Homogeneous hydrogenation of saturated bicarbonate slurry to formates using multiphase catalysis

Formic acid and formate salts are key intermediates along the pathways for CO2 utilization and hydrogen storage. Herein we report a highly efficient multiphase catalytic system utilizing ruthenium PNP pincer catalyst for converting supersaturated bicarbonate solutions and slurries to aqueous formate solutions up to 12M in molarity. The biphasic catalytic system delivers turnover frequencies up to 73 000 h-1 and remains stable for up to 474’000 turnovers once reaction conditions are optimized

Porphyrin Cobalt(III) “Nitrene Radical” Reactivity; Hydrogen Atom Transfer from Ortho-YH Substituents to the Nitrene Moiety of Cobalt-Bound Aryl Nitrene Intermediates (Y = O, NH)

In the field of cobalt(II) porphyrin-catalyzed metallo-radical reactions, organic azides have emerged as successful nitrene transfer reagents. In the pursuit of employing ortho-YH substituted (Y = O, NH) aryl azides in Co(II) porphyrin-catalyzed nitrene transfer reactions, unexpected hydrogen atom transfer (HAT) from the OH or NH2 group in the ortho-position to the nitrene moiety of the key radical-intermediate was observed. This leads to formation of reactive ortho-iminoquinonoid (Y = O) and phenylene diimine (Y = NH) species. These intermediates convert to subsequent products in non-catalyzed reactions, as is typical for these free organic compounds. As such, the observed reactions prevent the anticipated cobalt-mediated catalytic radical-type coupling of the nitrene radical intermediates to alkynes or alkenes. Nonetheless, the observed reactions provide valuable insights into the reactivity of transition metal nitrene-radical intermediates, and give access to ortho-iminoquinonoid and phenylene diimine intermediates from ortho-YH substituted aryl azides in a catalytic manner. The latter can be employed as intermediates in one-pot catalytic transformations. From the ortho-hydroxy aryl azide substrates both phenoxizinones and benzoxazines could be synthesized in high yields. From the ortho-amino aryl azide substrates azabenzene compounds were obtained as the main products. Computational studies support these observations, and reveal that HAT from the neighboring OH and NH2 moiety to the nitrene radical moiety has a low energy barrier

Two step activation of Ru-PN3P pincer catalysts for CO2 hydrogenation

Activation of homogeneous catalysts is an important step in ensuring efficient operation of any catalytic system as a whole. For the majority of pincer catalysts, the activation step leans heavily on the metal ligand cooperative chemistry that allows these complexes to react with small molecule substrates and engage in catalytic transformations. While the majority of such catalysts require a single activation event to become cooperative, herein we report an exception to this trend. Specifically, we demonstrate that a Ru-PN3P aminopyridine pincer catalyst, which lacks conventional reactivity with hydrogen upon typical one-fold activation, can exhibit this reactivity when a sequential two-step activation is performed. The resulting anionic complexes readily activate molecular hydrogen and react further with CO2 showing the previously unknown reactivity that is critical for CO2 hydrogenation catalysts. While active in CO2 hydrogenation, Ru-PN3Ps are significantly more efficient in hydrogenation of bicarbonates – a likely consequence of the chemistry of these pincers requiring formation of anionic complexes for hydrogen activation

Steric Protection of Rhodium-Nitridyl Radical Species

In an attempt to synthesize a mononuclear rhodium nitridyl complex with a reduced tendency to undergo nitridyl radical N-N coupling we synthesized a bulky analog of Milstein’s bipyridine-based PNNH ligand, bearing a tert-butyl group at the 6’ position of the bipyridine moiety. A three-step synthetic route toward this new bulky tBu3PNNH ligand was developed, involving a selective nucleophilic substitution step, followed by a Stille coupling and a final hydrophosphination step to afford the desired 6-(tert-butyl)-6\u27-((di-tert-butylphosphino)methyl)-2,2\u27-bipyridine (tBu3PNNH) ligand. This newly developed tBu3PNNH ligand was incorporated in the synthesis of the sterically protected azide complex [Rh(N3)(tBu3PNNH)]. We explored N2 elimination form this species using photolysis and thermolysis, hoping to synthesize a mononuclear rhodium complex with a terminal nitrido moiety. Characterization of the reaction product(s) using NMR, coldspray HR-ESI-MS and EPR spectroscopy shows that the material is both EPR and NMR silent, and data obtained by MS spectrometry revealed masses corresponding with both monomeric and dimeric nitrido/nitridyl complexes. The combined data point to formation of a paramagnetic [(tBu3PNN)Rh(µ-N)Rh(tBu3PNN)] species. It thus seems that despite its three tBu groups the new ligand is not bulky enough to prevent formation of Rh-N-Rh bridged species. However, the increased steric environment does prevent further reaction with carbon monoxide, which is unable to coordinate to rhodium.<br /

Two step activation of Ru‐PN3P pincer catalysts for CO2 hydrogenation

Activation of homogeneous catalysts is an important step in ensuring efficient operation of any catalytic system as a whole. For the majority of pincer catalysts, the activation step leans heavily on the metal ligand cooperative chemistry that allows these complexes to react with small molecule substrates and engage in catalytic transformations. While the majority of such catalyst require a single activation event to become cooperative, herein we report an exception to this trend. Specifically we demonstrate that Ru‐PN3P aminopyridine pincer catalyst, that lacks conventional reactivity with hydrogen upon typical one‐fold activation, can engage in this reactivity when a sequential two‐step activation is performed. The resulting anionic complexes readily activate molecular hydrogen and react further with CO2 showing the previously unknown reactivity that is critical for CO2 hydrogenation catalyst. While active in CO2 hydrogenation, Ru‐PN3Ps are significantly more efficient in hydrogenation of bicarbonates – a likely consequence of the chemistry of these pincers requiring formation of anionic complexes for hydrogen activation

Electrocatalytic Azide Oxidation Mediated by a Rh(PNP) Pincer Complex

One-electron oxidation of the rhodium(I) azido complex [Rh(N3)(PNP)] (5), bearing the neutral, pyridine-based PNP ligand 2,6-bis(di-tert-butylphosphinomethyl)pyridine, leads to instantaneous and selective formation of the mononuclear rhodium(I) dinitrogen complex [Rh(N2)(PNP)]+ (9+). Interestingly, complex 5 also acts as a catalyst for electrochemical N3− oxidation (Ep≈−0.23 V vs. Fc+/0) in the presence of excess azide. This is of potential relevance for the design of azide-based and direct ammonia fuel cells, expelling only harmless dinitrogen as an exhaust gas

Accessing the CpArNi(I) Synthon: Reactions with N-Heterocyclic Carbenes, TEMPO, Sulfur, and Selenium

A reactive "(CpNiI)-Ni-Ar" surrogate (Cp-Ar = C-5(C6H4-4-Et)(5)) is accessible via the reduction of the dimer [(CpNi)-Ni-Ar(mu-Br)](2) with two equivalents of KC8. A trapping reaction with TEMPO afforded the new nickel(II) complex [(CpNi)-Ni-Ar(eta(2)-TEMPO)] (3), while the addition of N-heterocyclic carbenes gave the new nickel(I) radicals [(CpNi)-Ni-Ar(IPr)] (4a, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and [(CpNi)-Ni-Ar(IiPr(2)Me(2))] (4b, IiPr(2)Me(2) = 1,3-diisopropy1-4,5-dimethylimidazol-2-ylidene). EPR spectra supported by DFT calculations on 4a and 4b indicate that the spin density mainly resides at the nickel center. The reaction of the "(CpNi)-Ni-Ar(I) source" with yellow sulfur gave the Ni2S6 complex [((CpNi)-Ni-Ar)(2)(mu-S-6)] (5); the "subselenide" [((CpNi)-Ni-Ar)(2)(mu-Se-2)] (6) was formed in the analogous reaction with grey selenium. All new complexes were characterized by NMR, EPR, and UV-vis spectroscopy; their molecular structures were determined by X-ray crystallography