Spectroscopy of End-On Copper(II) Superoxido Complexes: A Wave Function-Based Analysis.

Wave function methods are employed to analyze the ground and low-lying excited states of bipyramid trigonal copper(II) superoxido complexes, up to their characteristic ligand to metal charge transfer band. Several multireference methods have been combined to provide new insights into the interpretation of their experimental absorption spectra. We show that the intraligand transition on the dioxygen leads to a dark state. Among the results, we shall highlight the finding of doubly excited states in the region of the d-d transitions and the subtle interplay between Cu(I) and Cu(II) in the ground and excited states. Some of these findings could be obtained only with multireference methods.

Chiral Atropisomeric-NHC Catechodithiolate Ruthenium Complexes for Z-Selective Asymmetric Ring-Opening Cross Metathesis of Exo-Norbornenes.

A set of 16 chiral ruthenium complexes containing atropisomerically stable N-Heterocyclic Carbene (NHC) ligands was synthesized from prochiral NHC precursors. After a rapid screening in asymmetric ring-opening-cross metathesis (AROCM), the most effective chiral atrop BIAN-NHC Ru-catalyst (up to 97 : 3 er) was then converted to a Z-selective catechodithiolate complex. The latter proved to be highly efficient in Z-selective AROCM of exo-norbornenes affording valuable trans-cyclopentanes with excellent Z-selectivity (>98 %) and high enantioselectivity (up to 96.5 : 3.5 er).

Accurate computed singlet-triplet energy differences for cobalt systems: implication for two-state reactivity.

Accurate singlet-triplet energy differences for cobalt and rhodium complexes were calculated by using several wave function methods, such as MRCISD, CASPT2, CCSD(T) and BCCD(T). Relaxed energy differences were obtained by considering the singlet and triplet complexes, each at the minimum of their potential energy surfaces. Active spaces for multireference calculations were carefully checked to provide accurate results. The considered systems are built by increasing progressively the first coordination sphere around the metal. We included in our set two CpCoX complexes (Cp = cyclopentadienyl, X = alkenyl ligand), which have been suggested as intermediates in cycloaddition reactions. Indeed, cobalt systems have been used for more than a decade as active species in this kind of transformations, for which a two-state reactivity has been proposed. Most of the considered systems display a triplet ground state. However, in the case of a reaction intermediate, while a triplet ground state was predicted on the basis of Density Functional Theory results, our calculations suggest a singlet ground state. This stems from the competition between the exchange term (stabilising the triplet) and the accessibility of an intramolecular coordination (stabilising the singlet). This finding has an impact on the general mechanism of the cycloaddition reaction. Analogous rhodium systems were also studied and, as expected, they have a larger tendency to electron pairing than cobalt species.

Transition metal complexes bearing atropisomeric saturated NHC ligands.

From achiral imidazolinium salts, chiral transition metal complexes containing an N-heterocyclic carbene (NHC) ligand were prepared (metal = palladium, copper, silver, gold, rhodium). Axial chirality in these complexes results from the formation of the metal-carbene bond leading to the restriction of rotation of dissymmetric N-aryl substituents about the C-N bond. When these complexes exhibited a sufficient configurational stability, a resolution by chiral high-performance liquid chromatography (HPLC) on preparative scale enabled isolation of enantiomers with excellent enantiopurities (>99% ee) and good yields. A study of the enantiomerization barriers revealed the effect of the backbone nature as well as the type of transition metal on its values. Nevertheless, the evaluation of palladium-based complexes in asymmetric intramolecular α-arylation of amides demonstrated that the ability to induce an enantioselectivity cannot be correlated to the configurational stability of the precatalysts.

Simultaneous Control of Central and Helical Chiralities: Expedient Helicoselective Synthesis of Dioxa[6]helicenes.

An expedient synthesis of a new family of configurationally stable dioxa[6]helicenes was established using a sequential helicoselective organocatalyzed heteroannulation/eliminative aromatization via enantioenriched fused 2-nitro dihydrofurans featuring both central and helical chiralities. Starting from simple achiral precursors, a broad range of these previously unknown chiral heterocyclic scaffolds were obtained with good efficiency, and their aromatization proceeded with very high enantiopurity retention in most cases.

From Prochiral N-Heterocyclic Carbenes to Optically Pure Metal Complexes: New Opportunities in Asymmetric Catalysis.

Well-defined optically pure transition metal (TM) complexes bearing C1- and C2-symmetric N-heterocyclic carbene (NHC) ligands were prepared from prochiral NHC precursors. As predicted by DFT calculations, our strategy capitalizes on the formation of a metal-carbene bond which induces an axis of chirality. Configurationally stable atropisomers of various NHC-containing TM complexes were isolated by preparative HPLC on a chiral stationary phase in good yields and excellent optical purities (up to 99.5% ee). The carbene transfer from an optically pure Cu complex to a gold or palladium center reveals, for the first time, a full stereoretentivity, supporting the hypothesis of an associative mechanism for the transmetalation. The potential of these new chiral TM complexes was illustrated in asymmetric catalysis with up to 98% ee.

Recasting wave functions into valence bond structures: A simple projection method to describe excited states.

A method is proposed to obtain coefficients and weights of valence bond (VB) determinants from multi configurational wave functions. This reading of the wave functions can apply to ground states as well as excited states. The method is based on projection operators. Both energetic and overlap-based criteria are used to assess the quality of the resulting VB wave function. The approach gives a simple access to a VB rewriting for low-lying states, and it is applied to the allyl cation, to the allyl radical and to the ethene (notably to the V-state). For these states, large overlap between VB and multi reference wave functions are easily obtained. The approach proves to be useful to propose an interpretation of the nature of the V-state of ethene.

Large hyperconjugation in strained systems.

Carbocations can appear as transient species, for instance, in elimination reactions and various rearrangements. Hyperconjugation (or conjugation) can then stabilize the cationic character and form a partial π bond. The effect of the electronic delocalization from strained substituents to a carbocation part was calculated. Very large hyperconjugation was found, sometimes more than 80 kcal mol(-1) , which is much larger than typical conjugation effects (56 kcal mol(-1) for the allyl cation).

Padlocking dihydrofurannulation for the control of small degree of helicity built on a fused-tetracyclic core.

Enantioselective construction of small molecules displaying a configurationally stable helical shape built on a fused-tetracyclic core is a daunting synthetic challenge even more pronounced when five-membered rings are incorporated in the structure. The resulting higher configurational lability strongly hampers their access, and therefore the development of new efficient methodologies is timely and highly desirable. In this context, we describe a padlocking approach via the enantioselective organocatalytic domino furannulation of appropriately designed achiral fused-tricyclic precursors resulting in the synthesis of configurationally locked helically chiral tetracyclic scaffolds featuring one or two five-membered rings with the simultaneous control of central and helical chiralities.

C 2-Symmetric atropisomeric N-heterocyclic carbene-palladium(II) complexes: synthesis, chiral resolution, and application in the enantioselective α-arylation of amides.

The concept of atropisomeric N-heterocyclic carbene (NHC)-metal complexes was extended to NHCs possessing a C2-symmetry and implemented to prepare palladium-based complexes. An in-depth study of the NHC precursors and the screening of various NHC ligands enabled us to circumvent the issue associated with the formation of meso complexes. A set of 8 atropisomeric NHC-palladium complexes were prepared and then obtained with high enantiopurities, thanks to an efficient resolution by chiral HPLC at the preparative scale. These complexes displayed good activity in the intramolecular α-arylation of amides and various cyclic products were isolated with excellent enantioselectivities (up to 98% ee).

Successful MALDI-MS analysis of synthetic polymers with labile end-groups: the case of nitroxide-mediated polymerization using the MAMA-SG1 alkoxyamine.

A sample pretreatment was evaluated to enable the production of intact cationic species of synthetic polymers holding a labile end-group using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. More specifically, polymers obtained by nitroxide-mediated polymerization involving the MAMA-SG1 alkoxyamine were stirred for a few hours in trifluoroacetic acid (TFA) to induce the substitution of a tert-butyl group on the nitrogen of nitroxide end-group by a hydrogen atom. Nuclear magnetic resonance, electrospray ionization tandem mass spectrometry, and theoretical calculations were combined to scrutinize this sample pretreatment from both mechanistic and energetic points of view. The substitution reaction was found to increase the dissociation energy of the fragile C-ON bond to a sufficient extent to prevent this bond to be spontaneously cleaved during MALDI analysis. This TFA treatment is shown to be very efficient regardless of the nature of the polymer, as evidenced by reliable MALDI mass spectrometric data obtained for poly(ethylene oxide), polystyrene and poly(butylacrylate).

Bonding of gold with unsaturated species.

Interactions of gold(I) catalysts with alkenes and alkynes are analyzed. Neutral chlorido, and cationic phosphine and N-heterocyclic carbene complexes are studied. High-level ab initio calculations are performed to benchmark the accuracy of popular DFT methods. Donation and backdonation contributions in the bond between the gold fragment and the alkene/alkyne substrate are discussed. These contributions depend on the nature of the gold fragment, but also on the substituents on the alkene/alkyne.

Determination of dissolved nickel in natural waters using a rapid microplate fluorescence assay method.

A new microplate analytical procedure is described for the determination of nickel (Ni2+) ions in natural water samples. A lophine analogue fluorescent sensor was synthesized and a spectral study showed a selective fluorescence quenching effect of chemical sensor by Ni2+ under optimized conditions. Density functional theory (DFT) calculations confirmed the formation of a Ni(II)L3 complex obtained by the Job plot. The calculations showed that the fluorescence emission peak of L collapses due to the distortion of L in the complex. The simple and fast microplate procedure allowed us to quantify Ni2+ with a linear response from 1.6 to 40 µg L-1 and a quantification limit of 5.4 µg L-1 without the need of a preconcentration step. The optimized procedure using high-throughput microplate assay has been applied for the determination of Ni2+ in natural water samples with good analytical performances.

Modelling the two-dimensional polymerization of 1,4-benzene diboronic acid on a Ag surface.

Modelling of the two-dimensional polymerization of 1,4-benzene diboronic acid molecules on the Ag(111) surface, which leads to the formation of a covalent organic framework, is reported. An estimation of free enthalpy is given that takes into account the constraints induced by the molecular adsorption on the surface. The various thermodynamic functions, enthalpies, entropies, and free enthalpies, are obtained from DFT calculations. The entropic effect of the surface plays an important role in the polymerization free energy. A germination threshold is obtained.

The nature of resonance in allyl ions and radical.

A recent valence bond scheme based on Lewis structures, the valence bond BOND (VBB) method (BOND: Breathing Orbitals Naturally Delocalized) method (Linares, M.; Braida, B.; Humbel, S. J. Phys. Chem. A 2006, 110, 2505-2509), is applied to explore the nature of resonance in allyl systems. Whereas allyl radical is correctly described by the resonance between the two traditional Lewis structures, a third "long-bonded" structure, which apparently creates a pi bond between the two distant carbon atoms, appears to plays an important role in allyl ions description. The similar vertical resonance energy (VRE) for both allyl ions is rather moderate (approximately 37 kcal/mol) in the two-structure description but is significantly enhanced when the long-bonded structure is included into the VBB wave function (by up to 20 kcal/mol). The allyl radical is much less resonant and is correctly described by the traditional two-structure picture. The development of VBB Lewis structures into "pure" valence bond determinants enlightens the role of the third structure in the description of allyl ions. The existence of a long bond between the two distant carbon atoms is clearly ruled out. Charge equilibration effect is shown to be a minor factor. The third structure is finally attributed to one- and three-electron bonding character revealed in the pi systems of the cation and anion, respectively. This makes these systems two surprising examples of odd electron bonding within a singlet state. Last, the two-structure description of allyl radical is improved by addition of missing ionic structures.

Hückel-Lewis projection method: a "weights watcher" for mesomeric structures.

A new approach to extract the coefficients and weights of Lewis structures from the Hückel wave function is designed: Hückel-Lewis projection (HL-P). The weights are obtained by projection on overlapping Lewis structures. This straightforward alternative to ab initio approaches is detailed and used on typical cases, including acrolein, allyl radical, pyrrole-like systems, and imidazolylidene. A trust parameter is defined and shown as a guide to retrieve the most important Lewis structures. The emblematic examples of butadiene and benzene are chosen to illustrate the use of this parameter.

Methylenecyclopropene: local vision of the first 1B2 excited state.

The 1A1 ground and the first 1B2 excited states of the methylenecyclopropene (triafulvene) are described by localized wave functions, based on 20 structures valence bond structures. The results are compared to CASSCF(4,4) calculations for both the energetics and the dipole moment. Additional calculations with partial electronic delocalization are presented, and it is shown that the dipole moment modification does not correspond to a situation where the antiaromatic situation prevails (with 4n electrons in the cycle). Part of the analysis uses a "trust factor" that helps to decide if a wave function is appropriate to describe a given state. The trust factor compares the VB wave function to the CASSCF's with their overlap. Finally, the valence bond density is used to produce density maps that illustrate the electron transfer upon excitation. Graphical Abstract A projector-based method compares CASSCF wave functions to local wave functions, including Lewis structures as shown in the picture. A "trust factor" (τ) is obtained. Both the ground state and the first excited state of the methylenecyclopropene are discussed.

Hyperconjugation in Carbocations, a BLW Study with DFT approximation.

The geometry of ethyl cation is discussed, and the hyperconjugation effect in carbocations is evaluated at the B3LYP/6-311G(d) level. The Block Localized Wavefunction (BLW) method is used for all evaluations of the hyperconjugation, considered as the energy gained by the delocalization onto the C(+) atom. This energy is defined as the energy difference between the delocalized (standard) calculation, where the electrons are freely delocalized, and a localized form where the positive charge sits on the carbon center. It is evaluated for 18 carbocations, including conjugated systems. In these cases we were particularly interested in the additional stabilization brought by hyperconjugative effects. Among other effects, the ß-silicon effect is computed. Hyperconjugation amounts in several cases to an energy similar to conjugation effects.

Tandem mass spectrometry of trimethylsilyl-terminated poly(dimethylsiloxane) ammonium adducts generated by electrospray ionization.

Ammonium adducts of trimethylsilyl-terminated poly(dimethylsiloxane) (CH(3)-PDMS) produced by electrospray ionization were submitted to collision induced dissociation and revealed a particular MS/MS behavior: the same three main product ions at m/z 221, 295, and 369 were always generated in very similar relative abundances regardless of the size of the precursor ion. Combining accurate mass measurements and ab initio calculation allowed very stable cyclic geometries to be obtained for these ionic species. Dissociation mechanisms were proposed to account for the three targeted ions to be readily generated in a two-step or a three-step reaction from any CH(3)-PDMS ammonium adducts. A second set of three product ions was also observed with low abundance at m/z 207, 281, and 355, which were shown in MS(3) experiments to be formed in secondary reactions. An alternative dissociation process was shown to consist of a concerted elimination of ammonia and methane and the need for a methyl of an end-group to be involved in the released methane molecule would account for this reaction to mainly proceed from the smallest precursor ions.