Nitrate and Nitrite Reductions at Copper(II) Sites: Role of Noncovalent Interactions from Second-Coordination-Sphere

Abstract

Reductions of nitrate and nitrite (NO x –) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NO x – reductions are known, this report illustrates hydrazine (N2H4)-mediated transformations of NO x – to nitric oxide (NO)/nitrous oxide (N2O). For nitrate reduction to NO, initial coordination of the weakly coordinating NO3 – anion at [(mC)CuII]2+ cryptate has been demonstrated to play a crucial role. A set of complementary analyses (X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR), UV–vis, and NMR spectroscopies) on NO3 –-bound metal-cryptates [(mC)MII(NO3)](ClO4) (1-M, M = Cu/Zn) demonstrates the binding of NO3 – through noncovalent (NH···O, CH···O, and anion···π) and metal–ligand coordinate interactions. Subsequently, reactions of [(mC)CuII(14/15NO3)](ClO4) (1-Cu or 1-Cu/ 15 N) with N2H4·H2O have been illustrated to reduce 14/15NO3 – to 14/15NO. Intriguingly, in the absence of the second-coordination-sphere interactions, a closely related coordination motif [(Bz 3 Tren)CuII]2+ (in 3-Cu) does not bind NO3 – and is unable to assist in N2H4·H2O-mediated NO3 – reduction. In contrast, nitrite coordinates at the tripodal CuII sites in both [(mC)CuII]2+ and [(Bz 3 Tren)CuII]2+ irrespective of the additional noncovalent interactions, and hence, the N2H4 reactions of the copper(II)-nitrite complexes [(mC)CuII(O14/15NO)]+ and [(Bz 3 Tren)CuII(O14/15NO)]+ (in 2-Cu/4-Cu) result in a mixture of 14/15NO and N14/15NO.

NO Generation from Nitrite at Zinc(II): Role of Thiol Persulfidation in the Presence of Sulfane Sulfur

“…We also turned to probe the coformation of other minor gaseous products such as N 2 O and NH 3 . Analysis of headspace by FTIR spectroscopy on a gaseous sample shows characteristic vibrational features at 2207 and 2236 cm −1 (Figure S25), 40 thereby suggesting the generation of N 2 O from [Zn II ]−nitrite (1) + t BuBnSH + S 8 . Moreover, trapping of headspace in the presence of acid followed by 1 H NMR spectroscopic analysis in DMSO-d 6 shows a triplet centered at δ = ∼7.18 ppm ( 1 J NH = 50.3 Hz), which is assigned to NH 4 + (Figures S26 and S27).…”

Thiol and H2S-Mediated NO Generation from Nitrate at Copper(II)

“…Alternatively, OAT from nitrate may occur directly to a reductant. In some cases, carbon monoxide, phosphines, or the silyl-based Mashima reagent can abstract an O atom from a bound nitrate to give CO 2 , R 3 PO, or TMS-O-TMS. ,− Moreover, intramolecular O-atom transfer between metal-bound nitrate and nitrosyl can form nitrogen dioxide or nitrite. , Other methods include hydrazine-mediated nitrate reduction as well as electrocatalytic nitrate reduction using molecular complexes in homogeneous or heterogenized forms. ,− …”

“…25,30−34 Moreover, intramolecular O-atom transfer between metalbound nitrate and nitrosyl can form nitrogen dioxide or nitrite. 35,36 Other methods include hydrazine-mediated nitrate reduction 37 as well as electrocatalytic nitrate reduction using molecular complexes in homogeneous or heterogenized forms. 21,38−41 We previously described reduction of nitrite by thiols to nitric oxide and disulfides at the β-diketiminato-supported copper(II) nitrite complex [Cl 2 NN F6 ]Cu(κ 2 -O 2 N) (1), abbreviated as [Cu II ](κ 2 -O 2 N), and the THF-bound adduct 1-THF.…”

Nitrite and Nitric Oxide Interconversion at Mononuclear Copper(II): Insight into the Role of the Red Copper Site in Denitrification

“…[17][18][19][20] Moreover, reduction of nitrite-to-NO at copper(II) model complexes has been illustrated in the presence of various reductants such as thiol, [21,22] phosphine, [23][24][25] phenol, [26][27][28] catechol, [28] ascorbic acid, [29] and hydrazine. [30] In addition, reduction of a formal copper(III)-nitrite by phenol has been reported to follow proton-coupled electron-transfer (PCET) route. [31] On the other hand, aiming to understand the reactivity of NO with copper(II), [32][33][34][35][36] several examples demonstrate that NO leads to an inner-sphere electron transfer to reduce Cu II to Cu I through a {CuNO} 10 species.…”

“…A majority of the previous studies employing copper(I)‐nitrite complexes provide insights into the various mechanistic aspects of T2Cu nitrite reductases [17–20] . Moreover, reduction of nitrite‐to‐NO at copper(II) model complexes has been illustrated in the presence of various reductants such as thiol, [21,22] phosphine, [23–25] phenol, [26–28] catechol, [28] ascorbic acid, [29] and hydrazine [30] . In addition, reduction of a formal copper(III)‐nitrite by phenol has been reported to follow proton‐coupled electron‐transfer (PCET) route [31] .…”