Boron-complexation strategy for use with 1-acyldipyrromethanes

1-Acyldipyrromethanes are important precursors in rational syntheses of diverse porphyrinic compounds. 1-Acyldipyrromethanes are difficult to purify, typically streaking upon chromatography and giving amorphous powders upon attempted crystallization. A solution to this problem has been achieved by reacting the 1-acyldipyrromethane with a dialkylboron triflate (e.g., Bu2B-OTf or 9-BBN-OTf) to give the corresponding B,B-dialkyl-B-(1-acyldipyrromethane)boron(III) complex. The reaction is selective for a 1-acyldipyrromethane in the presence of a dipyrromethane. The 1-acyldipyrromethane-boron complexes are stable to routine handling, are soluble in common organic solvents, are hydrophobic, crystallize readily, and chromatograph without streaking. The 1-acyldipyrromethane can be liberated in high yield from the boron complex upon treatment with 1-pentanol. Alternatively, the 1-acyldipyrromethane-boron complex can be used in the formation of a trans-A2B2-porphyrin. In summary, the boron-complexation strategy has broad scope and greatly facilitates the isolation of 1-acyldipyrromethanes.     

Copper(I)-catalyzed disilylation of alkylidene malonates employing a Lewis base activation strategy

The disilylation of activated alpha,alpha-unsaturated esters has been accomplished by utilizing symmetrical disilanes and catalytic quantities of copper(I) salts in the presence of Lewis bases such as DMF. This new methodology with various alkylidene malonates is moderate to high yielding and affords alpha-silyl diesters, compounds that previously have not been readily accessible, that can be further manipulated to cleanly afford useful alpha-substituted alpha-hydroxy esters.     

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and M?ssbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the M?ssbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the M?ssbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.     

Orbital freezing and orbital glass state in FeCr2S4

Low-temperature specific heat measurements and dielectric spectroscopy have been performed on polycrystalline and single-crystalline FeCr2S4, the single crystals showing a transition into a low-temperature orbital glass phase. The freezing of the orbital moments is revealed by a glasslike specific heat anomaly and by a clear relaxational behavior of the dielectric permittivity, exhibiting several hallmark features of glassy dynamics. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunneling.     

Molecular structure of an Fe(IV) species:

The molecular structure of the formal iron(IV) porphyrinate derivative, [[Fe(TTP)]2N]SbCl6 (TTP = tetratolylporphyrinate), is reported. The structural parameters are compared to the previously reported species [Fe(TPP)]2N, in which the iron oxidation state is +3.5. Both the equatorial and axial bond distances in [[Fe(TTP)]2N]SbCl6 are slightly shortened and consistent with an increased formal charge on iron. The value for the axial Fe-N distance is 1.6280(7) A, and the average value of the equatorial Fe-Np distances is 1.979(5) A. The M?ssbauer isomer shift decreases upon oxidation, again consistent with an increase in formal charge. Values for the isomer shift at room temperature are -0.13 mm/s for [[Fe(TTP)]2N]SbCl6 and 0.04 mm/s for [Fe(TTP)]2N. Crystal data for [[Fe(TTP)]2N]SbCl6 are as follows: orthorhombic, space group Fddd, Z = 8, a = 23.689(2) A, b = 31.056(3) A, c = 22.7788(18) A.     

Stochastic eigenvalue problems for differential equations

We consider an eigenvalue problem for stochastic ordinary differential operators with stochastic boundary conditions. By replacing the random variables and stochastic processes by their mean values one gets the so-called averaged problem corresponding to the given stochastic problem. Let λ_i(ω) denote the stochastic eigenvalues and μ_i the eigenvalues of the averaged problem. In this paper the differences 〈λ〉?μ_i are calculated. The motive of such investigations was the effect of “bowing” in mixed crystal theory.     

A VUV isochromat spectrometer for surface analysis

A vacuum ultraviolet isochromat spectrometer employing an energy selective Geiger-Müller photon counter is described. The mean energy of the spectrometer is 9.7 eV with a standard deviation of ±0.23 eV. Together with the thermal energy distribution of the electrons emitted from a directly heated tungsten cathode, an overall resolution of ±0.33 eV is obtained. Pulsed operation of the x-ray tube avoids additional broadening due to the voltage drop across the filament. Further, pulsed operation of the accelerating voltage is employed to avoid counting losses due to the relatively large dead time of the radiation detector. Operating frequencies of up to 2 kHz are possible. With sensitivities of typically 10⁷ pulses per Coulomb, the anode currents are in the range of 100–200 μA. These attractive properties suggest to employ the method to study surface electronic properties of solids.