Selective Sensing of Phosphates by a New Bis-heteroleptic RuII Complex through Halogen Bonding: A Superior Sensor over Its Hydrogen-Bonding Analogue

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 H-1 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 center dot center dot center dot anion halogen-bonding interaction.