Sandra Signorella

Weak Interactions Modulating the Dimensionality in Supramolecular Architectures in Three New Nickel(II)-Hydrazone Complexes, Magnetostructural Correlation, and Catalytic Potential for Epoxidation of Alkenes under Phase Transfer Conditions

Three different ONO donor acetyl hydrazone Schiff bases have been synthesized from the condensation of acetic hydrazide with three different carbonyl compounds: salicylaldehyde (HL(1)), 2-hydroxyacetophenone (HL(2)), and 2, 3-dihydroxybenzaldehyde (HL(3)). These tridentate ligands are reacted with Ni(OOCCF(3))(2)·xH(2)O to yield three new Ni(II) complexes having distorted octahedral geometry at each Ni center: [Ni(L(1))(OOCCF(3))(CH(3)OH)](2) (1), [Ni(L(2))(OOCCF(3))(H(2)O)](2) (2), and [Ni(L(3))(L(3)H)](OOCCF(3))(H(2)O)(1.65)(CH(3)OH)(0.35) (3). The ligands and the complexes have been characterized by elemental analysis and IR and UV-vis spectroscopy, and the structures of the complexes have been established by single crystal X-ray diffraction (XRD) study. 1 and 2 are centrosymmetric dinuclear complexes and are structural isomers whereas 3 is a bis chelated cationic monomer coordinated by one neutral and one monoanionic ligand. O-H···O hydrogen bonds in 3 lead to the formation of a dimer. Slight steric and electronic modifications in the ligand backbone provoke differences in the supramolecular architectures of the complexes, leading to a variety of one, two, and three-dimensional hydrogen bonded networks in complexes 1-3 respectively. Variable temperature magnetic susceptibility measurements reveal that moderate antiferromagnetic interactions operate between phenoxo bridged Ni(II) dimers in 1 and 2 whereas very weak antiferromagnetic exchange occurs through hydrogen bonding and π-π stacking interactions in 3. All complexes are proved to be efficient catalysts for the epoxidation of alkenes by NaOCl under phase transfer condition. The efficiency of alkene epoxidation is dramatically enhanced by lowering the pH, and the reactions are supposed to involve high valent Ni(III)-OCl or Ni(III)-O· intermediates. 3 is the best epoxidation catalyst among the three complexes with 99% conversion and very high turnover number (TON, 396).

Synthesis, characterization and antioxidant activity of water soluble MnIII complexes of sulphonato-substituted Schiff base ligands.

Two new Mn(III) complexes Na[Mn(5-SO(3)-salpnOH)(H(2)O)]5H(2)O (1) and Na[Mn(5-SO(3)-salpn)(MeOH)]4H(2)O (2) (5-SO(3)-salpnOH=1,3-bis(5-sulphonatosalicylidenamino)propan-2-ol, 5-SO(3)-salpn=1,3-bis(5-sulphonatosalicylidenamino)propane) have been prepared and characterized. Electrospray ionization-mass spectrometry, UV-visible and (1)H NMR spectroscopic studies showed that the two complexes exist in solution as monoanions [Mn(5-SO(3)-salpn(OH))(solvent)(2)](-), with the ligand bound to Mn(III) through the two phenolato-O and two imino-N atoms located in the equatorial plane. The E(1/2) of the Mn(III)/Mn(II) couple (-47.11 (1) and -77.80mV (2) vs. Ag/AgCl) allows these complexes to efficiently catalyze the dismutation of O(2)(-), with catalytic rate constants 2.4x10(6) (1) and 3.6x10(6) (2) M(-1)s(-1), and IC(50) values of 1.14 (1) and 0.77 (2) muM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH 7.8. The two complexes are also able to disproportionate up to 250 equivalents of H(2)O(2) in aqueous solution of pH 8.0, with initial turnover rates of 178 (1) and 25.2 (2) mM H(2)O(2) min(-1)mM(-1)catalyst(-1). Their dual superoxide dismutase/catalase activity renders these compounds particularly attractive as catalytic antioxidants.

Kinetics and mechanism of the oxidation of D-galactono-1,4-lactone by CrVI and CrV

Abstract The oxidation of d -galactono-1,4-lactone by CrVI yields d -lyxonic acid, carbon dioxide and Cr3+ as final products when an excess of sugar acid over CrVI is used. The redox reaction occurs through CrVI → CrIII and CrVI → CrV → CrIII paths. The complete rate law for the CrVI oxidation reaction is expressed by −d [CrVI] \dt = (k0+kH [H+] ) [gal] [CrVI] , where k0 = (31±3) ×10−4 M−1 s−1 and kH = (99±5) ×10−4 M−2 s−1, at 40°C. CrV is formed in a rapid step by reaction of the CO·− 2 radical with CrVI. CrV reacts with the substrate faster than does CrVI. The CrV oxidation follows the rate law : −d [CrV] \dt = ( k ′ 0 +k ′ H [H+] ) [gal] , where k ′ 0 = (15±2) ×10−3 M−1 s−1 and k ′ H = (34±4) ×10−3 M−2 s−1, at 40°C. The EPR spectra show that several intermediate [Cr (O) (gala) 2] − linkage isomers are formed in rapid pre-equilibria before the redox steps.

Oxidation of l-rhamnose and d-mannose by chromium(VI) in aqueous acetic acid

Abstract The oxidation of l -rhamnose and d -mannose by Cr(VI) in aqueous acetic acid follows the rate law: −d[Cr(VI)]/d t = ( k 2 + k 3 [H + ] [ aldose ] [Cr(VI)]/{1 + [H + ]/ K a + K 1 [H + ][ aldose ]}, where k 2 = 3.5 +- 0.8 × 10 −3 s −1 and 8.6 +- 1.0 × 10 −4 s −1 , K 3 = 6.8 ± 0.5 × 10 −3 M −1 s −1 and 5.1 ± 0.5 × 10 −3 M −1 s −1 , K a = 1–4 M and K 1 = 13∓_ 2 and 17±5 M −2 for l -rhamnose and d -mannose, respectively. This rate law corresponds to the reaction leading to the formation of l -1,4-rhamnonalactone and d -1,4-mannonalactone. No cleavage to carbon dioxide takes place when a 30-fold or higher excess of aldose over Cr(IV) is employed. The free radicals formed in the slow electron-transfer steps react with Cr(VI) to yield two intermediate Cr(V) complexes with EPR signals at g 1 = 1.978 and g 2 = 1.973. The mechanism and associated reactions kinetics are presented and discussed.

The reduction of CrVI to CrIII by the α and β anomers of d-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study

Abstract The reduction of CrVI by α- d -glucose and β- d -glucose was studied in dimethyl sulfoxide in the presence of pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers reduce CrVI by formation of an intermediate CrVI ester precursor of the slow redox step. The equilibrium constant for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer. α- d -Glucose forms the CrVI–Glc ester with a higher equilibrium constant than β- d -glucose, but the electron transfer within this complex is slower than for the β anomer. The difference is attributed to the better chelating ability of the 1,2-cis-diolate moiety of the α anomer. The CrV species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of CrVI. The EPR spectra show that the α anomer forms several linkage isomers of the five-coordinate CrV bis-chelate, while β- d -glucose affords a mixture of six-coordinate CrV mono-chelate and five-coordinate CrV bis-chelate. The conversion of the CrV mono- to bis-chelate is discussed in terms of the ability of the 1,2-cis- versus 1,2-trans-diolate moieties of the glucose anomers to bind CrV.

OXIDATION OF 2-ACETAMIDO-2-DEOXY-D-GLUCOSE BY CRVI IN PERCHLORIC ACID

Abstract The oxidation of 2-acetamido-2-deoxy- d -glucose by CrVI in perchloric acid has been found to follow the rate law: −d[CrVI]dt = (a + b[H+]2) [CrVI]T where a = 7.37±0.35 × 10−5 s−1; b = 3.90±0.67×10−4 M−2s−1; and c = 1.18±0.01×10−3M−4s−1. This rate law corresponds to the reaction leading to the formation of 2-acetamido-2-deoxy- d -gluconic acid when a 20-fold or higher excess of aldose over chromium is employed. The results are discussed in terms of a possible mechanism with the associated reaction kinetics.

Chromic Oxidation of 2-Deoxy-d-Glucose. Comparative Study with Aldoses. I.

Abstract A rate law for the oxidation of 2-deoxy-d-glucose (2DG) by Cr(VI) in perchloric acid has been derived. This rate law corresponds to the reaction leading to the formation of 2-deoxy-d-gluconic acid (2DGA). No cleavage to carbon dioxide takes place when a twenty-fold or higher excess of aldose over Cr(VI) is employed. Kinetic constants are interpreted in terms of the absence of an hydroxyl group at C-2 on the stability of the chromic ester formed in the first reaction step. Free radicals formed during the reaction convert Cr(VI) to Cr(V). The latter species was detected by EPR measurements.

Degradative oxidation of d-ribono-1, 4-lactone by CrVI in perchloric acid

Abstract The oxidation of d -ribono-1, 4-lactone by CrVI yielded d -erythronic acid, erythrose, carbon dioxide and CrIII as final products when a 15-fold or higher excess of sugar over CrVI was used. The kinetics of the redox processes involving the CrVI/CrIII couple were determined and a mechanism is proposed. The complete rate law for the CrVI oxidation reaction is expressed by: −d[CrVI]/dt = [a[S][H+] + c[H+]3] [CrVI]T, where a = 3.09 × 10−3M−1s−1, b = 4.98 × 10−2M−2, s−1, c = 1.07 × 10−3M−3s−1 and S refers to the total reductant concentration, at 60°C. CrVI is formed in a fast step by reaction of the CO2− radical and CrVI, and CrV reacts with the organic substrate faster than CrVI does. The EPR spectra show that the intermediate CrV complex (g= 1.978) is formed and decays by a first-order process.

Synthesis, Characterization, and Catalase Activity of a Water-Soluble diMnIII Complex of a Sulphonato-Substituted Schiff Base Ligand: An Efficient Catalyst for H2O2 Disproportionation

A new diMn(III) complex, Na[Mn(2)(3-Me-5-SO(3)-salpentO)(μ-MeO)(μ-AcO)(H(2)O)]·4H(2)O (1), where salpentOH = 1,5-bis(salicylidenamino) pentan-3-ol, was synthesized and structurally characterized. The complex possesses a bis(μ-alkoxo)(μ-acetato) triply bridged diMn(III) core, the structure of which is retained upon dissolution. Complex 1 is highly efficient to disproportionate H(2)O(2) in an aqueous solution of pH ≥ 8.5 or in DMF, with only a slight decrease of activity. Electrospray ionization mass spectrometry, EPR, and UV-vis spectroscopy used to monitor the H(2)O(2) disproportionation in buffered basic medium, suggest that the major active form of the catalyst during cycling occurs in the Mn(III)(2) oxidation state and that the starting complex retains the dinuclearity and composition during catalysis, with the acetate that moves from bridging to terminal ligand. UV-vis and Raman spectroscopy of H(2)O(2) + 1 + Bu(4)NOH mixtures in DMF suggest that the catalytic cycle involves Mn(III)(2)/Mn(IV)(2) oxidation levels. At pH 10.6 in an Et(3)N/Et(3)NH(+) buffer, complex 1 catalyzes dismutation of H(2)O(2) with saturation kinetics on the substrate, first order dependence on the catalyst, and k(cat)/K(M) = 16(1) × 10(2) s(-1) M(-1). During catalysis, the exogenous base contributes to retain the integrity of the bis(μ-alkoxo) doubly bridged diMn core and favors the formation of the catalyst-peroxide adduct (low value of K(M)), rendering 1 a highly efficient catalyst for H(2)O(2) disproportionation.

Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) byD-Glucitol andD-Mannitol

The oxidation of D-glucitol and D-mannitol by CrVI yields the aldonic acid (and/or the aldonolactone) and CrIII as final products when an excess of alditol over CrVI is used. The redox reaction occurs through a CrVICrVCrIII path, the CrVICrV reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: a) −d[CrVI]/dt  {kM2 H [H+]2+kMH [H+]}[mannitol][CrVI], where kM2 H  (6.7±0.3)⋅10 M s−1 and kMH  (9±2)⋅10 M s−1; b) −d[CrVI]/dt  {kG2 H [H+]2+kGH [H+]}[glucitol][CrVI], where kG2 H  (8.5±0.2)⋅10 M s−1 and kGH  (1.8±0.1)⋅10 M s−1, at 33°. The slow redox steps are preceded by the formation of a CrVI oxy ester with λmax 371 nm, at pH 4.5. In acid medium, intermediate CrV reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxo-CrV intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-CrV species are present at any [H+], whereas hexacoordinate ones are observed only at pH<2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-CrV species are not observed, CrV complexes are stable enough to remain in solution for several days to months.