Metal-ligand synergy through electron transfer and its influence on the polymerization of olefins: the case study of iminopyridine chromium complexes

Beside the great relevance that chromium covers in the field of heterogeneous catalysis for ethylene polymerization as the active metal of the so-called Phillips catalyst, this transition metal has been deeply studied also for the preparation of various molecular complexes intended as homogeneous single-site catalysts for the production of linear alfa-olefins, and syndiotactic 1,2 poly(butadiene). Most of the recently reported Cr-complexes are enveloped by multidentate ligands with phosphine, amine, ether, and thioether donors because of their easy tuning of steric and electronic properties. In this context, a study of reactions between CrCl2 and CrCl3(THF)3 with three different iminopyridine ligands, was accomplished. Due to the non-innocent nature of the iminopyridine ligands, the characterization of the complexes required the aid of UV-vis-NIR spectroscopy to investigate the electronic properties and to better discriminate between the formal and the physical oxidation state of chromium. Indeed, in some cases an electron transfer from Cr to the ligand was possible, causing a variation of the oxidation state of the metal. In particular, the reaction of unsubstituted aniline L1 and L3 with CrCl2 gives rise to [(L1?)CrIIICl2(THF)]- (Cr1) and [(L3?)CrIIICl2(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 an one electron transfer from the metal to the ligand. Differently, the reaction of CrCl2 with the ortho-substituted L2, and CrCl3(THF)3 with the unsubstituted L1 gives rise to [(L2)CrIICl2(THF)]0 (Cr2) and [(L1)CrIIICl3(THF)]0 (Cr4) having the chromium in the divalent and trivalent oxidation state, 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 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, has proven particularly effective in the polymerization of ethylene, especially for Cr1, giving high molecular weight linear poly(ethylene)s. In contrast, 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. It is worth mention that the polymerization of cyclic olefins (i.e., norbornene and dicyclopentadiene) catalyzed by Cr1/MAO brought to the synthesis of a stereoregular polymers whose structure has been determined to be in both cases 2,3-exo-diheterotactic; in particular, the 2,3-exo-diheterotactic dicyclopentadiene oligomer is a novel material which was never reported before.