Palladium‐Catalyzed Ethylene/Methyl Acrylate Co‐Oligomerization: The Effect of a New Nonsymmetrical α‐Diimine with the 1,4‐Diazabutadiene Skeleton

Abstract

PdII complexes with a nonsymmetrical bis(aryl‐imino)diazabutadiene ligand (ArDAB) have been synthesized and characterized. The new ligand features one aryl ring substituted in the ortho positions with a methyl group and the other ring that bears a trifluoromethyl group on the meta positions, which leads to subtle steric and electronic differences at the two N donor atoms. This peculiar substitution makes the direct synthesis of the ligand unfeasible, and the relevant PdII complex, [Pd(CH3)Cl(ArDAB)], is obtained directly through a template reaction. The corresponding cationic complexes with either acetonitrile or DMSO were synthesized and characterized. The X‐ray crystal structure of a Pd complex with an α‐diimine and DMSO is reported. The monocationic complexes were tested as precatalysts in ethylene/methyl acrylate co‐oligomerization under mild reaction temperature and pressure conditions. A comparison of the catalytic behavior of the precatalysts with the corresponding nonsymmetrical aryl α‐diimines with an acenaphthene skeleton indicated that the active species with the new ligand is more productive and leads to the formation of ethylene oligomers and ethylene/methyl acrylate co‐oligomers.

The contradictory effect of the methoxy-substituent in palladium-catalyzed ethylene/methyl acrylate cooligomerization

“…The NMR spectra of 1b-3b showed sharp signals already at room temperature, as generally expected for Pd-acetonitrile derivatives with α-diimine ligands. 27,[33][34][35]37 For complex 1b, both trans and cis isomers were present in solution, in a 7 : 1 ratio (for 4b it was 10 : 1), 27 with the trans isomer being the major species, as identified from the 19 F, 1 H-HOESY spectrum (Fig. 6).…”

“…27 The 1 H NMR spectra of the dmso derivatives 1c-3c recorded in CD 2 Cl 2 at room temperature showed broad signals, which became sharper by decreasing the temperature, in agreement with our previous observations. 27,33,37 The decoalescence of the resonances of the aliphatic protons was reached at 263 K for 2c and at 233 K for 1c and 3c.…”

“…Recently, we demonstrated that the resonance of the Pd-CH 3 signal is a sensitive probe for the donating capability of the ligand, 27,36,37 and this claim is also confirmed for trans-1a, trans-2a and 3a: the Pd-CH 3 singlet moves from 0.71 to 0.63 and to 0.58 ppm on going from the CF 3 -disubstituted (1a) to the monosubstituted (2a) and to the methyl-methoxy (3a) aryl ring. In addition, the comparison of the resonance of this signal in 1a (0.71 ppm) and 4a (0.72 ppm) 27 suggests that the substitution of one methoxy with one methyl group on the same position of the Ar,Ar′-BIAN results in a ligand with very similar donating capability (1 vs. 4), despite the difference in the pK a value of the two native anilines (4.32 for the 2-methoxy-6-methylaniline vs. 3.91 for the 2,6dimethylaniline).…”

“…In addition, the comparison of the resonance of this signal in 1a (0.71 ppm) and 4a (0.72 ppm) 27 suggests that the substitution of one methoxy with one methyl group on the same position of the Ar,Ar′-BIAN results in a ligand with very similar donating capability (1 vs. 4), despite the difference in the pK a value of the two native anilines (4.32 for the 2-methoxy-6-methylaniline vs. 3.91 for the 2,6dimethylaniline). 38 Following the established procedure, 27,[33][34][35]37 the neutral complexes were converted into the monocationic derivatives [Pd(CH 3 )(L)(Ar,Ar′-BIAN)][PF 6 ] having either acetonitrile (L = CH 3 CN, 1b-3b) or dimethyl sulfoxide (L = dmso, 1c-3c) as the labile ligand (Fig. 4).…”

“…And, in the catalysts under investigation, this negative effect is so remarkable to overcome the positive effect associated with the presence of nonsymmetric ancillary ligands in the palladium coordination sphere, highlighted in our previous studies. 27,37 The GC/MS characterization of the volatile fraction pointed out that higher alkenes are produced by all the new catalysts (Table 1). Whereas this is expected for precatalysts 1b,c, 27,37 it is unusual for precatalysts 2b,c and 3b,c, which have both aryl rings of the ancillary ligand substituted in ortho positions.…”