Experimental and theoretical insights into palladium-mediated polymerization of para-N,N-disubstituted aminostyrene

Experimental and theoretical insights into polymerization of para-N,N-disubstituted aminostyrene monomers (St-4-NR2, R = Me, Et, Ph) using cationic α-diimine palladium complexes have been initially reported. The effects of the catalyst structure and monomer substituent were studied systematically. Polymerization turnover frequency (TOF) was shown to decrease in the order of monomer substituents Me > Et > Ph, whereas the molecular weight of the produced polymers showed an opposite trend (Me < Et < Ph). Methanol-mediated polymerization of para-N,N-dimethylaminostyrene (DMAS), along with polymer chain-end analysis, and palladium intermediate isolation proved that palladium-initiated DMAS polymerization obeyed a cationic mechanism. Comprehensive theoretical calculations further revealed that the carbocation was generated from the insertion of DMAS into the palladium center rather than the polarization of the methyl palladium intermediate with a coordinated DMAS. The produced amine-functionalized amorphous polystyrenes have low stereoregularity and exhibit good hydrophilic properties. The poly(para-N,N-disphenylaminostyrene) is a luminescent polymer and shows fluorescence properties, rendering this material a promising candidate for versatile potential applications.

Modulation of N^N'-bidentate chelating pyridyl-pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric N^N'-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast β-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the N^N'-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of α,β-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design.

One-Pot Cross-Coupling/C-H Functionalization Reactions: Quinoline as a Substrate and Ligand through N-Pd Interaction

Herein, we report a one-pot process that marries mechanistically distinct, traditional cross-coupling reactions with C-H functionalization using the same precatalyst. The reactions proceed in yields of up to 95%, in air, and require no extraneous ligand. The reactions are thought to be facilitated by harnessing the substrate quinoline as an N-ligand, and evidence of the palladium-quinoline interaction is provided by ¹H-¹⁵N HMBC NMR spectroscopy and X-ray crystallographic structures. Application of the methodology is demonstrated by the quick formation of fluorescent, π-extended frameworks.

Synthesis, spectral and luminescence study, crystal structure determination and DFT calculation of binuclear palladium(II) complexes

Binuclear palladium(II) complexes with metal-metal (d⁸-d⁸) bonding interaction were synthesized by reactions of the 1-methyl-1H-1,2,3,4-tetrazole-5-thiol (Hmtzt) or a mixture of Hmtzt and 1,3-propanediamine (1,3-pda) ligands. Complex [Pd₂(μ-mtzt)₄]·2CH₃CN (1) was synthesized by the reaction of Pd(OAc)₂ with Hmtzt dissolved in acetonitrile and complex [Pd₂(μ-mtzt)₂(mtzt)₂(1,3-pda)] (2) was synthesized by reaction of a mixture of Hmtzt and 1,3-propanediamine (dissolved in methanol) with PdCl₂ (dissolved in acetonitrile) and were identified through elemental analysis, IR, UV-Vis, ¹H NMR, luminescence spectroscopy as well as single-crystal X-ray diffraction method. A single-crystal of complex 1 shows that two Pd(II) centers are linked together by four bridging tetrazole ligands providing a paddle wheel-like arrangement. Also a crystal structure of complex 2 shows that this complex possesses a symmetric structure in which one Pd atom is tetra-coordinated by four sulfur atoms to forms PdS₄ and other Pd atom is tetra-coordinated by four nitrogen to forms PdN₄ coordination sphere. Density functional theory (DFT) was performed in this study for the Hmtzt ligand and binuclear palladium(II) complexes (1) and (2). The DFT calculation shows Pd^II-Pd^II bond lengths of 2.831 and 3.086Å in complex 1 and 2, respectively which are close to the observed bond lengths of 2.802(11) and 3.0911(17)Å from single-crystal X-ray structure. The optimized geometry of the complexes is shown good agreement by X-ray data. Structural properties and molecular descriptors including bond lengths, bond angles, chemical hardness, dipole moment, HOMO-LUMO energy levels, electron transfer were analyzed. The IR spectroscopy was performed using VEDA4 software and UV-Vis spectra were analyzed using time-dependent density functional theory (TD-DFT) method. The theoretical and experimental data were also compared with each other.

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

Three new α-diimines bearing at least one methoxy group on one aryl ring were investigated. The introduction of the methoxy substituent on the ancillary ligand had a remarkable effect on the catalytic performances of the relevant palladium complexes.