Studies on binuclear hydroxo/carboxylato-bridged manganese(III) complexes and a mononuclear manganese(III) complex involving salen type ligands

Synthesis of six hydroxo-bridged binuclear manganese(III) complexes of formulae [MnL-X-MnL](ClO₄) [X = OH (1–6)] along with a mononuclear manganese(III) complex (7) [Mn(L)(L′)(MeOH)₂] [HL′ = 2-(2-hydroxy-phen-yl)benzimidazole] and two carboxylate-bridged binuclear manganese(III) complexes (8) and (9) are described. The complexes have been characterized by the combination of i.r., u.v.-vis spectroscopy, magnetic moments and by their redox properties. The electronic spectra of all the complexes exhibit almost identical features consisting of two d–d bands at ca. 550 and 600 nm, one MLCT band at ca.400 nm, together with two π–π^* intra-ligand transitions at ca. 250 nm and ca.300 nm. Room temperature magnetic data range from μ = 2.7–3.0 BM indicates some super-exchange between the binuclear metal centers via bridging hydroxo/carboxylato groups. The X-ray crystal structure of the binuclear complex (5) revealed that it has a symmetric Mn^IIIN₂O₂ core bridged by a hydroxyl group. The X-ray analysis of the mononuclear complex (7) showed that the manganese-center possesses a distorted octahedral geometry. Electrochemical properties of hydroxo-bridged manganese(III) complexes (1–6) show identical features consisting of an irreversible and a quasi-reversible reduction corresponding to the Mn₂^III → Mn^IIMn^III → Mn^IIMn^II couples in the voltammogram. It was found that electron withdrawing substituents on the ligand result in easier reduction. Complex (7) displays an irreversible reduction at 0.08 V and a reversible oxidation at 0.45V assignable to the Mn^III → Mn^II reduction and Mn^III → Mn^IV oxidation, respectively. The carboxylate-bridged compound (8) exhibits two irreversible oxidations at 0.4 and 0.6 V, probably due to Mn₂^III → Mn^IIIMn^IV → Mn^IVMn^IV oxidations and shows a quasi-reversible reductive wave at −0.85 V, tentatively assigned to Mn₂^III → Mn^IIMn^III reduction.     

Reaction of superoxo Co(III) complex with quinones formation of a semiquinone Co(III) complex

The superoxo complex [Co(CN)₅O₂]^3- was found to act as a reducing agent towards quinones. One-electron reduction took place with $\text{o}$-quinones whereas two-electrons reduction with $\text{p}$-quinones. 3,5-Di-$\text{t}$-butyl-$\text{o}$-benzoquinone gave the corresponding semiquinone Co(III) complex quantitatively.     

Composition of a mixed-ligand gadolinium(III) complex in fluoride-dimethylformamide electrolyte

The gadolinium(III) complex formed in a fluoride-dimethylformamide (DMF) electrolyte was studied by linear-sweep voltammetry and IR spectroscopy. The composition of the mixed-ligand complex formed was found to correspond to the component ratios Gd: DMF: F = 1: 3: 4. It has been found that the coordination link in the complex is via two oxygen atoms and one nitrogen atom of dimethylformamide and involves four fluoride ions.     

Redox chemistry of mononuclear manganese(II), binuclear manganese(III) and binuclear mixed manganese(II)-manganese(III) complexes with 3-aminopyrazine-2-carboxylate in dimethylsulphoxide

Summary Mn^II forms a yellow mononuclear species with the title ligand having a 12 stoichiometry and whose conditional stability constant is 8.9 × 10¹⁰ m ^–2. The c.v. of this complex shows an oxidation at +0.78V versus s.c.e. Controlled-potential electrolysis at +0.80V versus s.c.e. yields a binuclear species of Mn^III with a 12 metal:ligand stoichiometry.The addition of Mn^III(urea)₆(ClO₄)₃ to a solution of the ligand produces a mononuclear complex of Mn^III if the concentration of the metal ion is less than 1 mM. At higher concentrations a binuclear species is obtained. The latter is reduced in two steps, at +0.24 and –0.58 V versus s.c.e. Controlled-potential electrolysis at 0.0 V produces a dark green complex after the transfer of 0.5 equivalents of charge per mole of Mn. This binuclear L₂Mn^II-Mn^IIIL₂ mixed-valence complex can be obtained only by electrolysis of the binuclear L₂Mn^IIIMn^IIIL₂ species. Attempts to prepare the complex chemically were unsuccessful - the binuclear Mn^III species was obtained in every case.Author to whom all correspondence should be directed.     

Vinyl polymerization by a manganese(III) complex

Bis(diethanolamine) manganate(III) was prepared. The polymerization of acrylamide and methacrylamide initiated by this complex in aqueous solution at pH 0.9 was studied at 45°. The rate of polymerization was followed by bromometry, the rate of complex disappearance spectrophotometrically and the molecular weights of the polymers were determined viscometrically. The rate of polymerization was found to be proportional to [Monomer]^1.0. The order with respect to initiator was found to be 0.5 for acrylamide and 0.3 for methacrylamide. The apparent overall activation energies for the polymerizations are ?87 kJ mol^?1 and ?59 kJ mol^?1 for acrylamide and methacrylamide respectively. A kinetic reaction scheme is proposed on the basis of the experimental data; kinetic parameters have been evaluated.     

A binuclear tantalum(III) complex with a bridging carboxylato ligand

Ta₂Cl₆(SMe₂)₃ reacts with one equivalent of C₄H₉CO₂Li to give a complex with a bridging carboxylate ligand, Ta₂Cl₅(O₂CC₄H₉)(SMe₂)(THF)₂. The product was isolated in two crystalline forms, 1 and 2, from a THF/hexane and benzene/hexane solvent mixture, respectively. The following are the unit cell parameters, for 1: monoclinic (P2₁/n), a = 10.537(5) Å, b = 22.015(4) Å, c = 11.663(4) Å, β = 107.80(3)°, V = 2576(3) ų, and Z = 4; for 2: monoclinic (P2₁/c), a = 15.584(4) Å, b = 15.647(4) Å, c = 11.275(3) Å, β = 106.04(5)°, V = 2642(3) ų and Z = 4. The complex is a dimer with a distorted octahedra-sharing-an-edge geometry. The TaTa distances in 1 and 2 were 2.766(1) Å and 2.779(1) Å, respectively, which is somewhat longer than in previously reported Nb(III) and Ta(III) dinuclear compounds. Diamagnetism of the complex is shown by NMR. Fenske—Hall calculations, which correctly predict an electronic transition at about 16,000 cm^?1, indicate a double TaTa bond. The observed elongation of the metalmetal bond is attributed mainly to steric crowding. The complex is the first proven low-valent Ta species with a coordinated carboxylate ion.     

Methods for analysis of the mononuclear and binuclear manganese(III) cyano-complexes

Simple titrimetric methods are described for the analysis of potassium hexacyanomanganate(III) and of heptapotassium mu-oxo-bis[pentacyanomanganate(III)] cyanide. Mn(2)O(CN)(6-)(10) is determined by potentiometric titration with hexacyanoferrate(III). Cyanide is determined in both complexes by back-titration with Hg(II) and SCN(-), and manganese(III) is determined by back-titration with Fe(II) and MnO(-)(4). The absorption spectra of both cyano-complexes in cyanide and in acidic solutions are also described, and their molar absorptivities are reported.     

Ni(III) vs. Ni(II)-thiyl radical: charge-delocalisation in a binuclear Ni(III)Ni(II)-dithiolate complex

Multi-frequency EPR spectroscopy on 61Ni-labelled samples of [Ni2(L)]3+ confirms extensive charge-delocalisation between the Ni(III) centre and thiolate donors in the Ni(II)Ni(III) complex.     

Formation of new monomeric Pd(III) complex species with 8-aminoquinoline

By means of EPR and magnetochemical methods the formation of 3 new Pd(III) complex species is proved resulting from the reaction of PdCl₄ ²⁻ and 8-aminoquinoline (8-AQ) in basic (aqueous or aqueous-methanolic) medium. Elemental analysis, visible and IR data for the complexes are also obtained.     

Formation of a Tc(III)-adenosine diphosphate complex

A⁹⁹Tc-ADP complex was prepared when KTcO₄ was reduced in aquous medium by SnCl₂, Na₂S₂O₄, NaBH₄ or Zn in the presence of ADP in excess. The resulting solution was studied by chromatography and spectrophotometry. Electrochemical reduction and substitution on [Tc^III(tu)₆]³⁺ were investigated as alternative synthetic routes. The anionic Tc-ADP complex was isolated as a solid. Cerimetric titrations confirmed the oxidation state +3 for the central atom. IR and¹H-NMR data showed that the purine base is bonded to the Tc central atom but not the ribose moiety. No oxo groups seemed to be directly bonded to the Tc atom. The complex is rather stable in neutral solutions. However, it decomposes to pertechnetate and TcO₂ at extreme pH values.     

Kinetics and mechanism of benzene oxidation by peroxymonosulfate catalyzed with a binuclear manganese(IV) complex in the presence of oxalic acid

It is established that Oxone (peroxymonosulfate, 2KHSO₅ · KHSO₄ · K₂SO₄) oxidizes benzene to p-quinone very efficiently and selectively in a homogeneous solution in aqueous acetonitrile in the presence of a catalyst, i.e., dimeric manganese(IV) complex [LMn(O)₃MnL](PF₆)₂ where L is 1,4,7-trimethyl-1,4,7-triazacyclononane, and a cocatalyst, i.e., oxalic acid. The dependences of the maximum rate of quinone accumulation on the initial concentrations of reagents are studied. It is proposed that benzene is oxidized by the manganyl particle containing the Mn(V)=O fragment that forms upon the reaction of the reduced form of the starting dimeric manganese complex with Oxone.     

Chlorination of alkenes by manganese(III) chloride species

Several manganese(III) chloride species have been prepared $\text{in situ}$ and used as effective chlorinating agents of alkenes.     

Aerobic catechol oxidation catalyzed by a bis(mu-oxo)dimanganese(III,III) complex via a manganese(II)-semiquinonate complex

A 3,5-di-tert-butyl-1,2-semiquinonato (DTBSQ) adduct of Mn(II) was prepared by a reaction between Mn(II)(TPA)Cl(2) (TPA = tris(pyridin-2-ylmethyl)amine) and DTBSQ anion and was isolated as a tetraphenylborate salt. The X-ray crystal structure revealed that the complex is formulated as a manganese(II)-semiquinonate complex [Mn(II)(TPA)(DTBSQ)](+) (1). The electronic spectra in solution also indicated the semiquinonate coordination to Mn. The exposure of 1 in acetonitrile to dioxygen afforded 3,5-di-tert-butyl-1,2-benzoquione and a bis(mu-oxo)dimanganese(III,III) complex [Mn(III)(2)(mu-oxo)(2)(TPA)(2)](2+) (2). The reaction of 2 with 3,5-di-tert-butylcatechol (DTBCH(2)) quantitatively afforded two equivalents of 1 under anaerobic conditions. The highly efficient catalytic oxidation of DTBCH(2) with dioxygen was achieved by combining the above two reactions, that is, by constructing a catalytic cycle involving both manganese complexes 1 and 2. It was revealed that dioxygen is reduced to water but not to hydrogen peroxide in the catalytic cycle.     

Formation of Tc(III)-EDTA complex

Tc(III)-EDTA complex has been synthesized by the ligand substitution reaction of hexakis-(thiourea) technetium(III) ion with EDTA. The complex exhibits absorption maxima at 368 and 470 nm. The formation reaction proceeds predominantly as follows:

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H₃bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.     

Thermodynamics of mixed-ligand complex formation of samarium(III) and cerium(III) ethylenediaminetetraacetates with various ligands in aqueous solution

The thermodynamic parameters of reaction (logK, Δ_r G ⁰, Δ_r H, Δ_r S) are determined for the mixed-ligand complex formation of SmEdta^? and CeEdta^? with glycinate, iminodiacetate, aspartate, and nitrilotriacetate ions in aqueous solution at 298.15 K and ionic strength of I = 0.5 (KNO₃). Based on these thermodynamic data, the most likely coordination modes are suggested for the complexone and the ancillary ligand in the mixed-ligand complexes.