Metallocenes stability

A wide variety of metallocenes containing different ligands based on indenyl (Ind) or fiuorenyl (El) derivatives, which are isoelectronic and equivalent to cyclopentadienyl (Cp) ligands, have since been synthesized. The Cp ligands in metallocenes stabilize and control the steric environment around to the metallic center, which is the active site for olefin coordination. 

In general, metallocarboranes are much less reactive (more stable) than the corresponding metallocenes and they tend to stabilize higher oxidation states of the later transition metals, e.g, [Cu (1,2-C2B9Hii)2]2- and [Cu (l,2-C2B9Hii)2] are known whereas cuprocene... 

As part of a multi-technique investigation (see also discussion under mid-infrared spectroscopy later), Corrales et al. [13] plotted the carbonyl index for films prepared from three grades of polyethylenes a high-density PE (HDPE), a linear low-density PE (LLDPE) and a metallocene PE (mPE) (see Figure 5). In this study, the data trend shown in Figure 5 correlated well with activation energies derived from the thermal analysis, which showed that the thermal-oxidative stability followed the order LLDPE > mPE > HDPE, whereas the trend... 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Related to these catalysts are the systems based on lanthanide metal systems or rare earth metal complexes [46, 47]. The main problem with these catalyst systems is their instability. When the catalyst solution is prepared by reachng a metallocene with an organolithium compound in a polar solvent, the prepared catalyst soluhon is unstable and decomposes quickly, even under a nitrogen atmosphere. The activity of these catalysts can be high only if the catalyst is added to the polymer soluhon immediately after preparation. Attempts have been made to overcome the stability problem by using an additive in the system to improve the stability and the activity of the catalyst [33-35, 41, 57, 58, 61]. Re-... 

Various phenyl-substituted ketimines and aldimines react with metallocenes 1 and 2, in a manner that depends on the substituents present [41]. In all cases, elimination of the al-kyne is observed. Complex 2b reacts with PhN=CMePh to give the r 2-complex 64, which is stabilized by an additional pyridine ligand [41a], In the reactions of 1 or 2a with the ketimine HN=CPh2, hydrogen transfer generates complexes 65. Two molecules of the aldimine PhN=CHPh are coupled by 2a to give the cyclic diamido complex 66 [41b]. 

Achiral, C -symmetric unbridged metallocenes, 16 104 Achiral hydrobora ting agents, 13 667 Achiral molecules, 6 73 Acicular reinforcement, 5 554 Acid acceptors, in VDC polymer stabilization, 25 719 Acid-activated bentonites, 6 680-681 Acid amide herbicides, 13 319-320 Acid anhydrides, 10 403-406, 484 reactions with alkanolamines from olefin oxides and ammonia, 2 127 Acid-base catalysis, 5 205-209... 

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

In a systematical study, Golovin et al. investigated a series of metallocene derivatives in terms of their redox potentials, mass transport properties, and chemical and electrochemical stabilities in both electrochemical test cells and commercial-size AA rechargeable cells.Figure 43 shows the complete voltammetric scan of the ferrocene-containing elec-... 

Whether or not such electrophilic organometallic species can be identified, or indeed isolated, depends primarily on the stability of the counteranion. The per-fluorophenyl boron compounds B(C,sF5)3 and [B(C6F5)4] , first prepared by Stone and co-workers in 1963 [33], proved particularly useful in this respect. Their use in metallocene polymerisation catalysis [34, 35] led to significantly more active catalysts and well-defined catalyst systems that proved mechanistically informative. These results have then enabled similar species to be detected in the more complex MAO-activated catalyst systems (vide infra). 

The structure of MAO is poorly defined and varies with preparation conditions, but it performs well in activating the metallocene initiator. The use of MAO is complicated by its lack of long-term storage stability. It is usually supplied by manufacturers as a cloudy solution of MAO in toluene MAO has very low solubility in aliphatic solvents. Precipitation is often observed on long standing, especially if the container is frequently opened and exposed to moisture and oxygen. This precipitation, if not too extensive, may not affect the utility of the MAO as a coinitiator. A modified MAO, known as MMAO, offers some improvement in storage stability and improved solubility in aliphatics. MMAO is prepared by controlled hydrolysis of a mixture of trimethylaluminum and triisobutylaluminum. 

Cyclopentadienyl compounds (i.e. metallocenes) (Fig. 5), which have at least one direct metal-carbon bond to the C5H5 ligand, were first synthesised in the 1950s [79,80]. Since then, reactions of cyclopentadienyl reagents have been applied for almost every element [123]. The main application of metallocenes is their use as catalysts in the polymerisation of olefins by Ziegler-Natta polymerisation processes. As many metallocene compounds are volatile and thermally stable, they are also suitable for use as precursors in MOCVD [124-127]. Although cyclopentadienyl compounds have attracted considerable interest as precursors in CVD depositions they are sometimes too reactive [128]. However, high reactivity and thermal stability make cy-... 

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