132 results found
Ho SKY, Ezeorah C, Chari S, et al., 2023, Monitoring light‐driven oxygen insertion reactions into metal carbon bonds by LED‐NMR spectroscopy, ChemPhotoChem, Vol: 7, Pages: 1-6, ISSN: 2367-0932
The facile light-driven insertion reaction of oxygen into metal carbon bonds of the BPI (1,3-bis(2-pyridylimino)isoindole) complexes [Pt(BPI)Me] and [Pd(BPI)Me] has been investigated by LED-NMR in CDCl3. The initial insertion reaction leads to peroxo complexes [Pt(BPI)OOMe] and [Pd(BPI)OOMe], which undergo further reactions over time. Spectra were recorded at 1 minute time intervals, which enabled the tracking of the methyl substituent, which eventually generates formaldehyde (and methanediol) and methanol in almost equal proportions. Degradation of the solvent CDCl3 to phosgene and DCl in the presence of oxygen and light leads to several side reactions. DCl reacts with [M(BPI)Me] and [M(BPI)OOMe] to form [M(BPI)Cl], whereas phosgene reacts with in situ generated methanol to chloro methylformate and dimethyl carbonate.
von Goetze R, Aljaber A, Lee K-Y, et al., 2022, Towards degradable polyethylene: end-functionalised polyethylene (PE-X) and PE-I/LDPE blends from iron-catalysed chain growth of ZnEt2 with ethylene, Polymer Chemistry, Vol: 13, Pages: 6377-6385, ISSN: 1759-9954
A series of end-functionalised polyethylene materials (PE-X) has been prepared via the catalysed chain growth (CCG) reaction of diethyl zinc with ethylene, catalysed by a bis(imino)pyridine iron catalyst activated by MAO. This CCG catalyst system enables the in situ formation of long alkyl chain zinc species Zn(PE)2, which are subsequently quenched to form PE-X. Quenching with oxygen results in PE-OH, but the functionalisation appears to be limited to approximately 80% due to the formation of mixed [ZnR(OR)]n clusters. Functionalisation with sulfur leads to polysulfides, PE-Sk-PE, whereby k is affected by temperature. Functionalisation with iodine leads to PE-I with high conversion, but the degree of functionalisation appears to be chain length dependant. PE-I has been blended with LDPE, either through solution mixing or via melt blending to give PE-I/LDPE blends with different chain lengths. Characterisation of the PE-I/LDPE blends has been carried by IR spectroscopy and thermal analysis (DSC). Surface analysis by FIB-SEM and EDX analysis up to 6 μm into the surface has shown a uniform distribution of PE-I within the LDPE matrix. The propensity of alkyl iodides to undergo photolytic cleavage makes these PE-I/LDPE materials interesting candidates for degradable PE.
Lo Q, Pye D, Gesslbauer S, et al., 2022, Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation, CATALYSIS SCIENCE & TECHNOLOGY, Vol: 12, Pages: 4544-4551, ISSN: 2044-4753
Several PNP-type diphosphine ligands have been synthesised and characterised, featuring a single or a double N-bridge between the P-donor atoms. PNP ligands 1 and 2 containing diazaphospholane donors have been prepared and reaction with [CrCl3(thf)3] results in coordination in a bidentate fashion to give dinuclear complexes [(1)CrCl3]2 and [(2)CrCl3]2 which have been characterised by scXRD analysis. In situ prepared catalysts using ligands 1 and 2 provide good activities and selectivities for the tri- and tetramerisation of ethylene reaching 35% 1-hexene and 61% 1-octene at 5400 g g−1 per Cr per h in the case of 1, and 42% 1-hexene and 55% 1-octene at 17 000 g g−1 (Cr) h−1 in the case of 2, comparable to standard iPrN(PPh2)2-type ligands under similar conditions. Chromium-catalysed ethylene oligomerisations with a doubly N-bridged cyclodiphosphazane ligand (4) result in a Schulz–Flory distribution of α-olefins with relatively low α values of 0.42 and 0.52. Computational studies using DFT on mononuclear chromium complexes of ligands 1 and 2 have shown that the binding of ethylene is favoured in these complexes compared to the benchmark PNP ligand iPrN(PPh2)2 and that the oligomerisation mechanism involves both single and double ethylene insertions.
Gong M, Guo Y, Malko D, et al., 2022, Using molecular oxygen and Fe-N/C heterogeneous catalysts to achieve Mukaiyama epoxidations via in situ produced organic peroxy acids and acylperoxy radicals, Catalysis Science & Technology, Vol: 12, Pages: 2978-2989, ISSN: 2044-4753
Under mild conditions of room temperature and pressure, and using either pure oxygen or air, aldehydes are converted using a heterogeneous Fe–N/C catalyst to produce the corresponding organic peroxy acid and acylperoxy radicals, which forms the epoxide from cyclohexene with high yield (91% for isobutyraldehyde in O2). Real-time monitoring of the rate of oxygen consumption and the electrochemical potential of the Fe–N/C catalyst has been used to study the formation of the peroxy acid and subsequent catalytic epoxidation of cyclohexene. Using isobutyraldehyde, it is shown that the aldehyde and the iron-based carbon catalyst (Fe–N/C) are involved in the rate determining step. Addition of a radical scavenger increases the induction time showing that radicals are initiated by the reaction between the aldehyde and the catalyst. Furthermore, UV-vis spectroscopy with 2,2′-azino-di-(3-ethylbenzthiazoline sulfonic acid) (ABTS) proved the in situ formation of peroxy acid. In the presence of cyclohexene, the peroxy acid leads to the corresponding epoxide with high yield. Monitoring the open circuit potential (OCP) and oxygen flow concurrently follows the production of the peroxy acid. The epoxidation reaction can take place only when the increase in open circuit potential is greater than 0.14 V, suggesting an in situ direct link between the relative oxidative strength of the peroxy acid and the likelihood of epoxidation.
Ho SKY, Lam FYT, de Aguirre A, et al., 2021, Photolytic activation of late-transition-metal-carbon bonds and their reactivity toward oxygen, Organometallics, Vol: 40, Pages: 4077-4091, ISSN: 0276-7333
The photolytic activation of palladium(II) and platinum(II) complexes [M(BPI)(R)] (R = alkyl, aryl) featuring the 1,3-bis(2-pyridylimino)isoindole (BPI) ligand has been investigated in various solvents. In the absence of oxygen, the formation of chloro complexes [M(BPI)Cl] is observed in chlorinated solvents, most likely due to the photolytic degradation of the solvent and formation of HCl. The reactivity of the complexes toward oxygen has been studied both experimentally and computationally. Excitation by UV irradiation (365 nm) of the metal complexes [Pt(BPI)Me] and [Pd(BPI)Me] leads to distortion of the square-planar coordination geometry in the excited triplet state and a change in the electronic structure of the complexes that allows the interaction with oxygen. TD-DFT computational studies suggest that, in the case of palladium, the Pd(III) superoxide intermediate [Pd(BPI)(κ1-O2)Me] is formed and, in the case of platinum, the Pt(IV) peroxide intermediate [Pt(BPI)(κ2-O2)Me]. For alkyl complexes where metal–carbon bonds are sufficiently weak, the photoactivation leads to the insertion of oxygen into the metal–carbon bond to generate alkylperoxo complexes: for example [Pd(BPI)OOMe], which has been isolated and structurally characterized. For stronger M–C(aryl) bonds, the reaction of [Pt(BPI)Ph] with O2 and light results in a Pt(IV) complex, tentatively assigned as the peroxo complex [Pt(BPI)(κ2-O2)Ph], which in chlorinated solvents reacts further to give [Pt(BPI)Cl2Ph], which has been isolated and characterized by scXRD. In addition to the facilitation of oxygen insertion reactions, UV irradiation can also affect the reactivity of other components in the reaction mixture, such as the solvent or other reaction products, which can result in further reactions. Labeling studies using [Pt(BPI)(CD3)] in chloroform have shown that photolytic reactions with oxygen involve degradation of the solvent.
Tay DWP, Nobbs JD, Aitipamula S, et al., 2021, Directing selectivity to aldehydes, alcohols, or esters with diphobane ligands in Pd-catalyzed alkene carbonylations, Organometallics, Vol: 40, Pages: 1914-1925, ISSN: 0276-7333
Phenylene-bridged diphobane ligands with different substituents (CF3, H, OMe, (OMe)2, tBu) have been synthesized and applied as ligands in palladium-catalyzed carbonylation reactions of various alkenes. The performance of these ligands in terms of selectivity in hydroformylation versus alkoxycarbonylation has been studied using 1-hexene, 1-octene, and methyl pentenoates as substrates, and the results have been compared with the ethylene-bridged diphobane ligand (BCOPE). Hydroformylation of 1-octene in the protic solvent 2-ethyl hexanol results in a competition between hydroformylation and alkoxycarbonylation, whereby the phenylene-bridged ligands, in particular, the trifluoromethylphenylene-bridged diphobane L1 with an electron-withdrawing substituent, lead to ester products via alkoxycarbonylation, whereas BCOPE gives predominantly alcohol products (n-nonanol and isomers) via reductive hydroformylation. The preference of BCOPE for reductive hydroformylation is also seen in the hydroformylation of 1-hexene in diglyme as the solvent, producing heptanol as the major product, whereas phenylene-bridged ligands show much lower activities in this case. The phenylene-bridged ligands show excellent performance in the methoxycarbonylation of 1-octene to methyl nonanoate, significantly better than BCOPE, the opposite trend seen in hydroformylation activity with these ligands. Studies on the hydroformylation of functionalized alkenes such as 4-methyl pentenoate with phenylene-bridged ligands versus BCOPE showed that also in this case, BCOPE directs product selectivity toward alcohols, while phenylene-bridge diphobane L2 favors aldehyde formation. In addition to ligand effects, product selectivities are also determined by the nature and the amount of the acid cocatalyst used, which can affect substrate and aldehyde hydrogenation as well as double bond isomerization.
Gragert MM, Tomov AK, Bettonville S, et al., 2020, Biaryl group 4 metal complexes as non-metallocene catalysts for polyethylene with long chain branching, European Journal of Inorganic Chemistry, Vol: 2020, Pages: 4088-4092, ISSN: 1434-1948
A series of biaryl Group 4 complexes with a bidentate and a tridentate pincer ligand have been synthesized and characterized. The complexes have been applied as metallocene analogues for the controlled polymerization of ethylene and the copolymerization of ethylene and 1‐hexene, with a particular focus on the control of the degree of long chain branching in these polymers.
Sang T, Wallis CJ, Hill G, et al., 2020, Polyethylene terephthalate degradation under natural and accelerated weathering conditions, European Polymer Journal, Vol: 136, Pages: 1-13, ISSN: 0014-3057
This review presents an overview of the state of the art in understanding of the chemistries behind polyethylene terephthalate (PET) degradation under hydrolytic and photolytic conditions, which are the main degradation processes that operate in the natural environment. Laboratory studies, carried out under so-called accelerated weathering conditions, have been reviewed and are compared to findings obtained from field studies through hydrolysis and photolysis from natural exposure. The review concludes by correlating the techniques used to date and our current understanding of the mechanisms and key intermediates involved in PET degradation.
Young CT, von Goetze R, Tomov AK, et al., 2020, The mathematics of ethylene oligomerisation and polymerisation, Topics in Catalysis, Vol: 63, Pages: 294-318, ISSN: 1022-5528
Linear α-olefins or LAOs are produced by the catalytic oligomerisation of ethylene on a multimillion ton scale annually. A range of LAOs is typically obtained with varying chain lengths which follow a distribution. Depending on the catalyst, various types of distributions have been identified, such as Schulz–Flory, Poisson, alternating and selective oligomerisations such as ethylene trimerisation to 1-hexene and tetramerisation to 1-octene. A comprehensive mathematical analysis for all oligomer distributions is presented, showing the relations between the various distributions and with ethylene polymerisation, as well as providing mechanistic insight into the underlying chemical processes.Linear α-olefins or LAOs are produced by the catalytic oligomerisation of ethylene on a multimillion ton scale annually. A range of LAOs is typically obtained with varying chain lengths which follow a distribution. Depending on the catalyst, various types of distributions have been identified, such as Schulz–Flory, Poisson, alternating and selective oligomerisations such as ethylene trimerisation to 1-hexene and tetramerisation to 1-octene. A comprehensive mathematical analysis for all oligomer distributions is presented, showing the relations between the various distributions and with ethylene polymerisation, as well as providing mechanistic insight into the underlying chemical processes.
Tay DWP, Nobbs JD, Romain C, et al., 2020, gem-dialkyl effect in diphosphine Ligands: synthesis, coordination behavior, and application in Pd-catalyzed hydroformylation, ACS Catalysis, Vol: 10, Pages: 663-671, ISSN: 2155-5435
A series of palladium complexes with C3-bridged bidentate bis(diphenylphosphino)propane ligands with substituents of varying steric bulk at the central carbon have been synthesized. The size of the gem-dialkyl substituents affects the C–C–C bond angles within the ligands and consequently the P–M–P ligand bite angles. A combination of solid-state X-ray diffraction (XRD) and density functional theory (DFT) studies has shown that an increase in substituent size results in a distortion of the 6-membered metal–ligand chair conformation toward a boat conformation, to avoid bond angle strain. The influence of the gem-dialkyl effect on the catalytic performance of the complexes in palladium-catalyzed hydroformylation of 1-octene has been investigated. While hydroformylation activity to nonanal decreases with increasing size of the gem-dialkyl substituents, a change in chemoselectivity toward nonanol via reductive hydroformylation is observed.
Vriamont CEJJ, Chen T, Romain C, et al., 2019, From lignin to chemicals: Hydrogenation of lignin models and mechanistic insights into hydrodeoxygenation via low-temperature C-O bond cleavage, ACS Catalysis, Vol: 9, Pages: 2345-2354, ISSN: 2155-5435
The catalytic hydrogenation of a series of lignin model compounds, including anisole, guaiacol, 1,2-dimethoxybenzene, 4-propyl-2-methoxyphenol, and syringol, has been investigated in detail, using a Ru/C catalyst in acetic acid as the solvent. Both hydrogenation of the aromatic unit and C–O bond cleavage are observed, resulting in a mixture of cyclohexanes and cyclohexanols, together with cyclohexyl acetates due to esterification with the solvent. The effect on product composition of the reaction parameters temperature (80–140 °C), pressure (10–40 bar), and reaction time (0.5–4 h) has been evaluated in detail. The lignin model compound 4-propyl-2-methoxyphenol was converted to 4-propylcyclohexanol in 4 h at 140 °C and 30 bar of H2 pressure with 84% conversion and 63% selectivity. Mechanistic studies on the reactivity of reaction intermediates have shown that C–O bond cleavage under these relatively mild conditions does not involve a C–O bond hydrogenolysis reaction but is due to elimination and hydrolysis reactions (or acetolysis in acetic acid solvent) of highly reactive cyclohexadiene- and cyclohexene-based enols, enol ethers, and allyl ethers.
Malko D, Guo Y, Jones P, et al., 2019, Heterogeneous iron containing carbon catalyst (Fe-N/C) for epoxidation with molecular oxygen, Journal of Catalysis, Vol: 370, Pages: 357-363, ISSN: 0021-9517
Pyrolized transition metal and nitrogen containing carbon materials (M-N/C) have shown promising activities as electrocatalysts for oxygen reduction reactions (ORR) in fuel cell cathodes. Similar materials have recently gained interest as heterogeneous catalysts. We report that ORR-active heterogeneous M-N/C materials can catalyze the chemical epoxidation of olefins with molecular oxygen and two equivalents of aldehyde at room temperature and ambient pressure. The observed yield and selectivity is higher than that for homogeneous analogues and the catalysts achieve TOF > 2700 h−1 and TON > 16,000. The ability to recycle the catalyst several times is also demonstrated.
Shahruddin S, Jimenez-Serratos G, Britovsek G, et al., 2019, Fluid-solid phase transition of n-alkane mixtures: coarse-grained molecular dynamics simulations and diffusion-ordered spectroscopy nuclear magnetic resonance, Scientific Reports, Vol: 9, Pages: 1-9, ISSN: 2045-2322
Wax appearance temperature (WAT), defined as the temperature at which the first solid paraffin crystal appears in a crude oil, is one of the key flow assurance indicators in the oil industry. Although there are several commonly-used experimental techniques to determine WAT, none provides unambiguous molecular-level information to characterize the phase transition between the homogeneous fluid and the underlying solid phase. Molecular Dynamics (MD) simulations employing the statistical associating fluid theory (SAFT) force field are used to interrogate the incipient solidification states of models for long-chain alkanes cooled from a melt to an arrested state. We monitor the phase change of pure long chain n-alkanes: tetracosane (C24H50) and triacontane (C30H62), and an 8-component surrogate n-alkane mixture (C12-C33) built upon the compositional information of a waxy crude. Comparison to Diffusion Ordered Spectroscopy Nuclear Magnetic Resonance (DOSY NMR) results allows the assessment of the limitations of the coarse-grained models proposed. We show that upon approach to freezing, the heavier components restrict their motion first while the lighter ones retain their mobility and help fluidize the mixture. We further demonstrate that upon sub-cooling of long n-alkane fluids and mixtures, a discontinuity arises in the slope of the self-diffusion coefficient with decreasing temperature, which can be employed as a marker for the appearance of an arrested state commensurate with conventional WAT measurements.
Ragauskas AJ, Williams CK, Davison BH, et al., 2018, The path forward for biofuels and biomaterials, Renewable Energy: Four Volume Set, Pages: 271-283, ISBN: 9781844078677
Fernandez-Alvarez VM, Ho SKY, Britovsek GJP, et al., 2018, A DFT-based mechanistic proposal for the light-driven insertion of dioxygen into Pt(ii)-C bonds, CHEMICAL SCIENCE, Vol: 9, Pages: 5039-5046, ISSN: 2041-6520
Shahruddin S, Jimenez-Serratos G, Britovsek G, et al., 2018, Exploring wax precipitation in crude oils through diffusion ordered spectroscopy nuclear magnetic resonance combined with molecular dynamics simulations, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Britovsek G, Tomov A, Muller E, et al., 2018, Ethylene oligomerisation beyond Schulz-Flory distributions, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Britovsek GJP, Merlini ML, Swart M, et al., 2018, Understanding the Catalase-Like Activity of a Bioinspired Manganese(II) Complex with a Pentadentate NSNSN Ligand Framework. A Computational Insight into the Mechanism, ACS Catalysis, ISSN: 2155-5435
Nobbs JD, Tomov AK, Young CT, et al., 2018, From alternating to selective distributions in chromium-catalysed ethylene oligomerisation with asymmetric BIMA ligands, Catalysis Science and Technology, Vol: 8, Pages: 1314-1321, ISSN: 2044-4753
The oligomerisation of ethylene with chromium-based catalysts containing asymmetric BIMA (bis(benzimidazole)methylamine) ligands produces linear alpha olefins (LAOs) that follow an alternating distribution. The catalytic activity and the degree of alternation is affected by the different ligands; in particular variations at the backbone of the ligand affect the nature of the distribution. For certain catalysts a deviation from regular alternating behaviour is observed, whereby increased amounts of 1-hexene and 1-octene (up to 29 mol%) are obtained compared to the amount expected from the distribution analysis based on C₁₀–C₃₄ LAOs. This behaviour towards more selective oligomerisation to 1-hexene and 1-octene can be explained by varying probabilities of single and double ethylene insertion. The deviations will depend on the size of the metallacycle and are most pronounced early on during the metallacycle growth.
Britovsek GJP, McGuinness DS, Tomov AK, 2016, Mechanistic study of ethylene tri- and tetramerisation with Cr/PNP catalysts: effects of additional donors, Catalysis Science and Technology, Vol: 6, Pages: 8234-8241, ISSN: 2044-4753
The mechanism of ethylene trimerisation and tetramerisation with chromium–diphosphinoamine (Cr–PNP)catalysts has been studied by experimental and theoretical (DFT) methods. The effects of a pendant etherdonor (ortho-methoxyaryl ligand substitution) and of anion coordination to the active species have beenstudied. In the former case, coordination of the ether donor to chromium favours 1-hexene by suppressingformation of the bisĲethylene) chromacyclopentane intermediate which is postulated to be the major routeto 1-octene. The effect of anion coordination is similar and as the coordination strength increases, displacementof the anion by a second ethylene ligand becomes more difficult, again favouring trimerisationover tetramerisation. Hence, the experimentally observed effects of pendant donor coordination andchanges in anion coordination strength can be rationalised.
Tomov AK, Nobbs JD, Chirinos JJ, et al., 2016, Alternating alpha-olefin distributions via single and double insertions in chromium-catalyzed ethylene oligomerization., Organometallics, Vol: 36, Pages: 510-522, ISSN: 0276-7333
The catalytic oligomerization of ethylene with chromium-based complexes containing bis(benzimidazolemethyl)amine (BIMA) ligands results in alternating distributions of linear α-olefins (LAOs). Extremely high activities are obtained (>100 000 g mmol–1 h–1 bar–1) with N-alkyl-substituted BIMA ligands, whereas bulky groups on the central nitrogen or alternative central donors result in much lower activities. Variations in the ligand backbone, as well as methylation of the benzimidazole units, lead to reduction in activity. The alternating LAO distributions have been mathematically analyzed using second-order recurrence relations. The shape of the distributions is affected by ethylene pressure (1–4 bar) and by the cocatalyst to some degree. On the basis of the results and analysis presented herein, we propose that the alternating behavior originates from the ability of these chromium BIMA catalysts to undergo single as well as double ethylene insertion reactions. A minor second distribution (<5 wt %) of 2-ethyl-1-alkenes is obtained under certain conditions, resulting from incorporation of 1-butene. DFT studies (M06L) and experimental observations regarding the reaction between AlMe3 and the N-methyl BIMA ligand 2 have shown that deprotonation of the benzimidazole N–H units can occur, which suggests a change in coordination of the BIMA ligand under oligomerization conditions.
Britovsek GJ, McGuinness DS, 2016, A DFT mechanistic study on ethylene tri- and tetramerization with Cr/PNP catalysts: single versus double insertion pathways, Chemistry - A European Journal, Vol: 22, Pages: 16891-16896, ISSN: 0947-6539
The mechanism of ethylene trimerization and tetramerization with a chromium-diphosphinoamine (Cr-PNP) catalyst system has been studied by theoretical (DFT) methods. Two representative ligands have been explored, namely Ph2 PN(Me)PPh2 and (o-MeC6 H4 )2 PN(Me)P(o-MeC6 H4 )2 . Calculations on the former ligand reveal how a combination of single and double ethylene insertion mechanisms may lead to 1-hexene, 1-octene and the major side products (cyclopentanes and n-alkanes). For the latter ligand, introduction of o-alkyl substitution leads to a more sterically congested active species, which suppresses the available pathways for tetramerization and side product formation. Hence, the high selectivity of o-aryl substituted PNP ligands for trimerization can be rationalized.
Petersen AR, White AJ, Britovsek GJ, 2016, Divergent reactivity of platinum(II) and palladium(II) methylperoxo complexes and the formation of an unusual hemi-aminal complex, Dalton Transactions, Vol: 45, Pages: 14520-14523, ISSN: 1477-9226
The 6,6′′-diaminoterpyridine palladium(II) methylperoxo complex eliminates methyl hydroperoxide and reacts with acetone to form a novel hemi-aminal palladium complex, whereas the analogous platinum(II) complex generates formaldehyde and a platinum(II) hydroxo complex.
Martinez-Ferrate O, Lopez-Valbuena JM, Martinez Belmonte M, et al., 2016, Novel iminopyridine derivatives: ligands for preparation of Fe(II) and Cu(II) dinuclear complexes, Dalton Transactions, Vol: 45, Pages: 3564-3576, ISSN: 1477-9226
A series of imino- and amino-pyridine ligands based on dihydrobenzofurobenzofuran (BFBF) and methanodibenzodioxocine (DBDOC) backbones have been synthesized. These ligands form exclusively dinuclear complexes with metals such as iron(II) and copper(II). The structures for complexes 15, 16, 18, 19, 20, 21, 23, and 24 were determined by X-ray crystallography. The complexes show large distances for the metal nuclei and different geometries depending on the nature of the metal. An octahedral geometry was observed for the iron(II) complexes, while copper(II) complex 24 showed a distorted trigonal bipyramidal geometry. The iron(II) complexes showed activity as catalysts in the cycloaddition of CO2 to epoxides, obtaining moderate yields of cyclic carbonates.
Britovsek GJP, Malinowski R, McGuinness DS, et al., 2015, Ethylene Oligomerisation Beyond Schulz-Flory Distributions, ACS Catalysis, Vol: 5, Pages: 6922-6925, ISSN: 2155-5435
The oligomerisation of ethylene producesα-olefin distributions ranging from Schulz-Flory distributionsto alternating and selective oligomer distributionswhich can be mathematically analysed and characterisedby recurrence relations.
Britovsek GJP, McGuinness DS, Wierenga TS, et al., 2015, Single- and Double-Coordination Mechanism in Ethylene Tri- and Tetramerization with Cr/PNP Catalysts, ACS Catalysis, Vol: 5, Pages: 4152-4166, ISSN: 2155-5435
The mechanism of ethylene trimerization and tetramerization with a chromium–diphosphinoamine (Cr–PNP) catalyst system has been studied with combined experimental and theoretical methods. Of the total product output, 1-octene, cyclopentanes, n-alkanes, and higher (C10+) olefins are formed with a fractional (∼1.4) order response to ethylene concentration, whereas 1-hexene formation is approximately first-order in ethylene. Theoretical studies suggest a mechanism involving a cationic monometallic catalyst in Cr(I) and Cr(III) formal oxidation states. A key feature of the developed model is the occurrence of a double-coordination mechanism in which a bis(ethylene) chromacyclopentane intermediate is responsible for 1-octene formation as well as the other coproducts that have a greater than first-order response to ethylene. In contrast, 1-hexene is formed primarily from a mono(ethylene) chromacyclopentane intermediate. The selectivity of catalysis is governed by the competition between single- and double-coordination pathways. The mechanistic model developed displays excellent correlation with experimental observations and is able to fully explain the formation of all products generated with this catalyst.
Britovsek GJP, Grau M, 2015, Iron Catalysis II, Publisher: Springer
Grau M, Britovsek GJP, 2015, High-valent iron in biomimetic alkane oxidation catalysis, IRON CATALYSIS II, Editors: Bauer, Publisher: SPRINGER-VERLAG BERLIN, Pages: 145-171, ISBN: 978-3-319-19395-3
The combination of iron salts or complexes with strong oxidants such as hydrogen peroxide results in the formation of high-valent iron oxo species, the nature of which has been under discussion in the chemical literature for more than a century. Recent advances in the design and development of molecular iron-based oxidation catalysts and their mechanistic understanding are summarised in this chapter, in particular iron complexes featuring tetradentate and pentadentate ligands. Inspired by enzymatic systems based on heme and nonheme ligand environments, the development of biomimetic iron-based catalysts for the selective oxidation of alkanes and alkenes can potentially be applied in a range of areas, from late stage functionalisation of natural product synthesis to large-scale oxidation of hydrocarbons.
Martínez-Ferraté O, Britovsek GJP, Claver C, et al., 2015, C–H benzylic oxidation promoted by dinuclear iron DBDOC iminopyridine complexes, Inorganica Chimica Acta, ISSN: 0020-1693
Petersen AR, Taylor RA, Vicente-Hernandez I, et al., 2014, Oxygen insertion into metal carbon bonds: formation of methylperoxo Pd(II) and Pt(II) complexes via photogenerated dinuclear intermediates, Journal of the American Chemical Society, Vol: 136, Pages: 14089-14099, ISSN: 1520-5126
Platinum(II) and palladium(II) complexes [M(CH₃)(L)]SbF₆ with substituted terpyridine ligands L undergo light-driven oxygen insertion reactions into metal methyl bonds resulting in methylperoxo complexes [M(OOCH₃)(L)]SbF₆. The oxygen insertion reactions occur readily for complexes with methyl ligands that are activated due to steric interaction with substituents (NH₂, NHMe or CH₃) at the 6,6″-positions on the terpyridine ligand. All complexes exhibit attractive intermolecular π···π or M···M interactions in the solid state and in solution, which lead to excited triplet dinuclear M–M complexes upon irradiation. A mechanism is proposed whereby a dinuclear intermediate is generated upon irradiation that has a weakened M–C bond in the excited state, resulting in the observed oxygen insertion reactions.
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