The coordination and atom transfer chemistry of titanium porphyrin complexes
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The preparation, characterization, and reactivity of a series of ([eta][superscript]2-alkyne)(meso-tetratolylporphyrinato)titanium(II) complexes are described. Also described are intermetal oxygen atom transfer reactions involving titanium(IV) and titanium(III) porphyrin complexes. The [eta][superscript]2-alkyne complexes, (TTP)Ti([eta][superscript]2-R-C≡C-R[superscript]') (where R = R[superscript]' = Me, Et, Ph; R = Me, R[superscript]' = Et), are prepared by treatment of (TTP)TiCl[subscript]2 with LiAlH[subscript]4 in the presence of the alkyne. The structure of (OEP)Ti([eta][superscript]2-Ph-C≡C-Ph) (OEP = octaethylporphryin) was determined by a single-crystal X-ray diffraction. The complex crystallizes in a monoclinic space group C2/c with a = 49.369(7) A, b = 13.734(9) A, c = 36.042(4) A, [beta] = 136.622(7)°, V = 16784(9) A[superscript]3, Z = 16, R = 0.060, R[subscript] w =.062. These compounds undergo simple substitution to displace the alkyne and produce doubly substituted complexes. Where R = R[superscript]' = Et or Me, the coordinated internal alkyne is displaced by a terminal alkyne, R[superscript]''-C≡C-H (R[superscript]'' = Ph, H). The structure of (TTP)Ti(4-picoline)[subscript]2 was determined by single-crystal X-ray diffraction. The complex crystallizes in the triclinic space group P1 with a = 9.764(2) A, b = 10.899(2) A, c = 13.530(2) A, [alpha] = 92.18(2), [beta] = 98.10(2)°, [gamma] = 114.14(2)°, V = 1293.6(4) A, Z = 1, R =.0517, R[subscript] w =.0542. The [eta][superscript]2-alkynes undergo redox reaction with diazo reagents and phosphine chalcogenides. They also abstract a chalcogenide from (TTP)Ti([eta][superscript]2-Ch[subscript]2) where Ch = O, S, Se;Treatment of (TTP)Ti=O with (OEP)Ti-Cl yields intermetal O/Cl exchange. This is formally a one electron redox process mediated by oxygen atom transfer. The equilibrium and forward rate constants at 20°C were determined to be 1.6 ± 0.4 and 2.4 ± 0.3 x 10[superscript]2 M[superscript]-1s[superscript]-1, respectively. Studies were performed to determine temperature, axial ligand, and solvent dependencies. Also, a zero-electron redox process mediated by atom transfer is observed when (TTP)TiCl[subscript]2 is treated with (OEP)Ti=O. The equilibrium constant was determined to be 47 ± 13 at 20°C. The forward rate constant was found to be 5 ± 1 at 20°C. Rate constants at various temperatures were determined to examine temperature dependence.