66 research outputs found
Spectroscopic, computational, and kinetic studies of the mu4-sulfide-bridged tetranuclear CuZ cluster in N2O reductase: pH effect on the edge ligand and its contribution to reactivity
Structural study of acidic metallocavitands and characterization of their interactions with Lewis bases
We report the synthesis and characterization of tantalumâboronate trimetallic clusters of the general formula {[Cp*Ta]3(ÎŒ2-RB(O)2)3(ÎŒ2-OH)(ÎŒ2-O)2(ÎŒ3-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(ÎŒ2-MesB(O)2)2[MesB(OH)(O)]2(ÎŒ2-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 tantalumâboronate 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 CâH Functionalization via a Concerted MetalationâDeprotonation 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 CâH palladation of both the indoles and simple arenes is best described as concerted metalationâdeprotonation, 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 15708â15709 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 anatĂŽmico e palinolĂłgico de AntĂŽnia ovata Pohl (Loganiaceae)
Nesta comunicação o autor considera a anatomia do caule, pecĂolo, lĂąmina foliar e madeira, alĂ©m dos aspectos morfolĂłgicos externo e palinolĂłgico, de espĂ©cimes de Antonia ovata, ocorrentes na floresta da regiĂŁo do rio JarĂ (Estado do ParĂĄ) e nos cerrados da AmazĂŽnia e do Brasil Central; nomeia os espĂ©cimes da mata como sendo uma variedade nova para a ciĂȘncia: 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 JarĂ river (AmazĂŽnia) 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 AmazĂŽnia, Brasil Central and forest of the region JarĂ 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 CâH bonds of free and Cu<sup>I</sup>-bound <i>N</i>-methylimidazole and oxazole via a concerted
metalationâdeprotonation
(CMD) pathway was evaluated, and the distortionâinteraction
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 CâH bond acidities and, thus, enhances
CMD reactivity for all CâH 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 (<sup>t</sup>Bu<sub>2</sub>PCMe<sub>2</sub>CH<sub>2</sub>)Â(OAc)]<sub>2</sub> complex in direct arylation of pyridine <i>N</i>-oxide suggested that the mechanism of this reaction may involve
a process in which CâH 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
CâH bonds of different (hetero)Âarenes via a concerted metalationâdeprotonation
(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 distortionâinteraction
analysis was performed to quantify the various contributions to the
CMD transition states. Calculated barriers of the CâH 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)Âarene
CâH 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 MetalâMetal Bond as a Sensitive Probe of Quality of Exchange-Correlation Functionals
The electronic structure of the vanadium dimer complex
[VÂ(C<sub>5</sub>H<sub>5</sub>)]<sub>2</sub>Pn with a single metalâmetal
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 singletâtriplet
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 metalâmetal
distance and the singleâtriplet energy gap in [VÂ(C<sub>5</sub>H<sub>5</sub>)]<sub>2</sub>Pn. 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 metalâmetal interactions.
Thus, these XC functionals are not quite suitable for computational
studies of multinuclear 3d transition metal complexes with weak-to-intermediate
metalâmetal bonding
The Equilibrium between the octahedral and square pyramidal form and the influence of an axial ligand on the molecular properties of V<sup>IV</sup>O 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 Kâ4.0 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 âadditivity ruleâ, 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)CâH Bond MetalationâDeprotonation Mechanism Spanning the Entire Spectrum of Arenes
A comprehensive understanding of the CâH bond
cleavage step
by the concerted metalationâdeprotonation (CMD) pathway is
important in further development of cross-coupling reactions using
different catalysts. Distortionâinteraction analysis of the
CâH bond cleavage over a wide range of (hetero)Âaromatics has
been performed in an attempt to quantify the various contributions
to the CMD transition state (TS). The (hetero)Âaromatics 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 CâH 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 Ï<sub>p</sub> parameters, correlates with each of the distortionâinteraction
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 CâH bonds catalyzed by transition
metal catalysts
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