Jindřich Poláček

Effects of methyl substituents at the cyclopentadienyl ligand on the properties of C5H5TiCl3 and C5H5TiAl2Cl8-x(C2H5)x (x = 0–4) complexes

The methyl substituents in the series of CpTiCl3 compounds (CP = Cp, MeCp, Me3Cp, Me4Cp, Me5 Cp and EtMe4Cp) shift the position of their CT absorption band from λ = 384 nm to max. 438 nm and decrease the rate of reduction of CpTiCl3 by ethylaluminium compounds yielding the trinuclear CpTiAl2Cl8 - xEtx (x = 0–4) complexes. In the CpTiCl3/excess Et2AlCl systems the rate of reduction was controlled by pseudomonomolecular decomposition of the proposed octahedral intermediate CpTiEt(Cl2AlEt2)(Cl3AlEt). The rate constants for reduction decreased in the above series of CpTiCl3 compounds from 1.10 × 10−3 to 6.15 × 10−5 s−1. The methyl substituents in the CpTiAl2Cl8-xEtx complexes shifted the charge transfer bands to longer wavelengths, the d-d transition to shorter wavelengths and the ESR g-value away from the free electron value. The opposite shifts were induced by the replacement of the outer chlorine atoms in the chloroaluminate ligands by ethyl groups. On going from Cp to Me5Cp the thermal stability of the CpTiAl2Cl8 complexes decreased while the complexes CpTiAl2Cl4Et4 became stable even with the excess of Et3Al. The CpTiAl2Cl8-xEtx complexes were also formed in the redox reaction of non-dimerizing methylcyclopentadienes (Me3CpH/EtMe4CpH) with bis(di-μ-chloroalane)(benzene)titanium(II) complexes C6H6 · TiAl2Cl8-xEtx (x = 0–2). The reaction was found stoichiometric except for the perchloro complexes forming diamagnetic byproducts.

Ethyl-substituted (π5-cyclopentadienyl)-bis(dihaloalanedi-μ-halo)titanium(III) and (η6-benzene)bis(dihaloalanedi-μ-halo)titanium(II) chloro and bromo complexes

Abstract Ethyl-substituted trinuclear complexes CpTiAl 2 Cl 8− n Et x ( x = 1-4) were prepared by by the reaction of CpTicl 3 with two equivalents of ethylaluminium compounds. The complexes were characterized by the half-width of their EPR single-line spectra, which decreased from 1.9 mT for x = 1 to 0.85 mT for x = 4, and by the position of their d-d absorption bands. From these data the composition of ethylated titanium complexes formed in the systems CpTiAl 2 Cl 8 -Et 3− x AlCl x ( x = 0, 1, 2) was determined. In the system C 6 H 6 · TiAl 2 Cl 8− Et 3− x AlCl x the formation of only the first two members of ethylated arenetitanium(II) series C 6 H 6 · TiAl 2 Cl 7 Et and C 6 H 6 · TiAl 2 Cl 6 Et 2 could be observed. The latter complexes differed from the parent complex in the position of the charge transfer band Ti II → C 6 H 6 and, after addition of an equimolar amount of cyclopentadiene, the EPR spectra of corresponding CpTi III trinuclear complexes were observed. In both, Ti III and Ti II series, the stability of the complexes decreased with increasing content of ethyl groups. The properties of CpTi III trinuclear complexes are compared with those of the binuclear complexes Cp 2 TiAlCl 4− x Et x ( x = 0, 1, 2). Results for the series of bromo complexes were mostly analogous.

Titanium-catalyzed cyclotrimerization of butadiene: I. Arenetitanium(II) chloro- and bromo-alane complexes

Abstract (η 6 -Benzene)bis(dichloroalanedi-μ-chloro)titanium(II) ( Ia ) and its bromo analogue (Ib) catalyze the cyclotrimerization of butadiene to ( Z,E,E )-1,5,9-cyclododecatriene (CDT) with an overall selectivity as high as 92% and 95%, respectively. The convenient catalytic properties of the complexes, i.e. the high catalytic stability of Ia and the high initial rate induced by Ib , have been optimally combined in mixed chloro-bromo complexes. Isothermal and isobaric kinetic measurements have revealed that the reaction order with respect to butadiene is close to 2.0 while the reaction order with respect to the catalyst approaches 1.0 at extreme dilution and decreases with increasing catalyst concentration. Deactivation of the catalyst results in the separation of insoluble titanium halides. The proposed kinetic scheme includes rapid equilibrium between arene- and bis(butadiene)-titanium(II) complexes, followed by the ratedetermining formation of the intermediate titanium complex with an openchain butadiene dimer and very fast cycloaddition of the third butadiene molecule. The formation of ( Z,E,E )-1,5,9-CDT is accounted for by the cycloaddition of butadiene in an s -( Z )- conformation to an intermediate, which is probably [1,2-bis( syn -η 3 -allyl)ethane]bis(tetrahaloalanate)titanium-(IV) or s η 1 -allyl analogue.

The crystal structure of (η6-C6Me6)Ti[(μ-Cl)2(AlClEt)]2 and the catalytic activity of the (C6Me6)TiAl2Cl8−xEtx(x 04) complexes towards butadiene

The composition of (C6Me6)TiAl2Cl8−xEtx complexes in (C6Me6)TiAl2Cl8 + n Et3Al (n = 0.5-6) systems was studied by UV-Vis spectroscopy and the X-ray crystal structure of one of them, (η6-C6Me6)Ti[(μ-Cl)2(AlClEt)]2 (IIa-2), has been determined. The complex crystallizes in the orthorhombic space group Pna21 with Z = 4 and lattice parameters a 15.634(3), b 11.355(2), c 14.417(2) A. The ethyl groups of IIa-2 reside in outer positions of aluminate ligands farther away from the C6Me6 ligand. The other part of the complex does not differ remarkably from structures of other (arene)TiII complexes. Negligible activity of (C6Me6)TiAl2Cl8 towards the butadiene cyclotrimerization is considerably increased by addition of 2.5–3.0 equivalents of Et3Al. As follows from UV-Vis spectra, such systems contain mainly the (C6Me6)TiAl2Cl5Et3 complex. It is suggested that the introduction of three Et substituents destabilizes the Ti-(η6-C6Me6) bond so that the replacement of hexamethylbenzene by butadiene in the first step of a catalytic cycle becomes more feasible.

Titanium-catalyzed cyclotrimerization of butadiene: Part II. The (C6H6)TiII(AlCl4)2-EtxAlCl3-x(x = 1–3) systems

Abstract The cyclotrimerization of butadiene to ( Z , E , E )-1,5,9-cyclododecatriene (( Z , E , E )-CDT) is catalyzed by the (C 6 H 6 )Ti(AlCl 4 ) 2 ( Ia )-Et x AlCl 3- x ( x = 1–3) systems which contain the Ia , (C 6 H 6 )Ti(AlCl 4 )(AlCl 3 Et) ( Ia-1 ) or (C 6 H 6 )Ti(AlCl 3 Et) 2 ( Ia-2 ) complexes as active species. The ethyl substituents bring about an increase in the initial reaction rate followed by an enhanced deactivation of the catalyst. In the highly ethylated systems, e.g. at a molar ratio of Et 2 AlCl: Ia >/ 6, the initial, very rapid reaction rate greatly decreases in later stages, and the reaction continues at a rate about one order lower. During the initial rapid stage, a precipitate of TiCl 2 is formed via deactivation of Ia-2 and decomposition of a complex derived from the catalytically ineffective (C 6 H 6 )Ti(AlCl 3 Et)(AlCl 2 Et 2 ) ( Ia-3 ) complex. The continued formation of ( Z , E , E )-CDT in the low-activity stage probably proceeds on Ia-2 , a low concentration of which is maintained by the reversible reaction between TiCl 2 and EtAlCl 2 present in a mixture with Et 2 AlCl. The reaction order with respect to butadiene, as determined from the initial reaction rates, decreases from 1.9 for Ia to 1.5 for Ia-1 and to a value between 1.0 and 1.5 for Ia-2 . The reaction order in the catalyst increases (probably to 1.0) with decreasing catalyst concentration and vice versa for all the systems; a value of 0.5–0.6 was obtained for [Ti] = 6.5 × 10 −4 − 3.9 × 10 −3 M. The observed reversible interconversions of the (C 6 H 6 )Ti II complexes Ia-Ia-2 and the Ti(II) complexes containing ( Z , E , E )-CDT, 1,5-cyclooctadiene or butadiene and the transformation of all of them to the (HMB)Ti II complexes by reaction with hexamethylbenzene (HMB) support our suggestion that ( Z , E , E )-CDT is selectively formed on the Ti(II) complexes with retained trinuclear structure. The catalyst deactivation is assumed to be initiated by an electron transfer from Ti(II) to an organic ligand, and the deactivation products differ in their dependence on the nature and the content of Et x AlCl 3- x

Titanium-catalyzed cyclotrimerization of butadiene Part III. Formation of catalytic bis(di-μ-chloroaluminate)-Ti(II) complexes in TiCl4EtxAlCl3-x (x=1–2) systems

Abstract The TiCl4+nEtxAlCl3-x and TiCl3+nEtxAlCl3-x (x=1–2) systems catalyze the cyclotrimerization of butadiene to (Z,E,E)-1,5,9-cyclododecatriene ((Z,E,E)-CDT) with a selectivity very similar to that exerted by the analogous (C6H6)TiII(AlCl4)2 (Ia)-based systems. The presence of (benzene)TiII complexes in the final reaction mixtures of the kinetic runs was observed by electronic absorption spectroscopy in the systems with x=1 for n⩾40, with x=1.5 for n=6–50 and with x=2 for n=6, using the high-intensity CT band at 400 nm. The content of (benzene)TiII complexes was also estimated from the intensity of the ESR signals of Cp*TiAl2Cl8−yEty (y=0–4) complexes rapidly formed in all the reaction mixtures after addition of pentamethylcyclopentadiene (Cp*H). It follows from spectroscopic observation of (benzene)TiII complexes after kinetic runs and comparison of the reaction rates in the TiCl4− and Ia-based systems that the concentrations of the catalytic (L)TiII (L = benzene, butadiene, intermediate dimer of butadiene, (Z,E,E)-CDT or 1,5-COD) complexes are generally lower in the former systems and decrease with increasing x. In the TiCl4 (TiCl3) + nEtxAlCl3-x (x = 1, 1.5) systems, an excess of the ethylaluminium components results in increased formation of the catalytic (L)TiII species; however, the reaction rate appropriate to the (L)TiII concentration is decreased by the retarding effect of EtAlCl2. The optimum catalysts, exerting the best selectivity for (Z,E,E)-CDT formation (90.5%), were found in the ethylaluminium sesquichloride systems with new maxima of activity discovered for the ratios Al/TiCl4 = 6 and Al/TiCl3 = 4. The TiCl4 (TiCl3) + nEt2AlCl systems have low activity, probably due to the low equilibrium concentration of the (L)TiII species. The presence of another catalytic species in these systems is manifested by the presence of byproducts (E,E,E)-CDT and 1,5-COD.

The structure of bis(Dihaloalane-di-μ-halo)(η-arene)titanium(II) complexes containing different halogen atoms

SummaryAn attempt was made to detect deviations from the random distribution of chlorine and bromine atoms in the series of bis(dihaloalane-di-μ-halo)(η6-arene) titanium(II) complexes of overall formula (C6H6) · TiAl2ClxBr8−x (1). The distribution of halogen atoms between bridge and outer positions of (1) was studied by the shift of g-values in the e.s.r. spectra of bis(dihaloalane-di-μ-halo)(η5-cyclopentadienyl)titanium(III) complexes obtained by the reaction of (1) with cyclopentadiene, and by the shift of thev2 band in the electronic spectra of (1). Both methods indicated the preferred occupation of bridge positions by chlorine atoms based on the assumption that the g-value and thev2 band position were linearly dependent on the chlorine content in the bridge positions.