Carbon dioxide oxygen atom coordination

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

The known carbon dioxide coordination complexes seem to fall into three categories. In the first, carbon dioxide functions as a discrete ligand the only interaction binding the C02 is with the metal itself. There is evidence for two such forms of unsupported coordination through the carbon-oxygen double bond, (21), and through the carbon atom alone, (22). 

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... 

A significant rate enhancement for the C02 insertion process was noted in the presence of alkali metal counterions (Table I), even in the highly coordinating THF solvent. This rate acceleration was not, however, catalytic in alkali metal counterion, since the once formed carboxylate was observed to form a tight ion pair (76, 77) via its uncoordinated oxygen atom with the alkali metal ion, as evinced by infrared spectroscopy in the v(C02) region. That is, the counterion was consumed during the carbon dioxide insertion reaction. 

Some complexes are known in which carbon dioxide is bonded to a transition metal. Aresia era/. [19) isolated the nickel complex (1)in which the CO ligand is coordinated througli the carbon atom and one of the oxygen atoms. Fachinetti et al. [20] found the polymeric cobalt complex (2), in which carbon dioxide is C bonded to cobalt and 0-bonded to two different potassium ions. Lappert isolated tlie remarkably stable niobium complex (3). which was the first authentic CO complex of an early d-block metal [21 j. 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

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