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

Abstract

Treatment of (Ind)(tBu 3 P = N)TiCl 2 (1; Ind = indenyl) with AlR 3 (R = Et, iBu, Me) affords heterobimetallic species of the form [(Ind)(tBu 3 PN)Ti(μ 2 -Cl) 2 AlR 2 ] (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 AlR 3 . Cyclic voltammetry studies indicate that 1 undergoes one-electron reduction at ca. −2.0 V vs ferrocenium/ferrocene; in the presence of AlR 3 , reduction initiates coordination of [AlR 2 ] to the nascent [Ti III ] 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 [Ti III , Al III ] species with each AlR 3 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.

Spectroscopic Interrogation of the Reduction of Model Chromium Precatalysts for Olefin Oligomerization

“…Collection of spectral data at 2 min intervals over a 10 min time course (see Figure ) revealed an increase in absorbance at λ = 227 nm, a decrease in absorbance at λ = 297 nm, and an increase in absorbance at λ = 342 nm. The series of collected spectra display two isosbestic points at 244 and 310 nm, demonstrating clean conversion of 1 to a new species that we hypothesize be a reduced Cr species (formed by Al–C bond cleavage and subsequent radical reactivity) or an Al-bound Cr species (bridged by alkyl or chloride bridging ligands), among other possibilities …”

“…Prior work with Ti IV/III complexes has implicated that reduction of Ti induces reactivity with AlEt 3 , a reactivity pattern that appears to be at work here as well. 27,28 Similarly, with 2 and 3, there is an observed larger shift in E p,a than E p,c (2: E p,c = +45 mV, E p,a = +96 mV; 3: E p,a = 642 mV, E p,c = 126 mV), indicating that there is a similar effect occurring with 2 and 3 upon addition of AlEt 3 (see Figures S13 and S14). This electrochemical behavior motivated us to study each of these complexes further with AlEt 3 to investigate the influence that the reagent has over the reactivity of these complexes under reductive conditions.…”

“…The series of collected spectra display two isosbestic points at 244 and 310 nm, demonstrating clean conversion of 1 to a new species that we hypothesize be a reduced Cr species (formed by Al−C bond cleavage and subsequent radical reactivity) or an Al-bound Cr species (bridged by alkyl or chloride bridging ligands), among other possibilities. 27 Encouraged by our initial finding of clean reactivity between 1 and AlEt 3 , we next titrated the resulting species with Cp* 2 Co to investigate possible further reduction chemistry accessible to the system. A solution of 1 in THF was thus mixed with AlEt 3 and allowed to stir for 10 min; the mixture was then titrated with Cp* 2 Co in increasing increments of 0.25 equiv (see Figure 10).…”

Transition Metal–(μ-Cl)–Aluminum Bonding in α-Olefin and Diene Chemistry

“…Both complexes catalyzed the polymerization of ethylene being activated by Et 3 Al in the presence of a solid superacid. During subsequent research [ 88 ], the complexes Ti14 and Ti15 of the formula (η 6 -C 9 H 7 )Ti (N=P t Bu 3 )(μ-Cl) 2 AlR 2 (R = Me and i Bu, respectively) were obtained, and the molecular structure of Ti15 ( Figure 5 b) was virtually identical to the molecular structure of Ti13 . However, in experiments on the polymerization of ethylene, catalytic activities of the complexes decreased in the order Ti15 > Ti13 > Ti14 .…”

“… Molecular structure of Ti–Al complexes ( a ) Ti13 and ( b ) Ti15 . Reprinted with permission from [ 88 ]. Copyright (2021) American Chemical Society.…”