The proton-coupled reduction of Cu-bound nitrite (NO) to nitric oxide (NO + 2H + e → NO(g) + HO), such as occurs in the enzyme copper nitrite reductase, is investigated in this work. Our studies focus on the copper(II/I) model complexes [(L2)Cu(HO)Cl] (1), [(L2)Cu(ONO)] (2), [(L2)Cu(CHCO)] (3), and [Co(Cp)][(L2)Cu(NO)(CHCN] (4), where HL2 = N-[2-(methylthio)ethyl]-2'-pyridinecarboxamide. Complex 1 readily reacts with a NO anion to form the nitrito-O-bound copper(II) complex 2. Electrochemical reduction of Cu → Cu indicates coordination isomerization from asymmetric nitrito-κ-O,O to nitro-κ-N. Isolation and spectroscopic characterization of 4 support this notion of nitrite coordination isomerization (ν ∼ 460 cm). A reduction of 2, followed by reaction with acetic acid, causes evolution of stoichiometric NO via the transient copper(II) nitrosyl species and subsequent formation of the acetate-bound complex 3. The probable copper nitrosyl intermediate [(L2)Cu(NO)(CHCN)] of the {CuNO} type is evident from low-temperature UV-vis absorption (λ = 722 nm) and electron paramagnetic resonance spectroscopy. A density functional theory (DFT)-optimized model of [(L2)Cu(NO)(CHCN)] shows end-on NO binding to Cu with Cu-N(NO) and N-O distances of 1.989 and 1.140 Å, respectively, and a Cu-N-O angle of 119.25°, consistent with the formulation of Cu-NO. A spin-state change that triggers NO release is observed. Considering singlet- and triplet-state electronic configurations of this model, DFT-calculated ν values of 1802 and 1904 cm, respectively, are obtained. We present here important mechanistic aspects of the copper-mediated nitrite reduction pathway with the use of model complexes employing the ligand HL2 and an analogous phenyl-based ligand, N-[2-(methylthio)phenyl]-2'-pyridinecarboxamide (HL1).

The synthesis, structure and properties of highest nuclearity spin delocalized mixed valence (MV) copper–sulfur clusters, a Cu8 nanowheel and a Cu16 nanoball containing Cu2S2 and (μ4-S)Cu4 units resembling CuA and CuZ sites, respectively, are reported.

Aliphatic thiolato-S-bridged tri-and binuclear nickel(II) complexes have been synthesized and characterized as models for the Ni p site of the A cluster of acetyl coenzyme A synthase (ACS)/carbon monooxide (CO) dehydrogenase. Reaction of the in situ formed N 2 S thiol donor ligands withrespectively. The X-ray crystal structures of 1−4 revealed a central Ni II S 4 moiety in 1 and 2 and a Ni II P 2 S 2 moiety in 3 and 4; both moieties have a square-planar environment around Ni and may mimic the properties of the Ni p site of ACS. The electrochemical reduction of both terminal Ni II ions of 1 and 2 occurs simultaneously, which is further confirmed by the isolation of [Ni{(L Me(S) ) 2 Ni(NO)} 2 ](ClO 4 ) 2 ( 5) and [Ni{(L Br(S) ) 2 Ni(NO)} 2 ](ClO 4 ) 2 (6) following reductive nitrosylation of 1 and 2. Complexes 5 and 6 exhibit ν NO at 1773 and 1789 cm −1 , respectively. In the presence of O 2 , both 5 and 6 transform to nitrite-bound monomers [(L Me(S−S) )Ni(NO 2 )]-(ClO 4 ) (7) and [(L Br(S−S) )Ni(NO 2 )](ClO 4 ) 2 (8). The nature of the ligand modification is evident from the X-ray crystal structure of 7. To understand the origin of multiple reductive responses of 1−4, complex [(L Me(SMe) ) 2 Ni](ClO 4 ) 2 ( 9) is considered. The central NiS 4 part of 1 is labile like the Ni p site of ACS and can be replaced by phenanthroline. The treatment of CO to reduce 3 generates a 3 red -(CO) 2 species, as confirmed by Fourier transform infrared (ν CO = 1997 and 2068 cm −1 ) and electron paramagnetic resonance (g 1 = 2.18, g 2 = 2.13, g 3 = 1.95, and A P = 30−80 G) spectroscopy. The CO binding to Ni I of 3 red is relevant to the ACS activity.

According to the well‐accepted mechanism, methyl‐coenzyme M reductase (MCR) involves Ni‐mediated thiolate‐to‐disulfide conversion that sustains its catalytic cycle of methane formation in the energy saving pathways of methanotrophic microbes. Model complexes that illustrate Ni‐ion mediated reversible thiolate/disulfide transformation are unknown. In this paper we report the synthesis, crystal structure, spectroscopic properties and redox interconversions of a set of NiII complexes comprising a tridentate N2S donor thiol and its analogous N4S2 donor disulfide ligands. These complexes demonstrate reversible NiII‐thiolate/NiII‐disulfide (both bound and unbound disulfide‐S to NiII) transformations via thiyl and disulfide monoradical anions that resemble a primary step of MCR's catalytic cycle.

To understand the electron transfer mechanisms (outer versus inner sphere) of catalytic superoxide dismutation via a Cu(ii/i) redox couple such as occur in the enzyme copper-zinc superoxide dismutase, the Cu(ii/i) complexes [(L1)2Cu](ClO4)2·CH3CN, (1·CH3CN) and [(L1)2Cu](ClO4), (2) supported by a bis-N2Sthioether ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine (L1) have been synthesized and structurally characterised. Both 1 and 2 display the same cyclic voltammogram (CV) featuring a quasireversible response at E1/2 = +0.33 V vs. SCE that falls in the SOD potential window of -0.04 V to +0.99 V. These complexes catalytically dismutate superoxide radicals at 298 K in aqueous medium (the IC50 for 1 is 2.15 μM). Electronic absorption spectra (233 K and 298 K), FTIR, ESI mass spectra, CV (233 K and 298 K) and DFT calculations collectively indicate formation of [(L1)2Cu(O2˙(-))](+), [(L1)2Cu(O2(2-))] and [(L1)2Cu(OOH(-))](+) species and help to elucidate the electron transfer mechanism for the SOD function of 1 and 2. Once O2˙(-) binds to Cu(II) (evident at 233 K), the first step of the catalytic cycle (Cu(II) + O2˙(-)→ Cu(I) + O2) does not follow but the second step (Cu(I) + O2˙(-) + 2H(+)→ H2O2 + Cu(II)) does follow. Therefore, the catalytic disproportionation of superoxide radicals via1 and 2 at 298 K indicates that the first and second steps of the catalytic cycle proceed through outer and inner sphere electron transfer mechanisms, respectively. Feasibility of the first step to occur in pure aprotic solvent (where 18-crown-6-ether is used to solubilise KO2) was tested and also supports the same notion of the electron transfer mechanisms as stated above.