Binuclear cobalt complexes

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

In acetic acid, the reaction of cobalt ions with ROOH proceeds via two channels through the mono- and binuclear cobalt complexes. 

The reaction of binuclear cobalt complexes is somewhat different. For Co2(CO)8 the overall mechanism proposed by Heck24 is... 

DR. DAVID STANBURY (Rice University) With regard to the problem of the electrocatalytic reduction of oxygen, I have attempted to formalize some ideas regarding the constraints of thermodynamics in order to elucidate the probable character of cobalt complexes which may catalyze the oxygen electrode via binuclear peroxo-bridged intermediates. The following gross mechanism is presupposed ... 

A chemical correlation 134,135) has also been achieved between the binuclear series of cobalt complexes and the monomeric ones. This was useful, since the signs of rotation and Cotton effects of the binuclear series show none of the regularities found for the monomers. 

The active site of methionine aminopeptidase contains a binuclear cobalt complex that is required for activity, although a number of divalent metal ions support turnover to varying degrees. X-ray crystallographic studies on the enzyme in complexes with transition state analogs suggests that the binuclear metal cluster serves to stabilize the tetrahedral intermediate in peptide hydrolysis. ... 

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],... 

Binuclear complexes of naphthalene (VIII) and of benzene (IX) with [Os(NH3)5]2+ and [Ru(NH3)5]2+ moieties were reported by Taube et al. (19), in which anti- //-1,2-rj2 3,4- 2) coordination of the arene is present. This bonding mode was first observed by Pasman et al. (20) in the binuclear Rhenium complex X. Wolczanski et al. (21) obtained complex XI from the mononuclear complex [Ta(0—Si Bu3)3 2-(N,C)-pyridine ] and benzene. Each tantalum atom is bound unsymmetrically to one C=C bond of benzene with a weak interaction to a third carbon atom the bonding may be described as a distorted tj3-enyl. In the dinuclear cobalt complex XII, the xylene functions as a bis(enyl)-ligand (12,22). 

However, the similarity in bond strengths of the peroxide linkage to molecular 02, the ease with which the known -peroxo Cobalt complexes liberate 02 (in contrast to /x-oxo bipyridyl Mn dimers) on photolysis, kinetic barriers on ju-oxo to peroxo dimer conversions led Sawyer et al.47 -49) to suggest peroxo binuclear complexes as the most probable intermediates. More studies with model compounds are needed to elucidate this point. Various mechanisms proposed for water oxidations are variations of these two principal types. 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

The binuclear cobalt complex 10.10, however, contains an NN bond that is not parallel, but normal to the bond between the two cobalt atoms and each of the azo N-atoms is bonded to both metal atoms (DeBlois et al., 1988). Some other interesting structures of binuclear complexes with diazenido and related ligands will be mentioned in the context of syntheses, as the bonding of the ligand is not fundamentally different from those summarized by the structures 10.3-10.10. 

Figure 6. EPR spectra of oxygen-containing samples of [sCImCo(II)]2 diporphyrins. (A) Upper trace complex of 1 with 1 atm of O2 at 77° K bottom trace after sample was evacuated at --20°C and recorded at 77°K. (B) Room temperature spectrum of the dioxygen adduct of cobalt complex of 3 after addition of small amount of I2. This is a typical binuclear fx-superoxo dicobalt spectrum. All experiments were carried out in CH2CI2/toluene mixtures.

Step is hydrogen addition to an unsaturated, mononuclear, acyl-cobalt complex have been challenged on the basis of in situ infrared spectroscopic studies/ ° The mechanism has been shown to include a binuclear reaction (Equation 3.7) as the major or only reductive elimination step. ... 

There have been two reports of fluxional dinuclear cobalt complexes. The first concerns the complexes [Co2(CO)4(/it-R2PCH2PR2)2] (R = Me, Ph). These exist in solution as equilibrium mixtures of CO bridged and nonbridged isomers which can be distinguished by variable-temperature NMR. The AG values for the process were 41.5 (R = Me) and 47.0 (R = Ph) kJ mol" in contrast to the very rapid fluxionality of [Co2(CO)g] for which AG is estimated to be ca 27 kJ mol from studies in a hexane matrix. The binuclear cobalt complexes are of type [ (i -Cp)Co 2(/it2- i -QHxSe)] (x = 6, 10, 12). The fluxional process... 

Certain cobalt complexes which form binuclear dioxygen adducts (6) are also known to oxidize phosphine ligands 34, 35). However,... 

The Cobalt Triad.— The first methylene-bridged binuclear cobalt complex [Ck)2O-CO)0i-CH2)(C5Mes)2] has been obtained by treatment of C0CI2 with lithium pentamethylcyclopentadienide and the lithium enolate of acetaldehyde (Li+CHaCHO-). The novel transannular electron-rich olefin (35) reacts with f RhCl(cod) 2] to form the chelating c -dicarbenerhodium(i) salt (36) [X=RhCla-... 

The di-ruthenium complexes Ru2(CO)2(/ -CO)(iM-CCH2)(i7-C5H5)2 and [Ru2(CO)2(/ -CO)( -CCH3)(i -C5H5)2][BF4] offer an ideal opportunity for comparison of the //-methylcarbene and the //-methylcarbyne ligands. The structure of the first binuclear methylene-bridged cobalt complex Coaf j -... 

Two known cobalt complexes of cyclooctatetraene are of the type CgHg(CoC5H5) , with = 1 or 2. In the mononuclear complex COT acts as a 1,5-diene with the eight-membered hydrocarbon in the tub configuration 153,154), while the binuclear complex probably has a central CgHg ligand with a plane tetragonal structure 147,154). 

However, the ligand environment substantially reflects the k value (cf. the data for Fe(P207) and Fe. In the acetic acid the reaction of cobalt ions with ROOH proceeds via two channels through the mono- and binuclear cobalt complexes... 

The oxygen-binding ability of a variety of simple cobalt(II) complexes in aqueous solution has been well studied [21,22]. An important class of such complexes is represented by the amine-cobalt complex and the polyamine chelates. Their oxygen adducts have been characterized as binuclear complexes with a peroxo group bridging the two cobalt atoms, as shown in Eq. (5), using the example of the hexammine-cobalt complex. This simple... 

---