Vojtech Varga

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.

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.

Permethyltitanocene-bis(trimethylsilyl) acetylene, an efficient catalyst for the head-to-tail dimerization of 1-alkynes

Abstract In the series of the (C 5 H 5− n Me n ) 2 Ti[ η 2 -C 2 (SiMe 3 ) 2 ] ( n = 0−5) ( 1A–1F ) complexes only (C 5 Me 5 ) 2 Ti[ η 2 -C 2 (SiMe 3 ) 2 ] ( 1F ) catalyzed the linear head-to-tail dimerization of 1-alkynes. A selectivity of better than 98% and a turnover number ( TN ) in the range 1200–1500 mmol of 1-alkyne per mmol of Ti was obtained after 14 d at 30°C for 1-pentyne, 1-hexyne, cyclohexylethyne, phenylethyne and trimethysilylethyne. Among other complexes in the series, 1A–1D attained TN ⩽ 25 after 5 d at 30°C and produced mainly a mixture of 1,3,5- and 1,2,4-trisubstituted benzene derivatives. The complex (C 5 HMe 4 ) 2 Ti[ η 2 -C 2 (SiMe 3 ) 2 ] ( 1E ) afforded ca. 20% of the head-to-tail dimers in addition to cyclotrimers but its activity was extremely low, corresponding to TN ∼ 7. The rate of dimerization by 1F is controlled by slow exchange rate of 1-alkynes with bis(trimethylsilyl)acetylene.

Dimeric titanocene hydride-hydridomagnesium chloride and bromide complexes. Crystal structures of the tetramethylcyclopentadienyl derivatives

Abstract Cp′2TiX2 -iPrMgX(Cp′  C5HMe4,C5H2Me3,C5H5; X  Cl or Br) systems afford blue crystalline products, with low solubility in diethyl ether. X-Ray single crystal analysis of the C5HMe4 derivatives revealed dimeric centrosymmetric structures of [(C5HMe4)2Ti(μ-H)2Mg(OEt2)(μ-Cl)]2 (Ia) and [(C5HMe4)2Ti(μ-H)2Mg(OEt2)(μ-Br)]2 (Ib). The solution EPR spectra of all the compounds (g = 1.9910–1.9934, aH = 0.66–0.75 mT, aTi = 0.54–0.66 mT, a(multiplet) = 0.04–0.1 mT) can be assigned either to the dimers or to the dissociated monomeric species.

Fermethyltitanocene(III) diacetylide - magnesium tweezer complexes, intermediates in the catalysis of linear head-to-tail dimerization of terminal acetylenes

The (C5Me5)2TiCl2/i-PrMgCl/Et2O (Mg:Ti  4–10) catalytic systems selectively catalyze the dimerization of terminal acetylenes RCCH to 2,4-disubstituted 1-buten-3-ynes for R  Me, Et, n-Pr, n-Bu, cyclohexyl, Ph, and SiMe3 with a turnover number 500–700 after 5 days. The ESR investigation of these systems has revealed that permethyltitanocene tweezer complexes with embedded Mg ion between the acetylene arms [(C5Me5)2Ti(η1-CCR)2]−[Mg(OEt2)X]+ (XCl or CCR) (1A–1G for R  Me, Et, n-Pr, n-Bu, cyclohexyl, Ph, and SiMe3) are intermediates in the formation of a dimerization catalytic center. Their ESR spectra are characterized by g-values in the range 1.990–1.993, coupling constant to the proton at α-carbon atom of the acetylene substituent 2.0–2.5 G and by coupling constants to 47Ti and 49Ti isotopes in the range 7.6–8.7 G. The tweezer complex [(C5Me5)2Ti(η1-CCSiMe3)2]−[Mg (THF)Cl]+ (2G) has been synthesized. It catalyzes the head-to-tail dimerizations with the same selectivity and similar turnover numbers as the above systems.

Synthesis and structure of titanium(III) tweezer complexes with embedded alkali metal ions: [(η5-C5HMe4)2Ti(η1-CSiMe3)2]−M+ (M Li, Na, K or Cs)

Abstract Complexes [( η 5 -C 5 HMe 4 ) 2 Ti( η 1 -CCSiMe 3 ) 2 ] − M + (M  Li ( 2 ), Na ( 3 ), K ( 4 ), and Cs ( 5 )) were prepared by the redox reaction of ( η 5 -C 5 HMe 4 ) 2 Ti( η 1 -CCSiMe 3 ) 2 ( 1 ) with the alkali metals in toluene. The X-ray crystal analysis of 4 (monoclinic, P 2 1 / c , a = 10.888(3) A, b = 18.047(8) A, c = 16.562(3) A, β = 104.10(2)°, Z = 4) revealed a tweezer structure with the potassium cation embedded between the trimethylsilylacetylide tweezer arms in a position which is slightly closer to the inner acetylide carbon atoms (2.946(4) A and 2.949(4) A) than to the outer ones (3.021(4) A and 2.961(4) A). The K + ion simultaneously binds a C 5 HMe 4 ligand of the adjacent molecule at the K-centroid distance of 3.186(5) A, thus forming a polymer chain arrangement of molecular units. The complexes 2–5 afford characteristic EPR spectra ( g = 1.9924–1.9935, Δ H = 2.0–2.4 G, a Ti = 7.8–8.4 G), infrared ( v (CC) at 1935–1948 cm −1 ) and UV-vis spectra (two CT bands in the region 355–390 nm). The molecular ions of 2–5 were observed in El mass spectra.

Synthesis and X-ray crystal structure of the permethyltitanocene hydride–magnesium hydride complex [{(C5Me5)2Ti(µ-H)2)2}2Mg]

The trinuclear hydride complex [{(C5Me5)2Ti(µ-H2)2}2Mg]1 was obtained from the [(C5Me5Ticl2]–PrImgcl–Et2O system; the X-ray crystal structure of 1shows two mutually perpendicular permethyltitanocene moieties coordinated through four bridging hydride bonds to the central pseudotetrahedrally coordinated magnesium atom.

A titanium(III) tweezer complex with an embedded alkali metal ion between diynyl ligands: [(C5HMe4)2Ti(η1−CCCCSiMe3)2]−[Li(THF)2]+

Abstract The complex [( η 5 −C 5 HMe 4 ) 2 Ti( η 1 −CCCCSiMe 3 ) 2 ] − [Li(THF) 2 ] + ( 1 ) was obtained by the reaction of (C 5 HMe 4 ) 2 TiCl with 2 equiv. of LiCCCCSiMe 3 in THF. X-Ray crystal structure determination of 1 revealed that the lithium ion is embedded between the inner triple bonds of 4-trimethylsilyl-1,3-butadiyn-1-yl tweezer arms. The EPR spectrum of 1 shows a single line, ΔH = 2.5 G at g = 1.9940 (±0.0003), and coupling to 49 Ti and 47 Ti isotopes a (Ti) = 7.5 G.

Effects of the Linking of Cyclopentadienyl and Ketimide Ligands in Titanium Half-Sandwich Olefin Polymerization Catalysts

The role of the ketimide ligand geometry in Ti half‐sandwich complexes and the consequent effects in olefin polymerization catalysis (ethylene, styrene, 1‐hexene polymerization, and ethylene/1‐hexene copolymerization) were investigated under various conditions. [CpTiCl2(N=CtBu2)] (1; Cp=η5‐cyclopentadienyl) was used as a reference compound for comparison with the recently described complex [{η5‐C5H4CMe2CMe2C(tBu)=N‐κN}TiCl2] (2 a) and a new derivative that has a longer linker between Cp and the ketimide, [{η5‐C5H4CH2CH2CMe2C(tBu)=N‐κN}TiCl2] (9). The presence of a distorted intramolecularly tethered ketimide moiety reduces the polymerization activity significantly in systems that contain Al‐based cocatalysts (methylaluminoxane, triisobutylaluminum). However, in Al‐free systems both types of compounds provided active polymerization catalysts. Notably, the recently reported activation system Et3SiH/B(C6F5)3 was for the first time demonstrated to activate Ti complexes for ethylene and 1‐hexene (co)polymerization catalysis by hydride transfer.