Synthesis and properties of alkyldicyclopentadienyl-vanadium(III) compounds

The compounds Cp₂VR (R = CH₃, C₂H₅, n-C₃H₇, n-C₄H₉, n-C₅H₁₁, CH₂C(CH₃)₃ or CH₂Si(CH₃)₃) have been prepared from Cp₂ VCl and RMgX in n-pentane. The air-sensitive compounds are stable at room temperature, but decompose between 65 and 138°C. The thermal stability decreases in the order R = CH₃ CH₂Si(CH₃)₃ > C₂H₅ > CH₂C(CH₃)₃ > n-C₅H₁₁ > n-C₄H₉ > n-C₃H₇. Compounds with R = i-C₃H₇ or t-C₄H₉ could not be obtained.     

Thermal decomposition of dicyclopentadienylarylvanadium compounds

The thermolysis of compounds of the type Cp₂VR (R = aryl) in the solid state has been studied. A distinct increase in thermal stability is observed upon substitution of the ortho-position of the aryl group. Thermal decomposition occurs with formation of RH, Cp₂ V, a vanadocene homologue with the group R substituted in one of the Cp rings and, probably, a vanadocene homologue with two substituted Cp rings. It is shown that the abstraction of the hydrogen atom from the cyclopentadienyl ring, necessary for the formation of RH, is an intermolecular process, whereas the substitution of the aryl group in the Cp ring is intramolecular. A decomposition mechanism is proposed in which the group R is transferred from the vanadium atom to the C₅H₅ ring of the same molecule by interaction with an aryl group of another molecule. The thermal decomposition of Cp₂VR is compared with that of the analogous titanium compounds.     

The effect of the amido substituent on polymer molecular weight in propene homopolymerisation by titanium cyclopentadienyl‐amide catalysts

In the homopolymerisation of propene by the cyclopentadienyl‐amide titanium catalyst systems [η⁵,η¹‐C₅H₄(CH₂)₂NR]TiCl₂/MAO and [η⁵,η¹‐C₅H₄(CH₂)₂NR]Ti(CH₂Ph)₂/B(C₆F₅)₃ (R = ^tBu, ⁱPr, Me), the catalyst with the smallest substituent (Me) on the amido moiety consistently gives the highest polymer molecular weight. This differs from the trend usually observed in related catalysts with tetramethylcyclopentadienyl‐amide ancillary ligands, where larger amide substituents result in higher molecular weights. Based on the present information a hypothesis is formulated in which an increased cation‐anion interaction for the less sterically hindered catalyst is responsible for disfavouring chain transfer relative to chain growth.     

Cationic and neutral diphenyldiazomethanerhodium(I) complexes as catalytically active species in the C-C coupling reaction of olefins and diphenyldiazomethane

Cationic rhodium(I) complexes cis-[Rh(acetone)2(L)(L')]+ (2: L = L'=C8H14; 3: L=C8H14; L'=PiPr3; 4: L=L'=PiPr3), prepared from [RhCl(C8H14)2]2] and isolated as PF6 salts, catalyze the C-C coupling reaction of diphenyldiazomethane with ethene, propene, and styrene. In most cases, a mixture of isomeric olefins and cyclopropanes were obtained which are formally built up by one equivalent of RCH=CH2 (R = H, Me, Ph) and one equivalent of CPh2. The efficiency and selectivity of the catalyst depends significantly on the coordination sphere around the rhodium(I) center. Treatment of 4 with Ph2CN2 in the molar ratio of 1:1 and 1:2 gave the complexes trans-[Rh(PiPr3)2(acetone)(eta1-N2CPh2)]PF6 (8) and trans-[Rh(PiPr3)2(eta1-N2CPh2)2]PF6 (9), of which 8 was characterized by X-ray crystallography. Since 8 and 9 not only react with ethene but also catalyze the reaction of C2H4 and free Ph2CN2, they can be regarded as intermediates (possibly resting states) in the C-C coupling process. The lability of 8 and 9 is illustrated by the reactions with pyridine and NaX (X=Cl, Br, I, N3) which afford the mono(diphenyldiazomethane)rhodium(I) compounds trans-[Rh(PiPr3)2(py)(eta1-N2CPh2)]PF6 (10) and trans-[RhX(eta1-N2CPh2)(PiPr3)2] (11-14), respectively. The catalytic activity of the neutral complexes 11 - 14 is somewhat less than that of the cationic species 8, 9 and decreases in the order Cl > Br> I > N3.     

Monoalkyl- and monoaryl-amidotitanium complexes

A series of new monoalkyl- and monoaryl-amido complexes, CpTiCl₂NHR (R = Et, i-Pr, t-Bu, Ph), has been synthesized and characterized. They were made in essentially quantitative yields by a very convenient reaction of CpTiCl₃ with Me₃SiNHR. The new complexes yield NMR and IR spectroscopic data for monoalkylamido complexes, such data being previously scarce. The compounds have very reactive amido hydrogen atoms, as is demonstrated by their thermal decomposition and their reaction with (Lewis) bases such as amines or organoalkali compounds. The products are the new bridged imido complexes, (CpTiCI)₂(μ-NR)₂.     

Binuclear dinitrogen complexe of aryl- and benzyldicyclopentadienyltitanium(III) compounds

The preparation of the deep-blue diamagnetic dinitrogen complexes (Cp₂TiR)₂N₂ with R=C₆H₅, o-, m-, p-CH₃C₆H₄, C₆F₅, CH₂C₆H₅ is described. Their chemical and physcial properties confirm the formulation in which the R groups are σ-bonded to the Cp₂Ti moiety, and the two nitrogen atoms are equivalent. The heats of formation of the complexes from Cp₂TiR and N₂ in toluene have been determined from spectrophotometric data; for R=C₆H₅, o-, m-, p-CH₃C₆H₄, C₆F₅, CH₂C₆H₅, the values are ?18, ?9, ?17, ?20, ?17 and ?14 kcal·mol^?1, respectively. The solid complexes vary markedly in thermal stability, and are extremely air sensitive. The complexed nitrogen can be completely reduced with sodium naphthalene; after hydrolysis of the products, NH₃ and N₂H₄ are obtained. In the thermolysis of the solids, some of the nitrogen is reduced.     

On the mechanism of the reduction of molecular nitrogen with aryldicyclopentadienyltitanium(III) complexes

The first step in the reduction of the dinitrogen ligand in (Cp₂TiR)₂N₂ (R  C₆H₅, m-, p-CH₃C₆H₄, C₆F₅, CH₂C₆H₅) by sodium napthalene (NaC₁₀H₈) involves the removal of one Cp group per titanium atom. The resulting diimide precursor reacts with a second mole of NaC₁₀H₈ with formation of a hydrazine precursor. This compound is thermally unstable and decomposes to an ammonia precursor. A minor part of the hydrazine precursor abstracts a proton from the solvent.