Andrea Meli

Alternating copolymerization of carbon monoxide and olefins by single-site metal catalysis

Abstract The alternating copolymerization of carbon monoxide and olefins is a reaction that transition metal complexes, generally containing palladium, catalyze in different phase variation systems. The polyketone products are not only low cost thermal plastics that can be made but also polymeric materials featured by unique chemical and physical properties. These properties can be finely tuned by an appropriate choice of the catalyst and olefinic comonomer. After introducing the topic, the specific metal catalysts for each type of olefinic substrate are described together with the reaction mechanisms involved.

A comparison between silica-immobilized ruthenium(II) single sites and silica-supported ruthenium nanoparticles in the catalytic hydrogenation of model hetero- and polyaromatics contained in raw oil materials

A comparative study of the hydrogenation of various heterocycles, model compounds in raw oil materials, by either Ru(II) complex immobilized on mesoporous silica or Ru(0) nanoparticles deposited on the same support has been performed. The single-site catalyst contains the molecular precursor [Ru(NCMe)3(sulphos)](OSO2CF3) tethered to partially dehydroxylated high-surface-area silica through hydrogen bonds between silanol groups of the support and SO3− groups from both the sulphos ligand [−O3S(C6H4)CH2C(CH2PPh2)3] and the triflate counter anion. Highly dispersed ruthenium nanoparticles were prepared by calcination/reduction of silica-supported Ru3(CO)12. The heterocycles (benzo[b]thiophene, quinoline, indole, acridine) are hydrogenated to cyclic thioethers or amines. The Ru(II) single-site catalyst is active for both benzo[b]thiophene and the N-heterocycles, while the Ru(0) catalyst does not hydrogenate the S-heterocycle, yet is efficient for the reduction of the N-heterocycles and simple aromatic hydrocarbons. The surface silanols promote the hydrogenation of indole via NH⋯ O(H)Si hydrogen bonds and can interact with the π-electron density of all substrates.

Modelling the Hydrodenitrogenation of Aromatic N‐Heterocycles in the Homogeneous Phase

Recent legislation directed at reducing nitrogen levels in fossil fuels has led to numerous studies of the hydrodenitrogenation (HDN) process, whereby nitrogen is removed from organic compounds in petroleum feedstocks. In this review a comprehensive study of the coordination, activation, hydrogenation, hydrogenolysis and denitrogenation of N-heterocycles assisted by transition metal complexes in either fluid solution or single-site systems, is reported. Furthermore, similarities to the reactions occurring in the HDN process over commercial heterogeneous catalysts are also discussed.

Dioxygen uptake and transfer by Co(III), Rh(III) and Ir(III) catecholate complexes

Abstract A large number of five-coordinate metal catecholate complexes of the general formula [(triphos)M(Cat)]Y have been synthesized and characterized by chemical, spectroscopic and electrochemical techniques (MCo, Rh, Ir; Cat=9,10-phenathrenecatecholate, 1,2-naphthalencatecholate, 3,5-di-tert-butylcatecholate, 4-methylcatecholate, 4- carboxycatecholate-ethylester, tetrachlorocatecholate; YBPh4, PF6; triphos=MeC(CH2PPh2)3). All of the compounds undergo electron-transfer reactions that encompass the M(III), M(II) and M(I) oxidation states of the metal, and the catecholate, semiquinone and quinone oxidation levels of the quinoid ligand. Paramagnetic Ir(III) semiquinonate complexes, [(triphos)Ir(SQ)]2+, and Ir(II) catecholates, [(triphos)Ir(Cat)], have been characterized by X-band ESR spectroscopy. The reactions of the metal catecholates in non-aqueous media with dioxygen have been investigated. With very few exceptions, all of the compounds react with O2 to give adducts of the general formula [(triphos) M(O O)(S Q)]Y. An X-ray analysis has been carried out on [(triphos) Ir(O O)(P henSQ)]BPh4, (Phen=9, 10-phenanthrenesemiquinonate). In the complex cation, the metal is octahedrally coordinated by the three phosphorus atoms of triphos and by three oxygen atoms, one from O2 and the other two from the catecholate ligand that has attained a semiquinoid character. The electrochemical behavior of the dioxygen adducts has been studied in detail. Depending on the E°′ values relative to the MIII(SQ)/MIII(Cat) couples of the parent metal catecholates, the dioxygen adducts undergo either a one-electron oxidation to give o-quinone complexes [(triphos) M(Q)]3+ and superoxide ion (O2−) or a two-electron oxidation to give [(triphos)M(Q)]3+ and O2. Several factors have been found to affect the O2 uptake by metal catecholates. Of particular importance are: (i) the coordination number of the metal; (ii) the basicity of either the catecholate ligand or metal; (iii) the temperature; (iv) the pressure of dioxygen. The role of each factor has been analyzed and rationalized. The transport of dioxygen from one metal catecholate to another has been studied. A mechanistic interpretation for the formation of the [(triphos) M(O O)(S Q)]+ complexes is proposed in light of a large crop of experimental data and molecular orbital considerations.

Isomerization of allylic alcohols to carbonyl compounds by aqueous-biphase rhodium catalysis

Isomerization of allylic and homoallylic alcohols is catalyzed by the zwitterionic Rh(I) complex (sulphos)Rh(cod) in water–n-octane to give the corresponding aldehyde or ketone in high yields and chemoselectivity. A π-allyl metal hydride mechanism is proposed on the basis of various independent experiments in both homogeneous and biphasic systems [sulphos=−O3S(C6H4)CH2C(CH2PPh2)3].

Hydrogenation of Quinoline by Rhodium Catalysts Modified with the Tripodal Polyphosphine Ligand MeC(CH2PPh2)3

As part of our modelling studies of the hydrodenitrogenation of N-heterocycles contained in raw oil materials, we investigated the selective hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline by rhodium catalysts modified with the tripodal polyphosphane ligand MeC(CH2PPh2)3. Experiments in standard autoclaves and in high-pressure sapphire NMR tubes, kinetic and isotope labelling studies, and independent reactions with isolated compounds have contributed to the elucidation of the catalytic mechanism as well as identification of the electronic requisites of the metal catalyst for selective and efficient hydrogenation.

Bis-alkoxycarbonylation of styrene by pyridinimine palladium catalysts

Pyridinimine-modified Pd(II) complexes of general formulae (N-N′)Pd(Y)2 catalyze the methoxycarbonylation of styrene to give dimethyl phenylsuccinate as the largely major product [N-N′ = py-2-C(R)N(2,6-R′C6H3), R = H, Me; R′ = Me, i-Pr; 6-Mepy-2-C(H)N[2,6-(i-Pr)2C6H3]; py-2-C(H)N(C6H5); Y = acetate, trifluoroacetate]. The influence of various catalytic parameters on the overall conversion of styrene to carbonylated products and on the product selectivity has been studied by systematically varying the type of palladium initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid (p-toluenesulfonic acid), and the CO pressure. By an appropriate choice of the structure of the pyridinimine ligand and of the reaction parameters, turn-over numbers as high as 96 and selectivities in dimethyl phenylsuccinate as high as 98% were obtained. In particular, the overall conversion of styrene is controlled by the steric properties of the alkyl substituents on the imine aryl group, while the nature of the substituent (H or Me) on the imine carbon influences the selectivity. The addition of 2 equivalents of TsOH to the catalytic mixtures generally increased the styrene conversion but lowered the selectivity in dimethyl phenylsuccinate due to greater production of methyl 3,6-diphenyl-4-oxohexanoate. Further additions of TsOH (up to 6 equivalents) resulted in better selectivities and lower conversions for all precursors.

Copolymerization of carbon monoxide with ethene catalyzed by bis-chelated palladium(II) complexes containing diphosphine and dinitrogen ligands

Several bis-chelated palladium(II) complexes, [Pd(P-P)(N-N)x](PF6)2, containing binary combinations of diphosphine and dinitrogen ligands have been prepared and characterized. The diphosphine ligands comprise 1,3-bis(diphenylphosphino)propane (dppp), meso-2,4-bis(diphenylphosphino)pentane (meso-bdpp), rac-2,4-bis(diphenylphosphino)pentane (rac-bdpp) and 2,2′-bis(diphenylphosphinoethyl)pentane (Etdppp), while the dinitrogen ligands are either 2,2′-bipyridine (bipy; x=1) or 1,8-naphthyridine (napy; x=2). The structure of [Pd(meso-bdpp)(N,N′-bipy)](PF6)2·CH2Cl2 has been determined by an X-ray structural analysis. All the Pd(II) complexes have been tested as catalyst precursors for the copolymerization of carbon monoxide and ethene in methanol solution in either autoclaves or high-pressure sapphire NMR tubes. The combination of meso-bdpp and bipy at palladium, in conjuction with both 1,4-benzoquinone and p-toluenesulfonic acid, has shown the best catalytic performance. The different catalytic activities exhibited by the stereoisomers [Pd(meso-bdpp)(N,N′-bipy)]- (PF6)2 and [Pd(rac-bdpp)(N,N′-bipy)](PF6)2 has been interpreted in terms of the different spatial distribution of the phenyl rings around the metal center determined by the conformation of the six-membered metallaring.

Reactivity of the triethylphosphinecarbon disulfide adduct Et3PCS2 toward iron(II) cations in the presence of the bis(tertiary phosphines), depe, and diphos, x-ray crystal structures of the complexes [(depe)2Fe(S2CPEt3)](BPh4)2 and [(depe)2Fe(S2CH)](BPh4)

Abstract Reaction of the triethylphosphinecarbon disulfide adduct Et 3 PCS 2 with iron(II) aquocations in the presence of the bis(tertiary phosphines) depe or diphos, and NaBPh 4 has given the monomeric complexes [(depe) 2 Fe(S 2 CPEt 3 )](BPh 4 ) 2 , 1 , and [(diphos)Fe(S 2 CPEt 3 ) 2 ](BPh 4 ) 2 · 0.5 (CH 3 ) 2 CO, 3 (depe = 1,2-bis(diethylphosphino)ethane; diphos = 1,2-bis(diphenylphosphino)ethane). Compound 1 undergoes nucleophilic attack by hydride ion on the coordinated Et 3 PCS 2 ligand to give the dithioformato derivative [(depe) 2 Fe(S 2 CH)](BPh 4 ), 2 . The structures of compounds 1 and 2 have been determined from counter diffraction X-ray data. Crystal data for 1 are: triclinic, space group P 1 , a 18.583(8), b 14.836(7), c 14.019(7) », α 94.38(4), β 102.56(4), γ 103.34(4)°, Z = 2, R = 0.080. Crystal data for 2 are: monoclinic, space group P 2 1 / a , a 28.912(10), b 11.949(6), c 13.386(7) », β 94.35(5)°, Z = 4, R = 0.076. In both structures the iron atom displays a distorted octahedral geometry, being linked to the phosphorus atoms of two depe molecules and to the two sulfur atoms of the zwitterion in compound 1 or of the dithioformato group in compound 2 .

Efficient rhodium catalysts for the hydrogenolysis of thiophenic molecules in homogeneous phase

Abstract In the presence of strong bases, the CS insertion complexes (triphos)Rh[ η 3 -S(C 6 H 4 )CHCH 2 ] and (triphos)Rh( η 3 -SCHCHCHCH 2 ) as well as the π-alkyne complex [(triphos)Rh( η 2 -MeO 2 CCCCO 2 Me)]PF 6 are catalyst precursors for the hydrogenation of thiophene (T), benzo[ b ]thiophene (BT) and dibenzo[ b , d ]thiophene (DBT) in tetrahydrofuran solution [triphos = MeC(CH 2 PPh 2 ) 3 ]. Both hydrogenolysis (thiols) and desulfurization (hydrocarbons) products are obtained. Among the substrates investigated, BT is the most reactive, whereas T is the easiest to desulfurize.