Phosphorus-nitrogen bonds double

Before 1964 no stable compound with a localized or delocalized carbon-phosphorus double bond was known. Indeed, it was generally assumed that the atomic radius of phosphorus, being larger than that of carbon or nitrogen, would not provide sufficient 2pn—3pir overlap for such a ir system to be stable Our first communication, written jointly with Peter Hoffmann which described the synthesis of a stable phosphamethin-cyanine 1 with a delocalized P—C double bond was therefore received with skepticism However, after Allmann confirmed the structure by X-ray analysis the existence of a new type of phosphorus bond in a cationic delocalized tt system was unambigously established... 

In 1981, the first stable compound containing a phosphorus-phosphorus double bond was prepared by Yoshifuji. The ensuing decades have witnessed a very rapid growth of almost all branches of phosphorus chemistry with the realisation that phosphorus is capable of forming multiple bonds to many elanents other than carbon and nitrogen. 

Poly(phosphazenes) are similar, partly inorganic polymers in that they consist of inorganic backbone, in this case of nitrogen and phosphorus atoms. They are separated formally by alternating single and double bonds and carry organic groups on the phosphorus atoms (10.3). 

The presence of heteroatoms usually provides a convenient feature for improving selectivity by employing selective detection mechanisms. GC may then use flame photometric detection (FPD) for S and P atoms and to a certain extent for N, Se, Si etc. thermoselective detection (TSD) and nitrogen-phosphorus detection (NPD) for N and P atoms electron capture detection (ECD) for halogen atoms (E, Cl, Br, and 1) and for systems with conjugated double bonds and electron-drawing groups or atomic emission detection (AED) for many heteroatoms. 

The lack of reactivity of (45) and CEP-pyrrole towards alcohols is attributed, at least partly, to the double bond character of the phosphorus-nitrogen bond as evidenced by the crystallographically determined abnormally short P-N bond lengths. 

Acyclic phosphazenes (phosphazo derivatives, phosphine imines, phosphoranimines) continue to attract interest. A review of the three coordinate materials, RN=PR =X has appeared. " Several molecular orbital calculations have been reported. An ab initio treatment of the PN energy surface suggests that this species is best regarded as having a dative phosphorus-nitrogen double bond rather than a triple bond and the phosphonitrene, once formed,... 

In all the above-mentioned examples, the greater capacity of phosphorus is explicable in terms of the double-bond character of the (P N) bond. In order to exclude such a possibility of n back-bonding, Miller et aL (68) used donor molecules in which the P and N atoms were separated by a CH2 group. Their n.m.r. results suggest that, even so, the coordination of BH3 occurs on the phosphorus site and not on the nitrogen site. 

The degradation of a-aminocarboxylic acids has proven to be a useful method for the preparation of iminium salts. For example, treatment of N- methylpipecolic acid (97) with phosphorus oxychloride gave 94% of the iminium salt (98) (Scheme 7) (76JA7448). The synthetic utility of this approach is illustrated by the synthesis of tetrahydroberberine (Scheme 8) (78JOC2115). The advantage of this method for the preparation of iminium salts is that the position of the carbon-nitrogen double bond can be controlled. 

Note that nitrates have three oxygen atoms surrounding each nitrogen whereas phosphates have four oxygens about each phosphorus. This is probably not a radius ratio effect but is instead connected with the inability of phosphorus to form double bonds of the usual type. When nitrogen, a first-row element, forms four bonds, one or two may be double, but when phosphorus forms four bonds, all must be single. A number of workers feel that there is some overlap between the 3d orbitals of phosphorus and the 2p orbitals of oxygen in phosphates, and that this leads to double-bond character" of a sort, but this question is still open. 

---