Research paperHeteroleptic triphenylacetate-tetramethylaluminate complexes of lanthanum and neodymium. Coordination of La3+ with the π-system as the reason for the low catalytic activity of lanthanum complexes
Graphical abstract
Similar La and Nd complexes of general formula [{Ln(Ph3CCOO)2}(AlMe4)]2 have different molecular structures and demonstrate different catalytic activity.
Introduction
Production of stereoregular rubbers plays an important role in the development of the modern tire industry. Such rubbers are obtained in the process of stereospecific polymerization of isoprene and butadiene. Three-component catalytic systems of a Ziegler-Natta type, derived from the neodymium carboxylates [1] are widely used in industry for this purpose over 20 years. However, the mechanism of this process remains unknown.
The main reason for this is the lack of structural data for neodymium complexes facilitating efficient polymerization processes. Even the structures of catalyst precursors remain unknown, since these neodymium-containing precatalysts normally form oligomers of uncertain composition [2].
It is assumed that a key role in the polymerization catalysis belongs to heteroleptic carboxylate-aluminate complexes. The latter complexes are formed when homoleptic acetate Ln(RCOO)3 reacts with an excess of organoaluminum compounds at the first stage of the catalytic system formation [3]. Relatively high tolerance of Nd-based catalytic system to the changes in the reaction stoichiometry supports this hypothesis [4]. Therefore, the study of AlR3 interactions with lanthanide carboxylates, as well as the investigation of structure and reactivity of heterometallic Ln-Al compounds produced during such interactions are of particular interest for understanding the route by which ternary catalytic system are being formed for stereospecific diene polymerization.
In spite of significant interest in heteroleptic aluminate complexes, mainly homoleptic tetraalkylaluminates of alkali and alkaline earth metals [5], [6], [7], [8], [9], [10], [11], [12], 4f- and 5f-elements [13], [14], [15], [16], [17], [18], [19] and some tetraalkylaluminates of transition metals [20] have been studied in details. Only a few heteroleptic lanthanide complexes containing both the tetraalkylaluminate and other ligands, including the carboxylates, were described [14], [21], [22], [23].
Earlier we suggested using bulky triphenylacetate ligands to design different types of 4f-carboxylate complexes. This approach allows stabilizing the coordination environment of lanthanide, and makes it possible to isolate and characterize carboxylate complexes, whose structures should be similar to the neodymium versatates being the main component of industrial ternary catalyst systems for stereospecific dienes polymerization. Using this approach, we synthesized a series of rare-earth triphenylacetates, and obtained Nd-based ternary catalytic system of stereospecific diene polymerization [24], [25]. The interaction of La and Nd triphenylacetates with an excess of AlEt3 led to the isomorphous heterometallic lanthanide-aluminum complexes [{Ln(Ph3CCOO)2}(AlEt4)]2 (Ln = Nd (1), La (2)) possessing a tetranuclear structure with four μ2-κ1:κ1-Ph3CCOO bridging ligands [24] (Scheme 1).
The present article is focused on the synthesis and study of the molecular structure of heteroleptic triphenylacetate-tetramethylaluminate complexes of lanthanum and neodymium, as well as the investigation of the reactivity of tetraethyl- and tetramethyl-aluminate complexes of lanthanum and neodymium, including diene polymerization catalysis.
Section snippets
General remarks
All described manipulations were conducted under argon using standard Schlenck and vacuum line techniques. Argon was purified, using columns with molecular sieves (4 Å) impregnated with Na/K alloy and vermiculite impregnated with MnII oxide [26]. AlMe3 and AlEt3 were purchased from Aldrich and used as received. Tetrahydrofuran was predried over NaOH and distilled from potassium/benzophenone ketyl under argon. Hexane was distilled from Na/K alloy/benzophenone ketyl under argon. Toluene was
Synthesis and structure of triphenylacetate-aluminate complexes
The similarity of molecular structures was observed for several types of analogous carboxylate complexes of lanthanum and neodymium: diphenylacetates of La and Nd [24], triphenylacetates of La and Nd (see supplement information, Fig. S1); complexes 1 and 2, obtained in the reactions of Ln(Ph3CCOO)3(THF)3 with an excess of AlEt3 [24]. It can be assumed that a similar molecular structure would lead to a similar catalytic activity of Nd and La triphenylacetates in diene polymerization.
Conclusions
Heteroleptic La and Nd complexes with triphenylacetate and tetraalkylaluminate ligands were obtained and structurally characterized. These Ln – Al complexes, which are formed by the interaction of lanthanide carboxylates with an excess of R3Al are not catalytically active species by themselves. However, they are the first intermediates in the formation of the catalyst system. Negligible catalytic activity of La-based catalyst systems in comparison to Nd -based systems is due to a tendency of
Notes
The authors declare no competing financial interests.
Acknowledgements
This research was supported by Russian Science Foundation (grant No. 17-13-01357).
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