Experimental and Computational Studies on Quadruply Bonded Dimolybdenum Complexes with Terminal and Bridging Hydride Ligands

This contribution focuses on complex [Mo₂(H)₂(μ-Ad^Dipp2)₂] ( 1 ) and tetrahydrofuran and pyridine adducts [Mo₂(H)₂(μ-Ad^Dipp2)₂(L)₂] ( 1⋅thf and 1⋅py ), which contain a trans-(H)Mo≣Mo(H) core (Ad^Dipp2=HC(NDipp₂)₂; Dipp=2,6-iPr₂C₆H₃). Computational studies provide insights into the coordination and electronic characteristics of the central trans-Mo₂H₂ unit of 1 , with four-coordinate, fourteen-electron Mo atoms and ϵ-agostic interactions with Dipp methyl groups. Small size C- and N-donors give rise to related complexes 1⋅L but only one molecule of P-donors, for example, PMe₃, can bind to 1 , causing one of the hydrides to form a three-centered, two-electron (3c-2e) Mo-H→Mo bond ( 2⋅PMe₃ ). A DFT analysis of the terminal and bridging hydride coordination to the Mo≣Mo bond is also reported, along with reactivity studies of the Mo−H bonds of these complexes. Reactions investigated include oxidation of 1⋅thf by silver triflimidate, AgNTf₂, to afford a monohydride [Mo₂(μ-H)(μ-NTf₂)(μ-Ad^Dipp2)₂] ( 4 ), with an O,O’-bridging triflimidate ligand.     

Regiospecific synthesis of 1-arylpyrazoles

A new regiospecific synthesis of 1-aryl-substituted pyrazoles by reaction of aryl-hydrazines with β-aminoenones is reported.     

Some comments on the study of iron-poor soils by Mössbauer spectroscopy

We discuss the use of MS and XRD in the study of an iron-poor soil. We have performed Mössbauer spectroscopy and careful XRD analysis on < 2 μm, < 50 μm and <2 mm sized samples, treated with different Fe-extraction methods. Since the iron content of the coarser fractions is comparable to that of the clay fractions, we show that the usual Mössbauer sample preparation procedures used to overcome the texture effects and to make background absorption corrections are insufficient and that additional homogenization is necessary. By milling the samples for a short interval we achieved spectral area values which are in agreement with Atomic Absorption results and allow to follow the behaviour of the extraction methods.     

Unfolding Meanings for Reflective Symmetry

In this paper, we analyse the processes through which students come to negotiate mathematical meanings for reflective symmetry. We describe a microworld, Turtle Mirrors, designed to provide tools to help students focus simultaneously on actions, visual relationships and symbolic representations. Through a detailed case study of one students thinking-in-change, we examine how the interactions with her partner and with the machine support a fusion of spontaneous and scientific concepts. Other examples of students work further illustrate how the microworld tools offer a means to supplement local understandings of symmetry with those with more explicit, mathematical formulations.This revised version was published online in September 2005 with corrections to the Cover Date.     

Spectral stability under tunneling

We study the spectral properties of multiple well Schrödinger operators on ⁿ . We give in particular upper bounds on energy shifts due to tunnel effect and localization properties of wave packets. Our methods are based on Agmon type estimates for resolvents in classically forbidden regions and geometric perturbation theory. Our results are valid also for an infinite number of wells, arbitrary spectral type and in non-semi-classical regimes.Laboratoire Propre, Centre National de la Recherche ScientifiquePhymat, Université de Toulon et du Var     

The shape resonance

For a class of Schrödinger operatorsH:=–(²/2m)+V onL ²( ⁿ ), with potentials having minima embedded in the continuum of the spectrum and non-trapping tails, we show the existence of shape resonances exponentially close to the real axis as 0. The resonant energies are given by a convergent perturbation expansion in powers of a parameter exhibiting the expected exponentially small behaviour for tunneling.     

Synthesis and aldol reactivity of o- and C-enolate complexes of nickel

The often facile C-/O-tautomerization of transition metal enolates is severely hindered in the cyclic Ni complexes 1 and 2, allowing the study of their individual reactivities. At room temperature only the O-bound tautomer, 2, reacts with aldehydes, giving rise to the corresponding addition products.     

Spectroscopic and theoretical studies of derivatives of 1,6- and 1,7-naphthyridines

The solvatochromism in 8-hydroxy-1,6-naphthyridin-5(6H)-one-7-carboxylic acid methyl ester (1), 5-hydroxy-1,7-naphthyridin-8(7H)-one-6-carboxylic acid methyl ester (2), and 4-hydroxy-2-methyl-1(2H)-isoquinolone-3-carboxylic acid methyl ester (3), has been studied in solvents of different polarity and hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) ability. The relative stabilities of isomers for these naphthyridine derivatives and their interaction with the solvent are reported. Two intramolecular hydrogen-bonded structures contribute to the ground state of compound 1. Temperature effects on the absorption bands were recorded to analyse the possible equilibrium between covalent and zwitterionic forms. The formation of zwitterionic species was observed only in HBD solvents, from which is inferred the solvent assistance in the proton transference. AM1 and PM3 semi-empirical calculations were used in support of the proposed interpretations.