A. Oskam

Characterization of chromium oxide supported on Al2O3, ZrO2, TiO2, and SiO2 under dehydrated conditions

Abstract In the present investigation, Raman and IR spectroscopy were used to study the surface structures of chromium oxide supported on alumina, titania, zirconia, and silica, as a function of the loading under dehydrated conditions. It was found, that the dehydrated surface structures of chromium oxide differ strongly from those previously reported under ambient conditions, in which the surfaces are hydrated. Two species, each possessing one short terminal CrO bond, and one (or more) oligomer (s) are proposed to be present on the dehydrated alumina, titania, and zirconia surfaces. The relative concentrations of these different chromium oxide species is independent of the surface coverage. The chromium oxide species present on the dehydrated silica surface are completely different from those observed on the other three supports. The Raman and IR spectra indicate the presence of an isolated chromium oxide species possessing two short CrO bonds together with a small amount of surface species possessing a terminal CrO 3 unit, isolated or not. The gradual disappearance of the surface hydroxyl groups of all four supports upon addition of chromium oxide, as monitored by IR spectroscopy, suggests that the chromium oxide species interacts with the surface by removal of the surface hydroxyl groups.

Bonding properties of Mo(CO)4-xL(PR3)x (x = 0, 1, 2; L =diazabutadiene, pyridine-2-carbaldehyde imine and 2,2′-bipyridine). Infrared, electronic absorption, 1H, 13C and 31P NMR and resonanceraman spectra

Abstract Infrared and electronic absorption, 1 H, 13 C and 31 P NMR and resonance raman spectra of the complexes Mo(CO) 4 L, Mo(CO) 3 L(PR 3 ) and Mo(CO) 2 L(PEt 3 ) 2 (L = 1,4-diazabutadiene, pyridine-2-carbaldehyde imine and 2,2′-bipyridine; R = OCH 3 , C 6 H 5 , C 2 H 5 , C 4 H 9 , c-C 6 H 11 ) are reported. The π-backbonding between Mo and L appears to be much stronger for L = DAB than for the other two ligands. The influences of basicity and bulkiness of the PR 3 group on the bonding properties of the complexes are discussed. Several Mo to L CT bands are observed and assigned. The character of these bands and the properties of the excited states are investigated with the use of the resonance Raman effect. The absorption spectra of some of the complexes show drastic changes in low temperature alkane glasses.

Structural determination of surface rhenium oxide on various oxide supports (Al2O3, ZrO2, TiO2 and SiO2)

In the present investigation Raman and IR spectroscopy were used to determine the structure of Re2O7/Al2O3, Re2O7/ZrO2, Re2O7/TiO2 and Re2O7/SiO2 as a function of loading under ambient and in situ dehydrated conditions. Under ambient conditions, the surface rhenium oxide is hydrated and possesses the structure of the ReO4 ion in aqueous solution, independent of coverage or support type. Under dehydrated conditions, the in situ Raman and IR spectra show that only one surface rhenium oxide species is present on the silica support, whereas two surface rhenium oxide species are present on the alumina, zirconia and titania supports. The concentration ratio of the two species is a function of the coverage, and their structures are similar, possessing three terminal ReO bonds and one bridging ReO-support bond (C3v symmetry). Differences in properties between the surface rhenium oxide species were determined by TPR as a function of coverage and support type. The in situ Raman, IR and TPR measurements suggest that the bridging ReO-support bond strength decreases with increasing coverage, while the TPR data further indicate that the ReO-support bond strength decreases in the order Al2O3 ⪢ SiO2 ≈ ZrO2 ⪢ TiO2 for a given rhenium oxide loading.

The syntheses and coordination properties of M(CO)3X(DAB) (M = Mn, Re; X = Cl, Br, I; DAB = 1,4-diazabutadiene)

Abstract M(CO) 5 X (M = Mn, Re; X = Cl, Br, I) reacts with DAB (1,4-diazabutadiene = R 1 N=C(R 2 )C(R 2 )′=NR′ 1 ) to give M(CO) 3 X(DAB). The 1 H, 13 C NMR and IR spectra indicate that the facial isomer is formed exclusively. A comparison of the 13 C NMR spectra of M(CO) 3 X(DAB) (M = Mn, Re; X = Cl, Br, I; DAB = glyoxalbis-t-butylimine, glyoxyalbisisopropylimine) and the related M(CO) 4 DAB complexes (M = Cr, Mo, W) with Fe(CO) 3 DAB complexes shows that the charge density on the ligands is comparable in both types of d 6 metal complexes but is slightly different in the Fe- d 8 complexes. The effect of the DAB substituents on the carbonyl stretching frequencies is in agreement with the A ′( cis ) > A ″ ( cis ) > A ′( trans ) band ordering. Mn(CO) 3 Cl(t-BuNCHCHNt-Bu) reacts with AgBF 4 under a CO atmosphere yielding [Mn(CO) 4 (t-BuNCHCHN-t-Bu)]BF 4 . The cationic complex is isoelectronic with M(CO) 4 (t-BuNCHCHNt-Bu) (M = Cr, Mo, W).

A study of the electronic properties of M(CO)4DAB (M = Cr, Mo, W; DAB = diazabutadiene). I. electronic `absorption, resonance raman, infrared, 13C- and 15N-NMR spectra

During the reaction of M(CO)5THF with DAB an intermediate M(CO)5DAB was formed, In which DAB is coordinated as a monodentate ligand. The complexes M(CO)4DAB posess structured absorption bands in the visible which were assigned, mainly on the basis of resonance Raman results, to different CT transitions from the metal to DAB. The resonance Raman spectra showed that orbitals of both DAB and the cis carbonyls are involved in the first excited states. A very pronounced enhancement of Raman intensity was found in polar solvents for a polarized band at about 215 cm−1, which could therefore be assigned to the νs(M-N). The character of this mode was investigated by 15N enrichment of DAB. 1H, 13C and 15NMR spectra show a large electron shift from DAB to the metal in polar solvents

Detection and assignment of different electronic transitions within the first CT band of M(CO)4L (M = Cr, Mo, W; L 1,4-diazabutadiene (DAB) and pyridine-2-carbaldehydeirnine (PyCa)) with the use of resonance raman and MCD spectra

Abstract The magnetic circular dichroism (MCD) together with the electronic absorption spectra of Mo(CO) 4)-pyridine-2-carbaldehyde-(N-phenyi -imine revealed that the first Mo to L CT band consists of different electronic transitions which could be asssigned with the help of resonance Raman spectra. The excited states of M(CO) 4-2 L(PR 3 x ) complexes (M = Cr, Mo, W; x= 0, 1; L = 1,4-diazabutadiene and pyridine-2-carbaldehyde-(R 1 )-imine, R 1 = i-prop, phenyl; R - phenyl and n-butyl are investigated with the resonance Ratnan effect and interpreted with the help of electronic absorption spectra and CNDO/S calculations. The resonance Raman spectra gave detailed information about the properties of the excited states of the various complexes.

Raman spectra of chromium oxide species in CrO3/Al2O3 catalysts

Abstract Raman spectra are reported of chromium(VI) oxide species in Cr/Al2O3 catalysts, and of aqueous solutions of chromium oxide used for the preparation of these catalysts. The Raman spectra reveal that after impregnation and drying at room temperature under vacuum, the chromium oxide is present as chromate submonolayer at 1 wt.% Cr/Al2O3, as dichromate between 5 and 15 wt.% Cr/Al2O3, as trichromate at 20 wt.% Cr/Al2O3 and as crystalline CrO3 and trichromate at 30 wt.% Cr/Al2O3. The supported chromium oxide species are in wet condition. The chromate in the 1 wt.% Cr/Al2O3 sample is stable upon calcination in air at 823 K. Preparation of the 1 wt.% Cr/Al2O3 catalyst from a Cr(NO3)3 solution gave the same result. It is shown that the molecular structure of the oxides is influenced by the chromium oxide concentration of the impregnation solutions, by the concentration in the catalysts and by the water content of the catalysts, which was varied by exposure to air and by using different laser powers.

He(I) and He(II) photoelectron spectra of some substituted ethylenes

Abstract The He(I) and He(II) photoelectron spectra of acrolein, acrylic acid, methyl acrylate, vinyl acetate, acrylamide and some of their methyl-substituted analogues are reported. Detailed assignments are proposed, mainly based on differences in intensity between the He(I) and He(II) spectra, on sum rule considerations and on the results of modified CNDO/S calculations. The assignment criteria are critically evaluated.

Photochemistry of metal—metal bonded complexes: III. MLCT photolysis of (CO)5MM′(CO)3(α-diimine) (M, M′ = Mn, Re) in 2-Me-THF and THF at 293 K; Evidence of photocatalytic formation of [(Mn(CO)3(α-diimine)(P(n-Bu)3)]+ [M(CO)5] - upon photolysis in the presence of P(n-Bu)3

Abstract Complexes op the type (CO) 5 MM′(CO) 3 (α-diimme) (M, M′ = Mn, Re) were photolyzed in 2-Me-THF at 293 K in the absence and presence of PR 3 by irradiation in the low energy M′ → α-diimine charge transfer band. In most cases the primary photoprocess appeared to be homolysis of the (M-M′) metal—metal bond. In the absence of PR 3 the radicals formed react with each other to give M 2 (CO) 10 and M′ 2 (CO) 6 (α-diimine) 2 . It is shown that the stabilities of these M′ 2 (CO) 6 (α-diimine) 2 complexes depend on the α-diimine ligand used. In the presence of PR 3 various reactions are observed, depending on the complex and the PR 3 ligand used. Thus, photolysis of (CO) 5 MnRe(CO) 3 (α-diimine) in the presence of PPh3 or P(n-Bu) 3 gives Mn 2 (CO) 10 and a reaction product of Re 2 (CO) 6 (α-diimine) 2 . However, (CO) 5 MnMn(CO) 3 (α-diimine) in the presence of PPh 3 gives Mn 2 (CO) 8 (PPh 3 ) 2 instead of Mn 2 (CO) 10 . A remarkable reaction is observed when a (CO) 5 MnMn(CO) 3 (α-diimine) complex is photolyzed in the presence of the basic phosphine P(n-Bu) 3 the ionic compound [Mn(CO) 3 (α-diimine)(P(n-Bu) 3 )] + [Mn(CO) 5 ] − being formed as the result of electron transfer from the radical Mn(CO) 3 (α-diimine)(P(n-Bu) 3 ) to Mn(CO) 5 and (CO) 5 MnMn(CO) 3 (α-diimine). This reaction is a photocatalyzed chain reaction with φ ∼ 10. All these reactions involve initial homolysis of the metal—metal bond. Only in the case of (CO) 5 ReMn(CO) 3 (R-DAB) in the presence of PR 3 was a carbonyl group of the Mn(CO) 3 (R-DAB) fragment photosubstituted. The mechanisms of the reactions are discussed.

Photoelectron spectra and Xα calculations of iron pentacarbonyl and ethyleneiron tetracarbonyl

Abstract The He 584 A photoelectron spectra of iron pentacarbonyl and ethyleneiron tetracarbonyl are presented. The ionization potentials are assigned using the results of discrete variational Xα calculations. The electronic structure of coordinated ethylene is discussed.