Rational synthesis and properties of new long-chain linear oligomeric germanium compounds

Abstract

Germanium compounds with single germanium-germanium bonds contain interesting optical and electronic properties such as absorption within the UV/visible range and conductivity, which can also be seen in other heavier Group 14 oligomers. In comparison to the tin and silicon analogues, the reported methods for synthesizing the germanium compounds are minimal. The longest linear oligogermane completely characterized, including its structure, is the perphenylated pentagermane, Ge5Ph12. Due to their inherent properties, it is of interest to synthesize longer, linear oligomeric germanium compounds. It is proposed that if a long enough chain of germanium atoms is obtained, properties characteristic of polygermane species may also be achieved. Protection/deprotection strategies, using a masked Ge-H bond, were initially attempted, however only a linear pentagermane could be obtain. All products were also liquid, preventing complete characterization using X-ray crystallography.

After synthetic efforts were shifted away from the protection/deprotection strategies, a successful strategy was achieved starting with a cyclic germanium species. The hexagermane Pri3Ge(GePh2)4GePri3 was synthesized in three steps from cyclic (GePh2)4 within reasonable overall yield. The hexagermane exhibits a ?max of 310 nm in its UV/visible spectrum, which is more red-shifted than any other discrete oligogermanes synthesized with six or less consecutive germanium atoms, and the DPV contained 5 consecutive, irreversible oxidation waves, the first of which was observed at 1080 mV. The hexagermane is the first discrete organogermanium compound to exhibit fluorescence, with a broad emission at 370 nm when excited at 312 nm. Crystals of the hexagermane were dichroic. Though colorless under ambient light, the hexagermane crystals appeared blue under "right" plane polarized light and pale yellow under "left" polarized light.

Since 1986 when the first pentagermane was synthesized, progress has not been successful at synthesizing a longer oligogermane. A linear hexagermane has finally been synthesized and completely characterized indicating that discrete oligomeric germanium compounds have the ability to mirror properties of polymeric germanium species given enough consecutive germanium atoms.