Synergistic Activation of Nitrite and Thiocarbonyl Compounds Affords NO and Sulfane Sulfur via (Per)thionitrite (SNO−/SSNO−)

(Per)thionitrite (SNO⁻/SSNO⁻) intermediates play vital roles in modulating nitric oxide (NO) and hydrogen sulfide (H₂S) dependent bio-signalling processes. Whilst the previous preparations of such intermediates involved reactive H₂S/HS⁻ or sulfane sulfur (S⁰) species, the present report reveals that relatively stable thiocarbonyl compounds (such as carbon disulfide (CS₂), thiocarbamate, thioacetic acid, and thioacetate) react with nitrite anion to yield SNO⁻/SSNO⁻. For instance, the reaction of CS₂ and nitrite anion (NO₂⁻) under ambient condition affords CO₂ and SNO⁻/SSNO⁻. A detailed investigation involving UV/Vis, FTIR, HRMS, and multinuclear NMR studies confirm the formation of SNO⁻/SSNO⁻, which are proposed to form through an initial nucleophilic attack by nitrite anion followed by a transnitrosation step. Notably, reactions of CS₂ and nitrite in the presence of thiol RSH show the formation of organic polysulfides R-S_n-R, thereby illustrating that the thiocarbonyls are capable of influencing the pool of bioavailable sulfane sulfurs. Furthermore, the availability of both NO₂⁻ and thiocarbonyl motifs in the biological context hints at their synergistic metal-free activations leading to the generation of NO gas and various reactive sulfur species via SNO⁻/SSNO⁻.     

Chiral Dicopper Complexes with a Doubly Asymmetric Ligand as Models for the Tyrosinase Active Site: Synthesis,Structure, O2‐Reactivity and Comparison with Their Symmetric Analogs

In order to model the asymmetric active site of the type‐3 copper enzyme tyrosinase the “doubly asymmetric” binucleating ligand 1‐[bis‐N,N‐(pyrid‐2‐ylmethyl)aminomethyl]‐3‐[N‐(pyrid‐2‐ylmethyl)‐N‐(2‐pyrid‐2‐ylethyl)aminomethyl]benzene (“unsDMPA”) is synthesized and coordinated to copper(I). The O₂‐reactivity of the Cu^I(unsDMPA) complex and its analog derived from the symmetric counterpiece of unsDMPA, DMPA, is investigated. Oxygenation in methanol leads to dicopper(II) bis(μ‐hydroxo) and bis(μ‐methanolato) complexes; the dicopper(II) bis(μ‐hydroxo) complex of the unsDMPA ligand is chiral. Oxygenation in dichloromethane leads to oxidative N‐dealkylation. This is attributed to a tendency of DMPA and unsDMPA complexes to form dicopper bis(μ‐oxo) intermediates, as evidenced by DFT. The implications of these results with respect to the design of tyrosinase model systems are discussed.     

Hydrocarbon Oxidation by a Porphyrin-π-Cation Radical Complex

Heme and chlorin π-cation radical oxidants are widely implicated in biological and synthetic oxidation catalysis. Little insight into the role of π-cation radicals in proton coupled electron transfer (PCET) oxidation is available. We prepared a Ni^II-porphyrin-π-cation complex ([Ni^II(P⋅⁺)]) and found it to be capable of the oxidation of a variety of simple hydrocarbon substrates. Interestingly, some of the products were hydroxylated, with ([Ni^II(P⋅⁺)]) working in concert with atmospheric O₂ to yield hydroxylated hydrocarbons. Kinetic data suggested that the porphyrin-π-cation radical species oxidised substrates through a concerted PCET mechanism, where the porphyrin-π-cation radical accepted the electron, and the proton was transferred to a free anion. Our findings highlight the potential role of π-cation radicals as hydrocarbon activators, demonstrating that porphyrin ligand non-innocence could be a readily manipulated resource for oxidation catalyst development.     

Methoxide Coordination at the Pocket of [CuIITpCum,Me] and a Simple Model for the Cu Center of Galactose Oxidase

An unusually negative oxidation potential is found for the tyrosine residue in the center of fungal galactose oxidase. The complex [Cu(Tp^Cum,Me){O(MeS)C₆H₄}] (see picture on the right; Tp^Cum,Me=hydrotris(pyrazolyl)borate) offers insight into the mode of (cysteinyl-tyrosine) coordination to the copper center, and the reason for the low oxidation potential.     

Why Does the Active Form of Galactose Oxidase Possess a Diamagnetic Ground State?

The relative orientation of the two magnetic orbitals , the Cu^II d orbital and the half-occupied π orbital of the tyrosyl radical, is the key to answering the question in the title. The arrangement shown (Cu^II-O-C bond angle of about 130° and a dihedral angle of about 90° between the x,y plane of the Cu^II polyhedron and the tyrosyl ring plane) leads to an overlap of the orbitals, which results in a singlet ground state.