Protonation Of Ligand Research Articles

Crystallographic and Computational Characterisation of the Potential PET Tracer 1,4,7‐Triazacyclononane‐1,4,7‐tri(methylenephosphonato)gallium(III)

AbstractThe macrocyclic chelator 1,4,7‐triazacyclononane‐1,4,7‐tri(methylenephosphonic acid) (H6NOTP) has shown significant promise for use in positron emission tomography with the positron‐emitting radionuclide 68Ga3+. We report the X‐ray crystal structure of the neutral complex 1,4,7‐triazacyclononane‐1,4,7‐tri(methylenephosphonato)gallium(III) ([Ga(H3NOTP)]), which crystallised in the centrosymmetric space group P21/n. The coordination geometry of the Ga3+ ion is distorted octahedral with three macrocyclic amine N donor atoms and three singly deprotonated phosphonato O donor atoms. Two discrete enantiomers, Δ(λλλ)‐SSS and Λ(δδδ)‐RRR, are present as a racemate within the unit cell. Intermolecular hydrogen bonds between phosphonic acid groups of adjacent complexes form a two‐dimensional sheet network. Density functional theory (DFT) calculations supported the observation of complexes with low electric field gradient values with slight but significant deviations from ideal octahedral geometry. The DFT calculations were also used to investigate the influence of intermolecular interactions and ligand protonation states on the coordination environment and surface charge distribution.

Regulating electron transfer over asymmetric low-spin Co(II) for highly selective electrocatalysis

Modulating the steric-electronic configuration of metal-organic centers is key for tuning the activity and selectivity of heterogeneous reactions, especially multi-electron transfer reactions. Here, three different asymmetric metal-organic complexes with unique steric-electronic structures are immobilized on nanocarbon for an electron-transfer-controlled oxygen reduction reaction. The strong-field ligand-induced low-spin ( LS ) Co II creates a necessary steric configuration for regulating reaction selectivity through ligand's proton transfer ability, for which acidic diamine ligands facilitate a four-electron transfer (94%), whereas basic ligands drive a highly selective two-electron route (97%). The steric-electronic regulation of the reaction selectivity at catalytic sites is characterized using X-ray absorption spectroscopy, reaction kinetic path analysis, and density functional theory calculation. Our results indicate that an LS state of Co II with asymmetric coordination is necessary to form a unique “flytrap” structure to promote O 2 capture for the subsequent proton-coupled electron transfer, which is regulated by the Brønsted acidity of coordinating ligands. • A series of asymmetric metal-organic complexes are immobilized on nanocarbons • Advanced characterizations are used to explore their structures at the atomic level • Selectivity of ORR is regulated by changing the ligand field strength of the active site • Operando X-ray spectroscopy and computational analysis confirm the reaction mechanism The majority of energy conversion processes such as hydrogen peroxide production, nitrogen fixation, and CO 2 reduction generally involve multi-electron transfer, resulting in multiple by-products and requiring tedious purification process. The development of highly selective electrocatalysts can not only reduce the energy consumption in the production process but also benefit to promote their large-scale application. Here, we report an asymmetric ligand design of molecular “Venus flytrap” at atomic Co II catalysts on a heterogeneous carbon surface for oxygen electrocatalysis with controlled selectivity. Our XAFS and DFT calculations show that the strong-field ligand-induced LS Co II centers offer inimitable steric-electronic advantages in capturing O 2 and the capability to regulate reaction pathways through an H-bonding interaction between the ligands and peroxo intermediate. The proposed concept completes the picture of designer single-atom catalyst by covering the often overlooked exterior coordination sites, and is expected to be generally applicable to other heterogeneous catalyst systems. Steric-electronic configuration regulation plays a key role in achieving highly selective electrocatalysis. As a proof of concept, a series of asymmetric metal-organic complexes were immobilized on a conductive substrate for oxygen reduction reaction (ORR). The strong-field ligand-induced low-spin cobalt sites exhibit an excellent selectivity for four-electron ORR, whereas the medium-field ligand linked Co sites display high efficiency for the two-electron reduction of oxygen into hydrogen peroxide.

Dual-lanthanide urea metal-organic framework based fluorescent traffic light microsensor for solvent decoding and visual trace water assay

A rapid and sensitive sensor for organic solvent decoding and water assay is crucial for human health and chemical industry. Herein, a dual-lanthanide metal-organic framework (Ln-MOF) Eu0.2Tb0.8-UL (UL: urea-containing ligand) was fabricated for 3-dimensional solvent decoding and ratiometric on-off water assay in organic solvents. According to innovative multiple response mechanisms including ligand protonation and -OH oscillation, the energy transfer of ligand to Ln3+ and Tb3+ to Eu3+ is precisely regulated by water content. Ultra-low limit of detection (0.004% v/v), broad response range (0–100%) and high-efficacy fluorescence color changing from red to green are achieved. A one-to-two logic device with single input (water content) and dual outputs (Eu3+ and Tb3+ emissions) was designed for the facile intelligent water detection, which is realized by 3 output results: YES (1, 0), PASS (0, 0) and NOT (0, 1), just like a smart traffic light sensor (red-yellow-green). The MOF-loaded paper microsensor was constructed for real-time and sensitive detection of trace water in organic solvents, coupled with a smartphone-assisted visual portable device. To our knowledge, this is the first case of Ln-MOF-based 3-dimensional solvent decoder, and traffic light-like logic analytical device and paper microsensor for real-time visual water assay in organic solvents.

The underappreciated influence of ancillary halide on metal-ligand proton tautomerism.

Syntheses of Vaska-type complexes [IrP2X(CO)] (P = phosphine, X = halide) with all four common halides (fluoride, chloride, bromide, and iodide) was attempted using a protic and hemilabile imidazolyl di-tert-butyl phosphine ligand. In the solid-state, all four complexes were found to be ionic with the halides in the outer-sphere, and the fourth coordination site of the square plane occupied by the imidazole arm of the ligand. In solution, however, the chloride complex was found to be in equilibrium with an octahedral IrIII–H species at room temperature. For the bromide and iodide analogs, the corresponding IrIII–H species were also observed but only after heating the solutions. The neutral IrI Vaska's analogs for X = Cl, Br, and I were obtained upon addition of excess halide salt, albeit heating was required for X = Br and I. The IrIII–H species are proposed to originate from tautomerization of minor amounts of the electron rich neutral Vaska analog (halide inner-sphere and phosphines monodentate) that are in equilibrium with the ionic species. Heating is required for the larger anions of bromide and iodide to overcome a kinetic barrier associated with their movement to an inner-sphere position prior to tautormerization. For the fluoride analog, the IrIII–H was not observed, attributable to strong hydrogen bonding interactions of the imidazolyl proton with the fluoride anion.

Determination of Stability Constant of The Complexes of Metal Ion Fe(III), Mn(II) And Cr(III) With Some Substituted 2-Oxo-2H-Chromene-3-Carbohydrazide Derivatives at 0.1 M Ionic Strength at 42oC

Proton ligand and metal ligand stability constant are measured for Fe(III) and 1, 2-dihydroxy benzene and 1, 5-disulphonic acid complexes . Determination of stability constant of substituted of pyrazoles with rare earth metals form complexes are reported . Effect of temperature on formation constants of 2 acetylpyridine(N benzoyl)glycine hydrazone with lanthanide(III) ions at different ionic strengths are reported. Irving and Rossotti, Herson and Gilbert , Wilkins and Lewis and Rossotti and Rossotti have determined stability constant by Bjerrum-Calvin titration technique . Kabadi , Jahagirdar and Narwade have determined pK values of salicylaldehyde, salicylic acid and sulphonic acid respectively by similar procedure. The metal ligand stability constant of some β-diketones are reported . Stability constants have investigated for some substituted pyrazolines, isoxalline and diketone . The method most frequently applied for study of complex equilibria is pH-metric titration technique . Stepwise formation of mononuclear binary complexes is described by set of equilibrium constants. For pH-metric measurements an electrode must be selected. According to Bjerrum, Martell and Calvin , the formation of complex MLN is stepwise process and one has to deal with a series of equilibria of the type: Irving and Rossotti, Herson and Gilbert , Wilkins and Lewis and Rossotti and Rossotti have determined stability constant by Calvin-Bjerrum titration technique . The value of nA,n, pK, log K1 and log K2 are evaluated by Irving and Rossotti's equation. The stability constant and thermodynamic parameters of complexes containing lighter lanthanides , copper(II) , calcium and magnesium are reported. It is also important in environmental studies , medicinal and industrial chemistry . Effects of transition metal on stability constant of complexes have studied by pH metric method . Bendi , Janrao and Ramteke have studied stability constant of ligands with lanthanide metals complexes.The present work involves pH metric study of substituted 2-oxo-2H-chromene-3-carbohydrazide derivatives with metal ions Fe(III), Mn(II), Cr(III) and Ti(III) at temperature42oC. The Bjerrum-Calvin titration technique as modified by Irving and Rossotti has been employed in the present study. The following three sets of titrations are carried out in sequence.

Axial-ligand-cleavable silicon phthalocyanines triggered by near-infrared light toward design of photosensitizers for photoimmunotherapy

Near-infrared photoimmunotherapy (NIR-PIT) is a novel phototherapy for the treatment of cancer that uses NIR light and conjugates of antibody-IR700, a silicon phthalocyanine photosensitizer. A key feature of NIR-PIT is light-induced axial ligand cleavage of IR700, which finally causes cytotoxicity. Here, we focused on protonation of the axial ligand on the IR700 anion radical during the photochemical process. The Gibbs energy in the protonation reaction of IR700 derivatives with different axial ligands was calculated. These calculations suggested the order of the cleavage efficiency corresponds to the basicity of the axial ligand (i.e. alkoxy > siloxy (IR700) > phenoxy > oxycarbonyl), which was confirmed by the photoirradiation experiments with synthesized compounds. Thus, axial ligand cleavage is significantly dependent on the basicity of the axial ligand. Our findings suggest that PIT reagent with an IR700 derivative bearing alkoxy group would show better efficacy than IR700.

Ligand Protonation at Carbon, not Nitrogen, during H2 Production with Amine-Rich Iron Electrocatalysts.

We present monometallic H2 production electrocatalysts containing electron-rich triamine-cyclopentadienyl (Cp) ligands coordinated to iron. After selective CO extrusion from the iron tricarbonyl precursors, electrocatalysis is observed via cyclic voltammetry in the presence of an exogenous acid. Contrary to the fact that amines in the secondary coordination sphere are often protonated during electrocatalysis, comprehensive quantum-chemical calculations indicate that the amines likely do not function as proton relays; instead, endo-Cp ring protonation is most favorable after 1e- reduction. This unusual mechanistic pathway emphasizes the need to consider a broad domain of H+/e- addition products by synergistically combining experimental and theoretical resources.

Synthesis of iron(III)-carbonyl complex with variable wavelength range for CO release depending on protonation and deprotonation of axial phosphorous ligands

N,C,S-pincer iron(III) carbonyl complexes with two phosphorus ligands—trans-[Fe(L-κ3N,C,S)(CO)(PR3)2]PF6 ([1]PF6, R = Me; PF6, R = OEt) are light-inducible CO releasers under ambient conditions. Optical wavelength ranges corresponding to such a CO release depend on the phosphorous ligands. Complex [1]PF6 responds to light with wavelengths shorter than 500 nm, while the photosensitivity of [2]PF6 is extended to wavelengths up to 800 nm, reaching the so-called phototherapeutic window. Theoretical and experimental studies suggest that the electron-donating ability of phosphorous ligands is essential to determine the wavelength range corresponding to photo-CO dissociation, although the steric difference in the phosphorous ligands for [1]PF6 and [2]PF6 hampers the exclusive evaluation of the electronic effect for CO dissociation from such complexes. A newly synthesized N,C,S-pincer iron(III) carbonyl complex with 1,3,5-triaza-7-phosphaadamantane (PTA) as axial ligands ([3]PF6) has shown variable photosensitivity for the dissociation of CO in response to protonation of the PTA ligand. Based on this finding, in the present study, [3]PF6 is used to confirm the photo-CO dissociation mechanism proposed for [1]PF6 and [2]PF6 and provide clues to molecular design for adjusting the ranges of optical wavelengths required for CO dissociation from trans-[Fe(PyBPT-κ3N,C,S)(CO)(PR3)2]PF6.

Redox-Induced Structural Reorganization Dictates Kinetics of Cobalt(III) Hydride Formation via Proton-Coupled Electron Transfer.

Two-electron, one-proton reactions of a family of [CoCp(dxpe)(NCCH3)]2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) form the corresponding hydride species [HCoCp(dxpe)]+ (dxpe = dppe (1,2-bis(diphenylphosphino)ethane), depe (1,2-bis(diethylphosphino)ethane), and dcpe (1,2-bis(dicyclohexylphosphino)ethane)) through a stepwise proton-coupled electron transfer process. For three [CoCp(dxpe)(NCCH3)]2+ complexes, peak shift analysis was employed to quantify apparent proton transfer rate constants from cyclic voltammograms recorded with acids ranging 22 pKa units. The apparent proton transfer rate constants correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants curiously plateau (kpl) as the reaction becomes increasingly exergonic. The absolute apparent proton transfer rate constants across both these regions correlate with the steric bulk of the chelating diphosphine ligand, with bulkier ligands leading to slower kinetics (kplateau,depe = 3.5 × 107 M-1 s-1, kplateau,dppe = 1.7 × 107 M-1 s-1, kplateau,dcpe = 7.1 × 104 M-1 s-1). Mechanistic studies were conducted to identify the cause of the aberrant kPTapp-ΔpKa trends. When deuterated acids are employed, deuterium incorporation in the Cp ring is observed, indicating protonation of the CoCp(dxpe) species to form the corresponding hydride proceeds via initial ligand protonation. Digital simulations of cyclic voltammograms show ligand loss accompanying initial reduction gates subsequent PCET activity at higher driving forces. Together, these experiments reveal the details of the reaction mechanism: reduction of the Co(III) species is followed by dissociation of the bound acetonitrile ligand, subsequent reduction of the unligated Co(II) species to form a Co(I) species is followed by protonation, which occurs at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)]+. Analysis as a function of chelating disphosphine ligand, solvent, and acid strength reveals that the ligand dissociation equilibrium is directly influenced by the steric bulk of the phosphine ligands and gates protonation, giving rise to the plateau of the apparent proton transfer rate constant with strong acids. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions.

Surfactant Removal for Colloidal Nanocrystal Catalysts Mediated by N-Heterocyclic Carbenes.

We report the facile removal of surfactants from colloidally synthesized nanocrystals via ligand exchange with N-heterocyclic carbenes (NHCs). Subsequent protonation of the NHC ligands in acid efficiently cleans the nanocrystals' surface while preserving their uniform morphology and structure for catalysis. The broad efficacy of this strategy is validated using monodisperse Pt, Pd, and Au nanocrystals, each prepared with strongly bound phosphine stabilizers. The surface-activated nanocrystals exhibit significantly improved catalytic activities, superior to those obtained with other surface cleaning methods, as demonstrated in two centrally important electrochemical reactions (glycerol oxidation and CO2 reduction). This work highlights a new surface activation strategy for catalysis and other applications that enables the efficient use of well-defined nanocrystal libraries prepared by colloidal chemistry.

A “nucleophilic” iodine in a halogen-bonded iodonium complex manifests an unprecedented I+···Ag+ interaction

Summary When an electron is removed from a halogen atom, it forms a halenium ion X+ (X = I, Br, Cl). In halogen bonding (XB), X+ is considered as a strong XB donor, and when interacting with two XB acceptors (e.g., pyridine), it forms a halonium XB complex with a [N–I–N] three-center-four-electron bond with the two XB acceptors. An unprecedented I+···Ag+ interaction occurs between a [L1–I–L1]+ halogen-bonded complex and a [L2–Ag–L2]+ complex in which the iodonium ion acts like a nucleophile and donates electrons to the silver(I) cation. The X-ray diffraction analysis reveals a short contact [3.4608(3) A] between the I+ and Ag+ cations, and ITC measurements give a ΔG of −6.321 kcal/mol and Ka∼37,000 M−1 for the 1:1 complex. The DFT computational study on the nature of the I+···Ag+ interaction indicates that the I+ is nucleophilic in character, manifesting an unusual and strongly attractive interaction between the I+ and Ag+ cations.

Switching Site Reactivity in Hydrogenase Model Systems by Introducing a Pendant Amine Ligand.

Hydrogenases are versatile enzymatic catalysts with an unmet hydrogen evolution reactivity (HER) from synthetic bio-inspired systems. The binuclear active site only has one-site reactivity of the distal Fed atom. Here, binuclear complexes [Fe2(CO)5(μ-Mebdt)(P(4-C6H4OCH3)3)] 1 and [Fe2(CO)5(μ-Mebdt)(PPh2Py)] 2 are presented, which show electrocatalytic activity in the presence of weak acids as a proton source for the HER. Despite almost identical structural and spectroscopic properties (bond distances and angles from single-crystal X-ray; IR, UV/vis, and NMR), introduction of a nitrogen base atom in the phosphine ligand in 2 markedly changes site reactivity. The bridging benzenedithiolate ligand Mebdt interacts with the terminal ligand’s phenyl aromatic rings and stabilizes the reduced states of the catalysts. Although 1 with monodentate phosphine terminal ligands only shows a distal iron atom HER activity by a sequence of electrochemical and protonation steps, the lone pair of pyridine nitrogen in 2 acts as the primary site of protonation. This swaps the iron atom catalytic activity toward the proximal iron for complex 2. Density-functional theory (DFT) calculations reveal the role of terminal phosphines ligands without/with pendant amines by directing the proton transfer steps. The reactivity of 1 is a thiol-based protonation of a dangling bond in 1– and distal iron hydride mechanism, which may follow either an ECEC or EECC sequence, depending on the choice of acid. The pendant amine in 2 enables a terminal ligand protonation and an ECEC reactivity. The introduction of a terminal nitrogen atom enables the control of site reactivity in a binuclear system.

Relation of Metal-Binding Property and Selective Toxicity of 8-Hydroxyquinoline Derived Mannich Bases Targeting Multidrug Resistant Cancer Cells

Simple SummaryEffective treatment of cancer is often limited by the resistance of cancer cells to chemotherapy. A well-described mechanism supporting multidrug resistance (MDR) relies on the efflux of toxic drugs from cancer cells, mediated by P-glycoprotein (Pgp). Circumventing Pgp-mediated resistance is expected to make a significant contribution to improved therapy of malignancies. Interestingly, MDR cells exhibit paradoxical hypersensitivity towards a diverse set of anticancer chelators. In this study we explore the relation of chemical and structural properties influencing metal binding and toxicity of a set of 8-hydroxyquinoline derivatives to reveal key characteristics governing “MDR-selective” activity. We find that subtle changes in the stability and redox activity of the biologically relevant metal complexes significantly influence MDR-selective toxicity. Our results underline the importance of chelation in MDR-selective toxicity, suggesting that the collateral sensitivity of MDR cells may be targeted by preferential iron deprivation or the formation of redox-active copper(II) complexes.Resistance to chemotherapeutic agents is a major obstacle in cancer treatment. A recently proposed strategy is to target the collateral sensitivity of multidrug resistant (MDR) cancer. Paradoxically, the toxicity of certain metal chelating agents is increased, rather than decreased, by the function of P-glycoprotein (Pgp), which is known to confer resistance by effluxing chemotherapeutic compounds from cancer cells. We have recently characterized and compared the solution’s chemical properties including ligand protonation and the metal binding properties of a set of structurally related 8-hydroxyquinoline derived Mannich bases. Here we characterize the impact of the solution stability and redox activity of their iron(III) and copper(II) complexes on MDR-selective toxicity. Our results show that the MDR-selective anticancer activity of the studied 8-hydroxyquinoline derived Mannich bases is associated with the iron deprivation of MDR cells and the preferential formation of redox-active copper(II) complexes, which undergo intracellular redox-cycling to induce oxidative stress.

Effect of N-salicylidene hydrazide protonation on the solid state structural diversity of its Cu(ii), Ni(ii) and Zn(ii) complexes

Study of the effect of ligand protonation on the structural diversity of Cu(ii), Ni(ii) and Zn(ii) compounds constructed from a N-salicylidenehydrazide Ligand.

Complexes of cyclen side-bridged with a methylene-bis(phosphinate) group

A macrobicyclic cyclen-based ligand containing a geminal bis(phosphinate) N,N′-bridge was synthesized via a Mannich-type reaction. The other two macrocycle amine groups were modified with two acetate pendants, yielding ligand H4L. The cyclen-bis(phosphinate) intermediate, the ligand salt Na0.5(H5L)(H3.5L)Cl·20H2O and CuII complex Li2[Cu(L)]·4H2O were characterized by single-crystal X-ray diffraction. The ligand protonation constants and the stability constants of H4L complexes with Cu(II) and Zn(II) and lanthanide(III) ions were determined by potentiometry. The ligand protonation constants and the stability constants of H4L complexes with Cu(II) and Zn(II) ions are similar to those of H4dota. Conversely, the stabilities of its Ln(III) complexes are ~ 7–10 orders of magnitude lower than those of H4dota complexes due to steric restrictions of the methylene-bis(phosphinate) side bridge. Therefore, these findings indicate that the ligand H4L is suitable for the complexation of transition metal ions.

Ligand Protonation Triggers H2 Release from a Dinickel Dihydride Complex to Give a Doubly “T”‐Shaped Dinickel(I) Metallodiradical

AbstractThe dinickel(II) dihydride complex (1K) of a pyrazolate‐based compartmental ligand with β‐diketiminato (nacnac) chelate arms (L−), providing two pincer‐type {N3} binding pockets, has been reported to readily eliminate H2 and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2Na, 2K) of L− are now synthesized via a direct reduction route. They feature two adjacent T‐shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e− substrate reductions as shown by the rapid reaction with H2 or O2. X‐ray crystallography reveals distinctly different positions of the K+ in 1K and 2K, suggesting a stabilizing interaction of K+ with the dihydride unit in 1K. H2 release from 1K is triggered by peripheral γ‐C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H2 elimination from the bimetallic cleft.

Ligand Protonation Triggers H2 Release from a Dinickel Dihydride Complex to Give a Doubly "T"-Shaped Dinickel(I) Metallodiradical.

The dinickel(II) dihydride complex (1K) of a pyrazolate‐based compartmental ligand with β‐diketiminato (nacnac) chelate arms (L−), providing two pincer‐type {N3} binding pockets, has been reported to readily eliminate H2 and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2Na, 2K) of L− are now synthesized via a direct reduction route. They feature two adjacent T‐shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local dx2-y2 type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e− substrate reductions as shown by the rapid reaction with H2 or O2. X‐ray crystallography reveals distinctly different positions of the K+ in 1K and 2K, suggesting a stabilizing interaction of K+ with the dihydride unit in 1K. H2 release from 1K is triggered by peripheral γ‐C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H2 elimination from the bimetallic cleft.

Synthesis and Biological Evaluation of Novel Zn(II) and Cd(II) Schiff Base Complexes as Antimicrobial, Antifungal, and Antioxidant Agents

(E)-N,N-Dimethyl-2-((E-1-(2-(p-tolyl)hydrazono)propan-2-ylidene)hydrazine-1-carbothioamide (DMPTHP) and their Zn(II) and Cd(II) complexes have been synthesized and characterized. Different tools of analysis such as elemental analyses, IR, mass spectra, and 1H-NMR measurements were used to elucidate the structure of the synthesized compounds. According to these spectral results, the DMPTHP ligand behaved as a mononegatively charged tridentate anion. Modeling and docking studies were investigated and discussed. Novel Schiff base (DMPTHP) ligand protonation constants and their formation constants with Cd(II) and Zn(II) ions were measured in 50% DMSO solution at 15°C, 25°C, and 35°C at I = 0.1 mol·dm−3 NaNO3. The solution speciation of different species was measured in accordance with pH. Calculation and discussion of the thermodynamic parameters were achieved. Both log K1 and –ΔH1, for M(II)-thiosemicarbazone complexes were found to be somewhat larger than log K2 and –ΔH2, demonstrating a shift in the dentate character of DMPTHP from tridentate in 1 : 1 chelates to bidentate in 1 : 2; M : L chelates and steric hindrance were generated by addition of the 2nd molecule. The compounds prepared have significant activity as antioxidants, similar to ascorbic acid. It is hoped that the results will be beneficial to antimicrobial agent chemistry. The formed compounds acted as a potent antibacterial agent. Molecular docking studies were investigated and have proved that DMPTHP as antibacterial agents act on highly resistant strains of E. coli and also as an anticancer agent.

Excited-state behavior and photoinduced electron transfer of pH-sensitive Ir(III) complexes with cyclometallation (C/N–) ratios between 0/6 and 3/3

The first coordination sphere of Ir(III) 2,2′-bipyridine / 2-phenylpyridine complexes can be tuned to achieve either C– or N–chelation in ratios that range between 0/6 and 3/3. Of particular interest is the synthesis of Ir(III) complexes bearing a 2,2′-bipyridine ligand coordinated in a N,C3 pattern, leaving an exposed pyridine moiety, accessible for acid-base chemistry or coordination to a second transition metal center. The protonated forms of these “rolled-over” Ir(III) complexes were isolated in a straight-forward procedure using trifluoroacetic acid. The photophysical, photochemical and electrochemical properties of both the protonated and unprotonated Ir(III) complexes were investigated by steady-state and time-resolved spectroscopies, as well as by density functional theory calculations. The nature of the excited states was shown to depend on both the ligand coordination pattern and protonation state of the complex. In addition, the unprotonated and protonated analogues were efficiently quenched by hydroquinone and benzoquinone in acetonitrile with quenching rate constants close to the solvent diffusion limit. The results presented herein have direct implications for proton sensitive photoredox chemistry and the development of photo-acids and photo-bases.

Octahedral Iron Complexes of Pyrazine(diimine) Pincers: Ligand Electronic Effects and Protonation.

Redox noninnocent ligands are known to be involved in altering the overall electronic nature of organometallic complexes by serving as an electron reservoir. Pyrazine(diimine) backbones in these complexes introduce enhanced π acidity over their more well-studied pyridine(diimine) analogues and open up the opportunity for functionalization of the nitrogen at the 4-position of the ring. Herein we report the characterization of bis-chelated pyrazine(diimine) [(PzDI)2Fe]n+ (n = 0, 1, and 2) complexes for electronic and structural comparison to pyridine(diimine) complexes (PDI) with similar architectures. Cyclic voltammetry studies show three reductions, two of which are ligand-based and reversible. Reduction of [(PzDI)2Fe]2+ (1) to [(PzDI)2Fe]+ (2) and (PzDI)2Fe (3) gives rise to characteristic structural changes, such as imine C═N bond lengthening, indicating the formation of a ligand radical, a conclusion which is further supported by electron paramagnetic resonance (EPR) and electronic structure calculations. Comparisons between the PzDI and PDI systems are highlighted. Complex 1 can be protonated at the uncoordinated pyrazine nitrogen, resulting in changes to its spectroscopic and redox properties; efforts to further functionalize the ligand are discussed.