The Cobalt cyclo-P₄ Sandwich Complex and Its Role in the Formation of Polyphosphorus Compounds

A synthetic approach to the sandwich complex [Cp'''Co(η⁴ -P₄ )] (2) containing a cyclo-P₄ ligand as an end-deck was developed. Complex 2 is the missing homologue in the series of first-row cyclo-P_n sandwich complexes, and shows a unique tendency to dimerize in solution to form two isomeric P₈ complexes [(Cp'''Co)₂ (μ,η⁴ :η² :η¹ -P₈ )] (3 and 4). Reactivity studies indicate that 2 and 3 react with further [Cp'''Co] fragments to give [(Cp'''Co)₂ (μ,η² :η² -P₂ )₂ ] (5) and [(Cp'''Co)₃ P₈ ] (6), respectively. Furthermore, complexes 2, 3, and 4 thermally decompose forming 5, 6, and the P₁₂ complex [(Cp'''Co)₃ P₁₂ ] (7). DFT calculations on the P₄ activation process suggest a η³ -P₄ Co complex as the key intermediate in the synthesis of 2 as well as in the formation of larger polyphosphorus complexes via a unique oligomerization pathway.