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 cof acial porphyrin H4DPB. Rhodium insertion and subsequent photolysis yie ld the cofacial dirhodium diporphyrin Rh2DPB. This cofacial dirhodium diporphyrin activates H-2 with the assistance of ancillary ligands whi ch attack the rhodium centers externally, weakening the metal-metal bo nd. The reaction of the dirhodium diporphyrin with H-2 and CO yields t he dirhodium diporphyrin dihydride Rh(H)Rh(H)DPB. The H-2 Cleavage rea ction seems to occur within the cavity of the cofacial diporphyrin, on the basis of the unusually high T1 values of the resultant hydrides a nd the predicted regiospecific bonding pattern of additional, bulky an cillary ligands. This reactivity differs from that of a monomeric rhod ium porphyrin system which gives a formyl complex. Phosphines have als o been employed in reaction with the dirhodium diporphyrin and H-2, re sulting in a different type of hydride complex, seemingly one having t hree-center two-electron bonds.