On the Brink of Decomposition: Controlled Oxidation of a Substituted Arsanylborane as a Way to Labile Group 13-15-16 Compounds

The reactivity of the organic-substituted arsanylborane tBuAsHBH₂NMe₃ ( 1 ) towards different elemental chalcogenes as well as organic oxidants such as O-NMe_3, Me₃Si−O−O-SiMe₃, MesCNO and cyclohexenesulfide is reported. By the reaction of 1 with grey selenium, the selenium oxidation product tBuAs(Se)HBH₂NMe₃ ( 2 ) was obtained. For the oxidation with sulfur, the two products tBuAs(S)HBH₂NMe₃ ( 3 a ) and tBuAs(S)SHBH₂NMe₃ ( 3 b ) could be isolated as oils. The structural characterization of As(tBuAs(S)SHBH₂NMe₃)₃ ( 4 ) as well as corresponding DFT computations allow insights into the decomposition behavior of 3 a and 3 b in solution. For the reaction of MesCNO with 1 , the formation of an unusual As−H activation product Mes-C(NOH)-AstBu-BH₂NMe₃ ( 5 ) is observed. In the reaction with Me₃N−O, the first isolatable oxo-arsanylboranes tBuAs(O)HBH₂NMe₃ ( 6 a ) and tBuAs(O)OHBH₂NMe₃ ( 6 b ) are obtained, with 6 b also being accessible via the controlled reaction of 1 with air.     

2‐[4‐Phenyl‐5‐(2‐pyridyl)‐4H‐1,2,4‐triazol‐3‐yl]nicotinic acid: a case of solvent‐dependent polymorphism

The title compound, C₁₉H₁₃N₅O₂, crystallizes in two monoclinic forms depending on the solvent used. From methanol or acetone, a yellow form [(Ia), m.p. 533 K] in the space group P2₁ is obtained, while with ethanol as the solvent, an orange form [(Ib), m.p. 541 K] in the space group Cc results. The conformers observed in the two polymorphs differ primarily in the relative orientation of pyridine/phenyl and triazole rings. Molecules of both polymorphs form chains through carboxyl O—H...N hydrogen bonding; however, in each crystal structure, a different group acts as acceptor, viz. a triazole and a pyridyl N atom for (Ia) and (Ib), respectively. This is the first case of polymorphism observed for crystals of a 3,4,5‐trisubstituted 1,2,4‐triazole derivative.     

Energetics and mechanism of ammonia synthesis through the Chatt Cycle: conditions for a catalytic mode and comparison with the Schrock Cycle

Through a series of DFT calculations the energy profile of the Chatt cycle is evaluated. This is the counterpiece of our earlier investigations of the Schrock cycle (Angew. Chem. 2005, 117, 5783; Angew. Chem. Int. Ed. 2005, 44, 5639), applying the same quantumchemical methodology and approximations. As for the Schrock cycle, decamethylchromocene acts as reductant. The protonation reactions are considered to be mediated by HBF4/diethyl ether or lutidinium. For all protonation and reduction steps the corresponding free reaction enthalpy changes are calculated. The derived energy profile and corresponding reaction mechanism bear strong similarities to the Schrock cycle. In particular, the most endergonic reaction is the first protonation of the N2 complex and the most exergonic reaction is the cleavage of the N--N bond. If lutidinium is employed as acid and Cp2*Cr as reductant, the reaction course involves steps that are not thermally allowed. For HBF4/diethyl ether as the acid and Cp2*Cr as reducant, however, a catalytic cycle consisting of thermally allowed reactions is principally feasible. This cycle involves a Mo I-fluoro complex as dinitrogen intermediate. It is shown that regeneration to the Mo 0-bis(dinitrogen) complex is thermally not accessible in this system. Moreover, the Mo I fluoro-dinitrogen complex is labile towards disproportionation. The implications of these results with respect to the realization of a catalytic system on the basis of Mo and W phosphine complexes are discussed.     

Origin of diastereocontrol in the oxy-Michael reactions of delta-lactol anions: a computational and experimental study

The diastereoselectivity in the alkylation and Michael addition of "naked" 6-substituted delta-lactolates has been studied by density functional (B3LYP) calculations with ab initio (MP2) energy refinements. The resulting proposed model for the origins of stereocontrol in this reaction has been tested by experiment. The reactions lead to a high cis diastereoselectivity across the THP ring due to the preference for both the alkoxide and the 6-substituent to sit equatorial in the alkylation transition structure. In the oxy-Michael addition of these lactolates to beta-substituted nitroolefins, we propose that the high diastereoselectivity beta- to the nitro group is a result of a combination of steric, stereoelectronic and solvation factors.     

Tetradehydroadamantane-1,3,5,7-di- and tetracations and their helium and hydride inclusion complexes: spherical aromaticity and evidence for a bonding interaction between carbon and helium

The unusual stability of the experimentally known 1,3-dehydro-5,7-adamantyl dication was previously explained by four-center two-electron aromaticity with three-dimensional (tetrahedral) topology. Magnetic criteria (ACID and ring-current analysis) now demonstrate that there is also a very strong contribution from hyperconjugation with all six methylene bridges. The delocalized system of electrons thus includes all valence electrons, and the structure, therefore, should rather be described as a spherically aromatic 50-electron system. The corresponding Td-symmetric tetracation with 48 electrons is antiaromatic and not a minimum structure. With a He atom or a hydride ion at the center of the cage, the tetracation is predicted to form a kinetically stable complex. Magnetic criteria demonstrate that the antiaromaticity is greatly reduced, and a bond analysis hints at bonding interactions between He (and H-) and the carbon atoms of the adamantane cage.     

Photochemical upconversion approach to broad-band visible light generation

The sensitized triplet-triplet annihilation (TTA) of 9,10-dimethylanthracene (DMA) upon selective excitation of [Ru(dmb)3]2+ (dmb = 4,4'-dimethyl-2,2'-bipyridine) at 514.5 nm in dimethylformamide (DMF) resulted in upconverted and downconverted DMA excimer photoluminescence. The triplet excited state of [Ru(dmb)3]2+ is efficiently quenched by 11 mM DMA in DMF resulting in photon upconversion but no excimer formation. The bimolecular quenching constant of the dynamic quenching process is 1.4 x 109 M-1 s-1. At 90 mM DMA, both upconversion and downconversion processes are readily observed in aerated DMF solutions. The TTA process was confirmed by the quadratic dependence of the upconverted and downconverted emission emanating from the entire integrated photoluminescence profile (400-800 nm) of DMA measured with respect to incident light power. Time-resolved emission spectra of [Ru(dmb)3]2+ and 90 mM DMA in both aerated and deaerated DMF clearly illustrates the time-delayed nature of both types of singlet-state emission, which interestingly shows similar decay kinetics on the order of 14 mus. The emission quantum yields (Phi) measured using relative actinometry increased with increasing DMA concentrations, reaching a plateau at 3.0 mM DMA (Phi = 4.0%), while at 90 mM DMA, the overall quantum yield diminished to 0.5%. The dominant process occurring at 3.0 mM DMA is upconversion from the singlet excited state of DMA, whereas at 90 mM DMA, both upconversion and excimeric emission are observed in almost equal portions, thereby resulting in an overall broad-band visible light-emission profile.