Electrochemical reduction of Ttz copper(II) complexes in the presence and absence of protons: Processes relevant to enzymatic nitrite reduction (TtzR,R′= tris(3-R, 5-R′-1, 2, 4-triazolyl)borate)
Abstract:Tris(triazolyl)borate (Ttz) is a proton responsive ligand, and the redox potential of Ttz complexes can be altered by protonation. Protonation events can therefore alter the thermodynamics of reduction of copper complexes, and this is relevant to nitrite reduction mediated by copper complexes wherein Cu(II) reduction to Cu(I) is the first step. The electrochemical behavior of tris(triazolyl)borate and the corresponding copper complexes, Ttz tBu,Me CuCl (1) and Ttz tBu,Me CuNO 2 (2), was investigated under both… Show more
“…On account of the comparatively simple active site structure of CuNIR suggested by the crystallographic studies, a number of groups have developed simplified analogues of this enzyme based on tripodal N-donor ligands coordinated to a Cu center. ,− In many cases, these analogues also encapsulate the functionality of CuNIR (and so can mediate the reduction of nitrite to NO), normally with the use of stoichiometric sacrificial electron donors. There is also a distinct subset of such Cu–N donor complexes that can mediate the electrocatalytic reduction of nitrite to NO. − In these latter cases in particular, the role of solution pH (or the presence of additional proton sources in nonaqueous media) has been shown to be critical, with acidic regimes being essential for catalytic reduction of nitrite to NO. This is perhaps not surprising, given the dependence of eq on the presence of protons.…”
The
selective and efficient electrocatalytic reduction of nitrite
to nitric oxide (NO) is of tremendous importance, both for the development
of NO-release systems for biomedical applications and for the removal
of nitrogen oxide pollutants from the environment. In nature, this
transformation is mediated by (among others) enzymes known as the
copper-containing nitrite reductases. The development of synthetic
copper complexes that can reduce nitrite to NO has therefore attracted
considerable interest. However, there are no studies describing the
crucial role of proton-coupled electron transfer during nitrite reduction
when such synthetic complexes are used. Herein, we describe the synthesis
and characterization of two previously unreported Cu complexes (3 and 4) for the electrocatalytic reduction of
nitrite to NO, in which the role of proton-relaying units in the secondary
coordination sphere of the metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the copper
center, while complex 3 lacks such functionality. Our
results suggest that complex 4 is twice as effective
an electrocatalyst for nitrite reduction than is complex 3 and that complex 4 is the best copper-based molecular
electrocatalyst for this reaction yet discovered. The differences
in reactivity between 3 and 4 are probed
using a range of electrochemical, spectroscopic, and computational
methods, which shed light on the possible catalytic mechanism of 4 and implicate the proton-relaying ability of its pendant
carboxylate group in the enhanced reactivity that this complex displays.
These results highlight the critical role of proton-coupled electron
transfer in the reduction of nitrite to NO and have important implications
for the design of biomimetic catalysts for the selective interconversions
of the nitrogen oxides.
“…On account of the comparatively simple active site structure of CuNIR suggested by the crystallographic studies, a number of groups have developed simplified analogues of this enzyme based on tripodal N-donor ligands coordinated to a Cu center. ,− In many cases, these analogues also encapsulate the functionality of CuNIR (and so can mediate the reduction of nitrite to NO), normally with the use of stoichiometric sacrificial electron donors. There is also a distinct subset of such Cu–N donor complexes that can mediate the electrocatalytic reduction of nitrite to NO. − In these latter cases in particular, the role of solution pH (or the presence of additional proton sources in nonaqueous media) has been shown to be critical, with acidic regimes being essential for catalytic reduction of nitrite to NO. This is perhaps not surprising, given the dependence of eq on the presence of protons.…”
The
selective and efficient electrocatalytic reduction of nitrite
to nitric oxide (NO) is of tremendous importance, both for the development
of NO-release systems for biomedical applications and for the removal
of nitrogen oxide pollutants from the environment. In nature, this
transformation is mediated by (among others) enzymes known as the
copper-containing nitrite reductases. The development of synthetic
copper complexes that can reduce nitrite to NO has therefore attracted
considerable interest. However, there are no studies describing the
crucial role of proton-coupled electron transfer during nitrite reduction
when such synthetic complexes are used. Herein, we describe the synthesis
and characterization of two previously unreported Cu complexes (3 and 4) for the electrocatalytic reduction of
nitrite to NO, in which the role of proton-relaying units in the secondary
coordination sphere of the metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the copper
center, while complex 3 lacks such functionality. Our
results suggest that complex 4 is twice as effective
an electrocatalyst for nitrite reduction than is complex 3 and that complex 4 is the best copper-based molecular
electrocatalyst for this reaction yet discovered. The differences
in reactivity between 3 and 4 are probed
using a range of electrochemical, spectroscopic, and computational
methods, which shed light on the possible catalytic mechanism of 4 and implicate the proton-relaying ability of its pendant
carboxylate group in the enhanced reactivity that this complex displays.
These results highlight the critical role of proton-coupled electron
transfer in the reduction of nitrite to NO and have important implications
for the design of biomimetic catalysts for the selective interconversions
of the nitrogen oxides.
“…The different order trend found for the catalytic cycle implies that reduction from Cu II to Cu I may be a controlling factor during the catalytic cycle. Moreover, Papish 42 and Dilworth 45 both reported that the reduction of Cu II to Cu I should be a key step in enzymatic nitrite reductions. Therefore, we were further interested in the reduction behaviors of the three LCu II complexes and Cu II (NO 3 ) 2 •3H 2 O.…”
Section: Papermentioning
confidence: 99%
“…38 These Cu I -nitro and Cu II -nitrito complexes are good bioinspired compounds and allow chemists to understand each step of the nitrite reduction mechanism; however, the catalytic aspects of copper-mediated nitrite reductions are rare, although there are a number of studies on bioinspired electro-catalysts used for nitrite reduction. [39][40][41][42][43][44] Dilworth described catalytic studies on Cu II complexes that converted NO 2 − into NO (g) and N 2 O (g) by using L-ascorbic acid. 45 Inspired by the reactivities of known Cu I -nitro complexes, 26,27 we chose three typical ligands (neutral Me2 Tpm (L1), anionic Me2 Tp (L2), and neutral iPr TIC (L3; tris(1-methyl-2-isopropyl-4imidazolyl)-carbinol) to explore how the electron-donating ability of the ligand affects catalytic nitrite reduction from Cu II sources in comparison with stoichiometric nitrite reduction from Cu I -nitro species (Scheme 2).…”
Catalytic nitrite reductions by CuII complexes containing anionic Me2Tp, neutral Me2Tpm, or neutral iPrTIC ligands in the presence of L-ascorbic acid, which served as an electron donor and proton source,...
“…In recent years, bio-inspired CuNiRs are also studied using electrochemical methods in both organic ,, and aqueous solutions. − ,,− The first electrochemical studies on nitrite reduction were reported in 1993 and 1995 by the group of Komeda. , Herein, the authors report that [Cu(tmpa)(OH 2 )] 2+ (tmpa = tris(2-pyridylmethyl)amine, Cu(tmpa) , see Figure b) reduces NO 2 – electrocatalytically to NO. Later, the Meyerhoff group reported its use in NO-releasing catheters, showing that Cu(tmpa) is a stable catalyst for more than 7 days .…”
Section: Introductionmentioning
confidence: 99%
“…The reactivity of various Cu-(I) 20−30 and Cu(II) 22,26−29,31−41 model compounds with nitrite was investigated, and several Cu(II)−NO 27,42−44 and Cu(I)−NO 45−47 studies by Fujii and co-workers showed that a stepwise protonation mechanism is operative in dichloromethane, 23,25 and work by Hsu and co-workers sheds light on the possible formation of HNO 2 at low pH. 38 In recent years, bio-inspired CuNiRs are also studied using electrochemical methods in both organic 34,48,49 and aqueous solutions. 11−14,37,50−52 The first electrochemical studies on nitrite reduction were reported in 1993 and 1995 by the group of Komeda.…”
Mononuclear copper complexes relevant to the active site of copper nitrite reductases (CuNiRs) are known to be catalytically active for the reduction of nitrite. Yet, their catalytic mechanism has thus far not been resolved. Here, we provide a complete description of the electrocatalytic nitrite reduction mechanism of a bio-inspired CuNiR catalyst Cu(tmpa) (tmpa = tris(2-pyridylmethyl)amine) in aqueous solution. Through a combination of electrochemical studies, reaction kinetics, and density functional theory (DFT) computations, we show that the protonation steps take place in a stepwise manner and are decoupled from electron transfer. The rate-determining step is a general acid-catalyzed protonation of a copper-ligated nitrous acid (HNO 2 ) species. In view of the growing urge to convert nitrogencontaining compounds, this work provides principal reaction parameters for efficient electrochemical nitrite reduction. This contributes to the investigation and development of nitrite reduction catalysts, which is crucial to restore the biogeochemical nitrogen cycle.
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