Tentative of determination of the acidity level in room temperature ionic liquids by electrochemical methods

peer reviewe

Alkene Oligomerization via metallacycles: Recent Advances and Mechanistic Insights

International audienceThe transformation of ethylene and alkenes is of high importance for the chemical industry. In this review, we focus on selective alkenes transformation where metallacyclopentane is suspected or demonstrated to be involved in the reaction mechanism. In addition to the alkenes, the "classical" products of ethylene oligomerization, we also cover articles dealing with the synthesis of cyclobutane and butadiene derivatives, through the common metallacycle intermediate. We also present studies that help decipher the precise mechanism of the transformations, i.e. involving synthesis of postulated intermediates, labelling experiments and DFT calculations

Role of Homogeneous Catalysis in Oligomerization of Olefins : Focus on Selected Examples Based on Group 4 to Group 10 Transition Metal Complexes

International audienceHomogeneous olefin oligomerization plays a pivotai role in the field of petrochemistry. Through catalysts, technology and process developments, market requirements in terms of productivity, selectivity and sustainability have been addressed. Over more than 50 years, an intensive research has been devoted to the design of new Group 4 to Group 10 transition metal complexes and to the study of their reactivity towards olefins leading to severa! breakthroughs of prime importance for academy as for industry. Since the early sixties, IFPEN contributed to bring innovative industrial solutions to different targets from gasoline production to alpha-olefin on purpose processes with over 100 production units built worldwide . Based on nickel, titanium, zirconium or chromium, the catalytic systems for such processes and their next generation are subject to a continuous research where the adaptation of the ligand architecture to the nature of the metal and its mode of activation, play a crucial role to control the reaction selectivity and the catalyst lifetime. lnteresting relationships between the complex structure and their reactivity have been drawn and will be discussed on selected examples

Hammett Acidity Scale in Ionic Liquids : An Indication of Their Weak Dissociating Character

Ionic liquids are salts with the particularity to exhibit melting points near room temperature (below 100°C, by convention) with no vapour tension. For this last reason, ionic liquids are sometimes called “green solvents”. In addition, their exclusive materials and solvent properties has led to an amazing increase of interest from both academic and industrial community, confirmed by the explosion of the number of published papers in the last decade. The many combinations of organic and inorganic cations and anions allow an infinity of new ionic solvents then permitting the selection of the desired properties for a given application. Nevertheless, it is impossible to investigate all these combinations and the unusual complexity of these new solvents gives rise to many controversies. Consequently, the development of the general rules for understanding the chemistry in ionic liquids is crucial. A fundamental property of solvent is its solvating power, for instance towards the proton. Therefore, we are interested to investigate the acid-base properties in ionic liquids in order to ultimately find a correlation with the acidic catalysis activity. We then have proposed a colorimetric method to determine the acidity levels accessible in these new media: the Hammett acidity function H0. This spectroscopic method is based on the protonation equilibrium for a family of coloured indicator with pKa’s assumed as solvent independent (following the Hammett proposition). This presentation will summarize our Hammett acidity measurements in several ionic liquids. - At first, we will show that it is possible to evaluate the Hammett acidity function with two different coloured indicators, in the same ionic liquid. Since the Hammett acidity of a given mixture was found to depend on the choosen coloured indicator, this suggests the formation of ions associations in ionic liquids. As a result, the ionic liquids are clearly not as dissociating as initially thought and the Hammett acidity function is in fact an apparent function, underestimating the real acidity level. - The apparent acidity functions have then been compared for several ionic liquids to which an acid has been added ([BMIm][NTf2], [BMIm][BF4], [BMIm][OTf], [BHIm][NTf2], [BMIm][PF6], [HNEt3][NTf2]). The conclusions are as follows: 1) the accessible acidity level is not influenced by the nature of the cation; 2) on the contrary, the nature of anion is very critical and the solvating power towards the proton follows the order: OTf- > NTf2- > BF4- > PF6-. The more the proton is solvated, the less it is acidic. - Finally, the difference of acidity of two acids, HOTf and HNTf2 (both strong acids in water), has been investigated in [BMIm][BF4], [BMIm][NTf2] and [BMIm][OTf]. In [BMIm][OTf], these two acids show the same acidity (they behave as strong acids) due to the solvent levelling effect; on the other hand, in [BMIm][NTf2] and [BMIm][BF4] allowing higher acidity levels, HNTf2 is stronger than HOTf . The observed difference is also another indication of the lower proton solvation in [BMIm][BF4] or [BMIm][NTf2] versus that in [BMIm][OTf]

A Comparison of the Acidity Levels in Room-Temperature Ionic Liquids

The Broensted acidity level was evaluated for several ionic liqs. ([BMIm][BF4], [BMIm][PF6], [BMIm][SbF6], [BMIm][OTf], [BMIm][NTf2], [HNET3][NTf2], and [HBIm][NTf2]) to which a strong acid, such as HNTf2 [NTf2=N(CF3SO2)2] and HOTf (OTf=CF3SO3), has been added. The main purpose is to tentatively measure the influence on the resulting acidity of (i) the solvent anion or cation structure and (ii) the added acid nature. The evaluation method is based on the detn. of the Hammett acidity functions H0 using UV/visible spectroscopy. The acidity of protons is mainly detd. by their solvation state, and consequently, the properties of protons depend on both the nature of the solvent and the nature and concn. of the acid. In practice, for the investigated ionic liqs., the cation as well as the added acid nature does not play a dominant role, whereas changing the anion nature may lead to very different acidities. Indeed, for a similar content of added acid, the measured acidity levels are in the order PF6- > BF4- > NTf2- > OTf-. The problems of the influence of impurities on the final acidity and of the dissocg. character of the ionic liq. are addressed

Synthesis of L2Ni(ORF)2 (RF=C(CF3)3) Complexes and their Reactivity in Ethylene Oligomerization

International audienceA family of L2Ni(ORF)2 (L2: (Cy3PO)24, dcpmS 5, dppf 9, bipyMe210; RF = C(CF3)3) complexes is synthesized via selective substitution of 2 equiv of (DME)NaORF from homoleptic [Ni(ORF)4][Na(DME)]2 complex 1, all characterized by 19F and 1H NMR and SCXRD analyses as well as elemental analyses. These L2Ni(ORF)2 precursors, activated by 2 equiv of PhF → Al(ORF)3, were active in ethylene oligomerization with selectivity toward butenes up to 97% and activities ranging from 10 to 50 kgC2H4·gNi·h–1. Mechanistic investigations, involving experiments with C2H4/C2D4 (1/1) coupled with GC-MS analysis, revealed the formation of a Ni–H fragment in the catalytic process. The L2Ni(ORF)2/2PhF → Al(ORF)3 catalytic system thus dimerizes ethylene through a Cossee–Arlman mechanism

About the Acidity Level in Room Temperature Ionic Liquids

peer reviewedThe Brønsted acidity level was evaluated for ionic liquids to which a strong acid has been added. The evaluation method is based on the determination of the Hammett acidity functions H0, using UV-Visible spectroscopy. The acidity of protons is mainly determined by their solvation state and consequently, the properties of protons depend on both the nature of the solvent and the nature and concentration of the acid. In practice, it was found that, for the investigated ionic liquids, the cation as well as the added acid nature does not play a dominant role, whereas changing the anion nature may lead to very different acidities. Indeed, for a similar content of added acid, the measured acidity levels are in the order: PF6- > BF4- > NTf2- > OTf-. The problems of the influence of impurities on the final acidity and of the weakly dissociating character of the ionic liquid are addressed

Taming the Lewis Superacid Al(OR F )3 (R F =C(CF3)3): DFT Guided Identification of the "Stable yet Reactive" Adduct S i Pr2→Al(OR F )3; Its Use as OR F-Abstractor from a "Ni-OR F " complex

International audienceA DFT study of several L→Al(ORF)3 (L=Lewis bases) adducts allowed the identification of (iPr2S)→Al(ORF)3 1-SiPr2 as a “stable yet reactive” adduct. 1-SiPr2 was shown to act as a masked Lewis superacid able to release Al(ORF)3 under mild conditions. It could be used to abstract a ORF− ligand from (bipyMe2)Ni(ORF)2 (bipyMe2 : 6,6’-dimethyl-2,2’-dipyridyl) and generate the nickel alkoxide complex [(bipyMe2)Ni(ORF)(iPr2S)]+[(RFO)3Al−F−Al(ORF)3]− 5. Ligand exchange of iPr2S by Ph3P yielded [(bipyMe2)Ni(ORF)(PPh3)]+[(RFO)3Al−F−Al(ORF)3]− 6

The Hydrogen Electrode in Ionic Liquids: Acidity Measurements and Titrations

The acidity level in ILs containing acid was first determined using the Hammett acidity function (H0)1-2 in our laboratory. It was demonstrated that this attainable acidity, extending from -3 to -8, is exclusively depending of the nature of anion and follow the order: PF6 > BF4 > NTf2 > OTf. Nevertheless, the Hammett acidity function is an apparent function in this media and must then be corrected for. Consequently, in a second step, we tried to evaluate directly the proton activity from the determination of a potentiometric acidity function (R0) based on the extrathermodynamic Strehlow assumption.3 Therefore, the equilibrium potential of the H+/H2 couple was measured with an hydrogen electrode versus the ferricinium/ferrocene couple for which the potential is considered as independent of the solvent