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Forschungsarbeit


Reactivity of P4 butterfly

von Rebecca Grünbauer, Manfred Scheer

The synthesis of phosphorus containing compounds (e.g. fertilizers) mainly utilizes white phosphorous, the most reactive modification of phosphorus, as a starting material. However, established synthetic routes require harsh reaction conditions, hazardous reagents and produce stoichiometric amounts of waste. [1] Therefore, the search for alternative and mild reaction pathways, called P4 activation, is of current interest.

The subject of P4 activation incorporates the activation and subsequent reductive P-P bond cleavage of the P4 tetrahedron found in white phosphorus, yielding interesting Pn (n?4) units stabilized in the coordination sphere of transition metal complexes or main group elements. [2] The tetraphospha-bicyclo[1.1.0]butane structural motif, which is obtained after the opening of one P-P bond of the P4 tetrahedron, represents the first step in the activation of P4. Due to its geometry, it is often called P4 butterfly motif.

We are mainly interested in the coordination chemistry originating from the P4 butterfly moiety towards different Lewis acids, since each P atom possesses a lone pair available for further coordination. In the following, two examples originating from this coordination chemistry will be discussed to elucidate the potential of the P4 butterfly unit as a starting material towards the synthesis of promising new compounds (Figure 1).

Supported by theoretical calculations, first results showed, that the P4 butterfly unit is able to coordinate in a chelating mode via the two wing tip P atoms displaying an exceedingly small bite angle (ca. 65 - 70°) compared to common bisphosphine ligands, which are an established class of co-ligands for various homogenous catalysts. [3] Consequently, the P4 butterfly ligand might display an interesting all-phosphorus alternative to i.e. bis(diphenylphosphino)methane (dppm), the bisphosphine ligand with the smallest bite angle known so far (72(2)°).

In contrast to the coordination of the P4 butterfly unit, rearrangement processes can be observed in some reactions as well. One example is the reaction of [{Cp*Cr(CO)3}2(u,n1:1-P4)] with [Cr(CO)4(nbd)] (nbd = norbornadiene) resulting in a complex with a kite-like distorted four-membered ring structure, which includes an uncommon 3e--donating P atom in a trigonal planar coordination geometry (Figure 1). Moreover, due to decarbonylation a formal P-Cr double bond is formed opening the way to various subsequent reactions.

In conclusion, the P4 butterfly unit is not only a promising starting material for the synthesis of complexes containing novel Pn ligands, but its reactivity towards compounds with variable Lewis acidity gives insight in the electronic properties of the central P4 scaffold. Hence, the observed reaction behaviour might contribute to our further understanding of the processes occurring during P4 activation improving the synthesis of essential phosphorus containing compounds.

References

1 (a) A. F. Holleman, N. Wiberg, E. Wiberg, Lehrbuch für Anorganische Chemie, 102. Edition, de Gryter, 2007, 746 - 749.

(b) D.E.C. Corbridge, Phosphorus: Chemistry, biochemistry & technology, 6. Edition, CRC Press, 2013

2 (a) B. M. Cossairt, N. A. Piro, C. C. Cummins, Chem. Rev., 110, 4164-4177, (2010).

(b) M. Caporali, L. Gonsalvi, A. Rossin, M. Peruzzini, Chem. Rev., 110, 4178-4235, (2010).

(c) M. Scheer, G. Balazs, A. Seitz, Chem. Rev., 110, 4236-4256, (2010).

(d) N. A. Giffin, J. D. Masuda, Coord. Chem. Rev., 255, 1342-1359, (2011).

3 P. W. N. M. van Leeuwen, P. C. J. Kamer, J. N. H. Reek, P. Dierkes, Chem. Rev. 100, 2741- 2770, (2000).

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