Computational study of the spin-state energies and UV-Vis spectra of bis(1,4,7-triazacyclononane) complexes of some first-row transition metal cations.

We report here computed spin-state energies and UV-Vis spectra for several transition metal complexes with a triazacyclononane ligand. Our results show that the spin ground-state is correctly obtained with either OPBE or SSB-D, except for the high-spin ground-state of the Co(ii) complex that was properly described only by SSB-D. The UV-Vis spectra from TD-DFT reproduce in general rather well the experimental spectra, but in cases of the Cr(III) and Co(II) complexes it clearly failed. Better results for the UV-Vis spectra have been obtained by using Ligand Field DFT.

Observation of Fe(V)=O using variable-temperature mass spectrometry and its enzyme-like C-H and C=C oxidation reactions.

Oxo-transfer chemistry mediated by iron underpins many biological processes and today is emerging as synthetically very important for the catalytic oxidation of C-H and C=C moieties that are hard to activate conventionally. Despite the vast amount of research in this area, experimental characterization of the reactive species under catalytic conditions is very limited, although a Fe(V)=O moiety was postulated. Here we show, using variable-temperature mass spectrometry, the generation of a Fe(V)=O species within a synthetic non-haem complex at -40 °C and its reaction with an olefin. Also, with isotopic labelling we were able both to follow oxygen-atom transfer from H(2)O(2)/H(2)O through Fe(V)=O to the products and to probe the reactivity as a function of temperature. This study pioneers the implementation of variable-temperature mass spectrometry to investigate reactive intermediates.

A multi-scale approach to spin crossover in Fe(II) compounds.

We report here for the first time a multi-scale study on the concept of spin-crossover compounds, which integrates improved density functionals, a polarizable force field and hybrid QM/MM calculations. This multi-scale setup is applied to the temperature dependence of spin states of a Fe(II) compound with trispyrazolylborate ligands that exhibits spin-crossover. Our study shows a transition temperature of around 290 K, which is in perfect agreement with experimental results. Moreover, based on our data we provide the origin of why spin transition occurs in this iron-compound: it results directly from spin-state changes in the iron-compound that lead to more favourable electrostatic interactions for the high-spin state.

Modeling the cis-oxo-labile binding site motif of non-heme iron oxygenases: water exchange and oxidation reactivity of a non-heme iron(IV)-oxo compound bearing a tripodal tetradentate ligand.

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe(IV)(O)((Me,H) Pytacn)(S)](2+) (2, (Me,H)Pytacn=1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S=CH(3)CN or H(2)O) is described. Complex 2 was prepared by reaction of [Fe(II)(CF(3)SO(3))(2)((Me,H) Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N(4) ligand that leaves two cis sites available for binding an oxo group and a second external ligand but, unlike the related iron(IV)-oxo species with tetradentate ligands, it is remarkably stable at room temperature (t(1/2)>2 h at 288 K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism is proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen-atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (bond dissociation energy=99.3 kcal mol(-1)).

Spin-state-corrected Gaussian-type orbital basis sets.

Recently, we reported that the basis set has a profound influence on the computed values for spin-state splittings [J. Phys. Chem. A 2008, 112, 6384]. In particular, small Gaussian-type orbital (GTO) basis sets were shown to be unreliable for the prediction of them. Here, we report simple modifications of the small Pople-type Gaussian-type orbital basis sets (3-21G, 3-21G*, 6-31G, 6-31G*), which correct their faulty behavior for the spin-state energies. We have investigated the basis sets for a set of 13 first-row transition-metal complexes for which reliable reference data have been obtained at the OPBE/TZ2P(STO) level. For several systems, we have used single and double spin-contamination corrections to avoid ambiguity of the results because of spin contamination, that is, the energies and geometries were obtained for the pure spin states. The spin ground states as predicted by the spin-state-corrected GTO basis sets (s6-31G, s6-31G*) are in complete agreement with the reference Slater-type orbital (STO) data, while those of the original basis sets and a recent modification by Baker and Pulay (m6-31G*) are not for all cases. The spin-state-corrected GTO basis sets also improve upon the original and modified basis sets for the accuracy of geometry optimization, while the accuracy of the vibrational frequencies is as good or better. At a limited additional cost, one therefore obtains very reliable results for these important spin-state energies.

Olefin-dependent discrimination between two nonheme HO-FeV=O tautomeric species in catalytic H2O2 epoxidations.

Wet oxygenase models: Unprecedented high levels of water incorporation into products (up to 75%) is observed in epoxidation reactions with H(2)O(2) by a bioinspired nonheme iron catalyst. A surprising substrate-dependent incorporation of water is observed, and is proposed to arise from fast equilibrium between two high-valent HO-Fe(V)=O isomeric species exhibiting different reactivity.

Structure, bonding, and relative stability of the ground and low-lying electronic states of CuO2. The role of exact exchange.

The C(2v) and C(s) ground and low-lying states of doublet CuO2 are examined for a series of different density functionals (pure, hybrid, and meta-hybrid) and CCSD(T) methods. The effect of changing the B3LYP functional a0 parameter is also explored. CCSD(T) results at the complete basis set limit show that the relative stability of the different electronic states is 2A(2)(C(2v)) < 2A''(Cs) < 2B(2)(C(2v)) < 2A'(C(s)) 2A(1)(C(2v)) < 2B(1)(C(2v)). Unlike CCSD(T), all DFT methods analyzed in this work erroneously predict the end-on 2A'' state as the ground state for CuO2 irrespective of the type of functional and percentage of Hartree-Fock (exact) exchange included in the B3LYP-like functional. Among the different functionals tested, B3LYP gives the best geometries and relative energies for the different electronic states when compared to CCSD(T) results. As for the effect of the a0 parameter, it is found that the B3LYP-like functional yielding better geometries contains 20% of exact exchange, although somewhat unexpectedly, the B3LYP-like functional with a larger contribution of exact exchange (90%) is the one that gives the smaller standard deviation for relative energies.

Theoretical study of the hydroxylation of phenolates by the Cu(2)O (2)(N,N'-dimethylethylenediamine) (2) (2+) complex.

Tyrosinase catalyzes the ortho hydroxylation of monophenols and the subsequent oxidation of the diphenolic products to the resulting quinones. In efforts to create biomimetic copper complexes that can oxidize C-H bonds, Stack and coworkers recently reported a synthetic mu-eta(2):eta(2)-peroxodicopper(II)(DBED)(2) complex (DBED is N,N'-di-tert-butylethylenediamine), which rapidly hydroxylates phenolates. A reactive intermediate consistent with a bis-mu-oxo-dicopper(III)-phenolate complex, with the O-O bond fully cleaved, is observed experimentally. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative mechanism to the concerted or late-stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase. In this work, the reaction mechanism of this peroxodicopper(II) complex was studied with hybrid density functional methods by replacing DBED in the mu-eta(2):eta(2)-peroxodicopper(II)(DBED)(2) complex by N,N'-dimethylethylenediamine ligands to reduce the computational costs. The reaction mechanism obtained is compared with the existing proposals for the catalytic ortho hydroxylation of monophenol and the subsequent oxidation of the diphenolic product to the resulting quinone with the aim of gaining some understanding about the copper-promoted oxidation processes mediated by 2:1 Cu(I)O(2)-derived species.

Theoretical study of the hydroxylation of phenols mediated by an end-on bound superoxo-copper(II) complex.

Peptidylglycine alpha-amidating monooxygenase and dopamine beta-monooxygenase are copper-containing proteins which catalyze essential hydroxylation reactions in biological systems. There are several possible mechanisms for the reductive O(2)-activation at their mononuclear copper active site. Recently, Karlin and coworkers reported on the reactivity of a copper(II)-superoxo complex which is capable of inducing the hydroxylation of phenols with incorporated oxygen atoms derived from the Cu(II)-O(2) (.-) moiety. In the present work the reaction mechanism for the abovementioned superoxo complex with phenols is studied. The pathways found are analyzed with the aim of providing some insight into the nature of the chemical and biological copper-promoted oxidative processes with 1:1 Cu(I)/O(2)-derived species.

Importance of the basis set for the spin-state energetics of iron complexes.

We have performed a systematic investigation of the influence of the basis set on relative spin-state energies for a number of iron compounds. In principle, with an infinitely large basis set, both Slater-type orbital (STO) and Gaussian-type orbital (GTO) series should converge to the same final answer, which is indeed what we observe for both vertical and relaxed spin-state splittings. However, we see throughout the paper that the STO basis sets give consistent and rapidly converging results, while the convergence with respect to the basis set size is much slower for the GTO basis sets. For example, the large GTO basis sets that give good results for the vertical spin-state splittings of compounds 1-3 (6-311+G**, Ahlrichs VTZ2D2P) fail for the relaxed spin-state splittings of compound 4 (where 1 is Fe-(PyPepS)2 (PyPepSH 2 = N-(2-mercaptophenyl)-2-pyridinecarboxamide), 2 is Fe(tsalen)Cl (tsalen = N, N'-ethylenebis-(thiosalicylideneiminato)), 3 is Fe(N(CH2-o-C6H4S) 3)(1-Me-imidazole), and 4 is FeFHOH). Very demanding GTO basis sets like Dunning's correlation-consistent (cc-pVTZ, cc-pVQZ) basis sets are needed to achieve good results for these relaxed spin states. The use of popular (Pople-type) GTO, effective core potentials basis set (ECPB), or mixed ECPB(Fe):GTO(rest) basis sets is shown to lead to substantial deviations (2-10 kcal/mol, 14-24 kcal/mol for 3-21G), in particular for the high spin states that are typically placed at too low energy. Moreover, the use of an effective core potential in the ECPB basis sets results in spin-state splittings that are systematically different from the STO-GTO results.

Nanosized trigonal prismatic and antiprismatic CuII coordination cages based on tricarboxylate linkers.

3D coordination cages have been synthesized via the supramolecular 3 + 2 self-assembly of macrocyclic dicopper molecular clips and tricarboxylate linkers, the nature of which allows the introduction of different functionalities to the 3D cages.

Alkane hydroxylation by a nonheme iron catalyst that challenges the heme paradigm for oxygenase action.

A nonheme iron catalyst catalyzed stereoselective oxidation of alkanes with H2O2 with remarkable efficiency and exhibiting an unprecedented high incorporation of water into the oxidized products. The present results challenge the canonical description of oxygenases, the standard oxo-hydroxo tautomerism that applies to heme systems and serves as a precedent for alternative pathways for the oxidation of hydrocarbons at nonheme iron oxygenases.

Theoretical study of the catalytic mechanism of catechol oxidase.

The mechanism for the oxidation of catechol by catechol oxidase has been studied using B3LYP hybrid density functional theory. On the basis of the X-ray structure of the enzyme, the molecular system investigated includes the first-shell protein ligands of the two metal centers as well as the second-shell ligand Cys92. The cycle starts out with the oxidized, open-shell singlet complex with oxidation states Cu(2)(II,II) with a mu-eta(2):eta(2) bridging peroxide, as suggested experimentally, which is obtained from the oxidation of Cu(2)(I,I) by dioxygen. The substrate of each half-reaction is a catechol molecule approaching the dicopper complex: the first half-reaction involves Cu(I) oxidation by peroxide and the second one Cu(II) reduction. The quantitative potential energy profile of the reaction is discussed in connection with experimental data. Since no protons leave or enter the active site during the catalytic cycle, no external base is required. Unlike the previous density functional theory study, the dicopper complex has a charge of +2.

Theoretical study of the reaction mechanisms involved in the thermal intramolecular reactions of 1,6-fullerenynes.

Substitution of a H atom by an alkyl group on the terminal carbon of the alkyne moiety of 1,6-fullerenynes has a strong impact on the products of the reaction undergone by this species after thermal treatment. While the reaction of 1,6-fullerenynes bearing an unsubstituted alkyne moiety results in the cycloaddition of the alkyne group to the fullerene double bond leading to cyclobutene-fused derivatives, the presence of an alkyl substituent leads to the formation of allenes. In the present work, we have performed an exhaustive theoretical analysis of all possible reaction mechanisms leading to cyclobutene-fused derivatives and allenes to offer an explanation of the reactivity differences observed. The results obtained show that formation of cyclobutene-fused derivatives occurs through a stepwise diradical reaction mechanism, while allene formation proceeds through a concerted way involving an uncommon intramolecular ene process. For the 1,6-fullerenynes bearing a substituted alkyne, the ene reaction path leading to allenes has an energy barrier somewhat lower than the stepwise diradical mechanism for the cyclobutene-fused derivative formation, thus explaining the outcome of the reaction.

Intramolecular ene reaction of 1,6-fullerenynes: a new synthesis of allenes.

[Structure: see text] Thermal treatment of 1,6-fullerenynes bearing an alkyl group on the terminal carbon of the alkyne moiety leads quantitatively to new allenes through a reaction mechanism involving an intramolecular ene process. This reaction outcome is in contrast to that recently found for free terminal alkynes which form cyclobutene derivatives through a [2+2] cyclization mechanism.

Redox-controlled molecular flipper based on a chiral Cu complex.

A molecular bipaddled flipper based on a tetradentate chiral Cu complex has been designed. The paddling motion of this unprecedented molecular-scale machine can be controlled by reversible oxidation of the metal center. Kinetic and computational (density functional theory) analyses provide a detailed picture of the flipper motion at the molecular scale, rationalize the switching role of the metal-ion oxidation state, and pose the basis for the fine-tuning of the dynamic motion of this new class of molecular-scale devices.

Analysis of electron delocalization in aromatic systems: individual molecular orbital contributions to para-delocalization indexes (PDI).

Our research group has recently defined two new aromaticity indexes based on the analysis of electron delocalization in aromatic species using the quantum theory of atoms-in-molecules. One of these indexes is the para-delocalization index (PDI) that measures the electronic delocalization between para-related carbon atoms in six-membered rings. In this paper, we show that this index can be partitioned into individual molecular orbital contributions. We have applied this PDI decomposition to several polycyclic aromatic hydrocarbons showing that this partitioning provides new insight into the origin of aromaticity.

Aromaticity analysis of lithium cation/pi complexes of aromatic systems.

The changes in the local aromaticity of aromatic rings on complexation with lithium cation are discussed for a series of polycyclic aromatic hydrocarbons by using, as probes of aromaticity, the para-delocalization index (PDI), the aromatic fluctuation index (FLU), the harmonic oscillator model of aromaticity index (HOMA), and the nucleus-independent chemical shift (NICS). In most cases, changes in the electronically based PDI and FLU indices are almost irrelevant. For this reason, the analysis is carried out mainly with the more sensitive HOMA and NICS descriptors. These indices indicate that changes in the local aromatic character of the different rings are small. In general, Li+ interacts more favorably with the ring having the largest pi charge, but there is no correlation between these magnitudes. The ring directly interacting with the lithium cation suffers a slight but significant reduction of aromaticity, while the reduction of the local aromaticity of the adjacent rings is minor. For those rings located further away from the ring directly connected to Li+, we found generally a small increase in aromaticity. Although there is no clear correlation between aromaticity and Li+ binding enthalpies, there is a rough correlation between the latter and the change in the aromaticity of the ring directly interacting with the metal cation.