1,720,974 research outputs found
ESR investigation of paramagnetic carbonyl-metal clusters of high nuclearity
No full textAn ESR study has been made of the high nuclearity paramagnetic metal cluster anions [Rh12(CO)13(μ2-CO)10(C)2]3-, [Co13(CO)12(μ2-CO)12(C)2]4- and [Co6(CO)8(μ2-CO)6C]-. The assignment of the HOMO is based on a mixed valence model which relates the g tensor components of cluster systems to those of an appropriate conventional paramagnetic center. With this model the HOMOs of [Rh12(CO)13(μ2-CO)10(C)2]3- and of [Co13(CO)12(μ2-CO)12(C)2]4- are found to be mainly comprised of metal dz2 atomic orbitals, while for [Co6(CO)8(μ2-CO)6C]- a large overlap between d atomic orbitals and ligand orbitals is suggested. The occupation of the valence molecular orbitals deduced from the ESR data is consistent with the variations in MM bond distance observed by X-ray analysis. © 1982
AN ELECTRON-SPIN-RESONANCE EVIDENCE OF ALUMINA SURFACE REDUCTION IN RU/GAMMA-AL2O3 CATALYSTS
No full textELECTRON-SPIN RESONANCE INVESTIGATION OF ZRO2-SUPPORTED RUTHENIUM - EVIDENCE OF STRONG METAL-SUPPORT INTERACTION
No full textAn e.s.r investigation of Ru/ZrO2 has shown that Ru centres are present in different oxidation states, two of which are paramagnetic: RuIII and RuI. The assignment of the electronic configuration follows from the results of the interaction of Ru/ZrO2 with CO, for the identification of RuIII and RuI, and with NO for the identification of RuII. Magnetic tensor components allow recognition of an RuIII-CO adduct (g∥ = 2.039, g⊥ = 2.003) and an RuI-CO adduct (g = 2.05) An RuII-NO species was also identified [g⊥ = 2.00, g∥ = 1.91, A⊥ (N) = 40 G]. The results of the interaction with O2 also have been discussed in order to give a fuller description of the electronic configuration of the surface ruthenium centres and of the interaction effects between metal and support centres. It appears that at the Ru-ZrO2 interface occurs an interaction stronger than in the Ru-γ-Al2O3 case, the strength depending also on the temperature of the pyrolysis used to obtain Ru/ZrO2 from the Ru3(CO)12/ZrO2 metal precursor. Also the analogous Rh/ZrO2 system was considered with respect to interaction with NO in order to compare the variation in the metal-support interaction owing to differences in the supported metal
INTERACTION OF CARBON-MONOXIDE WITH RU-GAMMA-AL2O3 - AN ELECTRON-SPIN-RESONANCE INVESTIGATION
No full textInteraction of carbon monoxide with Ru/ γ-Al2O3 leads to the formation of RuI and RuIII carbonyl derivatives. The relative amounts of the two species are a function of the temperature used in decarbonylating the Ru3(CO)12/ γ-Al2O3 precursor. The thermal and vacuum stabilities of the RuI derivative [g⊥ = 2.050, g∥ = 1.992, A∥(99Ru, 101Ru) = 28 G] and the RuIII derivative (g∥ = 2.046, g⊥ = 2.005) are different, the RuIII species being the more stable. The Ru-CO bond strength of these carbonyls is high in comparison with that of the analogous Rh/ γ-Al2O3 carbonyl derivatives. The enhanced bond strength may account for the difference in the catalytic behaviour between the Ru and Rh systems
ELECTRON-SPIN-RESONANCE INVESTIGATIONS OF RUTHENIUM SUPPORTED ON GAMMA-ALUMINA
No full textWe report an electron spin resonance investigation performed on alumina-supported ruthenium, Ru/γ-Al2O3, obtained by decarbonylation (pyrolysis or H2 reduction) of Ru3(CO)12/γ-Al2O3. The spectra observed after Ru/γ-Al2O3 was in contact with O2, CO, NO 'probe' molecules allow one to distinguish superoxide, carbonyl and nitrosyl paramagnetic derivatives of ruthenium, and show that ruthenium centers in formal oxidation states other than the zero metal state become stable by interaction with γ-Al2O3. The strength of the interaction between paramagnetic ruthenium centres and π*-acceptor molecules (O2, CO and NO) is dependent both on the π*-acceptor molecules and on the decarbonylation method
Paramagnetic metal and oxygen species observed with Rh/γ-Al2O3 and Rh/ZrO2.Dependence on the decarbonylation temperature of [Rh4(CO)12] on alumina and zirconia supports
No full textParamagnetic metal and oxygen species observed when alumina-(or zirconia-supported rhodium, Rh/γ-Al2O3(ZrO2), is obtained by the decarbonylation of [Rh4(CO)12]-Al2O3(ZrO2) are discussed as a function of the decarbonylation temperature (250–600 °C) and the method of decarbonylation (pyrolysis in vacuo or reduction in an H2 stream). On vacuum-pyrolysed samples, RhP(T)/γ-Al2O3, RhIIformal centres are stable only after decarbonylation at 250 °C; contact with O2 produces [RhIIIn–O2] ̇–(n= 1,2) and Al3+—O2–, with an increasing amount of Al3+—O–2 as the decarbonylation temperature increases. RhP(T)/ZrO2 does not show paramagnetic species before contact with O2: the treatment with O2 stabilizes both [Rh2III—O2] ̇– and Zr4+—O–2centres at a decarbonylation temperature of 250 °C, while essentially Zr4+—O2– alone is formed at higher temperature. On H2 reduced samples, RhH(T)/γ-Al2O3, formal RhO centres formed in the decarbonylation temperature range 250–500 °C, while no paramagnetic species were observed on RhH(T)/ZrO2. Contact with O2 shows behaviour of RhH(T)/γ-Al2O3(ZrO2) very similar to that of RhP(T)/γ-Al2O3(ZrO2). The stability of the Rh—O2 bond is a function of the metal positive charge and it is related to the decarbonylation temperature. The strength of the interaction between O2–and the support acidic centres, Al3+ and Zr4+, is always high inγ-Al2O3-supported samples, and dramatically increases in ZrO2-supported samples on increasing the decarbonylation temperature. Different pathways of electron transfer can explain the observed behaviour of Rh/ZrO2
Coordination polymers based on Rh-Ag carbide clusters
No full textCoordination polymers are nowadays intensively studied, for their potential application in nanoelectronics, and catalysis. They are typically composed by stereochemically rigid metallic ions (the linkers) and bifunctional ligands (the spacers). Organometallic linkers are very uncommon, probably because the soft transition metals in their low oxidation state do not bind stabily with the hard N- or O- donors. Howevwer, several organometallic clusters are known, which contain metal ions of the 11-12 groups. One example of this sort is [Rh6C(CO)15]2-, which has two nucleophilicic triangular faces which can be capped by [M-L] electrophilic groups (such as Ag-PPh3, Au-PPh3, Cu-NCMe). When both faces are capped, the neutral molecule [{L-M}2Rh6C(CO)15] are formed, which are stable and can be isolated in the solid state, and characterized by X-ray analysis. Moreover, it has been shown that bwo cluster unit can bind a single metal ion, as in the structurally caracterized [{Rh6C(CO)15}2Ag]-. Therefore, we tested the possibility to construct linear polymers, alternating cluster units, metal ions and, possibly, organic bifunctional spacers. As a matter of facts, the reaction between [Rh6C(CO)15]2- and [Cu(NCMe)4]PF6, or AgBF4 (in the presence of CH3CN) affords the known [{CH3CNM}2Rh6C(CO)15] (M = Cu, Ag), which were not isolated, but allowed to diffuse into a diluted solution of bifunctional heteroaromatic ligands (such as pyrazine or 4,4’ bipyridyl), obtaining cluster-containing insoluble materials. In the case of pyrazine, we also obtain single crystals suitable for X-ray analysis, and determined the solid state structure of the organometallic polymer of formula [N2C4H4-(C4H8O)Ag-Rh6C(CO)15-Ag(C4H8O)].
Suprisingly, the polymer is not linear, because the silver ions are trigonal planar, with the third coordination site occupied by a THF molecule. As an extension of this work, we aim to obtain analogous species with other metal ions, other spacers, or without solvent molecules
Highly dispersed Rh on -Al2O3, ZrO2, TiO2: morphological and electronic characterization
No full text8th and 9th group mixed metal clusters substituted with alkynes and their reactivity towards acids
No full textAlkynes have proved to be very versatile ligands for carbonyl clusters since they
can donate from 2 to 6 electrons to up to 4 metal atoms, giving rise to various
coordination modes. The existing literature concerns mainly the tetranuclear homoand
heterometallic clusters which show structural flexibility and can contain almost
all the elements of the 6th, 7th, 8th, 9th and 10th groups of the periodic table. We
decided to study the reactivity of tetrahedral mixed metal clusters (8th-9th group)
with disubstituted mono- and di-alkynes.
[Fe2Ir2(CO)12]2- reacts with diphenylacetylene in refluxing CH3CN loosing 2 COs
and giving [Fe2Ir2(CO)10(PhCCPh)]2- whose structure consists of a metal butterfly
arrangement surmounted by the alkyne; the C-C bond is parallel to the Ir-Ir hinge.
The dropwise addition of H2SO4 to a dark red CH2Cl2 solution of this compound
leads to the neutral species [FeIr2(CO)9(PhCCPh)], that can be crystallized from
hexane at low temperature.
X-ray analysis showed that the formal loss of a Fe(CO) fragment leaves the μ3-η2
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alkyne’s coordination mode unchanged; considering the two carbon atoms as a part
of the cage, the new cluster can be described as a pseudo square pyramid.
[CoRu3(CO)11(PhCCCCPh)]- is synthesized heating a CH3CN solution of
[CoRu3(CO)13]- and 1,4-diphenylbutadiyne at 80°C for 4h; only one of the C-C
triple bonds coordinates in the dihedral angle of the metallic butterfly, parallel to
the Ru-Ru hinge. Again, the titration of a CH2Cl2 solution of the
cluster with H2SO4 promotes the limited demolition of the
molecule, but X-ray diffraction revealed also the partial
hydrogenation of the former di-alkynic chain: the persisting μ3-
coordination of the C1=C2 bond is enforced by a Ru2-C3 σ
bond, whereas the PhC4H group doesn’t bind to the metal
triangle. So the cluster’s molecular formula is [CoRu2(CO)9(PhCCCC(H)Ph)].
Cyclovoltammetric studies will ascertain if the electrochemical oxidation products
are the same obtained in this chemical way
Paramagnetic metal and oxygen species observed with [CO4(CO)12] and [Rh4(CO)12] carbonyl clusters pyrolysed on γ-alumina and zirconia supports
No full textThe pyrolysis of γ-Al2O3- and ZrO2-supported [Co4(CO)12] and [Rh4(CO)12] gives highly dispersed metal systems whose electronic properties have been studied by e.s.r. spectroscopy. Paramagnetic formal RhII species (g∥ = 2.21, g⊥ = 2.11) were observed on Rh/γ-Al2O3 samples. The pattern of the hyperfine structure suggests that metal-metal interactions are present and that paramagnetic transition-metal carbonyl clusters can be assumed as model compounds. No paramagnetic species involving the metal were observed for either γ-Al2O3- or ZrO2-supported cobalt samples. This behaviour is consistent with a higher acidic character of γ-Al2O3 with respect to ZrO2. On contact with O2 and CO the following paramagnetic species were observed: O2- - Al3+ (g∥ = 2.034, g⊥ = 2.00) over Co/γ-Al2O3 samples, O2- - RhIII (g1 = 2.09, g2 = 2.03, g3 = 2.00) over Rh/γ-Al2O3 samples, O2- - RhIII and O2--Zr4+ (g⊥ = 2.03, g⊥ = 2.00) over Rh/ZrO2 samples and CO - RhII (ḡ = 2.034) over Rh/γ-Al2O3 samples. The O2 and CO bond strengths are higher for the γ-Al2O3-supported samples than for the ZrO2-supported samples and are tentatively related to the catalytic activity of these metal systems
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