Sepulchrates Cobalt complexes

Cobaltate, tris(l, 2-ethanediamine)-racemization solid state, 466 Cobaltate, tris(oxalato)-racemization solid state, 467 Cobaltates sepulchrates, 22 Cobalt complexes geometric isomerism, 11 hexaammine... 

Cobalt(III) sepulchrate (l)8 and tetrazamacrocyclic complexes of cobalt(II) (2)9 and nickel(II) (3) (6)9-11 catalyze the electroreduction of water to dihydrogen, at potentials ranging from - 0.7 V (complex (1)) to — 1.5 V (complexes (4)-(6)) vs. SCE in aqueous electrolytes, with current efficiencies as high as 95% for complex (4).9 It is noteworthy that the binuclear nickel biscyclam complex (6) is 10 times more active (at pH 7) than the mononuclear nickel cyclam complex (5). This behavior tends to indicate that some cooperativity between the two metal centers occurs in complex (6), as depicted in the possible reaction (Scheme 3) involving a dihydride intermediate.11... 

Sepulchrates, the polyaza cage macrobicycles analogous to the cryptates, were first synthesized in 1977.178 The cobalt(lII) complex shown in Figure 3 (19) is the octaazasepulchrate analog of the [2.2.2] cryptand (Figure 3 16), and is commonly written [Co(sep)]3+ (sep = sepulchrate). 

Figure 7-12. The reaction of [Co(en)3]3+ with formaldehyde and ammonia gives cobalt(m) sepulchrate 7.13. A representation of the complex ion in the solid state is also presented.

Fig. 8 Template synthesis of the cobalt(III) sepulchrate complex, [Coffl(sep)]3+. Step i complex formation and oxidation by air step ii Schiff base condensation step iii nucleophilic attack to the C=N double bond by the deprotonated forms of ammonia...

The corresponding macrobicyclic cobalt(II) [Co(sep)](ZnCl4) H20 complex was prepared by reduction of the [Co(sep)]Cl3 complex with zinc dust in aqueous HCl [95], A similar procedure was used for the synthesis of the dithionate [Co(sep)](S206) salt in the presence of Li2S206 [99], The optically active R- and S-isomers of cobalt sepulchrate were obtained from the optically active parent A-[Co(en)3]Cl3 and A-[Co(en)3]Cl3 complexes [94-95],... 

The platinum(IV), rhodium(III), and iridium(III) sepulchrates, dinitrosarcophaginates, and diaminosarcophaginates have been synthesized in high yields (45-65% for Pt(IV), 40% for Ir(III), and 90-100% for Rh(III)) starting from their tris-ethylenediaminates [94, 156, 157]. The rhodium (III) and iridium(III) complexes were prepared in a similar manner to that for cobalt (III) complexes, except of the elevated temperatures (Rh, 60°C Ir, 90°C) required for the quoted yields. Moreover, if chiral [Rh(en)3] cation was used initially, clathrochelate complexes were obtained in ca 100% chemical and chiral yields, despite the seven centres of chirality [157]. 

The macrobicyclic 3d-metal aza-capped l,3pn-sarcophaginates were studied by voltammetry. In contrast to sepulchrates and sarcophaginates, these complexes should favour to some degree the adoption of low oxidation states (+2 and +1). The oxidation waves are observed in DMF at 1380, 820, 227, 1180, and 1220 mV us SCE, respectively, for manganese, iron, cobalt, nickel, and copper complexes. The dependence of redox potentials on the number of d-electrons is the same as for [MCsar)] " " couple redox potentials. 

When immobilized on polymer surfaces, cage complexes may be utilized for modifications of electrodes. The increase in the electron-transfer rate on such surfaces is governed by two factors a high rate of electron transfer in cobalt clathrochelates and the regular disposition of these complexes on the surface. The properties of immobilized macrobicyclic complexes have been considered in Refs. 94, 410, and 411. Cyclic voltammetry has been used to characterize the incorporation of a range of structurally different d-metal sarcophaginates and sepulchrates into Nafion polymer [412]. 

Closely related to the football ligands are the so-called sepulchrate ligands. One can be formed by the condensation of formaldehyde and ammonia onto the nitrogen atoms of tris(ethylenediamine)cobalt(llI). This results in tris(methylene)amino caps on opposite faces of the coordination octahedron. If the synthesis utilizes one of the (A, A)-enantiomers, the chirality of the complex is retained. Furthermore, the complex may be reduced to the corresponding cobalt(II) cation and reoxidized to co-balt(III) without loss of chirality. This is particularly unusual in that, as we shall see in the following chapter, cobalt(ll) complexes are quite labile in contrast to the stability of cobalt(III) complexes. Once again the extra stability of polydentate complexes is demonstrated. 

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

Another way to achieve indirect chemical reduction in solution is represented by the use of organometallic complexes (e g., cobalt(lll) sepul-chrate trichloride). Often also redox partners and sometimes NAD(P)H are included in the reaction mixtures along with mediators [231, 232]. For example, application of a platinum wire working electrode supported the hydroxylation of lairric acid by recombinant CYP4A1 in the presence of rat CPR and cobalt(lll) sepulchrate trichloride in solution. The product formation rate obtained was comparable to that obtained with NADPH... 

The ions of cobalt(III), rhoditun(III) and iridium(III) are the best templates for assembling octahedral complexes with ligands of the sepulchrate type. To obtain macrocyclic polylactones and polylactams, templates with covalent character bonds, such as Sn, Si, Sb, or B, are used. Copper(I) is the most convenient centre for the synthesis of catenanes. 

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