NO Coupling at Copper to cis-Hyponitrite: N₂O Formation via Protonation and H-Atom Transfer

Abstract

Copper nitrite reductases (CuNIRs) convert NO 2 − to NO as well as NO to N 2 O under high NO flux at a mononuclear type 2 Cu center. While model complexes illustrate N−N coupling from NO that results in symmetric trans-hyponitrite [Cu II ]− ONNO−[Cu II ] complexes, we report NO assembly at a single Cu site in the presence of an external reductant Cp* 2 M (M = Co, Fe) to give the first copper cis-hyponitrites [Cp* 2 M]{[Cu II ](κ 2 -O 2 N 2 )[Cu I ]}. Importantly, the κ 1 −N-bound [Cu I ] fragment may be easily removed by the addition of mild Lewis bases such as CNAr or pyridine to form the spectroscopically similar anion {[Cu II ](κ 2 -O 2 N 2 )} − . The addition of electrophiles such as H + to these anionic copper(II) cis-hyponitrites leads to N 2 O generation with the formation of the dicopper(II)-bis-μ-hydroxide [Cu II ] 2 (μ-OH) 2 . One-electron oxidation of the {[Cu II ](κ 2 -O 2 N 2 )} − core turns on H-atom transfer reactivity, enabling the oxidation of 9,10dihydroanthracene to anthracene with concomitant formation of N 2 O and [Cu II ] 2 (μ-OH) 2 . These studies illustrate both the reductive coupling of NO at a single copper center and a way to harness the strong oxidizing power of nitric oxide via the neutral cishyponitrite [Cu](κ 2 -O 2 N 2 ).

Reductive NO Coupling at Dicopper Center via a [Cu2(NO)2]2+ Diamond-Core Intermediate

“…Given the ability of 2 to release N 2 O, we hypothesized that it might be either a dicopper hyponitrite or a dicopper nitrosyl species. Due to thermal sensitivity, only a few copper hyponitrite − ,, and copper nitrosyl complexes have been characterized. ,,− Considering literature precedence, we proposed several possible binding modes of NO: (i) dicopper (II,II) trans -hyponitrite, (ii) dicopper (II,II) cis -hyponitrite, (iii) dicopper bis-μ-NO [Cu 2 (NO) 2 ] 2+ , or (iv) dicopper mono-μ-NO [Cu 2 NO] 2+ (Scheme ).…”

“…First, complex 2 could release N 2 O in a rate-limiting step to generate a putative dicopper (II,II) oxo species that quickly captures another equivalent of NO to afford 3 (mechanism A, Figure A). Alternatively, complex 2 can react with another equivalent of NO in a rate-limiting step to release N 2 O and afford 3 in a kinetically invisible step via a [Cu 2 (NO) 3 ] 2+ -type of intermediates or transition states (mechanism B, Figure B). ,, These two scenarios differ in the kinetic dependence on NO concentration. Therefore, they may be distinguished by measuring k obs as a function of [NO].…”

“…(mechanism B, Figure 6B). 8,48,66 These two scenarios differ in the kinetic dependence on NO concentration. Therefore, they may be distinguished by measuring k obs as a function of [NO].…”

“…39−41 In another example by Agapie, 42 a trans-hyponitrite intermediate was also isolated when tricopper(I) precursor was treated with excess NO. Apart from NO coupling at dicopper sites, Tolman, 43−45 Dinca, 46,47 and Warren 48 mononuclear Cu cis-hyponitrite intermediates (Scheme 1C).…”

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

“…However, the observation of mixed-isotope-labeled nitrous oxide N 15 NO from the reaction of N 2 H 4 •H 2 O + {[Cu II ] -( 15 NO 2 )} + rules out the possible coupling or dimerization of NO/HNO generated from nitrite. 63,64 In an alternative mechanism, we propose a nucleophilic attack of N 2 H 4 on the N-site of the copper(II)-bound nitrite leading to the generation of NH 2 −NH−N=O, which decays to NH 3 and NNO (Figure 8). 62 The coformed [Cu II ]−OH complex is presumed to undergo N 2 H 4 -mediated reduction to [Cu I ] as described elsewhere in the literature.…”