Structure and Reactivity of a Cobalt(I) Phthalaldehyde Complex with Both σ- and π-Bonded Aldehyde Groups

A bidentate phthalaldehyde ligand with both σ and π coordination of the aldehyde groups is found in [(C₅ Me₅ )Co{(C(O)H)₂ C₆ H₄ }] (structure depicted). This complex is the "resting state" of the catalyst in the ring closure of the dialdehyde to give the lactone. Interchange of coordination modes occurs with a barrier of 70 kJ mol^-1 at 35°C. Investigation of other Co^I chelate complexes with a single aldehyde group shows that the coordination mode of the aldehyde is dictated by the nature of the bonding of the other ligating group.