Spectroscopy phosphasilenes

Other indications for the generation of phosphasilenes involved compounds 520 and 6.21 Both derivatives were generated by thermally induced elimination of LiF/THF from corresponding lithium(fluorosilyl)phospha-nylides. Whereas 5 was directly observed by means of 3,P-NMR spectroscopy, surprisingly, the sterically more hindered phosphasilene 6 was not detected due to its lower thermostability. Therefore, evidence for the existence of 6 was achieved solely by trapping experiments. Due to the more crowded substitution in 5 compared with 2, the dimerization process to give 7 is relatively slow (Eq. 1) but is complete within a few hours. 

The first moderately stable phosphasilene derivatives 12 were synthesized by Bickelhaupt et al. in 1984 (Eq. 2).l0a They possess limited stability (up to 60°C), and were characterized by NMR spectroscopy (see Section IV.C) and chemical reactions (see Section IV.E). 

In 1991 we reported on the synthesis of P-silyl and -phosphanyl substituted silylidenephosphanes (phosphasilenes) 2, i.e., compounds of type IIA and III, which are accessible by thermally induced elimination of LiF from corresponding / -lithium-(fluorosilyl)phosphanides IV [5, 6] and possess a remarkable thermal stability (up to 110°C). In 1993 the first crystalline phosphasilene 5 [7], a compound of type IV, had been prepared and its structure was established by X-ray diffraction [8], whereas the Si=P compounds of type V were merely characterized by means of NMR spectroscopy [9]. Compound 5 possesses a relatively long Si=P bond length and a non-planar geometry around the A, , a -Si atom, which can be explained in terms of steric hindrance or a second-order Jahn-Teller distortion [10, 11]. In comparison, the Si=P bond in 6 [12] is significantly shorter and the Si atom is trigonal planar coordinated. 

A transient phosphasilene 827, which was generated from 1,2-phosphasiletane 826, was first reported in 1979 (equation 286) . Just recently, this cycloreversion reaction was used to produce the phosphasilene in the gas phase and to prove its existence by high resolution mass spectrometry and photoelectron spectroscopy. Higher tempCTatures are needed for the decomposition of the diphosphadisiletane 828 to generate 829. The first ionization potentials are listed (equation 287). 

A sealed NMR tube was prepared by adding dry, degassed toluene-da (0.5 mL) to bis(trimethylsilyl)phosphino silylene (70 mg, 0.16 mmol) and stored at 100°C for 5 days in the dark. The initial yellow solution turned gradually pale yellow. The quantitative formation of phosphasilene was confirmed by NMR spectroscopy. The solvent was removed, and the remaining solid was crystallized from hexane at —30°C (61 mg, 0.14 mmol, 87%). Mp 230-23rC. 

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