Signet-Ring-Shaped Octaphosphorus–Cobalt Complexes: Synthesis, Structure, and Functionalization Reactions with Carbene Analogs

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Abstract

White phosphorus activation with low-valent metal species has proved to be very effective in converting P₄ into small P_n-containing molecules with n ≤ 4. Much less developed are metal-mediated P₄-coupling reactions that can yield selectively large polyphosphorus clusters. Herein, we report P₄-activation reactions with three-coordinate N-heterocyclic carbene(NHC)-cobalt(0)-alkene complexes, which produce the P₈ complexes of cobalt [(NHC)₂Co₂(μ-η6:η6-P₈)] (NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1; 1,3-bis(2,6-diethylphenyl)imidazol-2-ylidene (IDep), 2) in high yields. The P₈ ligand in 1 and 2 exhibits a signet-ring type structure that was predicted by theoretical study but has never been obtained in synthetic molecules. Theoretical studies suggest that [(NHC)₂Co₂(μ-η6:η6-P₈)] is best described as a Co0-[P₈]0-Co0 complex featuring a Co-Co σ-bond and discernible Co-to-P₈ back-donation. By treating with N-heterocyclic silylene, phosphinidene precursor, organic azides, and NHCs, complex 1 was transformed into [(IMes)₂Co₂(μ-η6:η6-P₈X)] (X = 1,3-bis(tert-butyl)-1,3-diaza-2-silacyclopent-4-en-2-ylidene, 3; PNMe₂, 4), [(IMes)₂Co₂(σ:(μ-η4:η6)-RNPPP₆)] (R = SO₂-C₆H₄-p-Me, 5; C(CF₃)₂Ph, 6), and [(IMes)₂Co₂(μ-η2:η2-cyclo-P₄P(IPri₂))(μ-η3:η3-P₃)] (7), respectively. Complexes 3-7 can be viewed as the adducts of 1 with the corresponding carbene analogous SiR₂, PR, NR, and CR₂, but their core structures are distinct. Under similar conditions, white phosphorus is inert toward these reagents. These cage functionalization reactions showcase the enhanced reactivity of the P₈ ligand in [(NHC)₂Co₂(μ-η6:η6-P₈)] over that of P₄, and indicate the amphiphilicity of the P₈ ligand toward electrophiles and nucleophiles.

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