Browsing by Author "Martins, Nuno M. R."
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- 1D Zn(II) coordination polymer of arylhydrazone of 5,5-dimethylcyclohexane-1,3-dione as a pre-catalyst for the Henry reactionPublication . Martins, Nuno M. R.; Mahmudov, Kamran T.; Silva, M. Fátima C. Guedes; Martins, Luisa; Guseinov, Firudin I.; Pombeiro, Armando J. L.Reaction of a new hydrazone of β-diketone, 5-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)isophthalic acid (H3L), with zinc(II) nitrate hexahydrate in a mixture of DMF and water (1:1) under hydrothermal conditions affords the 1D coordination polymer [Zn(μ3-HL)(H2O)2]n ∙ nH2O (1), where one of the carboxylic groups links two Zn(II) centers in a bridging bidentate (syn-syn-type) mode and the remaining one ligates a third metal cation in a monodentate fashion. It was characterized by IR and NMR spectroscopies, ESI-MS, elemental and single-crystal X-ray crystal structural analyses. 1 acts as an efficient pre-catalyst for the Henry reaction at 40 °C in aqueous medium, providing β-nitroalcohols with good yields (67–86%) and diastereoselectivities (syn:anti 77:23–69:31).
- 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.
- Catalytic oxidation of alcohols: recent advancesPublication . Kopylovich, Maximilian; Ribeiro, Ana P.C.; Alegria, Elisabete; Martins, Nuno M. R.; Martins, Luisa; Pombeiro, ArmandoThis review concerns metal-catalyzed reactions of oxidation of alcohols to the respective products, mainly ketones and aldehydes, mostly within the period of 2010–2014. Both conventional and unconventional systems, not only with usual reagents, but also with uncommon and prospective ones, are overviewed, with recently achieved developments.
- Copper(II) and iron(III) complexes with arylhydrazone of ethyl 2-cyanoacetate or formazan ligands as catalysts for oxidation of alcoholsPublication . Martins, Nuno M. R.; Mahmudov, Kamran; Guedes Da Silva, M. Fátima C.; Martins, Luisa; Pombeiro, ArmandoThe aquasoluble [Cu(1κN,O2:2κO-HL1)(S)]2 [S = CH3OH (1), (CH3)2NCHO (2)] and [Cu(κN-HL1)(en)2]·CH3OH·H2O (3) CuII complexes were prepared by the reaction of CuII nitrate hydrate with the new ligand (E/Z)-4-(2-(1-cyano-2-ethoxy-2-oxoethylidene)hydrazinyl)-3-hydroxybenzoic acid (H3L1), in the presence (for 3) or absence (for 1 and 2) of ethylenediamine (en), while the FeIII complex [Fe(κN3-HL2)2] (4) was isolated by treatment of iron(III) chloride hexahydrate with the new ligand (1E,1E)-N′,2-di(1H-1,2,4-triazol-3-yl)diazenecarbohydrazonoyl cyanide (H3L2), and characterized by elemental analysis, IR spectroscopy and single crystal X-ray diffraction. Cooperative E,Z → E isomerization of H3L1, induced by coordination and ionic interactions, occurs upon interaction with CuII in the presence of en. Complexes 1–4 act as catalyst precursors for the solvent-free microwave (MW) assisted selective oxidation of primary or secondary alcohols and diols to the corresponding aldehydes, ketones and diketones, respectively, with yields in the 5–99% range (TONs up to 4.96 × 102) after 60 min of MW irradiation at 120 °C. The influence of temperature, time and organic radicals was studied and also the regioselective oxidation of the catalytic systems involving the primary and secondary alcohols.
- Copper(II) tetrazolato complexes: Role in oxidation catalysis and protein bindingPublication . Saha, Manideepa; Vyas, Komal M.; Martins, Luisa; Martins, Nuno M. R.; Pombeiro, Armando; Mobin, Shaikh M.; Bhattacherjee, D.; Bhabak, Krishna P.; Mukhopadhyay, SumanThree new coordination complexes [Cu-2(phen)(2)(ptz)(4)] (1), [Cu(phen)(pmtz)(2)] (2) and [Cu(phen)(pytz)(2)] (3) (ptz = 5-phenyltetrazolate, pmtz = 5-(2-pyrimidyl)tetrazolate and pytz = 5-(2-pyridyl)tetrazolate) have been synthesized using metal mediated [2+3] cycloaddition reaction between copper bound azide polymer and different organonitriles. All complexes have been structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. Complex 1 shows a dinuclear structure via bridging ptz, while 2 and 3 reveal mononuclear structures. These complexes were explored as homogeneous catalysts for the selective peroxidative (with tBuOOH) oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA oil) under very mild and green (solvent-and additive-free) conditions using microwave irradiation. The catecholase-like activity of the synthesized Cu(II) complexes was investigated by employing 3,5-di-tert-butylcatechol (DTBC) as a convenient model substrate, which showed first order kinetics with respect to the substrate in the oxidation of 3,5-di-tert-butyl catechol (DTBC) to 3,5-di-tertbutyl benzoquinone (DTBQ). Further, the interaction of synthesized Cu(II)-tetrazolato complexes with bovine serum albumin (BSA) was studied experimentally as well as through molecular docking to obtain detailed binding information of the Cu(II) complexes with BSA, which revealed higher binding of complex 1 than 2 and 3
- 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.