Structural characterization and formation mechanism of sitting-atop (SAT) complexes of 5,10,15,20-tetraphenylporphyrin with divalent metal ions. Structure of the Cu(II)-SAT complex as determined by fluorescent extended x-ray absorption fine structure

The UV-vis absorption spectra and the formation kinetics of sitting-atop (SAT) complexes (M(H(2)tpp)(2+)) of 5,10,15,20-tetraphenylporphyrin (H(2)tpp) with a series of divalent metal ions (M2+ = Mn2+, Fe2+, Co2+, Ni2+ Cu2+, and Zn2+) in acetonitrile have been investigated. The structural characteristics of the SAT complexes for a series of M2+ ions are discussed on the basis of the UV-vis absorption spectra. The structure parameters around Cu2+ in the Cu(II)-SAT complex were determined by a fluorescent EXAFS method: the coordination number is 6 with three kinds of Cu-N interactions having bond lengths of 2.05, 1.98, and 2.32 Angstrom for pyrrolenine nitrogens of H(2)tpp, acetonitrile nitrogens at equatorial sites, and acetonitrile nitrogens at axial sites, respectively. The rate constants at 25 degreesC for the formation of the SAT complex in acetonitrile are as follows: k(f)/mol(-1) kg s(-1) = 3.4 x 10(2) for Mn2+, 0.18 for Co2+,1.6 X 10(-3) for Ni2+, and 61 for Zn2+. The finding;that the variation trends in the;ate constants for a series of M2+ ions for the SAT complex formation and the solvent exchange reaction in various solvents are very similar indicates that the-rate-determining step for the SAT complex formation is the interchange between the coordinating nitrogen of a bound acetonitrile and the pyrrolenine nitrogen of H(2)tpp. The fact that the values of kr are smaller by 4-6,orders of magnitude in comparison with those of the solvent exchange suggests that there is a large energetic loss due to the fast preeqdilibria prior-to the rate-determining step,such as deformation of a porphyrin ring rind outer-sphere encounter formation due to an electrostatic interaction between M2+ and the local negative charge on the pyrrolenine nitrogens. We observed the subsequent formation of the corresponding metalloporphyrins by deprotonation of the SAT complex and oxidation of the M2+ center, of which the dynamic behavior was significantly different for the different M2+ ions.