Iminophosphenium cations

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

While these reports are mainly concerned with an evaluation of the peculiar structure of the iminophosphenium cation, this species also reveals cycloaddition properties. The cation shows a [2-t-l] cycloaddition behaviour towards alkynes [50]. With iminophosphanes and alkylazides the cations react with the formation of four- and five-membered heterocyclic ring systems [51], as shown for example in Scheme 9. 

The iminophosphenium cation was also of interest for more physical investigations. It was noted to be a stable entity in gas phase experiments [52], the parameters were investigated in detail [53] and a systematic study of the nucleophilic addition of CH, NH and OH bonds was performed [54] with a concomitant interpretation of the chemical shifts (at the dicoordinate phosphorus centres). The latter authors also confirmed the loose interaction of a triphenylphosphine with the iminophosphenium cation (PP = 2.625 A). 

Reactions of the iminophosphenium cation tosylate with an equimolar amount of quinuclidine or 2,2 -dipyridyl proceed almost instantaneously and quantitatively to a bis-coordinated adduct 19 [56] (Scheme 11). 

A phosphine-donor stabilized adduct 29 of the methylenephosphenium ion, 7, was synthesized [70] in a way similar to the iminophosphenium cation 6 from the corresponding chloroderivative 28 and PPh3 with addition of AICI3. Interestingly the base-free methylenephosphenium cation has recently been observed in the gas phase [71] (Scheme 16). 

Iminophosphenium cations, [ P=N-Mes ] +, can be generated from ClP=NMes with anhydrous AICI3 or... 

Some cyclic four- and five-membered inorganic rings may contain two-coordinate phosphorus. Their synthesis is based on cycloaddition reactions of amino-iminophosphines or alkyl azides to iminophosphenium cations (Scheme 44). ... 

An unusual iminophosphenium cation is produced in a Staudinger-type reaction with phenyl azide [11],... 

Photolysis of tris(dimethylamino)azidophosphonium hexafluorophosphate (14) leads, via release of nitrogen, to the iminophosphenium salt 15 (synthesis of the first stable iminophosphenium salt36) (equation 8)37. The photoreaction of cation 14 shows a high dependence on the anion. The hexafluorophosphate achieves the results shown in equation 8, but the bromide yields the iminophosphorane 16. When the anion is a good... 

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