Water-Soluble Rhenium Nitride Complexes Supported by Multidentate Phosphinite Ligands Provide Insights into Aqueous Ammonium Synthesis Using Samarium Diiodide

Metal nitride complexes are key intermediates in ammonia synthesis. Although water would be an ideal source of protons, the synthesis of ammonia from well-defined nitride complexes in water is underexplored. The aqueous reactivity of rhenium nitride complexes supported by tridentate and tetradentate phosphinite-based ligands is examined here. The tetradentate ligand 2,9-bis(diisopropylphosphinitomethyl)-1,10-phenanthroline (⁶POphenOP) was found to support a nitride complex that is water-soluble and resists ligand hydrolysis from pH 2-13. Structural analysis enabled by hanging-drop crystallization was combined with NMR spectroscopic analysis to elucidate how the rhenium coordination sphere changes when dissolved in aqueous buffer and to identify networks of water molecules in the solid state, including an example of water hydrogen bonding with the nitride. Addition of SmI₂ to buffered water solutions of (⁶POphenOP)Re nitride complexes produced aqueous ammonium. Buffer concentration influences ammonium yields, an effect that is proposed to arise from coordination of buffer to Re and/or Sm, providing insight into the selection of appropriate conditions for aqueous N₂ fixation.