Weinert Lab

Research

OLIGOGERMANES

Research in the Weinert group is primarily focused on the synthesis and characterization of oligogermanes, which are the germanium analogues of hydrocarbons that contain germanium – germanium single bonds. The compounds are of interest due to their inherent σ-delocalization, where the electrons in the highest occupied molecular orbitals of these compounds are not localized between two individual atoms but rather are typically delocalized across the entire germanium – germanium framework. This occurs when the germanium atoms are disposed in a trans-coplanar orientation that then allows the overlap of the diffuse 4sp³ orbitals to occur (Figure 1).

FIGURE 1

The construction of these molecules is not trivial and previously reported synthetic methods were replete with complications that did not allow for a detailed study of these systems. We developed the hydrogermolysis reaction in 2006 for the synthesis of these molecules, and this serves as a useful tool for the construction of a wide variety of systems (Scheme 1). Using this method we have prepared numerous new oligogermanes having a variety of chain lengths and substituent patterns, and this has included linear, branched, and cyclic systems.

SCHEME 1

The ultimate goal of this work is to prepare molecules that have useful optical, electronic, or conductive properties.  Such physical attributes have been seen in polygermanes, which are systems with long chains and a dispersion of molecular weights, or germanium nanomaterials.  We are attempting to prepare fully characterized small molecules that exhibt such properties.  To this end, we reported the hexagermane Prⁱ₃Ge(GePh₂)GePrⁱ₃ and have studied its properties in detail (Scheme 2, Figure 2).  This molecule is thermochromic and also luminescent in both solution and in the solid state, and we postulate that the luminescence is due to a conformational change in the solid state upon excitation.  There is an intense blue emission in the soid state at 440 nm at 80 K when the molecule is excited at 300 nm, and this can be visually observed as well (Figure 3).

FIGURE 2

FIGURE 3

GERMYLAMINES AS AMIDATION AGENTS

 Recently, we have explored the use of germylamines, in particular Ph₃GeNMe₂, as versatile reagents for the conversion of acid fluorides to amides.  This is an important area of research since the peptide linkage is highly prevalent in biochemistry, protein chemistry, and pharmaceutical development and new methods to generate this moiety are always of interest.  We have found that Ph₃GeNMe₂ can be used to convert an extensive array of aromatic acid fluorides having substituents in the ortho-, meta-, or para- positions as well as those having multiple substituents (Figure 4)  We have modeled the transition state for this process and determined that it proceeds via a sigma bond metathesis pathway with a very low activation energy (Figure 5).  Thus, this process is complete at room temperature within 2 hours and conversions from the acid fluoride to the amide proceed completely.  The isolated yields of the amides are also high (greater than 90 %).  Two other advantages of this synthetic method are that the acid fluorides are more stable towards moisture than their corresponding acid chlorides and the Ph₃GeF byproduct can be isolated and converted back to the Ph₃GeNMe₂ starting material, making this method cost-effective.  A list of successfully synthesized amides is shown in Figure 6.  We are continuing to investigate the scope and limitations of this process, including the amidation of larger organic acid fluorides and the use of other germylamines having different substituents on the nitrogen atom.

FIGURE 4

FIGURE 5

FIGURE 6

Somewhat surprisingly, structurally characterized germylamines are quite rare.  Prior to our beginning our investigations, only two structurally characterized germylamines have been reported, Mes₃GeNH₂ and (Bu^tCC)₃GeNEt₂.  They typical method for the syntheses of these materials is the salt metathesis reaction shown in Figure 7.  During the course of our investigations, we obtained the X-ray crystal structures of both Ph₃GeNMe₂ and also the highly sterically encumbered germylamine Ph₃GeN(SiMe₃)₂ (Figure 8).  We are currently endeavoring to prepare and structurally characterize numerous other germylamines, but their preparation has been problematic.  Consequently, we are in the process of developing new synthetic techniques to prepare these molecules.

Figure 7

Figure 8