α-Diimine-Stabilized Gallylenes Can Activate CO2

The sequestration and transformation of carbon dioxide and its thio-analogues carbon disulfide (CS₂) and carbonyl sulfide (COS) are interesting research targets. Traditionally, transition metals and rare-earth metals have been the primary catalysts for activating these relatively inert substrates, while main-group elements have been less well-explored. Low-valent main-group complexes, e.g., gallylenes, can exhibit reactivities that are similar to transition metals. Ga(I) can facilitate, for example, oxidative additions and C–H activations, and is also a promising candidate for small-molecule activation.

Xiao-Juan Yang, Beijing Institute of Technology, China, and colleagues have investigated redox-noninnocent α-diimine ligands and found that gallylenes with such ligands can react with CO₂, CS₂, and COS (pictured). The team prepared the ligand via the condensation of benzoyl and 2,6-diisopropylphenylaniline, reduced it to its monoanion using sodium metal, and then reacted it with GaCl₃. This resulted in a chloro gallium(III) complex. Reductions with different alkali metals (Li, Na, or K) then gave anionic gallylenes (paired with the different alkali metal cations). The initially monoanionic α-diimine ligand was further reduced to the dianion, while the gallium(III) center accepted two electrons to become gallium(I).

The researchers found that a reaction of the potassium gallylene with CO₂ led to a trimeric carbonate complex and a dinuclear oxocarbonate complex (pictured above in the top row) via the reductive disproportionation of CO₂. Reactions with CS₂ gave dinuclear, bridged structures (pictured above in the center row). Depending on the reaction temperature, the gallylenes reacted with COS to give either a disulfide-bridged dimer or a dithiocarbonate complex (pictured above in the bottom row).

Overall, the work shows that anionic gallylenes with α-diimine ligands can readily activate CO₂, CS₂, and COS. It demonstrates that, with suitable ligands, main-group elements can be useful candidates for the activation of small molecules.