Anna M. Trzeciak

PEPPSI‐Type Palladium Complexes Containing Basic 1,2,3‐Triazolylidene Ligands and Their Role in Suzuki–Miyaura Catalysis

A series of PEPPSI-type palladium(II) complexes was synthesized that contain 3-chloropyridine as an easily removable ligand and a triazolylidene as a strongly donating mesoionic spectator ligand. Catalytic tests in Suzuki-Miyaura cross-coupling reactions revealed the activity of these complexes towards aryl bromides and aryl chlorides at moderate temperatures (50 °C). However, the impact of steric shielding was the inverse of that observed with related normal Nheterocyclic carbenes (imidazol-2-ylidenes) and sterically congested mesityl substituents induced lower activity than small alkyl groups. Mechanistic investigations, including mercury poisoning experiments, TEM analyses, and ESI mass spectrometry, provide evidence for ligand dissociation and the formation of nanoparticles as a catalyst resting state. These heterogeneous particles provide a reservoir for soluble palladium atoms or clusters as operationally homogeneous catalysts for the arylation of aryl halides. Clearly, the substitution of a normal N-heterocyclic carbene for a more basic triazolylidene ligand in the precatalyst has a profound impact on the mode of action of the catalytic system.

PERSPECTIVES OF RHODIUM ORGANOMETALLIC CATALYSIS. FUNDAMENTAL AND APPLIED ASPECTS OF HYDROFORMYLATION

Abstract Today’s hydroformylation process almost exclusively use rhodium homogeneous catalysts. Some domination of octacarbonyl dicobalt as the catalyst precursor was followed with broad application of rhodium based organometallic catalysts modified with mainly phosphorus ligands of different donor–acceptor properties and/or different cone/bite angles. Although there are many papers dealing with the comparative studies on structure-reactivity correlation, the effects of electronic and steric parameters of P-ligands on the catalytic activity of rhodium catalysts are not always predictable. Phosphines and phosphites as ligands, simple and structurally developed, bulky, mono and bidentate are still of great interest in the modification procedures of rhodium catalysts, especially when high regioselectivity in the hydroformylation is expected. Further development of the synthesis of phosphorus ligands led to the preparation of water soluble ligands and the creation of a new class of two-phase homogeneous catalysts, in this way solving problems of separation and/or catalysts reuse. Some new water soluble phosphines (PNS, PNa, PC) as well as N -pyrrolyl phosphines (PPh x (NC 4 H 4 ) 3− x ) of required electronic parameters applied for synthesis of new rhodium catalyst precursors will be discussed and the results of structural studies will be used in the explanation of the observed catalytic activity in the model hydroformylation reaction of olefins (1-hexene, unsaturated alcohols) as well as related reactions, i.e. isomerization and hydrogenation.

A new, highly selective, water-soluble rhodium catalyst for methyl acrylate hydroformylation

Abstract Hydroformylation of methylacrylate to α-aldehyde can be achieved in a two-phase system in the presence of two new water-soluble phosphines. High yields and selectivities of α-aldehyde (ca. 80% with a α/β ratio of 1:20) were obtained. Spectroscopic studies have been carried out and some new rhodium complexes formed in situ in catalytic systems have been identified.

Low pressure, highly active rhodium catalyst for the homogeneous hydroformylation of olefins

Abstract Hex-1-ene hydroformylation was examined at pressures of 1–11 atm and 40 °C, using Rh(acac)[P(OPh) 3 ] 2 + P(OPh) 3 as catalyst. The highest efficiency of aldehydes was achieved employing a small P(OPh 3 ) 3 excess-[P(OPh) 3 ]:[Rh] = 2:1. For reactions carried out at 1 atm, very high n/iso ratios i.e. 10–80 were obtained. Pressure increase caused a systematic drop of the n/iso ratio to ca. 5 at 11 atm. Simultaneously with hydroformylation, isomerisation of hex-1-ene to hex-2-ene occurs, but the contribution of the latter declines with increasing pressure. IR examination of the reaction mixture revealed that HRh(CO)[P(OPh) 3 ] 3 was the active form of the catalyst. The same catalytic system was applied in propylene hydroformylation at 5–10 atm pressure and 40 °C. In such conditions the yield of aldehydes was 10–70%, with a n/iso selectivity of 2–10.

New rhodium complexes as low pressure hydroformylation catalysts: effect of ligand on catalyst activity and selectivity

Hydroformylation of hex-1-ene in the presence of Rh(AA)(CO)(PPh3) or Rh(AA)[P(OPh)3]2 type complexes (where AA = β-diketone or 8-hydroxyquinoline) was examined at 12.6 atm pressure (CO/H2 = 1) and 85 °C, in benzene. An excess of the phosphite or the phosphine ligand improved the reaction yield and selectivity. The rate and selectivity of reactions catalyzed by the phosphite (Rh(AA)[P(OPh)3]2) complexes exceeded that of reactions catalyzed by the phosphine (Rh(AA)(CO)(PPh3)). The influence of the free ligand concentration on the reaction course was studied for two selected catalysts. Increasing the phosphine concentration in the system catalyzed by Rh(BAC)(CO)(PPh3) (BAC = benzoylacetone) results in a systematic increase in the reaction yield and selectivity; optimal reaction conditions were found at a ten-fold ligand excess. In the system catalyzed by Rh(ACAC)-P(OPh)3]2 (ACAC = acetylacetone), a small excess of phosphite ([P(OPh)3]: [Rh] < 10:1) favoured fast and efficient reaction, while further increase in the phosphite concentration was found to inhibit the hydroformylation process.

Palladium(0) nanoparticles encapsulated in diamine-modified glycidyl methacrylate polymer (GMA-CHDA) applied as catalyst of Suzuki–Miyaura cross-coupling reaction

Cyclohexyldiamine-modified glycidyl methacrylate polymer (GMA-CHDA) in the form of gel-type beads was used to encapsulate Pd(0) nanoparticles 4–15 nm in diameter and applied as a new, reusable catalyst for the Suzuki–Miyaura cross-coupling reaction of 2- and 4-bromotoluene with phenylboronic acid. It was found that the precatalyst preparation methodology strongly influenced its catalytic activity. The best results (100% yield of the product) were obtained when GMA-CHDA was first treated with hydrazine (reducing agent for Pd(II)) and next with PdCl2 solution. The new catalyst acts heterogeneously, and the post-reaction solution after catalyst separation is not catalytically active, suggesting that there is no leaching.

Rh(acac)(CO)(PR 3 ) and Rh(oxinate)(CO)(PR 3 ) complexes—substitution chemistry and structural aspects

The substitution of CO in Rh(acac)(CO) 2 by the phosphorus ligands P(OPh) 3 , P(NC 4 H 4 ) 3 , and PPh 2 (NC 4 H 4 ) has been studied kinetically by stopped-flow spectrophotometry as a function of temperature. With P(OPh) 3 and P(NC 4 H 4 ) 3 , both CO ligands are replaced in a stepwise fashion via the intermediate Rh(acac)(CO)(PR 3 ). However, the disubstituted complexes Rh(acac)(PR 3 ) 2 are thermodynamically unstable. Judged from the activation parameters, the individual steps are associative processes. In the case of PPh 2 (NC 4 H 4 ) only the monosubstituted complex is formed. The differences in the substitution rates as well as the stability of the various products are largely dominated by electronic (e.g. basicity) effects. X-ray structures of some of the mono-substituted complexes are given. In addition, also the reaction of Rh(oxinate)(CO) 2 with P(OPh) 3 has been studied kinetically showing that oxinate has a labilizing effect relative to acetylacetonate

Infrared and NMR, 1H, 19F, 31P studies of Rh(I) complexes of the formula: [Rh(β-diketone)(CO)X(P)Y] (x = 0, 1, 2; y = 0, 1, 2; × + y = 2; P = PPh3 or P(OP)3)

Products of substitution reactions of CO by PPH3 and P(OP)3 in Rh(β-diketone)(CO)2 complexes (where β-diketone: acetylacetone, thenoyltifluoroacetone, trifluoracetone, benzoyltrifluoroacetone, naphthoyltrifluoroacetone) were examined by IR and NMR. Reactions with PPh3 produced the compounds containing one CO group, i.e. Rh(β-diketone(CO)(PPh3). In the case of asymmetric β-diketones, two isomers were observed in solution. The presence of free phosphine caused labilization of the coordination sphere of complexes followed by fast exchange between the free and the coordinated phosphine. Reactions with P(OPh)3 produced Rh(β-diketone)-[P(OPh)3]2 or Rh(β-diketone)[P(OPh)3] complexes, depending on the amount of P(OPh)3 used. The NMR results indicate considerable delocalization of the electron density in these compounds.

Redox potential, ligand and structural effects in rhodium(I) complexes

The electrochemical behaviour of the set of tetracoordinate rhodium(I) complexes [Rh(O∩O)(CO)L] [O∩O=MeC(O)CHC(O)Me (acac), L=CO (1), P(NC4H4)3 (2), PPh(NC4H4)2 (3), PPh2(NC4H4) (4), PPh3 (5), PCy3 (6), P(OPh)3 (7) or PPh2(C6H4OMe-4) (8); O∩O=PhC(O)CHC(O)Me (bac), L=CO (9) or PPh3 (10); O∩O=PhC(O)CHC(O)CF3(bta), L=CO (11) or PPh3 (12)] and of the pentacoordinate [RhH(CO)L3] [L=P(NC4H4)3 (13), PPh3 (14), P(OPh)3 (15) or P(OC6H4Me-4)3 (16)] and [RhHL4] [L=PPh3 (17) or P(OC6H4Me-3)3 (18)] was studied by cyclic voltammetry and controlled potential electrolysis, in aprotic medium, at a Pt electrode. They present a single-electron oxidation wave (I) (irreversible or quasi-reversible) that can be followed, at a higher potential, by a second and irreversible one (II). The values of first oxidation potential for the tetracoordinate complexes fit the additive Levers electrochemical parameterisation, and the ligand electrochemical Lever EL and Pickett PL parameters were estimated for the N-pyrrolyl phosphines PPhn(NC4H4)3−n (n=0, 1 or 2) and for the organophosphines PCy3 and PPh2(C6H4OMe-4), the former behaving as weaker net electron donors (the electron donor ability decreases with the increase of the number of N-pyrrolyl groups) than the latter phosphines. The pentacoordinate hydride complexes 13–18 fit a distinct relationship which enabled the estimate of the EL ligand parameter for the phosphites P(OC6H4Me-3)3 and P(OC6H4Me-4)3. Electrochemical metal site parameters were obtained for the square planar and the pentacoordinate Rh(I)/Rh(II) couples and, for the former, the redox potential is shown to present a much higher sensitivity to a change of a ligand than the octahedral redox couples investigated so far. Linear relationships were also observed between the oxidation potential and the PL ligand parameter (for the series [Rh(acac)(CO)L]) or the infrared ν(CO) frequency, and a generalisation of the former type of correlation is proposed for series of square-planar 16-electron complexes [M′SL] with a common 14-electron T-shaped binding metal centre {M′S}. Oxidation of 5 by Ag[PF6] leads to the dimerisation of the derived Rh(II) species.

HYDROFORMYLATION AND ISOMERIZATION OF HEX-1-ENE CATALYZED BY RH(ACAC)(CO)(PPH3) : EFFECT OF MODIFYING LIGANDS

Abstract The application of the [Rh(acac)(CO)(PPh3)], [A], complex as a hex-1-ene hydroformylation catalyst (at 1 MPa and 353 K) produced 68% hex-2-ene and 20% aldehydes. Identified via IR in the post-reaction mixture were [Rh4(CO)12], [D], and [Rh6(CO)16], [E], carbonyls, which, applied as catalysts, produced hex-2-ene with a 70–90% yield. The presence of free triphenylphosphine changed the reaction course completely and increased the yield of aldehydes to 80% in all catalytic systems, independent of the catalyst precursor structure. The presence of amines [TBA (tribenzylamine), TFA (triphenylamine), and PhNH2 (aniline)] in reactions catalyzed by [A] and [A] + PPh3 leads to a decrease in the hex-2-ene yield and an increase in the yield of aldehydes.