A highly sensitive and selective rhodamine-semicarbazide based fluorescent sensor for Cu2+ detection in real water samples and fluorescence bioimaging in HepG2 cells

A colorimetric and fluorescent sensor, selective for Cu2+ ions, was synthesized in two steps using a rhodamine-based compound attached to the semicarbazide-picolylamine moiety (RBP). Spectroscopic measurements, including UV-Vis absorption and fluorescence emission, were conducted in the semi-aqueous medium containing acetonitrile/4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, denoted as MeCN/HEPES buffer (2:8, v/v, pH 7.0). The sensor exhibited high selectivity towards Cu2+ ions compared to other cations and demonstrated remarkable sensitivity towards Cu2+ ions, with a limit of detection at the nanomolar level. The calculated transitions indicated a 1:1 stoichiometric binding of RBP to Cu2+ ions based on a 4-coordination mode involving additional chelation in the semi-aqueous medium. The sensing mechanism for the detection of Cu2+ ions was investigated using high-resolution mass spectroscopy. The sensor could be employed as a real-time chemosensor for monitoring Cu2+ ions. Furthermore, the sensor has the potential for utilization in the detection of Cu2+ ions in actual water samples with the high precision and accuracy, as indicated by the small relative standard derivation values. The 50th percentile cytotoxicity concentration of RBP was found to be 22.92 μM. Additionally, the fluorescence bioimaging capability of RBP was demonstrated for the detection of Cu2+ ions in human hepatocellular carcinoma (HepG2) cells.

Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay

The hydrogen evolution reaction is an important process for energy storage. The six-coordinate cobalt complex [CoIII(L1-)(LH)]2+ (LH = N-(4-amino-6-(pyridin-2-yl)-1,3,5-triazin-2-yl)benzamidine) was found to catalyze photocatalytic hydrogen evolution. In this work, we performed density functional calculations to obtain the reduction potentials and the proton-transfer free energy of possible intermediates to determine the preferred pathways for proton reduction. The mechanism involves the metal-based reduction of Co(III) to Co(II) before the protonation at the amidinate N on the pyridinyl-substituted diaminotriazine benzamidinate ligand L1- to form [CoII(LH)(LH)]2+. Essentially, the subsequent electron transfer is not metal-based reduction, but rather ligand-based reduction to form [CoII(LH)(LH˙1-)]1+. Through a proton-coupled electron transfer process, the cobalt hydride [CoIIH(LH)(LH2˙)]1+ is formed as the key intermediate for hydrogen evolution. As the cobalt hydride complex is coordinatively saturated, a structural change is required when the hydride on Co is coupled with the proton on pyridine. Notably, the redox-active nature of the ligand results in the low acidity of the protonated pyridine moiety of LH2˙, which impedes its function as a proton relay. Our findings suggest that separating the proton relay fragment from the electron reservoir fragment of the redox-active ligand is preferred for fully utilizing both features in catalytic H2 evolution.

Tunable Metal-Free Imidazole-Benzimidazole Electrocatalysts for Oxygen Reduction in Aqueous Solutions

A series of metal-free imidazole-benzimidazole catalysts (ImBenz-H, ImBenz-NO2 , ImBenz-OCH3 ) for oxygen reduction reaction (ORR) were prepared. We demonstrate that the electrocatalytic O2 reduction by ImBenz-NO2 with the electron-withdrawing group showed high selectivity toward H2 O with the number of electrons transferred (n=3.7) in a neutral aqueous solution. The highest ORR selectivity toward H2 O2 was achieved using ImBenz-H (n=2.4) in an alkaline solution. Electrochemical studies of reaction kinetics disclosed that the highest turnover frequencies were obtained from ImBenz-H in both neutral and alkaline aqueous solutions. The results prove that the ORR selectivity is tunable by modulating the substituent of the ImBenz catalysts. Furthermore, DFT calculations suggested that the ORR mechanism of ImBenz-H involves the electron transfer from imidazole-benzimidazole to O2 resulting in the formation of H2 O2 which supports the redox active properties of the catalysts ImBenz.

Migration of Hydride, Methyl, and Chloride Ligands between Al and M in (PAlP)M Pincer Complexes (M = Rh or Ir)

Protolysis of AlMe3 or AlBui3 with 2-diisopropylphosphinopyrrole (1) yields molecules containing two flanking phosphines and a central Al-Me (2-Me), Al-iBu (2-iBu), or Al-H (2-H) unit. The reactions of 2-Me with [L2MCl]2 (L = cyclooctene or 1/2 1,5-cyclooctadiene and M = Rh or Ir) in the presence of pyridine produces PAlClP pincer complexes (3-Rh and 3-Ir) with Al-Cl and M-Me bonds. The analogous reaction of a mixture of 2-iBu and 2-H with [L2MCl]2 and pyridine resulted in the formation of analogous Rh-H (4-Rh) and Ir-H (4-Ir) complexes. Treatment of 3-Rh with NaBEt3H produced compound 5-Rh with an Al-Me and a Rh-H bond; the analogous reaction of 3-Ir did not result in a clean product. 4-Ir accepted an equivalent of H2 to produce 6-Ir with two terminal Ir-H bonds and one bridging Al-H-Ir moiety, whereas 4-Rh did not react with H2. The density functional theoretical treatment is in accord with this finding, highlights the likely mechanism for the H2 addition, and supports the bonding picture in 6-Ir arising from NMR and X-ray diffraction (XRD) observations. Spectroscopic data and XRD studies are consistent with distorted square-pyramidal structures (about Rh or Ir) for compounds 3-5, with an alane occupying the apical position. Complexes 3 and 4 possess some of the shortest known Rh-Al or Ir-Al distances.

Oudemansin and 9-methoxystrobilurin derivatives with antimalarial activity from cultures of the basidiomycete Favolaschia minutissima: assignments of the absolute configurations of the isoprene-derived units

Five undescribed polyketide metabolites, oudemansins E (1), M (2), P (3), and Q (4), and 9-methoxystrobilurin I (5), were isolated from cultures of basidiomycete Favolaschia minutissima TBRC-BCC 19434. A γ-lactone derivative (6) of noroudemansin A (8), which was previously reported as a semisynthetic compound, was also isolated. The absolute configuration of the isoprene-derived moiety of the known cometabolite 9-methoxystrobilurin E (9) was determined to be 2'R,6'S by comparison of the experimental and calculated ECD data, which was correlated to the new derivative 1. These compounds exhibited antimalarial activity against Plasmodium falciparum K1 (multidrug-resistant strain). A putative minor natural product, namely 9-methoxystrobilurin P (13), was prepared by semisynthesis, which exhibited significant antimalarial activity (IC50 0.086 μM).

Base-catalyzed diastereodivergent thia-Michael addition to chiral β-trifluoromethyl-α,β-unsaturated N-acylated oxazolidin-2-ones

Base-catalyzed diastereodivergent thia-Michael addition of thiols to chiral β-trifluoromethyl-α,β-unsaturated N-acylated oxazolidin-2-ones is reported. By tuning the base-catalyst (i-Pr2NEt, DABCO, or P2-t-Bu), a range of chiral thia-Michael adducts was synthesized in good yields with high diastereoselectivities. A plausible mechanism was proposed on the basis of the experimental results. This work is complementary to the existing methods offering advantages, e.g., switchable diastereoselectivity using a readily synthesized chiral starting material, a cheap and readily available base catalyst, and a simple and practical operation, enabling synthetic application in organic synthesis.

Amino-coumarin-based colorimetric and fluorescent chemosensors capable of discriminating Co2+, Ni2+, and Cu2+ ions in solution and potential utilization as a paper-based device

New chemosensors, L1-L3, based on the coumarin Schiff base scaffold with substituent modifications, have been designed and synthesized. The chemosensors L1-L3 exhibited the absorbance and fluorescence spectral changes that can discriminate Co²⁺, Ni²⁺, and Cu²⁺ ions. Sensor L1 demonstrated the ability to respond to Co²⁺, Ni²⁺, and Cu²⁺ ions. Remarkably, the slight modification of substituent on L2 has been observed to cause selective binding to Ni²⁺ and Cu²⁺ ions while L3 can specifically detect Cu²⁺ ions. The in-situ formation of metal and ligand complexes was determined by Job's plot analysis. The limit of detection and the sensing ability of all probes are estimated to be within the range of safe drinking water. Incorporation of the sensing compounds into a paper-based detection system using a laminated paper-based analytical device (LPAD) was demonstrated and found to be consistent to those obtained from the batchwise solution measurements.

Color-tunable emissive heptagon-embedded polycyclic aromatic dicarboximides

Heptagon-embedded polycyclic aromatic dicarboximides were developed as new push-pull fluorescent dyes through palladium-catalysed [4+3] annulation followed by nucleophilic substitution. The introduction of a seven-membered ring in these push-pull systems can efficiently modulate the optical properties leading to an enhancement of the fluorescence quantum yields up to 0.93 with color tunable emission covering the visible-NIR spectrum.

Incorporation of Cation Affects the Redox Reactivity of Fe-NNN Complexes on C-H Oxidation

Cations such as Lewis acids have been shown to enhance the catalytic activity of high-valent Fe-oxygen intermediates. Herein, we present a pyridine diamine ethylene glycol macrocycle, which can form Zn(II)- or Fe(III)-complex with the NNN site, while allowing redox-inactive cations to bind to the ethylene glycol moiety. The addition of alkali, alkali earth, and lanthanum ions resulted in positive shifts to the Fe(III/II) redox potential. Calculation of dissociation constants showed the tightest binding with a Ba²⁺ ion. Density functional theory calculations were used to elucidate the effects of redox inactive cations toward the electronic structures of Fe complexes. Although the Fe-NNN complexes, both in the absence and presence of cations, can catalyze C-H oxidation of 9,10-dihydroanthracene, to give anthracene [hydrogen atom transfer (HAT) product], anthrone, and anthraquinone [oxygen atom transfer (OAT) products], highest overall activity and OAT/HAT product ratios were obtained in the presence of dications, that is, Ba²⁺ and Mg²⁺, respectively.

QM/MM Molecular Modeling Reveals Mechanism Insights into Flavin Peroxide Formation in Bacterial Luciferase

Bacterial luciferase (Lux) catalyzes oxidation of reduced flavin mononucleotide (FMN) and aldehyde to form oxidized FMN and carboxylic acid via molecular oxygen with concomitant light generation. The enzyme is useful for various detection applications in biomedical experiments. Upon reacting with oxygen, the reduced FMN generates C4a-peroxy-FMN (FMNH-C4a-OO^-) as a reactive intermediate, which is required for light generation. However, the mechanism and control of FMNH-C4a-OO^- formation are not clear. This work investigated the reaction of FMNH-C4a-OO^- formation in Lux using QM/MM methods. The B3LYP/6-31G*/CHARMM27 calculations indicate that Lux controls the formation of FMNH-C4a-OO^- via the conserved His44 residue. The steps in intermediate formation are found to be as follows: (i) H⁺ reacts with O₂ to generate ⁺OOH. (ii) ⁺OOH attacks C4a of FMNH^- to generate FMNH-C4a-OOH. (iii) H⁺ is transferred from FMNH-C4a-OOH to His44 to generate FMNH-C4a-OO^- while His44 stabilizes FMNH-C4a-OO^- by forming a hydrogen bond to an oxygen atom. This controlling key mechanism for driving the change from FMNH-C4a-OOH to the FMNH-C4a-OO^- adduct is confirmed because FMNH-C4a-OO^- is more stable than FMNH-C4a-OOH in the luciferase active site.

Synthesis of indolo- and benzothieno[3,2-c]quinolines via POCl3 mediated tandem cyclization of o-alkynylisocyanobenzenes derived from o-alkynyl-N-phenylformamides

A synthesis of indolo[3,2-c]quinolines and benzothieno[3,2-c]quinolines has been developed employing o-alkynyl-N-phenylformamide derivatives as the substrates. The reaction proceeded via a tandem process involving POCl3‒assisted intramolecular cyclization of the firstly formed...

Charge neutral halogen bonding tetradentate-iodotriazole macrocycles capable of anion recognition and sensing in highly competitive aqueous media

A series of neutral tetradentate halogen bonding (XB) macrocycles, comprising of two bis-iodotriazole XB donors were synthesised in 60-70% yields via a stepwise CuAAC-mediated cyclisation strategy. Extensive ¹H NMR anion titration experiments reveal halide binding affinities are critically dependent on the substitution pattern of the xylyl spacer unit. The meta-substituted macrocycle remarkably displays cooperative tetradentate XB-halide anion recognition in highly competitive 40% aqueous-organic D₂O/acetone-d₆ (40 : 60, v/v) solvent mixtures. Integration of para-xylyl and naphthyl spacer units generates extended macrocyclic cavities, capable of selective oxalate recognition. Furthermore, preliminary fluorescence exeperiments reveal dicarboxylate specific sensing can be achieved through monitoring of the naphthylene centred emission.

Hydroboration of carbonyls and imines by an iminophosphonamido tin(II) precatalyst

A novel three-coordinated tin(II) chloride [Ph₂P(N^tBu)₂]SnCl (1) supported by an N,N'-di-tert-butyliminophosphonamide having two phenyl groups on the phosphorus atom was synthesized by the reaction of the starting lithium iminophosphonamide [Ph₂P(N^tBu)₂]Li with SnCl₂·(dioxane) in toluene. The molecular structure of 1 was established by X-ray diffraction analysis. Tin(II) chloride 1 can act as an efficient precatalyst for the hydroboration of a wide variety of aldehydes, ketones, and imines at -10 °C. DFT calculations propose that hydroboration involves hydride transfer from the corresponding tin(II) hydride intermediate [Ph₂P(N^tBu)₂]SnH (10) to the carbonyl substrates via four-membered transition states (TS-12), affording three-coordinated tin(II) alkoxide intermediates [Ph₂P(N^tBu)₂]SnOR (13), followed by the stepwise reaction of 13 with HBpin (pin = pinacolate) to release the boronate esters and regenerate the tin(II) hydride 10. The stoichiometric reaction of the in site-generated 10 with benzophenone 2a at -10 °C led to the formation of 13. Moreover, 13 also stoichiometrically reacted with HBpin at -10 °C, forming the corresponding boronate ester 3a and 10 based on the ¹H NMR spectrum of the reaction mixture.

Understanding the reactivity of geminal P/B and P/Al frustrated Lewis pairs in CO2 addition and H2 activation

Understanding CO₂ addition and H₂ activation by geminal P/Z (Z = B, Al) frustrated Lewis pairs (FLPs) is crucial for the development of transition metal-free catalysts for hydrogenation of unsaturated compounds and CO₂ conversion. The distortion-interaction energy decomposition and natural bond orbital were used to gain insights into the reactivity of CO₂ addition and H₂ activation. Remarkably, the orbital interactions in the reactions of CO₂ and H₂ with geminal P/B FLPs are stronger than those in the reactions with more reactive geminal P/Al FLPs, which contain a stronger Lewis acid. The fact that geminal P/B FLPs react with a higher energy barrier than geminal P/Al FLPs is actually due to the unfavorable core repulsion contribution to the interaction energy and the higher distortion energy found for the geminal P/B FLPs. In addition, we revealed that although the interaction energy is higher for H₂ activation, the distortion energy is significantly lower for CO₂ addition. Thus, geminal P/Z FLPs are more reactive toward CO₂ than toward H₂. Overall, the reactivity and thermodynamic stability for the reactions of geminal P/Z FLPs with CO₂ and H₂ are not affected by the type of bridging carbon (sp³-C or sp²-C) but mainly influenced by the substituents on P and Z (B or Al). An insight into the thermodynamic stability of the H₂ activation product is gained by quantifying the hydride affinity, the proton affinity, and the stability of the zwitterionic product relative to the prototypical Lewis acid-base pair, P^tBu₃/B(C₆F₅)₃.

Boesenmaxane Diterpenoids from Boesenbergia maxwellii

Three new diterpenoids, boesenmaxanes A-C (1-3), with an unprecedented core skeleton consisting of an unusual C-C bond between C-12 and an exo-cyclic methylene C-13, were isolated from the rhizome extracts of Boesenbergia maxwellii. The structures were elucidated by analysis of spectroscopic and X-ray diffraction data. Electronic circular dichroism spectra were used to determine the absolute configuration. All the isolates were evaluated for their cytotoxic effects, anti-HIV activity, and antimicrobial activity. Boesenmaxanes A and C (1 and 3) showed significant inhibitory activity in the syncytium reduction assay, with EC₅₀ values of 55.2 and 27.5 μM, respectively.

Dibenzopleiadiene-embeded polyaromatics via [4 + 3] annulative decarbonylation/decarboxylation

A novel and efficient sequential cross-coupling/annulation strategy is developed to construct structurally and optoelectronically diverse class of dibezopleiadiene-embeded polyaromatics.

Insights into the Regioselectivity of Hydroheteroarylation of Allylbenzene with Pyridine Catalyzed by Ni/AlMe3 with N-Heterocyclic Carbene: The Concerted Hydrogen Transfer Mechanism

The hydroheteroarylation of allylbenzene with pyridine as catalyzed by Ni/AlMe₃ and a N-heterocyclic carbene ligand has recently been established. Density functional calculations revealed that the common stepwise pathway, which involves the C-H oxidative addition of pyridine-AlMe₃ before the migratory insertion of allylbenzene, is unlikely as the migratory insertion needs to overcome a prohibitively high energy barrier. In contrast, the ligand-to-ligand hydrogen transfer pathway is more favorable in which the hydrogen is transferred directly from the para-position of pyridine-AlMe₃ to C2 of allylbenzene. Our distortion-interaction analysis and natural bond orbital analysis indicate that the interaction energy is strongly correlated with the extent of the charge transfer from the alkene (hydrogen acceptor) to the pyridine-AlMe₃ (hydrogen donor), which dictates the selectivity of the H-transfer to the C2 position of allylbenzene. Then, the subsequent C-C reductive elimination of the regioselective linear product is facilitated by the steric hindrance of the IPr ligand. Understanding these key factors affecting the product regioselectivity is important to the development of catalysts for hydroheteroarylation of alkenes.

Chalcogen Bond Mediated Enhancement of Cooperative Ion-Pair Recognition

A series of heteroditopic receptors containing halogen bond (XB) and unprecedented chalcogen bond (ChB) donors integrated into a 3,5-bis-triazole pyridine structure covalently linked to benzo-15-crown-5 ether motifs exhibit remarkable cooperative recognition of halide anions. Multi-nuclear 1 H, 13 C, 125 Te and 19 F NMR, ion pair binding investigations reveal sodium cation-benzo-crown ether binding dramatically enhances the recognition of bromide and iodide halide anions, with the chalcogen bonding heteroditopic receptor notably displaying the largest enhancement of halide binding strength of over two hundred-fold, in comparison to the halogen bonding and hydrogen bonding heteroditopic receptor analogues. DFT calculations suggest crown ether sodium cation complexation induces a polarisation of the sigma hole of ChB and XB heteroditopic receptor donors as a significant contribution to the origin of the unique cooperativity exhibited by these systems.

Resistance to the “last resort” antibiotic colistin: a single-zinc mechanism for phosphointermediate formation in MCR enzymes

Simulations show the mono-zinc form of MCR to be stable and competent for covalent phospho(ethanolamine) intermediate formation.

Synthesis of peri-Diarylated Naphthalimides via Double Decarboxylative Cross-Coupling Reaction

A facile method for the synthesis of peri-diarylated naphthalimides was developed by using a palladium-catalyzed double decarboxylative cross-coupling reaction. The present work offers a convenient approach for tuning the optical properties of naphthalimide derivatives.

Aryl C–O oxidative addition of phenol derivatives to nickel supported by an N-heterocyclic carbene via a Ni0 five-centered complex

An aryl ether with the assistance of organoaluminum, an aryl sulfonate/sulfamate and an ester/carbamate proceeds towards C–O bond cleavage via a Ni⁰ five-centered complex.

Oxidative dehalogenation and denitration by a flavin-dependent monooxygenase is controlled by substrate deprotonation

Enzymes that are capable of detoxifying halogenated phenols (HPs) and nitrophenols (NPs) are valuable for bioremediation and waste biorefining. HadA monooxygenase was found to perform dual functions of oxidative dehalogenation (hydroxylation plus halide elimination) and denitration (hydroxylation plus nitro elimination). Rate constants associated with individual steps of HadA reactions with phenol, halogenated phenols and nitrophenols were measured using combined transient kinetic approaches of stopped-flow absorbance/fluorescence and rapid-quench flow techniques. Density functional theory was used to calculate the thermodynamic and electronic parameters associated with hydroxylation and group elimination steps. These parameters were correlated with the rate constants of hydroxylation, group elimination, and overall product formation to identify factors controlling individual steps. The results indicated that the hydroxylation rate constant is higher when the pK a of the phenolic group is lower, i.e. it is more easily deprotonated, but not higher when the energy gap between the E LUMO of the C4a-hydroperoxy-FAD intermediate and the E HOMO of the phenolate substrate is lower. These data suggest that the substrate deprotonation has a higher energy barrier than the -OH transfer, and thus controls the hydroxylation step. For the group elimination, the process is controlled by the ability of the C-X bond to break. For the overall product formation (hydroxylation and group elimination combined), this analysis showed that the rate constant of product formation is dependent on the pK a value of the substrate, indicating that the overall reaction is controlled by substrate deprotonation. This step also likely has the highest energy barrier and thus controls the overall process of oxidative dehalogenation and denitration by HadA. This report is the first to identify a key mechanistic factor controlling the enzymatic processes of oxidative dehalogenation and denitration.

Electrophilic Aromatic Formylation with Difluoro(phenylsulfanyl) methane

Difluoro(phenylsulfanyl)methane (PhSCF2H) was found to undergo a reaction with aromatic compounds mediated by SnCl4, through a thionium intermediate characterized by NMR and TD-DFT analyses, leading to the formation of a mixture of S,S′-diphenyl dithioacetal and aromatic aldehyde which, after oxidative hydrolysis, provides the aromatic aldehyde in good to excellent yields. The salient feature of the present work is the reaction of activated aromatic compounds containing a deactivating ester functional group, leading to the formylated products in good yields.

Polycyclic polyprenylated acylphloroglucinols and biphenyl derivatives from the roots of Garcinia nuntasaenii Ngerns. & Suddee

Seven previously undescribed compounds, including three polycyclic polyprenylated acylphloroglucinols (garcinuntins A-C), three biphenyl derivatives (garcinuntabiphenyls A-C) and a lanostane triterpene (garcinuntine), along with thirteen known compounds were isolated from the root of Garcinia nuntasaenii Ngerns. & Suddee. Their structures were elucidated on the basis of spectroscopic techniques. For garcinuntins A-C, the absolute configurations were confirmed by the combination of single X-ray crystallography and ECD calculations. Anti-HIV activity using anti-HIV-1 reverse transcriptase and syncytium inhibition assays, and cytotoxic activity against a panel of cultured mammalian cancer cell lines of isolated compounds were investigated.

Mechanistic insights into HCO2H dehydrogenation and CO2 hydrogenation catalyzed by Ir(Cp*) containing tetrahydroxy bipyrimidine ligand: the role of sodium and proton shuttle

Catalytic HCO₂H dehydrogenation by Ir(Cp*) tetrahydroxy bipyrimidine is influenced not only by the protonation states but also by the involvement of Na⁺ and the availability of HCO₂H as a proton shuttle.

A Single-Site Mutation at Ser146 Expands the Reactivity of the Oxygenase Component of p-Hydroxyphenylacetate 3-Hydroxylase

The oxygenase component (C₂) of p-hydroxyphenylacetate (4-HPA) 3-hydroxylase (HPAH) from Acinetobacter baumannii catalyzes the hydroxylation of various phenolic acids. In this report, we found that substitution of a residue close to the phenolic group binding site to yield the S146A variant resulted in an enzyme that is more effective than the wild-type in catalyzing the hydroxylation of 4-aminophenylacetate (4-APA). Product yields for both wild-type and S146A enzymes are better at lower pH values. Multiple turnover reactions of the wild-type and S146A enzymes indicate that both enzymes first hydroxylate 3-APA to give 3-hydroxy-4-aminophenylacetate (3-OH-4-APA), which is further hydroxylated to give 3,5-dihydroxy-4-aminophenylacetate, similar to the reaction of C₂ with 4-HPA. Stopped-flow experiments showed that 4-APA can only bind to the wild-type enzyme at pH 6.0 and not at pH 9.0, while it can bind to S146A under both pH conditions. Rapid-quench flow results indicate that the wild-type enzyme has low reactivity toward 4-APA hydroxylation, with a hydroxylation rate constant (k_OH) for 4-APA of 0.028 s^-1 compared to 17 s^-1 for 4-HPA, the native substrate. In contrast, for S146A, the hydroxylation rate constants for both substrates are very similar (2.6 s^-1 for 4-HPA versus 2.5 s^-1 for 4-APA). These data indicate that Ser146 is a key catalytic residue involved in optimizing C₂ reactivity toward a phenolic compound. Removing this hydroxyl group expands C₂ activity toward a non-natural aniline substrate. This understanding should be helpful for future rational engineering of other two-component flavin-dependent monooxygenases that have this conserved Ser residue.

Mechanisms for dehydrogenation and hydrogenation of N-heterocycles using PNP-pincer-supported iron catalysts: a density functional study

The bifunctionality of Fe-PNP plays a role in stabilizing the ion-pair complex in the dehydrogenation via a hydrogen bonding and a C–H agostic interaction.

Pyridine–triazole ligands for copper-catalyzed aerobic alcohol oxidation

Effects of triazole configuration and substituents on reaction rates of aerobic oxidation of benzyl alcohol to benzaldehyde were investigated.

Mechanism of Ni N-heterocyclic carbene catalyst for C–O bond hydrogenolysis of diphenyl ether: a density functional study

Ni(SIPr)(η²-PhOPh) is the key active species for C–O bond hydrogenolysis of diphenyl ether.

Experimental and computational exploration of the dynamic behavior of (PNP)BF2, a boron compound supported by an amido/bis(phosphine) pincer ligand

The diarylamido/bis(phosphine) PNP pincer ligand (2-(i)Pr(2)P-4-MeC(6)H(3))(2)N has been evaluated as a scaffold for supporting a BF(2) fragment. Compound (PNP)BF(2) (6) was prepared by simple metathesis of (PNP)Li (5) with Me(2)SBF(3). NMR spectra of 6 in solution are of apparent C(2) symmetry, suggestive of a symmetric environment about boron. However, a combination of X-ray structural studies, low-temperature NMR investigations, and DFT calculations consistently establish that the ground state of this molecule contains a classical four-coordinate boron with a PNBF(2) coordination environment, with one phosphine donor in PNP remaining "free". Fortuitous formation of a single crystal of (PNP)BF(2)·HBF(4) (7), in which the "free" phosphine is protonated, furnished another structure containing the same PNBF(2) environment about boron for comparison and the two PNBF(2) environments in 6 and 7 are virtually identical. DFT studies on several other diarylamido/bis(phosphine) pincer (PNP)BF(2) systems were carried out and all displayed a similar four coordinate PNBF(2) environment in the ground state structures. The symmetric appearance of the room-temperature NMR spectra is explained by the rapid interconversion between two degenerate four-coordinate, C(1)-symmetric ground-state forms. Lineshape analysis of the (1)H and (19)F NMR spectra over a temperature range of 180-243 K yielded the activation parameters ΔH(‡) = 8.1(3) kcal mol(-1) and ΔS(‡) = -6.0(15) eu, which are broadly consistent with the calculated values. Calculations indicate that the exchange of phosphine donors at the boron center proceeds by an intrinsically dissociative mechanism.