Crowded organometallic compounds of the alkali metals with diphenylphosphino substituents in the organic group

Abstract

The lithium compound [LiC(SiMe₂CH₂PPh₂)₃] 1, obtained by metallation of the precursor HC(SiMe₂CH₂PPh₂)₃I with LiMe, crystallised from benzene as a solvate 1·1.5C₆H₆ that both in solution and in the solid state has an unusual tricyclic structure, with lithium bound to the carbanionic centre and the three phosphorus atoms. Lithiation of the related precursor HC(SiMe₃)₂(SiMe₂CH₂PPh₂) II under similar conditions gave the dilithium compound [Li(thf)C(SiMe₃)₂{SiMe₂CH[Li(thf)₂]PPh₂}] 2, which has a fluxional structure in solution. Metallation of the precursor HC(SiMe₃)₂(SiMe₂PPh₂) III gave the compounds MC(SiMe₃)₂(SiMe₂PPh₂) (M = Li 3 or Na 4). Compound 3 is fluxional in benzene with interchange of methyl groups between SiMe₂ and SiMe₃ fragments. Compound 4, in contrast, has a non-fluxional molecular structure. In the solid the molecules of 4 form chains in which the sodium is bound intramolecularly to the carbanionic centre and to phenyl and intermolecularly to phenyl and phosphorus. Attempts to make the potassium analogue of 3 or 4 led to cleavage of the P–Si bond and formation of KPPh₂. This has a complicated polymeric crystal structure in which molecules are linked by potassium–phenyl interactions.