Ralph Puchta

Dissected Nucleus-Independent Chemical Shift Analysis of π-Aromaticity and Antiaromaticity

Analysis of the basic π-aromatic (benzene) and antiaromatic (cyclobutadiene) systems by dissected nucleus-independent chemical shifts (NICS) shows the contrasting diatropic and paratropic effects, but also reveals subtleties and unexpected details.

Gutmann donor and acceptor numbers for ionic liquids.

We present for the first time Gutmann donor and acceptor numbers for a series of 36 different ionic liquids that include 26 distinct anions. The donor numbers were obtained by (23)Na NMR spectroscopy and show a strong dependence on the anionic component of the ionic liquid. The donor numbers measured vary from -12.3 kcal mol(-1) for the ionic liquid containing the weakest coordinative anion [emim][FAP] (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate), which is a weaker donor than 1,2-dichloroethane, to 76.7 kcal mol(-1) found for the ionic liquid [emim][Br], which exhibits a coordinative strength in the range of tertiary amines. The acceptor numbers were measured by using (31)P NMR spectroscopy and also vary as a function of the anionic and cationic component of the ionic liquid. The data are presented and correlated with other solvent parameters like the Kamlet-Taft set of parameters, and compared to the donor numbers reported by other groups.

Metal ion-catalyzed oxidative degradation of Orange II by H2O2. High catalytic activity of simple manganese salts

In an effort to develop new routes for the clean oxidation of non-biodegradable organic dyes, a detailed study of some environmentally friendly Mn(II) salts that form very efficient in situcatalysts for the activation of H2O2 in the oxidation of substrates such as Orange II under mild reaction conditions, was performed. The studied systems have advantages from the viewpoint of green chemistry in that simple metal salts can be used as very efficient catalyst precursors and H2O2 is used as a green oxygen donor reagent. Oxidations were carried out in a glass reactor over a wide pH range in aqueous solution at room temperature. Under optimized conditions it was possible to degrade Orange II in a carbonate buffer solution in less then 100 s using 0.01 M H2O2 in the presence of only 2 × 10−5 M Mn(II) salt. To gain insight into the manganese catalyzed oxidation mechanism, the formation of the active catalyst was followed spectrophotometrically and appears to be the initiating step in the oxidative degradation of the dye. High valent manganese oxo species are instable in the absence of a stabilizing coordinating ligand and lead to a rapid formation of catalytically inactive MnO2. In this context, the role of the organic dye and HCO3− as potential stabilizing ligands was studied in detail. In situUV-Vis spectrophotometric measurements were performed to study the effect of pH and carbonate concentration of the buffer solution on the formation of the catalytically active species. Electrochemical measurements and DFT (B3LYP/LANL2DZp) calculations were used to study the in situ formation of the catalytic species. The catalytic cycle could be repeated several times and demonstrated an excellent stability of the catalytic species during the oxidation process. A mechanism that accounts for the experimental observations is proposed for the overall catalytic cycle.

Novel Iron(III) Porphyrazine Complex. Complex Speciation and Reactions with NO and H2O2

The complex [iron(III) (octaphenylsulfonato)porphyrazine] (5-), Fe (III)(Pz), was synthesized. The p K a values of the axially coordinated water molecules were determined spectrophotometrically and found to be p K a 1 = 7.50 +/- 0.02 and p K a 2 = 11.16 +/- 0.06. The water exchange reaction studied by (17)O NMR as a function of the pH was fast at pH = 1, k ex = (9.8 +/- 0.6) x 10 (6) s (-1) at 25 degrees C, and too fast to be measured at pH = 10, whereas at pH = 13, no water exchange reaction occurred. The equilibrium between mono- and diaqua Fe (III)(Pz) complexes was studied at acidic pH as a function of the temperature and pressure. Complex-formation equilibria with different nucleophiles (Br (-) and pyrazole) were studied in order to distinguish between a five- (in the case of Br (-)) or six-coordinate (in the case of pyrazole) iron(III) center. The kinetics of the reaction of Fe (III)(Pz) with NO was studied as a model ligand substitution reaction at various pH values. The mechanism observed is analogous to the one observed for iron(III) porphyrins and follows an I d mechanism. The product is (Pz)Fe (II)NO (+), and subsequent reductive nitrosylation usually takes place when other nucleophiles like OH (-) or buffer ions are present in solution. Fe (III)(Pz) also activates hydrogen peroxide. Kinetic data for the direct reaction of hydrogen peroxide with the complex clearly indicate the occurrence of more than one reaction step. Kinetic data for the catalytic decomposition of the dye Orange II by H 2O 2 in the presence of Fe (III)(Pz) imply that a catalytic oxidation cycle is initiated. The peroxide molecule first coordinates to the iron(III) center to produce the active catalytic species, which immediately oxidizes the substrate. The influence of the catalyst, oxidant, and substrate concentrations on the reaction rate was studied in detail as a function of the pH. The rate increases with increasing catalyst and peroxide concentrations but decreases with increasing substrate concentration. At low pH, the oxidation of the substrate is not complete because of catalyst decomposition. The observed kinetic traces at pH = 10 and 12 for the catalytic cycle could be simulated on the basis of the obtained kinetic data and the proposed reaction cycle. The experimental results are in good agreement with the simulated ones.

Template and pH‐Mediated Synthesis of Tetrahedral Indium Complexes [Cs⊂{In4(L)4}]+ and [In4(HNL)4]4+: Breaking the Symmetry of N‐Centered C3 (L)3− To Give Neutral [In4(L)4]

There are two classes of well known T-symmetric complexes, in which four octahedrally coordinated metal ions are located in the apices of a tetrahedron, and each of the six edges are bridged by linear C2-symmetric bis(bidentate) chelators (L) and (L) . In [Cs {FeFe3(L)6}] (1), [M {Fe4(L )6}] + (2 ; M = NH4 , K, Cs), and [R4N {M4(L )6}] 11 (3 ; M = Fe, Ga), a cation is endohedrally encapsulated in the center of the tetrahedron, whereas in the complexes [M4\{M4(L)6}] (4) (M = NH4, RNH3: empty, K, Cs: H2O as guest; M 3 = Mg, Co, Ni, Mn), four cations are exohedrally centered above the four tetrahedral triangular faces (Figure 1). However, there are far fewer examples known of Tsymmetric complexes, in which the octahedrally coordinated metal centers in the vertices of the tetrahedra are linked by C3-symmetric tris(bidentate) chelators (L ) or (L) , which occupy the faces of the tetrahedra. Examples thereof

A brighter beryllium

Although it is mainly known for its toxicity, beryllium possesses an array of properties that makes it attractive for a variety of non-industrial purposes. Ralph Puchta discusses why it is not always best avoided.

Thermodynamic and Kinetic Studies on Novel Dinuclear Platinum(II) Complexes Containing Bidentate N,N-donor ligands

A series of novel dinuclear platinum(II) complexes were synthesized with bidentate nitrogen donor ligands. The two platinum centers are connected by an aliphatic chain of variable length. The selected chelating ligand system should stabilize the complex toward decomposition. The pK(a) values and reactivity of four synthesized complexes, viz. [Pt(2)(N(1),N(4)-bis(2-pyridylmethyl)-1,4-butanediamine)(OH(2))(4)](4+) (4NNpy), [Pt(2)(N(1),N(6)-bis(2-pyridylmethyl)-1,6-hexanediamine)(OH(2))(4)](4+) (6NNpy), [Pt(2)(N(1),N(8)-bis(2-pyridylmethyl)-1,8-octanediamine)(OH(2))(4)](4+) (8NNpy), and [Pt(2)(N(1),N(10)-bis(2-pyridylmethyl)-1,10-decanediamine)(OH(2))(4)](4+) (10NNpy), were investigated. This system is of special interest because only little is known about the substitution behavior of dinuclear platinum complexes that contain a bidentate chelate that forms part of the aliphatic bridging ligand. Spectrophotometric acid-base titrations were performed to determine the pK(a) values of the coordinated water ligands. The substitution of coordinated water by thiourea was studied under pseudofirst-order conditions as a function of nucleophile concentration, temperature, and pressure, using stopped-flow techniques and UV-vis spectroscopy. The results for the dinuclear complexes were compared to those for the corresponding mononuclear reference complex [Pt(aminomethylpyridine)(OH(2))(2)](2+) (monoNNpy), by which the effect of increasing the aliphatic chain length on the bridged complexes could be investigated. The results indicated that there is a clear interaction between the two platinum centers, which becomes weaker as the chain length between the metal centers increases. In addition, quantum chemical calculations were performed to support the interpretation and discussion of the experimental data.

The effect of perfluorination on the aromaticity of benzene and heterocyclic six-membered rings

Despite having six highly electronegative Fs, perfluorobenzene C(6)F(6) is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C(6)F(6) and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the Fs induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C(6)F(n)H((6-n)), n = 1-6) have similar ring-LMO-NICS(pi zz) values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C(5)H(5)Y heterocycles (Y = BH(-), N, NH(+)) are as aromatic as benzene, based on their ring-LMO-NICS(pi zz) and ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O(+)) are involved.

Fast substitution reactions of Pt(II) in different ionic liquids. reactivity control by anionic components.

The effect of several imidazolium-based ionic liquids on the rate and mechanism of the substitution reaction of [Pt(terpyridine)Cl](+) with thiocyanate was investigated as a function of thiocyanate concentration and temperature under pseudo-first-order conditions using stopped-flow and other kinetic techniques. The obtained rate constants and activation parameters showed good agreement with the ion-pair stabilization energies between the anions of the ionic liquids and the cationic Pt(II) complex derived from density-functional theory calculations (RB3LYP/LANL2DZp) and with parameters derived from the linear solvation energy relationship set by the Kamlet-Taft beta parameter, which is a measure of a solvents hydrogen bonding acceptor ability. In general, the substitution reactions followed an associative mechanism as found for conventional solvents, but the observed rate constants showed a significant dependence on the nature of the anionic component of the ionic liquid used as solvent. The second order rate constant measured in [emim][NTf(2)] is 2000 times higher than the one measured in [emim][EtOSO(3)]. This difference is much larger than observed for a neutral entering nucleophile studied before.

Mechanistic studies on fast ligand substitution reactions of Pt(II) in different ionic liquids: role of solvent polarity and ion-pair formation.

The effect of several imidazolium-based ionic liquids on the mechanism of a classical ligand substitution reaction of [Pt(terpyridine)Cl] (+) with thiourea was investigated. A detailed kinetic study as a function of the nucleophile concentration and temperature was undertaken under pseudo-first-order conditions using stopped-flow techniques. Polarity measurements were performed for the employed ionic liquids on the basis of solvatochromic effects, and they show similarities with conventional polar solvents. Density-functional theory calculations (RB3LYP/LANL2DZp) were employed to predict the ion-pair stabilization energy between the ionic components of the ionic liquids and/or between the anions of the ionic liquids and the cationic Pt (II) complex. These data illustrate how the anions of the ionic liquids can affect the investigated substitution reaction. In general, the substitution mechanism in ionic liquids was found to have an associative character similar to that in conventional solvents. The observed deviations reflect the influence of the ionic liquid on the interaction between the anionic component of the liquid and the positively charged complex.