RESUMEN
Some novel cobalt diphenylphosphine complexes were synthesized by reacting cobalt(II) chloride with (2-methoxyethyl)diphenylphosphine, (2-methoxyphenyl)diphenylphosphine, and 2-(1,1-dimethylpropyl)-6-(diphenylphosphino)pyridine. Single crystals suitable for X-ray diffraction studies were obtained for the first two complexes, and their crystal structure was determined. The novel compounds were then used in association with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene, and their behavior was compared with that exhibited in the polymerization of the same monomer by the systems CoCl2(PnPrPh2)2/MAO and CoCl2(PPh3)2/MAO. Some significant differences were observed depending on the MAO/Co ratio used, and a plausible interpretation for such a different behavior is proposed.
RESUMEN
The structural and electronic properties of silica-supported titanium chloride tetrahydrofuranates samples, obtained by impregnating a polymer-grade dehydroxylated silica with TiCl4(thf)2 and TiCl3(thf)3 complexes, precursors of Ziegler-Natta catalysts, are investigated by means of FT-IR, XAS, XES and diffuse reflectance UV-Vis spectroscopy, coupled with DFT calculations. The properties of the two silica-supported samples are very similar, irrespective of the starting precursor. In both cases, most of the chlorine ligands originally surrounding the Ti sites are substituted by oxygen ligands upon grafting on silica. As a consequence, the electronic properties of silica-supported Ti sites are largely different from those of the corresponding precursors, and in both cases most of the grafted Ti sites have a formal oxidation state of +4. The whole set of experimental data provide evidence that mono-nuclear Ti species are mainly present at the silica surface.
RESUMEN
X-ray powder diffraction (XRPD), Infrared, Raman, and UV/Vis spectroscopy have been used to investigate the structural, vibrational, and optical properties of Ti and Mg chloride tetrahydrofuranates as precursors of heterogeneous Ziegler-Natta catalysts for polyethylene production; as well as their interaction compound (pro-catalyst) and the final catalyst obtained after interaction with the AlR(3) activator. Although the structure of the precursors and of the pro-catalyst were well known, that of the catalyst (obtained by reaction of the pro-catalyst with AlR(3)) was not easily obtainable from XRPD data. IR and Raman spectroscopy provided important information on tetrahydrofuran (thf) coordination and on the ν(M-Cl) region; whereas UV/Vis spectroscopy gave the direct proof on both the formal oxidation state and the coordination environment of the active Ti sites. Those presented herein are among the first direct experimental data on the structure of the active Ti sites in Ziegler-Natta catalysts, and can be used to validate the many computational studies that have been increasing exponentially in the last few decades.
RESUMEN
Titanium tetrachloride reacts with 2,6-bis[(1-phenylimino)ethyl]pyridine, 1, and 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine, 2, giving the adducts of general formulae [Ti1Cl3]Cl, 3, and [Ti2Cl3]Cl, 6, the latter through the intermediacy of the covalently bonded [Ti2Cl4], 4. Heating 6 leads to reduction to the titanium(III) derivative [Ti2Cl3], 12, the latter characterized by X-ray diffraction methods. The reaction of [Ti1Cl3]Cl with a toluene solution of MAO proceeds with methylation at the ortho-position of the pyridine ring to give the titanium(iv) derivative [Ti(C22H21N3)Cl3], 8. The reaction of [Ti2Cl3]Cl with MAO gives a mixture of products containing [Ti2Cl2(OAlCl3)], 9. Compound 9, which has been prepared independently by reacting 6 with AlOCl, is a rare case of a compound containing the -OAlCl3 moiety, as shown by a single-crystal X-ray diffraction study. From the tetrachlorides of zirconium and hafnium with 1 or 2, the corresponding adducts [M(L)Cl4] have been obtained in high yields. These derivatives of Group 4 metals act as ethylene polymerization catalytic precursors: the substitution of the phenyl ring of the imino fragment strongly influences the catalytic activity which is 5,544 kg(PE) mol(Ti)(-1) h(-1) in the case of 3 and 267 kg(PE) mol(Ti)(-1) h(-1) with 6. Catalytic activity has been observed for zirconium and hafnium too, the activity decreasing from zirconium to hafnium, under comparable conditions.