Synthesis, Characterization, Substitution, and Atom-Transfer Reactions of (η²-Alkyne)(tetratolylporphyrinato)titanium(II). X-ray Structure of trans-Bis(4-picoiine)(tetratolylporphyrinato)titanium(II)

A general preparative method for (tetratolylporphyrinato)titanium(II) η²-acetylene complexes, (TTP)Ti(η²-RCCR′), (R = R′ = CH₃, CH₂CH₃, C₆H₅; R = CH₃, R′ = CH₂CH₃) is described. Displacement of 2-butyne from (TTP)Ti(η²-MeCCMe) with terminal acetylenes allows the preparation of (TTP)Ti(η²-HCCH) and (TTP)Ti(η²-PhCCH). The π complexes undergo simple substitution reactions with pyridine (py) and 4-picoline (pic) to afford the bis(ligand) complexes trans-(TTP)Ti(py)₂ and trans-(TTP)Ti(pic)₂. The structure of the bis(picoline) complex, C₆₆H₅₆N₄Ti, was determined by single-crystal X-ray diffraction (triclinic, P1̅, a = 9.764(2) Å, b = 10.899(2) Å, c = 13.530(2) Å, α = 92.18(2)°, β = 98.10(2)°, γ = 114.14(2)°, V= 1293.6(4) ų, Z = 1, R = 5.2%, and R_w = 5.4%). Crystallographic symmetry requires that the Ti atom resides in the center of the 24 atom porphyrin plane. The Ti-N_pic distance is 2.223(3) Å, and the average Ti-N_pyrrole distance is 2.047(8) Å. The two picoline ligands are coplanar, and the dihedral angle formed by the plane of the picoline rings and the Ti-N₁ vector is 43°. When (η²-PhCCPh)Ti(TTP) is treated with di-p-tolyldiazomethane, a diazo adduct (TTP)TiNNC(C₆H₄CH₃)2 is formed. Atom transfer occurs when (η²-PhCCPh)Ti(TTP) is treated with XPPh₃ (X = S, Se), resulting in a two-electron oxidized product, (TTP)Ti=X, PPh₃, and free PhCsCPh. Treatment of (TTP)Ti(η²-PhCCPh) with elemental sulfur or selenium produces the perchalcogenido complexes (TTP)Ti(S₂) and (TTP)Ti(Se₂). The chalcogenide ligand complexes (TTP)TiS and (TTP)TiSe were also electrochemically characterized for comparison with related derivatives of (P)Ti(S₂) and (P)Ti(Se₂). Each compound undergoes two reversible one-electron reductions which are located at E_1/2 = −1.07 ± 0.01 and 1.47 ± 0.01 V in CH₂C1₂ containing 0.1 M tetra-n-butylammonium Perchlorate. They also undergo two oxidations, the first of which is irreversible, consistent with an electrode reaction involving the axial ligand rather than the porphyrin macrocycle. A comparison of potentials for oxidation of (TTP)TiX and (TPP)Ti(η₂-X₂) indicates a stronger titanium-chalcogen bond in the case of the terminal selenide and sulfide derivatives as compared to the metal-chalcogen bond in the η²-X₂ complexes.