Intermetal Oxygen Atom Transfer Reactions of Titanium Porphyrins: Complete vs Incomplete Atom Transfer. X-ray Structure of (μ-Oxo)bis[(meso-tetra-p-tolylporphyrinato)titanium(III)]Inorganic Chemistry
Publication VersionPublished Version
AbstractTreatment of (meso-tetra-p-tolylporphyrinato)titanium(IV) oxide, (TTP)TiO, with (octaethylporphyrinato)titanium(III) chloride, (OEP)TiCl, in toluene-d8 results in the reversible exchange of axial ligands to form (TTP)TiCl and (OEP)TiO. The net result is a formal one-electron redox process. This occurs with a second-order rate constant of (2.4 ± 0.2) × 102 M-1 s-1 to form an equilibrium mixture with K = 1.7 ± 0.4 at 20 °C (ΔH⧧ = 10.8 ± 0.4 kcal/mol, ΔS⧧ = −10.7 ± 1.2 cal/mol·K). Use of pivalate in place of chloride on the Ti(III) complex causes no significant change in the rate of this one-electron redox process. In addition, excess chloride only partially inhibits the rate of oxygen transfer with chlorotitanium(III) complexes. A complete kinetic analysis indicates that these redox processes proceed through two parallel pathways, both involving an inner sphere μ-oxo intermediate. Competing associative and dissociative (chloride or pivalate loss) mechanisms are in effect. Reversible oxo/chloro exchange also occurs between (TTP)TiCl2 and (OEP)TiO in CHCl3 to produce (TTP)TiO and (OEP)TiCl2 with K = 47 ± 13 and kf = 5 ± 1 M-1 s-1 at 20 °C (ΔH⧧ = 11 ± 1 kcal/mol, ΔS⧧ = 11 ± 3 cal/mol·K, ΔH° = −3.7 ± 0.8 kcal/mol, and ΔS° = −5 ± 2 cal/mol·K). In contrast, when (TTP)TiO is treated with (TPP)Ti(η2-PhC⋮CPh), incomplete oxygen atom transfer occurs to produce the μ-oxo complex, [(TTP)Ti]2O. X-ray quality crystals of this complex can be prepared by the slow diffusion of O2 into a solution of (TTP)Ti(4-picoline)2. The structure of the μ-oxo complex, C114H90N8OTi2, was determined by single-crystal X-ray diffraction (crystal data: monoclinic, C2/c, a = 32.044(8) Å, b = 16.908(5) Å, c = 18.525(5) Å, β = 117.91(2)°, V = 8869(4) Å3, Z = 4, R = 5.88%, and Rw = 15.38%). Key metrical parameters include a Ti−O distance of 1.7941(8) Å, average Ti−N distances of 2.113(3) Å, and a Ti−O−Ti angle of 170.8(2)°. The Ti atom is displaced 0.66 Å out of the mean porphyrin plane toward the oxo bridge.
Copyright OwnerAmerican Chemical Society
Citation InformationJ. Alan Hays, Catherine L. Day, Victor G. Young and L. Keith Woo. "Intermetal Oxygen Atom Transfer Reactions of Titanium Porphyrins: Complete vs Incomplete Atom Transfer. X-ray Structure of (μ-Oxo)bis[(meso-tetra-p-tolylporphyrinato)titanium(III)]" Inorganic Chemistry Vol. 35 Iss. 26 (1996) p. 7601 - 7607
Available at: http://works.bepress.com/l-woo/82/