A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which is C5H5-) 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, 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. Metallocenes are a subset of a broader class of organometallic compounds called sandwich compounds.
In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are aromatically stabilized. Here they are shown in a staggered conformation.
The first metallocene to be classified was ferrocene, and was discovered simultaneously in 1951 by Kealy and Pauson, and Miller et al. Kealy and Pauson were attempting to synthesize fulvalene through the oxidation of a cyclopentadienyl Grignard reagent with anhydrous FeCl3 when they dicovered C10H10Fe as their product.[1] At the same time, Miller et al reported the same iron product from a reaction of cyclopentadiene with iron in the presence of alumninum, potassium, or molybdenum oxides.[2] The structure was determined by Wilkinson and Fischer and they were awarded in Nobel Prize for Chemistry in 1973 for the structural determination of Fe(C5H5)2.[3] They determined that the carbon atoms of the cyclopentadienyl (Cp) ligand contributed equally to the bonding and that bonding occured due to the metal d-orbitals and the π-electrons in the p-orbitals of the Cp ligands. This complex is now known as ferrocene and the group of transition metal dicyclopentadienyl compounds is known as metallocenes and have the general formula [(η5-C5H5)2M]. Fischer et al. first prepared the ferrocene derivitaves involving Co and Ni.
The general name metallocene is derived from ferrocene, (C5H5)2Fe or Cp2Fe , systematically named bis(η5-cyclopentadienyl)iron(II). According to the IUPAC definition, a metallocene contains a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure, i. e., the two cyclopentadienyl anions are co-planar with equal bond lengths and strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as η5, pronounced "pentahapto." There are exceptions, such as uranocene, which has two cyclooctatetraene rings sandwiching a uranium atom.
In metallocene names, the prefix before the -ocene ending indicates what metallic element is between the Cp groups. For example in ferrocene, iron(II), ferrous iron is present.
In contrast to the more strict definition proposed by IUPAC, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation -ocene, is applied in the chemical literature also to non-transition metal compounds, such as Cp2Ba, or structures where the aromatic rings are not co-planar, such as found in manganocene or titanocene dichloride (Cp2TiCl2).
Some metallocene complexes of actinides have been reported where there are three cyclopendadienyl ligands for a monometallic complex, all three of them bound η5.[4]
There are many (η-C5H5)-metal complexes and they can be classified by the following formulas: [5]
Formula | Description |
---|---|
[(η-C5H5)2M] | Symmetrical, classical 'sandwich' structure |
[(η-C5H5)2MLx] | Bent or tilted Cp rings with additional ligands, L |
[(η-C5H5)MLx] | Only one Cp ligand with additional ligands, L |
Metallocene complexes can also be classified by type: [5]
There are three main routes that are normally employed in the formation of these types of compounds: [5]
1. Using a metal salt and cyclopentadienyl reagents
2. Using a metal and cyclopentadiene
3. Using a metal salt and cyclopentadiene
A structural trend for the series MCp2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.[7]
M(C5H5)2 | rM-C (pm) | valence electron count |
---|---|---|
Fe | 203.3 | 18 |
Co | 209.6 | 19 |
Cr | 215.1 | 16 |
Ni | 218.5 | 20 |
V | 226 | 15 |
In metallocenes of the type (C5R5)2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal X-ray diffraction studies reveal both eclipsed or staggered rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few kJ/mol. Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimize steric hindrance between the methyl groups.
Spectroscopic properties: [5]
1. Vibrational (infrared and raman) spectroscopy of metallocenes.
Ferrocene (cm-1) | Ruthenocene (cm-1) | Osmocene (cm-1) | |
---|---|---|---|
C-H stretch | 3085 | 3100 | 3095 |
C-C stretch | 1411 | 1413 | 1405 |
Ring deformation | 1108 | 1103 | 1096 |
C-H deformation | 1002 | 1002 | 995 |
C-H out-of-plane bend | 811 | 806 | 819 |
Ring tilt | 492 | 528 | 428 |
M-ring stretch | 478 | 446 | 353 |
M-ring bend | 170 | 185 | - |
2. NMR (1H and 13C) spectroscopy of metallocenes.
3. Mass spectrometry of metallocenes.
Derivatives of Metallocenes:
After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.
1. Metallocenophanes
2. Polynuclear and heterobimetallic metallocenes
3. Multi-Decker sandwich compounds
4. Metallocenium cations
Olefin polymerization using metallocene catalysts has improved upon the traditional Ziegler–Natta catalysis by introducing polymer tacticity and increasing productivity.
The ferrocene/ferrocenium biosensor is used as a glucose sensor and determines the levels of glucose in a sample electrochemically through a series of connected redox cycles.
A group of bent metallocene dihalides [Cp2MX2] have shown anti tumor properties for M=Ti, Mo, Nb.[8]
Ferrocene exhibits flame-retardant and smoke suppressing properties for polymeric materials such as poly(vinylchloride).
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