organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

, a "constrained geometry complex" (CGC) is a kind of catalyst used for the production of

polyolefin A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...

s such as polyethylene and polypropylene. The catalyst was one of the first major departures from metallocene-based catalysts and ushered in much innovation in the development of new plastics.

Structure

CGC complexes feature a pi-bonded moiety (e.g.

cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...

) linked to one of the other ligands on the same metal centre in such a way that the angle at this metal between the centroid of the pi-system and the additional ligand is smaller than in comparable unbridged complexes. More specifically, the term CGC was used for ansa-bridged cyclopentadienyl amido complexes, although the definition goes far beyond this class of compounds. The term CGC is frequently used in connection with other more or less related ligand systems that may or may not be isolobal and/or isoelectronic with the ansa-bridged cyclopentadienyl amido ligand system. Furthermore, the term is frequently used for related complexes with long ansa-bridges that induce no strain. Ansa-bridged cyclopentadienyl amido complexes are known for the Group 3, 4, 5, 6 and some Group 8 metals, with the Group 4 congeners being the most studied ones.

Applications

Like Group 4 metallocenes, suitable Group 4 CGCs may be activated for the polymerisation of

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...

and alpha-olefins by reaction with co-catalysts, e.g. methylaluminoxane (MAO), tris(pentafluorophenyl)boranes, and trityl borates. The catalytic systems based on CGCs, however, display incorporation of alpha-olefin comonomers to a greater extend than comparable metallocene based systems. This superiority of CGCs in copolymerisation reactions is ascribed to (i) a high accessibility of the reactive centre and (ii) a low tendency of the bulk polymer chain to undergo

chain transfer reaction Chain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule. :P• + XR' → PX + R'• Chain transfer reactions reduce the average molecular weight of the final polymer. Chain ...

s. CGC derived polymers are currently marketed by The Dow Chemical Company as part of their INSITE technology. Beyond the use of CGCs for polymerisation reactions, a number of other transformations catalysed by CGCs (both of Group 3 and 4 metals) have been reported from academic laboratories. These include the application of CGCs as catalysts for hydrogenation of imines, hydroboration of alkenes, carboalumination of alkenes, hydrosilylation of alkenes, hydroamination/cyclisation of alpha ,omega-aminoalkenes and dimerisation of terminal

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

History

The first CGC was reported by Shapiro and Bercaw for a scandium complex. The following year patents were issued to The Dow Chemical Company and

Exxon ExxonMobil Corporation (commonly shortened to Exxon) is an American multinational oil and gas corporation headquartered in Irving, Texas. It is the largest direct descendant of John D. Rockefeller's Standard Oil, and was formed on November 30, ...

for applications in alkene polymerization. and today are made at the billion pound scale.Dow Chemical. Solutionism at Work: 2011 Databook, www.dow.com/financial, Form No. 161-00770, April 2012, page 62.

References

{{Reflist Organometallic chemistry Metal amides