Heterocycle-Fused Indenyl Silyl Amido Dimethyl Titanium Complexes as Catalysts for High Molecular Weight Syndiotactic Amorphous Polypropylene

Indenyl silyl amido titanium complexes based on indenoindoles, indenopyrrole, and
2-methylindenothiophene have been prepared and tested in propylene polymerization. The
indenoindole ligand precursors were prepared in high yields by the acid-catalyzed Fischer
condensation of indan-2-one and arylhydrazines. The Ti complexes dimethylsilyl(tert-butylamido)(N-methyl-2-methyl-5,6-dihydroindeno[2,1-b]indol-6-yl)dimethyl titanium (1),
dimethylsilyl(tert-butylamido)(N-ethyl-5,6-dihydroindeno[2,1-b]indol-6-yl)dimethyl titanium
(2), dimethylsilyl(tert-butylamido)(N-methyl-5,10H-indeno[1,2-b]indol-10-yl) dimethyl titanium (3), dimethylsilyl(tert-butylamido)(N-phenyl-5,10H-indeno[1,2-b]indol-10-yl)dimethyl
titanium (4), dimethylsilyl(tert-butylamido)(N-methyl-2-methyl-1,8-dihydroindeno[2,1-b]pyrrol-8-yl)dimethyl titanium (5), and dimethylsilyl(tert-butylamido)(2-methyl-8H-indeno[2,1-b]thiophen-8-yl)dimethyl titanium (6) were prepared by reacting the ligand, a 2-fold excess
of MeLi, and TiCl4. The molecular structures of the amidosilylindenyl titanium complexes
2 and 6 have been determined by single-crystal X-ray diffraction analysis:  the indenoindole
moiety in 2 and the indenothiophene moiety in 6 are perfectly planar, implying a certain
degree of delocalization of the heteroatom lone pairs into the aromatic moiety coordinated
to the metal atom. Catalysts 1, 2, and 5 produce syndiotactic amorphous polypropylenes
(sam-PP) of very high molecular weights, even at polymerization temperatures as high as
80 °C. Syndiotactic pentad contents range between 48 and 57% rrrr, and regioerrors are
close to or below the detection limit of the 13C NMR analysis (at 100 MHz). In toluene solution,
[Ph3C][B(C6F5)4] is a more efficient activator than MAO, [HNMe2Ph][B(C6F5)4], or B(C6F5)3.
The catalytic activity of 5/[Ph3C][B(C6F5)4] shows an approximate first-order dependence on
propylene concentration, while the tacticity and the molecular weight do not seem to change
noticeably with the monomer concentration. The obtained activation energy barrier for chain
release with this catalyst is ΔΔEr⧧ = 7.7 kcal/mol. The influence of cyclopentadienyl ligand
substituents on polymerization activity has been rationalized by means of the group
electronegativity of the [Me2Si(Cp‘)(t-Bu-N)TiMe] fragment, as defined within the framework
of density functional theory.