Chemistry of a cofacial dirhodium diporphyrin: synthesis and reactivity of Rh2DPB

Abstract

A novel cofacial dirhodium diporphyrin with a metal-metal single bond has been synthesized in two steps from the corresponding free base cofacial porphyrin H[sub 4]DPB. Rhodium insertion and subsequent photolysis yield the cofacial dirhodium diporphyrin Rh[sub 2]DPB. This cofacial dirhodium diporphyrin activates H[sub 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[sub 2] and CO yields the dirhodium diporphyrin dihydride Rh(H)Rh(H)DPB. The H[sub 2] cleavage reaction seems to occur within the cavity of the cofacial diporphyrin, on the basis of the unusually high T[sub 1] 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[sub 2], resulting in a different type of hydride complex, seemingly one having three-center two-electron bonds. 25 refs., 8 figs., 1 tab.