Reactivity of chloro(phthalocyaninato)iron(III) in pyridine–water

Abstract

The reaction of [Fe(pc)Cl](pc = phthalocyaninate) in water-containing pyridine (py) has been followed by spectrophotometric and conductometric techniques. It is a two-step process: the first step is first-order on [H₂O] and is accompanied by an increase in conductance, the second is kinetically independent of water and does not involve ionic species. The complex [Fe(pc)(py)₂] is formed during both steps and about 70–80% of the initial iron(III) is finally recovered in this form. The infinite-time conductance is quantitatively explained in terms of 100% formation of py·HCI. Addition of PPh₃ has a remarkable effect: when added to py before[Fe(pc)Cl] the second step tends to vanish as the concentration of PPh₃ increases and, for [PPh₃] 4 × 10^–3 mol dm^–3 the overall reaction appears as a one-step process; if the phosphine is added after the first step and before the second two consecutive reactions take place, the first kinetically phosphine-dependent, the second independent of PPh₃. Virtually quantitative oxidation to OPPh₃ occurs when the complex is allowed to react with a slight excess of PPh₃. The observations are explained in terms of a rapid Cl^– release from [Fe(pc)Cl(py)] to give [Fe(pc)(py)₂]⁺, Cl^– which, in turn, undergoes substitution of an axial py by water. A hydroxo species forms which readily dimerises and the dimer disproportionates to give [Fe(pc)(py)₂] and the iron(IV) species [Fe(pc)O(py)]. Considerations based on the instability of analogous oxoporphyrinatoiron(IV) species and on the nearly 75% overall yield of [Fe(pc)(py)₂] lead to the suggestion that the iron(IV) compound is in the form of a µ-peroxo or di-µ-oxo dimer.