Structural Studies Using X-ray Absorption Spectroscopy of Intermediates Formed by Reaction of Ozone with Halogeno(dodecaphenylporphyrinato)manganese(III) Derivatives
journal contributionposted on 16.02.2000, 00:00 by Vincent Gotte, José Goulon, Chantal Goulon-Ginet, Andrei Rogalev, Calogero R. Natoli, Karine Perié, Jean-Michel Barbe, Roger Guilard
Difference EXAFS and XANES studies at the Mn K-edge are reported which shed light on the mechanisms of activation of the title compounds by ozone. The reaction of ozone with either the chloro- or bromoporphyrinato Mn(III) derivatives yields the same intermediate in which the metal is in a formally tetravalent (IV) state. The coordination scheme includes the four equatorial pyrrolic nitrogen atoms of the porphyrin plus two ozone molecules arranged in a symmetrical configuration with respect to the porphyrin core. This geometry implies that the metal is shifted back into the mean plane of the porphyrin nitrogens. The Mn coordination with each ozone molecule results in two nonequivalent Mn···O distances: R(Mn···O1) = 2.09 ± 0.02 Å; R(Mn···O2) = 2.49 ± 0.05 Å. The second signal is assigned to a much weaker bond with an asymmetric (anharmonic) distribution of the (Mn···O2) distances. Whereas these intermediates have little or no catalytic activity regarding the epoxidation of olefins, catalytically active species were obtained when ozone reacts with the same precursors in an excess of pyridine. In the catalytically active species, the (Mn···O1) signal remains unaffected by the presence of pyridine whereas the (Mn···O2) signal is decreased by a factor of 2. Since there is no evidence of pyridine being coordinated to Mn, we suspect that pyridine is mediating a partial decomposition of one bound ozone molecule to release an active radical species. From Cl K-edge XAS studies performed on the chlorinated species treated with ozone and model compounds, we can definitely rule out the formation of chlorates or hypochlorite anions but both the XANES and EXAFS spectra show that Cl is oxidized during its substitution by ozone at the Mn site and could yield with ozone a covalent moiety Cl−O−O−O similar to the one which is presumed to form in the stratosphere by reaction of chlorine radicals with ozone. Some more speculative would be the formation of dimeric species in which a single Cl atom could bridge two ozone molecules. The radial distribution around Cl would then consist of two oxygen nearest neighbors at R(Cl···O1) = 1.69 ± 0.03 Å and four oxygen atoms at R(Cl···O2) = 2.20 ± 0.02 Å. This model is supported by ab initio MSW simulations of the XANES as well as by the EXAFS spectra.