Raman microscopy has been applied to study the preparation of shaped Mo/Al2O3 catalysts. The speciation of different Mo complexes over γ-Al2O3 support bodies was followed in time after pore volume impregnation with aqueous solutions containing different Mo complexes. The addition of NO3 -to the impregnation solutions allows for a quantitative Raman analysis of the distribution of different complexes over the catalyst bodies as this ion can be used as an internal standard. After impregnation with an acidic ammonium heptamolybdate (AHM) solution, the strong interaction between Mo7O24 6-and Al2O3 results in slow transport of this complex through the support and extensive formation of Al(OH)6Mo6O18 3-near the outer surface of the support bodies. This may be prevented by decreasing the interaction between Mo and Al2O3. In this way, transport is facilitated and a homogeneous distribution of Mo is obtained on a reasonable time scale. A decrease in interaction between Mo and Al2O3 can be achieved by using alkaline impregnation solutions or by the addition of complexing agents, such as citrate and phosphate, to the impregnation solution. In general, time-resolved in situ Raman microscopy can be a valuable tool to study the physicochemical processes during the preparation of supported catalysts.

Complexes of the type Re(X)(CO) 3 (R-diimine) (X ) Cl, Br, I; R-diimine ) bpy, iPr-PyCa, iPr-DAB) exhibit a significant influence of X on the energies and intensities of their lowest-energy electronic transitions. Resonance Raman experiments revealed a change in character of the lowest energy transitions of these complexes from Re f R-diimine (MLCT) to X f R-diimine (XLCT) upon going from Cl to Br. This halide influence can be explained by different extents of mixing of the d π (Re) and p π (X) orbitals. All complexes under study are emissive at 80 K in a glass; the bpy complexes are also emissive at room temperature in fluid solution. The emission from the XLCT excited state is characterized by a longer lifetime, due to smaller k nr and k r values, than the MLCT emission. Nanosecond time-resolved absorption spectroscopy of Re(X)(CO) 3 (bpy) revealed that the halide determines the excited state character of the complexes also in fluid solution. The transient absorption maximum shifts to lower energy going from Cl to Br to I, and the excited state lifetime increases from 50 to 57 to 79 ns, respectively. Variation of the R-diimine has less influence on the properties of the XLCT state than on those of the MLCT state.

MLCT excitation of the complexes [Re(R)(CO),(a-diimine)] (R = Me, Et, benzyl (Bz); a-diimine = iPr-PyCa, R-DAB) results in the homolysis of the Re-R bond leading to the formation of radicals R and [Re(CO),(a-diimine)]' as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low and depends on the temperature and excitation wavelength, whereas for R = Et and Bz the quantum yield is near unity and independent of T and A,,,. The reaction is shown to proceed via a o(Re-R)x* excited state that is rapidly (< 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time-resolved IR and UV/Vis absorption spectra studied in the ns-ps and ps-ps time domains, respectively, show that the ox* excited state is rather long-lived (z x 250 ns) in noncoordinating solvents; the dissociation of the Re-R bond from this state is strongly accelerated by polar or coordinating solvents (rent < 20 ps). The ox* excited state is spectroscopically characterized by a (presumably ox* -+ MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground-state values. The relative energies of the MLCT and reactive cm* states, controlled by the nature of the alkyl ligand, determine the photoreactivity of the complexes.

No abstract

Electrochemical and IR/UV-Vis Spectroelectrochemical Studies of fax-[MnX)(CO)3(iPr-DABGeneral rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. The title complexes [Mn(X)(CO) 3 (iPr-DAB)] n (n ) 0, X ) Br; n ) +1, X ) donor solvent) undergo a two-electron reduction according to an ECE sequence. The chemical step (C) involves prompt dissociation of the X ligand from the primary one-electron reduction product, followed by instantaneous one-electron reduction of the five-coordinate transient [Mn(CO) 3 -(iPr-DAB)] • producing the anion [Mn(CO) 3 (iPr-DAB)] -. The latter complex remains rather stable at T < 190 K, whereas at higher temperatures it undergoes an electron-transfer reaction with the parent complexes producing the dimer [Mn(CO) 3 (iPr-DAB)] 2 (the second C step in the overall ECEC sequence). The rate of this reaction decreases in the order THF • , is probably only a minor alternative route. In the presence of excess P(OMe) 3 , the principal oxidation product is the cation [Mn{P(OMe) 3 }(CO) 3 (iPr-DAB)] + . The fivecoordinate anions [Mn(CO) 3 (R-diimine)] -can be regarded as strongly π-delocalized complexes with the negative charge equally distributed over the R-diimine and CO ligands. The intriguing mechanism of their photochemical formation from fac-[Mn(Br)(CO) 3 (R-diimine)] at low temperatures has been rectified on the basis of this (spectro)electrochemical study. Download date: 13 May 2018 Electrochemical and IR/UV-Vis Spectroelectrochemical Studies of fac-[Mn(X)(CO)

No abstract

No abstract

No abstract