Synthesis and spectroscopic properties of Co(salen) derivatives containing pendant thioether groups and their dioxygen adducts

Two Co(salen) derivatives, Co(sal-ipsen) and Co(sal-bsen), containing pendant (CH₂)₂S(i-C₃H₇) and (CH₂)₂SC₆H₅ groups were synthesized. Electronic and ESR spectra in methylene chloride show that the former is five-coordinate with pendant thioether coordination at 198 K or below whereas the latter is four-coordinate at 198 K and becomes a mixture of the four- and five-coordinate species at liquid nitrogen temperature. Upon oxygenation at low temperatures, both complexes form dioxygen adducts in which the pendant thioether groups are coordinated to the trans position to dioxygen. Resonance Raman spectra show that Co(sal-ipsen) yields an equilibrium mixture of the 1:1 and 1:2(O₂/ Co) adducts at 190 K while Co(sal-bsen) forms only the 1:1 adduct under similar conditions. These differences between Co(sal-ipsen) and Co(sal-bsen) can be attributed to the variance in basicity of their pendant sulfur atoms.