Paola Belanzoni

Activity and degradation pathways of pentamethyl-cyclopentadienyl-iridium catalysts for water oxidation

The activity of three [Cp*IrLn] (Cp* = pentamethylcyclopentadienyl) archetypal catalysts ([Cp*Ir (bpy)Cl]Cl (1, bpy = 2,2′-bipyridine), [Cp*Ir(bzpy)(NO3)] (2, bzpy = 2-benzoylpyridine) and [Cp*Ir(H2O)3](NO3)2 (3)) for water oxidation to molecular oxygen was compared using cerium(IV) ammonium nitrate as a sacrificial oxidant. Kinetic studies were carried out by: i) measuring the depletion of Ce4+ through UV-Vis spectroscopy, ii) directly detecting the evolved oxygen through the Clark electrode and iii) measuring the volume of the evolved oxygen. The kinetics of Ce4+ consumption were zero-order in Ce4+ for catalysts 2 and 3, while they were first-order for 1. The order with respect to catalyst was 1 for 1 and 2 while it was 1.5 for 3. As a consequence, 2 (TOFmax = 14.4 min−1) and 3 (TOFmax = 50.4 min−1) were found to be the most active catalysts at low and high catalyst concentration, respectively, while the performance of 1 (TOFmax = 8.6 min−1) increased with increasing the concentration of Ce4+. 1 and 3 were found to be the most robust catalysts at low (3.1 μM, TON = 1240) and high (7.0 μM, TON = 4042) catalyst concentration, respectively. In situNMR studies were performed under exactly the same conditions of the catalytic experiments. It was observed that Cp* underwent an oxidative degradation, ultimately leading to acetic, formic and glycolic acids. Several Ir-containing intermediates of the degradation process were intercepted and fully characterized in solution through 1D- and 2D-NMR experiments. DFT and NMR studies indicated that the degradation proceeds via an initial double oxidative functionalization of both the quanternary carbon and proton of a Cp* C–CH3 moiety.

Activation of the C-H Bond by Electrophilic Attack: Theoretical Study of the Reaction Mechanism of the Aerobic Oxidation of Alcohols to Aldehydes by the Cu(bipy)(2+)/2,2,6,6-Tetramethylpiperidinyl-1-oxy Cocatalyst System

We have investigated the reaction mechanism of the selective aerobic oxidation of primary alcohols into aldehydes using a bipy-copper complex and the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical as cocatalysts (Gamez et al. Chem. Commun. 2003, 2412-2415) and compared it to the well-known oxidation by the TEMPO(+) ion. Our theoretical investigation shows that (a) the oxidation of alcohols to aldehydes by uncoordinated TEMPO(+) takes place by electrophilic attack on the C-H(alpha) bond of the alcohol; (b) the Cu(bipy)(2+) complex has two functions, namely, (1) it acts as a template that brings TEMPO and the (deprotonated) alcohol in proximity by coordinating these moieties in adjacent coordination sites, and (2) it oxidizes the TEMPO radical to (coordinated) TEMPO(+) ion. The H abstraction from alcohol by TEMPO(+) then proceeds as an intramolecular reaction, very much analogous to one of the reaction pathways with free TEMPO(+) and with a remarkably low barrier. We stress that compared to other A-H bonds (A = C, N, O, F), the relatively high-lying C-H bonds are particularly susceptible to electrophilic attack, and notably the C-H(alpha) bond next to the O in an alcohol is so because it is pushed up by an O lone pair. Electrophilic attack, being common to the particular catalytic system studied in this paper and the well-known biotic and abiotic oxidation catalysis by heme and non-heme complexes of the ferryl (Fe(IV)O(2+)) ion, appears to be a unifying electronic structure principle of C-H(alpha) hydroxylation and oxidation reactions.

An evaluation of the density functional approach in the zero order regular approximation for relativistic effects: Magnetic interactions in small metal compounds

The performance of the density functional approach in the relativistic zero order regular approximation for the evaluation of electron spin resonance (ESR) parameters in small metal compounds has been evaluated critically by comparison with experimental data and available theoretical results for 22 linear molecules, characterized by a 2Σ electronic ground state. For most of the molecules studied the calculated magnetic parameters are in good (A tensors) or reasonable (g tensors) agreement with experiment. Effects of spin-orbit coupling and spin polarization on the calculated hyperfine interaction are investigated. These two effects can only be evaluated separately, since the present method does not allow us to take spin-polarization effects into account in spin-orbit coupled density functional calculations. However, while spin-polarization effects are important for all the molecules investigated, spin-orbit effects are non-negligible only for the molecules containing heavier metal atoms. The ESR parameter...

Modeling solvent effects on electron-spin-resonance hyperfine couplings by frozen-density embedding.

In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [J. Phys. Chem. 97, 8050 (1993)] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfccs) of the H2NO molecule. The hfccs for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle.

An ab Initio Benchmark and DFT Validation Study on Gold(I)-Catalyzed Hydroamination of Alkynes.

High level ab initio calculations have been carried out on an archetypal gold(I)-catalyzed reaction: hydroamination of ethyne. We studied up to 12 structures of possible gold(I)-coordinated species modeling different intermediates potentially present in a catalytic cycle for the addition of a protic nucleophile to an alkyne. The benchmark is used to evaluate the performances of some popular density functionals for describing geometries and relative energies of stationary points along the reaction profile. Most functionals (including hybrid or meta-hybrid) give accurate structures but large nonsystematic errors (4-12 kcal/mol) along the reaction energy profile. The double hybrid functional B2PLYP outperforms all considered functionals and compares very nicely with our reference ab initio benchmark energies. Moreover, we present an assessment of the accuracy of commonly used approaches to include relativistic effects, such as relativistic effective potentials and a scalar ZORA Hamiltonian, by a comparison with the results obtained using a relativistic all-electron four-component Dirac-Kohn-Sham method. The contribution of nonscalar relativistic effects in gold(I)-catalyzed reactions, as we investigated here, is expected to be on the order of 1 kcal/mol.

Unexpected Anion Effect in the Alkoxylation of Alkynes Catalyzed by N‐Heterocyclic Carbene (NHC) Cationic Gold Complexes

The intermolecular alkoxylation of alkynes is the oldest application of cationic gold(I) catalysts; however, no systematic experimental data about the role of the anion are available. In this contribution, the role of the anion in this catalytic reaction as promoted by a N-heterocyclic carbene-based gold catalyst, [(NHC)AuX] (X=BARF(-) , BF4 (-) , OTf(-) , OTs(-) , TFA(-) , or OAc(-) ) is analyzed, through a combined experimental (NMR spectroscopy) and theoretical (DFT calculation) approach. The most important factor seems to be the ability to abstract the proton from the methanol during the nucleophilic attack, and such ability is related to the anion basicity. On the other hand, too high coordination power or basicity of the anion worsens the catalytic performance by preventing alkyne coordination or by forming too much free methoxide in solution, which poisons the catalyst. The intermediate coordinating power and basicity of the OTs(-) anion provides the best compromise to achieve efficient catalysis.

Relativistic calculation of hyperfine and electron spin resonance parameters in diatomic molecules

Abstract The four-component formulation of the Dirac–Hartree–Fock (DHF) approximation is used to calculate the electronic g-tensors and nuclear hyperfine interaction A-tensors for a number of open-shell diatomic species. The four-component approach makes it possible to include all relativistic single-particle effects involving the calculated magnetic interactions within the spinor structure, using a compact computational formulation. Results for most of the molecules investigated here are in good agreement with experiment.

Relativistic density functional theory using Gaussian basis sets

A four-component formulation of relativistic density functional theory is presented together with the details of its implemention using a G-spinor basis set. The technical features of this approach are compared to those found in the nonrelativistic density functional theory of quantum chemistry which employ scalar basis sets of Gaussian-type functions. Numerical results of the G-spinor expansion method are presented for a sequence of closed-shell atoms, and for a selection of relativistic density functionals, and are compared with finite difference benchmarks.

Hydroxylation Catalysis by Mononuclear and Dinuclear Iron Oxo Catalysts: a Methane Monooxygenase Model System versus the Fenton Reagent FeIVO(H2O)52+

Hydroxylation of aliphatic C-H bonds is a chemically and biologically important reaction, which is catalyzed by the oxidoiron group FeO(2+) in both mononuclear (heme and nonheme) and dinuclear complexes. We investigate the similarities and dissimilarities of the action of the FeO(2+) group in these two configurations, using the Fenton-type reagent [FeO(2+) in a water solution, FeO(H(2)O)(5)(2+)] and a model system for the methane monooxygenase (MMO) enzyme as representatives. The high-valent iron oxo intermediate MMOH(Q) (compound Q) is regarded as the active species in methane oxidation. We show that the electronic structure of compound Q can be understood as a dimer of two Fe(IV)O(2+) units. This implies that the insights from the past years in the oxidative action of this ubiquitous moiety in oxidation catalysis can be applied immediately to MMOH(Q). Electronically the dinuclear system is not fundamentally different from the mononuclear system. However, there is an important difference of MMOH(Q) from FeO(H(2)O)(5)(2+): the largest contribution to the transition state (TS) barrier in the case of MMOH(Q) is not the activation strain (which is in this case the energy for the C-H bond lengthening to the TS value), but it is the steric hindrance of the incoming CH(4) with the ligands representing glutamate residues. The importance of the steric factor in the dinuclear system suggests that it may be exploited, through variation in the ligand framework, to build a synthetic oxidation catalyst with the desired selectivity for the methane substrate.

O2 activation in a dinuclear Fe(II)/EDTA complex: spin surface crossing as a route to highly reactive Fe(IV)oxo species.

We study the cleavage of O2 in gas phase [(EDTAH)Fe(O2)Fe(EDTAH)]2-, a proposed intermediate in the aqueous Fe(II)-to-Fe(III) autoxidation reaction in the presence of atmospheric dioxygen and EDTA ligand. The role of the exchange coupling between the locally high-spin Fe centers in the O-O dissociation is investigated. Using results from broken symmetry (BS) density functional theory (DFT) calculations, we show that the system can be modeled as two high-spin (HS) S = 5/2 Fe(III) d5 centers coupled through a bridging peroxo O2(2-) ligand, consistent with hypotheses advanced in the literature. We show that in this electronic configuration the O-O cleavage reaction is forbidden by (spin) symmetry. Dissociation of the O2(2-) group to the product ground state may only take place if the system is allowed to undergo a transition to a state of lower spin multiplicity (S = 4) as the O-O bond is stretched. We show that the exchange coupling between the two Fe ions in [(EDTAH)Fe(O2)Fe(EDTAH)]2- plays only a minor role in defining the chemistry of O2 activation in this system. The peroxo/oxo interconversion involves a state outside the Heisenberg spin ladder of the initial S = 5 state. In this S = 4 state, the dinuclear complex evolves to two oxo complexes, [EDTAH x Fe(IV)O]-, with an overall energy barrier of only approximately 86 kJ mol(-1). According to recent theoretical work, the latter species are exceptionally strong oxidants, making them ideal candidate catalysts for organic oxidations (including C-H bond hydroxylation). We highlight the (spin) symmetry forbidden nature of the reaction on the S = 5 surface and its symmetry allowed character in the electronic configuration with S = 4.