Trifluoroethanol: key solvent for palladium-catalyzed polymerization reactions

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.

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.

Palladium complexes with simple iminopyridines as catalysts for polyketone synthesis

A systematic study on iminopyridine palladium complexes demonstrates the relationship among NMR chemical shifts, ligand donor capability and catalytic performances.

Activation of late transition metal catalysts for olefin polymerizations and olefin/CO copolymerizations

The late transition metal catalysts for homo- and copolymerizations of olefins expand the polymer properties and the usable monomer feedstock of early transition metal catalysts. A critical step for a polymerization is the activation of catalyst precursors and the generation of the actual active site. This perspective discusses possible activation protocols for two well-known catalyst systems, the a-diimine Ni or Pd olefin polymerization catalysts and the Pd catalysts for alkene/CO copolymerization. The general concept of catalyst activation is outlined and similarities as well as differences between single-component catalysts and in situ activated systems are highlighted.

The insertion reaction of acetonitrile on aryl nickel complexes stabilized by bidentate N,N′-chelating ligands

Organometallic complexes to be used as single component precursors in the catalytic dimerization/polymerization of olefins usually must contain a labile ligand that can easily be displaced by the olefin. This is the first step in the activation of the precursor. One commonly used labile ligand is a nitrile. Here we report an example of incompatibility between the nickel or palladium aryl bond and acetonitrile. Neutral [MBr(Mes)NN] complexes in which Mes=2,4,6-Me3C6H2, NN=diazabutadiene (DAD), pyridinylimine (PIM), 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) gave the expected [M(Mes)(3,5-lut)(NN)][BF4] compounds and the unexpected [Ni(Mes){NH=C(Me)(2,4,6-Me3C6H2)}(NN)][BF4] complexes in the presence of TlBF4 and 3,5-lutidine or acetonitrile. The sequence of reactions that leads to the imine ligand must include an initial insertion of the nitrile on the sigma(Ni-Mes) bond. These ionic complexes remain stable under 20 bar of ethylene.

Long-Lived Palladium Catalysts for CO/Vinyl Arene Polyketones Synthesis: A Solution to Deactivation Problems

A series of cationic palladium complexes of general formula [Pd(Me)(MeCN)(N-N)][PF(6)] (N-N = (phen) 1 a, 4,7-dichloro-1,10-phenanthroline (4,7-Cl(2)-phen) 2 a, 4,7-diphenyl-1,10-phenanthroline (4,7-Ph(2)-phen) 3 a, 4-methyl-1,10-phenanthroline (4-Me-phen) 4 a, 4,7-dimethyl-1,10-phenanthroline (4,7-Me(2)-phen) 5 a, 5,5,6,6-tetrafluoro-5,6-dihydro-1,10-phenanthroline (F(4)-phen) 6 a, containing different substituted phenanthroline ligands, have been prepared from the corresponding neutral chloro derivatives [Pd(Me)(Cl)(N-N)], (1 b-6 b). The X-ray crystal structure of [Pd(Cl)(2)(4,7-Cl(2)-phen)] (2 b') was determined. DFT calculations show that the electron density on the metal is tuned by the substituents on the ligands. The catalytic behavior of complexes 1 a-6 a in the CO/styrene and CO/p-Me-styrene copolymerizations was studied in detail, showing that the generated catalysts are active for at least 90 h, yielding copolymers of high molecular weight. A firm correlation between the electron density on palladium on the one hand and the catalytic activity of the complexes and the molecular weight and the stereochemistry of the polyketones synthesized on the other hand has been established: the catalyst containing the F(4)-phen is thus far the most active among those tested, yielding the syndiotactic CO/styrene copolymer with a stereoregularity of 96 % (uu triad) and with an M(w) value of 1 000 000.

Mono- and dinuclear bioxazoline-palladium complexes for the stereocontrolled synthesis of CO/styrene polyketones

The coordination chemistry of the chiral bioxazoline ligand (4S,4'S)-2,2'-bis(4-isopropyl-4,5-dihydrooxazole) to Pd(II) provides evidence that the ligand bonding can occur either through chelation of one Pd(II) ion leading to a mononuclear species with the expected cis geometry, or by double bridging of two Pd(II) ions giving a dinuclear complex with trans geometry. The species in solution are identified by 1H NMR spectroscopy. Both the mononuclear and the dinuclear complexes promote the CO/styrene copolymerization, yielding the corresponding polyketone with a fully or a predominantly isotactic microstructure, depending on the reaction medium. The nature of the anion present in the palladium precatalysts affects the polyketone stereochemistry. MALDI-TOF analysis of the copolymers synthesized reveals the presence of p-hydroxyphenolic end-groups, thus confirming and explaining the role of 1,4-hydroquinone as a molecular weight regulator.