SYNTHESIS AND CHARACTERIZATION OF SMALL MOLECULE ADDUCTS OF COBALT AND IRON PORPHYRINS

The reactions of cobalt and iron porphyrins with the series of diatomic molecule ligands carbon monoxide, nitric oxide and dioxygen have been systematically investigated to determine the influence of an increasing electron population of the ligand (\u27(pi)*) molecular orbital on the geometric and electronic properties of the resultant molecular species.^ The carbon monoxide adduct of tetraphenylporphyrin-cobalt(II) displayed an unusual optical absorption spectrum in the Soret region. The intense single Soret absorption of the parent compound was resolved into two components in a 60:40 ratio. The argument is made that the introduction of a charge transfer transition into the Soret region has resulted in an intensity borrowing from the original Soret absorption.^ The interaction of nitric oxide with tetraphenyl-porphyrincobalt(II) lead to the observation of an extended reactivity for the well characterized mononitrosyl derivative. The existence of a bisnitrosyl derivative has been detected by esr spectroscopy. Analysis of the esr spectrum suggests the complex possesses a highly delocalized unpaired electron which gives rise to hyperfine coupling from the cobalt center, both nitric oxide ligands which lie on either side of the porphyrin plane, and the pyrrole nitrogen atoms of the porphyrin molecule. Stabilization of this transient species through modification of the porphyrin allowed infrared analysis of the nitric oxide absorption region, where only a single stretching frequency was observed, implying dynamic equivalence of the nitric oxide ligands. A molecular orbital diagram which describes the origins of the equivalence of the nitric oxide ligands in this complex has been developed. This bisnitrosyl derivative is capable of further reaction with nitric oxide to produce a diamagnetic complex in which it is postulated that free nitric oxide has reacted with one of the bound nitric oxide ligands to yield coordinated N(,2)O(,2)(\u272-).^ The reaction of nitric oxide with phthalocyanato-iron(II) demonstrated that the same basic chemistry and molecular descriptions are associated with this system as apply to the well known iron porphyrins.^ A new approach to the study of dioxygen adducts of cobalt porphyrins was developed in which an internally coordinating nitrogenous Lewis base is now part of the porphyrin molecule. The interaction of the nitrogen base with the cobalt center could be demonstrated by esr spectroscopy. The dioxygen adducts of these cobalt porphyrins displayed esr and optical absorption spectra which were in direct correspondence with typical dioxygen-nitrogen base complexes of simple cobalt porphyrins. The simultaneous coordination of carbon monoxide and dioxygen on a simple cobalt porphyrin has also been achieved.^ A generalized molecular orbital diagram has been developed which unifies the various experimental observations made on this series of diatomic molecule complexes of square planar cobalt and iron porphyrins.