Synthesis, X‑ray Structural Analysis, and Ethylene Polymerization Studies of Group IV Metal Heterobimetallic Aluminum-Pyrrolyl Complexes

The synthesis, structural characterization, and ethylene
polymerization performance of heterobimetallic aluminum-pyrrolyl complexes
of group IV metals are described. The combination of MCl4 (M = Ti, Zr, Hf), aluminum alkyls and pyrroles leads, depending
on stoichiometry, to mono- and bis­(aluminum-pyrrolyl) complexes that
are remiscent of the corresponding mono- and bis- cyclopentadienyl
systems. The bis­(aluminum-pyrrolyl) complexes (η5-2,5-Me2C4H2NAlClMe2)2TiMe2 (1), zirconium (η5-2,5-Me2C4H2NAlClMe2)2ZrClMe (2), (η5-2,5-Me2C4H2NAlCl2Et)2ZrCl2 (3), and hafnium (η5-2,5-Me2C4H2NAlClMe2)2HfClMe (4) were found to be inactive toward ethylene
polymerization. By contrast, the electron-deficient mono­(aluminum-pyrrolyl)
piano stool complexes (η5-2,5-Me2C4H2NAlCl2Me)­TiCl2Me (5), (η5-2,3-Me2C8H4NAlCl2Me)­TiCl2Me (6), and
(η5-3,4,5,6-C12H12NAlCl2Me)­TiClMe2 (7), obtained by treatment
of TiCl4 with equimolar amounts of trimethyl aluminum and
the corresponding pyrrole ligands, were found to be moderately active
for ethylene polymerization with MAO or [Ph3C]+[B­(C6F5)4]−, in
all cases producing UHMWPE. The NMR scale reaction of 5 with B­(C6F5)3 showed the formation
of a solvent-separated ion pair, formed by the abstraction by B­(C6F5)3 of a methyl group from Al-CH3 rather than from Ti-CH3. 1H and 13C NMR analysis of 6 and 7 revealed
that several stable structural isomers exist in solution, with a slow
interconversion on the NMR time scale. The dimeric zirconium complexes
[(η5,κ1-2,5-Me2C4H2NAlClMe2)­ZrMeCl­(μ-Cl)]2 (8) and [(η5,κ1-2,5-Me2C4H2NAlCl2Et)­ZrCl2(μ-Cl)]2 (9), prepared by the reaction
of 2 equivalents of ZrCl4, 1 equivalent of 2,5-dimethylpyrrole,
and 1 equivalent of aluminum alkyl, showed structures containing a
bridging chloride between aluminum and zirconium, thereby giving structural
evidence for the lack of catalytic behavior of these complexes. A
possible explanation for the moderate or absence of catalytic activity
of the aluminum-pyrrolyl complexes was proposed based on DFT calculations.