A Copper(II)‐Nitrite Complex Hydrogen‐Bonded to a Protonated Amine in the Second‐Coordination‐Sphere

Abstract

Nitrous acid (HONO) plays pivotal roles in various metal‐free as well as metal‐mediated routes relevant to biogeochemistry, atmospheric chemistry, and mammalian physiology. While the metastable nature of HONO hinders the detailed investigations into its reactivity at a transition metal site, this report herein utilizes a heteroditopic copper(II) cryptate [oC]CuII featuring a proton‐responsive second‐coordination‐sphere located at a suitable distance from a [CuII](ONO) core, thereby enabling isolation of a [CuII](κ1‐ONO⋅⋅⋅H+) complex (2H‐NO2). A set of complementary analytical studies (UV‐vis, 14N/15N FTIR, 15N NMR, HRMS, EPR, and CHN) on 2H‐NO2 and its 15N‐isotopomer (2H‐15NO2) reveals the formulation of 2H‐NO2 as {[oCH]CuII(κ1‐ONO)}(ClO4)2. Non‐covalent interaction index (NCI) based on reduced density gradient (RDG) analysis on {[oCH]CuII(κ1‐ONO)}2+ discloses a H‐bonding interaction between the apical 3° ammonium site and the nitrite anion bound to the copper(II) site. The FTIR spectra of [CuII](κ1‐ONO⋅⋅⋅H+) species (2H‐NO2) shows a shift of ammonium NH vibrational feature to a lower wavenumber due to the H‐bonding interaction with nitrite. The reactivity profile of [CuII](κ1‐ONO⋅⋅⋅H+) species (2H‐NO2) towards anaerobic nitration of substituted phenol (2,4‐DTBP) is distinctly different relative to that of the closely related tripodal [CuII]‐nitrite complexes (1‐NO2/3‐NO2/4‐NO2).

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

“…Analysis of a 15 N-enriched HRMS sample from ( 1-Cu/ 15 N + N 2 H 4 ·H 2 O) reaction exhibits a 1 amu shift of the former peak to 590.3468, while the latter peak at 652.3030 is not sensitive to the 15 N-isotope originating from 15 NO 3 – in 1-Cu/ 15 N (Figures and , and S23). The former m / z feature is assigned as an N -nitrosated ligand ( mC -NO) (calcd m / z = 589.3497 for [ mC -NO + H] + ) (Figure C,D). , The m / z value and the isotopic distribution pattern of the 652.3022 peak is consistent with the formulation as [( mC )­Cu­(N 2 H 2 )] + ( 6a-Cu ) (calcd m / z = 652.3031). [( mC )­Cu­(N 2 H 2 )] + is postulated to form through N 2 H 4 to N 2 H 2 oxidation coupled to NO 3 – reduction.…”

“…However, FTIR analyses of the resultants obtained from the reaction of complex 3-M and NaNO 3 show the presence of free NO 3 – (Figure S16). Hence, the example of nitrate coordinated at the tripodal metal sites in [( Bz 3 Tren )­M II ] 2+ ( M = Cu/Zn) remains unknown to the best of our knowledge, while tripodal ligand tris -(3,5-dimethylpyrazol-1-ylmethyl)­amine is known to support a copper­(II)-nitrate complex in the absence of second-coordination-sphere interactions, perhaps due to different electronic effects of pyrazole relative to the amine donors in Bz 3 Tren . , Moreover, the examples of previously reported nitrite complexes [( Bz 3 Tren )­M II (NO 2 )]­(ClO 4 ) ( 4-M , M = Cu/Zn) , indicate that the rigid second-coordination-sphere as in the cryptate is not essential for the binding of nitrite at the Cu or Zn site in 3-M .…”

“…Hence, the example of nitrate coordinated at the tripodal metal sites in [(Bz 3 Tren)-M II ] 2+ (M = Cu/Zn) remains unknown to the best of our knowledge, while tripodal ligand tris-(3,5-dimethylpyrazol-1ylmethyl)amine is known to support a copper(II)-nitrate complex in the absence of second-coordination-sphere interactions, perhaps due to different electronic effects of pyrazole relative to the amine donors in Bz 3 Tren. 48,55 Moreover, the examples of previously reported nitrite complexes [(Bz 3 Tren)M II (NO 2 )](ClO 4 ) (4-M, M = Cu/ Zn) 31,35 indicate that the rigid second-coordination-sphere as in the cryptate is not essential for the binding of nitrite at the Cu or Zn site in 3-M. 56 Analysis of the resultant Fe complex by FTIR spectroscopy shows distinct ν(NO) features at 1709 cm −1 (for 1-Cu + N 2 H 4 •H 2 O) and 1680 cm −1 (for 1-Cu/ 15 N + N 2 H 4 •H 2 O), which corroborates the formation of (dtc) 2 Fe( 14/15 NO) (Figures 4 and 5A,B). The X-band EPR spectrum of the (dtc) 2 Fe product at room temperature shows a three-line spectrum with g iso = ∼2.04 with isotope-sensitive hyperfine coupling A iso ( 14 N) = ∼14.2 G and A iso ( 15 N) = ∼19.8 G (Figure S18), thereby confirming the formation of (dtc) 2 Fe(NO) from the headspace trapping experiments.…”

“…A set of recent reports illustrates deoxygenation of NO 3 – at relatively less oxophilic transition metal sites (e.g., Co and Ni) typically in the presence of a wide range of oxophilic reagents including CO, (Bpin) 2 (pyrazine), and VCl 3 . − While the reduction of NO 3 – at late transition metal sites remains rare and commonly realized with the usage of oxophilic reagents, − the developments of metal-based NO 3 – reducing processes are challenging due to weakly coordinating and the chemically inert nature of NO 3 – . In contrast, NO 2 – anion exhibits stronger binding affinity toward metal and offers more diverse reactivity patterns in comparison to NO 3 – . − Accordingly, the reductions of nitrite at metal sites have been demonstrated in the presence of a wide range of reducing agents including thiols, − H 2 S, phosphines, − phenols, − and ene-diols. − It is noteworthy that the reaction of hydrazine (N 2 H 4 ) and nitrate in the presence of copper­(II) salts has been known in the literature for several decades. , More specifically, N 2 H 4 in alkaline conditions leads to copper­(II)-mediated transformation of nitrate to nitrite in good yield, thereby enabling estimation of nitrate in water samples . While these results highlight the suitability of N 2 H 4 as a reductant for nitrate and the importance of copper­(II) salts, several factors such as pH, temperature, time, ultrasonic irradiation, and reaction stoichiometry have been known to affect the efficiency of the reductive transformation. , In the absence of molecular-level insights into the N 2 H 4 reactions of NO x – anions at the copper­(II) site, this reaction remains poorly understood.…”

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

“…A proton‐transfer assisted nucleophilic attack of phenol on a nitrite moiety coordinated to a Lewis acidic metal (e.g. Zn II ) site has also been proposed previously for the generation of ArONO [27,48,57] . Moreover, the previously reported [Cu II ]‐nitrite complex supported by a highly electron deficient β‐diketiminato ligand has been illustrated to react with Ph 2 NH or t BuOH following the nucleophilic attack mechanism [21] .…”