Nicolas Leconte

The structure of a one-electron oxidized Mn(III)-bis(phenolate)dipyrrin radical complex and oxidation catalysis control via ligand-centered redox activity

The tetradentate ligand dppH3, which features a half-porphyrin and two electron-rich phenol moieties, was prepared and chelated to manganese. The mononuclear Mn(iii)-dipyrrophenolate complex 1 was structurally characterized. The metal ion lies in a square pyramidal environment, the apical position being occupied by a methanol molecule. Complex 1 displays two reversible oxidation waves at 0.00 V and 0.47 V vs. Fc+/Fc, which are assigned to ligand-centered processes. The one-electron oxidized species 1+ SbF6- was crystallized, showing an octahedral Mn(iii) center with two water molecules coordinated at both apical positions. The bond distance analysis and DFT calculations disclose that the radical is delocalized over the whole aromatic framework. Complex 1+ SbF6- exhibits an Stot = 3/2 spin state due to the antiferromagnetic coupling between Mn(iii) and the ligand radical. The zero field splitting parameters are D = 1.6 cm-1, E/D = 0.18(1), g⊥ = 1.99 and g∥ = 1.98. The dication 12+ is an integer spin system, which is assigned to a doubly oxidized ligand coordinated to a Mn(iii) metal center. Both 1 and 1+ SbF6- catalyze styrene oxidation in the presence of PhIO, but the nature of the main reaction product is different. Styrene oxide is the main reaction product when using 1, but phenylacetaldehyde is formed predominantly when using 1+ SbF6-. We examined the ability of complex 1+ SbF6- to catalyze the isomerization of styrene oxide and found that it is an efficient catalyst for the anti-Markovnikov opening of styrene oxide. The formation of phenylacetaldehyde from styrene therefore proceeds in a tandem E-I (epoxidation-isomerization) mechanism in the case of 1+ SbF6-. This is the first evidence of control of the reactivity for styrene oxidation by changing the oxidation state of a catalyst based on a redox-active ligand.

Macro-modeling of spot weld strength and failure: Formulation and identification procedure based on pure and mixed modes of loading

Purpose This paper aims to propose a macro modeling approach to simulate the mechanical behavior and the failure of spot welded joints in structural crashworthiness computations. Design/methodology/approach A connector element is proposed to simulate the behavior and failure of spot weld joints. An elastic-plastic damageable model is used to describe the non-linear response and rupture. The connector model involves several parameters that have to be defined. Some are directly identified based on mechanical interpretations and experimental tests characteristics. The remaining parameters are identified through a finite element model updating approach using Arcan tests. Resulting from a sensitivity analysis, an original two steps optimization methodology, using the Modes I and II Arcan tests results sequentially, has been implemented to identify the remaining model parameters. Findings The numerical results for Arcan tests in mixed Modes I/II are in a good agreement with the experimental ones. The model is also validated on tensile pull-out, single lap shear and coach-peel tests. Originality/value By comparison with previous published results, the proposed model brings a significant improvement. The main innovative aspects of this work are as follows: the proposed formulation, a reduced number of parameters to optimize, an original sequential optimization methodology based on physical and mechanical analyses and a mesh size independent connector element.

Electronic Structure and Reactivity of One-Electron-Oxidized Copper(II) Bis(phenolate)–Dipyrrin Complexes

The sterically hindered bis(phenol)-dipyrrin ligands HLH3 and PhLH3 were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(HL)] and [Cu(PhL)]. Their X-ray crystal structures show relatively short C-O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl-phenoxyl radical species. Complexes [Cu(HL)] and [Cu(PhL)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (ε > 6 mM-1 cm-1) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave ( E1/21,ox = 0.05 and 0.04 V), as well as a reversible reduction wave ( E1/21,red = -0.40 and -0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(HL)]+ and [Cu(PhL)]+) and anions ([Cu(HL)]- and [Cu(PhL)]-) were generated. They all display an axial ( S = 1/2) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligand-centered redox processes in both reduction and oxidation. Complex [Cu(HL)](SbF6) was cocrystallized with [Cu(HL)]. Oxidation is accompanied by a slight contraction of both the C-O bonds (mean bond distance of 1.280 Å) and the C-C bonds connecting the peripheral rings to the dipyrrin. The cations show vis-NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)-dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions.