Sitting-atop complex formation of 2,3,7,8,12,13,17,18-octaethylporphyrin with copper(II) ion in acetonitrile

The reaction of 2,3,7,8,12,13,17,18-octaethylporphyrin (H2OEP) with copper(II) triflate and copper(II) perchlorate in acetonitrile was studied using spectrophotometry. The reaction product is the so-called sitting-atop complex where two pyrrolenine nitrogen atoms of the porphyrin coordinate to the incoming metal ion and two protons on the pyrrole nitrogen atoms still remain. The composition of the sitting-atop complex was determined by the mole ratio method, and it was found that the H2OEP molecule binds two copper(II) ions in the product. The mechanism of the reaction was confirmed to be a series of second-order reactions with the first and second step of the reactions being the outer sphere complex formation between the H2OEP molecule and copper(II) ion and the rate determining sitting-atop complex formation reaction, respectively, based on the mole ratio method. The reaction is relatively fast, and the second-order rate constants for the reaction of H2OEP with copper(II) ion was determined to be k=(3.2±0.3)×106 M−1 s−1 (T=25.0 °C) for the copper(II) triflate and k=(3.0±0.2)×106 M−1 s−1 (T=25.0 °C) for the copper(II) perchlorate under the second-order conditions. The pKa values of the mono- and diprotonated forms of the conjugate acid of several porphyrins including H2OEP were determined by spectrophotometric titration in acetonitrile. The higher reactivity of H2OEP toward copper(II) ion as compared with other porphyrins such as 5,10,15,20-tetraphenylporphyrin was attributed to its higher basicity.