Germanium(II) and Tin(II) Complexes of a Sterically Demanding Phosphanide Ligand

The reaction between PhPCl₂ and 1 equiv of RLi, followed by in situ reduction with LiAlH₄ and an aqueous workup yields the secondary phosphane PhRPH [R = (Me₃Si)₂CH]. Treatment of PhRPH with n-BuLi in diethyl ether generates the lithium phosphanide (RPhP)Li(Et₂O)_n [15(Et₂O)], which may be crystallized as the tetrahydrofuran (THF) adduct (RPhP)Li(THF)₃ [15(THF)]. Compound 15(Et₂O) reacts with 1 equiv of either NaO-tBu or KO-tBu to give the corresponding sodium and potassium phosphanides (RPhP)Na(Et₂O)_n (16) and (RPhP)K(Et₂O)_n (17), which may be crystallized as the amine adducts [(RPhP)Na(tmeda)]₂ [16(tmeda)] and [(RPhP)K(pmdeta)]₂ [17(pmdeta)], respectively. The reaction between 2 equiv of 17 and GeCl₂(1,4-dioxane) gives the dimeric compound [(RPhP)₂Ge]₂·Et₂O (18·Et₂O). In contrast, the reaction between 2 equiv of 15 and SnCl₂ preferentially gives the ate complex (RPhP)₃SnLi(THF) (19) in low yield; 19 is obtained in quantitative yield from the reaction between SnCl₂ and 3 equiv of 15. Crystallization of 19 from n-hexane/THF yields the separated ion pair complex [(RPhP)₃Sn][Li(THF)₄] (19a); exposure of 19a to vacuum for short periods leads to complete conversion to 19. Treatment of GeCl₂(1,4-dioxane) with 3 equiv of 15 yields the contact ion pair (RPhP)₃GeLi(THF) (20), after crystallization from n-hexane/THF. Compounds 15(THF), 16(tmeda), 17(pmdeta), 18·Et₂O, 19a, and 20 have been characterized by elemental analyses, multielement NMR spectroscopy, and X-ray crystallography. While 15(THF) is monomeric, both 16(tmeda) and 17(pmdeta) are dimeric in the solid state. The diphosphagermylene 18·Et₂O adopts a dimeric structure in the solid state with a syn,syn-arrangement of the phosphanide ligands, and this structure appears to be preserved in solution. The ate complex 19a crystallizes as a separated ion pair, whereas the analogous ate complex 20 crystallizes as a discrete molecular species. The structures of 19 and 20 are retained in non-donor solvents, while dissolution in THF yields the separated ion pairs 19a and [(RPhP)₃Ge][Li(THF)₄] (20a).