Structural Studies Using X-ray Absorption Spectroscopy of Intermediates Formed by Reaction of Ozone with Halogeno(dodecaphenylporphyrinato)manganese(III) Derivatives

Abstract

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.

Structural Investigation of High‐Valent Manganese–Salen Complexes by UV/Vis, Raman, XANES, and EXAFS Spectroscopy

“…In the manganese( V ) reference compound 3 the short manganese–nitrogen separation of the nitrido group was found at 1.51 Å, visible as a shoulder at 1.5 Å in the FT‐EXAFS spectrum at the Mn K‐edge (Figure 5 d). This value is in excellent agreement with those found for similar nitridomanganese( V ) complexes 14, 23a, 14, 29. Table 3 summarizes the structural parameters of the reaction products of 1 and 2 with 4‐PPNO ( 4 and 5 , respectively).…”

“…A fivefold coordination is also more probable as the additional backscatterer has a relatively short distance (≈1.8–1.9 Å). In an octahedral coordination the distance is expected to be longer 29. For example, for an octrahedral [(DPPF 20 )Mn IV (O 3 ) 2 ] complex, two MnO ozone bond lengths of 2.09 Å have been found.…”

Selective oxidation passing through η3-ozone intermediates: applications to direct propene epoxidation using molecular oxygen oxidant

“…A search for all organometallic (i.e., structures containing organic ligands bound to a metal atom) h 3 -ozone complexes returned only two studies describing organometallic complexes having a dihedral angle between the O 3 and O-metal-O planes less than 135 . 62,63 The rst was an experimental study of a Mn organometallic complex having a corresponding dihedral angle of 116 . 62 The second was a computational study of small metal complexes (e.g., O 3 Fe(CO) 3 and O 3 Ru(CO) 3 ).…”

Untitled

“…10 EXAFS studies showed that the compound formed when [MnCl(por)] [por 22 = 5,10,15,20-tetrakis(pentafluorophenyl)decaphenylporphyrinate] is exposed to ozone has two O 3 molecules coordinated in unsymmetric bidentate mode. 11 Studies of the new substituted salpn complexes of Mn(IV) of the type [Mn 4 (salpn)O] 2 , and a comparison with literature magnetic and structural data, established that the antiferromagnetic coupling constant J increased in magnitude in approximately linear fashion with the MnOMn bridging angle. This led to the suggestion that the dioxygen-evolving complex (OEC) of Photosystem II (PS II) is more complex than has been supposed.…”