Phosphazanes

Phosphazynes are rare. The bond length of the diatomic molecule P=N is 149 pm. The first stable compound containing the P=N group is [feN-R]+[A1C14] , [R = C6H2(2,4,6- Bu3)], in which the length of the feN bond is 147.5 pm. 

Some examples of phosphazanes are described below according to the types of P-N bonds. 

In this bonding type, the P atom uses its sp2 hybrid orbitals, one of which accommodates a lone pair. A bent molecular structure of this type 

The bond length of P-N is 161.2 pm, shorter than the standard P-N singlebond distance of 177 pm observed in H3N-PO3, which does not have a lone pair on theN atom. The bond angle N-P-N is 115°, smaller than idealized 120°, due to the repulsion of the lone pairs. 

The configuration of most R2P-NR2 compounds is represented by F2P-NMe2, which features a short P-N distance (162.8 pm) and trigonal planar arrangement (sp2 hybridization) at the N atom. The plane defined by the C2N unit bisects the F-P-F angle. This configuration minimizes steric repulsion between the substituents on P and N and also orients the lone pairs on the P and N atoms at a dihedral angle of about 90°, as shown in Fig. 15.5.1(a). 

Miscellaneous Compounds. The rate of the alkaline decomposition of [PPh4]+Cl in aqueous acetone increases rapidly with the acetone content of the solvent, a result similar to that observed previously with aqueous dioxan. Qualitatively similar results are found for [PhsPCHaPhl+Cl in aqueous THF. A virtually identical rate 

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In an excellent review by Roesky et al. in 1994 [70a] a vast number of examples for coordination complexes of cyclic phosphazanes and phosphazenes and other related systems have already been compiled. In the following section, an attempt is made to cover the latest features of group 13 systems along with some earlier examples with phosphorus-nitrogen based systems other than pyridyl phosphanes. 

The review by Roesky et al. [70] covered some of the earlier work related to the group 13 complexes of acyclic phosphazanes and phosphazenes. 

Tervalent organophosphorus compounds containing one single P-N bond with the valency of each atom saturated by protons or carbons (but no other heteroatoms) have been known since their discovery by MichaeUs more than one century ago [ 1 ] and named indistinctly as aminophosphanes, phosphanamines, phosphazanes, or phosphinous amides. This last chemical nomenclature is the one used by the Chemical Abstracts Service (CAS) for indexing these compounds and is also the one that best delimits the scope of this review those species derived from the parent H2P-NH2 (phosphinous amide in CAS nomenclature) by partial or total substitution of protons by hydrocarbon radicals (Table 1). 

Et), phosphazanes are obtained which contain P3N2 or Pi N3 skeletal units stabilized around the arene rings. 

Compounds XIV - XVII have potential for use in subsequent phosphazane ollgomer/polymer syntheses because of their available functionality. In fact, treatment of XVI with two equivalents of Ph2PCl in the presence of Et3N yields the P(III)-P(V)-P(III) triphosphazane XVIII quantitatively. Compounds XV and XVII are especially interesting because they are A-B type 1,4- stabilized reactive monomers (prepolymers) (14). Because A-B monomers contain both elements of functionality necessary for condensation, precise control of the reactant ratio can be maintained and products with the highest possible molecular weights can be expected (14). 

Reactions involving alkoxy- or aryloxyphosphines with amines might also be expected to yield phosphazane oligomers/polymers. However, PhP(0Ph)2 with 1,2-(NH2)2c6Hit produced only the Arbuzov type rearrangement product CgHi, (NH) 2P(0)Ph. 

Bis-phosphaallyl complexes of zinc have been prepared and characterized from cyclic amino-phosphazanes and aminoiminophosphanes. The structure of the bis(l,3-diaza-2-phosphaallyl)zinc complex [Zn(RNPNR )2] (R = R = C6H3Pr2i-2,6) was determined.311... 

The third chapter Recent Developments in the Chemistry of N-Heterocyclic Phosphines presents a survey on five- and six-membered phosphorus-nitrogen heterocyclic compounds whose rings combine a phosphazene or phosphazane unit... 

Previous attempts to synthesize benzo[l,3,2]diazaphosphole (59) in an analogous manner from benzene- 1,2-diimines by Malavaud et al. led to tetra-meric cyclo-phosphazanes, which existed in equilibrium with monomer only at... 

Verkade and co-workers have shown the usefulness of their phosphazanes in various stoichiometric as well as catalytic reactions <1999PS(144)101>. Compound 290 was used to promote the cyanohydration of benzaldehyde with trimethylsilyl cyanide (TMSCN). The cyanohydrin was isolated in 95% yield, but no enantioselectivity was noticed <2002JOM(646)161>. Compounds 291 and 292 were attached to dendrimers and shown to be effective in the catalysis of Michael reactions, nitroaldol reactions, and aryl isocyanate trimerizations <2004ASC1093>. 

The chemistry of cyclophosphazanes is dominated by four-membered rings six-membered P3N3 rings are far less common. The first examples of cyclotri-phosphazanes (MeNPX)3 (X = Cl, Br) were prepared by the cyclocondensation... 

Endocyclic P=N bonds tend to promote dimerization (Scheme 34) the phosphazane 132 is formed203 from the precursor 132a, and even if 134a is the result of the thermolysis of the phosphorane 133, as proved by the possibility of trapping it with a C=0 bond204, the product isolated is the dimer 134. 

The phosphatranes that have this bonding type are analogs of silatranes. The cage molecule shown on the right is a trigonal bipyramidal five-coordinated phosphazane with a rather long P-N bond. 

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