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)

Abstract

Treatment 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.