Pentamethylcyclopentadienyl ligand

Bis(trimethylsilyl)cyclopentadienyl Generic cyclopentadienyl ligand Pentamethylcyclopentadienyl ferf-Butylcyclopentadienyl... 

The solid-state structure of 166 shows that the pentamethylcyclopentadienyl ligands are symmetrically bound to the zinc atoms in a pentahapto fashion, with isometric Zn-C bonds, ranging from 2.27 to 2.30 A. 

Summarizing the available bonding information, decamethylsilicocene (1) is regarded as an electron-rich silicon(II) compound containing a hypercoordinated silicon atom which is sandwiched between two rather weakly 7i-bonded pentamethylcyclopentadienyl ligands and thus is effectively shielded the lone-pair orbital at silicon is part of the frontier orbitals of the molecule. 

The ability to insert in many element-element bonds is an important property of 1 the r -p1 rearrangement of the pentamethylcyclopentadienyl ligands during the reaction is a prerequisite to show a silylene-type reactivity. From a preparative point of view it is worth mentioning that element-silicon bonds which otherwise are difficult to form are easily accessible with the help of 1. In addition, the leaving group character of the pentamethylcyclopentadienyl substituents allows further chemical transformations (vide infra). 

After a decade of research the basic principles in the chemistry of decamethylsilicocene (1) seem to be understood. This compound shows the reactivity of a nucleophilic silylene due to the fact that the Tt-bonded pentamethylcyclopentadienyl ligands are easily transferred to a-bonded substituents during the reaction. The steric requirements of these substituents permit reactions with bulky substrates. The migratory aptitude and the leaving-group character of the pentamethylcyclopentadienyl groups... 

Pentamethylcyclopentadienyl group, 17, 20 leaving-group character, 13, 17, 31-32 ligands, 1, 9, 10... 

The molecular structure of 124 has been elucidated by an X-ray structural analysis. The central structural motif of 124 is an unsymmetrically substituted six-membered TisOs ring. Two pentamethylcyclopentadienyl ligands are coordinated to one titanium atom, while the other two are free of Cp. They are both part of eight-membered TiSi304 ring systems within the silsesquioxane frameworks. This results in an unusual bis(spirocyclic) inorganic ring system in the molecular structure of 124. 

Enolates of the related pentamethylcyclopentadienyl complexes, such as 8, may be generated by treatment with less bulky bases, such as butyllithium, since the methyl protons of the pentamethylcyclopentadienyl ligand are far less acidic than those of the cyclopentadienyl ligand. Alkylation of this enolate occurs with excellent diastereoselectivity to provide the expected product 9 with a d.r. of 96 4 (determined by HPLC). The configuration of the major diastereomer was not rigorously determined, but attack of the electrophile probably occurs from the same face of the enolate as in the analogous cyclopentadienyl complex l87. 

Their structures had been much less explored than those of their heavier congeners such as germylenes and stannylenes. Although dodecamethylsilicocene (19), formally also a silylene, has been known since 1986, compound 19 is stabilized by ri -coordination of pentamethylcyclopentadienyl ligands and is not a congener of a carbene. 

This is not surprising—there are many textbook discussions of the difference between transition and main group elements. Consonant with this is the finding that (Cp )2Mg (the sandwich species alternatively called bis(pentamethylcyclopentadienyl)magnesium and decamethyfinagnesocene) does not react with CO, unlike the corresponding species with Ca and some other metals and metalloids . Indeed, in (Cp )2Mg, there is little room for another ligand around the central metal and Mg seems electronically satisfied. 

The development of the chemistry of organometallic compounds containing the pentamethylcyclopentadienyl ligand has been hampered by the lack of a simple, high yield synthesis of 1,2,3,4,5-pentamethylcyclopentadiene (14). We have recently expanded the method first reported by Sorensen et al. (15) to a large scale preparation which follows Scheme 1 (16). This procedure is applicable... 

In 1976 Bercaw and co-workers reported the stoichiometric reduction of CO to methanol using derivatives of bis(pentamethylcyclopentadienyl)zir-conium (78). Bercaw had shown previously (79) that permethylated cyclo-pentadienyl ligands greatly enhance the stability of these complexes, permitting their isolation and/or in situ identification. In their study, these workers observed and partially characterized the novel hydride carbonyl species (13) which reacts with the dihydride complex Zr(C5Me5)2H2 at room temperature to form the methoxy species (14). 

The conversion of CO + H2 (syn-gas) to hydrocarbons and oxygenates (Fischer-Tropsch chemistry)119 is of considerable industrial importance and recently the activation and fixation of carbon monoxide in homogeneous systems has been an active area for research.120,121 The early transition elements and the early actinide elements, in particular zirconium124 and thorium,125 126 supported by two pentamethylcyclopentadienyl ligands have provided a rich chemistry in the non-catalytic activation of CO. Reactions of alkyl and hydride ligands attached to the Cp2M centers with CO lead to formation of reactive tf2-acyl or -formyl compounds.125,126 These may be viewed in terms of the resonance forms (1) and (2) shown below. 

F. Reactions of Pentamethylcyclopentadienyl Cobalt and Rhodium Complexes of OFCOT with Exogenous Ligands... 

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