Phosphorus, hexacoordinated

Chemical assemblies around hexacoordinated phosphorus atoms can be anionic, neutral or cationic depending upon the nature of the hgands. This is described in the following paragraphs. 

Akiba and coworkers reported a new type of anionic hexacoordinated phosphorus derivative exemplified by compound 29 bearing a stable three-membered dioxaphosphirane ring [48]. Phosphoranide 34, generated from P-H phosphorane 35 with KH in the presence of 18-crown-6, was exposed to dioxygen to give crystalline 29 (50%). Phosphate 29 was found to be quite stable as a solid to ordinary room light as well as to the air at ambient temperature for several months (Scheme 5). 

As mentioned, stabilization of neutral hexacoordinated phosphorus via nitrogen donation is possible and this topic has been widely studied in the past few years. As P-N bonds are weaker and longer than those of P-C and P-0, chemists have essentially relied on chelation to enforce their formation. Most structures involve five- and six-membered chelating rings and the compounds that have been reported are described in Schemes 7,8, and 9 and Figs. 7 and 8. 

Regitz has shown that the reaction of azaphosphole 38 with two equivalents of DEAD furnishes the zwitterionic 2 1 adduct 39 (Scheme 8) [52]. The extreme low frequency shift of the P NMR signal by more than 220 ppm in comparison to that of the azaphosphole confirms the formation of a betaine possessing a hexacoordinated phosphorus atom. 

Fig. 8 Some tricyclic neutral hexacoordinated phosphorus derivatives with P-N donations...

Since 1996, Holmes and coworkers have investigated further the formation of hexacoordinated phosphorus derivatives by virtue of sulfur donation [59-64]. 

Fig. 9 Tricyclic neutral hexacoordinated phosphorus derivatives (48a-48n) with P-S donation...

If the sulfur atom is oxidized to a sulfonyl moiety, then oxygen donation occurs to the phosphorus as observed in compounds 49a-49h (Fig. 10) [61, 64-66]. In most examples, the existence of an equilibrium between penta- and hexacoordinated phosphorus was demonstrated in solution. Introduction of electron withdrawing substituents on the ligands around the phosphorus increase its Lewis acidity and consequently enhance the proportion of P( VI) over P(V). 

Hexacoordinated phosphorus compounds derived from phthalocyanines have been similarly prepared. Hanack investigated the reaction of metal-free phthalocyanines with POBr3 or POCI3 in presence of pyridine [98]. The corresponding dihydroxyphosphorus phthalocyanine hydroxides 60 were characterized. 

Many types of phosphorus-phosphorus bonds are known, but it is rare to find such bonds in hexacoordinated phosphorus compounds (with the exception of 57). Cavell reported in 1998 the reaction of PCI5 with phenylbis(o-(trimethyl-siloxy)phenyl)phosphane, yielding the corresponding bischelate 61 in decent yield (52%) [99]. The octahedral nature of the central phosphorus atom was unambiguously determined by X-ray structural analysis. Two short axial bonds (2.202 A) lie perpendicular to the pseudo-octahedral equatorial plane. 

Finally, Lacour and coworkers reported the synthesis of hexacoordinated phosphorus cation 62 [100]. Tropolone, BINOL and PCI5 react in CH2Cl2at reflux to generate in one step a novel hexacoordinated phosphorus cation. 

Chirality is an important part of today s chemistry and, in this respect, the pseudo-octahedral geometry of hexacoordinated phosphorus derivatives is interesting as it suffices to coordinate to the central atom three identical sym-... 

Much effort has also been directed toward mimicking electron transfer on natural photosynthetic systems. Recently, the group of Harada has been able to prepare monoclonal antibodies against metallo porphyrins and show that the biological edifice can control photoinduced electron transfer from the porphyrin to organic acceptor molecules in solution. As it was important to design a biomolecule able to accommodate not only the metalloporphyrin unit but also organic substrates, Harada recently used a hexacoordinated phosphorus... 

The strong Bronstedt acid nature of some hexacoordinated phosphorus derivatives, [7",H ] (Et20)4 in particular, was recently used within the context of an industrial application [36]. The conjugated acid of tris(oxalato)phosphate anion 7 was found to effectively catalyze the ring-forming reaction of trimethyl-hydroquinone 63 with isophytol 64 to give (all rac)-a-tocopherol 65 (ethylene-carbonate/heptane 1 1,100 °C, 90%, Scheme 19). This process is particularly... 

Constant S, Lacour J (2005) New Trends in Hexacoordinated Phosphorus Chemistry. 250 1-41... 

The high diversity of hexacoordinated phosphorus compounds and their use as versatile reagents are fully illustrated in Chap. 1 by S. Constant and J. Lacour. A large number of new structures are reported with applications in different fields such as classical organic chemistry, bioorganic chemistry, electrochemistry and photochemistry, thus affording a general overview of the new trends in hexacoordinated phosphorus chemistry. 

Sheldrick, W. S. Stereochemistry of Penta- and Hexacoordinate Phosphorus Derivatives 73, 1-48 (1978). 

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