π-Complexes of metalloporphyrins as model intermediates in hydrodemetallation (HDM) catalysis

Transition metal porphyrinate complexes, especially those of nickel and vanadium, are removed from heavy petroleum fractions via hydrodemetallation catalysis (HDM). It has been proposed that HDM of metalloporphyrins proceeds via the partial hydrogenation of the porphyrin followed by demetallation. It is likely that the hydrogenation of the metalloporphyrin proceeds via its complexation to a metal center which transfers hydrogen to the porphyrin. In this paper we discuss recent efforts to simulate this important step. In this study we employ Zn(OEP) (OEP = dianion of octaethylporphyrin) as a model porphyrinate and (arene)Ru²⁺ in the role of an HDM catalyst site. The olive green salt [(cymene)Ru{Zn(OEP)OTf}]OTf (1) was characterized by optical and ¹H and ¹³C NMR spectroscopy, both of which indicate that the cation has C_s symmetry. A crystallographic study shows that the Ru center is π-bonded to one of the four pyrrolide rings, leading to an elongation of one Zn - N bond. It appears that 1 is similar to zinc complexes of N-methyl-porphyrins. Treatment of 1 with HOTf results affords H₄OEP²⁺ via the intermediacy of a species [(cymene)Ru(H_xOEP)]ⁿ⁺. We also prepared the new complex {(C₅Me₅)Ir[Zn(OEP)OTf]}OTf (2) which is fully analogous to 1.