Oxidation of Coordinated CO

Many examples of ligand substitution of stable metal carbonyl compounds have been induced by oxidation of coordinated CO by amine oxides. The prototypical examples are conducted with stoichiometric amounts of MejNO ZHjO. For example, the reaction of FefCOlj with MCjNO (Equation 5.42) forms Fe(CO)4(NMe3) at -30 °C, and the reaction of M(CO) (M = Cr, Mo, and W) with MOjNO forms M(CO)5(NMe3). Most often, this reaction is conducted in the presence of a ligand, such as a phosphine, ttiat displaces the amine from the initial product. The use of MOjNO to extrude CO has been reviewed.  

The ability of MOjNO to remove a CO ligand can be correlated with the v value. This correlation exists because complexes with higher values of v are more electrophilic, as noted in Chapter 2. As a result, complexes with higher v values are more susceptible to attack by the Me NO and oxidation of the CO ligand to CO3. 

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In general, the initially formed formyl, hydroxycarbonyl or nitrito-carbonyl complexes rapidly convert to hydrido or nitrosyl complexes with elimination of CO or C02 however, occasionally intermediates have been isolated. The oxidation of coordinated CO to C02 by amine oxides (Figure 3.15) may be considered a further example. The hydroxycarbonyl example underpins a technologically important process, the water-gas shift equilibrium, involving the catalytic conversion of CO and water into CO, and hydrogen (Figures 3.22, 3.11 and 2.13). 

The application of ESR to the ribonucleotide reductase system indicates that the catalytic intermediate is a Co(I)-species. The appearance of Cob(Il)alamin is probably caused by partial oxidation of the Co(I)-species. In the enzyme bound Co(II)-species the benzimidazole ligand is coordinated, and the corrin ring is bound so tightly that the enzyme produces a unique highly resolved ESR spectrum. 

The M(DLD)3" complexes (M = Co, Rh, Cr) can be isolated as crystalline, x-ray isomorphous salts of the Me3PhN+ cation with two waters of hydration. The diamagnetism and electronic spectra of the Co(lII) complex are consistent with a d6 octahedrally coordinated ion. Cyclic voltametry shows quasi-reversible oxidation at +0.14 V (in CH2C12 versus Ag AgI). A separation between the anodic and cathodic waves of 0.60 V suggests that a reversible rearrangement occurs upon oxidation of the Co(DED)3 anion. An investigation of the oxidation product is currently underway. 

Reaction of coordinated CO with less active nucleophiles can take place when the carbonyl ligand is sufficiently activated. The manner in which this activation occurs is by a reduction in the backbonding interaction (60) that may be achieved when the CO-bound metal ion is in a higher oxidation state. 

There are so far no examples of trigonally coordinated carbon atoms in Fe3C clusters, which is not surprising. However, the intermediacy of such a species is implied in the reported isolation of the ketenylidene cluster Fe3(CO)10(CCO), from the oxidation of [Fe4C(CO),2]2- in the absence of effective reagents. The identification of this novel cluster rests as yet on mass spectroscopic evidence (16). 

As regards the protecting effect, the complex is stable to Lewis acids. Also, no addition of BH3 occurs. As Co2(CO)6 can not coordinate to alkene bonds, selective protection of the triple bond in enyne 137 is possible, and hydroboration or diimide reduction of the double bond can be carried out without attacking the protected alkyne bond to give 138 and 139 [32], Although diphenylacetylene cannot be subjected to smooth Friedcl Crafts reaction on benzene rings, facile /7-acylation of the protected diphenylacetylene 140 can be carried out to give 141 [33], The deprotection can be effected easily by oxidation of coordinated low-valent Co to Co(III), which has no ability to coordinate to alkynes, with CAN, Fe(III) salts, amine /V-oxidc or iodine. 

Binuclear diazene complexes have been prepared in the meticulous and elegant work of Sellman and co-workers. The basis of the preparative route used in these studies is the oxidation of coordinated hydrazine by copper(II)-hydrogen peroxide mixtures] to yield dinitrogen complexes (291, 292). 

The oxidation of coordinated NCS in [Co(NH3)gNCS] + generally leads to both the NHg- and CN-substituted products for a given oxidant the proportion of these products is dependent on the concentrations of both oxidant and acid 168, 580, 670). 

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