35 research outputs found
POCN-type Pincer Complexes of NiII and NiIII : synthesis, reactivities, catalytic activities and physical properties
Cette thĂšse dĂ©crit la synthĂšse, la caractĂ©risation, les rĂ©activitĂ©s, et les propriĂ©tĂ©s physiques de complexes divalents et trivalents de Ni formĂ©s Ă partir de nouveaux ligands «pincer» de type POCN. Les ligands POCN de type amine sont prĂ©parĂ©s dâune façon simple et efficace via lâamination rĂ©ductrice de 3-hydroxybenzaldĂ©hyde avec NaBH4 et plusieurs amines, suivie par la phosphination de lâamino alcool rĂ©sultant pour installer la fonction phosphinite (OPR2); le ligand POCN de type imine 1,3-(i-Pr)2PC6H4C(H)=N(CH2Ph) est prĂ©parĂ© de façon similaire en faisant usage de PhCH2NH2 en lâabsence de NaBH4. La rĂ©action de ces ligands «pincer» de type POCN avec NiBr2(CH3CN)x en prĂ©sence dâune base rĂ©sulte en un bon rendement de la cyclomĂ©talation du lien C-H situĂ© en ortho aux fonctions amine et phosphinite. Il fut dĂ©couvert que la base est essentielle pour la propretĂ© et le haut rendement de la formation des complexes «pincer» dĂ©sirĂ©s. Nous avons prĂ©parĂ© des complexes «pincer» plan- carrĂ©s de type POCN, (POCNRRÎ)NiBr, possĂ©dant des fonctions amines secondaires et tertiaires qui dĂ©montrent des rĂ©activitĂ©s diffĂ©rentes selon les substituants R et RÎ. Par exemple, les complexes possĂ©dant des fonctions amines tertiaires ArCH2NR2 (NR2= NMe2, NEt2, and morpholinyl) dĂ©montrent des propriĂ©tĂ©s rĂ©dox intĂ©ressantes et pourraient ĂȘtre convertis en leurs analogues trivalents (POCNR2)NiBr2 lorsque rĂ©agis avec Br2 ou N-bromosuccinimide (NBS). Les complexes trivalents paramagnĂ©tiques Ă 17 Ă©lectrons adoptent une gĂ©omĂ©trie de type plan-carrĂ© dĂ©formĂ©e, les atomes de Br occupant les positions axiale et Ă©quatoriale. Les analyses «DSC» et «TGA» des ces composĂ©s ont dĂ©montrĂ© quâils sont thermiquement stables jusquâĂ ~170 °C; tandis que la spectroscopie dâabsorption en solution a dĂ©montrĂ© quâils se dĂ©composent thermiquement Ă beaucoup plus basse tempĂ©rature pour regĂ©nĂ©rer les complexes divalents ne possĂ©dant quâun seul Br; lâencombrement stĂ©rique des substitutants amines accĂ©lĂšre cette route de dĂ©composition de façon significative. Les analogues NMe2 et N(morpholinyl) de ces espĂšces de NiIII sont actifs pour catalyser la rĂ©action dâaddition de Kharasch, de CX4 Ă des olĂ©fines telles que le styrĂšne, tandis quâil fut dĂ©couvert que lâanalogue le moins thermiquement stable (POCNEt2)Ni est complĂštement inerte pour catalyser cette rĂ©action.
Les complexes (POCNRH)NiBr possĂ©dant des fonctions amines secondaires permettent lâaccĂšs Ă des fonctions amines substituĂ©es de façon non symĂ©trique via leur rĂ©action avec des halogĂ©nures dâalkyle. Un autre avantage important de ces complexes rĂ©side dans la possibilitĂ© de dĂ©protonation pour prĂ©parer des complexes POCN de type amide. De telles tentatives pour dĂ©protoner les fonctions NRH nous ont permis de prĂ©parer des espĂšces dimĂ©riques possĂ©dant des ligands amides pontants. La nature dimĂ©rique des ces complexes [P,C,N,N-(2,6-(i-Pr)2PC6H3CH2NR)Ni]2 (R= PhCH2 et Ph) fut Ă©tablie par des Ă©tudes de diffraction des rayons-X qui ont dĂ©montrĂ© diffĂ©rentes gĂ©omĂ©tries pour les cĆurs Ni2N2 selon le substituant N : lâanalogue (PhCH2)N possĂšde une orientation syn des substitutants benzyles et un arrangement ressemblant Ă celui du cyclobutane du Ni et des atomes dâazote, tandis que lâanalogue PhN adopte un arrangement de type diamant quasi-planaire des atomes du Ni et des atomes dâazote et une orientation anti des substituants phĂ©nyles. Les espĂšces dimĂ©riques ne se dissocient pas en prĂ©sence dâalcools, mais elles promouvoient lâalcoolyse catalytique de lâacrylonitrile. De façon intĂ©ressante, les rendements de ces rĂ©actions sont plus Ă©levĂ©s avec les alcools possĂ©dant 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 rĂ©actions dâalcoolyse procĂšdent par activation hĂ©tĂ©rolytique de lâalcool par lâespĂšce dimĂ©rique via des liaisons hydrogĂšnes avec une ou deux des fonctions amides du dimĂšre.
Les espĂšces dimĂ©riques de Ni (II) sâoxydent facilement Ă©lectrochimiquement et par reaction avec NBS ou Br2. De façon surprenante, lâoxydation chimique mĂšne Ă lâisolation de nouveaux produits monomĂ©riques dans lesquels le centre mĂ©tallique et le ligand sont oxydĂ©s. Le mĂ©canisme dâoxydation fut aussi investiguĂ© par RMN, «UV-vis-NIR», «DFT» et spectroĂ©lectrochimie.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 (POCNRRÎ)NiBr featuring tertiary or secondary amine moieties that exhibit different reactivities as a function of amine substituents R and RÎ. 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
Ruthenium complexes with an N-heterocyclic carbene NNC-pincer ligand: preparation and catalytic properties
1-Methyl-3-(2-((pyridin-2-ylmethylene)amino)ethyl)-1H-imidazol-3-ium bromide was prepared and used as an N-heterocyclic carbene NNC-pincer ligand precursor. Depending on the coordination strategy, a monometallic [Ru(NNC)(CO3)(PPh3)] complex, or the [Ru(ÎŒ-Cl)(NNC)]2(2Clâ) dimer, was obtained. A di-silver complex in which two ligands are monocoordinated to the metal center through the NHC groups was also obtained and characterised. The dimetallic ruthenium complex reacts with alcohols yielding a monohydride species. The preliminary studies on the catalytic activity of the ruthenium dimer indicate that the complex is active in the reduction of ketones and aldehydes under transfer hydrogenation conditions
On the Reactivity of P-Chloro Dithieno[3,2-b:2â,3â-d]phosphole Oxide
The P-functionalization of dithieno[3,2-b:2Ăą ,3Ăą -d]phosphole oxides via reaction of a P-chloro derivative with aromatic amines and alcohols is reported. The reactions proceed rapidly and provide the products in good yields, highlighting the synthetic versatility of the P-chloro dithienophosphole oxide toward effectively modifying the molecular scaffold without the need for elaborate phosphorus precursor syntheses. The resulting phosphinic amide and ester products show interesting photophysics that strongly depend on the nature of the P-substituent, such as high luminescence quantum yields for the ester derivatives, as well as Aggregation-Induced Enhanced Emission for the amide derivative. The phosphinic amide species also shows intriguing self-assembly in the solid state via hydrogen bonding.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
Acceptorless Dehydrogenative Coupling of Ethanol and Hydrogenation of Esters and Imines
This paper presents an outstanding air-stable ruthenium
catalyst that has unprecedented efficiency (TON up to 17â000)
for acceptorless dehydrogenative coupling of ethanol, yielding ethyl
acetate and hydrogen gas, and for hydrogenation of esters and imines
at 40 °C while using as low as 50 ppm [Ru]
Acceptorless Dehydrogenative Coupling of Ethanol and Hydrogenation of Esters and Imines
This paper presents an outstanding air-stable ruthenium
catalyst that has unprecedented efficiency (TON up to 17â000)
for acceptorless dehydrogenative coupling of ethanol, yielding ethyl
acetate and hydrogen gas, and for hydrogenation of esters and imines
at 40 °C while using as low as 50 ppm [Ru]
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 RuHÂ(CO)<sub>2</sub>[η<sup>5</sup>-C<sub>5</sub>(OH)ÂPh<sub>4</sub>] (<b>2</b>) and the naked 16-electron complex RuÂ(CO)<sub>2</sub>[η<sup>4</sup>-C<sub>5</sub>(î»O)ÂPh<sub>4</sub>] (<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 RuÂ(OR)Â(CO)<sub>2</sub>[η<sup>5</sup>-C<sub>5</sub>(OH)ÂPh<sub>4</sub>] 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 RuHÂ(CO)<sub>2</sub>[η<sup>5</sup>-C<sub>5</sub>(OH)ÂPh<sub>4</sub>] (<b>2</b>) and the naked 16-electron complex RuÂ(CO)<sub>2</sub>[η<sup>4</sup>-C<sub>5</sub>(î»O)ÂPh<sub>4</sub>] (<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 RuÂ(OR)Â(CO)<sub>2</sub>[η<sup>5</sup>-C<sub>5</sub>(OH)ÂPh<sub>4</sub>] 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 RuHÂ(CO)<sub>2</sub>[η<sup>5</sup>-C<sub>5</sub>(OH)ÂPh<sub>4</sub>] (<b>2</b>) and the naked 16-electron complex RuÂ(CO)<sub>2</sub>[η<sup>4</sup>-C<sub>5</sub>(î»O)ÂPh<sub>4</sub>] (<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 RuÂ(OR)Â(CO)<sub>2</sub>[η<sup>5</sup>-C<sub>5</sub>(OH)ÂPh<sub>4</sub>] 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