Germanes Achieve A Growth Spurt

The periodic nature of the elements foretells that silicon, germanium, and tin in group 14 should behave like carbon positioned above them and form linear chain molecules. Chemists have had success in making polymeric materials from silicon, germanium, and tin and oligomers of silicon and tin as short-chain models for the polymers. However, they have had limited luck in developing a suitable high-yield synthesis of germanium oligomers.

The promise of new optical and electronic properties of the germanium compounds has prompted one research team to keep trying. Kimberly D. Roewe and Charles S. Weinert of Oklahoma State University and coworkers have now reported a synthetic method for making linear germanes, including a hexagermane compound—the longest linear molecular germane characterized to date. Roewe and Weinert presented their work in talks last week in the Division of Inorganic Chemistry at the American Chemical Society national meeting in Indianapolis.

The researchers started by ring-opening a cyclic phenyl-substituted tetragermane they had synthesized previously and adding hydrogen to the ends of the Ge₄ chain. They subsequently treated the tetragermane with an isopropylgermanium amide, which enabled them to attach a germanium unit to each end of the chain to form the Ge₆ molecule (Chem. Commun. 2013, DOI: 10.1039/c3cc45450a).

“This is an important accomplishment,” said organometallic chemist Lawrence R. Sita of the University of Maryland, College Park. Germanium lies at an interesting part of the periodic table, Sita said. It is not quite a formal nonmetal like silicon or a metal like tin. “Chemists a generation ago first explored the related silicon and tin chemistry but left off without nailing down the germanium analogs,” Sita added.

The Ge₆ compound is the first germane long enough to exhibit luminescence, Weinert said. But more dramatic, he noted, is Ge₆’s dichroism. When polarized light is shined on the colorless material in one direction, the crystals appear off-white. But when polarized light is shined from a different direction, the crystals exhibit a striking blue color. The way the molecules stack together imparts a type of chirality responsible for the color shift, Weinert suggested. This property could allow the material to be used as an optical filter, he said.