Phosphenium ions

Synthesis and reaction chemistry of stable two-coordinate phosphorus cations (phosphenium ions). A. H. Cowley and R. A. Kemp, Chem. Rev., 1985,85, 367 (100). 

Useful reviews on areas of relevance for this chapter are one by Cowley and Kemp on the synthesis and reactions of phosphenium ions,... 

Phosphenium ion synthesis and reactions have been reviewed. A... 

Keywords 1,3,2-diazaphospholes 1,3,2-diazaphospholenes 1,3,2-diazaphosphinines 2,3-dihydro-l,3,2-diazaphosplinines diketiminto-phosphenium ions N-heterocyclic phosphines N-heterocyclc phosphenium ions 1,2,3-diazaphospholenium ions... 

Reactions of P-halogen-NHPs with Lewis acids leading to halide abstraction, or metathetic replacement of the halide by a nucleofugic, noncoordinating anion form by far the most important routes to access phosphenium ions in general [51, 52] and also 1,3,2-diazaphospholenium cations in particular. As Lewis acid assisted P-X... 

Quantitative assessment of the electrophilic character of various types of phosphenium ions has been attempted using computational studies on hydride and halide exchange reactions, and the results attribute to 1,3,2-diazaphospholenium ions a lower electrophilicity (and thus higher stability) than other types of phosphenium ions [20, 66], The gain in stability due to aromatic -delocalization is predicted to be somewhat larger than inductive stabilization resulting from exhaustive A-alkylation of the parent diaminophosphenium ion, [P(NH2)2]+. 

Apart from characterization of the individual types of six-membered heterocycles by routine spectroscopic methods, several cationic cyclic diketiminato-phosphenium ions and 1,8-diamidonaphthalene-derived P-chlorophosphines and phosphenium ions, respectively, were characterized by single-crystal X-ray diffraction studies. The P-halogen- and P-hydroxy-substituted cyclic diketiminato-phosphenium ions 28... 

Due to their isoelectronic relationship with Arduengo carbenes , stabilization of cyclic phosphenium ions by Ti-electron delocalization has been a matter of debate. 

Phosphenium Ions. In contrast to widely studied phosphonium ions (R4P+), the chemistry of the dicoordinate phosphenium ions was little known,719 but there have recently been significant developments. It has been recognized that phosphenium ions can only be generated if one of the substituents is a dialkylamino group723 [Eq. (4.176)]. Obviously, these ions are stabilized by the strong electron-donating amino function directly attached to phosphorus. On the... 

Structurally related cations are cyclic phosphenium ions 287,727,72S288,728-731 289,732-734 and 290735 with an NPN moiety. No interionic contacts were observed in the crystals of hexafluorophosphate, tetrafluoroborate, and triflate salts of cations 288. The planar ring structure and the ring bond distances being intermediate between single and double bonds lent evidence for n electron delocalization. This, however, is only a weak delocalization and the cations cannot be described as genuine aromatic... 

Veith et al.736 have prepared the unique four-membered cyclic phosphenium ion 291. X-ray crystal structure analysis of the tetrachloroaluminate salt clearly indicates the intramolecular backbonding from ring nitrogen atoms (average N—P bond distance = 1.633 A). 

Phosphenium ions can also be stabilized by intramolecular coordination to a Lewis base. In ion 294, which was prepared by hydride abstraction739 [Eq. (4.177)], intramolecular bis-coordination exists. The31P NMR spectrum of the corresponding P—H derivative740 exhibits one doublet at 831P 37.6 (JP N = 243 Hz). The X-ray structure, which shows no interaction between the cation and the hexafluorophosphate anion, clearly demonstrates an ionic structure. Both arms are coordinated to the phosphorus center. The N—P distances are 2.082 and 2.068 A, which are longer than the N—P o-bond (1.769 A) but significantly shorter than the sum of the two van der... 

Hz). Several pieces of spectroscopic evidence lead to the conclusion the Me5C5 ring of V is not bonded in the monohapto manner. First, the 1h and 13c spectra of V indicate that the ring and Me carbons of the Me5C5 moiety are equivalent moreover, the equivalence of the methyl groups persists to -100°C and -80°C in lH and 13c NMR experiments, respectively. Second, the 31p chemical shift of V (111.0 ppm) is 33.8 ppm upfield (i.e. shielded) compared to that of the phosphorus(III) chloride precursor, 2Z 1° aH cases reported to date, phosphenium ion for-... 

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