Structural study of acidic metallocavitands and characterization of their interactions with Lewis bases

We report the synthesis and characterization of tantalum−boronate trimetallic clusters of the general formula {[Cp*Ta]3(μ2-RB(O)2)3(μ2-OH)(μ2-O)2(μ3-OH)} (R = i-Bu (Ta3-iBu), C6H5 (Ta3-Ph), 2,5-(CH3)2(C6H3) (Ta3-2,5Me), 3,5-(CH3)2(C6H3) (Ta3-3,5Me), and 3,5-(CF3)2(C6H3) (Ta3-3,5CF3)). Three of these complexes feature a Lewis acid type cavity allowing for substrate binding in both the solid and the liquid state using a unique electrostatic interaction and a hydrogen bond. We also report the synthesis of {[Cp*Ta]2(μ2-MesB(O)2)2[MesB(OH)(O)]2(μ2-OH)2} (Mes = 2,4,6-(CH3)3C6H2 (Ta2-Mes)). All complexes have been characterized by NMR and X-ray diffraction studies, and the steric and electronic effects on the boronate ligands have been investigated. The physical properties of the interaction between the tantalum−boronate clusters and tetrahydrofuran and acetone have been studied by thermogravimetric analysis, FT-IR, and Density Functional Theory calculations to characterize the molecular interactions in the resulting adducts

Regioselective Oxidative Arylation of Indoles Bearing N-Alkyl Protecting Groups: Dual C−H Functionalization via a Concerted Metalation−Deprotonation Mechanism

The most direct method for synthesizing 2-arylindoles is oxidative coupling of an arene with an indole. We have shown that both the activity and regioselectivity of this cross-coupling reaction are correlated with the acidity of the medium. This insight has been applied to predict the best conditions for the oxidative cross-coupling of N-alkylindoles, an important class of substrates that has heretofore been incompatible with the harsh conditions required for oxidative cross-coupling. Both experimental and computational data indicate that the mechanism for C−H palladation of both the indoles and simple arenes is best described as concerted metalation−deprotonation, regardless of the substitution on the arene

Activation of N2O reduction by the fully reduced micro4-sulfide bridged tetranuclear Cu Z cluster in nitrous oxide reductase

J. Am. Chem. Soc., 2003, 125 (51), pp 15708–15709 DOI: 10.1021/ja038344nThe tetranuclear CuZ cluster catalyzes the two-electron reduction of N2O to N2 and H2O in the enzyme nitrous oxide reductase. This study shows that the fully reduced 4CuI form of the cluster correlates with the catalytic activity of the enzyme. This is the first demonstration that the S = 1/2 form of CuZ can be further reduced. Complementary DFT calculations support the experimental findings and demonstrate that N2O binding in a bent mu-1,3-bridging mode to the 4CuI form is most efficient due to strong back-bonding from two reduced copper atoms. This back-donation activates N2O for electrophilic attack by a proton

Estudo anatômico e palinológico de Antônia ovata Pohl (Loganiaceae)

Nesta comunicação o autor considera a anatomia do caule, pecíolo, lâmina foliar e madeira, além dos aspectos morfológicos externo e palinológico, de espécimes de Antonia ovata, ocorrentes na floresta da região do rio Jarí (Estado do Pará) e nos cerrados da Amazônia e do Brasil Central; nomeia os espécimes da mata como sendo uma variedade nova para a ciência: Antonia ovata Pohl var. excelsa Paula.In this paper the author studies extern morphological, palinological and anatomical aspects, aiming to put an end to the doubts in the taxonomic studies of the specimens of Antonia ovata Pohl (or aiming make clear the taxonomy of the specimens of Antonia ovata. Specimens of Antonia ovata from the woods of the region of Jarí river (Amazônia) are considered by the author as a new variety. With its description, the number of varieties of Antonia ovata rose to three: pilosa, ovata and excelsa (new variety). The extern morphological aspect is found among the individuals from three habitats: "cerrados" of Amazônia, Brasil Central and forest of the region Jarí river. The identification of the three varieties is based on the following characteristic. Presence or lack of hairs on the leaves and branches; microscopic structure of wood (see comparative table); height and diameter of the specimens; and finally the habitat. Pollen grains of these two varieties excelsa and ovata present polymorphism. The leaf of that species has structure of a higrophyllous plants. The stem is rich in mucilaginous cells; vascular bundles are bicollateral; the leafknot is bilacunar, and the trace is formed by two vascular bundles

Tuning the Regioselectivity of Palladium-Catalyzed Direct Arylation of Azoles by Metal Coordination

Cleavage of C–H bonds of free and Cu^I-bound <i>N</i>-methylimidazole and oxazole via a concerted metalation–deprotonation (CMD) pathway was evaluated, and the distortion–interaction analysis was performed to quantify the various contributions to the CMD transition states. Metal binding to the N3 atom for these azoles imparts an increase of C–H bond acidities and, thus, enhances CMD reactivity for all C–H bonds, leading to a reliable C2 > C5 > C4 reactivity compared to a bias for C5 > C2 arylation for noncoordinated azoles. This type of substrate activation and tuning of regioselectivity by metal coordination to heteroarenes can be used for many other classes of substrates for direct arylation reactions

Reactivity and Regioselectivity of Palladium-Catalyzed Direct Arylation in Noncooperative and Cooperative Processes

Recent discovery (J. Am. Chem. Soc 2012, 134, 3683) of the involvement of the cyclometalated [Pd (^tBu₂PCMe₂CH₂)­(OAc)]₂ complex in direct arylation of pyridine <i>N</i>-oxide suggested that the mechanism of this reaction may involve a process in which C–H activation occurs at one Pd center and the aryl group undergoes coupling with another aryl group at a second Pd center (a cooperative catalysis). In this work, cleavage of arene C–H bonds of different (hetero)­arenes via a concerted metalation–deprotonation (CMD) pathway was evaluated for both noncooperative and cooperative processes so that the two processes could be compared in terms of reactivity and regioselectivity. The distortion–interaction analysis was performed to quantify the various contributions to the CMD transition states. Calculated barriers of the C–H bond cleavage in the two processes indicate that the cooperative and noncooperative processes lead to the same regioselectivity of arylation. Differences in contributions to the activation barriers between the two processes are fairly minor. This allows us to use the existing data about (hetero)­arene C–H reactivity and regioselectivity in the noncooperative arylation and apply it to predict reactivity and regioselectivity of arylation in the cooperative process

Complexes with a Single Metal–Metal Bond as a Sensitive Probe of Quality of Exchange-Correlation Functionals

The electronic structure of the vanadium dimer complex [V­(C₅H₅)]₂Pn with a single metal–metal bond was characterized, and the energies of higher spin states were evaluated. To simplify evaluation of orbital contributions to bonding between atoms and fragments, occupancy-perturbed bond orders were introduced. The structure and experimentally determined singlet–triplet gap in this complex can be used to test the quality of modern exchange-correlation functionals. Most generalized gradient approximation (GGA) functionals were determined to be quite suitable to reproduce the metal–metal distance and the single–triplet energy gap in [V­(C₅H₅)]₂Pn. Further accuracy improvement can be achieved by using empirical dispersion corrections. Hybrid exchange-correlation functionals, including the B3LYP functional, performed poorly for both structural and energy predictions. The hybrid functionals significantly overestimate the stability of the singlet state with the antiferromagnetically coupled high-spin metal ions relative to the lowest-energy triplet state and the singlet state with stronger metal–metal interactions. Thus, these XC functionals are not quite suitable for computational studies of multinuclear 3d transition metal complexes with weak-to-intermediate metal–metal bonding

The Equilibrium between the octahedral and square pyramidal form and the influence of an axial ligand on the molecular properties of V^IVO complexes: a spectroscopic and DFT study

The previously unreported equilibrium in aqueous solution between the VIVO square pyramidal and trans octahedral form with an axial water molecule for a number of bidentate ligands with (N,O) and (O,O) donor sets (6-methylpicolinic (6-mepicH) and 6-methyl-2,3-pyridinedicarboxylic (6-me-2,3-pdcH2) acids, dipyridin-2-ylmethanol (Hdpmo) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (Hdhp)) has been demonstrated by the combined application of EPR spectroscopy and DFT methods. The EPR spectra suggest that, with increasing ionic strength, the equilibrium is shifted towards the formation of the pentacoordinated species and values of K≈4.0 and 7.0 for the systems containing 6-methyl-2,3-pyridinedicarboxylic acid and dipyridin-2-ylmethanol were measured. DFT calculations performed with Gaussian 03 and ORCA software predict the 51V anisotropic hyperfine coupling constant along the z axis (Az), which can be used to demonstrate the presence of an axially bound ligand trans to the V=O bond. The results suggest that an axial donor (charged or not) can lower |Az|, in contrast to what was previously believed on the basis of the “additivity rule”, and this explains the anomalous behaviour of the VIVO complexes formed by N-{2-[(2-pyridylmethylene)amino]phenyl}-pyridine-2-carboxamide (Hcapca) and several amidrazone derivatives. The decrease in |Az| for the axial binding of a solvent molecule is mainly a result of the reduction of |Aiso| and this was also observed when the solid [VO(6-methylpicolinato)2] was dissolved in DMSO or DMF. The variations in the structural (V=O, V-O and V-N distances, O-V-O and N-V-N angles, and the trigonality index t) and spectroscopic (|Az|, |Aiso| and v(V=O)) properties as a function of the axial VOH2 distance (R) are also presented. Finally, the electronic structures of the pentaand hexacoordinated complexes are discussed

Analysis of the Palladium-Catalyzed (Aromatic)C–H Bond Metalation–Deprotonation Mechanism Spanning the Entire Spectrum of Arenes

A comprehensive understanding of the C–H bond cleavage step by the concerted metalation–deprotonation (CMD) pathway is important in further development of cross-coupling reactions using different catalysts. Distortion–interaction analysis of the C–H bond cleavage over a wide range of (hetero)­aromatics has been performed in an attempt to quantify the various contributions to the CMD transition state (TS). The (hetero)­aromatics evaluated were divided in different categories to allow an easier understanding of their reactivity and to quantify activation characteristics of different arene substituents. The CMD pathway to the C–H bond cleavage for different classes of arenes is also presented, including the formation of pre-CMD intermediates and the analysis of bonding interactions in TS structures. The effects of remote C2 substituents on the reactivity of thiophenes were evaluated computationally and were corroborated experimentally with competition studies. We show that nucleophilicity of thiophenes, evaluated by Hammett σ_p parameters, correlates with each of the distortion–interaction parameters. In the final part of this manuscript, we set the initial equations that can assist in the development of predictive guidelines for the functionalization of C–H bonds catalyzed by transition metal catalysts