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Research

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© Julia Bader

Research

Don't just try it - do it!

Bert Hoge

The influence of electron-withdrawing substituents like perfluoroorganyl groups is essential for the stabilization of molecules with extraordinary bonding situations. Trifluoromethyl substituted compounds exhibit a very high Lewis acidity which enables the synthesis of extraordinary molecules such as phosphinous acids or stannylenes. However, these substances tend to decompose under elimination of difluorocarbene. The electronically very similar pentafluoroethyl group proves to be considerably more stable and the corresponding element compounds are available in good yields and large scales. This results in a manifold and unique chemistry of pentafluoroethyl element compounds.

Phosphinous Acids

31P   The stabilization of phosphinous acids

Diorganylphosphinous acids, R2P-O-H, are generally instable with respect to their tautomeric counterparts R2P(O)H. Phosphinous acids can be trapped by coordination to suitable transition metal complexes or stabilized by the introduction of strong electron-withdrawing groups.

Gleichgewicht Phosphinige Säure - Phosphanoxid

Bis(trifluoromethyl)phosphinous acid, (CF3)2POH, represents up to date the only literature-known example of a phosphinous acid which is stable towards tautomerization. Its transition-metal complexes of are in principle of great interest in homogeneous catalysis. However, they should not be used on a bigger scale, as bis(trifluoromethyl)phosphinous acid, (CF3)2POH, as well as tris(trifluoromethyl)phosphane, P(CF3)3, decompose explosively on contact with air.

Comparison between tris(trifluoromethyl)phosphane and tris(pentafluoroethyl)phosphane with oxygen

P(CF3)3 (left) and P(C2F5)3 (right)

Compared to P(CF3)3, the homologous compound P(C2F5)3 proves to be relatively inert towards aerial oxygen. Therefore (C2F5)2POH as well should be safer to handle than (CF3)2POH.
In cooperation with the Merck company, Darmstadt, we succeeded in the first synthesis of (C2F5)2POH, the second example of a phosphinous acid. This compound as well as its catalytically active transition-metal complexes are subject of a joint world patent with the Merck company.

119Sn   Electronically stabilized stannylenes

The majority of the monomeric stannylenes SnR2 is stabilized kinetically by sterically demanding substituents. Stannylenes with sterically less demanding groups tend to oligomerize. An electronic stabilization of SnR2 derivatives is little-known. Observations suggest a possible stabilization of stannylenes by electron-withdrawing groups, but only one synthesis of such a stannylene is literature-known: Sn(CF3)2 · n PMe3 could be identified spectroscopically, however, it decomposes at room temperature.
(C2F5)2SnH2 which is stable at room temperature eliminates reductively hydrogen on addition of a donor such as THF, forming selectively bis(pentafluoroethyl)stannylene, Sn(C2F5)2.

Reaktion zum Stannylen

In solution, the stannylene exists as a monomer and could be investigated by X-ray spectroscopy as its dimethylaminopyridine adduct Sn(C2F5)2 · 2 DMAP. The synthesis of bis(pentafluoroethyl)stannylene opens up a versatile coordination chemistry.

29Si   Silanes as strong Lewis acids

While (CF3)3SiNEt2 cannot be isolated as it eliminates difluorocarbene during the work-up, neat (C2F5)3SiNEt2 is available on a large scale and proves to be stable even at elevated temperatures. It turns out to be an excellent starting material for the synthesis of numerous tris(pentafluoroethyl)silicon derivatives. The fluorosilane (C2F5)3SiF features an extremely high Lewis acidity which even exceeds that of AsF5. The corresponding fluorosilicates [(C2F5)3SiF2]- and [(C2F5)3SiF3]2- are part of joint patents with Merck and BASF.

In addition to these elements, our attention is concentrated on pentafluoroethyl substituted germanium, lead and bismuth compounds.

Methods of Characterization

The compounds synthesized within our projects are characterized by multinuclear NMR spectroscopy and by the analysis of the mostly higher order spectra using the program gNMR. The infrared and Raman spectra are interpreted with respect to quantum chemical methods as they are implemented in the program package Gaussian 09.
The reliability of calculated molecular structures of the synthesized compounds has been proven by the excellent agreement with the results of single crystal X-ray structure analyses and by electron diffraction in the gas phase (GED).
Investigations of the temperature dependent relative concentrations of rotational isomers in the gas phase and solvent dependent tautomeric equilibria allow the experimental determination of the relative energies of the involved isomers. Because the determined relative energies are in excellent accordance with calculated data obtained by DFT methods, we are able to use quantum chemical calculations to evaluate for example which substituents R are appropriate for the stabilization of a phosphinous acid before synthesizing the corresponding compounds.

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