Binuclear Complexes of Cobalt III

Submitted by R. DAVIES, MASAYASU MORI.f A. C. SYKES, and J. A. WEILt 

The peroxo complex is brown in color and is diamagnetic. It decomposes slowly in the solid state and is stable in solution only in ca. 7 M ammonia. In acidic solutions it decomposes with the evolution of oxygen  

Impurities which are sometimes present in such samples can be detected most easily by adding acid and identifying the products. The O—0 bond distance has been shown to be 1.47 A., and the Co—0—0 angle is IIS0. A red hydroperoxo complex [(NHs)5Co(02H)Co(NHs)5]H3(S04)4 has been prepared by treating the sulfate salt of the peroxo complex with ice-cold 3 M sulfuric acid.  

The peroxo complex is first prepared as in Sec. A, but the product is not isolated from the final solution. The peroxo 

The chloride salt is prepared here, since it is more soluble than the nitrate or sulfate salts. In the presence of chloride a lower temperature (0°C.) is desirable for the first stage, so that side reactions in which chloropentaamminecobalt(III) is produced are minimized. 

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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... 

Haim, A. and W. K. Wilmarth Binuclear Complex Ions III. Formation of Peroxo and Cyano Bridged Complexes by Oxidation of the Pentacyano Complexes of Cobalt (II). J. Am. Chem. Soc. 83, 509 (1961). 

Mechanisms of the chromium(ii) reduction of binuclear and quadrinuclear cobalt(iii) complexes and the formation of cobalt(iii)-chromium(ill) intermediates (M. R. Hyde,... 

Picosecond spectroscopy sheds some light on the electron transfer behavior of the binuclear ion [(CN)5Fe(II)CNCo(III)(hedta)] ". Excited-state electron transfer takes place on a very short time scale giving iron(III) and cobalt(II) which undergoes a spin change and the back reaction is characterized by a 95-ps lifetime. The coordination of copper(I) to the olefinic chromophore of cobalt(III)-bound amino-alkene ligands results in binuclear complexes (1) which undergo intramolecular electron transfer... 

Barraclough CG, Eawrance GA, Fay PA. 1978. Characterization of binuclear /r-peroxo and /Li-superoxo cobalt(III) amine complexes from Raman spectroscopy. Inorg Chem 17 3317. 

While there have been a considerable number of structural models for these multinuclear zinc enzymes (49), there have only been a few functional models until now. Czamik et al. have reported phosphate hydrolysis with bis(Coni-cyclen) complexes 39 (50) and 40 (51). The flexible binuclear cobalt(III) complex 39 (1 mM) hydrolyzed bis(4-nitro-phenyl)phosphate (BNP-) (0.05 mM) at pH 7 and 25°C with a rate 3.2 times faster than the parent Coni-cyclen (2 mM). The more rigid complex 40 was designed to accommodate inorganic phosphate in the in-temuclear pocket and to prevent formation of an intramolecular ju.-oxo dinuclear complex. The dinuclear cobalt(III) complex 40 (1 mM) indeed hydrolyzed 4-nitrophenyl phosphate (NP2-) (0.025 mM) 10 times faster than Coni-cyclen (2 mM) at pH 7 and 25°C (see Scheme 10). The final product was postulated to be 41 on the basis of 31P NMR analysis. In 40, one cobalt(III) ion probably provides a nucleophilic water molecule, while the second cobalt(III) binds the phosphoryl group in the form of a four-membered ring (see 42). The reaction of the phosphomonoester NP2- can therefore profit from the special placement of the two metal ions. As expected from the weaker interaction of BNP- with cobalt(in), 40 did not show enhanced reactivity toward BNP-. However, in the absence of more quantitative data, a detailed reaction mechanism cannot be drawn. 

The examples of metal-chiral structures are mainly cationic mixed-ligand cobalt(III) complexes of ethylenediamine and its monodimethylphosphine analogue [Co(H2NCH2CH2NH2)3(H2NCH2CH2PMe3)3 J3+. Their synthesis, separation to enantiomers, and establishment of absolute configuration have been carried out for these compounds [276]. The binuclear cobalt(III) complexes 924 possess similar optical properties [277] ... 

Mast and Sykes (15) have recently investigated the kinetics of interconversion reactions of some binuclear cobalt(III) ammines. They have reported both chemical and kinetic evidence for the existence of the diaquo complex [(H20)(NH3)4CoNH2Co(NH3)4(H20)]+ a complex not previously prepared. Garbett and Gillard have reported similar interconversions with ethylenediamine ligands (8) and have assigned optical configurations ( ). 

The mononuclear cobalt complexes are stable and are able to be isolated in both 2+ and 3+ oxidation states. Cyclic voltammetric studies reveal reversible waves for both Co " 2+ and Co + i reduction couples. These redox couples are shifted anodically as the ligand substituents are changed from methyl to phenyl. Electrolytic and cyclic voltammetric studies before and after electrolysis support the idea that the integrity of the complexes is maintained during electrolytic cycles of the 2+/3+ oxidation states. The IpJIpa values of the Co + 2+ couple for the binuclear cobalt complexes are identical to those observed for the oxidation of the analogous iron complex. Attempts to produce the binuclear cobalt(III) species by exhaustive electrolysis have been limited by adsorption of the cobalt(III) complexes on the electrode surface [186, 187],... 

Apart from simple decaammine dimers, the macrobicyclic complex cation (l-amino-8-methyl-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eico-sane)cobalt(III) reacts with [(I o(NH3)5(OS02CF3)](CF3S03)2 to form a binuclear molecule in which the 1-amino group of the macrobicycle acts as the donor to the pentaamminecobalt(III) unit (191). Higher polymers may be accessible for example, we have some preliminary evidence for formation of a trimer with a Co—X—Co—X—Co center from reaction of imns-[Co(cyclam)(OS02CF3)2] with [Co(NH3)5X]" " species (23). 

The effectiveness of the binuclear complex 11 (Fig. 13), with two mononuclear cyclen-cobalt(III) units linked together by an anthra-cenyl spacer (cyclen = 1,4,7,10-tetraazacyclododecane), was compared with the monomer in the hydrolysis of phosphate monoesters (354). The reaction assisted by this rigid binuclear complex, having a phosphate-sized pocket, was 10 times faster than that promoted in the presence of two equivalents of the single cyclen-Co complex. In these experiments the substrate concentration was 25 pM and the total cobalt concentration was 2 mM at 25°C and neutral pH (354). No such cooperativity could be noted using a diester substrate because the pseudo-first-order rate constants were similar for both 11 and the mononuclear complex. With 11 as catalyst, an overall rate enhancement of 10 was achieved over the uncatalyzed hydrolysis of paranitrophenyl phosphate monoester as substrate. 

A binuclear cobalt(III) complex with two cyclic tetrammine (cyclen) ligands effectively catalyzes the hydrolysis of plasmid DNA. Rate enhancements of 10 at a concentration of 1 M are obtained (Hettich, 1997). 

Copper ions catalyze a variety of inorganic redox reactions. The kinetics and mechanisms of some of these reactions were analyzed in detail. Thus, the reduction of V(IV) by Sn(II) and Ge(II) is catalyzed by Cu ions in the presence of high concentrations of Cl. The mechanism involves the reduction of Cu(II) by Sn(II) or by Ge(II) followed by the reduction of V(IV) by Cu(I) (134). These reactions proceed via the inner-sphere mechanism (134). Also the copper-catalyzed reduction of peroxonitrite by sulfite (135), the copper-catalyzed reduction of a Ni(IV) complex by thiols (136), and the reduction of superoxide boimd to binuclear cobalt(III) complexes by thiols (137) and by ascorbate (138) follow analogous inner-sphere mechanisms. Copper ions also catalyze the reduction of peroxide-boimd Cr(IV) by ascorbate(i39). 

A mechanism involving the polarization of the ascorbate ligand by a Cu(II) central ion was proposed (138), though the involvement of Cu(I) cannot be ruled out (139). All these reactions proceed via the inner-sphere mechanism however, the copper-catalyzed reduction of superoxide boimd to a binuclear cobalt(III) complex by 2-aminoethanethiol proceeds via the outer-sphere mechanism (140). This is attributed to the effect of 2-aminoethanethiol as a hgand on the rate constant of the Cu(ll/1) electron self-exchange reaction which is suggested to proceed via the gated mechanism. 

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