Phosphinomethanides

Silicon and Phosphinomethanides A Novel Entry to Hypervalent and Low Valent Organosilicon Chemistry... 

A very special class of phosphorus based ligands are ambidentate, anionic phosphinomethanides I, since, due to the same number of valence electrons and bonds, both phosphorus and carbon are comparable in their reactivity and may compete for electrophiles. 

All these properties but the first and the last are also typical for cyclopentadienyl ligands, which are used extensively in organometallic chemistry. In organosilicon chemistry, the first stable monomeric silicon(II) derivative could be isolated with the aid of these ligands [3], It therefore seemed challenging to investigate the interaction of phosphinomethanides I with silicon centers. 

Me3SiCl reacts with phosphinomethanides I (R=Me) with at least one hydrogen as carbanion substituent (X = Y=H X=H, Y=SiMe3, PMej) via Si-C bond formation to give heteroelement substituted phosphinomethanides [4]. With fully C-heteroatom-substituted I, the reaction depends on the nature of X and Y, as shown by Eqs. (l)-(3) ... 

Si-P bonds are formed in the reaction with di- and tri-phosphinomethanides (Eqs.(l), (2)), and the resulting ylides 4 and 5 are fluctional in solution. In the case of monophosphinomethanide 3, both Si-C and Si-P bond formation is observed. The ylide 6 is rigid in solution on the NMR time scale, but it rearranges to the phosphinomethane derivative 7 within several days at 20 °C. 

For the rearrangement 6 —> 7, a ir-coordinate transition state III, similar to -complexes of phosphinomethanides with lanthanide metal centers [5], may be envisaged (see below). 

With very few exceptions, alkali metal complexes of phosphinomethanide ligands are synthesized by the deprotonation of a tertiary phosphine bearing an -CHR2 group with an alkali metal alkyl such as BunLi ... 

As has been observed for (di)organophosphide complexes of the alkali metals, the structures and aggregation states of alkali metal phosphinomethanide s are dramatically affected by the size and nature of the ligand substituents and the presence of additional coligands such as THF, tmeda, or pmdeta. The subtle interplay of these factors, and in particular the steric and electronic properties of substituents at both phosphorus and the a-carbon, defines the structures adopted by such complexes. 

The related tertiary phosphines PMe3 and PMe2Ph may also be met-alated by BunLi(tmeda), giving phosphinomethanide complexes that are shown by cryoscopy to be monomeric in benzene solution, but that crystallography confirms are dimeric in the solid state (126). Both complexes crystallize as dimers, [Li(CH2PMeR)(tmeda)]2 [R = Me (40), Ph (41)], with a similar structure to that of 39, containing a (CPLi)2 core. However, whereas 40 adopts a chair conformation, 41 is closer to a boat conformation with the phenyl groups in equatorial... 

Incorporation of additional donor functionality into the periphery of phosphinomethanide ligands also has dramatic consequences for the structures of their alkali metal complexes. The complex [Li C(SiMe2Ph)(PMe2)2 ]3 (49) crystallizes as solvent-free cyclic tri-mers (Fig. 18), in which each lithium is primarily coordinated by two P atoms from one ligand and the carbanion center of an adjacent ligand (138, 139). This is supplemented by an essentially -interaction with the ipso and an ortho-carbon of the phenyl ring associated with the carbanion bonded to lithium. Each Li is thus bound by two P atoms, two aryl carbons, and a central carbon of the phosphinomethanide ligands. 

This Li-phenyl interaction is similar to the r/ -Ph contact observed in the silicon-stabilized carbanion complex [Li C(SiMe2Ph)3 (THF)] (140). Addition of tmeda to 49 yields the monomeric compound [Li C(SiMe2Ph)(PMe2)2 (tmeda)l in which the lithium is bound by the two P atoms of the phosphinomethanide ligand and the two N atoms of the tmeda in a distorted tetrahedral geometry (139). There... 

Although potassium complexes of phosphinomethanide ligands have been used in the synthesis of lanthanide phosphinomethanides (143), it is only very recently that a heavier alkali metal phosphinomethanide complex has been isolated and structurally characterized. 

Whereas alkali metal phosphinomethanides have been well studied and a large number of complexes have been structurally characterized, there are apparently no reports of alkali metal complexes with arsenic-stabilized carbanions. 

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