Concerted Electron Transfer in Iminopyridine Chromium Complexes: Ligand Effects on the Polymerization of Various (Di)olefins

A study of reactions among CrCl₂, CrCl₃(THF)₃, and iminopyridine ligands differing in the nature of the substituents at the iminic carbon and at the ortho positions of the aryl ring (2,6-R¹₂C₆H₃NCR²(C₅H₃N) (R¹ = R² = H (L1); R¹ = iPr, R² = H (L2); R¹ = H, R² = CH₃ (L3)) but featuring close electron-accepting properties has provided a new example of the redox chemistry of chromium complexes. The reactions of unsubstituted aniline L1 and of L3 with CrCl₂ give rise to [(L1^•)­Cr^IIICl₂(THF)]⁻ (Cr1) and [(L3^•)­Cr^IIICl₂(THF)]⁻ (Cr3) complexes, respectively, containing chromium in the physical trivalent oxidation state and the ligand in the monoanionic radical state (L^•)⁻ as a result of a one-electron transfer from the metal to the ligand. In contrast, the reactions of CrCl₂ with the ortho-substituted L2 and of CrCl₃(THF)₃ with the unsubstituted L1 give rise to [(L2)­Cr^IICl₂(THF)]⁰ (Cr2) and [(L1)­Cr^IIICl₃(THF)]⁰ (Cr4) having the chromium in the divalent and trivalent oxidation states, respectively, and the unperturbed ligand in the neutral state. All four complexes were used, in combination with methylaluminoxane (MAO), as catalyst precursors for the polymerization of ethylene, cyclic olefins (i.e., norbornene and dicyclopentadiene), and 1,3-butadiene. A chromium to ligand synergy, coupled with a good stability of the active intermediate in the presence of the Al activator, proven particularly effective in the polymerization of ethylene, especially for Cr1, giving high molecular weight linear poly­(ethylene)­s. The formalism in the metal oxidation state does not affect the reactivity toward the cyclic olefins and 1,3-butadiene, while ligand steric effects emerge clearly. The use of bulky ortho substituents shuts down the activity in the polymerization of cyclic olefins, particularly for the bulkier dicyclopentadiene, and reverses the catalyst chemoselectivity in the polymerization of 1,3-butadiene.