Metallocenes actinide

Another combination catalyst that includes a Lewis acid component is Cp2MCl2/2n-BuLi/B(C6F5)3.60-61 Interest in such systems came about as a consequence of the successful use of well-characterized homogeneous cationic and cation-like Group 4 and actinide metallocene catalysts for olefin polymerization.62 63... 

The actinide/iron carbonyl complexes can be considered as partially ionic [An] " [Fe(CO)3] and [An] " "[Fe(CO)2] , in which there is An—Fe bonding as evidenced by CID elimination of CO molecules to ultimately yield AnFe. These reactions thus provided a route to actinide-transition metal bonding. In the reactions with Fe(CsH5)2, the actinide ion replaces the iron atom to produce the An cyclopentadienylide complexes, [An] " [(C5H5)3] , providing a new route to actinide metallocenes. The ThO" and UO ions reacted with Fe(CO)5 to yield formaUy [ThO] +[Fe(CO)4], [UO] +[Fe(CO)3] , and... 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

The first report of catalyzed chain growth on aluminum was presented by Samsel et al., using metallocenes of actinides [13] or hafnium [14, 15], Gibson et al. described a detailed study of chain growth using the bis(imino)pyridyl iron complex 1 in combination with several metal alkyls [16], Other systems reported to... 

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. 

The activation of y-CH bonds of the ligand N(SiMe3)2 has been observed in a number of early transition metal and actinide complexes.243 Attempts to prepare metallocene derivatives of this ligand for the Group IV metals lead to cyclometallation as shown (reactions 96 and 97).249,250... 

In Harrod s original report of the screening of metallocenes as dehydrocoupling catalysts, Cp MMe2 (M = U and Th) was studied and although these actinides did promote the reaction of PhSiH3, only dimers or small oligomers were produced.6b... 

A number of early transition metal compounds, e.g., Ti111 hydrides, have long been known as hydrogenation catalysts. Similarly, metallocene compounds of lanthanides and actinides can be extremely active, as a comparison of the turnover numbers of 1-hexene hydrogenations at 25°C (1 bar H2) show CpfLuH, 120,000 [Ir(COD)-(py)(PCy3)]PF6, 6400 [Rh(COD)(PPh3)2]PF6, 4000 RuHCl(PPh3)3, 3000 ... 

More recent studies have shown that a number of other mechanisms are operative in the hydrosilation process for different metals. Mechanistic proposals for early metals, lanthanides and actinides have been elaborated on. These involve a Chalk-Harrod like initial migratory insertion into a metal-hydride bond, followed by a a-bond metathesis step (Scheme 4). An alternative mechanism, however, was proposed for Group 4 metallocene catalysis, which involves a coordinated olefin, which undergoes a-bond metathesis with the hydrosilane. ... 

The analogous uranium(III) compounds also show unusual reactivity patterns. For example, addition of COT to (Cp )3U yields a mixed metallocene dimer (18) bridged by a COT ligand in this reaction, the (Cp )3U complex has effectively acted as a formal three-electron reductant (equation 3)." A variety of substituted (Cp )3U complexes form adducts with CO and CNR (isocyanides) - these are rare examples of actinide metals with r-acidic ligands." Paramagnetic uranium alkyl complexes are active in a range of catalytic processes." ... 

The cot clianion, like Cp, is aromatic but has ten electrons in eight tt orbitals. When it is allowed to react with tetrapositive actinides, such as Np, Th , Pa ", and a neutral metallocene results. --... 

Reactions of the monopositive actinide ions An+ with pentamethylcyclopentadiene, HCp, have been studied by mass spectrometry. This was the first study of the An+/HCp reaction for An+ = Am+, Cm+, Bk+, Cf/ and Es+. Each of the actinide ions reacted with HCp to produce [AnCp ]+ (+H), as well as additional products. Both Cf1-and Es+ have previously been found to be inert toward most alkenes, but efficiently reacted with HCp to induce (i) H-loss and [AnCp]+, (ii) H2-loss and [An(C5Me4CH2)]+, and (iii) CH3-loss and [An(C5Me4H)]+ (An = Cf, Es). These were the first organoeinsteinium complexes derived from activation of an organic substrate. Secondary products included [Cp 2An]+ (An = Am, Cm, Bk, Cf, Es), the compositions of which correspond to the metallocene sandwich complexes.93... 

Although the elements of the lanthanide and actinide series have long been known to exhibit a quite extensive organometallic chemistry, it is only within the last decade that typical sandwich species have been prepared and studied. These systems however, although resembling the familiar metallocene and bis-arene compounds of the d-block elements, are not strictly their analogues since in both f-orbital series the known sandwich complexes are derived only from the cyclooctatetraenyl dianion. 

The most famous of the sandwich compounds of cyclic CSHS is uranocene, Cns-CSHS)2U. This air-sensitive compound is actually a 22-electron species— clearly the 18-electron rule does not apply in the realm of the actinides. A schematic diagram of selected orbital interactions in uranocene is given in Figure 5-18. Similar metallocenes are also known for other actinides, although their study is inhibited somewhat by the radioactivity of these elements. 

The synthesis of the metallocene complexes 1 and 2 (2) was reported by Fagan et al. [27] and involves two major steps (1) reaction of the actinide chloride with 2 equiv of Cp MgCl to form the corresponding dichloride complex, and (2) reaction of the latter with 2 equiv of MeLi LiBr to obtain the corresponding dimethyl complex. The spectroscopic characterization of the complexes was fully disclosed however, the crystal structure for complex 2 was reported only recently [28]. 

Although the monomeric complexes of actinides with N, P or O donor Hg-ands are stable and could be synthesized without much problem, mono- and dithiol complexes are very difficult to synthesized because they tend to support a monomer-dimer equilibrium [244]. Only lately have a few complexes of bis(pentamethylcyclopentadienyl) metallocene dithiolates, (Ti -C5Me5)2 Th(SPr)2 [202] and (ii5-C5Me5)2U(SR)2 (where R = Me, f-Pr, t-Bu, Ph) [284] appeared. 

One of the most exciting and active areas of actinide research involves the development of novel catalysts. Thoriiun and uranium metallocene complexes have been shown to react in highly specific manners that in some cases parallel those of early transition metals, and in others the reactions are unique to the actinides. M. Sharma and M.S. Eisen s chapter details metallocene organoac-tinide chemistry with a special focus on novel reaction pathways that have in some cases been deduced from thermochemical studies. 

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