Axial coordination reactions with nitrogenous bases and determination of equilibrium constants for zinc tetraarylporphyrins containing four β,β′-fused butano and benzo groups in nonaqueous media

The axial coordination properties of six zinc tetraarylporphyrins with seven different nitrogenous bases were examined in CH₂Cl₂ for derivatives containing four β,β′-fused butano or benzo groups and the equilibrium constants (logK) determined using spectral titration methods. The examined compounds are represented as butano(YPh)₄PorZn and benzo(YPh)₄PorZn, where Por is the porphyrin dianion and Y is a CH₃, H or Cl substituent on the para-position of each meso-phenyl ring of the macrocycle. The initial four-coordinate butano- and benzoporphyrins will axially bind one nitrogenous base to form five-coordinate derivatives in CH₂Cl₂ and this leads to a 4–22 nm red-shift of the Soret and Q bands. The logK values range from 1.98 to 4.69 for butano(YPh)₄PorZn and from 3.42 to 5.36 for benzo(YPh)₄PorZn, with the exact value depending upon the meso and β-substituents of the porphyrin and the conjugate acid dissociation constants (pK_a) of the nitrogenous base.