Intermetal Oxygen Atom Transfer Reactions of Titanium Porphyrins: Complete vs Incomplete Atom Transfer. X-ray Structure of (μ-Oxo)bis[(meso-tetra-p-tolylporphyrinato)titaitium(III)]

Treatment of (meso-tetra-p-tolylporphyrinato)titanium(IV) oxide, (TTP)Ti=O, with (octaethylporphyrinato)titanium-(III) chloride, (OEP)Ti-Cl, in toluene-d₈ results in the reversible exchange of axial ligands to form (TTP)Ti-Cl and (OEP)Ti=O. 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)Ti=O in CHC1₃ to produce (TTP)-Ti=O and (OEP)TiCl₂with K = 47 ± 13 and k_f = 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)Ti=O 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) A, β= 117.91(2)°, V = 8869(4) ų, Z = 4, R = 5.88%, and A_w = 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.