scispace - formally typeset
Search or ask a question
Author

Quinton J. Bruch

Bio: Quinton J. Bruch is an academic researcher from University of North Carolina at Chapel Hill. The author has contributed to research in topics: Medicine & Catalysis. The author has an hindex of 5, co-authored 8 publications receiving 88 citations.
Topics: Medicine, Catalysis, Chemistry, Hydride, Pincer ligand

Papers
More filters
Journal ArticleDOI
TL;DR: A pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond is reported.
Abstract: The direct scission of the triple bond of dinitrogen (N2) by a metal complex is an alluring entry point into the transformation of N2 to ammonia (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl3 (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N2 to afford the trans,trans-isomer of the bimetallic complex [(PONOP)ReCl2]2(μ-N2) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N2 scission, and the trans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cis and cis,cis-isomers with blue light (405 nm) affords the octahedral nitride complex cis-(PONOP)Re(N)Cl2 in 47% spectroscopic yield and 11% quantum yield. The photon energy drives an N2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI2/H2O to produce a rhenium tetrahydride complex (38% yield) and furnish ammonia in 74% yield.

51 citations

Journal ArticleDOI
TL;DR: Despite advances in the development of molecular catalysts capable of reducing dinitrogen to ammonia using proton donors and chemical reductants, few molecular electrocatalysts have been discovered as discussed by the authors.
Abstract: Despite advances in the development of molecular catalysts capable of reducing dinitrogen to ammonia using proton donors and chemical reductants, few molecular electrocatalysts have been discovered...

47 citations

Journal ArticleDOI
TL;DR: Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride.
Abstract: The conversion of metal nitride complexes to ammonia may be essential to dinitrogen fixation. We report a new reduction pathway that utilizes ligating acids and metal–ligand cooperation to effect this conversion without external reductants. Weak acids such as 4-methoxybenzoic acid and 2-pyridone react with nitride complex [(H-PNP)RuN]+ (H-PNP = HN(CH2CH2PtBu2)2) to generate octahedral ammine complexes that are κ2-chelated by the conjugate base. Experimental and computational mechanistic studies reveal the important role of Lewis basic sites proximal to the acidic proton in facilitating protonation of the nitride. The subsequent reduction to ammonia is enabled by intramolecular 2H+/2e– proton-coupled electron transfer from the saturated pincer ligand backbone.

35 citations

Journal ArticleDOI
TL;DR: Crystallographic studies, infraredSpectroscopy, and 15N NMR spectroscopy indicate that N2 remains weakly activated in all cases, providing insight into the donor properties of the different pincer ligand states.
Abstract: A series of ruthenium(II) hydrido dinitrogen complexes supported by pincer ligands in different formal oxidation states have been prepared and characterized. Treating a ruthenium dichloride complex supported by the pincer ligand bis(di-tert-butylphosphinoethyl)amine (H-PNP) with reductant or base generates new five-coordinate cis-hydridodinitrogen ruthenium complexes each containing different forms of the pincer ligand. Further ligand transformations provide access to the first isostructural set of complexes featuring all six different forms of the pincer ligand. The conserved cis-hydridodinitrogen structure facilitates characterization of the π-donor, π-acceptor, and/or σ-donor properties of the ligands and assessment of the impact of ligand-centered multielectron/multiproton changes on N2 activation. Crystallographic studies, infrared spectroscopy, and 15N NMR spectroscopy indicate that N2 remains weakly activated in all cases, providing insight into the donor properties of the different pincer ligand s...

13 citations

Journal ArticleDOI
TL;DR: In this paper, a pincer-ligated rhenium complex (tBuPOCOP)Re(CO)2 ) was used to catalyze CO2 hydrogenation to formate with faster rates at lower temperatures.
Abstract: The catalytic hydrogenation of carbon dioxide holds immense promise for applications in sustainable fuel synthesis and hydrogen storage. Mechanistic studies that connect thermodynamic parameters with the kinetics of catalysis can provide new understanding and guide predictive design of improved catalysts. Reported here are thermochemical and kinetic analyses of a new pincer-ligated rhenium complex (tBuPOCOP)Re(CO)2 (tBuPOCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) that catalyzes CO2 hydrogenation to formate with faster rates at lower temperatures. Because the catalyst follows the prototypical "outer sphere" hydrogenation mechanism, comprehensive studies of temperature and solvent effects on the H2 splitting and hydride transfer steps are expected to be relevant to many other catalysts. Strikingly large entropy associated with cleavage of H2 results in a strong temperature dependence on the concentration of [(tBuPOCOP)Re(CO)2H]- present during catalysis, which is further impacted by changing the solvent from toluene to tetrahydrofuran to acetonitrile. New methods for determining the hydricity of metal hydrides and formate at temperatures other than 298 K are developed, providing insight into how temperature can influence the favorability of hydride transfer during catalysis. These thermochemical insights guided the selection of conditions for CO2 hydrogenation to formate with high activity (up to 364 h-1 at 1 atm or 3330 h-1 at 20 atm of 1:1 H2:CO2). In cases where hydride transfer is the highest individual kinetic barrier, entropic contributions to outer sphere H2 splitting lead to a unique temperature dependence: catalytic activity increases as temperature decreases in tetrahydrofuran (200-fold increase upon cooling from 50 to 0 °C) and toluene (4-fold increase upon cooling from 100 to 50 °C). Ramifications on catalyst structure-function relationships are discussed, including comparisons between "outer sphere" mechanisms and "metal-ligand cooperation" mechanisms.

11 citations


Cited by
More filters
Journal ArticleDOI
TL;DR: In this paper, a metal-organic framework-derived nitrogen-doped nanoporous carbon was used as an electrocatalyst for the nitrogen reduction reaction (NRR) by using renewable electricity.

373 citations

Journal ArticleDOI
TL;DR: In this paper, the earth-abundant dinitrogen (N2) in the aqueous phase was converted to ammonia (NH3) by electrochemically converting it to NH3 at ambient conditions.
Abstract: Ammonia (NH3) synthesis is an important industrial chemical process. Recently, electrochemically converting the earth-abundant dinitrogen (N2) in the aqueous phase to NH3 at ambient conditions has ...

133 citations

Journal ArticleDOI
TL;DR: Isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N2RR, and compare their activities with an isostructureural Fe complex are reported on.
Abstract: Despite the critical role Ru and Os complexes have played in the development of transition metal dinitrogen chemistry, they have not been shown to mediate catalytic N2-to-NH3 conversion (N2RR), nor have M-NxHy complexes been derived from protonation of their M-N2 precursors. To help delineate factors for N2RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N2RR, and compare their activities with an isostructural Fe complex. The Os system is most active, and liberates more than 120 equiv NH3 per Os center in a single batch experiment using Cp*2Co and [H2NPh2][OTf] as reductant and acid source. Isostructural Ru and Fe complexes generate little NH3 under the same conditions. Protonation of Os-N2– affords a structurally characterized Os=NNH2+ hydrazido species that mediates NH3 generation, suggesting it is a plausible intermediate of the catalysis. Inactive Os hydrides are characterized that form during catalysis.

127 citations

Journal ArticleDOI
TL;DR: The field of molecular transition metal complexes as well as recent boron examples for the formation of nitrogen-element bonds are surveyed, highlighting that the challenge for catalytic chemistry is not in the reactivity of coordinated dinitrogen but rather removal of the functionalized ligand from the coordination sphere of the metal.
Abstract: The functionalization of coordinated dinitrogen to form nitrogen–element bonds en route to nitrogen-containing molecules is a long-standing challenge in chemical synthesis. The strong triple bond a...

121 citations

Journal ArticleDOI
TL;DR: This study investigates how the selectivity of a tris(phosphine)borane iron(I) catalyst for catalyzing the nitrogen reduction reaction (N2RR, N2-to-NH3 conversion) versus HER changes as a function of acid p Ka, and finds that there is a strong correlation between p Ka and N2RR efficiency.
Abstract: Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N2, CO2, and O2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P3BFe+, for catalyzing the nitrogen reduction reaction (N2RR, N2-to-NH3 conversion) versus HER changes as a function of acid p Ka. We find that there is a strong correlation between p Ka and N2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N2 reduction (e.g., Fe(NNH) or Fe(NNH2)) in the presence of the metallocene reductant Cp*2Co. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for N2RR. Furthermore, DFT studies also suggest that the observed p Ka effect on N2RR efficiency is attributable to the rate and thermodynamics of Cp*2Co protonation by the different anilinium acids. Inclusion of Cp*2Co+ as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH3. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH3 per Fe at -2.1 V vs Fc+/0).

116 citations