NOVEL OSMIUM(IV) AND OSMIUM(V) PORPHYRINS - SYNTHESIS, SPECTROSCOPY, AND ELECTROCHEMISTRY

Reduction of [OsVI(P)(O)2] (P = porphyrin dianion) with PPh3, ascorbic acid, or N2H4•H2O in the presence of ROH and RSH yielded [OsIV(P)(OR)2] and [OsIV(P)(SR)2], respectively. [OsIV(OEP)Br2] (H2OEP = octaethylporphyrin) was prepared from [OsVI(OEP)(O)2] and Br2. The dialkoxyosmium(IV) and dibromoosmium(IV) porphyrin complexes are paramagnetic with measured µeff values of 2.27–2.79 µB, which are close to the spin-only values for two unpaired electrons and independent of temperature from –30 to 30 °C. The [OsIV(P)(SR)2] species are diamagnetic, suggesting that the RS- ligand is a poor p-donor. The 1H NMR spectral data for the paramagnetic [OsIV(P)(OR)2] and [OsIV(P)Br2] complexes are discussed. For the [OsIV(p-X-TPP)(OEt)2] system [H2(p-X-TPP) = para-substituted tetraphenylporphyrin], the relative isotopic shifts of the phenyl protons are mainly due to dipolar interactions. A large ring current effect on the methine C-H and pyrrolic methylene protons of [OsIV(OEP)X2] has been found. Except for [OsIV(P)(SR)2], the osmium(IV) porphyrins exhibit reversible Os(IV/III) and Os(V/lV) couples. For [OsIV(P)X2], the redox potentials of Os(IV/III) couples decrease in the order of X = Br > PhO > PhS > MeO, EtO > i-PrO. [OsIV(OEP)(OEt)2] can be oxidized to [OsV(OEP)(OEt)2]+ at a potential of 0.13 V vs Cp2Fe+/0. For the [OsIV(p-X-TPP)(OEt)2] system, the redox potentials of Os(V/IV) couples, which lie between 0.27 and 0.37 V, show a linear free energy relationship with the 4s constants of the para substituents on the four phenyl rings. Oxidation of [OsIV(OEP)(OEt)2] and [OsIV(TPP)(OEt)2] gave the respective [OsV(OEP)(OEt)2]+ and [OsV(TPP)(OEt)2]+, characterized by UV-vis spectroscopy. © 1990, American Chemical Society. All rights reserved.