Question

Synthesis of a Cobalt Complex that Mimics Oxygen Binding in Hemoglobin

Co(II) salen complex

Explain what happens when the oxygen adduct is dissolved in chloroform. Would the results be different if the reaction was performed in dichloromethane rather than DMSO? What about in acetonitrile?

Answer 1

In this experiment a simple cobalt complex is prepared which also reversibly binds dioxygen. Many complexes of this type have been used as model studies to understand the proteins function.
When Co(salen) ist the red-brown crystals which darkens on exposure to air. and the colour change is due to reversible uptake of dioxygen. H₂salen (I) is a Schiff-base ligand formed by the condensation of two molecules of salicylaldehyde with 1,2-diaminoethane (ethylenediamine).

Different crystalline forms existed depending on the solvent used in the preparation or for recystallisation and that these had varying capacity for oxygenation in the solid state. This variation in oxygenation has been related to the presence of voids in the crystal lattice, effective to allow the passage of oxygen. The X-ray crystal structure determination of theinactive form shows that the structure consists of dimeric units in [Cosalen]2.

The other active form of Cosalen contains dimeric units with the void lattices packing relative to the inactive form. The one particular form has one Co atom directly above the other..In solution, it has been found that, depending on the solvent, in the absence of dioxygen, the cobalt(II) may be four, five or six coordinate.Co(II)-salen is a four-coordinate complex that readily adds ligands to become five or six-coordinate. In a strongly coordinating solvent such as pyridine, both [Co(salen).pyr] and [Co(salen).2pyr] exist, while in CHCl3, the major species appears to be Co(salen).

The polar aprotic solvents are used in this experiment (like acetonitrile,dmso,dichloromethane) which are having negligible to little binding properties to bind to the inactive Co complex which can bind even before the O2 binds to the complex. Thus polar aprotic solvents are used.