Orazio Traverso

Ligand and medium controlled photochemistry of iron and ruthenium mixed-ligand complexes: prospecting for versatile systems

Abstract Selected Fe and Ru systems, whose photochemical behaviour is sensitive to numerous parameters, are presented. These systems, containing multiple species in equilibrium, are versatile enough to be adapted to special tasks and may also be used to model the phenomena and mechanisms occurring in nature. The role of various parameters is analysed and principal emphasis is given to the ligand sphere influence on the nature of the excited state and thereby on the photochemical mode. This is crucial in the case of Fe(II) complexes of the type [Fe(CN) 5 L] n − , whereas in the carbolyl–cyclopentadienyl complexes, represented by [cpRu(CO) 2 ] 2 , the nature of the excited state is of less importance than for pentacyanoferrates(II). The photochemistry of the carbonyl–cyclopentadienyl complexes is more susceptible to the impact of the medium and the role of the secondary processes is more significant.

Indenyl-amido titanium and zirconium dimethyl complexes: improved synthesis and use in propylene polymerization

Abstract The synthesis of a series of indenyl amido titanium dimethyl complexes, by means of the direct synthesis from the ligand, a 2-fold excess of MeLi, and TiCl 4 is reported. The 1 H NMR spectra of the complexes show a quartet structure for the metal-bound methyl groups, due to through-metal proton–proton coupling. Coupling of Ti-methyl protons with protons on the Cp ring is also revealed by COSY 2D-NMR. The performance of the Ti complexes in propylene polymerization, including [Me 2 Si(Me 4 C 5 )( t -BuN)]TiMe 2 ( 1-TiMe 2 ), [Me 2 Si(Ind)( t -BuN)]TiMe 2 ( 2-TiMe 2 ) and six other methyl titanium complexes bearing substituted indenyl ligands, has been investigated with different cocatalysts and at different polymerization temperatures and propylene concentrations. All complexes produce amorphous polypropylene ( am -PP). The catalytic activity and molecular weight strongly depend on the substitution of the Cp ring: 2-TiMe 2 gives polymers of lower molecular weight, while the presence of a methyl group in position 2 (as in 3-TiMe 2 ) determines up to 4-fold increase in molecular weight. The type of cocatalyst influences mainly the catalytic activity, the borates being better activators than MAO, but also molecular weight, with again the borates giving higher molecular weights than MAO. 5-TiMe 2 –Ph 3 CB(C 6 F 5 ) 4 shows an overall activation energy of polymerization of 7.35 kcal mol −1 . The rate of chain release is first order in monomer. The following activation energies for overall chain release have been calculated: ΔΔ E ‡ 2-TiMe 2 =3.4 kcal mol −1 , ΔΔE ‡ 5-TiMe 2 =3.8 kcal mol −1 , ΔΔ E ‡ 3-TiMe 2 =6.3 kcal mol −1 . Even if all the polymers produced are amorphous, 2-TiMe 2 and 5-TiMe 2 show a microstructure unbalanced towards isotacticity, while 3-TiMe 2 , 6-TiMe 2 and 8-TiMe 2 are syndiotactic-enriched. Chiral induction comes mainly from a weak enantiomorphic site control.

Photochemistry of ethylene-bis(triphenylphosphine)platinum

Abstract When [(Ph3P)2Pt(C2H4)] was irradiated in several solvents and at various wavelengths, the results differ with the conditions used. In particular irradiation at 254 nm gives a new alkylplatinum(II) complex by an insertion reaction.

The role of the excited states in the photochemical behavior of ruthenocene in halocarbon solvents

Abstract Halocarbon solvents appears to form charge-transfer complexes with ruthenocene. Ruthenocene in halo-carbon solvents exibits a new band of electron transfer to solvent type at 285 nm. Irradiation by light, which excites the complexes and not the components, caused the oxidation of ruthenocene to ruthenicenium cation. A possible mechanism of the thermal decomposition of the ruthenicenium cation is discussed.

Photoreduction of chlorohemin in pure pyridine

Abstract Chlorohemin (Fe(III)PPCl) undergoes photoreduction when irradiated in pure pyridine solution with 400–450 nm light. A thermal reduction is observed to occur simultaneously with the photochemical one, but after a one hour irradiation about 75% of the reduction product is formed in a photochemical way. Both five and six-coordinated species are observed to be present in solution; however, only the Fe(III)PPpy+ five coordinated complex is photoreducible. A mechanism is proposed whereby the primary photochemical act is an axial pyridine → iron electron transfer process yielding Fe(II)PP and py+ species. The Fe(II)PP moiety gives rise to the formation of the spectrophotometrically detectable Fe(II)(PP(py)2 complex. ESR spin trapping results are consistent with the formation of 2-pyridyl radicals from py+ cation by fast transfer of a proton to a pyridine molecule.

Thermodynamic properties of actinide complexes. Part. III. Uranyl(VI)-glycolate system

Abstract The changes in free energy, enthalpy and entropy for the formation of uranyl(VI)-glycolate complexes, have been determined at 25 °C and in aqueous perchlorate medium 1.00 M. All three complexes formed, are found to be stabilized by entropy, while the enthalpy term, excepting the third step, opposes the complex formation. The data are in agreement with the presence of chelate structures in a decreasing measure for each of the three successive steps of complexation.

Paramagnetic bis(amidinate) iron(II) complexes and their diamagnetic dicarbonyl derivatives

Reactions of two equivalents of the lithium amidinate salts Li[tBuC(NR)2] (R = iPr, cyclohexyl) with FeCl2 have been found to yield paramagnetic bis(amidinate) iron(II) compounds of the type [tBuC(NR)2]2Fe. In the case of R = cyclohexyl, the product has been characterized by single-crystal X-ray diffraction analysis as having a distorted tetrahedral geometry. The derivative with R = iPr is an oil, but its 1H and 13C NMR spectra indicate a similar monomeric structure. Both species have been found to react readily with CO to give the new diamagnetic FeII dicarbonyls [tBuC(NR)2]2Fe(CO)2. The compound with R = iPr has been structurally characterized, which showed it to have a strongly distorted octahedral structure with the carbonyls in a cis arrangement.

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.

Photochemistry of [(η5-C5H5)Ru(CO)2]2 in polar and non-polar solvents

Abstract The photochemistry of bridged and non-bridged isomers of the title complex was studied in various solvents by flash photolysis and continuous irradiation at different wavelength regions. Reactive decay of the excited non-bridged isomer produced either a polymer (in neat non-polar solvents) or monomeric [CpRu(CO)2Cl] complex (in non-polar solvents containing CCl4) via homolytic cleavage of the RuRu bond. Polymerization was preceded by generation of a transient species (IP1) with λ max ≈470 nm and τ 1 2 ≈4 ms , which was tentatively formulated as a trimer [CpRu(CO)2]3. Photolysis of the bridged isomer generated another transient species, IP2, characterized by the solvent-dependent absorption at 315–330 nm, assigned to the mono-bridged dimer with coordinated solvent L, [Cp(CO)2Ru(μ-CO)Ru(CO)LCp]. The IP2 properties and reactivity led to conclusion that photocleavage of the CO-bridge is the mode for the bridged isomer.

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.