化学专业英语之有机金属化合物——金属配合物

化学专业英语之有机金属化合物——金属配合物

ORGANOMETALLICS—METAL π COMPLEXES

Metal π complexes are characterized by a type of direct carbon-to-metal bonding that is not a classical ionic, σ, or πbond . Numerous molecules and ions, e.g., mono- and diolefins, polyenes, arenes, cyclopentadienyl ions, tropylium ions, andπ-allylic ions, can form metal πcomplexes with transition-metal atoms or ions. These are classified as organ metallic complexes, because of their direct carbon-metal bond, and as coordination complexes, because the nature and characteristics of the TT ligands are similar to those in coordination complexes. In 1827, Zeise reported that

ethylene reacts with platinum (II ) chloride to form a salt K (C

2H

4

)PtCl

3

(l),

but it was not until after the elucidation of the structure of ferrocene (2) in 1953 that attention was redirected to Ziese's salt, which was the first reported metal π complex.

Generally, metal TT complexes can be classified into three main groups; olefin-, cyclopentadienyl-, and arene-metal π complexes; mixed complexes are categorized according to structural or chemical analogies within these groups. Allyl π complexes are designated as olefin πcomplexes in this review. Study of metal πcomplexes has contributed to the elucidation of the mechanisms of Ziegler-Natta polymerization, the oxo reaction, and catalytic hydrogenation, and to the development of the Wacker process which is used for the oxidation of olefins1.

The following nomenclature for metal it complexes is used

:

(1) Organic πligands precede the metal atom. (2)Organic πligands precede inorganic 7t ligands. (3)Inorganic π ligands, e.g., carbonyl or nitrosyls, generally follow the metal atom; halides also follow the metal but precede carbonyls or nitrosyls. (4)A prefix, e.g., di, is preferred rather than bis in describing sandwich-typeπ complexes, e.g., dibenzenechromium.

(5) The symbol π can be used preceding a ligand in order to distinguish π-complex bonding from a, ionic, or other bonding. The symbol η(eta or hapto)precedes a ligand and indicates the number of C—M bonds in the ligand.

Monoolefins , dienes, polyolefins, and acetylenes serve as ligands to transition metals and form olefin πcomplexes. Typical examples of olefin πcomplexes are monoolefin ligands, e.g., potassium η2-ethyleneplatinum trichloride (1); and cyclopentadienylium. –η3-cycloheptatrienylium molybdenum dicarbonyl (3); diene ligands, eg, η4-butadieneiron tricarbonyl(4 ).

Certain of the delocalized π-electron ring systems of aromatic

molecules overlap with d

xy and d

y3

metal orbitals as do the π electrons

of alkenes with metal d orbitals2. The following aromatic rings can form π complexes;

The C

5H

5

- ,C

6

H

6

,and C

8

H

S

arenes are the most common in arene K complexes

that are characterized by π-bonded rings alone or π-bonded rings that are associated with one ring and other ligands, eg, halogens, CO, RNC, and R

3

P. Typical examples are the di-η5-cyclopentadienyl complexes , ie, metallocenes , eg , di-η5-cyclopentadienyliron (2 ). In

di-η4-5-cyclopentadienyliron ,ie, ferrocene, the 6-π-electron system of

the C

5H

5

- ion is bonded to the metal. Other aromatic ring systems are

mono-η5-cyclopentadienylmetal nitrosyl and carbonyl complexes.

Properties

The π-Complex Bond.Metal πcomplexes are among those that are least satisfactorily described by crystal-field theory (CFT) or valence-bond theory (VBT). The nature of the bonding can be treated more completely and quantitatively by molecular-orbital theory (MOT) or ligand-field theory (LFT). The ligand-field theory originally was advanced as a corrected CFT. The LFT relies on the use of molecular orbitals and often is used interchangeably with the MOT. The usual approach is to use the linear combination of atomic orbitals (LCAO) method. It is assumed that when an electron in a molecule is near a particular nucleus, the molecular wave function is approximately an atomic orbital that is centered at the nucleus. The molecular orbitals are formed by adding or subtracting the appropriate atomic orbitals. For transition metals .the "3d, 4s, and 4p orbitals are the atomic orbitals of interest. The ligands may have σ-and π-valence orbitals. Once the appropriate atomic orbitals have been selected for the metal and ligands, the proper linear combination of valence atomic orbitals is determined for the molecular orbitals. The determination of orbital overlaps that are possible, ie, meet inherent symmetry requirements, is done by application of the principles of group theory. At this point, the procedure becomes arbitrary in that approximate wave functions must be selected for use in the calculations of the overlap integrals and coulomb integrals3. Finally, an arbitrary charge distribution is chosen and the orbital energies and interaction energies are calculated, and a solution of the secular equation for the energies and coefficients of the atomic wave functions can be determined. A new initial charge distribution is repeated until consistent values are obtained.

Reactions

Metal πcomplexes react with a wide range of chemical reagents. However, the reactions of the π-olefin-, π-cyclopentadienyl-, and

it-arene-metal complexes are distinctly characteristic of each group, πCyclopentadienyl complexes, ie, metallocenes ,exhibit a high degree of aromaticity and undergo many typical aromatic substitution reactions. However, the π arene complexes do not exhibit a discernible degree of aromaticity.

Although most physical properties, particularly the structure of metal TT complexes, are interpreted by use of the basic principles of coordination chemistry, these established principles do not explain suitably some reaction anomalies of the different groups of metal π complexes.

Olefin πComplexes. Reactions involving olefin x. complexes similarly are characteristic of uncomplexed and complexed olefinic functions. Generally, reactions involving the former are not very different from those observed for free olefins. However, reactions of the latter are altered significantly by π-complex formation. Among the reactions of interest are addition, elimination, and substitution.

Cyclopentadienyl πComplexes. The most significant feature of the reactions of π-cyclopentadienyl complexes in general and ferrocene in particular involves their aromatic nature. The resonance stabilization energy for ferrocene is 210 kj/mol(50 kcal/mol). Ferrocene undergoes a large number of typical ionic aromatic substitution reactions, eg, Friedel-Crafts acylation, alkylation, metalation, sulfonation, and aminomethylation.

Friedel-Crafts Acylation. The acylation of metallocenes proceeds easily. The equimolar reaction of ferrocene and acetyl chloride in the presence of aluminum chloride yields monoacetylferrocene almost

exclusively. When an excess of acetyl chloride and aluminum chloride is used, a mixture of two isomeric diacetylferrocenes is produced. The heteroannular disubstituted derivative 1,1'-diacetylferrocene and the homoannular isomer 1,2-diacetylferrocene are obtained in a ratio of 60:1. The first acetyl group deactivates the π-cyclopentadienyl ligand toward further electrophilic substitution. Thus, the second acetyl group enters the other ring.

Sulfonation. Ferrocene can be sulfonated readily by sulfuric acid or cholrosulfonic acid in acetic anhydride to form ferrocenesulfonic acid and heteroannular disulfonic acid, π-Cyclopentadienylrhenium tricarbonyl can be sulfonated with concentrated sulfuric acid in acetic anhydride; the product is isolated as the p-toluidine salt. Formylation. Ferrocene is formylated with N-methylformanilide in the presence of phosphorus oxychloride. This reaction also is characteristic of highly reactive aromatic rings.

Arylation. The most significant radical substitution reaction of ferrocene is its reaction with aryl diazonium salts giving an arylation product.

Arene-Metal πComplexes.Generally, arene πcomplexes do not undergo the reactions that are characteristic of benzene and its derivatives. However, arene π complexes do undergo a limited number of substitution .addition .expansion, and condensation reactions.

Uses

Catalysis Involving Metal 7t-Complex Intermediates. Many

metal-catalyzed reactions proceed by way of a substrate metal π-complex intermediate. Commercially, the most-significant of these include the polymerization of ethylene,the hydroformylation of olefins yielding

aldehydes , ie , the oxo process (qv ), and the air oxidation of ethylene-producing acetaldehyde(qv) ,ie ,the Wacker process. Polymerization of Olefins. Ziegler-Natta Process. During the 1950s, ethylene was polymerized using a Ziegler-Natta catalyst, ie, a mixture of transition metal halides, eg, titanium halides, and trialkylaluminum (triethylaluminum commonly is used). The use of trialkylaluminum stimulated research into the use of organ metallic compounds in general. It has been determined that the Ziegler-Natta process involves a metal π-complex intermediate. A plausible mechanism for the polymerization can be formulated by applying typical organometallic and coordination reactions.

Oxidation of Olefins. Wacker Process. The oxidation of ethylene exclusively to ace-taldehyde and of other straight-chain olefins to ketones is achieved by the catalytic reaction of ethylene in an aqueous solution by palladium (II) or by oxygen in the presence of palladium( II ) chloride, copper (II)chloride,or iron(III)chloride. Generally, the oxidation of olefins by other metal ions ,eg ,Hg(II) ,Th(III) ,and

Pb( IV ) ,yields glycol derivatives as well as carbonyl products. The mechanism for the oxidation is postulated to include n-o rearrangements. Addition of Carbon Monoxide. Oxo Reaction. The oxo process has been developed extensively to produce primary alcohols by the reduction of the aldehydes which are formed in the process.

Health and Safety Factors

Some metal π complexes are air-sensitive and, therefore, their preparation requires an air-free reaction system. Their toxicity usually is based on the metal; however, organometallic compounds generally exhibit greater toxicities than their corresponding inorganic salts. The alkyl derivatives tend to be more toxic than the aryl complexes, which exhibit toxicities similar to those of the corresponding inorganic compounds.