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Pentacoordinated organoaluminum complexes: A computational insight
No full textThe geometry and the electronic structure of a series of organometallic pentacoordinated aluminum complexes bearing tri- or tetradentate N,O-based ligands have been investigated with theoretical methods. The BP86, B3LYP, and M06 functionals reproduce with low accuracy the geometry of the selected complexes. The worst result was obtained for the complex bearing a Schiff base ligand with a pendant donor arm, aeimpAlMe2 (aeimp = N-2-(dimethylamino)ethyl-(3,5-di-tert-butyl)salicylaldimine). In particular, the Al-Namine bond distance was unacceptably overestimated. This failure suggests a reasonably flat potential energy surface with respect to Al-N elongation, indicating a weak interaction with probably a strong component of dispersion forces. MP2 and M06-2X methods led to an acceptable value for the same Al-N distance. Better results were obtained with the addition of the dispersion correction to the hybrid B3LYP functional (B3LYP-D). Natural bond orbital analysis revealed that the contribution of the d orbital to the bonding is very small, in agreement with several previous studies of hypervalent molecules. The donation of electronic charge from the ligand to metal mainly consists in the interactions of the lone pairs on the donor atoms of the ligands with the s and p valence orbitals of the aluminum. The covalent bonding of the Al with the coordinated ligand is weak, and the interactions between Al and the coordinated ligands are largely ionic. To further explore the geometrical and electronic factors affecting the formation of these pentacoordianted aluminum complexes, we considered the tetracoordinated complex impAlMe2 (imp = N-isopropyl-(3,5-di-tert-butyl)salicylaldimine)), analogous to aeimpAlMe 2, and we investigated the potential energy surface around the aluminum atom corresponding to the approach of NMe3 to the metal center. At the MP2/6-31G(d) level of theory, a weak attraction was revealed only when NMe3 heads toward the metal center through the directions trans to the nitrogen atom. The analysis of the binding energies for this adducts revealed that the formation of the pentacoordinated derivative is a result of a subtle balance between the penalty paid to deform the impAlMe2 complex and energy gain resulting from interaction between the two fragments. © 2012 American Chemical Society
Comment on “LncRNA 00152 promotes the development of hepatocellular carcinoma by activating JAK2/STAT3 pathway”
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Aluminium complexes of a phenoxyimine ligand with a pendant imidazolium moiety: synthesis, characterisation and evidence for hydrogen bonding in solution
No full textTitanium complexes bearing a hemilabile heteroscorpionate ligand: Synthesis, reactivity, and olefin polymerization activity
No full textA series of titanium complexes (bpzmp)(TiRRR3)-R-1-R-2 (bpzmp = (3,5-Bu-t(2)-2-phenoxo)bis(3,5-Me-2-pyrazol-1-yl)methane; R-1 = R-2 = R-3 = Cl (2); R-1 R-2 R-3 = NMe2 (3); R-1 = Cl; R-2 = R-3 = NMe2 (4); R-1 = R-2 = Cl; R-3= NMe2 (5); R-1 = R-2 R-3 Me (6)) has been synthesized and characterized by VT NMR spectroscopy and X-ray diffraction analysis (2, 3, 5). The complexes 2-6 adopt in the solid state an octahedral structure in which the bpzmp ligand is kappa(3)-coordinated to the metal via the phenoxy group and the imino nitrogens of the two pyrazolyl rings. The investigation of the solution structure of 3 by means of VT H-1 NMR spectroscopy revealed a fluxional behavior of the bpzmp ligand that produces an equilibrium between the octahedral and tetrahedral form of the titanium complex, the latter resulting from the eta(1)-coordination of the ligand through exclusively the phenolate group. The thermodynamic and kinetic parameters of this process were evaluated by VT NMR spectroscopy. The selective replacement of chloride for dimethylamide in 4 and 5 shifts the equilibrium toward the octahedral complex, which is the favorite configuration at room temperature. Site exchange of the nonequivalent methyl groups in 6 was observed at room temperature in the slow-regime H-1 NMR time scale: Delta H double dagger and Delta double dagger S values of 22.3 +/- 1.1 kcal(.)mol(-1) and - 19.6 +/- 3.7 cal mol(-1) K-1 were respectively determined for the isomerization process occurring with a rate constant of 3 s(-1) and Delta G double dagger of 28.1 +/- 0.1 kcal(.)mol(-1) at 293 K. Complexes 2 and 6 are active olefin polymerization catalysts after activation with MAO or [Ph3C] [B(C6F5)(4)]. Linear polyethylene and atactic polypropylene were obtained in the polymerization experiments catalyzed by 6-[Ph3C][B(C6F5)(4)]. Highest polymerization activities were found with the 2-MAO catalyst, where leaching of the ligand due to MAO excess was suspected. Reaction of 6 with [Ph3C][B(C6F5)(4)] in the presence of THF readily produces the ionic complex [(bpzmp)TiMe2(THF)][B(C6F5)(4)], proposed as a model of the active species in this class of olefin polymerization catalysts
Electronic influence of ligand substituents in the ring-opening polymerization of L-Lactide promoted by OSSO-type zirconium complexes
No full textThe zirconium complexes (OSSO Cl )ZrO t Bu 2 (1), (OSSO Br )ZrO t Bu 2 (2) and (OSSO tBu )ZrO t Bu 2 (3) bearing OSSO-type ligands (OSSO Cl -H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dichlorophenol); OSSO Br -H = 1,4-dithiabutanedyl-2,2′-bis(4,6-dibromophenol); OSSO tBu -H = 1,4-dithiabutanedyl-2,2′-bis(4,6-di-tert-butyl-phenol)) were prepared and characterized. NMR spectroscopy revealed a rigid octahedral coordination geometry with a facial-facial coordination of the [OSSO] ligand to the metal complex. These compounds were active in the ring-opening polymerization of L-lactide exerting an effective control over the polymerization reaction. Activities correlated with the electron-withdrawing character of the ortho and para groups of the OSSO ligands. Density functional theory (DFT) investigations indicated that substitution with increasingly electron-withdrawing groups favors the monomer coordination and determines an earlier, more reactive transition state
Mediation of inflammation, obesity and fatty liver disease by advanced glycation endoproducts
No full textNSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l -Lactide
No full textA new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1-5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe2)4 or Hf(NMe2)4 [(NSSN-iPr)Zr(NMe2)2 (1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)Zr(NMe2)2 (3), (NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes present an octahedral coordination geometry with a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1-5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution
Aluminium complexes of a phenoxy-imine ligand with pendant imidazolium moiety: synthesis, characterisation and evidences for hydrogen bondings in solution
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