Reported are the syntheses, characterizations, and reactivities of the dinuclear nickel(II) complexes [Ni2(κ2-OOCR)3PNNPiPr]+ (R = Me or tBu, PNNPiPr = 2,7-bis-(di-iso-propylphosphino-methyl)-1,8-naphthyridine), isolated as the BF4- salts. Notably, the [Ni2(κ2-OOCR)3PNNPiPr]+ cations can be deprotonated reversibly at the methylene carbon of the ligand scaffold to form the neutral dinuclear Ni(II) complexes [Ni2(κ2-OOCR)3(*PNNPiPr)], where the *PNNPiPr anion is the deprotonated and dearomatized PNNPiPr. The latter complexes were also formed by hydrogen atom transfer (HAT) upon reaction of the mixed valent Ni2(I,II) species [Ni2(κ2-OOCR)3PNNPiPr] with hydrogen atom acceptors 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 2,4,6-triterbutyl phenoxyl radical (tBu3ArO˙) and 2,4-diethoxybenzoquinone (DEBQ) or with air. Based on the reduction potentials obtained via cyclic voltammetry and the pKa values tracked via UV-Vis spectroscopy, the bond dissociation free energies of the methylene C-H bonds were found between 59 and 61 kcal mol-1. The kinetics of the proton/electron abstraction from the mixed valent complexes by various equivalents of TEMPO were monitored by UV-Vis spectroscopy. This process is reversible upon reaction with 5,10-dihydrophenazine, serving as both an electron and proton donor. No reaction intermediates were detected.

Abstract The structural complexity and chemical heterogeneity of lignin present a formidable barrier to its use as a sustainable replacement for petroleum-derived aromatics. Here, we report a robust, earth-abundant copper/nitroxyl catalytic system that achieves the aerobic oxidative funneling of crude, non-pre-fractionated reductive catalytic fractionation (RCF) lignin oil into a single, high-value platform chemical: 2,6-dialkoxybenzoquinone. This transformation operates via a cooperative proton-coupled electron transfer (PCET) mechanism to cleave robust aromatic–aliphatic C–C bonds under mild aerobic conditions (O2 or open air). Using various alcohols as both solvent and nucleophile, the reaction provides modular access to a library of 2,6-dialkoxybenzoquinones in up to 86% isolated yield from model monomers/dimers and an impressive 27 wt % yield (gram product per gram lignin) directly from high-S poplar wood lignin oil (62 mol % convergent yield based on the S-phenol equivalents in the lignin oil). Crucially, we demonstrate that these tunable biomass-derived quinones serve as highly versatile synthons for advanced chemical synthesis to access precursors of bioactive natural product (scutellarein), organic electronics (TCNQ-type acceptors), platform building blocks (1,4-cyclohexanediol), cross-coupling reactions, and a modified high-performancepolyether ether ketone (PEEK) polymer. This work establishes a bridge connecting raw biomass fractionation to diverse applications in synthetic chemistry and materials science.

Described are multiple approaches using density functional theory to probe the acid catalyzed aquation of the hexaammineruthenium(II) cation (Ru(NH3)6 2+ + H3O+ -> Ru(NH3)5(H2O)2+ + NH4 +) reported initially by Taube and co-workers. These computations support the proposal that the initial step is protonation of the Ru(II) center and/or the metal-NH3 bond, thereby activating the latter toward dissociation. DFT analysis was also carried out for the hypothetical acid-mediated aquation of the isoelectronic hexaamminerhodium(III) complex, Rh(NH3)6 3+. The computations suggest a key mechanistic difference for the latter pathway, namely that protonation of the NH3 occurs late in a reaction coordinate involving dissociation of the Rh-NH3 with no direct interaction of H+ with the metal center. Furthermore, while the calculated activation energy is considerably higher in the latter case, the calculations suggest that protonation could play an important role in such ligand substitution reactions.

Lignin, a highly abundant component of lignocellulosic biomass, represents a promising sustainable source of aromatic platform chemicals. This work introduces a selective oxidative cleavage methodology to convert lignin into high-value para-benzoquinones under mild conditions. Employing Bobbitt's salt as the oxidant and leveraging a solvent-mediated side-chain exchange, the process achieves near-quantitative yields of 2,6-diethoxybenzoquinone (DEBQ) and related dialkoxybenzoquinones from 2,6-dimethoxy-4-propylphenol (DMPP). The method demonstrates exceptional selectivity, giving up to 97% DEBQ from lignin monomers and 42 wt% DEBQ from lignin oils derived by reductive catalytic fractionation (RCF) of high-S poplar biomass, surpassing previously reported approaches. The high selectivity towards oxidation of the syringol (S) unit ensures the isolation of DEBQ from a mixture of lignol monomers without chromatography, thereby enhancing potential scalability. Mechanistic studies reveal the critical roles of phenolic hydroxyl groups, benzylic hydrogens, dioxygen, and radical-mediated pathways in achieving oxidative cleavage. By coupling lignin valorization with tunable chemical derivatization, this strategy expands the utility of lignin as a renewable feedstock for pharmaceuticals, materials, and synthetic precursors and unlocks new possibilities for biomass-derived chemical production.

Described are multiple approaches using density functional theory to probe the acid catalyzed aquation of the hexaammineruthenium(II) cation (Ru(NH3)62+ + H3O+ → Ru(NH3)5(H2O)2+ + NH4+) reported initially by Taube and co-workers. These computations support the proposal that the initial step is protonation of the Ru(II) center and/or the metal-NH3 bond, thereby activating the latter toward dissociation. DFT analysis was also carried out for the hypothetical acid-mediated aquation of the isoelectronic hexaamminerhodium(III) complex, Rh(NH3)63+. The computations suggest a key mechanistic difference for the latter pathway, namely that protonation of the NH3 occurs late in a reaction coordinate involving dissociation of the Rh-NH3 with no direct interaction of H+ with the metal center. Furthermore, while the calculated activation energy is considerably higher in the latter case, the calculations suggest that protonation could play an important role in such ligand substitution reactions.

We report a photochemical investigation of the dinuclear metal carbonyl complex (CO)3(bpy)ReMn(CO)5, where bpy = 2,2 '-bipyridine. This investigation follows an earlier study of the photo-activated carbon monoxide release as the result of long-wavelength visible and near-infrared excitation of a several different (CO)5ReMn(CO)3(LL) complexes in aerobic media. These dinuclear species all show a strong metal - metal-bond (sigma MM)-to-ligand (pi L*) charge-transfer (MMLCT) bands as the lowest energy absorptions in their respective optical spectra. Photolysis of the MMLCT band leads to homolysis of the metal-metal bond to give mononuclear metal radicals that can be trapped with dioxygen. The species formed from the manganese carbonyl radical fragments undergo CO release via secondary reactions, while the mononuclear product(s) formed from the center dot Re(CO)3(bpy) radical is luminescent. In these contexts, we discuss potential applications of such dinuclear compounds as photoactivated CO-releasing moieties.

Five novel manganese (I) tricarbonyl complexes were synthesized by the reaction of Mn (CO)5Br with quinoline-2-carboxaldehyde and different anilines. The IR, UV, and myoglobin results indicate that these complexes can be used as photoCORMs (photo photoinduced carbon monoxide releasing molecules) activated by red light. The visible range spectra display strong charge transfer (CT) bands, the maxima of which are sensitive to the ligand substituents. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations show that these CT transitions are admixtures of metal-to-ligand and aniline-to-quinoline charge transfers and that increasing the electron-donating ability on the aniline functionality can lead to shifts in the absorption bands of the corresponding compounds to longer wavelengths and to enhanced band intensities, thus enhancing their photoactivity. Photolysis of these complexes in aerobic solution leads to CO photodissociation followed by oxidation of the Mn(I) center to Mn(II).

The endogenous reduction of nitrite to nitrosyl is drawing increasing attention as a protective mechanism against hypoxic injury in mammalian physiology and as an alternative source of NO, which is involved in a wide variety of biological activities. Thus, chemical mechanisms for this transformation, which are mediated by metallo proteins, are of considerable interest. The study described here examines the reactions of the biomimetic models Co(TTP)(NO2) (TTP = meso-tetratolylporphyrinato dianion) and Mn(TPP)(ONO) (TPP = meso-tetraphenyl-porphyrinato dianion) in sublimated solid films with hydrogen sulfide (H2S) and with ethanethiol (EtSH) at various temperatures from 77 K to room temperature using in situ infrared and optical spectroscopy. In both cases, the coordinated nitrite complex is eventually converted to the respective nitrosyl Co(TTP)(NO) and Mn(TPP)(NO); however, reaction at low temperature first gave a novel six-coordinate complex M(Por)(RSH)(nitrite). Warming these films in the presence of excess thiol resulted in the formation of the two nitrosyl complexes. Mass spectrometric analysis of volatile products and DFT computations of possible intermediates are reported, and potential mechanisms for reduction of the coordinated nitrite ions are discussed.

Mixed-valence multi-metallic complexes, in which the metal is present in more than one oxidation state, provide crucial insight into how electron transfer operates in both biological proteins/enzymes and synthetic inorganic compounds. Nature offers striking examples, such as the oxygen-evolving complex (OEC) of photosystem II, where mixed valency plays an essential role in facilitating proton-coupled electron transfer (PCET) The degree of electronic delocalization between redox sites is subdivided into three groups (Class I, Class II, and Class III) by the Robin-Day classification. Elucidating electronic structure and the function of such systems serves as a foundation for the design of bioinspired catalysts. Nickel, with its rich redox flexibility, is well positioned to form mixed-valent binuclear complexes across several oxidation states, including Ni₂(I,0), Ni₂(I,II), and Ni₂(II,III) dinuclear complexes. Several such systems mirror the redox profiles of enzymes like acetyl-CoA synthase, which is central to C1 metabolism. This perspective highlights the emerging landscape of multinuclear nickel complexes, focusing on their structural classification and redox behavior. Special attention is given to a newly characterized family of Class III Ni₂(I,II) complexes, which exhibit fully delocalized valency. Collectively, this work underscores how mixed-valent states not only advance our understanding of electron transfer mechanisms but can also guide the development of new redox-active materials for catalysis.

Light irradiation has been used in clinical therapy, including Photodynamic Therapy (PDT), for several decades. PDT favors cell death by combining the effect of a photosensitizer, light, and oxygen to generate reactive oxygen species (ROS). However, when one of these components is a limiting factor, instead of dying, the treated cells can activate biochemical defense pathways that promote cell growth, causing the tumor to recur. This work presents the development of new photo-sensitizers able to increase oxidative and nitrosative stress in addition to PDT as a strategy to potentiate tumor cell death. We evaluate the anticancer effects of a synergistic approach using a nitrosyl ruthenium phthalocyanine, trans-[Ru(Pc)(NO)(NO2)], that combine the production of reactive oxygen species (ROS) with the release of nitric oxide (NO) and compare it with its precursor [Ru(Pc)]. Photochemical and photophysical aspects were examined in terms of ROS and NO production by light irradiation and reductimetric processes. The cytotoxicity of the photosensitizers was assessed in both the absence and presence of irradiation against human melanoma cells, A375, and the non-cancer cells, 3 T3. The cytotoxic evaluation in A375 showed 1.0 mu M of [Ru(Pc] killed 63 % while trans-[Ru(NO)(Pc)(NO2)] eradicated 90 %, leading to a synergistic effect. The studies were extended regarding the relevance of the photosensitizer cellular sub-localization assessed by confocal microscopy, indicating their presence in the endoplasmatic reticulum (ER). The cell death biochemistry mechanism was investigated by Western blotting, which revealed expression of caspase-3-cleaved proteins, BAX, and cytochrome c, suggesting that apoptosis was stimulated by the intrinsic pathway initiated by oxidative stress in ER and mitochondria. The NO synergistic effect on PDT was studied by evaluating the cell recurrence by the Scratch Wound assay and the regulation of survival mechanisms related to the expression of the anti-apoptotic transcription factor NF-kB. NO appeared to inhibit NF-kB, suggesting that the synergistic effect was due to pro-apoptotic protein stimulation and anti-apoptotic protein inhibition.

Reported are stopped-flow kinetics studies of the hydrogen peroxide reaction with the water-soluble manganese(I) carbonyl complex. fac-[Mn(CO)3(Br)(bpCO2)]2- (I, bpCO22- = 2,2'-bipyridine-4,4'-dicarboxylate dianion) in alkaline solution. In an earlier study, the H2O2 oxidation of I was shown to be accompanied by the release of more than two equivalents of carbon monoxide. The stopped-flow studies here confirm the strongly pH dependence of the reaction rate, and a revised mechanism for this kinetics behavior is proposed. The larger interest in these systems derives from evaluating redox mechanisms for CO delivery from metal-based carbon monoxide releasing moieties (CORMs) under physiologically relevant conditions.

Hemes play key roles in enzymatic production of the mammalian gasotransmitter NO by nitric oxide synthase as well as in conversion from inorganic nitrite. In the present study, we report a hitherto unknown pathway of nitrosyl formation via thiol reduction of a iron porphyrin nitrate complex in the solid state.

We report the synthesis and characterization of a family of mixed-valent dinickel complexes, formally Ni23+, featuring a naphthyridine-based PNNPiPr pincer ligand (PNNPiPr = 2,7-bis(di-iso-propylphosphino-methyl)-1,8-naphthyridine). The mixed valent Ni23+ complexes, [Ni2(μ-Br)2PNNPiPr]2[NiBr4] (1), [Ni2(κ2-OAc)3PNNPiPr] (2), and [Ni2(κ2-OPiv)3PNNPiPr] (3), were synthesized from the addition of two equivalents of a nickel(II) salt followed by an additional equivalent of Ni(COD)2 (COD = cyclooctadiene) to the PNNPiPr ligand. The delocalized Ni23+ mixed valence assignment was corroborated by single-crystal X-ray diffraction, electron paramagnetic resonance, and magnetic susceptibility measurements. In addition to these measurements, the presence of the Ni-Ni bond and symmetrical structure qualifies these complexes as Class III mixed valence in the Robin-Day classification scheme. Cyclic voltammetry experiments used to characterize the redox properties of these complexes exhibited a metal-centered, quasi-reversible couple associated with the Ni2(II,II)/Ni2(I,II) reduction event, with E1/2 values ranging from -0.83 V to -1.05 V vs (C5H5)2Fe+/0 in acetonitrile for the series. Further tracking by UV-vis spectroscopy showed that the mixed-valent and (PNNPiPr)Ni2(II,II) complexes can be interconverted through either chemical oxidation or reduction using ferrocenium tetrafluoroborate ([(C5H5)2Fe][BF4]) or cobaltocene ((C5H5)2Co), respectively.

Cancer is one of the leading causes of death worldwide. Contemporary therapies do not achieve the expected effectiveness, the treatment is often non-selective, and its application is associated with several significant side effects, reducing the quality of life of patients who underwent surgery and their long and expensive hospitalization. Photodynamic therapy appears as a suitable clinical treatment once it is a non-invasive technique and uses a photosensitizer, oxygen, and light irradiation to kill cancer cells. Perhaps the main limitation of PDT is oxygen, once the tumor is mainly hypoxic. In this chapter, we propose the use of ruthenium-phthalocyanine compounds in anticancer therapy, essentially using the basic principles of photodynamic therapy. The evaluation of photocytotoxic effects is described as a function of the structure-activity relationship as well as the synergistic effect between nitric oxide and reactive oxygen species generated by light irradiation in the therapeutic window.

During the past few years, renewed interest in the luminescence properties of polynuclear copper(I) compounds based on Cu4I4L4 derivatives and salts of the Cu4I62- anion has arisen with regard to potential applications of these materials as radiation sensors. Here we review ongoing studies in this laboratory and in a number of others describing such systems. Investigations of the time-resolved radioluminescence of crystalline Cu4I62- salts reveal different temporal responses to excitation with various radiation types. The differences can be attributed to the generation of multiple excited states (or self-trapped excitons) in confined volumes of the solid salts, leading to non-exponential decays of these scintillators owing to bimolecular, nonradiative quenching.

Lignin-based syringol monomer, 2,6-dimethoxypropylphenol (DMPP), can be upgraded through two synthetic routes to multi-functional propylpyrogallol DMPPO and tri-epoxide which allows for various polymer applications.

The effects of the nitro-ruthenium porphyrin complex, RuNO 2 TPyP, in lung cells using 2D and 3D cell culture models.