Theoretical study of chloro-alpha, beta, gamma, delta-tetraphenylporphyrinato cobalt(III) dimer reaction: A reaction path to form pi cation radicals

摘要

Chloro-alpha, beta, gamma, delta-tetraphenylporphyrinato-cobalt(III) (Co(III)TPPCl) is a five-coordinate Co complex ([Co(III) (N)(4)Cl]) having square-pyramidal structure with chlorine atom at the apex. The compound is diamagnetic. Its H-1 NMR spectra in inert solvents show unusual signal broadening at higher temperatures. This was attributed to the partial transformation of the complex to paramagnetic species. To elucidate this specific phenomenon, we proposed a dimer reaction mechanism based on density functional theory calculations with M06-2X/segmented all-electron Gaussian (for Co atom) and 6-31G(d, p) (for C, H, N, Cl atoms) basis sets. The porphyrin dimer, [Co (III)TPPCl](2) (S = 0), is assumed as the important reaction intermediate. The optimized geometry of the dimer is stable compared to two optimized monomers (S = 0) by 22.27 kcal mol(-1) , but is unstable compared to one Co (III)TPPCl2 (S = 1/2) and one Co(II)TPP (S = 1/2) by 0.95 kcal mol(-1) . Using the vibrational frequencies from the geometry optimization, the sum of Co(III)TPPCl2 and Co(II)TPP species, the so-called pi cation radicals, is the overwhelming majority in the theoretical product distribution at 298.15 K and 1 atm. The dimer reaction can produce the paramagnetic species with a potential energy barrier of 1.65 kcal mol(-1).