Indolyl-based NCN-pincer titanium group metal complexes: Synthesis and catalytic application for styrene polymerization

Indole worked as the backbone and two side arms at the 1-, and 3-positions were introduced to obtain a 1, 3-di-functionalized indolyl compound. After the deprotonation by BuLi, it was reacted with MCl4 (M=Ti, Zr, Hf) to form a series of NCN-pincer type titanium group metal complexes (3-Ti, 3-Zr, 3-Hf). The complexes were characterized by NMR, elemental analysis, and single-crystal X-ray diffraction. The single-crystal structures revealed that the metal center bonds with the ligand in kappa(3)(NCN) modes and three chlorine atoms to form a six-coordinated distorted octahedral configuration. Styrene polymerization was investigated with 3-Ti, 3-Zr, and 3-Hf using aluminum alkyls and borate reagents as cocatalysts when the concentration ratio of the monomer to the complex was 300:1. It was found the complexes of different metal centers could efficiently catalyze styrene polymerization with high conversion under room temperature and using AlEt2Cl/(Ph3C)[B(C6F5)(4)] as cocatalysts, moreover, the hafnium complex displayed the higher activity. The obtained polystyrene was characterized by gel permeation chromatography (GPC) and C-13 NMR. The results showed the polymer was atactic and had a low number-average molecular weight ((M) over bar (n) = 4 000-8 000). CCDC: 2332412, 3-Ti; 2332412, 3-Zr; 2332414, 3-Hf.