Carbon Dioxide Jumps In To Form New Ligand

While studying how carbon dioxide reacts with certain types of main-group metal complexes, Diane A. Dickie, Marie V. Parkes, and Richard A. Kemp of the University of New Mexico, Albuquerque, discovered [N(CO₂)₃]³⁻, a previously unknown type of ligand (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200804218). Kemp's group is exploring potential methods for sequestering CO₂ or using it as a feedstock to make value-added chemicals such as organic isocyanates and carbodiimides. When the researchers reacted CO₂ with Sr[N(PPh₂)₂]₂(thf)₃, where Ph is phenyl and thf is tetrahydrofuran, an Sr₆ complex formed and trapped 12 equivalents of CO₂. Half of the CO₂ inserted into Sr–N bonds to form six bridging phosphino carbamate ligands, O₂CN(PPh₂)₂. But the researchers were surprised to find that the remaining CO₂ displaced PPh₂ groups to form two [N(CO₂)₃]³⁻ ligands. After searching chemical databases, they concluded that they had a new ligand on their hands. The [N(CO₂)₃]^3– ligand can be viewed as an anion derived from the hypothetical compound N(CO₂H)₃, Kemp notes. His group is now working to make this compound, possibly from known N(CO₂R)₃ esters, as a starting point for synthesizing [N(CO₂)₃]^3–.