Cobalt complexes, with binuclear

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. 

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 ... 

Recent reviews on the chemistry of metal-dioxygen complexes with particular relevance to cobalt systems include a number dealing with general properties,636,634 1 binuclear superoxo and peroxo complexes,642,643 reversible oxygenation,644-646 complex stability,647,448 catalytic oxidation649,650 and electronic651 and EPR652 spectral properties. 

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 reduction of imBT ligand with NaBH4 in methanol led to the formation of a saturated octaazamacrocyclic amBT ligand that forms binuclear complexes with zinc(II) and copper(II) [199] and mononuclear clathrochelates with manganese, iron, cobalt, nickel, and zinc (II) [203] by treatment of the free ligand with the corresponding metal ion salts. 

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. 

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. 

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... 

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). 

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.

Reaction of binuclear complexes of rhodium (O) with hydrogen to regenerate hydridorhodium (I) complexes under hydroformylation conditions has been established.The extension of these arguments to complexes with triphosphines or polymers with a high density of phosphine anchor-site has been made for complexes of cobalt and rhodium. ... 

Macrobicyclic ligands incorporating heterocyclic subunits. Vogtle (58) and Newkome (59) have successfully prepared cryptands containing respectively one (58) and three (59) pyridine subunits in the macrobicyclic framework (Figure 2.18). The ligand (58) forms complexes with many metal cations, and binuclear cobalt and copper complexes of (59) were isolated. 

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