Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO₂ Catalyzed by (salen)CrX Complexes

The most active and robust current catalysts for the copolymerization of carbon dioxide and epoxides or oxetanes, (salen)CrX in conjunction with PPNX (PPN+ = (Ph3P)2N+) or n-Bu4NX (X = Cl, N3, CN, NCO), are characterized both in solution by infrared spectroscopy and in the solid-state by X-ray crystallography. All anions (X) afford six-coordinate chromium(III) PPN+ or n-Bu4N+ salts composed of trans-(salen)CrX2− species. Of the X groups investigated in (salen)CrX, chloride is easily displaced by the others, that is, the reaction of (salen)CrCl with 2 equiv of N3−, CN−, or NCO− quantitatively provide (salen)Cr(N3)2−, (salen)Cr(CN)2−, and (salen)Cr(NCO)2−, respectively. On the other hand, addition of less than 2 equiv of azide to (salen)CrCl leads to a Schlenk (ligand redistribution) equilibrium of the three possible anions both in solution and in the solid-state as shown by X-ray crystallography and electrospray ionization mass spectrometry. It was further demonstrated that all trans-(salen)CrX2− anions react with the epoxide or oxetane monomers in TCE (tetrachloroethane) solution to afford an equilibrium mixture containing (salen)CrX·monomer, with the oxetane adduct being thermodynamically more favored. The ring-opening steps of the bound cyclic ether monomers by X− were examined, revealing the rate of ring-opening of the epoxides (cyclohexene oxide and propylene oxide) to be much faster than of oxetane, with propylene oxide faster than cyclohexene oxide. Furthermore, both X anions in (salen)CrX2− were shown to be directly involved in monomer ring-opening.