A lanthanocene is a type of metallocene compound that contains an element from the lanthanide series. The most common lanthanocene complexes contain two cyclopentadienyl anions and an X type ligand, usually hydride or alkyl ligand.
A lanthanocene is a type of metallocene compound that contains an element from the lanthanide series. The most common lanthanocene complexes contain two cyclopentadienyl anions and an X type ligand, usually hydride or alkyl ligand.
In 1954, Wilkinson and Birmingham described the tris(cyclopentadienyl)lanthanide complex, Ln(C5H5)3 (Ln = La, Ce, Pr, Nd, Sm). [1] [2] However, due to the highly moisture and oxygen sensitive character of organolanthanide compounds, as well as the incapability of the separation of simple alkyl and aryl derivatives of the type LnR3, this area of organomemtallic chemistry experienced a period of relative stagnation for two decades before lanthanocene complexes were prepared for some of the later lanthanides (Ln = Gd, Er, Yb, Lu). [3] [4] [5] [6]
The synthesis part will focus on lanthanide(III) metallocene complexes that contain Ln-C bonds, which are widely prepared from the corresponding Ln-Cl precursors as shown. [3]
The synthetic route leading to lanthanocene chlorides are summarized:
With the large 4f orbitals, lanthanide elements display properties significantly different from the common d-block transition metals. The large ionic radii limits the extent to which 4f orbitals can overlap with ligands, but at the same time allows the organolanthanide complexes to attain higher coordination numbers. [5] As such, cyclopentadienyl anions (abbreviated Cp) are typically used to occupy the unsaturated site as well as stabilize the metal complex. Some of the alkyl and hydride lanthanocene complexes exhibit unique activities towards C-H bond activation, [6] alkene functionalization, [7] and carbonyl activation. [8]
In 1983, Watson reported one of the first lanthanocene catalyzed C-H bond activation reactions. [6] The active catalysts are lutetium-methyl or lutetium-hydride complexes, which react at room temperature in hydrocarbon solvents with benzene, pyridine, and the ylide CH2PPh3 to give stable, isolatable products.
Studies have shown that organolanthanides are extraordinary catalysts for hydrofunctionalization reactions including hydrogenation, hydrosilylation, hydroboration, hydroamination, etc. Examples for each type have shown below. [9] [10] [11] [12]
Mechanism for hydrogenation is shown below, where the active catalyst is generated by sigma-bond metathesis, followed by olefin insertion and another sigma-bond metathesis to regenerate the catalyst. This is also the mechanism for other hydrofunctionalization.
Yet, as the large cyclopentadienyl ligand hinders the metal center, reactions with substituted alkenes are inhibited. Two general methods are used to overcome this difficulty. One is to increase the size of metal by incorporating lanthanides with larger ionic radii. Due to the lanthanide contraction, [13] this means replacing the late lanthanides with the early lanthanides. Another method is to decrease the size of the ligand by manipulating the geometry of ligands and substitutions on ligands. [7] For example, a hinged or ansa-bridged cyclopentadienyl ligand could be used to pull ligands closer to each other, and hence creating more open access to the metal center as shown on the right. [14]
A metallocene is a compound typically consisting of two cyclopentadienyl anions (C
5H−
5, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride or vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization.
Cyclopentadiene is an organic compound with the formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−.
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups. Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η5-) bonding mode. The metal–cyclopentadienyl interaction is typically drawn as a single line from the metal center to the center of the Cp ring.
Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.
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Rhodocene is a chemical compound with the formula [Rh(C5H5)2]. Each molecule contains an atom of rhodium bound between two planar aromatic systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has (haptic) covalent rhodium–carbon bonds. The [Rh(C5H5)2] radical is found above 150 °C (302 °F) or when trapped by cooling to liquid nitrogen temperatures (−196 °C [−321 °F]). At room temperature, pairs of these radicals join via their cyclopentadienyl rings to form a dimer, a yellow solid.
In organometallic chemistry, bent metallocenes are a subset of metallocenes. In bent metallocenes, the ring systems coordinated to the metal are not parallel, but are tilted at an angle. A common example of a bent metallocene is Cp2TiCl2. Several reagents and much research is based on bent metallocenes.
Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula [(η5-C5H5)Fe(CO)2]2, often abbreviated to Cp2Fe2(CO)4, [CpFe(CO)2]2 or even Fp2, with the colloquial name "fip dimer". It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but less soluble in carbon tetrachloride and carbon disulfide. Cp2Fe2(CO)4 is insoluble in but stable toward water. Cp2Fe2(CO)4 is reasonably stable to storage under air and serves as a convenient starting material for accessing other Fp (CpFe(CO)2) derivatives (described below).
In organometallic chemistry, f-block metallocenes are a class of sandwich compounds consisting of an f-block metal and a set of electron-rich ligands such as the cyclopentadienyl anion.
Jaqueline Kiplinger is an American inorganic chemist who specializes in organometallic actinide chemistry. Over the course of her career, she has done extensive work with fluorocarbons and actinides. She is currently a Fellow of the Materials Synthesis and Integrated Devices group in the Materials Physics and Applications Division of Los Alamos National Laboratory (LANL). Her current research interests are focused on the development of chemistry for the United States’ national defense and energy needs.
William J. Evans is a Distinguished Professor at the University of California, Irvine, who specializes in the inorganic and organometallic chemistry of heavy metals, specifically the rare earth metals, actinides, and bismuth. He has published over 500 peer-reviewed research papers on these topics.
Russell P. Hughes an American/British chemist, is the Frank R. Mori Professor Emeritus and Research Professor in the Department of Chemistry at Dartmouth College. His research interests are in organometallic chemistry, with emphasis on the chemistry of transition metal complexes interacting with fluorocarbons. His research group’s work in this area led to several creative syntheses of complexes of transition metal and perfluorinated hydrocarbon fragments.
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