Synthesis of new zirconium diketiminate complexes and catalytic applications

Résumé: Dans le but de préparer des complexes de Zr pour la catalyse homogène de la polymérisation des lactides et de l’hydroamination des olefines, l’elaboration et l’optimisation d’une méthode systématique et efficace de synthèse des ligands dikétimines ayant différents substituants alkyles (R) à la position N,N’ a été realisée. Des dikétimines (nacnacRH) symétriques ont été obtenus avec une pureté de plus de 95 % et un rendement de 65 % lorsque R = Me et des rendements allant de 80 à 95 % lorsque le groupe R = n-Pr, i-Pr, i-Bu, Bu, Cy et (+)-CH(Me)Ph. La synthèse des dikétimines ayant des substituants N-alkyls différents, dite asymétriques, donne toujours un mélange statistique de trois ligands: nacnacR,R’H, nacnacR,RH et nacnacR’,R’H qui n’ont pu être separés. Seuls les dikétimines asymétriques avec un substituant N-alkyl et un autre N-aryl (nacnacR,ArH) ont été obtenus avec des rendements plus élevés que celui du mélange statistique. Par la suite, la complexation de ces ligands bidentés au Zr, la caractérisation de ces complexes et l’investigation de la réactivité ont été étudiés. Les complexes de Zr de type (nacnacR)2ZrCl2 ont été obtenus par deux voies de synthèse principales: la première consiste à traiter le sel de lithium du ligand avec le ZrCl4. La seconde est la réaction du ligand avec les complexes neutres d’alkyl-zirconium(IV) par protonation de l'alkyle coordonné. En solution, les complexes obtenus de (nacnacR)2ZrX2 possèdent un comportement dynamique via un « Bailar-twist » et les paramètres d'activation de cette isomérisation ont été calculés. Le complexe octaèdrique (nacnacBn)2ZrCl2 n'est pas réactif dans la carbozirconation et son alkylation n'était pas possible par l’échange des chlorures avec les alkyles. L’analogue diméthylé (nacnacBn)2ZrMe2 peut être préparé par alkylation du ZrCl4 avant la complexation du ligand. On a également observé que ce dernier n’est pas réactif dans la carbozirconation. L‘analogue diéthoxyde (nacnacBn)2Zr(OEt)2 est obtenu par échange des diméthyles avec les éthoxydes. La polymérisation du lactide avec celui-ci en tant que précurseur est relativement lente et ne peut être effectuée que dans le monomère fondu. Par conséquent, pour résoudre les problèmes rencontrés avec les complexes de zirconium (dikétiminates non-pontés), un ligand dikétimines pontés par le diaminocyclohexane, (±)-C6H10(nacnacXylH)2, LH2, (Xyl = 2,6-diméthylphényle) a été préparé. La complexation de ce ligand tetradenté au metal a été réalisée par deux voies de synthèse; la première est la réaction du sel de lithium de ce ligand avec le ZrCl4(THF)2. La deuxième est la déprotonation du ligand neutre avec le Zr(NMe2)4 et l’élimination du diméthylamine. Des complexes du type: (±)-C6H10(nacnacXylH)2ZrX2 avec X = Cl, NMe2 ont été obtenus. Les ligands de chlorure sont dans ce cas facilement remplaçables par des éthoxydes ou des méthyles. On a observé l’activité la plus élevée jamais observée pour un complexe d’un métal du groupe 4 avec le complexe de (±)-C6H10(nacnacXylH)2Zr(OEt)2 dans la polymérisation de lactide. L'étude cinétique a montré que la loi de vitesse est du premier ordre en catalyseur et en monomère et la constante de vitesse est k = 14 (1) L mol-1 s-1. L'analyse des polymères a montré l’obtention de masses moléculaires faibles et l’abscence de stéréocontrôle. La réaction de (±)-C6H10(nacnacXylH)2ZrCl2 avec le triflate d’argent donne le (±)-C6H10(nacnacXylH)2Zr(OTf)2. Le complexe bis-triflate obtenu possède une activité catalytique elevée pour les additions du type aza-Michael. L’utilisation du R,R-C6H10(nacnacXylH)2Zr(OTf)2 énantiopur comme catalyseur, dans les additions du type aza-Michael asymétriques donne le produit desiré avec un excès énantiomérique de 19%.Abstract: In order to prepare the complexes of Zr targeted for homogeneous catalysis in polymerization of lactides and hydroamination of activated olefins, we focused on the elaboration and the optimization of a systematic and efficient method for the synthesis of diketimines ligands with a variety of substituted alkyl (R) on their position N,N'. Symmetrical diketimines (nacnacRH) were obtained with a greater than 95% purity and a yield of 65% when R = Me and yields ranging from 80 to 95% when R = nPr, iPr, iBu, Bn, and Cy (+)-CH (Me) Ph. The Synthesis of diketimines with different N-alkyl substituents, called asymmetric, always gives a statistical mixture of three ligands: nacnacR,R'H nacnacR,RH and nacnacR',R'H that made their isolation problematic. Yields greater than statistical mixtures were obtained only with asymmetric diketimines bearing N-alky and N-aryl substituents (nacnacR,ArH). Subsequently, we studied the complexation of these bidentate ligands with Zr, the characterization of these complexes and investigation of their reactivity. Zr complexes of type (nacnacRH)2ZrCl2 were obtained via two main synthetic routes: the first consists in treatment of the lithium salt of the ligand with ZrCl4. The second is the reaction of the ligand with neutral complexes of alkyl-zirconium (IV) by protonation of the alkyl coordinated. In solution, the obtained complexes (nacnacR)2ZrX2 showed dynamic behavior via a "Bailar-twist" isomerization and the activation parameters of the isomerization were calculated. Octahedral complex (nacnacBn)2ZrCl2, showed no reactivity in alkylation and carbozirconation was not possible by the exchange of alkyl with chlorides. The dimethyl analogue (nacnacBn)2ZrMe2, can be prepared by alkylation of ZrCl4 before ligand complexing. The diethoxide analogue (nacnacBn)2Zr(OEt)2 is obtained by exchange of dimethyls with ethoxides. The latter had slow reactivity in lactide polymerization under melt conditions. Consequently, to address the problems encountered with unbridged (diketiminate) zirconium complexes, a cyclohexanediyl-bridged diketiminate ligand, (±)-C6H10(nacnacXylH)2, LH2, (Xyl = 2,6-dimethylphenyl) is prepared. Complexation of the tetradentate ligand is realized via two synthetic routes; The first is reaction of the lithium salt of the ligand with ZrCl4(THF)2. The second is deprotonation of the neutral ligand with Zr(NMe2)4 and elimination of dimethylamine. Complexes of the type: (±)-C6H10(nacnacXylH)2ZrX2 with X = Cl, NMe2 are obtained. The chloride ligands are in this case readily replaceable with ethoxides or methyls. The (±)-C6H10(nacnacXylH)2Zr(OEt)2 complex showed the highest activity ever observed for any group 4 metal complex in lactide polymerization. The kinetic study showed that the rate law is first order in catalyst and monomer and the rate constant is k = 14(1) L mol−1 s−1. Analysis of the obtained polymer showed low molecular weight with no-stereocontrol. Reaction of the (±)-C6H10(nacnacXylH)2ZrCl2 with silver triflates yielded the (±)-C6H10(nacnacXylH)2Zr(OTf)2. The obtained bis-triflate complex showed to be a highly active catalyst for aza-Michael additions. The use of the enatiopure R,R-C6H10(nacnacXyl)2Zr(OTf)2 as catalyst for asymmetric aza-Michael additions of activated olefines gave the desired product with an enantiomeric excess of 19%

Synthesis of new zirconium complexes

L'étude suivante décrit la synthèse des ligands nacnacxylH, nacnacBnH, nacnacR,RH et nacnacCyH en utilisant une méthode générale qui implique des rendements élevés et des coûts raisonnables, la complexation de ces ligands au Zr, la caractérisation de ces complexes et l’investigation de leurs réactivités. Les complexes de zirconium ont été obtenus en utilisant deux méthodes synthétiques principales : la première consiste en traitement du sel de lithium du ligand avec le ZrCl4. La seconde est la réaction du ligand neutre avec les complexes d’alkyl-zirconium(IV) par protonation de l'alkyle coordonné. Le ligand adopte deux modes de coordination avec le Zr. Une coordination 2 est observée dans les complexes octaèdriques contenant un ou deux ligands nacnac. En présence d'un autre ligand ayant une coordonnation 5, par exemple Cp ou Ind, le ligand nacnac se trouve en coordination x avec le centre métallique de zirconium. En solution, les complexes obtenus de (nacnac)2ZrX2 montrent un comportement dynamique via un « Bailar-twist » et les paramètres d'activation de cette isomérisation ont été obtenus. Le complexe octaèdrique (nacnacBn)2ZrCl2, 2c, n'a pas montré de réactivité dans la carbozirconation et son alkylation n'était pas possible par l’échange des chlorures avec les alkyles. L’analogue dimethylé (nacnacBn)2ZrMe2, 2d, peut être préparé par alkylation du ZrCl4 avant la complexation du ligand. Ce dernier a été prouvé aussi de ne pas être réactif dans la carbozirconation.The present study describes the synthesis of ligands nacnacxylH, nacnacBnH, nacnacR,RH and nacnacCyH, using a general method of synthesis which affords high yields at affordable costs, the complexation of these ligands to Zr, the characterization of these complexes and the investigation of their reactivities. Zirconium complexes were obtained using two major synthetic routes: The first one consists of treatment of the previously prepared lithium salt of the ligand with ZrCl4. The second is the reaction of the neutral ligand with alkyl-Zr(IV) complexes by protonation of the coordinated alkyl(s). The nacnac ligand adopts two coordination modes with the Zr metal. 2-Coordination is observed in octahedral complexes containing one or two nacnac ligands. In the presence of another 5-coordinated ligand, such as Cp or Ind, the nacnac ligand is found to be 4/5-coordinated to the Zr center. The obtained complexes (nacnac)2ZrX2 showed a fluxional behavior in solution via a Bailar Twist and the activation parameters of this isomerisation were obtained. The cis octahedral dichloride complex (nacnacBn)2ZrCl2, 2c, showed no reactivity in carbozirconation and its alkylation was not possible by exchange of chlorides with alkyls. The dimethyl analogue (nacnacBn)2ZrMe2, 2d, could be prepared by alkylation of ZrCl4 prior to ligand complexation, but proved as well to be unreactive in carbozirconation

Syntheses, structures, and stabilities of aliphatic and aromatic fluorous iodine(I) and iodine(III) compounds::the role of iodine Lewis basicity

The title molecules are sought in connection with various synthetic applications. The aliphatic fluorous alcohols RfnCH2OH (Rfn = CF3(CF2)n–1; n = 11, 13, 15) are converted to the triflates RfnCH2OTf (Tf2O, pyridine; 22–61%) and then to RfnCH2I (NaI, acetone; 58–69%). Subsequent reactions with NaOCl/HCl give iodine(III) dichlorides RfnCH2ICl2 (n = 11, 13; 33–81%), which slowly evolve Cl2. The ethereal fluorous alcohols CF3CF2CF2O(CF(CF3)CF2O)xCF(CF3)CH2OH (x = 2–5) are similarly converted to triflates and then to iodides, but efforts to generate the corresponding dichlorides fail. Substrates lacking a methylene group, RfnI, are also inert, but additions of TMSCl to bis(trifluoroacetates) RfnI(OCOCF3)2 appear to generate RfnICl2, which rapidly evolve Cl2. The aromatic fluorous iodides 1,3-Rf6C6H4I, 1,4-Rf6C6H4I, and 1,3-Rf10C6H4I are prepared from the corresponding diiodides, copper, and RfnI (110–130 °C, 50–60%), and afford quite stable RfnC6H4ICl2 species upon reaction with NaOCl/HCl (80–89%). Iodinations of 1,3-(Rf6)2C6H4 and 1,3-(Rf8CH2CH2)2C6H4 (NIS or I2/H5IO6) give 1,3,5-(Rf6)2C6H3I and 1,2,4-(Rf8CH2CH2)2C6H3I (77–93%). The former, the crystal structure of which is determined, reacts with Cl2 to give a 75:25 ArICl2/ArI mixture, but partial Cl2 evolution occurs upon work-up. The latter gives the easily isolated dichloride 1,2,4-(Rf8CH2CH2)2C6H3ICl2 (89%). The relative thermodynamic ease of dichlorination of these and other iodine(I) compounds is probed by DFT calculations

Polyisobutylene (PIB)-NHC Supported Catalysts for Cross-Coupling Reactions: A Green and Sustainable Protocol

N-Heterocyclic Carbenes (NHCs): Over the last two decades N-Heterocyclic carbenes (NHCs) have immensely attracted chemists in nearly all fields of chemistry. N-Heterocyclic carbenes are commonly encountered in coordination chemistry, they are extensively used as ligands for organometallic complexes. Perhaps the biggest hit of NHCs ligands was their use in Grubbs II catalyst for olefin metathesis chemistry. It is noteworthy that the success of NHCs ligands in catalysis is due to several factors favoring their high activity, selectivity and stability when compared to the phosphine counterparts in Grubbs I catalyst [1]. Supported Catalysts: Increased environmental and health awareness requires that designing new metal-catalysts should focus not only on increasing activity and selectivity but also on finding new strategies that help chemists recycle and separate the metal-catalyst from the reaction mixture. In general, homogenous catalysis is preferred over heterogeneous catalysis. This is due to the higher turnover number, better selectivity and usually lower operating temperatures required. On the other hand, heterogeneous catalysis has the advantage of the ease of separation of the catalyst from the final products and is generally less expensive. One important strategy is to use catalysts attached to a heterogeneous support and separate them from the products by simple filtration. Alternatively, homogeneous catalysts that can self-separate from the products by selective solvent extraction would be of great interest. The frequency of their reuse would be environmentally beneficial and to a higher extent this should overcome the lower activity of conventional heterogeneous catalysts. Metal catalysts that can self-separate from the reaction mixture are of great importance due to the reduced metal leaching into the product mixture. In addition, their reuse and recovery make this overall process much greener compared to the conventional homogeneous/heterogeneous catalysis systems. Ever since Herrmann et al. [2] reported the polystyrene supported NHC-palladium catalyst, studies have largely been focused on the use of polymeric supports for NHC-palladium catalysts. While polyethylene-glycol-supported catalyst can be extracted with a polar solvent, Bergbreiter et al. [3] and others have showed that polyisobutylene (PIB) is a useful support for ligands and their metal catalysts (Pd, Ru...) having preferable solubility towards solvents with low polarities such as hexanes, heptanes and decanes. In all of these biphasic systems for cross-coupling/olefin metathesis, the design is mainly focused on the recovery and the reuse of the supported catalysts. Biphasic catalysis having thermomorphic behavior have witnessed great developments due to their temperature-dependent miscibility [4]. While reactions in these biphasic mixtures can be conducted under homogeneous conditions at high-temperatures, the supported catalysts and the products/by-products can be efficiently separated by restoring the biphasic conditions at a low-temperature (Scheme 1). Herein we report the synthesis of new PIB-supported N-heterocyclic carbenes ligands having two different frameworks and their Pd-complexes, 1 and 2. The use, recovery and effectiveness of catalysts are detailed in both Heck and Suzuki cross-coupling reactions (Scheme 2). Metal leaching to the polar phase will be discussed too. Scheme 2: Heck cross-coupling and Suzuki cross-coupling using catalysts 1 and 2.qscienc

Time reversed pulse waves study for wireless energy transmission in a low Q cavity

International audienceAn original approach based on time reversal technique is studied for wireless energy transmission with electromagnetic waves. Experiments around 2.45 GHz are performed in a low-Q cavity of human size approaching real indoor environment. It is shown that, in order to increase the amplitude of the focused signals on a specific spot using time reversal technique, expanding the bandwidth of the emitted signals is needed. In addition, focused spot dimension is theoretically and experimentally illustrated. Other potential benefits of the time reversal technique are discussed and the superiority of energy performance of this technique compared to others (e.g. Inverse Filter and Continuous Wave) is demonstrated

A Wideband V-Shaped Coplanar Patch Antenna for 2-6 GHz Designed for Energy Harvesting and Wireless Communications

International audienceThis article portrays a compact wideband coplanar-fed V-shaped antenna. It has been designed for harvesting energy from wireless local area networks (IEEE 802.11) and other wireless communication systems covering the 2–6 GHz frequency band. The Structure of this antenna comprises a rectangular patch and an isosceles trapezoid. A V-shaped slot is introduced on the rectangular patch in order to perturb the surface current path which contributes to the formation of the wide-band characteristics. A trapezoid is inserted between the coplanar feed and the patch for the purpose of impedance matching and resonance shifting. A parametric study has been carried out to understand the effects of various dimensional parameters and to optimize the performance of the antenna. The measurement results which are in agreement with Ansoft HFSS simulation results, showed an agreeable bandwidth and satisfactory characteristics for omnidirectional radiation

Selective and Nonselective Aza-Michael Additions Catalyzed by a Chiral Zirconium Bis-Diketiminate Complex

Reaction of the chiral bis-diketiminate complex <i>rac</i>- or (<i>R</i>,<i>R</i>)-C₆H₁₀(<i>nacnac</i>^Xyl)₂ZrCl₂ with AgOTf yielded the corresponding bis-triflate complex. The complex geometry changes from distorted octahedral in the dichloride complex to a pseudotetrahedral coordination involving π coordination of the diketiminate ligands. The bis-triflate complex is highly active for aza-Michael additions with turnover frequencies of 20000/h for the addition of morpholine to acrylonitrile and 1000/h for the addition of morpholine to methacrylonitrile. The enantioselectivities of the latter reaction in various solvents were low, never surpassing 19% ee. The reaction is first-order in olefin concentration and second order in amine concentration, which is explained by its participation as a base in the reaction mechanism. The presence of catalytic amounts of triethylamine slightly increases the observed rate constants and reduces the reaction order in amine to first order. Other activated alkenes such as methacrylonitrile, crotonitrile, methyl acrylate, and cyclohexenone can be employed, but no reactivity is observed toward styrene or vinyl ethers. Primary amines, secondary amines, and anilines can be employed as nucleophiles with activities correlating with their nucleophilicity, but the catalyst is unstable in the presence of alcohols

Selective and Nonselective Aza-Michael Additions Catalyzed by a Chiral Zirconium Bis-Diketiminate Complex

Reaction of the chiral bis-diketiminate complex <i>rac</i>- or (<i>R</i>,<i>R</i>)-C₆H₁₀(<i>nacnac</i>^Xyl)₂ZrCl₂ with AgOTf yielded the corresponding bis-triflate complex. The complex geometry changes from distorted octahedral in the dichloride complex to a pseudotetrahedral coordination involving π coordination of the diketiminate ligands. The bis-triflate complex is highly active for aza-Michael additions with turnover frequencies of 20000/h for the addition of morpholine to acrylonitrile and 1000/h for the addition of morpholine to methacrylonitrile. The enantioselectivities of the latter reaction in various solvents were low, never surpassing 19% ee. The reaction is first-order in olefin concentration and second order in amine concentration, which is explained by its participation as a base in the reaction mechanism. The presence of catalytic amounts of triethylamine slightly increases the observed rate constants and reduces the reaction order in amine to first order. Other activated alkenes such as methacrylonitrile, crotonitrile, methyl acrylate, and cyclohexenone can be employed, but no reactivity is observed toward styrene or vinyl ethers. Primary amines, secondary amines, and anilines can be employed as nucleophiles with activities correlating with their nucleophilicity, but the catalyst is unstable in the presence of alcohols