Silicon Surprises

Silicon chemistry is always full of surprises, and two research papers provide some new examples. Coffinite, USiO₄, is an abundant uranium mineral, but chemists have had a hard time reproducing it in the lab. A team led by Adel Mesbah of the Marcoule Institute for Separative Chemistry, in France, thinks it has figured out why. The researchers systematically studied the role of pH, temperature, heating time, and U:Si mole ratio in coffinite synthesis and found that pH is critical—a pH of 10 to 12 is necessary to form uranium silicate colloids that lead to coffinite (Inorg. Chem. 2015, DOI: 10.1021/ic502808n). The finding may prove useful in understanding the behavior of spent nuclear fuel placed in geological repositories. Meanwhile, Matthias Driess and coworkers at the Technical University of Berlin have isolated monomeric silicon disulfide, SiS₂, for the first time. The compound is an analog of CS₂, which is a common reagent and solvent. But the high polarity of the Si–S bond renders SiS₂ so reactive that under normal conditions it adopts a polymeric structure. The researchers found they could form monomeric SiS₂ by first trapping a naked, zero-valent silicon atom in the chelating arms of a bulky bis-N-heterocyclic carbene and then adding elemental sulfur (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201504489). The work could lead to further reactions involving silicon sulfide groups.