Synthesis, characterization, and butadiene polymerization of iron(III), iron(II) and cobalt(II) chlorides bearing 2,6-bis(2-benzimidazolyl)pyridyl or 2,6-bis(pyrazol)pyridine ligand

Abstract

Iron(III), iron(II) and cobalt(II) complexes bearing neutral N,N,N-tridentate ligand (Fe(III)L1, 1a; Fe(III)L2, 1b; Fe(III)L3, 1c; Fe(III)L4, 2a; Fe(III)L5, 2b; Fe(III)L6, 2c; Fe(II)L1, 3a; Fe(II)L2, 3b; Fe(II)L3, 3c; Fe(II)L4, 4a; Fe(II)L5, 4b; Fe(II)L6, 4c; CoL1, 5a; CoL2, 5b; CoL3, 5c; CoL5, 6b and CoL6, 6c) have been synthesized from the metal chlorides (FeCl3, FeCl2·4H2O or CoCl2) by treating the corresponding ligands (2,6-bis(benzimidazol-2-yl)pyridine, L1; 2,6-bis(1′-ethylbenzimidazol-2′-yl)pyridine, L2; 2,6-bis(1′-benzylbenzimidazol-2′-yl)pyridine, L3; 2,6-bis(pyrazol)pyridine L4; 2,6-bis(3-methylpyrazol)pyridine, L5 or 2,6-bis(3,5-dimethylpyrazol)pyridine, L6. The complexes are characterized by FTIR and elemental analyses. The structures of complexes 1a, 1b, 3a, 3b, 3c, 4a, 5a, 5b, 5c and 6c are further confirmed by X-ray crystallographic analyses. Six coordination iron(III) (1a, 1b and 1c) and iron(II) (3c*DMF) complexes adopt a distorted octahedral configuration with the equatorial plane formed by the three nitrogen atoms and one chlorine atom, while the iron(II) (3a, 3b and 4a) and cobalt complexes (5a, 5b, 5c and 6c) adopt a trigonal bipyramidal configuration with the equatorial plane formed by the pyridyl nitrogen atoms and the two chlorine atoms. All complexes are evaluated as precursors for 1,3-butadiene polymerization in the presence of cocatalyst in toluene at room temperature. Iron(III) and iron(II) bearing the same ligand show comparable catalytic performance. The catalytic activity and selectivity are significantly influenced by the ligand structure, with the latter being tunable within a wide range from cis-1,4 to trans-1,4. Cobalt complexes, in combination with either MAO or aluminum chlorides shown high activity, and high cis-1,4-selectivity irrespective of the structure of ligand backbone.