SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Chemical bonding

28

This tutorial review discusses the structural and electronic consequences of the Jahn-Teller effect in transition metal complexes, focussing on copper(ii) compounds which tend to be the most studied. The nature of a Jahn-Teller distortion in molecular complexes and extended lattices can be manipulated by application of pressure or temperature, by doping a molecule into a host lattice, or simply by molecular design. Many of these results have been achieved using compounds with a trans-[CuX(4)Y(2)] coordination sphere, which seems to afford copper centres that are particularly sensitive to their environment. Jahn-Teller distortions lead to some unusual phenomena in molecular magnetism, and are important to the functionality of important classes of conducting and superconducting ceramics.

Concepts: Electron, Oxygen, Chemical bonding, Zinc, Coordination chemistry, Inorganic chemistry, Silver, Jahn–Teller effect

28

Platinum(ii) complexes comprising abnormal diimidazolylidene ligands were synthesized from cis-PtMe(2)(DMSO)(2) using microwave-assisted double C-H bond activation. NMR analysis revealed an unusual solvolysis process, induced by coordinating solvents such as DMSO and MeCN, which has not been observed in related normal dicarbene complexes. NMR and IR spectroscopy and crystallographic analysis of the mono-substituted DMSO complex indicate a sulfur-bonding of the DMSO ligand to the platinum(ii) center. Analysis of the DMSO exchange kinetics provided for the first time a quantitative measure of the trans effect of abnormal carbene ligands. The kinetic exchange rate in these bidentate abnormal dicarbene complexes is 0.050(±2) s(-1) and thus similar to analogous platinum(ii) complexes containing phenylpyridine, yet significantly slower than that induced by pyridylidene pyridine. Reaction of the dicarbene platinum(ii) complexes with PhICl(2), Br(2) and I(2) afforded the corresponding platinum(iv) complexes. Linkage isomerism of the Pt(IV)-bound DMSO was observed when the bromination reaction was performed in DMSO solution. Moreover, solvolysis was less pronounced in the platinum(iv) complexes than in the corresponding platinum(ii) analogues.

Concepts: Ligand, Denticity, EDTA, Chemical bonding, Coordination chemistry, Solvent, C-H bond activation, Acetonitrile

28

Immobilized Cu(OAc)(2) -bis(oxazolines) via hydrogen bonding by SBA-15 was applied to asymmetric Henry reaction, and good enantioselectivities were obtained (up to 83% ee) between 2-methoxybenzaldehyde and CH(3) NO(2) in isopropyl alcohol (iPrOH). The catalyst could be reused seven times without any obvious loss in enantioselectivity. For the first time, this facile and clean immobilization method is applied to the use of bis(oxazolines) complexes. Chirality 24:1092-1095, 2012. © 2012 Wiley Periodicals, Inc.

Concepts: Oxygen, Chemical reaction, Hydrogen, Hydrogenation, Carbon, Chemical bonding, Covalent bond, Isopropyl alcohol

28

We report the high-pressure response of three forms (α, δ, and γ) of pyrazinamide (C5H5N3O, PZA) by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 14 GPa. These different forms are characterized by various intermolecular bonding schemes. High-pressure experimental results show that the γ phase undergoes phase transition to the β phase at a pressure of about 4 GPa, whereas the other two forms retain their original structures at a high pressure. We propose that the stabilities of the α and δ forms upon compression are due to the special dimer connection that these forms possess. On the other hand, the γ form, which does not have this connection, prefers to transform to the closely related β form when pressure is applied. The detailed mechanism of the phase transition together with the stability of the three polymorphs is discussed by taking molecular stacking into account.

Concepts: Spectroscopy, Diffraction, X-ray, Chemical bonding, Raman spectroscopy, X-ray crystallography, Tuberculosis treatment, Pyrazinamide

28

We have computationally studied para-X-substituted phenols and phenolates (X = NO, NO(2) , CHO, COMe, COOH, CONH(2) , Cl, F, H, Me, OMe, and OH) and their hydrogen-bonded complexes with B(-) and HB (B = F and CN), respectively, at B3LYP/6-311++G** and BLYP-D/QZ4P levels of theory. Our purpose is to explore the structures and stabilities of these complexes. Moreover, to understand the emerging trends, we have analyzed the bonding mechanisms using the natural bond orbital scheme as well as Kohn-Sham molecular orbital (MO) theory in combination with quantitative energy decomposition analyses [energy decomposition analysis (EDA), extended transition state-natural orbitals for chemical valence (ETS-NOCV)]. These quantitative analyses allow for the construction of a simple physical model that explains all computational observations. © 2012 Wiley Periodicals, Inc.

Concepts: Scientific method, Electron, Oxygen, Molecule, Chemical bonding, Covalent bond, Molecular orbital, Molecular orbital theory

28

By means of the joint use of electron localization function (ELF) and Thom’s catastrophe theory, a theoretical analysis of the energy profile for the hetero-Diels-Alder reaction of 4-methoxy-1,2-benzoquinone 1 and methoxyethylene 2 has been carried out. The 12 different structural stability domains obtained by the bonding evolution theory have been identified as well as the bifurcation catastrophes (fold and cusp) responsible for the changes in the topology of the system. This analysis permits finding a relationship between the ELF topology and the evolution of the bond breaking/forming processes and electron pair rearrangements through the reaction progress in terms of the different ways of pairing up the electrons. The reaction mechanism corresponds to an asynchronous electronic flux; first, the O1C5 bond is formed by the nucleophilic attack of the C5 carbon of the electron rich ethylene 2 on the most electrophilically activated carbonyl O1 oxygen of 1, and once the σ bond has been completed, the formation process of the second O4C6 bond takes place. In addition, the values of the local electrophilicity and local nucleophilcity indices in the framework of conceptual density functional theory accounts for the asychronicity of the process as well as for the observed regioselectivity. © 2012 Wiley Periodicals, Inc.

Concepts: Scientific method, Electron, Greek loanwords, Chemical bonding, Density functional theory, Quantum chemistry, Nucleophile, Catastrophe theory

28

The reactivity of the complexes [PtCl(2){Ph(2)PN®PPh(2)-P,P}] (R = -H, 3; R = -(CH(2))(9)CH(3), 8) toward group 6 carbonylmetalates Na[MCp(CO)(3)] (M = W or Mo, Cp = cyclopentadienyl) was explored. When R = H, the triangular clusters [PtM(2)Cp(2)(CO)(5)(μ-dppa)] (M = W, 4; M = Mo, 5), in which the diphosphane ligand bridges a Pt-M bond, were obtained as the only products. When R = -(CH(2))(9)CH(3), isomeric mixtures of the triangular clusters [PtM(2)Cp(2)(CO)(5){Ph(2)PN®PPh(2)-P,P}], in which the diphosphane ligand chelates the Pt center (M = W, 11; M = Mo, 13) or bridges a Pt-M bond (M = W, 12; M = Mo, 14), were obtained. Irrespective of the M/Pt ratio used when R = -(CH(2))(9)CH(3), the reaction of [PtCl(2){Ph(2)PN®PPh(2)-P,P}] with Na[MCp(CO)(3)] in acetonitrile stopped at the monosubstitution stage with the formation of [PtCl{MCp(CO)(3)}{Ph(2)PN®PPh(2)-P,P}] (R = -(CH(2))(9)CH(3), M = W, 9; M = Mo, 10), which are the precursors to the trinuclear clusters formed in THF when excess carbonylmetalate was used. The dynamic behavior of the dppa derivatives 4 and 5 in solution as well as that of their carbonylation products 6 and 7, respectively, is discussed. Density functional calculations were performed to study the thermodynamics of formation of 4 and 5 and 11-14, to evaluate the relative stabilities of the chelated and bridged forms and to trace a possible pathway for the formation of the trinuclear clusters.

Concepts: Ligand, Chemical bonding, Hapticity, Coordination chemistry, Chelation, Acetonitrile, Ethylenediamine, Cyclopentadienyl complex

27

Herein we report a rational design strategy for tailoring intermolecular interactions to enhance room-temperature phosphorescence from purely organic materials in amorphous matrices at ambient conditions. The built-in strong halogen and hydrogen bonding between the newly developed phosphor G1 and the poly(vinyl alcohol) (PVA) matrix efficiently suppresses vibrational dissipation and thus enables bright room-temperature phosphorescence (RTP) with quantum yields reaching 24 %. Furthermore, we found that modulation of the strength of halogen and hydrogen bonding in the G1-PVA system by water molecules produced unique reversible phosphorescence-to-fluorescence switching behavior. This unique system can be utilized as a ratiometric water sensor.

Concepts: DNA, Oxygen, Water, Hydrogen, Atom, Carbon, Chemical bonding, Covalent bond

27

Intermolecular interaction in the 1,2,5-chalcogenadiazole dimers was studied by ab initio molecular orbital calculations. Estimated CCSD(T) interaction energies for the thia-, selena- and tellura-diazole dimers are -3.14, -5.29 and -12.42 kcal/mol, respectively. The electrostatic and dispersion interactions are the major sources of the attraction in the dimers, although it was claimed that the orbital mixing (charge-transfer interaction) was the most prominent contribution to the stabilization. The induction (induced polarization) interaction also contributes largely to the attraction in the telluradiazole dimer. The large electrostatic and induction interactions are responsible for the strong attraction in the telluradiazole dimer. On the other hand, the short-range (orbital-orbital) interaction (sum of the exchange-repulsion and charge-transfer interactions) is repulsive. The directionality of the interactions increases in order of S < Se < Te. The electrostatic interaction is mainly responsible for the directionality. The strong directionality suggests that the chalcogen-nitrogen interaction plays important roles in controlling the orientation of molecules in those organic crystals. The nature of the chalcogen-nitrogen interaction in the chalcogenadiazole dimers is similar to that of the halogen bond, which is electrostatically-driven noncovalent interaction.

Concepts: Electron, Fundamental physics concepts, Maxwell's equations, Molecule, Chemistry, Van der Waals force, Chemical bonding, Macromolecule

27

Treatment of [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-CR)] [R = H (), Me ()] with AlPh(3) or AlMe(3) gives adducts [R'(3)Al(μ(3)-O)(μ-O)(2){Ti(η(5)-C(5)Me(5))}(3)(μ(3)-CR)] [R = H, R' = Ph (), Me (); R = Me, R' = Ph (), Me ()] that react with LiNMe(2) to give the contact ion pair complexes [Ph(3)Al(μ-NMe(2))Li(μ(3)-O)(3){Ti(η(5)-C(5)Me(5))}(3)(μ(3)-CR)] [R = H (), Me ()] or the solvent-separated ion pair compounds [Li{(μ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CR)}(2)][(Me(3)Al)(2)(μ-NMe(2))] [R = H (), Me ()]. Reactions of or with a mixture of AlPh(3) and LiCH(2)SiMe(3) lead to the solvent-separated ion pair complexes [Li{(μ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CR)}(2)][Al(CH(2)SiMe(3))Ph(3)] [R = H (), Me ()], presumably by evolution of redistribution intermediates containing the lithium dicubane cation [Li{(μ(3)-O)(3)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CR)}(2)](+) and [Li{Al(μ-Ph)(2)Ph(CH(2)SiMe(3))}(2)](-) anionic units. Surprisingly, reaction of with p-tolyl lithium gives the complex [{Me(2)Al(μ-Me)Li(p-MeC(6)H(4))}{(μ(3)-O)(2)(μ-O)Ti(3)(η(5)-C(5)Me(5))(3)(μ(3)-CH)}] in which the aryl lithium species is incorporated showing interactions with both the trialkyl aluminium unit and the organometallic oxide . X-ray diffraction studies were performed on , and .

Concepts: X-ray, Chemical bond, Chemical bonding, Metal, Oxide, Ion, Intimate ion pair, Solvolysis