Cobalt-corrinoids

Certain groups of organocobalt(III) complexes have been dealt with in previous reviews. The organo-corrinoids have been mentioned in all reviews on vitamin B, 2 since 1961, when the coenzyme form was identified as an organometallic compound [see, for example, (79, 178) and references therein]. The literature on the corrinoids is too extensive to be treated comprehensively here and for details and references readers are referred to the book on The Inorganic Chemistry of Vitamin B,2 (136)certain other aspects of the organometallic chemistry of cobalt corrinoids are treated elsewhere (137). The pentacyanides were reviewed in 1967 (105), the DMG complexes (cobaloximes) in 1968 (145), and some aspects of salen, BAE, and related complexes in 1970 (17). 

Only one optically active simple organocobalt complex has so far been reported, namely [RCo(DMG)2py], where R is the optically active 1-methylheptyl ligand 55). All cobalt corrinoids, where the ring has been... 

The reactions which are catalyzed by enzymes containing cobalt corrinoids can be classified into three groups. 

Lenhert and Hodgkin (15) revealed with X-ray diffraction techniques that 5 -deoxyadenosylcobalamin (Bi2-coenzyme) contained a cobalt-carbon o-bond (Fig. 3). The discovery of this stable Co—C-tr-bond interested coordination chemists, and the search for methods of synthesizing coen-zyme-Bi2 together with analogous alkyl-cobalt corrinoids from Vitamin B12 was started. In short order the partial chemical synthesis of 5 -de-oxyadenosylcobalamin was worked out in Smith s laboratory (22), and the chemical synthesis of methylcobalamin provided a second B 12-coenzyme which was found to be active in methyl-transfer enzymes (23). A general reaction for the synthesis of alkylcorrinoids is shown in Fig. 4. 

It is well known that the oxidation state of the cobalt atom changes in the enzymatic B12 dependent reactions. The redox chemistry of cobalt corrinoids received a great deal of attention in the seventies. The Co3 +/Co2 + reaction of different corroles was investigated in the past although no mechanistic details have been reported. The resulting chemistry has already been reviewed [11]. 

Table 20. Half wave potentials/V vs SCE of cobalt corrinoid complexes. According to Ref. [2]...

Cobamides consist of a cobalt corrinoid with a pendant nucleotide, which together occupy five of the six positions of an octahedral Co. The most important... 

Although the cobalt corrinoids have been studied extensively in. the last two decades (J), the significance of corrin as an equatorial ligand is not well understood. To characterize coenzyme B12 as an organocobalt derivative, a search for model cobalt complexes that can form a Co-C bond axial to a planar equatorial ligand has been stimulated. Studies on model systems (2-13), particularly on the cobaloxime derivatives (2-7), characterized their respective chemistry, but it is still not easy to establish a general correlation between the structure of an equatorial ligand and the properties of cobalt complex... 

The diamagnetic shielding effects provided by BDHC and corrinoid macrocycles are the same, based on NMR measurements of Co(III)(n-C3H7)(BDHC) and the corresponding n-propylated cobalt corrinoid (17, 21) (Figure 2). 

So far, for the highly oxygen-sensitive Co -forms of cobalt-corrinoids (such as Co -cobalamin, (6)), information from X-ray analysis on their three-dimensional structure is not available. However, in these strongly nucleophihc, highly reduced cobalt complexes, the diamagnetic Co -center (a d metal ion, isoelectronic with a Ni"-ion) presumably is ligated by the corrin ligand only, in a tetracoordinated, nearly square-planar fashion. As a consequence, the Co -forms of complete corrins would have to be base-off, that is, the nucleotide function would not be attached to the central cobalt ion. 

Molecular mechanics and molecular dynamics simulations of porphyrins, metal-loporphyrins, heme proteins, and cobalt corrinoids 02CCR(225)123. 

Our knowledge of the stereochemistry of porphyrins and related tetrapyrrole macrocycles has expanded rapidly since the first reported x-ray structure determination in 1959 The structures of metallotetrapyrrole complexes are of interest because of the common occurrence of this type of macrocycle in biological systems. As is well known, foremost among these are the heme proteins (iron derivatives), the various photosynthetic pigments (magnesium complexes), the vitamin Bn coenzyme (cobalt corrinoids), and coenzyme F430 (nickel corphinoids) of the methanogenic bacteria. 

With a one-electron transfer reaction of a cobalt-corrinoid complex cleavage or formation of a bond to an axial ligand occurs. A reduction is accompanied by an expulsion, and an oxidation by the coordination, of the ligand [90]. The electron transfer step accordingly takes place either in a concerted fashion or in a rapid associated step with coordination or dissociation of the axial Hgand. 

Most living organisms depend on catalysis by the transition element cobalt (1). The biological relevance of cobalt is due, mainly, to vitamin B12 derivatives (2,3), also known as cobalt corrinoids (4). In exceptional cases, enzymes may contain natural noncorrinoid cobalt ions (5), and structural Zn centers have been reconstituted with Co (and other divalent metal) ions, without significant loss of activity (1,4). Only some microorganisms are able to produce B12 derivatives naturally (6,7). Cobalt complexes similar to vitamin B12 (1) may have had fundamental roles in early forms of life on earth (1,8). 

An elegant model had been developed to describe the transfer of a methyl cation from the cobalt-corrinoid Fe-S protein to a proximal nickel center [116], Methyl transfer from a methyl-cobalt species to a Ni(I) complex yields a stable Ni(II)-Me species concomitant with oxidation of a second equivalent of the Ni(I) complex (Eq. 12.9). In this system, the methyl cobalt species is equivalent to a methyl cation source, although mechanistic studies revealed the involvement of methyl radicals formed via homolysis of an initially generated Co(II)-methyl species [117]. 

V. Further Transmethylations From Methyl-Cobalt—Corrinoid Compounds... 

Fig. 4. Summary of reactions of methyl-cobalt-corrinoids involved in the formation of methane and the synthesis of acetate from COi. This figure is diagrammatic only. The CHa-Co-corrinoid-enzyme complexes involved in methane formation and acetate synthesis are difierent from one another.

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