Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2

The rate constant of the comparably slow bimolecular NCN radical reaction NCN + O₂ has been measured for the first time under combustion relevant conditions using the shock tube method. The thermal decomposition of cyanogen azide (NCN₃) served as a clean high‐temperature source of NCN radicals. NCN concentration–time profiles have been detected by narrow‐bandwidth laser absorption at cm^?1. The experiments behind incident shock waves have been performed with up to 17% O₂ in the reaction gas mixture. At such high O₂ mole fractions, it was necessary to take O₂ relaxation into account that caused a gradual decrease of the temperature during the experiment. Moreover, following fast decomposition of NCN₃ and collision‐induced intersystem crossing of the initially formed singlet NCN to its triplet ground state, an unexpected and slow additional formation of triplet NCN has been observed on a 100‐μs timescale. This delayed NCN formation was attributed to a fast recombination of ¹NCN with O₂ forming a ³NCNOO adduct acting as a reservoir species for NCN. Rate constant data for the reaction NCN + O₂ have been measured at temperatures between 1674 and 2308 K. They are best represented by the Arrhenius expression . No pressure dependence has been observed at pressures between 216 and 706 mbar.     

The Interplay between Steric and Electronic Effects in SN2 Reactions

Myths of steric hindrance : In contrast with current opinion, energy decomposition analysis shows that the presence of bulky substituents at carbon leads to the release of steric repulsion in the transition state shown in the graphic. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier.

     

Pyridazine based scorpionate ligand in a copper boratrane compound

Reaction of potassium tris(mercapto-tert-butylpyridazinyl)borate K[Tn(tBu)] with copper(II) chloride in dichloromethane at room temperature led to the diamagnetic copper boratrane compound [Cu{B(Pn(tBu))(3)}Cl] (Pn = pyridazine-3-thionyl) (1) under activation of the B-H bond and formation of a Cu-B dative bond. In contrast to this, stirring of the same ligand with copper(I) chloride in tetrahydrofuran (THF) gave the dimeric compound [Cu{Tn(tBu)}](2) (2) where one copper atom is coordinated by two sulfur atoms and one hydrogen atom of one ligand and one sulfur of the other ligand. Hereby, no activation of the B-H bond occurred but a 3-center-2-electron B-H···Cu bond is formed. The reaction of copper(II) chloride with K[Tn(tBu)] in water gave the same product 2, but a formal reduction of the metal center from Cu(II) to Cu(I) occurred. When adding tricyclohexyl phosphine to the reaction mixture of K[Tn(R)] (R = tBu, Me) and copper(I) chloride in MeOH, the distorted tetrahedral Cu complexes [Cu{Tn(R)}(PCy(3))] (R = tBu 3, Me 4) were formed. Compound 4 is exhibiting an "inverted" κ(3)-H,S,S, coordination mode. The copper boratrane 1 was further investigated by density functional theory (DFT) calculations for a better understanding of the M→B interaction involving the d(8) electron configuration of Cu.     

Structures and bonding situation of Pb2X2 (X?=?H, F, Cl, Br and I)

Quantum chemical calculations using DFT (BP86) and ab initio methods (MP2, MP4 and CCSD(T)) have been carried out for the title compounds. The nature of the Pb?CPb interactions has been investigated with an energy decomposition analysis. The energy minimum structures of the halogen substituted Pb₂X₂ molecules possess a doubly bridged butterfly geometry A like the parent system Pb₂H₂. The unusual geometry can be explained with the interactions between PbX fragments in the X ²?? ground state which leads to one Pb?CPb electron-sharing ?? bond and two donor?Cacceptor bonds between the Pb?CX bonds as donor and vacant p(??) AOs of Pb. The energy difference between the equilibrium form A and the linear structure XPb??PbX (E) which is a second-order saddle point is much higher when X is a halogen atom than for X?=?H. This is because the a ⁴??^?????X ²?? excitation energies of PbX (X?=?F?CI) are higher than for PbH. The structural isomers B, D1, D2, E, F1, F2 and G of Pb₂X₂ are no minima on the potential energy surface.     

Aryl Triflates in On-Surface Chemistry

The reactivity of aryl triflates in on-surface C−C coupling is reported. It is shown that the triflate group in aryl triflates enables regioselective homo coupling with preceding or concomitant hydrodetriflation on Cu(111). Three different symmetrical π-systems with two and three triflate functionalities were used as monomers leading to oligomeric conjugated π-systems. The cascade, comprising different intermediates at different reaction temperatures as observed for one of the molecules, proceeds via initial removal of the trifluoromethyl sulfonyl group to give an aryloxy radical which in turn is deoxygenated to the corresponding aryl radical. Thermodynamically driven regioselective 1,2-hydrogen atom transfer leads to a translocated aryl radical which in turn undergoes coupling. For a sterically more hindered bistriflate, where one ortho position was blocked, dehydrogenative coupling occurred at remote position with good regioselectivity. Starting materials, intermediates as well as products were analyzed by scanning tunneling microscopy. Structures and suggested mechanism were further supported by DFT calculations.     

2‐Iminoimidazoline — starke Stickstoffbasen als Koordinationspartner in der Anorganischen Chemie

2‐Iminoimidazolines — Strong Nitrogen Bases als Ligands in Inorganic Chemistry Due to the tendency of the 5‐membered cyclic fragment to accept a positive charge which yields an ylide type bonding situation, 2‐iminoimidazolines are strong nitrogen bases. They can serve as neutral ligands being 2+2 electron donors. Deprotonation leads to the anions which are potential 2+4 electron donors. We describe first the synthesis and characterization of the title compound 2‐imino‐1, 3‐dimethylimidazoline (ImNH, 8 ) and its anion 9 . Next we demonstrate their properties as ligands in complexes of main group elements and transition metals. In a third chapter we describe attempts to functionalize iminoimidazolines with the goal to create neutral ligands that coordinate in a semistable fashion. On this way we want to make a contribution to the chemistry of complex compounds directed towards catalysis.