Percorrer por autor "Rapta, Peter"
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- A bis(mu-chlorido)-bridged cobalt(II) complex with silyl-containing schiff base as a catalyst precursor in the solvent-free oxidation of cyclohexanePublication . Zaltariov, Mirela-Fernanda; Vieru, Veacheslav; Zalibera, Michal; Cazacu, Maria; Martins, Nuno M. R.; Martins, Luisa; Rapta, Peter; NOVITCHI, Ghenadie; Shova, Sergiu; Pombeiro, Armando; Arion, Vladimir B.A new bis(µ-chlorido)-bridged cobalt(II) complex [Co2(µ-Cl)2(HL2)4][CoCl4] (1), where HL2 is a silyl-containing Schiff base, was synthesised. The structure of this compound was established by X-ray crystallography revealing a zwitterionic form adopted by the organic ligand. The temperature dependence of the magnetic susceptibility and the field dependence of the magnetisation indicate the presence of ferromagnetic interactions between paramagnetic d7 cobalt(II) centres (SCo = 3/2). The exchange coupling parameter J(Co1–Co2) = +7.0 cm–1 extracted from broken-symmetry (BS) DFT calculations agrees well with the value of +8.8 cm–1 determined from the experimental data by fitting them with the Hamiltonian math formula. Electrochemical studies indicate that complex 1 is inefficient as a catalyst in electrochemical reduction of protons. One of the reasons is the low stability of the complex in solution. In contrast, 1 acts as an effective homogeneous (pre)catalyst in the microwave-assisted neat oxidation of cyclohexane with aqueous tBuOOH (TBHP). The possible mechanism of catalytic oxidation and other advantages of using 1 in the oxidation of cycloalkanes are discussed.
- Copper(II) complex with a redox-noninnocent Schiff base bearing a tetraphenyldisiloxane unit: synthesis, structure and catalytic oxidation of cyclohexanePublication . Wittmann, Christopher; Palamarciuc, Oleg; Dascalu, Mihaela; Cazacu, Maria; Nesterov, Dmytro S.; Pombeiro, Armando; Rapta, Peter; Arion, Vladimir B.Abstract An organic–inorganic diamine, 1,3-bis(aminopropyl)tetraphenyldisiloxane, was prepared and introduced as a flexible spacer into the structure of a salen-type Schiff base (H2L7) extending the available small library of similar compounds derived from 1,3-bis(aminopropyl)tetramethyldisiloxane and substituted 2-hydroxybenzaldehydes (H2L1–H2L6). Like the previously reported mononuclear copper(II) complexes [CuL1]–[CuL6], the new copper(II) complex [CuL7], obtained by reaction of Cu(OAc)2·H2O with H2L7 in a mixture of organic solvents, has a tetrahedrally distorted square-planar (N2O2) coordination geometry. X-ray crystallography has shown that compared to [CuL1]–[CuL6] the Si–O–Si angle in [CuL7] is even closer to linear due to stronger intramolecular interactions between Ph groups than between Me groups in the central–R2Si–O–SiR2– fragment (R = Ph and Me, respectively). [CuL7] can be electrochemically reversibly oxidised by two successive one-electron processes, generating stable phenoxyl mono- and diradicals. Both oxidations are ligand-centred, leading to the formation of coordinated phenoxyl radicals. The UV spectrum of [CuL7] consists of π → π* and LMCT σ → d transitions. The low-energy d–d absorption is well described by AILFT CAS(9,5)/NEVPT2 calculations. The one-electron oxidised compound [CuL7]+ should exist in the triplet ground state as 3[CuL7]+ with one unpaired electron located on the dx2−y2 orbital of copper(II) (d9, SCu = ½) and another electron on the molecular orbital (MO) comprising pz oxygen and carbon atoms of the phenoxyl radical (Srad = ½). The broad absorption in the vis-NIR region of the optical spectrum of the one-electron oxidised complex is due to intervalence charge transfer in the triplet species 3[CuL7]+, but not in the [CuL7]2+ one. The doubly oxidised [CuL7] species shows very close doublet and quartet states, where the doublet state has an unpaired electron located on the Cu(II) d-orbital, while the quartet state has one unpaired electron on the Cu(II) d-orbital and two unpaired electrons on π-bonding orbitals. In all state-averaged CASSCF cases, the occupation of the Cu(II) d-orbital is nearly 1.0, indicating its limited involvement in the excited states. Catalytic studies showed that [CuL7] acts as a catalyst for the oxidation of alkanes with peroxides under very unusual solvent-free conditions, converting cyclohexane into cyclohexanol and cyclohexanone (with hydrogen peroxide or tert-butyl hydroperoxide as the oxidant) or into cyclohexanol and ε-caprolactone (with m-chloroperoxybenzoic acid as the oxidant). Theoretical investigations of the catalytic reaction mechanisms disclosed the principal intermediates.
- Vanadium(V) complexes with substituted 1,5-bis(2- hydroxybenzaldehyde)carbohydrazones and their use as catalyst precursors in oxidation of cyclohexanePublication . Dragancea, Diana; Talmaci, Natalia; Shova, Sergiu; NOVITCHI, Ghenadie; Darvasiova, Denisa; Rapta, Peter; Breza, Martin; Galanski, Markus; Kozisek, Jozef; Martins, Nuno M. R.; Martins, Luisa; Pombeiro, Armando; Arion, Vladimir B.Six dinuclear vanadium(V) complexes have been synthesized: NH4[(VO2)(2)((LH)-L-H)] (NH4[1]), NH4[(VO2)(2)((LH)-L-tBu)] (NH4[2]), NH4[(VO2)(2)((LH)-L-Cl)] (NH4[3]), [(VO2)(2)(VO) ((LH)-L-H) (CH3O)] (4), [(VO2) (VO) (t-BuLH) (C2H5O)] (5), and [ (VO2) (VO) (Cl-LH) (CH3O)(CH3OH/H2O)] (6) (where (LH4)-L-H = 1,5-bis(2-hydroxybenzaldehyde)carb ohydrazon e, t-BuLH4 = 1,5-bis(3,5-di-tert-butyl-2-hydroxybenzaldehyde) carbohydrazone, and (LH4)-L-cl = 1,5-bis(3,5-dichloro-2-hydroxybenzaldehyde)carbohydrazone). The structures of NH4[1] and 4-6 have been determined by X-ray diffraction (XRD) analysis. In all complexes, the triply deprotonated ligand accommodates two V ions, using two different binding sites ONN and ONO separated by a diazine unit -N-N-. In two pockets of NH4[1], two identical VO2+ entities are present, whereas, in those of 4-6, two different VO2+ and VO3+ are bound. The highest oxidation state of V ions was corroborated by X-ray data, indicating the presence of alkoxido ligand bound to VO3+ in 4-6, charge density measurements on 4, magnetic susceptibility, NMR spectroscopy, spectroelectrochemistry, and density functional theory (DFT) calculations. All four complexes characterized by XRD form dimeric associates in the solid state, which, however, do not remain intact in solution. Compounds NH4[1], NH4[2], and 4-6 were applied as alternative selective homogeneous catalysts for the industrially significant oxidation of cyclohexane to cyclohexanol and cyclohexanone. The peroxidative (with tert-butyl hydroperoxide, TBHP) oxidation of cyclohexane was performed under solvent -free and additive -free conditions and under low-power microwave (MW) irradiation. Cyclohexanol and cyclohexanone were the only products obtained (high selectivity), after 1.5 h of MW irradiation. Theoretical calculations suggest a key mechanistic role played by the carbohydrazone ligand, which can undergo reduction, instead of the metal itself, to form an active reduced form of the catalyst.