Impact of Organoaluminum Compounds on Phenoxyimine Ligands in Coordinative Olefin Polymerization. A Theoretical Study

Abstract

The reduction of the phenoxyimine moiety in three individual speciesnamely free ligand, aluminum complex, and titanium complexwith aluminum alkyls and aluminum hydride has been studied by means of DFT. It was demonstrated that the free phenoxyimine ligand in an equimolar mixture with trimethylaluminum does not undergo reduction. Instead, experimentally observed formation of the six-membered cyclic aluminum−phenoxyimine complex, useful in the ring-opening polymerization of lactones, takes place as the kinetically and thermodynamically favored process. However, it is anticipated that a 2-fold excess of the aluminum compound, especially aluminum hydride, acting on the resulting cyclic complex can convert the imine to the aluminum-subsituted amine functionality easily with an energetic barrier of approximately 10 kcal/mol. Finally, the propensity of the imine moiety in the titanium-based precursor of the coordinative olefin polymerization toward reduction with organoaluminum compounds is revealed and the mechanism of this reaction is also suggested.

Progression of Diiminopyridines: From Single Application to Catalytic Versatility

“…90 On the contrary, the formation of the cyclic complexes is clearly preferred over the imine group reduction in the phenoxyimines. 92,93 Diiminopyridines show even more diverse and puzzling reactivity. Because the cocatalysts applied in the coordinative polymerization process are customarily strong alkylating and reducing agents, the alkylation of the diiminopyridine ligand is invariably impossible to avoid, even though it might be considered as being an unwanted side reaction.…”

Donor Decomposition by Lewis Acids in Ziegler–Natta Catalyst Systems: A Computational Investigation

“…The primary function of the donor species appears to be to influence the stereo- and regioselectivity of polymer formation by binding in the vicinity of the active catalyst. − Other possible functions attributed to the donor include influencing the polymerization kinetics by coordinating to the titanium center , and making the (104) MgCl 2 surface more accessible by coordinating more to the more acidic (110) MgCl 2 surface, according to the former Corradini model, whose revisitation is still ongoing. It is, however, also known that the Lewis base donors can take part in undesired side reactions with Lewis acidic species present in the system, such as teal , a reaction that can lead to the decomposition of the donors. − The best donors are therefore those that are good at the primary function of influencing ZN catalysis, while showing a reduced tendency to take part in unwanted side reactions. Over the years, many donors − ,− have been tested, and currently, silyl ethers, , along with diethers, ,, are considered to be the most effective classes of donors for ZN catalysis.…”

Unexpected Precatalyst σ-Ligand Effects in Phenoxyimine Zr-Catalyzed Ethylene/1-Octene Copolymerizations

“…Using the solution, 3 + TMA (40 equiv) reacted at rt for 1 day, followed by BT addition, as a polymerization catalyst under otherwise identical conditions to entry 8 of Table shows significantly decreased activity, 1088 vs 5433 kg/(mol·h·atm), and similarly very low 1-octene incorporation (∼0 mol% vs 0.9 mol%). Thus, 7 , which derives from activation of 3 with BT in the presence of TMA, is likely the active species (Figure C); in contrast, the slow ancillary ligand transfer in the reaction of 3 + TMA ultimately affords an inactive catalyst structure which is currently unknown, ,,− ,− instead of the formation of a mono-FI-Zr complex observed in the analogous reaction with 1 , which leads to high activity and 1-octene incorporation.…”

“…It will be seen below that ancillary ligand transfer occurs during alkylation of 1 by TMA. Ti and Zr complexes having bidentate monoanionic ligands, including those having phenoxyimine ligands, have previously been reported to undergo ancillary ligand transfer reactions with trimethylaluminum (TMA), − forming bidentate monoanionic ligand coordinated Al complexes along with unidentified group 4 species. Such processes have been previously associated with catalyst deactivation pathways (Figure B). ,,− ,− Here, the proposed ancillary ligand abstraction scenario is confirmed by in situ NMR spectroscopy, precatalyst X-ray diffraction, and catalytic control experiments.…”

“…(A) Synthesis of key complexes relevant to trimethylaluminum (TMA) alkylation studies. (B) Previously reported ancillary ligand transfer from Ti or Zr to Al as a common catalyst deactivation pathway. ,− ,− (C) Reactions of precatalysts 3 and 1 with TMA, and proposed cationic active catalysts.…”