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(H2tpp)2+) of 5,10,15,20-tetraphenylporphyrin (H2tpp) 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 A for pyrrolenine nitrogens of H2tpp, acetonitrile nitrogens at equatorial sites, and acetonitrile nitrogens at axial sites, respectively. The rate constants at 25 degrees C for the formation of the SAT complex in acetonitrile are as follows: kf/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 rate 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 H2tpp. The fact that the values of kf 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 preequilibria prior to the rate-determining step, such as deformation of a porphyrin ring and 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.