Cobalt m Complexes

Cationic cobalt(m) complexes have successfully been applied to the asymmetric carbonyl-ene reaction.15 The yield and enantioselectivity were dependent, to a large extent, upon the counterion, with SbF6- giving the best results (16, Equation (9)). The conditions were general for a variety of alkenes, but only glyoxaldehydes were used as the carbonyl component. 

Figure 2-17. The first step in the SN1 ch mechanism for a kinetically inert cobalt(m) complex containing ammonia ligands involves a deprotonation of the co-ordinated NH3 to generate an amido complex.

Although very dramatic rate enhancements have been observed with labile metal ions such as copper(n) and nickel(n), most studies have involved kinetically inert d6 cobalt(m) complexes. In general, copper(n) complexes have been found to be the most effective catalysts for these reactions. 

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

In conclusion, the hydrolytic and other reactions of co-ordinated amino acid derivatives with nucleophiles may proceed by two major routes. The first involves a moderate acceleration by general acid catalysis of a monodentate TV-bonded ligand, whilst the second may involve very dramatic rate increases (by a factor of a million or so) associated with didentate chelating TV O-bonded ligands. There is little evidence for the widespread involvement of co-ordinated nucleophiles attacking the carbonyl in amino acid derivatives, although some special, and well characterised, examples with cobalt(m) complexes are considered in the next chapter. 

A typical example is seen in the addition of hydrogen cyanide to an imine to yield a cyanoamine (Fig. 4-29). Many of these reactions have been used to best advantage in the synthesis of macrocyclic ligands and complexes, and as such are considered in Chapter 6. A simple example of such a reaction is seen in the addition of HCN to the cobalt(m) complex indicated in Fig. 4-30. The starting complex is also readily prepared by a metal-directed reaction. 

Reactions involving the formation and hydrolysis of phosphate and polyphosphate esters are of vital importance in biological systems in which it is found that magnesium ions are almost invariably implicated. The formation and decomposition of adenosine triphosphate are the fundamental reactions involved in energy storage in living systems. In this context, it is perhaps relevant to note that the hydrolysis of ATP is enhanced, albeit in a very modest manner, by some cobalt(m) complexes. 

It is very often extremely difficult to demonstrate that a metal-co-ordinated hydroxide ion is involved in a particular reaction. Studies of kinetic behaviour provide one of the most powerful tools for the determination of reaction mechanisms. It is not, however, always easy to distinguish between intra- and intermolecular attack of water or hydroxide. The most unambiguous studies have been made with non-labile cobalt(m) complexes, and we will open this discussion with these compounds. 

We saw in Chapter 3 that the hydrolysis of chelated amino acid esters and amides was dramatically accelerated by the nucleophilic attack of external hydroxide ion or water and that cobalt(m) complexes provided an ideal framework for the mechanistic study of these reactions. Some of the earlier studies were concerned with the reactions of the cations [Co(en)2Cl(H2NCH2C02R)]2+, which contained a monodentate amino acid ester. In many respects these proved to be an unfortunate choice in that a number of mechanisms for their hydrolysis may be envisaged. The first involved attack by external hydroxide upon the monodentate A-bonded ester (Fig. 5-62). This process is little accelerated by co-ordination in a monodentate manner. 

A stimulating discussion of the chemistry cobalt(m) complexes of amino acid derivatives... 

Figure 7-2. A representation of the cobalt(m) complex 7.1 formed from the reaction of tris(di-methylglyoximato)cobaltate(in) with BF3 Et20. The three-dimensional structure observed in the crystal lattice of the tetrafluoroborate salt is also shown. The final view emphasises that the co-ordination geometry about the metal centre is distorted towards trigonal prismatic in these complexes.

Figure 7-9. The condensation of the cobalt(m) complex of the hexadentate ligand 7.4, which contains three primary amino groups, with formaldehyde and ammonia, gives the encapsulated complex 7.5. A view of the cation 7.5 as found in the solid state structure of its perchlorate salt is also presented.

As an example, consider the reaction of the cobalt(m) complex of 7.8 with formaldehyde and nitromethane to give 7.9, the cobalt(m) complex of ligand 7.10 (Fig. 7-10). Notice that 7.8 is the same precursor that we needed for the synthesis of 7.7. In the new ligand, 7.10, the cap is formed from three molecules of formaldehyde and one of nitromethane, and the capping atom is now the carbon of the CN02 group derived from the nitromethane. 

The oxidation of the metal complexes of l,10-phenanthroline-5,6-quinone is thought to proceed in a similar manner, with the first step being a benzilic acid rearrangement. Rearrangements of this type may also be followed directly in nickel(u) and cobalt(m) complexes of 2,2 -pyridil. The first step of the reaction involves nucleophilic attack on an O-bonded carbonyl group to form a hydrate, followed by a benzilic acid rearrangement. In this case, the benzilic acid rearrangement products may be isolated as metal complexes (Fig. 8-43). 

Figure 9-20. The cobalt(m) complex of 4-pyridylmethanol may be oxidised by cerium(iv) to 4-py ridinecarbal dehyde...

The mechanism will vary in precise detail according to the metal. In the case of ruthenium complexes, it is quite common to observe a conproportionation and the formation of a ruthenium(iv) intermediate. In other cases, the unavailability of the metal oxidation states precludes reaction. For example, cobalt(m) complexes of cyclam cannot be oxidised to imine species because although a cobalt(ii)/cobalt(m) couple is possible, the cobalt(n) oxidation state is not accessible under oxidative conditions. In the case of metal ions which can undergo two oxidation state changes, alternative mechanisms which do not involve radical species have been suggested. 

With mixed 1 2 metal complexes derived from two distinct unsymmetrical azo compounds four N-a,(5 isomers become possible. The N-a,[ > isomerism has been affirmed by NMR spectroscopy on diamagnetic 1 2 cobalt(m) complexes [35] and X-ray investigations [36],... 

Metal-Containing Tridentate Formazans. Formazans substituted with OH or COOH in the 2-position of the N1- or Ns-aryl group have the same complexing properties as 2-hydroxy- (or carboxy)-2-aminodiarylazo dyes. Similarly, they also form 1 1 complexes with four-coordinate metals, and 1 2 complexes with six-coordinate metals. Being N ligands formazans react more readily with cobalt salts than with chromium salts. The mostly blue 1 2 cobalt(m) complexes of type 34 [72] and the mostly gray-blue complexes of type 35 [73] are known. 

Macrocyclic N-Donors. Glick et al." have proposed that the greater difference in Co—X axial bond length between the cobalt(n) and cobalt(m) complexes of (16) compared with the corresponding complexes of (17) accounts for the unusually slow self-exchange rate of the former. The electronic spectra of the five-co-ordinate cobalt(n) complexes of the macrocycles (18) and (19) have been reported.100... 

Co-ordination of a free superoxide ion by the cobalt(m) complex of aquocobalamin (vitamin B12a) in DMF produces superoxocobalamin.227 The i.r. spectra228 of [(NH3)5CoOCo(NH3)5]+ have been examined laser-Raman spectra of the p-superoxobispentamminecobalt(m) and p-amido-p-superoxo-bis[tetrammine-cobalt(m)] have intense absorptions at ca. 1100 cm "1, attributed to O—O stretching.2 29... 

The application of 59Co n.m.r. spectroscopy to the study of cobalt(m) complexes is beginning to expand.285,286 An n.m.r. study of the stereoisomers of propane-1,2-diamine (pn), en, acac, CF3acac, etc., showed that the chemical shifts do not appear to be related to the absolute configuration, but that larger linewidths are observed with lower symmetry complexes 286 The chemical shifts are larger for ob-lel than for mer-fac isomerism in Co(pn)3 +. 285... 

A novel light-reversible redox system has been discovered381 with glycylglycine it is summarized in Scheme 7. Preparations of mixed cobalt(m) complexes of macrocycles and amino-acids, trans-[Co [ 14]aneN4 (amino-acid)2]3 + and trans-[Co Me4-[14]tetraeneN4](amino-acid)2]3+, have been reported using glycine, S-alanine, S-phenylalanine, and S-leucine.382... 

Cobalt(iv).—In addition to the oxocobaltates(iv) (q.v), cobalt(iv) is present in the trisdithiocarbamates [Co(S2NR2)3]BF4 (R = Me, Et, Pr, or Cy) formed by oxidation of the appropriate cobalt(m) complex in toluene or benzene with BF3. All the complexes are diamagnetic, which is thought to be because of spin-pairing (d5, CoIV) due to formation of associated species.421... 

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