Carbon dioxide insertion into metal-oxygen bonds

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

Basic ionic metal oxides or hydroxides such as CaO or KOH absorb CO2 to give metallic carbonates. Many covalent organometallic oxides or alkoxides similarly undergo insertion of carbon dioxide into the metal-oxygen bond to give the corresponding metal carbonates or alkyl metal carbonates ... 

Prior to a discussion of C02 insertion reactions into M-H and M-C bonds it is useful to review some of the known coordination chemistry of carbon dioxide, since activation of COz by metal centers is assumed to be of significance in most of these processes. Carbon dioxide can interact with metal centers by three functionalities. These include the Lewis acid site at carbon (1), the Lewis base sites of the terminal oxygen atoms (2), and the t]2 C=0 bond (3). It is possible as well that a combination... 

In the following section we shall consider the insertion of carbon dioxide into transition nieial carbon, metal hydrogen, metal -oxygen and metal -nitrogen bonds. 

If titanium-nitrogen and titanium-oxygen bonds arc available in the same molecule, CO insertion yields carbamates [104]. If carbon dioxide has the capability (0 insert into a metal-carbon or a metal-nitrogen bond, the insertion into M-N is preferred hafnium dialkyl diarnides react with CO at room temperature to yield the carbamato derivatives HfRjfOjCNR ) [105). 

Stepwise insertion of two moles of carbon dioxide into IrCl(C8Hi4)(PMe3)3 affords the five-membered ring metallacycle 12, in which one molecule of CO2 is bonded at the carbon center, while the other molecule of CO2 is bonded to the metal at oxygen . 

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