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

White phosphorus activation with low-valent metal species has proved to be very effective in converting P 4 into small P n -containing molecules with n ≤ 4. Much less developed are metal-mediated P 4 -coupling reactions that can yield selectively large polyphosphorus clusters. Herein, we report P 4 -activation reactions with three-coordinate N -heterocyclic carbene(NHC)-cobalt(0)-alkene complexes, which produce the P 8 complexes of cobalt [(NHC) 2 Co 2 (μ-η 6 :η 6 -P 8 )] (NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1 ; 1,3-bis(2,6-diethylphenyl)imidazol-2-ylidene (IDep), 2 ) in high yields. The P 8 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) 2 Co 2 (μ-η 6 :η 6 -P 8 )] is best described as a Co 0 -[P 8 ] 0 -Co 0 complex featuring a Co-Co σ-bond and discernible Co-to-P 8 back-donation. By treating with N -heterocyclic silylene, phosphinidene precursor, organic azides, and NHCs, complex 1 was transformed into [(IMes) 2 Co 2 (μ-η 6 :η 6 -P 8 X)] (X = 1,3-bis( tert -butyl)-1,3-diaza-2-silacyclopent-4-en-2-ylidene, 3 ; PNMe 2 , 4 ), [(IMes) 2 Co 2 (σ:(μ-η 4 :η 6 )-RNPPP 6 )] (R = SO 2 -C 6 H 4 - p -Me, 5 ; C(CF 3 ) 2 Ph, 6 ), and [(IMes) 2 Co 2 (μ-η 2 :η 2 -cyclo -P 4 P(IPr i 2 ))(μ-η 3 :η 3 -P 3 )] ( 7 ), respectively. Complexes 3 - 7 can be viewed as the adducts of 1 with the corresponding carbene analogous SiR 2 , PR, NR, and CR 2 , 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 P 8 ligand in [(NHC) 2 Co 2 (μ-η 6 :η 6 -P 8 )] over that of P 4 , and indicate the amphiphilicity of the P 8 ligand toward electrophiles and nucleophiles.