Synthesis, structures, electrochemistry and properties of dioxo-molybdenum(VI) and -tungsten(VI) complexes with novel asymmetric N2OS, and partially symmetric N2S2, NOS2N-capped tripodal ligands

A new class of asymmetric N-capped (dianionic/trianionic) tripodal proligands [H_x(Lⁿ)] (x = 2, n = 1-6; x = 3, n = 7, 8) which possess pendant arms with N₂OS, N₂S₂ or NOS₂ donor groups and with different chelate ring sizes {5,5,5} or {5,6,5} has been prepared. Treatment of these ligands with [WO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) in the presence of base (triethylamine or KOH) leads to the formation of cis-dioxotungsten(VI) complexes of the types [WO₂(Lⁿ)] (n = 1-6) and K[WO₂(Lⁿ)] (n = 7, 8). Reaction of these tetradentate ligands with [MoO₂(acac)₂] (acac = acetylacetonate) gives the corresponding Mo(VI) analogues [MoO₂(Lⁿ)] (n = 1-6) and K[MoO₂(Lⁿ)] (n = 7, 8). Moreover, a new five coordinate dioxomolybdenum(VI) complex with an NS₂ tridentate ligand [MoO₂(L⁹)] has been synthesised using similar procedure. All these compounds have been spectroscopically characterised and the molecular structures of [MoO₂(Lⁿ)] (n = 2, 6) and [WO₂(L⁶)] have been established by X-ray diffraction analysis. The electrochemistry and the catalytic activity for oxidation of allylic and benzylic alcohols of these dioxo complexes have also been investigated.     

Electrospray identification of new polyoxochromate species

Electrospray ionization mass spectrometry (ESI MS) has been conducted on the ammonium and alkali metal (A=Li⁺, Na⁺ and K⁺) dichromate systems. A large number of previously unknown polyoxochromate species have been characterized. Major series that have been identified include [A_x+1H_xCr^VI_xO_4x]⁺ (Li⁺, x=1-5; Na⁺, x=1-7; K⁺, x=1-4) and [A_2x−1Cr^VI_xO_4x−1]⁺ (Li⁺, x=2, 3; Na⁺, x=2-4; K⁺, x=2, 3) in the alkali metal dichromate systems, and [HCr^VI_xO_3x+1]⁻ (x=1-5) in the ammonium dichromate system. Several series also contain mixed oxidation state species, ranging from Cr(V) to Cr(II) in conjunction with Cr(VI), which is consistent with the ease of reduction of Cr(VI). Negative ion ESI MS spectra clearly demonstrate the existence of [HCrO₄]⁻ as the most abundant ion at −20 V, suggesting that its existence in solution is not just hypothetical, as was previously thought. The polymerization units for the series observed include {AHCrO₄}, {A₂CrO₄} and {CrO₃}, with the latter prominent in the alkali metal systems. This presumably arises from the fragmentation of dichromate, A₂Cr₂O₇→{A₂CrO₄}+{CrO₃}. Moreover, the ESI MS of the dichromate compounds have illustrated that the preservation of tetrahedral stereochemistry is of paramount importance for these systems, which leads to only limited polymerization compared to the related molybdate and tungstate systems.     

Chemical implantation of Group 4 cations on silica via cyclopentadienyl- and N,N-dialkylcarbamato derivatives

Chemical implantation of Group 4 cations [Ti(III), Ti(IV), Zr(IV), Hf(IV)] has been carried out under mild conditions by the reaction of polycyclopentadienyl- (MCp_n; M = Ti, n = 3, 4; M = Zr, Hf, n = 4), mixed cyclopentadienyl/N,N-dialkylcarbamato (ML_x(O₂CNEt₂)_y; M = Ti, L = Cp, C₅Me₅ (Cp*), x = 2, y = 1; M = Hf, L = Cp, x = 1, y = 3), and N,N-dialkylcarbamato (M(O₂CNR₂)_n, M = Ti, n = 3, R = ⁱPr; M = Ti, Hf, n = 4, R = Et; M = Zr, n = 4, R = ⁱPr) derivatives, with the silanol groups of amorphous silica. Cyclopentadiene/pentamethylcyclopentadiene and/or carbon dioxide and the secondary amine are released in the process. The amount of implanted cations depends on the metal and on the ligands, the pentamethylcyclopentadienyl complex being less reactive than the unsubstituted congener. The starting complexes and the final products have been characterized by EPR or by ¹³C CP-MAS NMR spectroscopy.     

Homogeneous oxidative coupling catalysts. Mechanism of catalysts formation by oxidation of [(Pip)nCuX]4 (n = 1 or 2, Pip = piperidine, X = Cl, Br or I) by dioxygen in aprotic media

This paper reports the mechanism of formation of oxidative coupling catalysts [(Pip)_nCuX]₄O₂, n = 1 or 2 and X = Cl, Br or I, which represent half of the catalytical cycle, Scheme 1. The mechanism has been described as a pre-equilibrium between [(Pip)_nCuX]₄ and O₂. K values are very sensitive to how strong the hydrogen-bonding between copper (I) tetranuclear and incoming dioxygen is, such association is also sensitive to the variation of X. The pronounced pre-equilibrium is the reason behind the oxidation of [(Pip)_nCuI]₄, which is not the case for pyridine type of ligands. The pre-equilibrium followed by rate determining step k₂, which is responsible to the formation of the oxidative coupling catalysts [(Pip)_nCuX]₄O₂. The overall reaction is a second-order process, first order in each [[(Pip)_nCuX]₄] and [O₂], with rate constant k_on (k_on = Kk₂) and exothermic ΔH^≠ varying from −3 to −12 kcal mol⁻¹ and ΔS^≠ varying from −87 to −65 cal deg⁻¹ mol⁻¹. k_on were found to be very sensitive to n value 1 or 2 and to the type of X (Cl, Br or I).     

Novel gaseous polyatomic binary and ternary lanthanide oxides

A variety of novel gaseous polyatomic binary and ternary oxides were observed at ambient temperature arising from lanthanide (Ln) nitrate Schiff base complexes, simple salts and sesquioxides, in an FAB mass spectrometer. The new binary oxides (as singly positive ions) detected are Ln₂O₃, Ln₃O₃, Ln₃O₄, Ln₄O₄, Ln₄O₅, Ln₄O₆, Ln₅O₆, Ln₅O₇, Ln₅O₈, Ln₆O₈, Ln₆O₉, Ln₇O₁₀, Ln₈O₁₁, Ln₈O₁₂ and Ln₉O₁₃; the ternary gaseous oxides are CeEuO₂, CeEu₂O₃ and Ce₂EuO₄, LaYbO₂, La₂YbO₄ and LaYb₂O₄; NdHoO₃, Nd₂HoO₄, and NdHo₂O₄; YTmO₃; Y_xTm_3−xO₄, x=1−2; Y_xTm_4−xO₆, x=1−3; Y_xTm_5−xO₇, x=1−4; Y_xTm_6−xO₉, x=1−5. Some of these oxides show the lanthanide cations in unusual oxidation states. Gadolinium-gallium ternary oxides, GdGaO₂, GdGaO₃ and Gd₂GaO₄ were also detected. The FAB MS environment is significantly reducing, yielding a homologous series Eu_nO_n where Eu²⁺ is dominant (E°(Eu³⁺/Eu²⁺)=−0.35 V) and no gallium or indium oxides (E°(M³⁺/M°=−0.34 V (In), −0.53 V (Ga)) were formed. The stoichiometry of the polylanthanide ternary oxides formed is determined largely by the chemistry of the major metallic component. The gaseous polyatomic oxides are probably formed through a reductive condensation process involving primary species Ln⁺ and LnO⁺ formed when the rare earth compounds are struck by fast Xe atoms. The demonstrated possibility of double component oxide formation broadens the number and types of gaseous lanthanide oxides which are accessible.     

Molecular structure and catechol oxidase activity of a new copper(I) complex with sterically crowded monodentate N-donor ligand

The attempted alkylation of 1,3-bis(2′-pyridylimino)isoindoline (indH) by the use of n-BuLi and subsequent alkyl halides led to quaternization of the pyridine nitrogens and the zwitterionic monodentate N-ligand (Me₂ind)I was formed. By the use of the ligand the copper(I) complex [Cu^I(Me₂ind)I₂] was prepared and its structure determined. It was found to be good catalyst for the oxidation of 3,5-di-tert-butylcatechol (DTBCH₂) to 3,5-di-tert-butyl-1,2-benzoquinone (DTBQ) and H₂O₂ by dioxygen. Detailed kinetic studies revealed first-order dependence on the catalyst and dioxygen concentration and saturation type behavior with respect to the substrate.     

Effect of dioxygen on copper(II) binding to α-synuclein

Using the fluorescent amino acid tryptophan (Trp), we have characterized the copper(II) binding of F4W α-synuclein in the presence and absence of dioxygen at neutral pH. Variations in Trp fluorescence indicate that copper(II) binding is enhanced by the presence of dioxygen, with the apparent dissociation constant (K_d(app)) changing from 100 nM (anaerobic) to 10 nM (aerobic). To investigate the possible role of methionine oxidation, complementary work focused on synthetic peptide models of the N-terminal Cu(II)-α-syn site, MDV(F/W) and M^∗DV(F/W), where M^∗ = methionine sulfoxide. Furthermore, we employed circular dichroism (CD) spectroscopy to demonstrate that the phenyl-to-indole (F→W) substitution does not alter copper(II) binding properties and to confirm the 1:1 metal-peptide binding stoichiometry. CD comparisons also revealed that Met1 oxidation does not affect the copper-peptide conformation and further suggested the possible existence of a Cu^II-Trp/Phe (cation-π) interaction.     

Assembly of two low-dimensional coordination polymers from Ag(I) and Cd(II) and 2,3-bis(2-pyridyl)pyrazine

Polydentate nitrogen heterocycle ligand 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) reacted with M(NO₃)_x (M = Ag, x = 1; M = Cd, x = 2) to give two new complexes [Ag(2,3-dpp)(NO₃)]₂ (1) and [Cd(2,3-dpp)(NO₃)₂]_n (2). Both complexes have been characterized by single-crystal X-ray diffraction, elemental analyses, IR and ¹H NMR spectroscopy. Single-crystal X-ray analyses showed that complex 1 crystallized in monoclinic, space group P2₁/n is a dimmer containing penta-coordinated Ag⁺ ion. While compound 2 has 1D chain-like structure with repeat unit Cd(2,3-dpp)(NO₃)₂, in which the Cd(II) presents octa-coordinated N4O4 donor set with two four-membered chelating rings and two five-membered chelating rings around Cd(II) ion. Meanwhile, every neutral chain [Cd(2,3-dpp)(NO₃)₂]_n is mutually connected by face-to-face π?π packing interactions to form a two dimensional layer. Furthermore, antibacterial activities of compound 1 and luminescent property of the compound 2 are also investigated.     

Oxidation of Manganous and Ferrous Metalloporphyrins by Dioxygen in Aqueous Solutions

The stoichiometry and rate of oxidation with dioxygen of tetra-(p-sulfonatophenyl)-porphinatomanganese(II) and the bisimidazole tetra(p-sulfonatophenyl)porphinato-iron(II) were studied in aqueous solutions at neutral pH. The stoichiometry for both complexes was determined; two molecules of metalloporphyrin reacted with dioxygen to produce the +3 oxidation state of the metalloporphyrins and hydrogen peroxide. The rate law for the oxidation of Mn(II)-TPPS is rate = k′[Mn(II)-TPPS][O₂], with k′ at 26.5° of 2.6 × 10⁵ M^?1 sec^?1. The rate law for the oxidation of Fe(II)-TPPS in the presence of imidazole is

with k″ = 10,100 sec^?1. Some possible mechanisms consistent with these data are discussed.     

Multidimensional profiling of components in complex mixtures of natural products for metabolic analysis,proof of concept: Application to Quillaja saponins

A method for separation and detection of major and minor components in complex mixtures has been developed, utilising two-dimensional high-performance liquid chromatography (2D-HPLC) combined with electrospray ionisation ion-trap multiple-stage mass spectrometry (ESI-ITMSⁿ). Chromatographic conditions were matched with mass spectrometric detection to maximise the number of components that could be separated. The described procedure has proven useful to discern several hundreds of saponin components when applied to Quillaja saponaria Molina bark extracts. The discrimination of each saponin component relies on the fact that three coordinates (x, y, z) for each component can be derived from the retention time of the two chromatographic steps (x, y) and the m/z-values from the multiple-stage mass spectrometry (z_n, n = 1, 2, …). Thus an improved graphical representation was obtained by combining retention times from the two-stage separation with +MS¹ (z₁) and the additional structural information from the second mass stage +MS² (z₂, z₃) corresponding to the main fragment ions. By this approach three-dimensional plots can be made that reveal both the chromatographic and structural properties of a specific mixture which can be useful in fingerprinting of complex mixtures.     

Influence of the sulfur species reactivity on biofilm conformation during pyrite colonization by Acidithiobacillus thiooxidans

Massive pyrite (FeS₂) electrodes were potentiostatically modified by means of variable oxidation pulse to induce formation of diverse surface sulfur species (S _n ^2?, S⁰). The evolution of reactivity of the resulting surfaces considers transition from passive (e.g., Fe_1?x S₂) to active sulfur species (e.g., Fe_1?x S_2?y , S⁰). Selected modified pyrite surfaces were incubated with cells of sulfur-oxidizing Acidithiobacillus thiooxidans for 24 h in a specific culture medium (pH 2). Abiotic control experiments were also performed to compare chemical and biological oxidation. After incubation, the attached cells density and their exopolysaccharides were analyzed by confocal laser scanning microscopy (CLMS) and atomic force microscopy (AFM) on bio-oxidized surfaces; additionally, S _n ^2?/S⁰ speciation was carried out on bio-oxidized and abiotic pyrite surfaces using Raman spectroscopy. Our results indicate an important correlation between the evolution of S _n ^2?/S⁰ surface species ratio and biofilm formation. Hence, pyrite surfaces with mainly passive-sulfur species were less colonized by A. thiooxidans as compared to surfaces with active sulfur species. These results provide knowledge that may contribute to establishing interfacial conditions that enhance or delay metal sulfide (MS) dissolution, as a function of the biofilm formed by sulfur-oxidizing bacteria.     

Homogeneous oxidative coupling catalysts: stoichiometry and characterization of the first stable oxotetranuclear solids [(Pip)nCuX]4O2 (n=1 or 2, Pip=piperidine, X=Cl, Br, I)

Copper(I) halides react quantitatively with piperidine (Pip) in dioxygen-free methylene chloride or nitrobenzene to form tetranuclear copper(I) complexes [(Pip)_nCuX]₄; n=1 or 2, X=Cl, Br or I. These complexes are very soluble and completely reduce dioxygen to dioxo bridging ligand, with stoichiometry, Δ[Cu(I)]/Δ[O₂]=4.0. The stable oxo solids [(Pip)_nCuX]₄O₂ mimic tyrosinase copper protein. They act as a homogeneous oxidative coupling catalysts for phenols. Electronic transition spectra in the near infrared with high molecular absorptivity are diagnostic for tetranuclear “Cu₄X₄” core structure. The electronic transitions are more likely due to charge transfer between a minimum of three halo ligands and copper(II) center. The room temperature EPR spectra of [(Pip)_nCuX]₄O₂ in methylene chloride are isotropic with four hyperfine lines. The room temperature solid-state EPR spectra of [PipCuX]₄O₂ show an axial spectra with d_x²−y² ground state, suggesting square pyramidal arrangement of the five coordinated ligands around copper(II) centers. Cyclic voltammetry measurements show that they are more likely irreversible in character and show slight quasi-reversability when X=Br or I. Constant potential electrolysis indicate that the number of electrons consumed are equal to four electrons which will be due to the reduction of four copper(II) to copper(I).     

Zinc(II) chlorido complexes of protonated kinetin and its derivatives: Synthesis, properties and X-ray structure of [Zn(Hkinetin)Cl3]·kinetin

The syntheses and characterization of five novel zinc(II) complexes with protonated kinetin (6-furfurylaminopurine) and its derivatives are described. Based on the results following from elemental analyses (C, H, N), FTIR, Raman, ¹H and ¹³C NMR spectroscopy, conductivity measurements, thermogravimetric (TG) and differential thermal analyses (DTA), and single crystal X-ray analysis, the complexes of the general composition [Zn(HLⁿ)Cl₃]·xLⁿ (1-5) have been prepared, where L¹ = kinetin (6-furfurylaminopurine), L² = 6-(5-methylfurfurylamino)purine, L³ = 2-chloro-6-furfurylaminopurine, L⁴ = 2-chloro-6-(5-methylfurfurylamino)purine and L⁵ = 2-chloro-6-furfurylamino-9-isopropylpurine, and x = 1/2-2. The structure of [Zn(HL¹)Cl₃]·L¹ (1) has been determined by single crystal X-ray analysis. The Zn(II) atom is tetrahedrally coordinated by three chlorido ligands and one N3-protonated organic molecule forming a ZnCl₃N donor set. The organic ligand L¹ is coordinated to the Zn(II) centre through the N7 atom of the purine moiety. NMR spectroscopic study confirmed the N3 and N7 atom to be the protonation, and coordination site, respectively.     

Cyclic and acyclic compartmental Schiff bases, their reduced analogues and related mononuclear and heterodinuclear complexes

[1+1] macrocyclic and [1+2] macroacyclic compartmental ligands (H₂L), containing one N₂O₂, N₃O₂, N₂O₃, N₄O₂ or O₂N₂O₂ Schiff base site and one O₂O_n (n=3, 4) crown-ether like site, have been prepared by self-condensation of the appropriate formyl- and amine precursors. The template procedure in the presence of sodium ion afforded Na₂(L) or Na(HL) · nH₂O. When reacted with the appropriate transition metal acetate hydrate, H₂L form M(L) · nH₂O, M(HL)(CH₃COO) · nH₂O, M(H₂L)(X)₂ · nH₂O (M=Cu²⁺, Co²⁺, Ni²⁺; X=CH₃COO⁻, Cl⁻) or Mn(L)(CH₃COO) · nH₂O according to the experimental conditions used. The same complexes have been prepared by condensation of the appropriate precursors in the presence of the desired metal ion. The Schiff bases H₂L have been reduced by NaBH₄ to the related polyamine derivatives H₂R, which form, when reacted with the appropriate metal ions, M(H₂R)(X)₂ (M= Co²⁺, Ni²⁺; X=CH₃COO⁻, Cl⁻), Cu(R) · nH₂O and Mn(R)(CH₃COO) · nH₂O. The prepared ligands and related complexes have been characterized by IR, NMR and mass spectrometry. The [1+1] cyclic nature of the macrocyclic polyamine systems and the site occupancy of sodium ion have been ascertained, at least for the sodium (I) complex with the macrocyclic ligand containing one N₃O₂ Schiff base and one O₂O₃ crown-ether like coordination chamber, by an X-ray structural determination. In this complex the asymmetric unit consists of one cyclic molecule of the ligand coordinated to a sodium ion by the five oxygen atoms of the ligand. The coordination geometry of the sodium ion can be described as a pentagonal pyramid with the metal ion occupying the vertex. In the mononuclear complexes with H₂L or H₂R the transition metal ion invariantly occupies the Schiff base site; the sodium ion, on the contrary, prefers the crown-ether like site. Accordingly, the heterodinuclear complexes [MNa(L)(CH₃COO)_x] (M=Cu²⁺, Co²⁺, Ni², x=1; M=Mn³⁺, x=2) have been synthesised by reacting the appropriate formyl and amine precursors in the presence of M(CH₃COO)_n · nH₂O and NaOH in a 1:1:1:2 molar ratio. The reaction of the mononuclear transition metal complexes with Na(CH₃COO) · nH₂O gives rise to the same heterodinuclear complexes. Similarly [MNa(R)(CH₃COO)_x] have been prepared by reaction of the appropriate polyamine ligand H₂R with the desired metal acetate hydrate and NaOH in 1:1:2 molar ratio.     

5f-Element complexes with a p-tert-butylcalix[4]arene bearing phosphinoyl pendant arms: Separation from rare earths and structural studies

Phosphinoylated calixarenes feature high coordination ability toward f elements and a great potentiality toward actinide/rare earth separation. Here, we report three characteristic properties of a tetra-phosphinoylated p-tert-butylcalix[4]arene, B₄bL⁴ functionalized with phosphinoyl pendant arms: (i) its coordination ability toward Th(IV) complexation in organic medium, (ii) its ability to separate thorium from yttrium, lanthanum, and europium in three different organic media, and (iii) the X-ray crystal structure of the La complex. Thorium(IV) forms 1:1 and 1:2 (M:L) complexes with B₄bL⁴: Th(NO₃)₄(B₄bL⁴)_n·xH₂O (n = 1, x = 1, 1; n = 2, x = 4, 2). Spectroscopic data point to the inner coordination sphere of 1 and 2 containing nitrate ions and water molecules. Molecular modeling of 1 yielded an 8-coordinate species and its coordination polyhedron can be described as a distorted square antiprism while that for 2, a 9-coordinate species, as a distorted tricapped trigonal prism. The extraction study of tetravalent thorium and trivalent rare-earth (Y, La, Eu) ions from acidic nitrate media by B₄bL⁴ in chloroform shows thorium being much more extracted than the rare earths, with selectivity close to 100%. The extraction behavior can be easily modulated by changing the initial conditions (pH, nitrate concentration). The X-ray structure of [LaB₄bL⁴(H₂O)₅] CH₃CN·(ClO₄)₃ points to the La^III ion lying on a C₄ axis and being 9-coordinated by the four O(P) atoms and five O atoms from water molecules. It is located in the middle of the void formed by the four O-CH₂-PO(Me)₂ pendant arms.     

Locating the uracil-5-yl radical formed upon photoirradiation of 5-bromouracil-substituted DNA

In a previous study, we found that 2-deoxyribonolactone is effectively generated in the specific 5-bromouracil (^BrU)-substituted sequence 5′-(G/C)[A]_{n = 1,2}^BrU^BrU-3′ and proposed that a formed uracil-5-yl radical mainly abstracts the C1′ hydrogen from the 5′-side of ^BrU^BrU under 302-nm irradiation condition. In the present work, we performed photoirradiation of ^BrU-substituted DNA in the presence of a hydrogen donor, tetrahydrofuran, to quench the uracil-5-yl radical to uracil and then subjected the sample to uracil DNA glycosylase digestion. Slab gel sequence analysis indicated that uracil residues were formed at the hot-spot sequence of 5′-(G/C)[A]_{n = 1,2}^BrU^BrU-3′ in 302-nm irradiation of ^BrU-substituted DNA. Furthermore, we found that the uracil residue was also formed at the reverse sequence 5′-^BrU^BrU[A]_{n = 1,2}(G/C)-3′, which suggests that both 5′-(G/C)[A]_{n = 1,2}^BrU^BrU-3′ and 5′-^BrU^BrU[A]_{n = 1,2}(G/C)-3′ are hot-spot sequences for the formation of the uracil-5-yl radical.     

Tetra-, penta- and hexacoordinate copper(II) complexes with N3 donor isoindoline-based ligands: Characterization and SOD-like activity

Reaction of 1,3-bis(2′-Ar-imino)isoindolines (HLⁿ, n = 1-7, Ar = benzimidazolyl, N-methylbenzimidazolyl, thiazolyl, pyridyl, 3-methylpyridyl, 4-methylpyridyl, and benzthiazolyl, respectively) with Cu(OCH₃)₂ yields mononuclear hexacoordinate complexes with Cu(Lⁿ)₂ composition. With cupric perchlorate square-pyramidal [Cu^II(HLⁿ)(NCCH₃)(OClO₃)]ClO₄ complexes (n = 1, 3, 4) were isolated as perchlorate salts, whereas with chloride Cu^II(HLⁿ)Cl₂ (n = 1, 4), or square-planar Cu^IICl₂(HLⁿ) (n = 2, 3, 7) complexes are formed. The X-ray crystal structures of Cu(L³)₂, Cu(L⁵)₂, [Cu^II(HL⁴)(NCCH₃)(OClO₃)]ClO₄, Cu^IICl(L²) and Cu^IICl(L⁷) are presented along with electrochemical and spectral (UV-Vis, FT-IR and X-band EPR) characterization for each compound. When combined with base, the isoindoline ligands in the [Cu^II(HLⁿ)(NCCH₃)(OClO₃)]ClO₄ complexes undergo deprotonation in solution that is reversible and induces UV-Vis spectral changes. Equilibrium constants for the dissociation are calculated. X-band EPR measurements in frozen solution show that the geometry of the complexes is similar to the corresponding X-ray crystallographic structures. The superoxide scavenging activity of the compounds determined from the McCord-Fridovich experiment show dependence on structural features and reduction potentials.     

Chemistry and reactivity of dinuclear iron oxamate complexes: alkane oxidation with hydrogen peroxide catalysed by an oxo-bridged diiron(III) complex with amide and carboxylate ligation

A new dinuclear iron(III) complex with the tetradentate ligand N,N^′-o-phenylenebis(oxamate) (opba) has been synthesised, and structurally, magnetically and electrochemically characterised. It possesses an unprecedented triply bridged Fe₂(μ-O)(μ-RCO₂?H₂O?O₂CR)₂ core, whereby two N-amides from the opba ligand complete the square-pyramidal coordination sphere of the O-carboxylate rich iron site (Fe-N=2.053 Å and Fe-O=2.015 Å). The antiferromagnetic exchange interaction between the two high-spin Fe^III ions through the oxo bridge (J=−190 cm⁻¹; H=−JS₁·S₂) is weaker than that found in related μ-oxo singly bridged diiron(III) complexes. The lessened antiferromagnetic coupling correlates to the remarkably long Fe-O distance (R=1.808 Å), which is associated to the relatively bent Fe-O-Fe angle (α=131.8°) in the Fe₂O unit. It experiences an irreversible one-electron oxidation process in acetonitrile at 25 °C (E=0.63 V versus SCE) to give the putative mixed-valent diiron(III,IV) species which is unstable under the experimental conditions. The reactivity of this unique oxo-bridged diiron(III) oxamate complex toward hydrogen peroxide activation and hydrocarbon oxidation in the presence of dioxygen has been examined. It catalyses the oxidation of alkanes like cyclohexane and adamantane to the corresponding alcohols and ketones by H₂O₂ and O₂ in acetonitrile at room temperature with limited catalytic activities (total yields of ca. 5% after 24 h) and modest selectivities (A/K=0.9, KIE=2.4, tert/sec=3.0). Overall, these results are more in line with a mechanism involving alkyl radicals as transient intermediates, although they do not exclude the possibility that a metal-based active oxidant is also involved in this C-H bond oxidation chemistry.