heteroleptic Ru^II complex, 1[PF₆]₂, which has a halogen-bonding iodotriazole unit, is demonstrated and is shown to be superior to its hydrogen-bonding analogue, 2[PF₆]₂. Complex 1[PF₆]₂, exploiting halogen-bonding interactions, shows enhanced phosphate recognition in both acetonitrile and aqueous acetonitrile compared with its hydrogen-bonding analogue, owing to considerable amplification of the Ru^II-center-based metal-to-ligand charge transfer (MLCT) emission response and luminescence lifetime. Detailed solution-state studies reveal a higher association constant, lower limit of detection, and greater change in lifetime for complex 1 in the presence of phosphates compared with its hydrogen-bonding analogue, complex 2. The ¹H NMR titration study with H₂PO₄⁻ ascertains that the binding of H₂PO₄⁻ occurs exclusively through halogen-bonding or hydrogen-bonding interactions in complexes 1[PF₆]₂ and 2[PF₆]₂, respectively. Importantly, the single-crystal X-ray structure confirms the first ever report on metal-assisted second-sphere recognition of H₂PO₄⁻ and H₂P₂O₇²⁻ with 1 through a solitary C−I⋅⋅⋅anion halogen-bonding interaction.