Reductive Nitric Oxide Coupling at a Dinickel Core: Isolation of a Key cis‐Hyponitrite Intermediate en route to N₂O Formation

Abstract

Reductive coupling of nitric oxide (NO) to give N2O is an important reaction in the global nitrogen cycle. Here, a dinickel(II) dihydride complex 1 that releases H2 upon substrate binding and serves as a masked dinickel(I) scaffold is shown to reductively couple two molecules of NO within the bimetallic cleft. The resulting hyponitrite complex 2 features an unprecedented cis‐[N2O2]2− binding mode that has been computationally proposed as a key intermediate in flavodiiron nitric oxide reductases (FNORs). NMR and DFT evidence indicate facile rotational fluxionality of the [N2O2]2− unit, which allows to access an isomer that is prone to N2O release. Protonation of 2 is now found to trigger rapid N2O evolution and formation of a hydroxido bridged complex, reminiscent of FNOR reactivity. This work provides fundamental insight into the biologically relevant reductive coupling of two NO molecules and the subsequent trajectory towards N2O formation at bimetallic sites.

Coupling of Nitric Oxide and Release of Nitrous Oxide from Rare-Earth-Dinitrosyliron Complexes

“…7 Evidence for each pathway has been obtained from studies of enzymes and of biomimetic compounds, 8,9 including structural authentication of bimetallic environments in which the metals are bridged by a hyponitrite ligand. 10 Dinitrosyliron complexes (DNICs) are important intracellular NO-containing species, small-molecule models of which abound. [11][12][13][14] The coupling of the two NO ligands in a monometallic model DNIC to produce N2O has, however, never been observed.…”

Nitric oxide reduction forming hyponitrite triggered by metal‐containing complexes

“…Nitric oxide (NO) reduction, known as the process of relieving nitrosative stress for pathogenic bacteria and fungi, is central to the regulation of NO x levels . Biomimetic model studies suggest that NO reduction yielding N 2 O may occur via hyponitrite [N 2 O 2 ] 2− ‐bound intermediate, generated by inter−/intra‐molecular (O)N–N(O) coupling reaction of two reduced metal‐NO units ({Fe(NO)} 7/8 ) or coupling of metal‐[NO] 2− unit with exogenous NO gas . It was demonstrated that NO ligands bound to Fe in mononuclear dinitrosyl iron complexes, existing in cells as intrinsic NO‐storage/−transport species in the biological systems, do not voluntarily form a reductive coupling product (hyponitrite), due to their spin‐parallel electronic structure of two NO‐coordinated ligands in [Fe(NO) 2 ] unit .…”

“…Also, NO reduction to N 2 O occurred upon addition of two equiv of decamethylcobaltocene into {Fe(NO)} 7 ‐{Fe(NO)} 7 [Fe 2 (BPMP)(OPr)(NO) 2 ](BPh) 2 (BPMP − = 2,6‐bis[[bis(2‐pyridylmethyl)amino]‐methyl]‐4‐methylphenolate) generating the proposed superreduced {Fe(NO)} 8 ‐{Fe(NO)} 8 intermediate, compared to addition of 1 equiv of decamethylcobaltocene into [Fe 2 (BPMP)(OPr)](OTf) 2 yielding the semireduced {Fe(NO)} 7 – {Fe(NO)} 8 intermediate at −80°C 4e. Additionally, Meyer et al developed a dihydride dinickel complex [KL(Ni‐H) 2 ] coordinated by ( N , N ′‐((((1H‐pyrazole‐3,5‐diyl)bis(methylene))bis(azanylylidene))‐bis(pent‐2‐en‐2‐yl‐4‐ylidene))bis(2,6‐diisopropylaniline)) (L) providing a pocket site for substrate reduction via reductive elimination of H 2 . Addition of NO into [KL(Ni‐H) 2 ] resulted in the formation of cis ‐hyponitrite‐ligated [KLNi 2 (N 2 O 2 )] via NO reductive coupling, as displayed in Scheme .…”

“…Additionally, Meyer et al developed a dihydride dinickel complex [KL(Ni‐H) 2 ] coordinated by ( N , N ′‐((((1H‐pyrazole‐3,5‐diyl)bis(methylene))bis(azanylylidene))‐bis(pent‐2‐en‐2‐yl‐4‐ylidene))bis(2,6‐diisopropylaniline)) (L) providing a pocket site for substrate reduction via reductive elimination of H 2 . Addition of NO into [KL(Ni‐H) 2 ] resulted in the formation of cis ‐hyponitrite‐ligated [KLNi 2 (N 2 O 2 )] via NO reductive coupling, as displayed in Scheme . The structural analysis reveals the cis ‐N 2 O 2 unit is perpendicular to the square‐planner LNi 2 plane with mutual interaction between potassium ion and the terminal O‐atom of cis ‐N 2 O 2 .…”

Molecularly Controlled Catalysis – Targeting Synergies Between Local and Non‐local Environments

“…Complex 21 constitutes a masked form of a dinickel(I) species (i. e., in 22 ) capable of undergoing pairwise H 2 /D 2 exchange [34a] . Besides, 21 can also reductively activate diverse small‐molecule substrates A−B (e. g., O 2 , [34b] NO, [34c] PhNO, [34d] HCCPh [34a] ) resulting in 23 (Scheme 10a). In contrast, when the pyrazolate‐based bis(β‐diketiminato) ligand features bulky m ‐terphenyl substituents, a reversible 2‐electron C−H reductive elimination/oxidative addition chemistry of the phenyl C(sp 2 )−H bond to the dinickel(I) intermediate occurs via metal−metal cooperativity [35] .…”