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 Keally and Pauson, and Miller et al. Keally and Pauson were attempting to synthesize fulvalene through the oxidation of a cyclopentadienyl Grignard reagent with anhydrous FeCl3 when they discovered C10H10Fe as their product. At the same time, Miller et al reported the same iron product from a reaction of cyclopentadiene with iron in the presence of aluminum, potassium, or molybdenum oxides. The structure was determined independently by Wilkinson and Fischer and they were awarded the Nobel Prize in Chemistry in 1973 for their work on sandwich compounds, including the structural determination of ferrocene. They determined that the carbon atoms of the cyclopentadienyl (Cp) ligand contributed equally to the bonding and that bonding occurred 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 derivatives 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).
There are many (η-C5H5)-metal complexes and they can be classified by the following formulas:
|[(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:
- Bent or tilted
- More than two Cp ligands
Synthesis of metallocenes
There are three main routes that are normally employed in the formation of these types of compounds:
1. Using a metal salt and cyclopentadienyl reagents
- The starting point is the cracking of dicyclopentadienyl, the dimer of cyclopentadiene.
- Working in a weak acidic environment, cyclopentadienyl can be deprotonated by strong bases or alkali metals. Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of reactions.
- MCl2 + 2 NaC5H5 → (C5H5)2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; Solvent = THF, DME, NH3)
- CrCl3 + 3 NaC5H5 → [(C5H5)2Cr] + ½ C10H10 + 3 NaCl
- NaCp acts as a reducing agent and forms cyclopentadienyl metal. Depending on the reaction condition it can form complexes containing σ- bonded cyclopentadienyl rings when added to metal salts.
2. Using a metal and cyclopentadiene
- This technique provides synthetic advantages of using metal atoms in the vapor phase rather than the solid metal. The highly reactive atoms or molecules are generated at a high temperature under vacuum and brought together with chosen reactants on a cold surface.
- M + C5H6 → MC5H5 + ½ H2 (M = Li, Na, K)
- M + 2 C5H6 → [(C5H5)2M] + H2 (M = Mg, Fe)
3. Using a metal salt and cyclopentadiene
- If the metal salt has poor basicity it is not able to deprotonate cyclopentadiene, a strong base can be used to produce cyclopentadienyl anions in situ.
- Tl2SO4 + 2 C5H6 + 2 OH− → 2 TlC5H5 + 2 H2O + SO42−
- Many other methods have been developed. Chromocene can be prepared from chromium hexacarbonyl by direct reaction with cyclopentadiene in the presence of diethylamine; in this case, the formal deprotonation of the cyclopentadiene is followed by reduction of the resulting protons to hydrogen gas, facilitating the oxidation of the metal centre.
- Cr(CO)6 + 2 C5H6 → Cr(C5H5)2 + 6 CO + H2
- Metallocenes generally have high thermal stability. Ferrocene can be sublimed in air at over 100 °C with no decomposition; metallocenes are generally purified by vacuum sublimation. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substituted derivative are particularly soluble, even in alkane solvents.
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.
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.
1. Vibrational (infrared and Raman) spectroscopy of metallocenes.
- Infrared and Raman spectroscopies have proved to be important in the analysis of cyclic polyenyl metal sandwich species with particular use in elucidating covalent or ionic M-ring bonds and distinguishing between centrally and coordinated rings. Some typical spectral bands and assignments of iron group metallocenes are shown in the following table:
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.
- Nuclear magnetic resonance (NMR) is the most applied tool in the study of metal sandwich compounds and organometallic species, giving information on nuclear structures in solution, as liquids, gases, and in the solid state. 1H NMR chemical shifts for diamagnetic organotransition-metal compounds is usually observed between 25 and 40 ppm, but this range is much more narrow for diamagnetic metallocene complexes, with chemical shifts usually observed between 3 and 7 ppm.
3. Mass spectrometry of metallocenes.
- Mass spectrometry of metallocene complexes has been very well studied and the effect of the metal on the fragmentation of the organic moiety has received considerable attention and the identification of metal-containing fragments is often facilitating by the isotope pattern of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak [C10H10M]+ and the fragment ions, [C5H5M]+ and M+.
Derivatives of metallocenes:
After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.
- Metallocenophanes feature linking of the cyclopentadienyl or polyarenyl rings by the introduction of a heteroannular bridge (bridges). Some of these compounds undergo thermal ring-opening polymerizations (ROP) to give soluble high molecular weight polymers with transition metals in the polymer backbone. Ansa-metallocenes are derivatives of metallocenes with an intramolecular bridge between the two cyclopentadienyl rings.
2. Polynuclear and heterobimetallic metallocenes
- Ferrocene derivatives: biferrocenophanes have been studied for their mixed valence properties. Upon one-electron oxidation of a compound with two or more equivalent ferrocene moieties the electron vacancy could be localized on one ferrocene unit or completely delocalized.
- Ruthenocene derivatives: in the solid state biruthenocene is disordered and adopts the transoid conformation with the mutual orientation of Cp rings depending on the intermolecular interactions.
- Vanadocene and rhodocene derivatives: vanadocene complexes have been used as starting materials for the synthesis of heterobimetallic complexes. The 18 valence electron ions [Cp2Rh]+ are very stable, unlike the neutral monomers Cp2Rh which dimerize immediately at room temperature and they have been observed in matrix isolation.
3. Multi-decker sandwich compounds
- Triple-decker complexes are composed of three Cp anions and two metal cations in alternating order. The first triple-decker sandwich complex,[Ni2Cp3]+, was reported in 1972. Many examples have been reported subsequently, often with boron-containing rings.
4. Metallocenium cations
- The most famous example is ferrocenium, [Fe(C5H5)2]+, derived from oxidation of ferrocene (few metallocene anions are known).
Many derivatives of early metal metallocenes are active catalysts for olefin polymerization. Unlike traditional and still dominant heterogeneous Ziegler-Natta catalysts, metallocene catalysts are homogeneous. Early metal metallocene derivatives, e.g. Tebbe's reagent, Petasis reagent, and Schwatrz's reagent are useful in specialized organic synthetic operations.
Metallocene dihalides [Cp2MX2] (M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.
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