Arene C−H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of SNAr Chemistry

The reactivity of the electron-rich anionic Al^I aluminyl compound K₂[(NON)Al]₂ (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) towards mono- and disubstituted arenes is reported. C−H activation chemistry with n-butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer-type transition state. Selectivity is therefore based on similar electronic factors to classical S_NAr chemistry, which implies the destabilisation of transition states featuring electron-donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent.     

A Phosphine Functionalized β-Diketimine Ligand for the Synthesis of Manifold Metal Complexes

A bis(diphenyl)-phosphine functionalized β-diketimine (PNac-H) was synthesized as a flexible ligand for transition metal complexes. The newly designed ligand features symmetrically placed phosphine moieties around a β-diketimine unit, forming a PNNP-type pocket. Due to the hard and soft donor atoms (N vs. P) the ligand can stabilize various coordination polyhedra. A complete series ranging from coordination numbers 2 to 6 was realized. Linear, trigonal planar, square planar, tetrahedral, square pyramidal, and octahedral coordination arrangements containing the PNac-ligand around the metal center were observed by using suitable metal sources. Hereby, PNac-H or its anion PNac⁻ acts as mono-, bi- and tetradendate ligand. Such a broad flexibility is unusual for a rigid tetradentate system. The structural motifs were realized by treatment of PNac-H with a series of late transition metal precursors, for example, silver, gold, nickel, copper, platinum, and rhodium. The new complexes have been fully characterized by single crystal X-ray diffraction, NMR, IR, UV/Vis spectroscopy, mass spectrometry as well as elemental analysis. Additionally, selected complexes were investigated regarding their photophysical properties. Thus, PNac-H proved to be an ideal ligand platform for the selective coordination and stabilization of various metal ions in diverse polyhedra and oxidation states.     

Nitrenium Salts in Lewis Acid Catalysis

Molecular compounds featuring nitrogen atoms are typically regarded as Lewis bases and are extensively employed as donor ligands in coordination chemistry or as nucleophiles in organic chemistry. By contrast, electrophilic nitrogen‐containing compounds are much rarer. Nitrenium cations are a new family of nitrogen‐based Lewis acids, the reactivity of which remains largely unexplored. In this work, nitrenium ions are explored as catalysts in five organic transformations. These reactions are the first examples of Lewis acid catalysis employing nitrogen as the site of substrate activation. Moreover, these compounds are readily accessed from commercially available reagents and exhibit remarkable stability toward moisture, allowing for benchtop transformations without the need to pretreat solvents.     

Aroylthioureas: new organic ionophores for heavy metal ion selective electrodes. A nuclear magnetic resonance study

The 1H and 13C NMR spectra of four series of 1-aroylthiourea derivatives in DMSO-d6 are reported. The NH signals for 3-alkyl substituted aroylthioureas are identified by their multiplicity and by homonuclear irradiation experiments. Correlation analyzes are made for NH, CO and CS signals in order to determine the best way to modulate the nucleophilic character of the CS group, as thioureas are well-known ionophore groups. Almost all 1,3-substituted thioureas (Series 2-4) show the reported chelated structure with the exception of those with CF3, CN and NO2 groups. Pyridine group promote a different equilibrium in solution. The fragment -CO-NH- transmits poorly the electronic effects of substituents in the aroylgroup.     

Sustained prediction ability of net analyte preprocessing methods using reduced calibration sets. Theoretical and experimental study involving the spectrophotometric analysis of multicomponent mixtures

A newly developed multivariate method involving net analyte preprocessing (NAP) was tested using central composite calibration designs of progressively decreasing size regarding the multivariate simultaneous spectrophotometric determination of three active components (phenylephrine, diphenhydramine and naphazoline) and one excipient (methylparaben) in nasal solutions. Its performance was evaluated and compared with that of partial least-squares (PLS-1). Minimisation of the calibration predicted error sum of squares (PRESS) as a function of a moving spectral window helped to select appropriate working spectral ranges for both methods. The comparison of NAP and PLS results was carried out using two tests: (1) the elliptical joint confidence region for the slope and intercept of a predicted versus actual concentrations plot for a large validation set of samples and (2) the D-optimality criterion concerning the information content of the calibration data matrix. Extensive simulations and experimental validation showed that, unlike PLS, the NAP method is able to furnish highly satisfactory results when the calibration set is reduced from a full four-component central composite to a fractional central composite, as expected from the modelling requirements of net analyte based methods.     

A highly distorted open-shell endohedral Zintl cluster: [Mn@Pb12]3-

Reaction of an ethylenediamine (en) solution of K(4)Pb(9) and 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) with a tetrahydrofuran (THF) solution of Mn(3)(Mes)(6) (Mes = 2,4,6-trimethylphenyl) yielded the anionic cluster [Mn@Pb(12)](3-). This species was observed in the positive and negative ion-mode electrospray mass-spectra of the crude reaction mixture. The crystalline samples obtained from such solutions allowed us to confirm the composition of the sample as [K(2,2,2-crypt)](3)[Mn@Pb(12)]·1.5en (1). Because of numerous issues related to crystal sample quality and crystallographic disorder a high-quality crystal structure solution could not be obtained. Despite this, however, the data collected permit us to draw reasonable conclusions about the charge and connectivity of the [Mn@Pb(12)](3-) cluster anion. Crystals of 1 were further characterized by elemental analysis and electron paramagnetic resonance (EPR). Density Functional Theory (DFT) calculations on such a system reveal a highly distorted endohedral cluster anion, consistent with the structural distortions observed by single crystal X-ray diffraction. The cluster anions are considerably expanded compared to the 36-electron closed-shell analogue [Ni@Pb(12)](2-) and, moreover, exhibit significant low-symmetry distortions from the idealized icosahedral (I(h)) geometry that is characteristic of related endohedral clusters. Our computations indicate that there is substantial transfer of electron density from the formally Mn(-I) center to the low-lying vacant orbitals of the [Pb(12)](2-) cage.     

Heteroatomic molecular clusters derived from Group 15 Zintl ion cages: synthesis and isolation of [M2(HP7)2](2-) (M=Ag, Au), two novel cluster anions exhibiting metallophilic interactions

Ethylenediamine (en) solutions of K(3)P(7) and 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) were reacted with the homoleptic group 11 complexes [M(nbe)(3)][SbF(6)] (M = Ag, Au; nbe = norbornene) yielding two novel cluster anions, [M(2)(HP(7))(2)](2-), both of which were isolated in low crystalline yields as [K(2,2,2-crypt)](2)[M(2)(HP(7))(2)] (M = Ag (1) and Au (2)). Optimization of the reaction conditions by incorporation of a proton source (ammonium tetraphenylborate) and the replacement of the light-sensitive nbe adducts of silver and gold with the chloride salts MCl (M = Ag, Au) was found to greatly increase the yield and purity in which 1 and 2 were isolated. Compounds 1 and 2 were characterized by single crystal X-ray diffraction, electrospray ionization mass-spectrometry (ESI- MS), elemental analysis, and (1)H and (31)P NMR spectroscopy. Density functional theory (DFT) calculations on the cluster anions were also conducted.     

Chemometric assisted solid-phase microextraction for the determination of anti-inflammatory and antiepileptic drugs in river water by liquid chromatography-diode array detection

In the present work, an analytical method for the simultaneous determination of seven non steroidal anti-inflammatory drugs (naproxen, ketoprofen, diclofenac, piroxicam, indomethacin, sulindac and diflunisal) and the anticonvulsant carbamazepine is reported. The method involves preconcentration and clean-up by solid-phase microextraction using polydimethylsiloxane/divinylbenzene fibers, followed by liquid chromatography with diode array detection analysis. Parameters that affect the efficiency of the solid-phase microextraction step such as soaking solvent, soaking period, desorption period, stirring rate, extraction time, sample pH, ionic strength, organic solvent and temperature were investigated using a Plackett-Burman screening design. Then, the factors presenting significant positive effects on the analytical response (soaking period, stirring rate, stirring time) were considered in a further central composite design to optimize the operational conditions for the solid phase microextraction procedure. Additionally, multiple response simultaneous optimization by using the desirability function was used to find the optimum experimental conditions for the on-line solid-phase microextraction of analytes in river water samples coupled to liquid chromatography and diode array detection. The best results were obtained using a soaking period of 5 min, stirring rate of 1400 rpm and stirring time of 44 min. The use of solid-phase microextraction technique avoided matrix effect and allowed to quantify the analytes in river water samples by using Milli-Q based calibration graphs. Recoveries ranging from 71.6% to 122.8% for all pharmaceuticals proved the accuracy of the proposed method in river water samples. Method detection limits were in the range of 0.5-3.0 microgL(-1) and limits of quantitation (LOQs) were between 1.0 and 4.0 microgL(-1) for pharmaceutical compounds in river water samples. The expanded uncertainty associated to the measurement of the concentration ranged between 8.5% and 29.0% for 20 microgL(-1) of each analyte and between 9.0% and 29.5% for the average of different concentration levels. The main source of uncertainty was the calibration step in both cases.