Search

THE reversible reaction of certain cobalt chelates with molecular oxygen can be considered as a simple model of the reversible oxygen binding by haemoglobin and myoglobin1,2. It was recently pointed out3–6 that in suitable experimental conditions synthetic oxygen carriers show a reaction stoichiometry corresponding to one oxygen molecule per metal atom. In both cases the six-coordinated complex with oxygen can be formed by an equatorial tetradentate chelating agent and a donor ligand in the axial position trans to oxygen4. The reversibility of oxygenation seems to depend on a non-polar environment of the metal provided by the aprotic solvents in the simple complexes and by the hydrophobic pocket accommodating the haem in haemoglobin7.

Die Reaktion von Co(II)-Komplexen der Schiffschen Basen mit entsprechenden Phosphinen in Methanol oder Ather bei Raumtemperatur in Gegenwart von Luftsauerstoff und die Behandlung mit wasriger Perchlorsaure fuhrt zu den funffach-koordinierten, quadratisch-pyramidalen Co(III)-Komplexen (I) bzw. zu den Co(III)-Komplexen (II).

INTEREST in the reversible reaction of molecular oxygen with certain chelates of transition metals has recently grown, especially in those synthetic carriers which form one-to-one adducts1-6, because these compounds may be thought of as simple models for the reversible oxygenation of haemproteins.

Novel cobalt(III) complexes of tetradenate chelating agents (chel) and monodentate tertiary phosphines of the type [CoIII(chel)PR3]+ClO4– and [CoIII(chel)PR3(H2O)]+ClO4– are reported. A five-co-ordinate square pyramidal structure is suggested for both types of compound in non-complexing solvents. The thermodynamic trans-effect of PR3 is indicated by the stoicheiometric formation constants of [CoIII(chel)(PR3)(NO2)]0 which can be correlated with σ* Taft constants of the phosphines. The trend of the trans-effect of L in [CoIII(chel)-L(H2O)]+ is found to be PR3 > organic group > OH– > H2O.

Three new di-metallorganic cobalt complexes of the type trans-(Bz)2Co(chel), where Bz is a benzyl group σ-bonded to cobalt atom and chel is an equatorial chelating system constituted by an amino-oximic ligand and its conjugated base, were synthesised. The protonated and the unprotonated ligands interact through an O–H ⋯ O bridge stabilising the entire structure. The complexes differ in the equatorial moiety which is derived from the following ligands: HLN-py=3-[(2-pyridyl)ethylimino]-butan-2-one oxime), HLN-Ph=3-[(2-phenyl)ethylimino]-butan-2-one oxime and the analogous HLN-PhCl=3-[(2-chlorophenyl)ethylimino]-butan-2-one oxime. Two of these compounds, namely those derived from HLN-py and HLN-PhCl were structurally characterised by means X-ray diffractometry. Data reveal that each complex is characterised by the presence of two unusually long cobalt–carbon bonds which are 2.120(4) A (mean value) in complex with HLN-py ligand and 2.119(4) A (mean value) in complex with HLN-PhCl. These data are consistent with a strong mutual trans-influence exerted by one ligand on the other.

The reaction of methylaquobis(dimethylglyoximato)cobalt(III) with 3-pyridylboronic acid in CH2Cl2/CH3OH affords a dinuclear complex containing two methylcobaloxime units held together by two B(OCH3)(3-Py) residues, which is very stable in solution. The dimer is arranged around a crystallographic symmetry center so that the pendant pyridine residue of one moiety coordinates to the Co atom of the other in one axial position, the two pyridine rings being nearly coplanar.

Axial alkyl or aryl groups, o -bonded to the metal atom in some tetradentate Co III complexes, are readily removed by iodine or iodine monochloride; electrophilic and radical attack on the carbon atom both can occur, depending upon the nature of the axial organic group and the chelating agent.

The mechanism of the reaction between the organocobaloximes RCo(DH)2H2O (DH = dimethylglyoximate) and iodine in methanol has been studied and a comparison made with the corresponding reaction in chloroform. The effect of the solvent on the reaction mechanism and the role of nucleophilic assistance are discussed.

The water substitution reactions of the monocation Co(III)(H 2 O) 2 (salen) + [salen = N,N′-ethylene-bis(salicylideneiminate)] were studied in aqueous solution. The nucleophiles used were pyridine, thiourea, morpholine, aniline, imidazole, NCO − , HSO 3 − , S 2 O 3 2− . The kinetics of mono-substitution reactions were found to be very rapid, with second order rate constants nearly independent of the nature of the incoming ligand. A comparison is made with the corresponding reactions of Co(III)hematoporphyrin and some other macrocyclic complexes.