A STABLE DINITROGEN COMPLEX OF A RUTHENIUM COFACIAL DIPORPHYRIN

An unusually stable, bridged dinitrogen complex of a biphenylene bridged diporphyrin is described. The complex (mu-N2)RU2DPB(1-tert-butyl-5-phenylimidazole)2 (DPB = diporphyrinatobiphenylene tetraanion) is proposed as a model for a dinitrogen reduction catalyst based on cofacial metallodiporphyrins. The differences in binding by bridged and nonbridged ligands to the two coordination sites between the porphyrin rings are discussed. The complex's preparation, structural characterization, ligand substitution reactivity, electrochemistry, and acid-base properties are reported. It is proposed that the complex's unusual stability is due to a chelation of the dinitrogen by the two tethered metal centers. Loss of dinitrogen from the complex occurs only through replacement of dinitrogen by an added ligand. The rate of replacement is dependent on the concentration of the added ligand. Advantages of cofacial diporphyrins as dinitrogen electroreduction catalysts compared to monomeric or untethered binuclear dinitrogen complexes are noted.