Metallocene

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metal
cation

A metallocene is a compound typically consisting of two

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
.

Some metallocenes consist of metal plus two cyclooctatetraenide anions (C
8H2−
8, abbreviated cot2−), namely the lanthanocenes and the actinocenes (uranocene and others).

Metallocenes are a subset of a broader class of compounds called sandwich compounds.[1] 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.

History

Ferrocene

The first metallocene to be classified was

cyclopentadienide, metallocenes of many elements have been prepared.[5]

One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado[6]

Definition

Ball-and-stick model of a metallocene molecule where the cyclopentadienyl anions are in a staggered conformation. The purple ball in the middle represents the metal cation.

The general name metallocene is derived from

planes 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 International Union of Pure and Applied Chemistry, 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 barocene (Cp2Ba), or structures where the aromatic rings are not parallel, such as found in manganocene or titanocene dichloride (Cp2TiCl2).

Some metallocene complexes of actinides have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η5.[7]

Classification

There are many (η5-C5H5)–metal complexes and they can be classified by the following formulas:[8]

Formula Description
[(η5-C5H5)2M] Symmetrical, classical 'sandwich' structure
[(η5-C5H5)2MLx] Bent or tilted Cp rings with additional ligands, L
[(η5-C5H5)MLx] Only one Cp ligand with additional ligands, L ('piano-stool' structure)

Metallocene complexes can also be classified by type:[8]

  1. Parallel
  2. Multi-decker
  3. Half-sandwich compound
  4. Bent metallocene or tilted
  5. More than two Cp ligands

Synthesis

Three main routes are normally employed in the formation of these types of compounds:[8]

Using a metal salt and cyclopentadienyl reagents

Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of reactions. It is most easily obtained by the reaction of molten sodium and dicyclopentadiene.[9] Traditionally, the starting point is the cracking of dicyclopentadiene, the dimer of cyclopentadiene. Cyclopentadiene is deprotonated by strong bases or alkali metals.

MCl2 + 2 NaC5H5 → (C5H5)2M + 2 NaCl            (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH3)
CrCl3 + 3 NaC5H5 → [(C5H5)2Cr] + 1⁄2 "C10H10" + 3 NaCl

NaCp acts as a reducing agent and a ligand in this reaction.

Using a metal and cyclopentadiene

This technique provides using metal atoms in the gas 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 + 1⁄2 H2            (M = Li, Na, K)
M + 2 C5H6 → [(C5H5)2M] + H2            (M = Mg, Fe)

Using cyclopentadienyl reagents

A variety of reagents have been developed that transfer Cp to metals. Once popular was

thallium cyclopentadienide. It reacts with metal halides to give thallium chloride, which is poorly soluble, and the cyclopentadienyl complex. Trialkyltin
derivatives of Cp− have also been used.

Many other methods have been developed.

oxidation of the metal centre.[10]

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 in the laboratory by vacuum

sublimation
. Industrially, sublimation is not practical so metallocenes are isolated by crystallization or produced as part of a hydrocarbon solution. For Group IV metallocenes, donor solvents like ether or THF are distinctly undesirable for polyolefin catalysis. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substitution on the metallocene increases the solubility in hydrocarbon solvents.

Structure

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.[11]

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

steric hindrance between the methyl groups
.

Spectroscopic properties[8]

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 central and coordinated rings. Some typical spectral bands and assignments of iron group metallocenes are shown in the following table:[8]

Spectral frequencies of group 8 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 –

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 paramagnetic 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.[8]

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 facilitated by the isotope distribution of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak, [C10H10M]+, and fragment ions, [C5H5M]+ and M+.

Derivatives

Main article: Sandwich compound

After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.

Metallocenophanes

bridge
between the two cyclopentadienyl rings.

Polynuclear and heterobimetallic metallocenes

  • Ferrocene derivatives: biferrocenophanes have been studied for their mixed
    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.
  • dimerize immediately at room temperature and they have been observed in matrix isolation
    .

Multi-decker sandwich compounds

Nickel triple-decker sandwich complex

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.[12] Many examples have been reported subsequently, often with boron-containing rings.[13]

Metallocenium ions

The most famous example is

ferrocenium, [Fe(C5H5)2]+, the blue iron(III) complex derived from oxidation of orange iron(II) ferrocene. The lithocene anion, [Li(C5H5)2]–[14]
, is the best-documented example of a metallocene anion; otherwise such ions are little known.

Applications

Many derivatives of early metal metallocenes are active catalysts for

Ziegler–Natta catalysts, metallocene catalysts are homogeneous.[8] Early metal metallocene derivatives, e.g. Tebbe's reagent, Petasis reagent, and Schwartz's reagent
are useful in specialized organic synthetic operations.

Potential applications

The ferrocene/

ferrocenium biosensor has been discussed for determining the levels of glucose in a sample electrochemically through a series of connected redox cycles.[8]

Metallocene dihalides [Cp2MX2] (M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.[15]

See also

References

  1. ^
    doi:10.1021/ja01128a527
    .
  2. ^
    S2CID 4181383
    .
  3. ^
    doi:10.1039/JR9520000632
    .
  4. doi:10.1515/znb-1952-0701
    .
  5. doi:10.1021/om100016p
    .
  6. ISSN 0003-2700
    .
  7. doi:10.1021/ic00231a008
    .
  8. ^
    ISBN 978-0632041626
    .
  9. doi:10.1021/om0207865
    .
  10. S2CID 209650632
    .
  11. doi:10.1016/0022-328X(95)05747-D
    . Discusses all metallocene structures available at that time.
  12. ISSN 0094-5714
    .
  13. doi:10.1021/ed081p657
    .
  14. doi:10.1002/anie.199417441
    .
  15. doi:10.1021/ja00024a002
    .

Additional references
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