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 neutral and acidic conditions. Upon protonation, reduction of 1 is shifted more positive (E pc = 290 mV) upon addition of 1.0 equiv. of acid. This result indicates that ligand protonation facilitates the reduction process, which is also evident from the UV-Vis spectral data. In contrast, with the Ttz tBuMe CuNO 2 (2) analogue, the reduction peak shifted towards a more negative potential while UV-Vis spectra shows no significant changes as acid is added. This suggests that the protons may not be involved in assisting the redox process but rather lead to decomposition events at reducing potentials. Other electrochemical control studies on a series of compounds, namely (Ttz tBu,Me)ZnCl (3), (Ttz tBu,Me)K (4), H(Ttz tBu,Me) (5), and Htz tBu,Me (6), were also conducted with and without acid present. These studies have shown that triazole rings, by themselves and in metal complexes, are not redox active under the conditions we have used. We therefore conclude that the Ttz ligand (including in 1 and 2) is not a site for reduction in our studies.

Proton-Coupled Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

“…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.…”

An investigation on catalytic nitrite reduction reaction by bioinspired CuII complexes

“…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.…”

“…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).…”

Elucidation of the Electrocatalytic Nitrite Reduction Mechanism by Bio-Inspired Copper Complexes

“…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 .…”

“…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.…”