Eleonora Polo

Photochemistry of iron-porphyrin complexes. Biomimetics and catalysis

Abstract An intramolecular electron transfer can occur in iron porphyrin complexes that are irradiated with light of suitable wavelength corresponding to axial ligand-to-metal charge transfer transitions. This process leads to the reduction of Fe(III) to Fe(II) and to the oxidation of the axial ligand to a radical species. The efficiency of the photoredox process is increased in the presence of species able of trapping the ferrous complex and/or the radical in competition with the back electron transfer process. Photoredox reactions of iron porphyrins and heme-proteins (cytochrome c) are examined in the framework of the fundamental role of electron transfer processes in biological systems. The products of the primary photoprocess can induce reactions leading to reduction or oxidation of various substrates with catalytic efficiency. This biomimetic aspect of the photoredox behavior of iron porphyrins is pointed out examining the results obtained in investigations concerning the iron-porphyrin based photoreduction of CCl4 in homogeneous solution, and photooxygenation of alkanes in both homogeneous and heteterogeneous systems.

Crystal structure and spectroscopic analysis of melamine hydrobromide. Evidence for iso-melamine cations and charge-transfer complexes in the solid state

The crystal structure and vibrational analyses of melamine hydrobromide by FTIR and FT-Raman spectra, are presented. The crystallographic data show π-electron delocalization towards the amino substituents with ring nitrogen protonation in the solid state. Additionally, the presence of intermolecular hydrogen bonding interactions occurring between amino substituents and ring nitrogen lone-pairs gives rise to charge–transfer complexes, as demonstrated by UV–VIS reflectance in the solid state. FTIR and FT-Raman spectra allow the assignments of the vibrational modes in melamine hydrobromide by comparison with the corresponding deuteriated molecules, and show the existence of iso-melamine cations in the solid state.

Synthesis of ring-substituted bis-η5-cyclopentadienyl derivatives of the Group IV elements containing the bicyclic ligands η5-C5H3(1,2-CH2–)n, n=4, 5, or 6

Abstract The synthesis of unsubstituted and substituted bicyclic η 5 -cyclopentadienyl ligands and their Group IV metal complexes [M{ η 5 -C 5 H 3 (1,2-CH 2 –) n } 2 Cl 2 ], where n =4, 5, 6 and M=Ti, Zr, Hf, is reported. An example of an ansa -bridged zirconium analogue is also described.

Spin trapping and some reactions of ruthenium-centred radicals

The generation of ruthenium-centred radicals by photolysis of dimeric ruthenium complex [Ru(CO)2Cp]2 (Cp=η5-C5H5) has been studied by EPR and spin trapping. Mechanistic studies reveal that in toluene homolytic cleavage of both the RuRu and RuCO bonds takes place yielding ·Ru(CO)2Cp radicals and an intermediate dinuclear ruthenium complex which is CO-bridged. This reacts with P[OCH(CH3)2]3 yielding the monosubstituted dinuclear compound {Cp2Ru2(CO)3P[COH(CH3)2]3}. The ruthenium-centred radicals react with dioxygen to form peroxyl radicals which may abstract hydrogen from CH bonds. The reactivity of the photogenerated ruthenium species with various substrates is reported.

Crystal structure and spectroscopic analyses of guanylurea hydrochloride. Evidence of the influence of hydrogen bonding on the π-electron delocalization

The crystal structure, and a complete vibrational analysis by FTIR and Raman spectra, of guanylurea hydrochloride are presented. The crystallographic results show a pronounced π-electron delocalization on the cation and the presence of strong intra- and inter-molecular hydrogen bonding interactions between amino groups belonging to the guanidine moiety and the carbonyl oxygen of the ureic group. This arrangement gives rise to a polymer-like structure, in which the guanylurea cation chains are laterally hydrogen bonded by Cl– anions and water molecules. The UV–VlS reflectance spectra are in agreement with the hypothesis of the existence of hydrogen bonding charge transfer complexes in the solid state. Other spectroscopic techniques, such as FTIR and Raman, have been employed to support the crystal data, and to investigate the vibrational assignments for this compound.

Unbridged bicyclic cyclopentadienyl zirconocene complexes: Their possible application as fluxional catalysts in propene polymerization

Three series of novel unbridged bicyclic cyclopentadienyl zirconium complexes, containing six-, seven- and eight-membered saturated rings fused to the cyclopentadienyl unit and bearing different substituents (-H, -CH 3 , -Ph) in position 2, were used in combination with methylalyminoxane (MAO) for the polymerization of propene at different temperatures. All the catalysts have been shown to be active, the lowest activity being observed with the eight-membered ring systems. With most catalysts the polymers obtained are fully atactic when the polymerizations are conducted at 30°C, and become partially isotactic at lower temperatures. The 13 CNMR analysis shows that in one case, that is, when the seven-membered ring is associated with the phenyl substituent in position 2, a preponderance of the isotactic heptad in the methyl region of the 13 C NMR spectrum is observed, which allows us to hypothesize the formation of stereoblocks.

Ethene/propene copolymerisations with rac-EBTHIZrR2/alumoxane: σ-ligands effect

Abstract We report here about the role played by the σ-ligands of a metallocene in ethene/propene (E/P) copolymerisations promoted by rac -[ethylenebis(4,5,6,7-tetrahydro-1-indenyl)]zirconium dichloride 1 , and its corresponding dimethyl derivative rac -EBTHIZrMe 2 2 and binaphtholate rac -EBTHIZr(BNP) 3 derivative, in the presence of several alumoxanes. Alumoxanes alternative to the traditional polymethylalumoxane (MAO) were synthesised and characterised. They are based on Al(2,4,4-trimethylpentyl) 3 (TIOA) 4 and were prepared by reaction of 4 with water to give either tetraisooctylalumoxane (TIOAO) 5 or polyisooctylalumoxane (TAO) 6 depending on the stoichiometry of the reaction. 1 H NMR spectra of these products are characterised by the presence of broad bands and of resolved multiplets. Their relative amount in the region between 1.9 and 2.5 ppm was identified as the “fingerprint” of the polymerisation activity. A comparison of the results obtained in polymerisation with metallocenes having different σ-ligands allowed us to shed more light on the role played by those species. In conclusion, the nature of the σ-ligands of a metallocene affects its catalytic activity in polymerisation, as a function of the alumoxane employed and of the relative ratio with the metallocene, and this effect is enhanced in “critical conditions,” i.e. with a weakly activating cocatalyst or at a low Al/Zr ratio.

Photooxidation of hydrocarbons on porphyrin-modified titanium dioxide powders

A composite catalyst consisting of an iron porphyrin covalently linked to TiO2 shows a new reactivity in the photochemical mono-oxygenation of hydrocarbons under mild conditions, with respect to the porphyrin and TiO2 used separately.

Oxidation of alkanes by dioxygen catalysed by photoactivated iron porphyrins

Cycloalkanes were oxidized by O2 itself under mild conditions (22 °C; 200 Torr of O2) in the presence of catalytic amounts of a polyhalogenated porphyrin–iron(III)—hydroxo complex irradiated with light of wavelength between 350 and 450 nm; these oxidations occurred without consumption of a reducing agent, selectively transformed cyclohexane into cyclohexanone under appropriate conditions (about 0. 2 turnover per min), and did not involve FeVO active species but, more probably, iron-alkylperoxo intermediates.

From zirconium to titanium: the effect of the metal in t-butylacrylate photoinitiated polymerisation

We report here the synthesis, photochemistry and photoinitiator activity of some titanocenes (1–4) and compare the results with those obtained for the corresponding zirconocenes (5–8). Analysis of the electronic spectra showed that the energy modulation of the lowest electronic transition, which appears to be LMCT in character, is driven both by the substituent on the cyclopentadienyl moiety and by the metal centre. Furthermore, the excited state resulting from irradiation of the complexes at the wavelength of the LMCT transition undergoes ligand-metal bond dissociation with formation of a radical pair, as evidenced by EPR spectroscopy coupled with spin trapping techniques. All the complexes were very active, compared with known organometallic photoinitiators, for the free-radical photopolymerisation of t-butylacrylate. The titanium complexes 1–4, which can be used with visible light, were more active than the zirconium derivatives 5–8. The better yields in photopolymerisation can be interpreted on the basis of the combined effect of two factors: (i) the photoreactivity of the complexes in solution and (ii) the high persistence of the Ti(III) radical species in solution, which guarantees high concentrations of initiators in the polymerisation process.