Pentacoordinated copper-sparteine complexes with chelating nitrite or nitrate ligand: Synthesis and catalytic aspects

Copper(II)-sparteine complexes, [Cu^II{(-)Sp}(NO₂)Cl] (1) and [Cu^II{(−)Sp}(NO₃)Cl] (2) (Sp = sparteine) with chelating nitrite and nitrate ligands, respectively, have been synthesized and structurally characterized. The penta-coordinated 1 or 2 exhibits distorted square pyramidal geometry and shows characteristic EPR spectra with g_||: 2.27 and g_⊥: 2.06. 1 and 2 behave similarly towards the catalytic epoxidation of alkenes as well as oxidation of alcohols. Though the epoxidation of cyclohexene using 1 or 2 as a catalyst and tertiarybutyl hydroperoxide (TBHP) as an oxidant at 298 K in acetonitrile results in 100% cyclohexene oxide product, under identical reaction conditions styrene selectively transforms to benzaldehyde (∼90%) instead of any styrene oxide. However, at higher temperature (353 K) the selectivity of cyclohexene to corresponding epoxide formation decreases appreciably and additional products, cyclohexanol and cyclohexanone are formed. Furthermore, 1 or 2 effectively catalyzes the oxidation of benzyl alcohol to benzoic acid and cyclohexanol to cyclohexanone in presence of molecular oxygen (O₂) as an oxidant at 353 K in acetonitrile.     

Synthesis, crystal structure and solid state photoluminescence of [Pt(trpy)(CCPh)]SbF6 (trpy=2,2′:6′,2″-terpyridine)

The synthesis and characterisation of an orange polymorph of [Pt(trpy)(CCPh)]SbF₆ is described where trpy = 2,2′:6′,2″-terpyridine. An X-ray crystal structure determination at 293 K reveals that the cations are planar and stacked head-to-tail with alternating Pt?Pt distances of 3.604(1) and 4.412(1) Å. The perpendicular distances between successive cation planes are constant along the stack at a value of 3.33 Å. Crystal structure determinations at 240 and 200 K show that reducing the temperature to 200 K has no significant effect on the cation arrangement. However, below 200 K there is a phase change that we have not been able to characterise, but which has an effect on the solid state photoluminescence exhibited by [Pt(trpy)(CCPh)]SbF₆. Thus, whereas at temperatures of ?200 K, a broad peak with two components at ca. 566 and 597 nm is observed, below 200 K a longer wavelength peak develops that red-shifts as the temperature is lowered [λ(em)_max = 637 nm at 80 K]. We assign the ?200 K emission as ³MLLCT in origin, since the X-ray data show that ligand-ligand (LL) and not metal-metal (MM) interactions are important at T ? 200 K. On the other hand, the long wavelength emission observed below 200 K is typical of ³MMLCT emission, suggesting that the phase change leads to d_z²(Pt)-d_z²(Pt) orbital interactions. Of particular interest is that the cation exhibits ³MLCT emission in dichloromethane that maximizes at 619 nm, i.e., the high temperature solid state emission occurs at a shorter wavelength, an unexpected result since intermolecular interactions in the solid usually cause the emission to occur at longer wavelengths. A possible explanation for this unexpected result is given.     

Femtosecond primary charge separation in Synechocystis sp. PCC 6803 photosystem I

The ultrafast (< 100 fs) conversion of delocalized exciton into charge-separated state between the primary donor P700 (bleaching at 705 nm) and the primary acceptor A₀ (bleaching at 690 nm) in photosystem I (PS I) complexes from Synechocystis sp. PCC 6803 was observed. The data were obtained by application of pump-probe technique with 20-fs low-energy pump pulses centered at 720 nm. The earliest absorbance changes (close to zero delay) with a bleaching at 690 nm are similar to the product of the absorption spectrum of PS I complex and the laser pulse spectrum, which represents the efficiency spectrum of the light absorption by PS I upon femtosecond excitation centered at 720 nm. During the first ∼ 60 fs the energy transfer from the chlorophyll (Chl) species bleaching at 690 nm to the Chl bleaching at 705 nm occurs, resulting in almost equal bleaching of the two forms with the formation of delocalized exciton between 690-nm and 705-nm Chls. Within the next ∼ 40 fs the formation of a new broad band centered at ∼ 660 nm (attributed to the appearance of Chl anion radical) is observed. This band decays with time constant simultaneously with an electron transfer to A₁ (phylloquinone). The subtraction of kinetic difference absorption spectra of the closed (state P700⁺A₀A₁) PS I reaction center (RC) from that of the open (state P700A₀A₁) RC reveals the pure spectrum of the P700⁺A₀⁻ ion-radical pair. The experimental data were analyzed using a simple kinetic scheme: An* [(PA₀)*A₁ P⁺A₀⁻A₁] P⁺A₀A₁⁻, and a global fitting procedure based on the singular value decomposition analysis. The calculated kinetics of transitions between intermediate states and their spectra were similar to the kinetics recorded at 694 and 705 nm and the experimental spectra obtained by subtraction of the spectra of closed RCs from the spectra of open RCs. As a result, we found that the main events in RCs of PS I under our experimental conditions include very fast (< 100 fs) charge separation with the formation of the P700⁺A₀⁻A₁ state in approximately one half of the RCs, the ∼ 5-ps energy transfer from antenna Chl* to P700A₀A₁ in the remaining RCs, and ∼ 25-ps formation of the secondary radical pair P700⁺A₀A₁⁻.     

Rapid synthesis of bis (2,2′-bipyridine) nitratocopper(II) nitrate using a hydrothermal method and its application to dye-sensitized solar cells

In this study we synthesized bis (2,2′-bipyridine) nitratocopper(II) nitrate in order to examine its the crystal structure, optical property and application to dye-sensitized solar cells (DSSCs). Single X-ray analysis results revealed that the acquired complex exhibited five-coordination with four nitrogen atoms of bipyridine and the oxygen bond of the ion. The reflectance UV-Vis absorptions showed three absorptions that were assigned to ligand-to-ligand at around 230-350 nm, metal-to-ligand charge transfer at around 350-600 nm, and d-d transfer at around ∼650 nm. Cyclic voltammetry in acetonitrile revealed a reversible Cu(I) → Cu(II) oxidation process at a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) levels of −4.692 and −4.071 eV, respectively. The photoelectric efficiency in DSSCs was approximately 0.032% with the nanometer-sized TiO₂ in the condition of an open-circuit voltage (V_oc) of 0.346 V, a short-circuit current density (J_sc) of 0.166 mA/cm² at an incident light intensity of 100 mW/cm².     

Functional phosphines. Part XIV. Cationic (P,N)2-coordinated hydrides of iridium(III): catalysts for CO hydrogenation or transfer hydrogenation?

Treatment of trans-[IrCl(CO)(PPh₃)₂] with Ph₂PCH₂CH₂NH₂ in refluxing para-xylene gave (OC-6-43)-[Ir(H)(Cl)(Ph₂PCH₂CH₂NH₂)₂]Cl (1) which interacted with K[BH(s-Bu₃)] to produce a mixture of (OC-6-22)-[IrH₂(Ph₂PCH₂CH₂NH₂)₂]Cl (2a) and (OC-6-32)-[Ir(H)(Cl)(Ph₂PCH₂CH₂NH₂)₂]Cl (2b). The trans-dihydride 2a was isolated in pure form from the reaction between 1 and KOH/i-PrOH. Different from its isoelectronic (P,N)₂-coordinated Ru^II analogues, the cationic chloro hydrido complex 1 does not act as a catalyst for the direct hydrogenation of acetophenone by molecular H₂, if activated by strong alkoxide base, but rather catalyzes the transfer hydrogenation of the CO bond with methanol or isopropanol as proton/hydride sources. Dihydrido complex 2a is ascribed the role of the actual catalyst as it supports the transfer hydrogenation reaction even in the absence of base. The crystal structure of the addition compound 1 · 2EtOH has been determined.     

Unprecedented rhodium-mediated catalytic transfer hydrogenation of a phosphonate functionalized olefin in ecofriendly media

RhCl₃ · xH₂O catalyst-mediated hydrogenation reactions of vinyl phosphonic diethyl ester H₂CCH-P(O)(OEt)₂ (1) have been investigated. Results demonstrate that the hydrogenation of H₂CCH-P(O)(OEt)₂ (1) to CH₃CH₂-P(O)(OEt)(OH) (2) proceeds in the presence of RhCl₃ · xH₂O catalyst, without any external hydrogen source and ancillary ligands, to near qualitative yields in ethanol and water media. ³¹P, ¹³C and ¹H NMR and deuterium-labeling experiments provide evidence for the non-concerted mechanistic pathway associated with the hydrogenation of 1 to 2.     

Synthesis, spectral and magnetical characterization of monomeric [Cu(2-NO2bz)2(nia)2(H2O)2] and structural analysis of similar [Cu(RCOO)2(L-N)2(H2O)2] complexes

A new complex of composition [Cu(2-NO₂bz)₂(nia)₂(H₂O)₂] (1) (nia = nicotinamide, 2-NO₂bz = 2-nitrobenzoate) has been prepared and its composition and stereochemistry as well as coordination mode have been determined by elemental analysis, electronic, infrared and EPR spectroscopy, magnetization measurements over the temperature range 1.8-300 K, and its structure has been solved, as well. The complex structure consists of the centrosymmetric molecules with Cu(II) atom monodentately coordinated by the pair of 2-nitrobenzoato anions and by the pair of nicotinamide molecules, forming nearly tetragonal basal plane, and by a pair of water molecules that complete tetragonal-bipyramidal coordination polyhedron about the copper atom. The complex 1 exhibits magnetic moment μ_eff = 1.86 B.M. at 300 K which decreases to μ_eff = 1.83 B.M. at 1.8 K. The magnetic susceptibility temperature dependence obeys Curie-Weiss law with Curie constant of 0.442 cm³ K mol⁻¹ and with Weiss constant of −1.0 K. EPR spectra at room temperature as well as at 77 K are of axial type with g_⊥ = 2.065 and g_∥ = 2.280 and exhibit clearly, but partially resolved parallel hyperfine splitting with A_II = 160 G, that is consistent with the determined molecular structure of 1. In order to analyze the factors influencing the degree of tetragonal distortion of coordination polyhedron, the dataset of 72 structures similar to that of 1 was extracted from CCD and analyzed. A significant correlation between the average Cu-O_ax bond length and tetragonality parameter τ which was found as a consequence of the Jahn-Teller effect.     

Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant

Fe(II)-tris(2-pyridylmethyl)amine complexes, Fe(II)-tpa, having different co-existing anions, [Fe(tpa)(MeCN)₂](ClO₄)₂ (1), [Fe(tpa)(MeCN)₂](CF₃SO₃)₂ (2) and [Fe(tpa)Cl₂] (3), were prepared. Effective magnetic moments (evaluated by the Evans method) revealed that while 1-3 in acetone and 3 in acetonitrile (MeCN) have a high-spin Fe(II) ion at 298 K, the Fe(II) ions of 1 and 2 are in the low-spin state in MeCN. The aerobic oxidation of 1-3 was monitored by UV-Vis spectral changes in acetone or MeCN under air at 298 K. Only the high-spin Fe(II)-tpa complexes were oxidized with rate constants of k_obs = 0.1-1.3 h⁻¹, while 1 and 2 were stable in MeCN. The aerobic oxidation of 1 or 2 in acetone was greatly accelerated in the presence of pure, peroxide-free cyclohexene (1000 equiv.) and yielded a large amount of oxidized products; 2-cyclohexe-1-ol (A) and 2-cyclohexene-1-one (K) (A + K: 23 940% yield based on Fe; A/K = 0.3), and cyclohexene oxide (810%). Besides cyclohexene, aerobic oxidation of norbornene, cyclooctene, ethylbenzene, and cumene proceeded in the presence of 1 in acetone at 348 K without any reductant. Essential factors in the reaction are high-spin Fe(II) ion and labile coordination sites, both of which are required to generate Fe(II)-superoxo species as active species for the H-atom abstraction of hydrocarbons.     

New polymeric thiocyanatoiron(II) complex with N,N′-diethylnicotinamide - Synthesis, structure, magnetic and spectral properties

The iron(II) compound of formula [Fe(NCS)₂(dena)₂]_n (dena = N,N′-diethylnicotinamide) has been prepared by the reaction between iron(III) thiocyanate and dena in ethanol solution. The complex was characterized by elemental analysis, spectral and magnetic measurements. Single-crystal X-ray diffraction methods show that the complex, crystallizing in the triclinic space group, undergoes a phase transition between 220 K and 230 K, connected with the doubling of cell volume. Crystal structures at 230 K (1a; HT phase) and 150 K (1b; LT phase) are described and a transition mechanism is discussed. In both phases the compound has an extended chain structure, in which the neutral molecule of N,N′-diethylnicotinamide acts as a bridging ligand binding through pyridine N atom to one centre and through amide O atom to the neighbouring Fe centre. The Fe²⁺ ion has a slightly distorted trans-octahedral environment with FeO₂N₄ chromophore, and all Fe-O and Fe-N bonds in the typical for high-spin iron(II) compounds range. Variable-temperature magnetic susceptibility data in the temperature range 1.8-300 K show that iron(II) is high-spin S = 2(⁵T_2g) and as a result effects due to zero-field splitting are anticipated at low temperatures. The IR spectrum suggested the coordination of N,N′-diethylnicotinamide to the central atom of iron(II) as a bridging ligand and NCS group as a monodentate ligand.     

Synthesis, crystal structure, magnetic properties and electrochemical behaviour of the mixed valence compound [Cu(CN)3Cu(C3H10N2)2]

The binuclear mixed valence copper(I/II) compound [Cu^I(CN)₃Cu^II(tn)₂] (1) (tn = propane-1,3-diamine) and its acetonitrile adduct [Cu^I(CN)₃Cu^II(tn)₂] · 2MeCN (2) have been synthesized. Complex 1 crystallizes triclinic, space group , a = 8.117(2) Å, b = 8.389(2) Å, c = 11.920(2) Å, α = 108.728(3)°, β = 100.024(3)°, γ = 104.888(4)°, Z = 2, and compound 2 monoclinic, space group P2₁/m, a = 8.752(2) Å, b = 13.243(3) Å, c = 9.549(2) Å, β = 114.678(4)°, Z = 2. In both crystal structures, the binuclear [Cu^I(CN)₃Cu^II(tn)₂] complex with slightly different bonding geometries is formed. One of the three nitrogen atoms of a Cu^I(CN)₃ moiety is coordinated to Cu(II) at the apex of a square-pyramid with two chelating ligands tn on its base. The shortest intramolecular Cu^II?Cu^II distance in 1 is 5.640(7) Å. The EPR behaviour of 1 has been investigated at room temperature and at 77 K. The magnetic properties were measured in the temperature range 1.8-300 K.     

Synthesis and characterization of a family of binuclear non-heme iron monooxygenase model compounds: Evidence for a “phenolate/amide carbonyl (PAC) shift” upon oxidation

A series of binuclear iron compounds has been synthesized using diamide, bis-phenolate ligands in which the carbon-linker between the amide nitrogen atoms has been varied. Two diferrous compounds in the series, along with their two-electron oxidized, di-μ-methoxy-bridged counterparts, have been crystallographically characterized, as have the di-μ-methoxy compounds (H₂Hbab = 1,2-bis(2-hydroxybenzamido) benzene, H₂Hbach = trans-1,2-bis(2-hydroxybenzamido) cyclohexane, H₂Hbame = 1,2-bis(2-hydroxybenzamido) ethane, H₂Hbap = 1,3-bis(2-hydroxybenzamido) propane, H₂Hbabn = 1,4-bis(2-hydroxybenzamido) butane, H₂Hbapen = 1,5-bis(2-hydroxybenzamido) pentane, N-MeIM = N-methylimidazole and OMe = methoxide). are structurally very similar to previously reported diferrous compounds of this family of ligands that have been shown to be active as oxygen atom transfer catalysts. Flexibility in the carbon-linker allows some variability in the orientation of the phenolate arms of the ligands in the diferric di-μ-methoxy compounds, but the Fe₂O₂ core remains largely unchanged across the series. Two-electron oxidation of the ferrous compounds in methanol shows a substantial ligand rearrangement that is consistent with other spectroscopic, electrochemical and kinetic investigations. The loss of both phenolate bridges upon oxidation is reminiscent of the “carboxylate shift” observed in binuclear non-heme enzymes and could provide insight into the driving force behind this family of compounds’ function as a catalyst.     

Synthesis, structural determination and magnetic properties of a heterobimetallic CuRe complex containing a macrocyclic ligand

Reaction of the rhenium(IV) compound, [Bu₄N]₂ReCl₄ox, with the highly unsaturated tetraazabismacrocyclic copper(II) complex cation [CuCuL]⁴⁺ (L = 6,13-Bis(dodecylaminomethylidene)-1,4,8,11-tetraazacyclotetradeca-4,7,11,14-tetraene) produced a new kind of heterobimetallic compound: [CuCuL][ReCl₄ox]₂ · 2DMF in which [ReCl₄ox]²⁻ anions and [CuCuL]⁴⁺ cations are linked by electrostatic forces. The crystal structure of this compound was determined at 173(2) K. It crystallizes triclinic, space group , with a = 9.441(4), b = 11.032(5), c = 15.261(7) Å, α = 89.05(1)°, β = 88.93(1)°, γ = 77.09(1)°, Z = 1, R₁ = 0.0557, wR₂ = 0.1332. The magnetic behavior of this compound has been investigated over the temperature range 1.72-300 K. The compound behaves as a ferrimagnetic Cu^IIRe^IV bimetallic, chain with intrachain antiferromagnetic coupling.     

The use of trans-4-cotininecarboxylate to construct a polymeric copper(II) azido complex: Hydro(solvo)thermal synthesis, structure and magnetic properties

A new pyridyl-carboxylate ligand, the anion of trans-4-cotininecarboxylic acid, HL, 1, has been used to prepare a new polymeric copper(II) complex, [CuLN₃]_2n, 2, based on a [CuLN₃]₂ dimeric building block. The single crystal structures of both 1 and 2 have been determined and 1 has been found to be in its zwitterionic configuration. The structure of 2 is a one-dimensional tape-like polymeric structure based on an end-on azido-bridged binuclear [Cu₂N₃]₂ backbone moiety. Magnetic studies reveal that 2 is close to paramagnetic from 2 to 300 K with a Curie constant of 1.094 emu K/mol, a Weiss temperature of 0.73 K and a corresponding μ_eff of 2.09 μ_B. A fit of χ_MT for 2 with S₁ = S₂ = ½, yields g = 2.441(6), J = −0.49(3) cm⁻¹, zJ = −0.38(2) cm⁻¹ and N(α) = 0.00053(12) emu/mol, a fit that indicates the presence of both very weak intramolecular intrachain antiferromagnetic exchange coupling within the one-dimensional tape-like chains and very weak interchain antiferromagnetic exchange coupling between these chains.     

Crystal structure and magnetic behaviour of a five-coordinate iron(III) complex of pyridoxal-4-methylthiosemicarbazone

The crystal structure and magnetic properties of a penta-coordinate iron(III) complex of pyridoxal-4-methylthiosemicarbazone, [Fe(H₂mthpy)Cl₂](CH₃C₆H₄SO₃), are reported. The synthesised ligand and the metal complex were characterised by spectroscopic methods (¹H NMR, IR, and mass spectroscopy), elemental analysis, and single crystal X-ray diffraction. The complex crystallises as dark brown microcrystals. The crystal data determined at 100(1) K revealed a triclinic system, space group (Z = 2). The ONSCl₂ geometry around the iron(III) atom is intermediate between trigonal bipyramidal and square pyramidal (τ = 0.40). The temperature dependence of the magnetic susceptibility (5-300 K) is consistent with a high spin Fe(III) ion (S = 5/2) exhibiting zero-field splitting. Interpretation of these data yielded: D = 0.34(1) cm⁻¹ and g = 2.078(3).     

[Mn2(Fpymo)4(H2O)4]: Synthesis, structure, magnetism and thermally induced solid-to-solid polymerisation reactions

The reaction of an aqueous solution of Mn(ClO₄)₂ · 6H₂O with 5-fluoro-2-hydroxypyrimidine (HFpymo) and NaOH in 1:2:1 ratio affords a species analysing as Mn(Fpymo)₂(H₂O)₂ (1) in 70% yield. Single crystal X-ray analysis reveals that 1 consists of [Mn₂(μ-Fpymo-N¹,O²)₂(Fpymo-O²)₂(H₂O)₄] dinuclear units, in which each Mn(II) ion shows a slightly distorted trigonal bipyramidal stereochemistry. Thermal treatment of 1 above 150 °C gives an anhydrous, amorphous material analysing as Mn(Fpymo)₂ (2a). Further heating of this compound above 250 °C results in the formation of the microcrystalline Mn(Fpymo)₂ species (2b). The thermal dependence of the magnetic susceptibility χ has been studied for species 1 and 2b in the 2-300 K temperature range at 100, 300 and 5000 Oe field strengths. The fitting of the χ values of 1 to the Curie-Weiss equation gives values of C = 2.450(2) and θ = 1.0(2) K, which is indicative of an almost negligible magnetic interaction between the Mn(II) centres. At variance, 2b shows a significant antiferromagnetic behaviour, with a decrease of the μ_eff values upon cooling. The fitting of the χ values of 2b to the Curie-Weiss equation gives the respective C and θ values of 4.26(1) and −14.8(3) K, which agrees with an efficient coupling of the magnetic Mn(II) centres, possibly through bridges of the Fpymo-N¹,N³ kind, within a polymeric network. The N₂ and CO₂ gas adsorption measurements at 77 K and 293 K, respectively, show that the 2b phase is not microporous, which is reflected in its low BET surface (19 m² g⁻¹) and its BJH pore size distribution.     

Binuclear manganese(III) complexes of an unsymmetric pyrazolate-based compartmental ligand with hard donor set

The synthesis, crystal structure and magnetic properties of manganese(III) binuclear complexes [Mn^III₂(L-3Н)₂(CH₃ОH)₄]·2CH₃ОH (1) and [Mn^III₂(L-3Н)₂(Py)₄]·2Py (2) (L = 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid) are reported. The ligand contains two distinct donor compartments formed by the pyrazolate-N and the oxime or the carboxylic groups. The complexes were characterized by X-ray single crystal diffraction, revealing that both 1 and 2 consist of dinuclear units in which the two metal ions are linked by double pyrazolate bridges with a planar {Mn₂N₄} core. Cryomagnetic measurements show antiferromagnetic interaction with g = 1.99, J = −3.6 cm⁻¹, Θ = −2.02 K for 1 and g = 2.00, J = −3.7 cm⁻¹, Θ = 1.43 K for 2.     

Molecular sensing behavior of the first Mn(I)-compound of di-2-pyridylketone-p-nitrophenylhydrazone (dpknph), fac-[Mn(CO)3(dpknph)Br]

The reaction between [Mn(CO)₅Br] and di-2-pyridylketone-p-nitrophenylhydrazone (dpknph) in diethyl ether under ultrasonic conditions gave fac-[Mn(CO)₃(dpknph)Br] in good yield. Optical and thermodynamic measurements on fac-[Mn(CO)₃(dpknph)Br] in non-aqueous polar solvents revealed reversible interconversion between two intense charge transfer absorption bands due to π-π* (dpk), followed by dpk → nitro intraligand charge transfer transition (ILCT), mixed with metal ligand charge transfer transition (MLCT) due to . In non-polar solvents, a single absorption band appeared. Extinction coefficients of 46 200 ± 2000 and 28 400 ± 2000 M⁻¹ cm⁻¹ were calculated in DMSO for the low- and high-energy electronic states of fac-[Mn(CO)₃(dpknph)Br] using excess NaBF₄. Changes in enthalpy (ΔH^ø) of +14.0 and −12.1 kJ mol⁻¹, entropy (ΔS^ø) of +28.65 and −64.30 J mol⁻¹ K⁻¹, and free energy (ΔG^ø) of +5.48 and +7.08 kJ mol⁻¹ at 298 K were calculated for the interconversion between the high and low energy electronic states of fac-[Mn(CO)₃(dpknph)Br]. These results allow for the use of these systems (fac-[Mn(CO)₃(dpknph)Br] and surrounding solvent or solute molecules) as optical sensors for a variety of physical and chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00 × 10⁻⁹ M can be detected and determined using fac-[Mn(CO)₃(dpknph)Br] in dmso in the presence and absence of NaBH₄.     

Relating catalytic activity and electrochemical properties: The case of arene-ruthenium phenanthroline complexes catalytically active in transfer hydrogenation

The electrochemical properties of cationic complexes [(η⁶-arene)Ru(N ∩ N)Cl]Cl (arene/N ∩ N = C₆H₆/1,10-phenanthroline (1), p-MeC₆H₄Prⁱ/1,10-phenanthroline (2), C₆Me₆/1,10-phenanthroline (3), C₆Me₆/5-NO₂-1,10-phenanthroline (4), and C₆Me₆/5-NH₂-1,10-phenanthroline (5)) were studied by cyclic voltammetry in order to rationalize catalytic activity in transfer hydrogenation of the respective aqua complexes [(η⁶-arene)Ru(N ∩ N)(OH₂)](BF₄)₂ (6-10). Complexes 1-5 were chosen because the ‘true’ catalysts 6-10 are unstable under the conditions of the measurement. The electrochemical behaviour of 1-5 in acetonitrile solution is rather complicated due to consecutive and parallel chemical reactions that accompany electron transfer processes. Nonetheless, interpretation of the electrochemical data allowed to assess the influence of the structure and substitution on the redox and catalytic properties: the catalytic ability correlates with the reduction potentials, indicating the decisive role of the η⁶-arene ring directly bonded to the catalytic centre (Ru).     

Magnetic and optical properties of trans-RSSR-[CrCl2(cyclam)]2ZnCl4 (cyclam = 1,4,8,11-tetraazacyclotetradecane) attributed to counterion via hydrogen bonding

This is the first reported example of a photochromic property presented by the change of a counterion in a coordination compound: the colour of the trans-[CrCl₂(cyclam)]₂ZnCl₄ (1) is dark green while the trans-[CrCl₂(cyclam)]Cl (2) is a reddish purple. The dark green colour of 1 under a fluorescent lamp changes to a deep purple when it is irradiated with an incandescent lamp; when the latter light is turned off, the dark green colour reappears instantaneously, this being a reversible process. Besides, the electron paramagnetic resonance (EPR) spectra of these polycrystalline samples show a very broad isotropic peak centred at g = 1.995 for 1 and for 2 a rhombic one at g = 4.309, 3.107 and 1.223. Their molar magnetic susceptibilities, χ_Mdc, against temperature (2-300 K) follow Curie Weiss behaviour. For 1, a low antiferromagnetic coupling (θ = −2.78 K) in the solid state was found as it approaches 2 K, while for 2, there was found a smaller antiferromagnetic coupling (θ = −0.40 K). From the luminescence studies at 17 K, the lifetime for 1 was found to be twice that for 2. The crystal and molecular structure of 2 were determined and discussed. Compounds 1 and 2 show the same trans-RSSR configuration with different hydrogen bonding networks. In 1 the supramolecular array includes intermolecular interactions in which the chromium atoms interact through the zinc atom of the tetrachlorozincate counterion via hydrogen bonding with the extraordinary consequence of showing the previously described physical properties. This has been supported by theoretical calculations, in which it is clearly observed that the HOMO orbital of 1 is a highly delocalised molecular orbital among Cr(III)?Zn(II)?Cr(III), thus giving even more evidence for the strong Cr(III)-Cr(III) interaction through the ZnCl₄ moiety via hydrogen bonding.     

Reaction of wild-type and Glu243Asp variant yeast cytochrome c oxidase with O2

We have studied internal electron transfer during the reaction of Saccharomyces cerevisiae mitochondrial cytochrome c oxidase with dioxygen. Similar absorbance changes were observed with this yeast oxidase as with the previously studied Rhodobacter sphaeroides and bovine mitochondrial oxidases, which suggests that the reaction proceeds along the same trajectory. However, notable differences were observed in rates and electron-transfer equilibrium constants of specific reaction steps, for example the ferryl (F) to oxidized (O) reaction was faster with the yeast (0.4 ms) than with the bovine oxidase (~ 1 ms) and a larger fraction Cu_A was oxidized with the yeast than with the bovine oxidase in the peroxy (P_R) to F reaction. Furthermore, upon replacement of Glu243, located at the end of the so-called D proton pathway, by Asp the P_R → F and F → O reactions were slowed by factors of ~ 3 and ~ 10, respectively, and electron transfer from Cu_A to heme a during the P_R → F reaction was not observed. These data indicate that during reduction of dioxygen protons are transferred through the D pathway, via Glu243, to the catalytic site in the yeast mitochondrial oxidase. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.