A new family of iron (II) complexes (2a-h) bearing tridentate 2-pyrazolyl substituted 1,10-phenanthroline ligands were successfully prepared and characterized by IR spectroscopy, elemental analysis, and mass spectra. Complexes 2a-f and 2h were further confirmed by the X-ray crystallographic analysis. 2a, 2b, 2e, and 2f adopted distorted trigonal bipyramidal configuration. 2c displayed a distorted octahedron formed by six coordinated nitrogen atoms of the two ligands. Linked by two bridged chloride atoms, complex 2d was a centrosymmetric dimmer, and complex 2h adopted a six-coordinate distorted octahedral geometry due to the coordination of two solvent molecules. These complexes activated by alkylaluminum were examined in butadiene polymerization. In combination with (AlBu3)-Bu-i, complexes 2a-c exhibited high catalytic activity (73.5%-94.3%) at 20 degrees C, whereas other complexes exhibited much lower activity. Interestingly, the activity and selectivity of the complexes increased as increasing polymerization temperature. In particular, 2b and 2c displayed both high activity (99% and 80%, respectively) and trans-1,4 selectivity (95.6% and 96.2%, respectively) at 60 degrees C The trans-1,4 selectivity of 2b varied as alkylaluminum used as a cocatalyst, in the following order: (AlBu3)-Bu-i > AlOct(3) > AlEt3 > AlMe3, whereas much lower trans-1,4 selectivity was observed in the cases of using MAO and MMAO. (C) 2013 Elsevier Ltd. All rights reserved.