A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

Using light energy and O₂ for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O₂ by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)₃]²⁺, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV^+.. MV^+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O₂ to O₂^.? in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.     

Conformational control of oxidation sites, spin states and orbital occupancy in nickel porphyrins

Ni(II) porphyrin π cation radicals are known to undergo an internal electronic isomerization to L₂Ni(III) cations upon complexation with ligands (L). Additional examples of the Ni(II) to Ni(III) conversion are presented for flexible, 'planar' NiOEP (2,3,7,8,12,13,17,18-octaethylporphyrin) and NiT(Pr)P (5,10,15,20-tetra-n-propylporphyrin) in which the Ni(III) orbital occupancy, d _z2 or d _x2-y2, is determined by the ligand field strength of the axial ligands (pyridine, imidazole, or cyanide). In contrast to these results, the nonplanar NiOETPP (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), which is easily oxidized because of its saddle-shape, yields a complex postulated to be a high spin Ni(II) π cation radical, based on crystallographic and optical data for (imidazole)2NiOETPP⁺ClO^4-, in which the electron of high spin Ni(II) in the d _x2-y2 orbital is antiferromagnetically coupled to the unpaired electron of the porphyrin radical leaving one electron in the Ni(II) d _z2 orbital, i.e. a pseudo Ni(III). The sterically encumbered, nonplanar NiT(t-Bu)P (5,10,15,20-tetra-tertiary-butylporphyrin) yields Ni(III) complexes when ligated by pyridine, imidazole or cyanide, but in all cases only the Ni(III) d _z2 orbital is occupied as evidenced by EPR spectroscopy. This anomalous chemistry is attributed to the fact that the macrocycle of NiT(t-Bu)P is so sterically constrained that it cannot readily expand to accommodate the longer equatorial Ni—N distances required by population of the d _x2-y2 orbital in Ni(III) or high spin Ni(II). Further support for this postulate derives from NiD(t-Bu)P (5,10-di-tertiary-butylporphyrin) which is less sterically constrained and in which the Ni(III) d _x2-y2 orbital is indeed occupied upon complexation with cyanide. These results thus illustrate the significant effects that the conformations, plasticity or rigidity of Ni porphyrin macrocycles can have on sites of oxidation (metal or porphyrin), spin states (low spin Ni(III) or high spin Ni(II)), and orbital occupancies (d _z2 or d _x2-y2 in Ni(III)).     

Controlling Interfacial Recombination in Aqueous Dye‐Sensitized Solar Cells by Octadecyltrichlorosilane Surface Treatment

A general and convenient strategy is proposed for enhancing photovoltaic performance of aqueous dye‐sensitized solar cells (DSCs) through the surface modification of titania using an organic alkyl silane. Introduction of octadecyltrichlorosilane on the surface of dyed titania photoanode as an organic barrier layer leads to the efficient suppression of electron recombination with oxidized cobalt species by restricting access of the cobalt redox couple to the titania surface. The champion ODTS‐treated aqueous DSCs (0.25 mM ODTS in hexane for 5 min) exhibit a V_oc of 821±4 mV and J_sc of 10.17±0.21 mA cm^?2, yielding a record PCE of 5.64±0.10 %. This surface treatment thus serves as a promising post‐dye strategy for improving the photovoltaic performance of other aqueous DSCs.     

Synthesis, characterization, electrochemical studies, and antibacterial activities of cobalt(III) complexes with Salpn-Tipe Schiff base ligands. Crystal structure of trans-[CoIII(L1)(Py)2]ClO4

The synthesis, characterization, spectroscopic and electrochemical properties of trans-[Co^III(L¹)(Py)₂]ClO₄ (I) and trans-[Co^III(L²)(Py)₂]ClO₄ (II) complexes, where H₂L¹ = N,N′-bis(5-chloro-2-hydroxybenzylidene)-1,3-propylenediamine and H₂L² = N,N′-bis(5-bromo-2-hydroxybenzylidene)-1,3-propylenediamine, have been investigated. Both complexes have been characterized by elemental analysis, FT-IR, UV-Vis, and ¹H NMR spectroscopy. The crystal structure of I has been determined by X-ray diffraction. The coordination geometry around cobalt(III) ion is best described as a distorted octahedron. The electrochemical studies of these complexes revealed that the first reduction process corresponding to Co(III/II) is electrochemically irreversible accompanied by dissociation of the axial Co-N(Py) bonds. The in vitro antimicrobial activity of the Schiff bse ligands and their corrsponding complexes have been tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, and Escherichia coli. The cobalt(III) complexes showed lower antimicrobial activity than the free Schiff base ligands.     

Synthesis,characterization, and ascorbic acid oxidase biomimetic catalytic activity of cobalt(III) oxime complexes

A new tetradentate tetraaza ligand was prepared via Schiff-base condensation of 3,4-diaminotoluene with 2,3-butandione monoxime in aqueous solution. This ligand coordinates cobalt(III) through nitrogen donors in equatorial positions with loss of one oxime proton with concomitant formation of an intramolecular hydrogen bond. A series of cobalt(III) complexes, [CoLX₂] (X?=?Cl^?, Br^?, or I^?), [SCNCoLBr], [CNCoLBr], [BF₂CoLBr], and [YCoLBr]ClO₄ (Y?=?pyridine, thiophene, triphenylphosphine, or n-pentylamine), was synthesized. The compounds were characterized based on the elemental analysis (C, H, N), electrical conductance, magnetic moment measurements, and spectral studies (IR, ¹H NMR, and UV-Vis). Thermal stabilities of representative complexes were examined by using thermal analysis (TGA and DTG). The reported complexes are d⁶ low-spin diamagnetic and a distorted octahedral environment was proposed. All complexes undergo tetragonal distortion as evidenced by splitting of ¹T_1g and ¹T_2g levels of the pseudo-octahedral symmetry. The ligand field parameters such as Dq^E , Dq^A , and the tetragonal splitting D_t have been computed and correlated with the nature of the coordinated axial ligands. The reported cobalt(III) complexes exhibit promising catalytic activity toward aerobic oxidation of ascorbic acid to the corresponding dehydroascorbic acid. The oxidase catalytic activity is linked to both the tetragonal splitting parameter D_t and the Lewis-acidity of cobalt(III) created by the nature of the coordinated axial ligands. The probable mechanistic implications of the catalytic oxidation reactions are discussed.     

Synthesis,electrochemical and in situ spectroelectrochemical studies of new transition metal complexes with two new Schiff-bases containing N2O2/N2O4 donor groups

New Schiff-base copper and cobalt complexes, [Cu(L¹)], [Cu(L²)] and [Co(L¹)], [Co(L²)] (where L¹ = N-N′-bis(3,5-di-tert-butylsalicylaldimine)-1,4-cyclohexane bis(methylamine) and L² = N-N′-bis(3,5-di-tert-butylsalicylaldimine)-1,8-diamino-3,6-dioxaoctane), were synthesized and characterized using elemental analysis, IR spectra, UV–Vis spectra, magnetic susceptibility measurements, ¹H and ¹³C NMR spectroscopy, thermal analysis and molar conductance (Λ_M). Their electro-spectrochemical properties were investigated using cyclic voltammetric (CV) and thin-layer spectroelectrochemical techniques in a dichloromethane solution (CH₂Cl₂). The CV of [Cu(L²)] showed a lower oxidation potential than that of [Cu(L¹)] under the same experimental conditions. The oxidation wave (II) of [Cu(L²)] was accompanied by an EC process (II′), which was not observed for [Cu(L¹)]. Also, [Cu(L²)] exhibited a reduction process, but [Cu(L¹)] did not. These results indicate that the Cu(II) ion in [Cu(L²)] is coordinated by N₂O₄ donor sites while [Cu(L¹)] presents a square-planar structure with N₂O₂ donor sites. Both oxidation processes for [Co(L¹)] and [Co(L²)] are based on the cobalt center, and they are assigned to Co(II)/Co(III) couples. The spectroelectrochemical results indicate that the oxidized species of [Cu(L²)] is similar to that of [Cu(L¹)], the only difference being that the absorption bands of the oxidized species for [Cu(L²)] shift to lower energy compared with those of [Cu(L¹)] because of their different coordination environment. The geometry of [Cu(L²)] changed into square-planar after the complex was totally oxidized and the neutral complex was only recovered following the EC process, as observed from the CV of [Cu(L²)]. For the two cobalt complexes, the bands corresponding to the π → π^∗transitions disappeared and new bands with small red shifts and of lower intensity were observed during the oxidation process. These new bands are attributed to the LMCT transition as observed in the case of the oxidation processes of the cobalt complexes.     

Kinetics of Reduction of Some Surfactant-Cobalt(III) Complexes by Iron(II)

The electron transfer kinetics of the reaction between the surfactant-cobalt(III) complex ions, cis-[Co(en)₂(C₁₂H₂₅NH₂)₂]³⁺, cis-α-[Co(trien)(C₁₂H₂₅NH₂)₂]³⁺(en:ethylenediamine, trien:triethylenetetramine, C₁₂H₂₅NH₂ : dodecylamine) by iron(II) in aqueous solution was studied at 298, 303, 308 K by spectrophotometry method under pseudo-first-order conditions using an excess of the reductant in self-micelles formed by the oxidant, cobalt(III) complex molecules, themselves. The rate constant of the electron transfer reaction depends on the initial concentration of the surfactant cobalt(III) complexes. ΔS^# also varies with initial concentration of the surfactant cobalt(III) complexes. By assuming outer-sphere mechanism, the results have been explained based on the presence of aggregated structures containing cobalt(III) complexes at the surface of the self-micelles formed by the surfactant cobalt(III) complexes in the reaction medium. The rate constant of each complex increases with initial concentration of one of the reactants surfactant-cobalt(III) complex, which shows that self micelles formed by surfactant-cobalt(III) complex itself has much influence on these reactions. The electron transfer reaction of the surfactant-cobalt(III) complexes was also carried out in a medium of various concentrations of β-cyclodextrin. β-cyclodextrin retarded the rate of the reaction.     

New tripodal N-heterocyclic carbene chelators for small molecule activation

In an effort to develop new tripodal N-heterocyclic carbene (NHC) ligands for small molecule activation, two new classes of tripodal NHC ligands TIME^R and TIMEN^R have been synthesized. The carbon-anchored tris(carbene) ligand system TIME^R (R = Me, t-Bu) forms bi- or polynuclear metal complexes. While the methyl derivative exclusively forms trinuclear 3:2 complexes [(TIME^Me)₂M₃]³⁺ with group 11 metal ions, the tert-butyl derivative yields a dinuclear 2:2 complex [(TIME^t-Bu)₂Cu₂]²⁺ with copper(I). The latter complex shows both “normal” and “abnormal” carbene binding modes and accordingly, is best formulated as a bis(carbene)alkenyl complex. The nitrogen-anchored tris(carbene) ligands TIMEN^R (R = alkyl, aryl) bind to a variety of first-row transition metal ions in 1:1 stoichiometry, affording monomeric complexes with a protected reactivity cavity at the coordinated metal center. Complexes of TIMEN^R with Cu(I)/(II), Ni(0)/(I), and Co(I)/(II)/(III) have been synthesized. The cobalt(I) complexes with the aryl-substituted TIMEN^R (R = mesityl, xylyl) ligands show great potential for small molecule activation. These complexes activate for instance dioxygen to form cobalt(III) peroxo complexes that, upon reaction with electrophilic organic substrates, transfer an oxygen atom. The cobalt(I) complexes are also precursors for terminal cobalt(III) imido complexes. These imido complexes were found to undergo unprecedented intra-molecular imido insertion reactions to form cobalt(II) imine species. The molecular and electronic structures of some representative metal NHC complexes as well as the nature of the metal–carbene bond of these metal NHC complexes was elucidated by X-ray and DFT computational methods and are discussed briefly. In contrast to the common assumption that NHCs are pure σ-donors, our studies revealed non-negligible and even significant π-backbonding in electron-rich metal NHC complexes.     

Photoreduction of chlorohemin in pure pyridine

Chlorohemin (Fe(III)PPCl) undergoes photoreduction when irradiated in pure pyridine solution with 400–450 nm light. A thermal reduction is observed to occur simultaneously with the photochemical one, but after a one hour irradiation about 75% of the reduction product is formed in a photochemical way. Both five and six-coordinated species are observed to be present in solution; however, only the Fe(III)PPpy⁺ five coordinated complex is photoreducible. A mechanism is proposed whereby the primary photochemical act is an axial pyridine → iron electron transfer process yielding Fe(II)PP and py⁺ species. The Fe(II)PP moiety gives rise to the formation of the spectrophotometrically detectable Fe(II)(PP(py)₂ complex. ESR spin trapping results are consistent with the formation of 2-pyridyl radicals from py⁺ cation by fast transfer of a proton to a pyridine molecule.     

Electron transfer reactions of tris(polypyridine)ruthenium(III) complexes with organic sulfides: importance of hydrophobic interaction

Ruthenium(III)-polypyridyl complexes, generated from the photochemical oxidation of Ru(II) complexes with molecular oxygen, undergo facile electron transfer reaction with dialkyl and aryl methyl sulfides. The rate controlling electron transfer process is confirmed from the absorption spectrum of the transient sulfide radical cation. The spectrophotometric kinetic study shows that the reaction is of total second order, first order in Ru(III) complex and in the organic sulfide. The reaction rate is susceptible to the change of ligand in [Ru(NN)₃]³⁺ and the structure of organic sulfide.     

Synthesis and characterization of cobalt(III) tetraaza macrocyclic complexes containing chloro and azido ligands

The complexes [Co(L)Cl₂]Cl · 4H₂O (1) and [Co(L)(N₃)₂]N₃ · 2H₂O (2) (L = 3,14-dimethyl-2,6,13,17-tetraazatricyclo [14,4,0^1.18,0^7.12]docosane) have been synthesized, and structurally characterized by X-ray crystallography, spectroscopy and cyclic voltammetry. The crystal structure of (1) is centrosymmetric and the cobalt(III) atom has an axially elongated octahedral geometry with four nitrogen atoms of the macrocycle and two chloride ligands. The cobalt(III) ion in (2) is coordinated to four nitrogen atoms from the macrocycle, and two azide ligands in an octahedral environment, which forms the 1D polymer through hydrogen bonding contacts involving the cation, azide anion and solvent water molecules. Electronic spectra of the complexes also exhibit a low-spin octahedral environment. Cyclic voltammetry of the complexes undergoes a one-electron wave corresponding to Co(III)/Co(II) processes. The electronic spectra and electrochemical behaviors of the complexes are significantly affected by the nature of the axial ligands.     

Resonance raman studies of oxygen binding in Cobalt(III)—salen complexes

Co-O and O-O bond stretching frequencies have been determined by oxygen isotopic substitution in a series of cobalt(III)—salen complexes. These all are of the binuclear type [Co(salen)L]₂O₂, with L being a basic ligand occupying an axial coordination position. The nature of the oxygen binding and the influence of the axial ligands are discussed.     

Mixed-ligand cyanide cobalt(III) complexes with 1,2-cyclohexanediaminetetraacetic acid: Synthesis and photochemical properties

[Co(CN)₂(H₃Cdta)] · 2H₂O (I) and Na₂[Co(CN)(Cdta)] · 3H₂O (II) (Cdta⁴⁻ is the 1,2-cyclohexanediaminetetraacetate) were synthesized. In I, the two cyanide ions and the Cdta nitrogen atoms are located in the equatorial plane, the carboxyl groups connected to different N atoms are in axial positions; in addition one of them, like two other uncoordinated carboxyl groups, is protonated. In II, Cdta is a pentadentate ligand coordinated in the cis-equatorial mode. UV excitation of the dicyano complexes at the ligand-to-metal charge transfer band gives organic derivatives of cobalt(III) stable enough for chromatographic isolation. Photolysis of II in neutral medium gives aqua(cyclohexanediaminetriacetato)cobalt (III) complexes as the final products.     

Synthesis,structure, and electrochemistry of <Emphasis Type="Italic">trans</Emphasis>-[Co<Superscript>III</Superscript>{(BA)<Subscript>2</Subscript>pn}(L)<Subscript>2</Subscript>]ClO<Subscript>4</Subscript> complexes

The structure, spectroscopic, and electrochemical properties of [Co{(BA)₂pn}(L)₂]ClO₄ complexes, where (BA)₂pn = N,N′-bis(benzoylacetone)-1,3-propylenediimine dianion and the two ancillary ligands (L) are pyridine, py (1), and 4-methylpyridine, 4-Mepy (2), have been investigated. These complexes have been characterized by elemental analyses, IR, UV–Vis and ¹H-NMR spectroscopy. The crystal structure of [Co{(BA)₂pn}(py)₂]ClO₄ (1) has been determined by X-ray diffraction. The coordination geometry around cobalt(III) is best described as a distorted octahedron. The electrochemical reduction of these complexes at a glassy carbon electrode in acetonitrile solution indicates that the first reduction process corresponding to Co^III–Co^II is electrochemically irreversible, which is accompanied by the dissociation of the axial N(py)–cobalt bonds. This process becomes quasi-reversible upon the addition of excess py ligands. The second reduction step of Co^II/I shows reversible behavior and is not influenced by the nature of the axial ligands. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Synthesis and characterisation of cobalt(II), cobalt(III) and rhodium(III) complexes ofN,N′, N″, N‴-tetra(2-cyanoethyl) cyclam [TCEC] andN,N′, N″, N‴-tetra)3-aminopropyl) cyclam [TAPC]

Summary Complexes of cobalt(II), cobalt(III) and rhodium(III) with TCEC and TAPC have been synthesised. TCEC with cobalt(II) gave [Co(TCEC)Br]Br and [Co(TCEC)Cl]Cl, five coordinate high spin square pyramid complexes, but the corresponding cobalt(III) complex could not be characterised. Rhodium(III) gave a six coordinate [Rh(TCEC)Cl₂]Cl complex, in which the two coordinated chlorides have acis-geometry and the four pendant arms lie on one side of the N₄ plane with none of the —CN groups coordinated TAPC on the other hand gives the cobalt(III) complex, [Co(TAPC)Br]Br₂, in which one of the amino groups of the four pendant arms is coordinated to cobalt. Rhodium(III) with TAPC gave [Rh(TAPC)Cl]Cl₂ in which one axial site is occupied by the amino group of one of the pendant arms and the other by Cl^–.     

Surface and Bulk Reactions of Solid Nickel Dimethylglyoximate with Electron-Active Gases

The action of ammonia on the surface of solid nickel dimethylglyoximate Ni(HDm)_d2 is reversible and is accompanied by a change in the electrophysical properties of the surface layer of the complex in proportion to the ammonia content in the air. The action of chlorine on the complex gives rise to bulk chemical reactions yielding the Ni(III) complex {[Ni^III(HDm)₂(H₂O)]₂Cl⁰₂}Cl^I₂ with a bridging μ-dichloride group, which is further oxidized to a substance presumably containing such ligands as one dimethylglyoxime group and products of its oxidation by chlorine.     

Dipole moment measurements on metal complexes of monothio-β-diketones

Dipole moments have been reported for over 200 monothio-β-diketone complexes, Met(RCSCHCOR′), (n  2, 3). The data show that the nickel(II), palladium(II), platinum(II), and copper(II) complexes are cis-square-planar, the zinc(II) complexes are tetrahedral, and the chromium(III), iron(III), ruthenium(III), cobalt(III), and rhodium(III) complexes are facial (cis)-octahedral. Measurements of the dipole moments of a considerable number of the metal complexes by both the static polarization and dielectric relaxation methods have shown that atomic polarization is ca. 0.3 D in square-planar, 0.5 D in tetrahedral, and 0.9 D in octahedral complexes.A large number of complexes, Met(RCSCHCOR?_n (R  phenyl or X-substituted phenyl, R′ CF₃) have been studied. The difference in the values of the dipole moments depend upon: (a) the magnitude and vector direction of the PhX bond moments; (b) the inductive effect arising from the difference in electron density of the C¹ and C⁵ carbon atoms of the ligand moiety — this is affected by the nature and position of the substituent X on the phenyl ring; (c) the change in moment brought about by the mesomeric effect of the substituent X.     

A mechanistic study on the disproportionation and oxidative degradation of phenothiazine derivatives by manganese(III) complexes in phosphate acidic media

The oxidative degradation of phenothiazine derivatives (PTZ) by manganese(III) was studied in the presence of a large excess of manganese(III)-pyrophosphate (P₂O₇ ²⁻), phosphate (PO₄ ³⁻), and H⁺ ions using UV–vis. spectroscopy. The first irreversible step is a fast reaction between phenothiazine and manganese pyrophosphate leading to the complete conversion to a stable phenothiazine radical. In the second step, the cation radical is oxidized by manganese to a dication, which subsequently hydrolyzes to phenothiazine 5-oxide. The reaction rate is controlled by the coordination and stability of manganese(III) ion influenced by the reduction potential of these ions and their strong ability to oxidize many reducing agents. The cation radical might also be transformed to the final product in another competing reaction. The final product, phenothiazine 5-oxide, is also formed via a disproportionation reaction. The kinetics of the second step of the oxidative degradation could be studied in acidic phosphate media due to the large difference in the rates of the first and further processes. Linear dependences of the pseudo-first-order rate constants (k _obs) on [Mn^III] with a significant non-zero intercept were established for the degradation of phenothiazine radicals. The rate is dependent on [H⁺] and independent of [PTZ] within the excess concentration range of the manganese(III) complexes used in the isolation method. The kinetics of the disproportionation of the phenothiazine radical have been studied independently from the further oxidative degradation process in acidic sulphate media. The rate is inversely dependent on [PTZ^+.], dependent on [H⁺], and increases slightly with decreasing H⁺ concentration. Mechanistic consequences of all these results are discussed.     

Oxidative dealkylation of 4a-alkylated 4a,5-dihydroflavins: Properties of 4a-alkylated flavin radicals

Covalent coenzyme substrate adducts (“σ-complexes”) are probable intermediates in flavin-dependent biological dehydrogenations. As chemical model reaction for the σ-complex decay, oxidative dealkylation of stable 4a-alkyl-4a,5-dihydroflavins was studied as a function of alkyl mobility and nature of the oxidizing agent. The alkyl groups studied were n-propyl, allyl and benzyl, the oxidizing agents ³O₂, ¹O₂^*, nitroxide radical, ferricyanide and light-excited flavin.For all three alkyl residues, the primary reaction is formation of the 4a-alkyl-4a-hydroflavin radical by le^?-abstraction. ³O₂ and ferricyanide are too weak to initiate this step. If, however, the radical 4a-RFl is once formed, at least five decay modes can be observed depending on the nature of R:(1) For saturated R the exclusive decay is back transfer of the electron initially abstracted. In this case, dealkylation can only be obtained with ¹O₂*, albeit with the relatively slow rate of < 10⁶ M^?1s^?1.(2) For unsaturated R further 1e^?-oxidation leads to quantitative formation of oxidized flavin, while the fate of the alkyl group is still uncertain: In any case, ROH and the corresponding aldehydes as well as the dimers R₂ can be excluded as products.(3) Further oxidation by ³O₂ again leads to a quantitative yield of oxidized flavin while the alkyl residues are converted to peroxy radicals. In an autocatalytic reaction they form the corresponding hydroperoxides with starting 4a-R-Fl_redH, leading to acrolein (R = allyl) or benzaldehyde (R = benzyl) as the major products.(4) In the absence of further oxidant, slow intramolecular alkyl migration is observed leading to the stable 5-alkyl-l,5-dihydroflavin isomer.(5) Competitively, alkyl migration occurs intermolecularly with the starting material as carbenium acceptor, resulting in formation of the stable 4a,5-dialkyl-4a,5-dihydroflavin and unsubstituted radical HFl, which disproportionates.