POCN-type Pincer Complexes of NiII and NiIII : synthesis, reactivities, catalytic activities and physical properties

Cette thèse décrit la synthèse, la caractérisation, les réactivités, et les propriétés physiques de complexes divalents et trivalents de Ni formés à partir de nouveaux ligands «pincer» de type POCN. Les ligands POCN de type amine sont préparés d’une façon simple et efficace via l’amination réductrice de 3-hydroxybenzaldéhyde avec NaBH4 et plusieurs amines, suivie par la phosphination de l’amino alcool résultant pour installer la fonction phosphinite (OPR2); le ligand POCN de type imine 1,3-(i-Pr)2PC6H4C(H)=N(CH2Ph) est préparé de façon similaire en faisant usage de PhCH2NH2 en l’absence de NaBH4. La réaction de ces ligands «pincer» de type POCN avec NiBr2(CH3CN)x en présence d’une base résulte en un bon rendement de la cyclométalation du lien C-H situé en ortho aux fonctions amine et phosphinite. Il fut découvert que la base est essentielle pour la propreté et le haut rendement de la formation des complexes «pincer» désirés. Nous avons préparé des complexes «pincer» plan- carrés de type POCN, (POCNRR΄)NiBr, possédant des fonctions amines secondaires et tertiaires qui démontrent des réactivités différentes selon les substituants R et R΄. Par exemple, les complexes possédant des fonctions amines tertiaires ArCH2NR2 (NR2= NMe2, NEt2, and morpholinyl) démontrent des propriétés rédox intéressantes et pourraient être convertis en leurs analogues trivalents (POCNR2)NiBr2 lorsque réagis avec Br2 ou N-bromosuccinimide (NBS). Les complexes trivalents paramagnétiques à 17 électrons adoptent une géométrie de type plan-carré déformée, les atomes de Br occupant les positions axiale et équatoriale. Les analyses «DSC» et «TGA» des ces composés ont démontré qu’ils sont thermiquement stables jusqu’à ~170 °C; tandis que la spectroscopie d’absorption en solution a démontré qu’ils se décomposent thermiquement à beaucoup plus basse température pour regénérer les complexes divalents ne possédant qu’un seul Br; l’encombrement stérique des substitutants amines accélère cette route de décomposition de façon significative. Les analogues NMe2 et N(morpholinyl) de ces espèces de NiIII sont actifs pour catalyser la réaction d’addition de Kharasch, de CX4 à des oléfines telles que le styrène, tandis qu’il fut découvert que l’analogue le moins thermiquement stable (POCNEt2)Ni est complètement inerte pour catalyser cette réaction. Les complexes (POCNRH)NiBr possédant des fonctions amines secondaires permettent l’accès à des fonctions amines substituées de façon non symétrique via leur réaction avec des halogénures d’alkyle. Un autre avantage important de ces complexes réside dans la possibilité de déprotonation pour préparer des complexes POCN de type amide. De telles tentatives pour déprotoner les fonctions NRH nous ont permis de préparer des espèces dimériques possédant des ligands amides pontants. La nature dimérique des ces complexes [P,C,N,N-(2,6-(i-Pr)2PC6H3CH2NR)Ni]2 (R= PhCH2 et Ph) fut établie par des études de diffraction des rayons-X qui ont démontré différentes géométries pour les cœurs Ni2N2 selon le substituant N : l’analogue (PhCH2)N possède une orientation syn des substitutants benzyles et un arrangement ressemblant à celui du cyclobutane du Ni et des atomes d’azote, tandis que l’analogue PhN adopte un arrangement de type diamant quasi-planaire des atomes du Ni et des atomes d’azote et une orientation anti des substituants phényles. Les espèces dimériques ne se dissocient pas en présence d’alcools, mais elles promouvoient l’alcoolyse catalytique de l’acrylonitrile. De façon intéressante, les rendements de ces réactions sont plus élevés avec les alcools possédant des fonctions O-H plus acides, avec un nombre de «turnover» catalytique pouvant atteindre 2000 dans le cas de m-cresol. Nous croyons que ces réactions d’alcoolyse procèdent par activation hétérolytique de l’alcool par l’espèce dimérique via des liaisons hydrogènes avec une ou deux des fonctions amides du dimère. Les espèces dimériques de Ni (II) s’oxydent facilement électrochimiquement et par reaction avec NBS ou Br2. De façon surprenante, l’oxydation chimique mène à l’isolation de nouveaux produits monomériques dans lesquels le centre métallique et le ligand sont oxydés. Le mécanisme d’oxydation fut aussi investigué par RMN, «UV-vis-NIR», «DFT» et spectroélectrochimie.This thesis describes the synthesis, characterization, reactivities, and physical properties of divalent and trivalent complexes of Nickel based on new POCN-type pincer ligands. The amino-type POCN ligands were prepared in a simple and efficient manner via reductive amination of 3-hydroxybenzaldehyde with NaBH4 and various amines, followed by phosphination of the resulting amino alcohol to install the phosphinite (OPR2) functionality. The imino-type POCN ligand 1,3-(i-Pr)2PC6H4C(H)=N(CH2Ph) was prepared similarly using PhCH2NH2 in the absence of NaBH4. Reaction of these POCN-type pincer ligands with NiBr2(CH3CN)x in the presence of a base results in the high yield cyclometalation of the C-H bond which is ortho to the amine and phosphinite functionalities. The base was found to be essential for a clean and high yield formation of the desired pincer complexes. We have thus prepared square planar POCN-type pincer complexes (POCNRR΄)NiBr featuring tertiary or secondary amine moieties that exhibit different reactivities as a function of amine substituents R and R΄. For instance, complexes bearing the tertiary amine moieties ArCH2NR2 (NR2= NMe2, NEt2, and morpholinyl) displayed interesting redox properties and could be converted into their trivalent analogues (POCNR2)NiBr2 when reacted with Br2 or N-bromosuccinimide (NBS). These 17-electron, paramagnetic trivalent complexes adopt a distorted square pyramidal geometry with Br atoms at axial and equatorial positions. DSC and TGA analyses of these compounds revealed them to be thermally stable up to ~170 °C; whereas absorption spectroscopy in solution showed that they undergo thermal decomposition at much lower temperatures to regenerate the monobromo divalent complexes; increased steric bulk of the amine substituents accelerate this decomposition pathway significantly. The NMe2 and N(morpholinyl) analogues of these NiIII species are active catalysts for the Kharasch addition of CX4 to olefins such as styrene, whereas the least thermally stable analogue (POCNEt2)Ni was found to be completely inert for this reaction. The complexes (POCNRH)NiBr featuring secondary amine moieties allow access to unsymmetrically substituted amine moieties via reaction with alkyl halides. Another important advantage of these complexes lies in the possibility of deprotonation to prepare amide-type POCN complexes. Such attempts at deprotonating the NRH moieties have allowed us to prepare dimeric species featuring bridging amido ligands. The dimeric nature of these complexes [P,C,N,N-(2,6-(i-Pr)2PC6H3CH2NR)Ni]2 (R= PhCH2 and Ph) was established through X-ray diffraction studies that showed different geometries for the Ni2N2 cores as a function of N-substituent: the (PhCH2)N analogue featured a syn orientation of the benzyl substituents and a cyclobutane-like arrangement of Ni and of the nitrogen atoms, whereas the PhN analogue adopted a nearly planar diamond-like arrangement of the Ni and of the nitrogen atoms and an anti orientation of the phenyl substituents. These dimeric species do not dissociate in the presence of alcohols, but they promote the catalytic alcoholysis of acrylonitrile. Interestingly, yields of these reactions are higher with alcohols possessing more acidic O-H moieties, with a catalytic turnover number reaching up to 2000 in the case of m-cresol. These alcoholysis reactions are believed to proceed through heterolytic activation of the alcohol by dimeric species via hydrogen bonding with one or two amido moieties in the dimer. The dimeric Ni (II) species were found to undergo facile oxidation both electrochemically and in reaction with NBS or Br2. Surprisingly, chemical oxidation led to isolation of new monomeric products in which both the metallic center and the ligand were oxidized. giving a trivalent species featuring an imine-type POCN ligand. Oxidation mechanism was investigated in detail by NMR, UV-vis-NIR, DFT and spectroelectrochemistry

Revised Mechanisms for Aldehyde Disproportionation and the Related Reactions of the Shvo Catalyst

It is widely believed that the Shvo catalyst (<b>1</b>) dissociates to form two active species in solution: the 18-electron hydride RuH­(CO)₂[η⁵-C₅(OH)­Ph₄] (<b>2</b>) and the naked 16-electron complex Ru­(CO)₂[η⁴-C₅(O)­Ph₄] (<b>3</b>). This combined experimental/computational study demonstrates that a sustained presence of <b>3</b> is not viable in the reactions of alcohols and organic carbonyls; thus, <b>3</b> is better treated as nonexistent under the typical catalytic conditions. We propose a modified view where the key catalytic species are the hydride <b>2</b> and the 18-electron metal alkoxide intermediate Ru­(OR)­(CO)₂[η⁵-C₅(OH)­Ph₄] existing in equilibrium with the corresponding alcohol complex. An X-ray crystallographic study of <b>2</b> revealed an interesting dihydrogen-bonded dimer structure in the solid state. The mechanistic ideas of this paper explain the highly efficient Tishchenko-like aldehyde disproportionation reaction with the Shvo catalyst. Additionally, our observations explain why <b>1</b> is inefficient for hydrogenation of ethyl acetate and for the acceptorless dehydrogenative coupling of ethanol. Our findings provide practical guidance for future catalyst design on the basis of the Shvo ruthenium dimer prototype

Revised Mechanisms for Aldehyde Disproportionation and the Related Reactions of the Shvo Catalyst

It is widely believed that the Shvo catalyst (<b>1</b>) dissociates to form two active species in solution: the 18-electron hydride RuH­(CO)₂[η⁵-C₅(OH)­Ph₄] (<b>2</b>) and the naked 16-electron complex Ru­(CO)₂[η⁴-C₅(O)­Ph₄] (<b>3</b>). This combined experimental/computational study demonstrates that a sustained presence of <b>3</b> is not viable in the reactions of alcohols and organic carbonyls; thus, <b>3</b> is better treated as nonexistent under the typical catalytic conditions. We propose a modified view where the key catalytic species are the hydride <b>2</b> and the 18-electron metal alkoxide intermediate Ru­(OR)­(CO)₂[η⁵-C₅(OH)­Ph₄] existing in equilibrium with the corresponding alcohol complex. An X-ray crystallographic study of <b>2</b> revealed an interesting dihydrogen-bonded dimer structure in the solid state. The mechanistic ideas of this paper explain the highly efficient Tishchenko-like aldehyde disproportionation reaction with the Shvo catalyst. Additionally, our observations explain why <b>1</b> is inefficient for hydrogenation of ethyl acetate and for the acceptorless dehydrogenative coupling of ethanol. Our findings provide practical guidance for future catalyst design on the basis of the Shvo ruthenium dimer prototype

Revised Mechanisms for Aldehyde Disproportionation and the Related Reactions of the Shvo Catalyst

It is widely believed that the Shvo catalyst (<b>1</b>) dissociates to form two active species in solution: the 18-electron hydride RuH­(CO)₂[η⁵-C₅(OH)­Ph₄] (<b>2</b>) and the naked 16-electron complex Ru­(CO)₂[η⁴-C₅(O)­Ph₄] (<b>3</b>). This combined experimental/computational study demonstrates that a sustained presence of <b>3</b> is not viable in the reactions of alcohols and organic carbonyls; thus, <b>3</b> is better treated as nonexistent under the typical catalytic conditions. We propose a modified view where the key catalytic species are the hydride <b>2</b> and the 18-electron metal alkoxide intermediate Ru­(OR)­(CO)₂[η⁵-C₅(OH)­Ph₄] existing in equilibrium with the corresponding alcohol complex. An X-ray crystallographic study of <b>2</b> revealed an interesting dihydrogen-bonded dimer structure in the solid state. The mechanistic ideas of this paper explain the highly efficient Tishchenko-like aldehyde disproportionation reaction with the Shvo catalyst. Additionally, our observations explain why <b>1</b> is inefficient for hydrogenation of ethyl acetate and for the acceptorless dehydrogenative coupling of ethanol. Our findings provide practical guidance for future catalyst design on the basis of the Shvo ruthenium dimer prototype

Tuning Iridium(I) PCcarbeneP Frameworks for Facile Cooperative N2O Reduction

A semiquantitative kinetic study correlates the rate of oxygen atom transfer from N2O to an iridium carbon double bond with the donor properties of six PCcarbeneP pincer ligand frameworks. Two new rigid, electron rich ligands are described and shown to be the most effective for rapid, selective reaction with N2O.<br /

Oxygen-Oxygen Bond Cleavage and Formation in Co(II) Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

Diprotonation of a remarkably stable, toluene soluble cobalt peroxo complex supported by a neutral, dianionic pentadentate ligand leads to facile O-O bond cleavage and production of a highly reactive Co(IV) oxyl cation intermediate that dimerizes and releases O2. These processes are relevant to both O2 reduction and O2 evolution and the mechanism was probed in detail both experimentally and computationally

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O-2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

International audienceIn reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O-2 to H2O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O-2 to water via Co(III)-O-O-Co(III) peroxo intermediates

Impact of Backbone Substituents on POCOP-Ni Pincer Complexes: A Structural, Spectroscopic, and Electrochemical Study

When treated at room temperature and in the presence of NEt₃ with {(<i>i</i>-PrCN)­NiBr₂}_<i>n</i>, the pincer-type ligands R-POC^HOP^{R^′} undergo direct C–H nickellation to give the pincer complexes (R-POCOP^{R^′})­NiBr in 45–92% yields (R-POCOP = κ^<i>P</i>,κ^<i>C</i>,κ^<i>P</i>-{R_<i>n</i>-2,6-(R′₂PO)₂C₆H_3–<i>n</i>}; R_<i>n</i> = 4-OMe, 4-Me, 4-CO₂Me, 3-OMe, 3-CO₂Me, 3,5-<i>t</i>-Bu₂; R′ = <i>i</i>-Pr, <i>t</i>-Bu). These complexes have been characterized by multinuclear NMR and UV–vis spectroscopy as well as single-crystal X-ray diffraction studies to delineate the impact of R and R′ on Ni–ligand interactions. The solid-state structural data have revealed slightly shorter Ni–Br bonds in the complexes bearing a 4-CO₂Me substituent, shorter Ni–P bonds in the complex bearing <i>t</i>-Bu substituents at the 3- and 5-positions, and longer Ni–P bonds in complexes featuring OP­(<i>t</i>-Bu)₂ donor moieties. The UV–vis spectra indicate that a 4-CO₂Me substituent causes a red-shift in the frequency of the MLCT bands (330–365 nm), whereas the ligand field transitions appearing in the 380–420 nm region are influenced primarily by the <i>P</i>-substituents. Cyclic voltammetry measurements have shown that the oxidation potentials of the title complexes are affected by <i>P</i>- and ring-substituents, oxidation being somewhat easier with <i>t</i>-Bu₂PO (vs <i>i</i>-Pr₂PO), OMe and Me (vs CO₂Me), and <i>t</i>-Bu (vs Cl). Moreover, oxidation potentials are affected more by the aromatic substituents at the 4-position vs those at the 3- and 5-positions

Systematic Dismantling of a Carefully Designed PCcarbeneP Pincer Ligand via C-C Bond Activations at an Iridium Centre

Accepted manuscript deposited April 11, 2016, as per: http://www.nrcresearchpress.com/page/open-access/optionsAn electron-rich PCsp2P ligand, incorporating N,N-dimethylamino groups para to the anchoring carbene donor of the ligand, was prepared and coordinated to iridium, producing the iridium carbene chloride 2. This species undergoes facile reaction with N2O to afford an iridaepoxide complex, 3, in which an oxygen atom has been transferred to the Ir=C bond. The rate of this reaction is significantly faster than that observed for the less electron rich, unsubstituted ligand. However, further reaction of 3 involving cleavage of one of the ligand C–C bonds was observed, producing the bis-phosphine chorido complex 4. This process was accelerated by the presence of H2. Heating 4 under H2 resulted in hydrogenolysis of the ortho-metalated phosphine ligand to give a hydrido complex (5) and decarbonylation of the acyl phosphine ligand to give, finally, Vaska’s complex analog 6. All compounds were fully characterized, and the sequence represents the dismantling of the PCsp2P ligand framework.NSERCYe

Activation of Si-H bonds across the nickel carbene bond in electron rich nickel PCcarbeneP pincer complexes

Accepted version of article deposited April 11, 2016 as per http://www.rsc.org/journals-books-databases/journal-authors-reviewers/licences-copyright-permissions/#deposition-sharingSilicon–hydrogen bonds are shown to add to a nickel carbon double bond to yield nickel α-silylalkyl hydrido complexes. Kinetic and isotope labeling studies suggest that a concerted 4-centred addition across the Ni[double bond, length as m-dash]C bond is operative rather than a mechanism involving Si–H oxidative addition. This constitutes an example of Si–H bond activation via ligand cooperativity.NSERCYe