Lithium phosphanide

Hitherto no monometalated molecular pnictide exists without solvation of the main group metal atom. Therefore, the monomeric species L (Fig. 2) can only be stabilized if the Li ion has its coordination sphere enlarged through donor solvation. More importantly, the lithium phosphanides of the type K undergo oligomerization processes to form dimer, tetramer, hexamer, or polymeric assemblies M—Q (Fig. 2), which dissociate in solution more easily than related amides (2, 11, 12). 

Considering the importance of alkali metal phosphanides it is not surprising that numerous review articles have dealt with this subject [34-36]. The solid state and solution structures vary from dimers with central M2 P2 cycles to larger rings and from chain to ladder structures as described for the lithium amides (see Sections 3.6.1 and 3.6.2). Cage compounds in the field of lithium phosphanides are unusual... 

Fig. 3.6-6. Molecular structures of the mixed lithium phosphanide-LiCI cluster 20 and the anionic cluster core in 21.

Fig. 3.6-7. Core structure of the lithium phosphanide cluster 28. Small dark circles lithium atoms, big dark circles phosphorus atoms.

Lithoxymethylidynephosphane The simple preparation of l-(l,2-dimethoxyethane-0,0 )lithoxy-alkylidenephosphanes from carboxylic esters and (l,2-dimethoxyethane-0,0 )lithium phosphanide (Eq. 8) motivated us to investigate the analogous reaction with carbonic acid esters very carfully. Whereas with the educt (dme)Li-PH2 several, difficult to separate by-products are formed, lithium bis(trimethylsilyl)phosphanide and diethyl carbonate react at 0 °C in 1,2-dimethoxyethane with elimination of two equivalents of ethoxy trimethylsilane to give the heteroatom substituted phosphaalkyne (dme)2Li-0-CsP in a nearly 80 % yield (Eq. 9) [13]. 

Another task was to synthesize a metastable 2-phospha-l,3-disilaallyl anion, which may be prepared by reduction of 37 with two mole equivalents elemental lithium and subsequent elimination of LiF. However, the reduction with Li metal, via the lithiumsilanidyl-lithium phosphanide 39 as reactive intermediate, does not fiirnish the desired allyl anion 40. Instead, the disilaphosphacyclopropanes 41, a valence isomer of 40, and 42, the protonated product, were formed but only 42 was isolated and structurally characterized by X-ray crystallography (Fig. 10) [18]. 

Covalent structures are adopted by LiP(SiMe3)2 and other lithium phosphanides and these may be dimeric, hexameric and so forth (8.46). The lithium compound can be used to obtain other compounds with P-Si linkages as indicated in Figure 9.11. Only the main reaction products are indicated by-products such as P(SiMe2)3 are also formed in some cases. Among the useful reactions is the production of isotetraphosphines. 

Oxygen and moisture has to be excluded carefully during the preparation procedures to avoid oxygen-centered cages. Then unusual metal deficient phosphanediides of lithium of the type [(Li2PR) (PR)m] with Li2 P( +m) cages are isolated. Investigations of phosphanides of the heavier alkali metals are far less common [37]. 

In contrast to the lithium amides and phosphanides, dimeric alkaline-earth metal bis(phosphanides) of the heavier group 2 metals show bicyclic structures of the... 

Surprisingly, the reaction of diethyl carbonate and 0,0 -diethyl thiocarbonate with lithium bis(trimethylsilyl)phosphanide has been found to give ethoxy trimethylsilane and the phosphaalkynes (dme)2Li-0-CsP and [(dme)3Li] [S-CsP] respectively. As known so far, lithoxymethylidynephosphane shows a great tendency to undergo [2+2] cycloaddition and reduction reactions. The NMR parameters of the [S-C=P] -anion resemble much more the values of diorganylamino-phosphaalkynes than those of its oxygen homologue. 

Through a rather complicated sequence of meanwhile fully understood reaction steps ethyl benzoate and lithium bis(trimethylsilyl)phosphanide form tris(l,2-dimethoxyethane-0,0 )-lithium 3-phenyl-l,3-bis(trimethylsilyl)-l,2-diphosphapropenide and 3,5-diphenyl-l,2-bis(tri-methylsilyl)-l,2,4-triphospholide. X-ray structure determinations on orange or green, metallically lustrous, crystals show the compoxmds to be ionic in the solid and to contain a 1,2-diphosphaallyl and a 2-phosphaallyl anion, respectively. Dark red tetrakis(tetra-... 

In contrast to phosphaalkynes, nitriles show quite a different chemical reactivity towards lithium trimethylsilylphosphanides. Whereas with benzonitrile and one equivalent of the lithium phenyltrimethylsilyl compound l-[(l,2-dimethoxyethane-0,0 )hthium-trimethyl-silylamido]benzylidenephosphane is formed, l-(l,2-dimethoxyethane-0,0 )lithium bis(trimethylsilyliminobenzoyl)phosphanide has been isolated from a similar reaction with lithium bis(trimethylsilyl)phosphanide in a molar ratio of 2 1. Solvent coordinate lithium is not bound to phosphorus, but to both the nitrogen atoms. Protonation gives the related bis(trimethylsllyliminobenzoyl)phosphane, which exists only as imino-enamine tautomer in the solid as well as in even very polar solvents. 

With respect to the mechanism just discussed, the statement of Cowley et al. [25] that merely Z-isomeric [2,2-dimethyl-l-(trimethylsiloxy)propylidene]trimethylsilylphosphane can eliminate hexamethyldisiloxane, does need further verification. From our point of view the E- and Z-isomer of the mesomeric enolate anion are readily interconverted by a rotation around the P-C bond of the keto form (Eq. 6) which is supposed to be an easily accessible transition state. At any rate, we were not able to confirm their results as the reaction of lithium bis(trimethylsilyl)phosphanide with 2,2-dimethylpropionyl chloride at -78 °C in cyclopentane solution gives exclusively the -isomeric phosphaalkene, whereas at room temperature the Z-isomer prevails. 

By analogy to Eq. 9 the new phosphaalkyne may also be obtained from 0,0-diethyl thio-carbonate and lithium bis(trimethylsilyl)phosphanide dissolved in 1,2-dimethoxyethane. Both educts react at about 0 °C to give ethoxy trimethylsilane and the finally isolated compound tris(l,2-dimethoxyethane-0,0 )-lithium 2-phosphaethynylsulphide (Eq. 12). Emphasis should be laid on its NMR parameters (5 P -121.3 S C 190.8 ppm Jcp 18.2 Hz [Dg]-THF solution) in that they correspond much more to the values of diisopropylaminophosphaalkyne (8 P -99.6 S C 152.2 ppm Jcp 14.7 Hz [31]) than to those of bis(l,2-dimethoxyethane-0,0 )lithoxymethylidynephosphane (5 P -384.2 166.6 ppm ... 

The first step - a 1,2-diphosphaallyl anion [49] In contrast to the syntheses of dimeric l-(l,2-dimethoxyethane-0,0 )lithoxyaikylidenephosphanes from ethyl formate or 2,4,6-trimethyl-benzoyl chloride and (l,2-dimethoxyethane-0,0 )lithimn phosphanide already mentioned (Eq. 13), the reaction of ethyl benzoate with lithium bis(trimethylsilyl)phosphanide at about 0 °C in... 

In order to understand the formation of compounds now being discussed one has to realize that tris(trimethylsilyl)phosphane originating in the desilylation of [l-(trimethylsiloxy)benzylidene]-trimethylsilylphosphane (Scheme 1) reacts slowly with lithium ethanolate to give lithium bis(trimethylsilyl)phosphanide again (Eq. 13). This compound must then be considered a continuous source for the phosphaalkyne H5Q-C P, provided that a sufficient amoimt of ethyl benzoate is present in solution. 

A 2-phosphaallyl anion as part of a five membered heterocycle [27, 35] When in the already discussed reaction between ethyl benzoate and lithium bis(trimethylsilyl)phosphanide the molar ratio of the educts is changed finally from 1 3 to 1 1, dark green, metallically lustrous, air sensitive crystals of... 

Surprisingly, the reaction of phosphaalkynes with lithium bis(trimethylsilyl)phosphanide may be transferred to alkynes. With diphenylethyne, for example, tetrakis(tetrahydrofuran)lithium 2,3,4,5-tetraphenylphospholide has been isolated in about 60 % yield from a THF solution (Eq. 17) again, the fate of both trimethylsilyl substituents remains unknown. The solid is built up of discrete tetrakis(tetrahydrofuran)lithium cations and planar phospholide anions (space group P1 w7 2-value 0.173 P-C 176 C-C 140 to 143 pm) the phenyl groups are tilted out of the C4P plane by angles of 47-65° [39]. 

Lithium bis(iminobenzoyl)phosphanides [41, 50] From the reaction of lithium bis(trimethylsilyl) phosphanide with two equivalents of benzonitrile at -50 °C the 1,2-dimethoxyethane complex of lithium bis(trimethylsilyliminobenzoyl)phosphanide is obtained in about 70 % yield (Eq. 19). and... 

When, however, benzonitrile is treated in diethylether with only one equivalent of lithium bis-(trimethylsilyl)phosphanide, [1 -(lithium-trimethylsilylamido)benzylidene]trimethylsilylphosphane, the product of the first reaction step, can be detected by its NMR spectra (6 P -11.7 5 C 222.6 ppm ... 

Jcp 53.5 Hz). The intermediate is formed by a nucleophilic attack of the phosphanide anion, combined with an 1,3-shift of one trimethylsilyl group from phosphorus to nitrogen (Eq. 20) it rearranges within three days at -50 °C to give lithium bis(trimethylsilyliminobenzoyl)- and lithium bis(trimethylsilyl)-phosphanide. In 1,2-dimethoxyethane the reaction is too fast to observe the intermediate. 

Fig. 11. Structure of the neutral complex l-(l,2-dimethoxyethane-0,0)lithium bis(trimethylsilyliminobenzoyl) phosphanide 30 % probability hydrogen atoms omitted measurement at -100 3 °C average values...

Treatment of chloromethylene phosphane C1P=C(TMS)2 with 2 equiv of lithium bis(trimethylsilyl) phosphanide in DME afforded the lithium salt of 1,2-diphosphapropenide 14 (Scheme 5). 

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