Heterobimetallic [Ti, Al] Complexes: Divergent Synthesis, Redox Properties, and Ethylene Polymerization Catalysis

Treatment of (Ind)­(tBu3P = N)­TiCl2 (1; Ind = indenyl)
with AlR3 (R =
Et, iBu, Me) affords heterobimetallic species of
the form [(Ind)­(tBu3PN)­Ti­(μ2-Cl)2AlR2] (R = Et, 2-Et; iBu, 2-iBu; Me, 2-Me). The formation of these compounds occurs chemically
or can be selectively triggered by electrochemical reduction of 1 in THF electrolyte solution containing AlR3.
Cyclic voltammetry studies indicate that 1 undergoes
one-electron reduction at ca. −2.0 V vs ferrocenium/ferrocene;
in the presence of AlR3, reduction initiates coordination
of [AlR2] to the nascent [TiIII] core and formation
of 2-Me, 2-Et, and 2-iBu. Oxidation of these heterobimetallic species occurs at
more positive potentials and leads to regeneration of 1. Single-crystal X-ray diffraction (XRD) analysis and magnetic resonance
experiments confirm the formation of paramagnetic [TiIII, AlIII] species with each AlR3 reagent. However,
spectroscopic results and gas chromatography show that 2-Me is significantly less stable than 2-iBu and that 2-iBu is less stable than 2-Et. 2-Me, 2-Et, and 2-iBu are all active precatalysts for ethylene polymerization,
but the diminished stability of 2-Me and 2-iBu in comparison with 2-Et is reflected
in unique polymerization results in each case. 2-iBu produces polyethylene with a particularly broad molecular
weight distribution (MWD), suggesting useful formation of multiple
active species during catalysis. Conversely, 2-Me is
a poor catalyst, suggesting detrimental speciation occurs with this
system under the polymerization conditions.