5,10,15,20-Tetrakis(α,α,α,α-o-pivalamidophenyl) porphinatoiron(II) bearing a covalently linked axial imidazole via m-aminobenzoic acid. Synthesis and influence of imidazole basicity on O₂-binding affinity

A 5,10,15,20-tetrakis(α,α,α,α-o-pivalamidophenyl) porphinatoiron(II) derivative bearing a covalently linked axial imidazole via a rigid benzoyloxy spacer (8c) has been synthesized. The O^- ₂ and CO-binding affinities of 8c (P^O2 _1/2: 213 Torr, P^CO _1/2: 0.17 Torr in toluene at 25°C) were significantly lower than those of a similar analog having a flexible ω-imidazolylalkyl group (9c). Kinetically, the low binding affinities of these gaseous molecules are attributed to the high dissociation rate constants. The stretching frequency of the CO bound to the central Fe(II), however, showed exactly the same value (v_CO: 1964 cm^-1) for both complexes. These results suggested that the intramolecular imidazole coordination was not disordered by the rigid benzoyloxy spacer in 8c. We concluded that the low O^- ₂ and CO-binding affinities of 8c were caused by the low σ-basicity (pK_a) of the imidazole ring.