Karel Mach

Preparation of μ-(η5:η5-Fulvalene)-di-μ-hydrido-bis(η5-cyclopentadienyltitanium) by the reduction of Cp2TiCl2 with LiAlH4 in aromatic solvents

Summaryμ-(η5:η5-Fulvalene)-di-μ-hydrido-bis(η5-cyclopentadienyltitanium) (1) can be prepared by the reduction of Cp2TiCl2 with LiAlH4 in methylbenzenes and in tetralin at their boiling temperatures in yields greater than 90%. The reduction proceedsvia the bis(η5-cyclopentadienyl)titanium(III) chloride dimer which is further transformed into the unstable [Cp2TiH] species. Thermal decomposition of the latter accompanied by hydrogen evolution gives rise to (1). μ-(η5:η5-Fulvalene)-μ-hydrido-μ-chloro-bis(η5-cyclopentadienyltitanium), the first fulvalene containing compound observed in the system is formed by hydrido-chloro exchange of (1) with (Cp2TiCl)2 and aluminium chlorohydrides.

Substituent effects in cyclic voltammetry of titanocene dichlorides

Abstract Methyl-substituted titanocene dichlorides (C5H5−nMen)2TiCl2 (n=0–5), [C5Me4(SiMe3)]2TiCl2, (C5Me4Ph)2TiCl2 (Ph=phenyl), [C5Me4(FPh)]2TiCl2 (FPh=para-fluorophenyl), [C5Me4(CH2Ph)]2TiCl2 and ansa-compounds Me2Si(C5H4)2TiCl2 and Me2Si(C5Me4)2TiCl2 were investigated by means of cyclic voltammetry at a mercury electrode in tetrahydrofuran. The standard potential (E°1) of the first electron uptake shifts to more negative values proportionally to the number of methyl groups in the (C5H5−nMen)2TiCl2 (n=0–3) compounds, with an increment of 0.093 V per one methyl group. A decline from this linear dependence is observed for (C5HMe4)2TiCl2 and a positive shift for (C5Me5)2TiCl2. The [C5Me4(R)]2TiCl2 (R=SiMe3, Ph, FPh and CH2Ph) compounds show even larger positive shifts of E°1. These positive shifts can be brought about by a steric strain between the cyclopentadienyl ligands which lowers the dihedral angle between cyclopentadienyl ring planes (φ) and thus decreases energies of bent titanocene 1a1 and b2 LUMO orbitals. This opinion is corroborated by the voltammetry of ansa-compounds Me2Si(C5H4)2TiCl2 and Me2Si(C5Me4)2TiCl2, having a large and fixed angle φ. Their E°1 values are close to those of (C5H5)2TiCl2 and (C5HMe4)2TiCl2, respectively.

Titanocene-bis(trimethylsilyl)acetylene complexes: effects of methyl substituents at the cyclopentadienyl ligands on the structure of thermolytic products

Abstract The (C5H5−nMenTi[η5-C2(SiMe3)2] (n = 0–5) (1A–1F) complexes were prepared by the reduction of corresponding titanocene dichlorides with magnesium in THF in the presence of bis(trimethylsily)acetylene (BTMSA). All of them were characterized by spectroscopic methods and (C5HMe2Ti[η5-C2(SiMe3)2] (1E) by the X-ray crystal analysis. The complexes decompose at temperatures in the range 100–200°C. Those with n = 0–3 yield (μ-η5 : η5-fulvalene)(di-μ-hydrido)bis (η5-cyclopentadienyltitanium) (2A) and its methylated analogues (2B–2D) whereas BTMSA is released. The crystal structure of 2D showed that the hexamethylfulvalene ligand contains non-methylated carbon atoms in inner alternate positions. Complex 1E afforded a mixture of products. Among them only volatile isomers (η3:η4-1,2-dimethyl-4,5-dimethylcyclopenteny)(η5-tetramethylcyclopentadienyl)titanium (2Ea) and (η3:η4-1,3-dimethyl-4, 5-dimethylenecyclopentenyl)(η5-tetramethylcyclopentadienyl)titanium (2Eb) have been so far isolated as minor products. The C5Me5 comples 1F yields quantitatively (η3:η4-1,2,3-trimethyl-4,5-dimethylenecyclopentyl)(pentamethylcyclopentadienyl)titanium (2F) and BTMSA is hydrogenated to a mixture of cis- and trans-bis(trimethylsilyl)ethene.

[6+2]Cycloadditions catalyzed by titanium complexes

Abstract The Ziegler catalyst TiCl4-Et2AlCl and the arenetitanium(II) complex (η6-C6H6)Ti(II)(AlCl4)2 induce [6 + 2]cycloaddition reactions of cycloheptatriene with dienes and acetylenes. Addition to 1,3-butadiene affords 7 - endo - vinyl - bicyclo[4.2.1]nona - 2,4 - diene (main product) and bicyclo[4.4.1]- undeca - 2,4,8 - triene, a product of [6+4]cycloaddition. Isoprene reacts similarly, yielding mainly 7- endo - isopropenyl - bicyclo[4.2.1]nona - 2,4 - diene. 2,3 - Dimethyl - 1,3 - butadiene gives 8,9dimethylbicyclo [4.4.1]undeca - 2,4,8 - triene, a product of [6 + 4]cycloaddition, while [6 + 2]cross-adducts are minor products. The reaction of cycloheptatriene with norbornadiene gives mainly hexacyclo[6.5.1.02,7.03,12.6,10.09,13]tetradec - 4 - ene via [6+2]cycloaddition followed by intramolecular Diels-Alder reaction. As a by-product, pentacyclo[7.5.0.02,7.03,5.048]tetradeca - 10,12 - diene is formed by a [2+2+2]mechanism. Addition of cycloheptatriene to diphenylacetylene and bis - (tri- methylsilyl)acetylene furnishes sustituted bicyclo[4.2.1]nona - 2,4,7 - trienes. Alkenes, E,E-2,4 - hexadiene and 1,3 - cyclooctadiene are unreactive. The [6+2]cycloaddition is made possible by coordination of cycloheptatriene to titanium, which changes the symmetry of the frontier orbitals in the triene. The reactivity of the trienophile is also enhanced by coordination to the catalyst.

Reduction of Bis[η5-(ω-alkenyl)tetramethylcyclopentadienyl]titanium Dichlorides: An Efficient Synthesis of Long-Chainansa-Bridged Titanocene Dichlorides by Acidolysis of Cyclopentadienyl-Ring- Tethered Titanacyclopentanes

The reduction of symmetric, fully-substituted titanocene dichlorides bearing two pendant ω-alkenyl groups, [TiCl2(η5-C5Me4R)2], RCH(Me)CH=CH2 (1 a), (CH2)2CH=CH2 (1 b) and (CH2)3CH=CH2 (1 c), by magnesium in tetrahydrofuran affords bis(cyclopentadienyl)titanacyclopentanes [TiIV{η1:η1:tlsb&endash;3%>η5:η5-C5Me4CH(Me)CH(Ti)CH2CH(CH2(Ti))CH(Me)C5Me4}] (2 a), [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)(CH2)2CH(Ti)(CH2)2C5Me4}] (2 b) and [TiIV{η1:η1:η5:η5-C5Me4(CH2)2CH(Ti)CH(Me)CH(Me)CH(Ti)(CH2)2C5Me4}] (2 c), respectively, as the products of oxidative coupling of the double bonds across a titanocene intermediate. For the case of complex 1 c, a product of a double bond isomerisation is obtained owing to a preferred formation of five-membered titanacycles. The reaction of the titanacyclopentanes with PbCl2 recovers starting materials 1 a from 2 a and 1 b from 2 b, but complex 2 c affords, under the same conditions, an isomer of 1 c with a shifted carbon-carbon double bond, [TiCl2{η5-C5Me4(CH2CH2CH=CHMe)}2] (1 c′). The titanacycles 2 a-c can be opened by HCl to give ansa-titanocene dichlorides ansa-[{η5:η5-C5Me4CH(Me)CH2CH2CH(Me)CH(Me)C5Me4}TiCl2] (3 a), ansa-[{η5:η5-C5Me4(CH2)8C5Me4}TiCl2] (3 b), along with a minor product ansa-[{η5:η5-C5Me4CH2CH=CH(CH2)5C5Me4}TiCl2] (3 b′), and ansa-[{η5:η5-C5Me4(CH2)3CH(Me)CH(Me)CH=CHCH2C5Me4}TiCl2] (3 c), respectively, with the bridging aliphatic chain consisting of five (3 a) and eight (3 b, 3 b′ and 3 c) carbon atoms. The course of the acidolysis changes with the nature of the pendant group; while the cyclopentadienyl ring-linking carbon chains in 3 a and 3 b are fully saturated, compounds 3 c and 3 b′ contain one asymetrically placed carbon-carbon double bond, which evidently arises from the β-hydrogen elimination that follows the HCl addition.

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.

SYNTHESIS, CRYSTAL STRUCTURES AND SOME PROPERTIES OF DIMETHYLSILYLENE-BRIDGED ANSA-PERMETHYLTITANOCENE TI(IV), (III) AND (II) COMPLEXES

Dimethylsilylene-bridged complexes Me2Si(C5Me4)2TiCl (2), Me2Si(C5Me4)2Ti[η2-C2(SiMe3)2] (3) and Me2Si(C5H4)2Ti[η2-C2(SiMe3)2] (4) have been prepared by the general methods which are known for obtaining of analogous non-bridged titanocene complexes. X-ray crystal structures of Me2Si(C5Me4)2TiCl2 (1), 2, and 3 reveal that the dihedral angle between the least-squares planes of cyclopentadienyl rings increases in the order 2 < 3 < 1. Comparison with the structures of analogous (C5HMe4)2Ti and (C5Me5)2Ti compounds shows that the value of increases in the series (C5Me5)2Ti < (C5HMe4)2Ti < Me2Si(C5Me4)2Ti, e.g. in the bis(trimethylsilyl)acetylene complexes from 41.1° for (C5Me5)2Ti[η2-C2(SiMe3)2] (9) to 50.0° for (C5HMe4)2Ti[η2-C2(SiMe3)2] (8) and to 53.5° for 3. Compounds 3, 8 and 9 induce the head-to-tail dimerization of 1-hexyne with the selectivity of 72%, 21% and ca. 100% respectively. The discrepancy between the selectivities and the values of for 3 and 8 is accounted for by a larger flexibility of the titanocene skeleton in 8, affording a larger space for a non-specific coordination of 1-hexyne. The effects of the μ-Me2Si group in 2 and 3 on some of their properties are compared with the effects of Me and H substituents in the non-ansa compounds with controversial results. For instance, the affinity of 2 to 2-methyltetrahydrofuran approaches that of (C5H2Me3)2TiCl whereas the shift of the v(C≡C) vibration in 3 indicates a stronger metal-acetylene bond than in 9.

Titanium-catalyzed head-to-tail dimerization of tert-butylacetylene. Crystal structures of [(C5HMe4)2Ti(μ-H)2Mg(THF)(μ-Cl)]2 (THF-tetrahydrofuran) and (C5HMe4)2TiOCMe3

Abstract tert-Butylacetylene (TBUA) is readily dimerized exclusively to 2,4-di-tert-butyl-1-buten-3-yne, head-to-tail dimer (HTTD), in the presence of the (η5-C5HMe4)2TiCl2/Mg/THF system. The ESR investigation revealed the formation of Ti–Mg hydride complexes [(η5-C5HMe4)2Ti(μ-H)2Mg(THF)(μ-Cl)]2 (2) and [(η5-C5HMe4)2Ti(μ-H)2]2Mg (3) in the absence of TBUA and a tweezer-type complex, probably [(η5-C5HMe4)2Ti(η1-CCCMe3)2]− [Mg(THF)Cl]+ (4) in its presence. The catalytic system as well as complexes 2–4 were deactivated by presumably tert-butanol contained in TBUA to give (η5-C5HMe4)2TiOCMe3 (5). Purification of TBUA improved the turnover by up to 8.8×103 mol TBUA per 1 mol Ti using complex 3 as a catalyst, however, complex 5 remained the only observable product of deactivation. The crystal structures of 2 and 5 were determined by X-ray diffraction analysis.

Direct proof of the molecular structure of dimeric titanocene; The X-ray structure of μ(η5:η5-fulvalene)-di-(μ-hydrido)-bis(η5-cyclopentadienyltitanium)· 1.5 benzene☆

Abstract The dimeric titanocene crystallizes as a benzene solvate with molar ratio 1: 1.5. The crystals are monoclinic, space group P 21 n / n with Z = 4 and lattice parameters α = 5.978(4), b = 14.541(6), c = 26.963(8) A and β = 92.11(2)°. In the series of (C 10 H 8 )(C 5 H 5 )TiX] 2 complexes, where X  H, H/Cl (1: 1), Cl or OH, the dihydrido complex has the shortest Ti-Ti distance (2.989 A) and largest dihedral angle between the planes of C 5 H 4 rings of the fulvalene ligand (17.7°).

The crystal and molecular structure of [(C5HMe4)TiBr(μ-O)]4 and [(C5Me5)TiBr(μ-O)]3, by-products from the preparation of titanocene dibromides

Abstract The compounds [(C 5 HMe 4 )TiBr(μ-O] 4 (I) and [(C 5 Me 5 )TiBr(μ-O)] 3 (II) result from hydrolysis of (C 5 HMe 4 )TiBr 3 and (C 5 Me 5 )TiBr 3 , respectively, in THF with aqueous HBr. They appear as byproducts if during the preparation of the corresponding titanocene dibromides air is accidentally introduced, or if substoichiometric amounts of reducing agents are present prior to addition of aqueous HBr. Such conditions cause formation of the cyclopentadienyltitanium dibromides or tribromides. The molecule of I forms a planar eight-membered titanoxane cycle with a two-fold symmetry axis perpendicular to the plane of the cycle. The TiOTi angles are alternately 149 and 176° whereas the OTiO angles are nearly constant at 103–106°. The six-membered titanoxane cycle in II is planar except for the oxygen atom which connects the titanium atoms bearing those C 5 Me 5 ligands orientated on the same side with respect to the plane; displacement of the oxygen atom is 0.4 A.