Hexacoordinate Phosphorus Compounds

A hexacoordinated macro-ring structure (111) has been synthesised by the 2 + 2 condensation of (109) with (110) in THF and in the presence of diethylamine. The compound was characterized by MS, elemental analysis and multinuclear ( H, P)NMR.  

The nucleophilic carbene (112) has been shown to react with pnicogen pentafluorides (113a-c) to form adducts (114ac) with octahedral geometry at P (P N = 1.898 A), As (As-N = 1.999 A) and Sb (Sb-N = 2.175 A). The structures are therefore those of internal zwitterions with an imidazolium cation and a pnictogen anion. The strong nature of the bonds to the carbene 

In conclusion, the realm of hypervalent phosphorus chemistry is becoming ever more complicated as it expands into the chemistry of As, Sb and Bi and as the coordination of phosphoranes with Lewis Acids and transition metal elements adds another facet to the burgeoning field of asymmetric synthesis. 

Chandrasekaran, R.O. Day, P. Sood, N.V. Timosheva, D.J. Sherlock and R.R. Holmes, Phosphorus, Sulfur, Silicon Relat. Elem., 2000,160, 1. 

Cavell and A. Jurgensen, J. Electron Spectroscopy and Related Phenomena, 1999,101-103, 125. 

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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. 

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. 

Table 6 Spirophosphoranides and other hexacoordinate phosphorus compounds containing a spiro phosphorus... 

Hexacoordinate phosphorus compounds are also considered as transition states in nucleophilic diplacements of phosphoranes in a manner similar to an 2 displacement in carbon chemistry (see Scheme 38) (72MI11704, 73AG(E)9l>. 

While the chemistry of pentacoordinated spirophosphoranes has been so far widely explored, cyclic hexacoordinate phosphorus compounds are much less studied because very often they are transient species and difficult to detect. However, an interesting example of a stable hexacoordinate phosphoranate bearing Martin ligands was recently described by Yamamoto. Another approach involved the participation of donor-acceptor nitrogen-phosphorus" and sulphur-phosphorus bonds in the formation of stable hexacoordinated compounds. 

Compound (17) bearing two unusual coordinate S P<-S linkages represents the first example of a hexacoordinated phosphorus compound of this type. The structure of (17) was established by X-Ray analysis and... 

Moreover, two equivalent S P bonds are quite strong [2.334(1)A] and are comparable to that in the chloro precursor (15) [2.317(1)A] but shorter than other hexacoordinate phosphorus compounds with only one S- P coordination. The presence of hydrogen bonded chloride (to pyrazole NH) ion also enhances the stability of hexacoordinated phosphate (17). 

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