Characterization of a tricationic trigonal bipyramidal iron(IV) cyanide complex, with a very high reduction potential, and its iron(II) and iron(III) congeners

Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [Fe(IV)(O)(TMG(3)tren)](2+) (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG(3)tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe(IV)(CN)(TMG(3)tren)](3+) (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe(II)(CN)(TMG(3)tren)](+) (2), via the S = 5/2 complex [Fe(III)(CN)(TMG(3)tren)](2+) (3), the progress of which was conveniently monitored by using UV-vis spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), Mo?ssbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe(IV/III) reduction potential of ～1.4 V vs Fc(+/o), the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t(1/2) in CD(3)CN solution containing 0.1 M KPF(6) at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to (13)C NMR at -40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG(3)tren ligand to support highly charged high-valent complexes.     

Sulfur K-edge X-ray absorption spectroscopy of 2Fe-2S ferredoxin: covalency of the oxidized and reduced 2Fe forms and comparison to model complexes

Ligand K-edge X-ray absorption spectroscopy (XAS) provides a direct experimental probe of ligand-metal bonding. In previous studies, this method has been applied to mononuclear Fe-S and binuclear 2Fe-2S model compounds as well as to rubredoxins and the Rieske protein. These studies are now extended to the oxidized and reduced forms of ferredoxin I from spinach. Because of its high instability, the mixed-valence state was generated electrochemically in the protein matrix, and ligand K-edge absorption spectra were recorded using an XAS spectroelectrochemical cell. The experimental setup is described. The XAS edge data are analyzed to independently determine the covalencies of the iron-sulfide and -thiolate bonds. The results are compared with those obtained previously for the Rieske protein and for 2Fe-2S model compounds. It is found that the sulfide covalency is significantly lower in oxidized FdI compared to that of the oxidized model complex. This decrease is interpreted in terms of H bonding present in the protein, and its contribution to the reduction potential E degrees is estimated. Further, a significant increase in covalency for the Fe(III)-sulfide bond and a decrease of the Fe(II)-sulfide bond are observed in the reduced Fe(III)Fe(II) mixed-valence species compared to those of the Fe(III)Fe(III) homovalent site. This demonstrates that, upon reduction, the sulfide interactions with the ferrous site decrease, allowing greater charge donation to the remaining ferric center. That is the dominant change in electronic structure of the Fe(2)S(2)RS(4) center upon reduction and can contribute to the redox properties of this active site.     

Catalytic activity and electrochemical properties of Cu(II)-Schiff base complex encapsulated in the nanocavities of zeolite-Y for oxidation of olefins and sulfides

Copper(II) complex of a Schiff base ligand (H₂L) was synthesized, characterized, and encapsulated in the cavities of zeolite-Y by a fixed ligand method. The zeolite encapsulated metal complex (CuL-Y) was characterized using FT-IR, UV–Vis and atomic absorption spectroscopy, thermogravimetric analysis (TGA), X-ray powder diffraction (XRD), scanning electron microscopy images (SEM), energy-dispersive X-ray spectroscopy (EDX), and Brunauer-Emmett-Teller (BET). The catalytic activity and electrochemical behavior of the encapsulated complex has been studied in the oxidation of a wide range of sulfides and olefins using H₂O₂ in ethanol. This heterogeneous catalytic system shows a dramatic increase in total turnover number (46,500) for oxidation of styrene. It could be readily reused for at least eight successive times without discernible activity and selectivity deterioration, which displays potential for practical applications.     

A pseudo trinuclear nickel–sodium complex containing tris(8-methyl-2-oxo-quinolidineamino ethylamine): Synthesis,spectral characterization,X-ray crystallography and in vitro biological evaluations

A new mixed nickel–sodium complex has been synthesized from Ni(ClO₄)₂ and tris(8-methyl 2-oxo-quinolidine amino ethylamine) with a 1:1 molar ratio in methanol and has been characterized by various analytical, spectroscopy and X-ray diffraction studies which confirmed an octahedral geometry around the nickel ion. Further, structural optimization of the complex was performed using DFT calculations. The ligand and complex were evaluated for their binding affinity with CT-DNA and an intercalative type of binding interaction was proposed from the absorption and fluorescence titration experiments. Albumin binding interaction of the ligand and complex was determined by absorption, fluorescence and synchronous spectral techniques at room temperature, suggesting the static quenching mechanism of BSA with the compounds. Antioxidant studies revealed the radical scavenging potential of Ni(II) complex. The anticancer activity of the ligand and complex was probed via in vitro cytotoxicity against human breast (MCF7) and lung (A549) cancer cell lines by MTT assay. Further, cytological changes observed in acridine orange/ethidium bromide and DAPI staining methods validated the cytotoxic potential of the complex.     

Trapping of tin(II) and lead(II) homologues of carbon monoxide by a benzannulated lutidine-bridged bisstannylene

The reaction of the benzannulated bisstannylene ligand 2 with Sn O or Pb O generated in situ gave the pincer complexes 3 and 4. Both complexes have been characterized by X-ray diffraction and multinuclear NMR spectroscopy. A divalent state has been found by M?ssbauer spectroscopy for the tin atoms in complexes 3 and 4.     

A square pyramidal copper(II) complex of a Schiff base ligand: synthesis,crystal structure,antibacterial and DNA interaction studies

A mononuclear Cu(II) complex [Cu(L)Cl₂] has been synthesized from a tridentate Schiff base ligand, piperidin-2-ylmethyl-pyridin-2-ylmethylene-amine (L). The single-crystal X-ray structure of the complex shows a square pyramidal geometry. The complex was tested against several bacteria and showed good antibacterial activities against almost all of the bacteria. The interactions of the title complex with calf thymus DNA (CT-DNA) have been investigated by electronic absorption and fluorescence spectroscopy, showing that the complex interacts with CT-DNA via partial intercalation.     

MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.     

Triphenylamine-substituted 2-pyridyl-1,2,3-triazole copper(I) complexes: an experimental and computational investigation

Abstract

Four new heteroleptic copper(I) complexes bearing either 2-pyridyl-1,2,3-triazole (pytri) or the related triphenylamine (TPA) substituted (TPA-tripy) ligands and the ancillary ligands 6,6′-dimesityl-2,2′-bipyridine (diMesbpy) or bis[(2-diphenylphosphino)phenyl] ether (POP) were synthesized in good yields (75-95%). All the complexes were extensively characterized using nuclear magnetic resonance (NMR) spectroscopies and electrospray ionization mass spectrometry (ESIMS) and in the case of the two pytri compounds the solid state structures were determined via X-ray crystallography. The pytri complexes showed MLCT absorption bands which shift from 433?nm for the diMesbpy complex to 347?nm for POP. TPA-pytri complexes introduce an ILCT band resulting in improved visible absorption (376?nm, 26,400 M^?1 cm^?1 for [Cu(TPA-pytri)(diMesbpy)](PF₆)). Emission from this ILCT state (470?nm, Φ?=?0.08) was red-shifted compared to the free ligand with negligible effects from ancillary ligands. Band assignments were confirmed with resonance Raman spectroscopy and TD-DFT calculations.     

15N HYSCORE characterization of the fully deprotonated, reduced form of the archaeal Rieske [2Fe-2S] center

The hyperfine couplings for strongly and weakly coupled 15N nuclei around a reduced Rieske [2Fe-2S] center of uniformly 15N-labeled, hyperthermostable archaeal Rieske protein at pH 13.3 were determined by hyperfine sublevel correlation (HYSCORE) spectroscopy and compared with those at physiological pH. Significant changes in the hyperfine couplings of the terminal histidine Ndelta ligands and Nepsilon nuclei were observed between them, which can be explained by not only the redistribution of the unpaired electron spin density over the ligands but also the difference in the mixed-valence state of the fully deprotonated, reduced cluster. These quantitative data can be used in theoretical analysis for the selection of an appropriate model of the mixed-valence state of the reduced Rieske center at very alkaline pH.     

Picosecond X-ray absorption measurements of the ligand substitution dynamics of Fe(CO)5 in ethanol

Ultrafast X-ray absorption near edge spectroscopy has been carried out for photo excited iron pentacarbonyl in ethanol with 2 picosecond resolution. A temporal resolution limited dissociation process was observed, followed by the formation of the mono-substituted complex Fe(CO)(4)EtOH within a few tens of picoseconds. The measurements have been carried out with a newly developed X-ray absorption instrument at station 7 ID-C of the Advanced Photon Source. The results show that single picosecond temporal resolution can be achieved at a synchrotron beam line.     

X-ray crystal structure of arsenite-inhibited xanthine oxidase: μ-sulfido,μ-oxo double bridge between molybdenum and arsenic in the active site

Xanthine oxidoreductase is a molybdenum-containing enzyme that catalyzes the hydroxylation reaction of sp(2)-hybridized carbon centers of a variety of substrates, including purines, aldehydes, and other heterocyclic compounds. The complex of arsenite-inhibited xanthine oxidase has been characterized previously by UV-vis, electron paramagnetic resonance, and X-ray absorption spectroscopy (XAS), and the catalytically essential sulfido ligand of the square-pyrimidal molybdenum center has been suggested to be involved in arsenite binding through either a μ-sulfido,μ-oxo double bridge or a single μ-sulfido bridge. However, this is contrary to the crystallographically observed single μ-oxo bridge between molybdenum and arsenic in the desulfo form of aldehyde oxidoreductase from Desulfovibrio gigas (an enzyme closely related to xanthine oxidase), whose molybdenum center has an oxo ligand replacing the catalytically essential sulfur, as seen in the functional form of xanthine oxidase. Here we use X-ray crystallography to characterize the molybdenum center of arsenite-inhibited xanthine oxidase and solve the structures of the oxidized and reduced inhibition complexes at 1.82 and 2.11 ? resolution, respectively. We observe μ-sulfido,μ-oxo double bridges between molybdenum and arsenic in the active sites of both complexes. Arsenic is four-coordinate with a distorted trigonal-pyramidal geometry in the oxidized complex and three-coordinate with a distorted trigonal-planar geometry in the reduced complex. The doubly bridged binding mode is in agreement with previous XAS data indicating that the catalytically essential sulfur is also essential for the high affinity of reduced xanthine oxidoreductase for arsenite.     

Synthesis,One‐ and Two‐Photon Photophysical and Excited‐State Properties,and Sensing Application of a New Phosphorescent Dinuclear Cationic Iridium(III) Complex

A new phosphorescent dinuclear cationic iridium(III) complex ( Ir1 ) with a donor–acceptor–π‐bridge–acceptor–donor (D? A? π? A? D)‐conjugated oligomer ( L1 ) as a N^N ligand and a triarylboron compound as a C^N ligand has been synthesized. The photophysical and excited‐state properties of Ir1 and L1 were investigated by UV/Vis absorption spectroscopy, photoluminescence spectroscopy, and molecular‐orbital calculations, and they were compared with those of the mononuclear iridium(III) complex [Ir(Bpq)₂(bpy)]⁺PF₆^? ( Ir0 ). Compared with Ir0 , complex Ir1 shows a more‐intense optical‐absorption capability, especially in the visible‐light region. For example, complex Ir1 shows an intense absorption band that is centered at λ=448 nm with a molar extinction coefficient (ε) of about 10⁴, which is rarely observed for iridium(III) complexes. Complex Ir1 displays highly efficient orange–red phosphorescent emission with an emission wavelength of 606 nm and a quantum efficiency of 0.13 at room temperature. We also investigated the two‐photon‐absorption properties of complexes Ir0 , Ir1 , and L1 . The free ligand ( L1 ) has a relatively small two‐photon absorption cross‐section (δ_max=195 GM), but, when complexed with iridium(III) to afford dinuclear complex Ir1 , it exhibits a higher two‐photon‐absorption cross‐section than ligand L1 in the near‐infrared region and an intense two‐photon‐excited phosphorescent emission. The maximum two‐photon‐absorption cross‐section of Ir1 is 481 GM, which is also significantly larger than that of Ir0 . In addition, because the strong B? F interaction between the dimesitylboryl groups and F^? ions interrupts the extended π‐conjugation, complex Ir1 can be used as an excellent one‐ and two‐photon‐excited “ON–OFF” phosphorescent probe for F^? ions.     

Stoichiometries and stability constants for the cerium(IV) carbonate complexes in dilute aqueous solution

Complexation between cerium(IV) and carbonate ions in aqueous solution has been studied by UV/Visible absorption spectroscopy, ¹⁷O NMR spectroscopy and potentiometric titration, and it is shown that in dilute solutions at pH 8.8 and 10, both 1?:?1 and 1?:?2 (metal?:?ligand) complexes are formed. This contrasts with the behaviour of the corresponding Th(IV) complex, where the dominant species is the pentacarbonato complex. From the NMR spectra, it is suggested that these species involve bidentate binding of carbonate ion by the metal, while potentiometric data provided accurate formation constants and indicated the dominant species to be the 1?:?2 complex.     

Femtosecond ligand/core dynamics of microwave-assisted synthesized silicon quantum dots in aqueous solution

A microwave-assisted reaction has been developed to produce hydrogen-terminated silicon quantum dots (QDs). The Si QDs were passivated for water solubility via two different methods: hydrosilylation produced 3-aminopropenyl-terminated Si QDs, and a modified St?ber process produced silica-encapsulated Si QDs. Both methods produce water-soluble QDs with maximum emission at 414 nm, and after purification, the QDs exhibit intrinsic fluorescence quantum yield efficiencies of 15 and 23%, respectively. Even though the QDs have different surfaces, they exhibit nearly identical absorption and fluorescence spectra. Femtosecond transient absorption spectroscopy was used for temporal resolution of the photoexcited carrier dynamics between the QDs and ligand. The transient dynamics of the 3-aminopropenyl-terminated Si QDs is interpreted as a formation and decay of a charge-transfer (CT) excited state between the delocalized π electrons of the carbon linker and the Si core excitons. This CT state is stable for ~4 ns before reverting back to a more stable, long-living species. The silica-encapsulated Si QDs show a simpler spectrum without CT dynamics.     

X-ray absorption spectroscopy to analyze nuclear geometry and electronic structure of biological metal centers—potential and questions examined with special focus on the tetra-nuclear manganese complex of oxygenic photosynthesis

X-ray absorption spectroscopy (XAS) has become a prominent tool for the element-specific analysis of transition metals at the catalytic center of metalloenzymes. In the present study the information content of X-ray spectra with respect to the nuclear geometry and, in particular, to the electronic structure of the protein-bound metal ions is explored using the manganese complex of photosystem II (PSIII) as a model system. The EXAFS range carries direct information on the number and distances of ligands as well as on the chemical type of the ligand donor function. For first-sphere ligands and second-sphere metals (in multinuclear complexes), the determination of precise distances is mostly straightforward, whereas the determination of coordination numbers clearly requires more effort. The EXAFS section starts with an exemplifying discussion of a PSII spectrum data set with focus on the coordination number problem. Subsequently, the method of linear dichroism EXAFS spectroscopy is introduced and it is shown how the EXAFS data leads to an atomic resolution model for the tetra-manganese complex of PSII. In the XANES section the following aspects are considered: (1) Alternative approaches are evaluated for determination of the metal-oxidation state by comparison with a series of model compounds. (2) The interpretation of XANES spectra in terms of molecular orbitals (MOs) is approached by comparative multiple-scattering calculations and MO calculations. (3) The underlying reasons for the oxidation-state dependence of the XANES spectra are explored. Furthermore, the potential of modern XANES theory is demonstrated by presenting first simulations of the dichroism in the XANES spectra of the PSII manganese complex.     

Synthesis,spectral characterization,and luminescence properties of a cup-like ligand and its magnesium(II) complex

A new tripodal ligand, N,N′,N″-tri(salicylaldehyde)triaminotriethylamine (1) has been synthesized and characterized by elemental analysis, IR and UV spectroscopy, MS, and X-ray crystallography. X-ray diffraction analysis reveals that the three chains of the ligand form a cup-like structure. The ligand’s magnesium(II) complex has been synthesized and characterized by elemental analysis, conductivity, and IR and UV spectroscopy. The luminescence properties of the ligand and its magnesium(II) complex were investigated in DMF, CH₃OH, and CH₃CH₂OH solution and in the solid state at room temperature.     

Picosecond X-ray absorption spectroscopy of a photoinduced iron(II) spin crossover reaction in solution

In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron-nitrogen (Fe-N) bond is lengthened by 0.21+/-0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change in the metal-ligand interactions as reflected by the altered density of states of the unoccupied metal orbitals. Our results constitute the first direct measurements of the dynamic atomic and electronic structural rearrangements occurring during a photoinduced FeII spin crossover reaction in solution via picosecond X-ray absorption spectroscopy.     

In situ spectroscopy and spectroelectrochemistry of uranium in high-temperature alkali chloride molten salts

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.     

Facile preparation of a three-coordinate copper(I) complex: Steric hindrance,supramolecular structure,optical property and TD-DFT study

The solution reaction of Cu(CH₃CN)₄(PF₆) with a NN ligand 2-(2′-quinolyl)benzimidazole and a sterically bulky P ligand tris(2-methylphenyl)phosphine facilely yielded the three-coordinate copper(I) complex [Cu(2-QBI)(o-Tol₃P)](PF₆) (1). The complex has been characterized by single-crystal X-ray diffraction, Fourier Transform infrared spectroscopy and elemental analysis, UV–Vis (ultraviolet–visible) and photoluminescent spectroscopy studies. Time-dependent density functional theory has been used for calculating the electronic origin of the low-lying excited states, which were unexpectedly assigned mainly as a ligand-to-ligand or an intra-ligand charge transfer state instead of the metal-to-ligand charge transfer transition. Interestingly, 1 exhibits a concentration-dependent absorption in solution. This absorption behavior is interpreted as some excimer's formation based on the study of supramolecular structure, spectroscopy and calculations.     

Preparation,characterization and use of new lignocellulose-based bio nanocomposite as a heterogeneous catalyst for sustainable synthesis of pyrimido benzazoles

A sustainable combinatorial synthesis of densely substituted pyrimido [1,2-b] benzazole derivatives in water under microwave irradiation was performed using a new lignocellulose-based bio nanocomposite (BNC) as heterogeneous catalyst. The lignocellulosic waste peanut shells (LCWPS) were turned into a value-added product, a new BNC PS/ZnO, which was prepared via in situ hydrothermal synthesis. The as-prepared BNC was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction spectrum. PS/ZnO has been successfully used in a sustainable catalytic method for the synthesis of pyrimido [1, 2-b] benzazole derivatives in water under microwave irradiation. The time of this reaction was significantly reduced. This catalytic system has a very high turnover number (TON?～?10³) and turnover frequency (TOF?～?10^5?h^?1). This paper presents the benefit of sustainable management of LCWPS, a bio-sourced polymeric carbohydrate for production of new nanocatalyst.