Metallocenes.pdf

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METALLOCENES 35

METALLOCENES

Scope

Although the traditional usage of the term metallocene has encompassed sand-

wich structures containing two rings of ﬁve carbons each bound through all atoms

to a central metal atom, common parlance in the ﬁeld of oleﬁn polymerization

has expanded this deﬁnition to include structures having only one C 5 ring or

none. The term is now applied to all single-site catalysts (qv) (ie, all catalysts

having a single, well-deﬁned active-center structure), in some cases restricted to

complexes of the early- and middle-transition metals and lanthanides, to distin-

guish the catalysts from the recent families discovered by Brookhart and others

based on late-transition metals. For the purposes of this publication, we equate

the word metallocene with the family of compounds containing at least one C 5

ring (or analogous heterocycle) bound to an early-transition-metal or f-block atom

that, in the presence of a cocatalyst if required, effect the polymerization of

ethylene and other

Historical Background

Although the discovery of (C 5 H 5 ) 2 Fe (ferrocene), ﬁrst reported by Kealy and

Paulson in 1951 (1) but recognized as a sandwich compound by Wilkinson and

co-workers (2), brought into being a new family of complexes that contain the

Cp 2 M (Cp

-oleﬁns. We will concentrate on those complexes, predomi-

nantly bis(cyclopentadienyl)zirconium(IV) compounds, that polymerize ethylene

and propylene with high efﬁciency.

C 5 H 5 ) fragment, it was not until the discoveries of Sinn and Kaminsky

in the late 1970s that metallocenes, and speciﬁcally zirconocenes, were recognized

36 METALLOCENES

Vol. 7

Ti, Zr) (4,5), which had begun

soon after the ﬁrst reports of titanocene dichloride; and ( 2 ) the observation that

small amounts of water added to the medium of certain Ziegler–Natta ethylene

polymerizations caused marked increases in the rate of enchainment. It was the

ﬁnding (by many accounts serendipitous) that large amounts of moisture, when

added to the conventional Ziegler–Natta cocatalyst trimethylaluminum, dramat-

ically improved the activity of mixtures of the aluminum alkyl with metallocenes,

that set into motion the wave of industrial and academic research in this area.

Although the activities of the new metallocene catalysts were quite remark-

able (

10 6 g(PE)/mmol(Zr)), they would have probably remained a labora-

tory curiosity had they not exhibited a crucial characteristic: they were homoge-

neous. (Here we understand homogeneous to mean soluble and also having a single

active-site structure.) Single-site catalysts were known, of course, for oleﬁn poly-

merization. Catalysts based on vanadium had produced polymers with molecular

weight distributions ( M w / M n ) of about 2, the value expected for ideal single-site

behavior in polymerization and chain transfer. However, the vanadium family of

single-site catalysts had aspects of its chemistry that made it less attractive to

academic work and also to commercialization. First, these catalysts are generally

mixtures of vanadium complexes of hard inorganic ligands such as halides, oxides,

or acetylacetonate groups, most if not all of which would be expected to be lost

upon reaction with a large excess of trialkylaluminum or chlorinated aluminum

alkyl compound. Because of this, the precursor to the active catalyst generally

lacks modiﬁable substituents that persist in the active complex, making group

substitution and structure–property trend analysis futile. The paramagnetism of

vanadium catalysts also precludes NMR spectroscopy, a key tool for characteriza-

tion. Also, because of the generally low activity and temperature sensitivity of the

vanadium-based single-site catalysts, use of these catalysts in modern large-scale

industrial settings has been difﬁcult, because such important processes as Union

Carbide’s UNIPOL or the slurry process of Phillips Petroleum have eliminated

ash removal, and also require fairly high reaction temperatures (40–110 ◦ C) for a

commercially acceptable operation, as dictated by the economics of heat transfer

from the reactor. The early non–aluminoxane-based titanium catalysts (such as

Cp 2 TiCl 2 /(C 2 H 5 ) 2 AlCl (4)) were homogeneous, but not extremely active.

In one stroke, the discovery of methylaluminoxane (the product of the par-

tial hydrolysis of trimethylaluminum) gave to researchers a system of interest to

both the synthetic chemist and the reaction engineer. Thus we have seen a ﬂower-

ing of chemical discovery and innovation in open and patent literatures alike, as

investigators have taken advantage of the stability of the Cp M linkage during

polymerization and have introduced substituents that change the active-site be-

havior in myriad ways. Driven as much by the desire for a degree of novelty that

would confer patent protection as by the thirst for knowledge, researchers have

extended the range of single-site catalysts far beyond those having Cp groups. The

rise of metallocenes has in large part legitimized the study of oleﬁn polymerization

among “serious” organometallic chemists, and in turn, has forced captains of in-

dustry to become more ﬂuent with the fundamentals of synthetic and mechanistic

as powerful oleﬁn polymerization catalyst precursors. (For a good discussion of

the succession of discoveries leading to the Kaminsky system, see Refs. 3) This

work brought together two strands of research: ( 1 ) the investigation of the alkyla-

tion and polymerization chemistry of Cp 2 MX 2 (M

Vol. 7

METALLOCENES 37

chemistry than seemed possible only decades ago. One suspects that the ramiﬁ-

cations of both changes will be felt for years to come, regardless of the ultimate

commercial acceptance of metallocene polymerization technology.

Properties of Metallocenes

Structure and Bonding. Both bis- and mono-Cp complexes of early tran-

sition metals are marked by a pseudo-octahedral environment, which is evident

from the bond angle between the halide atoms of zirconocene dichloride, for ex-

ample, of 90 ◦ . The angle formed from the metal atom and the two centroids of

the Cp rings in bis-Cp complexes is generally about 120 ◦ , although this can be

raised or lowered, with the latter effect being more common, through the addi-

tion of bridging groups between the rings. Metallocenes useful for polymerization

generally have d 0 electronic conﬁgurations, and are thus diamagnetic. Complexes

containing Cp rings alone are usually weakly colored, often yellow–gold, but the

fusion of an aromatic ring to the Cp ring results in a much stronger coloration,

presumably because of L–M (

π ∗ orbitals of ethylene or CO.

Chemical Reactivity. Group IVB metallocenes are moderately air-stable

compounds unless ( 1 ) there is a highly basic carbanion attached, either in the form

of a highly alkylated Cp ring or a polycyclic C 5 -containing ligand, or ( 2 ) there is

an alkyl group attached directly to the metal; in the latter case the compound

is extremely water- and oxygen-sensitive, and often decomposes upon heating or

exposure to strong light. The hydrolysis products of Group IVB metallocene dichlo-

rides are generally HCl and zirconia, as well as a mixture of organic compounds.

Preparation. Group IVB metallocenes and Lanthanocenes are convention-

ally prepared by ligand exchange between lithium or Grignard reagents and metal

chlorides (eqs. 1a and 1b). Often the metal chlorides are present as etherate com-

plexes; such complexes need to be prepared carefully, as the reaction is unusually

exothermic.

m Cp

MX n

Cp′ m MX ( n − m )

m LiX

(1a)

m Cp

′

MgX

MX′ n

′ m MX

′ ( n − m )

m MgXX

′

[X,X′= halide; M = transition metal; Cp′= (substituted) cyclopentadienyl]

(1b)

Milder preparatory schemes include the reaction of silyl or stannyl cyclopen-

tadienides with metal halides, as shown in equation 2. (For the application of this

– d ∗ ) transitions.

The molecular orbitals involved in bonds between the Cp 2 M fragment and

the remaining, nonring ligands are shown in Figure 1, adapted from Lauher and

Hoffmann (6). In d 0 systems such as Cp 2 ZrCl 2 , the electrons ﬁlling these orbitals

are derived solely from the non-Cp ligands. Alteration of the metallocene geome-

try, or in the electronic properties of any of the ligands will obviously change the

energies of the molecular orbitals, with consequences for metal–alkyl reactivity.

Because of the lack of metal-based electron density, however, Group IVB metal-

locenes are not strongly inﬂuenced by acceptor orbitals on non-Cp ligands such as

the

′

38 METALLOCENES

Vol. 7

2a 1

b 2

a 1

1a 1

a 1 + b 2

( a )

( b )

Fig. 1. ( a ) Orbitals responsible for metal–hydride bonding in Cp 2 TiH 2 . Left-hand orbitals

derive from Cp 2 Ti 2 + , while the in-phase and out-of-phase combinations of the two H −

1 s orbitals are on the right. Coordinates deﬁned in ( b ). Reprinted with permission from

Ref. 6. Copyright (1976) American Chemical Society.

reaction to bridging ligands, see Ref. 7.) This method is especially well suited to

the preparation of titanocenes, as the use of a strongly reducing metal alkyl such

as LiCp is avoided. Another route to metallocenes involves the reaction of the

neutral mono- or bis-Cp ligand with a metal tetraamide (eq. 3). This pathway has

been demonstrated by Jordan (8) to allow the preparation of racemic bis(indenyl)

zirconium compounds in high selectivity. Aluminum complexes of bis(indene) lig-

ands will also undergo transmetallation reactions with zirconium tetraamides in

high yield (9).

R m

EMe 3

R m

MX n

MX ( n − m )

n XEMe 3

(2)

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METALLOCENES 39

R m

n HNR

′ 2

M(NR

′ 2 ) n

MX ( n − m )

halide; M

transition metal; E

Si, Ge, Sn]

(3)

Ti, Zr).

Titanocene dichloride has been reported (10) to be a potentially useful anticancer

agent, while the zirconium analogue has been identiﬁed as a mutagen in at least

one toxicological report (11). Prudence dictates that extreme care should be exer-

cised with metallocenes, particularly those with polycyclic aromatic groups.

Cocatalysts for Polymerization

Aluminoxanes.

Health and Safety Considerations. Solutions of MAO are generally non-

pyrophoric because of the low volatility of toluene. However, if toluene is removed,

a ﬁnely divided white powder remains that is extremely air- and water-reactive,

and will often spontaneously burst into ﬂame. Contact of MAO or its solutions with

water usually results in an immediate explosion, but caution must be maintained

even after the initial reaction, because, similar to other aluminum alkyl reagents,

MAO often forms a crust of alumina when hydrolyzed quickly. This crust tends to

protect the remaining alkyl, and disturbing it without care may cause a second

violent reaction.

Similar safety considerations apply to modiﬁed MAO, although when solvent

has been removed these materials are often glasses or viscous oils. Modiﬁed MAO

solutions in light alkanes such as isopentane may form dangerous mixtures with

air because of the volatility of the solvent and the presence of an intrinsic source

of ignition.

Methylaluminoxane. Methylaluminoxane (MAO) is prepared from the con-

trolled hydrolysis (eq. 4) of trimethylaluminum (TMA), usually in toluene.

2 n H 2 O

2 n CH 4

2 n

(4)

This is an extremely exothermic reaction and poses great hazards, as large

amounts of heat and gas must be safely removed. Early workers employed hy-

drated inorganic salts as a controllable method for addition of water, but it appears

that this procedure is not practiced commercially. The patent literature suggests

that the main suppliers of MAO (Albemarle, Akzo Nobel, and Witco) add water

directly, either by use of moist nitrogen or a marginally wet aromatic solvent. The

precautions necessary for the synthesis of MAO add to the cost of this material,

which suffers as well from the expense of TMA, which cannot be made by the

normal routes available for the higher trialkylaluminum compounds, such as the

Toxicity. There is little known of the effects on living organisms of metal-

locene precursors beyond those of the parent compounds Cp 2 MCl 2 (M

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