DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 1805–1812 1805 Saddle-shaped dioxo-ruthenium(VI) and -osmium(VI) 2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin (H 2 dpp) complexes. Synthesis, spectral characterisation and alkene oxidation by [Ru VI (dpp)O 2 ] † Chun-Jing Liu, a Wing-Yiu Yu, a Shie-Ming Peng, b Thomas C. W. Mak c and Chi-Ming Che * ,a a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong b Department of Chemistry, National Taiwan University, Taipei, Taiwan c Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong An improved procedure for the preparation of the saddle-distorted porphyrin 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 phenyl- boronic 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 H 2 dpp, 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)O 2 ] 2, prepared by m-chloroperoxybenzoic acid oxidation, is an active oxidant for alkene epoxidations. In CH 2 Cl 2 [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 CH 2 Cl 2 (with 2% Hpz) follow second-order kinetics: rate = k 1 [2][alkene]. For norbornene and styrene, the second-order rate constants, k 1 , in CH 2 Cl 2 at 25.9 °C are (3.79 ± 0.04) × 10 -3 and (4.78 ± 0.09) × 10 -3 dm 3 mol -1 s -1 respectively. The role of non-planar conformation in modifying the bio- logical properties and functions of some metalloporphyrins has recently come under rigorous investigations. 1 Out-of- plane distortion is observed in the crystal structures of the bacteriochlorophylls in the photosynthetic reaction centres of Rhodopseudomonas viridis 2 and the light-harvesting antenna bacteriochlorophyll a of Prosthecochloris aestuarii. 3 Variations in the extent of the distortion have been proposed as a possible reason for the unidirectionality of electron transfer. 4 On the other hand, the steric congestion among the peripheral sub- stituents in some synthetic polyhalogenated porphyrins such as octa-β-halogenotetrakis(pentafluorophenyl)porphyrins (X = Cl or Br) has created an out-of-plane distortion of the porphyrin ligands, which is proposed to play a crucial role in enhancing the catalytic activities of the iron() complexes by lowering their oxidation potentials. 5 In this respect, we are interested to examine how the reactivities/catalytic activities of ruthenium/ osmium complexes could be influenced by the conformational distortion of the porphyrin macrocycles. 6 Here we report the first synthesis and characterisation of the saddle-distorted dioxo-ruthenium() and -osmium() complexes of 2,3,5,7,8, 10,12,13,15,17,18,20-dodecaphenylporphyrin (H 2 dpp), and the † Supplementary data available: rate constant concentration dependen- cies, cyclic voltammograms. For direct electronic access see http:// www.rsc.org/suppdata/dt/1998/1805/, otherwise available from BLDSC (No. SUP 57369, 7 pp.) or the RSC Library. See Instructions for Authors, 1998, Issue 1 (http://www.rsc.org/dalton). Non-SI units employed: μ B ≈ 9.27 × 10 -24 J T -1 , atm = 101 325 Pa, cal = 4.184 J. reactivities, as well as the catalytic activities, of the dioxo- ruthenium() complex towards alkene oxidations. Experimental Materials All solvents were purified by the standard procedures prior to use. Benzaldehyde, tert-butylamine, propionic acid and pyrrole were either distilled or purified by standard methods before use. Bromine and m-chloroperoxybenzoic acid (Merck), palladium() chloride, dodecacarbonyltriruthenium and dodecacarbonyltriosmium (Aldrich) were used as received. Tetrakis(triphenylphosphine)palladium(0) was prepared according to the reported procedure. 7 2,3,7,8,12,13,17,18- Octabromo-5,10,15,20-tetraphenylporphyrin, [H 2 obtpp], was obtained by bromination of [Cu II (tpp)] 8 following the pro- cedures described by Bhyrappa and Krishnan. 9 Physical measurements The UV/VIS spectra were acquired on a Perkin-Elmer Lambda 19 spectrophotometer and infrared spectra on a Shimadzu IR- 470 spectrophotometer. Cyclic voltammograms were recorded on a Princeton Applied Research model 273 potentiostat using a glassy carbon electrode and Ag–AgNO 3 (0.1 mol dm -3 in MeCN) as the reference electrode. Gas chromatography was performed on a Hewlett-Packard 5890 Series II gas chromato- graph equipped with a SPB-5 capillary column (30 m) using nitrogen as the carrier gas, a flame ionisation detector and a 3396 Series II integrator. The 1 H and 13 C NMR spectra were