Cobalt protonation/deprotonation

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

The molecules of all the apically and ribbed-carboxymethylated cobalt(III) sarcophaginates (N-protonated, neutral, and carboxy-deprotonated, and the nitrozation product) as well as their hexacoordinate 1,8-bis- and 1,3,8-tris-carboxymethylated analogs adopt the leh configuration [117, 232]. The chirality of the coordination polymeric cobalt(III) l,8-bis(carboxymethylamino)sarco-phaginate / Mn2+, Mn22+, Co +, and Zn + systems was studied by X-ray crystallography [233]. 

Macrobicyclic cobalt compounds satisfy all these requirements. Their additional advantage is that the redox potentials and electron-transfer rates may be varied on introduction of different apical substituents, by changing the charge of the complexes via protonation or deprotonation, or by altering steric factors. This allows one to select the most suitable complexes as ETAs. 

In addition to reactions involving cobalt species, protonation (3) and deprotonation (4), together with surface association of nitrate (8) and sodium ions (9) are taken into account. In doing so one deals with seven standard equilibrium constants. The parameters of basic surface reactions (3,4,8,9) were obtained by separate potentiometric experiments in absence of cobalt ions. 

A reversible deprotonation for a 17-electron hydride complex was shown for the stable cationic cobalt(II) hydride complex [CoH CH3C(CH2PPh2)3 (PEtj)]BPh4, yielding a presumed Co(0) neutral species. Although the latter species could not be isolated and fully characterized, it was shown that it can be reprotonated to yield the Co(II) hydrido species back [40]. Polyhydride complexes have also been shown to readily lose protons. Early examples have been provided by Walton on rhenium compounds, e.g. ReH5(PR3)3 derivatives [101], on the basis of electrochemical investigations. 

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