Synthèse, chimie de coordination et applications en catalyse de nouveaux systèmes NHC-Ylure de phosphonium

This work is part of the chemistry of electron-rich neutral carbon ligands represented on the one hand by N-heterocyclic carbenes (C-sp2) and on the other hand by phosphonium ylides (C-sp3). The main objective is to develop a general method for the synthesis of polydentate ligands incorporating these two strongly coordinating carbon units and to prepare corresponding stable complexes with a view to applications in homogeneous catalysis. The first chapter proposes a bibliographical study on the electronic/steric properties, the different synthesis methods and coordination modes, as well as the main applications in homogeneous catalysis of these two families of carbon ligands. In the second chapter, a general synthesis method based on the introduction of a flexible propyl spacer connecting the two coordinating NHC and phosphonium ylide donors and allowing access to new C,C- chelating NHC-phosphonium ylide ligands and their related complexes is described. This strategy is successfully applied in both bidentate and tetradentate series. Taking advantage of this straightforward strategy, the third chapter is devoted to the preparation of new pincer complexes whose backbone is built from a NHC core associated with a phosphonium ylide end, the second coordinating end being either identical to the first one or of a different nature, such as a NHC, phosphine, or phenolate. The electronic properties of these pincer ligands are then analyzed using experimental and theoretical tools and the activity of the corresponding palladium complexes evaluated in the catalytic allylation of aldehydes. The last part of this chapter describes access to a new family of pincer complexes based on the presence of a phosphonium ylide located in a central position. This unique architecture leads in particular to the formation of a stable palladium(II) carbonyl complex, the exact structure of which was determined by an X-ray diffraction study.Ce travail s'inscrit dans le cadre de la chimie des ligands carbonés neutres riches en électrons représentés d'une part par les carbènes N-hétérocycliques (C-sp2) et d'autre part par les ylures de phosphonium (C-sp3). L'objectif principal est de développer une méthode générale de synthèse de ligands polydentes incorporant ces deux motifs carbonés fortement coordinants et de préparer des complexes stables dans la perspective d'applications en catalyse homogène. Le premier chapitre propose une étude bibliographique sur les propriétés électroniques/stériques, les différentes méthodes de synthèse et modes de coordination, ainsi que les principales applications en catalyse homogène de ces deux familles de ligands carbonés. Dans le deuxième chapitre, une méthode générale de synthèse basée sur l'introduction d'un espaceur propyle flexible reliant les deux unités coordinantes NHC et ylure de phosphonium et permettant d'accéder à de nouveaux ligands C,C- chélatants NHC-ylure de phosphonium et leurs complexes est décrite. Cette stratégie est appliquée avec succès aussi bien en série bidente qu'en série tétradente. Tirant profit de cette méthode de synthèse directe, le troisième chapitre est consacré à la préparation de nouveaux complexes pinces dont le squelette est construit à partir d'un cœur NHC associé à une extrémité ylure de phosphonium, la deuxième extrémité coordinante pouvant être identique à la première ou bien de nature différente de type NHC, phosphine, ou phénolate. Les propriétés électroniques de ces ligands pinces sont ensuite analysées à partir d'outils expérimentaux et théoriques et l'activité des complexes de palladium correspondants évaluées en catalyse d'allylation d'aldéhydes. La dernière partie de ce chapitre décrit l'accès à une nouvelle famille de complexes pinces basée sur la présence d'un ylure de phosphonium situé en position centrale. Cette architecture unique conduit notamment à la formation d'un complexe de palladium(II) carbonyle stable à température ambiante et dont la structure exacte a été déterminée par une étude par diffraction des rayons X

Synthesis, Coordination Chemistry and Catalysis Applications of New NHC-Phosphonium Ylide Systems

Ce travail s'inscrit dans le cadre de la chimie des ligands carbonés neutres riches en électrons représentés d'une part par les carbènes N-hétérocycliques (C-sp2) et d'autre part par les ylures de phosphonium (C-sp3). L'objectif principal est de développer une méthode générale de synthèse de ligands polydentes incorporant ces deux motifs carbonés fortement coordinants et de préparer des complexes stables dans la perspective d'applications en catalyse homogène. Le premier chapitre propose une étude bibliographique sur les propriétés électroniques/stériques, les différentes méthodes de synthèse et modes de coordination, ainsi que les principales applications en catalyse homogène de ces deux familles de ligands carbonés. Dans le deuxième chapitre, une méthode générale de synthèse basée sur l'introduction d'un espaceur propyle flexible reliant les deux unités coordinantes NHC et ylure de phosphonium et permettant d'accéder à de nouveaux ligands C,C- chélatants NHC-ylure de phosphonium et leurs complexes est décrite. Cette stratégie est appliquée avec succès aussi bien en série bidente qu'en série tétradente. Tirant profit de cette méthode de synthèse directe, le troisième chapitre est consacré à la préparation de nouveaux complexes pinces dont le squelette est construit à partir d'un cœur NHC associé à une extrémité ylure de phosphonium, la deuxième extrémité coordinante pouvant être identique à la première ou bien de nature différente de type NHC, phosphine, ou phénolate. Les propriétés électroniques de ces ligands pinces sont ensuite analysées à partir d'outils expérimentaux et théoriques et l'activité des complexes de palladium correspondants évaluées en catalyse d'allylation d'aldéhydes. La dernière partie de ce chapitre décrit l'accès à une nouvelle famille de complexes pinces basée sur la présence d'un ylure de phosphonium situé en position centrale. Cette architecture unique conduit notamment à la formation d'un complexe de palladium(II) carbonyle stable à température ambiante et dont la structure exacte a été déterminée par une étude par diffraction des rayons X.This work is part of the chemistry of electron-rich neutral carbon ligands represented on the one hand by N-heterocyclic carbenes (C-sp2) and on the other hand by phosphonium ylides (C-sp3). The main objective is to develop a general method for the synthesis of polydentate ligands incorporating these two strongly coordinating carbon units and to prepare corresponding stable complexes with a view to applications in homogeneous catalysis. The first chapter proposes a bibliographical study on the electronic/steric properties, the different synthesis methods and coordination modes, as well as the main applications in homogeneous catalysis of these two families of carbon ligands. In the second chapter, a general synthesis method based on the introduction of a flexible propyl spacer connecting the two coordinating NHC and phosphonium ylide donors and allowing access to new C,C- chelating NHC-phosphonium ylide ligands and their related complexes is described. This strategy is successfully applied in both bidentate and tetradentate series. Taking advantage of this straightforward strategy, the third chapter is devoted to the preparation of new pincer complexes whose backbone is built from a NHC core associated with a phosphonium ylide end, the second coordinating end being either identical to the first one or of a different nature, such as a NHC, phosphine, or phenolate. The electronic properties of these pincer ligands are then analyzed using experimental and theoretical tools and the activity of the corresponding palladium complexes evaluated in the catalytic allylation of aldehydes. The last part of this chapter describes access to a new family of pincer complexes based on the presence of a phosphonium ylide located in a central position. This unique architecture leads in particular to the formation of a stable palladium(II) carbonyl complex, the exact structure of which was determined by an X-ray diffraction study

NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

International audienceThe chemistry of NHC core pincer ligands of LX2 type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R3P+–CR2−) or by a heteroatom through amide (R2N−), oxide (RO−), or thio(seleno)oxide (RS−, RSe−) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species

Diverse C -Coordination Modes of NHC-Tricyclohexylphosphonium Ylide Ligands in Palladium(II) Complexes

International audienceThanks to the +PCy3 substituents which are unlikely to undergo intramolecular C–H activation, the Pd(II) complex [(C,C,C)PdCl](OTf) having a neutral LX2-type NHC, diphosphonium bis(ylide) pincer ligand was selectively prepared by a double deprotonation of the [(NHC)PdCl2(Py)] precursor with tBuOK. The influence of the +PCy3 substituents on the overall electronic properties of this pincer scaffold was evaluated by IR spectroscopy data of the corresponding Pd–CO adduct, revealing its stronger electron-donating character in comparison to the structurally related NHC core pincer bearing +PPh3 extremities. Treatment of the electron-rich pincer complex [(C,C,C)PdCl](OTf) with AgOTf led to the C,C-chelating NHC-phosphonium ylide Pd(II) complex [(C,C)Pd(OTf)Cl](OTf) via the oxidatively induced homolytic cleavage of a Pd–ylide bond. In the presence of a stronger base (KHMDS or LDA) cyclometalation of a +P–Cy substituent was observed, affording a constrained Pd(II) complex featuring the unique anionic LX3-type C,C,C,C-NHC, diphosphonium tris(ylide) ligand

Direct Access to Palladium(II) Complexes Based on Anionic C , C , C -Phosphonium Ylide Core Pincer Ligand

International audienceThe reaction of readily available imidazolium–phosphonium salt [MesIm(CH2)3PPh3](OTf)2 with PdCl2 in the presence of an excess of Cs2CO3 afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)](OTf) exhibiting an LX2-type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C–H bond activation. The replacement of labile MeCN in the latter by CNtBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [(C,C,C)PdBr] and [(C,C,C)Pd(COnBu)], respectively

Palladium( ii ) pincer complexes of a C , C , C -NHC, diphosphonium bis(ylide) ligand

International audienceA new family of pincer palladium(ii) complexes bearing an electron-rich C,C,C-NHC, diphosphonium bis(ylide) ligand of LX2-type was prepared through the dual N-functionalization of 1H-imidazole by (3-bromopropyl)triphenylphosphonium bromide. Selected basic conditions allowed the sequential coordination of the NHC and phosphonium ylide moieties to Pd(ii). This strategy led to an original ortho-metallated complex where the Pd center is bonded to four carbon atoms of three different natures: carbenic (sp2), arylic (sp2), and chiral ylidic (sp3). Protonation of the latter afforded NHC, diphosphonium bis(ylide) pincer Pd(ii) complexes as a mixture of meso- and dl-diastereomers (de = 50%). The selectivity of C-coordination was rationalized on the basis of DFT calculations, evidencing the quasi-degeneracy of the two diastereomeric forms

NHC Core Phosphonium Ylide-based Palladium(II) Pincer Complexes: The Second Ylide Extremity Makes the Difference

International audienceThe coordinating properties of N-heterocyclic carbene (NHC) (A), phenolate (B), and phosphonium ylide (C) moieties were investigated systematically through the preparation of a family of NHC, phosphonium ylide-based pincer ligands, where the third donor extremity can be either an NHC, a phenolate, or a phosphonium ylide. The overall donor character of such ligands [NHC(AaBbCc)] (a + b + c = 2) was analyzed by comparison of the molecular orbitals (energy and shape), oxidation potentials (Epox), and IR νCO and νCN stretching frequencies of their isostructural pincer Pd(II) complexes [NHC(AaBbCc)PdL][OTf] (L = NCCH3, CO, or CNtBu). The three categories of pincer complexes based on phosphonium ylides were easily obtained by acidic treatment of their highly stable ortho-metalated Pd(II) precursors prepared in a single step from readily available N-phosphonio-substituted imidazolium salts. Analysis of IR data indicated that NHC and phenolate ligands have a similar donor character but which remains lower than that of the phosphonium ylide. The impact on catalytic performance of the incorporation of a second strongly donating phosphonium ylide into the ligand architecture was illustrated in the Pd-catalyzed allylation of aldehydes

Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional P-Coordination of the Ylide Moiety

An efficient synthesis of two pincer pre-ligands [Ph2PCH(R)ImCH2CH2CH2PPh3]X2 (R = H, X = OTf; R = Ph, X = BF4) was developed. Subsequent reactions with PdCl2 and an excess of Cs2CO3 lead to the formation of highly stable cationic orthometalated Pd(II) complexes [(P,C,C,C)Pd]X exhibiting phosphine, NHC, phosphonium ylide and σ-aryl donor extremities. The protonation of one of the latter complexes with R = H affords the Pd(II) complex [(P,C,C)Pd(MeCN)](OTf)2 bearing an unprecedented nonsymmetrical NHC core pincer scaffold with a 5,6-chelating framework. The overall donor properties of this phosphine-NHCphosphonium ylide ligand were estimated using experimental nCN stretching frequency in the corresponding [(P,C,C)Pd(CNtBu](OTf)2 derivative and was shown to be competitive with related bis(NHC)-phosphonium ylide and phenoxy-NHC-phosphonium ylide pincers. The presence of a phenyl substituent in the bridge between phosphine and NHC moieties in orthometalated complex [(P,C,C,C)Pd](BF4) makes possible the deprotonation of this position using LDA to provide a persistent zwitterionic complex [(P,C,C,C)Pd] featuring a rare P-coordinated phosphonium ylide moiety in addition to a conventional C-coordinated one. The comparison of the 31P and 13C NMR data for these C- and P-bound phosphonium ylide fragments within the same molecule was performed for the first time and the bonding situation in both cases was studied in detail by QTAIM and ELF topological analyses