Title: Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical
Abstract Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth’s most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table. more »« less
Wong, Anthony; Chakraborty, Arunavo; Bawari, Deependra; Wu, Guang; Dobrovetsky, Roman; Ménard, Gabriel
(, Chemical Communications)
null
(Ed.)
We report the facile activation of aryl E–H (ArEH; E = N, O, S; Ar = Ph or C 6 F 5 ) or ammonia N–H bonds via coordination-induced bond weakening to a redox-active boron center in the complex, (1 − ). Substantial decreases in E–H bond dissociation free energies (BDFEs) are observed upon substrate coordination, enabling subsequent facile proton-coupled electron transfer (PCET). A drop of >50 kcal mol −1 in H 2 N–H BDFE upon coordination was experimentally determined.
Stephens, David_N; Mock, Michael_T
(, European Journal of Inorganic Chemistry)
Abstract The molecular complexes described herein use main‐group elements or transition metals to control the stoichiometric cleavage of N−H bonds of ammonia (NH3) and/or catalyze chemical and electrochemical NH3oxidation to dinitrogen (N2). We highlight the phenomenon of coordination‐induced bond weakening and a variety of N−H bond cleavage mechanisms of NH3including H atom abstraction, inter‐ and intra‐molecular deprotonation reactions, oxidative addition, andσ‐bond metathesis that have been demonstrated with molecular systems. We provide an overview of the molecular complexes reported for the rapidly developing field of NH3oxidation catalysis to form N2. These systems exhibit several diverse structure types and innovative ligands to support transition metals capable of activating NH3and mediating a challenging chemical transformation that requires breaking strong N−H bonds and forming an N−N bond en route to N2formation.
Hooe, Shelby L.; Cook, Emma N.; Reid, Amelia G.; Machan, Charles W.
(, Chemical Science)
The two-electron and two-proton p -hydroquinone/ p -benzoquinone (H 2 Q/BQ) redox couple has mechanistic parallels to the function of ubiquinone in the electron transport chain. This proton-dependent redox behavior has shown applicability in catalytic aerobic oxidation reactions, redox flow batteries, and co-electrocatalytic oxygen reduction. Under nominally aprotic conditions in non-aqueous solvents, BQ can be reduced by up to two electrons in separate electrochemically reversible reactions. With weak acids (AH) at high concentrations, potential inversion can occur due to favorable hydrogen-bonding interactions with the intermediate monoanion [BQ(AH) m ]˙ − . The solvation shell created by these interactions can mediate a second one-electron reduction coupled to proton transfer at more positive potentials ([BQ(AH) m ]˙ − + n AH + e − ⇌ [HQ(AH) (m+n)−1 (A)] 2− ), resulting in an overall two electron reduction at a single potential at intermediate acid concentrations. Here we show that hydrogen-bonded adducts of reduced quinones and the proton donor 2,2,2-trifluoroethanol (TFEOH) can mediate the transfer of electrons to a Mn-based complex during the electrocatalytic reduction of dioxygen (O 2 ). The Mn electrocatalyst is selective for H 2 O 2 with only TFEOH and O 2 present, however, with BQ present under sufficient concentrations of TFEOH, an electrogenerated [H 2 Q(AH) 3 (A) 2 ] 2− adduct (where AH = TFEOH) alters product selectivity to 96(±0.5)% H 2 O in a co-electrocatalytic fashion. These results suggest that hydrogen-bonded quinone anions can function in an analogous co-electrocatalytic manner to H 2 Q.
Cooney, Shannon E.; Schreiber, Eric; Brennessel, William W.; Matson, Ellen M.
(, Inorganic Chemistry Frontiers)
Anionic dopants, such as O-atom vacancies, alter the thermochemical and kinetic parameters of proton coupled electron transfer (PCET) at metal oxide surfaces; understanding their impact(s) is essential for informed material design for efficient energy conversion processes. To circumvent challenges associated with studying extended solids, we employ polyoxovanadate–alkoxide clusters as atomically precise models of reducible metal oxide surfaces. In this work, we examine net hydrogen atom (H-atom) uptake to an oxygen deficient vanadium oxide assembly, [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 . Addition of two H-atom equivalents to [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 results in formation of [V 6 O 5 (MeCN)(OH 2 )(OCH 3 ) 12 ] 0 . Assessment of the bond dissociation free energy of the O–H bonds of the resultant aquo moiety reveals that the presence of an O-atom defect weakens the O–H bond strength. Despite a decreased thermodynamic driving force for the reduction of [V 6 O 6 (MeCN)(OCH 3 ) 12 ] 0 , kinetic investigations show the rate of H-atom uptake at the cluster surface is ∼100× faster than its oxidized congener, [V 6 O 7 (OCH 3 ) 12 ] 0 . Electron density derived from the O-atom vacancy is shown to play an important role in influencing H-atom uptake at the cluster surface, lowering activation barriers for H-atom transfer.
Ghosh, Pokhraj; Ding, Shengda; Chupik, Rachel B.; Quiroz, Manuel; Hsieh, Chung-Hung; Bhuvanesh, Nattami; Hall, Michael B.; Darensbourg, Marcetta Y.
(, Chemical Science)
Experimental and computational studies address key questions in a structure–function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN 2 S 2 or [(NO)Fe]N 2 S 2 as donors to (η 5 -C 5 H 5 )Fe(CO) + or [Fe(NO) 2 ] +/0 generate a series of four bimetallics, gradually “softened” by increasing nitrosylation, from 0 to 3, by the non-innocent NO ligands. The nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels, demonstrating required features of electrocatalysis. Computational modeling of experimental structures and likely transient intermediates that connect the electrochemical events find roles for electron delocalization by NO, as well as Fe–S bond dissociation that produce a terminal thiolate as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation is via proton/hydride coupling by internal S–H + ⋯ − H–Fe units of the “harder” bimetallic arrangements with more localized electron density, while softer units convert H − ⋯H − via reductive elimination from two Fe–H deriving from the highly delocalized, doubly reduced [Fe 2 (NO) 3 ] − derivative. Computational studies also account for the inactivity of a Ni 2 Fe complex resulting from entanglement of added H + in a pinched –S δ− ⋯H + ⋯ δ− S− arrangement.
Sinhababu, Soumen, Singh, Roushan Prakash, Radzhabov, Maxim R., Kumawat, Jugal, Ess, Daniel H., and Mankad, Neal P. Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical. Nature Communications 15.1 Web. doi:10.1038/s41467-024-45721-1.
Sinhababu, Soumen, Singh, Roushan Prakash, Radzhabov, Maxim R., Kumawat, Jugal, Ess, Daniel H., and Mankad, Neal P.
"Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical". Nature Communications 15 (1). Country unknown/Code not available: Nature Publishing Group. https://doi.org/10.1038/s41467-024-45721-1.https://par.nsf.gov/biblio/10490682.
@article{osti_10490682,
place = {Country unknown/Code not available},
title = {Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical},
url = {https://par.nsf.gov/biblio/10490682},
DOI = {10.1038/s41467-024-45721-1},
abstractNote = {Abstract Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth’s most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table.},
journal = {Nature Communications},
volume = {15},
number = {1},
publisher = {Nature Publishing Group},
author = {Sinhababu, Soumen and Singh, Roushan Prakash and Radzhabov, Maxim R. and Kumawat, Jugal and Ess, Daniel H. and Mankad, Neal P.},
}
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