Saddle-shaped dioxo-ruthenium(VI) and -osmium(VI) 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H2dpp) complexes. Synthesis, spectral characterisation and alkene oxidation by [RuVI(dpp)O2]

Abstract

An improved procedure for the preparation of the saddle-distorted porphyrin 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H₂dpp) (yield = 75%) based on the Suzuki cross-coupling reaction between phenylboronic acid PhB(OH)₂ and [2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin] 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 ruffle distortions. In both 1 and 3 the pyrrole rings are alternately tilted up and down with respect to the least-squares plane of the 25-atom porphyrin core, and the pyrrole carbons experience an average displacement of 0.769 Å from the least-squares plane compared to 0.78 Å for free H₂dpp, whereas the Ru and Os atoms are displaced by 0.1006 and 0.0792 Å from the 25-atom porphyrin core respectively. The complex [Ru^VI(dpp)O₂] 2, prepared by m-chloroperoxybenzoic acid oxidation, is an active oxidant for alkene epoxidations. In CH₂Cl₂ [containing 2%(w/w) pyrazole], styrene, norbornene and cis-stilbene were oxidised selectively to their respective epoxides in excellent yield. Complete stereoretention was observed for the oxidation of cis-stilbene, however oxidation of cis-β-methylstyrene afforded significant 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 determined. Magnetic susceptibility measurement (µ_eff = 3.24 µ_B) suggests the formulation of Ru^IV with two unpaired electrons in its electronic ground state. The Ru–N (pz) bond distances are 2.022(13) and 2.083(12) Å. The reactions of 2 with alkenes in CH₂Cl₂ (with 2% Hpz) follow second-order kinetics: rate = k₁[2][alkene]. For norbornene and styrene, the second-order rate constants, k₁, in CH₂Cl₂ at 25.9 °C are (3.79 ± 0.04) × 10^–3 and (4.78 ± 0.09) × 10^–3 dm³ mol^–1 s^–1 respectively.