CHEMISTRY OF A COFACIAL DIRHODIUM DIPORPHYRIN - SYNTHESIS AND REACTIVITY OF RH2DPB

A novel cofacial dirhodium diporphyrin with a metal-metal single bond has been synthesized in two steps from the corresponding free base cofacial porphyrin H4DPB. Rhodium insertion and subsequent photolysis yield the cofacial dirhodium diporphyrin Rh2DPB. This cofacial dirhodium diporphyrin activates H-2 with the assistance of ancillary ligands which attack the rhodium centers externally, weakening the metal-metal bond. The reaction of the dirhodium diporphyrin with H-2 and CO yields the dirhodium diporphyrin dihydride Rh(H)Rh(H)DPB. The H-2 Cleavage reaction seems to occur within the cavity of the cofacial diporphyrin, on the basis of the unusually high T1 values of the resultant hydrides and the predicted regiospecific bonding pattern of additional, bulky ancillary ligands. This reactivity differs from that of a monomeric rhodium porphyrin system which gives a formyl complex. Phosphines have also been employed in reaction with the dirhodium diporphyrin and H-2, resulting in a different type of hydride complex, seemingly one having three-center two-electron bonds.