Phosphinines structures

Figure 2. B3LYP-6-311+G(d,p) structure of phosphinine (3), with calculated bond lengths and experimental H NMR chemical shifts.

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

The generally accepted view about the aromaticity of A -phosphinines as summarized in the recent book is that A -phosphinines are not aromatic but they clearly are highly stable substances exhibiting very little ylidic character ." In accordance with the description used for the four-membered ring phos-phetes and diphosphetes, the resonance structures 33a—c are usually considered to describe the electronic structure." ... 

V. B.7S The 1.707 A bond length (MP2/6-31G(d)) is somewhat shorter than that for the analogue. However, the calculated structure is not entirely planar the hydrogens on the carbon atoms move somewhat out from the plane. The X-ray structure of the heavily substituted l.l,3,3,5,5-hexakis(di-methylamino)-lA .3A ,5A -triphosphinine was planar. In all cases the PC bond lengths were equal and in the range of the aromatic PC bonds (1.687 A from the X-ray structure). It is worth noting that no NIGS data has been reported for y , -phosphinines as yet. 

The alkylation on the P atom of substituted phosphinines resulted in anionic /.4-phosphinines [315], The aromaticity of the ring is significantly reduced according to NMR chemical shift values. Similarly, the adduct of triphosphabenzene with ethyl-magnesium-bromide has been reported to yield an ion pair, which resulted in a nonplanar structure with nonequalized bond lengths [316],... 

In two recent works the phosphorus analogues of pyridones 109-111 (Scheme 58) have been studied computationally [317, 318], 110 is planar, while for 109 and 111 the planar structures are transition states with 11.0 and 19.8 kcal moH above the nonplanar minimum, respectively. The low barriers to inversion indicate that the planar structures have considerable aromaticity, which is shown by the NICS values exhibiting 67%, 51%, and 33% of the benzene value for 109,110, and 111, respectively [317], ELF analysis also indicates a certain aromaticity in these compounds [317], The bonding situation in 109 is similar to the cyclic phosphinocarbene isomer of phosphinine 112 [319] (Scheme 59), which was shown to be planar and aromatic. Stabilization of this carbene can be achieved by incorporating it to an anne-lated ring system 113, and also by the electron donor effect of an ylide formed with an additional o4,l5-P (114) [319],... 

Other multidentate ligands have been prepared and their structures determined. The bisphosphinine 64 crystallized with the two phosphinine planes almost orthogonal (91.59°), and the two phosphorus atoms oriented away from each other (P-P distance 5.8 A). However, even at low temperature (—95 °C) there was no evidence in either the 31P... 

The phosphinine rings (PC5R5, P2C4R4,... P6) are analogs of benzene and form sandwich structures with the transition metals. In r 6-phosphinine complexes, the phosphinine rings are all planar. 

Both C5H5P and C5H5As have planar structures with C—C and C—E bond lengths indicative of aromatic delocalization of the ir electrons. However, the scope of their aromatic organic chemistry is not nearly as extensive as that of pyridine. Phosphinine and its derivatives form both type complexes, for example, 10-XXXVI and 10-XXXVII. 

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