LOW-SPIN FIVE-COORDINATE Co(II) COMPLEXES OF A 14-MEMBERED MACROCYCLIC TETRAAMINE CONTAINING TWO DEPROTONATED AMIDES—A NEW REVERSIBLE OXYGEN CARRIER

A new 1:1 complex formed between Co(II) and doubly deprotonated 1,4,8,11-tetraazacyclotetradecane-5,7-dione exhibits an ESR spectrum typical of a low-spin square-planar Co(II) complex with the unpaired electron occupying dz2 orbital. Addition of excess imidazole or pyridine yields five-coordinate square-pyramid complexes, which reversibly bind to O2 at −70°C. The rigid planarity of the tetraamine macrocycle is compared with porphyrin chelates using ESR parameters.