Abstract
β-ketoimines are extensively applied for the synthesis of organometallic complexes intended as (pre)catalysts for a variety of chemical transformations. We were interested in the synthesis of two Cr complexes bearing a simple bidentate β-ketoimine (L), with different ligand binding modes, as well as their application as a precatalyst in the polymerization of olefins. Complex 1 (L2CrCl3) was obtained by direct reaction of L with CrCl3(THF)3, while, for the synthesis of complex 2 (LCrCl2), the ligand was first deprotonated with nBuLi, giving the β-ketoiminato ligand L─Li+, and then reacted with CrCl3(THF)3. Characterization of the complexes proved that the Cr(III) ion is coordinatively bonded to L in 1, while it is covalently bonded to L in 2. The complexes were then used as precatalysts for the polymerization of ethylene and various cyclic olefins. Upon activation with methylaluminoxane, both the complexes exhibited poor activity in the polymerization of ethylene, whilst they exhibit good productivity in the polymerization of cyclic olefins, affording semicrystalline oligomers, without a significant difference between 1 and 2. To gain more insight, we investigated the reaction of the complexes with the Al-cocatalyst by IR and UV-Vis spectroscopies. The results proved that, in case of 1, the Al-activator deprotonates the ligand, bringing to the formation of an active species analogous to that of 2.
Highlights
For the polymerization of cyclic olefins, the monomer and toluene were transferred into the reactor, the solution was brought to the desired polymerization temperature, and the cocatalyst and a toluene solution (2 mg mL−1) of the chromium complex were added in that order
Two chromium complexes bearing a β-ketoimine ligand were synthesized from CrCl3(THF)3 according to two different routes: in complex 1 the ligand is coordinated to the metal in the neutral form, while complex 2 was obtained with the lithium salt of the same ligand
Characterization of the two complexes by elemental analysis, IR and UV-Vis spectroscopies, revealed that complex 1 is surrounded by two ligands, with the two O atoms coordinated to Cr, while in complex 2 only one ligand is present, covalently bonded to Cr through the O atom and with the N atom chelated to the metal by coordination
Summary
The spectrum is characterized by two very intense bands at 1510 cm−1 (ν(C=N) vibration) and 1394 cm−1 (ν(C–N) vibration of the N–Ph group), both enhanced in intensity by the presence of the covalently bonded Cr cation, while no bands are observed in the spectral region characteristic for the ν(C=O) vibration of the metal chelated carbonyl. The spectrum of complex 2 shows a broad band centered at about 37,000 cm−1, which is tentatively assigned to an O → Cr ligand-to-metal charge-transfer transition, which can be observed only when Cr is covalently bonded to O.
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