Web Date: September 9, 2013
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, which includes a hexagermane compound—the longest linear molecular germane characterized to date. Roewe and Weinert presented their work in talks this week in the Inorganic Chemistry Division 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). “I’m pretty certain we can get to Ge7 and Ge8 compounds using similar methods,” Weinert told C&EN.
“This is an important accomplishment,” commented 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. Being able to prepare and study the germanium oligomers “is not as easy.”
Although the germanes are structurally analogous to saturated alkanes, they act electronically more like alkenes, Weinert noted. For example, they exhibit σ-electron delocalization, leading to potentially useful optical and electronic properties.
The Ge6 compound is the first germane long enough to exhibit luminescence, Weinert said. But more dramatic, he noted, is Ge6’s dichroism under different orientations of polarized light. When shining polarized light on the colorless material in one direction, the crystals appear off-white. But when shining polarized light from a different direction, the crystals exhibit a striking blue color. “We haven’t seen anything quite this graphic in the germanium compounds we have studied before,” Weinert said.
Working with crystallographer Arnold L. Rheingold of the University of California, San Diego, Weinert and Roewe determined that the four central germanium atoms are aligned in the same plane, whereas the two terminal germanium atoms are displaced out of the plane. When the molecules stack together to form crystals, this structural feature imparts a type of chirality in the molecules responsible for the color shift, Weinert suggested. This property could allow thin films of the materials to be used as optical filters, he said.
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