SADDLE-SHAPED DIOXO-RUTHENIUM(VI) AND DIOXO-OSMIUM(VI) 3,5,7,8,10,12,13,15,17,18,20-DODECAPHENYLPORPHYRIN (H(2)DPP) COMPLEXES - SYNTHESIS, SPECTRAL CHARACTERIZATION AND ALKENE OXIDATION BY [RU-VI(DPP)O-2]

An improved procedure for the preparation of the saddle-distorted porp hyrin 2,3,5,7,8,10,12,13,15, 17,18,20-dodecaphenylporphyrin (H(2)dpp) (yield = 75%) based on the Suzuki cross-coupling reaction between phen ylboronic acid PhB(OH)(2) and 3,17,18-octabromo-5,10,15,20-tetraphenyl porphyrin] has been developed. X-Ray diffraction studies of[M-II(dpp)( CO)(py)] (M = Ru 1 or Os 3) showed that 1 and 3 are isostructural, and the porphyrin macrocycles exhibit severe out-of-plane saddle and ruff le distortions. In both 1 and 3 the pyrrole rings are alternately tilt ed up and down with respect to the least-squares plane of the 25-atom porphyrin core, and the pyrrole carbons experience an average displace ment of 0.769 Angstrom from the least-squares plane compared to 0.78 A ngstrom for free H(2)dpp, whereas the Ru and Os atoms are displaced by 0.1006 and 0.0792 Angstrom from the 25-atom porphyrin core respective ly. The complex [Ru-VI(dpp)O-2] 2, prepared by m-chloroperoxybenzoic a cid oxidation, is an active oxidant for alkene epoxidations. In CH2Cl2 [containing 2%(w/w) pyrazole], styrene, norbornene and cis-stilbene w ere oxidised selectively to their respective epoxides in excellent yie ld. Complete stereoretention was observed for the oxidation of cis-sti lbene, however oxidation of cis-beta-methylstyrene afforded significan t amounts of trans-epoxide suggesting that a carboradical mechanism is operative. The crystal structure of the complex [Ru-IV(dpp)(pz)(2)] ( 5), the product of the stoichiometric alkene oxidations, was determine d. Magnetic susceptibility measurement (mu(eff) = 3.24 mu(B)) suggests the formulation of Ru-IV with two unpaired electrons in its electroni c ground state. The Ru-N (pz) bond distances are 2.022(13) and 2.083(1 2) Angstrom. The reactions of 2 with alkenes in CH2Cl2 (with 2% Hpz) f ollow second-order kinetics: rate = k(1)[2][alkene]. For norbornene an d styrene, the second-order rate constants, k(1), in CH2Cl2 at 25.9 de grees C are (3.79 +/- 0.04) x 10(-3) and (4.78 +/- 0.09) x 10(-3) dm(- 3) mol(-1) s(-1) respectively.